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. This consideration includes a thorough examination of the European regulations (legislation and standardization) currently in force concerning the topic.

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 work space, 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. Appropriate solutions which respect human factors and ergonomic design principles will not be possible, however, unless they take account of 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. Future developments in human-system interaction with regard to complex work systems are addressed in Miller et al. (2012) ^[7].

The legislation in force in the European Union and its member countries (e.g. Machinery Directive 2006/42/EC; Work with display screen equipment Directive 90/270EEC) requires human factors and ergonomic design strategies (e.g. task orientation) and principles (e.g. compatibility)^[8] 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 ^[9] ,^[10],^[4],^[11].

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. The objective is optimum productivity coupled with assurance of the user's health and well-being. This means, however, that the work equipment, in this case machinery, and the interface must be designed to be ergonomic.

The human-machine interface 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 etc., and further 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 (Fig. 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 (Fig. 1). Sometimes the control loop is open: the decision based on presented information is “no operation is now required”.

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 itself states that ergonomic principles have to be taken into account such as: "allowing for the variability of the operator's physical dimensions, strength and stamina" (Directive 2006/42/EC, Annex I, 1.1.6 "Ergonomics"). A guide has been developed to further support the application of the Directive including^[13] and guidance for the application of ergonomics standards in the design of machinery is provided by EN 13862012^[14]. The EN 614-1 ^[11] European standard, "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^[11].
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 ^[15].

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^[16], "Safety of machinery – Human physical performance".

Cognitive ergonomics

Long and Whitefield (1989) define cognitive ergonomics as the application of the cognitive sciences to human-machine interaction. Cognitive processes that are relevant during consideration of human-machine interaction have been referred to, for example, in EN 614-2:2008^[11] for task interfaces and in EN 894-1 ^[17] for interaction interfaces. Among these processes are 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) ^[18],^[19]. 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

In accordance with recent models, human memory may be split into three entities. These entities are however not separated from each other but work together and interfere with each other^[20]. 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"^[21]. 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^[22]. For this reason, engineers and designers generally make use of both principles.

Motor response

According to the EN 894-1 standard ^[17], 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 work-load (ISO 10075)

EN 894-1 ^[17] 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 workload ^[9], a general concept of mental workload has been agreed upon internationally by the parties concerned with occupational safety and health, and is presented in ISO 10075-1:2000. The relationship between mental workload and mental strain is described in ISO 10075-1:2000; 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 ISO 10075-2:2000 (see also^[23]) (cf.^[24]], see also the article on “Job Demands"). 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" (ISO 10075-2:2000^[25]). 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^[26]. 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 to it, 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 decreasing involvement of users leads to monotony and reduced vigilance. 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, occupational safety and health is governed within two separate legal areas. Where products are concerned, identical requirements based upon directives under Article 114/115 of the EC Treaty apply throughout the entire European Single Market. In accordance with the New Approach, the basic safety requirements are formulated in the directives (e.g. Machinery Directive 2006/42/EC). The technical specifications of products meeting the essential requirements set out in the directives are laid down in harmonized standards (such as EN 614-1:2009^[11] and EN ISO 894-3:2010 ^[17] for machinery). Application of harmonized or other standards remains voluntary, and the manufacturer may always apply other technical specifications in order to meet the requirements. Products manufactured in compliance with harmonized standards benefit from a presumption of conformity with the corresponding essential requirements^[27]. By contrast, provision is not made for complete harmonization of occupational health and safety at the workplace. Each member state is free to adopt regulations beyond the minimum requirements laid down in the European directives based upon Article 153 (e.g. the Directive on Work with display screen equipment, 90/270/EEC), provided such regulations do not conflict with the provisions of the EC Treaty. National legislation intended for employers is therefore needed in this area.

Both areas are relevant for the design and use of a human-machine interface. Firstly, 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 614-2:2008^[11], 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], ^[9]. EN 614-2:2008^[11] 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^[11]. For an example of the task interface design of a drilling machine, please refer to EN 614-2:2008^[11].

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^[28],^[16],^[29],^[30]. 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 the human being imparts instructions to a machine (see Fig. 1, "Actions") by using manual actuators to set parameters on it and by obtaining information (see Fig. 1) 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 894-3 ^[31] 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 on the machine's states 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 894-2 ^[31]): • 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 (ISO 9241-110)

The ISO 9241-110^[32] international standard deals with usability aspects of display design. These principles, identified as being important for the design and evaluation of such user interfaces, are:

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 fulfillment of their tasks is to provide macros and defaults for recurring tasks. There is then no need to enter repeated data by hand.

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 characterized 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.

