Wednesday, May 23, 2018

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Human Machine Interfaces

Many modern PLC systems are equipped with computer-based interface panels which human operators use to observe data gathered by the PLC and also enter data to the PLC. These panels are generally referred to as Human1-Machine Interfaces, or HMI panels.

HMIs may take the form of general-purpose (“personal”) computers running special graphic software to interface with a PLC, or as special-purpose computers designed to be mounted in sheet metal panel fronts to perform no task but the operator-PLC interface. The first photograph shows an example of the former, and the second photograph an example of the latter:

HMI screen showing Generation Oxygen Vacuum Swing Adsorption
Allen-Bradley RSView

Modern HMI panels and software are almost exclusively tag-based, with each graphic object on the screen associated with at least one data tag name, which in turn is associated to data points (bits, or words) in the PLC by way of a tag name database file resident in the HMI. Graphic objects on the HMI screen either accept (read) data from the PLC to present useful information to the operator, send (write) data to the PLC from operator input, or both. The task of programming an HMI unit consists of building a tag name database and then drawing screens to illustrate the process to as good a level of detail as operators will need to run it.

Like programmable logic controllers themselves, the capabilities of HMIs have been steadily increasing while their price decreases. Modern HMIs support graphic trending, data archival advanced alarming, and even web server ability allowing other computers to easily access certain data over wide-area networks. The ability of HMIs to log data over long periods of time relieves the PLC of having to do this task, which is very memory-intensive. This way, the PLC merely “serves” current data to the HMI, and the HMI is able to keep a record of current and past data using its vastly larger memory reserves2.

1An older term for an operator interface panel was the “Man-Machine Interface” or “MMI.” However, this fell out of favor due to its sexist tone.

2If the HMI is based on a personal computer platform, it may even be equipped with a hard disk drive for vast amounts of data storage.


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