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PID variable not centred on setpoint with only proportional control

Dear all,
 
I have a heater controlling the temperature in a room using the LabVIEW PID.vi. The room setpoint and the average room temperature are the inputs to the PID program, and a 4-20 signal, to control the heater, is the output. With just Proportional control (P value only; I and D set to zero), the room temperature oscillates, but NOT about setpoint. It is in fact more than a degree (on average) below setpoint. Is this normal? I would have thought the process variable should vary about setpoint.
 
I have noticed that the output signal to the heater, stays at zero even when the room tempearature falls below setpoint, and doesn't start to ramp up until it has fallen a bit below the setpoint value. Why should this be?
 
Incidentally, no value of integral constant "I", seems to make any difference to the signal. Any help on this would be greatly appreciated, as I have spent some time trying to tune these.
 
Regards,
 
Will
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If it oscillates you probably have a problem with your tuning parameters. Or maybe other things that induce instability like dead zone, time delay, etc. It's hard to diagnose this without (much) more detail.

By the way, you WILL have a steady state error with only proportional control. Adjusting the integral factor is the right way to address this, but it appears not to be working in your system. This suggests that there may be other issues.
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Will,
 
Also, oscillations about a specific setpoint could indicate a limit cycle where you controller is unstable and it is "banging" on upper and lower limits of the saturation of the signal (4-20 mA) in this case. You can easily fix this reducing the gain of the controller. Try to apply 50% of the actual gain that you have and, if oscillating, try 25% and so on. At one point you should be able to avoid the oscillation and you can tune the Integral gain.
 
Barp - Control, Simulation, RTT and HIL - National Instruments
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Hi Barp,

Thank you for the mail. With lower P values, we have been able to eliminate the oscillation, but we have found that adding in different values of I makes no difference whatsoever. This is the most puzzling aspect of all of this. Do you think it might have anything to do with the beta setting on the PID parameters. Also, a guy in NI technical support suggested that if the error is too big, the I value is limited in some way and doesn't have an effect. Do you know anything about this?

Will

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