Controllability

“A dialog supports controllability, if the user is able to maintain direction over the whole course of the interaction until the point at which the goal has been met."^[32]

One example for the support of controllability is to enable users to cancel or undo actions or to interrupt and continue partial dialogs.

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.

Error tolerance

“A dialog supports error tolerance if, despite evident errors in input, the intended results may be achieved with either no or minimal corrective action having to be taken. Errors should be explained to the user for him or her to correct them."^[32]

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.

Suitability for individualization

“A dialog supports suitability for individualization, if the dialog system is constructed to allow for modification to the user's individual needs and skills for a given task."^[32]

Individualization exists if for example the user can choose his preferred language or if he is able to adapt the user interface to his deficits (e.g. larger font size in the event of defective vision).

Suitability for learning

“A dialog supports suitability for learning, if it guides the user through the learning stages minimizing the learning time."^[32]

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.

Information presentation

Principles regarding information presentation are referred to in ISO 9241-12^[32]. This standard includes principles on how information may be organized, and on on the integration of graphical objects and codes.

Usability testing (expert and user methods) as part of the developmental process

Definition

The ISO 9241-11:1998 standard^[32] defines the term “usability" as the “extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use"^[32]. According to Chao^[33] the aim is for a certain product to be evaluated by users or experts with regard to different criteria, e.g. whether it is effective, efficient and easy to learn and whether it satisfies human requirements.

Usability testing methods

Unfortunately, it is very difficult and expensive to recruit sufficient users to examine all relevant aspects regarding the interaction with a specific machine. Nielsen^[34] describes inspection methods (the informal way of employing experts) “as a way to save users". According to the author, usability inspection is “a set of cost-effective ways of evaluating user interfaces to find usability problems".

While users should be involved in all stages of the developmental process^[35] , inspection methods are particularly suitable for the early stages^[36]. They include for example heuristic evaluation and cognitive walkthrough. These two methods are the most researched and actively used techniques^[36].

According to Nielsen^[34], heuristic evaluation “is a method for finding usability problems in a user interface design by having a small set of evaluators examine the interface and judge its compliance with recognized usability principles (the “heuristics")". This method is cost-efficient and therefore valuable in the event of time or budgetary constraints^[34],^[36]. In addition, experts are much better at finding usability problems by using the heuristic evaluation than users without usability expertise^[37]].

Since it does not require a fully functioning prototype or the participation of users, cognitive walkthrough is also suitable for the early stages in the evaluation process. The experts take the perspective of the user and identify problems that might occur during interaction. During evaluation, particular attention is paid to the support of “exploratory learning" (first-time use without training). Prerequisites for cognitive walkthrough are a description of the users and their knowledge, a description of tasks that are performed with the system and a list of correct actions needed to complete these tasks^[38].

The use of inspection methods alone is not a substitute for empirical user testing^[36],^[34]). The best result can be achieved when the two methods are combined, since the inspection methods and empirical methods both enable problems to be found that are overlooked when only one method is used^[34]. Methods of empirical user testing are for example the think aloud protocol, interviews, questionnaires and usability tests in the laboratory.

The think aloud protocol is a frequently used method, especially during usability tests ^[39]. It is cheap and can be used when only a few participants are available. The aim is to gain new insights into the design under test. During a usability test, the participants are asked to express their thoughts while they complete their normal tasks. In this way the usability experts may discover problems in practice and learn how the users feel and think about the design.

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^[40]. 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 Fig. 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 summarized under accessible design ^[41]. 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.

Certain basic principles applicable to the design of work equipment, particularly to the design of human-machine interfaces, will be stated here as examples^[42]: • Alternative format: this refers to a "different presentation which may make products and services accessible by the use of another mobility or sensory ability". Acoustic signals used as alarms may for example be accompanied by visual stimuli such as flashing lights. • Position: the areas of reach of persons with physical disabilities, such as wheelchair users, differ in their heights and ranges. It is important for displays and control actuators to be positioned such that they are accessible to as many different people as possible. • Lighting, glare, colour and contrast: these are important aspects for easy recognition and comprehension of information. Some colours, such as red and green, cause difficulties in distinguishing from each other to a substantial proportion of the population (nearly 1/10 of males).

Practical examples

The following illustrations (1 & 2) show examples of the implementation of ergonomic principles for human-machine interaction.

Compatibility of location

Practical examples

The following illustrations (figures 2-7) show examples of the implementation of ergonomic principles for human-machine interaction.

Compatibility of location

Compatibility of direction

Conclusion

During the design of human-machine interfaces in companies a structured process should be implemented that routinely considers the norms and guidelines that have been demonstrated 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 emphasized 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.