Design, development and construction of a monitoring table tennis net

Transcription

1 Available online at Procedia Engineering 13 (2011) th Asia-Pacific Congress on Sports Technology (APCST) Design, development and construction of a monitoring table tennis net Roman Gastinger *, Stefan Litzenberger, Anton Sabo University of Applied Sciences Technikum Wien, Institute for Sports Engineering & Biomechanics, Höchstädtplatz 5, 1200 Vienna, Austria Received 19 March 2011; revised 11 May 2011; accepted 12 May 2011 Abstract This study s objective was to design, develop and construct a monitoring table tennis net to minimize mistakes in adjusting the net and achieve a constant and highly precise net height, net tension as well as detection of ball-net contacts during service (so called: let). Furthermore, it should improve and expand the standards for net adjustment which are unclear and not exactly reproducible at the moment. The monitoring device consists of a grey consoling housing, a custom-made force sensor, an electronic circuit, two graphical user interfaces, two height sensors, a let sensor, an electronic circuit and two graphical user interfaces. The proposed device provides a way for measuring net height and tension to ensure similar playing conditions (worldwide) and can be used for the detection of ball-net impacts during service Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of RMIT University Keywords: Table tennis net; monitoring; net height; net tension; let 1. Introduction The standards in table tennis request that the top of the net should be 152.5mm above the playing surface [1]. Currently the height is adjusted with a normal light net gauge [2]. The disadvantage of this method is that subjective mistakes can occur. The required net tension, which affects net height is described by the International Table Tennis Federation (ITTF) by giving the following two recommendations: * Corresponding author. Tel.: address: roman.gastinger@gmx.at Published by Elsevier Ltd. Open access under CC BY-NC-ND license. doi: /j.proeng

2 298 Roman Gastinger et al. / Procedia Engineering 13 (2011) The tension of the net cord may be checked in the middle of the table either by fingers, or much better by a 100g heavy tension gauge. The 142.5mm high part of the gauge should hang on the net; the tension is good if the bottom of the gauge comes next to the tabletop surface, but without touching it. Otherwise the devices to adjust the net tension must be operated [2]. These requirements are rather insufficient and not exactly reproducible, because of a missing objective measurement method. Furthermore the detection of ball-net contacts (so called: let) during service is often very tricky for the umpire. High velocities and short ball-net contacts are complicating the referee s decision. In such cases subjective mistakes can occur too. This study s objective was to design, develop and construct a monitoring table tennis net to minimize mistakes in adjusting the net and to support the referee in critical decisions. The values of interest were net height, net tension and detection of ball-net contacts during service. This should lead to a constant and highly precise net height and net tension by measuring and displaying the results for the players and the umpire. Furthermore, it should improve and expand the net assembly standards which are often unclear and not exactly reproducible. The monitoring table tennis should be used as referee support as a LED starts flashing when a let occurs. This can lead to more fairness in table tennis State of the art In table tennis an objective method for measuring the net height and net tension does not exist so far. In related sports such as tennis an electronic device for measuring the net height was developed by Anderson in 2009 [3]. In 2001 a mechanical measuring system was developed to adjust the net tension in tennis [4]. Bräuer & Naber [5] and Pinney [6] patented their inventions for detecting ball net contacts during service. In table tennis only a few innovations were done and these were focused on electronic scoring devices. The systems are only useable for hobby and semi-professional players where no referee is prescribed by the regulations. The patent of Yiu [7] describes that the system includes a sensor for detecting ball-net contacts. 2. Methods 2.1. Calculation The first step was to acquire the boundary conditions for the dimensioning of the force sensor. The main parameter was the net force which is required for a properly adjusted net. The disadvantage is that no information is given for the required net force by the ITTF. Therefore, the boundary conditions of the second recommendation [2], for adjusting the net force, were used to perform a calculation. The table tennis table has to be cm ± 1.5 mm in width [8] and the net assembly cm ± 2 mm in length [2]. That leads to a total net width of 183 cm ± 3.5 mm. Figure 1a shows the table tennis net with 183 cm in width and cm in height. A net tension gauge (weight: 100g) is positioned in the middle of the table. The ITTF recommendation states that the cm high part of the gauge should hang on the net (Figure 1(b)). The tension is correct if the bottom of the gauge comes next to the surface of the table, but without touching it [2]. That implies a net deflection less than one centimeter.

