This figure shows the pin diagram of the FB_PID function block:
The FB_PID function block is a standard PID function block with manual tuning, hold function, bumpless transfer and damping time for derivative action.
This function block provides following features:
oDifferent modes such as P, PI, PD, and PID.
oManual mode operation to control the PID output in manual mode.
oAnti reset wind-up to avoid the saturation or wind-up in integral action: If the control variable reaches actuator limit, process error will continue to integrate very large integral term is called as windup.
oDamping time (Td) to filter the overshoot due to derivative action.
oBump less transfer is activated when mode changes from manual to auto.
Bump less transfer avoids sudden change in PID output when mode changes.
oDetected error status is generated by function block to display detected errors.
oInner and outer window functions are used in integral calculations.
If absolute value of process error is less than inner window then integral part is scaled with a factor [ABS (err)/Inner Window].
This minimizes the overshoot in the PID output.
If absolute value of process error is greater than inner window and less than outer window then normal integral calculations are done.
If absolute value of process error is greater than outer window then anti reset windup is active and integral output will hold the last value.
The following equation shows the PID output:
Where:
y (t) = PID Output
Kp = Proportional gain
Tn = Integral time
Tv = Derivative time
Td = Filter time for derivative
e (t) = Process error between set point and feedback value.
This figure shows the block diagram for the FB_PID function block:
This figure shows normal behavior diagram of the FB_PID function block:
This figure shows the FB_PID function block diagram with detected error:
