Magnetoelastic sensor using strain-induced magnetic anisotropy to measure the tension or compression present in a plate

A magnetoelastic sensor. The magnetoelastic sensor uses strain-induced magnetic anisotropy to measure the tension or compression present in a plate. During construction, an annular region of the plate is magnetized with a circumferential magnetization. Magnetic field sensors are placed near this magnetized band at locations where the magnetization direction is non-parallel and non-perpendicular to the axis of tension. The strain-induced magnetic anisotropy caused by tension or compression then produces a shift in the magnetization direction in the plate regions near the field sensors, thereby causing magnetic field changes which are detected by the magnetic field sensors. The magnetic field sensors are connected to an electronic circuit which outputs a voltage signal which indicates the tension or compression in the plate.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a magnetoelastic sensor and, more specifically, to a magnetoelastic sensor for sensing tension or compression.

Description of Related Art

Conventional tension and compression sensors use strain gauges to produce electrical signals which indicate the tension or compression present. Illustrated inFIG. 13is a conventional strain gauge, generally designated as1300. The strain gauge1300comprises an input1310and an output1320connected by a plurality of windings1330. The input1310, output1320, and plurality of windings1330are formed from a thin-film conductor1340, such as a metal foil. The input1310, output1320, and plurality of windings1330are disposed on an insulative substrate1350.

The insulative substrate1350is adhered to a surface for which strain is desired to be measured. Strain is measured by sensing a resistance of the thin-film conductor1340as the strain gauge1300is deformed when under tension or compression. When stretched in a direction indicated by A or B inFIG. 13, the resistance of the thin-film conductor1340increases. Thus, by measuring the increase in resistance, the tension of the surface to which the strain gauge1300is attached may be inferred. When compressed in a direction opposite to that indicated by A or B inFIG. 13, the resistance of the thin-film conductor1340decreases. Thus, by measuring the decrease in resistance, the compression of the surface to which the strain gauge1300is attached may be inferred.

S-shaped tension or compression sensors, also known as load cells, typically incorporate one or more conventional strain gauges1300to sense tension or compression. Illustrated inFIG. 14is a conventional S-shaped load cell, generally designated as1400. The load cell1400comprises a first arm1410, a second arm1420, and a body1430. Disposed on the body is a plurality of strain gauges1440A through1440D. Each strain gauge1440may be a strain gauge1300.

The load cell1400detects an amount of force applied in directions generally designed as C inFIG. 14. When the force is applied in the directions C, the strain gauges1440A and1440D undergo compression, and the strain gauges1440B and1440D undergo tension. By measuring the tension and compression, the size of the force can be calculated.

Conventional tension sensors using magnetoelastic effects are described in U.S. Pat. Nos. 5,195,377 to Garshelis, and U.S. Pat. No. 6,220,105 to Cripe. A conventional Villari effect tension sensor is described in U.S. Pat. No. 5,905,210 to O'Boyle et al.

BRIEF SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, there is provided a tension sensor comprising a plate comprising a magnetoelastic region. The tension sensor further comprises at least one pair of sensors disposed above the magnetoelastic region. The at least one pair of sensors are configured to sense a change in a magnetic field produced by the magnetoelastic region in response to a strain in the plate imposed by a tension on the plate.

In accordance with another aspect of the present invention, there is provided a compression sensor comprising a plate comprising a magnetoelastic region. The compression sensor further comprises at least one pair of sensors disposed above the magnetoelastic region. The at least one pair of sensors are configured to sense a change in a magnetic field produced by the magnetoelastic region in response to a strain in the plate imposed by a compression on the plate.

In accordance with yet another aspect of the present invention, there is provided method of manufacturing a magnetoelastic sensor. The method comprises steps of forming a plate from an austenitic non-magnetic stainless steel alloy, cold-working an area of the plate to convert the austenitic non-magnetic stainless steel alloy in the area of the plate to martensite, rotating the plate, bringing a magnet near a surface of the plate and near the area of the plate converted to martensite to magnetize the area, and mounting at least one pair of magnetic field sensor assemblies above the surface of the plate near the magnetized area.

DETAILED DESCRIPTION OF THE INVENTION

Reference to the drawings illustrating various views of exemplary embodiments of the present invention is now made. In the drawings and the description of the drawings herein, certain terminology is used for convenience only and is not to be taken as limiting the embodiments of the present invention. Furthermore, in the drawings and the description below, like numerals indicate like elements throughout.

