Emerson Oxymitter 5000 Instruction Manual page 152

Model 4081FG Oxygen Analyzer with F
Status Handling
Application Information
Closed Loop Control
C-10
fieldbus Communications
OUNDATION
If the input status on the PID block is Bad, the mode of the block
reverts to Manual. In addition, you can select the Target to Manual if
Bad IN status option to direct the target mode to revert to manual. You
can set the status option in Manual or Out of Service mode only.
NOTE
Target to Manual if Bad IN is the only status option supported by the
PID function block. Unsupported options are not grayed out; they
appear on the screen in the same manner as supported options.
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.
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.
• 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.