PID question. Sampling frequency?

valemike · Guest

I'm using pin RB0 as a zero cross detector from the AC line.
My period for the heater is 8 cycles (thus 8 x 16.6ms = 133ms).
The duty cycle is 0 thru 8, giving me a fraction of n/8.

If doing PID regulation, how often do I have to sample the temperature? Everytime the 8 cycles has elapsed? Or more often, less often?

Ttelmah · Guest

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

In your case once per eight cycles would be ideal, preferably near the end of cycle eight when the heater is almost allways off. For a heater it doesn't matter much but if it was an electrically noisy load like a motor it is especially good to sample when the load is off.

If you were to sample every 7 or 9 cycles you could get a "beat note" between the PWM cycles and the sampling cycles that can cause problems. Keep the sampling a constant multiple (or sub-multiple) of the PWM cycle.
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