The user interface (UI), in the industrial design field of human–machine interaction, is the space where interactions between humans and machines occur. The goal of this interaction is to allow effective operation and control of the machine from the human end, whilst the machine simultaneously feeds back information that aids the operators’ decision-making process. Examples of this broad concept of user interfaces include the interactive aspects of computer operating systems, hand tools, heavy machinery operator controls, and process controls. The design considerations applicable when creating user interfaces are related to or involve such disciplines as ergonomics and psychology.

Generally, the goal of user interface design is to produce a user interface which makes it easy (self-explanatory), efficient, and enjoyable (user-friendly) to operate a machine in the way which produces the desired result. This generally means that the operator needs to provide minimal input to achieve the desired output, and also that the machine minimizes undesired outputs to the human.

With the increased use of personal computers and the relative decline in societal awareness of heavy machinery, the term user interface is generally assumed to mean the graphical user interface, while industrial control panel and machinery control design discussions more commonly refer to human-machine interfaces.

Other terms for user interface are man–machine interface (MMI) and when the machine in question is a computer human–computer interface.

Overview

The user interface or human–machine interface is the part of the machine that handles the human–machine interaction. Membrane switches, rubber keypads and touchscreens are examples of the physical part of the Human Machine Interface which we can see and touch.

In complex systems, the human–machine interface is typically computerized. The term human–computer interface refers to this kind of system. In the context of computing the term typically extends as well to the software dedicated to control the physical elements used for human-computer interaction.

The engineering of the human–machine interfaces is enhanced by considering ergonomics (human factors). The corresponding disciplines are human factors engineering (HFE) and usability engineering (UE), which is part of systems engineering.

Tools used for incorporating human factors in the interface design are developed based on knowledge of computer science, such as computer graphics, operating systems, programming languages. Nowadays, we use the expression graphical user interface for human–machine interface on computers, as nearly all of them are now using graphics.

Interface design

Primary methods used in the interface design include prototyping and simulation.

Typical human–machine interface design consists of the following stages: interaction specification, interface software specification and prototyping:

• Common practices for interaction specification include user-centered design, persona, activity-oriented design, scenario-based design, resiliency design.
• Common practices for interface software specification include use cases, constrain enforcement by interaction protocols (intended to avoid use errors).
• Common practices for prototyping are based on interactive design based on libraries of interface elements (controls, decoration, etc.).

Quality

All great interfaces share eight qualities or characteristics:

1. Clarity The interface avoids ambiguity by making everything clear through language, flow, hierarchy and metaphors for visual elements.
2. Concision^[11] It’s easy to make the interface clear by over-clarifying and labeling everything, but this leads to interface bloat, where there is just too much stuff on the screen at the same time. If too many things are on the screen, finding what you’re looking for is difficult, and so the interface becomes tedious to use. The real challenge in making a great interface is to make it concise and clear at the same time.
3. Familiarity^[12] Even if someone uses an interface for the first time, certain elements can still be familiar. Real-life metaphors can be used to communicate meaning.
4. Responsiveness^[13] A good interface should not feel sluggish. This means that the interface should provide good feedback to the user about what’s happening and whether the user’s input is being successfully processed.
5. Consistency^[14] Keeping your interface consistent across your application is important because it allows users to recognize usage patterns.
6. Aesthetics While you don’t need to make an interface attractive for it to do its job, making something look good will make the time your users spend using your application more enjoyable; and happier users can only be a good thing.
7. Efficiency Time is money, and a great interface should make the user more productive through shortcuts and good design.
8. Forgiveness A good interface should not punish users for their mistakes but should instead provide the means to remedy them.

Principle of least astonishment

The principle of least astonishment (POLA) is a general principle in the design of all kinds of interfaces. It is based on the idea that human beings can only pay full attention to one thing at one time,^[15] leading to the conclusion that novelty should be minimized.

