Abstract:
The disclosed systems and methods allow the weight and relative position of an object on a weighing surface to be simultaneously determined using a circuit that does not require pre-programmed tables and that can be used in an analog or digital environment. One example system includes first, second, third, and fourth load cells having respective first, second, third, and fourth strain gauges. The strain gauges are configured to measure strain at the load cells caused by the object on the weighing surface. The system also includes circuitry configured to simultaneously determine weight and position of the object on the weighing surface, and a display that reports the weight of the object, a longitudinal position of the object, and a lateral position of the object.

Description:
BACKGROUND OF THE INVENTION 
     It is sometimes necessary to know not only the weight of an object but also the relative position of the center of gravity of the object on a particular surface. For example, it would be desirable to know the position of the center of gravity of a load in a truck to alert a driver if a shift of the mass in a truck during transport creates a risk of the truck tipping when traveling around corners. It would also be desirable to know the relative location of the center of gravity of an object on a conveyer system to ensure that the object is centered for processing or packaging or is at risk of falling off the conveyor without visual monitoring. It is also desirable to know the relative position or shift of position of a patient in a hospital bed. Patients are often unable to be cognizant of their location relative to the edge of a hospital bed. In these instances, it is desirable to know when the patient approaches the edge of a bed to alert a nearby caregiver that the patient is in danger of falling off the bed. Some prior patents have attempted to address this problem in different ways. 
     U.S. Pat. No. 5,276,432 to Travis (“Travis”) discloses that a patient&#39;s weight is determined by locating four force cells in a supporting relation at the corners of the patient&#39;s bed. First, the bed is tared by measuring the bed weight from the summed output of the four force cells. Then, the weight from each of the four load cells is measured with the patient in place and the tare value is subtracted to obtain a net patient weight. Then, a center of gravity is calculated from one particular load cell. From this point of reference, the position of the patient can be calculated. To estimate the position, all load cell measurements are needed. The data must be fed into a multi-purpose computer and processed. Thus, an instantaneous position indication is not possible with this system. Moreover, the patient&#39;s position can only be estimated when the patient is in the region defined by the four corners of the bed. 
     U.S. Pat. No. 7,253,366 to Bhai (“Bhai”) uses four load cells located at the corners of the bed. The signals from the load cells are summed and the bed tare is subtracted to indicate the patient&#39;s weight. From a series of prior calibration measurements, a table of data is stored. The measurements are compared to the lookup table and stored, and a variation of the patient&#39;s weight from the preset table is used to indicate a change in position. Again, a multi-purpose computer is required to perform a variety of calculations. 
     SUMMARY OF THE INVENTION 
     Despite the above prior approaches, there exists a need to determine the weight and relative position of an object on a weighing surface simultaneously using a circuit that does not require pre-programmed tables and that can be used in an analog or digital environment. One or more embodiments of the present invention address these and other needs by providing a different approach that involves a method that provides instantaneous weight and weight position relative to a neutral plane and that may be used to provide an alarm whenever the weight distribution approaches a preset boundary. 
     One example embodiment of the present invention is a system for determining weight and position of an object on a weighing surface. The system includes first, second, third, and fourth load cells having respective first, second, third, and fourth strain gauges. The strain gauges are configured to measure strain at the load cells caused by the object on the weighing surface. The system also includes circuitry configured to simultaneously determine weight and position of the object on the weighing surface. The circuitry determines the weight and position by obtaining a first electrical signal produced by the first strain gauge, a second electrical signal produced by the second strain gauge, a third electrical signal produced by the third strain gauge, and a fourth electrical signal produced by the fourth strain gauge. The circuitry simultaneously adds the first electrical signal, the second electrical signal, the third electrical signal, and the fourth electrical signal, to produce a sum electrical signal proportional to a total weight on the load cells, and subtracts from the sum electrical signal a tare value proportional to the total weight of the weighing surface to produce a tared electrical signal. The circuitry further simultaneously subtracts a sum of the third electrical signal and the fourth electrical signal from a sum of the first electrical signal and the second electrical signal to determine a longitudinal value indicative of the position of a center of gravity of the object on the weighing surface relative to the locations of the first and third load cells and relative to the locations of the second and fourth load cells. The circuitry further simultaneously subtracts a sum of the second electrical signal and the fourth electrical signal from a sum of the first electrical signal and the third electrical signal to determine a lateral value indicative of the position of a center of gravity of the object on the weighing surface relative to the locations of the first and second load cells and relative to the locations of the third and fourth load cells. The system also includes a display that reports the weight of the object based on the tared electrical signal, a longitudinal position of the object based on the determined longitudinal value, and a lateral position of the object based on the determined lateral value. 
