Shifting Gears
 

Hello all. I'll keep this short and sweet.
My team and I have decided that we are going to build a multi gear gearbox so that we can shift gears on the fly. We have 2 solid ideas on how too do it and were currently doing all the math. I would just like to here from others any difficulties you had with changing gears. Any words of advice or ideas would be greatly appreciated.

Keep it simple :D so when it do brake down it would be easier to fix.

Make it robust so it won't break down. (then it can be as complex as you want:p)

Shifting on the fly is a challenge.
Bevel the teeth (grind them off at a 45 degree angle) of the mating gears. It makes the shifting smoother. Not all the way, mind you. Just on the edges.
We have tried programming a "synchro-mesh" system that would match the motor speeds. But up to now the computer power wasn't there. Maybe this year.
And you don't have to really shift on the fly. Think about what you are doing at the time you want to shift from high gear to low gear and low gear to high gear. You might be surprised what's really going on. I was.

You don't have to mesh gears. You could have a shifter dog mechanism.

^could you elaborate:D

Gregory Frechou:

http://www.chiefdelphi.com/forums/sh...ht=Technok at

I second the whole bevel-gears thing... we had lots of problems with the gears meshing until we did this.

Another thing I suggest goes to the programmers. When shifting, even our team's better drivers had problems getting the speed right. In the heat of the battle, it's hard NOT to be going in full throttle. As a result, when we were shifting, all of our drivers were putting too much speed in between gears, and the gears were making the clang-clang-clang noise of dhoom from sliding past each other and not meshing. What we did was we used two digital inputs and four limit switches (wired as shown in the 1-min Paint diagram below) to tell us when we were engaged and when we weren't engaged. At any point in time, the High-Gear OR the Low-Gear input must be 1. If both are 0, that means we're not engaged. When we're not engaged, the program limits the speed to something like 1/5th or 1/6th of the maximum speed - slow enough for the gears to finally mesh. This system is nice because its fully automatic - the driver can push as hard as he wants on the joysticks because the robot automatically limits the max speed.

Depending on how easily your system shifts, you might also consider having the program automatically go forward and backwards. We found that if you jiggle between forwards and backwards, it shifts faster. It's something you need to test out on your system and see how it works for you.

Hope all this made sense... it's 1am and I'm falling asleep, so my grammar probably isn't up to par. If you don't understand something, post and I'll get back to you in the morning.

Perform all the math, figure out exactly how strong it needs to be, then build it 3x stronger.

Shifting Gears
 

http://jaw.iinet.net.au/crustyquinns/gears.html

http://www.chiefdelphi.com/forums/sh...ht=Technok at

I'd like to mention that this design is courtesy of the technokats.

Here are some ideas that we have. Please offer any and all words of advice it's greatly appreciated. We are rookies at building gear boxes. We're not sure which equations to use or even were to start. Please help.

Re: Shifting Gears
 

I would hate to have to make that. Although I would feel very proud after I did.

Why shift gears when you can shift wheels? :)

Yeah kickers. 190 did this very sucessfuly in 2001. But i think it takes up amuch more weight than a transmission.

It is heavier, usually, but can offer more advantages as well. Not only does it allow you to change speed and output torque, but other things like wheel base, tread, width and type.

I've helped do it very successfully in 2002 and, with luck, I'm going to do it even better in 2004.

Re: Shifting Gears
 

I just wanted to point out, that on the shaft we had bend, when we redesigned it to avoid the bending, we made it 8x stronger. ;)

Re: Shifting Gears
 

Alrighty, it appears that you're in sort of a bind (bad pun intended) with where to start, so I'll sort of give you some (or a lot) of background on transmissions. Please don’t take my words as absolute truths, but instead use them balanced with some other veterans to come to your own conclussions. Some people more talented than myself may disagree with some of what I have to say.

Thoughts on Transmissions in FIRST.

When choosing a transmission design, you have to weigh a few different aspects before making your choice: The most important aspect is your justified purpose of having the transmission in the first place. Robots can maneuver reasonably well without the potential headache, cost, and design time needed to create a transmission.

The foremost purpose of any transmission is to effectively provide an adequate balance of speed and torque for a given situation. Typically in FIRST, a transmission implies more than one torque and speed setting which is adjustable during a round.

In FIRST robotics, your drive train must be able to do two things effectively:

1. Move from one point to another as quickly as possible, typically in a specific orientation.
2. Maneuver in "high torque situations" against field playing objects and opposing robots.

Without going into the details of the analyses that I’ve performed, the amount of torque required in "high torque situations" is approximately 2.5 to 3 times as much torque as required for standard and high speed maneuvering situations. Essentially, pushing a robot requires significantly more torque than is required for simple turning, which requires more torque than just moving forward and back.

Some quick facts on transmissions are written below. When I spit out numbers, these are calculated. Please ask questions about any and all of it if you’d like more information.

1. The proper gear ratio for using the drill motor in low speed, with 6 inch wheels, a 130 lbs robot with a coefficient of friction of 1.2, while not pulling more that 37 Amps is about 2.5 to 1. Using any less than this ratio with those assumptions will lead your robot to trip the 40 Amp circuit breaker while in pushing matches, no questions asked.

2. The limiting factor in almost all drive trains using a single pair of motors is tripping the circuit breakers of the individual motors. The circuit breakers for the chips and drill motors are 40 amps, while the stall current, (the amount of current that a motor is using while it is exerting is maximum (stall) torque), is over twice that of the breaker. Thus, even if the wheels are spinning due to slip, you can still be pulling more current than is permitted by the breakers.

3. If you choose to couple a drill motor and an Atwood motor in single side of a drive train design, (using 4 motors total) your danger lies in pulling more than 120 Amps which would trip the master circuit breaker.

4. Transmissions are (relatively) engineering and manufacturing intensive, as gears have much tighter tolerances than most other features of a robot, such as some sort of macro level manipulator. They are also much more detrimental when the fail, since when your drive train is not working, neither is your robot. Hence, special care and adequate resources must be applied such that a transmission performs effectively. Failure of your drive train and transmission can completely ruin an entire competition or potentially an entire season.

5. Transmissions, obviously, have a weight associated with them. Typical transmissions have weights between 3 and 8 lbs. per side. Often, this is primarily determined by the quantity of steel required in the gearing and meshing components. Using more than a single pair of motors can further increase weight by 8 to 10 lbs., depending on mounting and gearing required.

6. Transmissions have relatively high financial costs. Since each gear average ~$25, and you’ll be requiring between 6 and 10 gears per side, the total cost adds up quickly. Often times, for fiscially conservative teams, this means that having replacement gears may not be an option.

With those facts put out there in the open, I’ll talk about my personal thoughts on transmissions. Again, please comment as you agree or disagree.

It should be noted that there is a finite (limited) amount of force that a robot can push on the field. In a four-wheel drive system, this is equal to the weight of the robot times the coefficient of friction (stickiness of the wheels to the ground). Even if you have a max output torque by your motors equal to that of a V8 automotive engine, you won’t be able to push more than the weight of the robot times that coefficient of friction. (which is typically in ideal situations, about 1.1 to 1.5)

With all of this being said, I have calculated that you can move at a relatively high speed, ~10-12 feet per second, as well as push other robots with the use of only a single pair of motors, and a pair of properly chosen gear ratios.


To further extrapolate on this, I believe that it is (though admittedly very debatable) wasteful to use a second pair of motors. Below is my justification.

