Feedback systems in table tennis

Abstract

According to Farfel’s principle (Farfel, 1977) effective feedback systems applied in training should provide feedback information rapidly and objectively. Powerful information and communication technology simplify the development of sports specific feedback systems of that kind. Special focus may be put on the acquisition and presentation of performance relevant parameters. In the case of table tennis factors affecting the quality of the ball played are the spin, the position, where the ball hits the table, and the time left for the opponent to react properly (Baca, Baron, Leser & Kain, 2004; Hohmann, Zhang & Koth, 2004). Systems that give immediate feedback on certain performance parameters have been developed to assist training. Besides of directing and conditioning the technique some motivational effects can be expected. Two types of feedback systems have been built and are applicable in table tennis training. The first variant is based on the detection of impact positions of the ball on the table, the second on the acquisition of ball impact intervals.

Introduction

According to Farfel’s principle (Farfel, 1977) effective feedback systems applied in training should provide feedback information rapidly and objectively. Powerful information and communication technology simplify the development of sports specific feedback systems of that kind. Special focus may be put on the acquisition and presentation of performance relevant parameters. In the case of table tennis factors affecting the quality of the ball played are the spin, the position, where the ball hits the table, and the time left for the opponent to react properly (Baca, Baron, Leser & Kain, 2004; Hohmann, Zhang & Koth, 2004). Systems that give immediate feedback on certain performance parameters have been developed to assist training. Besides of directing and conditioning the technique some motivational effects can be expected. Two types of feedback systems have been built and are applicable in table tennis training. The first variant is based on the detection of impact positions of the ball on the table, the second on the acquisition of ball impact intervals.

Methods

Impact position detection A schematic of the setup is presented in Figure 1. A detailed description can be found in Baca & Kornfeind (2004). Four accelerometers (Kistler 8632C10; four-channel amplifier 5134A1; Kistler, Winterthur, Switzerland) are fixed on the underside of one half of the table and connected to an amplifier, which itself is connected to a DAQ-system consisting of a notebook computer and a data acquisition card (NI-6062E, National Instruments, Austin, USA). Vibration signals produced by the ball hitting the table are registered by the four sensors. A trigger impulse, generated from an electronic circuit, starts the recording of a specified number of samples repeatedly after every ball impact (1000 Samples@125 kHz). A threshold algorithm determines the four instants of time, when the vibration signal arrives at the sensors. Software (LabVIEW®, National Instruments, Austin, USA) has been programmed for this purpose. A triangulation algorithm, which is also implemented in this software, calculates the impact position from the four instants of time. To determine the coordinates of the impact position, xT and yT, only three instants of time are required. The fourth, redundant sensor is used to increase accuracy. A least square method has been selected to determine the impact point coordinates. xT and yT are calculated, which minimize

where xi, yi and zi are the coordinates of the i-th sensor with regard to the table coordinate system, ti, the instants of times, the vibration signal arrives at sensor i (i=1,…,4), and v is the velocity of signal propagation, which depends on material properties of the table.

Figure 1. Setup for detection of ball impact positions. S1 – S4 denote the positions of the accelerometers fixed on the underside of one half of the table.

If metallic parts are coupled to the table top, as is the case for the type of tables used by Baca and Kornfeind (2004), vibration damping material has to be used for a mechanical decoupling of the table top and the metallic parts. Thus, faster signal propagation towards the sensors through the metal resulting in noise signals can be prevented. The program developed displays the reconstructed impact points immediately after the impact. A circle representing the ball is drawn onto the calculated position into a graphic presentation of the table half. In addition, the numerical values of the coordinates are shown (Figure 2).

Figure 2. Computer screen presenting a series of ball impact positions.

