Sensor networks with battery-powered nodes can seldom simultaneously meet the design goals of lifetime, cost, sensing reliability and sensing and transmission coverage. Energy-harvesting, converting ambient energy to electrical energy, has emerged as an alternative to power sensor nodes. By exploiting recharge opportunities and tuning performance parameters based on current and expected energy levels, energy harvesting sensor nodes have the potential to address the conflicting design goals of lifetime and performance. This paper surveys various aspects of energy harvesting sensor systems- architecture, energy sources and storage technologies and examples of harvesting-based nodes and applications. The study also discusses the implications of recharge opportunities on sensor node operation and design of sensor network solutions.

Energy Harvesting Sensor Nodes: Survey and Implications

Wireless power technology offers the promise of cutting the last cord, allowing users to seamlessly recharge mobile devices as easily as data are transmitted through the air. Initial work on the use of magnetically coupled resonators for this purpose has shown promising results. We present new analysis that yields critical insight into the design of practical systems, including the introduction of key figures of merit that can be used to compare systems with vastly different geometries and operating conditions. A circuit model is presented along with a derivation of key system concepts, such as frequency splitting, the maximum operating distance (critical coupling), and the behavior of the system as it becomes undercoupled. This theoretical model is validated against measured data and shows an excellent average coefficient of determination of 0.9875. An adaptive frequency tuning technique is demonstrated, which compensates for efficiency variations encountered when the transmitter-to-receiver distance and/or orientation are varied. The method demonstrated in this paper allows a fixed-load receiver to be moved to nearly any position and/or orientation within the range of the transmitter and still achieve a near-constant efficiency of over 70% for a range of 0-70 cm.

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Analysis, Experimental Results, and Range Adaptation of Magnetically Coupled Resonators for Wireless Power Transfer

We present the design of a communication system that enables two devices to communicate using ambient RF as the only source of power. Our approach leverages existing TV and cellular transmissions to eliminate the need for wires and batteries, thus enabling ubiquitous communication where devices can communicate among themselves at unprecedented scales and in locations that were previously inaccessible.To achieve this, we introduce ambient backscatter, a new communication primitive where devices communicate by backscattering ambient RF signals. Our design avoids the expensive process of generating radio waves; backscatter communication is orders of magnitude more power-efficient than traditional radio communication. Further, since it leverages the ambient RF signals that are already around us, it does not require a dedicated power infrastructure as in traditional backscatter communication. To show the feasibility of our design, we prototype ambient backscatter devices in hardware and achieve information rates of 1 kbps over distances of 2.5 feet and 1.5 feet, while operating outdoors and indoors respectively. We use our hardware prototype to implement proof-of-concepts for two previously infeasible ubiquitous communication applications.

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Ambient backscatter: wireless communication out of thin air

This paper presents the wireless identification and sensing platform (WISP), which is a programmable battery-free sensing and computational platform designed to explore sensor-enhanced radio frequency identification (RFID) applications. WISP uses a 16-bit ultralow-power microcontroller to perform sensing and computation while exclusively operating from harvested RF energy. Sensors that have successfully been integrated into the WISP platform to date include temperature, ambient light, rectified voltage, and orientation. The microcontroller encodes measurements into an electronic product code (EPC) Class 1 Generation 1 compliant ID and dynamically computes the required 16-bit cyclical redundancy checking (CRC). Finally, WISP emulates the EPC protocol to communicate the ID to the RFID reader. To the authors' knowledge, WISP is the first fully programmable computing platform that can operate using power transmitted from a long-range (UHF) RFID reader and communicate arbitrary multibit data in a single response packet.

http://www.alansonsample.com/publications/docs/2008%20-%20TIM%20-%20Design%20RFID%20Based%20Battery%20Free%20Programmable%20Sensing%20Platform.pdf

Design of an RFID-Based Battery-Free Programmable Sensing Platform

Recently, Wireless Sensor Networks (WSNs) have attracted lot of attention due to their pervasive nature and their wide deployment in Internet of Things, Cyber Physical Systems, and other emerging areas. The limited energy associated with WSNs is a major bottleneck of WSN technologies. To overcome this major limitation, the design and development of efficient and high performance energy harvesting systems for WSN environments are being explored. We present a comprehensive taxonomy of the various energy harvesting sources that can be used by WSNs. We also discuss various recently proposed energy prediction models that have the potential to maximize the energy harvested in WSNs. Finally, we identify some of the challenges that still need to be addressed to develop cost-effective, efficient, and reliable energy harvesting systems for the WSN environment.

