From Kindles to Protecting Tanks: The Different Uses for Electrophoretic Displays

From Kindles to Protecting Tanks: The Different Uses for Electrophoretic Displays

Dr Matthew Roberts - Senior Consultant, Data Exploitation

By: Matthew Roberts
Senior Consultant, Data Exploitation

6th September 2017

Home » Insights » Sensors

Most people have heard of a Kindle or an e-reader: a device that uses an electronic paper display to allow users to read on a paper-like display in bright sunlight. What many people won’t have heard of is the other uses for this technology.

The display technology used in e-readers is usually an electrophoretic display (an ‘EPD’). EPDs work by suspending charged pigments in a fluid contained within a capsule. A voltage can be applied between electrodes on either side of the capsule in order to move the pigment. This configuration can vary, but typically a clear fluid is used with black and white pigments. The white pigments are positively charged and the black pigments are negatively charged. Varying the voltage moves the pigments to control how much of each pigment type is at the visible surface of the microcapsule. The pigments at the visible surface determine how much light is reflected and therefore how white that part of the display looks.

This is a very different approach to producing an image compared to the display technology used in TVs, laptops, and mobile phones which typically use liquid crystal displays (LCDs) or organic light-emitting diode (OLED) displays. LCD and OLED technologies alter the amount of light that is emitted. An EPD, on the other hand, is a reflective technology.

Reflective display technologies don’t need to compete with sunlight in order to be visible outdoors. This is why it is much easier to read an e-reader than a smartphone when in direct sunlight. In addition to this, EPD technology allows text and images to be displayed in such a way that doesn’t require power to maintain the image (you only need power to move the pigments).

The combination of these two properties makes for a very compelling technology for low power displays that can be used both indoors and outdoors. The applications of this technology are more varied than some people might realise. For example, EPD technology has been used in electronic supermarket price labels, indoor signs, bus timetables, bracelets, and watches. There have even been attempts to incorporate EPD display technology into phone cases and shoes.

Plextek has experimented with using the same technology to create an adaptive visual camouflage system for vehicles. We essentially use thin and flexible EPD panels to cover a vehicle with displays that are low power and visible in daylight conditions. To use an emissive display to achieve this would require huge amounts of power (and produce a lot of heat)! It would also need careful control of the brightness to blend in, whereas, the reflective nature of EPDs naturally varies in brightness as lighting conditions change.

Most EPDs create a greyscale image. We have used a colour filter array to convert black and white into shades of green and yellow. It’s a bit like putting colour overhead projector acetate over a piece of paper. The colour gamut that is produced is surprisingly flexible, ranging from light green and cream to dark green and dark brown.

This allows us to display a wide variety of camouflage schemes that are similar to those found on military vehicles. We can even display pictures and text, such as messages relating to humanitarian aid. A scheme can be changed in seconds. The versatility that this provides is very different to the traditional method of repainting a vehicle in order to change the scheme. The new capability that it provides allows schemes to be chosen that work well in one environment rather than finding a compromise for the range of environments that might be encountered.

The possibilities don’t stop there. Colour EPD technology is currently being developed, where more pigment colours are used in each capsule instead of an overlay. This will enable EPDs to cover a much richer colour gamut enabling new applications such as tablet PCs with daylight readable low power screens and large colour billboards that can be updated remotely and consume significantly less power than emissive versions.

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Most people have heard of a Kindle or an e-reader: a device that uses an electronic paper display to allow users to read on a paper-like display in bright sunlight. What many people won’t have heard of is the other uses for this technology.

The display technology used in e-readers is usually an electrophoretic display (an ‘EPD’). EPDs work by suspending charged pigments in a fluid contained within a capsule. A voltage can be applied between electrodes on either side of the capsule in order to move the pigment. This configuration can vary, but typically a clear fluid is used with black and white pigments. The white pigments are positively charged and the black pigments are negatively charged. Varying the voltage moves the pigments to control how much of each pigment type is at the visible surface of the microcapsule. The pigments at the visible surface determine how much light is reflected and therefore how white that part of the display looks.

