A team led by Khurram Afridi, associate professor of electrical and computer engineering, is pioneering an innovative way to wirelessly charge electric cars, automated forklifts and other mobile machines while keeping them in motion.

Imagine you are driving a Tesla or equivalent electric car on the highway. Your battery is low. Of course, you can get off at the next exit and spend your time and energy looking for a charging station. Or you can simply change lanes and drive through a special charging belt embedded in the road.

This is the vision of Khurram Afridi, associate professor of electrical and computer engineering in the School of Engineering. He pioneered an innovative method for wirelessly charging electric cars, automated forklifts and other mobile machines while keeping them in motion.

PhD students (from left) Sounak Maji and Maida Farooq and postdoctoral researcher Sreyam Sinha develop wireless power transmission systems in the laboratory of Khurram Afridi, associate professor of electrical and computer engineering on the far right.

This technology not only saves time for drivers, but also improves warehouse productivity. It will also pave the way for more sustainable transportation.

"When you say,'Okay, we will enable electric vehicles,' you will be asked a lot of infrastructure issues." Afridi said. "How does that society work? If every car in this country is electric, you need a lot of sockets to plug them in. We don't have that kind of power source for fast charging in our house."

The origin of the Afridi project can be traced back to the original Tesla more than 100 years ago-Serbian-American inventor Nikola-who used AC electric fields to illuminate unplugged fluorescent lights in the 1890s, amazed audiences. Inspired by Tesla's ideas, French scientists designed their own version of wireless power that will transmit energy through an alternating magnetic field instead of an electric field to provide power to the tram. They soon discovered that their method was impractical, and their interest in the technology faded.

Over the past few decades, various groups have re-examined the idea of ​​providing wireless power to moving vehicles, starting from California in the 1980s as part of its Advanced Transportation and Highway Partnership (PATH) program trying to test road-powered electric Car, developed conveyor system power supply technology for New Zealand researchers in the 1990s. However, even with the scientific advancement of electric and electronic technologies, commercial attempts to apply this method to road-driven vehicles have proven difficult and expensive.

The most popular wireless vehicle charging proposals focus on using the power of alternating magnetic fields, because these magnetic fields can be generated using off-the-shelf currents. If the magnetic field changes rapidly, the transmitted power will increase. Therefore, compared to the hundreds of hertz used in the PATH project, recent work (achieved through improved technology) has focused on frequencies of several tens of kilohertz.

However, the problem remains that the magnetic fields are bulky because they form a complete loop and they must be directed to keep them away from certain areas. This is because the high-intensity alternating magnetic field can harm passengers in the car or heat the steel bars on the road, so it is necessary to block at these points. The material that guides the magnetic field-ferrite-is fragile, bulky, expensive, and loses a lot of energy when the magnetic field changes rapidly.

The wireless charging project combines two distinct communities—high-frequency electronics and high-power electronics—to create a new field and realize new applications.

Although bulky and expensive wireless chargers can be used for stationary charging, the large-scale deployment of magnetic field-based systems to charge mobile vehicles is too costly.

Afridi discovered a unique solution to these challenges through a field of vision far beyond the magnetic field and kilohertz: outer space.

Afridi said: "Wireless power transmission is based on the same basic physical principles used to send information to deep space spacecraft (such as Voyager) via radio waves." "Except for now, we send more to moving vehicles over shorter distances. energy of."

As an undergraduate at the California Institute of Technology, Afridi has a keen interest in radio frequency electronics for deep space communications. At the end of his sophomore year, he worked with NASA's Jet Propulsion Laboratory to design the 8 GHz and 32 GHz transmitters of the SURFSAT 1 satellite, which was launched in 1995 as the predecessor of the Cassini Saturn satellite two years later.

At MIT's graduate school, Afridi turned to research power electronics, which operates at much lower frequencies with a difference of six orders of magnitude, but handles much higher power levels. In fact, Afridi gave up his gigahertz for kilohertz. But he always wanted to know how to push power electronic equipment to the highest frequency.

He said: "What really drives me fundamentally is these two very different communities-high-frequency electronics and high-power electronics-they have never talked to each other, they do not speak the same language, they are essentially very different. Solving problems in different ways and merging them together, essentially creating a whole new field and enabling new applications."

From deep space to highways and warehouses

In 2014, Afridi began to explore the potential of restoring Nikola Tesla's original living room technique of manipulating electric fields, but the frequency and power were much higher.

In the system designed by Afridi's team, two insulated metal plates on the ground are connected to the power line through a matching network and a high-frequency inverter to generate an oscillating electric field that attracts and repels the charges in a pair of matching metal plates connected to it. The bottom of the vehicle. This drives the high frequency current through the circuit on the vehicle and rectifies it. The rectified current then charges the battery.

Compared with the circular arc of the magnetic field, a huge advantage of electric fields is that they have more linear and more directional characteristics. Therefore, they do not require magnetic flux guiding materials, such as ferrite, and can work at higher frequencies. The main challenge is that the electric field generated by the available voltage is very weak. Afridi's team compensated by increasing the voltage and operating the system at a very high frequency to achieve high power transmission.

"The operating frequency of the latest magnetic field system developed for electric vehicle charging is 85 kHz. The operating frequency of the electric field system we developed in the laboratory is 13.56 MHz. So it runs nearly 200 times faster, which makes up for it to some extent It has five orders of magnitude shortcomings that it needs to overcome," Afridi said. "It turns out that you can handle higher voltages more easily than higher currents, which helps to further bridge the difference in power transfer capabilities."

The team's ferrite-free system is expected to be smaller, lighter, cheaper and easier to embed on roads. However, the system is not easy to develop.

Wireless charging technology uses the energy of the electric field, but will increase the voltage and work at high frequencies to achieve a large level of power transmission.

In order to overcome many technical obstacles, Afridi's team-including colleagues Brandon Regensburger, MS '19, Ph.D. '20, Postdoctoral researcher Sreyam Sinha, Ph.D. '20, Ph.D. student Sounak Maji-Collaborated with several collaborators at Cornell University. Francisco Monticone, assistant professor of electrical and computer engineering, helped the team develop a charging board based on engineering metamaterials to better focus the electric field. Debdeep Jena and Huili Grace Xing are both professors of electrical and computer engineering and materials science and engineering, and they are also collaborating to develop wide band gap materials and equipment that can handle high-voltage and high-frequency operation.

The team’s most notable innovation is the Active Variable Reactance (AVR) rectifier, which allows the vehicle to obtain full power when passing the charging plate, even if the charging plate pair (approximately every few meters on the road) is not perfectly aligned. AVR also helps to power larger vehicles, which have a greater clearance between the landing gear and the ground.

If it is difficult to create a wireless charging system, it is equally difficult to implement it on a large scale.

Afridi believes that one way is to electrify high-traffic roads first, especially to support large long-distance trucks. Another option is to focus on the city and install charging bars at stop signs and traffic lights so that drivers can charge while waiting.

The technology can also be used in manufacturing warehouses and fulfillment centers, so autonomous robots can work around the clock. Afridi is currently working with Toyota Material Handling North America to develop dynamic charging for forklifts and material handling mobile robots. He is also a member of the International Research Center funded by the National Science Foundation, which is dedicated to advancing sustainable electrified transportation.

"Wireless charging may sound crazy at first," he said. "But if we really have this technology, it will make a lot of sense."

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