Just one thing to note if you implement PI or PID control digitally - make sure the calculations have adequate precision. Ideally use floating point, if you have to use integer arithmetic, 16 bits may be marginal, in a HLL best to use 32 bit (int or long int).. Also, having a loop sample rate that is too fast exacerbates the precision problem. For the response plot in the earlier post, a loop sample time of a few hundred mS should be more than adequate.
Precision problems generally show as 'flat spots' in integral action, ie. it stops working before the error is driven to zero.
For the loop you seem to have, I'd suggest starting with PI control only because (1) the algo is simpler, (2) derivative is troublesome of there's any noise on the measurement and (3) tuning derivative can be tricky even for experienced control engineers.
I've found that doing these sorts of loops in analog is often quicker and easier, a good MOS input opamp such as the TLC2272 will do PI easily. The only challenge is to get long integral times, circuit layout becomes critical in order to avoid leakage problems. Also, caps for the integrator need to be very low leakage.
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