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.
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 SystemA 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.General Functionality
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.
Operator Feedback
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.
Environmental ConsiderationsThe 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. For example, a Magnetic Resonance Imaging (MRI) HMI System interface should last at least 10 years.
Style
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.
Regulatory/Standards Considerations
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.
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 (momentary, maintained, rotary, etc.).
- Physical configuration and mounting needs.
- Special requirements such as illumination, marking, environmental sealing, etc.
Color scheme
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.
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.
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.
Look for part two of this article in tomorrow's MBT newsletter.
