Alanson P. Sample

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

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.

Experimental results with two wireless power transfer systems

This paper describes two wireless power transfer systems. The Wireless Identification and Sensing Platform (WISP) is a platform for sensing and computation that is powered and read by a commercial off-the-shelf UHF (915MHz) RFID reader. WISPs are small sensor devices that consume on the order of 2uW to 2mW, and can be operated at distances of up to several meters from the reader. The second system harvests VHF or UHF energy from TV towers, with power available depending on range and broadcast transmit power. We report on an experiment in which 60uW is harvested at a range of about 4km.

Enabling Seamless Wireless Power Delivery in Dynamic Environments

Effective means of delivering wireless power to volumes of spaces will enable users the freedom and mobility to seamlessly power and recharge their devices in an unencumbered fashion. This has particular importance for consumer electronic, medical, and industrial applications, where usage models focus on unstructured and dynamic environments. However, existing wireless power technology falls short of this vision. Inductive charging solutions are limited to near-contact distances and require a docking station or precise placement for effective operation. Far-field wireless power techniques allow much greater range, but require complicated tracking systems to maintain a line-of-sight connection for high-efficiency power delivery to mobile applications. Recent work using magnetically coupled resonators (MCRs) for wireless power delivery has shown a promising intersection between range (on the order of a meter), efficiency (over 80%), and delivered power (up to tens of watts). However, unpredictable loads rapidly change system operating points, and changes in position disrupt system efficiency, which affects the ultimate usability of these systems. Dynamic adaptation to these changes in operating conditions and power transfer range is a critical capability in developing a fully functional and versatile wireless power solution. This paper provides an overview of methods used to adapt to variations in range, orientation, and load using both wideband and fixed-frequency techniques.

A wirelessly-powered platform for sensing and computation

We present WISP, a wireless, battery-free platform for sensing and computation that is powered and read by a standards compliant Ultra-High Frequency (UHF) RFID reader. To the reader, the WISP appears to be an ordinary RFID tag. The WISP platform includes a general-purpose programmable flash microcontroller and implements the bi-directional communication primitives required by the Electronic Product Code (EPC) RFID standard, which allows it to communicate arbitrary sensor data via an EPC RFID reader by dynamically changing the ID it presents to the reader. For each 64 bit “packet,” the WISPs microcontroller dynamically computes the 16-bit CRC that the EPC standard requires of valid packets. Because the WISP device can control all bits of the presented ID, 64 bits of sensor data can be communicated with a single RFID read event. As an example of the system in operation, we present 13 hours of continuous-valued light-level data measured by the device. All the measurements were made using power harvested from the RFID reader. No battery, and no wired connections (for either power or data) were used. As far as we are aware, this paper reports 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 response to a single RFID reader poll event.

Evaluation of Wireless Resonant Power Transfer Systems With Human Electromagnetic Exposure Limits

This study provides recommendations for scientifically sound methods of evaluating compliance of wireless power transfer systems with respect to human electromagnetic exposure limits. Methods for both numerical analysis and measurements are discussed. An exposure assessment of a representative wireless power transfer system, under a limited set of operating conditions, is provided in order to estimate the maximum SAR levels. The system operates at low MHz frequencies and it achieves power transfer via near field coupling between two resonant coils located within a few meters of each other. Numerical modeling of the system next to each of four high-resolution anatomical models shows that the local and whole-body SAR limits are generally reached when the transmit coil currents are 0.5 ARMS - 1.2 ARMS at 8 MHz for the maximal-exposure orientation of the coil and 10-mm distance to the body. For the same coil configurations, the exposure can vary by more than 3 dB for different human models. A simplified experimental setup for the exposure evaluation of wireless power transfer systems is also described.

