Minibot climb rate

[billbo911]

We have not received our FTC parts yet, obviously because the orders went in today :o . I have a couple quick questions and then a result of quick calculations based on assumptions.

What are the weights of the following items?:

NXT controller, Tetrix motor, 12v battery pack, HiTechnic motor controller.

I did some very ballpark calculations based on ideal situations. If we can build a robot weighing less than 5 Lbs, it should be able to climb the pole in less than 7 seconds.

Does that sound right?

[Brandon Holley]

We have not received our FTC parts yet, obviously because the orders went in today :o . I have a couple quick questions and then a result of quick calculations based on assumptions.

What are the weights of the following items?:

NXT controller, Tetrix motor, 12v battery pack, HiTechnic motor controller.

I did some very ballpark calculations based on ideal situations. If we can build a robot weighing less than 5 Lbs, it should be able to climb the pole in less than 7 seconds.

Does that sound right?


What are you basing your calculations on? Overall output of the motors (ie: power)?

-Brando

[billbo911]

What are you basing your calculations on? Overall output of the motors (ie: power)?

-Brando

Yep! I chose to run the calculations close the motor's max efficiency. I also chose to use the largest wheel I could find that appeared to be legal.

[Chris Hibner]

This calculation is actually very easy using the work-energy thorem.

Power = Work / Time

Work = weight * height

Therefore:

Time = (weight * height) / Power


Example:

Motor power: 8.43 W (Tetrix motor)
Efficiency: 0.85
Weight: 5 lb * 4.4545 N/lb = 22.27 N
Height to climb: 2.1 m
assume gearing for peak power.

then:

Time = (22.27 N * 2.1 m) / (8.43 W * 0.85)

Time = 6.53 sec

This is the FASTEST time. If you make the wheel big enough that the torque on the motor causes the motor to move away from the peak power point, the minibot will climb SLOWER, not faster. In other words, bigger wheels aren't always better.

[billbo911]

This calculation is actually very easy using the work-energy thorem.

Power = Work / Time

Work = weight * height

Therefore:

Time = (weight * height) / Power


Example:

Motor power: 8.43 W (Tetrix motor)
Efficiency: 0.85
Weight: 5 lb * 4.4545 N/lb = 22.27 N
Height to climb: 2.1 m
assume gearing for peak power.

then:

Time = (22.27 N * 2.1 m) / (8.43 W * 0.85)

Time = 6.53 sec

This is the FASTEST time. If you make the wheel big enough that the torque on the motor causes the motor to move away from the peak power point, the minibot will climb SLOWER, not faster. In other words, bigger wheels aren't always better.

I looks like your numbers and mine are spot on. I took a different approach, but the result is the same within .1 seconds.:yikes:

Just so others can see where I got my numbers, I ran it through the JVN Calculator. (http://www.chiefdelphi.com/media/papers/2059)

[BornaE]

Seems like everyone is forgetting the possible use of surgical tubing to shoot that bot up the pole much much faster.

Also you can use 2 TETRIX motors.

[billbo911]

Seems like everyone is forgetting the possible use of surgical tubing to shoot that bot up the pole much much faster.

Also you can use 2 TETRIX motors.

Who says we are forgetting?

[Brandon Holley]

Yep! I chose to run the calculations close the motor's max efficiency. I also chose to use the largest wheel I could find that appeared to be legal.

That's how I would it approach it as well. 7 seconds doesn't leave much time for lining up/deployment.... *Must* *Climb* *Faster*

-Brando

[billbo911]

That's how I would it approach it as well. 7 seconds doesn't leave much time for lining up/deployment.... *Must* *Climb* *Faster*

-Brando

Again, you are spot on. With only 10 seconds to deploy the Minibot and get it to climb the pole..... Every millisecond you shave off the climb rate, the better chance you have. Deployment has to be perfect. No slippage can be tolerated in the drive. It all has to be done right if you intend to grab the 30 points!

Think about dragsters. Getting off the line is just as important as how fast you go down the track. It's all about "elapsed time".

[Doug G]

Again, you are spot on. With only 10 seconds to deploy the Minibot and get it to climb the pole..... Every millisecond you shave off the climb rate, the better chance you have. Deployment has to be perfect. No slippage can be tolerated in the drive. It all has to be done right if you intend to grab the 30 points!

Think about dragsters. Getting off the line is just as important as how fast you go down the track. It's all about "elapsed time".

Yesterday we built a simple pole climber out of Vex parts with two 3-wire motors, 4 wheels, old vex microcontroller, and a battery. I'm hoping the FTC motors are more powerful (they're specs say they are) because it takes it about 10 seconds to climb 10 ft.

The minibot challenge will be more difficult than people think this year, mostly due to the timing allowed for it.

[Brandon Holley]

The minibot doesnt have to get there in those 10 seconds of alloted end game time. remember 469's robot last year?
<G68> Scores will be assessed when the MATCH ends and all objects in motion come to rest, or 10 seconds elapses, whichever comes first.

You might be right, but it's still a bit ambiguous. I'll refer to Gary's post (http://www.chiefdelphi.com/forums/showpost.php?p=995572&postcount=6)
in this thread (http://www.chiefdelphi.com/forums/showthread.php?threadid=88636)


I'm gonna need a clarification from the Q&A before I'd feel comfortable committing to getting an extra 10 seconds.

On the other hand, I want our minibot to be the first one to the top every single time, thus I'm not planning on our minibot needing an extra 10 seconds.

-Brando

[JesseK]

I think this shines the light on deployment mechanisms just as much as it shines the light on the climbing minibot. Teams won't know if they're lined up properly until the 10 second mark, which means that a missed deployment is as disasterous as a slow robot.

Good catch with the <G68> rule, though I don't think it effects who wins the race. It would simply dictate whether or not last place always got the 10 points, even after the buzzer.

[RyanCahoon]

During brainstorming, my team used the minibots in the kickoff video as a reasonable estimate, which appeared to climb the pole in 4-5sec.

--Ryan

[Jibs]

Seems like everyone is forgetting the possible use of surgical tubing to shoot that bot up the pole much much faster.

Also you can use 2 TETRIX motors.
So the calculation above is based off one motor?

[TGA Reaper]

Question: What is the estimated ideal weight for the mini bot?

[Doug G]

Question: What is the estimated ideal weight for the mini bot?

The first one we built was about 4 pounds. I think the final version we settle on will be more like 6-7 pounds.

[Hawiian Cadder]

i think my team intends to use surgical tubing to launch the mini-bot to the top in less time than 1 second. last years kicker on our robot only contacted the ball for 3 inches and was able to kick pretty far. i think we will be doing something similar for this.

[TGA Reaper]

The first one we built was about 4 pounds. I think the final version we settle on will be more like 6-7 pounds.
have you tried the heavier weight yet?

[indubitably]

What are the odds that a minibot launched by a slingshot will recieve a penalty?

I feel <G20> might be at risk as the slingshot might tap it after its initial release.

[JesseK]

Hmm, the calculations I'm getting from both of my 'drag race' calculators, adjusted for straight vertical ascent, comes out to just over 5 seconds. I'll investigate...

What are the odds that a minibot launched by a slingshot will recieve a penalty?

I feel <G20> might be at risk as the slingshot might tap it after its initial release.

Please use the other minibot threads for this question...

[billbo911]

The first one we built was about 4 pounds. I think the final version we settle on will be more like 6-7 pounds.

I'm curious Doug, what differences would cause your weight to increase 50-75%?

[mechE131]

I would reccomend someone go and look back at their statics, dynamics, mechatronics and material science textbooks; these calculations are a little on the flimsy side.

If I were a student, I would start with a free body diagram and account for all of the forces involved. Then factor the motor in with all included drivetrain factors.

You must then base a more practical time compared to your calculated "theoretical" time. Factor things in like manufacturing tolerances, material fatigue, material temperature change, etc. and read up on how they would effect your climb rate.

Then, after your minibot is built, compare the two calculated times with the experimental time; the time it actually takes the minibot to assend to the top of the both. Run several trials and make improvements.

[Sean Raia]

i think my team intends to use surgical tubing to launch the mini-bot to the top in less time than 1 second. last years kicker on our robot only contacted the ball for 3 inches and was able to kick pretty far. i think we will be doing something similar for this.

WHAT is this obsession with using surgical tubing to launch your minibot?

You can use springs to launch it just as well if not better, they just have to be part of your main robot.

[MikeE]

I would reccomend someone go and look back at their statics, dynamics, mechatronics and material science textbooks;

This problem is a perfect example of one of the more selfish reasons I enjoy being a mentor on an FRC team: I have a reason to learn about other branches of Engineering outside of my original EE/ECE/CS training.

[My original back-of-the-envelope calculations gave a time of 8s with one motor, but I can now see how some of my initial assumptions were flawed.]

[GaryVoshol]

WHAT is this obsession with using surgical tubing to launch your minibot?

You can use springs to launch it just as well if not better, they just have to be part of your main robot.

Provided the rules allow the HOSTBOT to provide upward impetus to the MINIBOT. That is not yet established (either way) in the rules.

[Mike Schreiber]

Question: What is the estimated ideal weight for the mini bot?

Ideally, I'd say massless...but I don't think even the best teams can pull that off, plus I'm not sure a massless minibot could press the sensor at the top... You want the minibot as light as you can make it while still having the mechanism you need to climb.

[JesseK]

I applied the equations in this (http://www.chiefdelphi.com/media/papers/2405) whitepaper to JVN's Calculator (with some other mods as well) last night and came up with some interesting calculations. Over lunch today I further tested things, and have reached some conclusions:

A 5-lb minibot can reach the top in under 6 seconds
A 10-lb minibot will struggle to reach the top under 10 seconds
A 15-lb minibot probably won't make it to the top under 20 seconds ("probably" because there's a 10% fudge factor here)
There are only 5 gearing/wheel combinations that will even lift a 10-lb robot to the top in under 15 seconds, and all of those are well behind any of the 5-lb robot combinations.

Thus, the lighter the better.

[mechE131]

That model is only good for a situation where the normal force is exactly opposite of the force caused by weight (mass X acceleration due to gravity) and perpendicular to the resulting force (driving forward).

In the case of a "climbing" robot, the most suitable analytical situation would assume the normal force is perpendicular to the pole the robot is latched to.

I would recommend to all students draw a diagram of all the forces on their robots before using any calculators to see if the same situation applies.

[Gary Dillard]

So now that update 1 says we have to use the motors and controller, can someone answer the original question - does anyone have the weight of the required components (battery pack, controller, motor) so we can start doing some real calcs?

[billbo911]

So now that update 1 says we have to use the motors and controller, can someone answer the original question - does anyone have the weight of the required components (battery pack, controller, motor) so we can start doing some real calcs?

Gary,
Which rule says that using the NXT is required?
<R92> does not say that currently.

<R92> The following items are the only permitted materials for use on the MINIBOTS:
A. TETRIX components,
B. no more than two motors (PN W739083),
C. exactly one 12V rechargeable NiMH battery pack identical to those supplied in the FTC kit of parts (PN W739057)
D. No more than one HiTechnic DC motor controllers,
E. No more than one NXT controller with the Bluetooth functionality disabled,....

Both D and E say "No more than one...". It does not say "One and only one".
I will admit, the rule can change. If they do, we will adjust.

[Gary Dillard]

You're right, the update only says motor and battery are required; it doesn't say controller although it says "appropriate circuitry", so what would that be other than through the controller? You can't follow the robot electrical rules and direct wire it because that would require the power panel and fuses which aren't on the allowable material list. So I still need those weights.

[Alan Anderson]

You can't follow the robot electrical rules and direct wire it because that would require the power panel and fuses which aren't on the allowable material list.

My impression is that the ROBOT electrical rules don't apply to the MINIBOT. It has its own rules. This appears to another case of confusion with when the manual should say ROBOT and when it should say HOSTBOT.

I expect great things from the Q&A. Terrible, yes, but great.

[Doug G]

I'm curious Doug, what differences would cause your weight to increase 50-75%?

The first prototype was pretty flimsy. We are looking adding some more structural support and are assuming the Tetrix DC motors and battery are heavier. Once the design is optimized, we'll go through and lighten it up. We're 90% sure now that we'll build our own frame out of aluminum and not use the Tetrix materials. Hard to plan this out without the components (motor and battery). It is sad that they didn't put those in the KOP.

I'm even more convinced now, that the real challenge with the minibot is your deployment system. I have a feeling that at competition, we'll see some great minibots, but not as many great deployment systems.

[billbo911]

I'm even more convinced now, that the real challenge with the minibot is your deployment system. I have a feeling that at competition, we'll see some great minibots, but not as many great deployment systems.

I couldn't agree more!

[JamesBrown]

Correct me if I am wrong but isn't it possible to use the Tetrix continuous rotation servos in addition to (or instead of) the allowed motors per rule <R92> A?

