Cascade Control - Mitsubishi Electric MELSEC FX Series User Manual

FX -4LC User's Manual
3U
6.7

Cascade control

6.7.1 Cascade control
Cascade control combines master channel control and slave channel control into one control loop.
Cascade control is suitable for situations where there is a large time delay between the temperature near a
heat source and the temperature of the object to be controlled.
The master channel performs PID arithmetic operation using the measured value (PV) of the control target or
an external input value as the input, converts the control output into the cascade signal using the cascade
bias and cascade gain, and then corrects the set value (SV) of the slave channel.
The slave channel performs PID control using the set value (SV) converted into the cascade signal.
Up to 2 control loops are available in the 4LC.
Set the control mode to "0" to "3" for performing cascade control.
The combination of available master channel and slave channel is determined for each control loop as shown
below.
Control loop
1
2
Cascade control block diagram (reference)
This diagram shows an example of the control loop 1.
Control loop1
Master channel side (channel 1)
Measured value (PV)
or
External input value
Set value (SV)
Slave channel side (channel 2)
Measured value (PV)
or
External input value
Set value (SV)
Master channel
Channel 1
Channel 3
→ For details on the control mode switch, refer to Section 6.2.
-
PID arithmetic
operation
+
Set value monitor
-
PID arithmetic
operation
+
Set value monitor
Cascade bias
+ +
Cascade monitor
6 Introduction of Functions
Slave channel
Channel 2
Channel 4
Control output (MV)
Control output (MV)
Cascade gain
6.7 Cascade control
Control output
OUT2
OUT4
Cascade OFF
Control output
OUT 1
Cascade ON
Control output
OUT 2
10
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1
2
3
4
5
6
7
8
9