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Description
Function Notes
R400
Experion PKS Series A Fieldbus Interface Module User's Guide
July 2010
The Proportional/Integral/Derivative function block provides
classic three-mode control function for closed-loop control
applications. When the Process Variable deviates from the
Set point, the PID function acts upon the error to move the
output in a direction to correct the deviation. PID blocks
support cascade applications to compensate for the
difference in process time constants of a primary and
secondary process measurement. A functional schematic of
the block is shown in the previous illustration for reference.
Supports Out of Service (OOS), Initialization Manual
(IMan), Local Override (LO), Manual (Man), Automatic
(Auto), Cascade (Cas), Remote Cascade (RCas) and
Remote-Out (ROut) modes.
The input (IN) passes through a filter with a time
constant (PV_FTIME). The filtered value becomes the
Process Variable (PV) to be used with the Set point
(SP) in the block's algorithm. A PID algorithm will not
integrate, if the limit status of the input (IN) is constant.
The full cascade SP sub-function is used, with rate
and absolute limits. Additional control options are
available to have the SP value track the PV value,
when the block's actual mode is IMan, LO, Man, or
ROut. Limits do not cause SP-PV tracking.
A BYPASS switch function is available for operators to
use, when secondary cascade controllers have a bad
PV and the Bypass Enable (LSB) CONTROL_OPTS is
ON. The Bypass Enable option is required, since some
control schemes may become unstable when BYPASS
is ON. An operator can only set the BYPASS switch,
when the block is in the Man or OOS mode. While
BYPASS is ON, the SP value, in percent of range, is
passed directly to the target output, and the value of
OUT is used for BKCAL_OUT. When block mode
switches to Cascade, the upstream block is requested
to initialize to the value of OUT. Upon transition to
bypass OFF, the upstream block is requested to
initialize to the PV value, regardless of the Use PV for
BKCAL_OUT CONTROL_OPTS status.
The tuning constant used for the Proportional term is
GAIN, RESET is used for the Integral term, and RATE
is used for the Derivative term. Both RESET and RATE
are time constants expressed in seconds. Some
controllers use the inverse values of Proportional Band
Honeywell
10. Appendix A
10.2. Standard Function Blocks
409
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