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Command law

When process identification has been done, the controller command law is choosen from the following criteria:

  • θ/τ for stable process, also said with a proportional response,
  • K×τ for unstable process, also said with integral response.
Command law for stable process, proportional response
Process: Gs static gain, τ pure lag, θ first order time constant, calculate θ/τ
θ/τ max θ/τ min
Command law
>20 Switch (open/close)
20 10 P
10 5 PI
5 2 PID
<2 Closed loop insufficient
Command law for unstable process, integral response
Proces: K static gain, τ pure lag, calculate K×τ
K×τ max K×τ min
Command law
>0.5 Closed loop insufficient
0.2 0.5 PID
0.1 0.2 PI
0.05 0.1 P
<0.05 Switch (open/close)