Widespread use of the 2.4GHz radio frequency due to the growth of WiFi sources means excess signals are currently inactive and available for alternative uses when not being utilised to access the Internet.
NUS researchers have now worked with Japan’s Tohoku University (TU) on the development of a technology that uses tiny smart devices known as spin-torque oscillators (STOs) to harvest and convert these wireless radio frequencies into energy.
“In this way, small electric gadgets and sensors can be powered wirelessly by using radio frequency waves as part of the Internet of Things,” said project leader Prof. Yang Hyunsoo, NUS Department of Electrical and Computer Engineering. “With the advent of smart homes and cities, our work could give rise to energy-efficient applications in communication, computing and neuromorphic systems.”
The application of STOs in wireless communication systems is often hindered by low output power and broad linewidth. This can be overcome by mutual synchronisation of multiple STOs, but current schemes, such as long-range magnetic coupling between multiple STOs, have spatial restrictions.
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Meanwhile, long-range electrical synchronisation using vortex oscillators is limited in frequency responses of only a few hundred MHz, and requires dedicated current sources for the individual STOs which can complicate the overall on-chip implementation.
To overcome these limitations, the team developed an array of eight STOs connected in series. The 2.4GHz electromagnetic radio waves used by WiFi was converted into a direct voltage signal using the array, then transmitted to a capacitor to light up a 1.6-volt LED. When the capacitor was charged for five seconds, it was able to light up the LED for one minute after the wireless power was switched off.
In their study, published in Nature Communications, researchers highlighted the importance of electrical topology for designing on-chip STO systems and compared the series design with the parallel one.
They found that parallel configuration is more useful for wireless transmission due to better time-domain stability, spectral noise behaviour and control over impedance mismatch — while series connections have an advantage for energy harvesting due to the additive effect of the diode-voltage from STOs.
“Aside from coming up with an STO array for wireless transmission and energy harvesting, our work also demonstrated control over the synchronising state of coupled STOs using injection locking from an external radio-frequency source,” said first author Dr Raghav Sharma. “These results are important for prospective applications of synchronised STOs, such as fast-speed neuromorphic computing.”
Researchers are next looking to increase the number of STOs in their array to enhance energy harvesting ability, and plan to test their energy harvesters for wirelessly charging other electronic devices and sensors. They also hope to work with industry partners on development of on-chip STOs for self-sustained smart systems.
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