[engine1320]

Where do you order these kits. We have never done anything with FTC. It is a stretch for us to do FRC. This is a challenge we have. Where do I order the 2010 FTC Base Kit. I looked on line but all are Back ordered. Where have you bought these from. Thanks

[JesseK]

For the original question:

Last night we weighed 2 minibot motors, 2 minibot motor mounts, the FTC battery, 1 TETRIX 18" Rail, and (2) 4" wheels. Total weight was between 4 & 4.5 lbs (scale only does 0.5-lb increments, but rounds up).

I will note that we do not plan to use the NXT or the motor controllers. We cannot find specs regarding the NXT's shock rating. Without that data, and given the open-field nature of this year's game, we will not put a $200 device on the bot and call it "reliable".

[Travis Hoffman]

I just received my (free, with hundreds left in stock - I will not be providing details [again] - search for it - I feel like Gary and Eric now) FIRST Choice FTC Mini Kit. Rushing (imagine me doing that) back to one of our labs, I measured the following weights for the FTC DC motor and battery pack:

Motor: 210.35 g
Battery Pack: 599.69 g

What are these strange Copiolian measurements of which I speak?

Motor: 0.464 lb.
Battery Pack: 1.322 lb.

Where do you order these kits. We have never done anything with FTC. It is a stretch for us to do FRC. This is a challenge we have. Where do I order the 2010 FTC Base Kit. I looked on line but all are Back ordered. Where have you bought these from. Thanks

Also, enclosed within my effectively-packaged AndyMark box was a sheet proclaiming a 30% discount on additional *in-stock* TETRIX parts for FRC teams via www.legoeducation.us/FRC (http://www.legoeducation.us/FRC). No blar comments from the peanut gallery - this is one thread that hasn't given me a headache yet.

[Gary Dillard]

ALL RIGHT! Travis is the go-to guy for FTC questions

A couple quick ones -

Weight of the wheels?

Thickness of the gears? (and weight, but I can calculate close enough)

Verify 32 diametral pitch on the gears (is the 120 tooth gear 3.75 PD?)

Thanks

[Travis Hoffman]

ALL RIGHT! Travis is the go-to guy for FTC questions

A couple quick ones -

Weight of the wheels?

Thickness of the gears? (and weight, but I can calculate close enough)

Verify 32 diametral pitch on the gears (is the 120 tooth gear 3.75 PD?)

Thanks

Travis is surely not the only one with an FTC mini kit handy, as at least 800+ other teams have ordered these. ;)

Did you know that the gears were "Assembled in USA"? Probably the reason behind the cost.... Go go gadget globalization.

Mr. Ruler sez that the 120 tooth gear's PD is 3.75", and its thickness is 0.25"-ish.

Mr. Scale sez that the 120 tooth gear weighs 114.71 g / 0.25 lb. (in its thin plastic packaging - didn't want to mess up the shiny finish).

Mr. Scale also sez that the 4" wheel weighs 117.24 g / 0.26 lb.

[Madison]

ALL RIGHT! Travis is the go-to guy for FTC questions

A couple quick ones -

Weight of the wheels?

Thickness of the gears? (and weight, but I can calculate close enough)

Verify 32 diametral pitch on the gears (is the 120 tooth gear 3.75 PD?)

Thanks

The gears are .25" thick. They do have a diametral pitch of 32.

They have a bore of 8mm with four 3.5mm holes on a 16mm bolt circle. There are also eight 8mm holes on a ~59.58mm bolt circle. I think these are decorative. :)

I can corroborate Travis' weight measurements.

[billbo911]

Travis is surely not the only one with an FTC mini kit handy, as at least 800+ other teams have ordered these. ;)

Did you know that the gears were "Assembled in USA"? Probably the reason behind the cost.... Go go gadget globalization.

Mr. Ruler sez that the 120 tooth gear's PD is 3.75", and its thickness is 0.25"-ish.

Mr. Scale sez that the 120 tooth gear weighs 114.71 g / 0.25 lb. (in its thin plastic packaging - didn't want to mess up the shiny finish).

Mr. Scale also sez that the 4" wheel weighs 117.24 g / 0.26 lb.

Did you get a weight on the 80 tooth gear? Or would I be close enough by extrapolating it from the 120 tooth?

By extrapolation I'm guessing the 80 tooth gear is 76.47 g.

[Travis Hoffman]

Did you get a weight on the 80 tooth gear? Or would I be close enough by extrapolating it from the 120 tooth?

By extrapolation I'm guessing the 80 tooth gear is 76.47 g.

I'm not in a position to do any more measurements today. Perhaps another can help out with additional measurements?

Is your math right? Gears are same thickness and have the same number/size of holes.

120T area = pi * ([120/32] / 2) ^ 2 = 11.045 sq. in.
80T area = pi * ([80/32] / 2) ^ 2 = 4.91 sq. in.

Density Ratio = 0.444

80T Weight estimate = 0.444 * 114.71 = 50.93 g

[DonRotolo]

Correct me if I am wrong but isn't it possible to use the Tetrix continuous rotation servos in addition to (or instead of) the allowed motors per rule <R92> A?Interesting, but <R92>B states B. no more than two motors (PN W739083) No more than seems to exclude a third motor.

(Team Update 1 changed A to read "A. TETRIX components that are not in violation of any other rules", and B is an other rule)

[ebarker]

it doesn't say controller although it says "appropriate circuitry", so what would that be other than through the controller?

A computer is nothing more than an arrangement of switches. It can be mathematically proven that if you properly arrange switches, you can build memories, arithmetic logic units, registers, and many more things needed to build a modern computer. That is why you see binary, or base 2 arithmetic in computers. It is their native language.

From the robot rules - "An unlimited number of limit switches and two ordinary household light switches." is allowed on the minibot.

So it is possible to create a "computer" that has a hardwired program to perform the necessary algorithms for machine control. After all Von Neumann said that hardware and software was equivalent ! Really.. He did !

.

[zbanks]

Light switch also seems vague.

http://www.google.com/imgres?imgurl=http://www.electrical-online.com/wp-content/uploads/2010/09/light-switch.jpg&imgrefurl=http://www.electrical-online.com/lightsandswitches/&usg=__NMeq9qV53Ij4lRggfPTxfSQzvyo=&h=1000&w=618&sz=11&hl=en&start=0&zoom=1&tbnid=dN9womQOcb0QzM:&tbnh=117&tbnw=77&prev=/images%3Fq%3Dlight%2Bswitch%26um%3D1%26hl%3Den%26s a%3DN%26biw%3D1024%26bih%3D514%26tbs%3Disch:1&um=1&itbs=1&iact=hc&vpx=216&vpy=123&dur=89&hovh=286&hovw=176&tx=102&ty=225&ei=F3EuTf6yEcP38AaD4uWyCg&oei=F3EuTf6yEcP38AaD4uWyCg&esq=1&page=1&ndsp=13&ved=1t:429,r:7,s:0

http://t3.gstatic.com/images?q=tbn:ANd9GcQ_V8NRaODQDJv6L1781YkWZmPenyruo tUF4nZNCpHRGzXlZwFe

The latter seems more useful, but seems less "standard." How much can we take the switch apart? If all we need is the mechanism, can we tear away at the innards?

[Kevin Sevcik]

You haven't even approached non-standard light switches. I'm busy pondering whether they think three way light switches are "common" or not. Cause it sure would be nice to reverse those motors.

[billbo911]

Is your math right? Gears are same thickness and have the same number/size of holes.

120T area = pi * ([120/32] / 2) ^ 2 = 11.045 sq. in.
80T area = pi * ([80/32] / 2) ^ 2 = 4.91 sq. in.

Density Ratio = 0.444

80T Weight estimate = 0.444 * 114.71 = 50.93 g

Fat finger on the calculater some where. Yours looks accurate..
Thank!

[Travis Hoffman]

Fat finger on the calculater some where. Yours looks accurate..
Thank!

80T Weight = 46.00 g (in packaging) - checked last night

[Gary Dillard]

A computer is nothing more than an arrangement of switches. It can be mathematically proven that if you properly arrange switches, you can build memories, arithmetic logic units, registers, and many more things needed to build a modern computer. That is why you see binary, or base 2 arithmetic in computers. It is their native language.

From the robot rules - "An unlimited number of limit switches and two ordinary household light switches." is allowed on the minibot.

So it is possible to create a "computer" that has a hardwired program to perform the necessary algorithms for machine control. After all Von Neumann said that hardware and software was equivalent ! Really.. He did !

.

If you can do that within the 12 pound weight limit using limit switches and two light switches, I'll bow to you in honor.

[jblay]

Cause it sure would be nice to reverse those motors.

why would you want to be able to run them backwards?

[JB987]

why would you want to be able to run them backwards?

Makes for easy/gentle retrieval if you allow your robot to climb back down to the base to patiently await you!

[Kevin Sevcik]

why would you want to be able to run them backwards?What JB said. If I can put a lightweight system on there to reverse the motors, I don't have to put on a robust retrieval system. Nor do we have to cart a reachin' stick out on the field after every match to get our minibot down. Mind you, we don't have our kit yet. The motors might backdrive quick enough that opening the circuit is adequate. I'd just like to keep my options open.

[jblay]

What JB said. If I can put a lightweight system on there to reverse the motors, I don't have to put on a robust retrieval system. Nor do we have to cart a reachin' stick out on the field after every match to get our minibot down. Mind you, we don't have our kit yet. The motors might backdrive quick enough that opening the circuit is adequate. I'd just like to keep my options open.

The top of the pole is 122 inches off the ground. your mini bot is twelve inches so the bottom of your minibot is 112 inches off the ground. the cylinder is 12 inches off the ground so if you stand on it you only need to reach 100 inches in the air or 8 feet 4 inches. so long as you have someone on your driveteam than can reach that high you will be fine.

[JesseK]

The top of the pole is 122 inches off the ground. your mini bot is twelve inches so the bottom of your minibot is 112 inches off the ground. the cylinder is 12 inches off the ground so if you stand on it you only need to reach 100 inches in the air or 8 feet 4 inches. so long as you have someone on your driveteam than can reach that high you will be fine.

I'm not so sure they'll allow us to stand on the base to get the MINIBOTS down. Interesting nonetheless.

[Mark McLeod]

I'm not so sure they'll allow us to stand on the base to get the MINIBOTS down. Interesting nonetheless.
You don't think the Field Staff will be using them as benches during long timeouts?

[mechE131]

I'm not so sure they'll allow us to stand on the base to get the MINIBOTS down. Interesting nonetheless.

Yeah, standing on the base will probably get a team member a good dope slap from a ref.

[AndrewMcRadical]

What JB said. If I can put a lightweight system on there to reverse the motors, I don't have to put on a robust retrieval system. Nor do we have to cart a reachin' stick out on the field after every match to get our minibot down. Mind you, we don't have our kit yet. The motors might backdrive quick enough that opening the circuit is adequate. I'd just like to keep my options open.
Coming from someone who is also on an FTC team, if you build your minibot light enough to be competitive(~5lb), I doubt the motors will backdrive at all.

[vamfun]

I applied the equations in this (http://www.chiefdelphi.com/media/papers/2405) whitepaper to JVN's Calculator (with some other mods as well) last night and came up with some interesting calculations.

In case anyone is interested, I posted my modification of this program for the Minibot climb here (http://vamfun.wordpress.com/2011/01/16/minibot-model/).

Added the FTC motor specs, some efficiency losses and Normal force effects.

Robodox 599 Numerology 3x3x3x3x3.3 minibot @ .8 efficiency

(wheel_dia_in) x (gear_ratio) x (time_to_climb) x (speed_fps)x(weight_lbs) :)

[DonRotolo]

You haven't even approached non-standard light switches. I'm busy pondering whether they think three way light switches are "common" or not. Cause it sure would be nice to reverse those motors.
Q&A defined 'light switches' at the kind you'd find at Home Depot or Lowes,. meant to switch lights in the house on or off,, and mounted in a standard wall electrical box.

My house has several 3-way switches so mounted, I bought them at Lowes, so they're OK to use.

[tagayoff]

A house "4 way" switch will reverse the motors...

[Kevin Sevcik]

A house "4 way" switch will reverse the motors...
*blinks*
I hadn't even heard of four way switches. That's even better!

[tagayoff]

4 way switches are use in rooms where lights are controlled from more than two places at once. The first two locations use 3 way's and the 3rd "or more" use 4 way's. The switch has 4 terminals on it . In one position it connects across. In the other position in cross the terminals. Great for reversing DC motors.

[Duke461]

Our minibot got to the top of the pole today in 4 seconds. I'd love to share the video, but its "Top secret Confidential" stuff right now :yikes:

[JB987]

4 way switches are use in rooms where lights are controlled from more than two places at once. The first two locations use 3 way's and the 3rd "or more" use 4 way's. The switch has 4 terminals on it . In one position it connects across. In the other position in cross the terminals. Great for reversing DC motors.

Food for thought...4 way switches will indeed help you reverse your motors but there is a cost. Even stripped down they are relatively heavy and also one must consider the rate of the descent and the endpoint velocity as your bot strikes the base platform. Not sure what the effect on the field element will be. Has anyone tested a motor aided return to the base platform?

