A human machine interface (HMI) is an interface which permits interaction between a human being and a machine. Human machine interfaces vary widely, from control panels for nuclear power plants to the screen and input buttons on a cell phone. Designing such interfaces is a challenge, and requires a great deal of work to make the interface functional, accessible, pleasant to use, and logical. Some engineers specialize in developing human machine interfaces and changing the ways in which people interact with machines and systems.

Two components are needed in a human machine interface. The first is an input. A human user needs some way to tell the machine what to do, to make requests of the machine, or to adjust the machine. Examples of input devices include keyboards, toggles, switches, touch screens, joysticks, and mice. All of these devices can be utilized to send commands to a system or even an interlinked set of systems.

The interface also requires an output, which allows the machine to keep the human user updated on the progress of commands, or to execute commands in physical space. On a computer, for example, users have a screen which can display information. A robot, on the other hand, may move in response to commands and store data on a hard drive so that people can see how the robot responds, learns, and navigates the world. Outputs can also include things as simple as status lights which alert people when toggles or switches have been activated

Also in addition the HMIs are used in Explosive Areas (e.g. Zone 2)and are designed & selected as per the process requirements !