Appendix
examining the differences over a number of samples. A tangent is constructed to the point where the rate of
change is a maximum, and its intersection with the time axis is found. The delay time, Tu, is the time from
the start of the measurement up to the intersection of the inflection tangent and the time axis. Knowing Tu
and Vmax, the Chien, Hrones and Reswick formula yields the controller parameters for suppression of
disturbances with 20% overshoot. The parameters for the pre-controller can easily be derived from the
parameters for the main controller with the aid of heuristic formulae. After completion of the self-tuning these
parameters are used in an automatic switch to closed loop operation.
Requirements
Development environment target platform PLC libraries to include
TwinCAT 3.1.4006 PC or CX (x86) Tc2_TempController
7.4 Disturbance Compensation
Disturbance Compensation
A disturbance signal has significant effect on the quality of the controller and eventually on the controlled
process. A PID controller can passively overcome the effect of a disturbance signal by increasing the
controller output. But this is an inefficient way of handling the compensation.
The function block FB_CTRL_TempController_DistComp offers an addional Lead-Lag compensation to
actively compensate the disturbance signal. It is assumed that the disturbing signal in question is measured
and fed to the function block. Following block diagram explains the structure of the disturbance
compensation:
The disturbance compensation is actually a Lead-Lag compensator. A Lead-Lag compensator is a versatile
component, which can be used to achieve I, D, PI, PD and PID type compensators by carefully choosing its
gain and time constants. The compensator is useful to reduce stead state error, peaks, and improving the
dynamic response to the disturbance. More information about the Lead-Lag compensator can be found here.
TC3 Temperature Controller46 Version: 1.1
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