176461 User Manual.book

Process Control Instruction

PD Data File

The PID instruction implemented by the MicroLogix 1500 is virtually identical in function to the PID implementation used by the Allen-Bradley SLC 5/03 and higher processors. Minor differences primarily involve enhancements to terminology. The major difference is that the PID instruction now has its own data file. In the SLC family of processors, the PID instruction operated as a block of registers within an integer file. The Micrologix 1500 PID instruction utilizes a PD data file.

You can create a PD data file by creating a new data file and classifying it as a PD file type. RSLogix automatically creates a new PD file or a PD sub-element whenever a

PID instruction is programmed on a rung.

Each PD data file has a maximum of 255 elements, and each PID instruction requires a unique PD element. Each PD element is composed of 20 sub-elements, which include bit, integer and long integer data. All of the examples in this chapter use PD file 10 sub-element 0.

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MicroLogix 1500 Programmable Controllers User Manual

PID Instruction

Normally, you place the PID instruction on a rung without conditional logic. If conditional logic is in front of the PID instruction, the output remains at its last value when the rung is false. The integral term is also cleared when the rung is false.

Note:

In order to stop and restart the PID instruction, you need to create a false-to-true rung transition.

The example below shows a PID instruction on a rung with RSLogix 500 programming software.

When programming, the setup screen provides access to the PID instruction configuration parameters. The illustration shows the RSLogix 500 setup screen.

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