User's Manual

Automation and Drives - SCE
T I A Training Document Page 23 of 64 Module
The figure below shows the performance of the P-controller:
Time
System
Deviation
Setpoint
A
ctual
Value
Controlled
Variable
The advantages of this controller type are, on the one hand, its simplicity (the electronic
implementation can, in the simplest case, consist of merely a resistor); on the other hand, in its
prompt reaction in comparison to other controller types. The main disadvantage of a P-controller is
its lasting system deviation. The setpoint is never completely reached, even over long periods of
time. This disadvantage as well as a reaction speed that is not yet ideal can be minimized only
insufficiently by using a larger proportional factor, since otherwise, the controller will overshoot; that
is, it will overreact so to speak. In the worst case, the controller oscillates continuously, whereby the
controlled variable is moved by the controller itself away from the setpoint -instead of by the
disturbance variable.
The problem of continuous system deviation is best solved by the integral action controller.
Preface Fundamentals Discontinuous Action Controller Controller Block (S)FB41 Setting the System Appendix
B3
Issued: 02/2008 Control Engineering with STEP 7