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.