Abstract:
A magnetometer for a force sensor assembly includes first inner and outer coils that are spaced axially apart from second inner and outer coils. Between the inner and outer coils is a plurality of magnetic strips. These magnetic strips are disposed at axially distinct portions within the inner and outer coils to detect magnetic field asymmetry. A drive circuit is connected to the inner coils and the outer coils generate an alternating magnetic field. The magnetometer of this invention creates an artificially enhanced demagnetization field reducing hysteresis within the sensor assembly.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to U.S. Provisional application No. 60/707,927 filed on Aug. 12, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention generally relates to a magnetometer for a torque sensor. More particularly, this invention relates to a magnetometer including several coils disposed relative to each for measuring torque related divergent magnetic fields.  
         [0003]     A non-contact force sensor includes a force transducer element that responds to the application of force by generating a magnetic field. The generated magnetic field is then detected by a magnetometer. The force transducer element typically includes a magnetoelastic material that responds to the application of force by generating a magnetic field. The application of force to the magnetoelastic material creates shear stresses within the magnetized regions causing the direction of the magnet field generated by the force transducer element to shift from a substantially circumferential direction to a helical direction. The helical shifting of the magnetic field is detected as an axial component of the magnetic field. The axial component of the magnetic field is proportional to the applied force and provides an accurate and reliable indication of torque applied to a torque element.  
         [0004]     Sensing of the magnetic field and specifically the axial components of the distortions in the magnetic field caused by force is accomplished through the use of a magnetometer or magnetic field sensor. A commonly used type of magnetic field sensor is a flux gate sensor, which is fabricated as a coil of magnet wire that generates a magnetic field to magnetically saturate a core of magnetically saturatable material. The coil is energized by an alternating current that provides periodic magnetic saturation of the core. The magnetic field produced by the force transducer is superimposed on the periodic magnetic field generated by the coils. The superimposing the magnetic field produced by the torque transducer shaft creates an asymmetry in the magnetic saturation of the coils. Changes in the inductance of the coils due to the magnetic saturation results in a voltage that is induced to the coils. It is this voltage that is measured to determine the amplitude and direction of force applied to the force transducer element.  
         [0005]     It is commonly observed within magnetoelastic torque transducers a magnetic hysteretic effect wherein the zero-torque magnetic field emanating from the transducer does not return to zero amplitude following the application and removal of a stimulus torque. It is observed that the amplitude of the magnetic hysteresis resulting from the application of a force varies with the square of the magnetic flux passing within the transducer element, as a consequence of the magnetic field generated in the transducer from this force.  
         [0006]     Consequently it is an object of the current invention to provide a means of attenuating the effects of magnetic hysteresis within the transducer through the operation of the magnetometer.  
       SUMMARY OF THE INVENTION  
       [0007]     An example magnetometer according to this invention includes first inner and outer coils that are spaced axially apart from second inner and outer coils. Between the inner and outer coils is a plurality of magnetic strips. These magnetic strips are disposed at axially distinct portions within the inner and outer coils to detect magnetic field asymmetry.  
         [0008]     The example magnetometer device according to this invention includes first and second inner coils disposed about a common axis and spaced axially apart. The first outer coil is disposed concentrically relative to the first inner coil and the second outer coil is also disposed concentrically about the second inner coil. Between the first inner coil and the first outer coil is a first plurality of magnetically saturatable elements. These magnetically saturatable elements are disposed in a parallel manner with respect to the axis. Between the second inner coil and the second outer coil is a second plurality of magnetically saturatable elements. These elements become saturated in the presence of a magnetic field.  
         [0009]     A drive circuit is connected to the inner coils and the outer coils to generate an alternating magnetic field. This alternating magnetic field selectively and alternatively magnetically saturates each of the first and second plurality of magnetic saturatable elements. The inner coils generate a magnetic field in a first direction and the outer coils generate a magnetic field in a second direction that is opposite the first direction. The opposing magnetic fields created by the inner and outer coils concentrate the emitted field into the magnetically saturable elements, and largely cancel any externally magnetic field.  