[obsidianjeff]

Food for thought...4 way switches will indeed help you reverse your motors but there is a cost. Even stripped down they are relatively heavy and also one must consider the rate of the descent and the endpoint velocity as your bot strikes the base platform. Not sure what the effect on the field element will be. Has anyone tested a motor aided return to the base platform?

wouldn't a simple resistor on the switch be enough to make it drive slower in one direction but not effect the upwards movement?

[JesseK]

wouldn't a simple resistor on the switch be enough to make it drive slower in one direction but not effect the upwards movement?

If resistors & diodes were allowed, perhaps when combined with a diode this would work fine.

[obsidianjeff]

gah! of course in my haste to develop a simple easy solution i forget to check the list of acceptable parts... ::rtm::

[vamfun]

If resistors & diodes were allowed, perhaps when combined with a diode this would work fine.

If you are intent on driving it down, you can switch the motors to be in series and this will at least cut the voltage in half for each motor which effectively cuts the power by 1/4 th.

But I can't imagine that the drag torque of these motors when properly geared for speed will be enough to hold the bot up the pole with no power. I don't have a number for the FTC motors but the vex 393 motors have about 1 inlb of drag torque or about 7 % of the Max torque. If the FTC are about the same % then you would only get about 21 oz in of drag per motor or a total of 42 oz in (2.6 inlb) for two motors. . The load torque for a 4 lb minibot would be 4*radius*gear_ratio which for a typical mini bot is 4*2*2 = 8 inlb. So I think the minibot should come down by its self with my assumptions.

[tagayoff]

Food for thought...4 way switches will indeed help you reverse your motors but there is a cost. Even stripped down they are relatively heavy and also one must consider the rate of the descent and the endpoint velocity as your bot strikes the base platform. Not sure what the effect on the field element will be. Has anyone tested a motor aided return to the base platform?

I was thinking,
On->forward...contact->up... contact->off...gravity-> down.
on/off .12 lb 4 way .18 lb

[jnapp18]

4.5 second climb on first proto... time to lose some weight...

[Greg McKaskle]

Perhaps it would also be useful to monitor wheel encoders as well. If the wheels travel 12 ft climbing a 10 ft pole, your problem isn't weight. Viewing the velocity and other derived values per wheel would be pretty interesting to evaluate different prototypes.

Greg McKaskle

[perry3018]

are you allowed to use the surgical tubing on the mini bot????

[billbo911]

Perhaps it would also be useful to monitor wheel encoders as well. If the wheels travel 12 ft climbing a 10 ft pole, your problem isn't weight. Viewing the velocity and other derived values per wheel would be pretty interesting to evaluate different prototypes.

Greg McKaskle

You are absolutly correct that this information would be very usefull. The problem is, if you added the encoders and an NXT to capture that information, you would be radically changing the characteristics of the minibot. The data would not be relative to the actual performance of the bot in it's final configuration.

[tagayoff]

<R92> The following items are the only permitted materials for use on the MINIBOTS:
U. Rubber bands,
V. Surgical tubing,

[Chris is me]

Use encoders with a 15 foot cable and leave the NXT at the ground for testing purposes. Minimal impact on the robot, enough to get good data.

[billbo911]

Use encoders with a 15 foot cable and leave the NXT at the ground for testing purposes. Minimal impact on the robot, enough to get good data.

Good suggestion!
Alternatively, shorter wires and move the NXT along side by hand. Either way, any data you get is better than guessing.

[bearbot]

Our mini bot got to the top of the pole today in 4 seconds. I'd love to share the video, but its "Top secret Confidential" stuff right now :yikes:

wow congrats that quick ha ha top secret ::rtm:: what about gracious professionalism just kidding just wondering what parts you are using

[Aren_Hill]

wow congrats that quick ha ha top secret ::rtm:: what about gracious professionalism just kidding just wondering what parts you are using

Nothing in GP says a team has to publicly share designs, keeping a competitive edge will be especially crucial in regards to the minibot as many will be easily within range of being duplicated.

[budzilla]

I have a question regarding the minibot and this seems to be the best place to ask it. Sorry if that's not correct.

R92 specifically mentions the following 2 adhesives.
G. Polycarbonate glue
R. PVC cement or cleaner

Does this really mean nothing else can be used? I'm thinking that exactly what that means, but I would like some more input.

If we indeed can only use these two adhesives and nothing else... In my limited experience, the PVC cement is not very useful on anything but PVC and/or CPVC. The "Polycarbonate glue" seems have different varieties available and some of those look like they might be useful for attaching other items on the minibot list to each other. However, it sure would be nice to just whip out the contact cement or silicone if it's kosher to do so.

Thanks for any input in advance.

[Duke461]

wow congrats that quick ha ha top secret ::rtm:: what about gracious professionalism just kidding just wondering what parts you are using

I will give one hint: Surgical Tubing is being used. And yes, it does comply with the rules :yikes:

[billbo911]

I have a question regarding the minibot and this seems to be the best place to ask it. Sorry if that's not correct.

R92 specifically mentions the following 2 adhesives.
G. Polycarbonate glue
R. PVC cement or cleaner

Does this really mean nothing else can be used? I'm thinking that exactly what that means, but I would like some more input.

If we indeed can only use these two adhesives and nothing else... In my limited experience, the PVC cement is not very useful on anything but PVC and/or CPVC. The "Polycarbonate glue" seems have different varieties available and some of those look like they might be useful for attaching other items on the minibot list to each other. However, it sure would be nice to just whip out the contact cement or silicone if it's kosher to do so.

Thanks for any input in advance.

Welcome to Chiefdelphi!

As you will see mentioned in many threads here, the official answers to rules questions can only be provided through the Q&A system. Additionally, refinements to the rules come out in the Team Updates as well. All we can do here is give our opinions based on the rules.

Strictly speaking, I do not believe <R92> allows the use of any glue other than the two mentioned. One other place to check is in the Tetrix catalog (http://www.tetrixrobotics.com/Building_System/Downloads/default.aspx?moid=533) linked in Team Update #3. If it is not listed in either place, then it most likely will not be allowed.

[EagleEngineer]

Question: What is the estimated ideal weight for the mini bot?

less than 5 pounds, so somewhere between 1-4 pounds will be good.:cool:

[budzilla]

Strictly speaking, I do not believe <R92> allows the use of any glue other than the two mentioned. One other place to check is in the Tetrix catalog (http://www.tetrixrobotics.com/Building_System/Downloads/default.aspx?moid=533) linked in Team Update #3. If it is not listed in either place, then it most likely will not be allowed.

I agree. BTW, thanks for the welcome. I'm a long time lurker, 1st time poster.

You are welcome for all the fish and thank you for all the input.

[Patrick Seeney]

But wouldn't surgical tubing that was wound up before deployment be considered stored energy? Otherwise you could just have a motor running and solely rely on the surgical tubing to launch you up the pole.

[Chris is me]

But wouldn't surgical tubing that was wound up before deployment be considered stored energy? Otherwise you could just have a motor running and solely rely on the surgical tubing to launch you up the pole.

Surgical tubing that does not contribute to the vertical motion of the robot is allowed. The best exampe I can think of is a surgical tubing tied gate latch.

[bspymaster]

huh?

[Patrick Seeney]

In that case I get it, it is going to come down to the deployment. Our current version of the mini bot can get up the pole in under 4 seconds, but it is still in the early testing stages. We have talked using surgical tubing in the delivery method...but like I said we are still working on which of our two gripper prototypes we want to proceed with.

[Frenchie461]

Our minibot got to the top of the pole today in 4 seconds. I'd love to share the video, but its "Top secret Confidential" stuff right now :yikes:

we're now down to the 3-3.5 second range, and I think we could get under three with some more design tweaks and lightening. The design is still top secret, but I will tell you that it uses parts in intresting ways...:D

[JesseK]

I'm calling shenanigans on a minibot under 3 seconds. Reliably deploying and shutting off while making the climb in under 3.5 seconds is even improbable @ 14V, perfect peak power, and the minimum 3.5lbs it takes for the battery, motors, switches, [custom] wheels, and just 1 ft. of attachment structure held together with magic (or rivets...).

[Doug G]

I'm calling shenanigans on a minibot under 3 seconds. Reliably deploying and shutting off while making the climb in under 3.5 seconds is even improbable @ 14V, perfect peak power, and the minimum 3.5lbs it takes for the battery, motors, switches, [custom] wheels, and just 1 ft. of attachment structure held together with magic (or rivets...).

3 seconds is doable. I don't think Frenchie is implying that the deployment time is included in their claim.

[perry3018]

Is the mini bot allowed to have springs on it

[yp673mp]

We just got 2.93 seconds with our mini bot...in heavy mode. We haven't even started shaving weight off.

[big1boom]

our minibot is going up in about 2.8 seconds. Not including deployment time, so it is doable, but deployment is where the race will be won.

[acrease77]

theres no springs allowed on the minibot or for deployment--team update #1 said no stored energy in deployment.
and how did you ever get it in under 3 seconds if not with a spring/launcher thing??

[artdutra04]

I'm calling shenanigans on a minibot under 3 seconds. Reliably deploying and shutting off while making the climb in under 3.5 seconds is even improbable @ 14V, perfect peak power, and the minimum 3.5lbs it takes for the battery, motors, switches, [custom] wheels, and just 1 ft. of attachment structure held together with magic (or rivets...).The TETRIX motors are actually about 16W at peak power, not 8.43W as the spec sheet suggests. Surprisingly fast times are feasible.

[JB987]

Getting similar times on direct drive prototypes and 2-1 gear ratio prototypes... we are assuming similar times is due to gearing inefficiency and the added weight of all the extra gears, spacers, etc... The question is, how much better can we expect if we lighten up the Tetrix gears and improve gear alignment???

[billbo911]

Getting similar times on direct drive prototypes and 2-1 gear ratio prototypes... we are assuming similar times is due to gearing inefficiency and the added weight of all the extra gears, spacers, etc... The question is, how much better can we expect if we lighten up the Tetrix gears and improve gear alignment???

Strictly based on calculations and the specifications available, the difference in times when directly driven and when a 1:2 gearing increase is added is only 1 second.

The calculation with the gearing did not include the additional weight of the extra hardware and gears requires to add the gears.
When I calculated with the weight added, I only see a difference of .8 seconds.

Therefore, I am not surprised in the least that your times are very close. Add in a factor of human error if manually timing, and you could easily have a dead heat.

So, would lightening the hardware help? You bet!

[chemipanda]

I know this is a bit off topic but what size wire do you happen to use for wiring the motors?

thanks

[Chris is me]

So... we saw smoke when we made our bare bones minibot try to climb, with a modest overgear (0.5:1). These things can't be stalled for even a half second, apparently? I have no idea what smoked or whether or not the motors still worked, but if the motors are that delicate...

[E2207]

Surgical tubing that does not contribute to the vertical motion of the robot is allowed. The best exampe I can think of is a surgical tubing tied gate latch.

Does rule include using surgical tubing as a drive belt and a friction surface on the wheel?

[boomergeek]

So... we saw smoke when we made our bare bones minibot try to climb, with a modest overgear (0.5:1). These things can't be stalled for even a half second, apparently? I have no idea what smoked or whether or not the motors still worked, but if the motors are that delicate...

We smoked a motor hitting the top of the pole and not shutting off...probably a maximum of about 3 seconds of full voltage on a jammed motor.

This happened on the very first occurrence.

My suggestion is make sure you put in the motor cutoffs at the end of the race or make sure your wheel contacts are slippery enough to keep that motor turning.

I'm glad we ordered two extra motors. I expect many teams will fry motors before getting a working minibot. ::ouch::

---

BTW, our prototype uses solder on the motor contacts and uses standard tab and receptacle electrical connectors- not wire nuts.

If they didn't want us to use solder, then it would have been nice for them to supply Tetrix "DC Motor power cables". http://www.legoeducation.us/store/detail.aspx?ID=1629

[Chris is me]

Does rule include using surgical tubing as a drive belt and a friction surface on the wheel?

Nope - I meant "surgical tubing to store energy" rather than "surgical tubing". If the tubing isn't storing energy there's no rule broken.

[Bochek]

as a referance, Today, team 2200 made it to the top (no top plate yet) of the pole in 2.6 seconds, with a minibot that weighed 4.9lbs. and we still have LOTS of room to save weight.

[Bruceb]

What type of gearing are most of you advanced minibotters using?
Bruce

[klmx30302]

What type of gearing are most of you advanced minibotters using?
Bruce

I would also like to know what gearing others are using. Our minibot is taking 4-4.5 seconds to climb the pole with no extra gearing.

[rbaker8734]

I looks like your numbers and mine are spot on. I took a different approach, but the result is the same within .1 seconds.:yikes:

Just so others can see where I got my numbers, I ran it through the JVN Calculator. (http://www.chiefdelphi.com/media/papers/2059)

I think that you are forgetting about torque!!!

[Gary Dillard]

Nor do we have to cart a reachin' stick out on the field after every match to get our minibot down.

It would appear that a "reachin' stick" is not allowed per the rules

http://forums.usfirst.org/showthread.php?t=16331

You can't carry anything functional into the arena except the operator console.