Energy harvesting in wireless sensor networks: A comprehensive review

This paper presents a wireless, battery-free, platform for sensing and computation that is powered and read by a standards compliant ultra-high frequency (UHF) radio frequency identification (RFID) reader. The WISP (wireless identification and sensing platform) includes a fully-programmable 16 bit microcontroller with analog-to-digital converter. The microcontroller firmware implements portions of the electronic product code (EPC) class 1 generation 1 protocol. When queried, the platform communicates arbitrary sensor data by emulating an EPC tag whose ID encodes the desired sensor data; the required 16-bit CRC is computed dynamically by the microcontroller. The RFID reader reports the received tag ID to application software which can interpret the information contained in the tag ID. The programmability of the WISP along with its implementation as a PCB allows for flexible integration of arbitrary low-power sensors. Furthermore, sensors are also exclusively powered from the RFID reader resulting in a completely battery free device. Sensors integrated into the WISP platform so far include light, temperature, and rectified voltage, and are shown experimentally to have an operating range of up to 4.5m. To the authors' knowledge, WISP is the first fully programmable computing platform that can operate using power transmitted from a long-range (UHF) RFID reader and communicate arbitrary, multi-bit data in a single response packet.

Design of a Passively-Powered, Programmable Sensing Platform for UHF RFID Systems

A wirelessly powered 0.125 mm2 65 nm CMOS IC for Brain-Machine Interface applications integrates four 1.5 μW amplifiers (6.5 μVrms input-referred noise with 10 kHz bandwidth) with power conditioning and communication circuitry. The multi-node backscatter frequency locks to a wireless interrogator using a frequency-domain multiple access communication scheme. The full system, verified with wirelessly powered in vivo recordings, consumes 10.5 μW and operates at 1 mm range in air with 50 mW transmit power.

A Fully-Integrated, Miniaturized (0.125 mm²) 10.5 µW Wireless Neural Sensor

We present the WISP passive data logger (PDL), an RFID sensor data logging platform that relies on a new, wirelessly-charged power model. A PDL has no battery yet (unlike a passive sensor tag) is able to collect data while away from an RFID reader. A PDL senses and logs data using energy stored in a capacitor; the capacitor can be wirelessly recharged (unlike active tags), and data can be uploaded whenever the PDL is near a reader. Standard EPC generation 2 readers are used for WISP-PDL charging, ID-reading, and sensor data transfer. This allows WISP-PDLs to operate using commercial RFID readers as the only support infrastructure (for both data and power), and allows WISP-PDLs to co-exist with standard RFID tags. We describe the design and implementation of a prototype WISP-PDL, and report results from a short demonstration study that shows it can monitor the temperature and fullness of a milk carton as it is used over the course of a day.

Wirelessly-Charged UHF Tags for Sensor Data Collection

Biosensors present exciting opportunities in novel medical and scientific applications. However, sensor tags presented to date cannot interface with practical sensors, lack addressability, and/or require a custom (high-cost) interrogator. Our tag provides these features via ultra-low-power circuitry including a low-noise biosignal amplifier, unique tag ID generator, calibration-free 3 MHz oscillator, and EPC C1 Gen2 protocol compatibility. In addition to design details and measurement data from the fabricated IC, we present in vivo muscle temperature measurement from an untethered in-flight hawkmoth.

Daniel J. Yeager

Papers

Design of an RFID-Based Battery-Free Programmable Sensing Platform

Design of a Passively-Powered, Programmable Sensing Platform for UHF RFID Systems

A Fully-Integrated, Miniaturized (0.125 mm²) 10.5 µW Wireless Neural Sensor

Wirelessly-Charged UHF Tags for Sensor Data Collection

A 9 $\mu$ A, Addressable Gen2 Sensor Tag for Biosignal Acquisition