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4 Auto-Tuning

FX

3G

/FX

3U

/FX

3UC

 PLC User's Manual - Analog Control Edition

PID Instruction (FNC 88)

For acquiring satisfactory control results during PID control, it is necessary to obtain the optimal value of each
constant (parameter) suitable for the control target.
This paragraph explains the step response method to obtain three constants in the PID control (proportional
gain (K

P

), integral time (T

I

) and differential time (T

D

)).

What is the step response method
In this method, by giving stepped output from 0 to 100%

*1

 to the control system, three constants in the PID

control are obtained from the operation characteristics (maximum ramp (R) and dead time (L)) and the input
value variation.

*1.

The stepped output may be obtained from 0 to 75% or from 0 to 50%.

When the input value (PV) does not change normally due to factors such as wire breakage in an analog input
line, auto-tuning is not finished.
Detect and avoid such occurrences by introducing a sequence to monitor the input value or the elapsed time
from the start of auto-tuning.

×100ms]

×10ms]

Only proportional

control (P operation)

⎯

⎯

PI control

(PI operation)

33 L

⎯

PID control

(PID operation)

20 L

50 L

0%

100%

Output value

Output value (MV)

Time

Input value

variation

Dead time (L)

[s]

Time (s)

1 (s)

Maximum ramp (R)

Output value

(MV)

1

RL

×

×100

0.9

RL

Output value

(MV)

×

×100

1.2

RL

Output value

(MV)

×

×100