Servos hum and studder

D. Schewe

We have just started to use parallax robots in our newly formed robots club. A few of the robots have servos that humm and move in a studdering motion. Is this a sign of a defective servo or did we damage them (or hook them up wrong)?

dandreae

You'll need to verify that the servos are properly centered and that the batteries are fresh.· If this doesn't the problem please contact Parallax at support@parallax.com and put attention "Dave" and I will help you with·this problem.

Dave

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Dave Andreae

Tech Support
dandreae@parallax.com
Http://www.parallax.com



Post Edited (Dave Andreae (Parallax)) : 12/17/2005 12:02:05 AM GMT

Vern Graner

D. Schewe said...
We have just started to use parallax robots in our newly formed robots club. A few of the robots have servos that hum and move in a shuddering motion. Is this a sign of a defective servo or did we damage them (or hook them up wrong)?

It would be easier to answer this question if we had some additional details on how your are driving the servos. For example, are you driving the servos with a Basic Stamp directly (i.e. the SERVO sockets of a BOE?)? If so, if you would post the BSII code you are using to drive the servo and the circumstances under which the servo begins to vibrate or hum this would be useful.

Are you using a serial servo controller? If so, the code you are using to drive it and the configuration of your robot when it demonstrates the behavior would again be useful.

In general, a standard RC "hobby" servo will "seek" to the position dictated by the PWM pulse the servo is recieving. If the servo is placed under "load" (i.e. trying to hold a position against gravity) it will sometimes HUM or vibrate as the PID circuit in the servo attempts to move the load into the commanded position. If it overshoots (usually due to the kinetic energy stored in the load), the servo will attempt to correct for the over-shoot and then over compensate in the opposite direction. In this situation, it will never reach the commanded position. This compensation/overshoot (aka "seeking" behavior) is seen as vibration or "shuddering" of the servo. A good example of this is shown in this little demo video of a Robo-1 robot that has a rather heavy load on the servo that runs the arm:

http://www.spiderspreyground.com/jitter.wmv

As you can clearly see, the servo is trying to position the arm, but it is unable to do so simce the PID control in the servo is not calibrated to operate with that level of "load" on the arm. There was a good article in the January Issue of Nuts and Volts magazine that detailed the mathmatics behind PID (Proportional, Integral, Derivative) control. There's also an excellent online article here:

http://www.expertune.com/tutor.html

Seatlle robotics also has an interesting article on servo position tuning:

http://www.seattlerobotics.org/encoder/200010/servohac.htm

Hope this gives you a starting point on resolving this issue.

Vern

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Vern Graner CNE/CNA/SSE    | "If the network is down, then you're
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Post Edited (Vern) : 12/16/2005 9:00:29 PM GMT

D. Schewe

Thanks for the information. I will look into the details of this problem next week and get back to you. I appreciate the help.