So now, getting the robot to come down off of the tower seems like a pretty important part of the design.

[Aren_Hill]

You can't carry anything functional into the arena except the operator console.


So you have a reaching stick on a tether attached to the operator console :rolleyes:

In all seriousness, i think last year 1114 had a special fixture device to let them pull the bot off the post.
I think in 2007 648 had a hand crank to move the arm down after a match

In 2007 we used my multitool to release a little catch and lower the lift

Hope they don't go TSA style for people entering the playing area :rolleyes:

[awesomegeek166]

Team 2130 is right on par with other times posted. :) Only tested the prototype though!

[Numbers]

So... we saw smoke when we made our bare bones minibot try to climb, with a modest overgear (0.5:1). These things can't be stalled for even a half second, apparently? I have no idea what smoked or whether or not the motors still worked, but if the motors are that delicate...

Same here, the motors start to smoke pretty fast. Wonder if any real damage was done... check the resistance of the non-smoked motor, and then check the motor that smoked for any difference? this might work...hmm...

[JesseK]

So... we saw smoke when we made our bare bones minibot try to climb, with a modest overgear (0.5:1). These things can't be stalled for even a half second, apparently? I have no idea what smoked or whether or not the motors still worked, but if the motors are that delicate...

Let's take a second and do some *wait for it* real engineering.

:ahh: :ahh:

To get the climb times we want, we have to run the TETRIX motors at 50% of their stall torque. Depending on gearing and weight, the robot will not run at perfectly 50%, so most bots are either on the close-to-stall or further-from-stall side of the power curve. A 0.5:1 ratio with 3" wheels is slightly on the closer-to-stall side for a 5-lb minibot, by my calculations anyways.

Given the experiences we've had with Banebots 550/545 and Fischer-Price 9012 motors, I'd say the burn out at stall of an TETRIX motor is perfectly aligned with what should happen. If we run the FP9012 at 50% of stall 100% of the time, it seems like simply thinking bad thoughts at the motor will burn it out when it hits a bump.

So Chris, you should be happy that your TETRIX motor smoked. It means you didn't do the engineering first -- and that fact will inevitably be a invaluable lesson at some point in your engineering career. It also means the sky isn't falling (whew).

If we're so concerned about motor (and wallet) longevity, the perfectly viable option is to not gear the thing for peak power. On the field, I'm sure we'd get 10 points every time we deployed; in the Elims, that's typically much better than a 50% chance the minibot nets either 30 points or 0 points (depending on how angry the motors are you keep running them so hot).

[cmass]

We are right at 3 seconds with 5# minibot.

On our way to a 3.5# minibot and less slip. I think 2 seconds just might be necessary to win the race.

Will start to play with the 4-way switches tomorrow cause right now we have to catch the bot while the gears are turning .... not a good answer!

[chris janney]

Ok I know this is coming a little late, but i don't know if the whole shoot the bot up the pole is completely legal. I say this b/c i found out that compressing air inside a tube of PVC isnt legal. i dont know if shooting the bot will work well at all, simply because you need to line the shoot up, and get a correct power/weight ratio. mess that up and you wont hit the button. too much and you could miss. also with surgical tubing, it wears out after awhile. go into too many rounds and you minibot will start to fail. all in all, if you can create a system that would shoot the bot up, and then catch and climb the pole, shoot me a line @ Chris@Team3266.org

[Duke461]

I have a hard time believing a lot of these times (no pun intended). I really don't think this many teams could reach a time of under 3 seconds already unless you've only focused on the minibot. i also believe that
A. Some people have been hitting stop on their stopwatches before it reaches the top
B. The "2 second minibot"'s will probably be hard to deploy.
C. Those minibot's do not/ cannot consistently get under 3 seconds.
Right now our best time is 3.57 seconds. Weight is roughly 4 pounds, consisting of two motors, chains, surgical tubing, wheels, etc.
Very very very reliable deployment, now just working on deploying mechanism placement on the bot.

[GaryVoshol]

Ok I know this is coming a little late ...Yes, it is a little late, about 3 or 4 Team Updates late. Flinging the MINIBOT has been completely outlawed. You can only use the Tetrix motors as a source of propelling the MINIBOT up the TOWER.

You need to keep watch on Team Updates: http://www.usfirst.org/roboticsprograms/frc/content.aspx?id=450 Else, you are going to be building a robot for playing a game that doesn't exist anymore.

[Chris is me]

Let's take a second and do some *wait for it* real engineering.

So Chris, you should be happy that your TETRIX motor smoked. It means you didn't do the engineering first -- and that fact will inevitably be a invaluable lesson at some point in your engineering career. It also means the sky isn't falling (whew).

If we're so concerned about motor (and wallet) longevity, the perfectly viable option is to not gear the thing for peak power. On the field, I'm sure we'd get 10 points every time we deployed; in the Elims, that's typically much better than a 50% chance the minibot nets either 30 points or 0 points (depending on how angry the motors are you keep running them so hot).

Don't worry, it wasn't my engineering.. :)

I'm thinking along those lines - winning the minibot race isn't as important as being in it!

Direct driving two 4" wheels (not sure on one for each wheel or both mechanically linked to a lower "gearbox") seems simple, robust, lighter, and less likely to blow up Tetrix motors. Now to get the darn thing to come DOWN the pole...

[klmx30302]

With 4" wheels the motors will backdrive easily, the way we get ours down is by putting some long bolts on the bottomside of the robot. next get a pole and attach a ziptie in a large loop at the top, use it to gently pull on the bottom of the minibot and it should backdrive all the way down.
Here is a video of our minibot prototype:
http://www.youtube.com/watch?v=8IKLSgfXqcM

[Doug G]

With 4" wheels the motors will backdrive easily, the way we get ours down is by putting some long bolts on the bottomside of the robot. next get a pole and attach a ziptie in a large loop at the top, use it to gently pull on the bottom of the minibot and it should backdrive all the way down.
Here is a video of our minibot prototype:
http://www.youtube.com/watch?v=8IKLSgfXqcM

Right now our robot uses just one motor and 4" wheels and back drives on the way down way too fast. However when we shunt the motor, after it switches off at the top, it comes down nice and slow. If teams haven't figured out how to get it back down safely, I suggest you add a dpst switch that turns off motor power and at the same time connects the two ends of the motor together for a slow return.

[McGurky]

Our minibot prototype is getting to the top in just about 4 seconds (3.9ish)

now while we realize there is quite a bit of human error involved with the timing devices, it is a pretty solid baseline to work off of.

Here is the video(yes we are actually providing proof of the climb:rolleyes: )
http://www.youtube.com/watch?v=R24GD8FIt9Q

[PVCMike]

I too am curious to know what the sub 3 second mini-bots are using for gearing. Thank you to the few people who shared videos! No proof like video!

[wireties]

I think that you are forgetting about torque!!!

And gravity!

[inkspell4]

Can anyone give hints on haw to make the climb rate as fast as possible any ideas will bee much appriciated

[liam.larkin]

Some quick math and some analysis of the torque/stall curves for the motors have shown us that the most optimal gearing rate is .5/1 please check my math. Additionally using this gearing rate we found that a robot if carefully constructed can weight about 5-6 pounds. This gets you to the top in about 4.0-4.75 seconds. This has been pretty much confirmed by our present prototype minibot design.

These individuals claiming 2-3 seconds I have questioned unless the robot is like 3 pounds or they are direct driving but with that said you direct drive the motors get really close to stalling after only a few pounds. I would love to see a video of a team beating 4.0 seconds

[cmass]

your 5-6 pounds is way wrong.

Start with only what you need and then make that lighter. I'm certain that 4# is possible and teams will do better than that.

With standard out of the kit parts and not modified we got to 3 seconds and < 5#.

You are missing something.

[cmass]

With 4" wheels the motors will backdrive easily, the way we get ours down is by putting some long bolts on the bottomside of the robot. next get a pole and attach a ziptie in a large loop at the top, use it to gently pull on the bottom of the minibot and it should backdrive all the way down.
Here is a video of our minibot prototype:
http://www.youtube.com/watch?v=8IKLSgfXqcM

No ladders, no poles on the field, look at the Q&A forum. Keep working!

[Gary Dillard]

Some quick math and some analysis of the torque/stall curves for the motors have shown us that the most optimal gearing rate is .5/1 please check my math. Additionally using this gearing rate we found that a robot if carefully constructed can weight about 5-6 pounds. This gets you to the top in about 4.0-4.75 seconds. This has been pretty much confirmed by our present prototype minibot design.

These individuals claiming 2-3 seconds I have questioned unless the robot is like 3 pounds or they are direct driving but with that said you direct drive the motors get really close to stalling after only a few pounds. I would love to see a video of a team beating 4.0 seconds

OK let me check your math, 2 ways

Suppose your minibot weighs 5 pounds, you use 2 motors with a .5:1 gear ratio, and you use 4 inch diameter wheels (using nominal numbers and ignoring start up and inefficiencies for now). The robot exerts a torque on the wheel of 10 inch pounds (weight times radius), and the gear ratio makes the torque on the motors 20 inch pounds, or 10 inch pounds on each motor. Looking at the motor curve, they run at 77.8 rpm at 10 inch pounds load (sorry, I always work in English units). 77.8 rpm on the motor divided by the gear ratio and divided by 60 gives you 2.593 revs per second at the wheel, times pi times the diameter gives you 33 inches per second up the pole. If you start at the top of the line (30 inches off of the floor) you have to travel 92.25 inches (122 minus 30 plus 1/4), which equals 2.83 seconds.

Now for optimum performance (tweaking the gearbox and/or the wheel diameter), you want to run the motors at peak power which is 9.36 Watts times 2 or 18.72 watts, which converts to 165.67 inch pounds per second. Moving 5 pounds times 92.25 inches and dividing by the power gives you 2.784 seconds feasible.

So you can see that for a robot less than 5 pounds, even including some inefficiencies 3 seconds is absolutely feasible.

[billbo911]

Do the math. Save the world!

[IKE]

The "little bits" here and there can kill you on the mini-bot. Our initial prototypes were 0.5 lbs heavier than we estimated (5 lbs). We had "geared" to be on peak power for those runs, but the 10% heavy also meant we needed about 10% more torque which took us down about 10% in power. Thus the 10% more weight would have added about 22% (1.1/0.9=1.21), but weight there is more... This 10% more mass also added additional friction which meant more torque, and again lower speeds therefore (1.15/0.85=1.35). Thus a mini-bot that we thought would climb in a little over 3 seconds ended up taking over 4 seconds (3.25 * 1.35 = 4.4 seconds). With certain mini-bot designs, a little weight can kill the performance for that design. This was initially very frustrating, but after review it is much more encouraging.

Back to some more iterations.

[JesseK]

So you can see that for a robot less than 5 pounds, even including some inefficiencies 3 seconds is absolutely feasible.

Check your math. The short story is -- equations only tell us where to start with MINIBOT prototyping, yet only real prototypes will tell us if the theory is sound.

The problem with such simple Power->Torque conversion in this case is that it does not account for the increased time the MINIBOT will take to get to its max speed under a higher torque load. Kinematic equations are non-linear with that respect when combined with the inverse relationship of an electric motor's speed-versus-torque relationship, thus analyzing a distance-versus-time chart between the two options may surprise you.

The problem is, distance-versus-time isn't quite as straight forward as it seems. In my analysis, I have to piecewise the graphs into acceleration sections and max-speed sections based upon the calculated time it take to accelerate to max speed under load.

So while I don't claim to have my calculations be 100% precise to real-world conditions, they do show that MINIBOTS with a 0.5:1 ratio with 4" wheels and a 5-lbs of weight will spend over 75% of their climb time in the acceleration phase, resulting in a 6+ second climb. 0.5:1 ratios with 3" wheels have 4-4.5 second climbs (50% of which is acceleration). Direct-drive 4" wheels have about the same times due to greater efficiency motor-to-pole coupled with less mass due to no gearing (all else equal), even though its max speed is technically slower than 0.5:1x3" wheels on a flat field.

...or they are direct driving but with that said you direct drive the motors get really close to stalling after only a few pounds
Actually, putting a 0.5:1 ratio on the MINIBOT puts it's torque load twice as close (ish) to stall as direct drive does.

[Gary Dillard]

... will spend over 75% of their climb time in the acceleration phase,

That doesn't pass the sanity check. If it takes 3-4 seconds to get to 33 inches per second, that's an acceleration of about 10 inches per second^2, or .03 g's. It doesn't start with a load of 0 g's, it starts with 1g plus acceleration, so that's about 3% reduction in available power using your numbers.

[Kevin Sevcik]

... 5-lbs of weight will spend over 75% of their climb time in the acceleration phase, resulting in a 6+ second climb....I'll ditto Gary here. Check your math on this one. I was thinking along similar lines two weeks ago and whipped up a spreadsheet to do all the numerical integration for me. The acceleration phase was stupidly short, in the tenths of a second. But we don't need a complicated spreadsheet to tell us that. If we gear for peak power, that's 1/2 stall torque. So full stall torque is 2 times the weight of the bot, is 1 g instantaneous acceleration at the beginning. When you get up to half your final speed, you're still doing 0.5g. At 3/4 final speed, it's still 1/4g, and so on.

