Abstract:
A mobile target screen is described for ball game practicing and simulation. Tow force sensors are mounted at each of the four corners of the frame which holds a target screen. Measurements form the force sensors are used to compute and display a representation of ball speed, the location of the ball on the target screen, and the direction of the ball motion. These parameters can be used to predict the shooting distance and the landing position of the ball. It also provides enough information to predict the trajectory of the ball which can be displayed on a video screen which communicates with the sensors through a wireless transceiver.

Description:
RELATED APPLICATIONS 
       [0001]    The present US non-provisional patent application is related to and claims priority benefit of an earlier-filed provisional patent application titled “Mobile golf training device” 61/491,360. May 5, 2011. The identified earlier-filed application is hereby incorporated by reference into the present application, as though fully set forth herein. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates generally to golfing practice devices which predict the trajectory of a golf ball or any other type of ball hit, thrown or kicked into the target. 
       BACKGROUND OF INVENTION 
       [0003]    The ability to predict the trajectory of a golf ball is important in sport training as well as family entertainment. This type of apparatus is commonly referred to as a golf simulator. The prior art describes a number of techniques for predicting the trajectory of a golf ball hit from a specific location. 
         [0004]    The first category of golf simulator is based on video cameras with high frame rates [1994/U.S. Pat. No. 5,342,051, 2011/U.S. Pat. No. 7,959,517], and the speed and the position of the golf ball can be measured through video capturing and image processing. Usually, a single video camera is not able to precisely measure the direction of the ball movement, so that arrays of video cameras have to be used, each filming from a different angle and position, to capture both the speed and the direction of the golf ball. In practice, the relative positions and angles of these video cameras have to be carefully arranged in order to correlate the images and compute the angle of the ball movement using image processing algorithms. Not only the computing software and image processing algorithms are complicated, the installation of multiple cameras also requires professional accuracy to assure the correct correlation of these images. 
         [0005]    The second category of golf simulator is based on two-dimensional arrays of light sources and detectors in the tee area [1995/U.S. Pat. No. 5,437,457, 1998/U.S. Pat. No. 5,718,639]. The moving golf ball sequentially blocks the light from the sources to detectors and therefore the position of the ball can be detected as the function of time, so that the speed can be calculated. The complexity of this type of system is relatively high due to the use of large numbers of light sources and detectors, and the setup procedure also has stringent requirements on the positions of sources and detectors. 
         [0006]    The third category of golf simulators is based on a mechanical setup in which a golf ball is hanging on the distal end of a rotating drum through an elongated cord [2012/U.S. Pat. No. 8,137,207]. When the golf ball is hit with a golf club, the impact force on the rotating drum and the frame which holds the drum can be measured so that the speed and the direction of the ball can be determined. In this case, the golf ball is mechanically tied to the rotating drum with a cord. Since the golf ball is not completely free, the experience of practicing would not be satisfactory. 
         [0007]    The fourth category of golf simulators is based on measuring the impact of a golf ball hitting a screen. Miyahara [1995/U.S. Pat. No. 5,478,077] uses 4 microphones to detect collision sound when a ball hits a screen. Another microphone is used at the shooting point to detect the sound of the ball being hit. Based on the relative time of the sound signals received by these 5 microphones, the speed and the moving direction of the ball can be determined. In this technique, the 4 microphones on the screen can only determine the location of the ball on the screen, while the calculation of the speed and the direction of the ball rely heavily on the location of the tee with respect to the screen. The material to make the screen may also have to be special in order to produce the required sound. 
