Electronic – Algorithms for automatic PID tuning

Please note the following: As you know, the PI controller is responsible in modifying the steady state response. That is improving the steady state values and settling time. You are simply adding a pole at the origin.

On the other hand, the PD controller will help you in improving the transient response. That is overshoot, rise time and peak time.

Now it is important to remember that adding a PI controller will 'usually' increase the order of the system which will cause a change in the previous steady state value.

Similarly, adding PD controllers will change the transfer function of your original system due to exposing the system to a zero. This means that the root locus 'if plotted' will be different.

Therefore; there will always be a compromise -trade off-. The best you can do is add margins of errors or inequality ranges for accepted values for both steady state and transient parameters.

Long story short, adding any controller to the system to improve the steady state will effect the transient and vice verse.

Without the integral term being used within the feedback it could be nonsense - any error will accumulate in the integral term and just appear illogical. However it could show something useful....

For instance, at the start of your graph the integral term develops a value because there is a small error between demand and actual. As you approach t=2500 it seems to be drifting down slightly as the actual psi drifts slightly up - this makes sense.

When demand changes about t=2500, the integral ramps up (as you'd expect) and keeps ramping because actual isn't attaining demand.

This tells me it is working, however at 3200 something goes wrong - I'd expect integral to keep ramping up but it appears to flat-line then disappear about 4000.

I'd be suspicious of what happens after about 3200.

Hope this helps.