Pid Control Overview; Forward Operation And Reverse Operation - Mitsubishi MELSEC-Q series Programming Manual

1.2

PID Control Overview

PID control is applied to the process control of flow rate, speed, air volume, temperature, tension, compounding or like.
In the following application, a value of a control target system can be kept at a set value with PID control.
Set value
PID control compares the value measured in the detection section (process value: PV) with the preset value (set value: SV)
and adjust the output value (manipulated value: MV) to eliminate the difference between the process value and set value.
In PID control, proportional operation (P), integral operation (I) and derivative operation (D) are combined to calculate the
manipulated value that will make the process value equal to the set value fast and precisely.
• If the difference between the process value and set value is large, the manipulated value is increased to make it close to
the set value fast.
• When the difference between the process value and set value has reduced, the manipulated value is decreased to make it
equal to the set value slowly and precisely.
1.3

Forward Operation and Reverse Operation

(1) Forward operation is the action that increases the manipulated value when the process value increases more than the
set value.
(2) Reverse operation is the action that increases the manipulated value when the process value is decreasing more than
the set value.
(3) Forward operation and reverse operation make the manipulated value larger as the difference between the set value and
the process value becomes larger.
(4) The following figure shows an example of process control in forward operation and reverse operation.
Temperature
Process control instructions
are used
SV
PID
operation
PV
Process value
Set value
Time
Forward operation (for air conditioning)
Subdivided (micro-blocked) processings
of PID control
Manual
MV
D/A
MV
Automatic
converter
MV
module
A/D
converter
module
Temperature
Process value
Reverse operation (for heating)
Controlled
system
Sensor
Set value
Time
13
1
2
3
4
3
6
7
8