As I stated, a drive train with a single motor pair using proper gear ratios can reach the maximum amount of torque required, which means that a second pair of motors will only provide a different (higher) maximum speed. With that said, I would like to state the primary disadvantages of using a second motor pair.

1. The additional actual weight of a motor pair, the gearing needed to mesh with the first set of motors, as well as the required mounting of these motors could add between 8 and 15 lbs to your robot.

2. You run the risk of tripping your main (120 A) circuit breaker, which means that the entire robot will shut down for the rest of the round. Two motors could pull a maximum current no higher than 80 Amps, while 4 motors could pull 160 amps, dangerously above what will trip the main breaker.

3. At higher speeds, it is more difficult for the driver to accurately control the robot.

4. Twice the motor count means at least twice the complexity. Calculation requirements double, nearly twice the machining for motor mounts double, and at least twice as many pieces will require the high tolerances needed for an efficient drive train.

Hence, it is in my opinion that with the current rules that are in place with FIRST robotics, a second motor pair is a waste of resources, since the primary purposes of a transmission can be achieved with only one pair.

However, YOU need to decide: is it worthwhile to spend twice (or more) the machining, twice (or more) the cost, twice (or more) the design time, and twice (or more) the weight simply so that your robot can move FASTER. You need to decide what is fast enough.

Currently, Team 461 is working on a design that could potentially completely level the playing field for many financially strapped and machining limited teams by designing a transmission that:

1. Requires no machining beyond a drill press and bandsaw.
2. Has a total cost for a PAIR of transmissions less than $250
3. Has weight total less than 4.5 lbs per side.

More details are coming soon regarding that design.

Feel free to ask questions, make comments, or corrections.

Matt

Re: Shifting Gears
 

I agree with 80-90% of what Matt has to say, but the above quoted comment is one that I feel I need to say something about.

In the 2002 season, many of the matches ended up being 120 second pushing matches. In addition, many teams lifted the goal(s) in order to get a higher normal force with the floor (resulting in more pushing force for the same coef. of friction). Finally, FIRST provided 80 Amp household breakers (read: "not rated for mechanical impacts") for the main power.

This combination resulted a number of teams with N motors per side (N>1) having issues with the breaker tripping, especially if they got bumped during or shortly after a pushing match.

Last year, FIRST switched to 120 Amp automotive aftermarket breakers. Between the higher current rating and the designer's expectation that cars see more shock and vibration than typical homes (West Coast teams excluded ;-), I don't know of any teams that had problems with the main breaker tripping last year, whether they had N motors per side or not.

This is not to say that there are not multiple reasons NOT to use N motors per side -- there are many, but tripping the 120 Amp breaker is not one of them (imho).

By the way, I have it from usually reliable sources that given the current breaker and the individual 40 or 30 or 20 amp circuit breakers, it is a very close call as to whether it is even possible to trip the breaker short of a metal bar shorting your main power feeds.

Depending on the state of the charge of your battery and the variation limits of your particular battery's internal resistance and the lengths of your wiring harnesses, it may be almost impossible to trip the main breaker.

Having said I agree with Matt, I will point out one reason for multiple motor drives that he ALMOST points out but does not quite complete the thought.

As Matt points out, in many cases, the 40 Amp breaker is the limited factor on the output of your drive system.

The consequence of this is that if you have 2 motors per side, you have 2X the 40 Amp current limit.

This was the case with our robot last year. In normal driving (i.e. practicing), our robot worked just fine. It had a good balance between top speed and turning/pushing torque. But after a while of "competition driving" the breakers would poop out on us at the whimpiest of pushing matches.

So... ...we retrofitted an extra set of motors per side for the sole purpose of pushing back the limits due to the single 40 Amp current path.

For what it is worth...


Joe J.

Re: Shifting Gears
 

I'll take an 80-90% from Joe any day. :)

To comment on what Joe is saying, I understand his point. In addition, I have a few hunches as to why there were teams that used multiple motors and didn't trip the 120 Amp breakers:

Most of the time, teams who are using multiple pairs of motors have excellent designers in the first place, or copied the designs of excellent designers. In pushing matches, the teams were in their properly designed high torque gear set. This should have caused them little fear of tripping much of anything.

However, I would bet the farm that a robot in their high speed gear set pushing up against a wall would trip the 120 Amp breaker before any individual motors would give out.

So I'll definitely agree that Joe has a viewpoint that is very valid. My point was just a heads up: It theoretically possible to push that 120 Amp limit if your gear ratios are not designed properly. You must design your ratios with a limiting factor of the 120 Amp breaker if you use more than two motors.

If you assume that since you're using twice the number of motors, you can make your speed ratio twice as high, you'll be in trouble, since you don't have twice the power due to the 120 Amp circuit breaker. If the game changes this year to requiring some sort of manipulator that could be pulling another 10 or 20 amps... be alert!

To summarize, I'm giving a theory viewpoint for the new designer, while Joe is calling it like he's seen it (which is essentially that teams have designed multiple motor drive trains well! Woo hoo! :)).

Thanks for the comments, Joe!

Re: Shifting Gears
 

Also don't totally forget about another reason for multiple motors -

Redundancy.

We used 2 per side last year and had the drills fail twice in the season, the previous year the fisher price failed 3 times. However the robot kept on moving since more than one motor was linked to the drivetrain (Although one side had less power we were not a sitting duck that drives in circles)

For our robot last year with Tank Treads, we were able to use a version of the 45 design that gave us the power we needed to push hard and spin the tracks against an immovable object and in high cover the field at a very quick pace. Both of these would have been somewhat less if we didn't have both the motors teamed together. We still would have had a high and low but they would have been slower. Also anyone considering building a transmission should be lightening out all of the gears to a geat extent to reduce the weight so that the total added is much less than 8-9 pounds per motor. We have shown that 20 pitch gears can be lightened nearly 80% and still hold up to the duty required of them in a 130 lb Drivetrain for an entire season.

So I guess I disagree that more than one motor per side is a total waste in a FIRST robot, it all depends on what you want your robot to do. Too fast for some people is not too fast for others. I have seen many people in complete control of RC Nitro cars at over 50 MPH and I have seen others that can't drive straight a 5MPH.

Redundancy is a 2 edged sword...
 

One thing about redundancy: It can hurt you some times too.

On several occassions we have had one of our multiple motors fail or partially fail and not discover it for several matches.

The machine works, but it works at a lower level of performance.

This year, our team is planning on putting in a "diagnostic mode" in which every motor is tested while monitoring sensors (current, speed, etc.) for out of range values. We are also planning on having sensors similarly checked automatically.

I don't suppose that it will find 100% of the problems, but it could help us avoid a full speed crash into the wall during autonomous simply because of an unpluged cable (this really happened last year).

Joe J.

P.S. Breaker ratings have safety factors in them. Essentially, they can carry the rated current for infinite time. At some percentage above that rating, they trip after some time depending on the percentage of over current.

Bottom line, I estimate that a robot could probably dead short the battery for 5 or 10 seconds before it would actually trip the 120 Amp battery (don't try this at home as batteries do not like such dead shorts). Given our current battery, with the motors we have and the required secondary breakers, I think it would be hard to trip that main breaker.

two separate decisions
 

I think you made a great point Matt about each team needing to decide if they really need to add the complexity of a shifting system. It depends on the game, the field, and your strategy (none of which we know until after Jan 10th). You shouldn't add it unless you really need it. The simplicity and reliability gained by having a non-shifter is at the very least worth considering, especially for teams who are doing it for the first time.