Illustrated inFIG. 1is a top view of an exemplary embodiment of a magnetoelastic sensor, generally designated as100, in accordance with an exemplary embodiment of the present invention.FIG. 2Aillustrates a right-side view along a cross-section of the magnetoelastic sensor100at a section line180, andFIG. 2Billustrates a left-side view along a cross-section of the magnetoelastic sensor100taken at a section line170.

Referring toFIGS. 1, 2A, and 2B, the magnetoelastic sensor100comprises a plate110, a first distribution bar120connected to the plate110at a first end111of the plate110, and a second distribution bar130connected to the plate110at a second end112of the plate110. Disposed in the plate110is a magnetic band140. In the exemplary embodiment of the magnetic band140illustrated inFIG. 1, the magnetic band140is an annulus. In other exemplary embodiments of the magnetic band140, different shapes of the magnetic band140are contemplated. For example, the magnetic band140may be diamond shaped. It is to be understood that the plate110may have various dimensions, may not be perfectly planar on either surface, and may not have a perfectly uniform thickness across its entire length.

Disposed above the magnetic band140are a plurality of sensor assemblies150A,150B,150C, and150D. Each of the sensor assemblies150A,150B,150C, and150D comprises, respectively, a sensor platform152A,152B,152C, and152D on which a respective sensor154A,154B,154C, and154D is disposed. The sensors154A and154C are disposed along the section line170(also referred to herein as “centerline170”). The sensors154B and154D are disposed along the section line180(also referred to herein as “centerline180”). The centerline170longitudinally bisects the sensor assemblies150A and150C and their respective sensors154A and154C. The center line170longitudinally bisects the sensor assemblies150B and150D and their respective sensors154B and154D. The sensors154A,154B,154C, and154D are disposed symmetrically about a center point165of the plate110, which center point165is also the center point of the magnetic band140. The sensors154A,154B,154C, and154D are disposed over the magnetic band140such that a centerline145of the magnetic band140laterally bisects the sensors154A,154B,154C, and154D.

The sensor assemblies150A,150B,150C, and150D are disposed on the magnetic band140each at a respective angle, −α, α, −α, and α, relative to a longitudinal axis160of the plate110. The angles, α and −α, are chosen so that the centerlines170and180are neither parallel to the longitudinal axis160nor perpendicular thereto.

In an exemplary embodiment, the angles, α and −α, are chosen so that the centerlines170and180intersect the magnetized band140perpendicularly to a tangent of the centerline145of the magnetic band140, and where the magnetic field produced by the magnetic band140at the points of intersection is neither parallel nor perpendicular to the centerline160of the plate110.

In another exemplary embodiment, the magnitude of angle, α, −α, is chosen to be greater than or equal to 30° and less than or equal to 60°.

In yet another exemplary embodiment, the magnitude of angle, α, −α, is chosen to be greater than or equal to 40° and less than or equal to 50°.

In still another exemplary embodiment, the magnitude of angle, α, −α, is 45°.

The magnetic field sensors154A,154B,154C, and154D each produce an output signal that changes when a magnetic field produced by the magnetized band140in a direction parallel to the centerlines170and180changes. The magnetic field sensors154A and154C have high sensitivity to magnetic fields parallel to the centerline170, and the magnetic field sensors154B and154D have high sensitivity to magnetic fields parallel to the centerline180.

The first and second distribution bars120,130at the top111and the bottom112of the plate110are thicker than the plate110. Thus, as forces, F1and F2, are applied to the distribution bars120,130, respectively, an even amount of strain or compression is produced in the plate110, rather than a large amount of strain or compression along the center line160of the plate110and less elsewhere.

In an alternative exemplary embodiment of the magnetoelastic sensor100, the first and second distribution bars120,130are formed integrally with the plate110and are areas of the plate that are thicker than the portion of the plate110in which the magnetized band140is disposed. In such embodiment, as forces, F1and F2, are applied to the distribution bars120,130, respectively, an even amount of strain or compression is produced in the plate110, rather than a large amount of strain along the center line160of the plate110and less elsewhere.

In the exemplary embodiment of the magnetoelastic sensor100described above, the magnetic band140is formed within the plate110. In such embodiment, the magnetic band140may be formed from a magnetized band that is molded within a nonmagnetized or nonmagnetizable, e.g., non-ferromagnetic, plate110.

In another exemplary embodiment of the magnetoelastic sensor100, the magnetic band140may be a magnetized region of the plate110, in which case the plate110is formed entirely from a ferromagnetic material. It is to be understood that other exemplary embodiments of the magnetoelastic sensor100in which the magnetic band140is disposed above or on a top surface113of the plate110are contemplated. In such other embodiments, the plate110is not magnetized and may be formed from a material that is not capable of being magnetized.

