7-18
Section
Data Control Instructions
428
The following diagram shows the relationship between the sampling period and
PID processing. PID processing is performed only when the sampling period
(100 ms in this case) has elapsed.
70 ms
1 cycle
60 ms
70 ms
70 ms
PID processing
with initial values
(0 ms)
No processing
(70 ms)
PID processing
(70+30=100 ms,
no carryover)
No processing
(60 ms)
PID processing
(130 ms, 30 ms carryover)
PID CONTROL Action
Execution Condition OFF
All data that has been set is retained. When the execution condition is OFF, the
manipulated variable can be written to the output word (OW) to achieve manual
control.
Rising Edge of the Execution Condition
The work area is initialized based on the PID parameters that have been set and
the PID control action is begin. Sudden and radical changes in the manipulated
variable output are not made when starting action to avoid adverse affect on the
controlled system (bumpless operation).
When PID parameters are changed, they first become valid when the execution
condition changes from OFF to ON.
Execution Condition ON
The PID action is executed at the intervals based on the sampling period, ac-
cording to the PID parameters that have been set.
Sampling Period and PID Execution Timing
The sampling period is the time interval to retrieve the measurement data for
carrying out a PID action. PID(––), however, is executed according to CPU
cycle, so there may be cases where the sampling period is exceeded. In such
cases, the time interval until the next sampling is reduced.
PID Control Method
PID control actions are executed by means of PID control with feed-forward con-
trol (two degrees of freedom).
When overshooting is prevented with simple PID control, stabilization of distur-
bances is slowed (1). If stabilization of disturbances is speeded up, on the other
hand, overshooting occurs and response toward the target value is slowed (2).
With feed-forward PID control, there is no overshooting, and response toward
the target value and stabilization of disturbances can both be speeded up (3).
Simple PID Control
Feed-forward PID control
As the target response is slowed,
the disturbance response worsens.
As the disturbance response is speeded
up, the target response worsens.
Overshoot
Target response
Disturbance response
(1)
(2)
