Automationdirect.com DL05, User Manual

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PID Loop Operation
Maintenance
8–19
PID Loop Operation
Basic Loop Operation
Each PID loop is dependent on the instructions and data values in its respective loop
table. The following diagram shows an example of the loop table locations
corresponding to the main three loop variables: SP, PV, and Control Output. The
example below begins at V2000 (you can use any memory location compatible with
Loop Table requirements). The SP, PV and Control Output are located at the
addresses shown.
Process Variable V+03
Loop
Calculation
S
Error
Term
+
–
Control Output V+05 Setpoint V+02
Loop Table
V2002 Setpoint XXXX
V2003 Process Variable XXXX
V2005 Control Output XXXX
The data for the SP, PV, and Control Output must interface with real-world devices.
In the figure below, the sources or destinations are shown for each loop variable. The
Control Output and Process Variable values move through the analog input/output
combination module to interface with the process itself.
A few rungs of ladder logic are required to copy data from the analog module to the
loop table, or vice versa. Refer to the analog module chapter of this manual for an
example of the required ladder logic.
Process Variable V+03
Loop
Calculation
S
+
–
Control Output V+05 Setpoint V+02
Analog
Output
Analog
Input
Setpoint Sources:
Operator Input
Ramp/soak generator
Ladder Program
Another loop’s output (cascade)
Process
The Setpoint has several possible sources, as listed above. Many applications will
use two or more of the sources at different times, depending on the loop mode. In
addition, the loop control strategy and programming method also determine how the
setpoint is generated.
When using the built-in Ramp/Soak generator or when cascading a loop, the PID
controller automatically writes the setpoint data in location V+02 for you.
If you want
to use a setpoint from any other source, the ladder program must write that
setpoint
to the loop table location V+02 .
Each of the three main loop parameters can have only one source or destination at
any given time. During the application development, it is a good idea to draw loop
schematic diagrams showing data sources, etc., to help avoid mistakes.
Data Locations
Data Sources