Overshoot Prevention Function - Mitsubishi Electric MELSEC FX Series User Manual

FX -4LC User's Manual
3U
Associated buffer memory
BFM No.
CH1 CH2 CH3 CH4
#58 #98 #138 #178 Proportional band (P)/Heating proportional band (P)
#60 #100 #140 #180 Integral time (I)
#61 #101 #141 #181 Derivative time (D)
#62 #102 #142 #182 Control response parameter
#64 #104 #144 #184 Output limiter upper limit/Heating upper output limiter setting
#65 #105 #145 #185 Output limiter lower limit
#67 #107 #147 #187 Output change ratio limiter
#240 #241
6.4.2

Overshoot prevention function

When a deviation continues for a long time, the PID arithmetic operation result exceeds the effective range of
the control output. At this time, even if the deviation becomes smaller, it takes some time until the output
comes within the effective range because of the integral operation. As a result, execution of an actual
correction operation is delayed, and overshoot/undershoot occurs.
In order to prevent overshoot, the 4LC is equipped with the RFB (reset-feedback) limiter function.
The RFB limiter function feeds back the excessive quantity to the integral value and makes the arithmetic
operation result be held at the limit point when the PID arithmetic operation result exceeds the limit point
(upper/lower limit of the output limiter) so that the PID arithmetic operation result always remains inside the
effective range. Accordingly, when a deviation becomes small, the correction operation is immediately
performed.
The RFB limiter function is automatically activated while PID control is performed, and does not require
setting.
Operation
(When the RFB limiter function is not provided)
Control output
Temperature
Set value
(SV)
Control mode switch
Output limiter upper limit
Output limiter lower limit
Large overshoot
Description
(When the RFB limiter function is provided)
The excessive quantity above the output limiter
upper limit is fed back to the integral value, and
the arithmetic operation result is held at the
upper limit value.
Control output
Temperature
Set value
(SV)
6 Introduction of Functions
6.4 PID control
Reference
Subsection 8.2.22
Subsection 8.2.24
Subsection 8.2.25
Subsection 8.2.26
Subsection 8.2.28
Subsection 8.2.29
Subsection 8.2.31
Subsection 8.2.62
Output limiter upper limit
Output limiter lower limit
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