Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 38)

Publication 1757-UM006A-EN-P - May 2002
1-24 The Fieldbus Communication Model
Function Notes
(cont.)
• 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 and repeats per minutes for their tuning constants. Users can choose which
tuning constants they want to display.
• 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.
• Use the Direct Acting CONTROL_OPTS to define how a change in PV relative to the SP affects the output. When
Direct Acting is ON, the output increases when the PV exceeds the SP. When Direct Acting is OFF, the output
decreases when the PV exceeds the SP. Be sure this option is set correctly and never changed while in the
Automatic mode, since it makes the difference between positive and negative feedback. This option setting also
affects the calculation of the limit states for BKCAL_OUT.
• This block includes a Feed Forward algorithm. It accepts a value that is proportional to some disturbance in the
control loop as its FF_VAL input. The FF_SCALE values convert the FF_VAL to a percent of output span value. The
converted value is multiplied by the FF_GAIN and added to the target output of the block's algorithm. If the status
of FF_VAL is Bad, the last usable value will be used to prevent a bump in the output. When the status returns to
Good, the block adjusts its Integral (RESET) term to maintain the previous output.
• The TRK_VAL input brings in an external value or uses a constant. The TRK_SCALE values convert the TRK_VAL to
a percent of output span value. If the CONTROL_OPTS Track Enable selection is true and TRK_IN_D is true, the
converted TRK_VAL replaces the output (OUT), when the block is in Automatic (Auto), Cascade (Cas), Remote
Cascade (RCas), or Remote-Out (ROut) mode. The CONTROL_OPTS Track in Manual selection must be true for this
to occur in Manual mode. If the actual mode is OOS or IMan, the track request is ignored.
• If the TRK_VAL replaces the OUT, its status becomes Locked Out with Limits set to Constant. The actual mode goes
to LO. The status of BKCAL_OUT, RCAS_OUT and ROUT_OUT goes to Not Invited (NI), if not already there.
• If the status of TRK_IN_D is Bad, its last usable value will be maintained and acted upon. If the device restarts,
losing the last usable value, it will be set to false.
• If the status of TRK_VAL is Bad, the last usable value will be used. If there is no last usable value, the present value
of the OUT will be used.
• Use the Obey SP limits if Cas or RCas CONTROL_OPTS to use SP value after limiting in Cas or RCas mode.
• Use the Use PV for BKCAL_OUT CONTROL_OPTS to the PV value for the BKCAL_OUT value.
Parameters
(see Appendix A
for definitions of
each parameter) ACK_OPTION
ALARM_HYS
ALARM_SUM
ALERT_KEY
BAL_TIME
BKCAL_HYS
BKCAL_IN
BKCAL_OUT
BLOCK_ALM
BLOCK_ERR
BYPASS
CAS_IN
CONTROL_OPTS
DV_HI_ALM
DV_HI_LIM
DV_HI_PRI
DV_LO_ALM DV_LO_LIM
DV_LO_PRI
FF_GAIN
FF_SCALE
FF_VAL
GAIN
GRANT_DENY
HI_ALM
HI_HI_ALM
HI_HI_LIM
HI_HI_PRI
HI_LIM
HI_PRI
IN
LO_ALM
LO_LIM
LO_LO_ALM LO_LO_LIM
LO_LO_PRI
LO_PRI
MODE_BLK
OUT
OUT_HI_LIM
OUT_LO_LIM
OUT_SCALE
PV
PV_FTIME
PV_SCALE
RATE
RCAS_IN
RCAS_OUT
RESET
ROUT_IN
ROUT_OUT SHED_OPT
SP
SP_HI_LIM
SP_LO_LIM
SP_RATE_DN
SP_RATE_UP
ST_REV
STATUS_OPTS
STRATEGY
TAG_DESC
TRK_IN_D
TRK_SCALE
TRK_VAL
UPDATE_EVT
Table 1.L Proportional/Integral/Derivative Block Specifications