G-22
4 Auto Tuning
FX
3U
/FX
3UC
Series PLC User's Manual - Analog Control Edition
PID Instruction (FNC 88)
4.3 Cautions on Auto Tuning Execution
4.2.3 Reference: How to obtain three constants in PID control (step response method)
For acquiring satisfactory control results in PID control, it is necessary to obtain the optimal value of each
constant (parameter) suitable to 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 (TD)).
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)) acquired from
the input value variation.
*1. The stepped output may be obtained from 0 to 75% or from 0 to 50%.
Operation characteristics
Operation characteristics and three constants
4.3 Cautions on Auto Tuning Execution
1. Countermeasures in program when the input value (PV) does not change
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 phenomenon by introducing a sequence to monitor the input value or the elapsed time
from the start of auto tuning.
Control type Proportional gain (KP) [%] Integral time (TI) [×100ms] Differential time (TD) [×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
×
0.9
RL
Output value
(MV)
×
1.2
RL
Output value
(MV)
×
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