LED ergonomic displays for Industrial measurement and process control
Few would argue that LEDs today are ubiquitous devices. For many years, however, LEDs were little more than indicator lights. They were predominantly red, although green and yellow versions later became popular, enabling a stop, go, and caution meaning in their use. While LEDs can compete with neon indicators in main powered equipment, it was almost certainly their ability to operate at lower voltages that facilitated their widespread adoption in a multitude of electronic systems, from professional equipment through to consumer products. LEDs were also low-power devices, so they were also favored in battery-powered designs.
Single LEDs were essentially status or threshold indicators, i.e. on/off or “low battery”, but their ease of use quickly expanded their use into a variety of applications. Multiple LEDs were arranged to provide displays mimicking analog meters or thermometer-style gauges, such as volume or recording level indicators in consumer audio equipment. In some instances, there was a novelty aspect to such designs as the coarse granularity of their measurement hardly compares with the meters or gauges they replace. Where they often win out is in providing a bold, bright, easy-to-read display when absolute accuracy or greater resolution is not required. Equally, the intrinsically rugged nature of solid-state LED technology suited portable applications, where delicate analog meters could be easily damaged and therefore prove unreliable.
The use of multiple LEDs, as described above, rapidly led to the development of specific LED bar-graph displays with a number of LEDs encapsulated in a single, dual-in-line package. Apart from displaying pseudo-analog level information, these were often used as ‘progress’ indicators in process control systems. From here it was a short step to arranging LEDs in a package to create 7-segment numeric and 16-segment alphanumeric character readout displays. Prior to this, electronic character displays used ‘Nixie’ tubes, but these were typically limited to early digital instruments such as voltmeters and frequency counters, and certain public display applications in clocks, elevators, and departure boards.
Consequently, this simple evolution in LED display technology, coupled with the digital revolution that was happening in late 1970s, saw an explosion in the use of LED character displays and the demise of ‘Nixie’ tube applications. More significantly, it spawned many new applications, especially in consumer products such as LED digital watches and calculators and other products such as microwave ovens and video recorders. Clearly, size and cost were also factors in this booming use of LEDs, enabling designs that were not previously practical or cost-effective.
Since then, we have seen LED displays superseded by newer, alternative technologies in a number of areas and for a variety of reasons. Liquid crystal displays (LCDs) were quick to replace LEDs in watches because low power, reflective TN LCDs meant the display was always visible without pressing a button. Similar, “always-on” battery-powered devices also adopted LCDs, while mains-powered equipment typically favored vacuum fluorescent displays (VFDs) that offered bright, multi-character displays in various colors, which were attractive and more readable than the monochrome LCD technology of the time.
Technology does not stand still, however, and LEDs, LCDs, and VFDs have all improved markedly in the last three decades. Each technology has its pros and cons for different applications, but these also depend on the target market for its potential volume. At one end of the spectrum, color LCD panels (albeit increasingly with LED backlighting) dominate the television and computer display market. Conversely, for simple indicators, LEDs remain king. In between, the choice for multi-segment bar, character, or other graphical display is more complex. Without a doubt, VFDs offer a highly customizable solution that can incorporate alphanumeric characters with other symbols, indicators, and bar-graph style readouts. But such dedicated customization is typically only warranted for high volume applications such as automobile dashboards or consumer appliances. For lower volumes, where bright, highly visible, easy-to-read displays are required, the design flexibility offered by LEDs is unbeatable, as we will see in the industrial data acquisition systems considered below.
Why choose LEDs for industrial measurement and process control displays?
Industrial measurement and process control systems come in all shapes and sizes. Some are highly computerized, with data captured remotely for display on a regular computer screen, or perhaps using a dedicated program that also provides a control interface. Other systems, such as that shown in Figure 1, may be more traditional, featuring dedicated indicators, displays, and controls.