Digital RC Servo Jitter

Jun 24, 2005 1 Replies

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I'm working on building a low-cost precision 3 axis stage actuated by small digital RC servos. It seems that the digital servos I'm using (JR DS368's) don't run smoothly between steps. Even when I drive them with a 32 bit PWM controller, the motion appears choppy, and this choppiness is effectively transmitted directly to the stage (current implementation has no damping built in).

My question is, is this jitter/choppiness simply inherent in digital servos because of the fast response time, or is there some way to mitigate it? Has anyone seen a digital RC servo that runs as smoothly as an analog servo? The reason for choosing the DS368 is the high resolution (at relatively low cost) - approximately 5900 steps over 120 degrees.

Thanks, Aaron

Well digital servo's jitter all the time but not for the reason you state. It's due to the internal feedback so that almost any error on the output will cause the motor to activate (or make the servo buzz). They typically work nice and smoothly when used in R/C applications as that is what they were designed for, you may have to adjust your frame rate up or down a bit to more closeley match the original R/C specs (normal analogue servos require 20ms frames, digital servo's can go to higher frame rates if they are quick response e.g. KO Propo's range of servo's for R/C cars).

This question might be better answered in comp.robotics.misc as there a lot of them have interfaced servo's and microcontrollers and might have a better idea on where you might be going wrong or have some tips for you. It could also be just interference caused by spikes in the power supply feeding the servo's. Have you made sure you're using a separate power source for the control circuit and servo's connected only by the ground wire and signal lead all with plenty of decoupling?

Chris

Digital RC Servo Jitter

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