- High-efficiency buck DC/DC converter for energy harvesting eliminates need for batteries in wearables.
- Energy harvesting development kit captures power from multiple sources.
- Flexible starter kit speeds demonstration of shoe-based proximity sensor powered by both vibrational and solar energy.
Energy harvesting starter kit makes it easy to go battery free
In recent years, new wearable devices have been incessantly introduced to our markets, as observed in the cases of smartwatches and smartglasses. To illustrate the potential for wearable device applications, one has only to observe the number of units in markets worldwide, which is expected to grow from 5.6 million in 2013 to 124 million by 2020 (see figure 1). Several factors are driving this growth. Smartphones have almost saturated the markets in most developed nations, and manufacturers have started searching for new trend-creating devices like wearables to be connected as peripherals. Other elements that are driving this development include the emergence of the Internet of Things (IoT), as well as continued advances in semiconductor technologies that have enabled the production of smaller devices with higher speed and functionality.
Figure 1: The number of wearable devices should reach 124 million by the year 2020; smartwatches and smartglasses will account for nearly 60% of that total.
Two thirds of wearable devices are terminals worn on the body, like watches and glasses, while the remainder includes those attached to shoes, clothing and bags. The possibility of implanting devices in human bodies and connecting to their brains is also under discussion. In the future, wearable devices are expected to bring the scenarios depicted in science fiction movies into reality. The potential is enormous, but there is one big problem—how do we power those devices without having to change or recharge the batteries frequently? The solution is energy harvesting and the Cypress development board provides an easy point of entry.
While innovations of wearable devices are accelerating, the area of battery technology lags behind. In particular, battery life poses a serious concern. Unlike smartphones, items like glasses, watches, shoes and clothing are not charged in our daily life. If all these items were required to be charged frequently in the future, people would find it annoying. Wearable devices require a battery with a lifetime that is so long that we will hardly need to take our devices off, making us almost unaware of their presence on our body.
Wearable devices in general need to be small and lightweight. The problem is that current batteries are constrained by a fundamental characteristic: The smaller they are, the smaller their energy densities become. Semiconductors are still able to keep up with Moore's Law, accelerating miniaturization speed at the pace of 200% annually. Meanwhile, in the 20 years since lithium ion secondary batteries were introduced, the technology has only improved energy density per unit volume by about 350% over the entire time span. That equates to less than 10% of the miniaturization speed of semiconductors.
If the advance of battery technology has proven to be slow, it makes perfect sense to find other solutions. That prompted us to look into the recently emerged technology known as energy harvesting. Energy harvesting involves gathering, or harvesting, small amounts of environmental energy produced by sources that surround us such as light, vibration and heat, and converting it to electrical energy. We need to investigate whether the technology can provide us with a mechanism for prolonging the battery life for wearable devices, or better yet, for running them without a battery. It is important to note that energy harvesting technology is still far from mature. It cannot currently run a wrist-watch type wearable device without an additional battery, for example. It is nevertheless ideal for devices such as those that transmit simple data from sensors to smartphones to keep an every-day activity log.
Speeding energy harvesting adoption
Energy harvesting provides a promising power solution for many wearables applications. It’s unfamiliar to many engineers, though, and implementing it can consume valuable time that development teams could otherwise devote to enhancing their product’s core value proposition. The Cypress MB39C811-EVBSK-02 Bluetooth® Smart Beacon starter kit provides an easy, quick way for embedded engineers to experiment with energy harvesting.
The kit includes Cypress MB39C811 power management IC as a standard feature, as well as a Bluetooth® Smart module that allows program re-writing in the user areas and a solar panel for indoor use for operation check (see figure 2). The MB39C811 is a high-efficiency buck DC/DC converter IC that adopts an all-wave bridge rectifier using low-dissipation and a comparator system. When connected to a separately provided vibration harvester such as a piezoelectric device, the starter kit can operate in a hybrid mode using vibrational energy and indoor optical energy.
Figure 2: The Cypress MB39C811-EVBSK-02 Bluetooth® Smart Beacon starter kit features a high-efficiency buck DC/DC converter designed for use with piezoelectric or solar cell energy harvesters.
The kit also includes a board with a pre-written sample program for operating the Bluetooth Smart beacon, which automatically transmits the Bluetooth Smart beacon packet to a smartphone when the board is placed under office illumination condition (normally 400 to 500 lux on a desk) using only the energy from its solar panel (see figure 3). The transmission interval is set at 1 s under a 500-lux environment.
Figure 3: Starter kit automatically transmits Bluetooth Smart Beacon packet to a smart phone when solar panel is illuminated with 500 lux.
Smart shoes—energy harvesting in action
To test out the kit, we applied energy harvesting techniques to a wearable proximity sensor designed to transmit a child’s location. We embedded the board and a power-generating device in a child’s shoe and asked our test subject to walk out and back a distance of 50 m to see if location data could be obtained. First, radio field intensities in terms of received signal strength indication (RSSI), which is used in the iBeacon standard, were measured with a smartphone. Next, the distance r between the smartphone and the child was determined using the following formula:
where RSSI is received intensity at 1 m (fixed value) and Tx is actual intensity received (measured value). For our vibration harvesting device, we chose an electromagnetic induction type so that the vibration generated by the motion of the child’s foot was converted to electrical energy for transmitting the radio waves (see figure 4). In addition, the design features a solar panel to allow operation under conditions in which energy could be obtained from either vibration or light. With this hybrid design, the child’s location could be detected even when the child was not moving.
Figure 4: Starter kit allows location sensor mounted in child’s shoe to be powered by energy harvested from vibration and solar sources.
The results showed that the device successfully transmitted data to the smart phone via the Bluetooth® Smart beacon (see figure 5). Powered only by the vibrational energy generated by a walking child, the sensor was able to detect and transmit the child’s location. Experiments like this example are expected to contribute to addressing the battery issues associated with wearable devices, with the use of energy harvesting technology offered by Cypress.
Figure 5: In tests, the subject walked a distance of 50 m from the receiver before returning. Plots of distance versus time show how effectively the sensor can operate on a combination of photovoltaic and vibrational energy (left) or vibrational energy alone (right).
Energy harvesting shows enormous promise for addressing the challenges of the wearables market. Development of real-life equipment incorporating Cypress’s power management ICs for energy harvesting is under way in various areas. It is mainly aimed at battery-free wireless sensor terminals used in such applications as machine-to-machine (M2M) communications, implemented as part of the industrial IoT. Developers and makers should watch for upcoming releases. Likewise, the company plans to offer in near future power-management ICs and low-power-consumption MCUs that are specifically designed to address battery issues in the area of wearable devices.
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