4 - 4
MELSEC-Q/QnA
4. FUNCTIONS OF PID CONTROL
4.2.3 Proportional operation (P operation)
The control method for proportional operation is described below.
(1) In proportional operation, an MV (manipulated value) proportional to the deviation
(the difference between the set value and process value) is obtained.
(2) The relationship between E (deviation) and the MV is expressed by the following
formula:
MV=Kp • E
Kp is a proportional constant and is called the "proportional gain".
Condition
Proportional Operation
When proportional gain Kp is
smaller
Control operation gets slower.
When proportional gain Kp is
larger
Control operation gets faster.
However, hunting is more likely to occur.
(3) The proportional operation in step response with a constant E (deviation) is
illustrated in Fig. 4.2.
De
via
tio
n
MV
E
Time
Time
Kp. E
Fig. 4.2 Proportional Operation with a Constant Deviation
(4) A certain error produced relative to a set value is called an offset.
An offset is produced in proportional operation.
t
t
Set value
Offset
Set value
Offset
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