In yet another exemplary embodiment of the magnetoelastic sensor100, the plate110is made from a non-magnetic material where the region140can be subjected to a process to change its metallurgical phase. A type of austenitic non-magnetic stainless steel alloy is selected to form the plate110. The area corresponding to the region140is cold-worked to convert it to martensite, which is ferromagnetic. The plate110is rotated around an axis perpendicular to the center point165of the plate110, and then while it is rotating, a permanent magnet is brought close to the surface113of the plate110near the area of the plate110corresponding to the region140for a large number of revolutions. The permanent magnet is removed after a magnetization direction has been imparted in the region140. This approach is beneficial because forming the plate110from a homogeneously ferromagnetic material could lead to problems, and molding or attaching the region140could be problematic because of the extremely high interface shear stresses in the plate110in certain applications. Sensor assemblies in accordance with the exemplary embodiments described herein are then mounted above the surface113of the plate110.

In an exemplary embodiment in which the plate110is formed from a ferromagnetic material, the magnetized band140having a circumferential magnetization direction indicated by the arrowed centerline145inFIG. 1is produced by rotating the plate110around an axis perpendicular to the center point165of the plate110, and then while it is rotating, bringing a permanent magnet close to the surface113of the plate110for a large number of revolutions. The permanent magnet is removed after a magnetization direction has been imparted in the magnetic band140, which is a magnetized region of the plate110. In this exemplary embodiment, the plate110is formed from a ferromagnetic material. It is to be understood that reference number145also refers to the magnetic field produced by the magnetic band140. Sensor assemblies in accordance with the exemplary embodiments described herein are then mounted above the surface113of the plate110.

AlthoughFIG. 1illustrates a single magnetic band140, it is to be understood that other exemplary embodiments in which a plurality of permanent magnets placed at various azimuthal locations in the plate110can also be used. In other exemplary embodiments, more than one magnetic band may be formed in or on the plate110, in which case the magnetoelastic sensor comprises four sensor assemblies for each ring. In still other exemplary embodiments, instead of a permanent magnet forming the magnetic band140, an electromagnet is used to produce the magnetized band140.

FIG. 2Aillustrates the relative positions of the sensor assemblies150B and150D and the plate110.FIG. 2Billustrates the relative positions of the sensor assemblies150A and150C and the plate110.

As illustrated inFIGS. 2A and 2B, the sensor platforms152A,152B,152C, and152D comprise, respectively, inside surfaces151A,151B,151C, and151D on which the sensors154A,154B,154C, and154D are respectively disposed. The sensor platforms152A,152B,152C, and152D further comprise, respectively, outside surfaces153A,153B,153C, and153D. The inside surfaces151A,151B,151C, and151D face the magnetic band140such that the sensors154A,154B,154C, and154D, as disposed on the respective inside surfaces151A,151B,151C, and151D, are between the magnetic band140and the respective sensor platforms152A,152B,152C, and152D.

FIG. 3illustrates a detailed view of a region300of the plate110under the sensor assembly150C and specifically a region300of the magnetic band140under the sensor assembly150C, in accordance with an exemplary embodiment of the present invention. Inside this region300, there are illustrated a tension axis (also referred to as a “magnetoelastic anisotropy axis”)310, a first effective anisotropy axis320, and a second effective anisotropy axis330.

The first effective anisotropy axis320is the direction of the magnetic field145produced by the magnetic band140when the forces, F1and F2, are not present. The second effective anisotropy axis330is the direction of the magnetic field145produced by the magnetic band140when the forces, F1and F2, are present. The second effective anisotropy axis330is a result of the combination of the tension axis310and the first effective anisotropy axis320and is proportional to the strength of the forces, F1and F2. The first effective anisotropy axis320is offset from the second effective anisotropy axis330by an angle β, which changes as the magnitude of the forces, F1and F2, change. The angle, β, increases as the magnitude of the forces, F1and F2, increase and decreases at the magnitude of the forces, F1and F2, decrease.

FIG. 4illustrates another detailed view of the region300of the plate110, in accordance with an exemplary embodiment of the present invention. The view of the region300inFIG. 4illustrates an effect of the tension caused by the forces, F1and F2. The magnetic field145in the magnetic band140is represented inFIG. 4by a vector410. As the direction of the effective first anisotropy axis320changes to the direction of the second anisotropy axis330, the magnetic field410inside the magnetic band145changes direction to a direction represented by a vector420. The change in magnetic field is represented by a vector430, which is perpendicular to the vector410, the sum of the vectors410and430being the vector420.

