Chapter 7 Usage of Various Functions
7-
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
g)
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.
The Fig. 2-12 shows the PV and MV of PI control system when the windup occurs. As shown as the Fig. 2-12, the
actuator is saturated because of the large initial deviation. The integral term increase until the PV reaches to the
SV (deviation = 0), and then start to decrease while the PV is larger than SV (deviation < 0). However, the MV
keeps the saturated status until the integral term is small enough to cancel the windup of actuator. As the result of
the windup, the actuator will output positive value for a while after the PV reached to the SV, and the system show
a large overshoot. A large initial deviation, load disturbance, or mis-operation of devices can cause windup of
actuator.
Time
PV
b=1
b=0.7
b=0.5
b=0.1
SV
