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B
asi
s
Chapter 6 Built-in PID Function
6-29
6.6 PID Auto-tuning
6.6.1 Basic Theory of PID Auto-tuning
It describes the function of PID auto-tuning.
The performance of PID controller is very different according to P, I, D coefficient. Generally, It is very difficult and takes long time
to predict the system and set P, I, D coefficient because of non-periodical disturbance, interference of other control loop, dynamic
characteristic of control system though the engineer is good at handling the PID controller. So auto-tuning that sets the PID
coefficient automatically is very useful. Generally, there are many methods in setting the PID coefficient. Here, it will describe
Relay Auto-tuning.
(1) PID coefficient setting by Relay auto-tuning
It makes critical oscillation by force and uses the width and period of oscillation to specify the PID coefficient. It applies max.
output and min. output to control system for auto-tuning. Then, oscillation with steady period and steady width occurs
around the Set value like figure 6.15, and it can calculate the boundary gain by using it like expression (6.3.1).
< Figure 6.15 Relay auto-tuning >
width
output Min output Max
K
u
×
− ×
=
p
) . . ( 4
(6.4.1)
At this time, oscillation period is called boundary period. If boundary gain and period is specified, use table 6.9, Ziegler &
Nichols tuning table to specify the PID coefficient. This Relay tuning is relatively simple to configure and easy to know the
boundary gain and period so it is used frequently and XGB built-in PID auto-tuning uses this method.
Controller
Proportional gain
(Kp)
Integral time(Ti) Differential time(Td)
P
u
K 5 . 0
- -
PI
u
K 45 . 0 2 . 1 /
u
P
-
PID
u
K 6 . 0 2 /
u
P 8 /
u
P
< Table 6.9 Ziegler & Nichols tuning table >