The change of the magnetic field, i.e., the magnetic field component430, produces a change in the magnetic field outside the plate110in the region300. The sensor154C is positioned to detect the change in the magnetic field430outside the plate140. The sensor154C is positioned to be especially sensitive to magnetic fields in an outwardly radial direction, i.e., in a direction parallel to the centerline170. Thus, the sensor154C is positioned to sense the component of the magnetic field outside the plate140caused by the magnetic field component430. The sensor154C is configured to output a signal indicative of the magnetic field430when the tension caused by the forces, F1and F2, is present.

The sensors154A,154B, and154D are positioned similarly to the sensor154C. Thus, the sensor154A is positioned to be especially sensitive to magnetic fields in an outwardly radial direction, i.e., in a direction parallel to the centerline170. The sensors154B and154D are positioned to be especially sensitive to magnetic fields in a direction parallel to the centerline180. The sensors154A,154B, and154D are positioned to sense a component of the magnetic field outside the plate140caused by a change of the magnetic field outside of the plate110because of tension in the plate110.

AlthoughFIGS. 3 and 4are described with reference to a tension in the plate110, it is to be understood that such description is applicable to an instance in which the forces, F1and F2, cause compression in the plate110. Under compression, however, the changes in the anisotropy axis and the magnetic fields are opposite to the changes described with reference toFIGS. 3 and 4when tension is present.

Referring now toFIG. 5, there is illustrated the magnetoelastic tension sensor100ofFIG. 1annotated to show strain axes510and520, in accordance with an exemplary embodiment of the present invention. The strain axis510passes through a center point of the sensor154C and the center point of the sensor154B. The strain axis520passes through a center point of the sensor154D, and the center point of the sensor154A.

The sensor assembly150A is positioned to sense a portion145A of the magnetic field145; sensor assembly150B is positioned to sense a portion145B of the magnetic field145; sensor assembly150C is positioned to sense a portion145C of the magnetic field145; and sensor assembly150D is positioned to sense a portion145D of the magnetic field145. The sensors154A through154D produce respective signals indicative of the magnetic fields that they sense.

Each sensor signal produced by the sensors154A through154D comprises a first component resulting from the tension or compression in the plate110caused by the forces, F1and F2, and a second component resulting from environmental magnetic field(s). When connected correctly to electronic circuitry (described below with reference toFIG. 11), the first components of the sensor signals provided by the magnetic field sensors154A,154B,154C, and154D in response to the tension or compression created by the forces, F1and F2, add constructively. The second component of the sensor signals provided by the magnetic field sensors154A,154B,154C, and154D in response to environmental magnetic fields largely add destructively. Thus, the final sensor output (described below with reference toFIG. 11) is mostly insensitive to environmental magnetic fields.

Referring now toFIG. 6, there are illustrated various directions of the changes in the magnetic fields produced at the locations of the sensor assemblies150A,150B,150C, and150D as a result of tension in the plate110, in accordance with an exemplary embodiment of the present invention. When the plate110is placed under tension, the magnetic field145A under the sensor assembly150A changes, as represented by a vector650A; the magnetic field145B under the sensor assembly150B changes, as represented by a vector650B; the magnetic field145C under the sensor assembly150C changes, as represented by a vector650C; and the magnetic field145D under the sensor assembly150D changes, as represented by a vector650D.

The angles of the vectors650A,650B,650C, and650D are −α, α, α, and −α relative to the centerline160of the plate110(illustrated inFIG. 1). Providing for the magnetic field sensors154A,154B,154C, and154D to have identical polarity of sensitivity to the changes650A,650B,650C, and650D in the magnetic field145produced by the magnetized band140causes the sensitivity of the final sensor output to the tension to be high. Note that the direction of the vector650C is the same as the vector430.

In one exemplary embodiment, the magnetic field sensors154A,154B,154C are fluxgate magnetometers. In another exemplary embodiment, the magnetic field sensors154A,154B,154C are Hall sensors.

The various embodiments of the magnetoelastic sensor100described herein are advantageous in that the magnetic field sensors154A,154B,154C, and154D sense very little magnetic field when the tension or compression is not present. This is the result of the magnetic band140being ring shaped or generally symmetrical about the center point165. Thus, the magnetoelastic sensor100ideally has no unpaired magnetic poles where the sensor assemblies150A,150B,150C, and150D are disposed.

