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LED ergonomic displays for industrial apps

14 Mar 2013  | Digi-Key Corp.

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Having established the need for a measurement and control system using a dedicated interface, the design process then needs to look at what data need be displayed, how the process is to be controlled, and what feedback is needed to confirm control operation and status.

Control status indicators
Controls can take the form of switches, dials, and push buttons, and may also operate directly on the circuit or signal concerned or indirectly where a low-voltage control system operates a relay that switches on the power to a pump or motor. Direct controls are electro-mechanical devices, ranging from simple on-off switches through multi-position rotary or slider switches, and include dial or slider potentiometers. The nature of many of these devices means that their status may be apparent from their position, as is often the case with a toggle switch. This is fine for a prominently positioned switch in a well-lit location, but for many applications, the provision of a status indicator is generally expected. With indirect controls, an indicator becomes desirable, especially if the requirement is to show the actual status of a remote device. Indeed this requirement is more akin to simple data monitoring and little different to displaying a threshold measurement such as 'low battery'.

The characteristics on an indicator will of course depend on the application and the environment where it is deployed. The layout of the control panel is also vitally important to ensure the indicator is correctly associated with the right control. This may be achieved by simple physical proximity with the indicator immediately adjacent to the switch or even built into it. In other cases, it may be that the indicators need to be logically arranged and provided with clear labels, as is the case with the laminated membrane-switch type shown in the figure. As the purpose of an indicator is to be seen, design considerations must include ambient lighting levels, and the distance from which it will be viewed. Size, colour, brightness and even shape are some of the selection criteria with most requirements being readily met by the wide variety of LEDs available today.

A number of manufacturers provide LED options. Lumex Inc., for example, has a range of styles that embraces most of the types found in measurement and process control equipment considered here. The first one, the SSI-LXH312ID, is a 3 mm diameter, high-intensity, red LED in a black plastic-moulded case with bezel, suitable for push-fit mounting in 4.32 mm diameter panel cutout. This is perhaps the most common style of LED, and is familiar to almost every designer. Identical in style, dimensions and specification, but offered with 6" flying leads rather than wire terminals, is the SSI-LXH312ID-150. Round LEDs are very traditional, but the SSI-LXH072YD is an example of a 7 mm x 2.3 mm rectangular-format yellow LED that fits into a 11 mm x 5.3 mm panel cutout. While the first three panel indicators shown are designed to push-fit into the front of panels, they can equally well be fitted to a sub-panel located behind a laminated or glazed front panel. However the last style shown, the SSI-LXR1612GD, is a 3 mm diameter green LED that is clearly intended for external use as it features an attractive chromed-finish bezel. It has a metal threaded body that fits into a 6.2 mm diameter panel cutout and is retained by a nut.
Data readouts using bar and character indicators
Moving from simple threshold measurements, some data requires displays that are more complex. Multi-segment LEDs can readily address this requirement, either with bar or character displays. In this case, Lite-On Inc.'s LTA-1000HR 10-segment, red, bar graph display is typical of many such products. These devices can be stacked end-to-end for longer displays, or arrayed in two dimensions, if required, to create a more complex display, such as a 2-D bar chart. Bar graphs displays measured data in a pseudo-analogue fashion, but are also used to display a control setting. For example, two bar graph displays might be arranged side by side, with one showing the target setting and the other showing the actual reading. An interesting variation on the bar-graph type LED is a curved display made by Lumex Inc., the SSA-LXB10SRW-10652, which can be stacked end-to-end and can mimic a dial-type meter.

Character displays are typically used for digital meters to present measured data such as voltage, current, temperature or pressure. The number of digits will depend on the application. Given purely numeric data, a 2-character, 7-segment display such as the HDSP-5623 from Avago Technologies, shown in Figure 4, can provide the appropriate solution. These displays can also be stacked side-by-side if four or more characters are required. This device features 14.2 mm high characters formed using green LEDs, but red and yellow versions are also available. This size character is great for viewing from a Cdistance or when a bold display is required. For some instruments, the smaller, 3.8 mm high characters found in Avago's HCMS-2902 4-character dot matrix display may be more appropriate. Being a dot-matrix type, this display can represent standard alpha-numeric characters, so may have applications where text information needs to be presented, e.g. simple messages like operator prompts or error codes.

Ready to go display and control solutions
Finally, not every measurement and process control application requires a custom designed display and control interface. Alternative, off-the-shelf solutions, such as that by Matrix Orbital are designed to decrease development time by integrating simple input and display interfaces into a robust external enclosure. The ELK series enclosure display comes in four versions, each combining the advantages of 20-character, 4-line LCDs with a 7-key tactile keypad and three tri-colour LED indicators. All use LED backlighting for the LCD display, with different colour options to ensure optimum readability in the target application environment. These enclosure display units have a threaded camera mount on the back and an attached USB cable, so they can be easily tied into any application and placed in the optimum location for ease of viewing and control operation.

LEDs satisfy a number of display requirements across a range of industrial data acquisition and measurement systems. Ranging from simple threshold indicators or readouts of temperature or pressure, to more sophisticated gauges and bar-graph type displays, there are many LED products to choose from. LED considerations include size, colour, brightness and other parameters that contribute to an effective and ergonomic solution in these types of application. The article represents a roundup of specific LED products for use in industrial settings, available from a variety of manufacturers, both basic components for original designs (Lumex, Lite-On, and Avago) and a ready-to-go display solution from Matrix Orbital.

About the author
Digi-Key Corp. contributed this article.

To download the PDF version of this article, click here.

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