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User Interface

Smartpedia: A user interface enables interaction between man and machine or system, product or software.

User interface – the interaction between man and machine

Machines are part of our lives. They facilitate our work and our interaction. We communicate via smartphone and watch series on internet-enabled TV sets. The dishwasher performs valuable services and we get from A to B with the help of cars. This is made possible by interfaces through which a human being interacts with a concrete machine. This is called an user interface.

The way user interfaces work – sometimes also called man-machine interface (MMS) or human machine interface (HMI) – varies depending on the machine or system, the product or the software. This can be easily understood by looking at various examples of user interfaces:

  • We unlock our smartphone by fingerprint, facial recognition or numerical code.
  • We use a remote control to change TV stations on the television,
  • we activate functions in software programmes via individual menu items,
  • and we use a control panel to set the duration of a dishwasher cycle.

In cars, there are many user interfaces (often abbreviated to UI) with which a human being – the driver – interacts with the machine – the car:

  • The car is accelerated by pressing the accelerator pedal,
  • and brakes are applied by pressing the brake pedal.
  • The gear is changed by simultaneously pressing the clutch and operating the gearstick,
  • and the indicator of a change of direction is activated by the indicator lever.
  • The high beam is switched on and off by the lever and
  • and a suitable radio station is selected by turning a knob.
  • The temperature of the seat heating can be influenced by pressing a button several times and
  • and the room temperature is adjusted by means of a slider.

Obviously, a car is a system with many components that can be activated via different user interfaces. Usually, these components are described by many requirements and the implementation of these requirements has a great influence on the user experience, i.e. on the perceptions and reactions of a person when using the car or the components.

Types of user interfaces

There are different types of user interfaces. Here you will find a selection:

  • The Command Line Interface (CLI) is the classic computer user interface. It is part of a computer program and accepts input from users via a text line. It is considered the precursor of the GUI.
  • The Graphical User Interface (GUI) is the graphical user interface found in most common software programs. Primarily, the user interacts with the mouse and keyboard, with the actual interaction elements being menus, icons or toolbars that users recognise. Example: A floppy disk is the icon for storing information.
  • The Tangible User Interface (TUI) interacts with a physical object as an input or output medium. This is found, for example, in museums that want to encourage visitors to take active actions. Example: In the Senckenberg Museum in Frankfurt/Main, the earth’s plates or continents can be moved on a display by means of a large steering wheel. This shows how the earth changes over millions of years.
  • A Text User Interface (also TUI) is a purely text-based interface that, in contrast to the CLI, makes full use of the screen or a screen window. It is therefore also called a Terminal User Interface or Character-based User Interface (CHUI). Mainframe computers often have corresponding user interfaces.
  • A Voice User Interface (VUI) is a voice-controlled interface that allows both voice input and voice output. Examples are Siri from Apple, Alexa from Amazon or speech-to-text apps.

In addition to these types, there are also combined approaches such as the Natural User Interface (NUI) as a combination of GUI, VUI and facial recognition, or the Perceptual User Interface (PUI) as a combination of GUI, VUI and gesture recognition. With the help of the Brain Computer Interface (BCI), a programme is supposed to react directly to the thoughts of the user without a “diversion” via an input device such as a keyboard or mouse.³

Requirements for user interfaces

When it comes to the topic of what requirements user interfaces should fulfil, aspects such as “intuitive operation” or a “clear arrangement of controls” are often mentioned.¹ Of course, such requirements are not wrong per se, but they are also not very meaningful. What user A finds intuitive, user B may find confusing and illogical. Such different perceptions are influenced by personal preferences, knowledge and experience, for example. Example: On an Apple computer, external data media can be “ejected” by dragging and dropping them into the recycle bin. What is common and logical for Apple users might surprise some Windows users, because the recycle bin in the Microsoft world is merely a place where things end up before they are finally deleted.

Furthermore, the nature of a user interface determines the requirements for human-machine interaction. Example: A voice user interface must understand speech input in different pitches, speech rates, dialects or distances from the microphone before it can implement instructions accordingly. Whatever is considered “intuitive” in such an interface, it is certainly something different from a Graphical User Interface.

So when it comes to requirements for user interfaces, there are various approaches to finding out what these are in concrete terms:

  • Concrete requirements elicitation and analysis in the course of a requirements engineering process.
  • The study of standards and standards series such as DIN EN ISO 9241.²
  • The early use of tools such as click dummies, with which layouts and designs can be tested and users better understood, or wireframes, which help in the design of websites, web applications or software screens.
  • Collaboration with user interface designers or user experience experts.


User interface - the interaction between man and machine

Impulse to discuss:

Should manufacturers pay attention to the spirit of the times when designing user interfaces or would they rather stay true to their – possibly somewhat outdated – form of operation?


[1] Often also the

  • consistency of operation,
  • the use of standards,
  • universal usability,
  • informative feedback,
  • simple error handling and error prevention measures,
  • self-contained dialogues,
  • the reversal of actions,
  • an aesthetic and minimalist design,
  • a user help system, and
  • useful documentation

are mentioned as typical requirements for a UI.

[2] The DIN EN ISO 9241 specifies requirements for the ergonomic design of electronic visual displays. These include, among others, recommendations for the visual presentation of information, guidelines for the individualisation of software, interaction principles, contactless counter control for human-system interaction, guidelines for the design of user interfaces for the World Wide Web, accessibility, guidelines for the accessibility of devices and services in information and communication technology or the human-centred design of interactive systems.

[3] Earlier systems – so-called batch machines – had a Batch Interface that knew punch cards or paper tape on the input side, and line printers on the output side.

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