Emerson Rosemount OCX8800A Manual page 184

F Fieldbus
™
OUNDATION
December 2022
The derivative term RATE applies a correction based on the anticipated change in
error. Derivative control is typically used in temperature control where large
measurement lags exist.
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 blocks 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, you
can manage control manually until a Good input status is reestablished.
8.9.10 Application examples
Basic proportional/integral/derivative (PID) block for
steam heater control
Situation
A PID block is used with an analog input (AI) block and an analog output (AO)
block to control the flow steam used to heat a process fluid in a heat exchanger.
Figure 8-17
184
illustrates the process instrumentation diagram.
Manual
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