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Unread 10-12-2003, 00:33
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Re: Shifting Gears

Quote:
Originally Posted by Matt Adams
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.
Emphasis to PAIR and OR added by me.

Quote:
Originally Posted by Rob Colatutto
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.
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
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Unread 10-12-2003, 01:49
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Re: Shifting Gears

Quote:
Originally Posted by Matt Adams
Emphasis to PAIR and OR added by me.
I definite "competitive" as a max speed greater than 10 feet per second, and having a maximum applied force of 150 lbs.
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).


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
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Unread 09-12-2003, 14:39
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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.
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Unread 10-12-2003, 02:05
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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.
Quote:
Originally Posted by Joe Johnson

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...
<SNIP>
...if you need or want more power than that, more motors are your only real solution.

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
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Unread 10-12-2003, 13:34
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"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.

Last edited by Joe Johnson : 10-12-2003 at 13:44.
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Unread 10-11-2003, 12:26
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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.
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Unread 05-12-2003, 02:37
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Post Re: Shifting Gears

Quote:
Originally Posted by MBosompra
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.
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
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Unread 05-12-2003, 10:23
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Re: Shifting Gears

Quote:
Originally Posted by Matt Adams
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.

Matt
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.

Last edited by Joe Johnson : 05-12-2003 at 10:28.
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Unread 05-12-2003, 12:18
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Re: Shifting Gears

Quote:
Originally Posted by Joe Johnson
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.
I'll take an 80-90% from Joe any day.

Quote:
Originally Posted by Joe Johnson
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).

<snip>

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.
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!
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Unread 05-12-2003, 14:53
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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.
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Unread 05-12-2003, 15:43
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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.

Last edited by Joe Johnson : 05-12-2003 at 15:47.
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Unread 05-12-2003, 17:11
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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
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Unread 05-12-2003, 19:57
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Re: two separate decisions

Quote:
Originally Posted by Ken Patton
Yes, it draws more current.
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.

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Unread 05-12-2003, 21:31
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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
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Unread 08-12-2003, 01:24
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Thumbs down Re: Cooling Breakers

Quote:
Originally Posted by Tytus Gerrish
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
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.
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