This is a very different approach to producing an image compared to the display technology used in TVs, laptops, and mobile phones which typically use liquid crystal displays (LCDs) or organic light-emitting diode (OLED) displays. LCD and OLED technologies alter the amount of light that is emitted. An EPD, on the other hand, is a reflective technology.

Reflective display technologies don’t need to compete with sunlight in order to be visible outdoors. This is why it is much easier to read an e-reader than a smartphone when in direct sunlight. In addition to this, EPD technology allows text and images to be displayed in such a way that doesn’t require power to maintain the image (you only need power to move the pigments).

The combination of these two properties makes for a very compelling technology for low power displays that can be used both indoors and outdoors. The applications of this technology are more varied than some people might realise. For example, EPD technology has been used in electronic supermarket price labels, indoor signs, bus timetables, bracelets, and watches. There have even been attempts to incorporate EPD display technology into phone cases and shoes.

Plextek has experimented with using the same technology to create an adaptive visual camouflage system for vehicles. We essentially use thin and flexible EPD panels to cover a vehicle with displays that are low power and visible in daylight conditions. To use an emissive display to achieve this would require huge amounts of power (and produce a lot of heat)! It would also need careful control of the brightness to blend in, whereas, the reflective nature of EPDs naturally varies in brightness as lighting conditions change.

Most EPDs create a greyscale image. We have used a colour filter array to convert black and white into shades of green and yellow. It’s a bit like putting colour overhead projector acetate over a piece of paper. The colour gamut that is produced is surprisingly flexible, ranging from light green and cream to dark green and dark brown.

This allows us to display a wide variety of camouflage schemes that are similar to those found on military vehicles. We can even display pictures and text, such as messages relating to humanitarian aid. A scheme can be changed in seconds. The versatility that this provides is very different to the traditional method of repainting a vehicle in order to change the scheme. The new capability that it provides allows schemes to be chosen that work well in one environment rather than finding a compromise for the range of environments that might be encountered.

The possibilities don’t stop there. Colour EPD technology is currently being developed, where more pigment colours are used in each capsule instead of an overlay. This will enable EPDs to cover a much richer colour gamut enabling new applications such as tablet PCs with daylight readable low power screens and large colour billboards that can be updated remotely and consume significantly less power than emissive versions.

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Further Reading

No longer anything to fear from wireless charging

No longer anything to fear from wireless charging

Loek Janssen - Project Engineer, Sensor Systems

By: Loek Janssen
Project Engineer, Sensor Systems

1st March 2017

Home » Insights » Sensors

Recently, I have been working on a project where we really wanted wireless charging for the device. Being only in the prototype phase, it was considered more a “nice to have” than a complete requirement, with several engineers putting it into the “requires a lot of work” category. However, after doing some research, I decided that the standards (such as Qi and PMA) were developed enough, with excellent support, to give it a try.

So how does it work?
Most wireless charging is based around the idea of inductive charging, i.e. inducing a current using a magnetic field. In many ways, the idea is similar to that used in transformer, current in one coil, induces current in a second coil. However, transformers share the same magnetic core and when the two coils are separated by air the method is horrendously inefficient with most of the power wasted instead.

When the two coils are, in combination with capacitors, used to form resonant circuits, the efficiency over air increases dramatically. The range is only a few centimetres but reasonable amounts of power can be safely and easily transferred.

As only alternating signals can be used in the inductive coupling, any receiver then needs to rectify the signal, filter and provide the output to power the system or charge a local battery. A control loop usually exists in the receiver to limit the overall current drawn. Additionally, the transmitter and receiver can also communicate by gently modulating the signal, while only a small amount of data can be transferred; it is more than enough to allow basic control messages between the two devices.

wireless

Standards
Several standards now exist using this idea of inductive charging, using different frequencies, voltages and modulations for power transfer and communication. While none have yet won out, multiple mature ICs, which handle much of the heavy lifting of the receiver (and transmitter) side, happily exist for the QI, PMA and Airfuel standards. The Qi standard, as an example, uses a frequency of 100-200 kHz and ASK (amplitude shift key) modulation for communication.

