CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/948,724 entitled “Force Sensing System for Exercise Equipment”, which was filed Jul. 10, 2007. 
     
    
     BACKGROUND 
       [0002]    A force sensing system is a system comprising one or more load sensors with the necessary electronics to acquire and process various signals. The signals may provide force measurement data for activities performed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    Embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like reference numerals indicate corresponding, analogous or similar elements, and in which: 
           [0004]      FIG. 1  is a conceptual diagram of an exemplary force sensing system for a tensioned flexible member; 
           [0005]      FIG. 2  is a cross-sectional illustration of a portion of the force sensing system adapted for a belt type of flexible member; 
           [0006]      FIG. 3  is a cross-sectional illustration of a portion of the force sensing system adapted for a cable type of flexible member; 
           [0007]      FIG. 4  is a flowchart illustrating an exemplary method for sensing and measuring a tension load generated in the flexible member of the force sensing system; and 
           [0008]      FIG. 5  is a flowchart further illustrating the exemplary method for sensing and measuring a tension load generated in the flexible member of the force sensing system that further incorporates measuring and storing the distance travelled and time elapsed. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    The force sensing system is described here in an exemplary context of being coupled to a weight stack exercise machine, although other weight lifting applications, commercial or industrial, are also contemplated. A weight stack exercise machine comprises one or more lifting arms that are moved by the person exercising on the exercise equipment. The lifting arm(s) are coupled to the weight plates of the weight stack by a weight lifting member that may be a flexible member such a belt or cable. The amount of weight lifted by a user during exercise is determined by the number of weight plates selected from the weight stack. 
         [0010]    The rest of this description is based on an embodiment in which the flexible member is a belt. However, it will be obvious to a person of ordinary skill in the art how to modify the described embodiment to apply a different weight lifting member, for example, one involving a cable or a band. The belt is attached to the weight stack to lift the weights and to translate the load to one or more of the lifting arms. The weight lifting member used in commercially available weight stack machines may comprise the use of one or more cables, belts, or bands, or a combination thereof. A variety of materials, including composite materials, for the cable, belt and/or band are also contemplated. 
         [0011]      FIG. 1  is a conceptual diagram of an exemplary force sensing system for a tensioned flexible member. As the weight plates of weight stack  112  is being lifted by a person exercising on the exercise equipment, belt  101  connecting the lifting arms of the exercise equipment to weight stack  112  begins to experience a tension load  113 . The greater the weight of weight stack  112  being lifted the greater the tension load  113  on belt  101 . As the weight is lifted, belt  101  moves in translation and changes its position accordingly. Belt  101  runs directly through force sensing module enclosure  102  following a specific travel path, wrapped around and engaged at load pulley  104 . A pre-tensioning pulley  111  may be employed between the weight stack  112  and load pulley  104  in order to dampen the effects of any sudden perturbations experienced during lifting of weight stack  112 . Pre-tensioning pulley  111  may be adapted to conform to the shape of belt  101  to securely engage and contain belt  101  while belt  101  moves. 
         [0012]    As the weight stack exercise machine is being used, directing belt  101  to travel through and partially wrap around load pulley  104  forces belt  101  to transfer its static tension load  113  directly to load pulley  104 , whereby load pulley  104  experiences direct radial force  103 . Load pulley  104  is able to rotate substantially freely about its fixed axis  116  that is anchored, either directly or indirectly, within force sensing module enclosure  102 . As belt  101  moves, load pulley  104  upon which it is engaged rotates correspondingly. 
         [0013]    Radial force  103  exerted on load pulley  104  stresses the fixed axis  116  of load pulley  104 , causing a physical strain thereupon. Force sensor cell  105  is in contact with the fixed axis  116  of load pulley  104  and is configured to sense a change in physical strain thereupon via strain sensor  115 . Strain sensor  115  may be a commercially available strain gauge arrangement comprising one or more strain gauges. Force sensor cell  105  is further electrically configured to generate a change in output voltage proportional to the change in physical strain that the strain sensor  115  experiences. Force sensor cell  105  further includes amplifier circuit  109  for amplifying and conditioning the output voltage from strain sensor  115  for further processing via analog to digital converter  114 . Amplifier circuit  109  may have programmable amplifier gain to enhance its versatility in handling a wide range of tension loads  113  from weight stack  112 . 
         [0014]    Other types of force sensors instead of a strain gauge arrangement, such as optical strain sensors, are contemplated and may alternatively be deployed in force sensor cell  105 . 
         [0015]    Printed circuit board  106  is electrically connected to force sensor cell  106 , and includes electrical circuitry and components for processing output voltage signals therefrom. Analog to digital converter  114  converts the amplified output voltage signal from amplifier circuit  109  into a digital output voltage signal for processing at processor  108 . Analog to digital converter  114  may alternatively be located within force sensor cell  105  instead of on printed circuit board  106 . Memory  110  is coupled to processor  108  and stores a predetermined correlation between the output voltage signal of force sensor cell  105  and tension load  113 , the correlation having been determined as the result of a calibration process. Memory  110  also stores selected results of computations performed by processor  108 . 
