Abstract:
An electromechanical device that provides a spot to a weightlifter performing a weightlifting exercise. The device has a metal frame that supports two arms, called spotter arms. The two spotter arms extend out so as to be able to support a barbell. The arms raise and lower on the frame, remaining parallel with the floor and perpendicular to the frame and are powered by a motor driven lead screw. The operation of the motor is controlled by a computer control system. The input to the control system is provided by a load-cell mounted in one of the two spotter arms. When the control system is operating in the spot mode, the load-cell measures the amount of force that is placed on the spotter arm by the barbell held by the weightlifter. The control system then moves the spotter arms in accordance with the amount of force on the load-cell in order to provide a safe spot thereby increasing the effectiveness of the exercise. The load-cell is mounted within a hollow interior of one of the spotter arms. A mechanical assembly that transfers the force of the barbell supported by the arms to the load-cell is also contained within the hollow interior of the spotter arm.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of athletic and exercise equipment. More particularly, this invention relates to the field of devices and methods for assisting individuals with performing weight lifting exercises in order to prevent injury and to increase the effectiveness of the exercise. 
     BACKGROUND OF THE INVENTION 
     A significant problem for free weight lifters is that, in the absence of a human spotter, it is difficult to derive the maximum benefits from lifting the weights. The reason is that it is dangerous to continue an exercise to the point of fatigue, which is the very time when maximum benefit is derived. In other words, the lifter lifting alone must stop lifting when any doubt creeps into his or her mind whether they can perform the next repetition. In lifts where the bar would not be a danger to the lifter if he or she could not do an extra repetition, such as a curl, the lifter still does not derive maximum benefit from the exercise because of the lack of a spotter. Doing bench presses, where the weight lifter lays underneath the free weights, is particularly dangerous without a spotter. 
     At present, there are numerous mechanical devices that are designed to assist individuals with performing weightlifting exercises. The prior art includes the following United States Patents: Tanski, U.S. Pat. No. 4,807,875; Ryan, U.S. Pat. No. 5,048,826; and Coleman, U.S. Pat. No. 5,407,403. The apparatus disclosed in the &#39;875 patent issued to Tanski has two arms that extend from the sides of a bench press device. These two arms extend underneath an Olympic weight lifting bar. A chain and sprocket assembly, driven by an electric motor, raises and lowers these arms. This device is operated by a switch positioned at the foot of the athlete. In addition, the device is provided with switches that limit the raising and lowering of the arms. 
     A safety apparatus for use with a barbell assembly is taught by the &#39;826 patent issued to Ryan. This assembly includes a support frame, a pair of cables that extend to engage the barbell, and a winch assembly on the support frame that extends and retracts the cables. In addition, the device is provided with sensors to measure the tension on the cables. Also, the device has sensors to measure the direction and the velocity of the movement of the cable. A controller, such as an Intel 8087 micro-controller, is used to control the operation of the winch assembly. 
     The &#39;403 patent issued Coleman teaches a weight lifting safety device that has a computerized control system. This device contains a motor driven cable and a sprocket assembly that can be connected to either a barbell or a pair of dumbbells. The device is provided with sensors to track the speed of the motion of the cable. The control system is programmed with the desired velocity profile of the motion of the bar for the exercise. If the weightlifter moves at a pace that is faster or slower than this profile, the control system activates the motor driven cable assembly and takes control of the weight. 
     At present, the use of electromechanical devices to provide a spot to free-weightlifters is uncommon. One major reason for this uncommon use is the inability of the current technology to provide a mechanical spot that matches the quality of a human spot in connection with the use of free-weights. 
     SUMMARY OF THE INVENTION 
     The present invention is an electromechanical device that provides a spot to a weightlifter performing a weightlifting exercise that is similar in quality to a spot given by a human spotter. The invention has a metal frame that supports two arms, called spotter arms. The two spotter arms extend out so as to be able to support a barbell. The arms raise and lower on the frame, remaining parallel with the floor and perpendicular to the frame. Thus, this device provides a free weight lifter with assistance in lifting weights when, during the course of the exercise, the muscles are fatigued and the lifter cannot lift the amount of weight on the bar by themselves. This assistance is called a “spot.” A control system operates the movement of the two spotter arms. The movement of these two spotter arms is caused by a motor-driven lead-screw. This electro-mechanical device is provided with an electro-optical sensor that provides feed-back information to the control system. 