     In many embodiments, the weighing surface is a hospital bed and the object is a patient. Some embodiments also include an alarm configured to indicate if the object shifts to an unwanted position. In such embodiments, the alarm may be triggered to indicate by comparing the determined longitudinal value to a longitudinal alarm value to determine whether the determined longitudinal value exceeds the longitudinal alarm value, where the longitudinal alarm value is indicative of the object shifting to a an unwanted position. The alarm may also, or in the alternative, be triggered to indicate by comparing the determined lateral value to a lateral alarm value to determine whether the determined lateral value exceeds the lateral alarm value, where the lateral alarm value is indicative of the object shifting to a an unwanted position. In some embodiments the circuitry is further configured to normalize the longitudinal value by linearly reducing longitudinal value by the absolute value of tared electrical signal. The circuitry may also, or in the alternative, be further configured to normalize the lateral value by linearly reducing lateral value by the absolute value of tared electrical signal. 
     Another example embodiment of the present invention is method of determining weight and position of an object on a weighing surface, where the weighing surface has first, second, third, and fourth load cells with respective first, second, third, and fourth strain gauges, and where the strain gauges are configured to measure strain at the load cells caused by the object on the weighing surface. The method involves simultaneously adding a first electrical signal produced by the first strain gauge, a second electrical signal produced by the second strain gauge, a third electrical signal produced by the third strain gauge, and a fourth electrical signal produced by the fourth strain gauge, to produce a sum electrical signal proportional to a total weight on the load cells, and subtracting from the sum electrical signal a tare value proportional to the total weight of the weighing surface to produce a tared electrical signal. The method further involves simultaneously subtracting a sum of the third electrical signal and the fourth electrical signal from a sum of the first electrical signal and the second electrical signal to determine a longitudinal value indicative of the position of a center of gravity of the object on the weighing surface relative to the locations of the first and third load cells and relative to the locations of the second and fourth load cells. The method further involves simultaneously subtracting a sum of the second electrical signal and the fourth electrical signal from a sum of the first electrical signal and the third electrical signal to determine a lateral value indicative of the position of a center of gravity of the object on the weighing surface relative to the locations of the first and second load cells and relative to the locations of the third and fourth load cells. The method further involves displaying a weight of the object based on the tared electrical signal, displaying a longitudinal position of the object based on the determined longitudinal value, and displaying a lateral position of the object based on the determined lateral value. 
     Yet another example embodiment of the present invention is a system for determining weight and position of an object on a weighing surface. The system includes first and second load cells having respective first and second strain gauges. The strain gauges are configured to measure strain at the load cells caused by the object on the weighing surface. The system also includes circuitry configured to simultaneously determine weight and position of the object on the weighing surface. The circuitry determines the weight and position by obtaining a first electrical signal produced by the first strain gauge, and a second electrical signal produced by the second strain gauge. The circuitry simultaneously adds the first electrical signal and the second electrical signal to produce a sum electrical signal proportional to a total weight on the load cells, and subtracts from the sum electrical signal a tare value proportional to the total weight of the weighing surface to produce a tared electrical signal. The circuitry further simultaneously subtracts the second electrical signal from the first electrical signal to determine a position value indicative of the position of a center of gravity of the object on the weighing surface relative to the locations of the first and second load cells. The system also includes a display that reports the weight of the object based on the tared electrical signal, and a position of the object based on the determined position value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
         FIG. 1  is a schematic of a mechanism for measuring strain caused by an object on a weighing surface. 
         FIG. 2  is a graph of example output of the mechanism of  FIG. 1 . 
         FIG. 3  is a schematic of another mechanism for measuring strain caused by an object on a weighing surface. 
         FIG. 4  is a graph of example output of the mechanism of  FIG. 3 . 
         FIG. 5A  is a schematic of a system for determining weight and position of an object on a weighing surface, according to an example embodiment of the present invention. 
         FIG. 5B  is a graph of example output of the system of  FIG. 5A . 
         FIG. 6A  is a schematic of a system for determining weight and position of an object on a weighing surface, according to an example embodiment of the present invention. 
         FIG. 6B  is a graph of rectified output of the system of  FIG. 5A . 
         FIG. 6C  is a graph of rectified output of the system of  FIG. 6A . 
         FIG. 7  is an electrical drawing corresponding to the schematic of  FIG. 6A . 
         FIGS. 8A-8D  are detailed electrical drawings corresponding to the schematic of  FIG. 6A . 
         FIG. 9  is a flow diagram illustrating a method of determining weight and position of an object on a weighing surface, according to an example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of example embodiments of the invention follows. 
