LS Industrial Systems SV-iS7 PLC, Руководство пользователя

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Chapter 6 Usage of Various Functions
6-11
(d) PID action
① PID action controls the control object with the manipulation quantity produced by
(P+I+D) action.
PID action when a given deviat ② ion has occurred is shown as the following Fig. 7.7.
Fig. 7.7 PID action with a constant deviation
(e) 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 of 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. 7.8 shows the PV and MV of PI control system when the windup occurs. As
shown as the Fig. 7.8, 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 shows a large overshoot. A large initial deviation, load
disturbance, or miss-operation of devices can cause windup of actuator.