National Instruments Xmath Interactive Control Design Module ICDM - Manual (Page 45)

Chapter 4 PID Synthesis
© National Instruments Corporation 4-5 Xmath Interactive Control Design Module
Time Versus Frequency Parameters
Notice that the sliders and variable-edit boxes use time parameters,
whereas the Bode plot handles use frequencies, that is, the inverses of the
time parameters. If you think of integral action as being parameterized by
a characteristic time, then you may prefer to use the slider. If you think of
integral action as being parameterized by a characteristic frequency (reset
rate), then you may prefer to manipulate the Bode plot handle.
Ranges of Sliders and Plots
The ranges for the sliders and plots can be changed in several ways. If you
enter a value that lies outside the slider range in the corresponding variable
edit box, the range of the slider will automatically adjust to accommodate
the new value. You also can change the range of a slider using the Ranges
window, which appears when you select View»Ranges or press <Ctrl-R>
in the PID window. Selecting View»Auto Scale will cause ICDM to select
sensible values for the slider and plot ranges based on the current controller.
The ranges for the plots also can be changed interactively. Refer to the
General Plotting Features section of Chapter 2, Introduction to SISO
Design .
Controller Term Normalizations
Each of the controller terms is normalized in a way that is convenient for
most PID design tasks as described in the following sections.
Integral Term Normalization
The integral term is high-frequency normalized, which means that it is
approximately one for frequencies above 1/T
int
. Therefore, you can adjust
the integral time constant 1/T
int
without significantly affecting the
controller transfer function at high frequencies. For example, you can add
integral action to a controller without significantly affecting the stability
margins or closed-loop dynamics by adding the integral term with 1/T
int
well below the crossover frequency, that is, 1/T
int
large. In this case, your
controller will enforce steady-state tracking, but over a time period longer
than the closed-loop system dynamics. You then can slowly decrease 1/T
int
until you get a good balance between fast integral action and the
degradation of stability margins.