METHOD OF MAKING A PERMANENT MAGNET SENSOR ELEMENT WITH A SOFT MAGNETIC LAYER

Abstract: A position sensor (10) is made by assembling a permanent magnet (14), a soft magnetic layer (16) on the magnet (14), a magnetoresistive sensor element (18) on the soft magnetic layer (16), and a toothed ferritic member (20) mounted for movement past the sensor element (18). The permanent magnet (14) is surface-treated to form, in situ, the soft magnetic layer (16) comprising a thin layer of high-permeability, low-coercivity material. An Nd-Fe-B alloy magnet (14) is surface-treated by a laser or electron beam to melt a thin layer (16) which is then cooled to re-solidify into a soft magnetic material. Rapid cooling achieves an amorphous surface layer having the desired properties. Slow cooling results in the formation of alpha-iron grains which enhance the soft magnetic properties. Iron formation is aided by reacting the molten layer with an oxidizing atmosphere. Iron alloying by adding iron powder to the molten layer is also feasible.

Description: Field of the Invention This invention relates to a method of making a magnetic sensor and particularly to a method of making a permanent magnet with an integral thin surface layer of soft magnetic properties as specified in the preamble of claim 1, for example as disclosed in EP-A-0 357 200. Background of the Invention A sensitive magnetic position sensor has been proposed in EP-A-0 357 200 which incorporates a permanent magnet, a flux density sensor mounted on the magnet, and a toothed excitation element movable past the magnet to vary the magnetic flux affecting the flux sensor. To optimize sensitivity, a thin intermediate layer (about 0.1 mm thick) of ferromagnetic material with soft magnetic properties is sandwiched between the magnet and the flux sensor. This enables the flux to more readily shift with respect to the sensor upon movement of the toothed element. It is proposed in that disclosure to provide the soft magnetic layer by securing a foil of such material to the magnet or by adding a layer of iron powder to powdered magnet material and forming both by compressing and/or sintering. It is further proposed that the permanent magnet be made of MQ2 magnet material which is a hot pressed Nd-Fe-B alloy having a high energy product and is a trademarked product of General Motors Corporation. U.S.A-4 792 367 (Lee), discloses the MQ2 material which is a hot-pressed rare-earth-iron alloy further including boron. The preferred methods of forming permanent magnets from that material include overquenching a molten mixture of the components thereof by melt-spinning to obtain an amorphous material having little intrinsic coercivity and then hot-pressing to promote crystal growth to the optimum size for the desired hard magnetic properties. (An even higher energy product magnet can be formed by hot-pressing and hot-working such amorphous material to form a deformed fine grain microstructure consisting predominantly of an Nd₂Fe₁₄B phase and an intergranular phase.) An intrinsic room temperature coercivity exceeding 15 kOe is obtained in the hot-pressed-only material, along with very high remanence and high energy product. The magnet material, when in the overquenched amorphous condition, exhibits the high magnetic permeability required of the intermediate layer of the sensor. Summary of the Invention A method of making a position sensor according to the present invention is characterised by the features specified in claim 1. It is an object of the invention to improve a method of making a magnetic sensor by treating a thin surface layer of a permanent magnet material to obtain soft magnetic properties in that layer. A further object is to provide such a method which causes the layer to revert to an amorphous state having the desired properties. It is another object to provide such a method of making a permanent magnet with a soft magnetic layer by treating the magnet surface to change the structure or chemistry of the surface layer to enhance the soft magnetic properties. The invention is carried out in a method of making a position sensor which comprises assembling a permanent magnet, a high-permeability layer on the magnet, a magnetic sensing element on the high-permeability layer, and an exciter element mounted for movement relative to the sensing element, by the improvement comprising the steps of: forming the permanent magnet of an iron-based alloy having high coercivity, and forming the high-permeability layer in situ by treating a surface of the magnet to form a thin layer of low coercivity and high permeability by rapidly melting the layer by a high-energy beam and cooling the layer to destroy the coercivity therein. The invention involves changing the surface layer to an amorphous structure having soft magnetic properties. The invention is also carried out by a method of forming a permanent magnet with a high-permeability and low-coercivity surface layer comprising the steps of: forming a permanent magnet of an Nd-Fe-B alloy having a fine grain microstructure yielding high coercivity, melting a surface layer of the magnet by an energy beam with sufficient beam power density to melt a thin layer of the magnet, and slowly cooling and re-solidifying the layer to cause phase separation and the formation of alpha-iron grains as well as an Nd-Fe-B phase in the surface layer microstructure, thereby enhancing the soft magnetic properties of