Revision 4 Foundation Fieldbus RMA803 Remote Indicator User's Guide Page 55
Execution
The Process Value to be controlled is connected to the IN input. The value is passed through a filter,
and its time constant is PV_FTIME. The value is then shown as the PV, which is used in
conjunction with the SP in the PID algorithm. A PID does not integrate if the limit status of IN input
is constant, or if further control action based on the PID error proceeds IN input further towards its
active status limit. A full PV and DV alarm sub-function is provided. The PV has a status, although
it is a contained parameter. This status is a copy of IN’s status, unless IN is Good and there is a PV
or block alarm.
The full cascade SP sub-function is used with rate and absolute limits. The block has additional
control options which cause the SP value to track the PV value. The SP value tracks the PV value
while the block is in Actual mode of IMan, LO, or ROut, or when the target mode of the block is
MAN.
The block provides a switch for BYPASS, which is available to the operator if the Bypass Enable
control option is set as True. BYPASS can be used in secondary cascade controllers that have a Bad
PV. The BYPASS Enable option is required, so if BYPASS is set as True, not all cascade control
schemes are stable. BYPASS can only be changed when the block mode is in MAN or OOS mode.
When BYPASS is set, the value of SP, in percent of range, is passed directly to the target output,
and the value of OUT is used for BKCAL_OUT. When the mode is changed to Cas, the upstream
block is requested to initialize to the value of OUT. When a block is in Cas mode, on the transition
out of BYPASS, the upstream block is requested to initialize to the PV value, irrespective of the
“Use PV for BKCAL_OUT” option.
GAIN, RESET, and RATE are the tuning constants for the P, I, and D terms, respectively. The
block provides existing controllers that are tuned by the inverse value of some or all of them, such as
proportional band and repeats per minute. The human interface to these parameters must be able to
display the user's preference.
BAL_TIME parameter can be used to set the rate at which the I term moves towards balancing the
difference between the previous integral term and the limited output. The Direct Acting control
option, if set as True, causes the output to increase when the PV exceeds the SP. If set as False, the
output decreases when the PV exceeds the SP. The Direct Acting control option must be set
carefully, as it can cause a difference between positive and negative feedback.
ATTENTION
The Direct Acting control option can never be changed while in AUTO
mode. The setting of the option must also be used in calculating the limit
state for BKCAL_OUT.
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