Imagine coming home and dropping your phone, laptop, and Bluetooth® headset on your kitchen table so that they all recharge simultaneously. What if you could drive your electric car into a garage, park above a mat, and know it will be charged in the morning? What if there was 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?

An electric vehicle charging wirelessly, parked above a charging pad.

Wireless power transfer is making these scenarios and other applications a reality by delivering a cordless way to charge electronic devices. WiTricity, a Watertown, MA-based company that develops wireless charging technology based on magnetic resonance, has launched a new method for the wireless transfer of electrical power. Invented at the Massachusetts Institute of Technology (MIT) by Professor Marin Soljacic and a team of researchers, WiTricity’s technology has the ability to charge multiple devices at once, over distances and through materials like wood, plastic, granite, and glass. Companies such as Toyota, Intel, and Thoratec have already licensed the technology for use in hybridelectric vehicles, smartphones, wearable electronics, and heart pumps.

The Power of Magnetic Resonance

Other options for wireless energy transfer require precise device positioning on a pad or holder, very close proximity to (often resting directly on) the charging source, and the source can only charge a single device with a single coil. The engineers at WiTricity have leveraged the power of magnetic resonance to rethink these limitations.

Highly resonant wireless power transfer relies on oscillating time-varying magnetic fields generated by alternating current passing through a coil that functions as a power source. A power amplifier connected to this source coil controls the power levels and operating frequency, driving the magnetic field levels. A capture device, which acts as a receiver and captures the magnetic field, contains another coil tuned to the same frequency as the source. The field converts the magnetic energy back to RF alternating current in the receiver, which can then be used as a new local power source after being rectified and regulated by power electronics.

Figure 1. (Left) A capture resonator, a resonant repeater, and a source resonator. (Right) A WiTricity source resonator designed for consumer electronics applications.

The notable difference between WiTricity’s technology and other approaches is the use of magnetic resonance. With both coils tuned to the same resonant frequency, the receiving coil is able to capture maximum power through the magnetic field with very low losses, and power can be transmitted without the source and capture device sitting next to each other or being perfectly aligned.

“One major advantage is the flexibility of motion and positioning. The receiving coil doesn’t have to be in direct contact with the device; for instance, while driving your car you could drop your phone into a cup holder positioned near the capture device, rather than arranging it on a charging pad,” explained Andre Kurs, co-founder of WiTricity. “And you can charge everything together, including electronics that have different power requirements.”

Resonant repeaters that each contain another circuit and coil may be placed between the source and receiver, allowing power to hop over greater distances (Figure 1). Transfer occurs effectively even with barriers (such as people and concrete walls) between the power source and the receiver.

Figure 2. Simulation results showing the magnetic field levels (left) and power dissipated (right) in a source resonator for consumer electronics applications.

In designing for maximum efficiency using coils with the same resonant frequency, the team had to account for variables such as number of coil turns, diameter, and necessary power input. From the early stages of development, they relied on computer simulation to test key details, verify designs, and optimize the system. Using a COMSOL Multiphysics® software model, the electromagnetic and thermal behavior of different coil configurations was analyzed, and new designs were validated.

One challenge lay in making the technology scalable for a wide range of devices; a car, for instance, needs a different charging configuration than a smartphone. “Design validation in COMSOL was cost-effective and time-saving, and allowed us to virtually test our concepts before building the real device.”

Figure 3. COMSOL simulation showing the specific absorption rate (SAR) in a hand above a charging cellphone. SAR is a measurement of electromagnetic energy absorbed and turned into heat. Results are in dB relative to the FCC limit (a value of zero represents the limit).

Simulations were created with different setups for each application, and included electromagnetically relevant components such as coil windings, specially shaped ferrites and metal surfaces used to guide the electromagnetic field, plates for shielding sensitive electronics, and large objects that might perturb the field, such as a car chassis. A multiphysics study was run to analyze the resulting electromagnetic and thermal performance as a function of power drawn by the devices, coil displacements, and the effects of perturbing objects (Figure 2, left). Circuit parameters were extracted from the results to guide the design of the electronics, as well as predictions of power dissipation and thermal loading on different components (Figure 2, right). The team adjusted their designs accordingly, determining the viable range of coil displacements and power levels as a function of size, weight, and thermal constraints.

Since such devices are near to or in contact with people’s bodies, electronics manufacturers must adhere to safety limits on the electromagnetic fields emitted by their products. The magnetic fields needed for WiTricity’s wireless transfer are usually fairly weak, but each new application needs to be checked for compliance. To make sure that the field levels and resulting body temperatures would meet regulations, the team ran several more COMSOL simulations to study different body tissues in close proximity to the device (Figure 3). Their models calculated the electric field based on the operating frequency of the charging system, and confirmed that the results were well within FCC safety guidelines.

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