Long range battery-free wireless power transfer testbed for underground mines IoT and LPWAN devices

Underground mines are susceptible to occasional roof falls and cave-ins, temporarily destroying the existing wireless communications and telemetry infrastructure. During this temporary outage, intermittent provision of electrical energy wirelessly to the already deployed low-power wireless area networks (LPWAN) and Internet of Things (IoT) devices assumes a fundamental requirement. In this article, we propose and design a long-range far-field radio frequency (RF) wireless power transfer (WPT) testbed to power LPWAN and IoT devices at 35 m in an underground mines facility. Class AB external power amplifier (PA) was introduced to achieve a long-distance RF WPT, in the 880 MHz band. Thus, considerably reducing the adverse effects of the high signal attenuation power loss of about 50 dB. With the aid of a commercially available off-the-shelf (COTS) TPS61030 boost converter, a 2.5 V/25F supercapacitor was boosted to provide the energy needed to the Heltec ESP32 system-on-chip (SoC) microcontroller unit (MCU). The Heltec ESP32 integrates various components, including a dual-core CPU, LoRa, Wi-Fi, and Bluetooth radio connectivity modules. Thus, realizing far-field, IoT, and LPWAN battery-free technology in underground mines environment. This novel research exploited the concept of utilizing supercapacitors for the storage of harvested energy. To the best of our knowledge, this is the first long-range far-field WPT testbed specifically designed for underground mines facilities at 880 MHz band. In this study, we also investigated the impact of using various modulation schemes. The schemes evaluated were of amplitude modulation (AM), frequency modulation (FM), and pulse width modulation (PWM). Empirical results indicate that the most suitable scheme for RF-WPT in the underground mine is FM. In addition, we performed extensive analysis to understand the energy and current consumption profiles of the Heltec ESP32 LoRa, WiFi, and BLE transceiver using Power Profiler Kit II under different configurations and scenarios. The experimental results indicate that the WiFi connectivity is not energy efficient compared to the BLE transceiver.