         [0008]    Curchod [1993/U.S. Pat. No. 5,221,082] uses 4 force sensors, one on each corner of the screen, to measure the pressure force introduced when a golf ball hits the screen. A relatively large force is expected on a pressure sensor when the hitting point of the ball on the screen is close to that sensor. That is, dr·F r =dl·F l , where dr and dl are distances between the ball hitting point and the right and the left sensor, respectively, and F r  and F l  are pressure forces measured on the right and the left sensor, respectively. The same relation also holds for the top and the bottom sensors. Based on this, relative distance between the hitting point on the screen and each sensor can be calculated, so that the location of hitting point on the screen can be determined. Although the forces measured on the screen can determine the location of the ball hitting on the screen, its direction has to be calculated based on the location of the tee which is the starting point. Therefore, the measurements of the 4 force sensors on the screen have to be combined with the measurements of other optical sensors in front of the screen to predict the direction and the trajectory of the golf ball. 
         [0009]    The purpose of the present invention is to provide a ball target screen which by itself, is able to determine both the position and the flying direction of a ball when it hits the screen. The apparatus will be simple enough for backyard practicing, requiring minimum setup complexity. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0010]    The current invention is intended to provide a target screen which has the ability to detect the speed and location of the ball hitting on it, as well as the flying direction of the ball. This allows the reconstruction of the trajectory of the ball and the prediction of the shooting range and the location of the landing point. 
         [0011]    In one embodiment the four corners of the target screen are tied to a rigid frame. Two force sensors are used at each corner. Upon the event when a ball hits the target screen, the momentum exerted on the target screen creates a vector force on each corner. The two force sensors are amounted in the way they measure forces in the orthogonal (vertical and horizontal) directions. This allows the determination of the force vector produced at each corner of the target screen. Based on the force vectors simultaneously measured from the four corners of the target screen, the location, the speed and the flying direction of the ball can be calculated. There is no need to know the location of the tee where the ball started. This is fundamentally different from the prior art [1993/U.S. Pat. No. 5,221,082], where one sensor is used on each corner which is only able to measure the scalar force, and as a consequence the flying direction of the ball cannot be determined. 
         [0012]    The force vectors measured by the 8 sensors are collected and communicated with a calculation and display unit through a wireless transceiver. Since the trajectory of the ball can be determined by the target screen and force sensors alone and no other information, such as the location of the tee or the initial speed of the ball after impact from the golf club, this invention is especially suitable for low budget and mobile applications requiring fast setup and easy operation. 
         [0013]    These and other features of the present invention are discussed in detail in the section titled DETAILED DESCRIPTION, below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The drawings constitute a part of this specification and include exemplary embodiments of the disclosed subject matter illustrating various objects and features thereof, wherein like references are generally numbered alike in the several views. 
           [0015]      FIG. 1 , Illustrates the configuration of the target screen which is composed of a rigid frame, a soft screen and 8 sensors at the 4 corners. 
           [0016]      FIG. 2 , Illustrates the embodiment of converting the pulling force from the target screen into force on sensors using a mechanical structure. 
           [0017]      FIG. 3 , Illustrates the embodiment of converting the pulling force from the target screen into force on the sensors using a spherical or round structure inside an enclosed box. 
           [0018]      FIG. 4 , Illustrates the static force f DC  versus dynamic force (f(t)−f DC ) when a ball hits the screen. 
           [0019]      FIG. 5 , Illustrates the forces measured by the force sensors and the location of the ball on the screen. 
