In the game of tennis, the tennis ball can travel at speeds well in excess of 100 mph. When the ball bounces off the ground on or near a boundary line of the tennis court, it is often difficult to determine with the human eye exactly where the ball struck the ground since it is in contact with the ground only for a split second. Obviously, it is of great importance to the play of the game to make this determination as accurately and as quickly as possible. In a professional tennis match, there can be as many as ten officiators whose primary responsibility is to determine whether the tennis ball bounced in or out of bounds. Due to the difficulty of accomplishing this task using only the naked human eye, dozens of automated systems for determining whether a tennis ball has struck the ground in or out of bounds have been proposed and/or developed.
Many of the systems are electromagnetic in nature and require a specially designed tennis ball having specific electromagnetic materials therein which allow the position of the ball to be detected when it is in the vicinity of a boundary line. The boundary line is equipped with electro-magnetic field detecting equipment to detect the position of the ball when it is near the boundary line. Examples of such systems are disclosed in U.S. Pat. Nos. 5,342,042, 5,303,915, 4,859,986, 4,664,376, 4,432,058, and 4,092,634. These types of systems are disadvantageous because they require special balls to be effective. The addition of electromagnetic materials to the ball will change the properties of the ball in a manner relevant to play of the game. It also will increase the cost and complexity of the ball.
Other proposed systems employ optical sensors positioned near the boundary lines. Two such systems are disclosed in U.S. Pat. Nos. 5,059,944 and 4,866,414. These systems are disadvantageous because the optical sensors are very expensive and need to be placed in positions that may interfere with play. Further, the players' bodies may block the optics, causing the system to be unable to detect the ball.
U.S. Pat. No. 5,394,824 discloses a thermo-chromic system in which the boundary lines are coated with a thermo-chromic material that changes color when it is struck by the tennis ball. The change in color is dependent at least in part on the speed of the ball when it contacts the thermo-chromic material. The change in color remains for a limited period of time after contact and, thus, gives an officiator a finite amount of time to run to the spot and examine the line after a ball strike to determine where on the thermo-chromic line the ball strike occurred. The time required to run to the spot and observe the line adds delay to the game.
U.S. Pat. Nos. 3,982,759 and 4,365,805 disclose systems in which the tennis court boundary lines are outlined with a laminate comprising two conductors separated by an insulating layer. When a ball strikes the laminate, the two conductors will be compressed towards each other, triggering a pressure sensitive switching means, thereby generating a particular electrical signal indicating that the ball has struck the laminate. Depending on whether the laminate is positioned just out of bounds or just in bounds, the generation of the signal will indicate that the ball strike was out of bounds or in bounds, respectively.
U.S. Pat. No. 4,365,805 also discloses a method and apparatus by which the location of the ball strike in the direction parallel to the boundary line also is determined. Particularly, the two parallel conductive strips are separated by an insulating strip with holes therethrough. When the ball strikes the laminate, it will cause the two separated conductive layers to compress towards each other and contact each other in the positions adjacent the holes, but will remain separated by the insulating material in the positions corresponding to where there are no holes. Thus, the holes and the adjacent conductor portions form switches that selectively couple the two conductive layers when pressure is applied to the strip. These switches are all interconnected by a series of thin-film resistors or resistive wires. By connecting the switches with resistors, a switch closure, rather than resulting in a complete electrical short, instead creates an electrical circuit having a unique resistance profile for each switch. By measuring the resistance of the circuit when a switch is closed due to a ball strike, the unique switch which was closed is determinable. Accordingly, this system not only detects whether the ball struck the laminate, but where in the direction parallel to the laminate strip (i.e., parallel to the boundary line) the ball struck.
U.S. Pat. No. 4,855,711 also discloses a pressure sensitive switch type system. However, in this system, two conductive layers are separated by a continuous resilient insulating layer. When the ball strikes the laminate strip, the two parallel separate conductors do not contact each other, but merely move closer to each other due to the compression of the resilient insulating layer. The change in distance between the two conductive layers yields a displacement current in the conductive layers which is detected and indicative of the fact that the ball struck the laminate strip. Processing of the displacement current is performed to distinguish whether the compression was due to spurious impact, e.g., a foot or a racket striking the laminate strip, or a ball strike. The different types of impacts can usually be easily distinguished by the shape and duration of the displacement current. For instance, a footstep on the line typically will cause a compression of much longer duration than a ball strike and thus create a displacement current of much longer duration.
One disadvantage of all of the aforementioned prior art line calling systems is that they are expensive to manufacture, comprising specially made laminates (in the case of the pressure sensitive type systems or electromagnetic systems) or optical equipment (in the case of the optical systems). The systems disclosed in U.S. Pat. No. 4,365,805 is particularly expensive in that the conductors in the laminate include thin film resistors or resistive wires between all of the switches.
Accordingly, it is an object of the present invention to provide an improved, instant sport line calling system.
It is another object of the present invention to provide a low cost sport line calling system.
It is yet another object of the present invention to provide a sport line calling system which can detect a ball impact that occurs contemporaneously with a separate, spurious impact on the boundary.
It is a further object of the present invention to provide a low cost tennis line calling system which determines not only whether a ball has struck the ground in or out of bounds but also where in the direction parallel to the boundary the ball strike occurred.