Getting it into a product
While the datasheets require some maths to determine the correct component values, it is fairly straightforward, and once a suitable charging coil has been chosen, I quickly got around to prototyping the design. With the right evaluation board, I was able to get a nice receiver working over short distances, allowing me to test various coils to see which would work best through different plastic materials. I had chosen a QI receiver IC, as the QI protocol seemed very mature, and looking ahead to the future, plans were in place for extensive improvements from the recently released V1.2 standard.

Now the prototyping was done, I designed the IC, components and charging coil into our system and once the final PCB was back, tested the wireless charging system. As expected, power could be drawn through the system, with the IC internally controlling the voltage to produce a nice 5V DC signal to power to the system.

Despite not having any personal experience in the area, the wealth of information and useful components allowed me to quickly prototype and design a circuit with wireless charging. It is a seriously useful component of the system, allowing devices to be sealed and protected, while still being easy to recharge.

Regularly, people are surprised by the addition of successful wireless charging to the system but I think this is a hangover from the past; when implementing such a design was difficult, complex and required a lot of effort to get working. Now we have the addition of excellent standards and mature ICs in mass production, it is definitely time to stop being afraid of wireless charging.

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Not every snake is the same

Not every snake is the same…

Glenn Wilkinson - Senior Consultant, Sensor Systems

By: Glenn Wilkinson
Senior Consultant, Sensor Systems

15th February 2017

Home » Insights » Sensors

They live behind cupboards, skulk under the bed, lurk at the back of desks and hide in car glove boxes. They lay in wait across the world. Coiled and ready to unleash their potentially deadly power in an instant. And we wouldn’t have it any other way.

USBPhoneThe now ubiquitous USB charger lead has increasingly littered our lives for nigh on 10 years now. Adopted by portable device manufacturers near unanimously as soon as their handheld marvels had a need for a power and wired data connection. So given how common the USB charger is – it’s surprisingly misunderstood.

Stay with me now – it might stop you getting bitten…

Start looking into the subject and you’ll find forums, reviews and help boards littered with common complaints, comments and questions (sad face emoji’s removed to avoid repetition);

“I bought a 2 amp charger, but my phone doesn’t charge any faster.”
“I left it connected overnight and it was still flat in the morning.”
“Will this phone charger work on my tablet?”
“My Galaxy charges faster on my PC than with this supply.”
“I get a ‘charger unrecognised’ message when I connect it.”

So what’s going on?

First off, your charger doesn’t determine how much power it is going to supply – your device decides what it is going to consume. It’s a self-protection mechanism. The batteries you are charging need a limited charge current to prevent overheating and maximise cell lifetime, so your device contains a charge circuit which implements this limit. Go and buy the beefiest charger you want, but it will only charge your handheld quicker if the device allows it.

So if the charging smarts are in your device, why can it perform differently with some chargers, and not at all with others, even though they have sufficient power capability?

We could start digging into history here, pointing fingers and quoting standards – but let’s not go there. Simplistically put; somewhere between standards being insufficient or optional, and manufacturers implementing in a manner to suit their own needs, we end up with the ‘mish-mash’ of solutions we have now amalgamated over time.

USBslotsStrictly a USB-compliant host port can support a number of power modes (USB2.0 high/low, BC, PD for those curious). The host USB port and device need to indicate to each other across the USB data lines and agree the most suitable mode to operate in.

Some devices ignore any form of convention completely and just draw a fixed current. This is relatively common in legacy smartphones and is usually around 500mA, not unintentionally the same as the original USB2.0 high power spec.

Others create their own hybrid language solutions, predominantly by bias voltages on data lines to indicate whether faster charge modes beyond USB low or high power current limits can be used.

It has to be said that things are getting better with the widespread adoption of the USB battery charging spec (BC1.2), and particularly the simplicity of the direct charge port (DCP). But the delay in it being published means there are still a huge number of devices out there with their own niche charging characteristics and languages.

