Patent Application
20080168844
1. Field of the Invention
The present invention relates to a magnetostrictive strain sensor with an airgapless core.
2. Description of the Background Art
Magnetostrictive strain sensors are known and examples are disclosed and claimed in U.S. Pat. Nos. 6,941,824, issued Sep. 13, 2005, and U.S. Pat. No. 6,993,983 issued Feb. 7, 2006, both assigned to the assignee of the subject invention. An inductance coil extends around an axis for establishing a loop of magnetic flux looping axially through the core and extending around the coil to define a donut shaped ring of magnetic flux surrounding the coil. A core made of a magnetostrictive material, such as a Nickel-Iron alloy, provides a primary path for the magnetic flux in a first portion of the loop of magnetic flux. A magnetic carrier provides a return path for the magnetic flux in a second portion of the loop of magnetic flux as the magnetic flux circles the coil through the core and the carrier. The permeability of the magnetostrictive core, thus the inductance of such a device, is a function of the strain applied to the core along the axis. The coil inductance therefore provides a useful signal.
The coil can be excited with an AC voltage or AC current to induce an alternating magnetic field in the core. This field loops around the coil, and will possibly travel through other elements and materials, such as airgaps, and other matter.
Magnetostrictive sensor concepts are described in Publication US 2006/0150743 A1, published Jul. 13, 2006, U.S. Pat. No. 7,104,137, issued Sep. 12, 2006, and Publication U.S. 2006/0086191, published Apr. 27, 2006.
Magnetostrictive materials have a permeability that varies with stress (Villari effect). Usually, stress is sensed by measuring the inductance of a coil (usually of a copper wire) wound around a core made of a magnetostrictive material such as a nickel-iron alloy. These conventional sensors have at least 2 parts, a coil and a core. The core is made of at least two parts, so that the coil can be both wound around the inner part of the core, and surrounded by the outer part of the core.
The inductance depends not only on the permeability of the core material(s), but also on any airgaps between the various parts of the core. The airgaps are problematic because the relative permeability of air (μo=1) is orders of magnitude smaller than the permeability of likely core materials, so the reluctance of the airgaps is large compared to that of the core even if the airgap is small, even as small as a fraction of a millimeter. This invention overcomes this problem by using a seamless, one-piece core.
U.S. Pat. Nos. 4,541,288 and 4,561,314 show a coil wound around a toroidal core made of amorphous material, a type of material that exhibits magnetoelasticity. Despite a superficial resemblance (magnetoelasticity, toroidal core), there are fundamental differences between these patents and the present invention. These patents use a different aspect of the magnetoelastic effect, where the saturation flux density of a material is affected by stress (see
The present invention overcomes deficiencies found in the prior art by measuring the inductance of a coil for a strain sensor comprising a monolithic magnetostrictive material core while essentially eliminating the effect of airgaps upon inductance.
This invention relates to a strain sensor comprising a monolithic magnetostrictive material airgapless core, wherein permeability of the material depends on stress; the core having an aperture therein; and a coil, wound about the core and through the aperture; the core and the coil being configured such that when the coil is connected in circuit, it establishes a loop of magnetic flux that circulates through the core and about the coil, whereby impedance of the core is measured.
The invention further relates to a strain sensor comprising a monolithic magnetostrictive material airgapless core, wherein permeability of the material depends on stress; the core having a groove therein; and a coil, wound about the core and through the groove; the core and the coil being configured such that when the coil is connected in circuit, it establishes a loop of magnetic flux that circulates through the core and about the coil, whereby impedance of the core is measured.
The invention also relates to the above strain sensor integrated in a sensor plug useful for sensing the pressure of diesel fuel in common rail diesel engine systems, oil pressure in engine oil systems, oil pressure in hydraulic actuators for backhoes and other earth moving and construction vehicles.
The invention further relates to a method for measuring stress utilizing the mentioned strain sensor in which impedance of the core is measured in the substantial absence of the effect of airgaps between coil and core upon impedance.
Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following non-limiting detail description when considered in conjunction with the accompanying drawings wherein:
Magnetostriction denotes the following physical effect: The change in the dimension of a body when it is magnetized. More specifically, Joule magnetostriction is the change in the length of a body when it is magnetized. Joule magnetostriction can be positive (the change in length with field is positive), or negative (the change in length with field is negative), depending on the material.
