User`s manual

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MELSEC-Q

1 OVERVIEW

1.4 PID Control

This section explains "proportional operation", "integral operation" and "derivative operation"

performed for PID control using the process control instructions.

1.4.1 Proportional operation (P operation)

This section explains the control method using proportional operation.

(1) Proportional operation is the action that compares the deviation (difference between the set

value and the process value) to find the manipulated value.

(2) The change in relationship between deviation (DV) and manipulated value (MV) using

proportional operation is shown using the following numeric expression.

MV Kp • DV

Kp is called the proportional gain or proportional constant.

(3) The proportional operation when the deviation is a constant stepped response is shown in Fig.

1.3.

Time

K DV

Deviation

Manipulated

value

Time

Fig. 1.3 Proportional operation when deviation is constant

(4) The manipulated value fluctuates between

10 and 110%.

As Kp becomes larger the manipulated value corresponding to the deviation also becomes

larger making the compensation operation stronger.

(5) Offset occurs in proportional operation.