Proportional-on-PV Controller

The most misunderstood and most underutilised version of the PID algorithm is the proportional-on-PV type. Taking the algorithm developed as Equation (3.25) we modify the proportional action so that it is based on PV rather than error. 

Rather than consider SP changes, we should give more attention to load changes. Load changes are process disturbances which cause the PV to move away from the SP. On om 

Figjure 3.14 Response to SP change (proportional-on-error algorithm) 

Switching the algorithm between proportional-on-error and proportional-on-PV has no affect on the way it responds to load changes. The difference we see is due to the difference in tuning. The more tightly tuned algorithm deviates from SP by less than half and the duration of the upset is also halved. 

This opportunity for substantial improvement is often overlooked by control engineers. Preoccupied with tuning controllers for SP changes they rarely appreciate how much faster the controller can be made to react to process disturbances. 

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