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Thanks all for the suggestions. I tried a few values. Then I realized the setpoint is not an input to this differential equation.... So I am now putting the error PV-SP into X.

If dT/Tn is 0.33 and K = 2.0 it appears to behave, that is it doesn't skyrocket over small error. Smaller or larger dT/Tn, order of magnitude then any tiny error causes output values to be crazy high.

But it's so poorly defined I don't think I can use it.

A peer implements Autosar routines for ECU's and tells me how great they are, but won't dig into his (=employer's) code to see how he did it. He uses them for throttle and cruise control, and tells me the integrator windup is better dealt with using freq. domain math and the equations/algorithms are safe, the consortium includes over a dozen car makers. Simpler routines look really good, like the "safe" math routines (no division by zero, no variable overflow/wraparound etc.).

But the documents are unbearable. I'll look if I am missing a definition doc with units etc. but I'm out of time to get this going.

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