Human Machine Interface (HMI) Systems provide the controls by which a user operates a machine, system, or instrument and includes all the elements a person will touch, see, hear, or use to perform control functions and receive feedback. Sophisticated HMI Systems enable reliable operations of technology in every application, including high-speed trains, CNC machining centers, semiconductor production equipment, and medical diagnostic and laboratory equipment.

Today’s HMI Systems can include supervisory control and data acquisition (SCADA) and alarming, as well as deliver information  to and receive information from, other networked systems, such as materials-handling or enterprise-resource planning systems (ERP).

A well-designed HMI System does more than just present control functions and information; it provides an operator with active functions to perform, feedback on the results of those actions, and information on the system’s performance. It is the task of everyone involved in the HMI design, the engineers, management, HMI consultant, and industrial designer, to meet the defined usability requirements for a specific HMI System.

HMI solutions are functionally critical to major industry sectors – machinery, transportation, electronics, medical, audio/video, telecommunications, process control, life sciences, lifting and moving machinery, unattended payment terminals, and public access. Depending on application requirements, an HMI System can be anything from a panel with a set of electromechanical controls such as a pushbutton, keylock, or rotary switch, to a multi-layered graphical touchscreen interface networked to production and/or corporate systems.

Why is Well-Considered HMI Design Important?

The interactive impact of the human/machine interface is much more significant than its basic functionality. HMI Systems are the principal point of contact between the user and a machine or process. A good HMI System makes this interaction seem intuitive. A poor HMI System can alienate users or potential customers, encourage users to circumnavigate the system, or result in poor or unsafe system performance.

As the direct link to the user, HMIs directly represent the core system’s quality and value. A sophisticated mix of design and layout considerations, such as contemporary style, color, and tactile response coupled with ergonomic and intuitive operation, create an optimal user experience that determines a customer’s satisfaction with the core product.

An example of the impact of effective HMI System design can be found in the iPhone developed by Apple. Here, an innovative design based on capacitive technology enhanced the usability of a smartphone by introducing an elegant user interface that also increased functionality and productivity. The combination of quality, innovation, and intuitive design proved to be an extremely effective marketing strategy, illustrating the enhanced value of elegant control functionality applied to a commonplace product.

How Do You Design an HMI System?

A highly-reliable HMI System that delivers safe, cost-effective, consistent and intuitive performance relies on the application of engineering best practices throughout design and panel layout, production, testing, and quality assurance processes.

Defining the Operational/Functional Requirements

The tools needed for effective operator control of the equipment as well as the requirements of the overall application determine the selection of interface functions. There are many factors to consider in the initial design phase that are critical to both the HMI and the core system to which it is interfaced.

