PID Control with quantized output

assuming a classical PID temperature control driving an SSR which has a zero-crossing feature effectively quantizing the power output to 1/2 AC
cycle resolution, and thermal time constants on the order of 1-10 seconds, and control resolution of better than 0.5% full scale desired, is there any hope for classical PID here? Do the same tuning rules apply? Or, is there a better control architecture for this application?
tia!
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snipped-for-privacy@yahoo.com wrote:

The better control architecture is to apply some dithering to the PID output so the SSR duty cycle is held more or less constant. With those time constants you should be able to get fairly good control, although you'll see some ripple and other quantization effects with a 1 second time constant. If you linearize your output this way then yes, the same tuning rules apply.
I've published an article in Embedded Systems Programming magazine which deals with just this subject. You can get to it from http://www.wescottdesign.com/articles/sigmadelta.html .
I wouldn't use the 1/2 cycle resolution, though. If you are consistently turning on for the same half cycle then you'll build up a healthy DC current and saturate a core in some transformer somewhere which isn't a good thing. You could conceivably correct this with a fancier sigma-delta algorithm, but personally I'd just avoid the whole issue by using whole AC cycles.
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Tim Wescott
Wescott Design Services
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snipped-for-privacy@yahoo.com wrote:

To avoid heating transformers with DC, use whole cycles. There are good ways to convert control magnitude to duty cycle linearly. 1/60th of a time constant is close enough to continuous to be difficult to distinguish from it.
Most heaters depart from classic PID performance because control isn't symmetric; cooling is passive. http://users.rcn.com/jyavins/servo.html might interest you.
Jerry
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Tim, Jerry, thanks very much for the feedback! This is very helpful, the Delta-Sigma idea sounds like the way to go!
Jerry Avins wrote:

has a

1/2 AC

desired,
good
isn't
http://users.rcn.com/jyavins/servo.html
get.

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