         [0010]     The magnetometer is disposed about a force-receiving element that includes a magnetoelastic material. The magnetoelastic material generates a magnetic field responsive to application of a force. This magnetic field is super-imposed on the magnetically saturatable elements and creates an imbalance in a voltage that is measured at an inner node and an outer node. The generation of a magnetic field by the magnetoelastic material causes an asymmetric magnetic field that in turn creates the voltage signal indicative of the applied force.  
         [0011]     Magnetometer accuracy is affected by magnetic hysteresis that can result in an undesirable amount of inaccuracy within the sensor assembly. Further, the magnetometer of this invention creates an artificially enhanced demagnetization field that results in a reduction of hysteresis within the torque transducer. It does this by generating a counter-magnetic field opposing that generated by the transducer so that the net magnetic field within both the magnetically saturable elements and the magnetoelastic transducer so that the magnetic flux within the transducer is kept to a minimum of amplitude.  
         [0012]     Accordingly, the magnetometer device according to this invention reduces the effects of hysteresis to provide an overall improvement and accuracy to the measurements obtained by a torque sensor assembly.  
         [0013]     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a schematic illustration of a portion of a magnetometer device according to this invention.  
         [0015]      FIG. 2  is an enlarged cross-sectional view of a portion of a magnetometer device according to this invention.  
         [0016]      FIG. 3  is a cross-section of an example coil assembly according to this invention.  
         [0017]      FIG. 4  is a schematic representation of a torque sensor assembly including an example drive circuit according to this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     Referring to  FIGS. 1 and 2 , a torque sensor assembly  10  includes a torque transducer  12 . The torque transducer  12  includes a shaft  14  disposed about an axis  20 . The shaft  14  supports a ring of magnetoelastic material  16 . The ring of magnetoelastic material  16  generates a magnetic field  18  responsive to the application of force as is indicated at  22 . A bobbin  24  is disposed concentrically about the axis  20  and also the torque transducer  12 .  
         [0019]     The bobbin  24  supports a first inner coil  36 , a first outer coil  38  and a second inner coil  40  and a second outer coil  42 . The first inner coil  36  and the first outer coil  38  are disposed within a first axial region  26 . The second inner coil  40  and the second outer coil  42  are disposed in a second axial region  28 . The first and second axial regions  26 ,  28  are separated an axial distance apart.  
         [0020]     Disposed between the first inner coil  36  and the first outer coil  38  is a first plurality of magnetically saturatable elements  44 . The magnetically saturatable elements  44  comprise a wire that includes a very high length to diameter ratio. The first plurality of magnetically saturatable elements  44  are dispersed in equal angular segments about the bobbin  24  and concentrically with the axis  20 . A second plurality of magnetically saturatable elements  46  is disposed between the second inner coil and the second outer coil  42 .  
         [0021]     The second plurality of magnetically saturatable elements  46  is also comprised of a plurality of wires that is disposed equal angularly about the bobbin  24 . The bobbin  24  itself includes a first flange  30 , a second flange  32  and middle flange  34 . The middle flange  34  provides for the separation of the first inner and outer coils  36 ,  38  from the second inner and outer coils  40 ,  42 .  
         [0022]     Referring to  FIG. 3 , a schematic representation of the example bobbin  24  illustrates the relative position of the first inner and outer coil assemblies  36 ,  38  relative to the second inner and outer coil assemblies  40 ,  42 . Note that the first inner and outer coil assemblies  36 ,  38  are disposed such that the first plurality of magnetically saturatable elements  44  are disposed there between. The second plurality of magnetically saturatable elements  46  are disposed between the second inner and outer coils  40 ,  42 . The first inner coil  36  is coupled electrically to the second inner coil  40  and generates a magnetic field orientated in a first direction. The second outer coil  42  is coupled to the first outer coil  38  and generates a second magnetic field orientated in a second direction that is opposite the first direction. The opposing magnetic field generates a much lower inductance as compared to coils that are mounted within the bobbin  24  individually.  