           [0020]      FIG. 6   a , Illustrates a 3-dimensional description of the speed vector. Where φ V  is the angle between {right arrow over (v)} 0  and the horizontal xy plane and φ H  is the angle projected on the horizontal plane and the z-axis. 
           [0021]      FIG. 6   b , Illustrates a 2-dimensional description of the speed vector {right arrow over (v)} 0  projected on the yz′ plane. Where z′ is the direction of {right arrow over (v)} 0  projected on the horizontal xy plane. 
           [0022]      FIG. 7 , Illustrates the 3-D trajectories of the ball with the initial velocity V 0 =20 m/s, be h 0 =0.5 m and the angles are (f V =45°, f H =18°), (f V =45°, f H =0°), and (f V =30°, f H =−18°), respectively. 
           [0023]      FIG. 8 , Illustrates the spin of the ball through the difference of two speed vectors {right arrow over (v)} 0  and {right arrow over (v)} a . 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The purpose of the present invention is to provide a simple and mobile target screen which is able to detect the speed, the location and the flying direction of a ball  11  when it hits the screen  10 . As shown in  FIG. 1 , the screen  10  is tied on a rigid frame  9  through the four corners. A total of eight force sensors ( 1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 ,  8 ) are affixed to the frame  9 , two at each corner and operably attached to the screen  10 . Force sensor  1  measures the force of the screen applied on the top left corner of the frame  9  towards the horizontal right (+x) direction, whereas force sensor  2  measures the force at the same corner of the frame  9  but towards the veridical downward (−y) direction. Similarly, pressure sensor pairs ( 4 ,  3 ), ( 5 ,  6 ), and ( 8 ,  7 ) measure forces of the screen exerted on the other 3 corners of the frame  9  in the horizontal and vertical directions respectively. 
         [0025]    An electro-mechanical force sensor is a device whose conductance or electrical current is proportional to the mechanical force applied on its sensing area. Force sensors can be low-cost and miniature in size. In order to utilize force sensors in the current apparatus, the stretching forces of the strings  25 ,  26 ,  27  and  28  which hold the screen  10  have to be converted into forces which can be measured by respective force sensors  1 - 8 . One embodiment for such configuration is illustrated in  FIG. 2 , which illustrates the top right corner configuration of the frame  9 . Each of the strings  25 ,  26 ,  27  and  28  which holds the corner of the screen  10  in a certain direction (horizontal or vertical) passes through a via  15  in frame ( 9 ) and is secured to a short piece of rigid bar  24 . The forces pulling on the strings  25  and  26  are transferred to the pressure force on sensors  1  and  2 . 
         [0026]    Another embodiment of the force sensor configuration is shown in  FIG. 3 , which illustrates the top right corner configuration of the frame  9 , in which a solid ball or a solid disk  34  is enclosed inside a box  33 . One end of a string  29  is attached to the ball/disk  34  and the other end of the string  29  is attached on the corner of the screen  10 . Force sensors  1  and  2  are mounted on the inside walls of the box  33  in the vertical and horizontal directions, respectively. The pulling forces of the screen in the vertical and horizontal directions are transferred to pressure force on sensors  1  and  2 . The force sensor can be a low-cost force-sensitive resistor (such as standard 402 FSR from Interlink Electronics, or Flexiforce A201 pressure sensor from Tekscan). In these semiconductor-based force-sensitive resistors, the conductance is proportional to the force applied on the active area. With a proper electrical biasing, the force can be converted to an electrical voltage signal. Other types of force sensors can also be used. 
         [0027]    At the moment when a golf ball hits the screen, the force produced on all of the force sensors  1 - 8  can be simultaneously measured. The speed of the ball, the location of the ball on the screen and the flying direction of the ball can all be determined by the force values measured on the sensors  1 - 8 , which is further described below. 
       Determining the Speed of a Flying Ball 
       [0028]    Assume a golf ball  11  has a mass m and a speed V 0 . When it hits the screen, its momentum will be reduced from mV 0  to zero within a relatively short time interval, that is, 
         [0000]        F=∫f ( t ) dt=mV   0    Equation (1)
 
