Mario. wrote: > I think the point was more like: running a 10kHz control loop with > 10-cycle delay instead of running direct 1kHz control loop. I'm not > being specific here, just the picture. The reason for a filter is that the encoder is a doubly-quantized mechanism. First, it only counts discrete positions, second, the count is sampled at some fixed rate. No matter what that rate is, you get a flicker in the number of counts that occur in each sample period. If you have a differential term in your PID control loop, it tends to magnify this flicker, and the worst case is at 1/2 the sample rate. This causes the servo amp to produce a lot of amplitude at 1/2 that sample rate, whetever the rate is. Mostly it makes annoying whistling noises at certain speeds. A filter that notches out that 1/2 sample rate frequency seems like a good idea.
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