You are using PI control on a temperature loop, and it just does not want to...

You are using PI control on a temperature loop, and it just does not want to stabilise. Any ideas what you might want to set / adjust, in order to stabilize the loop, and get the PV to stop oscillating, and settle down at the Set Point?

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Expert Solution

A PI controller is proportional + Integral controller. A proportional term largely defines or shapes the transient behaviour of the response while the Integral term is to bring the output of the system to the desired set point i.e. to take care of the steady state behaviour or eliminate the the error in steady state value. The transfer function of a PI controller is given by,

C_pi(s) = Kp + Ki/s = (Kp s +Ki) /s = Kp(s+Ki/Kp) /s

Clearly the transfer function of the PI controller has one zero and one pole at origin. The pole at origin is marginally stable and causes instability to the system if not compensated by an accompanying zero. Nonetheless a PI controller has a zero add stability to the system. If the location of zero is far from the jw axis, the effect of the pole at origin will not get compensated properly. Every pole added to the system pulls the rootlocus towards right side which makes the closed loop system unstable. Every zero added, pulls towards left that makes it stable. Hence for achieving the stability, the PI controller that you have designed, may be added one more zero that makes it a PID controller. A PID has two zeros and one pole at origin. One zero nullifies the effect of pole at origin while the other shapes the response taking care of the stability of the closed loop system. By carefully adjusting the location of the other zero ( using rootlocus technique in Matlab) you will have the response which is free of oscillations and smooth as you desired.

Manojponduru answered 1 year ago

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