         [0016]    Optionally, a position sensing system may be incorporated into the force sensing module enclosure  102 . The position sensing system may comprise an encoder, potentiometer or other position sensing components in communication with load pulley  104 . Exemplary optical encoder  107  is in optical communication with load pulley  104  and senses rotation of load pulley  104  as belt  101  moves during lifting and lowering activity. Optical encoder  107  transmits load pulley  104  rotation information to processor  108  which in turn computes the corresponding linear distance travelled by belt  101 . The distance travelled by belt  101  which is computed by processor  108  may be stored in memory  110 . 
         [0017]    Although printed circuit board  106  is depicted at a specific location within force sensing module enclosure  102 , it may alternatively be located elsewhere while being electrically accessible thereto. Furthermore, all separate components including processor  108 , amplifier circuit  109 , analog to digital converter  114  and memory  110 , or any combination thereof, may be incorporated into a single integrated circuit device to be deployed for use with force sensor cell  105 . 
         [0018]      FIG. 2  is a cross-sectional illustration of a portion of the force sensing system, at load pulley  104 , adapted for a belt type of flexible member. Belt channel  202  is designed to geometrically conform to the shape of belt  201  in order to securely retain belt  201  within load pulley  104 . 
         [0019]      FIG. 3  is a cross-sectional illustration of a portion of the force sensing system, at load pulley  104 , adapted for a cable type of flexible member. Cable channel  302  is designed to geometrically conform to the shape of cable  301  in order to securely retain cable  301  within load pulley  104 . 
         [0020]      FIG. 4  is a flowchart illustrating the exemplary method for sensing and measuring a tension load generated in the flexible member of the force sensing system. At  401 , a tension load  113  is generated in belt  101  when the selected weight plates of weight stack  112  are lifted by a user of the exercise machine. At  402 , a corresponding radial force exerted at load pulley  104  by belt  101  stresses the fixed axis of load pulley  104  to cause a physical strain thereon. At  403 , force sensor cell  105  senses the physical strain at the axis of load pulley, and generates an amplified output voltage signal which is proportional thereto, having been amplified by amplifier circuit  109 . At  404 , analog to digital converter  114  converts the amplified voltage signal to a digital output voltage signal suitable for processing by processor  108 . 
         [0021]    At  405 , the force sensing system is calibrated to establish and predetermine a correlation between tension load  113  and output voltage signal of force sensor cell  105 . The correlation between correlation between tension load  113  and output voltage signal of force sensor cell  105  may be of a linear form F=Av+B, where F is the tension load equal to the weight stack being lifted, A and B are numerical constants, and v is the output voltage of force sensor cell  105 . The correlation between tension load  113  and output voltage signal of force sensor cell  105  is not limited to a linear form, and may comprise alternate forms, such as a polynomial form. By observing the output voltages corresponding to at least two predetermined weights, numerical constants A and B are determined. More than two predetermined weights may be used for the calibration. For instance, one weight at the lower end of the range of weights in weight stack  112 , another weight at the upper end of the range of weights, and then other intermediate weights within the upper and lower end of the range of weights for the weight stack exercise machine. For each weight used in the calibration, the corresponding output voltage is recorded. Processor  108  may then be used to compute numerical constants A and B based on the formula F=Av+B. Numerical constants A and B are stored in memory  110  along with the formulaic relationship F=Av+B for correlating tension load F with output voltage v. 
         [0022]    At  406 , the value of tension load  113  caused by the selected weight plates of weight stack  112  is calculated based on the output voltage of force sensor cell  105 , based on the formula F=Av+B, numerical constants A and B having been predetermined and stored in memory  110 . 
         [0023]      FIG. 5  is a flowchart further illustrating the exemplary method for sensing and measuring a tension load generated in the flexible member of the force sensing system that further incorporates measuring and storing the distance travelled and time elapsed. At  510 , the rotation of load pulley  104  is sensed by optical encoder  107 , which transfers the rotation information to processor  108  for calculating the corresponding linear distance travelled by belt  101 . The time elapsed during the rotation of load pulley  104  may also be measured by processor  108 , and communicated to memory  110 . At  411 , the distance travelled and the time elapsed are stored in memory  110 . 
         [0024]    The force measurement data and/or position measurement data may be used to determine the amount of range of motion performed by the person exercising on the exercise machine in real time. The measurements, range of motion, and/or information gleaned from processing the measurements may be sent from memory  110  to a display or reporting system. Possible forms of display and reporting may include visual display, audio or other user output. For example, a numeric display viewable by the user may indicate the precise weight being lifted. Lights of different colors (e.g. red, yellow and green) in a display viewable by the user may indicate the actual range of motion relative to a possible range of motion, and may indicate whether the load being lifted is at, below or above a prescribed weight. 
         [0025]    Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as exemplary forms of implementing the claims.
A force sensing system comprising a force sensor cell with the necessary electronics to acquire and process various signals. The signals may provide force measurement data for activities performed, for instance, on a weight stack exercise machine.