[Schmitty!]

We had a similar issue, not with the FP motors, but with the Mabuchi motor. We purchased a Banebots gearbox for a gear reduction and use this to pull balls from our lower vertical feed up to the shooter wheels. This put the Mabuchi motor in fairly close proximity (within two feet) of the camera servo. Every time the Mabuchi motor was energized the servo for the camera would go nuts. We took the following steps to narrow down the problem and I actually captured video of part of the debugging. I’m including it with this post.
The steps we took were:
1) We initially saw the issue when we operated the camera servo normally and energized the Mabuchi motor with a speed controller.
2) We took the motor leads from the Mabuchi motor and ran them to a completely different battery. As soon as I connected the motor leads to the battery the servo went nuts again. This is what I got video of.
3) We checked for a low impedance (resistance) between the motor case and either of the motor leads and verified it was an open (greater then 10 Meg Ohms).
4) We removed the Mabuchi motor entirely from the robot and operated it from an external 12V battery in the air near the camera servo. Again we saw problems with the servo.

This led us to believe that the problem is caused by electromagnetic interference (EMI). When the motor is operated the brushes internal to the motor cause a great deal of arcing, which is similar to a bunch of small lightning strikes. There isn't a great deal of energy in these arc’s but it's very high frequency and can capacitively and inductively couple onto other parts of the motor (like the case and back down the motor leads). Once the energy is outside the case it can radiate out very easily and interfere with the operation of neighboring sensitive electronics (like the feedback loop in servos).

The goal was to give the energy on the motor case and leads a path back to the source. A very effective way to do this is to attach small (< 1uF) ceramic or polyester capacitors from the motor case to the positive and negative leads of the motor. Aluminum electrolytic or tantalum capacitors should not be used because they have a fairly high series resistance and inductance and can be damaged if the motor voltage was reversed.

We connecting a 0.1uF 50V ceramic capacitor (available at any Radio Shack) from the positive motor lead to the case and a second capacitor from the negative motor lead to the case. The problem instantly went away and we've never seen it since.

From my interpretation of <R55> they are trying to protect against DC current through the frame like typically done in an automobile. This practice can be very damaging if it’s not handled properly. I believe connecting a small capacitor between the motor leads and the motor case (which may be connected to the frame) should not cause problems and shouldn’t be covered by this rule. Is this correct?