Human machine interface

Introduction

Accidents involving machines or vehicles are often attributed to human error. However, when these accidents are investigated in detail, it can often be concluded that they were in fact caused by poor ergonomic design of the interface or by problems of interaction between the machine and the user rather than by negligent or careless use. It is therefore essential that the design of interaction and of the interfaces be based on principles governing the dialogue and human physical and mental characteristics. The following article will illustrate these basic human characteristics and show how they may best be considered by the design of interfaces based on the European legislation and standardisation.

What does human-machine interaction mean?

In ergonomics, human factors and the related disciplines of human-machine interaction or human-system interaction are described in terms of joint performance and of the communication and feedback processes between humans and technical systems^{[1][2] [3]}. Since the terms "humans" and "systems" are somewhat generic, "interaction" refers in the context of design and evaluation to human-system interfaces such as the task and the interaction interface. The concept of the work system^[4] [5] provides a suitable framework for the design and evaluation of human-system interaction. Interaction is essential for system performance, since work systems comprise "one or more workers and work equipment acting together to perform the system function, in the workspace, in the work environment, under the conditions imposed by the work tasks"^[4]. 

When human-system interaction focuses on more specific issues, it may sometimes seem reasonable or appropriate to address interaction more specifically, i.e. with regard to processes between the human and the machine, the environment, the software, the job or the organisation^[6]. Interaction design may therefore concentrate on interaction interfaces such as machinery displays and controls. However, appropriate solutions which respect human factors and ergonomic design principles are only possible if they take into account the tasks to be performed. Interaction design and evaluation must always focus on the system as a whole, in order to avoid improvements being made to individual system components to the detriment of compatibility with other components. The goal of human-system interaction is to create a seamless and natural interaction between humans and machines (systems), improving the efficiency and effectiveness of the interaction^[7]. As technology continues to evolve and especially with the role of Artificial Intelligence (AI), human-machine interaction is becoming increasingly important. AI and Machine Learning (MI) integrated into smart devices and applications can anticipate user needs, adapt to preferences, and continuously improve the user experience, thus reshaping the design of human-system interaction. In this context both AI and MI are key technologies of human–machine interaction enabling machines to learn from data and adapt to user behaviour, making interactions more personalised and efficient. Also other technologies such as natural language processing and computer vision allowing machines to resp. understand and respond to human language and perceive and interpret visual information, make interactions more natural and intuitive7. 

Legislation such as the Machinery Directive 2006/42/EC^[8] (repealed by Regulation 2023/1230/EU on machinery^[9] applying from 20 January 2027) requires human factors and ergonomic design strategies (e.g. task orientation) and principles (e.g. compatibility^)[10] to be considered during the design of work systems. This also applies to the design of effective, efficient and safe human-system interaction. Workload assessment is considered a crucial criterion, especially from an occupational safety and health perspective^{[11] [12] [4] [13]}

For reasons of both safety and cost, it is essential that the technical system (the machine) and the human being are able to work together smoothly. Designing work systems in accordance with ergonomic requirements enables to create working conditions that^[14]:

• result in optimal operator workload,
• ensure human safety, health and well-being, and
• optimise overall system performance.

The human-machine interface enables interaction between a human operator and a machine or system. It constitutes the connection and communication point between the user and the system and provides an intuitive and efficient way for operators to control, monitor and configure devices and processes. It includes:

• Elements with which the human being operates the machine, i.e. is able to execute functions. Such elements particularly include control actuators such as buttons, levers, adjusting wheels, keyboards, touch screens, voice controls, 3D-glasses, etc., and other elements which facilitate the machine's use, such as toolholders and chucks for the workpiece.
• Facilities for the exchange of information, such as displays which inform the user of the functional states of the machine. Interaction between the human being and the machine can be regarded as a closed control loop (figure 1). Sometimes the control loop is open: the decision based on presented information is “no operation is now required".

Interaction between the human being and the machine can be regarded as a closed control loop (figure 1). Sometimes the control loop is open: the decision based on presented information is “no operation is now required”.

Figure 1: Human-machine system shown as a closed control loop.