Regarding whether or not there are benefits to multiple motors: there are. Adding the extra motor per side allows you to pick a numerically lower gear ratio that allows a higher top speed. Or if you choose to leave the ratio the same you will have more torque. Yes, it draws more current, but since a lot of the time you are in transient high current situations (i.e., just passing through a high current operating condition), you don't risk tripping the breakers. There are some teams that play it riskier than others, and those of us that have taken the risks can usually recall instances where we paid the price (ours was in 2002 where our 6-motor drive was in high gear and we tried to turn the goal 180 degrees - the main breaker tripped; this would not have happened in low gear).

2002 was the killer year. You needed speed (to get to the goals) and torque+traction (to move the goals). If ever there was a drivetrain-intensive year that was it. Multiple motors and shifting transmissions were very helpful.

I'd consider adding an extra motor before considering adding an extra gear ratio.

Ken

Re: two separate decisions
 

If Im not mistaken, while it draws more current, you are drawing less current per motor since the load is being shared between four motors rather than two. Since weve all basically agreed that the main breaker isnt going to trip, this would help keep individual motors from tripping their respective 40 amp breakers.

Cory

Re: Shifting Gears
 

there was an idea floating around to blast the breakers with some liquid nitro or pack em with dry ice so they wouldnt trip. i dont know if anyone did it tho or even if they were allowed to

Re: Cooling Breakers
 

I know that in the past there have indeed been teams that have done this. While it's not forbidden in the rules, what you're really talking about is trying to get around a safety issue by modifying your breakers to a lower temperature so they don't trip. This is NOT a good idea. Tripping breakers shouldn't happen in the first place if everything is properly designed. The one exception may be during finals where you have back to back to back matches and breakers and motors heat up over extended use.

The most common way to cool your breakers is to take some of that keyboard cleaning compressed air. For long days of demonstrations, we've used compressed air to cool our motors down. However, this is honestly a pretty terrible engineering practice, and is NOT recommended.

Re: Shifting Gears
 

we had a 2 shift transmission last year which could shift on fly...

low gear- 4 ft. per sec. 960 lbs of torque...
high gear- 10 ft. per sec. 416 lbs of torque...

Re: Shifting Gears
 

The biggest problem we had with our dual motor, dual speed gear boxes were stripping the 15t gears on the drill motor sides when going back and forth, it cost us a lot of matches in Annapolis.

Re: Shifting Gears
 

in-lbs. or ft-lbs.?

Re: Cooling Breakers
 

They actually make something called "freeze spray" - it comes in the same size and shaped bottle as the keyboard cleaning stuff, though. Yeah, you get the same effect, but since freeze spray is designed for cooling rather than blowing, you could probably get more for your money or cooler temps. It's also antistatic.

Can't give you any product brand or where my team got it... look around - I'm sure you'll find it.

Re: two separate decisions
 

You are right Cory, 2002 was the last time a lot of teams had problems with this. Although, I'm not sure that its simply a matter of us agreeing that it doesn't happen anymore. I remember an extremely fast machine (25) that appeared to have breaker problems in the finals at Houston last year. I don't know for sure if this was a main breaker problem or not.

Ken

Re: two separate decisions
 

That was them tripping thier main breaker. What team 25 had in 02 and in 03 was a servo actuated break that pushed into a whole drilled into one of thier drive gears that they used as a break since they were geared high both years so not much pushing power. The pin didn't come out all the way and thus when trying to moved they pulled a nice 220A or so and popped the breaker.

Matt, what method of shifting does that transmission that 461 is working on use? The only thing I could think of that does not require more than a lathe or mill would be a mesh shifter.

Re: Cooling Breakers
 

The only difference between "canned air" and "freeze spray" is that on the inside of the can one is up-side-down.

Re: Shifting Gears
 

design it as simple as you can, it makes it easier to fix.

If it is complex, try to design it so you can repair it easily, or have spare trannies made, so that a tranny repair wont take you hours.

Re: Shifting Gears
 

This is absolutely true. Ideally, having a transmission that is modular is the way to go. In addition, in my humble opinion, your transmission shouldn't have reductions on the order of 1:50 and 1:20 for instance, they should be just an initial step, of say 1:1 and 1:3 so that if for some reason the whole thing explodes, you can just pull it out, move a motor and be at that 1:1 ratio, which is something on the order of magnitude that your robot will move.

Secondly, having a tranmsission that you can easily modify is very ideal. For example: don't weld gears to shafts! If something strips or breaks, you'll have to replace the entire shaft. Bad news, especially if there are multiple gears on the same shaft! Not only obnoxious, but time consuming and expensive.

Matt

Re: Shifting Gears
 

<rant>
There is something that i feel every member of a FIRST team and anyone involved in engineering in general should read and take to heart:
http://www.seattlerobotics.org/encod.../classics.html

That said i agree with previous posts, which state that the benefits of shifting should be carefully weighed against its disadvantages. As hard as it may be, decisions should be made not on emotion, but rather objective analysis and common sense. One thing is see all to often is something which is done merely because it is cool.

The first step in determining whether a shifting transmission (or any feature) is right for your team is to ask why do we need one? Every good design should start by clearly stating what problem(s) it is meant to solve (not the inverse!).

Are you trying to achieve higher speeds? higher torque? faster accereration? A better balance between the three that does not involve compromising at least one of the others?

After assessing the need for a feature, one must then ask "do we have the resources to design, fabricate, test, and pay for this feature all within the set time constraints?"

If the above criterion are met then i would say go for it!... But remember a good bot, or anything else for that matter, is built just as much if not more by careful planning at the systems and functional level just as much as by engineering prowess. This is a lesson that our team learned the hard way last year, which could have easily been avoided.
</rant>


this thread seems to have forked off into a discussion of two discrete but related topics and thus i will address them induvidually.

>1 motor per side
If weight permits and current is not an issue, i see no reason not to use multiple motors on each side. There are 2 advantages i see in using multiple motors. First and most obvious, you get more power to work with. Depending on how your drivetrain is designed this can translate into more speed or greater acceleration and pushing power. Depending on the game this year speed and or torque may be of great advantage. Secondly, someone earlier mentioned redundancy. If One motor fails, your robot will still be able to move (to some extent) rather than being completely disabled. Given the high mortality rate of the drill motors redundancy is definately a good thing( the reasons for this are another topic). In the IT world, redundancy is pretty standard on "mission critical" servers etc. The disadvantages of multiple motors seem minimal. Yes there is added weight, but not much. <rant>Most of the gearboxes i have seen in FIRST are ridiculously overengineered and much heavier than need be. using 1/2" aluminum plates and gears capable of transmitting in excess of like 50hp under shock loads is just not necesary. Look at the gears in the drill gearboxes and the Technokats transmission. Have you ever seen one of these break?</rant> The weight added by multiple motors and gearboxes to couple them really isn't that much.

Shifting Transmissions
I think that most people would agree with me that a good shifting transmission has many benefits, for example your bot can have a normal gear for general driving and then a low gear for pushing matches and more delicate menuvers. There can also be big disadvantages associated with shifting. Shifting gearboxes are (generally) heavy, difficult to engineer and fabricate, and possibly unreliable. That said, if there is good reason to have a shifting and your team has the resources to implement such a design, i see no reason not to have one.

Re: Shifting Gears
 

Absolutely correct. However, I think the past two years have really demanded a hearty drive train. See below.


To clarify, if you are referring to the part of my previous post where I brought up using multiple motors (which you very well may not be), I'd just like to clarify that I believe that multiple motors on a shifting transmission is unnecessary. However, I whole heartedly believe that it's very practical and essentially required to have multiple pairs of motors if you do not have a way to change your gear ratios.

