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‘Keeper Coach – a complexengineering challengeTake a really simple idea, like building a machine that will allowgoalkeepers to practise their techniques in goal and you wouldthink that it would be a fairly simple task of firing a ball in randomdirections at different intervals. Then take that idea, hand it over toa group of engineers and you start to discover just how remarkablycomplicated such a project becomes.That’s essentially what Calvin Cunniffe, Cameron Israel,Leemeshen Naidoo and Friedrich WH Schulenberg discovered whenthey embarked on the quest to design an automated soccer ballkickingmachine to train goalkeepers. The four students are studyingengineering at the University of KwaZulu-Natal in Durban.In fact, in their concluding comments in the report submittedfor the PneuDrive Challenge, the students wrote: “The PneuDriveChallenge has been an amazing learning curve, even more so as itserves to form part of the final year design and research project forthe University of KwaZulu-Natal. Designs have been altered in wayswhich would simplify processes using the best engineering methods.”Of course, their comments are what makes this practicalcompetition such an excellent training device for engineeringstudents who face perhaps, for the first time, the reality of devisingpractical engineering solutions that will be applied in the real world.The theme for the PneuDrive Challenge was Motion in Sport andthe group looked at the various options that had been suggested andchose the automated goalkeeper training device, which they promptlynamed the ‘Keeper Coach.The device needed to be capable of launching a soccer ball from thepenalty box into the goal at various positions. Ideally the device hadto randomly select a position to launch the ball to an unpredictableposition for the goalkeeper. It was intended as a supplementarytraining apparatus for goalkeepers.Moreover it would allow the goalkeeper to practise independentlyfrom the rest of the team as the machine would cycle through a preprogrammedshot at goal without being able to predict where thenext shot would go. When the ball is returned (by the goalkeeper) tothe ball retrieval system, a sensor will be tripped and the next shotplayed. This allows the goalkeeper to determine the cycle time.Having designed the machine, the students realised that therewere a number of design enhancements that could be added to itsoperation but these were relegated to future developments ratherthan incorporated into the final submission to the judges.The design criteria stipulated that the shot selections had to bemade according to the standard goalmouth size of 7,32 metres wideand 2,44 metres high and the maximum speed at which the ballcould travel was 120 km/h.The machine would have to be set up on the penalty spot in themiddle of the goalmouth 11 metres from it. Five predefined positionscould be randomly selected but the ball had to strike within thegoalmouth every time. The goalkeeper had to control the cycle timeof the machine.The students considered various different solutions that mightmeet the criteria such as a machine with a swivel arm that ‘kicked’the ball with its ‘foot’ and by controlling the direction in which thefoot was aiming, a different spot in the goalmouth could be achieved.The students felt that there were too many variables in this designand in any event it was similar to an existing and patented productanyway.All in all, seven different concepts were carefully considered,analysed and examined before a final design (the eighth concept)was chosen and even then it was still an extremely complicateddevice to make.The final design comprised a rotational positioning system, avertical positioning system, a horizontal positioning system, apowerful ram, a ball lifting system and a ball retrieval system. Whatseemed quite simple at the outset was quickly becoming more andmore complicated.The first problem was to find a way to kick the ball with a forceof 1 400 Newtons or more. Initially a pneumatic drive appeared tosolve the problem but the repetitive shock loading on the pneumaticdrive would eventually cause it to fail.The next option was to use an electric cylinder operated by a servomotor turning a ball screw or lead screw to extend a piston. However,while an electric motor can deliver great forces, it can only operateat a maximum piston speed of 0,75 m/s, which is too slow.So the students considered the option of using a spring to impartforce onto the ball. The spring would be compressed and thenreleased and when it made contact with the ball the spring forcewould be large enough to impart the necessary force to launch theball at the correct speed.To compress the spring posed a number of additional considerationssuch as using an electric cylinder coupled to the front of the springby an electromagnet. The cylinder would extend until it reached thenatural length of the spring. The electromagnet would then activateand the direction of the electric cylinder would be reversed to drawback the spring.32
