Biaxial load cell with highly anisotropic force resolutions

A load cell for measuring extremely low level, anisotropic biaxial loads is disclosed. The system includes at least one load cell of a specific geometry and dimension which, when mounted, provides a rotationless system accurately transmitting both normal and shearing strains to strain gauges mounted thereon. More specifically, by shaping the load cell so as to concentrate the strains imposed thereon, detection of extremely minute loads, such as coefficients of friction, can be measured.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to measurement devices, and more particularly relates to stress and strain measurement systems.

BACKGROUND OF THE DISCLOSURE

When external forces are applied to a stationary object, stress and strain in that object result. In general, stress is the internal resistance force of the object, while strain is the displacement and deformation that result. More specifically, strain is defined as the amount of deformation per unit length of an object when a load is applied. It can be calculated by dividing the total deformation of the original length by the original length of the object.

Over time, many devices have been devised to measure strain which itself can be either compressive or tensile. One simple type of strain gauge detects a change in an electrical characteristic of the object placed under strain be it in the form of capacitance, inductance, or resistance. If the strain of a particular composition is to be measured, the strain gauge itself is typically mounted to the material by epoxy bonding techniques or the like. Accordingly, when forces applied to the material are to be measured, a resulting strain is necessarily transmitted to the strain gauge and thus any change in its electrical characteristic will in turn be usable in determining the strain of the material being measured.

In one particular area of current interest, the properties of biological articular cartilage need to be measured. This is particularly beneficial in research being conducted with respect to osteoarthritis, since if the characteristics of the cartilage and its interaction with the lubrication provided by the human body, can be determined, advances in medical technology such as treatments for osteoarthritis can be made. Research has shown that the articular cartilage is naturally lubricated through a protein found in synovial fluid called lubricin. Further research may show the serviceable life for the cartilage may be increased if the protein structure can be modified or altered with synthetic agents. However, a difficulty encountered with measuring such materials and thus being able to accurately test agents and lubricants is that their coefficients of function are so small, current measurement technology is insufficient. In addition, such materials have extreme aspect ratios and typically exhibit highly anisotropic characteristics.

In light of the foregoing, a need exists for a newly developed load cell able to measure loads in both a normal and shearing direction, to eliminate the torsional effects of the measuring beam on the data, to provide a rotationless mounting system for the beam, and to concentrate the stress for the biaxial measurements such that measurements of extremely low loads can be detected.

SUMMARY OF THE DISCLOSURE

In accordance with one aspect of the disclosure, a biaxial load cell is disclosed able to measure extremely low coefficients of function and achieve highly anisotropic load resolution in at least two directions. Specifically, the load cell is able to measure frictional coefficients of articular cartilage which are on the order of 0.001.

In accordance with another aspect of the disclosure, a load cell is disclosed that can be used to measure biaxial loads in many different applications

In accordance with one aspect of the disclosure, one cell is disclosed which includes a tower, a beam extending from the tower, a strain gauge mounted on the beam, and a processor connected to the strain gauge.

In accordance with another aspect of the disclosure, a load cell measurement system is disclosed which comprises a platform, a moveable table mounted on the platform, and a plurality of load cells extending upwardly from the platform

In accordance with another aspect of the disclosure, the load cell measurement system is disclosed which comprises a platform, a moveable table mounted on the platform, a plurality of load cells extending upwardly from the platform, each of the load cells including a tower extending upwardly from the platform, a beam extending from the tower, strain gauges, and a processor. The beam includes a base, a tapered section extending from the base, and an arm extending from the tapered section. Each of the base, tapered section, and arm include cut-out sections, the base including top, central and bottom strips. The strain gauges are mounted on the top, central and bottom strips while the processor is connected to the strain gauges

In accordance with a still further aspect of the disclosure, a load cell is disclosed having a base, a tapered section extending from the base, and an arm extending from the tapered section, the aim being of a lesser width than the base, the base, tapered section, and arm each including cut-out sections.

These and other aspects and features of the disclosure will become mole readily apparent upon reading the following detailed description when taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DISCLOSURE

Referring now toFIG. 1, a measurement system constructed in accordance with the teachings of the disclosure is generally referred to by reference numeral20. While the system20will be described herein in connection with measuring coefficients of function in articular cartilage, it is to be understood that the system20, particularly the load cell described in further detail herein, can be used in many other applications including, but not limited to, testing material strengths, fatigue points, durability and the like. In fact, the load cell can be used in any situation where there is a need to measure extremely low strain loads and/or biaxial loads which are very anisotropic.

