At 16:35 04/22/99 -0500, John Payson wrote:
>|I've been struggling a little with a PID control scheme (actually it's just
>|PI, I've been warned that D terms promote instability!)  I've got a
>|proportional scheme that looks like this:
>
>For your system, I think you may be best off trying to estimate, at any
>given time, the peak temperature the system would reach if you were to
>completely cease heating it.  If that temperature is below your target,
>switch the heater on.  If the temperature is above your target, switch
>the heater off.  If done properly, this should yield good performance
>without too much work.

in a sense (you've got to stretch it a bit here :), that's what a correctly
tuned pid algorithm does. and as you say correctly, for slow processes like
an oven heating, it's often the 'i' part which introduces instability and
not the 'd' part. often the 'd' part is so small that it gets almost
negligible when you're close to the setpoint (and the temperature changes
are small); it serves primarily to avoid overshooting when approaching the
setpoint.

ge