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Many of us are familiar with e-paper readers such as the Amazon Kindle and the low power daylight-readable screens they offer. As is often the case, what starts out as a premium technology will often find its way into other applications as it matures and gets cheaper.
A major development for e-paper has been electronic supermarket price labels, allowing the dynamic labelling of products. This is an extremely cost-conscious but high volume area. E-paper price labels are connected to the shop’s main computer, making re-labelling goods far less time consuming, and on-the-spot promotional pricing easy because the till knows the correct price straight away. The displays used are small and extremely thin, which Lascar realised made them ideal for use as instrumentation displays; combine this with the large range of low-cost 32-bit processors available, such as the ARM M0, and the traditional panel meter could be replaced by one with full-graphics capability for around the same price.
What is e-paper?
E-paper makes use of electrophoretic ink, which consists of many tiny microcapsules, each with a set of black and white ink particles. The particles carry a charge, with black positive and white negative. Applying an electric field to a specific region (i.e. pixel) of the display will make either the black or white particles rise to the top, depending on the field’s direction, giving the appearance required for that pixel.
Whilst energy is needed to move the particles, once the electric field has been turned off they remain in place. This phenomenon can be exploited by instrument manufacturers to create a low power full-graphics display. Traditional segment LCD displays are very low power, and that’s why the battery in an LCD watch or clock lasts such a long time. However, as soon as you move up to full-graphic pixel-based displays, the power consumption rises significantly. When it comes to instrument displays, the display does not need to be updated nearly as fast as for other graphics applications such as video. This means for an instrument, e-paper display power usage can be kept down to close to that of a traditional LCD panel meter, yet you still get a full-graphics, high-contrast and sunlight readable display. Another benefit of e-paper is that the display panel is thinner than all LCD technologies, being only around 1mm (0.025”) thick compared with traditional LCD glass panels that typically have a thickness of 2 to 3mm.
What is the benefit of full-graphic e-paper for use in instrumentation?
There is more to an instrument reading than just a number, there are units and the name of a process or condition to which the reading refers. Often, a simple numerical readout may not be the optimal way to communicate the process state to the reader; for instance, in the case of showing battery charge, a bar graph might be better. For other types of reading, e.g. the level of a fuel tank, other graphical representations such as a moving-needle output might be best. You may also want to show further information on your display such as alarm conditions and warnings. The full-graphics capability of e-paper makes all of this possible, with the further advantage that when the power is turned off, the information is still displayed.
Ease of use
If a display technology offers plenty of possibilities, it should also be easy to setup. For its SGD 21-B voltmeter display, Lascar provides a free PanelPilot B setup application, which makes configuration straightforward; the user can select from a range of meter styles and then sets up their scaling to suit their application.
The use of a microcontroller in the SGD 21-B instead of a panel meter ASIC opens up new possibilities. It means different applications for use on the meter can be selected via USB. The meter can be calibrated digitally instead of using a potentiometer, so it can be done without physical access to the meter and minimises the problems of ageing and temperature drift associated with potentiometers. Calibration is not just about trimming a reading though, the processor can be employed to linearise sensor curves such as those with thermocouples or thermistors. Offsets can be applied, when reading 4-20mA loop outputs for example.
Connecting to the outside world
A microcontroller offers many input and output pins, which can be used for control and indication. For example, the user might want to set an output when a given level is exceeded (a high alarm condition) or receded from (a low alarm condition). Industry-standard interfaces like SPI allow other devices to be connected to the meter and not just analogue signals. Furthermore, text for labelling or information can be uploaded to the meter via USB.
During the development of the SGD -B series meters it became apparent that having a display that retained its display information after the power was turned off could lead to a confusing if not dangerous condition. Imagine the meter was being used to measure a voltage of a process which could be at mains potential (200V+). If the power was removed from the meter when the process was at a low potential, and then later the process voltage returned to a high level, the user might think it was safe because the meter would still show the low reading. To prevent this, a method had to be developed to show when the meter was powered down and the reading was invalid. To change an e-paper display, power is required, so the solution was to use a super capacitor (also known as an ultra-capacitor or dual-layer capacitor) which could store enough energy to enable the meter to change its display to a “No Power” symbol when its power supply voltage was dropping below a certain level. Although the reading will no longer be relevant, any alarm conditions at the point of power failure might still be.
E-paper cannot readily be made translucent or transmissive, so a backlit display is not possible, and a frontlit solution is needed; this is in the form of a very thin layer of acrylic, side-lit by an array of white LEDs. Front lighting retains all the advantages of the e-paper display, i.e. the high contrast and sunlight readability. Whilst the SGD 21-B does not have front lighting, it may be available on a future product version.
Tri-colour e-paper displays are now available. These consist of black and white particles, plus particles of a third colour, often yellow or red. Full colour e-paper is already being demonstrated although the price and size are prohibitive for small instrumentation, but as can be expected with technological developments, that is sure to change.
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