Benjamin H. Waters

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.

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%.

Power Delivery and Leakage Field Control Using an Adaptive Phased Array Wireless Power System

Efficient wireless power transfer and precise control of power delivery and leakage field strength can be achieved using a phased array wireless power transfer system. This has particular importance for charging multiple devices simultaneously, or charging devices in environments where humans or foreign objects will be in close proximity. The phased array wireless power system consists of two or more phase-synchronized power amplifiers each driving a respective transmit coil. The system can maximize power delivery to an intended receiver in one location while simultaneously minimizing power delivery and leakage fields in other locations. These functions are possible by varying the amplitude and phase of each transmitter. This paper provides an analysis of a phased array wireless power transfer system using near-field magnetically coupled resonators, and derives parameters that can be used to automatically determine the optimal magnitude and phase of each transmitter to deliver power to one or more receivers. Experimental results verify the theoretical analysis and additional features of the full system are demonstrated.

Innovative Free-range Resonant Electrical Energy Delivery system (FREE-D System) for a ventricular assist device using wireless power.

Technological innovation of a smaller, single moving part has an advantage over earlier large pulsatile ventricular assist devices (VADs) prone to mechanical failure. Drivelines limit the potential for extended patient survival durations with newer pumps and act as source for infection, increased morbidity, rehospitalizations, and reduced quality of life. The Free-range Resonant Electrical Energy Delivery (FREE-D) wireless power system uses magnetically coupled resonators to efficiently transfer power. We demonstrate the efficiency over distance of this system. The experimental setup consists of an radiofrequency amplifier and control board which drives the transmit resonator coil, and a receiver unit consisting of a resonant coil attached to a radiofrequency rectifier and power management module. The power management module supplies power to the axial pump, which was set at 9,600 rpm. To achieve a seamless wireless delivery in any room size, we introduced a third relay coil. This relay coil can be installed throughout a room, whereas a single relay coil could be built into a jacket worn by the patient, which would always be within range of the receive coil implanted in the patient’s body. The power was delivered over a meter distance without interruptions or fluctuations with coil, rectifier, and regulator efficiency more than 80% and overall system efficiency of 61%. The axial pump worked well throughout the 8 hours of continuous operation. Having same setup on the opposite side can double the distance. A tether-free operation of a VAD can be achieved by FREE-D system in room-size distances. It has the potential to make the VAD therapy more acceptable from the patient perspective.

Optimal coil size ratios for wireless power transfer applications

Optimization of coil sizes for wireless power transfer applications has a significant impact on the range and efficiency of the wireless power system. Often it is difficult to accurately analyze how a set of coils will perform before they are constructed due to the complexity of approximating the parasitic components and coupling coefficients associated with the coils. Also, for certain wireless power applications such as consumer electronic devices, implanted medical devices and industrial equipment that have physical constraints on the size and shape of the coils, it can be a difficult and time-consuming process to design and evaluate several different coil configurations for optimal range and efficiency. This paper provides simplified design equations to accurately calculate the self inductance, capacitance, resistance, quality factor and coupling coefficient in terms of coil geometries for flat, spiral coils. Experimental results validate the analysis and a design example is provided to optimize the size of a transmit coil for maximum range and wireless power efficiency to a 5.8cm receive coil used in a four-element wireless power transfer system.

A wirelessly powered, biologically inspired ambulatory microrobot

Onboard power remains a major challenge for miniature robotic platforms. Locomotion at small scales demands high power densities from all system components, while limited payload capacities place severe restrictions on the size of the energy source, resulting in integration challenges and short operating times when using conventional batteries. Wireless power delivery has the potential to allow microrobotic platforms to operate autonomously for extended periods when near a transmitter. This paper describes the first demonstration of RF wireless power transfer in an insect-scale ambulatory robot. A wireless power transmission system based on magnetically coupled resonance is designed for the latest iteration of the Harvard Ambulatory MicroRobot (HAMR), a piezoelectrically driven quadruped that had previously received power through a tether. Custom power and control electronics are designed and implemented on lightweight printed circuit boards that form a part of the mechanical structure of the robot. The integration of the onboard receiver, power and control electronics, and mechanical structure yields a 4cm, 2.1g robot that can operate autonomously in two wireless power transmission scenarios.

A Reconfigurable Resonant Coil for Range Adaptation Wireless Power Transfer

To widen the range of a magnetically coupled wireless power transfer system, we propose a novel reconfigurable resonant coil. This device consists of a series of subcoils that use switches to control the number of turned-on subcoils. With this design, both the coupling coefficient between coils and the quality factor of the proposed coil can be dynamically tuned when the distance between the transmit and receive coils changes. Thus, the proposed coil system increases the efficient power transfer range compared to a conventional resonant system. In this work, we replace the first coil in a four-coil system with the proposed coil. This coil uses six switches to change the number of series subcoils from six to one as the distance between the transmit and receive coils increases. A theoretical analysis demonstrates how to properly design and configure the proposed coil. Experimental results show the range at which the system achieves 70% efficiency or higher increases by 120% compared to the conventional system without the proposed method. The proposed system keeps high efficiency when the load varies from 40 to 300 Ω.

Toward Total Implantability Using Free-Range Resonant Electrical Energy Delivery System: Achieving Untethered Ventricular Assist Device Operation Over Large Distances

Heart failure is a terminal disease with a very poor prognosis. Although the gold standard of treatment remains heart transplant, only a minority of patients can benefit from transplants. Another promising alternative is mechanical circulatory assistance using ventricular assist devices. The authors envision a completely implantable cardiac assist system affording tether-free mobility in an unrestricted space powered wirelessly by the innovative Free-Range Resonant Electrical Energy Device (FREE-D) system. Patients will have no power drivelines traversing the skin, and this system will allow power to be delivered over room distances and will eliminate trouble-prone wirings, bulky consoles, and replaceable batteries.

SAR distribution for a strongly coupled resonant wireless power transfer system

Tissue heating is a key safety consideration in wireless power transfer (WPT) systems. Heating is regulated in the form of specific absorption rate (SAR) limitations to prevent dangerous conditions when wireless power transfer is used in proximity to people. Implanted biomedical devices which depend on wireless power transfer for their operation are particularly of interest, as a high potential for tissue heating exists in these systems. Finding ways to reduce SAR for a given load power requirement enables reduced tissue heating and/or increased limits on power transmission. This work explores SAR heating in the two resonant modes (in-phase and out-of-phase) of a strongly coupled wireless power transfer system, where the power receiver is implanted in tissue. Results based on full EM simulation with realistic planar transmit/receive coil model near 13.56 MHz and simplified tissue model indicate that the higher frequency mode (out-of-phase mode) of strongly coupled wireless power transfer results in significantly lower peak and average SAR heating.