Witricity Leverages Magnetic Resonance for Flexible
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WIRELESS CHARGING WITRICITY LEVERAGES MAGNETIC RESONANCE FOR FLEXIBLE WIRELESS CHARGING Engineers at WiTricity have used multiphysics simulation in the innovative development of wireless power transfer technology that extends efficiency and charging ranges beyond the reach of existing methods. By LEXI CARVER IMAGINE COMING HOME and dropping your phone, laptop, and Bluetooth® headset on your kitchen table so that they all recharge, simultaneously. Or driving your electric car into a garage, parking above a mat, and knowing it will be charged in the morning. Or being told by your doctor that there is a new medical implant to replace the one you wear—and the new version does not include power cords or the need to replace batteries. Wireless power transfer is making these scenar- ios and other applications a reality by delivering a cord- less way to charge electronic devices. WiTricity, a Watertown, MA-based company that develops wire- less charging technology based on magnetic resonance, has launched the most consumer-friendly method avail- able for the wireless transfer of electrical power. Invented at the Massachusetts Institute of Technology (MIT) FIGURE 1: Top: Concept behind WiTricity’s technology, includ- by Professor Marin Soljacic and a team of research- ing coils, electronics that control power output and regulation, and ers, WiTricity’s technology has the ability to charge mul- wireless communication between the source and capture devices to tiple devices at once, over distances and through mate- ensure the correct power output. Bottom: The technology supports charging through surfaces of different materials (left); the wireless rials like wood, plastic, granite, and glass. Companies range may be extended using resonant repeaters (right). such as Toyota, Intel, and Thoratec have already licensed the technology for use in hybrid-electric vehicles, smart- phones, wearable electronics, and heart pumps. that functions as a power The field converts the mag- source. A power ampli- netic energy back to radio- » MAGNETIC RESONANCE WIDENS THE fier connected to this frequency alternating cur- TRANSMISSION GAP source coil controls the rent in the receiver, which OTHER OPTIONS FOR WIRELESS ENERGY TRANSFER require pre- power levels and oper- can then be used as a new cise device positioning on a pad or holder, very close prox- ating frequency, driving local power source after imity to (often resting directly on) the charging source, and the magnetic field levels. being rectified and regu- the source can only charge a single device with a single coil. A capture device, which lated by power electronics. Now, the engineers at WiTricity have leveraged the power of acts as a receiver and cap- The notable differ- magnetic resonance to rethink these limitations. tures the magnetic field, ence between WiTricity’s Their system, dubbed “highly resonant wireless power contains another coil tuned technology and other transfer”, relies on oscillating time-varying magnetic fields to the same frequency as approaches is the use of generated by alternating current passing through a coil the source (see Figure 1). magnetic resonance. With 8 COMSOL MULTIPHYSICS SEPTEMBER 2015 Bluetooth is a registered trademark of Bluetooth SIG, Inc. FIGURE 2: Left: A capture resonator, a resonant repeater, and a source resonator. Center: A WiTricity source resonator designed for consumer electronics applications. Right: An electric vehicle charging wirelessly, parked above a charging pad. both coils tuned to the Transfer occurs effec- ing and testing is time- Then he ran a multiphy- same resonant frequency, tively even with barriers consuming and expen- sics study to analyze the the receiving coil is able to (such as people and concrete sive, and design mistakes resulting electromagnetic capture maximum power walls) between the power can set you back a lot—in and thermal performance through the magnetic field source and the receiver. such a competitive mar- as a function of power with very low losses, and ket, we can’t afford that,” drawn by the devices, power can be transmit- » MODELING THE he said. “Design valida- coil displacements, and ted without the source ELECTROMAGNETICS tion in COMSOL was cost- the effects of perturbing and capture device sit- OF WIRELESS POWER effective and time-saving objects (see Figure 3, top). ting next to each other or TRANSFER and allowed us to virtually He extracted circuit being perfectly aligned. IN DESIGNING FOR MAXI- test our concepts before parameters from the “One major advantage MUM EFFICIENCY using coils building the real device.” results to guide the design is the flexibility of motion with the same resonant fre- He created simula- of the electronics, as well and positioning. The quency, Kurs and his team tions with