     The input to the control system is provided by a load-cell mounted in one of the two spotter arms. When the control system is operating in the spot mode, the load-cell measures the amount of force that is placed on the spotter arm by the barbell held by the weightlifter. The control system then moves the spotter arms in accordance with the amount of force on the load-cell in order to provide a safe spot thereby increasing the effectiveness of the exercise. 
     The load-cell is mounted within a hollow interior of one of the spotter arms. A mechanical assembly that transfers the force of the barbell supported by the arms to the load-cell is also contained within the hollow interior of the spotter arm. In a preferred embodiment, only one spotter arm is hollow and contains the load-cell and force transferring mechanism in order to reduce manufacturing costs. The other arm is merely a solid arm that supports the barbell in parallel with the hollow arm. However, in an alternative embodiment, both spotter arms are hollow and contain a load-cell and force transferring mechanism. 
     The object of the present invention is to provide an improved electromechanical device that will aid free-weightlifters in performing their exercises. More specifically, the object of the present invention is to increase the safety of weightlifting exercises by providing a computer controlled device that can provide assistance to weightlifters when they are no longer able to complete the exercise themselves. Another object of the invention is to provide a mechanical spot that mimics that quality and nature of a human spot. A still further object of the invention is to provide an electromechanical spotting device that has a structure that closely resembles current weightlifting structures. 
     Further objects and advantages of the invention will become apparent as the following description proceeds and the features of novelty which characterize this invention are pointed out with particularity in the claims annexed to and forming a part of this specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features that are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself; however, both as to its structure and operation together with the additional objects and advantages thereof are best understood through the following description of the preferred embodiment of the present invention when read in conjunction with the accompanying drawings wherein: 
     FIG. 1 consists of a side view of the spotter system; 
     FIG. 2 consists of a top view of the spotter system; 
     FIG. 3 consists of a front view of the spotter system; 
     FIG. 4 shows the electrical cabling of the spotter system; 
     FIG. 5 shows the process of the setup phase; 
     FIG. 6 shows the process of the exercise phase; 
     FIG. 7 shows the process of the spot phase; 
     FIG. 8 shows a sectional view of one of the spotter arms revealing the load-cell and associated force transferring system; 
     FIG. 9 shows a side sectional view of the spotter arm; 
     FIG. 10 shows a top sectional view of the spotter arm; and 
     FIG. 11 shows a front sectional view of the spotter arm. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In this specification, all elements that are described in the Figures have three digit numbers. Additionally, this specification uses equations to describe the operation of the invention. All equations are numbered using only one or two digits. 
     FIG. 1 gives a side view of the spotter system  100 . FIG. 2 gives a top view and FIG. 3 gives a frontal view. A pair of horizontal beams  101  forms a base for a pair of vertical beams  102 . Two beams  103  each form a stabilizing triangle joint between each horizontal beam  101  and vertical beam  102 . Each of two top beams  104  are held in place by vertical beams  102 . A pair of beams  105  also form stabilizing triangle joints between top beams  104  and vertical beams  102 . Vertical beams  102  are spaced apart by upper lateral beam  151  and lower lateral  152 . For stability, two beams  153  each form a stabilizing triangle joint between lower lateral beam  152  and each vertical beam  102 . Beams  154  also each form a stabilizing triangle joint between lower lateral beam  152  and horizontal beam  101 . 
     The spotter system  100  can have a bench press board  110 , to support the weightlifter for various exercises, such as the bench press. Board  100  is supported by uprights  111  and  112 . Upright  112  can be secured in holes  113  in horizontal beams  101  by pins  114 , to secure the location of board  110  relative to spotter system  100 . By securing the location of board  110  relative to spotter  100 , the safety of the weightlifter can be enhanced and the efficiency of the exercise improved. 
     A pair of vertical smooth shafts  106  are fixedly held in parallel between horizontal beams  101  and top beams  104 . Shafts  106  each have stop ring  107  to hold the shaft in place against top beams  104 . A pair of brackets  108  each support a clamp  109 . Each clamp  109  grips one of shafts  106 . 
