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PID Control Loops
If an override loop is implemented, it is configured for either a high select or a low select. The PID
algorithm is executed for both the primary loop and the override loop, and the calculated changes are
compared. The process is controlled by the loop that requests either the greater (high select) or lesser
(low select) adjustment. In other words:
•
If a high select is configured:
-
Process control switches from the primary loop to the override loop at the point where the
adjustment calculated by the override loop is greater than the adjustment calculated by the
primary loop.
-
Control returns to the primary loop when the adjustment calculated by the override loop is
less than the adjustment calculated by the primary loop.
•
If a low select is configured:
-
Process control switches from the primary loop to the override loop at the point where the
adjustment calculated by the override loop is less than the adjustment calculated by the
primary loop.
-
Control returns to the primary loop when the adjustment calculated by the override loop is
greater than the adjustment calculated by the primary loop.
Note: Because the loops calculate an adjustment rather than an absolute value, no reset wind-up
occurs.
PID control is implemented through a single analog output or two discrete outputs. Both the primary
loop and the override loop use the same outputs.
10.2.1
PID control loop example
This PID control loop is designed to allow pressure control to override flow control when the pressure
exceeds a setpoint value, e.g., 700 psig:
•
A flow variable (e.g., mass flow rate) is assigned to the primary loop (the flow loop), with a
setpoint appropriate to the process.
•
Pressure is assigned to the override loop (the pressure loop), with a setpoint of 700 psig.
•
The override loop is configured for a low limit.
The process is monitored and modified by the flow loop until the pressure input approaches 700 psig.
At the point where the adjustment calculated by the pressure loop is less than the adjustment
calculated by the flow loop, the pressure loop tries to close the valve and take control of the process.
Control returns to the flow loop when the adjustment calculated by the pressure loop is greater than
the adjustment calculated by the flow loop.
10.3
Configuring a PID control loop
To configure a PID control loop:
1. Ensure that
Section 3.4).
2. Click
Figure 10-1.
102
is set to
Active PIDs
Configure > Control > PID Loop
on the
1
Device Information
. The
PID Loop
Micro Motion
window,
Points
window is displayed, as shown in
®
Model 3711 Gas Flow Computers
panel (see
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