I would fight to the tooth (with numbers of course!) that you can not have a robot that is both competitively fast and competitively strong using only one pair of drill motors or the chips without a mulit-ratio transmission.

Perhaps that'd be a post for later.

Re: Shifting Gears
 

That statement depends very highly on the game that you are playing. For example, last year you did not need to have pushing power if you weren't solely a ramp dominator. Take team 25, one speed, 4 motors, 12.5ft/s. All you need to move someone off of something, top of the ramp for example, is momentum (p=mv). No where in the formula for liner momentum is torque taken into account. Then you could also argue that team 25 was a strong robot for defense because of thier brake system. Or like any other robot with a high coef. of static friction, you can just turn sideways for defending a stack, or in previous years a goal. A 4 motor drive geared correctly for one speed can always be competitive.
Take last year for example, FIRST made it a little easier on everyone by making the ramp top out of hdpe so that way everyone would have a fair fighting chance for it at the end of the match. In most cases a fast robot could charge the ramp at the last few seconds and push down a stronger robot with thier momentum,which lead other teams to develope a method to keep thier robot attached to the hdpe by means other than just a drive system. Innovative new ideas can beat widely spread old ideas.
When picking your drive and wether or not your team wants to shift gears, analize the game first and then decide wether or not you actually need to shift to accomplish all the things you want to do in the game. If you can think of more reasons not to shift gears and concentrate somewhere else on an arm or such, then you should probably put a 2 speed tranny off to the side untill needed. If need the most pushing power or the most speed to accomplish your goal, then concentrate in there if you do not have the resources to make a shifting tranny. If you make a list of things you want your robot to do and high mobility is on the list, then I would suggest investing time in a swerve/crab drive rather than a 2 speed 1 dirrection drive.
One thing to keep in mind for everyone designing your multispeed tranny's: the shifting mechanisms can wear down to the point where they become unshiftable. You may want to keep that in mind with some methods so that if you are forced to lock it into one speed, you'll still be able to be competitive in that speed. Meaning don't set a goal of your robots high speed to go as fast as possible without tripping breakers, but then also try to be fast enough where you can be competitive if need be to lock into low gear. For that reason you also may want to make your multispeed tranny have 4 motors, since in many cases adding in another motor will only require 1 or 2 extra gears and so the power added is well worth it. You can also design it so if need be, you can add a motor or easily take out a motor if you decide to use it for a mechanism or if you just can't make weight. Just some of my thoughts on the subject...

I need more power Cap'n!
 

I disagree with the statement that there is no reason to have multiple motors if you have a shifter.

They cover similar problems, but not EXACTLY the same problems.

If you need or want more power (e.g. for faster accelerations) multiple motors gives you that. Notice that I use the term "power" in the engineering sense. I do NOT mean "more low end torque" as many do when they use the term on these fori.

POWER = WORK PER SECOND <-- SPEED X TORQUE in our case.

Switching gear ratios can help you get more power up to a point by choosing to load the motor such that it is providing its peak power (i.e. at 1/2 its stall torque) but if you need or want more power than that, more motors are your only real solution.

Joe J.

Re: Shifting Gears
 

Our team actually takes the opposite approach to this, we try to weld anything that we can but we start with robust materials that are highly unlikely to break. Welding the gears to the shaft after everything is the way you want it takes one more failure point out of the equations (Assuming you have someone who is excellent at welding versus other options such as set screws, pins etc) Again this past year in debug of the Team45 transmission it was a hare narrow and ended up being in high & low at the same time in a high power shift. This immediately locked up the transmission and send a shockwave through the gears, shearing the pinion off the drill motor, all other welded gears were unharmed and lasted the entire year and are still going strong. As stated above, many people use gears that are more than up to the task of anything the FIRST robot can throw at them. Personally, I like 20 pitch gears that have been lightened to an extreme amount, the teeth show little if any wear after a season. Since the drivetrain is arguably the most important part of the robot (Manipulation can be useless if you robot is stranded on the wrong end of the field) I try to have the team place a priority in power, modularity, and robustness there.

As for replacing a broken shaft, it should be a rule of thumb that you make a spare of anything that is custom and has a chance of failing. For transmissions I would strongly recommend making one that is similar left to right and fabricate a spare at the same time as you are fabricating the mains.

Re: Shifting Gears
 

Emphasis to PAIR and OR added by me.

Emphasis to 4 motors (implying more than a single pair) added by me.

I think that Rob just read my post a little too quick. (No biggie ;)) I think you can easily be competitive if you have:
1. More than one pair of motors (4+)
or
2. A shifting transmission

My thesis is this (restated):
You can not be competitive using only two motors total and no shifting mechanism.

I definite "competitive" as a max speed greater than 10 feet per second, and having a maximum applied force of 150 lbs.

Using the above "competitive" criteria, this is simply not possible, and not really a really up for debate. Perhaps my definition of competitve could be debated, as it does depend on each game.

If you'd like some quick numbers, just let me know!

Matt

Re: Shifting Gears
 

I guess I ought to clarify my point here. The reason that I suggested against welding gears to shafts is essentially for a couple of reasons:

1. If you break teeth on any gear on that shaft, all the gears on that shaft are essentially garbage because you can't remove them.

2. Welding around gearboxes has potential for splashing of metal in between teeth inadvertently, and that's BAD NEWS.

You said that you had to replace a drill motor, but your gears were unharmed. Personally, I'd much rather replace a gear than an entire motor since they're cheaper and easier to attain, and easier to have spares of too.

To follow with that, I always try to use keys since the allow you to salvage your gears after a critical failure, but have shear strength much higher than set screws or pins. I'd agree they don't have the strength of a nice solid weld, but read on...

If you were careful and took some time to calculate the absolute max torque that you would have on a properly functioning gear box (and add a little safety factor for good measure), I think you'd really like to have whatever is holding your gears fail first, rather than the teeth on the gear or a shaft within the box. I'd much rather replace a key, pin or screw before anything else.

Just a few more cents on some really good thoughts flowing from a lot of folks. Keep 'em flowing.

Matt

Re: Shifting Gears
 

Well... of course!

As Joe mentioned above... that's what Power is all about... and any single pair of our motors simply don't have enough to handle the speed, AND the torque you mention simultaneously.

All you rookies listening?
This is one of the most common "errors" people make:
"Our robot is fast as hell! And it can also push the world! We don't need a shifter, or other motors."

*bzzzzzzt* Try again.

Unless you shift, or have extra motors... my robot will always either be able to out push you, or out run you. NO ARGUMENTS WILL CHANGE THAT.

Physics, does not lie (only FIRST team representatives do). :rolleyes:


I agree 100% with what Matt stated (and more importantly, so does Physics).
I also typically use a ~1.2 coeff of friction (130 lb robot * 1.2 coeff ~ 150 lbs pushing force) and 10-11 feet per second as my benchmark points for low and high gear. It is NOT physically possible to have both using the drills, chips, FP individually. (Unless.... you shift!)

*phew*
This is a sore subject for me. I'm sick of 80%+ of FIRSTers not understanding this MOST FUNDAMENTAL concept of drivetrain physics. Everyone should take the time to understand the principles behind the robots... (it makes scouting a heck of a lot easier when you can recalculate a robot's theoretical performance based on some quick questions, and a mental calculation).


Again... if anyone has questions on stuff like this, you are more than welcome to email or IM me. There are also plenty of whitepapers, and threads covering topics like this. Or... ask your friendly neighborhood FIRST mentor/engineer!