In addition, while the system20illustrated herein depicts a total of five different measurement instruments22, it is to be understood that the teachings of the disclosure can be used in conjunction with measurement systems employing only one or any other number of instruments22. In fact, it is a unique benefit of the present disclosure that using a single measurement system20, multiple readings can be simultaneously taken

For example, in the sample indicated above, if the coefficient of friction of biological cartilage is to be measured, in the embodiment depicted inFIG. 1, five different samples of cartilage could be provided on the measurement table24and using a different load cell22for each sample (not shown) upon movement of table24, measurements of each sample can be taken. As one of ordinary skill in the art will readily understand, such an advance will greatly expedite the measurement and research process, as well as reduce the expense of construction and measurement. Again, function coefficient measurement is but one example of how the disclosed technology can be employed.

Referring again toFIG. 1, as shown therein, the measurement system20may include one or more individual measurement instruments22positioned to measure items placed upon table24. Each instrument22may include a mounting towel28from which a load cell30orthogonally extends. The mounting towers28may include adjustment mechanisms32to adjust the relative height of the load cell30to the table24. Such adjustment mechanisms could be provided in a variety of fashions including the threaded rod34and threaded thru-hole36arrangement depicted inFIG. 1. In so doing, the rod34extends through a threaded hole36provided in carriage38and upon rotation of rod34, carriage38is carried up and down mounting tower28. In one embodiment, the threaded rod may be provided as a compound screw formed from two screws of slightly different pitches and soldered together. This allows, in such an embodiment, resolution of 200 micrometers per revolution of the screw. In still a further embodiment, a motor such as, but not limited to, a servo-motor may be used with a closed-loop feedback system to automatically translate the load cell30up and down, thus allowing the instrument22to work in either load or displacement control.

Attached to the carriage38is the load cell30. In older to securely attach the load cell30to the carriage38, it can be seen that the carriage38includes a recess40sized to receive a proximal end42of the load cell30. Such nesting of the load cell30within the carriage38ensures accurate positioning A mounting plate44is then positioned over the proximal end42and, with one of more fasteners46, the mounting plate. In so doing, the load cell30is attached securely to the carriage38. In the depicted embodiment, four thru-holes48(seeFIG. 7) are provided in the proximal end42of the load cell30, with the fasteners46being threaded screws countersunk therethrough, but other attachment mechanisms, including but not limited to, unitary construction, welding, adhesives, and the like are possible. Moreover, any number of mounting arrangements can be employed to position the load cell30as needed with the above-mentioned towel configuration being only one example.

Referring now not only toFIG. 1, but toFIG. 7as well, it can be seen that the load cell30is of a specific geometry and relative dimensions. As will be described in further detail herein, the geometry and relative dimensions have been selected for multiple reasons including the prevention of torsional forces at the distal end50being transmitted back to the proximal end42and thus inaccurately effecting the measurements made by the instrument22. In addition, the geometry of the load cell30acts to concentrate the biaxial forces resulting from the normal and shearing strains on the load cell30thus making for measurements of minute loads more readily attainable.

More specifically, the load cell30is shown to include the aforementioned proximal end42and distal end50of substantially different widths α and β, respectively. The distal end50is narrower than the proximal end42and separated therefrom by a central section52which tapers from the proximal end42to the distal end50. The load cell30further includes a number of removed areas or cut-outs, with the proximal end42including two six-sided polygonal cut-outs54, while the central section52includes a triangular cut-out56and the distal end50includes a rectangular cut-out58. Through the use of such a web-like structure, positioning the cut-outs appropriately, and providing the mounting end at a wider dimension than the measuring end, stresses can be concentrated, and thus measured more effectively. In other words, when in use, the distal end50of the load cell30deflects and the resulting strain placed on the load cell30is transmitted back and concentrated at specific locations in the proximal end42where strain gauges described later herein are mounted. While the embodiment depicted includes the aforementioned six-sided, triangular and rectangular cut-outs, it is to be understood that while effective, other shapes are included within the scope of this disclosure.

