How many motors can you run at once?
 

Last year (2012), our team had a problem of running too many motors at once, we had 2 motors for our intake, and 3 for our shooter, and when we would drive (2 motors), we would stall out one of our motors. How many motors did your team run at once? and what techniques did you use to prevent something like this?

Re: How many motors can you run at once?
 

If I remember correctly our 2012 bot gets the award for most motors
4 motors for conveyer belts/pick up
2 motors for shooting
2 motors to drive
1 motor to tilt shooter/shift electronics board and battery (for weight movement during balance)
1 motor for bridge device

Thats 10 (I might be missing one because I remember the number 11).

We placed bets to see which would burn out first. We NEVER changed one.

no problems, with so many motors. most were even running the whole time, and many under ALOT of wieght

Re: How many motors can you run at once?
 

That's so interesting, because for some reason our robot would always kill one motor even though we were only running 7-8 at one time

Re: How many motors can you run at once?
 

Do you know what motor specifically was stalling? It sounds more like a mechanical issue.

The only issue that I have seen with running too many motors (more than what you are describing here) is tripping the main robot breaker.

Re: How many motors can you run at once?
 

It was one of our banebot motors for our intake

Re: How many motors can you run at once?
 

Almost certainly mechanical. This year we had 4 CIMs and 4 MiniCIMs running at the same time with no problems.

Re: How many motors can you run at once?
 

We had 6 cims on drive, 2 banebots for shooter, 1 minicim for deploying the climber, and a compressor running at the same time. In the past, we've had more. Its probably a mechanical issue.

Re: How many motors can you run at once?
 

As others have pointed out, it's not the quantity of motors, but the current that they draw. Do you have a way of measuring? Any motors getting overly warm?

Re: How many motors can you run at once?
 

In 2012 we had 12 motors

4 550s on drive
2 cim shooter
4 motors for intake
1 motor for bridge tipping(don't remember motor)
window motor for turret

Re: How many motors can you run at once?
 

you drove a 150lb robot with 4 550's?

how did that work out?

Re: How many motors can you run at once?
 

We had 14 in 2012. Our only confirmed failures were mechanical (or programming).

- (4) CIMs - 1 per swerve wheel, driving
- (4) BB 395s - 1 per swerve wheel, steering
- (2) AM-0912s - ball acquisition & lift
- (1) FP 9015 - shooter drive
- (1) Denso Throttle - shooter tach
- (1) FP 9013 - barrier crossing + ball intake
- (1) Window Motor L - bridge lowering

I'm not saying it a was a good idea, but it worked out ok. No systemic electrical issues with respect to it.

Re: How many motors can you run at once?
 

The limiting factor is how much current can you pull from the battery before the voltage drop across the sum of all resistance exceeds the drop out for the voltage regulators. The internal resistance on a new, fully charged battery is .011 ohms. At 400 amps that will drop almost 5 volts. Figure another 2 volts for other resistance and you now are below the threshold at which the voltage can be maintained in the DSC. when the voltage falls down to 4.5 volts, the 24 volts and the 12 volts for the radio will drop out.
My recommendation is to limit motor usage to below 300-350 amps at any one time. You can do this by ramping up the motors you are turning on and using delays to prevent them from all starting at the same time. A motor that is not turning is in stall and will draw full stall current limited only be the series resistance of the circuit feeding the motor. Typical stall currents of 115 amps for CIM motor is common on an average robot.

Re: How many motors can you run at once?
 

I'd say you can run no more than the amount specified in the rules. :rolleyes:

Re: How many motors can you run at once?
 

Technically, this was unlimited in 2012 (and 2013), because there was no cap on the allowed number of drive motors in COTS computing devices. :rolleyes: :rolleyes: You would hit the weight, cost or size limit eventually, though.
Section in full: K. drive motors or fans that are part of a speed controller or COTS computing device and fans included in the 2012 Kit of Parts.

There were also an unlimited number of certain servos and electrical solenoid actuators, which, while not "motors", do seem to fall into the OP's electrical concerns. However, it'd take some doing to compare them with a CIM, and I don't think that's actually what caused the referenced problem anyway. Electrical load issues can manifest in many ways--listen to Al (always, but particularly about the radio dropping out). But it's not the most likely explanation for stalling the same motor over and over again.

Re: How many motors can you run at once?
 

Thanks for the input guys, I'll have to do some testing over the summer!

Re: How many motors can you run at once?
 

I also suspect that part of the problem is that you were only running 2 motors on drive. It seems paradoxical, but if you put 4 motors on drive, each motor will have a much easier time accelerating your robot, reducing your overall current draw.

Re: How many motors can you run at once?
 

Also, can you describe your drive setup in more detail? It sounds like you might not have been running your drive at the right ratio. Also, what was the ratio and load on the motor younwere stalling? Like others, this sounds much more like a mechanical issue than an electrical one. If there's no smoke or popping circuit breakers when the problem occurs... you can usually eliminate electronic issues.

Re: How many motors can you run at once?
 

Can you (or someone on your team) show the math for this ?

Re: How many motors can you run at once?
 

Not to steal the fun from anyone who might be working on Ether's challenge above, only to offer a hint: The CIM performance data (available at the FRC website under Technical References not there any more, but can be found here) indicates that at 12V the motor reaches peak efficiency of about 65% at about 20 Ampere current draw. When the current draw reaches 27 Ampere, efficiency is still about 63%. Operating beyond that current draw penalizes efficiency, and of course exceeding about 65 Ampere current draw put the motor behind the power curve (i.e., past peak).

Re: How many motors can you run at once?
 

This year we ran 10 motors (because we were limited to a single digital I/O module in our cRIO):
5 CIM Motors (4 drive and 1 pyramid climbing)
1 Mini CIM (shooter)
4 Banebot 540s (steering)
No problems.

Last year, with two digital I/O modules and two digital sidecars, we ran 13 motors without problems.

High overall current draw, however, can lead to low supply voltage (exacerbated by batteries in poor condition) and this can cause control problems consistent with those you experienced. Do you check battery condition? Also, using just (2) CIMs for drive is light, and CIMs draw an awful lot of current if run close to stall.

Re: How many motors can you run at once?
 

Not to derail the thread, but if more than one of your motors is doing the same thing (drivetrain, for example), you can splice PWM cables together so a single output on the sidecar gives signals to multiple speed controllers. We were planning to do this at one point, but it turned out to be unnecessary.

Re: How many motors can you run at once?
 

Yeah, we considered this for our climber and shooter, but with fully independent swerve drive there aren't many motors doing the same thing. (On the other hand, we were also at max weight.)

We're also really hoping the 2014 rules allow you to change which controller gets the PWM signal by using a spike relay (basically put a switch in the Y splitter), so teams don't have to run two motors together while only using one at a time. The Q&A turned us down, but it would provide for another interesting take on the OP's question if allowed next year.

Re: How many motors can you run at once?
 

Let's say I want to push with 100lbs of force, with a single speed drivetrain with a free speed of 11ft/s on 4in wheels. This means an overall gear reduction of 8.43:1. The wheels exert a total of 200lbs-in of torque, meaning the motors exert a total of 23.7lbs-in, or 380oz-in. With two CIMs, each is drawing 74.8A, for a total of 149.6A. With four CIMs, each is drawing 38.7A, for a total of 154.8A. (Figures obtained from your motor calculator.) These two numbers are nearly identical.


With more pushing force, I suspect that four CIMs will do better than two, but I'm too lazy to do the math right now. As for simple acceleration without pushing, it is unfortunately too difficult to factor in all the forces resisting the acceleration, and probably more useful to test such things out in the field.