A computer-aided user interface, actually a usage interface – also known as a human machine interface (HMI), is the part of computer program that communicates with the user. In ISO 9241-110, the term user interface is defined as "all parts of an interactive system (software or hardware) that provide information and control that is necessary for the user to complete a certain task with the interactive system." The user interface / human machine interface (HMI) is the point of action where a human is in contact with a machine. The simplest case is a light switch: It is neither a human, nor a "machine" (lamp), but an interface between the two. In order for a human machine interface (HMI) to be usable and meaningful for people, it must be adapted to its requirements and abilities. For example, programming a robot to turn on the light would be too complicated and a switch on the roof would not be practical for a light in the cellar.
Classification of human machine interface (HMI):
Systematically considered, the user interface is one of the human machine interfaces (HMI): Human ↔ human-machine interface ↔ machine. Different sciences dedicate themselves to the topic, such as IT, cognition research and psychology. The basic knowledge for a user-friendly interface design is gleaned in the scientific discipline of ergonomics. The actual areas of activity are cognitive ergonomics, system ergonomics and software ergonomics (usability engineering).
Operating and observing
The user interface is, in addition to a "human machine interface" (HMI), also called a "man machine interface" (MMI) and allows the operator to, under certain circumstances, go beyond operating the machine and to observe the status of equipment and intervene in the process. The information ("feedback") is provided either by means of control panels with signal lamps, display fields or buttons or by means of software using a visualization system that runs on a terminal, for example. With a light switch, the visual feedback comes from the impression of "light" and the switch setting of "on, and "dark" and "off". The driver's compartment of a car also features numerous user interfaces – from the controls (pedals, steering wheel, switch and indicator stalks etc.) through to visual acknowledgments from the "machine", the car (display of speed, range, radio channel, navigations system etc.).
User-friendliness of the human machine interface
The success of a technical product depends on more than factors such as price, reliability and lifecycle; it also depends on factors such as manageability and user-friendliness (usability). Ideally, a human machine interface (HMI) explains itself intuitively, without any need for training. The light switch is, despite its popularity and simplicity, not an ideal user interface, but a compromise between two contradictory objectives. This is that the switch must be located close on the device to be turned on, i.e. on a lamp itself (so that you do not need to search for it). Otherwise it should be close to the door (which is where it generally is), so that you don't have to feel around for it in the dark. Another popular, but also not ideal interface, is a touch-sensitive screen: Here, you can call up a program for email, for example, by touching the screen, which then gets the email. However when you press the icon, the finger covers the icon itself. This does not generally create problems, but it is not possible to draw or write precisely on the screen using your fingers.
Evolution of human machine interfaces (HMI)
For products with a long lifecycle, the human machine interfaces (HMI) have been optimized over the years. There are two switches that are missing for audio and video playback devices that were common in the eighties: The function of the switch to jump to the previous take or the next take was integrated into the switches for fast forwarding and rewinding. To do this, the user interface became more complex, because each of the two knobs has two functions. For the developers of user interfaces, reduction such as this plays a central role: The reduction of access to a complex machine to just a few controls may make basic operation easier, but is usually not appropriate for the complexity. With the very complex operating systems of modern computers, this competing objective is solved by means of two categories of user interfaces / human machine interfaces (HMI): One shows the user the everyday icons, the trashcan, the folder, etc., which they understand and can can operate immediately without having to spend time learning them: For example, clicking on a link opens the target website. The other allows them to use the command line interface to access the computer system at a low-level, however this requires a great deal of learning. For example taskkill /F /IM iexplore.exe ends all processes in relation to Internet Explorer on a Windows system.