the layer. Brief Description of the Drawings The above and other advantages of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings wherein like references refer to like parts and wherein: Figure 1 is a schematic diagram of a position sensor to be made by the method of the invention, and Figure 2 is a graph of demagnetization curves of a permanent magnet material and an amorphous form of the same material having soft magnetic properties. Description of the Invention The magnetic position sensor 10 shown in Figure 1 comprises a monolithic body 12 combining a permanent magnet 14 and a soft magnetic material layer 16 on one surface of the magnet 14. A flux density sensor 18 mounted on the layer 16 is connected by leads, not shown, to external circuitry to provide an output signal indicative of flux changes. An exciter element 20 of magnetic material, spaced from the sensor 18 and layer 16, is movable in a path generally parallel to the layer 16 and has a series of teeth 22 which affect the flux path from the magnet as the exciter element moves. The movement of the exciter teeth little affects the total flux density but does vary the spatial distribution of the flux density along the width of the magnet 14, creating sharp local flux density variations that can be sensed by the sensor 18. The effect of the soft magnetic material layer 16 is to greatly increase the sensitivity of the device. A layer 0.03 to 0.05 mm thick is considered to be sufficient. This device is further detailed in EP-A-0 357 200, which is incorporated herein by reference. U S-A-4 792 367 (Lee), which is incorporated herein by reference, describes the making of the preferred magnetic material. The precursor material, a molten mixture of neodymium (Nd), iron (Fe) and boron (B) in the correct proportions to yield a predominant Nd₂Fe₁₄B phase, is rapidly quenched by melt-spinning to form an amorphous material having low coercivity and high permeability. This material is transformed into a permanent magnet having very high coercivity and low permeability. These material properties are contrasted in Figure 2 which shows demagnetization curves for the permanent magnet material 24 and the amorphous or soft magnet material 26. To transform a thin layer of the permanent magnet material to soft magnetic properties which may or may not be the same as the amorphous material characterized in Figure 2, a change of microstructure and/or a chemical change in the layer can be made. Surface heating by a laser beam or electron beam of sufficient power density can melt a thin layer of the material. Then, by controlling the cooling rate of the layer, the material re-solidifies to form a microstructure dependent on the cooling rate. Both melt depth and cooling rate are controlled by beam power density. For example, when treating a magnet 8 mm thick, a beam power density of 1.26 x 10⁴ W/cm² from a CO₂ laser was sufficient to produce surface melting and yielded a layer 0.3 mm thick. Greater power density would increase the thickness and result in slower cooling. The melting step can comprise scanning the surface by a laser beam in a pattern to progressively melt the desired area of the magnet surface. The surface layer 16 can be repetitively melted and re-solidified to progressively increase the soft magnetic properties of the layer 16. The magnetic permeability of the surface layer is increased by promoting the formation of ferrite in the re-solidified microstructure. Melting and re-solidification of the alloy under near-equilibrium conditions allows phase separation resulting in a significant volume fraction of relatively large alpha-iron grains in the surface layer microstructure. To attain this, a cooling rate of 10⁴°C/sec is appropriate. This is a 100 times slower rate of cooling than that used to form the amorphous material from a melt. A thicker molten layer leads to a slower cooling rate. If the resulting alpha-iron layer is then thicker than desired for the end product, surface grinding is used to attain a thinner layer and also results in a smooth finish. One embodiment of the method of the present invention includes the step of chemically altering the surface layer 16 of the magnet 14 by placing the magnet 14 in the presence of constituents which combine with the surface layer 16 when it is treated by an energy beam with sufficient beam power density to melt the thin layer 16. One way of doing this is to add iron powder to the surface layer 16 and then to melt the surface layer 16 with a laser beam so that the added iron alloys with the molten surface layer material. To increase the volume fraction of alpha-iron and reduce the amount of the hard magnetic Nd₂FE₁₄B phase formed during re-solidification, melting in an oxidizing atmosphere will tend to preferentially oxidize the highly-reactive Nd, to thereby limit the formation of the Nd₂Fe₁₄B phase and to increase the permeability of the re-solidified layer. Multiple passes, i.e., repeatedly scanning and melting the same area, may be useful to maximize the intended effect. To make the magnetic position sensor, a permanent magnet 14, preferably an Nd-Fe-B alloy, is surface-treated by a high-energy beam to form a thin layer 16 of magnetically soft material by one of the processes described above, a magnetic sensing element such as a magnetoresistor 18 is mounted on the thin layer, and an exciter element 20 is movably mounted for movement past the sensing element.

