Joshua F. Ensworth

Magnetic metamaterial superlens for increased range wireless power transfer.

The ability to wirelessly power electrical devices is becoming of greater urgency as a component of energy conservation and sustainability efforts. Due to health and safety concerns, most wireless power transfer (WPT) schemes utilize very low frequency, quasi-static, magnetic fields; power transfer occurs via magneto-inductive (MI) coupling between conducting loops serving as transmitter and receiver. At the “long range” regime – referring to distances larger than the diameter of the largest loop – WPT efficiency in free space falls off as (1/d)6; power loss quickly approaches 100% and limits practical implementations of WPT to relatively tight distances between power source and device. A “superlens”, however, can concentrate the magnetic near fields of a source. Here, we demonstrate the impact of a magnetic metamaterial (MM) superlens on long-range near-field WPT, quantitatively confirming in simulation and measurement at 13–16 MHz the conditions under which the superlens can enhance power transfer efficiency compared to the lens-less free-space system.

Every smart phone is a backscatter reader: Modulated backscatter compatibility with Bluetooth 4.0 Low Energy (BLE) devices

In this work, we show how modulated backscatter signals can be crafted to yield channelized band-pass signals akin to those transmitted by many conventional wireless devices. As a result, conventional wireless devices can receive these backscattered signals without any modification (neither hardware nor software) to the conventional wireless device. We present a proof of concept using the Bluetooth 4.0 Low Energy, or BLE, standard widely available on smart phones and mobile devices. Our prototype backscatter tag produces three-channel bandpass frequency shift keying (FSK) packets at 1 Mbps that are indistinguishable from conventional BLE advertising packets. An unmodified Apple iPad is shown to correctly receive and display these packets at a range of over 9.4 m using its existing iOS Bluetooth stack with no changes whatsoever. We create all three BLE channels by backscattering a single incident CW carrier using a novel combination of fundamentalmode and harmonic-mode backscatter subcarrier modulation, with two of the band-pass channels generated by the fundamental mode and one of the band-pass channels generated by the second harmonic mode. The backscatter modulator consumes only 28.4 pJ/bit, compared with over 10 nJ/bit for conventional BLE transmitters. The backscatter approach yields over 100X lower energy per bit than a conventional BLE transmitter, while retaining compatibility with billions of existing Bluetooth enabled smartphones and mobile devices.

Ultra-low power 2.4GHz RF energy harvesting and storage system with −25dBm sensitivity

An RF energy harvesting and storage system is described that trickle charges a battery from incident power levels as low as -25 dBm referred to the feedpoint of an 8 dBi patch antenna. The circuit is optimized for the indoor ambient power range typically observed in the 2.4 GHz ISM band so that we can harvest the energy provided by nearby Wi-Fi, Bluetooth and other devices. In this incident power regime, rectified voltages are low, so power management circuit operation in the 100mV regime is critical. We present several improvements to our prior work that significantly improve its performance, including a novel wideband multi-element antenna array, an improved boost converter, and a redesigned battery charger. At -25dBm RF input power, the new harvesting system sources 150μJ into a rechargeable battery after 1 hour. We believe that this work represents the lowest reported startup power yet achieved in battery-storage RF energy harvesting systems.

Quasi-Static Magnetic Field Shielding Using Longitudinal Mu-Near-Zero Metamaterials.

The control of quasi-static magnetic fields is of considerable interest in applications including the reduction of electromagnetic interference (EMI), wireless power transfer (WPT), and magnetic resonance imaging (MRI). The shielding of static or quasi-static magnetic fields is typically accomplished through the use of inherently magnetic materials with large magnetic permeability, such as ferrites, used sometimes in combination with metallic sheets and/or active field cancellation. Ferrite materials, however, can be expensive, heavy and brittle. Inspired by recent demonstrations of epsilon-, mu- and index-near-zero metamaterials, here we show how a longitudinal mu-near-zero (LMNZ) layer can serve as a strong frequency-selective reflector of magnetic fields when operating in the near-field region of dipole-like sources. Experimental measurements with a fabricated LMNZ sheet constructed from an artificial magnetic conductor – formed from non-magnetic, conducting, metamaterial elements – confirm that the artificial structure provides significantly improved shielding as compared with a commercially available ferrite of the same size. Furthermore, we design a structure to shield simultaneously at the fundamental and first harmonic frequencies. Such frequency-selective behavior can be potentially useful for shielding electromagnetic sources that may also generate higher order harmonics, while leaving the transmission of other frequencies unaffected.

