Abstract:
Single coil magnetostrictive force or strain sensors. The single coil sensors have one electrical coil to both excite the magnetrostrictive circuit and detect changes in the permeability of the circuit arising from forces applied to the circuit or induced to strain a portion of the circuit, The sensor may be instructed to sense either or both compressive or tensile forces applied to the magnetostrictive circuit. Thin foils or plating applied to non-magnetically permeable materials may comprise part or all of the magnetostrictive circuit to provide an exceptionally inexpensive and light weight structure for the sensor. A permanent magnet may be incorporated into the structure to provide a sensor capable of measuring dynamic force, especially those of impact, explosion and relatively high frequency.

Description:
REFERENCE TO RELATED APPLICATION 
     This application is based on provisional application No. 60/061,292, filed Oct. 7, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The field of the invention pertains to sensors wherein changes in the physical size of a magnetic circuit are driven by or sensed by electric coils through which the magnetic circuit passes. Typically, such a device comprises an exciting coil and a sensing coil joined by a magnetic circuit of high permeability combined with physical stress/strain characteristics that measurably affect the physical dimensions of the magnetic circuit and the magnetic permeability. 
     Magnetostrictive sensor structures are disclosed in applicant&#39;s previous U.S. Pat. No. 5,437,197 wherein an magnetic circuit passes through two coils. One coil excites the magnetic circuit by means of an alternating electric current and the second coil senses changes in magnetic flux as changes in physical forces applied to the magnetically permeable circuit occur. In alternative embodiments dynamic sensors comprise a direct current applied to the exciting coil or the substitution of a permanent magnet for the exciting coil to produce a constant exciting flux in the magnetic circuit. 
     U.S. Pat. No. 5,297,439 discloses a sensor comprising a magnetostrictive material inductively coupled to a simple electric resonance circuit. Changes in strain of the magnetostrictive material cause changes in the resonance frequency of the electrical signal which, in turn are picked up by a nearby aerial and transmitter/receiver. 
     U.S. Pat. No. 4,955,241 discloses a magnetoelastic force measuring device comprising a soft magnetic measuring film in a configuration that provides good thermal error compensation. In another manner U.S. Pat. No. 5,007,295 discloses a magnetoelastic force transducer configured to compensate for non-force induced changes in the magnetic permeability of the device. U.S. Pat. No. 4,823,621 discloses a magnetoelastic force transducer having similar compensation means as the two patents above but with an asymmetric center pole configured to compensate for any measurement signal arising under zero applied force on the device. U.S. Pat. Nos. 4,802,368 and 4,825,709 both disclose configurations to compensate for temperature changes and other non-force induced changes in the magnetic permeability of a thin walled component of the magnetic circuit. 
     Also, of background interest are three papers co-authored by applicant and published by the American Institute of Physics. A Noncontacting Magnetostrictive Strain Sensor, Darrell K. Kleinke and H. Mehmet Uras, Rev. Sci. Instrument 64(8), August 1993, discloses a sensor wherein the portion of the magnetic circuit subject to strain is separated from the rest of the magnetic circuit by air gaps. 
     A magnetostrictive Force Sensor, Darrell K. Kleinke and H. Mehmet Uras, Rev. Sci. Instrument 65(5), May 1994, discloses a sensor wherein the cores of the exciting coil and sensing coil are the principal magnetostrictive elements of the permeable circuit. Modelling of Magnetostrictive Sensors, Darrell K. Kleinke and H. Mehmet Uras, Rev. Sci. Instrument 67(1), January 1996, discusses the mathematical modelling of the sensors disclosed in the above two papers. 
     With a view toward simplifying the above sensors and, therefore, providing more economically constructed sensors for mass production and installation, the applicant has developed the following improved sensors. 