Claims: A method of making a position sensor (10) comprising assembling a permanent magnet (14), a high-permeability layer (16) on the magnet (14), a magnetic sensing element (18) on the high-permeability layer (16), and an exciter element (20) mounted for movement relative to the sensing element (18), characterised in that the method includes the steps of: forming the permanent magnet (14) from an iron-based alloy having high coercivity, and forming the high-permeability layer (16) in situ by treating a surface of the magnet (14) by rapidly melting the surface with a high-energy beam and then cooling the surface to decrease the coercivity therein so as to form a thin layer (16) of low coercivity and high permeability. A method of making a position sensor (10) according to claim 1, in which the permanent magnet (14) is formed from an Nd-Fe-B alloy having a fine grain microstructure yielding high coercivity, characterised in that the high-permeability, low-coercivity layer (16) is formed in situ on the magnet by rapidly melting a surface layer of the magnet by an energy beam with sufficient beam power density to melt a thin layer (16), and then cooling the layer (16) to rapidly re-solidify the surface to form a substantially amorphous microstructure with said low coercivity and high permeability. A method of making a position sensor (10) according to claim 1 or 2, characterised in that the melting step comprises scanning the surface by a laser beam in a pattern to progressively melt the desired area of the magnet surface. A method of making a position sensor (10) according to claim 1 or 2, characterised in that the surface layer (16) is repetitively melted and re-solidified to progressively increase the soft magnetic properties of the layer (16). A method of making a position sensor (10) according to claim 1, in which the permanent magnet (14) is formed from an Nd-Fe-B alloy having a fine grain microstructure yielding high coercivity, characterised in that the high-permeability, low-coercivity layer (16) is formed in situ on the magnet by rapidly melting a surface layer of the magnet by an energy beam with sufficient beam power density to melt a thin layer (16), and slowly cooling and re-solidifying the layer (16) so as to cause phase separation and the formation of alpha-iron grains as well as an Nd-Fe-B phase in the surface layer microstructure, thereby enhancing the soft magnetic properties of the layer (16). A method of making a position sensor (10) according to claim 5, characterised in that the melting step comprises scanning the surface by a laser beam in a pattern to progressively melt the desired area of the magnet surface. A method of making a position sensor (10) according to claim 5, characterised in that the surface layer (16) is repetitively melted and re-solidified to progressively increase the soft magnetic properties of the layer (16). A method of making a position sensor (10) according to claim 1, in which the permanent magnet (14) is formed from an Nd-Fe-B alloy having a fine grain microstructure yielding high coercivity, characterised in that the method includes chemically altering said surface layer (16) of the magnet (14) by placing the magnet (14) in the presence of constituents which combine with the surface layer (16) when the surface layer (16) is treated by said energy beam with sufficient beam power density to melt said thin layer (16), the subsequent cooling and re-solidifying of the molten layer (16) causing the formation of iron grains in the surface layer microstructure, thereby enhancing the soft magnetic properties of the layer (16). A method of making a position sensor (10) according to claim 8, characterised in that the melting and cooling steps are carried out in a reactive atmosphere to oxidize the Nd, so as to decrease the amount of the Nd-Fe-B phase and to increase the alpha-iron content of the re-solidified layer (16). A method of making a position sensor (10) according to claim 8, characterised in that the step of chemically altering said surface layer (16) comprises surface alloying of the surface with a laser beam and includes the step of adding iron powder to the surface layer so that the added iron alloys with the molten material of the surface layer (16).

Inventors: BRADLEY, JOHN R., HEREMANS, JOSEPH P., SCHROEDER, THADDEUS
Applicants: GENERAL MOTORS CORPORATION
Application year: 1990
Main classification: G01D5
IPC codes: G01B7, G01D5, H01F41, B23K26, C22C1