A 2.4GHz ambient RF energy harvesting system with −20dBm minimum input power and NiMH battery storage

We describe a radio frequency (RF) energy harvester and power management circuit that trickle charges a battery from incident power levels as low as -20dBm. We designed the harvester for the 2.4 GHz RF band to leverage the ubiquity of energy that is produced by Wi-Fi, Bluetooth, and other devices. This paper reports on the design and current status of the harvester and compares our performance to other published results. In this incident power regime, rectified voltages are low, so power management circuit operation in the 100mV regime is critical. This paper describes a novel rectenna design, boost converter, and battery charger for RF energy harvesting specifically tuned to this low-power regime. At -20dBm RF input power, the harvesting system (rectenna, boost converter, and battery charger) sources 5.8μJ into a rechargeable battery after 1 hour.

Waveform-aware ambient RF energy harvesting

In this paper we suggest a new class of RF energy harvesters, which we call “waveform aware harvesters”. In contrast to traditional rectenna designs, which are usually designed for high efficiency with continuous wave (CW) signals, waveform aware harvesters are RF to DC converters which are optimized for their performance with non-CW signals. We suggest that waveform aware harvesters may have significant advantages in ambient energy harvesting, where the available RF energy is in the form of communication waveforms of a variety of types. We present an initial proof-of-concept demonstration of a waveform aware harvester optimized for harvesting energy from 2.4 GHz Wi-Fi (802.11b/g) signals with a realistic traffic model. Under realistic traffic conditions, 802.11b/g client transmissions are bursty, with a high peak-to-average ratio and a low duty cycle. We demonstrate optimized recovery of harvested energy from single 802.11b/g transmission bursts on the order of 1 ms in duration. We present an expression for maximizing usable energy stored in an energy reservoir given a signal model and parameters of the energy-harvester circuit. In contrast to other work where assumptions of CW sources lead to the desirability of a large storage capacitor, our approach considers the existing communication signal model and optimizes capacitor size to maximize the stored usable energy for a short transmission burst.

BLE-Backscatter: Ultralow-Power IoT Nodes Compatible With Bluetooth 4.0 Low Energy (BLE) Smartphones and Tablets

Backscatter communication promises significant power and complexity advantages for Internet of Things devices such as radio frequency identification (RFID) tags and wireless sensor nodes. One perceived disadvantage of backscatter communication has been the requirement for specialized hardware such as RFID readers to receive backscatter signals. In this paper, we show how backscatter signals can be designed for compatibility with the Bluetooth 4.0 low energy (BLE) chipsets already present in billions of smart phones and tablets. We present a prototype microcontroller-based “BLE-Backscatter” tag that produces bandpass frequency-shift keying modulation at 1 Mb/s, enabling compatibility with conventional BLE advertising channels. Using a +23-dBm equivalent isotropically radiated power continuous wave (CW) carrier source, we demonstrate a range of up to 13 m between the tag and an unmodified Apple iPad Mini as well as a PC with the Nordic Semiconductor nRF51822 chipset. With the tag 1 m from the receiver, we demonstrate a range of up to 30 m between the CW carrier source and the tag. In both cases, the existing Bluetooth stack was used, with no modifications whatsoever to hardware, firmware, or software. The backscatter tag consumes only 1.56 nJ/b, over

Ultra-low power autonomous 2.4GHz RF energy harvesting and storage system

6\times

A low power 2.4 GHz superheterodyne receiver architecture with external LO for wirelessly powered backscatter tags and sensors

less than the lowest power commercial Bluetooth transmitters.

Full-duplex Bluetooth Low Energy (BLE) compatible Backscatter communication system for mobile devices

We present an autonomous end-to-end 2.4GHz RF energy harvesting and storage system suitable for harvesting energy from WiFi and similar devices. The system is designed to collect ultra-low power ambient RF input energy and automatically store it into a battery. Previous work in this area required a pushbutton switch for full functionality - RF energy is first stored in a capacitor and manually transferred into a battery. In this work, in addition to the main harvester, an auxiliary control system is developed to replace the pushbutton and achieve fully autonomous energy harvesting and storage. Two design approaches are proposed for the control system. The tradeoffs of each technique will be discussed. The designed control systems were integrated into the main harvesting structure. Experimental results indicate that the system autonomously stores 241μJ into a NiMH battery after 30 minutes with an incident power (at the feedpoint of the antenna) of -21dBm.