John

Re: Shifting Gears
 

Not in the case of the Technokats tranny. From what I've read in the past they went through a lot of trouble to get that dang gear, but the motor was no problem. The drill motor is actually very easily obtainable.

Re: I need more power Cap'n!
 

Quote:

Originally Posted by Joe Johnson I disagree with the statement that there is no reason to have multiple motors if you have a shifter.

Everything you said is completely correct, as I would expect out of any Purdue alum... :)

I guess that my two cents come in that I think that you can have "enough acceleration" and your max speed can be "fast enough" and your max torque can certainly be calculated to be sufficient with one pair of motors and a well designed shifting gear box. And by "enough", I mean reasonably competitive.

Obviously a robot with 4 motors will be faster and accelerate more quickly than one with only a single pair. My opinion comes in saying that the estimated 10-14 feet per second you can get out of a high speed ratio gear set gives sufficiently competitive speed and acceleration for most applications.

This is only an opinion. I belive that often for many teams, the weight required and additional engineering time could be better used elsewhere. Certainly that is debatable, and your point is definitely well taken.

Matt

"Enough" is relative...
 

I agree that depending on the game, your choosen strategy and the skill of the operator, it is quite possible to make competitive robots with only one motor per side and a shifter. I'll even go farther than that. It is possible to make competitive robots without a shifter and only one motor per side (again, depending on the game, your strategy and the skill of your drivers).

My point was that power is a main reason for adding motors, even with a shifter.

How much power is "enough" is up to each team to figure out.

One more comment on this more power issue. When I say "more acceleration" I do not just mean quicker changes in velocity. While it is possible that this will be an important design consideration, I think that it is quite unlikely that the time savings alone are sufficient to justify needing more power in your drive system. Think about it. Is it likely that the half second or so you saved because you got up to full speed in 1 second rather than 2 seconds is going to win you a match? I doubt it.

Having more power for acceleration is more than just having a high DV/DT. It also includes moving at a constant speed up a ramp for example.

Another reason that acceleration is important is that until your robot gets moving, your motors are effectively stalled. The breakers cannot tell the difference between stalled motor current and current from accelerating your robot. So... ...by having more acceleration your motors are operating at this "stalled" condition for a shorter time period. Translation: your breakers will not trip as often from hard accels and decels. Cooler Breakers give you more margin for operation. You will not have to shift out of high gear for a minor skirmish with an opponent because your breakers will have that reserve of current for just that much longer than they would have.

Margin of error for the operators. Now THAT wins matches!

By the way, I am not a huge fan of multiple motors per side, but I just think that the decision should be made with a fair accessment of the merits of each case rather than a hard a fast rule that shifting is the answer, or multiple motors is the answer, or that multiple motors + shifting is the answer, etc.

Just some more things to ponder as you weigh things in the balance.

Joe J.

Re: Shifting Gears
 

in-lbs....

Re: Shifting Gears
 

Wow. This thread has been great for me. The sensors and coding I can handle with our students, but I'm very worried about our drive system for our team.

If I have followed this thread correctly, experience suggests that wise goals for a drive are...

a) Top speed of 10-12 ft/s and
b) A pushing force of 150 lbs.

These goals are a good target for most of the prior years of FIRST competition relative to the max amps and motors in the FIRST kit. Did I read correctly?

Now, given the above, possible schemes for achieving these goals (again with the restrictions of amps and motors) is to...

A) Use a pair motor configuration (one on each side),
B) Use A) with a shifter,
C) Use mutliple pair motor configuration (more than one motor on each side), or
D) Use C) with a shifter.

After reading the thread, my comments are that A) does not lead to competitive capabilities but what is the easiest for a Rookie team to achieve. Some form of B) and C) are the suggested routes for a competitive robot but require significant but equal investiment in terms of design, fabrication, blood, sweat, and tears. Finally, D) offers no competitive advantage over B) or C) and only compounds the blood, sweat, and tears.

Using my active listening (reading) skills, I've wanted to echo this back to the thread to see if I've digested the thread properly and also to refocus the thread/debate to more of a tutorial/guidance/sage suggestions for Rookie teams.

Re: Shifting Gears
 

I am very glad to have the summary. Thanks.

Except, I disagree with the declarative statement that multiple motors combined with shifters provides NO competitive advantage.

They clearly can provide benefits -- there are 2 questions you need to ask:

#1 Is there any benefit in your particular situation, given your robot and game strategy, ?
#2 Are the extra pain and suffering required for your team to implement the system worth the possible benefit?

I think that the answer is very likely that for most teams and most game strategies, the extra benefit is not worth the cost, but that is not always the case.

Ask the TechnoKats how important a shifting multimotor drive was to them the year they had to auto-balance with two goals on the on the ramp (2001)?

Ask Las Guerrillas how important it was to them the year they grabbed & lifted the goals (2002)?

It can be EXTREMELY important.

But perhaps not for your team depending on how you choose to play next year's game.

Joe J.

Re: Shifting Gears
 

Since I'm the one who made the strong and somewhat intentionally controversal comments regarding multiple motors, shifters, "competitive" definition, etc... I suppose I should comment.

To meet the two "competitive" objectives:
A. Top speed of 10-12 ft/s and
B. A pushing force of 150 lbs.

This can be accomplished with any of the following:
1. 4 or more motors coupled together
2. 2 motors coupled with a multi-gearset transmission
3. 4 or motors coupled with a multi-gearset transmission

What is factual is that you can accomplish A and B with either 1 or 2.
Dr. Johnson has no qualms with this.

His concern from my understanding is that "competitive" is not universal. He gave two perfect examples of sitautions where the needs of certain teams exceeded that of A and B, and 1 and 2 would not suffice.

i.e.
If your requirements exceed that of A and B, or if you would like to be "extra competitive", option 3 can (and will) provide that.

My thesis is that for MOST teams, MOST of the time, in MOST games of the past, you could be "competitive" using MAINSTREAM strategies without using method 3 above. Because you could do it without 3, anything more than 1 or 2, I feel, is wasteful (over engineered).

Matt

Re: Shifting Gears
 

I admit that the qualifications of A and B went past me upon first reading. Given those qualifications, then I agree, there is no reason to have BOTH multiple motors and shifters.

Joe J.

Re: Shifting Gears
 

Disclaimer: Beware this post... actual math may be involved!

Allright... I'm going to have to humbly disagree, and challenge this statement.

When I read this... it essentially shattered my little "shifting is necessary" paradigm, and it scared me enough that I actually decided to check the numbers. (School is out... I actually have time to run some numbers!) So... let's walk through the calculations I did, and see the results. Does the above statement hold up?

Sorry Joe, Sorry Matt... my numbers say you are both wrong.

The 3 major factors here are:
1. 10 fps max speed
2. 150 lb linear bot pushing force
3. 4 Motors, no shifter.

First off... Set up some assumptions:
I'm gonna say, let's be nice. I'll assume 100% efficiency for the whole gearbox, (cuz I'm a nice guy so I'll break the laws of physics for you gentleman ;)). Notice, I'm also using the SLOWEST "competitive max speed" you mentioned (10fps). I'll also design to achieve this at the motor's free speed (again, cuz I'm a nice guy... and we'll assume no speed loss anywhere in the gearbox).

What does this mean?

Basically... I'm giving Joe and Matt the benefit of the doubt. I'm setting things up so they are COMPLETELY in Matt/Joe's favor. I WANT to prove myself wrong.



So, let's begin...
First, we'll design our combiner.

