IDEC MicroSmart Pentra - Manual (Page 38)

PID M ODULE M AIN F UNCTIONS
FC5A MicroSmart PID Module User’s Manual FC9Y-B1283 4-15
Cascade Control
The cascade control combines two PID controls to form one feedback loop to control a single target. The
cascade control is effective for applications in which the delay time or dead time is considerably large. When
delay time is large, it takes a long time for the process variable (PV) to change after the output manipulated
variable (MV) is changed. By using the cascade control, highly stable control can be realized for such
applications, though it takes time for the process variable (PV) to reach the set point (SP).
CH1 control is used as the master and CH0 control as the slave of the cascade control. The output
manipulated variable (MV) of the master (CH1 control) becomes the set point (SP) of the slave (CH0 control),
and the control result of CH0 is outputted from the CH0 output. The output manipulated variable (MV) (0 to
100%) of the master (CH1 control) is corresponded to the set point (SP) of the slave (CH0) according to the
external SP input linear conversion minimum and maximum values. For example, when the external SP input
linear conversion minimum value is 100°C and the maximum value is 400°C, the output manipulated variable
(MV) (0 to 100%) of the master (CH1 control) is converted as follows: 0% is converted to 100°C, 50% is
converted to 250°C, and 100% is converted to 400°C.
When a system using the cascade control is designed, it is required that the slave (CH0 control) have smaller
delay time and faster response comparing to the master (CH1 control).
Example: The cascade control is used for an application in which the heat quantity of a heater is controlled
using a power controller in order to control the temperature of the control target as shown in the figure below.
It is also possible to utilize the heating/cooling control to prevent a rapid temperature rise of the control target
by using a fan as the cooling output.
System Configuration and Wiring
Wiring Example of the FC5A-F2M2 [Current Output Type]
PV (M) SP (M)
PID (M)
MV (M)
AT (M)
PV (S) SP (S)
PID (S)
MV (S)
AT (S)
Master (Primary Control)
Slave (Secondary Control)
PID Module (with Cascade Control)
Power Controller
Control Target
Heater
CH1 Input
CH0 Input
CH0 Output
Cooling Equipment such as Fan
CH1 Output
Single Phase
200V
Control
Target
CH0 Thermocouple
CH1 Thermocouple
+
-
+ -
Power Controller
Heater
u
U
+
-
Power
Supply
PID Module (FC5A-F2M2)
IN0
IN1
24V DC
OUT0
OUT1
Channel
Terminal
No.
Terminal
No.
Channel