[Richard Wallace]

Originally Posted by EricVanWyk

...
For the experts here: Can you please comment on how this balancing scheme might be impacted by motor variation? drive train variation? phase of the moon?

In my day job I design electric motors. My designs are not the same type that we use on FRC robots, but their basic physics is similar.

One of the most significant characteristics of any electric motor is its unloaded (free) speed at rated voltage. Let's take the CIM motor as an example: per its manufacturer's data sheet its shaft will spin freely at 5310 RPM when 12V dc is supplied to its electrical terminals. Zoom in to view the table in the lower left corner of the data sheet linked above, and you will see that the manufacturer also states that the variation on the given free speed is +/- 10%. This variation is caused by differing strengths of the motor's permanent magnet material from one batch to the next, and by angular misalignment of the motor's commutation brushes with respect to those magnets.

The graphs shown next to the table on the data sheet linked above indicate that the CIM's current draw is proportional to the torque developed on its shaft. The constant of proportionality (i.e., the slope of the current vs. torque line on that graph) is determined by the same design factors, and the same variations, that determine the motor's free speed. So the stated +/- 10% free-speed variation applies to torque-per-Ampere also.

Because the causes of the variation are the same, using current sensors to 'correct' for free speed variation is probably not going to be very effective.