Torsional Sensing Load Cell

The present invention relates to load cells and more specifically to an apparatus and method for a torsional sensing load cell.

Load cells are used to provide accurate measurements of compressive or tensile forces. Typically, the force creates a strain in the load cell which is measured by strain gage transducers. Accurate measurements, however, require that the force be applied along the same axis as the transducers. An off-axis force has a perpendicular force component, resulting in bending and/or shear loads which would be sensed by the transducers and be mistakenly taken as part of the axial load.

An example of such a load cell which is sensitive to the effects of off-axis loading is known as a compression washer. U.S. Pat. No. 4,212,360 discloses such a load cell (FIGS. 4a and 4b), an example of which is shown in a product brochure entitled "Compression Only/Thru Hole Load Washer". As the name implies, this type of load cell is configured as a washer so that it can be mounted by securing a bolt through the device.

Other compressive load cells which are less sensitive to off-axis loading are characterized by several types. One such load cell employs a shear web design disclosed in U.S. Pat. No. 5,461,933. This design consists of a pair of concentric rings joined by two or more web members. Transducer elements disposed on the webs provide compression and tension force measurements. Although this design reduces sensitivity to off-axis loads, the web assembly must be accurately machined to tight tolerances. In addition, the transducers must be carefully bonded to the walls of the webs in a symmetrical manner in order to ensure accuracy in the measurements.

A ball and socket assembly design serves to maintain the loading force along the principle axis and exactly centered on the load cell. Such a design is shown in Photo 2 in the article by Clegg entitled "Bonded Foil Strain Gauge Force Transducers", Sensors, October 1996, pp. 68-75. As can be seen from the photograph, the manufacture of such a device can be quite expensive.

A shear beam load cell, such as the one disclosed in U.S. Pat. No. 5,220,971, can also be used in tension and compression applications. The transducer elements are diagonally placed on a machined web in the center portion of the device and measure the shear force of the applied load. Such devices are expensive to machine and accurate measurements depend on consistently centered and axially constrained forces. A similar load cell is disclosed U.S. Pat. No. 3,376,795 comprising an annularly shaped shear-beam load cell wherein the force-measuring transducers are located in webs formed into the body of the load cell.

What is needed is an easy to manufacture load cell which can deliver accurate measurements and which is insensitive to the forces present under off-axis loading conditions. It is also desirous to construct a load cell having the above characteristics, yet is compact in size and has low manufacturing costs.

A torsional sensing load cell in accordance with the present invention comprises an active load sensing member having first and second deflecting arms. A portion of each deflecting arm receives the force resulting from an applied load. Upper and lower loading surfaces are coupled to the force-receiving portions so that the force of an applied load and its corresponding reactive force are directed only to the force-receiving portions of the deflecting arms. A pair of transducers are mounted on the load sensing member to sense torsional stresses resulting from the applied load.

In a preferred embodiment of the invention, the load sensing element is a curved member having a longitudinal gap. Various cross sectional shapes are possible; for example, the curved member may have a cylindrical shape, a rectangular shape, and so on. The open area formed within the curvature, likewise, may take on any shape. Thus in the preferred embodiment, the load sensing member is a cylinder having a gap formed along its longitudinal length and having a cylindrical opening therethrough.

The loading surfaces serve as means for mounting the load cell to receive a load. As such, the loading surfaces may be solid members having threaded openings for mounting purposes. Alternatively, the loading surfaces may be hollowed out so that the load cell can be mounted by passing a mounting bolt through the entire structure of the load cell.

In the preferred embodiment, each of the two transducers comprises two sets of strain elements arranged in a chevron pattern, one set being disposed ninety degrees relative to the other. The two sets of strain elements comprising each transducer are mounted to the load sensing member on a forty-five degree angle relative to a neutral axis of the load sensing member. In a variation of the preferred embodiment, each transducer may comprise only one such set of strain elements.

FIG. 1 is a perspective view of a load cell according to the present invention.

FIG. 2 is an exploded view of the load cell shown in FIG. 1.

FIG. 3 shows a low capacity load cell.

FIGS. 4A-4B, 5A-5C are schematic diagrams of the bridge circuitry coupling the transducers used in the present invention.

FIGS. 6A-6B, 7A-7B illustrate the underlying concepts of the present invention.

FIG. 8 is a plan view of a load cell of the present invention.

FIGS. 9A-9F are top views of the load cell of FIG. 8 shown along the view line 9--9, illustrating alternate profiles of the active load sensing portion of the load cell.

FIGS. 10A-10B show an alternate configuration of the load sensing member.

FIGS. 11A-11B illustrate alternate gap profiles for the load sensing member .

