Smart, connected rackets are changing the game of tennis. Up until now, players have settled for making a rough guess as to how hard, with how much and what type of spin and how closely to the sweet spot near the center of the strings they hit the ball. They have no way to make quantitative goals or assess if they are making progress in their game.
Today, smart, connected tennis rackets accurately measure these critical parameters to determine how well the ball was hit and many other statistics. The information is transmitted from the tennis racket to an app on the player’s smartphone giving the player the ability to analyze and track his or her progress and performance.
Putting all of this intelligence inside a tennis racket was a major engineering challenge, according to Jean-Marc Zimmermann, Chief Information Officer of Babolat, the company that introduced the first and so far the only smart, connected tennis rackets, called the Play series. “The first challenge we faced was to make the racket handle and play exactly like our existing rackets because if we did not the best players would never use them.”
Babolat engineers needed to find a sensor that could completely and accurately track the motion of the tennis racket while being small and light with low enough power consumption and the ability to withstand large enough shock loads and be unobtrusively designed into the racket.
Engineers selected the InvenSense MPU-6000 integrated 6-axis motion tracking device that combines a three-axis gyroscope, three-axis accelerometer and associated electronics in a 4 by 4 by 0.9 mm package that weighs only 1 gram. The accelerometer tracks the acceleration of the racket in 3D space, making it possible to determine the speed of the racket as it contacts the ball which in turn can be used to estimate the ball speed. The gyroscope determines the orientation of the racket in 3D space which, in combination with the racket speed, can determine the spin imparted by the racket to the ball.
The MPU-6000 also includes analog-to-digital converters that convert analog signals from the sensors to digital signals and a 1024 byte first in first out (FIFO) buffer for storing the data. Babolat connected rackets also include a microprocessor that collects data from the MPU-6000, on-board data storage and a Bluetooth module that transmits it to the player’s smartphone.
To add these devices to the handle, Babolat engineers had to reduce the weight of the other components to compensate and position the sensors so the racket’s center of gravity would be unchanged. This required redesigning the handle and particularly changing the composite layup design. The layup design is the characteristics – including the material, thickness, order and direction of the different layers -- of the composite materials that are wound onto a mold as part of the process of building the handle.
Another major challenge was developing the algorithms that translate sensor data into tennis data that players can understand and use to improve their games. “We started by spending many hours filming players hitting the ball with an early prototype of the racket,” Zimmermann said. “We mapped the information to a database that we could use to quickly test the accuracy of the algorithms we were developing under a wide range of conditions.”
Perhaps the most interesting algorithm is the one that determines the location of impact of the ball on the racket. It was obviously impossible to put sensors in the strings of the racket that would determine the impact location directly. After performing many tests, engineers found that the impact location could be inferred by combining the readings of the gyros and accelerometers. The further the impact is from the sweet spot, the greater the magnitude of vibration caused by the impact. Also, an impact that is not at the sweet spot will cause the racket to rotate immediately after the impact. The vibration of the racket is measured with the accelerometers and the rotation of the racket is measured with the gyros. Based on large amounts of physical testing, Babolat engineers wrote an algorithm that combines these measurements to accurately determine the impact location.
The player pairs the racket to his or her phone like any other Bluetooth device and downloads and opens the Babolat app to sync the data to the phone. After uploading the data from the racket, the player views high-level information such as the length of each rally, shots per minute, number of forehands, backhands and serves. The player can drill down to see the power, ball speed, percentage of spin and impact location for the entire session or for a particular type of shot. The player can compare his or her performance to previous games to see if their overall game or a particular type of shot is improving. Finally, the player can look at individual shots to see how well they were played. The historical section of the app makes it possible to call up previous sessions to measure progress against the player’s goals.
“Amateur players are actually the ones that will benefit the most from smart, connected rackets because they have the least coaching and the least awareness of how they are hitting the ball,” Zimmerman concluded. “Often players find that they are not hitting the ball as hard and not putting as much spin on the ball as they think. The Play series provides accurate and objective feedback that helps these players improve their game.”