Sensor Systems Laboratory

Inventing new technology for sensing, robotics, wireless power, and medical devices

Joshua R. Smith, Ph. D.
Milton and Delia Zeutschel Professor
Allen School of Computer Science and Engineering, University of Washington
Department of Electrical and Computer Engineering, University of Washington

WREL (Wireless Resonant Energy Link)

WREL Researchers

Joshua R. Smith, Principal Investigator

Ben Waters, MS Student, EE

Brody Mahoney, MS Student, EE

Xingyi Shi, MS Student, EE


WREL enables transfer of large amounts of power (up to 50W) over medium distances (1m, depending on coil size). Our key contribution is the development of adaptive tuning techniques that enable high efficiency power transfer, even as transmit-receive range, orientation, or load vary. WIth most wireless power systems, the further the receiver is from the transmitter, the less power can be transferred. The WREL system has a "magic regime" in which efficiency does not fall with distance. We are currently working to power implanted medical devices using these techniques. Find out more about the FREE-D system, our wirelessly powered heart pump.

Our recent work in WREL investigates the next generation of wireless power for long range, adaptive wireless power transfer for both high power (100s of watts) and low power (milli-watt) consumer electronic, biomedical, and military applications. Some of the previous applications for the WREL system include:

WREL Publications

Adaptive Impedance Matching for Magnetically Coupled Resonators, Benjamin H. Waters, Alanson P. Sample, and Joshua R. Smith. PIERS Proceedings; pp. 694-701, Moscow, Russia, August 19023, 2012.

Numerical Electromagnetic Analysis of Human Exposure for Wireless Power Transfer Systems, Andreas Christ, Mark G. Douglas, John Roman, Emily B. Cooper, Alanson P. Sample, Joshua R. Smith, Niels Kuster. Proceedings of the Tenth International Congress of the European Bioelectromagnetics Association (EBEA 2011), Rome, Italy Feb 21-24, 2011.

Evaluation of Wireless Resonant Power Transfer Systems with Human Electromagnetic Exposure Limits, Andreas Christ, Mark G. Douglas, John Roman, Emily B. Cooper, Alanson P. Sample, Joshua R. Smith, Niels Kuster, submitted for publication.

Analysis, Experimental Results, and Range Adaptation of Magnetically Coupled Resonators for Wireless Power Transfer, Alanson P. Sample, David T. Meyer, Joshua R. Smith, IEEE Transactions on Industrial Electronics

Magnetic Resonant Coupling

Coupled magnetic resonance is a near-field phenomenon where power transfer can be highly efficient between the source and receiver with negligible coupling to environmental objects.

In conventional inductive coupling, efficiency drops off as 1/distance3. Charging mats and wireless toothbrushes use these methods and can only transfer appreciable power when the receiving device is within a few millimeters. In contrast, WREL can deliver tens of watts of power over several feet with > 80% efficiency.

An unusual property of our double resonant wireless power transfer system is the presence of what we call the "magic regime." Magnetically coupled resonators with sufficiently high quality factor (Q) coils exhibit a surprising behavior. In ordinary inductive power transfer, efficiency decreases whenever transmit-to-receive distance increases; suprisingly, in this system maximum power transfer efficiency can be achieved over a broad range of distances and orientations. Increasing distance does not always decrease efficiency. The following figure shows this behavior:

The axis labeled k23 above is related inversely to distance; larger values of k23 (toward the left side of the figure) correspond to smaller TX-RCV distances. The vertical axis measures power transfer efficiency. The figure shows that maximum power transfer efficiency can be achieved for any distance inside the dotted red box, which indicates the magic regime. The extent of the magic regime, which is bounded by a critical coupling point, is a key figure of merit which can be used to compare performance of disparate implementations.

In order to achieve this constant efficiency behavior of the overcoupled regime, we have developed an auto-tuning algorithm that tracks the optimal frequency corresponding to maximum S21. Auto-tuning is performed by using a directional coupler to detect the forward and reflected waves passing from a power amplifier to the transmit resonator. A micro-controller unit can detect these changes and the WREL controller can automatically tune the system to achieve maximum power transfer efficiency. The block diagram below shows this topology:

The following two videos demonstrate the benefits of using auto-tuning to maximize efficiency and using relay resonators to increase range of the wireless power system, as well as showing that various objects interferring with the medium between the transmit and receive resonators have a minimal affect on the wireless power transfer.