Abstract:
A low cost strain gage load cell made without compromising accuracy and stability by a composite structure using a sensing element formed of a load cell quality material, such as metal or a metal alloy, and adjoining non-sensing elements formed of a molded plastic material. Stable and secure joints between the load cell sensing element and the plastic non-sensing element of such a load cell are provided using various structures and related structural manufacturing methods. For example, non-sensing elements, such as a mounting block to mount the load cell to a base support and a load application block to receive a load platform, are formed of an injection molded plastic and sensing elements, such as first and second parallel beams of a load cell quality metal alloy, have ends embedded in the injected molded plastic non-sensing elements. The composite load cell structure is applicable to many different types of load cell designs.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to strain gage load cells for weighing, force measurement, and pressure measurement. 
     2. Description of the Related Art 
     A strain gage load cell is basically a metal structure subject to a load to be measured, with strain gages sensing strain in the loaded structure. The strain gages are connected in an electrical bridge circuit. Power is supplied acros one bridge diagonal, and an output signal across the other diagonal is used as an indication of the load on the metal structure. Strain gage load cells have been used extensively for weighing, force measurements, and pressure measurements since the middle of the 20 th  century. 
     The output signal from a strain gage load cell derives entirely from changes in the resistance of the strain gages. Accurate load measurements require that the strain gages sense the strain in the loaded structure accurately, and also that the strains in the loaded structure are true measures of the load on the metal structure. Only certain specially heat treated metal alloys, selected for low mechanical hysteresis and low creep are suitable for use in the loaded metal structure in high accuracy strain gage load cells for “legal for trade” weighing. It is also important to insure that the load is transferred to the metal structure without errors caused by slip or shifting of the point of load application. 
     Extensive efforts to make strain gage load cells more accurate have led to hundreds of different strain gage load cell designs now known in the art. Known strain gage load cells are all-metal designs. Most high accuracy strain gage load cells have both the loaded metal structure and the load application parts machined from one piece of load cell quality alloy. This provides ideal load introduction and measurement of applied strains, but the cost for material and machining are high. 
     Some planar gage type load cells for low load ratings have sensing structures machined from a flat sheet of metal, with fold-back arms and other load application and support parts of metal bolted to the sensing structure. This lowers the cost of material and machining, but the assembly cost is increased, and the repeatability is generally impaired. 
     SUMMARY OF THE INVENTION 
     A main object of the present invention is to provide strain gage load cells that are less expensive to make than prior art strain gage load cells, while maintaining high accuracy and repeatability. 
     This object is obtained by a strain gage load cell with a load sensing structure, comprising a load cell sensing element of load cell quality material and a non-sensing element of a plastic material joined to said sensing element. 
     Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1A is a top view of a planar gage strain gage load cell with a single fold-back arm according to a preferred embodiment of the invention. 
     FIG. 1B is a side view of the load cell shown in FIG. 1A when unloaded. 
     FIG. 1C is a side view of the load cell shown in FIG. 1A when loaded. 
     FIG. 2 is a top view of a planar gage strain gage load cell with lateral fold-back arms according to a preferred embodiment of the invention. 
     FIG. 3 is a top view of a planar gage strain gage load cell with two central fold-back arms according to a preferred embodiment of the invention. 
     FIG. 4A is a top view of a membrane type load cell according to a preferred embodiment of the invention. 
     FIG. 4B is a vertical section along the line “IVB—IVB” through the center of the load cell shown in FIG.  4 A. 
     FIG. 5A is a side view of a load cell with a bending beam sensing structure of metal and additional metal flexures according to a preferred embodiment of the invention. 
     FIG. 5B is a vertical section along line “VB—VB” through the load cell shown in FIG.  5 A. 
     FIG. 5C is a top view of the load cell shown in FIG.  5 A. 
     FIG. 6A is a top view of a planar beam type strain gage load cell for compression or tension loading according to a preferred embodiment of the invention. 
     FIG. 6B is a vertical section along line “VIB—VIB” through the load cell shown in FIG.  6 A. 
     FIG. 7A is a top view of a sealed dual bending beam strain gage load cell according to a preferred embodiment of the invention. 
