Note: This is my first post on Chief Delphi and also my first year in FRC, so please bear with me as I may use some wrong terminology as well as have posted this in the wrong thread. It seems to fit into many threads and this seemed to be overall technical so that is my thought process. Feel free to correct me.
So we went to Northern Lights Regional and our robots two sides of the drive train were not running at the same speed. We use two CIM motors per side going into a Tough Box Mini. We looked on the driver station logs to find that the two left side CIM Motors had a bigger gap in amps compared to the other two CIM motors on the right side. We also seemed to be drifting to the left while driving. I’m wondering if this could be caused by one of the following and if one of them, which one:
How big is the drift? And if you’re using the kop chasis which Im assuming since you said you have tough box minis it could be that the two outer axles are tightened more on one side then the other
We used the KOP chassis with the wide chassis configuration. It is not a huge drift but messes with our auto. We hoped to have fixed this problem by using a gyro but want to be safe and kill the issue for sure. Also a gyro doesn’t help much during teleop. As for if we tightened them the same I’m not 100% sure. Will have to look into that. I would assume we did though.
This seems like a great first post! Welcome to Chief Delphi!
I would guess that your problem is some extra mechanical drag in one side of the drive train. You could verify this by turning each side of the drive train by hand.
You might also try running the drive with one CIM at a time in order to verify that they have similar performance, because it could be a bad motor, as you stated.
Sometimes a CIM motor’s mounting holes are not located symmetrically. Try removing the CIM that is drawing higher current, and re-installing it with the mounting screw positions flipped 180 degrees. That might relieve a too-tight fit of the motor pinion to the 50 tooth gear.
Also, I agree with Bryce – this is a great first post! Welcome to CD!
The mechanical drag might be from the robot weighting more on one side than the other, which is something you can’t tell by free-spinning the drivetrain sides.
If the drifting is very small, it could just be small imperfections that are not really worth the time to get rid of. If you really want to drive straight, use encoders.
It would help is to know what the current gaps on each side were. Like 10% or 50%? On the lower end, effects like the one Richard mentioned are very possible. At the higher end, it could be one motor that’s burnt out* or simply bad. Do you have any other information you could share that could help us out in helping you?
*The burnt out one would be the one with higher current, but this is only likely if you’re using old motors
We have put the robot up off the ground on a cart and noticed that they still turn at riders to speeds so I don’t believe it is the weight. Thanks though!
The two motors on the right side are showing maximum spikes of about 72-75A. One of the two on the left is in the same range, but the one that concerns us is maxing out around 60-65A.
Just to be clear, they are not continuously drawing that much current, but the gap is roughly proportional (1 motor 10-20% below the other three) at lower current as well.
The side with the motor that draws less current is slower, and thus is the side that we drift towards, at a rate of about 6 inches per 10 feet.
Just to eliminate other options, could this be caused by the CIM being too loose, rather than too tight, in the gearbox?
Could there be something wrong with the motor controller or associated PDP port that would cause this?
I trust the collective judgement of those who have said that it is a bad CIM, and in fact we had discussed it as a possibility earlier, but on the off chance that it isn’t the problem, I would like to know what other avenues we could pursue.
If a CIM were loose, the gear meshes would bind and show an increase in current. It’s easy enough to check though.
You can check to see if it’s a controller problem if you’re able to simply swap the controllers where they attach to the motors. In my experience it is more likely to be the motor than the motor controller, but that also supposes you’ve gone through the calibration routines of the controllers.
I was referring to the issue that Richard Wallace addressed in post 5 in this thread, if the mounting holes are too far away instead of too close. Not likely at all I don’t think, but if it could be the problem it’s worth knowing about.
we attempted to calibrate them, but only one of the four was successful. dooodsdare can probably tell you if it was the one corresponding to this motor or not, I don’t do electrical/programming for us.
We found a problem in the code with the calibration. Had it set up to calibrate Victors not Talons. Would this cause a problem with the amps in the pictures shown above?