     Spotter-arms connecting-plate  131  is slidably attached to vertical smooth shafts  106  via top ball bushing  136  and bottom ball bushing  137 . The spotter arms  132  are held in place via this spotter-arms connecting-plate  131 . Each spotter arm  132  has a forward vertical protrusion  133  and a rearward vertical protrusion  134 , to keep the barbell confined on the spotter arms  132  when the barbell is resting on spotter arms  132 . One or both of spotter arms  132  are hollow and contain a force transferring system  800 . The force transferring system  800  transfers the weight of the barbell supported on the arms  132  to a load-cell  807  thereby allowing a computer control system  200  to determine the extent to which the weightlifter requires a spot. The majority of the force transferring system  800  is contained within the hollow interior  132 A of one of the spotter arms  132 . The force transferring system  800  includes a top plate  801 . The top plate  801  extends between the forward vertical protrusion  133  and the rearward vertical protrusion  134  and supports the weight of the barbell. Internally threaded nut  125  is attached to nut mount  130 . Nut mount  130  is connected to spotter-arm connecting-plate  131  via reinforcing plate  138 . 
     Direct current (DC) motor  120  rests on motor mount  121 . DC motor  120  is chosen among motors commercially available, to deliver torque to gear box  122 . Gear box  122  can have two purposes. The first purpose may include providing a mechanical advantage (MA) to the torque of DC motor  120 , at the expense of motor RPM (revolutions per minute). Thus, the gear box  122  will multiply the torque of motor  120  by a factor of MA while dividing the RPM of DC motor  120  by the same factor of MA. This torque-speed tradeoff can provide increased torque to lead screw  124 . It is possible that gear box  122  may simply have a mechanical advantage of unity. Gear box  122  has a second purpose, in FIG. 1, which is to provide a right-angle change to the direction of the torque generated by DC motor  120 . 
     Gear box  122  is connected to flexible coupling  123 , to accommodate misalignment between lead screw  124  and gear box  122 . Flexible coupling  123  is then connected to lead screw  124 . Lead screw  124  is an externally-threaded shaft. Lead screw  124  may also be called a power screw. Lead screw  124  passes through internally threaded nut  125 . The external threads of lead screw  124  and internal threads of nut  125  are identical in pitch and thread profile, to allow these two members to be in mating rotational contact. Lead screws can have threads with profiles including square threads, modified square threads, Acme threads, stub Acme threads, 60-degree threads, or national buttress threads. Both nut  125  and lead screw  124  should have the same direction of thread, either right-handed or left-handed. Thus, it is critical that the nut and the lead screw have both the same pitch, the same thread profile, and the same right or left handedness of the thread. 
     The rotation of lead screw  124  about the vertical axis moves mating nut  125  either up or down, depending on the rotation of lead screw  124 . DC motor  120 , gear box  122 , flexible coupling,  123 , lead screw  124 , and mating nut  125  form a power-train subassembly. Since nut  125  is mechanically connected to the spotter arms  132 , rotation of DC motor  120  raises or lowers the spotter arms  132 , depending on the direction of rotation of DC motor  120 . 
     The above power-train subassembly is the preferred embodiment. However, other power-train subassemblies could be used in spotter system  100 . DC motor  120 , gear box  122 , flexible coupling,  123 , lead screw  124 , and mating nut  125  could be inverted from that shown in FIGS. 1-3, where the DC motor could be suspended from the top of the frame and the gear box and lead screw could be underneath the motor. Alternately, DC motor  120 , gear box  122 , flexible coupling,  123 , lead screw  124 , and mating nut  125  could be replaced by a hydraulic cylinder or a pneumatic cylinder. Either the hydraulic or pneumatic cylinders would serve the purpose of elevating or lowering spotter arms  132 . 