I'm going to use the Drill and the Chip, the 2 most powerful motors in our kit (again, just to be mean to myself, and nice to Matt/Joe). I design to match free speeds at 3688 RPM. (A 15:80 reduction off the Drill, and a 12:18 reduction off the Chip... gives us about a 99% speed match). I'm happy.
Based on my combiner theory this means the new "super motor" has a stall torque of 7.97N*m, a stall current of 234 Amps, and a free speed of 3688RPM. All of this is done at 12V for both motors.



Now, let's find our "10fps" ratio.

I'm going to choose a 6" dia wheel, simply because it's common. If we want our robot to go 10 feet per second, using a 6" wheel the wheel needs to be spinning at about 382 RPM. Now, we know our "super motor" free speed is 3688 RPM, so what reduction do we need between the motor and the wheel?
(382/3688 = .104).
So, we'll use this as our gearbox reduction.



Now... let's see what kinda max torque our new box is capable of.

Our "super motor" stall torque is 7.97N*m, which means (1 side of) our robot has a stall torque of (7.97 / .104 = 76.63 N*m), again with a 6"dia wheel, this is about 113 lbs of linear pushing force. Using 2 gearboxes this yields 226 lbs of pushing force (at stall). So yes... your gearbox has fulfilled the design requirement of 150lbs. However... the super motor is at stall, and drawing 234 Amps... which is bad.
I'm not happy with it, are you? Let's see how bad it is.



So, let's see what kinda current our robot draws if it's pushing 150 lbs.

150/2 = 75 lbs per gearbox. Using a 6" wheel we can see that we need 50.84 N*m of output torque from each box to achieve 150lbs of linear pushing force. To calculate the torque load on our "super motor" we simply say: 50.84 *.104 = 5.287 N*m. Now... how will our super motor perform under a 5.287N*m load? Well... by distributing the load, and calculating, I show that the Drill Motor is drawing 85.09 Amps, and the Chiaphua is drawing 71.76 Amps. These values are unacceptable for our competition. You'd burn up breakers if you tried to push for any extended amount of time (which is often necessary).

Simple solution to this?
Add a shifter to your robot.


Okay Joe/Matt... am I missing something?
I come from the "shifting side of the tracks" and am a huge fan of using a shifter to solve all my drivetrain problems. I'm also super-conservative when it comes to motor loading, and I always worry I'm going to draw too much current.

Based on my calculations, your above statement is wrong.
If you want to achieve Matt's "competitive speed" and his "competitive pushing force" in the same bot, using only 4 motors... you NEED a shifter. Otherwise... you're going to pop the breaker and sit there dead the whole match.

Prove me wrong... ;)
John (who sometimes has issues with arithmatic and algebra)


PS - I want to emphasize this was all in good fun, and in the interest of learning. Can anyone show me if I went wrong somewhere?

Man.. this was fun! A real live enginerding debate on CD that involves... *gasp* NUMBERS!

Re: Shifting Gears
 

No, you wouldn't be dead the whole match. It would be more like a sputtering car that is in the process of running out of gas. The robot would go full speed for about 30 seconds, you would get an advantage in the match, and then it would start cutting out on you. It would go for about 4-5 seconds and then stop. This would repeat until the end of the match. ... unless you had a shifter. Then you could just shift into low and continue on with the match.

I agree with the above analysis that John made, and I agree that shifters are extremely beneficial. We on team 45 have shifted gears every year since 1999 and we have learned that the #1 advantage to shifting is saving your breakers... and the side benefits are the immediate response of power or speed.

A circuit breaker on a FIRST robot can handle amps above it's rated limit, but just not for too long. If a robot is in high gear for 10-15 seconds, under harsh driving conditions, the breakers will hold up. However, after those 10-15 seconds, the robot driver better start thinking about shifting to low, because the breakers are getting low and starting to give.

The trick with these little circuit breakers is that they don't trip immediately at 40.1 amps. You can overload them for a while, but not too long.

In many matches over the past 5 years, we would run in high gear for the first 40-60 seconds (depending what we were doing) and then shift to low for the latter part of the match. This relief to the breakers would enable us to run in high during the last 5-10 seconds, if needed.

At least, that is from my experience.

Also, on a different note, I would contend that 150 lbs of pushing force may not be enough for a top-tier robot. There were robots which could push with more force than that during the last 2 years. Last year, many robots had unbelievable pushing power on the grating. I would up that number to 200-250 lbs. to be conservative.

Andy B.

Re: Shifting Gears
 

As I made that final blunt conclussion in my last post, I thought to myself, "You know what Matt "Number Cruncher" Adams... you never really did crunch numbers on using 4 motors. "

Furthermore, I think I concluded in my powerpoint presentation that using both motors will get you about 85% of 461's max speed last year with no worries of tripping breakers... which was theoretically 7 ft/sec.

Just now I cranked some numbers, and found the "best you can do" with only 4 motors and no shifter is about 6 feet per second without tripping breakers.

Of course, I also briefly ran over John's numbers and as best as I can tell...

John is correct.



Furthermore, I'd like to say in some respects, this is actually pretty obvious; 4 motors won't have the same advantages as shifting, and hence why John probably said, "Wait a second..."

If you look at both the drill and the chips, they're reasonably close, in terms of output torque at 40 AMPs at the same RPM. Since you have an argueable limit near 120 AMPs on your main breaker, you can only add an additional 40 amps in addition to the 80 A max you have from two drills (due to thier individual 40 breakers). Essentially, that's just like making your drivetrain 50% better. That's not enough to be truly competitive, though it's admittedly better than just a single pair with a single gear ratio. With a shifter, you can have torque ouput (or max speed) increase by factors of say 2.5 or 3. That's a big difference.

Thanks John!

Matt

P.S. To redeam myself, I'll crank out some numbers again with a shifter sometime soon to show how this is a different for those who are wondering.

Re: Shifting Gears
 

I also want to re-emphasize something that Joe mentioned: the definition of "competitive advantage" really depends upon the game.

I agree that in your typical game, the numbers you present above (especially the 150 lb pushing force number) would be about your pushing limit. However, in the 2002 game, we determined that our robot would need to push with a force of about 720 - 960 lbs before our treads would start to slip. For this game, having 150 lb. of pushing force would not be an advantage at all.

The point is: assuming a peak pushing force number before the game is announced can get you in trouble... unless you have a shifter. Then you can just pop in a new ultra-low ratio. However, your strategy may be such that a shifter is irrelevant.

Re: Shifting Gears
 

The what-if discussion has been very important for me (as a rookie mentor). I appreciate the hashing out of the various "theories". It may seem irrelevant to discuss strategies without knowing the game, but the experience that each of you have with regard to prior years and the debate relative to those years is very enlighting.

With regard to shifters, I'm assuming that teams do not design to shift on the fly. This thread has mainly discussed the design arguments and complexity of multi-motor/shifters.

My question would be what are the operational "gotchas" and "goodies" about the various theories. How well do the student operators handle shifting during competitions? How much awareness (training) does shifiting require? How does shifting affect autonomous mode?

Transmission Styles
 

Actually, many top tier teams do design to shift on the fly. One of the primary reasons teams use transmissions that do not shift on the fly is is due to the additional complexity in machining that's typically required for these sorts of designs.

However, if I had more time, I'd press the arguement that though most definitely a nice feature, shifting on the fly isn't a game breaking feature.

However let it be known that there's a difference between:

1. Shifting on the fly.
2. Shifting after being stopped momentarily.
3. Stoping, shifting, and slowly moving forward for components to engage.

