Measuring and testing – Dynamometers – Responsive to force
Reexamination Certificate
2000-03-17
2001-04-24
Noori, Max (Department: 2855)
Measuring and testing
Dynamometers
Responsive to force
C073S862333
Reexamination Certificate
active
06220105
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to load cells and, more particularly, to non-contacting magnetoelastic load cells for use in measuring applied force.
BACKGROUND OF THE INVENTION
The strain gage load cell is one of the more common means of measuring force. It is a precisely machined structure which responds with a deformation to a given applied force. A strain gage bonded to a deformable element exhibits a change in resistance according to the degree of deformation, which results in an electrical signal indicative of the magnitude of the applied force. However, since the strain gage is bonded directly to the load cell, it does not permit rotation of the load cell with respect to the sensing electronics. Unless slip rings or some form of isolated electronics are utilized, it is impossible with a strain gage load cell to measure axial loads on a rotating shaft. Moreover, strain gage sensors are very expensive and are thus commercially impractical for competitive use in many load cell applications.
It is well known that the axial deflection of a helical spring produces a torsional strain in the spring material. For this reason, when a solenoidal coil spring constructed of a magnetoelastically active steel and circumferentially magnetized around the axis of the wire forming the coil is placed under axial loading, the coil wire twists, causing a reorientation of the magnetization in the wire, with the magnetization becoming increasingly helical as the axial loading increases. As a result, the helical magnetization has both a circumferential component and an axial component, i.e., parallel to the wire in the coil. The axial component of magnetization causes a magnetic field to arise which extends in a direction parallel to the axis of the coil spring. This magnetic field can be sensed and an electrical signal developed which ideally should be proportional to the magnitude of the axial loading. However, due to the forming process of straight wire into a coil spring, internal residual stresses are created within the coil which develop their own magnetic fields under stress and which contribute to the net field sensed by the magnetic field sensors. As a result, the electrical signal which is developed is not solely a function of the applied axial load but also reflects the contribution of the internal residual stresses developed during processing. Accordingly, such solenoidal coil springs do not provide inherently accurate and reproducible results and do not represent a good choice for measuring axial loading, such as in load cells.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a magnetoelastic load cell which permits the measurement of axial loads on a rotating shaft and which provides an output signal accurately correlated to the applied load.
It is another object of the present invention to provide a magnetoelastic load cell which depends upon the magnetocrystalline anisotropy of the shaft itself as the primary source of anisotropy for returning the magnetization to its previously established circumferential direction when the applied load is reduced to zero.
It is still another object of the invention to provide a magnetoelastic load cell which depends for its operation on the sensing of a quantity that is inherently zero when the applied load being measured is zero and which changes in both direction and magnitude in a correlative manner with the load being measured.
It is yet another object of the invention to provide a magnetoelastic load cell which requires no external exciting field for its operation and which requires neither exciting currents nor coils.
A still further object of the invention is to provide a non-contact method for measuring a force applied to a surface comprising the steps of providing a magnetoelastically active, ferromagnetic, magnetostrictive disk-shaped member to which the force is applied, the member having an upper surface and a lower surface and comprising a central hub, an annular rim and at least two slots formed through the member between the hub and the rim for defining at least two generally spiral-shaped spokes extending between the upper surface and the lower surface of the member, each of the spokes traversing approximately 360° between its origin at the hub and its termination at the rim, the spokes being magnetically polarized in a single circumferential direction and possessing sufficient magnetic anisotropy to return the magnetization in the spokes, following the application of a force to the surface, to the single circumferential direction when the applied force is reduced to zero; causing a magnetic field to arise from the member as a consequence of the application of force to the surface; and sensing the magnitude of the magnetic field at a position proximate to the magnetoelastically active member as an indication of the magnitude of the force applied to the surface; the magnetoelastically active member being formed of a polycrystalline material wherein at least 50% of the distribution of local magnetizations lie within a 90° quadrant symmetrically disposed around said single circular direction and having a coercivity sufficiently high that the field arising from said magnetoelastically active region does not magnetize regions of said member proximate to said magnetoelastically active region to give rise to parasitic magnetic fields which are of sufficient strength to destroy the usefulness, for force sensing purposes, of the net magnetic field seen by said magnetic field sensing means.
These objects and others are achieved by providing a load cell comprising a magnetoelastically active region including a ferromagnetic, magnetostrictive diskshaped member having an upper surface and a lower surface and comprising a central hub and an annular rim, the member having at least two slots formed therethrough between the hub and the rim for defining at least two generally spiral-shaped spokes extending between the upper surface and the lower surface of the member. Each of the spokes traverses approximately 360° between its origin at the hub and its termination at the rim, the spokes being magnetically polarized in a single circumferential direction and possessing sufficient magnetic anisotropy to return the magnetization in the spokes, following the application of a force to the region, to the single circumferential direction when the applied force is reduced to zero. Magnetic field sensors, such as a flux-gate inductors, are mounted proximate the member and are responsive to the active region field which arises as a result of the application of a stress in the region. The member is desirably formed of apolycrystalline material wherein at least 50% of the distribution of local magnetizations lie within a 90
20
quadrant symmetrically disposed around the single circular direction and has a coercivity sufficiently high, most preferably greater than 35 Oe, that the field arising from the member does not magnetize proximate regions of the member to give rise to parasitic magnetic fields which are of sufficient strength to destroy the usefulness, for force sensing purposes, of the net magnetic field seen by the magnetic field sensors.
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Gars
Friedman Stuart J.
Magna-Lastic Devices, Inc.
Noori Max
Peabody LLP Nixon
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