         [0023]     Referring to  FIG. 4 , a schematic representation of the example sensor assembly  10  according to this invention includes a drive circuit  50 . The drive circuit  50  provides the current input that excites the inner coil assemblies  36 ,  40  and outer coil assemblies  38 ,  42 . The drive circuit  50  provides an alternating current through each of the coil assemblies  36 ,  38 ,  40  and  42  to generate the desired opposing magnetic fields. As the drive alternating current alternates between peaks and valleys, so does the generated alternating magnetic field. The alternating magnetic field results in magnetic saturation of each of the pluralities of inductors  44 ,  46 . The magnetic saturation of the inductors  44 ,  46  are evenly distributed due to the orientation and magnetic fields generated by the identically configured inner coils  36 ,  40  and outer coils  38 ,  40 . This provides for the magnetic saturation of the magnetically saturatable elements in an even symmetrical manner.  
         [0024]     Upon the application of a torque  22  to the torque transducer  12 , the magnetoelastic material  16  generates a magnetic field  18 . This magnetic field  18  is in direct proportion to the application of force  22 . The magnetic field  18  generated by the magnetoelastic material  16  superimposes itself on the magnetically saturatable elements  44 ,  46 . This imposition of the generated magnetic field  18  on the magnetically saturatable elements  44 ,  46  produce asymmetric magnetic field saturation. This asymmetric magnetic field saturation is detected through the measurement of voltage as an inner node  64  and an outer node  66 .  
         [0025]     The asymmetry in the magnetic field within the saturatable magnetic elements relative to the top and bottom axial regions  26 ,  28  of the magnetometer result in a voltage output at the node  64 ,  66 . This voltage output is measured by the drive circuit  50 .  
         [0026]     Observing the voltage at the common nodes  64 ,  66  between the top and bottom axial regions  26 ,  28  generate an observed voltage waveform possessing an even order harmonics of the excitation current. The observed voltage waveform possesses phase and amplitude characteristics indicative of the amplitude of the magnetic field  18  and thus the torque applied to the shaft.  
         [0027]     A signal related to the amplitude phase of the second harmonic content of this waveform is used as an input to a feedback loop  60 . The feedback loop  60  feeds a current output to an inner amplifier and is delivered to the outer coil windings  38 ,  42 . This generates an additional magnetic field that is equal and opposite of the magnetic field  18 . This equal and opposite magnetic fields generated by the outer coils  38 ,  42  causes the magnetically saturatable elements  44 ,  46  to operate at a zero state of net flux and provides a demagnetization field that reduces the magnetic flux within the transducer element  12  such that little residual magnetism remains.  
         [0028]     The drive circuit receives a drive clock input signal at  52  that aids in driving the inner and outer coils  36 ,  38 ,  40 ,  42 . A signal  62  is input into a demodulator  58  that is in receipt of the voltage signal measured at the inner node  64  and the outer node  66 . This signal is then input into an error integrator  56  to produce the output or feedback signal that is fed back to the first and second outer coils  38 ,  42  through the feedback circuit  60 . Another output from the demodulator  58  results in an output  54  that utilizes an indication of applied force to the torque transducer  12 .  
         [0029]     Accordingly, the flux gate magnetometer of this invention includes magnetically saturatable elements  44 ,  46  driven by current from a feedback loop that generates a magnetic field in opposition to that generated by the torque transducer. This causes the saturatable elements to operate at zero net flux. This zero net flux provided by the magnetic field enhances the demagnetization of the torque transducer and specifically of the magnetoelastic material to minimize residual magnetic flux and improve and reduce magnetic hysteresis improving the accuracy of the torque transducer.  
         [0030]     Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.