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Chapter 10
PID Control Loops
10.1
About this chapter
This chapter discusses the following topics:
•
PID control loops overview
•
Configuring PID control loops
•
Tuning PID control loops
•
Monitoring PID control loops
10.2
PID control loops overview
Proportional, Integral, and Derivative (PID) control loops are used to provide smooth and stable
operation of the feedback loops that control a regulating device such as a control valve or a motor.
PID control loops are frequently used to keep a process variable at or near a setpoint.
The Model 3711 supports either one analog PID control point or two discrete PID control points. If
two DOs are used, one provides open/forward control and the other provides close/reverse control.
Note: Discrete control is available only if the 6-point I/O expansion board is installed and the AO/DO
option is set to DO, so that there are two DOs in the system (see Section 7.3). In addition, sampler
control must be disabled (see Section 6.4.2). For both DOs, DOUT Type must be set to Time Duration
(see Section 7.7).
Note: If analog control will be used, ensure that AO Assignment is disabled (see Section 6.5).
The PID control points manage two separate loops: the primary loop and the override loop. You may
use the primary loop only, or both loops.
For each loop to be used, a process variable is assigned and a setpoint value is configured. For each
execution of the PID algorithm, the Model 3711 reads the current value of the assigned process
variable, and:
•
If Output Type is set to Analog, calculates the change in the analog output required to maintain
the configured setpoint. The calculated change is added to the current value of the analog
output.
•
If Output Type is set to Discrete, calculates the discrete output activation period required to
maintain the configured setpoint, then:
-
If the correction is positive, routes the action to the open/forward DO.
-
If the correction is negative, routes the action to the close/reverse DO.
Configuration and Use Manual
101
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