Mitsubishi Electric FX5 User Manual page 102

• Overshoot suppression by using sampling time
When the response speed of the control target is high, shorten the sampling time ((s3)+0) to control finely. Overshoot may be
suppressed. However, if the sampling time is too short, it is easily affected by momentary fluctuation of noise.
Control result before the change
Input value
(PV)
Target value
(1) Overshoot
Hunting suppression
When suppressing hunting, the operation is as follows.
■When auto-tuning is executed
When hunting occurs or the output is too large during the PID control using parameters obtained by the step response
method, the parameter may be not appropriate because auto tuning is completed before the maximum ramp value that
describes characteristics of the control target is obtained correctly.
Change the following setting. The correct maximum ramp value will be obtained and the result may improve.
• Hunting suppression setting
When the maximum ramp value cannot be obtained even if the settings of the sampling time and filter input value are
changed, use the hunting suppression setting (turn ON b8 of (S3)+1). Timeout time setting value after maximum ramp
detection ((S3)+25) is set so that auto tuning completion caused by a temporary ramp decrease can be avoided. Also, the
timeout time (R) after maximum ramp detection setting value varies depending on the response speed of the control target.
Control result when the maximum ramp value cannot be
obtained
Input value
(PV)
Target value
Output value
(MV)
(1) Hunting
(2) The output fluctuation is large.
• Sampling time
When the sampling time ((S3)+0) is short, it may be determined that the ramp does not increase because of the difference of
the variation between the ON part and OFF part of the output period. Set the sampling time to be not less than the time of
output period.
10 PID CONTROL FUNCTION
100
10.9 Example of parameter adjustment and the effect on PID control operation
(1)
Time
(1)
Time
(2)
Time
Control result when the sampling time is shortened
Input value
(PV)
Target value
(1)
(1) Overshoot is suppressed.
Control result when the maximum ramp value can be obtained
Input value
(PV)
Target value
Output value
(MV)
(2)
(1) Hunting is suppressed.
(2) The output fluctuation is small.
Time
(1)
Time
Time