Why am I telling you all this? Because it’s another one of those hidden little hurdles that us engineers have to overcome. Plextek recently created an award winning commercial device featuring USB charging ports. The brief required those ports to support charging of any common smart device at a rate equal to the manufacturer’s charger. This means speaking every language, and knowing which to use at any particular time.

Of course, the main fun of this is in the testing. After a good day swamped with more tech than a Gadget Show giveaway, and kind co-operation of a very nervous man at PC World (after some negotiation) – job done, a truly multilingual solution.

So hopefully you may have gained some small appreciation of the engineering behind something the vast majority of people take for granted – and there may just be fewer sad faces on those internet message boards.

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Sensor Systems – Getting to Market… Quickly!

Chris Roff - Head of Smart Sensors

By: Chris Roff
Head of Smart Sensors

1st February 2017

Home » Insights » Sensors

As an engineer, when somebody comes to see you with an exciting technical idea, you are always keen to listen. But, when the first meeting is on Halloween, and the business case requires complex prototypes deployed before Christmas you’re allowed to worry that it could be a little scary! But, with careful planning, lots of experience and a team who know what’s what, it’s possible to deliver without getting spooked.

This particular project was in the smart sensors area and required an array of about 20 different environmental sensors (some of which needed creation from scratch), a pair of backhaul radio options (cellular and ISM) as well as a secure backend interface. Security was extremely important and the device had to operate in a harsh environment. Oh – and it all had to run from a battery with tough lifetime specs and sit inside a unique custom enclosure…

We responded to this design and timescale challenge by bringing the customer’s technology leads into our offices for a week-long workshop. The week’s agenda started at high-level aims and finished with electronic schematics being sketched out – progress!

The next phase of the programme saw high-risk sensors being prototyped in the laboratory whilst the electronic schematics were being fine-tuned. Meanwhile Printed Circuit Board (PCB) layout engineers were creating component footprints and doing everything possible to be ready to route the schematic as soon as it was released. Having a good view of the components that exist in the sensor market is vital to allow rapid down-selection of appropriate devices (from the often intimidating range of options).

At the same time, software engineers were devising the message protocol for the radio communications and working on low-level drivers for the chosen components. Embedded device software suitable for battery powered sensor systems requires working with limited memory space and real pressure to control current consumption.

Another parallel effort was looking at the mechanics, designing enclosures, IP68 seals, cable assemblies and the like. Modern 3D printing and soft tooling facilities allow large complex designs to be realised within even the most aggressive schedules. And watching the machines in action is a sight to behold.

Once the electronic hardware is nearly designed in CAD, the supply chain must jump into action, securing components “kitting up” and arranging access to pick and place assembly lines. Supply chain guys are an engineer’s best friend when time is tight – they always manage to locate that rare component just when you think the lead time is unworkable.

After assembly, when the product returns from the factory, it can be a tense time. Everyone is proud to see their hard work made real, but equally tense that one mistake overlooked during design could render the boards useless. Usually there’s a deep sigh of relief when the first LEDs flash and no smoke appears!

The next stage is to bring the board up, test hardware functionalities and software control. Typically a plan for these activities is made beforehand and a burndown list is worked through to ensure nothing is missed. Ticking off working functionalities is very satisfying for engineers but the thrill is perhaps greatest for the project managers!

Inevitably along the way some modifications will be required when bits of circuitry don’t work quite as they did in the simulator or on paper. The trend for smaller and smaller electronic components means that this work requires sharp eyes and a steady hand with soldering irons or hot air guns. When you have a deadline, having technicians or “wire men” with nerves of steel on your side is invaluable. Having patient ones is a bonus too!

Once the hardware is functioning as desired and the software has been developed to beta level you can begin system tests to verify that the product will do what it needs to in the field. The key here is to plan a suite of test conditions that cover both normal operation and all likely corner cases. At this stage modifications may again be required to arrive at the stable end product.

Once the design passes test it’s ready for showtime. Final assembly, shipping and installation of prototype units before Christmas? No problem for this team! Now there’s just time to monitor that sensor data over another portion of Christmas pudding…

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