TWO SYSTEMS FOR PROVIDING KR FEEDBACK IN TABLE TENNIS TRAINING

In table tennis, performance parameters describing the outcome of one stroke are the spin, the position, where the ball hits the table, and the ball velocity. Systems that give immediate feedback on the quality of the ball just played are applicable in training. Besides of directing and conditioning the technique some motivational effects can be expected. The application of two types of KR (knowledge of results)-feedback systems shall illustrate the concept. The first type is based on detecting impact positions of the ball on the table in almost real time, the second on the acquisition of ball impact intervals. The impact point detecting system is capable to give feedback on the accuracy of the placement when performing certain tasks and to give feedback on impact positions during service. The system neither disturbs the players nor does environmental noise influence the system. Four accelerometers are fixed onto the underside of one half of the table. A triangulation method is applied to calculate the impact position from the vibration signal registered by the accelerometers. 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. After each series of trials, the player gets visual feedback on the ball impact positions. To give feedback on the quality of service techniques, a low cost device was developed. Ball impacts on the table cause typical acoustic signals. Two microphones are used for recording these signals. A microcontroller based system allows determining and displaying the time interval between first and second impact of the ball on the table immediately after service execution. In the case of short services, it also determines and displays the time interval between the second and the third impact. Obviously, the time intervals are strongly affected by the degree of spin of the serviced ball.