     DC motor  120  has an integral encoder  360  for the purposes of providing rotation feedback to DC motor servo control  126 . This same feedback is used for determining the rotational motion of lead screw  124  and, hence, the position of spotter arms  132 . Such an encoder  360  is well known in the industry and typically has an internal disk which is either transparent or opaque. If this internal disk is transparent, it is typically made of glass with uniformly spaced dark radial lines etched on it. If this internal disk is opaque, it is typically stainless steel foil with uniformly spaced open radial slots etched in it. Either way, the internal disk is typically interposed between an internal light source and a light detector. As the internal disk rotates, it thus passes or blocks light and this is detected by the light source. One pair of alternating light and dark as detected by the light detector is called a count. If there is a pair of light sources and light detectors, the encoder is said to have quadrature, which means that the encoder can tell both the direction (clockwise or counterclockwise) as well as the magnitude (count) of the rotational motion of the internal disk. Typically counts in one rotational direction are considered positive and counts in the opposite rotational direction are considered negative. So, by summing the positive and negative counts, the sum of these counts gives the desired rotational position. By measuring the time duration between counts, the rotational velocity of the internal disk in revolutions per second, and hence the lead screw  124 , can also be determined by the DC motor servo control  126 . 
     This internal disk is connected to shaft of DC motor  120 . Typically, encoders are classified by the number of lines per revolution, regardless of whether these lines are dark radial lines on transparent glass or open radial slots in opaque stainless steel foil. A 100 line encoder would have 100 uniformly spaced lines in the internal disk. Thus, if controller  126  measured 550 line counts, it would know that the internal disk and hence the DC motor  120  made 5.5 revolutions (550/100). If the mechanical advantage (MA) of the gear box  122  was unity, then the lead screw  124  would have also made 5.5 revolutions. In all subsequent example calculations, it will be assumed that the mechanical advantage of the gear box  122  is unity. 
     The external threads of lead screw  124  have a pitch p which is the amount of distance a point moves along the threads for one revolution of the lead screw  124 . The units of pitch p are typically inches per revolution. The angular rotation and angular velocity of lead screw  124  are known by servo controller  126  via (a) the encoder feedback from DC motor  120  and (b) the known mechanical advantage of gear box  122 . DC motor servo control  126  can convert these angular rotation and angular velocity quantities into linear vertical position and linear vertical velocity by multiplying these angular quantities by the pitch of the lead screw p and then dividing by the mechanical advantage of the gear box  122 . Assuming that the gear box has a mechanical advantage of unity, if the count of a 100 line encoder is +550, the lead screw  124  has turned +5.5 revolutions (+550/100). If the pitch p is 1 inch, then the lead screw  124  has raised nut  125  and spotter arms  132  +5.5 p or 5.5 inches. Similarly, if the encoder disk, and hence the lead screw  124 , is rotating at +10 revolutions per second, the vertical velocity of the nut  125  and spotter arms  132  are equally +10 p or +10 inches per second. This is summarized in the following equations, equations 1-2, which would be calculated by DC motor servo control  126 . Control  126  would need to have the number of lines of encoder  360  and the mechanical advantage of gear box  122  stored in its memory to convert the line count into revolutions. 
      Vertical position of spotter arms  132 , in inches=(motor revolutions)*(inches/screw-revolution)/(mechanical advantage)  (eq.1) 
     
       
         Vertical velocity of spotter arms  132 , in inches/second=(motor revolutions/second)*(inches/screw-revolution)/(mechanical advantage)  (eq.2) 
       
     
     In FIG. 4, this cabling diagram shows that DC motor servo control  126  provides current and voltage to DC motor  120  via power cable  195 . Cable  196  provides the encoder signal from the rotation of DC motor  120  to DC motor servo control  126 . This DC motor servo control  126  could be a dedicated unit, as shown in FIGS. 2-4, or it could be a card inside of a personal computer  200  or a laptop or other microprocessor configuration. Alternately, computer  200  could be resident inside of DC motor servo control  126 . Computer  200  and DC motor servo control  126  communicate via cable  201 . 
     Either DC motor servo control  126  or computer  200  holds key parameters, such as spot position A, the low limit of exercise motion B, and the upper and lower limits of permitted-travel of spotter arms  132 . The upper and lower limits of travel of spotter arms  132  are needed so that the spotter arms will not collide with beams  104  or  101  when positions A and B are being defined by the weightlifter. Other key parameters would include how far to lower the spotter arms  132  from the lower limit of exercise B, so that spotter arms are out of the way during the free-weight exercise period. There may be a database for the weightlifter which stores the positions A and B for that person, based on the exercise done. Thus, the weightlifter would not have to reenter positions A and B every time an exercise was done. 