3 Roman Gastinger et al. / Procedia Engineering 13 (2011) (a) (b) Fig. 1. (a) Adjustment of the net force with a 100 g heavy net gauge positioned in the middle of the net. (b) 100 g tensionn gauge with two parts, the cm one for the net height and the cm part for the net tension [2]. y x F S1 183 cm 1cm F S2 F Net gauge = 0,981N Fig. 2. Acting forces F S1, F S2, F Net gauge, deflection and bending angle alpha ( ) at the rope of the table tennis net. Figure 2 displays the acting forces at the rope when loading the net with a 100 g (0.981 N) heavy net gauge in the middle which is represented by the force F Net gauge. In order to be able to conduct the calculation, the table tennis net s top tape was described as a simple rope which was horizontally fixed on the left and right side. The bending angle alpha could be calculated by Equation 1. (1) A deflection of 1 cm and a net length of 183 cm results in a maximum angle alpha of As a rigid body is in mechanical equilibrium when the sum of all forces and the sum of all torques of the system is zero, the total sum x-direction showed that F S1 is equal to F S2 (Equations 2-3). (2) (3) Equation 4 represents the total sum of all forces in the y-direction. In this case, a rope force F Rope of 44.9 N (4.58 kg) can be calculated (Equation 5).

4 300 Roman Gastinger et al. / Procedia Engineering 13 (2011) = 0 F - F S1 * sin - F S2 * sin = 0 (4) 2 * F S1 * sin = F Net gauge (5) F S1 = F Rope = F 2* sin = 0,981 N 2* sin ( ) = N (6) A force of 44.9 N is necessary to achieve a net deflection less than one centimeter. However, this value will be smaller in practice, due to the mesh which supports the rope against deflection Force sensor Construction and dimensioning of the sensors was carried out with Pro/ENGINEER Wildfire 4.0 (Parametric Technology Corp., USA). The dimensions of the aluminium sensor are 40 mm x 15 mm x 50 mm (A x B x C) which can be seen in Figure 3(a). The essential element is the bar (D) where the strain gauges are applied. The finite element analysis (FEA) shows an optimal cross section of 2.5 mm x 8 mm (D). This fulfils the specified criteria which will be described in the following section. To minimize local stress peaks, curvatures (E) were dimensioned between the rectangular body and the bar. The drill hole with a diameter of 11 mm (F) is necessary for the adjustment of the tension force sensor below the net assembly. The circular recess (G) allows an adjustment of the net force by fixing the table tennis net rope. B C A (a) (b) Fig. 3. (a) The dimensioning of the tension force sensor with Pro/ENGINEER Wildfire 4.0 (Parametric Technology Corp., USA). The letters A, B and C are indicating the dimension values and D, E, F and G the particular characteristics. (b) Finite element analysis (FEA) with ANSYS Workbench 11.0 (ANSYS Inc., USA) maximum effective strain (von Mises). The calculations based on the FEA, showed detailed information of the occurring strains and tensions which were necessary for the application of the strain gauges. Figure 3(b) shows the fixed bearing (B) loaded with 45 N in y-direction (A), which was based on the results of the theoretical calculation. The company HBM (Hottinger Baldwin Messtechnik GmbH, GER) suggests strains from 500 to 2000 μm/m for a strain gauge application. The simulation shows a maximum effective strain (von Mises) of

5 Roman Gastinger et al. / Procedia Engineering 13 (2011) μm/m (C), which fits in the preferred area. Furthermore, a comparison stress (von Mises) of MPa could be measured. After the simulation, the aluminium body of the tension sensor was manufactured. This was followed by the application of two strain gauges, one on the upper- and one on the lower side of the bar Height sensor The height of the net is measured by linear slide potentiometers with a travel distance of 30 mm (ALPS Electric Inc., USA). To fit the potentiometers to the table tennis net, a height adjustment was dimensioned in Pro/ENGINEER Wildfire 4.0 and built subsequently. The dimensions of the height adjustment are 72 mm x 44 mm x 40.5 mm (A x B x C) which can be seen in Figure 4(a). The material is Polyamide 6 (Haberkorn Holding AG, Austria). The essential element is the groove for the height sensor (D). The drill holes on the top of the adjustment are used to fix the device below the net assembly (E). Figure 4b shows the height adjustment (A) with the mounted potentiometer (B). Moreover, the conversion of the rotation into a translation (C) can be seen. B C A (a) (b) (c) Fig. 4. (a) Dimensioning of the height adjustment for the height sensor with Pro/ENGINEER Wildfire 4.0 (A, B and C are indicating the dimensions, D and E the particular elements). (b) Height adjustment with the sensor and the conversion of the rotation into translation. (c) Final monitoring net with the grey consoling housing including the electronic circuit, the A/Dconverter, the graphical user interface the height sensors and the let sensor Let sensor The let sensor has to fulfill specific criteria to make an adjustment on the middle of the net possible. These included low weight, high sensitivity and small dimensions. Therefore a vibration sensor (weight: 3g, dimensions: 12 mm x 7 mm x 5 mm (width x height x depth)) was used to detect vibrations due to ball-net contacts during service. The verification of the let-sensor s accuracy to detect ball-net contacts was done with two different methods. First three star certificated table tennis balls were dropped down from a fall height of 25 cm above the table thus resulting in a relative fall height to the net s top-rope of 10cm.