Looking to the top view of the load beam ofFIG. 10, it can be seen that the dimension of the load cell30also varies in thickness. More particularly, the load cell30includes a uniform thickness but for a concentration zone60which extends through the proximal end40in alignment with the polygonal cut-outs54. As will be pointed out in more detail herein, this reduction in thickness of the load cell30also contributes to the concentration of the biaxial stresses and thus enables more accurate measurement of very low coefficients of function or other low level loads. While the dimensions of the load cell30can be altered, certain dimensions are particularly important to ensure non-torsional effects, and the ability to resolve anisotropic loadings. The symmetry of the load cell30and the relative positions and relationships within and between the cut-outs54-58also assists in this regard. Another important dimension is the thickness of the concentrate zone60. If the concentration zone60is made thicker, more stability against normal and torsional loads is achieved, but at the expense of shear load resolution. Conversely, if the concentrate zone60is made thinner, the ability of the load cell30to measure shear loads with accuracy increases, but the load cell would be less able to withstand normal loads. By way of example, the load cell30may have a thickness of 1.59 millimeters while the concentration zone60may have a thickness of 0.99 millimeters, but other dimensions are certainly possible.

Referring now to the distal end50of the load cell30, it can be seen that it further includes a plurality of thru-holes62which facilitate attachment of pin mounting shoe64. More specifically, the mounting shoe64can be seen to include a longitudinal groove66for receipt of the distal end50. By providing thru-hole68through the shoe64as well, fastener's70can be used to attach the shoe securely to the load cell30. Extending from the shoe64is a pin72having a rod-like overall configuration terminating in a conical tip74(seeFIG. 2). In alternative embodiments, the shoe64and pin72may be integrally formed with the load cell30or otherwise connected thereto as by welding, adhesives, or the like.

Referring now toFIG. 5, a single load cell22operating in conjunction with a measurement sample is provided in more detail. As shown therein, the table24includes a polymeric slab76attached to the glass surface of the measurement table24. The polymeric slab includes an elongate slot or well78allowing for movement of the measurement sample therein. The well78is filled with a lubricant of interest to be measured and sealed against both the glass of the measurement table24and the inside surface of the well78by way of an elastomeric seal or gasket80

It can also be seen fromFIG. 5, as well asFIG. 6, that the sample is attached to a sample holder82which may be provided in brass or some other suitable material. The sample of cartilage is glued or otherwise adhered to a bottom surface85of the sample holder82. The sample holder82further includes a conical depression (not shown) into which the conical tip74of the pin72fits loosely, and thus ensures that pin72moves wherever the sample holder82moves. Such a loose fit is important in that it allows no rotational moments to be applied to the sample, but rather only compressive loads and resulting shearing loads. In one embodiment, the conical depression has an angle roughly two times that of the conical tip74of the pin72.

As also shown inFIG. 5, the table24itself is mounted for movement at the direction of motor86. The table24can be so mounted upon rails88or the like to allow for longitudinal, lateral, and vertical movement i.e., three-axis movement. Accordingly, it can be seen that when the motor86is activated, a shaft89connected to the motor86and mounted within pillow block91moves causing the table24to move as desired. While not shown, an electronic drive or programmable logic controller may be used to coordinate movement of the motor86, as well as provide closed loop feedback by way of position sensors that the desired movement has been attained. One of ordinary skill in the art will understand many activating mechanisms can be employed to move the table26including, but not limited to, piezoelectric linear actuators, geared motors coupled to rack and pinion drive arrangements, and the like.

As the cartilage sample rests within lubricant well92, movement of the table24causes the sample to move accordingly. Any resistance to that movement, depending upon the characteristics of the lubricant and the cartilage itself, is transmitted by way of first the sample holder82, then to the pin72, then to the pin mounted shoe64, and then to the load cell30. Such deflection of the distal end50will result in either normal or shearing strains which the measurement system20is designed to measure. Moreover, by way of the unique geometry of the load cell30itself, such strains are concentrated at predetermined locations, so as to make measurement of even minute coefficients of function possible, when strain gauges94are mounted at those locations.

