Mitsubishi FX3U-4AD User Manual page 454

FX /FX /FX PLC User's Manual - Analog Control Edition
3G 3U 3UC
PID Instruction (FNC 88)
4.2.3 Reference: How to obtain three constants in PID control (step response method)
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 ), integral time (T
P
What is the step response method
In this method, by giving stepped output from 0 to 100%
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%.
Operation characteristics
100%
Output value
0%
Input value
variation
Operation characteristics and three constants
Control type
Only proportional
control (P operation)
PI control
(PI operation)
PID control
(PID operation)
4.3 Cautions on Auto-Tuning Execution
1. Countermeasures with in the 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 occurrences by introducing a sequence to monitor the input value or the elapsed time
from the start of auto-tuning.
L-22
) and differential time (T
I
Output value (MV)
1 (s)
Dead time (L)
[s]
Proportional gain (K ) [%]
P
1 Output value
×
RL (MV)
0.9 Output value
×
RL (MV)
1.2 Output value
×
RL (MV)
)).
D
*1
to the control system, three constants in the PID
Time
Maximum ramp (R)
Time (s)
Integral time (T ) [×100ms]
I
⎯
33 L
20 L
4 Auto-Tuning
4.3 Cautions on Auto-Tuning Execution
Differential time (T ) [×10ms]
D
⎯
⎯
50 L