M5d - module 5d PDF

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module 5d...

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9. Practical issues of feedback controllers 1. Direct and reverse action of controller For the controller, if the gain of its transfer function Kc is positive, it is called “reverse-acting” controller. It is given such a name because for a controller with positive gain (Kc>0), the controller output p(t) increases as the measurement ym(t) decreases. This kind of relation can be easily seen from a proportional controller with transfer function Gc=Kc:

p (t )  p  K ce (t ) p (t )  p  K c ( y sp (t )  ym (t )) Similarly, if controller gain Kc is negative (Kc0) air-to-open valve is referred to as “fail-close”

Air-to-close:   

to close the valve more, need higher air pressure; the process gain of the valve is negative (Kv0). Assume process gain (Kp>0) and sensor/transmitter gain (Km>0):  

If the valve is air-to-open (Kv>0), then reverse acting controller (Kc>0) should be used; If the valve is air-to-close (Kv>0), then direct acting controller (Kc...