     Uninterruptable power supply (UPS)  190  provides backup power to DC motor servo control  126  via power cable  191 . UPS  190  is connected to a standard wall outlet or other power outlet via power cable  192 . DC motor  120  could have an internal brake which locks the motor from further rotation once power is cut to it. In case of a power outage, DC servo control  126  would first move spotter arms  132 , and hence the exercise weights, to spotter position A before cutting power to such a DC motor with an internal brake. 
     UPS provides backup power to computer  200  via power cable  193 . Computer  200  normally gets its power from a standard wall outlet or other power outlet via power cable  199 . Similarly, controller  126  normally gets its power from a standard wall outlet or other power outlet via power cable  198 . 
     The amount of current needed to be supplied by DC motor servo control  126  to DC motor  120  to raise or lower the barbell can be estimated by the following screw-torque equations for a single-threaded lead-screw.                Lift                 screw        -        torque     =           FT   *   d     2     *       p   +     PI   *   u   *   d           PI   *   d     -     u   *   p           +       FT   *   u   *   d     2               (     eq   .              3     )                 Lower                 screw        -        torque     =           FT   *   d     2     *       p   -     PI   *   u   *   d           PI   *   d     +     u   *   p           +       Ft   *   u   *   d     2               (     eq   .              4     )                                
     where p=pitch of single threaded lead screw  124   
     d=diameter of lead screw  124   
     PI=3.14159 
     u=coefficient of friction between lead screw  124  and mating nut  125   
     FT=weight of the barbells borne by spotter arms  132  plus the weight of the spotter arms  132 , back plate  131 , nut mount  130 , and reinforcing plate  136   
     Dividing the screw-torque in equations 3-4 by (a) the torque constant Kt of DC motor  120  and (b) by the mechanical advantage of gear box  122  gives (c) the current needed to be provided by control  126  to DC motor  120  during the normal operation of the spotter arms  132 . This calculation is shown in equation 5. This same current would have to be provided via UPS  190  to DC motor servo control  126  during emergency operation of the spotter arms  132 . 
     
       
         Motor current=Screw-torque/(Torque constant*Mechanical advantage)  (eq.5) 
       
     
     Equation 5 can be used to estimate the current required to lift, I(lift), and lower, I(lower), the spotter arms and the barbells being spotted. I(lift) is given in equation 6 and I(lower) is given in equation 7. 
     
       
         I(lift)=Lift screw-torque/(Torque constant*Mechanical advantage)  (eq.6) 
       
     
     
       
         I(lower)=Lower screw-torque/(Torque constant*Mechanical advantage)  (eq.7). 
       
     
     Paddle board  250  has setup button  251 , exercise button  252 , and spot button  253 . Paddle  250  is connected to computer  200  via cable  210 . Buttons  251 - 253  could be foot activated, if paddle  250  resides on the floor and the weightlifter is using his or her hands to hold the weights. However, if the weight lifer is using the spotter for leg exercises, the buttons  251 - 253  could be hand operated. Paddle  250  could be complimented by voice input  270  to computer  200 , via cable  271 . Alternately cables  210  and  271  could be an infrared “wireless” link to computer  200 . 
     Computer  200  could display activity items to the weightlifter via display  260 . Display  260  could be a liquid crystal display (LCD) or a common cathode ray tube (CRT) display. Display  260  is electrically connected to computer  200  via cable  261 . Contact sensors  135  are connected to computer  200  via cables  280 . 
     Position A shown in FIG. 1 is the upper limit of spot desired by the weightlifter. Position B is the lower limit of spot desired by the weightlifter. Positions A and B will vary from exercise to exercise for an individual. Positions A and B will vary from individual to individual for a given exercise. Thus, a setup phase is recommended to establish positions A and B for each user for each desired exercise. 
     FIG. 5 shows the beginning of the setup phase  500  of the use of spotter system  100 . In step  502 , the user enters his or her name and optional PIN (personal identification number) into computer  200 . If the user has already established positions A and B for various exercises in step  504 , the user picks which exercise he or she wants to perform in step  506 . Then the process jumps to the exercise phase in step  508 . 