With reference to FIGS. 1 and 2, a load cell 100 comprises a central load sensing body 60 which is a cylindrical member. Transducers 202, 204 are mounted to the central body, and as will be discussed in greater detail the transducers are positioned about the central body 60 in diametrically opposed relation. Such placement of the transducers is indicated in FIG. 1, showing transducer 202, and in the exploded/rotated view of the load cell shown in FIG. 2.

The cylindrical central body 60 is further characterized by having a gap or slotted opening 50 formed through the thickness of the cylindrical member, thus forming a "split" in the cylinder. Splitting the cylinder in this manner results in the cylinder having two deflectable arms 65, 67.

The load cell 100 further comprises upper and lower loading members 20, 40 which serve as the load bearing surfaces. Loading members 20, 40 are coupled to the central body 60 at force-receiving portions 61, 63 of the deflectable arms as shown in FIG. 2. Specifically, upper load bearing member 20 is coupled to force-receiving portion 61 located on a portion of the upper surface of central body 60. Similarly, lower loading member 40 is coupled to force-receiving portion 63 located on a portion of the bottom surface of central body 60. The central body 60 is thereby said to be "suspended" at portions 61, 63.

The preferred embodiment of the load cell 100 further includes a through hole 30 formed through each of the upper and bottom loading surfaces 20, 40, forming upper and lower ring-shaped load-bearing members. The openings provide a means for mounting the load cell to the mounting surface of a load producing structure, in this case by passing a mounting bolt through the cell. Note that the particular structure of the loading surfaces 20, 40 is not relevant to the practice of the invention beyond providing a means for mounting the load cell. Thus, the loading surfaces 20, 40 may be solid members with mounting holes drilled and tapped thereinto for mounting, instead of having through holes 30 as shown. Or, the load cell may be attached by welding or gluing the loading surfaces to an appropriate load producing structure.

The embodiment depicted in FIGS. 1 and 2 show the loading surfaces 20, 40 and the central body 60 as a single element, such as might result from being cast in a mold. Being able to form the load cell in a single casting step greatly reduces the cost of manufacture. This is not necessary to the practice of the invention, however. In an alternative embodiment, the load cell can be assembled from separate elements. For example, FIG. 3 shows a load cell 100' which comprises a weighing plate 20' spot welded to a load sensing element 60', which in turn is spot welded to a base member 40'. The load sensing element 60' can be a loop of wire formed as shown in FIG. 3 for use in a low capacity application such as a postal scale or a gram scale. The gap 50' in the loop of wire corresponds to the split 50 of the central load sensing body 60 of the cylindrical embodiment shown in FIGS. 1 and 2.

Continuing with FIGS. 1 and 2, under static conditions the force of a load applied to one loading surface, say loading surface 20, will produce an opposing reactive force on loading surface 40. Recall that the loading surfaces direct the forces to force-receiving portions 61, 63 of the central load sensing body 60. Since the central body is split, these forces produce a torsional stress which is sensed by the transducers 202, 204. Moreover, since the central body is suspended (i.e. supported) only at the force-receiving portions 61, 63, the torsional stress is transmitted through the central body from one end to the other. This is shown more clearly in FIG. 3 where the central load sensing element 60' can be seen to be supported only at two points 61', 63'. This is an important aspect of the invention because such an arrangement of the sensing member subjects both of the transducers to the same torsional stress. If, on the other hand, the central body 60 included an additional supporting member interposed between the force-receiving surfaces 61, 63, each transducer would be subject to different, independent torsional stresses. However, as will become clear in the discussion below, it is necessary in the present invention that both transducers detect the same torsional stress in the central body arising from the same torsion-producing forces. An inspection of the embodiment of FIG. 3 shows that a similar torsional stress arises in the central load sensing element 60' where transducers 202', 204' are subject to the same torsional stress by virtue of the load sensing element being supported only at force-receiving portions 61', 63'.

In FIG. 4A, transducers 202, 204 each comprises two sets of strain elements R.sub.T, R.sub.C arranged in a ninety degree chevron pattern, which is a standard configuration for strain gauges used to measure strain on the surface of a member under torsional load, and are mounted relative to a neutral axis of the sensing member. In the presence of a torsional stress, each transducer 202, 204 will be subject to a stress.tau..sub.A, .tau..sub.B respectively, such as shown in FIG. 4A. The actual direction of the stresses will vary depending on the position of the load with respect to locations A and B. For any loading situation, however, one set of strain elements of transducer 202 (e.g. R.sub.TA) will be in tension when the other set of strain elements (e.g. R.sub.CA) is in compression. The strain elements R.sub.TB, R.sub.CB comprising transducer 204 are similarly stressed. The strain elements of both transducers are coupled in a Wheatstone bridge, such as shown in FIG. 4B, where similarly strained elements are located on opposite legs of the bridge. The differential output signal V.sub.o is characterized by ##EQU1## However, since R.sub.CB and R.sub.TB are oppositely strained, the difference signal actually represents a summation of the torsional stresses sensed at both transducers 202, 204, namely .tau..sub.A +.tau..sub.B.