     FIG. 7B is a vertical section, partially exploded, along line “VIIB—VIIB” through the load  1 cell shown in FIG.  7 A. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
     FIGS. 1A-1C show different views of a load cell  10  according to a preferred embodiment of the invention. The load cell  10  comprises two flat parallel and coplanar bending beams  12 ′ and  12 ″ of a load cell quality material, such as a load cell quality aluminum or stainless steel alloy. The ends of the bending beams  12 ′,  12 ″ are joined to end pieces  14  and  15  of plastic by injection molding. One end piece  14  has through holes  16  for bolts (not shown) for mounting the load cell  10  as a cantilever on a base  100  via threaded holes  102 . The other end piece  15  has a fold-back arm  13  extending between the two bending beams  12 ′,  12 ″. A threaded load application hole  18  in the fold-back arm  13  is centered between the midpoints of the bending beams  12 ′,  12 ″. The load application hole  18  is provided for mounting of a load platform (not shown). The geometry of the load cell  10 , with one end  14  mounted on a bending resistant support  100 , and a fold-back arm  13  with a loading hole  18  between the midpoints of the bending beams  12 ′,  12 ″, makes the bending beams  12 ′,  12 ″ bend in a controlled mode, so they form a shallow S-shape as shown in FIG. 1C when a load “F” is applied, as is well known in the art. 
     Strain gages  121 - 124  are bonded to the bending beams  12 ′ and  12 ″ near the junction. between the bending beam  12 ′ and the end pieces  14 ′ and  15 , as is common in the art. Strain gages  121  and  122  are shown bonded to the top surface of bending beam  12 ′, while strain gages  123  and  124  are shown bonded to the underside of bending beam  12 ″. This arrangement of the strain gages simplifies correction for differences in strain gage sensitivities by resistive de-sensitizing, e.g. as described in U.S. Pat. No. 4,979,580. 
     When the bending beams  12 ′ and  12 ″ flex as shown in FIG. 1C, strain gages  121  and  123  will sense tensile strain, and strain gages  122  and  124  will sense compressive strain, so a strain gage bridge containing the four strain gages  121 - 124  will become unbalanced in proportion to the magnitude of the load on the load “F”, as is well known in the art. If all four strain gages  121 - 124  are accurately placed and have the same sensitivities, the output from the bridge will be insensitive to positional variations of load placement on a load platform. 
     The end piece  14  and end piece  15  with fold-back arm  13 , are plastic material parts, fastened to the ends of bending beams  12 ′,  12 ″ by injection molding. The ends of the beams  12 ′,  12 ″ are inserted into molds, which are filled by a plastic compound under high temperature and pressure. After curing and cooling, the plastic material will enclose the ends of the beams  12 ′,  12 ″ tightly, but it may not adhere well to the metal beams. In order to get permanent, creep free joints between the plastic material and the metal beams, even when the plastic material will not adhere reliably to the metal beams  12 ′,  12 ″, the ends of the beams  12 ′,  12 ″ can be provided with spaced apart gripping surfaces for the plastic material, for instance by punching or machining holes  19  in the ends of the metal beams  12 ′,  12 ″ as shown in FIG. 1A, or by cutting notches in the edges of the beams  12 ′,  12 ″. During the injection molding process, the plastic material is forced to fill such holes  19  and notches under high pressure, thereby forming a matrix of plastic and metal which keeps the ends of the beams  12 ′,  12 ″ firmly joined to the plastic end pieces  14 ,  15  when the plastic has cured. In most cases, the plastic material will shrink or expand slightly during curing and cooling. A typical injection molding material, such as polypropylene sulfide with 40% glass fiber for reinforcement, shrinks about 0.1% during curing. This causes firm prestress forces between the plastic material and the gripping surfaces, which further enhances the solidity of the joints. Various injection molded plastic compounds with strength data similar to aluminum are readily available in today&#39;s market. 
     Through holes  16  for mounting bolts in the left hand end piece  14  are formed in the plastic during the molding process by inserts in the injection mold, with matching holes in the bending beams  12 ′,  12 ″. One or more threaded holes  18  for mounting of a load platform on the right hand fold-back arm  13  are formed by similar inserts in the mold. 
     FIGS. 2 and 3 are top views of planar bending beam load cells  10 ′ and  10 ″ according to further preferred embodiments of the invention. These load cells  10 ′,  10 ″ are functionally equivalent to the load cell  10  shown in FIGS. 1A-1C, and equivalent parts are designated by the same reference numbers. The differences between load cells  10 ,  10 ′ and  10 ″ are in the mounting methods employed. 
