Energy-Neutral Devices: Can Hybrid RF-Acoustic Signals Point Them Out?

Abstract

We present a hybrid signaling approach to position energy-neutral devices. Our method combines the best of two worlds: instant RF signals for both communication and energy transfer, and slower propagating acoustic waves for accurate distance measurements. We introduce advanced energy loading of an 'E-buffer' through beamformed RF signals. The scientific contribution of our approach is twofold, advancing both the positioning performance and the energy harvesting efficiency. On the one hand, we overcome current distance limitations in RF backscattering-based indoor localization. On the other hand, it enhances the energy harvesting by using the calculated position and beamforming a higher amount of directed RF energy into the mobile node. We provide a functional architecture for both sides of the system and present a proof-of-concept. Practical measurements in representative use cases show an update rate of 10 positions per hour within the regulatory constraints in the 868MHz band for distances up to 4.5 m. An energy and power model are drawn up to provide insights into the performance trade-offs.

Techtile: a Flexible Testbed for Distributed Acoustic Indoor Positioning and Sensing

“…Within the Techtile environment, a number of Kema L010 ultrasonic speakers combined with off-the-shelf amplifiers with a frequency range over 45 kHz are mounted on the tile's overlays. These speakers are currently used for RF-acoustic indoor positioning of energy neutral devices [13], [14]. Vice versa, if an ultrasonic speaker based mobile node needs to be localized, a more compact, energy-efficient and omnidirectional speaker is desired, which is not the case with the Kema L010.…”

“…Mobile nodes in acoustic positioning often consist of microphone modules instead of speakers, as receiving requires less energy than transmitting acoustic signals. Energy-wise, adding RF to the system is a dreadful decision but can be countered by adding RF-energy harvesting hardware on the mobile node side [14]. Another solution can be found by (almost) passive RF-backscattering, where communication is acquired by either reflecting or absorbing the incoming RF waves.…”

A Multi-band Solution for Interacting with Energy-Neutral Devices

“…These two voltages are the boundaries on which power will be delivered and are subject to the hybrid RF-acoustic hardware. The capacitance is set to 22 µF according to [11] with V chrdy = 3.10 V, V ovdis = 2.8 V and worst case LDO efficiency η LDO of 74 %. The necessary storage energy to be harvested can be calculated with:…”

“…In previous research [11], we showed that directional, single band wireless power transfer in combination with RF backscattering enables the full passive positioning of a hybrid RFacoustic mobile tag. However, two problems raised, namely the low update rate and the position presumption for directional power transfer.…”