John Kasunich wrote: > Jon Elson wrote: > >>Peter C. Wallace wrote: >> >>>If you are just driving a bare motor, try raising your sampling rate... >> >>Geez, the resonance seems to be about 40 Hz, my sampling rate is >>already 1 KHz, 25 X higher. But, yes, I can go to at least 5 >>KHz and see what happens. >> >>Jon >> > > > Do you have current feedback or a current loop? (I doubt it.) No. This is an absolute bare-bones drive. Measuring current is fairly easy on a brush drive, and I've done that before. But, I don't even know how to measure the current on a 3-phase motor. My brush solution was to put a current sense resistor in series with the output right after one half-bridge (before the output filter) and read it with an Analog Devices AD620 instrumentation amp. This works amazingly well reading 100 mV drop off a resistor while the common-mode is a 70 V 100 KHz square wave! But, I really don't see how to do that and decipher four-quadrant motor current from the motor lines. The problem then is you need to get the current reading into an equation, while EMC is already putting out a digital PWM signal. I only have a 36 macrocell CPLD on the brushless servo amp board and 4000-series CMOS on the brush model, so DSP computations are out of the question. If you > control current, you more-or-less eliminate one pole from the system > response. Yes, maybe this whole PWM idea has a fundamental flaw, especially with larger motors. But, now I've backed myself into a corner, building a whole product line around a possibly flawed concept.
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