Illustrated inFIG. 7is an exemplary alternative embodiment of the magnetoelastic sensor100, generally designated inFIG. 7as700, in accordance with an exemplary embodiment of the present invention. In the magnetoelastic sensor700, the sensor assemblies150A through150D are replaced with sensor assemblies750A through750D. The magnetoelastic sensor700is otherwise similar to the magnetoelastic sensor100.

Illustrated inFIGS. 8A and 8Bare cross-sectional views of the magnetoelastic sensor700, in accordance with an exemplary embodiment of the present invention.FIG. 8Aillustrates a right-side view along a cross-section of the magnetoelastic sensor700at the centerline180, andFIG. 8Billustrates a left-side view along a cross-section of the magnetoelastic sensor700taken at the centerline170.

The sensor assemblies750A through750D comprise respective sensor platforms752A,752B,752C, and752D, respectively, having inside surfaces751A,751B,751C, and751D and outside surfaces753A,753B,753C, and753D. The sensor assemblies750A through750D further comprise, respectively, first sensors754A,754B,754C, and754D disposed, respectively, on the inside surfaces751A,751B,751C, and751D and second sensors755A,755B,755C, and755D disposed, respectively, on the outside surfaces753A,753B,753C, and753D. The first sensors754A,754B,754C, and754D and the second sensors756A,756B,756C, and756D are symmetrically disposed about the center point165of the plate110.

The second sensors756A,756B,756C, and756D are disposed near the first sensors754A,754B,754C, and754D but at a distance greater from the magnetic band140than the first sensors754A,754B,754C, and754D. The first sensors754A,754B,754C, and754D are chosen to have a direction of sensitivity opposite (180°) from their respective paired second sensors756A,756B,756C, and756D. The pairing reduces the sensitivity of the magnetoelastic sensor700to ambient magnetic fields compared to the magnetoelastic sensor100.

The first sensors754A and754C are disposed above the magnetic band140along the centerline170, and the first sensors754B and754D are disposed above the magnetic band140along the centerline180. The centerline170longitudinally bisects the first sensors754A and754C, and the centerline180longitudinally bisects the first sensors754B and754D. The sensors754A,754B,754C, and754D are disposed over the magnetic band140such that a centerline145of the magnetic band140laterally bisects the sensors754A,754B,754C, and754D.

The second sensors756A and755C are respectively disposed above the first sensors754A and754C along the centerline170, and the second sensors756B and756D are respectively disposed above the first sensors754B and754D along the centerline180. The centerline170longitudinally bisects the second sensors756A and756C, and the centerline180longitudinally bisects the second sensors756B and756D. The sensors755A,755B,755C, and755D are disposed over the magnetic band140such that a centerline145of the magnetic band140laterally bisects the sensors755A,755B,755C, and755D.

FIGS. 9, 10A, and 10Billustrate an exemplary alternative embodiment of the magnetoelastic sensor700, generally designated inFIGS. 9, 10A, and 10Bas900, in accordance with an exemplary embodiment of the present invention.FIGS. 10A and 10Billustrate cross-sections of the magnetoelastic sensor900taken along the centerlines180and170, respectively. The magnetoelastic sensor900comprises the elements of the magnetoelastic sensor700. In the magnetoelastic sensor900, the sensor assemblies750A through750D are replaced with sensor assemblies950A through950D.

The sensor assemblies950A through950D are similar to the sensor assemblies750A through750D, but they differ in that the second sensors956A,956B,956C, and956D are inset radially relative to the center point165of the magnetic band140compared to the second sensors755A,755B,755C, and755D. This inset is best seen inFIG. 9. The magnetoelastic sensor700includes no such inset.

Referring now toFIG. 11, there is illustrated a schematic drawing of an exemplary embodiment of a sensor assembly, generally designated as1100, in accordance with an exemplary embodiment of the present invention. The sensor assembly1100comprises a magnetoelastic sensor100,700, or900connected to circuitry1110via a communications link1115. The magnetoelastic sensor100,700, or900outputs the signals from its sensor assemblies via the communications link1115to the circuitry1110. The circuitry1110combines the signals provided by the sensor assemblies and outputs the combined signal via an output1120. The output1120indicates the amount of tension or compression sensed by the magnetoelastic sensor100,700, or900.

Referring now toFIG. 12, there is illustrated a graph of data from a test of an exemplary implementation of the magnetoelastic sensor100. Weights were hung from the exemplary implementation of the magnetoelastic sensor100, and the output voltage was recorded. The slope in the graph shows a sensitivity of 0.56 mV/pound.