FIG. 5A shows an embodiment using transducers 206, 208 which have a simpler construction. Here, each transducer 206, 208 comprises only a single set of strain elements. FIG. 5B shows a voltage divider circuit used for such transducers. In this case, the output signal is defined by the voltage divider equation which characterizes the circuit. Unlike the bridge circuit of FIG. 4B, the circuit of FIG. 5B provides an output signal that is proportional to the torsional stress experienced by a single transducer, namely ##EQU2## This embodiment produces a smaller output signal than the embodiment of FIG. 4A. However, a differential signal can be generated by using the circuit of FIG. 5C which comprises a pair of fixed resistances R used in conjunction with R.sub.TA and R.sub.CB in a bridge configuration. The equation which characterizes this circuit is ##EQU3## This alternate embodiment, however, lacks the sensitivity afforded by that shown in FIG. 4A. The embodiment of FIG. 5A, nonetheless, offers the advantage of being simpler and less costly to manufacture, and therefore under the right circumstances may be preferable over the embodiment of FIG. 4A.

The description will now turn to a discussion of the forces involved in the load cell of the present invention with reference to the free body diagrams of FIGS. 6A and 6B. The cross sectional views shown in the figures are taken at points A and B, the locations on the central load sensing body 60 where the transducers 202, 204 are mounted. See FIGS. 1-3. Consider a load F.sub.load applied to the loading surface 20. Under static conditions, the load F.sub.load is counteracted by reactive forces at points A and B. These torsional and shear forces may be determined by analyzing equilibrium at these points respectively. ##EQU4## Similarly, the total shear and torsional stresses at point B are: ##EQU5##

The gages are placed to measure the total reactive torsional force: ##EQU6##

It can be seen that the sum of the torsional reactions at A and B, T.sub.total is directly proportional to the loading force F.sub.load where the constant of proportionality is D, the distance between the two points A and B. This relation holds true even if the load is applied outside of points A and B, such as in the loading situation of FIG. 6B, which can be shown by a similar analytical treatment of the force vectors. Thus, a given load applied anywhere (i.e. off-axis loading) upon the load bearing surfaces will yield the same .tau..sub.reaction. This is a substantial improvement over prior art systems which cope with off-axis loads using more expensive designs. The present invention has the double advantage of being extremely simply in design (and therefore is a low-cost item to manufacture) and is insensitive to the position of an applied load.

To complete the discussion, it is noted that in addition to the total reactive torsional stress there is a total reactive shear stress which is simply the sum of the shear forces at points A and B, namely 2 * F.sub.load.

An aspect of the present invention is that the actual shape of the load sensing member (e.g. central body 60, FIGS. 1 and 2) is unimportant. This is shown in FIG. 7A, showing a generalized form of a load sensing member 6. Arbitrary portions 5, 7, and 9 emphasize the notion that sensing member 6 can take on any shape. An example is illustrated by the arbitrarily shaped sensing member 6' of FIG. 7B. A loading force F.sub.L is shown bearing downwardly into the plane of the drawing sheet upon an upper force-receiving section of the sensing member 6', and a counteracting reactive force F.sub.R is shown bearing upwardly upon a bottom force-receiving surface. The resulting action of the two forces is a torque which is transmitted through the body of sensing member 6', and which is detected by transducers 2, 4.

Returning to FIG. 7A, a requirement of the present invention is that the transducers be arranged upon load sensing member 6 such that Equations 4-6 are satisfied. This can be accomplished by mounting the transducers on load member 6 wherein a tangent to the member at the location of each transducer is perpendicular to the line drawn between the transducers. Thus in FIG. 7A, tangent lines X indicate the locations on sensing member 6 where transducers 2, 4 are positioned in accordance with the requirements set forth in Equations 4-6. Thus, transducers arranged in diametrically opposed fashion, such as shown in FIGS. 3, 9A-9F, and 10, satisfy the requirements of Equations 4-6. In addition to the side-mounted arrangement shown in FIG. 7A, the transducers may be mounted to the upper surface of sensing member 6 or to its bottom surface and still satisfy Equations 4-6. Depending on the situation, such alternative mounting arrangements may be desirable over the side mounting configuration.

With reference to FIG. 7B, several locations A, B, C have been identified that would be suitable for mounting transducers, each of the locations indicating the required tangent lines X. The distance between the neutral axes of the sensing member at the point of application of the transducers is referred to as the "effective diameter" of the sensing member. As can be seen in Eqn. 6, sensitivity is increased as the effective diameter D increases. Thus, in FIG. 7B a more sensitive load cell is achieved by placing the transducers at location A than at either of locations B or C.

