We are building a center-drive 4-caster bot. It seems to be tracking left in about a 5 or 6 ft. radius. We tried running it on tank mode to see if we were able to compensate for the differing output but being center-drive with 4 caster but the problem persists and it is ever harder to compensate. We put tracking wheels on it to see if that would remedy the situation but it pulls the same, but turns much slower and remains very sensitive. We recalibrated the PMW’s and also tried to fix the problem with trim, but no luck. 
:mad:
You might check the casters for jamming. Also, the CIMs do have a bias. One side (in this case, the left one) is probably traveling against the bias. I am not sure how to compensate for the bias.
Sounds to me like your left gearbox has much more drag in it than the right gearbox. You may want to look at the gearbox and drivetrain on the left side to see if you are binding there.
Also, all motors have a bias. Since 1 side is turning 1 way and the other side is turning opposite, the same offset PWM from center (or equal PWM with wiring reversed) will produce different speeds and will need to be compensated for.
Our team was originally going for a ‘center-drive 4-caster bot’. When we made the prototype, we took it out into the parking lot to try to drive it. The robot ended up doing donuts. After a while we found that one of the gearboxes had a problem, so they fixed it. After that, it no longer did donuts, but it still tracked left. We found that the problem was the distribution of weight on the robot. It tracked to whichever side that the battery (and therefore, most of the weight) was on.
Our team ended up abandoning casters because we found it to be too complicated and too prone to problems.
The most common cause of this is that the CIM motors will be more powerful in one direection than in the other. This is common to most all DC electric motors and has to do with the timing of the commutator brushes. The motors are designed to be efficient in one direction.
The easiest way to fix this is to use software to “dial back” full throttle on the motor that is more powerful in a given direction.
For example: If the robot veers left while driving forward modify the code to have the right motor see a lower number than 256 when the joystick is fully forward. Continue to change the value down until the robot no longer veers off. While some may argue that your motors will now be going slower it’s not really a problem because you will never be able to faster than your slowest motor.
You have probably noticed that the robot veeers in the other direction when you go in reverse. You can use the above example to adjust for that as well.
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Being that you are center drive with 4 casters the problem could be that 1 of the casters is jamming up a bit.
Also the center drive wheel “may” be slightly off center and causing it to veer slightly as well.