Jeff Epler wrote: > On Mon, Nov 17, 2008 at 10:50:32PM +0800, Howard Chan wrote: > >> http://myweb.polyu.edu.hk/~icwfchan/P1.png >> > > This graph does not show measured noise in the DAC output. To measure > that you would have to hook a scope or other diagnostic equipment to the > physical DAC output pin on the mesa servo daugher card. > > This graph shows DAC output commanded by emc2, which is presumably the > output of the pid algorithm mentioned by seb. He is correct that it's a > tuning issue. In addition to the P, I, D, and FF parameters you may > wish to adjust deadband. deadband can be used to get a PID output of > exactly 0 when within a specified range of the commanded position. > (However, if there is an uncompensated voltage offset in the DAC or > amplifier, this may not fix the problem entirely) > I wonder if Mr. Chan is having problems with the D term magnifying the quantizing error in the encoder count. I find I have to use very small amounts of D or the quantized noise dominates the system and causes horrible squealing of the servo. Sometime I want to see if I can splice a simple digital filter into the D term to taper off the frequency response of the D term. Possibly a filter could just be applied between the PID output and the DAC, PWM or other output device.
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