Application Information - Emerson Rosemount OCX 8800 Reference Manual

Foundation Fieldbus
Note
The instrument must be in Manual or Out of Service mode to set the status option.
Note
The AI block only supports the BAD if Limited option. Unsupported options are not grayed out; they
appear on the screen in the same manner as supported options.
7.9.9

Application information

The PID function block is a powerful, flexible control algorithm that is designed to work in a
variety of control strategies.
The PID block is configured differently for different applications. The following examples
describe the use of the PID block for closed-loop control (basic PID loop), feedforward
control, cascade control with master and slave, and complex cascade control with
override.
Closed loop control
To implement basic closed loop control, compute the error difference between the
process variable (PV) and setpoint (SP) values and calculate a control output signal using a
PID (proportional integral derivative) function block.
The proportional control function responds immediately and directly to a change in the PV
or SP. The proportional term GAIN applies a change in the loop output based on the current
magnitude of the error multiplied by a gain value.
The integral control function reduces the process error by moving the output in the
appropriate direction. The integral term RESET applies a correction based on the
magnitude and duration of the error. Set the RESET parameter to zero for integral only
control. To reduce reset action, configure the RESET parameter to be a large value.
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:
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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.
OCX 8800