     Embodiments of the claimed invention can be explained with reference to  FIG. 1 , showing an object (weight)  10  on a surface  12  supported by a cantilever beam  13  containing a strain gauge  14  that may be connected to a power source  15 , amplifying means  16 , and output  17 . As the force translates across the surface  12  in the direction of arrow  18   a , a torque on cantilever beam  13  is produced. This bends the beam  13 . The output  17  of the strain gauge circuit  14  will show a responsive reading reflecting the output  17 . 
       FIG. 2  represents an output curve for an object of fixed weight that moves along surface  12  in the directions of arrows  18   a  and  18   b . The torque when the object is centered on the cantilever beam is zero. As the object  10  moves in the direction of arrow  18   a , a positive output is observed. The farther the object  10  is moved from the cantilever beam, the greater the output despite the object  10  being constant in mass. As the object  10  moves in the direction of arrow  18   b , a negative output that is proportional in magnitude but opposite in charge is observed. The farther the object  10  is moved from the cantilever beam, the greater the negative output despite the object  10  being constant in mass. A zero or neutral point  20  is shown in  FIG. 2  representing a position of the center of gravity of the object  10  being positioned over the cantilever beam  13 . 
     This same logic can be applied to an object  30  on a planar surface  32  supported by a cantilever beam  34  as shown in  FIG. 3 . The cantilever beam can bend in two orthogonal directions. The first direction is represented by arrows  36   a  and  36   b . The second direction is represented by arrows  38   a  and  38   b.    
     As shown in  FIG. 4 , movement of the object  30  in the direction of arrow  36   a  will produce a positive output  40   a . Movement of the object  30  in the direction of arrow  36   b  produces a negative output  40   b . Centering the object  30  along the line  38   a,b  produces a zero output  40   c . Centering the object  30  along line  36   a,b  produces a zero output  42   c . The position of the object  30  can be calculated and identified relative to the cantilever beam  34 . It is the purpose of the claimed invention to simultaneously measure the weight of the object and its location on a surface. One example way to accomplish this is to provide a force transducer (not shown in  FIG. 3 ) supporting the cantilever beam  34  of  FIG. 3 . 
     With reference to  FIG. 5A , surface  50  is supported by two load cells  52  and  54 . The load cell  52  and  54  have respective strain gauges  58  and  56  that are powered by respective power sources  62  and  60  and amplifiers  66  and  64 . The output from strain gauges  56  and  58  are added to calculate the total weight  70  and subtracted to determine the relative position  68 . The addition of the signals results in a constant reading regardless of the position between load cell  52  and load cell  54 . On the other hand, if the signals from strain gauge  56  are subtracted from the signals of strain gauge  58 , a curve such as shown in  FIG. 5B  may be produced. When an object  51  is positioned over load cell  54 , the difference between the positive strain gauge reading from  56  and the negative strain gauge reading  58  results in a large positive reading. When the object  51  is positioned equidistant between load cell  52  and load cell  54 , the reading on strain gauges  56  and  58  will be equal and their difference is zero. When the object  51  is positioned over the load cell  52  the positive reading from strain gauge  58  can be subtracted from the already negative reading of the strain gauge  56  resulting in a large negative reading. Thus, a curve is observed, as illustrated in  FIG. 5B , that is proportional to the relative position of the object  51  along the supporting surface  50 . 
       FIG. 6A  shows a weighing surface  80 , such as a weighing platform or a hospital bed that is supported by four load cells  81 ,  82 ,  83  and  84 , each with strain gauges  85 ,  86 ,  87 , and  88  that measure the strain at each of the load cells  81 ,  82 ,  83  and  84 . If the load cells have similar outputs, object  89  can be positioned at any location on the weighing surface  80 . The sum of all the output of all strain gauges  85 ,  86 ,  87  and  88  will be relatively constant regardless of where the object  89  is located on the surface  80 . However, the difference between the output of strain gauges  85  and  88 , for example, can reveal the position of the weight relative to axis  90  extending between load cells  81  and  84 . Thus, a signal can be produced for the location of the center of gravity simultaneously with the total weight. 
