Execution - Honeywell ST700 User Manual

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.
Page 54 FOUNDATION Fieldbus ST 700 Pressure Transmitter User's Guide Revision 2.0