     The reason for steps  502 ,  504 , and  506  is that the user would not have to repetitively define exercise positions A and B each and every time the user desired to exercise. Weightlifters can be short or tall and exercises can range from squats (low exercises), to bench presses and curls (middle height exercises), to military presses done overhead (high exercises). Thus, positions A and B have to be defined. 
     If positions A and B are not already defined, the step  510  checks to see if setup button  251  was pushed. If not, step  510  cycles back to itself. If setup button  251  was pushed, step  510  jumps to step  512 , where spotter arms  132  are elevated. Step  514  checks to see if setup button  251  was pushed again because spotter arms  132  are at the desired position A. If not, the process cycles back to step  512  and spotter arms  132  are elevated more. However, if setup button  251  is pushed in step  514 , signifying the location of position A, spotter arms  132  are now lowered in step  518 . Step  520  checks to see if setup button  251  was pushed again. If not, the process goes back to step  518  and spotter arms  132  are lowered more. If setup button  251  is pushed again in step  520 , signifying the location of position B, the process goes to step  524 , where the newly defined positions A and B for this exercise are stored in the computer  200 . By storing values A and B, they will not have to be continually be redefined for this weightlifter. Then step  524  flows to step  508 , to begin the exercise phase. 
     In FIG. 6, the free-weight exercise phase begins with step  600 . In step  602 , the process checks to see if exercise button  252  was pushed. If not, the process cycles back to step  602 . However, once exercise button  252  is pushed in step  602 , spotter arms  132  are dropped below position B in step  604 . How far spotter arms  132  are dropped below position B could be user-adjustable. Dropping spotter arms  132  to their lowest possible position could be done. Alternately, spotter arms  132  could be dropped a fixed distance below position B, such as 6 inches below position B. After the spotter arms are dropped out of the way, the user may engage in free weight lifting until he or she presses spot button  253 , in step  606 . If spot button  253  is not pressed, step  606  cycles back to itself. However, once spot button  253  is pressed in step  606 , the process flows to step  608  and the spot phase begins. 
     The spot phase begins in step  700 . The process moves to step  702 , where spotter arms  132  lift at vertical velocity V 1 . Vertical velocity V 1  may be set in computer  200  or DC motor servo control  126  by either the factory or by the weightlifter. Step  704  checks to see if contact has been made with the barbells yet. Contact would be determined by either (a) a change in the force on the load-cell  807  or (b) a jump in the motor current provided to DC motor  120  by DC motor servo control  126  once the weight of the barbells is engaged by spotter arms  132 , per equations 3 and 5. Once contact is made by spotter arms  132  with the barbells, the barbells are raised to position A at a velocity V 2  until N percent of the weight is on the arms  132  in step  706 . The N percent of weight on the arms  132  is measured by the load-cell  807 . The information gathered by load-cell  807  is transmitted to the computer  200 . N is a constant programmed by the user into the computer  200 . Velocity V 2  is preferably less than velocity V 1 , or may be equal to it. The user may set velocity V 2  to his or her preference and store it in computer  200  in her user profile. 
     Once at position A, step  708  checks to see if the weight on the arms  132  is increasing by determining the force on the load-cells  807 . If the weight on the arms  132  is not increasing, the spotter arms  132  are sufficiently spotting the weightlifter. In this case, the spotter arms  132  are raised to position A. In step  713 , the computer  200  checks to see if the spotter arms  132  have reached position A. If the arms  132  have not reached position A, the computer  200  cycles through steps  706  and  708  again. Once the arms  132  have reached position A, the computer  200  proceeds back to step  600 . Referring again to step  708 , if the weight on the arms  132  is increasing, the computer  200  interprets the increasing weight as evidence that the weightlifter is not managing to lift the barbell successfully with the present spot. In this case, in step  710 , the computer  200  increases the amount of the spot given to the weightlifter. Specifically, in step  710  the percentage of weight supported by the arms  132  in the spot phase is increased by N percent. Again, the computer  200  determines the amount of weight supported by the arms  132  through measuring the force on the load-cells  807 . In step  714 , the computer  200  determines if the weight supported by the arms  132  has been adjusted four times in step  710 . If the computer  200  has adjusted the weight supported by the arms four times in step  710 , the computer proceeds to step  717  where the arms  132  are automatically raised to position A. After step  717 , the computer  200  stops the exercise in step  718 . Step  714  and  717  are a safety feature. If it is necessary to adjust the weight supported by the arms four times in step  710 , the weightlifter is no longer in a position to lift the weight in a meaningful sense. If N is equal to 10 percent of the weight of the barbell, four increments of step  710  will cause the spotter arms to support 50 percent of the weight. At this point, it is clear that the mechanical spotting device, and not the weightlifter, is performing the exercise. The computer  200  therefore just removes the weight. If the computer  200  has not reset the weight supported by the spotter arms four times in step  710 , the computer  200  cycles again to step  706 . 