In fact, if you toss out friction, this is a very nice linear system to model. The fact that your torque/force decreases linearly with your speed makes it just another damping term, so it actually works out to a simple mass-damper system with an external force. So if I wasn't so lazy, I could tell you the exact time constant based solely off the robot's mass and motor characteristics.

[Steve_Alaniz]

...... And then... there was this

http://www.youtube.com/watch?v=sO4uNj44oZE

I'm SOOOOOOoooo envious!

Steve

[JB987]

Don't believe everything you see...all may not be as it seems. See the other related thread highlighting 1625's minibot climb. Do you clearly see the 1.5 lb. battery on the skyrocketing minibot? Do you vaguely see it?

[ttldomination]

Don't believe everything you see...all may not be as it seems. See the other related thread highlighting 1625's minibot climb. Do you clearly see the 1.5 lb. battery on the skyrocketing minibot? Do you vaguely see it?

Right towards the end of its climb, you can see the battery fly up past the robot and fall down.

Or some other relatively heavy objected connected via wires.

- Sunny

[Steve_Alaniz]

Don't believe everything you see...all may not be as it seems. See the other related thread highlighting 1625's minibot climb. Do you clearly see the 1.5 lb. battery on the skyrocketing minibot? Do you vaguely see it?

You are correct... could be an "illegal" minibot.... I STILL envy that speed...Or maybe it's the Mythbuster-ish destruction that occurs when it slams into the top! COOL!

Steve

[Tobot]

Direct driving two 4" wheels (not sure on one for each wheel or both mechanically linked to a lower "gearbox")

It is my assumption that "direct drive" in this thread refers to driving a wheel directly from the motor shaft. In other words, without the Tetrix gearbox attached. Is this correct?

My team is considering the idea of doing away with the tetrix gearbox. But I don't like the looks of that 2.5 mm motor shaft.

[Cyberphil]

It is my assumption that "direct drive" in this thread refers to driving a wheel directly from the motor shaft. In other words, without the Tetrix gearbox attached. Is this correct?

My team is considering the idea of doing away with the tetrix gearbox. But I don't like the looks of that 2.5 mm motor shaft.

I believe when everyone says direct drive, they mean with the tetrix gearbox, considering you cannot alter the motors (which I assume means the gearbox connected also).

If I am wrong, please smite me. Thats what I meant whenever I said directly driven.

[Chris27]

I believe when everyone says direct drive, they mean with the tetrix gearbox, considering you cannot alter the motors (which I assume means the gearbox connected also).

If I am wrong, please smite me. Thats what I meant whenever I said directly driven.

*smite*

http://forums.usfirst.org/showthread.php?t=16187

Don't believe everything you see...all may not be as it seems. See the other related thread highlighting 1625's minibot climb. Do you clearly see the 1.5 lb. battery on the skyrocketing minibot? Do you vaguely see it?

ye of little faith...:rolleyes:

[Chris is me]

It is my assumption that "direct drive" in this thread refers to driving a wheel directly from the motor shaft. In other words, without the Tetrix gearbox attached. Is this correct?

My team is considering the idea of doing away with the tetrix gearbox. But I don't like the looks of that 2.5 mm motor shaft.

I meant a direct drive off the stock gearmotor, not motor sans gearbox.

The latter could be done but for my team at least it's beyond our capabilities.

[Tobot]

I propose that from here on in we specify what we mean when we say "direct drive."

My team is in the same boat regarding technical ability to use the motor shaft. On the other hand, I cracked open the tetrix gear box last night. With some moderate machining capability, one could signicantly modify the ratio of the gearbox. I took a couple of photos that I will try to put up this evening (CST).

I would love to use the internal gearbox alone for several reasons. Not the least of which is that two of our wheels and a driven tetrix gear came loose on our first climb test.

[Tobot]

Here's an internal view of the Tetrix gearbox:

http://farm6.static.flickr.com/5140/5394176791_0a8c324550.jpg (http://www.flickr.com/photos/ukweli/5394176791/)

It looks possible to mate the brass helical gear to the 40T gear on the output shaft, with a little bit of machining. Sorry this post is a day late.

[JB987]

*smite*

http://forums.usfirst.org/showthread.php?t=16187



ye of little faith...:rolleyes:

Or a just a case of skepticism with the treatment being a clearer video with close up? ;)

[Gary Dillard]

Here's an internal view of the Tetrix gearbox:

http://farm6.static.flickr.com/5140/5394176791_0a8c324550.jpg (http://www.flickr.com/photos/ukweli/5394176791/)

It looks possible to mate the brass helical gear to the 40T gear on the output shaft, with a little bit of machining. Sorry this post is a day late.

Thanks for the pic. How many teeth on the gear on the input shaft that mates to the 26 tooth gear?

[IKE]

Thanks for the pic. How many teeth on the gear on the input shaft that mates to the 26 tooth gear?

Math would say 10. (40/10)*(25/15)*(30/10)*(26/10)=52:1

[Tobot]

How many teeth on the gear on the input shaft that mates to the 26 tooth gear?

10 teeth on the helical gear on the motor shaft. This makes the ratio input 52:1 output for the gearbox and motor as assembled. Mating the 10 tooth element of the 26:10 brass gear to the 40 tooth output shaft gear would yield
input 10.4:1 output for the same assembly.

[Jared Russell]

Here's an internal view of the Tetrix gearbox:

(Picture Omitted)

It looks possible to mate the brass helical gear to the 40T gear on the output shaft, with a little bit of machining. Sorry this post is a day late.

That is exactly what we did (using only an inexpensive mill and lathe and a dremel), and it works great.

[Chris is me]

That is exactly what we did (using only an inexpensive mill and lathe and a dremel), and it works great.

Do the gains in efficiency offset the lower torque output of the motor, or is this an "experts only" mod for teams that make custom 2" wheels?

How dramatic is the speed gain? 1625-dramatic?

[Chris Hibner]

Do the gains in efficiency offset the lower torque output of the motor, or is this an "experts only" mod for teams that make custom 2" wheels?

How dramatic is the speed gain? 1625-dramatic?

The dyno data showed a ~60% increase in output power. That makes it 1625 fast. You would definitely have to do some custom gearing and probably custom wheels (custom wheels will remove the necessity for some of the gear ratio). The big thing is to get the gear/wheel combo right so your reflected torque to the motor is ~50% of stall.

[Gary Dillard]

Here's a tip, whether or not you chose to modify the gearbox. We found in years past that cleaning the stock grease from our NBD gearboxes and using light lithium grease had a noticeable improvement in output (efficiency). It looks like this gearbox is pretty clean - don't know if it was that way when you opened it, but even if it was well lubed it might be a good idea to clean it out and put in lithium grease. (which per update 5 is an allowable material).

[Chris is me]

Hmm....

I imagine for a 1:1 (pre mod) reduction it's more efficient to leave the gearbox as is than to take out half the stages and compensate for the speed increase with Tetrix gears.

But for a 0.5:1 (pre mod) - that could be interesting.

[Grim Tuesday]

7 seconds is not too fast considering 1625 can climb in 3 seconds

Wow.

We are 4.5 with a 1 second deployer. We havent tested our other two mini bots yet. 2:1 gear ratio, magnet holding method.

[jnapp18]

not to show off or anything... but this is our minibot... water bottles attached to simulate battery weight.. 3.4 seconds!

http://www.youtube.com/watch?v=UqPyHQqQtYk

[jnapp18]

http://www.youtube.com/watch?v=UqPyHQqQtYk

water bottles attached to simulate battery weight

[waialua359]

nice job.

[pfreivald]

Looks like a nice start!

[kialogical310]

Does anyone know the weight of the individual tetrix parts like the large gear=xlbs, motors=ylbs sort of thing?

[Alpha Beta]

According to our scale in the shop battery is 1 lb 5.1 oz and motor is 7.0 oz.

[JB987]

Does anyone know the weight of the individual tetrix parts like the large gear=xlbs, motors=ylbs sort of thing?

Answer to this question is in this thread. It would be helpful to read the whole thread.

[omricohen]

our minibot climb the pole in approximately 2.5 sec , a little less.
with all the wight on it , the battery is attached to the minibot.
you welcome to see it in the post:
http://www.chiefdelphi.com/forums/showthread.php?threadid=90373
hope for comments on it .:)

[Tobot]

...It looks like this gearbox is pretty clean - don't know if it was that way when you opened it...

The photo was taken seconds after opening the box for the first time. It was quite clean and lubrication was pretty light.

[Chris is me]

Considering the use of tiny helical gears I think adding more lubrication will help even unmodified gearmotors.

[thefro526]

The photo was taken seconds after opening the box for the first time. It was quite clean and lubrication was pretty light.

Considering the use of tiny helical gears I think adding more lubrication will help even unmodified gearmotors.

I looked at one of the transmissions we had, and I didn't notice any lubrication. It may have been there, but I couldn't see it or really feel it.

Does anyone have a good grease for these guys? I'd imagine you'd want a pretty light grease.

[Joe Ross]

Has anyone tried the Thermal-Protected DC Motor Power Cable yet? If my memory is correct it's set for 2.4 amps, which is lower then some of the motor current draw that I've seen people post. I assume it will work for a short time at a current draw of above 2.4 amps, so I'm curious whether it works for other people.

[IKE]

Has anyone tried the Thermal-Protected DC Motor Power Cable yet? If my memory is correct it's set for 2.4 amps, which is lower then some of the motor current draw that I've seen people post. I assume it will work for a short time at a current draw of above 2.4 amps, so I'm curious whether it works for other people.

We used the tetrix motor cables last night. They worked great, and did not trip to the best of my knowledge. Judging by time, it was pretty close to the peak power point.

[mac]

We are right at 3 seconds with 5# minibot.

On our way to a 3.5# minibot and less slip. I think 2 seconds just might be necessary to win the race.

Will start to play with the 4-way switches tomorrow cause right now we have to catch the bot while the gears are turning .... not a good answer!

Be careful, you can not use one four-way switch to reverse your motors. You need to use two Three-Way Switches or one Double pole-Double throw Switch. I did electrical work for 9.5 years.
Mentor Mac mccubbin99@live.com
240-405-9213 301-831-0407
Mt.Airy,Md. Team 686

[Gary Dillard]

Be careful, you can not use one four-way switch to reverse your motors. You need to use two Three-Way Switches or one Double pole-Double throw Switch. I did electrical work for 9.5 years.


Apparently the GDC thinks otherwise

From Team Update #7:

Although we are not in the business of designing MINIBOTS for teams, we do wish to point out that there are many, many ways to have a MINIBOT descend the pole after TRIGGERING the target. To name a few: mechanically reducing the friction against the pole upon hitting the target; turning off the motors using a wall switch or NXT logic; reversing the motors using a 4-way switch or NXT logic. We are sure you will think of many more

[dsmith14469]

This circuit should be correct for using a 4 way switch to reverse the motors.

Dean

[Mr V]

Yes a 4 way switch will reverse your motors, the key is to put the battery connections on one color screws (we used the black) and the motor on the other color screws. Doesn't matter which is which just that both leads of a respective item are connected to the same color screws. If you want to change which direction of the sw is up and which is down reverse the motor leads. It is also possible to wire it so that one of the motors works in brake mode while the other has power in the "down" mode.

[billbo911]

Here's a couple updates. This is a picture of the bot in it's functional state. We'll be tweaking on it a bit, but it works as planned.

http://billbo911.smugmug.com/Hobbies/2011/unused/1185682320_SyRJr-L.jpg

Our goal was a sub 5 second robot. Currently, and without much fine tuning yet, we are at 4.5 second from the platform to the top of the pole without the sensoe plate.

http://www.youtube.com/watch?v=HnKO-nry2EA

[pfreivald]

We hit 2.71 seconds today (including the battery and all necessary switches), and that's without our new custom wheels (which aren't back from the CnC mill group yet). We were very excited, as prior to this we had been stuck at around 4.5 seconds!

[Da Kid]

what did you use to have the minibot turn on when it hits the pole. we're using magnets also.

[billbo911]

what did you use to have the minibot turn on when it hits the pole. we're using magnets also.

There is a polycarbonate rod the extends past the magnet. When the magnet pulls the bot to the pole, it pushed the rod back, which in turn closes the main electrical switch.

[atinylittlemuon]

Here's a couple updates. This is a picture of the bot in it's functional state. We'll be tweaking on it a bit, but it works as planned.

http://billbo911.smugmug.com/Hobbies/2011/unused/1185682320_SyRJr-L.jpg

Our goal was a sub 5 second robot. Currently, and without much fine tuning yet, we are at 4.5 second from the platform to the top of the pole without the sensoe plate.

http://www.youtube.com/watch?v=HnKO-nry2EA
um... where's your battery? we tried magnets in the early stages of our design and they would work just fine untill we put the battery on and the whole thing would fall right off.

