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Every control system has to fulfil the requirements of its working cycle but there are another two members of the "essential trio" - HMI and Safety. They are always present, even when ignored by the author of the system.

Even the simplest control systems have to provide some form of Human Machine Interface. A few LEDs connected in parallel to output loads (or the sound of clicking relays) are a crude form of HMI. For anything a bit more complex - or anything that is to be used professionally - the HMI needs to be designed carefully. For many designs it may be biggest part of the project. It is also the part which sells, because it is visible.

It often helps if the HMI layout and function is designed in parallel with the control algorithm. The HMI has to be able to interface to the operation itself, but the design of HMI often reveals some aspects of operation which need to be controlled and which may not be otherwise obvious at early stages of the project.

A good HMI:

A good HMI should provide on-screen all information needed for control of operation, but low priority data may not be always visible. It is important to prioritize data for complex systems. Failure scenarios in particular have to be assessed, and the HMI should provide clear descriptions of problems and preferably offer possible solutions to the operator.


$Date: 2004/12/28 05:32:12 $