Re: Q about Motion Control system, noise & sampling rates.

From: Jay (127.0.0.1_at_garbage.gar)
Date: 06/18/04

  • Next message: Paul Burke: "Re: Q about Motion Control system, noise & sampling rates." 
    Date: Thu, 17 Jun 2004 22:03:39 -0500

    Hi Jack,

    In article <60l4d0dmn92bsh7jvnkaasgd11h14s6ian@4ax.com>,
    jackklein@spamcop.net says...
     
    > I have two questions:
    >
    > 1. How many encoder counts are you dealing with per second?
    >
    > 2. Why are you working with instantaneous frequency?

    Answer to 1: The speed of the motor is controllable between 0 to 200 Hz,
    2000 counts/rev with quadrature counting (500 line).

    Answer to 2: I had a bit of a brain hickup. I'm actually doing something
    like what you outlined where I use a counter, acrue pulses and see how
    many have come in my sampling interval.

    The timing technique I mentioned in my original mail is something I
    tried to use to measure the jitter of the encoder signal numerically
    within the FPGA/uC circuit (i.e. see how the period changes from encoder
    pulse to encoder pulse). This method is NOT used my control-loop.

    > I don't know why you think you need some sort of band pass filter
    > either.

    I wanted to low-pass filter the error signal since the variance in how
    many counts I get per sample interval can be large. My issue was dealing
    with the fact that I'm recieving data from a system that's purely
    digital in nature and I didn't want to get any aliasing if I implemented
    a filter.

    > The usual strategy to use with a discrete digital input signal like an
    > encoder signal is to count them. Every 10 ms (100 Hz), you take the
    > count since the last time, and use that directly for your velocity.

    Ok, I understand what you're saying. With respect to applying a
    traditional digital filter (FIR/IIR/moving average) should I just "do
    it" and not worry about aliasing?

    Regards,
    Jay.

  • Next message: Paul Burke: "Re: Q about Motion Control system, noise & sampling rates."

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