The transducers are preferably mounted to the surface of the load sensing member, rather than to a web structure formed in the member. This affords maximum sensitivity to torsional stresses since torsion at the surface is at its greatest. It is noted, however, that placement of transducers within web structures may be convenient for a given situation, so long as the web is not so deep as to lie along a neutral axis where the torsion becomes zero.

The arbitrarily shaped sensing member 6' shown in FIG. 7B was used to illustrate that its actual shape is unimportant. In practice, however, such a structure is impractical. The load cell shown in profile in FIG. 8 and its variations shown in FIGS. 9A-9F illustrate more realistic embodiments. FIGS. 9A-9F show various top-view profiles of the load sensing member 60 taken from view line 9-9 in FIG. 8. The transducers 202, 204 are shown arranged in diametrically opposed fashion. FIGS. 9A-9C show typical profiles for the sensing member. However, as shown in the profiles of FIGS. 9D-9F, the outer and inner perimeters of the sensing member need not be similarly shaped.

FIGS. 10A and 10B show a load cell 100 comprising a solid block of material formed by a series of slotted cutouts. A central vertical slot 71 defines the deflecting arms 65, 67 of the load sensing portion. Upper and lower horizontal slots 73, 75, which are formed perpendicularly relative to vertical slot 71, respectively define the upper and lower load bearing members 20, 40. Torsional sensing strain gage transducers 202, 204 are affixed to the sensing member. Note that the through hole 30 has been reduced to the intermember space formed by extending gap 71 through the center of the part to create an effective diameter. The result is a maximum cross sectional-size torsional sensing load cell with a minimum effective diameter. This structure results in a very high capacity load cell; for example, a 10,000 lb. load capacity can be achieved in a cube-shaped load cell that is 1.5 inches wide and 1.3 inches tall. Mounting holes can be drilled and tapped in upper and lower load bearing members 20, 40 for mounting. Conversely, low capacity load cells, such as the one shown in FIG. 3, can be constructed with a large effective diameter to provide the higher sensitivities generally required in low load capacity applications. Although not shown, it is clear that the present invention includes other solid shapes (e.g. solid cylinder, sphere, prism, etc.) having similarly slotted cutouts and is not limited to the cubical embodiment shown in FIG. 10A.

FIGS. 11A and 11B show that the gap 50 need not be a vertical gap as shown in the earlier embodiments. The embodiment of FIG. 11B, for example, shows a gap that includes a notched portion 55 which serves as a stop to prevent the load cell from being deflected beyond its yield point under loading conditions which exceed the rated capacity of the load cell.

Referring again to FIGS. 1 and 2, typical dimensions for a load cell in accordance with the present invention will now be discussed. In practice, the gap width D.sub.1 is substantially equal to the height of the slots D.sub.2, D.sub.3 which separate the load bearing members 20, 40 from the load sensing member 60. This is the result of the preferred mode of manufacture wherein the slots of the load bearing members 20, 40 are formed with the same tool used to form the gap 50. Through hole 30 has a dimension D.sub.4 that is defined by the size of the mounting bolt used to mount the load cell. The thickness D.sub.5 of the load sensing member 60 is generally defined by the desired size of the through hole 30 and the outside dimension of the load cell itself. The height D.sub.6 of the load sensing member 60 is generally dictated by the desired loading capacity of the load cell; a greater height provides an increased load capacity. Following are typical dimensions of load cells manufactured in accordance with the present invention.

A 3000 lb. capacity load cell is constructed with a 3/16" gap width D.sub.1 and loading members 20, 40 which are separated from load sensing member 60 by a distance D.sub.2, D.sub.3 of 3/16". Such a load cell is mounted with a 3/4 bolt with a 0.80" diameter D.sub.4 through hole 30. The thickness D.sub.5 and height D.sub.6 of the load sensing member are 0.5".

As another example, a 6000 LB rated load cell has a 3/16" gap width D.sub.1 with loading members 20, 40 which are separated from load sensing member 60 by a distance D.sub.2, D.sub.3 of 3/16". A 0.921" diameter D.sub.4 through hole 30 permits mounting of the load cell with a 7/8" bolt. The thickness D.sub.5 and height D.sub.6 of the load sensing member are 0.66".

As a final example, an 18,000 lb. load cell has a 1/4" gap width D.sub.1 with similarly spaced D.sub.2, D.sub.3 load bearing members. A 1" diameter D.sub.4 through hole provides mounting with a 11/32" bolt. The load sensing member 60 has a thickness D.sub.5 and height D.sub.6 of 1".