• General functionality: How many functions will be controlled by this interface? What kind of visual, auditory, or tactile feedback will best serve the operator in performing the defined functions? The goal is to provide an HMI System that clearly communicates the information necessary to accomplish the specific task assigned to the defined component, system, or equipment.
• Degree of input complexity: Input can be as simple as an on/off switch or a touchscreen display, but their use depends on environmental factors. Defining input requirements will help decide which control technology is best suited for a specific application.
• Operator Feedback: Feedback is critical to operator effectiveness and efficiency. It is essential in systems that have no mechanical travel, such as a touchscreen or a capacitive device that when triggered has no moving parts. In some cases feedback provides confirmation of an action, while in others it adds to the functionality.
• Interface/Interconnection with other systems: HMI Systems must be able to interface/interconnect with the system under control as well as other related systems. Additionally, it might be networked to a manufacturing execution system and a supply logistics/inventory system.
• Environmental considerations: The application environment – encompassing both physical location and vertical industry environment – determines HMI System durability requirements. Environmental stresses include exposure to moisture and the elements, temperature extremes, wear and tear, vandalism, and general rough use characteristic of harsh environments such as an industrial production floor.
• Lifecycle durability: Not only should the HMI System be rugged enough to withstand the elements and heavy use, but it should also last for the duration of the equipment lifecycle. A product is as good as its weakest link. If the HMI System fails, it is most often perceived as a failure of the core system itself. Therefore the operator interface should be designed to an even higher level of reliability, because it is the critical link between the operator and the equipment.
• Style: HMI System style is a high priority for many consumer goods and especially luxury products. HMI style considerations are effective when they create a level of product differentiation that delivers a unique selling proposition.
• Regulatory/standards considerations: A thorough knowledge of technical ergonomic, design, and manufacturing standards is fundamental to HMI System design. Depending on the ultimate product application, observing appropriate standards assures that a product will meet industry criteria.
• Panel layout: The panel layout should be designed to provide the operator functional groups of related information in a predictable and consistent manner. In addition, the system must require an operator to initiate action and keep the operator informed by providing timely feedback on those actions. The layout should be organized so that the operator is clearly prompted in advance when the next operator action is required.
• HMI component selection: HMI designers can simplify their search for the appropriate switch or HMI component by carefully analyzing their application requirements then determining the following: electrical ratings, actuation preferences, physical configuration and mounting needs, and special requirements such as illumination, marking, and environmental sealing.
• Color scheme: The key to effective use of color is simplicity. Avoid too many colors or flashing alarms. Keep colors bold and bright and use a neutral background if necessary to make them stand out. Use colors conservatively, conventionally, and consistently. Color should never be the sole source of information.
• Information presentation: Once again, simplicity is the key. Don’t crowd a screen – avoid cluttering it with irrelevant data. Forcing an operator to search for the required information increases response time and potential errors. Have a consistent set of menu buttons and functions from screen to screen. If you have multiple screens of information, make the operator’s progress through them intuitive and logical. To indicate changing states, use changing icons.
• User feedback: Feedback is critical to ergonomic industrial design. Make sure the results of pressing a control button, toggling a switch, or entering a command are absolutely clear. Determine if operator feedback is visual, auditory, tactile, or a combination of multiple techniques.

Define the Operator

Know your operators – the key to a successful HMI System implementation requires a well-grounded definition and understanding of the operators.

Will the operator be a passive/intuitive user? If so, commands/functions should be simple with an easy-to-comprehend interface. For this type of user, repeatability is also important – information and actions should appear consistently from use to use.

For an expert user, where more sophisticated control is desirable, there may be multiple layers or levels for interfacing with equipment.

Typically there are three general categories of users (whether they are novices or experts): operators, supervisors, and maintenance personnel.

• Operators: The primary concern is providing the operator with intuitive access to the subset of controls necessary for daily production tasks on the equipment. In general, the idea is to minimize unnecessary data while keeping detailed data available upon request.
• Supervisors: A higher level of control is generally granted to supervisors and access may be controlled by a password/log-in procedure. This may include separate screens of detailed information and offer more data entry options.
• Maintenance: Maintenance personnel can be given full access to machine control and data displays. These capabilities are often inaccessible by operators and supervisors.

How Do You Choose the Best Control Technologies Appropriate to the Application?

Once you have defined HMI functionality, you are ready to investigate control technologies. Each technology has advantages and disadvantages related to the HMI System, equipment, and application:

• Cursor Control: The selection between different control technologies is primarily determined by the resolution of control that is required by the application. A trackball or joystick enables granular, pixel-by-pixel control, a far higher resolution than possible with a typical PC point-and-click controller.
• Switches: Pushbutton switches allow the option of illumination to indicate open/close switch status when a quick visual indication is desired. Rocker/toggle switches provide higher current capability and are also used when a very quick visual indication of “on” or “off” is necessary. Rotary-switch and keylock technologies serve best when the application requires position indicators such as those used in heater or fan control. Slide switches are the technology of choice when ease-of-use and low-cost switching is desirable – commonly found on notebook cases and handheld on/off functionality. A slide switch can take computer users from operational to programming mode quickly and intuitively.
• Short travel technologies: Short travel technologies have been developed for industries where ease of cleaning or disinfecting is mandatory, for example pharmaceutical, chemical, and food processing, or in a hazardous environment where a sealed system is required.
• Touch and switching technologies: Applications operating in aggressive environments such as public access or, for example, soda dispensing, where the syrupy liquid tends to get into crevices and gum up the machinery – require a rugged, completely sealed surface.
• Display technologies: The basic function of displays in HMI applications is to provide an information source – operators interact to obtain information or to prompt for the next screen. Display technology choices are dictated by the HMI System environment and its degree of ambient illumination, as well as by color requirements.
• Interactive Displays, Touchscreen: Touchscreen technologies offer a range of functionalities and characteristics that govern HMI Systems choice according to application and environment. It is important to determine which touch technology will be used in the early stages of the design cycle as the different options offer quite unique electrical and mechanical requirements. Examples include capacitive touchscreens, infrared touchscreens, resistive touchscreens, or surface acoustic wave.
• Motion Control: Motion control most often employs joystick technology for applications requiring macro control, such as controlling the bucket on a payloader, a robotic arm, or directional control for a piece of materials handling equipment, or pull mechanisms. A joystick can also be used for higher-resolution applications as illustrated by the medical application example above, under “Cursor Control.”