     With reference to  FIG. 7 , a diagram is illustrated showing circuitry that calculates the weight and position of the center of gravity (eccentricity) of an object  89  on a surface  80 , as shown in  FIG. 6A . Outputs  110 ,  112 ,  114  and  116  from strain gauges  85 ,  86 ,  87 , and  88  ( FIG. 6A ) may be assumed to have equal output and polarity for a given force. For the purpose of this discussion, the output from each strain gauge is assumed to be positive. The output from these strain gauge is summed in module  118  together with a tare input  120 . For analog signals, module  118  can be a summing amplifier and tare adjustment  120  can be a potentiometer or the like to cancel the weight of bed and associated equipment. A patient&#39;s weight can be displayed on an indicating device such a digital display  122 . Positive outputs  110  and  112  are summed with outputs  114  and  116  after outputs  114  and  116  are sent through inverting devices  124  and  126  in module  130  and displayed on a similar display  132  as the relative position along the longitudinal axis  94  (of  FIG. 6A ) of the weighing surface  80 . The longitudinal axis will be the centerline of a bed if the bed is the weighing surface. Preferably, the weight of the weighing surface  80  is symmetrical around the longitudinal axis  94 . If the weight is not symmetrical, a tare adjustment can be introduced upon initial set up as indicated by  134 . The output from module  130  may also be sent to a circuit  136  that inverts negative signals to a positive signal. Such a circuit  136  is called full wave rectification in power supply circuits and can be simulated by operational amplifiers with diode feedback to improve linearity. In digital circuits, a value is sampled and if negative, its sign is inverted. In this manner, a patent&#39;s position on either side of a bed will indicate a positive signal when it is off the centerline in the longitudinal direction. It can be compared with a preset value. This is shown schematically as comparer  139  with set device  138  and sent to an alarm  140 . The output is shown in  FIG. 6B  and  FIG. 6C . 
     In a like manner, positive outputs  112  and  116  are summed with outputs  110  and  114  after outputs  112  and  116  are inverted in modules  126  and  144 , along with a tare signal from tare device  144  if needed to correct the position of the neutral plane along the lateral axis  96  (see  FIG. 6A ). The output is displayed on indicator  146 . The signal may also be rectified by circuit  148  and compared to a preset signal from set device  150  in comparer  152 . The signal can operate alarm  154  or the signal may be combined with the alarm signal from  140  to operate a single alarm device. 
     It should be noted that when the signals are combined as described above, the object&#39;s weight is monitored in the longitudinal and lateral direction and the position can operate an alarm whenever its weight is displaced left or right of a longitudinal axis, or if above or below a lateral axis. If the alarm signal is divided by the weight signal derived from module  118 , then a patient&#39;s position on the surface  80  can be indicated as unit or 1 when it is on the edge of the bed. This is achieved by linearly reducing (dividing) the output of the patient&#39;s position along the lateral axis by the absolute value of output of the patient&#39;s weight, producing a normalized output. Similarly, the patient&#39;s position along the longitudinal axis may be normalized. This can simplify setting of the alarm, since if it is desired to keep the patient in the center one half of the bed, the alarm may be set to 0.5 on a scale of 0 to 1. In many applications it may not be necessary to measure the weight deviation toward the top or bottom of the bed, thus summing module  130  may be omitted. 
       FIGS. 8A-8D  are detailed electrical drawings illustrating a detailed circuit diagram, similar to  FIG. 7 , that calculates the weight and position of the center of gravity (eccentricity) of an object  89  on a surface  80  shown in  FIG. 6A . 
       FIG. 9  is a flow diagram illustrating a method  900  of determining weight and position of an object on a weighing surface, according to an example embodiment of the present invention. According to the method  900 , the weighing surface has first, second, third, and fourth load cells with respective first, second, third, and fourth strain gauges, and where the strain gauges are configured to measure strain at the load cells caused by the object on the weighing surface. A first electrical signal produced by the first strain gauge, a second electrical signal produced by the second strain gauge, a third electrical signal produced by the third strain gauge, and a fourth electrical signal produced by the fourth strain gauge are obtained ( 905 ). The method involves simultaneously ( 910 ) adding the first electrical signal, second electrical, third electrical signal, and fourth electrical signal to produce a sum electrical signal proportional to a total weight on the load cells ( 915 ), and subtracting from the sum electrical signal a tare value proportional to the total weight of the weighing surface to produce a tared electrical signal ( 920 ). The method further involves simultaneously ( 910 ) subtracting a sum of the third electrical signal and the fourth electrical signal from a sum of the first electrical signal and the second electrical signal to determine a longitudinal value indicative of the position of a center of gravity of the object on the weighing surface ( 925 ). The method further involves simultaneously ( 910 ) subtracting a sum of the second electrical signal and the fourth electrical signal from a sum of the first electrical signal and the third electrical signal to determine a lateral value indicative of the position of a center of gravity of the object on the weighing surface ( 930 ). The method further involves displaying a weight of the object based on the tared electrical signal, a longitudinal position of the object based on the determined longitudinal value, and a lateral position of the object based on the determined lateral value ( 935 ). 
     While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.