[R3P0]

Seems like everyone is forgetting the possible use of surgical tubing to shoot that bot up the pole much much faster.
.

I think this was discussed as not allowed on several other threads. Plus team updates it was also clarified. Maybe someone could help post those rules. ( psst I am at work)

[BornaE]

I think this was discussed as not allowed on several other threads. Plus team updates it was also clarified. Maybe someone could help post those rules. ( psst I am at work)

If you look at the date on the original post (10th of jan) you should notice that team update one which disallowed the use of surgical tubing for upward movement was released the day after I posted that.

and yes, surgical rubber is not a legal source of energy to climb the pole per the current rules as G19 states.

[billbo911]

um... where's your battery? we tried magnets in the early stages of our design and they would work just fine untill we put the battery on and the whole thing would fall right off.

Watch the video. The battery is opposite the motors.

[Douglas Rohrer]

I have taken a different approach to estimating the theoretical minimum time possible for the mini-bot to reach the top of the 10 ft pole.

Rather than the using the power output from the motor, I used the rated no-load speed of the Tetrix motors (152 to 154 rpm) to estimate the shortest time possible. This rpm for the motor would produce a Motor Rotation Time for one revolution of 0.39 seconds in an idea world with no load (battery, motors, framework, etc.).

This means that a 3 inch wheel directly attached to the motor would require 4.96 seconds to reach the top of the 10 foot pole. Similarly, a 4 inch wheel would reduce this time to 3.72 seconds. Using a 1:3 gear train would reduce these times to 1.65 and 1.24 seconds respecively, but reduce the effective power of the motors to lift.

Motor rotation time = (60 sec/ 1 minute)* (1 Minute / 154 revolutions)
= 0.39sec/rev

Revolutions to top = (Pole Hieght) / (diameter of Wheel * Pi)
= (120 inches) / (4 inches * 3.1415)
= 9.55 revolutions of the 4" wheel

Time to top = (Revolutions to Top) * (Motor Rotation Time)
= (9.55 rev) * (0.39 sec/rev)
= 3.72 sec for a 4" wheel to reach the top of the poe.

[Tyler Hicks]

Clocked in at 1.7 Seconds tonight! :D

[jimwick]

our minibot goes up the pole in just under 6 seconds if the wheels don't slip, actual test of real minibot and real pole.

we can deploy and climb the pole in under 10 seconds under software control; it works about half the time. the other half the alignment isn't quite right and it doesn't work.

jw

[Tobot]

We are flirting with 3.5 second climbs on a fresh battery.

http://www.flickr.com/photos/ukweli/5466703933/

[Gary Dillard]

That is exactly what we did (using only an inexpensive mill and lathe and a dremel), and it works great.

We were able to modify the Tetrix gearbox with no special tools / machines - we just had to stake the ID of the large gear to get it to press on to the top of the output shaft at the position of the gear attached to the brass gear

[wireties]

We were able to modify the Tetrix gearbox with no special tools / machines - we just had to stake the ID of the large gear to get it to press on to the top of the output shaft at the position of the gear attached to the brass gear

What was the resulting gear ratio? And what does "stake the ID mean"?

TIA

[Mr V]

What was the resulting gear ratio? And what does "stake the ID mean"?

TIA

If you look at the output shaft you'll see that in the area where the gear is pressed on the shaft is "knurled" or has ridges formed in it to create the press fit. So I assume he means that they used a chisel or punch to deform (stake) the ID (inside diameter) of the gear so it will press on the un-knurled portion of the shaft where it will mesh with the input gear.

http://farm6.static.flickr.com/5140/5394176791_0a8c324550.jpg

The mod of making the first gear mesh with the gear on the output shaft results in a 10.4 to 1 ratio instead of the factory 52 to 1.

Another way to accomplish this IF you have a lathe is to machine a spacer that fits on the output shaft between the gear and case so that it meshes with the input gear. On the motor end of the output shaft you need to turn it down the same amount as the length of the spacer so it will fit in the case.

In addition to increasing the output speed ~5x it reduces the drag in the trans aprox 50% by removing 2 gear interfaces. Make sure you don't loose the spacer on top of the input gear and remove the spacers on the other shaft, there is a small one on the shaft under the 25:10 gear.

[Grim Tuesday]

Why keep that silly gearbox? Take it all the way off, and you can be seeing times in the 1.4-1.8 range. Most of the gains are due to the loss of weight of the huge wheels, and the gearboxes, not the raising of efficiency.

[pfreivald]

Most of the gains are due to the loss of weight of the huge wheels, and the gearboxes, not the raising of efficiency.

That is fundamentally untrue, and easy to demonstrate as untrue.

Our current minibot is in the 1.8 second range (and very consistent, thank you), and even when we add excess weight just to see what it does with it, it's still faster.

Reducing weight matters, but increasing efficiency matters a whole lot, too. It's not a dichotomy, here -- both are important!

[Grim Tuesday]

Thanks for proving me wrong. I was just inferring, so apologies, and I learned something!

[billbo911]

I know it's not fast, but it's reliable. We are thrilled it is ready for Sacramento.
http://www.youtube.com/watch?v=AVx7oFAyfjg

[ngoc0623]

in order to climb the pole, you dont need the wheels and with this you can cut the weight of the minibot. Just suggesting. :)

[jskene]

We were consistently getting ~1.2 seconds. We got our minibot measured by team 1279 at the DC regional and got a higher accuracy measurement:

http://www.chantillyrobotics.org/documents/2011/Minibot-time-612.jpg

Our deployment arm, when it was working properly :(, was taking ~0.2 seconds to get the bot on the pole, so total time was ~1.27 seconds.

Below is a photo of the bot, which uses a cnc machined chassis. Our goal was to use only the minimum of materials necessary to hold the components in place. Weight came in at 2.5 lb.

http://www.chantillyrobotics.org/documents/2011/Minibot-612.jpg

[WileyB-J]

meet richard.
http://farm6.static.flickr.com/5295/5575850142_4358852dcc.jpg (http://www.flickr.com/photos/wileyb-j/5575850142/)
not the fastest liger in the forest but hes around 2.2s and stupidly reliable.
it seems like its one of the most machined minibots out there!

[theschell]

our mini bot helped us get up the pole in under 2 seconds with deployment so that got us back to back wins

[EagleEngineer]

I know 968's minibot is exteremly fast. They get like .7 seconds with the dployment. Well that is what was said, but if you saw it in really time their minbot was up when the clock still said 9. This minibot helped them win the LA regional. their minibots somewhere around a one seconds minibot.

[Marc S.]

We were consistently getting ~1.2 seconds. We got our minibot measured by team 1279 at the DC regional and got a higher accuracy measurement:

http://www.chantillyrobotics.org/documents/2011/Minibot-time-612.jpg

Our deployment arm, when it was working properly :(, was taking ~0.2 seconds to get the bot on the pole, so total time was ~1.27 seconds.

Below is a photo of the bot, which uses a cnc machined chassis. Our goal was to use only the minimum of materials necessary to hold the components in place. Weight came in at 2.5 lb.

http://www.chantillyrobotics.org/documents/2011/Minibot-612.jpg

Thats prety impressive. If you don't mind, what roller size did you use?

[jskene]

The axles are 3/8" aluminum, drilled to fit the motor shaft (we removed the spiral gear). The "tires" are 1/2" OD surgical tubing.

[Ether]

meet richard.
http://farm6.static.flickr.com/5295/5575850142_4358852dcc.jpg (http://www.flickr.com/photos/wileyb-j/5575850142/)


What's the blue broom-hook thing at the top made of ?

[WileyB-J]

What's the blue broom-hook thing at the top made of ?




if you're referring to the clip, its actually blue pvc pipe!

[Ether]

if you're referring to the clip, its actually blue pvc pipe!

are you using the stock 52:1 gearbox? and are those 4" wheels ?

[billbo911]

are you using the stock 52:1 gearbox? and are those 4" wheels ?


Based on a quick comparison in proportions to the opening in their deployment system, my guess is they are 4.6 inch. It looks like they are made of PVC and wrapped with surgical tubing.

[WileyB-J]

are you using the stock 52:1 gearbox? and are those 4" wheels ?




Based on a quick comparison in proportions to the opening in their deployment system, my guess is they are 4.6 inch. It looks like they are made of PVC and wrapped with surgical tubing.

Stock gearboxes, I modded some gearboxes to 10:1 but I didn't invest much thought into them after. The wheels are actually around 5.5", the picture angle is misleading. They use a 4" to 3" PVC pipe fitting for the wheel rim and its 3/8's surgical tubing. Im pleased with the way they came out, and can safely say its a unique design (unlike many little screamer bots out there).

[Ether]

pi*5.5" wheel diameter = 17.3" wheel circumference.

90"/17.3" = 5.2 wheel revs to climb pole

(5.2 revs)/(2.2 seconds)*(60 seconds/minute) = 142 rpm

At that speed, the motors are generating virtually no torque.

Somebody please check my math.

[Kevin Sevcik]

pi*5.5" wheel diameter = 17.3" wheel circumference.

90"/17.3" = 5.2 wheel revs to climb pole

(5.2 revs)/(2.2 seconds)*(60 seconds/minute) = 142 rpm

At that speed, the motors are generating virtually no torque.

Somebody please check my math.


He noted in passing that he modded the gearboxes to 10:1. Which probably puts him on the wrong side of peak power, but makes a little more sense.

[Ether]

He noted in passing that he modded the gearboxes to 10:1. Which probably puts him on the wrong side of peak power, but makes a little more sense.

OK, is that what he meant? I thought that was an aside. The way I read it, he modded some gearboxes but then decided to use stock (as-is) ones.

Your interpretation makes more sense I guess.

[Mr V]

OK, is that what he meant? I thought that was an aside. The way I read it, he modded some gearboxes but then decided to use stock (as-is) ones.

Your interpretation makes more sense I guess.





I also took it to mean that the gear boxes on this bot were the original 52 to 1 ratio.

The 10.4 to 1 boxes, based on our experiences and pile of smoked motors, won't work with a 5.5" diameter wheel. We ended up with ~3" wheels with our 10.4 to 1 gear boxes.

The math doesn't add up to a 2.2 sec time with 52 to 1 boxes either, unless that bot's weight is less than the total of the battery and motors, let alone the wheels and frame, again based on our experiences.

[WileyB-J]

We used stock (52:1) gearboxes on the minibot. The average time was 2.2-2.3 seconds from bottom of the pole to the top. It weighs around 4.5-5 pounds.

If this helps:
http://a1.sphotos.ak.fbcdn.net/hphotos-ak-snc6/197410_10150449454615702_774960701_17962725_488661 0_n.jpg

[AlDee]

We were consistently getting ~1.2 seconds. We got our minibot measured by team 1279 at the DC regional and got a higher accuracy measurement:

Our deployment arm, when it was working properly :(, was taking ~0.2 seconds to get the bot on the pole, so total time was ~1.27 seconds.

Below is a photo of the bot, which uses a cnc machined chassis. Our goal was to use only the minimum of materials necessary to hold the components in place. Weight came in at 2.5 lb.

http://www.chantillyrobotics.org/documents/2011/Minibot-612.jpg

Very Impressive. What type of switch are you using for the trip down?

[Garret]

Our minibot had a 3.5 second climb. Unlike a lot of robots that I saw that used magnets to hold on to the pole. Our magnets were located in our wheels and then covered in electrical tape for traction.

I have attached a picture of one of the early forms of the minibot. the magnets are underneath the thin layer of black electrical tape.

We had some problems with deployment early on, mainly due to the fact that the robot was driving down the pole, but other than that it worked reasonably well.

[Ether]

Our magnets were located in our wheels and then covered in electrical tape for traction.

I have attached a picture of one of the early forms of the minibot. the magnets are underneath the thin layer of black electrical tape.

Is there any gap between the magnets, or are they butted right up against each other?

How did you secure them to the wheel?

Did you alternate them N/S ?

[wireties]

Late in the process we got a 1.7s minibot working with 1/4" tubes covered with surgical tubing. I never understood why everyone thinks magnets are necessary. If you center the minibot's mass left to right and balance the top to bottom mass difference by angling the minibot a bit, magnets are not necessary. Sure the minibot is not trying to climb straight up but it is also not trying to pull the magnets up with it - and its simpler.

[Garret]

Is there any gap between the magnets, or are they butted right up against each other?

How did you secure them to the wheel?

Did you alternate them N/S ?

The magnets are butted right up against eachother and are alternated. To stick them to the wheel we took electrical tape and made a loop to make it act like double-sided tape. Then we used a layer of electrical tape on top of the magnets to hold them to the wheel. Overall the minibot worked well, the main issues were silly mistakes made when hooking the battery up or when setting it up before a match (forgetting to set it to turn on). but overall when we didn't mess up before the match the minibot worked well.

[Ether]

I never understood why everyone thinks magnets are necessary.

What method do you use keep the bot from falling off the pole?

[wireties]

What method do you use keep the bot from falling off the pole?


if the mass if distributed left and right and the angle of the bot as it climbs the pole adjusted to account for top to bottom weight distribution (hang the minibot from a string and check the angle it wants to hang and almost duplicate that) it will just climb straight up. Ours did that though we did add a small cross-section of a pvc pipe to make it fall straight.