Connecting/Communicating with an HMI System

Once you have established how your HMI will look, feel, and operate, you need to consider how the HMI will connect to and communicate with the core equipment or system under control. Typically, communication can be achieved through several approaches: hard wired connection, serial bus connection, or wireless connection. Each approach has pros and cons – selection will depend on how your HMI needs to fit within your application.

Selecting the appropriate communications technologies may include combining some or all of these approaches.

Conventional, hard wired systems are still used in many transportation and industrial legacy systems. Hard wired systems require no special tools and are simple, visible, and easy to understand, especially where the HMI interface controls a single machine.

There are many drawbacks, including difficulty integrating changes or new features – new features require new wiring. Conventional wiring also requires more space due to the number of wires and the actual size of the wires and larger connectors due to higher pin counts. A hard-wired system is typically heavier and more expensive, which can be detrimental in some applications, such as transportation.

As an example, an application requiring a hard-wired assembly or panel might consist of a metal panel plate with 10 switches connected to two wires apiece, 20 wires in all. Each of these wires must be conjoined with 10 application connectors beneath the panel plate. An added illumination requirement would double the wire count, resulting in 40 wire connections to the application.

In some industries, such as rolling stock, users prefer hard-wired HMI Systems, in many instances because of an attachment to legacy technology, but also because of the ripple effect impacting documentation, maintenance, service, and the effect on training operational personnel across the vast scale of the fleet. Such a change represents a very substantial challenge in terms of time, effort, and cost that may not be offset by enhanced efficiency, performance, and revenues.

Serial bus systems provide many advantages over hard wired connections, including easy addition of new functionality – typically through software – without adding or replacing hardware. Wiring is much simpler and more flexible with smaller cables and connectors allowing for more compact design, and easier hardware updating and relocation.

Wireless connections have been used in industrial applications over the last 20 or so years, primarily to take advantage of real-time data transmission, application mobility, and remote management capabilities.

A WWAN utilizes mobile communication networks such as cellular, UMTS, GPRS, CDMA2000, GSM, CDPD, Mobitex, HSDPA, 3G, and WiMax. All of these networks offer wide service coverage and are normally used for citywide, nationwide, or even global digital data exchange.

WLANs transmit data over a shorter distance, normally 100 meters or so. In terms of transmission technology, WLAN uses spread-spectrum or OFDM (orthogonal frequency-division multiplexing) modulation technology to provide the convenience of exchanging data without the limitation of cables. Popular wireless communication technologies being applied to industrial applications include WiFi, Bluetooth, and ZigBee.

Safety Considerations

For HMI Systems design, safety considerations are a critical part of the system. Human error is a contributing factor in most accidents in high-risk environments. Clear presentation of alarms as well as the ability to report errors, are crucial elements in any HMI.

In addition, emergency stop switches, generally referred to as E-Stops, ensure the safety of persons and machinery and provide consistent, predictable, failsafe control response. A wide range of electrical machinery must have these specialized switch controls for emergency shutdown to meet workplace safety and established international and domestic regulatory requirements.

E-Stops differ from simple stop switches (that merely turn equipment off) in that they offer “foolproof” equipment shutdown. This is accomplished through advanced switch design that requires a twist, pull, or key to release electrical contacts to allow machinery restart.

Designers should be aware of international and U.S. standards and regulations that impact the design and use of E-Stops.

