Light reflection-based system for spin measurement of table tennis balls
Résumé
INTRODUCTION:
Mastering spin is crucial to win a match in table tennis. Spin is one of the keys performance indicators for a
player, alongside with placement, ball speed, technique and precision. Therefore, it is crucial to measure
performance.
However, spin measurement is hard to perform in real match conditions because the speed is high (+/-800
rad/s)[1], unless using a high-speed cameras and using image processing algorithms.[2]–[5]
We proposed a new low-cost tool for spin measurement based on observation of light reflection on balls. The
proposed system is also able to measure linear speed from a moving object in its field of view (FOV).
METHODS:
To calibrate accurately our system, comparisons are made with a high-speed camera Miro C210 from Phantom.
First experiment is the measure in controlled conditions. The system is used to measure a spinning ball
mounted on a motor with constant speed (to ensure the measurement quality of the device).
Second experiment is the observation of the spin on real strokes. Strokes were performed by an experienced
senior player. Services and hits were realized several times with increasing spin given by the player. Balls were
delivered by a ball thrower robot (Amicus Expert from Butterfly).
Spin estimation with camera is obtained through frame by frame analysis, while our system is expressing spin
by frequency and waveform analysis.
RESULTS:
Experiment 1 shows a measured spin at 468,7 rad/s (+/-18,8 rad/s).
Experiment 2 shows various spin measures, for strokes and services. Services observed spin range goes from
174 rad/s to 329.23 rad/s.
Strokes performed for topspin forehand shots range goes from 391.75 rad/s to 808.6 rad/s.
Strokes performed for topspin backhand shots range goes from 569.8 rad/s to 769 rad/s. Linear speed measured
on forehand & backhand shots is really close from the one measured by camera (+/3.46%).Estimated error
between High speed camera and our system is +/-5.6%.
CONCLUSION:
Although results are consistent with high-speed camera, we observe that the system is light-sensitive. It would
not be able to detect in a fully lightened or a fully dark environment or at least with a larger error.
We are using bicolored balls which won’t be allowed in real match conditions. Traditional match balls, with their
natural brands are not reflective enough to get a meaningful output.
Nonetheless, the presented system, shows strong results in accordance with a high-speed camera for both
measuring linear and rotational speed.
[1] P. Blank, B.H. Groh, B.M. Eskofier, in Proceedings of the 2017 ACM International Symposium on Wearable
Computers, Association For Computing Machinery, New York, NY, USA 2017, 2.
[2] T. Tamaki, H. Wang, B. Raytchev, K. Kaneda, Y. Ushiyama, 2012, 1457.
[3] J. Tebbe, L. Klamt, Y. Gao, A. Zell, 2020, 9694.
[4] C. Liu, Y. Hayakawa, A. Nakashima, SICE Journal of Control, Measurement, and System Integration 2012, 5,
233.
[5] Y. Zhang, R. Xiong, Y. Zhao, J. Wang, IEEE Transactions on Instrumentation and Measurement 2015, 64, 2280.
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