Source:^[15]

Ergonomic basics of human-machine interaction

Physical ergonomics

The ergonomic design of work systems in which for example work equipment is used is based upon the dimensions and characteristics of the human body. The Machinery Directive and also the Machinery Regulation state that ergonomic principles have to be taken into account. A guide has been developed to further support the application of the Directive^[16] as well as a specific guidance document on the ergonomic requirements^[10].  The Regulation on Machinery has extended the requirements on ergonomics especially to take into account machinery with self-evolving behaviour or logic that is designed to operate with varying levels of autonomy.
Guidance for the application of ergonomics standards in the design of machinery is provided by EN 1386^[17]. The European standard EN 614-113 "Safety of machinery – Ergonomic design principles" specifies these requirements: "The objective is to design the machinery in its context with the work system to be consistent with human capabilities, limitations and needs. This requires an analysis of the work tasks that operators have to carry out and the effect of any constraints that the design and its influence to the environment (e.g. noise, vibration) is likely to have on the operators' health, safety and well-being. Machinery shall be designed to take account of the variability in operators' characteristics. These include:
• body dimensions,
• posture,
• body movements,
• physical strength,
• mental abilities^[13] .
The standard further contains the requirement that machines must be designed for use by persons with body dimensions between the 5th and 95th percentile. In particular, this means that all elements required for use of the machine must actually be suitable for operation by 90% of the expected users. The users may include old, young, healthy and/or disabled persons.

Consideration of body dimensions is however the basis by which the user is prevented from reaching dangerous points on a machine, in order for accidents to be avoided. For this purpose, certain distances must be observed from the danger zone of the machine. Safety devices such as light barriers or safety mats may have to be installed, or alternatively access points designed so small that no body part is capable of passing through them^[18].

Body posture, body movements and the exertion of physical force during use of a machine must be considered during the machine's design in order to prevent them from causing excessively high stresses. In particular, the posture to be adopted must not be monotonous or constrained. Instead, changes in body posture must be possible; ideally, the task should even permit movement. The weight of body parts and of equipment should be supported, and the force to be exerted must be appropriate in consideration of the posture. Angles of comfort are a factor here: they must not be exceeded too frequently during use of the machine. Momentary and total forces exerted under the working shift must also not lead to excessive stress. Guidance for acceptable manual handling, forces, working postures and movements, as well as repetitive handling at high frequency is provided for example by European standards in the EN 1005 series, "Safety of machinery – Human physical performance"^[19].

Cognitive ergonomics

Cognitive ergonomics is a part of ergonomics and can be describes as ‘a discipline and practices that aim to ensure ‘appropriate interaction between work, product and environment, and human needs, capabilities and limitations’[20] . In this human-system interaction, cognitive ergonomics focuses on mental processes, such as attention, perception (including memory and reasoning), information processing and motor response. Some aspects of these basic cognitive processes are explained below in further detail.

Attention

Human attention may be selective or divided, depending upon the task itself and the associated factors. The ability of humans to divide attention between several processes at the same time is very limited and prone to failure. Control and display design must therefore ensure that information that must be processed in parallel is presented very close together, or that it addresses different senses (e.g. vision, hearing, touch, smell).

Perception

When human beings absorb information through their senses, they classify and group it in accordance with certain rules. These rules have been termed the "Gestalt laws" (of grouping)^[21] [22]. Elements will for example be classified in the same group when they move in the same direction (law of common fate), are similar in shape (law of similarity), or form a pattern that is simple, regular and orderly (law of good gestalt). These laws must be considered during the design of human-machine interfaces, particularly manual controls and displays.

Memory

Human memory may be split into three types or entities. These entities are however not separated from each other but work together and interfere with each other^[23]. Stimuli that have been received by the senses first pass the sensory memory. Information in the sensory memory may be retained for as little as a few tenths of a second and for no more than three and a half seconds, depending upon the sensory modality of the stimulus. Sensory memory stores information in order to permit pattern detection. All information that is not considered relevant is lost. The relevant information is passed on to the short-time or working memory. This entity may store a very limited amount of information for a short time (several seconds). Relevant information is then passed on to the long-term memory. The capacity of the long-term memory is unlimited, but information may be difficult to retrieve from it. Retrieval is easier when information need only be recognized rather than recalled. Users therefore find it easier to choose from a number of key combinations when using a machine than to remember the actual key required.

In consideration of these characteristics of human memory, all information that users are required to include within their concept of a machine must be presented in a way that emphasizes the importance of the information; the information must be repeated or presented for a sufficient length of time; and if possible, it should correspond to the patterns of the machine's functionality.

Reasoning

The way in which human beings classify and process information depends strongly upon their experience. If they have no concept of how a machine works, they must gather information and create a concept or pattern in their minds. This form of processing is termed "bottom-up"^[24]. It is time-consuming and demands effort on the part of the user. Conversely, "top-down" processing may be used when the user already possesses a concept of the machine's system of operation: in this case, users can exploit their experience, enabling them to grasp quickly how the machine functions. Use of the system concept may however also lead to mistakes when the concept does not correspond sufficiently to the actual situation. The risk of selecting an incorrect action on the basis of the system concept is particularly high when users are under time pressure^[23]. For this reason, engineers and designers generally make use of both principles.

