How a control system may become unstable

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Presentation transcript:

How a control system may become unstable Course PEF3006 Process Control Fall 2017 How a control system may become unstable By Finn Aakre Haugen (finn.haugen@hit.no) PEF3006 Process Control. USN. F. Haugen. 2017

PEF3006 Process Control. USN. F. Haugen. 2017 Control loop: 2 Typical causes of reduced stability or even instability of a control loop: Increase of loop gain Increase of loop dead-time Increase of loop time-constants PEF3006 Process Control. USN. F. Haugen. 2017

Let’s impose various parameter changes on this simulator: Level control of wood-chip tank 3 And we will use Skogestad's PI tuning formula to readjust the PI settings to cope with stability problems! Recall the PI formulas: Kp = 1/(2*Ki*tau), Ti = 4*tau Increase of loop gain: Controller: Controller gain (observe the effect of increase) Integral time (decrease) Actuator: Feed screw gain (increase) Process: Cross sectional area (decrease) Sensor: Sensor gain (increase) Increase of time-delay: Process: Transportation time (increase) Increase of time-constant: Measurement filter: Time-constant in filter (increase) PEF3006 Process Control. USN. F. Haugen. 2017