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C-11
PID Function Block
The MODE parameter is a switch that indicates the target and actual
mode of operation. Mode selection has a large impact on the operation
of the PID block:
• Manual mode allows the operator to set the value of the loop
output signal directly.
• Automatic mode allows the operator to select a setpoint for
automatic correction of error using the GAIN, RESET, and RATE
tuning values.
• Cascade and Remote Cascade modes use a setpoint from
another block in a cascaded configuration.
• Remote Out mode is similar to Manual mode except that the
block output is supplied by an external program rather than by
the operator.
• Initialization Manual is a non-target mode used with cascade
configurations while transitioning from manual operation to
automatic operation.
• Local Override is a non-target mode that instructs the block to
revert to Local Override when the tracking or fail-safe control
options are activated.
• Out of Service mode disables the block for maintenance.
Abrupt changes in the quality of the input signal can result in
unexpected loop behavior. To prevent the output from changing
abruptly and upsetting the process, select the SP-PV Track in Man I/O
option. This option automatically sets the loop to Manual if a Bad input
status is detected. While in manual mode, the operator can manage
control manually until a Good input status is reestablished.
Application Example: Basic PID
Block for Steam Heater Control
Situation A PID block is used with an AI block and an AO block to control the
flow steam used to heat a process fluid in a heat exchanger. Figure C-3
illustrates the process instrumentation diagram.
Figure C-3. PID Function Block Steam
Heater Control Example.
Steam Supply
TCV
101
Steam Heater
Condensate
TC
101
TT
101
TT
100
FIELDBUS-FBUS_14A