HMI Design Guidelines

Good interface design is significantly more difficult than one might grasp intuitively. It entails removing the irrelevant artifacts from Human Machine Interfaces (HMIs) reducing the number actions required to complete a task and eliminating unnecessary alternatives in order to achieve improved efficiency and reduced decision time.

Although this sounds simple these principles are in direct contrast to the common principle of adding one more button, another data point or something flashy that will impress users when the encounter a piece of software of HMI for the first time.

A sharp distinction also needs to be drawn between usability and utility. Usability is aimed at solving a specific and immediate need whereas utility is concerned with the additional features and design elements that are often aimed at creating “wow” designs without functional benefit. This practice of form over function may work well for many consumer applications but could have severely detrimental impacts for Industrial Automation applications.

Two useful frameworks for thinking about software usability include the ISO-9241 standard for ergonomics of human-systems interaction and the Nielsen Heuristics.

Nielsen’s 10 Usability Heuristcs for interface design

1.      Visibility of System Status

The system should always keep users informed about what is going on through appropriate feedback within a reasonable time. Users need to know whether their interaction with the system was successful – i.e. did the system know that I just pressed a button. This can be realized with a progress indicator or color change on press etc.

Providing immediate feedback also allows users to catch errors and correct them timeously. User should not be left wondering whether they need to repeat the action because there is no visual feedback or long-time delays

2.      Match between the system and the real world

As UX professionals, we should never assume that our own interpretations and understanding of words or objects match those of our users. (You ≠ user).

The system should speak the users' language, with words, phrases, and concepts familiar to the user, rather than system-oriented terms. Follow real-world conventions, making information appear in a natural and logical order.

3.      User Control and Freedom

Users often choose system functions by trial and error and will need a clearly marked "emergency exit" to leave the unwanted state without having to go through an extended dialogue.

In some instances, undesirable actions need to be prevented to ensure that there is no damage to property, plant and equipment.

4.      Consistency and standards

A standard ensures that your users can understand the individual interface elements in your design and that they know where to look for what features.

5.      Error prevention

Even better than good error messages is a careful design which prevents a problem from occurring in the first place. Either eliminate error-prone conditions or check for them and present users with a confirmation option before they commit to the action.

6.      Recognition rather than recall

Minimise the user's memory load by making objects, actions, and options visible. The user should not have to remember information from one part of the dialogue to another. Instructions for use of the system should be visible or easily retrievable whenever appropriate.

7.      Flexibility and efficiency of use

Accelerators — unseen by the novice user — may often speed up the interaction for the expert user such that the system can cater to both inexperienced and experienced users. Allow users to tailor frequent actions.

8.      Aesthetic and minimalist design

Interfaces should not contain information which is irrelevant or rarely needed. Every extra unit of information in a dialogue competes with the relevant units of information and diminishes their relative visibility.

Cognitive overload can also result in important factors being overlooked or diminished relative to less important factors on your HMI.

9.      Help users recongnise, diagnose and recover from errors

Error messages should be expressed in plain language (no codes), precisely indicate the problem, and constructively suggest a solution. The bain of industrial automation applications has long been matrix menu structures that make little or no sense without a manual and previous experience.

10.  Help and documentation

Even though it is better if the system can be used without documentation, it may be necessary to provide help and documentation. Any such information should be easy to search, focused on the user's task, list concrete steps to be carried out, and not be too large.

ISO-9241 Guidelines

Part 10 of the ISO-9241 Standard defines the following ergonomics principles for Human Machine Interfaces:

1. suitable for the task
2. self-descriptive
3. user controllable
4. conformable with user expectations
5.  error tolerable
6. suitable for individualisation
7. suitable for learning.

Part 11 of the guideline describes usability being the extent to which the product can be used by specified users to achieve specified goals with:

i.                    efficiency, being the accurateness and completeness of results achieved

ii.                  effectiveness, being the resources used to reach goal

iii.                satisfaction, being user acceptance and ability to work in comfort with the system in a specified context of use.

Usability Testing

The most effective way to determine whether these design principles have been incorporated ito your HMI design is through usability testing

The goals of usability testing vary by study, but they usually include:

• Identifying problems in the design of the product or service
• Uncovering opportunities to improve
• Learning about the target user’s behavior and preferences

Conclusion

Good user interface design is more challenging that might be initially understood by the various role players in the industrial automation applications however the cost of poor design can have very costly consequences for the users of HMIs. Investing a little more time and resources upfront might well increase the immediate cost of developing human machine interfaces but the life cycle cost will pay positive dividends.

References:

• Nielsen Norman Group Articles
• Progea – Usability and Guidelines Designing HMI