I would strongly suggest that transmissions of type 1 or 2 be used...

If I didn't have finals this week.. there'd be much more to this post. I'm sure others will follow through with their thoughts as they have done so well before.

Let me also note that most of the time shifting is actually done through software, so "learning how to shift" for drivers isn't exactly the same difficulty of learning to drive stick on a car. When to implement it in a match, however, is a different matter.

Matt

Re: Shifting Gears
 

What if you had treads driven by a 3" pully instead of 6" wheels? I'm pretty sure that would satisfy the 10 fps and 150 lbs requirements without burning up.

Re: Shifting Gears
 

Not at all!
If we used the above designed gearbox with a 3" pully... it would be going half the speed, with double the output torque.

We're not talking about coeff of friction at all... sure the treads might end up being a little more "grippy" but... that doesn't matter in this debate. You can have all the traction in the world, but unless your gearbox can output the torque to back it up... you're just wasting your time. Just like you can have all the output torque in the world, but without traction, you're doing the same.

Given a set amount of power output, there is only a certain speed you can go while comfortably (without drawing too much current) pushing with a given force. The above proof I did, was to show that the drill and Chip alone, geared at 10fps cannot produce a 150lb linear force "comfortably".

Sure, gearing the system down to 5 fps would probably do it... but that's besides the point of this debate.

You can't have your cake and eat it to.
You can't have "tons of speed" and "tons of pushing force" at the same time, unless you have more power than our current motors allow (unless... you SHIFT!)



Again, the "competitive" level of speed and pushing force varies each year depending on the game. Some seasons 10 fps is enough, some seasons it isn't (like in 2002 if you wanted to beat 60/71 to the goals). Some seasons 150 lbs of linear pushing force is enough, some seasons it isn't (like in 2002 if you wanted to fight back against 308 when they had the goals lifted).

At the beggining of every drive-design process, we all have to set ourselves some specifications we want to fulfill, decide what the drive must accomplish to play the game to the level we want to play it. We basically need to do some guesswork about what our robots will need to deal with during the upcoming season. Any team in 2002 that thought "wow... our robot can push with 100lbs of force... we're unstoppable!" was... mistaken.

Also, if we decide "we are going to lift the goals so we can push harder". We then need to determine exactly how much torque the drive needs to be able to output, in order to back that up. Otherwise... as stated above, we're wasting our time. Imagine if 60 hadn't downshifted after lifting the goals in 2002... a pushing robot would have gotten in their way, and their motors would have stalled. No good! They needed their low-gear torque.

For those who are having trouble understanding some of the basic principles invovled in this debate, I recommend this presentation:
http://www.teamfordfirst.org/_docume...0(12-7-02).ppt

It's the best one I've seen in terms of throughly explaining drivetrain theory. (Paul's the man. :)) If you have any other questions, search around this forum, ask your friendly-neighborhood-FIRST-mentor, or drop me an email/IM.

John
(Who last season was on here learning for himself about drive physics, and never thought he'd be the one doing the ranting/teaching.)

Re: Shifting Gears
 

Actually, the above clips brings up a question....perhaps maybe should be a new thread....but what the pros/cons and arguments for tract vs wheeled drives (or other configurations)?

Re: Shifting Gears
 

Ahh... you're right... I'm an idiot.
::Kicks himself for forgetting elementary physics::

Re: Shifting Gears
 

I would like to STRONGLY echo what IrisLab says ... Although this is my third year as a mentor, it is the first year I am heavily involved in drivetrain design. While my career experience has been limited to static structures, these discussions, and especially this thread, have greatly increased my awareness of the important details to consider.

THANKS GUYS !!

And you can fly if you pull hard enough on your bootstraps!
 

The size of the wheel is immaterial to the calculations.

If John wanted to do ALGEBRA rather than ARITHMATIC (he did not do MATH as he claimed ;-) he could have made the exact same arguments with the size of the wheel being any arbitrary size.

I will answer his comments in another message, but rest assured that the size of the wheel is not part of the answer.

Joe J.

You ME guys should get out more...
 

John's analysis is right as far as it goes in that his arithmatic is correct.

What he shows is that a 2 motor solution is going to be MARGINAL at meeting the 10fps top speed and the 150 lbs pushing force criteria.

His main objection is that of current draw exceeding the rated current of the breakers at the 150 lbs rating. Yes this is true. But this is where you have to balance your particular plan for the game and your drive skill.

While you cannot stall for long periods of time, it the breakers can go a quite a bit longer than many of you folks seem to realize.

I don't have the Snap Action data sheet handy, but from the Bussman website, you can clearly see that a 40 Amp fuse can carry 80 Amps for 20 seconds before tripping.

20 seconds of pushing is a LOT of time if your driver is trained to know where the edge of the envelope is.

If you are Wildstang or Chief Delphi or one of the other teams that love Swerve, you can win not by brute force but by pulling at angles.

So... ...I feel I am just justified in the 10fps & 150 lbs with 2 motors per side. It is a close thing but not out of the bounds (especially if you recall that I discussed acceleration as essential a stall condition for the motors in an earlier part of this thread -- this is why you will likely have less than the calculated times to trip at the end of the match even if you did not get in any pushing fights during the earlier part of ther match).

FINALLY, I want to emphasize that I am NOT saying that I agree that 10fps and 150lbs is acceptable. But if is were, it is (just) possible with a shifter OR with 2 motors.

That's my story and I am sticking to it...

Joe J.

Silly SparkE's making gearboxes... ;)
 

Joe, you are 100% correct.
I myself never double checked the breaker specs. While I DID know they were capable of exceeding their rating, I had no idea for how long. You are right, 10fps and 150lb are possible to achieve from the same gearbox, using nothing but 4-motors and a dream. ;)

However... I should now revise my previous statement.
It may be possible, but it is not practical. (or at least... I'll NEVER do it).

Now this is more of a "personal preference" type debate... but when I design drivetrains/gearboxes: I keep my current draws down as far as possible. (Slower is safer!)

I almost always design for continual usage, because... who knows. In 2002, we saw matches where teams were forced to go into extended pushing matches for close to the full 2 minutes... since we're dealing with a hypothetical game... I guess you are correct. However, my read on Matt's statement is for 150 lbs of pushing continuously...

When I am designing, marginal is NEVER good enough.
I like my drivers as far away from the "edge of the envelope" as possible, at all times.

Why limit yourself to 20 seconds? Sometimes that extra second of pushing can mean the difference between victory and defeat, and a popped breaker (even one that resets soon after) can often mean you watch the rest of the competition from the stands.

Whenever one designs to live on the edge, he might eventually find himself plummeting over the side. This is why, I never push my motors too hard. (Those FP's sure have a distinctive smell when they're overworked and underpaid!)

In this competition, reliability is key.

So, you've gotta ask yourself...
Why not just downshift and call it a day? ;)


John, who wonders if CD9 will have a shifter under the hood. ;)

I did the math, and all I got was this lousy T-shirt.
 

Been there, done that... made a spreadsheet that does it for me. :D

Stay tuned folks... depending on how fast this break goes by, there may be a new whitepaper or two coming out.

In this particular instance, I thought a plug and chug solution could prove my point effectively. Remember folks: it's not the size of your formula, it's how you use it.

Re: I did the math, and all I got was this lousy T-shirt.
 

All in fun John, All in fun. I had no doubt that you actually did the algebra and you are right that your example was better illustrated with a specific wheel diameter rather than the generalized solution. I was just having a bit of fun with you since you put your ***DANGER MATH ALERT*** disclaimer at the front of your posting.