Motor response

According to the EN 894-1 standard^[25] , a simple reflex takes 0.04 seconds. Conscious reactions that include cognitive processing in the brain take at least 0.15 seconds. When a signal is not expected, the response time increases to at least 0.5 seconds.

Ergonomic principles related to mental workload

EN 894-125 also refers to mental strain as a factor that affects the performance of machine users. Another important aspect in the design of human-machine interaction is therefore prevention of the adverse consequences of mental strain, possible long-term negative health effects, and higher risks of accidents during machine operation.

While a broad range of definitions exists for workload and specifically for mental workload11, a general concept of mental workload has been agreed upon internationally by the parties concerned with occupational safety and health, and is presented in EN ISO 10075-1:2017. The relationship between mental workload and mental strain is described in EN ISO 10075-1:2017; ergonomic design principles that assist in avoidance of the impairing consequences of mental strain, such as mental fatigue, monotony, reduced vigilance and mental satiation, are presented in EN ISO 10075-2:2000. The aim is to create "optimum working conditions with respect to health and safety, well-being, performance, and effectiveness, preventing over- as well as underload"^[26]. The design principles may be applied to human-machine interaction. Selected examples are given below.

Mental fatigue is influenced by the intensity and duration of mental workload and by its distribution over time. If, for example, too much information is provided too quickly (mental stress factor of the task), the limited capacity of the working memory is exceeded (individual characteristic of humans) leading to mental strain, particularly mental fatigue (impairing effect of mental strain). Another example is the presentation of numerous irrelevant parameters which the operator is required to check^[27]. Instead of presenting only relevant information, the operator must filter relevant information from the total information provided, leading to increased mental workload and mental fatigue. To reduce mental fatigue, it is important to reduce the intensity of the load, to limit the duration of exposure, or to change its distribution by permitting rest periods.

Monotony is caused by insufficient variation in the work task and work environment, repetitive operations and low task difficulty, especially over a long period. Increasing the variety of tasks, providing workers with autonomy in their working speed, permitting rest periods and providing optimum illumination (preferably adjustable by users) are ways in which monotony can be reduced. Lack of stimulation owing to increasingly automatic processes and robotisation decreasing involvement of users leads to monotony, reduced vigilance and loss of job control^[28]. Sustaining attention is another problem, since reduced vigilance can lead to a decline in performance (e.g. poor detection of critical signals) after as little as ten to twenty minutes. If possible, the need for sustained attention should therefore be avoided, or at least technical support given.

Mental satiation arises when the performance of repetitive, similar or identical tasks is required. To prevent this, functions must be appropriately divided between the machine and the user: whereas the machine assumes the simple and repetitive tasks, the user focuses on complete tasks (characterized by planning, executing and controlling elements).

Design strategies and principles

In the European Union, legislation for placing products on the market including safety and health requirements  is based upon legislative acts under Article 114/115 of the EC Treaty and applies throughout the entire European Single Market. The essential safety and health requirements are formulated in the directives or regulations (e.g. Machinery Directive 2006/42/EC, Machinery Regulation 2023/1230/EU). Harmonised standards lay down the technical specifications for products to meet the essential requirements of the directives or regulations. Products manufactured in compliance with harmonised standards benefit from a presumption of conformity with the corresponding essential requirements^[29]. 

The human-machine interaction requires the task interface to be designed according to design strategies and principles such as those laid down and illustrated in EN 614-213, in order for humans to be able to perform safely and efficiently and without damage to their health or impairment in their well-being^[5] [11] EN 614-2 guides the designer in a task and function analysis of the work system for the purpose of task interface design. Such a procedure assists in designing the allocation of functions between operator and machinery, the level of automation of the machinery, and specific operator tasks. In addition, ten characteristics of well-designed tasks are presented to support ergonomic design of the task interface, ranging from consideration of operator experience and capabilities and task feedback to avoidance of multi-tasking and over- and underload^[13] . 

