Denso Window Motors, Warning - Don't use with Jaguars !

[diviney]

Important warning to FRC teams.

During this year's 2010 competition at the DC regional we were struggling to solve an intermittent problem with our arm. The arm is powered with one of the Denso window motors that came in this years KOP. This motor used a Jaguar controller, commanded with a joystick. We initially suspected an electrical problem, but the symptoms were mechanical also.

What happens is that the motor will occasionally simply fail to respond to the controls. When the motor fails to respond, there is a voltage present on the input to the motor. We [well, our students with young ears] could hear a high-pitched sound from the motor as further indication that it was being powered. There was no mechanical interference, yet we found that there was sort of a mechanical hysteresis taking place. If the arm was commanded to go down, but was unresponsive, we could gently push it up with our finger and that would cause it to properly drive down in response to the command. Likewise, if the arm was commanded to go up, but became unresponsive, we could nudge it down and it would respond by driving up as commanded. We tried a spare motor, but if anything it was worse with the replacement motor.

After the competition, we took the arm back to the shop, and were able to re-create the problem by connecting it to another robot. We looked at the output of the Jaguar on a scope both when in this frozen state and not. The Jaguar outputs a pulse-width modulated waveform that toggles between -12V and +12V. The frequency of the PWM waveform is around 15 KHz (hence the audible tone). When the arm entered this frozen state, the output waveform stopped toggling altogether and went to some intermediate voltage with a small amount of 15 KHz superimposed.

This is not a fluke due to a bad Jaguar or a bad motor because we saw it with 2 different motors and 2 different Jaguars. Once we observed the failure in the shop, we substituted a Victor speed controller, and everything then worked perfectly. The Victor has an output waveform that's very different than the Jaguar. Instead of toggling between +12 and -12, it toggles between +12 and zero (one direction), and between -12 and zero for the other direction. More importantly, it operates at around 117 Hz instead of 15 KHz. It is our belief that there is something about the Jaguar that is not unconditionally stable. The particular combination of the Denso motor and the Jaguar seems to be unstable. We are guessing that the impedance presented by the Denso motor at 15 KHz causes the Jaguar output to become unstable. Nudging the motor a little affected the instability allowing it to recover for a bit.

One final observation - we were not able to get the motor to fail by simply commanding it on and off. By using the joystick to vary the speed up and down, forward and backward, it would fail readily. If you are not varying the speed and just want forward / reverse on / off, you [might] be OK.

Note: Both of the Jaguars we tried are the older brown ones. We have not tried this on one of the newer black ones. Our advice is to NOT use a Jaguar to power a Denso window motor. If I get a chance, I may measure the impedance of the motor to see if there is some kind of a resonance near the 15 KHz Jaguar frequency.

This problem can be quite elusive as it behaves as though you have a loose connection or similar.

If I have a chance to check the motor impedance characteristics, I will post a follow-up