LSIS XBE-DC08A, Руководство пользователя, Страница: 336 / 1284

B
asi
s
Chapter 6 Built-in PID Function
6-27
(2) P IDCAS
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.