Chapter 15 PID Function (Built-in function)
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(2) PIDCAS
PIDCAS is a command to execute CASCADE control.
- Operand M and S mean master loop and slave loop respecively and available only for
constant(0~15).
- If start junction is on, cascade control is executed through master loop and slave loop.
- In case of IEC type, PIDCAS function block is used for cascade control.
Cascade control is called a control method which is intended to increase control stability through
quick removal of disturbance by connecting two PID control loops in series and is structured as
follows.
[Figure 15.13 Comparison of single loop control and cascade control]
Looking at the figure, it is found that cascade control contains slave loop control within external
control loop. That is, the control output of external loop PID control is entered as SV of the
internal loop control. Therefore, if steam valve suffers from disturbance in the figure, single loop
PID control may not be modified until PV, y(s) appears while cascade control is structured to
remove any disturbance by the internal PID loop control before any disturbance that occurs in
its internal loop affects the PV, y(s), so it can early remove the influence from disturbance.
XGB internal PID control connects two PID control loops each other, making cascade control
possible. At the moment, MV of external loop is automatically entered as the SV of internal loop,
so it is not necessary to enter it through program.
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