servo jitters

[jon-s]

Our Hi-Tech servo from the KOP jitters when it's programmed to stay still. It does this regardless of whether we run our shooter motors (2 rs550s from banebots).

We were told by another team that the jittering might be caused by electrical noise in the digital sidecar.

The servo is placed at the same height as the shooter motors, about 6 inches away, along the same piece of c-channel. We had to extend the cable for the servo to reach the digital sidecar. At one point the cable for the servo crosses paths with the wires powering the motors , without touching.

Has anyone else experienced this problem?
Does anyone know of a way to remedy this?

[CalTran]

While we have not had issues with servos "jittering", we have had problems with electrical noise interfering with our signals from the Digital Sidecar. At the point at which the wires cross, try and ensure that they cross perpendicular, rather than running parallel. Also, we found out that even the slightest separation (About 2 inches minimum) between the pwm and the powers to other things will vastly improve any interference problems

[Nikkocharger]

A couple of my R/C cars do that when they go out of range or there is some type of interference. I'm guessing your servo is plugged into the the digital side car. Do you also have a jumper installed next to it? You could also check the wires.

[Alan Anderson]

Originally Posted by jon-s

Our Hi-Tech servo from the KOP jitters when it's programmed to stay still.

Can you describe the jitter? It might be premature to dismiss a programming issue as a possible cause.

[fox46]

Describing how bad this jitter is can help provide a more complete diagnosis. Is it just a jittering sound or how many degrees of movement are you seeing in either direction? Servos will sometimes hunt or occasionally "buzz" when trying to find their center.

My bet would be that since these kinds of servos have a definite lifespan and are not particularily high quality, the potentiometer inside them can develop wear, particularily around the center of their travel (home position) as this is where the servo spends most of its time. If this servo has been used previously it is possible that the potentiometer has become worn and thus the electronics are "hunting" trying to find the home position or center. If this is a new servo, it is possible that you just have a defective unit. Try adjusting the center position in the code so that it is no longer in it's current spot. This should move the potentiometer onto a less-used or "good" portion of its travel thus hopefully providing a cleaner feedback signal to the controller which will stop it from hunting. I have seen servos develop these symptoms quite a bit with radio controlled aircraft and cars as they can often experience a fair bit of mechanical vibration during use which wears the potentiometer very quickly.

Try swapping a new servo (preferrably one that is brand new out of the box)in its place and see if the problem is still present.

[Whippet]

Are your servos connected to comparatively long pieces of metal? If this is the case, the servo just might be moving past the position it is supposed to and overcorrecting due to the inertia of the metal. This problem plagued our FTC robot, and there wasn't really anything we could do to fix it. If this is the case, you might consider having another servo hold this metal stationary when it's not in use. If this is not the case, I have no idea how to help you.

[adf0221]

we had this same issue during the middle of this season. We found out that it was an issue with the radio losing connection and then gaining it back very quickly. We solved the issue by replacing the Ethernet cable between the C-Rio and the radio. The jittering stopped after that!

[jon-s]

When the servo jitters (I don't recall any sound), it moves back and forth, up to about 50 degrees. It does this regardless of what position it starts in. It has done this on both of our robots.

It is plugged into the sidecar and should have a jumper on it. The servo moves a thin 2 inch long arm. I think this is the first year our team has used the servos, they've only been moved maybe a dozen times. People have backdriven them, but not very often. The wires and cable cross at about 60 degrees to each-other.

[Alan Anderson]

Originally Posted by jon-s

When the servo jitters (I don't recall any sound), it moves back and forth, up to about 50 degrees. It does this regardless of what position it starts in. It has done this on both of our robots.

That's not jitter. I believe that much motion has to be commanded. How is the robot code controlling the servo position?

[JohnGilb]

As a note, I have seen a servo drift by ~30 degrees due to electrical interference from nearby high-current motor cables (though this was back in 2004).

[nitneylion452]

Originally Posted by jon-s

When the servo jitters (I don't recall any sound), it moves back and forth, up to about 50 degrees. It does this regardless of what position it starts in. It has done this on both of our robots.

It is plugged into the sidecar and should have a jumper on it. The servo moves a thin 2 inch long arm. I think this is the first year our team has used the servos, they've only been moved maybe a dozen times. People have backdriven them, but not very often. The wires and cable cross at about 60 degrees to each-other.

50 degrees to either side is a lot of motion to be a jitter. The HS-322HD's full range is only ~170 degrees (based on us eyeballing it). You should take a look at your code and make sure you aren't sending conflicting commands. If everything looks ok, try a new servo. You can order a HiTec HS 485HB which offers an additional 30oz-in of torque and is rated at 3.9 watts (322 is only 2.5).

As far as the jumper suggestion, that cannot be the case. If there were no jumper, the servo would not get power and not move at all.

EDIT:

It is also worth noting that (in Java, I'm not familiar with the other languages esp LabView) the method to tell the servo where to go is called setSpeed(). This is very misleading. setSpeed() tells the servo where to go, not how fast to move, so calls to it with different values, like 0.0 and 0.5, will cause it to fluctuate between the two positions and may be the root of your problem.

[jon-s]

Originally Posted by jon-s

When the servo jitters (I don't recall any sound), it moves back and forth, up to about 50 degrees.

Edit: Typo: the servo jitters about 5 degrees. The amount of jittering seems to vary.

[nitneylion452]

That changes things a lot. It is a very good possibility that your servo is defective and is fighting to get to its home position. Try a new servo, if the problem persists, increase the angle at which your signal wires cross others.

[jonboy]

Why not unplug the fuses/cktbkrs to all the high power items like the motors to help eliminate high current EMI and then check for servo jitter? Better yet just plug another servo into the same port on the DSC and chk for jitter.

[Al Skierkiewicz]

[R66]
A noise filter may be wired across motor leads or PWM leads. For the purposes of Inspection and rules compliance,such filters will not be considered custom circuits, and will not be considered a violation of Rule [R49] or Rule [R65].
Acceptable signal filters must be fully insulated and are:
A one microfarad (1 μF) or less non-polarized capacitor may be applied across the power leads of any motor on your Robot (as close to the actual motor leads as reasonably possible).
A resistor may be used as a shunt load for the PWM control signal feeding a servo.
It is not uncommon for servos to react to RF noise generated in the motors and motor wiring. It is best to run the PWM cables away from high current wiring and when it must cross, cross at right angles to minimize mutual coupling. the resistor lowers the impedance of the cable and generally, keeps the RF from overcoming the servo command signal.