Abstract:
A weighing device designed to accurately measure an object&#39;s weight that is easier and more cost effective to produce. In particular, the device consists of a strain gage load cell having a pair of substrates mounted in parallel. The first and second substrates each contain a pair of resistors which are embedded in the surface of the substrate pair, forming a Wheatstone Bridge configuration. The substrates may be formed of ceramic.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to a mechanoelectric converter, and more particularly, to a weighing device designed to convert mechanical loads into electric signals using a ceramic shear beam load cell. 
     2. Related Art 
     Heretofore, strain gage load cells used in scales for weighing applications consisted of a complex detection circuit made of resistors, which were mounted on the surface of a solid metal substrate. Referring to FIG. 1, a prior art strain gage load cell  10  is shown. In this device, a complex thin film or metal foil strain gage circuit  14  is mounted, typically by a glueing process, to a metal substrate  12 , such as stainless steel or aluminum. Load cell  10  is mounted to a base (not shown) via through-holes  16 . 
     FIG. 2 depicts another type of prior art strain gage load cell used in the industry. In this version, substrate  12  is hollowed out at midsection  18  in order to focus the stress in the area where the strain gage circuit  14  is mounted, thereby increasing the sensitivity accuracy of the device. 
     However, the machining associated with constructing strain gage load cells in either manner is quite costly. This may be partially attributed to the inherent cost of machining a substrate made of stainless steel. 
     Another costly disadvantage to the current methods used in the industry is the attachment process. Although gluing the strain gages to the surface of the substrate has recently become a reliable means of attachment, the expense remains high. 
     Based on the above, there exists a need to provide an accurate weighing device that is easier and less costly to produce. 
     SUMMARY OF THE INVENTION 
     The present invention provides a device that overcomes the above-identified problems of the related art through the use of, in general, a weighing device utilizing a strain gage load cell. The load cell is constructed of two substrates mounted in parallel, each having a pair of resistors embedded on opposing faces of the substrate pair. In the alternative, the resistors may be embedded on opposing faces of a single substrate. The substrates are preferably ceramic. 
     The first general aspect of the present invention provides for a weighing apparatus, comprising: a) a substrate pair including a first and a second substrate positioned substantially parallel to each other; b) a first and a second strain sensor mounted on opposing faces of the substrate pair; and c) a first and a second spacer, coupled to the first and second substrates at opposite ends thereof, for maintaining the first and second substrates in a spaced apart parallel position. This aspect allows for an accurate weighing apparatus that is cheaper and easier to construct than prior art weighing devices because of the two substrate configuration, having resistors mounted thereon. This aspect also allows for a weighing apparatus that does not deform or “creep” in response to cyclic loading. 
     The second general aspect of the present invention provides for a weighing apparatus, comprising: a) a first and a second substrate positioned substantially parallel to each other; b) a first and a second strain sensor mounted on opposite surfaces of the first substrate; and c) a first and a second spacer, coupled to the first and second substrates at opposite ends thereof, for maintaining the first and second substrates in a spaced apart parallel position. This allows for similar advantages as those mentioned in the first aspect. 
     The third general aspect of the present invention provides for a weighing apparatus comprising: a first spacer mounted to a base and a second spacer mounted to a weighing substrate; a first substrate fastened to the first and second spacers, the first substrate having a pair of resistors and a pair of trimming resistors mounted on a first side of the first substrate; a second substrate fastened to the first and second spacers and spaced from and in parallel with the first substrate, the second substrate having a pair of resistors mounted to a second side of the second substrate; and a set of traces connecting the first and second pair of resistors and the pair of trimming resistors to a microprocessing unit which is connected to an output unit. This aspect provides similar advantages as those associated with the first aspect. It also provides for a trimming device that is easier to manufacture, has better thermal tracking, eliminates the need for additional wiring, and allows for easier balancing via a laser trimming process. 
     The foregoing and other features and advantages of the present invention will be apparent from the following more particular description of preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein: 
     FIG. 1 depicts a prior art load cell constructed of a solid substrate; 
     FIG. 2 depicts a prior art load cell wherein the substrate contains a hollowed mid-section; 
     FIG. 3 depicts a cross-sectional view of a weighing device in accordance with the present invention; 
     FIG. 4 depicts a split view of the strain gage load cell in accordance with the present invention; 
     FIG. 5 depicts the electrical configuration of the trimming resistors in accordance with the present invention; 
     FIG. 6 depicts a cross-sectional view of a loaded weighing device in accordance with the present invention; and 
     FIG. 7 depicts a variation of the present invention. 
    
    
     It should be noted that the drawings are not to scale. They merely depict schematic representations of the invention, and are not intended to portray specific parameters of the invention. The drawings are intended to show only typical embodiments of the invention, and therefore, should not be considered as limiting the scope of the invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although certain preferred embodiments of the present invention will be shown and described in detail, it should be understood that various changes and modifications may be made without departing from the scope of the appended claims. The scope of the present invention will in no way be limited to the number of constituting components, the materials thereof, the shapes thereof, the relative arrangement thereof, etc., and are disclosed simply as an example of the preferred embodiment. 
     FIG. 3 depicts a weighing device  20  in accordance with the present invention. Weighing device  20  includes a base  22 , a spacer  26 , a spacer  28 , a weighing plate or substrate  24 , a top substrate  30 , a bottom substrate  32 , and resistors  34 ,  36 ,  38  and  40 . Top substrate  30  and bottom substrate  32  are fastened, substantially in parallel, to spacers  26  and  28 , typically via an adhesive, such as an epoxy or pressure sensitive tape, etc. Top substrate  30  and bottom substrate  32  are made of ceramic, but may be any other similar elastic material possessing the desired properties, such as a polymer material, stainless steel, etc. 
