Best Practices in High-Performance HMI Design

Having a Solid Foundation to Build On Is Extremely Important, and Having an Enforced Policy That Ensures Compliance Is Just As Important

By Ian Nimmo

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Regardless of your starting point for a human-machine interface (HMI) project, we recommend following the ISA-101 Lifecycle Model. Every project should be based on the "HMI Philosophy and Style Guide" document. The role of the philosophy document is to confirm what good looks like and make sure that the process control operator's desktop has consistent and common HMIs across all platforms and manufacturers' equipment in use (Figure 1).

The ISA-101 Lifecycle Model

The role of the style guide is to interpret the philosophy in the language of a given vendor, such as Honeywell Experion PKS or Emerson Delta V, etc. The style guide will have a lot of commonality with the philosophy, but the style guide will have vendor-specific language on template windows and color selection.

Hence, it is extremely important to the success of a project that the philosophy document addresses many of the problems described in our August column ("What Is High-Performance HMI?"). I have many customers who claim to have a philosophy document for their HMI that is about six pages long.

Our philosophy document spends more than six pages just talking about color, and it's more  than 75 pages and covers:

  • Display design—human factors engineering principles and functional requirements,
  • Display hierarchy,
  • HMI elements,
  • Alarm depiction and alarm management, which is harmonized with the alarm management philosophy document,
  • Guidance on the HMI design process,
  • Purpose and use of  the HMI style guide and toolkit or object library,
  • How to measure HMI performance,
  • Management of change of HMIs,
  • The impact of control rooms on the HMI, and
  • Large-screen display considerations.

It is important that the document is good enough to be the rules for an HMI gap analysis, allowing users to compare their HMI graphics against a detailed guideline with easy-to-use key performance indicators (KPIs).

Having a solid foundation to build on is extremely important, and having an enforced policy that ensures compliance is just as important. Many HMI projects start out with good intentions. However, somewhere along the way, they get derailed, and any coding or design principles get lost in the mix of getting the plant working—and the preference is to do it "my way."

The design process should be guided down the path of getting good requirements capture. This can be done by a variety of techniques, the simplest being a very basic task analysis to the more involved hierarchical task analysis (HTA) promoted by the HMI experts and academia.

This is another important step often left out or substituted by just copying P&IDs and sprinkling live data on top. Doing so leads to information and data being distributed over many pages of graphics, and makes a simple adjustment very complex because of the navigation issues. For an operator to adjust four controllers (A, B, C, & D), he or she currently has the navigation nightmare illustrated in Figure 2.

The task analysis should identify the hierarchical information required for each of the displays from Overview, Unit View, Detail View and Diagnostic View. The philosophy and style guide will dictate how the information, based on priority and importance, will be displayed.

The philosophy also dictates how to take advantage of color, brightness, contrast, salience and line thickness, combined with graphical objects designed to transform data into information. In the past, HMI schematics have relied on operators interpreting lots of data and using high levels of cognition to put the data into context.

One of the tools control systems consultants, Lin & Associates of Phoenix, has developed is an analog gauge that supports easy identification of an abnormal condition (Figure 3). When the process being measured moves outside of predetermined or operator-set operating parameters, the gauge changes to a number, indicating that an operator action is required.

Once the schematic graphics have been developed, tested and reviewed by operations, they should be
enhanced using an iterative process aimed at continuous improvement. Development of the displays is one activity; delivery of the displays to a console with screens is another important activity.

Console development is an important part of the high-performance HMI, and should be based on solid ergonomics and human factors defined in the International Standard ISO 11064. Most modern displays today use large-screen displays for overview information and four other screens for more detailed information, which includes change zones and diagnostic trending.

This change in approach to graphical schematic design has led to improvements in an operator's ability to detect problems before alarms are activated. It is easy to achieve a 50% improvement in operator performance, which can provide significant ROI and move today's operators from a "reactive" operating stance to a "predictive" or "proactive" operating stance.

Operator Response to an Abnormal Condition

When an operator leaves it until the alarm has sounded, often there is a domino effect, and many alarms initiate in sequence. The operator can only manage these alarms sequentially, so we often get a second alarm and a poor response as shown in Figure 4 by trend 1. The best possible response we can get is shown by trend 2. Either way, the more time we spend in this abnormal band, the more costly it is to our operations. It affects quality, equipment reliability, economics and production rates. Obviously, the more cost-effective response is to detect, diagnose and respond before the alarm condition occurs and being proactive by using trends and the analog meter object described earlier.

Alarms, Waiting too long
Waiting Too Long
Figure 4. When an operator does not respond until the alarm sounds, it can
cause a domino effect, initiating additional alarms and increasing costs by
affecting quality, equipment reliability, economics and production rates. 

This is why we call them high-performance graphics; they improve the operator's performance and ultimately the plant performance by avoiding abnormal conditions. 

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