         [0000]    Where, f(t) is the instantaneous force on the target screen  10  which is, in general, a function of time t, and F is the integrated value of the force over time. In practice, the force values measured by the force sensors  1 - 8  will not be zero even without the ball  11  hitting the screen  10 . The static force f DC  on each force sensor is determined by the tightness of the screen  10  fixed to the frame  9  which often depends on installation.  FIG. 4  illustrates the force f(t) on a pressure sensor as the function of time during the event when a ball  11  hits the screen  10 . As long as the force sensor ( 1 - 8 ) response is linear, the static force f DC  can be subtracted in signal processing in which only the time varying component of the force is considered in the integration.
 
Based on equation (1) the speed of the ball  11  can be found by the overall force exerted on the screen  10 , that is,
 
         [0000]        V   0   =B ( F   Ax   +F   Ay   +F   Bx   +F   By   +F   Cx   +F   Cy   +F   Dx   +F   Dy )   Equation (2)
 
         [0000]    Where, B is a proportionality factor depending on the force sensor  1 - 8  characteristics, and the mass of the ball  11 , which can be calibrated. Assume F Ax , F Ay , F Bx , F By , F Cx , F Cy , F Dx , and F Dy  are the integrated force values measured by the 8 sensors  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 , and  8  at the four corners as shown in  FIG. 5 , the position of the ball on the screen can be deduced from these values as described below. 
       Position of the Ball on the Screen 
       [0029]    As shown in  FIG. 5 , if the bottom-left corner of the screen is used as the origin (0, 0), the position (x p , y p ) of the ball on the screen can be calculated based on the force values F Ax , F Ay , F Bx , F By , F Cx , F Cy , F Dx , and F Dy  measured by the sensors. Suppose the length and the width of the screen are L x  and L y , respectively, when a golf ball  11  hits the screen  10  at the location (x p , y p ), the angles shown in  FIG. 5  can be calculated based on the following basic trigonometry relations, 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    the vertical position of the ball  11  on the screen  10  is, 
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         [0000]    Similarly, the horizontal position of the ball  11  on the screen  10  is 
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         [0000]    The location of the ball  11  (x p , y p ) on the screen  10  can also be found using: 
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         [0000]    In fact, equations (4) and (5) are redundant; they calculate the same location parameters but using force values measured from different sets of sensors. The average of these two sets of measurements allows reduction in the impact of the measurement errors. 
       Traveling Direction of the Ball 
       [0031]    First consider the simplest condition that a golf ball  11  hits the screen  10  perpendicularly (to the forward direction). There is no momentum change in the horizontal direction when the ball  11  is stopped by the screen, and therefore the sum of the measured vector forces on the screen in both the horizontal (x) direction and the vertical (y) direction should be zero, that is, F Ax +F Dx =F Bx +F Cx  and F Ay +F By =F Cy +F Dy . 
         [0032]    In general, the speed vector of a ball {right arrow over (v)} 0  can be defined by its speed V 0  and an angle φ with respect to the forward direction z (where z is perpendicular to the xy plane). As illustrated in  FIG. 6 , this angle can be further decomposed into a horizontal angle φ H  and a vertical angle φ V . 
         [0000]    Obviously, φ H  is determined by the horizontal momentum of the ball, which is proportional to the normalized differential force in the horizontal direction. The horizontal angle φ H  can be found as, 
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         [0000]    Where, M is a proportionality constant, which depends on the friction between the golf ball  11  and the target screen  10 , as well as the fidelity of the force sensors  1 - 8 . This proportionality constant can be calibrated experimentally after the mobile target screen system is fabricated. Similarly, the vertical angle can be found as, 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    Note that the angles φ H  and φ V  may either be positive or negative representing the case when the ball  11  travels to the left/right or high/low with respect to the surface normal to the target screen  10 . 
       Flying Trajectory Prediction of the Ball 
       [0033]    As illustrated in  FIG. 6 , assume the ball  11  has a mass m, a velocity V 0  and a height h 0  upon hitting the screen, its vertical velocity is, 
         [0000]        v   y ( t )= V   0  sin(φ V )− gt    Equation (7)
 
         [0000]    Where, g=9.8 m/s 2  is the gravity. The distance traveled in the vertical direction is then, 
         [0000]        y ( t )= h   0   +∫v   y ( t ) dt=h   0   +V   0  sin(φ V ) t− ½ gt   2    Equation (8)
 
         [0000]    After a time T, the ball  11  falls to the ground, that is, h 0 +v 0  cos(φ V )T−½gT 2 =0 
         [0000]    
       
         
           
             
               
                 
                   T 
                   = 
                   
                     
                       
                         2 
                          
                         
                             
                         
                          
                         
                           V 
                           0 
                         
                          
                         
                           sin 
                            
                           
                             ( 
                             
                               φ 
                               V 
                             
                             ) 
                           
                         
                       
                       + 
                       
                         
                           
                             4 
                              
                             
                                 
                             
                              
                             
                               V 
                               0 
                               2 
                             
                              
                             
                               
                                 sin 
                                 2 
                               
                                
                               
                                 ( 
                                 
                                   φ 
                                   V 
                                 
                                 ) 
                               
                             
                           
                           + 
                           
                             8 
                              
                             
                                 
                             
                              
