Emerson Rosemount OCX 8800, Instruction Manual

Page: 166 / 306

Section 7: Foundation
Fieldbus
April 2017
Instruction Manual
IM-106-880C, OI
14
0
Status
Handling
7.12.8 Status Handling
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.
7.12.9 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.