26 Jan 2017

E-paper: displays for low-power IoT devices

Scott Soong, Pervasive Displays looks at how E-paper can provide displays for battery-powered or batteryless devices for Internet of Things and other applications.

E-paper displays have a lot to offer, especially for applications like IoT where power consumption is key. In fact, wherever you look these days, you’ll see a display. From our phone and watch screens to our UHD TVs, we’ve quickly become accustomed to devices having vivid and detailed displays. And while these displays are great for user experience, they’re notoriously power-hungry. Think how often you need to charge a modern smartphone, compared to an early 2000s feature phone: one of the biggest contributors is the screen.

In the Internet of Things, IoT, world, there are lots of situations where it would be beneficial for the connected device to have a screen providing direct user feedback, without having to send data via the cloud to another device. A smart energy meter or temperature sensor could show the current reading, for example. Moreover, a built-in display enables designers to offer additional features and value in a range of applications, including retail shelf labels.

But given that many IoT devices have extremely limited power budgets, TFT LCDs aren’t suitable. Instead, bistable technologies such as e-paper provide the ideal combination of screen characteristics and energy-efficiency for IoT use. Let’s look at how bi-stable displays like E-paper displays work.

Why built-in displays can add value to IoT devices

The key difference between many consumer electronics devices and IoT equipment is that the former are designed to be user-facing and offer rich user experiences, while the latter are typically cloud-facing, their sole purpose being to record and transmit data.

Usually optimized for a specific use case, IoT kit must often be small, mobile and battery-powered. And to achieve the longest-possible battery life, many IoT device designers avoid TFT displays. In solely machine-to-machine situations, this lack of an onboard display is no big deal. But in the growing number of IoT applications where at least some human interaction is involved, there are numerous benefits to be had from a built-in display.

We’ve already touched on the value of smart meters and other sensors displaying their current reading. Another example would be personal glucose monitors showing the current blood sugar level, meaning the wearer doesn’t need to open an app on their smartphone to check. Or imagine RFID logistics tags that display shipping information, without needing additional equipment to scan and extract the data.

Introducing very low-power bistable displays: e-paper

‘Bistable’ displays – particularly e-paper – are the perfect match for the type of state information IoT devices work with. This type of screen only uses power when you change what it’s displaying. Once it’s there, it remains visible, without consuming further power. Moreover, unlike TFT LCD screens, e-paper is reflective, so doesn’t require a backlight.

Traditional Liquid Crystal Display

Fig 1. TFT LCDs produce a visible image by shining light through a layer of liquid crystal. (Source: Pervasive Displays)

How are bistable displays including e-paper displays different from TFT LCDs? Simply put, TFT LCDs create a visible image by passing light from a backlight through a layer of liquid crystal. An e-paper display, on the other hand, is made up of millions of tiny capsules, each containing positively charged white particles and negatively charged black ones, suspended in clear fluid. When you apply a charge to the top and bottom of each capsule in the e-paper display, the ink particles rearrange accordingly, meaning you can create an image by applying appropriate charges across the full display. Ambient light reflects off the image, meaning you don’t need a backlight.

E-paper display

Fig 2. E-paper displays create the image by applying appropriate charge across a matrix of tiny capsules, each containing black and white ink particles. Unlike TFT LCDs, there’s no need for a backlight. (Source: E Ink)

Given that the main consumer of power in a TFT LCD is the backlight, the fact you don’t need one with e-paper means this technology instantly requires significantly less energy. Instead, screen refresh rate becomes the primary power consumer, and here again, e-paper is much more energy-efficient, thanks to its bistable properties. Where TFT LCDs must continually be refreshed to keep the crystals correctly aligned, e-paper only needs to be refreshed when you change what’s on the display. Between refreshes, e-paper requires no extra power.

  Screen update operation Power consumed per day w / 6 updates (mAh) Power Consumed per year (mAh)
  Consumption (mA) Duration (s)    
2" EPD module (V231, eTC/G2) 2.33 2.32 0.01 3.29
2" TFT LCD module 30 0.02 720 262 800

Fig 3. This table shows the comparative power use of e-paper and a TFT LCD in the same use case. In a year, the e-paper display consumes just over 0.001% of the power required by the TFT LCD.

