servo jitter when running Jaguar on RS-775

[Brandon Holley]

We are experiencing the same exact issue you described.

We use 2 servos, one to release the deployment system, and one to release the actual minibot. In both cases the servos would jitter as we ran our two 775s for our elevator system.

After discovering our 2nd set of motors both had internal shorts (the 1st set did as well), we managed to scrounge up 2 more 775s that were sans-short. Since replacing those motors the problem has gone away.

It was an extremely frustrating problem because we had no idea when/if the deployment system would trigger as we ran our elevator.

I've noted the other suggestions in this thread, but I know for a fact the wires running to the motors are not close to the PWM servo cables. The capacitor may be a helpful solution, we'll have to look into it as a fall back.

-Brando

[kamocat]

Interesting.

As for induction from wire to wire, that's a definite no. The wire runs don't even come near eachother.
However, I suppose (if the 775 is shorted to the frame) it could be induction (or capacitance) from the frame onto the wire. I suppose the way to tell would be to power the DSC from a seperate battery, and see if the problem still persists.

Tom:
Yes, I ended up setting the Servos to "disabled" in Begin.vi. I had assumed they were initialized to that automatically, but apparentally not. (I'll admit the way I'm controlling them is kind of weird. Since we were prototyping our grabber using the FTC servo controllers, I made the FRC code use the same values, by converting the servo reference to a PWM reference, and setting that value directly. However, this does give me another advantage: I can disable the servo by setting it a PWM value of 0)

[EricVanWyk]

Originally Posted by Brandon Holley

After discovering our 2nd set of motors both had internal shorts (the 1st set did as well), we managed to scrounge up 2 more 775s that were sans-short. Since replacing those motors the problem has gone away.

Further evidence that the Banebots chassis fault issue is actually a bigger deal than Banebots wants to admit.

Teams, please check your `bots for this.

[Tom Bottiglieri]

Originally Posted by EricVanWyk

Further evidence that the Banebots chassis fault issue is actually a bigger deal than Banebots wants to admit.

Teams, please check your `bots for this.

When we were having the issue, I was able to isolate it to the 775s by pulling their breakers and disconnecting the leads (separate actions- as to isolate the jag). Both of these things did away with the jitter.

Interestingly enough, of the two servos we had connected, only one PWM port was driving the jittery one. Switch PWM port, the other jitters.

[Tom Line]

Yeah, not to pile on, but we just pulled 4 banebots new out of the box. 1 had a case short, and 2 had bad bearings - with a slight outward load on the shaft the motors would lock up hard and not rotate at all.

[Al Skierkiewicz]

I am going to throw a guess on this one where the Jag breaker isolates it and switching PWM servo outputs moves the problem. The Jag is capable of very high rise times and therefore is likely to put some wild RF signals on the motor leads. If the 775 has a case short, caps across the motor leads will not help. I have experienced noise currents traveling on wire and then crossing over to another wire and causing problems. Sometimes, RF ferrite beads around the conductors can shunt the noise. My suspicion on the 775 problem is that you will find it to be a greater problem when driving in one direction vs. the other. If that is indeed the case, then there are significant currents driving through the case to the common electrical lead on the Jag and back out to anything connected to the Jag with the same common terminal.

[Brandon Holley]

Originally Posted by Al Skierkiewicz

If that is indeed the case, then there are significant currents driving through the case to the common electrical lead on the Jag and back out to anything connected to the Jag with the same common terminal.

Your guess seems right to me. Our servos only showed the issue when the elevator was driving up (aka: under the largest load).

-Brando

[Gdeaver]

Has any one tried adding a cap to the servo wire as close to the servo as possible? While it may not work in this case, It has solved twitchy servo problems on some of my projects in the past. Cap on the + and - .

[kamocat]

I've taken of the RS775. It did indeed have a short to its case. (About 1 ohm)
There was nothing else connected to the frame, but considering the long servo runs, I don't doubt that the servos were jittering from induction onto their signal line.

I'll get around to testing sometime.

[EricVanWyk]

Originally Posted by Tom Bottiglieri

Interestingly enough, of the two servos we had connected, only one PWM port was driving the jittery one. Switch PWM port, the other jitters.

Tom -

Do you remember what port jittered and what port was good?