Instruction Manual

Instruction Manual
D103434X012
Blocks
November 2014
85
PID Function Block
The PID function block (figure B‐4) combines all of the necessary logic to perform proportional/integral/derivative
(PID) control. The block supports mode control, signal scaling and limiting, feedforward control, override tracking,
alarm limit detection, and signal status propagation.
Figure B‐4. Proportional+Integral+Derivative (PID) Function Block
B2720
BKCADLOUT
OUTDD
CAS
IN
PID
BKCAL IN
FF VAL
IN
TRK
IN  D
TRK
VAL
BKCAL_IN
= THE ANALOG INPUT VALUE AND STATUS FROM ANOTHER BLOCK'S BKCAL _OUT OUTPUT THAT
IS USED FOR BACKWARD OUTPUT TRACKING FOR BUMPLESS TRANSFER AND TO PASS LIMIT STATUS.
CAS_IN
= THE REMOTE SETPOINT VALUE FROM ANOTHER FUNCTION BLOCK.
FF_
VAL = THE FEEDFORWARD CONTROL INPUT VALUE AND STATUS.
IN
= THE CONNECTION FOR THE PROCESS VARIABLEFROM ANOTHER FUNCTION BLOCK.
TRK_
IN_D = INITIATES THE EXTERNAL TRACKING FUNCTION.
TRK_
VAL = THE VALUE AFTER SCALING APPLIED TO OUT IN LOCAL OVERRIDE MODE.
BKCAL_
OUT = THE VALUE AND STATUS REQUIRED BY THE BKCAL_IN INPUT OF ANOTHER FUNCTION BLOCK
TO PREVENT RESET WINDUP AND TO PROVIDE BUMPLESS TRANSFER TO CLOSED LOOP CONTROL.
OUT
= THE BLOCK OUTPUT AND STATUS.
The block supports two forms of the PID equation: Standard and Series. You can choose the appropriate equation
using the MATHFORM [70] parameter. The Standard ISA PID equation is the default selection.
S
tandard Out + GAIN ex
ǒ
1 )
1
t
r
s ) 1
)
t
d
s
T t
d
s ) 1
Ǔ
) F
Series Out + GAIN ex
ǒ
1 )
1
t
r
s
Ǔ
)
ǒ
t
d
s ) 1
T t
d
s ) 1
Ǔ
)F
where
GAIN: proportional gain value
τ
r
: integral action time constant (RESET parameter) in seconds
s: laplace operator
τ
d
: derivative action time constant (RATE parameter)
α: fixed smoothing factor of 0.1 applied to RATE
F: feedforward control contribution from the feedforward input (FF_VAL)
E: error between setpoint and process variable
To further customize the block for use in your application, you can configure filtering, feedforward inputs, tracking
inputs, setpoint and output limiting, PID equation structures, and block output action. Table B‐12 lists the PID block