PID change range change P param too?

[Blokk]

Hi,

I have a quite general question about PID control. (i have pid toolkit)

I measure a process value (PV, it is temperature), and I have a fixed set-point. The output of the PID controller drives a current source (Keithley 2400 sourcemeter). When the PV is far (starting up state) from the setpoint, the output should be quite high to decrease transient time. So lets say, we start the control with range 1Amp in the Keithley. Of course, when the PID does not need too high output, I can go down, first to the 100 mA range, and so on. Also, when I change the range of the Keithley, I change the maximum/minimum range terminal value on the PID vi.

(Of course, the resolution much better of the Keithley at 1mA compared to 1Amp. )

I PID tune such a system at range 1Amp for example, and I have the P, I, D params. If I move from the 1Amp range to the 100mAmp range, do I need to change the P parameter?

Thanks!

[nathand · Accepted solution]

It depends.  If you feed the value to the Keithley in absolute real units (amps, or milliamps) then there's no need to change the PID gains.  If the value to the Keithley is a relative unit - for example a percentage of full scale - then yes, you will need to adjust your P gain as the full scale range changes.

[Blokk]

Thank you very much, this makes sense 🙂 (I use absolute amp values)!

Now it is clear 🙂