IDEC MICROSmart FC6A Series Ladder Programming Manual page 403

4. Proportional Band (S1+8)
The proportional action changes the output proportional to the deviation between the set point and process variable.
If the process variable is in the range of the proportional band, the control output (S3+6) turns on or in proportional to the
deviation. If the process variable is out of the proportional band, the control output (S3+6) is always on or off.
When the proportional band is made larger, the control output (S3+6) turns on and off from a state where the deviation is large,
so overshooting (process variable goes above the set point), undershooting (process variable falls below the set point), and
hunting (process variable is in an unstable undulating state) decrease, but it takes time until the process variable reaches the set
point and the offset between the set point and the process variable becomes larger.
When the proportional band is made smaller, the control output starts turning on and off at near the set point, so the time until
the process variable reaches the set point decreases and the offset also becomes smaller, but hunting increases. If the
proportional band is set extremely small, the control is the same as on/off operation.
When auto tuning is used, the appropriate proportional band can be automatically set for the target application. For details, see
"Auto Tuning (AT)" on page 19-24.
The proportional band can be set from ±0.01 to ±100.00% in 0.01% increments.
5. Integral Time (S1+9)
With only the proportional action, an offset occurs between the set point and the process variable even when the target
application is in a stable state. In order to bring this offset close to 0, the integral action is required. Integral time is a factor that
determines the manipulated variable by the integral action. If the integral time is too short, the integral action becomes too large,
which causes hunting of a long cycle. Conversely, if the integral time is too long, the processing until the set point is reached will
take time. The integral execution range is - proportional band to + proportional band. If the process variable goes out of the
proportional band due to a change in the set point or a disturbance, the integration calculation stops. As a result, the response of
the output manipulated variable against the change in set point improves and undershoot and overshoot can be reduced.
When auto tuning is used, the appropriate integral time can be automatically set for the target application. For details, see "Auto
Tuning (AT)" on page 19-24.
The integral time can be set from 0.1 to 6,553.5 seconds in 0.1 second increments.
6. Derivative Time (S1+10)
When the set point changes or when the difference between the set point and the process variable increases due to a
disturbance, the derivative action quickly brings the process variable close to the set point by increasing the manipulated
variable. When the derivative time is decreased, the derivative action becomes weaker and the response to rapid temperature
changes becomes slower. The action to control rapid temperature increases also becomes weaker, so the time to reach the set
point will become shorter, but overshoot will occur more easily. When the derivative time is increased, the derivative action
becomes large and the response to rapid temperature changes becomes faster. The action to control rapid temperature
increases also becomes more powerful, so the time to reach the set point will become longer, but overshoot will occur less easily.
When auto tuning is used, the appropriate derivative time can be automatically set for the target application. For details, see
"Auto Tuning (AT)" on page 19-24.
The derivative time can be set from 0.1 to 6,553.5 seconds in 0.1 second increments.
7. Derivative Gain (S1+11)
When the derivative gain is set to a small value, the output manipulated variable is more easily affected by noise and changes in
the set point. When the derivative gain is set to a large value, the output manipulated variable is less easily affected by noise
and changes in the set point, but stability at normal times decreases. When noise or changes in the process variable occur,
normally set this to 20 to 30%.
The derivative gain can be set from 0 to 100% in 1% increments.
8. ARW (Anti-Reset Windup) (S1+12)
Set the point to start the integral action. If the integral term is enabled from the start of execution of the PIDA instruction,
overshoot may be caused. Overshoot can be controlled by delaying the integral action with ARW linked to the proportional band.
ARW is normally 100%, which results in appropriate control with little overshoot. If ARW is set too small, overshoot can be
eliminated, but an offset may occur.
The ARW can be set from 0 to 100%.
For example, when ARW=50% and proportional band=20.00%, the integral action starts from when the deviation becomes
smaller than 10%.
9. AT (Auto Tuning) Bias (S1+13)
Set the bias value for the auto tuning.
The auto tuning start point is determined by the AT bias value in 1 increments.
When process variable ≤ (set point - AT bias value)
When process variable ≥ (set point + AT bias value)
When (set point - AT bias value) < process variable < (set point + AT bias value)
For details, see "Auto Tuning (AT)" on page 19-24.
The AT bias can be set in the range of process variable minimum value < (set point - AT bias setting value) or process
variable maximum value > (set point + AT bias setting value) in units of 1.
10. Advanced/Basic
Click this button to show or hide the advanced settings.
On the Control tab, Derivative Gain (7) can be set as an advanced setting.
Process Variable Value
FC6A S MICROS L
ERIES MART
P M
ADDER ROGRAMMING ANUAL
19: PID C
ONTROL
AT Start Point
Set point - AT bias value
Set point + AT bias value
Set point
FC9Y-B1726
I
NSTRUCTION
19-13