     Load cell  10 ′, as shown in FIG. 2, is designed to be mounted on separated supports (not shown) via holes  16 ′ in two parallel fold-back arms  17 ′,  17 ″ of plastic material extending from plastic end piece  14 ′. As embodied herein, fold-back arms  17 ′,  17 ″ and end piece  14 ′ are formed of one integral injection molded plastic material. The fold-back arms  17 ′,  17 ″ eliminate bending stresses on the supports. Similar to the embodiment of FIGS. 1A-1C, fold-back arm  13 ′ and end piece  15 ′ are also formed of an injection molded plastic material. 
     Load cell  10 ″, as shown in FIG. 3, has a central fold-back arm  17 ″′ extending from end piece  14 ″ and a central fold-back arm  13 ″ extending from end piece  15 ″ with corresponding mounting holes  116  and  118  equidistant from the end pieces  14 ″ and  15 ″. Two pairs of further mounting holes  116  and  118  are arranged resepctively in the end pieces  14  and  15 . Each set of three mounting holes  116 ,  118  form a triangle, which provides stable support and mounting on a base (not shown) at one end, and for support and mounting of a load platform (not shown) at the other end. As embodied herein, end piece  14 ″ with fold-back arm  17 ′″ and end piece  15 ″ with fold-back arm  13 ″ are formed of an injection molded plastic material. The load platform is preferably centered over the gap between the two fold-back arms  17 ″′,  13 ″, so the beams  12 ′,  12 ″ bend in a symmetrical shallow S-shape. Load platform mounting holes  118  also may be formed as threaded holes, such as shown for load cell  10  in FIG.  1 . 
     Side views of load cells  10 ′,  10 ″ shown in FIGS. 2 and 3 would be essentially similar to FIGS. 1B and 1C described above. The bending beams  12 ′,  12 ″ are in both cases forced into a controlled bending mode, forming shallow S-shapes, and the strain gages  121 - 124  will in both cases sense strains equivalent to the strains sensed in load cell  10 . 
     A membrane type load cell  40  according to another preferred embodiment of the invention is shown in FIGS. 4A and 4B. In this case, the sensing member is a flat membrane  42  of a load cell quality material, such as a load cell quality aluminum or stainless steel alloy. The membrane  42  is joined at its rim to a plastic cylinder  44 , and a metallic load button  45  is suitably fastened to a central hole  48  in the membrane. When a load is applied to the load button  45 , the membrane  42  will deflect in a controlled mode with an inflection zone midway between the inner edge of the cylinder  44  and the outer edge of the load button  45 . A number of strain gages  121 - 128  arranged to sense radial strain are bonded to the membrane  42  on each side of the inflection zone, as shown in FIG.  4 A. The strain gages are preferably bonded to the underside of the membrane  42 , where they are protected. All odd numbered strain gages will sense compressive strain, while all even numbered strain gages will sense tensile strain. Alternatively, the even numbered strain gages may be arranged to sense tangential strain, while the odd numbered strain gages remain arranged to sense radial strain, as is well known in the art. The strain gages  121 - 128  are connected in a bridge circuit, which will provide an output signal indicating the load on the load button. 
     The membrane  42  is preferably joined to the cylinder  44  by injection molding. To assure firm joining of the injection molded plastic cylinder  44  to the metal membrane  42 , gripping surfaces are provided by punched or machined holes  49  inside the rim of the membrane  42 , as described above with reference to element  19  of load cell  10  as shown in FIGS. 1A-1C. 
     The load cell  40  may also be used as a pressure gage instead of as a load cell. When used as a pressure gage, the device  40  needs not have a load button  45 , but the bottom of the cylinder  44  must be sealed so a reference pressure can be maintained on the underside of the membrane  42 . A pressure differential between the two sides of the membrane  42  will cause a net force to act on the membrane  42 , causing a deflection similar to the one described above. 