The inverse effect, called inverse magnetostriction or Villari effect, is the change in permeability of a magnetic material with an applied stress.
Materials which exhibit a magnetostrictive effect, especially those exhibiting a large magnetostrictive effect, are called magnetostrictive materials.
A magnetostrictive stress sensor (such as the one described in our invention) therefore uses the Villari effect on magnetostriction, and Villari effect are often overlooked and all are denoted “magnetostrictive devices.” Reference: Bozorth, R. M., Wiley-Interscience, IEEE Press Reissue, 1993.
A typical strain sensor is shown in
The type of magnetostrictive material used in the present sensors has a permeability that depends on stress. Examples of such materials are nickel-iron alloys (both maraging steels, with lower nickel content, around 18%, and regular nickel-iron alloys, with nickel content between 20% and 100%), galfenol, terfenol, etc. So the changing permeability can be sensed by measuring the inductance of the coil for instance by exciting the coil with a small, constant amplitude current and reading the voltage across the coil.
More particularly, in an example of useful magnetostrictive materials are: Nickel-iron alloys (alloys with more than 20% nickel content), maraging steels (nickel alloys with less than 20% nickel), cobalt-iron alloys, Terfenol, or gallium-iron alloys (known as Galfenol), especially for sensors used in compression mode; pure nickel alloy with a large percentage of nickel, especially for sensors used in tension mode.
The most useful materials are nickel-iron alloys in general, and maraging steels for those instances where the stress level is particularly high. Galfenol is a new and promising material. Steels in general exhibit some magnetostriction but much smaller.
The reasons to reduce or eliminate all airgaps are developed in details in US Patent Application Publication 2006/0150743 A1. Because the relative permeability of air is one and that of the core and return-flux carrier are 100 or more, even a small airgap (fraction of a millimeter) contributes to the inductance of the coil, therefore lowering useful signal. Moreover, if the airgap changes from about 0.1 to 0.2 mm, then a significant change in inductance is observed that may be at least as large as the inductance change resulting from the stress in the core. In other words, the airgaps can be a source of noise which hide the useful sensor signal. Reduction and elimination of airgaps must be devised.
In '743 publication, a solution is suggested whereby the interfaces between the various constituting parts of the core are placed normal to the applied force. The idea is that, with proper machining the airgap can be reduced initially, and the force when applied will tend to further close the airgap, thus guaranteeing that it is always close. While this approach has been successful, it still relies on matched surfaces for good contact, and this good contact must be maintained over time, temperature fluctuation, etc.
The present invention overcomes this difficulty using a single-piece core 22, and winding a coil 20 through an aperture 28, as shown in
Monolithic means made of a single, solid piece can be machined down from a larger piece, or molded into shape, from a single mold. Includes no airgap, separating film, or mating surface within, in any plane, whether in the direction parallel to the magnetic flux, normal to the magnetic flux, etc.
Some processes such as welding two separate parts together will yield a piece which has the appearance of being monolithic. However, the welding process cannot be guaranteed to leave no separating airgap, or separating film of welding material within, therefore it cannot product a monolithic piece.
The monolithic core can be implemented in various geometries. Various examples are shown in the drawings, all in the context of force sensing.
In
Generally, depending on the material, magnetostriction has either a positive or negative coefficient. The coefficient of magnetostriction which exhibits a positive or negative sign is the ratio of change of permeability for a given change in stress applied, or coefficient λ:
where μ and σ are the material permeability and applied stress, respectively. See R. M. Bozorth, Ferromagnetism, IEEE Press, Wiley-Interscience, John Wiley & Sons, Inc., 3rd Ed. (2003).
Materials with a negative coefficient of magnetostriction when used in compression must have the flux lines generally aligned with the stress, as summarized in Table I. For materials with a positive coefficient of magnetostriction, when in compression, the magnetic flux lines are desirability normal to the stress lines, see Table II. Tables I and II also show tensile stress, for completeness.
Now, looking at desirable magnetostrictive materials: A desirable type of material is a nickel-iron alloy, because it exhibits a large coefficient λ, and is relatively strong and inexpensive. Nickel-iron alloys, however, can exhibit either a positive or negative coefficient of magnetostriction, depending on the nickel content of the alloy. R. J. Bozorth, p. 616.
So, for force sensors where the stress is compressive, and for nickel-iron alloys with nickel content between 40% and 70%, the magnetic flux lines must be in line with the stress lines. For nickel-iron alloys with nickel content between 85% and 95%, the flux lines must be normal to the stress.
