Battery lifetime is an important issue in wearable devices. Ideally, they should always be ready to receive control signals without consuming a lot of power. Researchers at ETH Zurich have now developed a zero-power receiver for touch communication that harvests its energy directly from the signal. Having electronic gadgets always ready to function at our command comes at a cost.
According to some estimates, the power consumption of TV sets, DVD players, washing machines and other appliances in standby mode accounts for up to a quarter of the total electric energy used in an average household.
While that is certainly a deplorable waste, the problem becomes even more crucial for battery-operated devices, especially the new generation belonging to the “Internet of Things”, and wearable devices such as fitness trackers or health monitors.
We want them to be constantly ready to receive input, but that means that they will continuously consume power, and batteries will drain fast. Also, radio-transmitted control signals cause problems regarding privacy and security. Michele Magno, a researcher with the Department of Information Technology and Electrical Engineering at ETH Zurich, has now found a clever way to address these issues.
“The trick lies in harvesting the necessary energy to receive a wake-up command directly from the transmitter through a touch”, explains Magno, who has been working on wake-up radio and energy harvesting technologies for many years. The idea for the new device originated from a chance encounter with researchers at the Disney Research lab in Zurich, who were interested in a touch-activated switch to implant into their toys.
“They had an approach involving a receiver that would drain the batteries in a few hours, so in the end nothing came of that collaboration”, Magno says, but adds: “Still, my curiosity was piqued, and I really believed that the receiver could only be successful if it needed zero power. So, I started working on a prototype of the device I had in mind in my spare time, and later with the help of my master’s students Philipp Mayer and Raphael Strebel.”
In the meantime, Magno’s idea has been submitted as a patent to the European Patent Office. Its principle is simple but challenging: the receiver, which does not have its own battery, captures signals on an electrode when it is “touched” by a human body.
In order to get the receiver to wake up, before sending the actual command signal – a modulated electromagnetic wave at a frequency of a few Megahertz - the transmitter adds a “preamble” lasting a few milliseconds that doesn’t contain information.
The energy absorbed by the receiver during that time is stored in a capacitor, which acts as a power source for receiving and decoding the actual control signal that follows. Other power-hungry devices in sleep mode can then be woken up by the receiver provided that the correct identification was received.
“In this way, we have a true zero-power receiver that can be used in a multitude of ways”, Magno explains, “such as touch sensors on your car that recognise you and open the doors for you”. This could be much safer than current radio-wave based technologies like RFID, which broadcast their signals over a distance and can, therefore, be hacked.
Another interesting area is intra-body communication, in which wearable devices placed, for example, on both arms of a person, communicate with each other, or handshake information exchange between two users.
Magno and his colleagues have shown that their prototype receiver has an on-body range of more than 1.7 meters, allowing communication between the wearer’s wrist and any other part of their body.
When Michele Magno speaks about his latest work, his enthusiasm is palpable, and he has ambitious plans for the future. With the help of the ETH transfer office he plans to create start-up companies that will develop his prototype into commercial devices for applications ranging from touch communication to monitoring trains.
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Image credit: ETH Zurich.