     A reinforced double bending beam load cell  50  according to another preferred embodiment of the invention is shown in FIGS. 5A-5C. FIG. 5A is a lateral view of the load cell  50 , FIG. 5B is a vertical section through the load cell  50  along line “VB—VB” in FIG. 5A, and FIG. 5C is a top view of the load cell  50 . The sensing member of this load cell  50  is a double-bending beam structure  51  machined from a rectangular-bar of load cell quality metal alloy. A large hole  58  and four surrounding notches  57  are machined at the center of the metal bar so two bending beams  52 ′ and  52 ″ are formed. Two additional bending beams  53 ′,  53 ″ are arranged one above and one below the bending beam structure  51 , and plastic end pieces  54  and  55  are injection molded over the ends of the bending beam structure  51  and the ends of the additional bending beams  53 ′,  53 ″. A number of small holes  59  are machined through the ends of the bending beam structure  51  as shown in FIG. 5A, and similar holes  59 ′ are punched or machined through the ends of the additional bending beams  53 ′,  53 ″ as shown in FIG. 5C to provide gripping surfaces for the injection molded plastic in the end pieces  54 ,  55 . These gripping surfaces ensure firm, slip free joints between the plastic end pieces  54 ,  55  and the metal bending beams  51 ,  53 ′,  53 ″, as explained in the text above. 
     One end block  54  will be bolted to a base via mounting holes  56 , so the load cell  50  extends as a cantilever from the base. When a load is placed on the other end piece  55  with mounting holes  56 ′, the loaded end piece  55  will deflect, but it is forced to remain vertical by the parallelogram action of the two sensing bending beams  52 ′ and  52 ″ and the two additional bending beams  53 ′,  53 ″. The bending beams  52 ′,  52 ″,  53 ′,  53 ″ will thus be forced to form shallow S-shapes when the load cell  50  is loaded. 
     Four strain gages (not shown) are bonded to the inside of the large hole  58  near the notches  57  to sense the strain in the bending beam structure  51 , and are connected in a bridge providing an output signal from the load cell  50 , as is common in the art. 
     The large hole  58  in the sensing bending beam structure makes it possible to provide hermetic sealing of the strain gages in load cell  50 . This is achieved by inserting a tube  110  of very thin metal through the hole  58 , inside the strain gages, as indicated in FIGS. 5A and 5B, and then flanging the ends of the tube in place. The flanges can then be welded to the rims of the hole  58  by plasma welding without damage to the strain gages. 
     The additional bending beams  53 ′,  53 ″ carry no strain gages. They serve only as flexures to make the load cell  50  more resistant to bending moments and side forces. 
     A planar gage load cell  60  according to a further preferred embodiment of the invention is shown in FIGS. 6A and 6B. FIG. 6A is a top view of the load cell  60 , and FIG. 6B is a vertical section through the load cell  60  along line “VIB—VIB” in FIG.  6 A. In this case, the sensing structure is a planar bending beam gage  61  similar to the complete load cell  10  shown in FIG.  2 . The complete planar bending beam gage  61  is machined from a sheet of load cell quality metal alloy, and it comprises two parallel and coplanar bending beams  62 ′,  62 ″ with end pieces  64  and  65 . The end piece  65  is connected to a fold-back arm  67 , which includes a loading hole  68 . The end piece  64  extends into two parallel fold-back arms  64 ′. The fold-back arms  64 ′ and  67  are formed by narrow cuts  65 ′ and  67 ′ in the sheet forming the gage  61 . Strain gages (not shown) are bonded to the bending beams  62 ′,  62 ″ in the same way as strain gages  121 - 124  are bonded to bending beams  12 ′,  12 ″ in the load cell  10  shown in FIGS. 1A-1C. 
     The entire planar bending beam gage  61  is injection molded into a plastic box  120 , which serves as a base and a protective cover for the sensing gage  61 . Through holes  126  can be used for bolting of the load cell  60  to a foundation in certain applications. An opening  122  in the top of the plastic box  120  is an entrance opening for a load application rod (not shown), and a threaded hole  123  at the bottom of the box  120  can be used in conjunction with the threaded hole  68  to form an in-line load cell arrangement. The hole  123  also provides access to the lower end of the load application rod. When a downward or an upward directed load is applied to the threaded hole  68 , the bending beams  62 ′,  62 ″ will flex to form shallow S-shapes, and the output from a bridge circuit including the strain gages provides a measure of the applied load. Creep free joints between the gage  61  and the plastic box  120  are not required in this design, because all critical joints are formed in the solid metal plate forming the gage  61 . 