Types

• Direct manipulation interface is the name of a general class of user interfaces that allow users to manipulate objects presented to them, using actions that correspond at least loosely to the physical world.
• Graphical user interfaces (GUI) accept input via devices such as a computer keyboard and mouse and provide articulated graphical output on the computer monitor. There are at least two different principles widely used in GUI design: Object-oriented user interfaces (OOUIs) and application oriented interfaces.^[16]
• Web-based user interfaces or web user interfaces (WUI) that accept input and provide output by generating web pages which are transmitted via the Internet and viewed by the user using a web browser program. Newer implementations utilize PHP, Java, JavaScript, AJAX, Apache Flex, .NET Framework, or similar technologies to provide real-time control in a separate program, eliminating the need to refresh a traditional HTML based web browser. Administrative web interfaces for web-servers, servers and networked computers are often called control panels.
• Touchscreens are displays that accept input by touch of fingers or a stylus. Used in a growing amount of mobile devices and many types of point of sale, industrial processes and machines, self-service machines etc.
• Command line interfaces, where the user provides the input by typing a command string with the computer keyboard and the system provides output by printing text on the computer monitor. Used by programmers and system administrators, in engineering and scientific environments, and by technically advanced personal computer users.
• Touch user interface are graphical user interfaces using a touchpad or touchscreen display as a combined input and output device. They supplement or replace other forms of output with haptic feedback methods. Used in computerized simulators etc.
• Hardware interfaces are the physical, spatial interfaces found on products in the real world from toasters, to car dashboards, to airplane cockpits. They are generally a mixture of knobs, buttons, sliders, switches, and touchscreens.
• Attentive user interfaces manage the user attention deciding when to interrupt the user, the kind of warnings, and the level of detail of the messages presented to the user.
• Batch interfaces are non-interactive user interfaces, where the user specifies all the details of the batch job in advance to batch processing, and receives the output when all the processing is done. The computer does not prompt for further input after the processing has started.
• Conversational interfaces enable users to command the computer with plain text English (e.g., via text messages, or chatbots) or voice commands, instead of graphic elements. These interfaces often emulate human-to-human conversations.^[17]
• Conversational interface agents attempt to personify the computer interface in the form of an animated person, robot, or other character (such as Microsoft’s Clippy the paperclip), and present interactions in a conversational form.
• Crossing-based interfaces are graphical user interfaces in which the primary task consists in crossing boundaries instead of pointing.
• Gesture interfaces are graphical user interfaces which accept input in a form of hand gestures, or mouse gestures sketched with a computer mouse or a stylus.
• Holographic user interfaces provide input to electronic or electro-mechanical devices by passing a finger through reproduced holographic images of what would otherwise be tactile controls of those devices, floating freely in the air, detected by a wave source and without tactile interaction.
• Intelligent user interfaces are human-machine interfaces that aim to improve the efficiency, effectiveness, and naturalness of human-machine interaction by representing, reasoning, and acting on models of the user, domain, task, discourse, and media (e.g., graphics, natural language, gesture).
• Motion tracking interfaces monitor the user’s body motions and translate them into commands, currently being developed by Apple.^[18]
• Multi-screen interfaces, employ multiple displays to provide a more flexible interaction. This is often employed in computer game interaction in both the commercial arcades and more recently the handheld markets.
• Non-command user interfaces, which observe the user to infer his / her needs and intentions, without requiring that he / she formulate explicit commands.
• Object-oriented user interfaces (OOUI) are based on object-oriented programming metaphors, allowing users to manipulate simulated objects and their properties.
• Reflexive user interfaces where the users control and redefine the entire system via the user interface alone, for instance to change its command verbs. Typically this is only possible with very rich graphic user interfaces.
• Search interface is how the search box of a site is displayed, as well as the visual representation of the search results.
• Tangible user interfaces, which place a greater emphasis on touch and physical environment or its element.
• Task-focused interfaces are user interfaces which address the information overload problem of the desktop metaphor by making tasks, not files, the primary unit of interaction.
• Text-based user interfaces are user interfaces which output a text. TUIs can either contain a command-line interface or a text-based WIMP environment.
• Voice user interfaces, which accept input and provide output by generating voice prompts. The user input is made by pressing keys or buttons, or responding verbally to the interface.
• Natural-language interfaces – Used for search engines and on webpages. User types in a question and waits for a response.
• Zero-input interfaces get inputs from a set of sensors instead of querying the user with input dialogs.
• Zooming user interfaces are graphical user interfaces in which information objects are represented at different levels of scale and detail, and where the user can change the scale of the viewed area in order to show more detail.