The task interface design serves in turn as a framework or an outline for the design of the interaction interface. Human-system interaction refers to the interaction interface, to be designed according to principles of human information processing, i.e. referring to all senses serving data acquisition, to reasoning and decision-making, and to the taking of action with regard to both physical and cognitive dimensions^{[19] [25] [30] [31]}. An actuator will serve as a simple example for illustration of the relevant stages in human information processing during the design of an interaction interface. If the operator's task is to switch off a machine, location and design of the actuator and the operator's field of vision and his or her size will determine the data acquisition stage of human information processing. Presentation of information on the actuator and the shape of the actuator are relevant to the reasoning and decision-making stage, even if physical interaction is not relevant. The force required and the feedback given by the interaction interface are associated with the action-taking stage.

In order to perform his or her task, humans give instructions to a machine (see figure 1, Action) by using manual actuators to set parameters on it and by obtaining information on the machine's status in the various processing areas in the form of displays or signals, and also directly from machine functions.

Control actuators and displays

The EN 894-325  European standard provides information on the type of control actuator that is suitable for performance of particular tasks. A machine manufacturer can make his selection in consideration of the procedure described in this standard. For the control actuators to be used safely and effectively, their design should in turn satisfy ergonomic criteria. These relate to the design of the control actuator itself, i.e.: 

• The dimensions of the control actuator and its distance from other controls in consideration of how it is gripped: with a clench grip involving the whole hand; with a pinch grip involving several fingers; or with the contact grip involving contact with individual fingers. The possible precision of adjustment differs according to the form of grip. 

• The structure of the surface and the material from which it is manufactured. 

• The position of the control actuators in relation to each other. 

• The position of the control actuators in relation to the user: the control actuators should be arranged in the user's close or distant area of reach depending upon the frequency or importance of their use or, where required by the workplace, in the radius of activity of the user's body or upper body. 

• The exertion of force required for operation of the control actuator: on the one hand, a minimum value must be assured serving as a threshold, in order to prevent inadvertent actuation if at all possible; on the other hand, the reset force of the control actuator for example can provide the user with feedback of its position.

The user receives information about the state of the machine directly through the senses, or indirectly via signals, gauges and displays. The indirect information can be transmitted visually (for example by signal lamps, analogue displays, digital displays) or acoustically (for example by warning tones or the speed of the machine), and sometimes also haptically (e.g. vibration in control).

With regard to the elements of the system as a whole, ergonomic design in turn means consideration for the criteria derived from the human characteristics (refer for example to EN 894-225 ): 

• The displays must be visible to the user in a comfortable posture, without requiring extreme movements or twisting of the head or neck, i.e. the dimensions of the fields of vision and fixation must be considered. 

• The information on the displays must be recognizable, i.e. the size of the digits and symbols, contrast, resolution and the use of colour must be appropriate. 

• The form of the display must satisfy the requirements in terms of precision, speed and recognition of the values. 

• The frequencies and sound levels of acoustic signals must be appropriate for their urgency (warning tone or status indication).

Software design and dialogue principles

The EN ISO 9241-110^[32] describes interaction principles and general design recommendations that are applicable in the analysis, design and evaluation of interactive systems. The priority with which the interaction principles are applied depends on the purpose of the system, the users of the system, the tasks, the environment, the specific interaction technique used and the consequences arising from use. They have also been proven to be an adequate tool for user evaluation of robotic systems^[28].

These seven principles are:

1. Suitability for the task

A dialog supports suitability for the task, if it supports the user in the effective and efficient completion of the task. The dialog presents the user only those concepts which are related to the user's task^[32]. One way to support users in the effective and efficient fulfilment of their tasks is to provide macros and defaults for recurring tasks. There is then no need to enter repeated data by hand. 

2. Self-descriptiveness

A dialog supports self-descriptiveness, if each dialog step is immediately comprehensible through feedback from the system or is explained to the user on his or her requesting the relevant information^[32].

A dialog is characterised as self-descriptive if there is, for example, minimal need for reading the manual. Possible actions should be clarified to the user and context-sensitive help should be given.

3. Controllability

The interactive system allows the user to maintain control of the user interface and the interactions, including the speed and sequence and individualisation of the user-system interaction32. One example for the support of controllability is to enable users to cancel or undo actions or to interrupt and continue partial dialogs.

4. Conformity with user expectations

A dialog supports conformity with user expectations, if it corresponds to the user's task knowledge, education, experience, and to commonly held conventions^[32].

This would be the case if, for example, moving a control for temperature from left to right led to an increase in the temperature or if user terminology were used in all dialogues. Temperature declining from left to right, in turn, would be a lack of consistency.

