PID CONTROL SYSTEMS
Proportional plus reset plus rate controllers combine proportional control actions
with integral and derivative actions.
DESCRIBE the characteristics of the following types of
automatic control systems:
Proportional plus reset plus rate control
For processes that can operate with continuous cycling, the relatively inexpensive two position
controller is adequate. For processes that cannot tolerate continuous cycling, a proportional
controller is often employed. For processes that can tolerate neither continuous cycling nor offset
error, a proportional plus reset controller can be used. For processes that need improved stability
and can tolerate an offset error, a proportional plus rate controller is employed.
However, there are some processes that cannot tolerate offset error, yet need good stability. The
logical solution is to use a control mode that combines the advantages of proportional, reset, and
rate action. This chapter describes the mode identified as proportional plus reset plus rate,
commonly called Proportional-Integral-Derivative (PID).
Proportional Plus Reset Plus Rate Controller Actions
When an error is introduced to a PID controller, the controllers response is a combination of the
proportional, integral, and derivative actions, as shown in Figure 30.
Assume the error is due to a slowly increasing measured variable. As the error increases, the
proportional action of the PID controller produces an output that is proportional to the error
signal. The reset action of the controller produces an output whose rate of change is determined
by the magnitude of the error. In this case, as the error continues to increase at a steady rate,
the reset output continues to increase its rate of change. The rate action of the controller
produces an output whose magnitude is determined by the rate of change. When combined, these
actions produce an output as shown in Figure 30.