                             
                               gh 
                               0 
                             
                           
                         
                       
                     
                     
                       2 
                        
                       
                           
                       
                        
                       g 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     9 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    Within this time, the ball  11  travels in the z′-direction for a distance of, 
         [0000]    
       
         
           
             
               
                 
                   
                     L 
                     
                       z 
                       ′ 
                     
                   
                   = 
                   
                     
                       
                         v 
                         0 
                       
                        
                       
                         cos 
                          
                         
                           ( 
                           
                             φ 
                             V 
                           
                           ) 
                         
                       
                        
                       T 
                     
                     = 
                     
                       
                         V 
                         0 
                       
                        
                       
                         cos 
                          
                         
                           ( 
                           
                             φ 
                             V 
                           
                           ) 
                         
                       
                        
                       
                         
                           
                             2 
                              
                             
                                 
                             
                              
                             
                               V 
                               0 
                             
                              
                             
                               sin 
                                
                               
                                 ( 
                                 
                                   φ 
                                   V 
                                 
                                 ) 
                               
                             
                           
                           + 
                           
                             
                               
                                 4 
                                  
                                 
                                     
                                 
                                  
                                 
                                   V 
                                   0 
                                   2 
                                 
                                  
                                 
                                   
                                     sin 
                                     2 
                                   
                                    
                                   
                                     ( 
                                     
                                       φ 
                                       V 
                                     
                                     ) 
                                   
                                 
                               
                               + 
                               
                                 8 
                                  
                                 
                                     
                                 
                                  
                                 
                                   gh 
                                   0 
                                 
                               
                             
                           
                         
                         
                           2 
                            
                           
                               
                           
                            
                           g 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     10 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    During this time interval T, the ball  11  travels in the x-direction for a distance of, 
         [0000]    
       
         
           
             
               
                 
                   
                     L 
                     x 
                   
                   = 
                   
                     
                       
                         V 
                         0 
                       
                        
                       
                         sin 
                          
                         
                           ( 
                           
                             φ 
                             H 
                           
                           ) 
                         
                       
                        
                       T 
                     
                     = 
                     
                       
                         V 
                         0 
                       
                        
                       
                         sin 
                          
                         
                           ( 
                           
                             φ 
                             H 
                           
                           ) 
                         
                       
                        
                       
                         
                           
                             2 
                              
                             
                                 
                             
                              
                             
                               V 
                               0 
                             
                              
                             
                               sin 
                                
                               
                                 ( 
                                 
                                   φ 
                                   V 
                                 
                                 ) 
                               
                             
                           
                           + 
                           
                             
                               
                                 4 
                                  
                                 
                                     
                                 
                                  
                                 
                                   V 
                                   0 
                                   2 
                                 
                                  
                                 
                                   
                                     sin 
                                     2 
                                   
                                    
                                   
                                     ( 
                                     
                                       φ 
                                       V 
                                     
                                     ) 
                                   
                                 
                               
                               + 
                               
                                 8 
                                  
                                 
                                     
                                 
                                  
                                 
                                   gh 
                                   0 
                                 
                               
                             
                           
                         
                         
                           2 
                            
                           
                               
                           
                            
                           g 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     10 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    If the initial height is negligible and let h 0 =0, this distance expression can be simplified as, 
         [0000]    
       
         
           
             
               
                 
                   
                     L 
                     
                       z 
                       ′ 
                     
                   
                   = 
                   
                     
                       
                         2 
                          
                         
                             
                         
                          
                         
                           V 
                           0 
                           2 
                         
                          
                         
                           cos 
                            
                           
                             ( 
                             
                               φ 
                               V 
                             
                             ) 
                           
                         
                          
                         
                           sin 
                            
                           
                             ( 
                             
                               φ 
                               V 
                             
                             ) 
                           
                         
                       
                       g 
                     
                     = 
                     
                       
                         
                           V 
                           0 
                           2 
                         
                          
                         
                           sin 
                            
                           
                             ( 
                             
                               2 
                                
                               
                                 φ 
                                 V 
                               
                             
                             ) 
                           
                         
                       
                       g 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     12 
                     ) 
                   