     Spacer  26  is mounted to base  22  via bolts, screws, glue, welding, etc. Using similar methods, spacer  28  is mounted to weighing plate  24  at a midpoint M. Spacers  26  and  28  are typically made of aluminum, but may be any hard material, such as plastic, metal, ceramic, etc. Resistors  34  and  36  are embedded into the top side of top substrate  30 , and resistors  38  and  40  are embedded into the bottom side of bottom substrate  32 . Resistors  34 ,  36 ,  38  and  40  are electrically connected to form a Wheatstone Bridge configuration, commonly used in the industry. Resistors  34 ,  36 ,  38  and  40  are embedded into their respective substrates via a screen printing process, baked on using a firing kiln, or by other comparable method. These methods eliminate the expense and unreliability associated with the prior art gluing process. 
     FIGS. 4 and 5 depict a split view of strain gage load cell  48  in accordance with the present invention, wherein FIG. 4 shows the top side of top substrate  30  and FIG. 5 shows the bottom side of bottom substrate  32 . Load cell  48  includes spacers  26  and  28 , top substrate  30 , bottom substrate  32 , resistors  34 ,  36 ,  38  and  40 , a trimming resistor  42 , electrical connections or traces  50 , microprocessing unit  54  and display unit  56 . Top substrate  30  includes trimming resistors  42   a  and  42   b  (not found on bottom substrate  32 ), which is mounted on top substrate  30  over spacer  26 . The electrical configuration of trimming resistor  42  in relation to resistors  34 ,  36 ,  38  and  40  is shown in FIGS. 4 and 5. Trimming resistor  42  is designed to electrically balance the Wheatstone Bridge. Unlike the unbalanced prior art devices, which typically add discrete trimming resistors at a location adjacent to the device, the present invention mounts the trimming resistor to top substrate  30 . This allows for easier manufacturing, better thermal tracking, the elimination of additional wiring, and allows for easier balancing via a laser trim process. 
     Electrical connections or traces  50  are composed of a thick film material and are preferably screen printed onto top substrate  30  and bottom substrate  32 . This process reduces the cost of electrical connections and eliminates any reliability problems associated with the prior art gluing process. Electrical connections or traces  50  are soldered to wires  51 , which connect load cell  48  to a microprocessing unit  54 , which is then electrically connected to a display unit  56 , i.e., a computer screen, a printer, etc. Microprocessing unit  54  calibrates and converts the resistance measurement (volts) received from load cell  48  into the desired units of measure, i.e., pounds, ounces and so on, which is then displayed on display unit  56 . 
     Spacer  26  contains threaded through-holes  44  to fasten, via bolts, screws, etc., the left side of load cell  48  to base  22  (as shown in FIG.  3 ). Similarly, spacer  28  contains threaded through-holes  46  to fasten, via bolts, screws, etc., the right side of load cell  48  to weighing plate  24  (as shown in FIG.  3 ). It should be noted that the placement of the threaded through-holes  44  and  46  reduces the amount of torque on the holes and bolts used for connection, as compared to the in-line placement in the prior art (see FIGS.  1  and  2 ), which tends to shear the connecting bolts over time. Spacers  26  and  28  may also be connected to base  22  and weighing plate  24 , respectively, via glue, welding, etc. 
     It should be noted that resistors  34  and  38  are to be mounted over the inside edge  27  of spacer  26 , as depicted in FIG. 4, since this is a location of high stress. Consequently, this is a good place to detect a distortion in the elements upon the application of weight. Likewise, resistors  36  and  40  should be mounted over the inside edge  29  of spacer  28 . 
     FIG. 6 shows an enlarged view of weighing device  20  to illustrate the displacement of substrates  30  and  32  by weight. When a weight W is applied to weighing plate  24 , spacer  28  is forced downward toward base  22 . Substrates  30  and  32  are distorted at their mid-sections  52 , between spacers  26  and  28 , while the portions of the substrates extending beyond the spacers remain in parallel. Resistors  34 ,  36 ,  38  and  40  undergo corresponding variations which in turn provides a measure of the amount of distortion in substrates  30  and  32  at inner surfaces  27  and  29 . 
     FIG. 7 depicts a variation of load cell  48  in accordance with the present invention, wherein resistors  34  and  36  are embedded into the top surface of top substrate  30  and resistors  38  and  40  are embedded into the bottom surface of top substrate  30 . It should also be noted that although bottom substrate  32  is shown, it is not necessary when resistors  34 ,  36 ,  38 , and  40  are embedded into top substrate  30 . 
     The construction methods as described in this disclosure utilize at least four resistors to make up a full Wheatstone bridge circuit. It should be noted that less than four resistors may be used with the construction but would cause some sacrifice in performance. The number of resistors, and their locations are not limited by the present disclosure. 
     It should also be noted that the sensitivity of weighing device  20  can be altered by varying the horizontal distance between spacers  26  and  28 , or by altering the dimensions of top and bottom substrates  30  and  32 , respectively. For example, as the distance between the spacers increases, mid-section  52  of the device will be less rigid therefore more responsive to the application of a weight. Whereas decreasing the distance between spacers increases the rigidity of mid-section  52 , therefore making the device less sensitive to the application of weight. Similarly, by increasing the thickness or width, or decreasing the length of substrates  30  and  32  they become less flexible and device  20  becomes more rigid, thus less sensitive to the application of weight. 
     It should be appreciated that the present invention may be used in a variety of weighing applications, such as weighing letters, weighing passengers sitting in a car seat, etc. 
     While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.