When the ball is about to be fired the electromagnet would simplybe deactivated. While this system would work, the students chosewhat they believed was a more elegant solution to use one of Festo’sfluidic muscles.The fluidic muscle uses compressed air to expand a rubber hose inits peripheral direction. Due to conservation of volume, the increasein diameter causes the muscle’s length to decrease, generating tensileforce. High-strength fibres support the rubber material, used to formthe muscle.The fluidic muscle offers up to ten times the force of a standardcylinder and has good dynamic response at high loads and so thestudents chose to use a muscle to compress the spring. In operation,compressed air will fill the muscle, reducing its length and drawingback the spring and when the spring is about to be released, air in themuscle is released rapidly.Another advantage of the fluidic muscle is that it weighs just1,245 kg compared with an electric cylinder that weighs about ninekilograms. The reduction in mass allowed smaller components to beused to move the ram (that would ‘kick’ the ball) and as the momentof inertia would decrease so would the forces it experienced.The next task was to find a suitable spring with an internal diameterof at least 43,9 mm to contain the fluidic muscle. A coil diameterof 14 mm was selected on the assumption that the wire thicknessneeded to be fairly large and by using a mean diameter of 100 mma spring index of 7,14 could be achieved. The design of the springwas conducted through design analysis equations outlined in theFundamentals of Machine Component Design by Robert C Juvinall.According to Juvinall this index was in a good range. The number ofcoils was calculated at 13. The fluidic muscle was designed with theforce needed to propel the ball as one of the parameters. A deflectionat which the muscle was to operate was used as the second definingparameter. The muscle was to deflect the spring by 60 mm and thespring would have a force of 1 800 N at this deflection.A nominal length of 350 mm was selected and the requireddeflection of 60 mm was specified. The operating pressure, usingFesto’s fluidic muscle design program, MuscleSim, was set to 5,3bars. The resulting force was 1 787 N and a contraction of 17percent. A disadvantage of using the fluidic muscle is that the degreeof contraction must be less than 25 percent of the nominal lengthdue to limitations on the degree to which the diameter can expandbefore failure.This resulted in a large spring length and affected the momentof inertia of the ram. The MuscleSim program recommended thatfor optimal usage the degree of contraction should be 15 percentor less but the students felt that a contraction of 17 percent wouldbe acceptable as the extra length required to reduce the contractionratio would increase the size and weight of the ram without any realbenefit.A quick-exhaust value was incorporated into the design to reduceany dampening effects of escaping air when the muscle was rapidlyelongated. It operates best when fitted directly to the muscle but thiswas not possible in terms of the design and so a fitting was connectedto the quick-exhaust valve instead.The mounting configuration allowed one of the muscles to beblocked so that air would enter and release through one connectionand this meant that only one solenoid valve would be necessary.The ram had to be mounted inside a casing to support thespring and prevent vertical deflection as it released or was beingcompressed. By using two individual cylindrical sleeves and allowingthem to move relatively close to each other, the fluidic muscle andspring will function as required.One of the cylindrical sleeves needs to be larger in diameterthan the other to allow the sleeves to slide relative to each other.The sleeves are attached to the spring and as the spring varies itslength, the sleeves adjust by sliding one over the other. So the ramdesign comprises a swivel shaft, an outer and inner sleeve and twoend covers.The ram rotates to the left or right on the swivel shaft with abearing at each end that is capable of withstanding the recoil forceof 1 800 N. To prevent the sleeves from sliding off each other,rubberised stopper units with good impact absorbing properties areused to absorb the spring’s kinetic energy and stop the inner andouter sleeves.Attached to the end of the ram is a device called a ‘shoe’ madefrom a rubberised cricket ball. The shoe makes contact with thesoccer ball and needs to be hemispherical and must be joined to theram. This was achieved by gluing a circular aluminium plate to analuminium rod that is attached to the outer sleeve’s end cover.The hemispherical shape means that the spring’s force is notconcentrated on one point when the ram is fired as, if it was, thismight puncture the ball. Moreover, rubber materials have highfriction factors that reduce any slippage between the ball and theshoe on contact.So now the team had worked out how to fire a soccer ball and thenext problem was to position it accurately when it was slammed atthe goalmouth. So the physics of a soccer ball had to be considered.The effect that spin has on the ball’s motion was used to control andvary the trajectory of the ball in the vertical plane.However, horizontal positioning of the ball was aimed at notintroducing spin and the ball was to be hit along the centre axisin the direction that it should fly to eliminate curvature within thehorizontal plane. An electric drive would be used to move the ballvertically and the distance away from the ram would be controlledby varying the length of the electric drive shaft.The vertical positioning system allows the ball to be accuratelylocated in front of the ram and it is broken down into two sub-systemsthat are used to provide accuracy. The first sub-system controls thedistance between the soccer ball and the front end of the ram. Thesecond sub-system controls various vertical offset displacementsbetween the centre of the soccer ball and the ram. The two subsystemsare integrated.The design of the first sub-system uses a pneumatic DNCI standardcylinder with an integrated displacement encoder and was designedusing Softstop, Festo cylinder design program. The cylinder has aAugust 2010 33
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