Applications

Manufacturing and Process Industries

Manufacturing production floors – and particularly machine tool manufacturing environments present a number of challenges for HMI Systems. Requirements include environmental sealing (IP 65 or greater) against moisture, cutting fluids, oil, and dirt. HMI Systems must also be able to withstand temperature variations, excessive heat and cold, etc., as well as shock, vibration, and high duty cycle.

The interface functionality requires a broad selection of switching technologies that address the range of applications that might be present. Options range from different types of actuator functions and illumination to push buttons, keylocks for security, and emergency stop buttons for safety—all in varying shapes, colors, and sizes to make the HMIS easily operable.

Transportation Industry

There are two distinct categories of HMI Systems related to transportation: operator controls and passenger controls.

For operators of rail vehicles, buses, and emergency vehicles, the key to an effective HMI System is consistent and predictable performance with time-proven controls that are familiar to multiple operators. As transportation systems grow more complex, operator controls should be easier to understand and use in order to reduce the risk of human error.

Passenger applications often use audible feedback such as voice/sound indicators for door open and close functions. In a stop-request application, passengers can alert the operator by pushing a button when approaching a desired stop. They receive immediate confirmatory feedback via a sound, or visually via LED illumination on a control panel where LEDs illuminate requested stops in the same way that interior elevator panels illuminate selected floors. Other passenger HMI controls include override systems, emergency-call equipment, prompted by audible, visual, and hidden indicators and programmable acoustic warning signals.

Controls must also be durable and as tamper-resistant as possible. Durable mechanical stops protect against excessive force on both the operational and passenger sides. Controls on vehicle interiors must be able to withstand low-pressure hose downs, and high-pressure hosing on the outside.

Semiconductor Production

Applications in semiconductor tend to have many operator terminals. They often consist of touchscreen displays that are essentially flat-screen computers of various sizes interfaced to production machines. The environment is generally extremely clean.

Communications use a range of hardwired/bus control and wireless for the tethered applications. A specific version of Ethernet has been developed for the semiconductor environment. Called EtherCAT Industrial Ethernet, is in use in a wide variety of semiconductor and flat panel display manufacturing operations.

Medical Equipment

The medical environment is quite broad with applications that include clinical, diagnostic, patient use, or skilled operator use equipment such as Magnetic Resonance Imaging (MRI). Diagnostic equipment manufacturers focus on patient-facing priorities including ergonomics and reassurance.

Large diagnostic equipment like MRI, Computed Tomography (CT), or other Diagnostic X-Ray machines, where the patient is being scanned from a stationary position, have both operator and patient controls to stop the process in case of patient discomfort. The operator is highly skilled, and the control station is very similar to a computer workstation. The controlling interface keyboard is typically comprised of a cursor control device and short travel technology, which may be located outside of the equipment suite.

Clinical equipment could be an infusion or blood pump, or machine for dialysis control. Cleanliness is a key priority, incorporating antimicrobial surfaces with the ability to be sterilized. Most functions are in the hands of a skilled operator but clinical equipment may be operated by a skilled patient/user as well. Displays are used to show key data, alarming, and system status. Touchscreens may be used but must be resistant to cleaning solutions to limit the spread of infection.

Public Access

Public use equipment – ATMs, kiosks, gas pumps and self-checkouts, etc. – is one of the most difficult environments. The controls must be rugged enough to survive harsh use but also as simple as possible. Operators are assumed to be unskilled.

Control panels use big buttons and target areas, and display menus should require a minimal learning curve with quick input. Touchscreens are widely used because they present a quick graphical display and menu to walk the user through the process. Intuitive operation prevails. If the human factor is poor or it is intimidating to use, the product will not be successful – intuitive operation is a feature of the marketing strategy. If the application requires considerable input, a touchscreen is not optimal. A keyboard and sequenced menus may be better alternative.

Public applications represent a security risk, so they have to be sufficiently ruggedized to withstand the environment. ATMs are almost impenetrable. Illumination and audio considerations are also critical. An outdoor environment might provide excessive ambient light during the day and no light at night – selecting a display that can work in both sets of conditions is key. In the same way, if audio cues are used, the impact of ambient noise should be considered.

An operator interface should provide the ability to safeguard information. ATM debit transactions require that personal information be entered, resulting in a higher level of security and standards. Financial institutions must use high level 3DES encryption and not allow personal data to be stored in local or remote locations.

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