Keep up the good work.

Joe J.

Ah ha!
 

Ah ha!

John is using the ole.... "I'll state a forumla and then use different units on the output" trick.

You need 75 lbs on each gearbox. You have a 6 inch wheel, with a 3" moment arm, which means you need 225 in-lbs, or 25.42 N-m. Maybe John used the diameter instead.

This halves the rest of the current requirements down to around 42.5 amps on the drill and 35 amps on the atwood... with a little tweaking they could be both modified to get under the 40 amp limit.

Admittedly, the numbers show that this will be pushing 160 amps altogether... which is a little too high for my blood.

and so continues the thread that never ends...

Matt

Re: Ah ha!
 

Whoops...
Matt IM'd me earlier tonight, and YES... it's true. I was off by a factor of 2, early on in the calculation -- I used the diameter of the wheel for one of my calculations (not all of them) by accident.

Although... even with the correct calculations, the drivetrain is still under more load than I (personally) would find acceptable during a pushing match. However, it is now much closer to being arguably "good enough".

So Joe, Matt... I admit defeat.
As usual... a factor of 2 is the death of me!
(This is why I triple check ACTUAL design calculations I'm doing).

Eating his words,
John

Dr. Joe 'fesses up...
 

Don't feel so bad. I didn't even do the calculations...

...when I am actually designing a robot, I do the calculations (though I sometimes still get it wrong -- ask me why our robot had a 6 tooth sprocket last year ;-), but for this thread I just relied on my experience with 2 motors per side in 1999, 2001, & 2002. I was confident that it could be done and just signed on the bottom line -- skipping the fine print.

I should have suspected something was wrong when John's calcs showed the system to be so marginal (which was not my experience), but I was blinded by the circuit breaker concern and I just missed it.

Thanks to Matt for checking the calcutions (by the way, everyone should take the time right now to give positive reps to both John and Matt for the work they have put into this entire thread -- yes do it right now -- it only take a few seconds).

Joe J.

Shifting Gears
 

I'd just like to say that I really appreciate all of the contributions that so many people have made to this thread. I know that it was difficult for me to find comprehensive data on a lot of the subjects, but it seemed where I was missing out, there was always someone to fill in.

This post covered a lot of different topics, from transmission theory, to combining motors, to defining "competitive" robot capabilities, and some nice insight on how those 120 Amp breakers work in REALITY...

Big props to everyone involved. I'm really impressed and excited to have some good insight brought to this fundamental topic for everyone to see.

Matt

Re: Shifting Gears
 

Sit right back folks, it's story time from Matt Adams.

Sit, honest. This might get long.

I'm rehashing one of my very first posts (my very first?) and this reply made by Dr. Joe.

First off, I'd like to say that I absolutely respect Dr. Joe and many of the brilliant things that he's shared with the first community through Chief Delphi and other venues. He's a brilliant guy. So brilliant, in fact, that when he said the above statement back sevearl months ago, I really appreciated the engineering experience ahead of the the theoretical calculations I do in school. I used that advice while doing calculations for our machine, and we didn't have a single problem with our 120 Amp breaker over the course of the entire season, minus a single match. That single match is why I'm posting now.

Let me immediately remark that I am in no way trying to put any blame on Dr. Joe for the success or lacktherof of 461 this season. Ultimately any sort of machine failures are the responsibility of the design engineers and the people who give designs the stamp of approval (or signature in our case). That person was me.

With that all said...
461 had 4 motors (no transmission) for their drive train this past year, as well as a few supplimental motors to control other functions, namely a fisher price for a small ball roller and a van door for a big ball grabber.

The reason I post now is to share some more real-life experience using this breaker, in hopes that similar mistakes are not made in the future.

Curie Field, 2004. Last match of the division finals. 461 had played at least 8 consecutive matches without swapping out with an alliance member for strategic reasons... both joysticks were pushed forward, and the robot does too... for about 5 seconds after autonomous before tripping the 120 Amp breaker... costing the alliance the match and perhaps winning the division or more.

The 120 Amp breakers are thermal devices... over the course of several matches, they heat up, and become more likely to trip. While several teams know to use compressed air to cool motors during the final rounds, most teams don't think about the main breaker. If you're pulling 160 amps or so through it for several consecutive matches, this is an important fact to remember.

The FIRST finals are truly a test for machines, and all of the design parameters need to be built around this. From personal experience, there's no feeling worse than losing a match because of a human error or an obvious potential failure that wasn't planned for. I hope that everyone who reads this post ensures they think carefully about power management, thermal conditions of their machine, and designing machines around their shining moments in the finals.

Good luck everybody!

Matt

Re: Shifting Gears
 

Sit right back folks, it's story time from Matt Adams.

Sit, honest. This might get long.

I'm rehashing one of my very first posts (my very first?) and this reply made by Dr. Joe.

First off, I'd like to say that I absolutely respect Dr. Joe and many of the brilliant things that he's shared with the first community through Chief Delphi and other venues. He's a brilliant guy. When he said the above statement back several months ago, I really appreciated the engineering experience instead of the theoretical calculations I do in school. I used that advice while doing calculations for our machine, and we didn't have a single problem with our 120 Amp breaker over the course of the entire season, minus a single match. That single match is why I'm posting now.

Let me immediately remark that I am in no way trying to put any blame on Dr. Joe for the success or lacktherof of 461 this season. Ultimately any sort of machine failures are the responsibility of the design engineers and the people who give designs the stamp of approval (or signature in our case). That person was me.

With that all said...
461 had 4 motors (no transmission) for their drive train this past year, as well as a few supplimental motors to control other functions, namely a fisher price for a small ball roller and a van door for a big ball grabber.

The reason I post now is to share some more real-life experience using this breaker, in hopes that similar mistakes are not made in the future.

Curie Field, 2004. Last match of the division finals. 461 had played at least 8 consecutive matches without swapping out with an alliance member for strategic reasons... both joysticks were pushed forward, and the robot does too... for about 5 seconds after autonomous before tripping the 120 Amp breaker... costing the alliance the match and perhaps winning the division or more.

The 120 Amp breakers are thermal devices... over the course of several matches, they heat up, and become more likely to trip. While several teams know to use compressed air to cool motors during the final rounds, most teams don't think about the main breaker. If you're pulling 160 amps or so through it for several consecutive matches, this is an important fact to remember.

The FIRST finals are truly a test for machines, and all of the design parameters need to be built around this. From personal experience, there's no feeling worse than losing a match because of a human error or an obvious potential failure that wasn't planned for. I hope that everyone who reads this post ensures they think carefully about power management, thermal conditions of their machine, and designing machines around their shining moments in the finals.

Good luck everybody!

Matt

Re: Shifting Gears
 

Let me just start by saying that all my remarks more or less come with a "your mileage may vary" disclaimer. I don't take blame or credit so easily.

Now to the 120Amp breaker issue. I am surprised that even with this condition you tripped the breaker, but I suppose that with all the improbablities piled up on top of eachother, it is possible. Well... ...to say "I suppose" is wrong. I should say, now I realize that given a long enough time of stalling 4 motors plus others you can trip the 120A breaker.

But evenso, I pretty much stand by my thought that it is almost impossible to trip it in a 2 minute match.

If anything, Matt's story is a cautionary tale to spray the 120A breaker with "cool in a can" while you are spraying your motors, especially if you are stalling many of the drive motors for significant portions of the match (search for my messages on "Cool in a Can" if you want to know my feelings about the importance of this product -- especially during elimination rounds where you play many matches in a row).

Joe J.