Powering a Ventricular Assist Device (VAD) With the Free-Range Resonant Electrical Energy Delivery (FREE-D) System

Wireless data communication technology has eliminated wired connections for data transfer to portable devices. Wireless power technology offers the possibility of eliminating the remaining wired connection: the power cord. For ventricular assist devices (VADs), wireless power technology will eliminate the complications and infections caused by the percutaneous wired power connection. Integrating wireless power technology into VADs will enable VAD implants to become a more viable option for heart failure patients (of which there are 80 000 in the United States each year) than heart transplants. Previous transcutaneous energy transfer systems (TETS) have attempted to wirelessly power VADs ; however, TETS-based technologies are limited in range to a few millimeters, do not tolerate angular misalignment, and suffer from poor efficiency. The free-range resonant electrical delivery (FREE-D) wireless power system aims to use magnetically coupled resonators to efficiently transfer power across a distance to a VAD implanted in the human body, and to provide robustness to geometric changes. Multiple resonator configurations are implemented to improve the range and efficiency of wireless power transmission to both a commercially available axial pump and a VentrAssist centrifugal pump [3]. An adaptive frequency tuning method allows for maximum power transfer efficiency for nearly any angular orientation over a range of separation distances. Additionally, laboratory results show the continuous operation of both pumps using the FREE-D system with a wireless power transfer efficiency upwards of 90%.

RFID: From Supply Chains to Sensor Nets

The next generation internet will be the internet of things (and not just of computing devices like PCs, PDAs); this is presumed to be enabled by integrating simple computing plus communications capabilities into common objects of everyday use. Radio-frequency identification (RFID) is a compelling technology for creation of such pervasive sensor networks due to its potential for ubiquitous, low-cost/low-maintenance use. However, the current drivers for RFID deployment emphasize supply chain management using passive tags, implying that RFID sensor nets require advances beyond the components and system designs aimed at supply chain applications. This work provides a glimpse of how this may be achieved.

A capacitive touch interface for passive RFID tags

This paper presents a novel method for incorporating a capacitive touch interface into existing passive RFID tag architectures without additional parts or changes to the manufacturing process. Our approach employs the tags antenna as a dual function element in which the antenna simultaneously acts as both a low-frequency capacitive fringing electric field sensor and also as an RF antenna. To demonstrate the feasibility of our approach, we have prototyped a passive UHF tag with capacitive sensing capability integrated into the antenna port using the WISP tag. Finally, we describe how this technology can be used for touch interfaces as well as other applications with the addition of a LED for user feedback.

IDSense: A Human Object Interaction Detection System Based on Passive UHF RFID

In order to enable unobtrusive human object interaction detection, we propose a minimalistic approach to instrumenting everyday objects with passive (i.e. battery-free) UHF RFID tags. By measuring the changes in the physical layer of the communication channel between the RFID tag and reader (such as RSSI, RF phase, and read rate) we are able to classify, in real time, tag/object motion events along with two types of touch events. Through a user study, we demonstrate that our real-time classification engine is able to simultaneously track 20 objects and identify four movement classes with 93% accuracy. To demonstrate how robust this general-purpose interaction mechanism is, we investigate three usage scenarios 1) interactive storytelling with toys 2) inference of daily activities in the home 3) identification of customer browsing habits in a retail setting.

Photovoltaic enhanced UHF RFID tag antennas for dual purpose energy harvesting

The most significant barrier to improving passive RFID tag performance for both fixed function ID tags and enhanced RFID tags is the limitation on the amount of power that can be harvested for operation. This paper presents a novel approach for incorporating solar harvesting capability into existing passive RFID tags without increasing the parts count or changing the tag assembly process. Our approach employs the tags antenna as a dual function element in which the antenna simultaneously harvests RF energy, communicates with the RFID reader, and harvests solar energy for auxiliary power. This is accomplished by using low cost, printable photovoltaics deposited on flexible substrate to form part of the antennas radiating structure. Several prototype UHF RFID antennas are demonstrated using commercially available thin film, amorphous solar cells. To quantify the improvement in tag performance, Intels WISP was used as an initial test vehicle. The effective read range of the tag was increased by six times and exceeded the readers sensitivity limitations. Additionally, the new antenna allowed for sensing and computing operations to take place independent of the RFID reader under typical office lighting conditions.