Thus, those skilled in the art know how this relationship depends on the material (some have positive, other negative, magnetostrictive coefficients), and depends on whether the stress is compressive or tensile. Therefore, they will know which part of the core should be longer. In
For the toroidal shape, the coil 20 must be recessed, as shown on
The sensing unit may include more than one coil. Having several coils 30, 32 can be useful for redundancy, to cancel EMI noise, or for cancellation of disturbances such as temperature. Another coil may be wound on the same core 22, see
Stress can be caused by force, pressure, torque, acceleration, etc., so the sensor concept described here can be applied to sense force, pressure, torque, acceleration, combinations thereof, etc. The subject figures concern force sensor applications. An example of a pressure sensor application is shown in
Wall 39 is different from the diaphragms used in other pressure sensors such as strain gauge sensors. Strain gauge pressure sensors are known and commercially available, see for instance U.S. Pat. No. 7,131,334. In strain gauge sensors, a diaphragm is used, like wall 39, to isolate the pressurized fluid from the attached strain gauge, which serves as the sensing element. However, unlike wall 39, a diaphragm must be sufficiently flexible and deformable to apply stress to the strain gauge. Wall 39 does not deform. The force exerted on wall 39 by the pressure is directly transmitted to the sensor core 22.
The sensing unit consists of a single-piece core 22 with a hole through which the coil(s) is wound.
This example is useful for sensing the pressure of diesel fuel in common rail diesel engine systems, oil pressure in engine oil systems, oil pressure in hydraulic actuators for back hoes and other earth moving and construction vehicles, etc.
The single-piece core system presented here offers a unique opportunity to integrate the sensor 36 physically with the system where it is used. For instance, the pressure sensor 36 shown in
Provided the material of the pipe 46 or system being monitored for pressure is sufficiently magnetostrictive, the present concept can be implemented by a single step of making one through hole 48 close to the area the pressure of which is monitored, and winding a coil 20 through that hole 48. An example is shown in
This invention concerns development of a pressure sensor for withstanding higher pressures. Examples are common rail diesel fuel sensors, and combustion chamber pressure sensors (gasoline or diesel). In these applications, aside from the single-piece core, a distinctive feature is the blind hole 38 in which the sensor 36 is fitted (see
The outside diameter of the cylinder of
To handle higher pressures, maraging steel is used as a material combining both strength and magnetostriction.
For common materials, frequencies as low as 100 Hz are sufficient. Desirable values of frequencies can be either in the ranges 1 to 15 kHz or 15 to 50 kHz. Both of these ranges would provide adequate sensor dynamic response, and allow the force sensor to follow fast motion. For example, the force pattern experienced during the motion of a vehicle brake system, if the sensor is part of such a system. The lower range (1 to 15 kHz) has the advantage of avoiding the range of frequencies usually selected for motor control, thus minimizing interference if the sensor is close to a motor. The higher range (15 to 50 kHz) has the advantage of being inaudible for humans. It would also allow for yet higher dynamic response.
A sensor assembly for measuring force along an axis in accordance with the subject invention is shown in various embodiments in figures wherein like parts or portions are indicated with like numerals.
At least one inductance coil having multiple turns or coils, or multiple coils each having one or more turns or coils, extends around the force axis for establishing a loop of magnetic flux (shown by the arrows) looping axially through the coil and extending around the axis to define a ring of magnetic flux surrounding the coil. In the exemplary embodiments, only one coil is shown, and the self-inductance of the coil is calculated and measured. Alternative embodiments may include several coils, either connected in series or separately, and “inductance” should be understood as, more generally, self-inductance or mutual inductance.
In the exemplary figures shown in this application, the force axis happens to coincide with a geometrical axis of symmetry. However, the word “force axis” should be understood broadly as the direction of the force, or the direction of the force path, through the core. In fact, the force axis or force path may, or may not be an axis of symmetry; it may, or may not be, a line, and one could envision situations where this path or axis is not straight but curved. It could also be a surface rather than a line.
Obviously, many modifications and variations of the present invention are possible in light of the above teachings. The invention may be practiced otherwise than as specifically described within the scope of the appended claims. In addition, the reference numerals in the claims are merely for convenience and are not to be read in any way as limiting. The foregoing references are hereby incorporated by reference.
