
The individual components in a linear feedback control system such as sensors, controllers, and
power converters are examples of continuous-time filters, which transmit the range of frequencies in
the input according to their frequency response function. Control system design based on frequency
response relies on assumptions related to the frequency content of the command inputs and the
uncontrollable inputs, referred to as load variables or disturbances.
A simple unity feedback control system is shown in Figure 4.26. R(s) and D(s) are the reference
(command) and disturbance inputs.
The open-loop system model is
Y(s) ¼ G
P
(s)[U(s) þ G
D
(s)D(s)] (4:326 ...