HTH

[Tom Line]

if the mass if distributed left and right and the angle of the bot as it climbs the pole adjusted to account for top to bottom weight distribution (hang the minibot from a string and check the angle it wants to hang and almost duplicate that) it will just climb straight up. Ours did that though we did add a small cross-section of a pvc pipe to make it fall straight.

HTH

Am I right in assuming that you are using a wheel on each side of the pole, with the battery on the same side as one wheel, so that you're using torsion to lock the minibot 'on'?

[Ether]

if the mass if distributed left and right and the angle of the bot as it climbs the pole adjusted to account for top to bottom weight distribution (hang the minibot from a string and check the angle it wants to hang and almost duplicate that) it will just climb straight up. Ours did that though we did add a small cross-section of a pvc pipe to make it fall straight.

HTH

I've never seen your bot so I cannot quite picture what you are describing.

I guess what I was asking was do you have some sort of snap-action device that springs shut and locks the bot to the pole, or do you "press-fit" the bot to the pole, or some other method?

[wireties]

Am I right in assuming that you are using a wheel on each side of the pole, with the battery on the same side as one wheel, so that you're using torsion to lock the minibot 'on'?


Close - we did have a wheel on both sides but the weight was even left and right but the bottom (below the wheels) was heavier than the top (above the wheels). We launched it with the wheels at a 45 angle, it pulled itself onto the pole (righting itself in the process) and then fights (a little, maybe 2 or 3 degrees) against the cantilever on the way up. We depended mostly on flex in the motor mounts and sticky wheels to create the rolling friction against the pole.

HTH

[wireties]

I've never seen your bot so I cannot quite picture what you are describing.

I guess what I was asking was do you have some sort of snap-action device that springs shut and locks the bot to the pole, or do you "press-fit" the bot to the pole, or some other method?


Press fit is the best description, flex in the motor mounts and sticky wheels created most of the rolling friction. We used a small cross section of PVC but that was needed to make it fall straight down

http://flic.kr/s/aHsjuentrP

[Ether]

Press fit is the best description, flex in the motor mounts and sticky wheels created most of the rolling friction. We used a small cross section of PVC but that was needed to make it fall straight down

http://flic.kr/s/aHsjuentrP

See attached JPG.

1) The rollers look angled. Is that intentional?

2) It seems like the weight of the battery pack (2) would cause the bot to fall away from the pole in the direction indicated by the "->" arrow (assuming that the picture is right-side-up) ?

3) Is this part of the minibot?

Have you burned up any Tetrix motors with this configuration?

[wireties]

See attached JPG.

1) The rollers look angled. Is that intentional?

2) It seems like the weight of the battery pack (2) would cause the bot to fall away from the pole in the direction indicated by the "->" arrow (assuming that the picture is right-side-up) ?

3) Is this part of the minibot?

Have you burned up any Tetrix motors with this configuration?


1) They do look a little angled. The frame must have bent some last time it came down, it is quite thin. They are supposed to be normal to the plate everything is mounted on.

2) Indeed it would if you did not tilt it fwd a wee bit, which means we are sacrificing some power I know but it was still quite fast

3) Yes, we had a slower minibot (very reliable) which launched from a 45 degree ramp and we wanted to use both minibots interchangeably. That is the only purpose for that piece of the assembly.

We burned up one motor after running it up many dozens of times during testing but that motor smoked some while testing an interim design so it was a little suspect.

We tried button magnets but this configuration seemed to work better. Perhaps a nicer shaped magnet would have worked better, maybe we'll try it before the off-season events.

HTH

[billbo911]

I never would have guessed when I started this thread that it would have grown to 220+ posts. I also didn't consider one thing, this is FIRST. When I did the very first rough calculations, I had to make a lot of assumptions. My first guesstimate of climb time was in the ball park of 7 seconds. I should have known that the envelope would definitely be pushed, heck, almost to the breaking point.

The first minibot we built was very reliable and won a lot of races. It is not fast, just reliable, 4.5 seconds consistently. We rode that little bot all the way to the finals in Sacramento. It will be with us in St. Louis and be available for any team that needs it. It will have a deployment plate with it already set up, all that is needed will be a set of rails to send it out on and a battery. Her name is Miracle.

Now, just so you don't get confused, Miracle still works just fine, but we on Eagle Force don't like to sit by when we know we can do better.

Meet Miracle 2 v2.1a.
http://billbo911.smugmug.com/Hobbies/2011/2011-04-1118-09-34824/1248995057_ds9R8-L.jpghttp://billbo911.smugmug.com/Hobbies/2011/2011-04-1118-10-33884/1248995231_WeZAi-L.jpg

Miracle 2 v2.1a was inspired by a post by Sanddrag (http://www.chiefdelphi.com/forums/showthread.php?t=93796), along with several other designs we have seen.

When we originally set out to build a new mini to take to St. Louis. we were shooting for a sub 2 second mini. That is a far cry from the first estimate I made of ~7sec. way back when this thread started. So how does Miracle 2 v2.1a perform? Lets just say, < 1.2sec is pretty darn close to acceptable.
There is one issue we still need to solve, see if you can identify it.
http://www.youtube.com/watch?v=nvXV8_8UQGk

BTW,
I have this Minibot all set up in SolidWorks. There are a couple minor items missing from the CAD, like the nylon screws, but otherwise, it is fairly complete. If you want a zipped version of the project, just shoot me a PM.

[slijin]

Wow, that's a pretty frightening suicide leap. Maybe adding a couple magnets would help rectify that.

[SM987]

Looks awesome. We have several videos with kids making sure to catch the minibot after hitting the plate. Our solution was a piece of PVC a bit larger in OD to the pole cut to around 200 degrees so that the chaotic "landings" were minimized.

[billbo911]

Wow, that's a pretty frightening suicide leap. Maybe adding a couple magnets would help rectify that.

Looks awesome. We have several videos with kids making sure to catch the minibot after hitting the plate. Our solution was a piece of PVC a bit larger in OD to the pole cut to around 200 degrees so that the chaotic "landings" were minimized.

Well, the plan for us is to reduce weight even more, not add more. So, we are going to try a little physics first.

As far as we can tell, the battery is the main issue. It's momentum is causing the back of the mini to continue moving up, even though the frame has stopped against the top plate. To remedy this, we are going to flip the battery over and move it up until it is parallel with the top of the frame. That way, it has no where to travel once contact with the plate is made. (If that isn't enough energy into the sensor to trigger it, nothing will be.) Maybe some padding on top will help as well.

[R1ffSurf3r]

Well, the plan for us is to reduce weight even more, not add more. So, we are going to try a little physics first.

As far as we can tell, the battery is the main issue. It's momentum is causing the back of the mini to continue moving up, even though the frame has stopped against the top plate. To remedy this, we are going to flip the battery over and move it up until it is parallel with the top of the frame. That way, it has no where to travel once contact with the plate is made. (If that isn't enough energy into the sensor to trigger it, nothing will be.) Maybe some padding on top will help as well.

What is the diameter and material of those wheels? That sucker is quick!

[billbo911]

The axles are .375" and made of Polycarbonate. The tires are a silicon based surgical tubing material.

If you notice, this thing is accelerating almost the entire time. With slightly smaller axles, say .036", it might accelerate a bit quicker, but could reach max velocity too soon and increase the overall time to climb. The only way to tell is to try it, and we are happy right where it is.

From my experience with R/C car racing, I find the best approach is to be accelerating all the way until just before you start the next turn. In this case, that point is the top of the pole. So, it looks like this design is pretty close to optimal with the weight we currently have. Reducing the weight further will only improve things.

[Richard Wallace]

Wow, that's a pretty frightening suicide leap. Maybe adding a couple magnets would help rectify that.Minibot suicide is only one of the concerns about that leap -- the other is human safety. UL Safety Advisors at MSC made pit announcements, and distributed printed notices, requiring the use of gloves to catch minibots that regularly fell from the many practice poles teams had brought. That safety guidance is wise; at two events, I saw students in the first-aid stations being treated for bad cuts they had sustained by trying to catch falling minibots.

[Kevin Sevcik]

Well, the plan for us is to reduce weight even more, not add more. So, we are going to try a little physics first.

As far as we can tell, the battery is the main issue. It's momentum is causing the back of the mini to continue moving up, even though the frame has stopped against the top plate. To remedy this, we are going to flip the battery over and move it up until it is parallel with the top of the frame. That way, it has no where to travel once contact with the plate is made. (If that isn't enough energy into the sensor to trigger it, nothing will be.) Maybe some padding on top will help as well.
Have you considered shifting the impact point behind the battery? If you're correct in your guess that inertial load from the battery is pulling you away from the pole, then you could just shift your pivot point at the top of the minibot. If your impact point is farther from the pole than the battery, then the inertia of the battery will push you more into the pole. It might not be the optimal solution, but it might be quicker to implement. Plus it'd preserve your current weight distribution.

If none of that made sense, sketch up a free-body diagram of the minibot as it makes contact with the pole. Assign an arbitrarily large upwards force at the center of mass of your minibot, balanced by a downwards force at your point of impact, and a horizontal force centered on your magnets. See what happens to the horizontal force as you move the impact point farther from the pole.

[wireties]

From my experience with R/C car racing, I find the best approach is to be accelerating all the way until just before you start the next turn. In this case, that point is the top of the pole. So, it looks like this design is pretty close to optimal with the weight we currently have. Reducing the weight further will only improve things.

Provided it is reaching the same max velocity (no matter how long it accelerates), doesn't reaching max velocity sooner yield the fastest time up the pole? If it is linearly accelerating the whole way up, the average velocity is only 1/2 the max velocity. RC cars don't have gravity to consider I reckon.

HTH

[Ether]

From my experience with R/C car racing, I find the best approach is to be accelerating all the way until just before you start the next turn. In this case, that point is the top of the pole. So, it looks like this design is pretty close to optimal with the weight we currently have.

...

If you notice, this thing is accelerating almost the entire time. With slightly smaller axles, say .036", it might accelerate a bit quicker, but could reach max velocity too soon and increase the overall time to climb. The only way to tell is to try it...

I could be wrong, but I don't think in this case the time-to-climb is minimized by making the wheel diameters so large that the minibot is accelerating all the way to the top.

Since you have a working minibot with a measured time-to-climb, you could easily plug your weight and wheel diameter into this model (http://www.chiefdelphi.com/media/papers/2470) and adjust the friction value until the model matches your measured time-to-climb. Then change the wheel diameter in the model and see what diameter results in minimum time-to-climb.

A side benefit of smaller wheel diameter is less motor current.


Reducing the weight further will only improve things.

Absolutely.

[billbo911]

Have you considered shifting the impact point behind the battery? If you're correct in your guess that inertial load from the battery is pulling you away from the pole, then you could just shift your pivot point at the top of the minibot. If your impact point is farther from the pole than the battery, then the inertia of the battery will push you more into the pole. It might not be the optimal solution, but it might be quicker to implement. Plus it'd preserve your current weight distribution.



Kevin,
Your suggestion is actually quite easy to follow and might be the final solution. Currently, the initial impact to the sensor plate is the tip of the actuator arm of our Stop/brake limit switch. That is followed by the impact of the frame of the minibot.
To shift the initial impact point behind the battery should be as simple as placing a small amount of surgical tubing across the back of the top of the battery and then moving the battery up so that it is flush with the top of the frame. This will do two things:

1) Pad the battery with a bumper that is made of legal material, and reduce the possibility of damage to the battery from being slammed into the sensor plate.
2) It will shift the pivot point outside the center of mass of the minibot and thus cause it to rotate into the pole instead of away from it, as you suggested.

Provided it is reaching the same max velocity (no matter how long it accelerates), doesn't reaching max velocity sooner yield the fastest time up the pole? If it is linearly accelerating the whole way up, the average velocity is only 1/2 the max velocity. RC cars don't have gravity to consider I reckon.

HTH

There is a minor problem with this. To get the minibot to accelerate faster, you have to go with smaller "wheels". This is direct drive with no transmission or gearing to deal with.

As you stated, average velocity can be determined by max/2. But, with smaller wheels, max velocity goes down. (Here is where practice makes perfect. If we really wanted the absolute fastest minibot possible, we would have to make several different sizes of axles to find which gave the best time.)

Let me try a little back of the envelope discussion to help explain this.

Let's assume that max velocity is reached just as you reached the top of the pole. Max velocity reached was 8 ft./sec. So average would be 4 ft./sec and the 10 foot pole would have been climbed in 2.5 seconds.

Now, we reduce the "wheel size" so that we accelerate to max velocity quicker, but in doing so, also reduce our max velocity. Now max velocity is 6 ft./sec and is attained at 6 feet up the pole (ball park for discussion). The average velocity for that 6 feet ( (6 ft./sec)/2) = 3 ft/sec, and therefore, 2 seconds to travel. The remaining 4 feet of distance is traveled at a velocity of 6 ft./sec and takes .667 seconds.