     SUMMARY OF THE INVENTION 
     The invention comprises a magnetostrictive force or strain sensor having one electric coil to both excite the magnetostrictive circuit and detect changes in the permeability of the circuit arising from forces applied to the circuit or induced to strain a portion of the circuit. The new sensor may be constructed to sense either or both compressive and tensile forces applied to the magnetostrictive circuit. Thin foils or plating applied to non-magnetically permeable materials may comprise part or all of the magnetostrictive circuit to provide an exceptionally inexpensive and light weight structure for the sensor. A permanent magnet may be incorporated into the structure to provide a sensor capable of measuring dynamic forces, especially those of impact, explosion and relatively high frequency. 
     The new sensors have a wide range of applications including but not limited to occupant sensing and weighing in automobile airbag systems, engine, powertrain and suspension controls, and monitoring highway bridge loads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side cross-sectional view of the new sensor in simplest form; 
     FIG. 1A is a side cross-sectional view of a modification of the new sensor of FIG. 1; 
     FIG. 2 is a side cross-sectional view of the new sensor with an air gap in the magnetostrictive circuit; 
     FIG. 2A is a side cross-sectional view of a modification of the new sensor of FIG. 2; 
     FIG. 3 is a side cross-sectional view of the new sensor with an enclosed coil; 
     FIG. 4 is a side cross-sectional view of the new sensor with a center air gap, self-excitation and axial sleeve load carrying member; 
     FIG. 5 is a side cross-sectional view of the new sensor with a double layer core inside the coil; 
     FIG. 6 is a side view of a simple adjustable adapter to measure tensile forces with the new sensor; 
     FIG. 7A is a side cross-sectional view of the new sensor with multiple coils arranged for series electrical connection in a compact configuration; 
     FIG. 7B is a top view of the sensor of FIG. 7A; 
     FIG. 8A is a side cross-sectional view of the new sensor of FIG. 7 with a central load carrying member added; 
     FIG. 8B is a top view of the sensor of FIG. 8A; 
     FIG. 9A is a side view of the new sensor comprising a pair of E-cores in facing arrangement; 
     FIG. 9B is a bottom view of the new sensor of FIG. 9A; 
     FIG. 10A is a side cross-sectional view of the new sensor of FIG. 9 including load carrying members inside the coils; 
     FIG. 10B is a bottom view of the new sensor of FIG. 10A; 
     FIG. 11A illustrates in block diagram a constant current and voltage change sensing circuit for the new sensor; and 
     FIG. 11B illustrates in block diagram a constant voltage and current change sensing circuit for the new sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates in simplest configuration the new sensor comprising a hollow tubular member  20  and upper  22  and lower  24  face plates. Passing through apertures  26  in the face plates  22  and  24  and the center of member  20  is a bolt and nut assembly  28  which retains the sensor assembly mechanically together. 
     Wound about the member  20  is a coil  30  to which is applied an alternating current (AC) to excite the coil and thereby create a magnetic flux about the coil. The upper  22  and lower  24  face plates are constructed of ferromagnetic material which may be magnetostrictive. The hollow tubular member  20  is of a ferromagnetic material such as ferrite and acts as the compression/extension member when forces are applied externally to the sensor. 
     In response to the external forces shown by arrows  32 , which may be tensile if the sensor is preloaded by tightening the bolt and nut assembly  28 , the reluctance of the hollow tubular member  20  changes. Thus, by either holding the AC electric current constant a change in AC voltage results from the change in reluctance or by holding the AC electric voltage constant a change in AC current results from the change in reluctance. The change in strain due to external force  32  in the magnetostrictive material of the member  20  causes a change in reluctance which can be easily measured as the change in AC current or AC voltage in the same coil as that excited. 
     As shown in FIG. 1A, the upper  22  and lower  24  face plates and the hollow tubular member  20  may be substantially constructed of a non-magnetically permeable substrate material such as a plastic. Onto the non-magnetically permeable material is plated or otherwise formed a magnetostrictive film or coating  25  which changes in reluctance in response to strain of the film coincident with strain of the substrate. 
     Although shown in FIG. 1 with a centerline or axis  34  implying a circular configuration in plan view, the hollow tubular member  20  may be square or rectangular in plan view with the coil  30  wrapped thereabout. 
