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, and 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.
The task of an HMI System is to make the function of a technology self-evident to the user. A well-designed HMI fits the user’s image of the task he or she will perform. The effectiveness of the HMI can affect the acceptance of the entire system; in fact in many applications it can impact the overall success or failure of a product. The HMI System is judged by its usability, which includes how easy it is to learn as well as how productive the user can be.
It is the mission 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.
A well-designed HMI System does more than just present control functions and information; it provides an operator with intuitive active functions to perform, feedback on the results of those actions, and information on the system’s performance.
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 and 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 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 a single 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?
Does the operation require real-time indicators? Multiple data-entry points? How many times is a button pressed? Are there safety considerations? Are emergency stop switches required? Which standards apply – industry, safety, international?
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 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 I/O points that provide machine status. Additionally, it might be networked to 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.
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. For example, a Magnetic Resonance Imaging (MRI) HMI System interface should last at least 10 years.
HMI System style is a high priority for many consumer goods and especially luxury products. In the marine industry, the consoles for high-performance racing boats feature contemporary styling and an array of ergonomic technologies. HMI style considerations are effective when they create a level of product differentiation that delivers a unique selling proposition.
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, subsystems, 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, ISO, and others.
Define the Operator
Know your 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.
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.
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.
Choosing the Best Control Technologies
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 (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.
Rotary-switch and keylock technologies serve best when the application requires position indicators such as those used in heater or fan control. Keylocks provide an additional layer of security to the application. Rotary switches also can be used for an application requiring multiple positions.
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.
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, for example pharmaceutical, chemical, and food processing, or in a hazardous environment where a sealed system is required. Short travel technology can include cost effective, conductive rubber keys in a typical keyboard, dome keys under an overlay, or a multi-layer 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.
Capacitive or high-frequency signals electronically activate an on/off function by changing capacitive load. Capacitive/high-frequency technologies require the use of nonconductive front panel materials which can be up to 15 mm thick, for example those operating under protective glass within hazardous environments.
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. OLEDS, organic (carbon-based) light-emitting diodes 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.
Capacitive touchscreen transmit 75 percent of the monitor light (compared to 50 percent by Resistive touchscreens), resulting in a clearer picture. They use only conductive input, usually a finger, in order to register a touch.
Infrared touchscreen technology projects horizontal and vertical beams of infrared light over the surface of the screen. When a finger or other object breaks those beams, the X/Y coordinates are calculated and processed. These cost-effective touchscreens can also be used by workers with gloves and are relatively impervious to damage.
Resistive touchscreen technology offers cost-effective, durable performance in environments where equipment must stand up to contaminants and liquids, such as restaurants, factories, and medical environments. When touched, the conductive coating on the screen makes electrical contact with the coating on the outer layer, the touch coordinates are registered by the controller to activate the on/off function.
Surface Acoustic Wave (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 if 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 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.
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.
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.
Field bus protocols evolved for interconnecting industrial drives, motors, actuators and controllers. Field buses include: PROFIBUS, DeviceNet, ControlNet, CAN/CANOpen, InterBus, and Foundation Field Bus.
Higher level networks connect with field bus 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 last 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 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 EU provides specifications in its EU Machinery Directive for any equipment for 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.
There are also standards for public access HMI Systems, including security and cryptography standards for systems that handle payment cards; specific flammability standards and test procedures for transportation systems, and medical device and equipment standards.
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.
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