     FIGS. 7A and 7B show an encapsulated double bending beam load cell  70  according to another preferred embodiment of the invention. FIG. 7A is a top view of the load cell  70 , and FIG. 7B is a vertical section through the load cell  70  along line “VIIB—VIIB” in FIG.  7 A. The load cell  70  comprises two bending beams  72 ′,  72 ″ of load cell quality metal alloy, with two end pieces  74  and  75  of plastic. The two bending beams  72 ′,  72 ″ are arranged on top of each other at a constant distance defined by the end pieces  74 ,  75 , which are injection molded over the ends of the beams  72 ′,  72 ″. The end piece  74  is part of a frame  130  around the bending beams  72 ′,  72 ″, and the second end piece  75 , which is movable up or down. The frame  130  has mounting holes  76 , and the movable end piece  75  has a pair of threaded holes  78  for a load application device (not shown). 
     The ends of the bending beams  72 ′,  72 ″ are shaped as shown in FIG. 7A, and have holes  79  or notches providing gripping surfaces for the plastic in the end pieces  74 ,  75 , as explained earlier. When a load is applied to the movable end piece  75 , the beams  72 ′,  72 ″ will deflect in the form of shallow S-shapes, forming inflection zones near the joints with the end pieces  74  and  75 . Strain gages (not shown) are bonded to the bending beams  72 ′,  72 ″ near these inflection zones, and are connected in a bridge circuit providing an output signal proportional to the load on the movable end piece  75 . 
     The plastic frame  130  has a depression  132 ′ at the bottom for sealable mounting of a removable cover  134 , and a second depression  132 ″ on the top for mounting of a soft seal  137  with a seal frame  135 . The seal frame  135  has a central through opening to allow access to the soft seal  137 . The soft seal  137  is preferably made from a thin fabric, such as silk, impregnated by nitrite rubber. Mounting bolts for a load application device (not shown) are screwed into the mounting holes  78  through the seal  137 , so a tight seal is formed over the inner parts of the load cell  70 . The soft seal  137  allows the movable end piece  75  to move in response to an applied load, while adding only negligible force components in the load direction. This type of load cell has important applications in the food industry, where the load cell  70  and all other devices must be cleaned frequently, such as by water spray. 
     The external shapes of the injection molded plastic parts shown in all the examples above are determined by the molds used during the injection molding process. The mold will include protrusions on the inside in non-critical parts of the plastic pieces, as is well known in the art, both to ensure even curing of the plastic material, and also to reduce the amount of plastic used. The pockets formed in the finished plastic pieces by such protrusions in the mold are not shown in the figures described above, because their depiction would obscure details important for an understanding of the invention. 
     Injection molded blocks being part of load cells according to the preferred embodiments of the invention also provide an inexpensive way to provide cavities integrally formed as part of the injection molded blocks for housing of interconnections between strain gages, and for trimming resistors, amplifiers, A/D converters, etc., as will be readily understood by those skilled in the art. Such cavities are also not shown in the figures described above. 
     The function of load cells according to the invention is the same as corresponding load cells made entirely from metal. A main advantage of the invention is low cost, especially for the preferred embodiments of load cells  40 ,  50  and  70  as shown in FIGS. 4A-4B,  5 A- 5 C, and  7 A- 7 B, where equivalent prior art load cells required costly machining from a solid block of expensive load cell quality metal alloy. Load cells  10 ,  10 ′, and  10 ″ as shown in FIGS. 1A-1C,  2  and  3 A have a further advantage that the fold-back arms are much more rigid than in prior art load cells where the fold-back arms were machined from thin sheets of metal. And, in all cases, there is the advantage of being able to provide mounting cavities for interconnections and the like in and as an integral part of the injection molded plastic pieces at minimal cost. 
     The advantages of the invention are most apparent in load cells for small rated loads, where there are large markets if the price is right. The preferred embodiments of the invention described above have been with various types of strain gage load cells used for small rated loads, but the invention is applicable to any type of load cell. 
     The scope of the invention shall not be limited by any statement in the text above, nor by any detail of the accompanying figures. For example, in the embodiments presented above the sensing elements have been described as made of a load cell quality material, embodied herein as a load-cell quality metal or metal alloy, but this is not a requirement of the invention. Today, there are new developments in the fields of ceramics and glasses, which in the future may make such materials, or related materials, suitable for load cell sensing elements. 
     Although preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principle and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.