1. Less is more. It's important to keep the HMI simple and with the operator in mind. It's best when it's self-explanatory and easily understood. Also, try to make the pages similar and follow the same page layout throughout. Avoid making the display too technical. It's normal for engineers to try to give the customer everything, but with HMI, less really is more.
2. Right-size displays. Don't try to save money by selecting an HMI display screen that's too small. It's also important not to cram too much information onto a screen. Size the display according to the amount of information that is most important for the operator to see. Always discuss requirements with the equipment's operators well ahead of time, not just with their managers. Operators usually have different needs and the success of your system depends on their usage.
3. Design tips. A good design requires careful use of layout, color and content. If you get it wrong, your operator misses an indication, you lose money, or worse, someone is injured. The 'bad' screen is less than satisfactory: the layout is poor, the plant representation isn't logical and the screen layout makes it difficult to locate the data. Poor selection of colors, excessive use of capitals in a serif font and repetitive use of units with all data values, makes this a really difficult screen to read -- especially at a glance or from a distance. Avoid colors that could create problems for people with color blindness. Minimize the use of colors to allow actual device state and alarms to stand out. For alarming, choose colors that contrast with the normal process view so the operator will notice the change.
4. Plant review forum. Hold a design review with a group of plant personnel to discuss any status notifications, events, alerts and alarms that need to be programmed, both from the perspective of an audio-visual action and an operations response. Step through the intended functional system, once as the designer, once as the user and then invite at least two levels of users who will be interfacing with the HMI. Doing this prior to specifying equipment helps to identify the features that users will want in the HMI station. It also avoids surprises at point of commissioning.
5. Location, location, location. Real estate can be prime in a busy production area. Locate the HMI in a practical place, out of heavy traffic areas but accessible. Be aware of near-future projects in the area. Guard the HMI location so others don't park or configure something else on top of the station.
6. Back-up work periodically. Back-ups are especially before implementing upgrades or changes. Software such as Norton's Ghost Image can be invaluable to support and maintain HMI systems.
7. Visualize the process. HMI graphics should illustrate the production process in the plant to provide better visualization to the operators, giving them a sense of the action that's required. The use of hardware that meets minimum requirements and provides keeps the number of failure points low and assures high availability of the system.
8. Only essential data. Make control and monitoring of the process simpler by selecting only the most essential information from the process database for the historian. This will reduce the load on the system and keep it from stalling or failing. Don't forget the need for maintenance and make sure you schedule periodic backups.
9. Think about flow. It is essential to have a clear design approach to the HMI. Decide how the display blocks naturally flow and how sections need to be grouped together for the operator. Do not blindly follow P&I diagrams. The S88 functional hierarchy is a good place to start. Make paper-based designs to get a feel for screens, navigation and other requirements, and review with client prior to designing and making electronic screens.
10. Alarm strategy. Alarming needs to have a well-articulated strategy. Alarms must be used for conditions that require intervention and must have a clear corrective action associated with each one. Anything else should not be an alarm.
The key aspect of an HMI display is the dynamic plant data. There are two basic types of dynamic data: Alarms and normal Plant Status data.
1. Alarm status for the overall plant, preferably organized into groups, should be visible on every screen and there should be a simple navigation route to access the screen containing additional details about the alarm.
2. The alarm colors should follow the safety convention: Red = stop, prohibition, danger; Yellow = caution, risk of danger; Green = safe condition; Blue = mandatory action. As one in 12 men have some form of color-blindness, which can affect the perception of red and green, color cannot be used as the sole indicator of alarms.
3. Any color change must be supplemented with a pictorial change and, if the alarm is critical, an audible alert. Pictorial changes could include shape changing, a change in the position of an indicator, having additional text or objects appear on an alarm. Flashing of alarms that have not been accepted is very irritating and stressful and should be avoided.
4. Similarly, automatic changing of displays should be avoided - to have a page disappear when you are working on it is irritating and, in extreme cases, a cascade of alarms can produce so many screen refreshes, an operator can be locked out of the system.
5. Audible alerts can be very useful, especially if the system is able to create multiple tones and pitches. These tones can be used to transmit the importance of an alarm. Research has shown that a high-pitched, fast-pulsing sound automatically conveys urgency; a lower pitched, slow-pulsing sound is less urgent.
6. Whatever convention is used, alarms should be placed where they can be easily seen, such as along the top of the screen.
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