As a result, in situations where the display won’t be updated regularly, e-paper requires significantly less power than a TFT LCD of the same size. By implementing external timing control (eTC) and power-optimized waveforms, e-paper provider Pervasive Displays has created offerings that use just 2 mA of current to update the contents of the display.

If you imagine an always-on, two-inch display that’s updated six times a day, daily power consumption could be as low as 0.01 mAh for the e-paper version, whereas a similarly sized TFT would need 720 mAh. Looked at in another way, the e-paper display would consume 3.29 mAh in a year – less than 2% of the capacity of a 220 mAh CR2032 coin cell battery. In contrast, the TFT would chew through 262,800 mAh – or nearly 1,200 CR2032 batteries.

The ideal display technology for low-power IoT devices

E-paper isn’t right for every situation. If a display needs to be updated frequently, or used in dark environments (and therefore requires a backlight), the advantages of e-paper diminish. But for large numbers of IoT applications, where what you need is to display infrequently changing state information without consuming very much power, e-paper makes this feasible in devices where it previously wasn’t. In other cases, designers can use it instead of less energy-efficient display technologies to extend battery life.

Moreover, e-paper’s power consumption is so low that it can be driven using harvested energy. For example, an e-paper display built into an RFID or NFC device can run on solar energy or scavenged RF energy, neither of which would be powerful enough to drive other types of display. We’ll look at this in more detail below.

New opportunities to add displays

At present, designers of wirelessly connected IoT devices tend to prioritize battery life, at the expense of user interfaces and displays. The only link to the wider world is via the cloud, typically using ZigBee, Bluetooth low energy or some form of IoT-optimized narrowband technology.

However, lots of IoT applications have at least some human-facing element, and removing the human interface can affect the user experience, forcing someone to use a second device to access data via the cloud – when the first device is there in front of them.

It’s in these types of wireless, battery-powered devices that e-paper can restore display and human interface capabilities, while having only a very small impact on battery life.

The possibilities are many: the display could show the current state values the device is sending to the cloud, or it could show maintenance and debugging data, such as battery life or even a full memory dump if the device crashes. Remember: because e-paper is bistable, whatever’s on the screen will stay there, even if the power source fails, making it ideal for this kind of troubleshooting.

And where the connected device is able to receive data from the cloud as well as sending it, e paper can further transform customer experiences. Imagine a payment card with a built in e paper display: the card could be used to pay for travel or venue entry, then receive and display an image of the ticket, including a scannable barcode.

Powering a display with harvested energy

As we touched on above, e-paper can run almost indefinitely by harvesting all the power it needs to update its display, including from solar and RF energy scavenging. This enables designers to create so-called ‘no-power displays’ in devices without batteries.

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About the author

Scott Soongis CEO of Pervasive Displays and has over a decade of experience in software in addition to 12 years working in displays businesses. During his career, Scott has been a founding partner at four start-up companies, including Pervasive Displays. Scott sits on the board of several other technology businesses as a consulting partner. He was a board member of One Laptop Per Child (OLPC), which looks to provide kids in developing countries with a rugged, low-cost, low-power, connected laptop. Scott has an MBA from the Haas School of Business at the University of California, Berkeley as well as a BA from the University of Michigan at Ann Arbor, US.

Pervasive Displays designs, manufactures and markets e-paper displays for applications requiring high resolution, good readability and low power consumption. The company focuses on commercial and industrial applications including retail, logistics, healthcare, security and manufacturing. Its mature TFT, integrated circuit and e-paper platforms reduce risks and improve product value. Pervasive Displays simplifies the work of designers by providing both standard and custom displays and offering extensive technical support through an ecosystem of partners. Design resources include detailed product specifications, application notes, reference designs, source code and applications support. The company is based in Tainan City, Taiwan and has offices in The Netherlands and Oregon, USA. More information: www.pervasivedisplays.com

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