How Much Information Is Enough?

Our control panels include five panel meters that receive from the PLC and display analog parameters like machine speed and temperature to machine operators. Our panel meter manufacturer is going out of business, and we’re wondering if we should just switch to another manufacturer or go to a solid-state graphics display. We know the graphics display could show lots of information besides just five meter readings, but we’re concerned about up-front hardware costs and about controlling software development costs. What's the best way to go?

—from August ’08 Control Design

ANSWERS

It’s All About the Operator

Solid-state graphic display costs have dropped considerably in recent years. The competitive HMI market, coupled with Asian manufacturing, has made it possible to get small LCD micro HMI panels for less than $200, which is less than many panel meters currently on the market. Some suppliers also offer free programming software to reduce development costs. Most of the micro HMI graphic panels support hundreds of screens and also display dynamic alphanumeric text as well as graphical objects such as meters, bar graphs and alarming. You also could consider using objects that allow operators to adjust set points for connected devices that control variables such as temperatures, speed, timing and flow rates. This most often is done using software objects such as a numeric entry keypad or slider objects, greatly expanding the amount of data displayed and the operator’s ability to control the process. If you can improve productivity by allowing more data to be displayed and provide better control for the operators, then a micro LCD HMI panel might be worth using. It is possible to replace your panel meters with lower up-front cost and development time than expected.

Which reading best describes the forthcoming problem? Source: Yokogawa

Consider whether the HMI you choose meets your application’s requirement for environmental standards and agency approvals. Most panel meters meet NEMA 4X (IP65) requirements for wash-down applications and have UL, CE or similar agency approvals. You should be aware that, even in the industrial automation market, there are some HMI graphical displays that do not meet these standards, so you should review the manufacturer’s specifications carefully. You also need to make sure the HMI graphical panel supports the communication protocol used by the PLC controller being used. Most HMI panels support multiple serial protocols and can be configured easily with the programming software.

Choose how the operator will interface with the application. Many HMI panels allow the operator to press a built-in function key based on what is being displayed on the screen. This can be confusing to the operator if there are several objects on the screen at one time. You might consider an HMI panel that has a touchscreen overlay, allowing the operator to interface with the application by simply touching the object. Touchscreens tend to be more intuitive than function keys. Operators quickly see what needs to be done and instinctively touch the screen to initiate an action.

Since the HMI graphical panels require software configuration, make sure the supplier has a technical support group that provides help in dealing with the learning curve your programmers may have. Also, make sure the supplier stocks the product you choose and carries a solid warranty.

Greg Philbrook
HMI product manager
AutomationDirect

Digital Vs. Analog

It is important to consider what the return on your investment will be and what you require from the instrument. Humans experience the world analogically. Vision, for example, is an analog experience. So, when metering is required, do not eliminate the analog option, but consider the actual application.

We live in a digital society. Decisions often are made to install a digital solution without giving the analog solution consideration because we assume that the digital meter to be more advanced. While the analog readout is harder to read at a glance, it still holds a significant place in an instrument panel. For almost 50 years, generator builders, welder builders, switchgear manufacturers and automobile and aviation groups have argued the differences. In most instances, they side with analog displays, augmented by a digital readout of numerical information. The absolute accuracy of an instrument depends primarily on the transducer and circuitry that measures the physical quantity at interest and not on the display. Also, consider the purpose of the information supplied to the user. Sometimes, a display can be useful primarily as status indicators, simply to tell at a glance if the system is in proper range, or even approaching an undesirable condition. For this purpose, analog instruments are far better than digital.

Keep in mind that the purpose of an instrument is to let you build an image of the world from an abstract output. People are best when dealing with pictorial information. Numerical outputs have to be mentally translated into appropriate visual representation.

As an exercise, try describing a spiral staircase in numerical form or without using your hands. Wouldn’t a simple graphic display be easier?