different set- as predictions of power receiving coil doesn’t have had to account for variables ups for each application, dissipation and thermal to be in direct contact with such as number of coil turns, and included electromag- loading on different com- the device; for instance, diameter, and necessary netically relevant compo- ponents (see Figure 3, bot- while driving your car you power input. From the early nents such as coil wind- tom). The team adjusted could drop your phone stages of development, they ings, specially shaped their designs accord- into a cup holder posi- relied on computer simula- ferrites and metal surfaces ingly, determining the via- tioned near the capture tion to test key details, ver- used to guide the electro- ble range of coil displace- device, rather than arrang- ify designs, and optimize the magnetic field, plates for ments and power levels as ing it on a charging pad,” system. Using a COMSOL shielding sensitive elec- a function of size, weight, explained Andre Kurs, co- Multiphysics® software tronics, and large objects and thermal constraints. founder of WiTricity. “And model, Kurs analyzed the that might perturb the “The simulation allowed you can charge everything electromagnetic and thermal field, such as a car chassis. us to disentangle various together, including elec- behavior of different coil con- tronics that have differ- figurations, and was able to ent power requirements.” quickly validate new designs. Design validation in COMSOL was Extending the wireless One challenge lay in cost-effective and time-saving and range is also easy enough: making the tech nology allowed us to virtually test our concepts resonant repeaters that scalable for a wide range before building the real device.” each contain another cir- of devices: a car, for cuit and coil may be placed instance, needs a differ- —ANDRE KURS, CO-FOUNDER, WITRICITY between the source and ent charging configura- receiver, allowing power tion than a smartphone. to ‘hop’ over greater dis- “We have such a wide range tances (see Figure 2). of applications, prototyp- SEPTEMBER 2015 COMSOL MULTIPHYSICS 9 WIRELESS CHARGING To make sure that the field levels and resulting body temperatures would meet regulations, the team ran several more COMSOL simulations to study dif- ferent body tissues in close proximity to the device. Their models calculated the electric field based on FIGURE 4: COMSOL simula- tion showing the specific absorp- the operating frequency of tion rate (SAR) in a hand above the charging system, and a charging cell phone. SAR is a confirmed that the results measurement of electromagnetic were well within FCC safety energy absorbed and turned into guidelines (see Figure 4). heat. Results are in dB relative to the FCC limit (a value of zero represents the limit). » RETHINKING A GROWING INDUSTRY FOR WIRELESS top system. The A4WP is CHARGING an innovative group com- WITRICITY’S DESIGNS BASED prising leading com panies on magnetic resonance are that are ushering in a new a major improvement over way of thinking about other wireless charging wireless power: they are methods, allowing reliable imagining a future where wireless power transfer everyday surfaces—desks, in a flexible, consumer- cup holders, and even friendly product. Thanks your kitchen countertop— to their simulation work become zones for charg- in COMSOL Multiphysics, ing the electronic devices the WiTricity team opti- we depend on so much. FIGURE 3: Simulation results showing the magnetic field levels (top) mized their designs for and power dissipated (bottom) in a source resonator for consumer better efficiency and lon- electronics applications. ger ranges before building costly prototypes. effects that we couldn’t iso- » KEEPING A SAFE In addition to being late just by testing, like DISTANCE, RIGHT UP frontrunners in game- power dissipation and heat CLOSE changing wireless transfer,” Kurs remarked. SINCE SUCH DEVICES are power transfer technol- “The flexibility of COMSOL near to or in contact with ogy, WiTricity is on the was particularly useful; people’s bodies, electronics board of the Alliance for we built a suite of simu- manufacturers must adhere Wireless Power (A4WP), lation apps around it that to safety limits on the elec- an organi zation dedicated would allow any of our tromagnetic fields emit- to building a “global wire- engineers—whether they ted by their products. The less ecosystem” and creat- knew how to use COMSOL magnetic fields needed for ing standards for wireless or not—to quickly test WiTricity’s wireless transfer charging. Another board and validate designs are usually fairly weak, but member, Intel, has licensed even if they didn’t under- each new application needs WiTricity’s technology to Andre Kurs, co- stand the entire model.” be checked for compliance. develop a wireless desk- founder, WiTricity. 10 COMSOL MULTIPHYSICS SEPTEMBER 2015.