Self-powered surface may evaluate table-tennis play

Zhong Lin Wang started playing table tennis only four years ago. But last year, he and other researchers came up with a clever way to up their game: Build a smart table.

Now they’ve just unveiled a prototype. It can measure where a ball lands, how fast the ball’s going and where it’s headed. The table can do that because its innovative wood surface forms the top layer of a novel self-powered sensor. The data it acquires could guide players to perform better, says Wang.

Wang is a materials scientist at the Georgia Institute of Technology, in Atlanta. He describes the invention as a machine that can learn to “analyze the behavior” of how someone plays.

Importantly, the new smart table won’t need a battery to detect the ball. When a ball hits the wood, it sets off a chain of events that can both generate a small electric current and record measurements of the ball’s behavior.

Hassan Askari says he is “mostly amazed” at this — and at the number of materials that turn out to have this property. Askari is a mechanical engineer at the University of Waterloo, in Canada. He didn’t work on the new sensor, but he does study self-powered materials.

Tapping electricity to study game play

Wang is an expert at inventing devices that provide their own power. In 2012, he invented a triboelectric nano generator. He calls it TENG, for short. Even if you’ve never heard of “triboelectricity,” you know the phenomenon. Most of us call it static electricity.

On dry winter days, if you shuffle across a carpet in your socks and then touch a doorknob, you may feel a shock. That shuffling caused negatively charged particles called electrons to build up on your body. To balance things out, when you touched the doorknob, the electrons jumped from you to that knob. That stream of electrons — an electric current — supplied that static-electric jolt you felt. 

The first TENG that Wang put together in 2012 looked like a sandwich. It had layers of charged materials separated by a small gap. When the sandwich bent or changed shape, its layered materials rubbed against each other. This caused electrons to start building up on one side. That left a positive charge on the other side of the sandwich. When the negatively and positively charged sides later separated and moved away from each other, they generated a small electric current. It traveled through a copper wire on the negative side.

Over the years, Wang has worked with researchers to build many devices using TENGs. They made plastic balls, for instance, that can float in the ocean, creating small currents as they bob in the waves. And some keyboards they built harvest energy from typing.

Why wood?

What makes the new game table truly unique is its use of wood as one of the TENG’s layers. As it turns out, Wang explains, “wood is an excellent triboelectric material.”

Wang’s group realized early on that it couldn’t rely on wood that comes directly from a tree. It’s too hard and too easily broken. They needed wood that could bend.

Lignin is the stuff that makes wood and other plants rigid and hard. It’s a natural polymer, or a material made from long chains of molecules. To remove the lignin, the scientists boiled balsa wood in chemicals for seven hours. Then they dipped the wood in boiling water to rinse out the chemicals. (If you follow such steps long enough, wood eventually becomes transparent. Another team used steps something like that to make window “glass” out of wood.) Once it came out of the boiling baths, the wood now flexed and bent easily.

The scientists then cut the treated wood into small squares. Each measured just 4.5 centimeters (1.8 inches) on a side. Those squares became the top layer of a TENG. Beneath this wood, they added a layer of copper to conduct electricity. They attached that layer to a copper wire. Then, instead of using a table-tennis ball, they hit the surface with a type of hard plastic. When it registered an electric current, they moved on to building a test table.

Wang’s team also published its findings on the new sensors November 26, 2019, in Nature Communications.

From TENG tables to more?

As a ball strikes the wood surface, that wood pushes against the copper layer. Electrons accumulate. When the wood bends back to its original position, a small amount of current travels through the wire. In lab tests, the engineers showed that a grid made of wood TENGs could be used to measure where the ball hit, how fast it was going and the angle it was traveling. Table-tennis players can use such data to learn more about their game and how to play better, says Wang. (Fact: He’s been using it to improve his own technique.)

Building a full-size game table would require a large grid of wood TENGs. Wang says that’s on his drawing board as a project for the future.

“To me, this is the first move toward using energy harvesting and sensing in sports,” says Askari. And it need not end with table tennis, he adds.

Self-powered sensors of wood could be useful in sports such as volleyball, he says. “In volleyball, officials use video checks to look at the lines around the court to make sure the ball is inside the court,” he notes. Sensors built into those lines could do that job. The sensors’ data could then be sent to the referees, he says. 

Concludes Askari: “This technology will give us lots of options for future applications.”

This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.