Chapter 15. The PID functions
15-10
The figure 2.11 shows the variation of PV according to the several different reference
values (b). As shown in the Fig. 2.11, the small reference value produces small
deviation value, and it makes the control system response be slow.
In general, control system is required to be adaptable to various external / internal
changes. Especially, it should shows a stable transient response with the sudden
change of the SV to be robust to load disturbances and/or measurement noise.
Figure 2-11 The PI control with several reference values
15.2.1.7
Integral windup
All devices to be controlled, actuator, has limitation of operation. The motor has speed
limit, the valve can not flow over the maximum value. When the control system has
wide PV range, the PV can be over the maximum output value of actuator. At this time,
the actuator keeps the maximum output regardless the change of PV while the PV is
over the maximum output value of actuator. It can shorten the lifetime of actuator.
When the I control action is used, the deviation term is integrated continuously. It
makes the output of I control action very large, especially when the response
characteristic of system is slow.
This situation that the output of actuator is saturated, is called as ‘windup’. It takes a
long time that the actuator returns to normal operating state after the windup was
occurred.
T i m e
P V
b=1
b=0.7
b=0.5
b=0.1
S V
