Rosemount OCX 8800, Instruction Manual

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Instruction Manual
IM-106-880, Rev 2.1
April 2017
OCX 8800
7-44
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:
•
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.