driverless car driving

Prototype system enables wireless charging for electric cars in motion

Image credit: Dreamstime

Engineers have demonstrated a wireless charging solution that can power objects in motion which could one day allow electric cars to be charged while they are driving.

Although wireless charging pads already exist for smartphones, they only work if the phone is sitting still.

Stanford electrical engineers Shanhui Fan and Sid Assawaworrarit built the first system that could wirelessly recharge objects in motion three years ago but the technology was too inefficient to be useful outside the lab.

They have now demonstrated a technology that could potentially be scaled up to power a car moving down the road.

In the near-term, the system could make it practical to wirelessly recharge robots as they move around in warehouses and on factory floors, eliminating downtime and enabling robots to work almost around the clock.

“This is a significant step toward a practical and efficient system for wirelessly recharging automobiles and robots, even when they are moving at high speeds,” Fan said.

“We would have to scale up the power to recharge a moving car, but I don’t think that’s a serious roadblock. For recharging robots, we’re already within the range of practical usefulness.”

Wireless chargers transmit electricity by creating a magnetic field that oscillates at a frequency that creates a resonating vibration in magnetic coils on the receiving device. This frequency changes if the distance between the source and receiver changes by even a small amount.

In their first breakthrough, the researchers developed a wireless charger that could transmit electricity even as the distance to the receiver changes by incorporating an amplifier and feedback resistor that allowed the system to automatically adjust its operating frequency.

That initial system wasn’t efficient enough to be practical as the amplifier uses so much electricity internally to produce the required amplification effect that the system only transmitted 10 per cent of the power flowing through the system.

The researchers have now managed to boost the system’s wireless-transmission efficiency to 92 per cent by replacing the original amplifier with a far more efficient 'switch mode' amplifier.

Such amplifiers aren’t new, but they are finicky and will only produce high-efficiency amplification under very precise conditions. It took years of tinkering, and additional theoretical work, to design a circuit configuration that worked.

The new lab prototype can wirelessly transmit 10 watts of electricity over a distance of around 70cm.

Fan says there aren’t any fundamental obstacles to scaling up a system to transmit the tens or hundreds of kilowatts that a car would need.

He says the system is more than fast enough to re-supply a speeding automobile. The wireless transmission takes only a few milliseconds - a tiny fraction of the time it would take a car moving at 70 miles an hour to cross a four-foot charging zone. The only limiting factor will be how fast the car’s batteries can absorb the power.

The wireless chargers shouldn’t pose a health risk because even those that are powerful enough for cars would produce magnetic fields that are well within established safety guidelines.

In 2016, researchers demonstrated a wireless charging system for electric vehicles that is 90 per cent efficient, although designed specifically for parked cars.

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