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(3) PID control (simple two-degree-of-freedom)
The Q64TCN operates in "simple two-degree-of-freedom". In this form of PID control, parameters are simplified
compared to the two-degree-of-freedom PID control.
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. ( Page 188, Section 4.7)
The following explains 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 to improve "response to the change of the set value (SV)" are set, "response
to the disturbance" degrades. Conversely, when PID constants to improve "response to the disturbance"
are set, "response to the change of the set value (SV)" degrades.
• In the two-degree-of-freedom PID control, a manipulated value (MV) is determined considering the set
value (SV) or 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.
By setting , , and above properly, optimum control can be achieved.
Note that required parameter settings increase and PID constants can hardly be auto-set by the auto tuning
function for complete two-degree-of-freedom PID control. Therefore, the Q64TCN operates in the simple two-
degree-of-freedom PID control for which parameters are simplified.
Set value
(SV)
Normal
Fast
Slow
Response to the change
of the set value (SV)
Set value
(SV)
Response to the disturbance
PID control
Object to be
controlled
Disturbance D
1
1 + T
I s
1 + TD s
KP TD s
1
T
I s
KP (1 + )
Added function for two-degree-of-freedom
Manipulated
value (MV)
Set value
(SV)
G(s)
Temperature
process value (PV)
-
+
-
+
+
+
+
+
+
-