     FIG. 8 shows a sectional view of one of the spotter arms revealing the force transferring system  800 . The sectional view shown in FIG. 8 is taken along section B shown in FIG.  2 . The force transferring system  800  is comprised of a top plate  135  that is rigidly connected to three supporting bars  802 . Each supporting bar  802  is rotationally mounted to a pivot wheel  803  with a set of pins  808 . There are three slots  810  that are cut into the top of the arm  132  that the supporting bars  802  pass through. There are a total of three pivot wheels  803 . The three pivot wheels  803  are rotationally mounted to the sides of the interior hollow portion of the spotting arm by pivot rods  804 . A bottom push-rod  805  connects the three pivot wheels  803  together thereby causing the pivot wheels  803  to rotate in unison. An end  806  of the push-rod  805  is mechanically engaged to the load-cell  807 . The load-cell  807  is rigidly mounted within the interior of the hollow portion  809  of the spotting arm  132 . The load-cell  807  measures the amount of force that is placed on the top plate  135 . The load cell  807  is electrically connected to the computer  200  via an electrical cable or wireless transmitter. The barbell rests upon the top plate  135 . The weight of the barbell pushes the top plate  135  down thereby causing the pivot wheels  803  to rotate. The rotation of the pivot wheels  803  caused by a downward movement of the top plate  135  pushes the bottom push-rod  805  into the load cell  807 . This force transferring system  800  thereby enables the load cell  807  to measure the amount of force placed upon the top plate  135  by the barbell. One advantage of the force transferring system  800  is that the barbell can rest anywhere along the top plate  135  and the load cell  807  will measure the same amount of force. Therefore, the weightlifter can conduct his exercise routine normally without worrying about the position of the barbell along the spotter arm  132 . 
     As described in FIG. 1, FIG. 8 also shows how the arms  132  are moveably secured to the shaft  106  by a top bushing  136  and a bottom bushing  137 . The bushings  136  and  137  allow the arm  132  so slide along the shaft  106 . Note that the load-cell  807  is rigidly secured to the arm  132 . In addition, the pivot rods  804  are rigidly secured to the sides of the arm  132 . 
     FIGS. 9,  10 , and  11  show the side, top, and front views of the force transferring system  800 . In FIG. 9, note that there are ridges  133  and  134  that are formed in the arm  132  to prevent the barbell from sliding off the arm  132 . The side view shown in FIG. 9 is taken along section B shown in FIG.  2 . 
     Referring to FIG. 10, a top view of the arm  132  and the force transferring mechanism  800  is shown. The top view shown in FIG. 10 is taken along section C shown in FIG.  1 . Note that each wheel  803  is actually a pair of wheels  803 . The pivot shaft  804  extends through both wheels  803  and is rigidly secured to the sides of the arm  132  within the hollow interior  809 . The wheels  803  are free to rotate about pivot rods  804 . Each of the three supporting bars  802  are rotationally mounted to the pair of wheels  803  by a pair of pivot pins  808 . A second set of pivot pins  808  rotationally mount the push-rod  805  to the wheels  803 . Note that the entire force transferring system  800  is contained within the hollow interior  809  of the arm  132  except for the top plate  135  and the supporting bars  802 . 
     A front sectional view of the arm is shown in FIG.  11 . The front sectional view shown in FIG. 11 is taken along section A shown in FIG.  2 . The rear ridge  134  restricting the movement of the barbell is shown. The push-rod  805  is rotationally mounted to the pair of wheels  803  by a pair of pins  808 . The top plate  135  is rigidly fixed to the rod  802  that is rotationally mounted to the wheels  803  by a pair of pins  808 . Note that the pivot rod  804  that rotationally secures the two wheels  803  is rigidly secured to the sides of the arm  132  within the hollow interior  809 . 
