Aging tennis star Andre Agassi and upstart Sebastien Grosjean made it to the quarterfinals of the French Open this past week on a gritty combination of sweat, determination and piezoelectric technology.
Both players looked to a Head Sport AG racket integrated with a microchip and piezo composites to boost their games. While it wasn't the first time smart rackets have been used and while veteran Agassi was eliminated before the finals endorsement of the technology by two top-seeded players at a high-profile tournament marked a milestone for the proliferation of intelligent electronics in sporting goods.
Using piezoelectric materials in conjunction with a custom-designed analog chip, the racket "transforms the mechanical energy from ball impact into electrical energy," said Herfried Lammer, who is racquet-sport R&D director at Head. The microchip, integrated in the racket handle, then "orients the direction of the force, stiffens the racket for ultimate power and eliminates vibrations for comfort."
The so-called Head ChipSystem was the brainchild of 10 Head researchers in Kennelbach, Austria, working in partnership with developers from MIT spin-off Continuum Control Corp. (now Continuum Photonics; Billerica, Mass.) and fiber supplier Advanced Cerametrics Inc. (Lambertville, N.J.). The team completed development of its first model two years ago.
Lammer told EE Times that the biggest challenge in engineering the racket was the "limitations on power." The International Tennis Federation has a relatively permissive record of accepting advances in racket shapes and materials that boost the level of play, but its rules prohibit the use of external batteries in rackets.
The partnership with Continuum made it possible to develop "self-powered electronics," in which the electricity needed to power the device is entirely generated by the user. Because the ChipSystem's technology is self-powered, the Head racket's design stays on the fair side of federation rules, Lammer said.
The self-powered electronics are embedded in the racket handle and Continuum's active-fiber composites (AFCs) are integrated into the yoke, said George Rossetti, principal materials scientist at Continuum.
Intellectual-property rights to the piezoelectric technology and microchip designed by Continuum belong to Head inasmuch as they apply to sporting goods, said Lammer. But Rossetti said Continuum retains rights to a number of adjacent technology components for use outside tennis equipment.
Subsequent to its work with Head, Continuum ceased developing technology for sporting goods and now is pursuing optical-networking components and subsystems, including optical switches. Rossetti would not say whether the company will leverage its existing piezoelectric technology for its optical-networking play.
Technology in play
Head's ChipSystem comprises the AFCs, flex circuitry and the microchip. The AFCs, called Intellifiber by Head, consist of piezoelectric composite materials that are situated between electrodes on both sides of the racket's throat. The flex circuitry connects the piezoelectric fibers in the frame with the chip in the handle, transmitting voltage and current to and from the microchip. The chip simultaneously controls the power boost and eliminates vibration.
The piezoelectric composites convert mechanical vibrations caused by the ball's impact into electrical energy. The chip then "captures the energy and gives out information on when to release [it]," Lammer said. Within milliseconds, the electronics send the electrical energy back to the piezoelectric composites to optimize racket response.
As a result, he said, the ChipSystem creates an "active counterforce" in the throat area of the racket to boost power and "electronically creates a canceling countervibration to provide dampening effects."
The piezoelectric effect is a well-understood property that exists in many materials. In a piezoelectric material, the application of a force or stress results in a charge in the material. Applying a charge to the same material, in return, will result in a change of mechanical dimensions or strain.
Piezo-based intelligent systems have been used in aircraft and advanced car designs. Where Head's ChipSystem differs is in "our self-powered approach," claimed Lammer. "We are using only the energy that is created by the incoming ball and the vibrations occurring due to the impact."
With the advanced piezoelectric fiber now being perfected at Head, "we can get higher voltage up to 600 V," Lammer claimed. Further, while conventional piezoelectric ceramic plates offer only a flat surface, the piezoelectric fibers tailored for the Head application can hug a tennis racket's curves and thus allow "us to work with a force and a movement in a certain, desired direction," he said.
Head is not disclosing details of its microchip; Lammer would say only that it is "principally a MOSFET chip." The device resides on a circuit board in the racket handle, with the inductance acting as the storage mechanism. Graf Elektronik GmbH (Dornbirn, Austria) manufactures the board for Head.
Next up is to make the Intellifiber-plus-microchip technology a fixture on the slopes. Earlier this year, Head introduced Intelligence Ski, which it bills as able to sense and adapt to changes in snow conditions, speed and terrain.
Just as in the tennis racket, the Intellifiber used in the ski transforms mechanical energy into electricity. The electric energy generated by the fiber is sent to a microchip embedded in the ski and then through a resistor, where disruptive frequencies are filtered out. The remaining energy is fired back to the fiber, and the torsional stability of the ski is instantly increased, according to the company. "This essentially allows people to make a clear turn," said Lammer.
Active vs. passive skis
Head has competition in the application of piezo technology to ski equipment. A company called K2, in conjunction with Active Control (Cambridge, Mass.), has applied it to ski gear as a passive dampening device.
Head's Lammer said his company's ski differs because it is based on an active piezoelectric system. While the passive system uses only the dissipation of the energy created, the active system means "we store the energy and there is additional energy we can use to act again," said Lammer. Further, "we can control the timing of releasing the energy."
Head continues to work with Advanced Cerametrics on fiber technology. Its partnership with Continuum ended when the team completed the development of the electronic racket. Much of the improvements made in future rackets will stem from software development "to improve the timing" Lammer said.