How to best design an HMI system
A proper interface between a machine and its human operator greatly impacts efficiency and ease of use and should promote a harmonized connection between the two. This tutorial provides guidance on how to best build that connection though a human-machine interface (HMI).
Human-machine interface (HMI) systems provide the controls by which a user operates a machine, system, or instrument. Sophisticated HMI systems enable reliable operations of technology in every application, including high-speed trains, CNC machining centers, semiconductor production equipment, as well as medical diagnostic and laboratory equipment. HMI systems encompass all the elements a person will touch, see, hear, or use to perform control functions and receive feedback on those actions.
How to 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, panel layout, production, testing, and quality assurance processes. Just as critical, in-depth knowledge of and compliance with all relevant ergonomic, safety, and industry standards must inform each step of the design and manufacturing cycle. Clear definitions of the functional requirements, the operator's level of expertise, and any communications/interactions with other systems provide the starting point in the knowledge-intensive design process.
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.
How many functions will be controlled by this interface? Where one function might be served by pushbutton, keylock, and rotary switches, multiple functions could require several screen displays to cover operator functions and options. What kind of visual, auditory, or tactile feedback will best serve the operator in performing the defined functions?
Degree of input complexity
Input can be as simple as an on/off switch or a touchscreen display. Touchscreen HMI systems are increasingly popular in public transaction applications, because they can simplify complex operations and tolerate a moderate degree of rough use. Defining input requirements will help decide which control technology is best suited for a specific application.
Feedback is critical to operator effectiveness and efficiency. Feedback can be visual, auditory, tactile, or any combination of these that is necessary for the application. Feedback 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. For example, in an industrial setting the HMI might connect via hardwire or a serial bus to input/output (I/O) points that provide machine status. Additionally, it might be networked into a manufacturing execution system and a supply logistics/inventory system.
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.
The HMI system should be rugged enough to withstand the elements and heavy use, but it also should last for the duration of the equipment lifecycle. For example, a magnetic resonance imaging (MRI) HMI system interface should last at least 10 years.
A thorough knowledge of technical ergonomic, design, and manufacturing standards is fundamental to HMI system design. These include engineering standards, such as MIL-STD-1472F, which establishes human engineering design criteria for military systems, sub-systems, equipment, and facilities; Federal standards set by the Americans with Disabilities Act; and industry guidelines, such as those from SEMI S2-93, the global semiconductor industry association, covering HMI for semiconductor manufacturing equipment. Additional HMI specifications are defined by ANSI, IEEE, International Organization for Standardization (ISO), and others.
Define the operator
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.
For any user along the range from intuitive to expert, interface ergonomic considerations should include: panel layout, HMI component selection, information presentation, feedback, and safety considerations.
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 (momentary, maintained, rotary, etc.)
- Physical configuration and mounting needs
- Special requirements such as illumination, marking, environmental sealing, etc.
The key to effective use of color is simplicity. Avoid too many colors or flashing alarms. Stick with the "traffic light" model for key actions:
- Red for stop/failure/fault
- Yellow for warning
- Green for OK/start/go/pass.
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.
Cursor control (Trackball, joystick, keypad, touchpad, etc.)
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, rocker, slide, keylock, rotary, etc.)
Pushbutton switches allow the option of illumination to indicate open/close switch status when a quick visual indication is desired. They are also useful in machinery and machine tools, electronic production, rail and bus transportation, medical treatment and diagnostics, or other environments for easier manipulation when gloves are worn.
Short travel technologies (Conductive rubber, membrane, keyboard, keypad, etc.)
Short travel technologies have been developed for industries where ease of cleaning or disinfecting is mandatory. Short travel technology can include cost-effective, conductive rubber keys in a typical keyboard, dome keys under an overlay, or a multilayer membrane.
Touch and switching technologies (Capacitive, Piezo, high frequency, etc.)
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. Piezo, capacitive, and high frequency technologies all offer rugged switch technology with long life cycles and low maintenance costs.
Display technologies (LCD, active matrix, OLED, FED, plasma, etc.)
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. Active matrix LCD technologies are commonly used for color functionality, while legacy LCD technology is used in applications where monochromatic feedback is sufficient. Organic light-emitting diodes (OLEDS) (carbon-based) can currently support smaller displays.
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.
Surface acoustic wave (SAW) touch technology
SAW touch technology sends acoustic waves across a glass surface from one transducer to another positioned on an X/Y grid. The receiving transducer detects whether a wave has been disrupted by touch and identifies its coordinates for conversion to an electrical signal. SAW serves well in outdoor and harsh environments because it can be activated by a heavy stylus or gloved fingers.
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.
Connecting/communicating with an HMI system
Once you have established how an HMI will look, feel, and operate, 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.
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 one 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.
Serial bus systems
As equipment and control systems became more complex and data hungry, transmission of data became a critical issue. To facilitate faster data transmission rates, devices incorporated serial bus connections-especially in electronics, semiconductor, machining, industrial, process, and transportation. A serial bus approach eliminated data transmission slowdowns due to cable length and delivered reliable, real-time operations and work-in-process feedback.
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.
Fieldbus protocols evolved for interconnecting industrial drives, motors, actuators, and controllers. Fieldbuses include: Profibus, DeviceNet, ControlNet, CAN/CANOpen, Interbus, and Foundation Fieldbus.
Higher level networks connect with fieldbus protocols primarily across variations of Ethernet. These include: Profinet, EtherNet/IP, Ethernet Powerlink, EtherCAT, Modbus-TCP, and SERCOS III.
Industrial applications have employed wireless technologies over the past 20 or so years, primarily to take advantage of real-time data transmission, application mobility, and remote management capabilities. Interference, reliability, and security continue to present difficulties for wireless connections in the HMI universe.
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.
International and U.S. standards for HMI systems
Key to the entire HMI system design cycle is a thorough knowledge of federal, industry, ergonomic, safety, and design standards. These include Human Engineering standards, such as MILSTD-1472F, which establishes human engineering design criteria for military systems, subsystems, equipment, and facilities; federal standards like those set by the Americans with Disabilities Act; and industry guidelines such as those from SEMI, the global semiconductor industry association, covering HMI for semiconductor manufacturing equipment.
Additional HMI specifications are furnished by ANSI, IEEE, ISO, and others.
The European Union (EU) provides specifications in the EU Machinery Directive for any equipment in domestic, commercial, or industrial applications that have parts actuated by a power source other than manual effort. Meeting this directive earns the equipment a CE mark.
Depending on the ultimate product application, observing appropriate standards assures that a product will meet industry criteria. This includes placement of components, legend size and color, emergency stop switch configuration and guards, and other ergonomic factors that improve usability, efficiency, and safety.
- Efficiency and ease of use rely on the design of the human-machine interface
- Application, best practices, standards, and operator skill sets influence HMI designs.
- Guidelines and experience help with HMI designs.
- See additional articles on HMI system design below.