So, 2.667 seconds to cover the same distance instead of 2.5 seconds. That is why you "should" accelerate the entire way.

These numbers are for discussion only and do not represent the actual values.


Either,

I will definitely run these values through your model and see what the result are.

As I said, the real test would be to test with multiple different "wheel" sizes and measure the actual result.
There are so many variables in this project, battery voltage, wheel tackiness, wheel friction against the pole etc., that sometimes trial and error just works out better.

[Kevin Sevcik]

And now I'll turn around and doubt your back of the envelope calculations. I keep seeing people in this thread declaring that the average velocity is half the maximum velocity if you reach the maximum at the top of the pole. But this is only true if you're accelerating at a constant rate.

This is not the case with a permanent magnet dc motor. With a pmdc, as speed increases, produced torque decreases. As torque decreases, your acceleration decreases. So you're going to spend longer accelerating from max/4 to max/2 than you did accelerating from 0 to max/4. This is going to make a significant difference in your calculations. I'd recommend playing with the model Ether linked to and seeing the effect of changing wheel diameters, because it's not as straightforward as you think.

[billbo911]

And now I'll turn around and doubt your back of the envelope calculations. I keep seeing people in this thread declaring that the average velocity is half the maximum velocity if you reach the maximum at the top of the pole. But this is only true if you're accelerating at a constant rate.

This is not the case with a permanent magnet dc motor. With a pmdc, as speed increases, produced torque decreases. As torque decreases, your acceleration decreases. So you're going to spend longer accelerating from max/4 to max/2 than you did accelerating from 0 to max/4. This is going to make a significant difference in your calculations. I'd recommend playing with the model Ether linked to and seeing the effect of changing wheel diameters, because it's not as straightforward as you think.

I believe you are spot on, but, for the sake of discussion, I believe the example I used is close enough to understand the principal.

Now, for complete accuracy, or theoretically accurate results, then Either's model should definitely be considered.

I will do a couple more measurements for accuracy tonight and then run them through the model and see what happens. (Honestly, with Spring Break being next week and the school being closed, I am fairly certain the only changes we will be making are those that keep the bot on the pole :yikes: )

Based on the values I recall for weight and wheel diameter, the Either Model shows that a wheel diameter of .436" is optimal. Currently we have axles that are .375" in dia. and then there is a very pliable Silicon Surgical tubing over that. Uncompressed, the entire wheel is fairly close to .5" in dia. So, under the load of compression into the pole, we are fairly close to the given number. Again, I will verify this tonight.

[Ether]

Based on the values I recall for weight and wheel diameter, the Either Model shows that a wheel diameter of .436" is optimal.


Did you adjust the friction until the model agreed with your time-to-climb, and THEN adjust the wheel diameter to find the optimum? See excerpt below:

plug your weight and wheel diameter into this model (http://www.chiefdelphi.com/media/papers/2470) and adjust the friction value until the model matches your measured time-to-climb. Then change the wheel diameter in the model and see what diameter results in minimum time-to-climb.

[billbo911]

Did you adjust the friction until the model agreed with your time-to-climb, and THEN adjust the wheel diameter to find the optimum? See excerpt below:






I set up the model by entering in values for weight that are fairly close, but probably not exact. I then entered my best guesstimate for our current wheel diameter. I set the time to 1.2 sec, which is our measured time. Once these values were set, I adjusted the friction value until the distance value matched our climb distance.

Your Excel file does not modify the TIME value, TIME is an input, but it does modify the DISTANCE value as each input parameter is modified.

Once I found a friction value that matched our climb distance, I left that value alone. After that , I re-iteratively modified the wheel diameter until I maximized the distance traveled in the same time of 1.2 sec.

[wireties]

These numbers are for discussion only and do not represent the actual values.


Its all about the numbers I reckon. You are trying hit the sweet spot on the motor curve. The fastest minibot I've seen is from FIRST Team 148. I didn't measure it but it seemed to be in the 1s range. It sure looked like that thing hit max V quickly.

Good luck in St Louis!

[Ether]

I set up the model by entering in values for weight that are fairly close, but probably not exact. I then entered my best guesstimate for our current wheel diameter. I set the time to 1.2 sec, which is our measured time. Once these values were set, I adjusted the friction value until the distance value matched our climb distance.

That should be OK.

Once I found a friction value that matched our climb distance, I left that value alone. After that , I re-iteratively modified the wheel diameter until I maximized the distance traveled in the same time of 1.2 sec.

That should get you in the ballpark. If you are game, try this instead: once you've found the friction value as described above, modify the wheel diameter and observe the graph to read the climb-time* for the given (fixed) distance. Find the wheel diameter which minimizes the climb-time for that given distance.

If you don't want to mess with that, would you mind posting your present observed values for bot weight, wheel diameter, and climb-time so I can run them?


* you can change the value in cell A7 to change the scale of the graph if necessary

[billbo911]

That should be OK.



That should get you in the ballpark. If you are game, try this instead: once you've found the friction value as described above, modify the wheel diameter and observe the graph to read the climb-time* for the given (fixed) distance. Find the wheel diameter which minimizes the climb-time for that given distance.

If you don't want to mess with that, would you mind posting your present observed values for bot weight, wheel diameter, and climb-time so I can run them?


* you can change the value in cell A7 to change the scale of the graph if necessary


I don't mind at all. I believe the values I used were as follows:
2.35LB.
.436 dia (final), .5 original to find friction.
1.2 sec. climb time.

[Ether]

I don't mind at all. I believe the values I used were as follows:
2.35LB.
.436 dia (final), .5 original to find friction.
1.2 sec. climb time.

Oops, forgot to ask: what did you use for the climb distance (ie the distance climbed in 1.2 seconds). Also, just to be sure: you didn't use a "ramp" launcher did you? In other words, the minibot started out from a dead stop and climbed in 1.2 sec correct?

[billbo911]

Oops, forgot to ask: what did you use for the climb distance (ie the distance climbed in 1.2 seconds). Also, just to be sure: you didn't use a "ramp" launcher did you? In other words, the minibot started out from a dead stop and climbed in 1.2 sec correct?




I should have known that was needed :o
95 inches.

[Ether]

95 inches.


Given your inputs of 2.35 lb minibot weight, 0.5 inch shaft diameter, 1.2 seconds climb time, and 95 inches climbed distance, here's what I came up with:

friction: 0.83 pound

optimum diameter: 0.43 inches

... which tends to support the conclusion that you are pretty close to the optimum diameter. The only puzzling thing is that the graph in the model looks like your bot velocity with your original .5 inch diameter has pretty much reached its peak value half way up the pole. Was your assessment that the bot continued substantial acceleration all the way to the top a subjective judgment or did you analyze data from a video?

(see attachments)

[billbo911]

Given your inputs of 2.35 lb minibot weight, 0.5 inch shaft diameter, 1.2 seconds climb time, and 95 inches climbed distance, here's what I came up with:

friction: 0.83 pound

optimum diameter: 0.43 inches

... which tends to support the conclusion that you are pretty close to the optimum diameter. The only puzzling thing is that the graph in the model looks like your bot velocity with your original .5 inch diameter has pretty much reached its peak value half way up the pole. Was your assessment that the bot continued substantial acceleration all the way to the top a subjective judgment or did you analyze data from a video?

(see attachments)



Very much subjective. I based the comment on visual and auditory observation.

The bottom line is, we are really close. Lightening will only help, and deployment will be the biggest key to consistent success.

[Mark Sheridan]

Hey Bill

Perhaps there is wheel slip in the beginning of the climb, thus prolonging the acceleration in the beginning of the climb. 0.83 lbs is a lot of friction for the minibot. My guess is the the friction is lower and the minibot can accelerate quicker, but slippage losses in the beginning of the climb slow down the time. It overcomes the slip when motor toque is lower and the rpm higher, thus higher up on the pole. From here is starts accelerating with its full potential. Overall it appears to accelerate the entire length. It would be like pushing (or distorting) the velocity curve to the right.

On our revised minibot, it appears to slip a bit in the beginning but accelerates quickly after it. We run a 2.5 pound minibot with a 3/8 od wheel. It climbs in about 1.5 seconds. We are defiantly running higher magnets forces then yours. By increasing the magnet force, you can prevent wheel slip and prevent the robot from flying off.

I will see if your numbers compare well to my calculator later tonight.

[Ether]

Perhaps there is wheel slip in the beginning of the climb

I think you may be right:

http://www.chiefdelphi.com/forums/showthread.php?p=1045576#post1045576

[boomergeek]

We finished our last regional and did not implement our secret weapon:
(although we did tell the Boston Regional judges about it)
Rubber on rubber (1.2 CoF sliding friction) for the first 3 inches of vertical climb before switching to rubber to steel (0.3-0.6 CoF sliding friction).
We projected that would save us 0.2 to 0.3 seconds and still keep normal force low.

The rubber on rubber is minibot contact to a hostbot ramp that runs vertically parallel to the tower pole and then switches onto the pole before the minibot crosses the 18" line.

[billbo911]

OK, more info and a video.

Kevin,
Shifting the battery up to the top and placing some surgical tubing at the back of the battery did the trick. Now our pivot point is the farthest point from the pole. This video is taken with a fairly depleted battery.

http://www.youtube.com/watch?v=P89k0hlHd1E

Either,

More accurate measurements:

Weight is 10052g, or, 2.32 lb.
Wheel diameter is effectively .45" when compressed against the pole.

So, based on our earlier discussions, it is really close to ideal.

[crazyStone]

2.215 lbs right now but we are a little slow at the beginning of our climb at about 1 second, we are going to tinker with our magnets to try and decrease the amount of time. We shall see how well this works in the next few days.

[sumginazu]

billbo911 - Team 1351's minibot had the exact same problem with falling off the pole. I'll try to implement changes similar to what you did; hopefully we can avoid catastrophe at Calgames. :) We already came close to destroying our extremely fragile Minibot at the Silicon Valley regional. It fell off of the pole twice during matches, where nobody could catch it - the .090 aluminum that forms its chassis barely survived.

Mark (or anyone else) - Other than increasing magnet strength, how can one reduce the wheel slip? I know there are obvious options like changing the tread material, but I'm wondering if any other techniques could also work.

[billbo911]

billbo911 - Team 1351's minibot had the exact same problem with falling off the pole. I'll try to implement changes similar to what you did; hopefully we can avoid catastrophe at Calgames. :) We already came close to destroying our extremely fragile Minibot at the Silicon Valley regional. It fell off of the pole twice during matches, where nobody could catch it - the .090 aluminum that forms its chassis barely survived.


Take a close look at the top of the battery. That "bump" is just a couple pieces of surgical tubing under some electrical tape. The battery is also raised up so that it is level with the robot frame. By doing this, the point that hits the sensor plate first is the bumper on the battery. The tubing compresses helping to dissipate some energy and also forces the minibot into the poll instead of away from it.

http://billbo911.smugmug.com/Hobbies/2011/2011-04-1317-47-53188/1251505732_gUgeS-L.jpg

[Mark Sheridan]

Mark (or anyone else) - Other than increasing magnet strength, how can one reduce the wheel slip? I know there are obvious options like changing the tread material, but I'm wondering if any other techniques could also work.

This sounds like the best solution:


We finished our last regional and did not implement our secret weapon:
(although we did tell the Boston Regional judges about it)
Rubber on rubber (1.2 CoF sliding friction) for the first 3 inches of vertical climb before switching to rubber to steel (0.3-0.6 CoF sliding friction).
We projected that would save us 0.2 to 0.3 seconds and still keep normal force low.

The rubber on rubber is minibot contact to a hostbot ramp that runs vertically parallel to the tower pole and then switches onto the pole before the minibot crosses the 18" line.

I like this idea a lot. It is unfortunate that you did not have time to implement it. I hope your team will be able to try it out at an off season competition.

Bill our calculations show that your minibot should climb in about 1.3 seconds. I think its a little off, I am going to tinker with it a bit. Its set up to have drive-train efficiency different at low rpm than high rpm. Just to be clear, this is a separate efficiency from motor efficiency.

[jacob9706]

You can also modify the transmission or just take it off all together. Do the calculations with no transmission and a 3/8 shaft with surgical tubing as a wheel. I got under 1.5 seconds at max efficiency.

[AlDee]

Late in the process we got a 1.7s minibot working with 1/4" tubes covered with surgical tubing. I never understood why everyone thinks magnets are necessary. If you center the minibot's mass left to right and balance the top to bottom mass difference by angling the minibot a bit, magnets are not necessary. Sure the minibot is not trying to climb straight up but it is also not trying to pull the magnets up with it - and its simpler.

At the beginning of the build season, someone had suggested magnets and we quickly dismissed the idea, thinking with the potential weights involved, magnets would not be practical, except perhaps to hold a latching mechanism closed. As the build season progressed we experimented with many differnt concepts and prototypes. I favored a balanced approach as you describe. I found that anytime we would get close, one tweak would throw the whole thing off. When someone put together a prototype with a rare earth magnet, it became very simple. No muss no fuss, just get it to the pole and climb. That was the direction we went and it worked very well.