     To measure dynamic forces the sensor may be constructed with a permanent magnet material comprising the hollow tubular member  20 . Such a configuration provides a static magnetic flux through the magnetostrictive circuit and the coil  30 . Dynamic forces resulting from impact, explosion or high frequency applications can be measured without pre-excitation of the coil  30  with an AC current. 
     FIG. 2 illustrates a sensor similar to the sensor of FIG. 1, but modified to provide a better flux path, shielding and an air gap  136  in the magnetostrictive circuit. The upper face plate  138  is extended down about the coil  130  in the manner of a cover  140  to form the air gap  136  with the lower face plate  142 . The other elements of the sensor may be the same as in FIG.  1  and are numbered accordingly with the prefix “1”. 
     As the hollow tubular portion  120  is strained by the applied force  132  the physical change in the air gap  136  between the cover  140  and lower face plate  142  together with reluctance change in  120  causes a significant change in the magnetic flux passing through the entire circuit of the face plates  138  and  142 , hollow tubular portion  120  and cover portion  140 . The change in the air gap  136  effectively acts as a magnetic amplifier within the sensor resulting in an amplified change in reluctance and in the electric detection signal. 
     FIG. 2A illustrates the modification of the sensor of FIG. 2 with a magnetostrictive film  135  applied to the upper face plate  138  and cover  140 , and separately to the lower face plate  142  and hollow tubular portion  120 . Most importantly the plating extends into the gap  136 . 
     As with the above described sensor the cross-section of the sensors in FIGS. 2 and 2A may be circular, square, rectangular or similar cross-section. 
     FIG. 3 illustrates a sensor similar to the sensor of FIG. 1, but modified to completely enclose the coil  230  with the magnetostrictive circuit. The magnetostrictive circuit pieces may be constructed of a ferromagnetic material such as ferrite and comprise a pair of mirror image pieces joined along a central line  243 . The mirror image pieces comprise an upper face plate  222  and lower face plate  224 . Integral with the upper face plate  222  are an upper tubular portion  220  surrounding aperture  226  and upper outside cover  240 . Likewise, integral with the lower face plate  224  are lower tubular portion  221  and lower outside cover  241 . 
     As with the above described sensor the cross-section of the sensor in FIG. 3 may be circular, square, rectangular or another similar cross-section. The magnetostrictive circuit may comprise a plastic substrate with a magnetically permeable film applied to the interior or exterior surfaces of the sensor such as at  245 . The forces  232  applied to the sensor for measurement may be compressive only or with precompression as noted above, tensile forces may also be measured. 
     In FIG. 4 the upper and lower mirror image pieces are separate to form an air gap  336  at the horizontal central line  343 . A load carrying sleeve  325  surrounds the central aperture  326 . An integral external flange  327  supports and separates the upper tubular portion  320  and the lower tubular portion  321  to form the air gap  336 . 
     For dynamic force measurements the sleeve  325  may be formed from a permanent magnet material as above. Or, as noted above the upper and lower pieces comprising the upper and lower face plates  322  and  324 , tubular portions  320  and  321  and covers  340  and  341  may be constructed with a plastic substrate and plated with the magnetostrictive material. The magnetostrictive material may also be applied to a sleeve  325  of plastic or other non-magnetically permeable material. The coil  330  is wound about the tubular portions  320  and  321 . 
     As with the above sensors the horizontal cross-section may be circular or non-circular and with precompression of the sensor both compressive and tensile forces  332  may be applied to the sensor and measured. 
     FIG. 5 illustrates a modification of the sensor to separate the load carrying members from the magnetostrictive circuit. The sleeve  425  and tubular member  420  extend axially beyond the upper face plate  422  and lower face plate  424 . As above the exciting and detecting coil  430  is wound about the tubular member  420  and enclosed by a cover  440 . The face plates  422  and  424 , the tubular member  420  and the cover  440  all comprise ferromagnetic material such as ferrite or may be constructed of plastic and plated, coated or otherwise covered with ferromagnetic material as necessary to form the magnetostrictive circuit. The sleeve  425  may also form a part of the magnetostrictive circuit or merely be a load carrying member along with the tubular member  420 . With this construction the face plates  422  and  424  and the cover  440  may be much lighter in construction. For measurement of dynamic forces either the sleeve  425  or the tubular member  420  may be constructed of a permanent magnet material. And, the sensor may be precompressed to permit measurement of both tensile and compressive forces  432 . 