5. Use error robustness

Systems should be designed in such a way that user errors do not occur at all, if possible. In case of identifiable errors, the system should treat them tolerantly and assist the user when recovering from errors. If errors do occur, the system should react tolerantly to them and, ideally, correct them itself so that users can still reach their goal. Corrective hints, alternatives or even automatic corrections are provided to the user. Additional, informative error messages including information on which error occurred, and why, are better than error messages informing the user only that his action has failed.

6. Learnability

An interactive system should make it easy for users to discover and understand the system's functions. It should also be possible to try things out without serious consequences. Easy recognition of previously learned functions is also part of learnability.

A dialog supports suitability for learning if it supports relevant learning strategies such as learning by doing or learning by example. In addition, in order to prevent an increase in mental workload and with regard to the constraints of human characteristics in information processing, recognition rather than memorization should be preferred.

7 User engagement

The system should be designed to be appealing and inviting in order to motivate users and provide them with a positive experience. It must be trustworthy so that it is used without hesitation. Active participation, e.g. in the form of feedback, can also strengthen user engagement^[33].

Human-centred design

Human-centred design is an approach to systems design and development that aims to make interactive systems more usable by focusing on the use of the system and applying human factors/ergonomics and usability knowledge and techniques^[34]. Standard EN ISO 9241-210 provides guidance on human-centred design processes for interactive systems. It emphasises designing systems that are usable and provide a positive user experience. Basic principles include the active involvement of users and a deep understanding of users, tasks as well as the context of use, including physical, social, and organisational environments^[34]. Human-centred design plays a crucial role in shaping the human-system interface in modern workplaces. Its principles ensure that the design of tools, systems, and environments is focused on the needs, capabilities, and limitations of the users. Human-centred design starts by identifying stakeholders including the workers and their needs, and then actively involves the stakeholders in the co-design and co-production of technology and systems^[28].

Human-machine interfaces and specific user groups

Older users

Many human characteristics, particularly physiological aspects, change during the ageing process. Sensory performance (sight, hearing, touch) deteriorates, as does physical strength and the performance of the cardiovascular system^[35]. These deficits, which have long been at the centre of discussion of the demographic shift, contrast with characteristics which are enhanced in old age or do not emerge before it is reached. Such characteristics primarily include abilities relating to experience, such as social and communication skills. Some of the abilities that deteriorate with age can be trained; for example, it is known that regular training of muscles enables their performance to be maintained at a high level, despite ageing. The emerging deficits can in some cases also be compensated for by experience or newly acquired skills. Older persons’ experience may even give them an advantage over younger colleagues.

Physical impairment or changes in sensory performance in particular can be compensated for by very good ergonomic design of the elements making up the human-machine system and by the form in which information is presented (see figure 1). Suitable illuminance or supplementary lighting can for example meet the need for greater brightness. High contrast and large digits on displays facilitate their recognition. Acoustic signals which provide feedback from machine functions can be adjusted in their frequency and volume such that they can be heard equally well by younger and older users. A well-designed human-machine interface with consideration for all ergonomic findings helps users in all age groups to work equally well, without suffering health complaints.

Disabled workers

Greater impairment of physical or sensory performance may lead to impaired work performance or disability. Principles for the design of products relating to the facility of their use by persons with impaired performance or disabilities are summarised under accessible design^[36] . In this context, not least with regard to the safe use of work equipment, accessible design does not mean that anybody, without exception, should be able to use the item of equipment concerned without modification or adjustment. For example, a CNC machine or gantry crane is not intended to be operated by anyone, without prior training. This would contradict the safety objective. The broadest possible user group should nevertheless be considered during design.

Accessible design

Accessible design principles focused on diverse users should be applied to maximize the number of potential users who can readily use a system in various contexts^[37]. Accessible design principles focus on creating environments, systems, and products that are usable by people with a wide range of abilities and disabilities. These principles ensure that all employees can perform their tasks efficiently, comfortably, and safely. Therefore, accessible design ensures that interactions between humans and machines are intuitive, efficient, and inclusive. Interfaces should be designed to accommodate different users. For example, providing both visual and audio feedback ensures that people with visual or hearing impairments can effectively use the system. Integrating assistive technologies into the human-machine interface like screen readers, speech recognition software, and alternative input devices, supports designing systems that are easily usable by a wide and diverse user base.

Conclusion

During the design of human-machine interfaces in companies a structured process should be implemented that considers the standards and guidelines that have been discussed in this article. Experience in the field also shows that it is good practice to involve future users of machinery in the design process. The article has emphasised that consideration for the physical and mental limitations of humans is an essential aspect in the design of interfaces. Failure to consider these aspects may lead to a lack of motivation and productivity and to adverse health implications for machine users at workplaces.