Wireless charging with a laser
The team mounted a thin PV cell to the back of a smartphone, illuminated by an infrared laser. To make the system safe, the team designed safety features such as flat plate heatsink on the smartphone to dissipate excess heat from the laser, as well as a reflector-based mechanism to shut off the laser if a person tries to move in the charging beam’s path.
The safety system that shuts off the charging beam centres on low-power, harmless laser “guard beams,” which are emitted by another laser source co-located with the charging laser-beam and physically “surround” the high power charging beam. Custom 3D printed reflectors around the power cell on the smartphone reflect the guard beams back to photodiodes on the laser emitter. The guard beams deliver no charge to the phone themselves, but the reflection from the smartphone back to the emitter allows them to serve as a sensor, stopping the charging beam when any object crosses a guard beam.
“Safety was our focus in designing this system,” said Shyam Gollakota, an associate professor in the UW’s Paul G. Allen School of Computer Science & Engineering. “We have designed, constructed and tested this laser-based charging system with a rapid-response safety mechanism, which ensures that the laser emitter will terminate the charging beam before a person comes into the path of the laser.”
“In addition to the safety mechanism that quickly terminates the charging beam, our platform includes a heatsink to dissipate excess heat generated by the charging beam,” said Arka Majumdar, a UW assistant professor of physics and electrical engineering. “These features give our wireless charging system the robust safety standards needed to apply it to a variety of commercial and home settings.”
“The guard beams are able to act faster than our quickest motions because those beams are reflected back to the emitter at the speed of light,” said Gollakota. “As a result, when the guard beam is interrupted by the movement of a person, the emitter detects this within a fraction of a second and deploys a shutter to block the charging beam before the person can come in contact with it.”
A narrow beam delivers a steady 2W of power to the PV cell from a distance of up to 4.3m. But the emitter can be modified to expand the charging beam’s radius to an area of up to 100cm2 from a distance of 12m. This extension means that the emitter could be aimed at a wider charging surface, such as a counter or tabletop, and charge a smartphone placed anywhere on that surface.
The researchers programmed the smartphone to signal its location by emitting high-frequency acoustic chirps that were picked up by small microphones on the laser emitter.
“This acoustic localization system ensures that the emitter can detect when a user has set the smartphone on the charging surface, which can be an ordinary location like a table across the room,” said Vikram Iyer, a UW doctoral student in electrical engineering.
When the emitter detects the smartphone on the desired charging surface, it switches on the laser to begin charging the battery.
“The beam delivers charge as quickly as plugging in your smartphone to a USB port,” said Elyas Bayati, a UW doctoral student in electrical engineering. “But instead of plugging your phone in, you simply place it on a table.”
To ensure that the charging beam does not overheat the smartphone, the team also placed thin aluminum strips on the back of the smartphone around the power cell. These strips act as a heatsink, dissipating excess heat from the charging beam and allowing the laser to charge the smartphone for hours. They even harvested a small amount of this heat by mounting a nearly-flat thermoelectric generator that is 8mm thick and 40mm wide above the heatsink strips.