Mitsubishi Electric NZ2GF2B-60TCTT4 User Manual page 26

(3) PID control (simple two-degree-of-freedom)
In the simple two-degree-of-freedom, the module controls the target subject using not only PID constants but also
the control response parameter. The parameter can be set to "fast", "normal", or "slow". This setting enables the
form of "response to the change of the set value (SV)" to change maintaining "response to the disturbance" in a
good condition.
Set value
The following describes the difference between the one-degree-of-freedom PID control, two-degree-of-freedom
PID control, and simple two-degree-of-freedom PID control.
(a) One-degree-of-freedom PID control and two-degree-of-freedom PID control
• General PID control is called one-degree-of freedom PID control. In the one-degree-of freedom PID
control, when PID constants that improve "response to the change of the set value (SV)" are set,
"response to the disturbance" worsens. Conversely, when PID constants that improve "response to the
disturbance" are set, "response to the change of the set value (SV)" worsens.
• In the two-degree-of-freedom PID control, a manipulated value (MV) is determined considering the set
value (SV) and variations. In this form of PID control, "response to the change of the set value (SV)" and
"response to the disturbance" can be compatible with each other.
(b) Two-degree-of-freedom PID control and simple two-degree-of-freedom PID control
The following figure is a block diagram of the two-degree-of-freedom PID control.
Added function for two-degree-of-freedom
Set value
(SV)
-
+
The appropriate setting of , , and  in the above figure can achieve optimum control. However, a complete
and perfect two-degree-of-freedom PID control needs many parameters to be set and the automatic
adjustment by auto tuning is also difficult to perform. The temperature control module is therefore equipped
with the simple two-degree-of-freedom PID control with simplified parameters.
24
Fast
Normal
(SV)
Slow
Response to the change
of the set value (SV)
+
+
1
1 + T s
I
Set value
(SV)
Response to the disturbance
PID control
+ +
1
K (1 + )
P
T s
I
- +
+ K T
P
1 +
-
Object to be
controlled
Disturbance D
G(s)
Manipulated
value (MV)
s
D
T s
D
Temperature
process value (PV)