Referring now toFIGS. 8-10, the particular locations of the strain gauges94are described in detail. While any number of different types of strain gauges can be used, the preferred embodiment of the pending disclosure uses. Wheatstone bridges positioned in four different locations upon the load cell30. Particular model numbers which can be used are SS-060-022-500P and SS-060-033-2008V manufactured by Micron Instruments, but other types of strain gauges, which may or may not be Wheatstone bridges, can be used as well. As will be noted fromFIGS. 9 and 10, the strain gauges94are mounted within the concentration zone60. Both the reduced thickness of the concentration zone60and the unique geometry of the load cell30concentrate stresses from the distal end50back through the central section52and to the proximal end42. These stresses are further concentrated by the provision of, and the unique shape of, the relatively large polygonal cut-outs54within the concentration zone so as to concentrate the stresses both at a central strip96, top web98, and bottom web100.

As is conventional, the strain gauges94are adhered to the concentration zone60by way of an epoxy so as to ensure transmission of the strain of the distal end50through the strain gauges94, themselves. Moreover, the strain gauges94are mounted, as can be seen inFIGS. 9 and 10about mid-lines102and104dissecting the load cell30. More specifically, first with respect toFIG. 9, a mid-line102dissecting the load cell along a horizontal axis runs directly between the mid-section of the strain gauge94. Similarly, the mid-line104which divides the thickness of the load cell30, runs between strain gauges94as well. In so symmetrically mounting the strain gauges, more accurate measurements, such as for example of the coefficients of function of articular cartilage can be attained. Once connected as shown inFIG. 9, leads106are connected, as by soldering, to the strain gauges94, with mounting pads108being provided to facilitate such attachment.

As indicated above, the Wheatstone bridges are comprised of semiconductor strain gauges bonded on to the surface of the load cell30at specific locations. The high resistance and gauge factors of the strain gauges allow for minute strains to be detected. That corresponding strain data, in the form of a voltage, is collected, and a decoupling algorithm is run on the data to distinguish the forces applied in the normal and shearing directions. Samples of the decoupled raw data in both the shearing and normal directions can then be chatted as shown, by way of example only, inFIG. 11. This data reflects the linear translation of the table24with a constant frequency, and behavior consistent with the sample tested against glass. The design of the system enables standard excitation voltages and any commercially available signal conditioning equipment to be used to collect the data.

The fact that bending of the beam occurs in two dimensions requires that care must be taken to overcome the mutual interaction of the two applied loads with respect to the strain that is measured with the strain gauges mounted on the load cell. The aforementioned decoupling algorithm is therefore used. The load cell is calibrated using known masses applied in one or both directions simultaneously. The voltages from the transducers (strain gauges) due to the applied load, when taken together, represent a unique loading. Due to the elastic nature of the material of the beam (aluminum), a linear relationship can be created to relate the output voltages to applied loads using the following equation:

[LShearLNormal]=[k11k12k21k22]⁡[VShearVNormal]where matrix L is the respective loads;matrix V is the respective output voltages from the load cell; andmatrix k is the decoupling coefficient matrix allowing a unique shear and normal load to be calculated from the output voltages for a particular load cell.

In one preferred embodiment, the measurement system is designed to measure forces that are up to 1,000 times greater in a normal direction of (Z axis) as shown inFIG. 1, as opposed to a shearing direction (X axis) shown inFIG. 1as well. The load ranges in each direction enable the effective measurement of coefficients of friction or anisotropy index from 3×10−6to 250. The range of friction coefficients which is able to be measured is also dependent on the specifics of the signal conditioning protocol, and this range may be even wider than described herein. Again, coefficients of frictions are but one example of an anisotropic biaxial load which may be measured by the load cell30.

Based on the foregoing, it can be seen that the present disclosure provides a measuring device with significant advantages over known load cells. One advantage is that separate load cells need not be used for each direction of measurement in that each load cell30is able to measure biaxially making the system less bulky and less expensive to manufacture. Additionally, since a normal load can be applied with the disclosed load cell, the ability arises to have multiple load cells in parallel, as shown inFIG. 1, testing samples simultaneously on one common substrate. The ability to test samples in parallel significantly cuts down on experiment time and cost of the production of the unit. The system was also designed for the purposes of characterizing tribology where there are normally anisotropic properties. In addition to characterization of frictional properties, the load cell also has applications in many other areas including, but not limited to, tissue engineering bioreactors. Due to the ability to apply loads through translation of the load cell, the ability exists to impose known normal strains allowing for the ability to measure tissue samples with very different material properties.

While the foregoing was written with reference to specific examples and embodiments, it is to be understood that the scope of the invention is not to be limited thereby, but rather are provided to satisfy best mode and enablement requirements while providing support for any and all claims which may issue herefrom.