A battery-less BLE smart sensor for room occupancy tracking supplied by 2.45-GHz wireless power transfer

Abstract

Wireless power transfer (WPT) has emerged as a solution for supplying smart sensors for long-term battery-less deployment. Because the amount of power harvested by the smart sensor is limited due to WPT path loss, the optimization objective is twofold: achieving ultra-low-power operation for the sensing task and improving the harvesting efficiency even at low incident power. In this paper, we focus on the use case of a Bluetooth LE-connected motion detection system supplied by 2.45-GHz RF power. The full system (RF energy harvester, power management, sensor transducer and interface, control, data processing and wireless transmission) is implemented using low-power off-the-shelf components. In the sensing sub-system, ultra-low-power operation is achieved by the duty-cycling of the sensor interface and by an event-driven scheme for communication. In the harvesting sub-system, the design of the matching network and rectifier, combined with maximum power point tracking (MPPT), is optimized for increasing the power harvesting efficiency (PHE) at low incident power. Measurements show a total reduction in the power consumption for the sensing sub-system by a factor 20. When using custom WPT waveform with high peak-to-average power ratio, the RF energy harvester is functional with an incident RF power starting from −20 dBm. The smart sensor is able to perform its motion-detection task with an incident power as low as −17.3 dBm.