Simatic 505 SoftShop
9 - PID Loops
••••
157
The control, equation with error squared is based on signed error squared,
instead of the error alone.
To implement a high gain for large errors, and no gain for small errors,
incorporate an error deadband. When error deadband is selected, the
controller does not take any action on the output if the process variable is
within the yellow deviation limits.
Squared error calculation:
e
n
= (SP - PV
n
) x abs (SP - PV
n
)
Deadband error calculation:
e
n
= 0 if abs (SP - PV
n
) < YDEV
e
n
= (SP - PV
n
) - YDEV if (SP - PV
n
) > YDEV
e
n
= (SP - PV
n
) + YDEV if (SP - PV
n
) < - YDEV
No error calculation:
e
n
= (SP - PV
n
)
Reverse Acting
Select
YES
for reverse-acting loop in the REVERSE ACTING field.
Select
NO
for a direct-acting loop.
A reverse acting loop is defined to have a negative gain; i.e., a positive
change in error (SP - PROCESS VARIABLE ) results in a negative
change in the output from the controller. The value of the output signal
decreases as the value of the error increases.
A direct acting loop is defined to have a positive gain; i.e., a positive
change in error (SP - PROCESS VARIABLE ) results in a positive
change in the output from the controller. The value of the output signal
increases as the value of the error increases.
Monitor Deviation
To have the controller monitor the deviation Loop limits, select Yes in
the Monitor Deviation field; otherwise, select No.
Deviation Yellow Alarm
Enter values in engineering units for the setpoint deviation limits. The
deviation Alarm bands are always centered around the target or setpoint;
i.e., the deviation Alarm test is actually on the control error. This value
indicates the maximum allowable error (SP-PROCESS VARIABLE ) that
sets the yellow Alarm deviation Alarm. The yellow deviation limit must
be within the span of the process variable, and it must be less than or
equal to the orange deviation Alarm.
Deviation Orange Alarm
Enter values in engineering units for the setpoint deviation limits. The
deviation Alarm bands are always centered around the target or setpoint;
i.e., the deviation Alarm test is actually on the control error. This value
indicates the maximum allowable error (SP-PROCESS VARIABLE ) that
sets the orange Alarm deviation Alarm. The orange deviation limit must
be within the span of the process variable, and it must be greater than or
equal to the yellow deviation Loop.
