Human machine interface (HMI) design: It takes more than just pretty pictures
In order to insure successful implantation, effective HMI design should start with the same proper planning as any other project. Just making an HMI look like the real think is not always the best answer. HMIs are present in a great deal of applications, which leads to multiple lessons that should be taken into consideration when starting a new design project. Often times, designers overlook what they could have learned from past designs (and even mistakes) until it’s too late. It is imperative that accidents from the past are used to insure success in the future. Arming yourself with simple rules from lessons learned on the how to communicate to operations is extremely important to the success of your project.
Remember the need of the audience
The primary HMI points are not for engineering, maintenance, or management. It is for the main line of control the operator. Evaluate what would should be showing to operations. Not that these other groups do not deserve to have a part of the HMI, they should be secondary to the main need and that this operator. Operations will spend most all their time watching the same things over and over again. So items such as eye strain are a large part to design consideration. Alerts to changes should stand out, process holdups should be able to be evaluated quickly and all systems that are same should look that way. Control functions should be standard and the same across the board. The secondary group will probably want to see a different layer of the system. For management it will be high level overviews and production graphs and data. For maintenance they are going to want to have some help points to navigate into the PLC or DCS to make troubleshooting issues easier.
Screen layout is important for good HMI interaction. Screens should have set layers, as too many drill downs will lead to distraction. One good method for design is an overview that does not have a lot of process detail, but more red light green light indication for the areas that are built underneath. From the overview within one click the secondary main layer of detail should be shown. This would include device control.
A third layer maybe used beyond this that would have additional piping detail and process representation but should be done sparingly. Device information is also great to have and be accessible from this third click level. Remember operators will do this every day and know the equipment. What one thinks is important when doing the initial design because everything is new to everyone will change with exposure. This will help keep the screens clean which becomes imperative to bring attention to anomalies in the process. Single quick navigation should be used when possible. Menu bars and active alarm banners help keep the process within one click. Also, with screen layout screen pop-ups need to be kept to a minimum. Pop-ups during a critical situation will take focus away from the operator.
Keep it simple and boring, in this case boring means effective. It is all about being able to show the anomaly in the process. Background colors should be kept to a gray on gray scale is possible. Primary colors such as red, green, or blue should not be used as a background color. Black and white do show a great contrast, but it will be too much for long periods of monitoring. Gray on gray is very effective for both eye fatigue and contrast. Three dimensional graphics where fun at one point with cutaways and moving parts but these are all distractions.
Pick a format and stick with it. In most cases: Red – Stop, Green – Good, Yellow- Warning, Blue – Mandatory Operation are the core colors to use. Pipe color can also be added sometimes but should be left to the overview with active representation of the process.
Displaying data or setpoints
When displaying data or setpoints is a good rule of thumb to show what the range of the variable should be expected. The data value when evaluated can easily be checked against the normal range without having to look up or remember what is acceptable. Detailed device information that is shown on the third level of display may include drawing references or PLC information. Other forms of effective data representation can be graphical in nature with health gauges (similar to a car speedometer) that go back to the green is good red is bad style representation. The HMI should always be about the ease of access to the process.
Looking towards the future
Just as phones have become main stream there are plants that are starting to lean to remote HMI view only access for these devices as well. Tablets and phones are mobile and effective in the field. These applications will take a change in the approach to the current model just as web pages first started out they had to adapt for the phone user. So keeping the page layout and data informative with effective layers will make the transition an easier step as they are the building blocks to effective representation.
The topic of alarms was left on purpose as that demands a topic all its own. Alarms are the first line of defense and effective alarm management is even more critical. Just remember, HMI design is creative and critical to effective process management.
This post was written by Dave Cortivo. Dave is a senior engineer at MAVERICK Technologies, a leading automation solutions provider offering industrial automation, strategic manufacturing, and enterprise integration services for the process industries. MAVERICK delivers expertise and consulting in a wide variety of areas including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, business process optimization and more.
MAVERICK Technologies is a CSIA member as of 3/20/2015