What is a Human Machine Interface?

A Human Machine Interface or HMI is the graphical display that represents a machine, process, or facility so that the user can easily, intuitively control and monitor the status of such a system.

A simple machine will require a simple interface for the user and not require a HMI. A few indicator lights, some push buttons and switches will be all that is needed to operate a simple machine, say one that presses parts into a casting of some sorts. To quickly give you an idea of what the lights and push buttons would do, is to inform the operator that a part was in place and ready for the next step along the line to a finished piece. The operator would know now the machine is ready, add another component to the part and push the appropriate button to begin the next action.

Way before there where the great color displays of today, a panel (metal , plastic or even wood) would be created and imprinted with a simplified version of the actual process. At key points on this panel, holes wold be drilled and indicators and pilot lights installed. This panel would be mounted on the wall and the panel for the switchgear such as push buttons and selector switches, mounted below. Typically placed at desk height to allow the operator to control the machine or process.

Keep in mind that the Human Machine Interface is just...well... an interface. What I mean is that the real control and intelligence is handled by a controller, usually a PLC, although integrated HMI/PLCs are available. The HMI sends a request for an action by a graphic push button pressed by the operator to the controller. In turn a action is performed though the intelligence programmed into the controller and the machine actual producing the action. The PLC through sensors placed on the machine can send this status to the HMI.

The HMI can be a self contained touch screen device made specifically to be used with a particular brand of PLC or controller. While others are more universal and can be used with many controllers since they have most communication protocol included or communication drivers available. Usually, you will see these mounted right on the enclosure door of a control cabinet. The HMI is also the user's front end to a SCADA system's controls and databases. Essentially SCADA is the top level of control of a network of PLCs controlling a large, complex process.

Other HMIs make use of a software package and adapter for a common PC. The adapter card is inserted into the PC to enable connection to a specific communication medium and protocol of the control system. Depending on the supplier and the power and complexity, the software package will contain a development system and run-time license. In this way a developer can have one system for creating the design of the HMI and multiple less costly run-time licenses for use on every machine.

Good design is necessary to obtain the most benefit from an Human Machine Interface. The Screens that are created, how they are laid out and how it all flows together can make a huge difference. Generally a Main Screen, Overview screen for each main area or system, a Current Alarms and Alarms History screen are a good start. The Overview screen will provide access to more detailed sub screens through a grouping of push buttons and/or logical drill down points on the Overview screen. Easy navigation to and from connected processes is necessary as well as to the main screens.

Choosing the correct graphics from the included library and creating your own is important for depicting your machine or process so that operator will learn quickly and not become confused.

The included graphic libraries have cool looking indicators, push-buttons, meters, numerical displays, tanks, piping, valves, motors, blowers, etc.

Creating a great HMI is part art, part engineering, part psychology. A thorough understanding of the process is necessary and it can be quite a bit fun, especially for the creative types among us.