Mitsubishi Electric MELSEC FX2N-2LC, User Manual

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FX2N-2LC Temperature Control Block Introduction of Functions 5
5-2
5.1.2 Overshoot prevention function
Generally in PID control, when a deviation continues for a long time, the PID arithmetic opera-
tion result exceeds the effective range (from 0 to 100%) of the operation quantity. 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 FX
2N
-2LC 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 opera-
tion result exceeds the limit point (upper/lower limit of the output limiter) so that the PID arith-
metic operation result always remains inside the effective range. Accordingly, when a deviation
becomes small, the correction operation is immediately performed.
<When the RFB limiter function is not provided>
Operation
quantity (%)
<When the RFB limiter function is provided>
Output limiter upper limit
Output limiter lower limit
Temperature
Target
value
Large overshoot
Operation
quantity (%)
Output limiter upper limit
Output limiter lower limit
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.
Temperature
Target
value