Inventor: Robert A. Beken, San Diego, California, U.S.A
USPTO Provisional Patent Application, March 28, 2004
Testing tennis racquets and tennis balls is difficult and expensive. Most such systems use computer controlled racquet swinging machines and computer controlled ball pitching machines. These systems are quite expensive and not all that reliable. The expense and reliability issues are not with the hardware but rather with the fact that these machines require intensive human intervention for their operation. What is needed is a machine that can perform large series of tests automatically and with no human intervention.
The present invention is a complete system which tests tennis racquets and balls. The system is quite inexpensive and even a money magnet for university grad students and professors alike. The system can attract researchers in the fields of electronics, mechanics, statistics, physics.
The system depends not on fine accuracy for single bounces of a ball from a racquet but instead upon thousands of automated firings of balls and the measured returns. By repeating the same test several times the accuracy is increased far beyond that available from single shot machines.
Further, this is the first system that can actually monitor the actions of a single string as the ball strikes it.
This is an automated system. The device can be set up to process 5,000 impacts in a single day’s session and all without human intervention. The system can provide two million separate tests in a single year and all under computer control.
The testing floor is a calibrated concrete floor which has lock down plates at various positions. The racquet mount is fixed at the far end of this floor. The ball gun and force plate mounts can be moved and aligned and affixed to this floor as needed to provide the vast array of ball flight angles required when testing racquets and balls.
The testing floor also has a ball return system so as to capture the discharged balls and return them to the ball gun.
The racquet mount consists of a floor mounted table constructed of concrete and weighing at least 500 pounds and which has attached to its upper surface a Stewart Platform. The Stewart Platform is a device which can be moved in six degrees of freedom and which is renown for its rigidity. Between the Stewart Platform and the racquet are three sets of triple accelerometers. These accelerometers measure the push, pull, and rotation of the racquet in all axes. The Stewart Platform is controlled by a PC.
The optical system consists of a small X, Y platform, a video camera, red laser and a CCD photo array. The X, Y platform is mounted to the Stewart Platform and thus is coplanar with the rear face of the racquet. The camera is used to located a single racquet string under computer control. The laser is then used to illuminate a selected string. The CCD photo array is used to image the movement of that string at 2,000 samples a second as the ball affects the position of that string.
The ball gun is a tube five feet long with an exhaust port at the rear which exhausts compress air into the tube. Within the tube is a piston. At 90 degrees spaced around the inner surface of the tube are brake pads. These pads are composed of rubber and when engaged — pressed into the inner space of the tube — will come in contact with the ball and cause the ball to spin. The more contact the more effect the brake will have on the spin of the ball.
The force plate is a triangular aluminum plate three feet on a side which has been imparted with a rough surface finish on its front face. The force plate is equipped with three sets of accelerometers — one set at each corner. The accelerometers are packaged in clusters of three — one for Y one for X and one for Z axes. The rear of the force plate is a second triangular aluminum plate and the accelerometers see the difference in acceleration between the front plate and its twin. The rear plate is attached to the force plate mount.
The force plate mount is a frame 10 feet across and 10 feet high. This frame has two sliding arms — one in X and one in Y. These arms move the force plate in X and in Y as needed. The simplest method of movement is by extended length garage door openers. The exact position of the force plate is derived from the a potentiometer attached to the end of each screw. The potentiometer reads full rotations (going from zero resistance to maximum and to zero again) and reads partial rotations (any portion of the potentiometer rotation is indicated by a known variation in resistance).
In operation, the technician sets up a test envelope through which the racquet and ball are to be tested. Further, the technician sets up the areas and even specific strings of the racquet to be closely examined at ball impact.
In operation the technician mounts the racquet on the Stewart Platform and aligns the position and angle of the racquet at its starting position for the day.
The technician then moves the ball gun so that it aligns with the racquet and where the ball return will hit the force plate.
The technician then aligns the force plate so as to be the target for the ball upon its reflection from the racquet.
The angles of the racquet are then incremented under computer control so as to allow the ball fired by the ball cannon to impact all programmed areas of the racquet’s surface.
The camera system incrementally moves the laser and CCD as needed and measures the activity of each and every string in each and every position on the racquet. These measurements are taken from a location to the rear of the racquet and do not affect the racquet of the ball. These measurements can be set to any resolution from every .001of an inch to every .25 of an inch over the entire surface of the racquet.
The ball is launched by the ball cannon throughout the specified ranges of speeds and spins.
The ball impacts the racquet and then is returned toward the force plate. The force plate measures the impact energy and these measurements include position, speed, angle, and spin.
This is an automated system. The device can be set up to process 5,000 impacts in a single day’s session and all without human intervention.
The key is that the system — while sensitive and accurate — depends upon huge volumes of shots to refine that testing.
It can take even years with a conventional racquet swinging machine to accomplish what this machine can do in a single afternoon.
Further, this machine provides data for each string on the racquet and at every position on that racquet and measures that movement to better than a thousandth of an inch. This machine provides data for each shot and how that shot impacting the stings affects the position of the racquet (grip push, pull, twist).
This machine also tests the ball in how it reacts with a racquet and how any spin imparted on that ball affects its impact with the racquet.
The machine can thus be used as a ball tester — using a known “racquet” as the test surface.