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litre accumulator are mounted to the distribution box.The Tshabalala uses a standard synchronous timing belt to minimiseelongation, which is important for positional control of the trolley onthe track and is critical for accuracy. To ensure the Tshabalala iseasy to transport, a standard ATV (quad-bike) trailer was chosenand modified to include two guide channels for the trolley’s wheels.As part of the project, the Stellenbosch University students thencarefully investigated the physics of a soccer ball, particularly whenit is in flight. This gave them the necessary understanding for theautomatic and semi-automatic operation of the Tshabalala and ledto ball velocity being restricted to between 20 m/s and 30 m/s andspin limited to between 0 and 0,3.The calculations were done from empirical data gathered using theTeamgeist ball that was selected for the 2006 World Cup. Similardata was not available for the Jabulani ball that was used in SouthAfrica this year.The students realised that there was a total of 28 611 possiblecombinations of user input if each possible shot on goal was to becalculated manually, which was not practical and would negativelyaffect the viability of the machine.To overcome this the students wrote a Matlab program to determinethe flight path of a ball for a set of given inputs in Cartesian coordinatesfor the semi-automatic and automatic modes of operation.The set of initial conditions used in the program were taken from thepoint when the ball and the belts no longer make contact.The Matlab program determines the sum of the forces on theball to calculate the acceleration and then determine the speed andposition of the ball at the end of the time. This is done repeatedlyuntil the ball in the goalmouth reaches a specified final position.There are nine goal positions that can be selected by the user. Eachgoal position relates to a Y and Z co-ordinate. The Matlab programconsiders every combination of spin, ball velocity, track position andinitial azimuth and elevation angles of the turret to determine thefinal position of the ball.The Matlab progam essentially fixes the spin, ball velocity, trackposition and elevation angle and then scans the goal for toleranceblocks. The elevation angle is then changed to the next increment anda scan of the goal is done again. When the final position correspondsto the co-ordinates of the centre of a tolerance block, the set of initialconditions for that shot are saved. This is done for all combinationsand then a table is generated which relates to the initial conditionsfor every possible shot. This table is then included in the programmingof the PLC. The table corresponds to one complete scan of allnine shot positions while keeping track position, ball velocity andspin constant. The complete table is an extended version of this,containing all possible shots on goal. For wireless communicationwith the Tshabalala, a hand-held console is used. It has incrementalsliders for ball velocity, and selection buttons for position and spinand push buttons for the ‘On’ and ‘Off’ function.The console also contains a transmitter that communicates witha receiver on the Tshabalala to activate the controls on the servomotors, mounted on the trolley and mechanically rotate rotarypotentiometers which are connected to a 24 V DC power supply. Bydoing this the source voltage can be scaled according to the angleof rotation of the potentiometer and used as an analogue input tothe PLC. Each scaled voltage corresponds with the magnitude ofan input.The receiver, transmitter and servo motors are supplied by Futabaand potentiometers from Bourns. A Pulse Code Modulation (PCM)system was used for the design, as it will not transmit to the servosif it encounters interference. Standard servo motors were chosen tominimise costs and power consumption.The rotary potentiometers were connected directly to the servomotors with a gear ratio of five meaning that a servo motor witha rotation of 60 degrees will cause a potentiometer to rotate 300degrees. Input from the console is received via a radio receiveronboard the trolley and relate to output on servo motors, whichdisplace potentiometers to generate an output voltage. These outputvoltages are used as input voltages on the PLC and range from 0 to10 V and is divided into five two volt increments.Thus an input voltage of between 0 and 2 V will relate to the firstincrement, between 2 V and 4 V the second increment and so forth.The potentiometer is set to give an output voltage in the middle orthe range (1 V, 3 V and so on) to ensure the correct voltage levelsare achieved.Using these systems, the control of the Tshabalala is effectiveand simple for the end user (although not nearly as simple for thedesigners). The students chose a Siemens Simatic S2-700 PLC forthe system and all control logic is programmed into the controller,which also contains the Matlab extended table.As energy saving was a requirement of the project, the studentsrealised that a significant amount of energy is dissipated during thedeceleration process when the trolley stops. This kinetic energy canbe recovered using a regenerative power supply and used to powerother components.Power losses occur when converting electrical energy to mechanicalenergy and through friction.The Tshabalala is an elegant and highly practical machine thatmight prove invaluable to any soccer team and considering that itcosts just over R500 000 to build, it should represent a viable optionfor the sophisticated training of soccer players in various differentsituations. It has a high degree of repeatability making it ideal forgoalkeeper training or for playing of set-pieces such as a free-kickor a corner. Essentially the Tshabalala can replace any player thatkicks a ball on the field and can do so with greater accuracy andrepeatability than any player as well.That makes it an invaluable asset to any coach, anywhere in theworld.August 2010 27