Motor Speed offset

Hello all,

On our robot we have run into an interesting problem. We are running the kit of parts frame from AndyMark AM143U. We have the standard gearbox (ToughBox mini’s) hooked up into VEX 217-2000 12V Motors.

When we drive the robot, one side spins faster than the other. Which in turn leads to our robot not driving in a straight line, even though the joystick is pressed into a straight line. So far, weight is pretty much evenly distributed. When we have our robot lifted, and the joystick pushed 100% forward, our robot will run for about 10-15 seconds before one full side stops moving completely. Our battery voltage drops from 12.5 volts to about 10.5-9.5v or so during the continuous run. Our robot registers no error codes during this time. I have a feeling that the problem is electrical, but I am not sure where to look. Any suggestions would be greatly appreciated.


Team 5720

Are you saying that when you have the robot “sitting on jacks” and with the wheels spinning in mid-air, one side of the drivetrain will lock up?

I’m guessing you have an overwhelming amount of drag in one side of your drivetrain. If possible, measure the current draw on each motor on both sides of the drivetrain. You should only be drawing a few amps per motor per side. If you’re drawing more than a few amps per motor, I’d recommend removing that gearbox and confirming if it’s assembled correctly.

The “curves to one side” happens pretty often, I’ve seen mechanical inconsistancies playing a big part (does one gearbox have more friction than another?). However, the “stops running” is interesting…

When the side of the drivetrain stops running, do the lights on the motor controller indicate that it should still be running, or do they go back to indicating zero or no-signal?

The first thing to check is mechanical. It is very easy to build the gearbox with a gear backwards (the raised bosses around the bores should be on the bearing side) or with other binding or dragging problems. Sometimes something just doesn’t fit properly and binds, even when you’ve assembled it correctly. We’ve had binding at the cluster shaft (replaced shaft), with the large cluster gear rubbing the output shaft’s rear bearing (replaced housing), and with CIM pinions pushed too tightly against the large cluster gear (solved by rotating the CIM 180 degrees around its axis).

Try backdriving the gearbox by hand (a hub is handy for this) and feel for spots where it sticks. If you find any take pieces in and out and rearrange until you find where it’s binding.

Edit: Our “first time” smooth-running success with TB-minis has been no better than 50%. After a particularly bad year of it in 2016, this year we have been verifying that our gearboxes run smoothly before we ever put them under load (that is on the floor) or install encoders.

Edit2: If the gearboxes are OK, check that the belts are lined up and snug and that the idle (corner) wheels rotate freely when not attached to the belt, and that the direct drive (center) belt is no harder to backdrive than your initial tests.

Make sure all of your motors are actually running. We have had a bad connection to a motor cause this. Just disconnect one motor on each side and make sure it still works. Also make sure both of the motors are spinning the wheels the same way.

It sounds to me like you have one CIM/motor controller combination that is not driving. When you run that side, if the motor controller on the unpowered CIM is in brake mode, the other CIM will pop its breaker trying to drive it.

Have your controls guys test each motor in code individually to prove that they are all wired correctly and getting power.

Yes thats exactly what I am saying.

I’m guessing you have an overwhelming amount of drag in one side of your drivetrain. If possible, measure the current draw on each motor on both sides of the drivetrain. You should only be drawing a few amps per motor per side. If you’re drawing more than a few amps per motor, I’d recommend removing that gearbox and confirming if it’s assembled correctly.

We are currently taking apart the gearboxes to see if they are not assembled correctly. After running them, one of the motors seemed warmer than another to the touch. But nothing else is sticking out

CIMs really should only start to feel warm after heavy use over a sustained period of time… If one motor felt “room temperature” and another felt warm in the same gearbox, it could very likely be a control problem where one motor was being driven and the other was coasting or braking. If both felt warm in the same gearbox, then there was probably significant wear in your drivetrain (gearbox is probably best place to look, at first).

Following on to Nathan… if you don’t find any gearbox issues, try running each CIM with no load (nothing but possibly a pinion on the shaft). If the CIM gets warm with no load, toss it out if it’s an old one, or see if you can get a replacement from whomever sold it to you if it’s a new one.

Are the speed controllers used for each motor showing proper status lights?
Depends on what type of speed controller, but generally a solid light on all four when the robot is Enabled but not moving.

When moving, you should see blinking red on both of the speed controllers on one side, and both green on the opposing side.

Thanks for your guys replies. We took apart the gearboxes and found no problems in there. We also rewired the entire robot with new wires to make sure no issues were found there. When running the robot afterwards, we still have the same stopping problem on one side. We noticed a clicking sound, upon further investigation we found one of our Talon SR motor controllers status light while running was orange. And would then completely click off. After we stopped, the status light would click back on in time.

Any ideas what to try next?

OK, there’s definitely something fishy on that Talon or something feeding it.

Try swapping the PWM cables between that Talon and the Talon which controls the other motor on the same gearbox. If the problem moves, it’s your PWM cable, RIO, or software (in decreasing likelihood). If cable or software, the fix is easy and not too expensive, and hopefully obvious. If the RIO, your best bet for now is to move the control to another DIO port, both in software and hardware.

If the problem stays with the same Talon, try swapping it to another PDP port, and (separately) to another breaker.

Edit: Try swapping the motors on the same gearbox between the talons to make sure you don’t have a motor overloading the breaker.

If none of these fix the issue, sounds like a bad Talon. Fixing that is probably beyond online advice. Replace it; if it’s new, try to get the replacement from the vendor.

Thanks for the advice. Using your idea we found the electrical gremlin. We had a few bad PMW cables, and one that was crossed in the wrong direction. I appreciate everyone’s help in the matter.

Thank you!

A quick dividing line in troubleshooting these kinds of issues: Mechanical issues doesn’t come & go. Compare the drag of one side to the other by turning the wheels through a couple of rotations. If one side is harder to turn than the other, you have mechanical issues. Otherwise it is probably electrical.

Get in the habit of observing the status lights on the controllers. Motors pairs should be blinking at roughly the same rate and color. If you are consistent with your motor wiring, Colors on the right side & left side will be different with a non zero command. I have occasionally found CIMs to spin backwards. --With red positive it spins CW rather than the normal CCW