User's Manual

Automation and Drives - SCE
T I A Training Document Page 22 of 64 Module
2.7.3.1 Proportional Controllers (P-Controller)
In the case of a P-controller, the controller output y is always proportional to the recorded system
deviation (y ~ e). The result is that a P-controller responds to a system deviation without a delay, and
generates a controller output only if there is a deviation e.
The proportional pressure regulator sketched in the figure below compares the force FS of the
setpoint spring with the force FB that pressure p2 generates on the spring-elastic metal bellows. If
the force is not in balance, the lever rotates around the pivot point D. The valve position
ñ changes, and accordingly the pressure p2 to be controlled until a new power balance is
established. The figure shows the behavior of the P-controller as a sudden system deviation occurs.
The amplitude of the controller output step y depends on the extent of the system deviation e, and
the amount of the proportional coefficient Kp: To keep the system deviation low, a proportional
factor has to be selected that is as large as possible. Increasing the factor accelerates the reaction
of the controller; however, a value that is too high may cause overshoot, and a considerable
oscillatory tendency of the controller.
Metal Bellows
Setpoint Spring
Preface Fundamentals Discontinuous Action Controller Controller Block (S)FB41 Setting the System Appendix
B3
Issued: 02/2008 Control Engineering with STEP 7