                 
               
             
             
               
                 
                   
                     L 
                     x 
                   
                   = 
                   
                     
                       2 
                        
                       
                           
                       
                        
                       
                         V 
                         0 
                         2 
                       
                        
                       
                         sin 
                          
                         
                           ( 
                           
                             φ 
                             H 
                           
                           ) 
                         
                       
                        
                       
                         sin 
                          
                         
                           ( 
                           
                             φ 
                             V 
                           
                           ) 
                         
                       
                     
                     g 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     13 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    The detailed trajectory of the ball  11  can be found by its position at any time described as, 
         [0000]        y ( t )= h   0   +V   0  sin(φ V ) t− ½ gt   2    Equation (14a)
 
         [0000]        z ( t )= V   0  cos(φ H )cos(φ V ) t    Equation (14b)
 
         [0000]        x ( t )= V   0  sin(φ H )cos(φ V ) t    Equation (14c)
 
         [0000]      FIG. 7  shows a 3-dimensional display of the ball  11  trajectories with the initial velocity V 0 =20 m/s and the angles are (f V =45°, f H =18°), (f V =45°, f H =0°), and (f V =30°, f H =−18°), respectively. The height of the ball on the screen was assumed to be h 0 =0.5 m. 
         [0034]    In one embodiment of the present invention, the coordinates of the golf ball  11  landing location is provided on a simple digital display, and the deviation from the putting-hole location will also be displayed. In yet another embodiment, the calculated ball trajectory will be presented on a computer or TV screen with the background of the green golf course field and the putting-hole location. 
       Force Calculation and Sensor Selection: 
       [0035]    The maximum force on the sensor  1 - 8  depends on the mass and the speed of the ball  11 . This maximum force value is required in selecting the force sensors which should have the appropriate dynamic range. 
         [0036]    In the case of a golf ball, its mass is 45.9 g, which is m=0.0459 kg. The speed of a golf ball is usually not more than 80 miles per hour, which is V 0 ≦36 m/s. If the golf ball  11  is stopped by the screen  10  within Δt=0.1 seconds, the force exerted on the screen should be, 
         [0000]        F=ma=mV   0 /0.1≦0.0459×36/0.1=16.5 kg·m/s 2 =16.5N=1.65 kg   Equation (15)
 
         [0000]    Although there are 8 force sensors  1 - 8 , the force on each force sensor  1 - 8  is not equal, depending on the position of the ball  11  on the target screen  10 . Therefore, the safe estimation for the maximum force on each force sensor should be 1.65 kg, so that it will not be damaged. 
       Estimation of Ball Spin: 
       [0037]    Based on the force measurements from the eight force sensors  1 - 8 , it is possible to determine the full speed vector of a flying ball  11  when it hits the screen  10 . However, the spin of the ball  11  cannot be determined by this information alone. If one wants to further determine the spin of the ball, the origin or location of where the ball  11  starts just prior to being hit by a golf club has to be predetermined. As illustrated in  FIG. 8 , assume the tee  12  is located at a distance d from the bottom center of the target screen  10 . If there is no spin, the speed vector of the ball  11  when it hits the target screen  10  can be calculated by drawing a straight line from the location of the tee  12  to the measured ball hitting location on the target screen  10 , which is shown as {right arrow over (v)} 0  in  FIG. 8 . If the ball has spin, the trajectory will be curved before hitting the screen and the direction of the speed vector, shown as {right arrow over (v)} a  in  FIG. 8 , measured by the force sensors will be different from that of {right arrow over (v)} 0 . The angular difference between {right arrow over (v)} a  and {right arrow over (v)} 0  can be used to evaluate the speed and the orientation of spin. 
         [0038]    It will be appreciated that the mobile ball target screen of the present invention can be used for applications other than golf training and entertainment. Furthermore, the mobile ball target screen can be fabricated in various sizes and from a wide range of suitable components and materials, using various manufacturing and fabrication techniques accommodating different types of balls. Thus, although the invention has been disclosed with reference to various particular embodiments, it is understood that equivalents may be employed and substitutions made herein without departing from the contemplated scope of the invention.