LSIS XBE-DC08A, Руководство пользователя, Страница: 321 / 1284

B
asi
s
Chapter 6 Built-in PID Function
6-12
(c) Prevention of dual integral accumulation
It makes dual integral accumulation function enabled/disabled. To understand integral accumulation prevention function, it is
necessary to explain the phenomenon of integral accumulation first of all. Every drive has a limit. That is, a motor is limited to
the speed and a valve can become status overcoming the complete open/close. If it happens that MV output from a control is
beyond the output limit of a drive, its output is maintained as saturated, which may deteriorate the control performance of a
system and shorten the life of a drive. Formula (6.2.3) shows that the integral control among PID control output components
accumulates errors as time goes on, from which it may take more time to return the normal status after the actuator is
saturated in a system of which response characteristically is slow. It is so called integral accumulation phenomenon as
illustrated in Fig. 6.9, which shows that if the initial error is very large, the error is continuously accumulated by integral control.
Accordingly, a drive is saturated within its output upper limit while the control signal is getting larger, keeping being saturated for
a long while until the drift becomes negative and the integral term turns small enough. Due to the operation, the PV may have
a large over-shoot as seen in the figure. Such a wind-up phenomenon may occur if the initial drift is large or by a large
disturbance or due to malfunction of a device.
The PID function of XGB series is basically with the integral accumulation prevention function, cutting off any integral
accumulation phenomenon. In addition, it can detect a time when SV is suddenly decreased, providing a more strong dual
integral accumulation prevention function.
< Figure 6.9 Integral accumulation phenomenon >
(d) PWM Output Enabled
PWM output means an output method to turn a junction on – off with a duty proportional to control output calculated by a
uniform output cycle. If PWM output is enabled, it realizes PWM output in accordance with PWM output cycle set in the
parameter of PWM output junction(P20 ~ P3F) designated in the parameter. At the moment, the PWM output cycle follows
the PWM output cycle separately set in PID operation cycle. figure shows the relation between PID control output and PWM
output.
Ex) if PWM output cycle: 1 second, PWM output junction: P20, max. output: 10000, min. output: 0
Time Output P40 junction operation
0 sec 5000 0.5 sec On, 0.5 sec Off
1 sec 3000 0.3 sec On, 0.7 sec Off
Time
Target
value
Upper limit of drive output
PV
MV
Drive saturation
Integral accumulation phenomenon