     In FIG. 6 a simple device is shown for precompressing any of the sensors shown in FIGS. 1 through 5. The device comprises a pair of complementary eye bolts  450  and  452  having a threaded engagement therebetween  454 . Each eyebolt  450  and  452  includes a flange  456 . With the threaded engagement  454  positioned within central aperture  26  of any of the above sensors, precompression can be applied to the sensor by tightening the threaded engagement and tensile forces subsequently applied through attachments to the eyes of the eyebolts  450  and  452 . Depending on the application the device of FIG. 6 may be constructed of ferromagnetic material or non-ferromagnetic material. 
     Illustrated in FIGS. 7A and 7B is a modification of the sensor for applications where the axial dimension is limited. Ferromagnetic face plates  522  and  524  are spaced apart and retained by three or more (4 as shown) ferromagnetic pins  540  to comprise the magnetostrictive circuit. The pins  540  are shouldered at each end and press fit or otherwise attached in holes  528  in the face plates  522  and  524 . 
     Although shown round, the face plates  522  and  524  may be triangular, cruciform or of other geometric shape. Each pin  540  has wound thereabout a coil  530 . The coils  530  are electrically connected in series and may be multiple layer wound on each pin  540 . Using a device such as shown in FIG. 6 the sensor of FIGS. 7A and 7B may be precompressed and used for tensile force measurements. 
     As above parts of the magnetic circuit may incorporate a permanent magnet material for dynamic force measurements or a plastic material with a ferromagnetic film applied thereto. With this configuration a relatively large coil  530  can be accommodated in a limited axial space. 
     FIGS. 8A and 8B illustrate a modification of FIGS. 7A and 7B wherein a sleeve  625  and center tubular member  620  are added to provide a load carrying member and magnetostrictive member respectively between the upper  622  and lower  624  face plates. The sleeve  625  and tubular ember  620  surround the central aperture  626 . Outside of the tubular member  620  are free or more pins  640  (4 pins shown) each wound with a coil  630 . The coils  630  are electrically connected in series making this sensor configuration also useful where axial space is limited. The pins  640  are press fit or otherwise attached to the upper  622  and lower  624  face plates in holes  628 . 
     As above all of the components except the coils  630  may be made of ferromagnetic materials or ferromagnetic materials formed on the surfaces of non-permeable materials. The sleeve  625  and tubular member  620  may be constructed of permanent magnet material to sense dynamic loading. And, by precompression tensile as well as compressive forces  632  may be measured. 
     Illustrated in FIGS. 9A and 9B is a substantially planar form of the sensor. E-cores  722  and  724  symmetric about a center plane  743  comprise a center piece  720  and side pieces  740  above and below the center plane. Wrapped about the center piece  720  is the coil  730 . The E-cores may be constructed of ferromagnetic material, permanent magnet material for dynamic force measurements or magnetostrictive film material on a non-magnetic substrate. The force  732  to be measured is applied in the plane of the sensor as shown. 
     In FIGS. 10A and 10B the sensor of FIGS. 9A and 9B is further modified by parallel apertures  826  surrounded by load carrying sleeves  825  in the E-cores  822  and  824 . The apertures  826  permit eyebolt devices similar to FIG. 6 to apply precompression and permit tensile forces and compressive forces  832  to be conveniently measured. Coils  830  are wrapped about all three sleeves  825  and apertures  826  and connected electrically in series. 
     The block diagrams of FIGS. 11A and 11B illustrate the sensor is electrically connected to a source of well regulated constant current or constant voltage. By measuring changes in the voltage or current respectively the changes in magnetic permeability arising from strain of the magnetostrictive components under load can be measured and continuously monitored in real time. 
     Having described my invention, many modifications thereto will become apparent to those skilled in the art to which it pertains without deviation from the spirit of the invention as defined in the appended claims.