An average accuracy of 0.020± 0.011 m was obtained by Baca and Kornfeind (2004) within the area of the table at least 0.25 m away from the net. Impact time interval detection A low cost system has been developed to determine time intervals between ball impacts after the serve in table tennis. In the case of long serves the time interval between first (own side) and second (opponent’s side) impact is determined. In the case of short serves, the ball bounces on the opponent’s side twice resulting in a second time interval to be calculated. Two microphones are used for recording the acoustic signals caused by the ball impact on the table. Both are fixed in metallic boxes. The boxes are put onto both halves of the table. The signals from the microphones are preprocessed electronically and then fed to a microcontroller (PIC16F628; Microchip, USA), which is also connected to the serial port of a PC, notebook or PDA (Figure 3).

Figure 3. Left: Schematic presentation of the system for calculating impact time intervals. Right: Complete system without PC/PDA.

A LabVIEW® application program acquires the data from the serial port and displays the results on the computer screen. In addition to a numerical presentation of the time intervals a speedometer informs on the player’s performance graphically (Right: Green area – good; Left: Red area – bad; Figure 4). The overall system is not bound to a specific table tennis table and can easily be transported to the environment (table, hall, etc.), where it is used. The system operates self-controlled. Because of an automated system reset into a “wait state” after a short period without acoustic impact signal, no user intervention is required between successive serves. If connected to two monitors, the system may be used by two players standing on both sides of the table, who serve alternately.

Figure 4. Presentation of impact time intervals. Time 1: First to second impact, Time 2: Second to third impact (short serves only).

Results

Both types of systems have successfully been applied to give feedback in youth training. Usability and system stability were considered satisfactory by the users. Feedback based on impact position detection The feedback system neither disturbs the players nor does environmental noise influence the system. Feedback on the accuracy of the placement when performing certain tasks may be given. In addition, a series of trials may be evaluated and summary feedback may be given. In a typical application of the system in training a table tennis robot serves the ball in short intervals. The player has to return each ball into a marked area or to return one ball cross and the following into the marked area alternately. After each series of trials the player gets visual feedback on the ball impact positions (Figure 2, Figure 5). The system may also be used to give feedback on impact positions and impact time intervals in serve training. Players are thereby able to study the variability of different serve techniques. Feedback based on impact interval detection Typical exercises performed by the players include the task to minimize the impact intervals in order to decrease the reaction time of the opponent. Obviously the time interval is strongly affected by the degree of spin of the serviced ball. Youth players utilizing the system in serve training enjoyed this kind of aid and were highly motivated. A kind of competition situation can be observed. A first study showed tendencies that training with the system might be useful in shortening the impact time intervals of short services (i. e. Time 2 in Figure 4).

Discussion

Rapid feedback systems utilizing powerful sensor and information technology provide innovative and effective support to coaches and athletes. Mighty IT-tools facilitate the development of user-friendly systems, which are specifically oriented towards their needs.

Figure 5. Feedback training using impact position detecting system.

In the development of the systems used to give feedback in table tennis special care has been taken to measure and/or calculate the characteristics of interest accurately and to present the results to the users (coaches and athletes) fast and comprehensive. Graphic visualisation forms have therefore been implemented in both cases in addition to the presentation of the numerical values (Figure 2 and Figure 4). It is expected that novel and rapid performance measurement and feedback tools based on modern information technology will become more and more pervasive in daily table tennis training.

Bibliografía

• Baca, A., Baron, R., Leser, R. & Kain, H. (2004). A process oriented approach for match analysis in table tennis. In Science and Racket Sports III, A. Lees, J.-F. Kahn, I. Maynard (Eds.), Routledge, London, 214-219.
• Baca, A. & Kornfeind, P. (2004). Real time detection of impact positions in table tennis. In The Engineering of Sport 5, Vol. 1, M. Hubbard, R. D. Mehta, J. M. Pallis (Eds.), Sheffield: ISEA, 508-514.
• Farfel, W. S. (1977). Control over movement in sport. [In German: Bewegungssteuerung im Sport], Berlin: Sportverlag.
• Hohmann, A., Zhang, H. & Koth, A. (2004). Performance diagnosis by mathematical simulation in table tennis in left and right handed shakehand and penholder players. In Science and Racket Sports III, A. Lees, J.-F. Kahn, I. Maynard (Eds.), Routledge, London, 220-226.