6 302 Roman Gastinger et al. / Procedia Engineering 13 (2011) Electronics The main parts of the electronic circuit were a circuit board, a DC/DC converter, a bridge amplifier, resistors, two strain gauges, capacitors, light emitting diodes, a push button, a switch and block terminals. The two strain gauges were connected into a Wheatstone bridge circuit with two resistors and two potentiometers to balance the bridge. The bridge circuit causes an output signal related to the stress value [9]. The amplification was realized with an instrument amplifier INA 125p (Burr and Brown Corp., USA). The whole circuit was put into a grey console housing ( grey box ), which includes an aluminium front panel. On the panel the LEDs, a switch, a push button, input and output connector and three different sockets for the power supply were mounted. The user has the possibility to power the grey box in three different ways. These are a common 9V Battery, an external power supply and to power it via USB-connection. The green LEDs on the front panel are showing the user which mode is used. In addition a 500 ma fuse is included which prevents the whole electronic circuit from wrong powering Software The visualization and further data processing was done with the software LabVIEW 8.6 (National Instruments Corp., USA).The powered and amplified analog signals from the grey box are A/D-converted by a NI USB 6008 (National Instruments Corp., USA), which is connected to the computer via USB. Two different graphical user interfaces (GUI) were created, a development- and an expert version. The first one can be chosen to calibrate height and force sensors and to record data for further analysis. The purpose of the second one was to provide an easy to use interface for referees and users as well providing a simple and quick handling. In the GUI of the expert version two blue bars indicate net height on each side to ensure a horizontal net and highly precise net height, an indicator is lit in case of ball-net contacts during service and a pointer diagram shows the rope force. User interaction (push bottom on the front panel of the grey box) is demanded to confirm a let-event. 3. Results The calibration measurement showed that the sensors, electronics and data acquisition worked correctly and reliably. The rope force was measured 10 times by loading the net with a 100 g heavy net gauge in the middle. The results are displayed in Figure 5. Rope Force [N] is displayed on the y-axis, measurement number on the x-axis. Figure 5 also includes the mean value of the measurement (black horizontal line) and the standard deviation (light grey horizontal area). A rope force of N with a standard deviation of ± 1.47 N was measured (1.84 kg ± 0.15 kg). For the verification of the let sensor 15 measurements were carried out, five at each side and five at the middle of the table. Furthermore, intentional ball/net contacts during service were executed by table tennis players. The measurements showed a detection rate of one hundred percent.

7 Roman Gastinger et al. / Procedia Engineering 13 (2011) Rope force [N] Rope force [N] Standard deviation Net force [N] Mean value Measurement Fig. 5. Net force with a 100 g load (net gauge) in the middle of the table. The deflection had to be less than one centimeter. 4. Discussion and Conclusion The operational testing was done in a sports hall of a table tennis club. The digital and monitoring table tennis net was mounted at a JOOLA World Cup table tennis table (JOOLA table tennis GmbH & Co. KG, GER). The measuring equipment which consists of the grey box including the electronic circuit, NI USB A/D-converter and laptop-computer was positioned next to the net on a referee table (Fig. 4c). A total power consumption of less than 1 W was measured. If there is no external power supply the grey box and the sensors can be powered by a 9 V battery for more than four hours. A rope force of N ± 1.47 (1.84 kg ± 0.15) was measured. This is more than 50 % smaller than the calculated value. The main reason is the mesh which wasn t included in the calculation and supports the rope against deflection. The findings could result in rope force recommendations of 15.7 N to 20.6 N (1.6 kg to 2.1 kg). This monitoring device makes it possible to standardise net tension in table tennis and thus can provide similar playing conditions worldwide. Moreover, the monitoring table tennis net can be used as referee support indicating a let by a flashing LED. References [1] Michaelis & Sklorz. (2004). Richtig Tischtennis. 6th edition. München: BLV. p.16 [2] ITTF-T2. (2004). The Net Assembly - Technical Leaflet T2 [online] Available from (Accessed: 15th of October 2009). [3] Anderson, P. J. (2009). Patent No United Kingdom. [4] Pierce, G. N. (2001). Patent No. 6,248,030. United States of America. [5] Bräuer, D., & Naber, D. (1994). Patent No. 5,585,778. Germany. [6] Pinney, M. (2004). Patent No United Kingdom. [7] Yiu, C.-H. (1996). Patent No. 5,733,210. Taiwan. [8] ITTF-T1. (2004). The Table - Technical Leaflet T1 [online] Available from (Accessed: 18th of October 2009). [9] Hoffmann, K. (1987). Eine Einführung in die Technik des Messens mit Dehnungsmessstreifen. Darmstadt: Hottinger Baldwin Messtechnik GmbH.