[dtengineering]

While feedback loops are the best way to get perfectly even performance on both sides, if you are experiencing severe undesired turning on a CIM drive system I suspect the problem is mechanical, not electrical.

My understanding is that the CIMs are very symmetric motors and exhibit almost exactly equal performance in each direction. (unlike some other DC motors that are timed for higher performance in one direction)

Check your drive train... gearboxes (does one turn more easily than the other?), chains (differing chain tensions can cause extra drag on one side), sprocket alignment, and bearing alignment for your axles. When you power down and turn the wheels by hand can you feel a different "stiffness" from one side to the other? Is your battery mounted to one side, or do you have other weight distribution differences? That might make a difference, too.

If you have access to a clamp on ammeter (or other convenient means of measuring current), put your robot up on blocks and run both sides full-forward and full-reverse. Does one side draw significantly more curent than the other? If so, that is a sign of a mechanical friction-type problem more so than an electrical problem.

For three years now we have used open loop control with the small CIMs and while the robots have not travelled perfectly straight, they have been good enough to be competitive.... once we got the tensions equalized and the transmissions broken in. There is, however, a limit to how good you can go with this, and if you want perfection you are going to need some form of feedback.

Jason