     The coefficient of friction in equations 3-4 can vary with temperature, age, and environment. However, equations 3-7 can provide the background for estimating the percentage of the weight being spotted by the spotter system  100  and the amount being actually lifted by the weightlifter without precise knowledge of the coefficient of friction. By controller  126  cycling the spotter system (a) with the barbell through a spot cycle, steps  700  through  714 , and (b) without the barbell through an identical spot cycle, (c) each time without the weightlifter touching the barbell or the spotter arms, then (d) the current supplied to DC motor  120  to lift the barbell during a 100% spot-lift ILIFT( 100 ) and a 0% spot lift ILIFT( 0 ) can be empirically measured. This is called the 100% spot-calibration and the 0% spot-calibration. 
     By measuring the current ISPOT during the actual lift portion of step  706 , the percent of the weight of the barbell being spotted is defined by equation 8 and the percent of the weight of the barbell being supported by the weightlifter is defined by equation 9.                Percent                 spotted     =                      ISPOT   -     ILIFT        (   0   )               ILIFT        (   100   )       -     ILIFT        (   0   )           *   100      %             (     eq   .              8     )                 Percent                 lifted     =                        ILIFT        (   100   )       -   ISPOT           ILIFT        (   100   )       -     ILIFT        (   0   )           *   100      %             (     eq   .              9     )                                
     For example, if ILIFT( 100 ) equals 12 amperes, ILIFT( 0 )=2 amperes, and ISPOT during step  706  is 6 amperes, then the percent lifted is 60%, [(12-6)/(12-2)]. Similarly, the percent spotted is 40%, [(6-2)/12-2)]. 
     The sum of equations 8-9 is unity, meaning that the percent spotted plus the percent lifted add up to 100%, as expected. It should be noted that ILIFT( 0 ) the current necessary to lift the weight of just the articulated portion of spotter system  100 , namely (a) spotter arms  132 , (b) spotter-arm connecting-plate  131 , (c) reinforcing plate  138 , (d) internally threaded nut  125 , and (e) nut mount  130 , could be measured at the beginning of the exercise period for that day or at some other convenient time. ILIFT( 0 ) need not be measured for each exercise. However, ILIFT( 100 ) would have to be measured each time the weight of the barbell changed. 
     The results of the estimated percentages of (a) weight spotter and (b) actually lifted during the spot phase, step  706 , could be displayed on display  260 . The calculations required by equations 8-9 would be done by computer  200 . Computer  200  would know the current used during step  706  by querying DC motor servo control  126 , both during the 100% spot-calibration and during the actual spotting of the weightlifter. As previously described, it is DC motor servo control  126  which is providing that current to DC motor  120 . 
     Equations 1-9 could equally be solved in System International (SI) units, which are commonly called metric units in the United States. 
     One last feature of this invention has to do with designing lead screw  124  to be self-locking, meaning that in the event of a compound failure, namely a power outage of normally available power and the failure of the UPS  190 , that the barbell and spotter arms  132  do not descend down upon the weightlifter. The term self-locking does not mean that the lead screw  124  and nut  125  “freeze.” Rather, the term self-locking means that the coefficient of friction between the lead screw  124  and nut  125  is sufficient that the barbell and spotter arms  132  stay in place based on friction alone, without the assistance of electrical power to DC motor  120 . If lead screw  124  has a square thread, the condition for self-locking is that the pitch p of lead screw  124  is equal to the diameter d of the lead screw  124  times PI times the coefficient of friction u between lead screw  124  and nut  125 . This is given in equation 10. Thus, by prudent selection of the lead screw  124  and nut  125 , additional safety can be designed into spotter system  100 . 
     
       
         pitch  p =diameter  d *PI*coefficient of friction  u   (eq.10) 
       
     
     While the invention has been shown and described with reference to a particular embodiment thereof, it will be understood to those skilled in the art, that various changes in form and details may be made therein without departing from the spirit and scope of the invention.