Abstract:
The magnetometer assembly according to this invention includes a first coil assembly and a second coil assembly for detecting separate magnetic fields along an axial portion of a torque transducer. The first and second coil assemblies are driven by independently adjustable circuits. Each of the circuits are independently adjustable to adjust the gain that is distributed to each of the first and second coils. This provides for an adjustment and precise alignment of a magnetic field generated by the first coil and the second coil assembly such that external magnetic fields can be cancelled providing increased accuracy and reduced hysteresis effects.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to U.S. Provisional application No. 60/707,635 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, magnetoelastic type torque sensor typically includes a torque transducer element that responds to the application of torque by generating a magnetic field. Such generated or changed magnetic fields are detected by a magnetometer. The torque transducer element typically includes a magnetoelastic material that responds to the application of torque by generating a corresponding magnetic field. The application of torque to the magnetoelastic material creates shear stresses within the magnetized regions causing the direction of the magnet field generated by the torque 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 torque 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 torque is accomplished through the use of magnetic field sensors. A commonly used type of magnetic field sensors is a flux gate sensor, which is fabricated as a coil of fine wire surrounding a core of magnetically saturatable material, and is supplied with an alternating current. The alternating current provides for the periodic magnetic saturation of the magnetic elements. The magnetic field produced by the torque transducer shaft is superimposed on the periodic magnetic field generated by the coils. 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 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 torque applied to the torque transducer element.  
         [0005]     A known prior art magnetic field sensor includes a bobbin having upper and lower axial sections separated by a central flange. The upper and lower coils are isolated from each other and are induced with an alternating current to produce a magnetic field. Magnetically saturatable strips are disposed between the coil and the torque transducer element. These magnetic strips are magnetically saturated by the alternating current that is produced within the coils. The magnetic strips are disposing parallel to the shaft and the axis of rotation. The magnetic strips are fabricated from a material that possesses a very abrupt magnetic saturation characteristic, meaning that the magnetic strips are saturatable through a small change in applied magnetic field and in the absence of the magnetic field quickly demagnetize.  
         [0006]     Disadvantageously, manufacturing tolerances of each of the flux gates and magnetic asymmetries of the torque transducer and magnetoelastic materials can result in an incomplete cancellation of the external magnetic fields such as the earth&#39;s field effects.  
         [0007]     Accordingly, it is desirable to design and develop a device and method for independently adjusting the sensitivities of each flux gate sections to permit precise cancellation of the effects of external magnetic fields.  
       SUMMARY OF THE INVENTION  
       [0008]     An example magnetometer according to this invention includes a first coil assembly and a second coil assembly disposed about a common axis and spaced an axial distance from the first coil. The first coil and the second coil are driven by a magnetometer circuit that includes a first circuit for driving the first coil and a second independent circuit for independently driving the second coil.  
         [0009]     The first coil assembly includes an inner coil and an outer coil and a first plurality of magnetically saturatable elements disposed therebetween. The second coil assembly includes a second inner coil and a second outer coil and a second plurality of magnetically saturatable elements disposed therebetween.  
         [0010]     The first circuit and the second circuit are driven independently of each other and also include an independently adjustable gain. The gain of the first circuit and the second circuit may be adjusted independently of each other so that the transducer is given the ability to reject the effects of common mode signals or external magnetic fields such as those generated by the earth&#39;s magnetic field. The particular phasing of the first and second circuits generates drive signals to their respective first coil and second coils that are separated by 90°. This separation causes a ripple current at each of the first coils and the second coils that are out of phase with each other. This out of phase ripple current attenuates the ripple that is present at the circuit power supply.  
         [0011]     Accordingly, the magnetometer of this invention provides dual identical circuits that drive different fluxgates about a torque transducer that are independently adjustable to accommodate differences caused by manufacturing tolerances or other external and internal inconsistencies to provide for the effective cancellation of the effects generated by external magnetic fields.  
         [0012]     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  
       [0013]      FIG. 1  is a schematic illustration of a coil assembly according to this invention.  
         [0014]      FIG. 2  is a schematic representation of a magnetometer circuit according to this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     Referring to  FIGS. 1 and 2 , a torque sensor assembly  10  according to this invention includes a torque transducer  12  comprised of a shaft  14  disposed about an axis  24 . Affixed to the shaft  14  is a ring  16  of magnetoelastic material. The magnetoelastic material generates a magnetic field  18  in response to the application of a torque  26 . As appreciated, although a torque transducer is illustrated other force sensors using magnetoelastic material to generate magnetic fields responsive to the application of torque would also benefit from the disclosures of this invention.  
         [0016]     Disposed concentrically about the torque transducer  12  is a bobbin  28 . The bobbin  28  includes an upper region  30  and a lower region  32 . The upper region  30  and the lower region  32  are separated and spaced apart axially by a middle flange member  34 . Each of the first region  30  and the second region  32  includes a coil assembly. The upper region  30  includes a first coil assembly  31  that comprises a first outer coil  40  that is disposed about a first inner coil  42 .  
         [0017]     Disposed between the first outer coil  40  and the first inner coil  42  is a plurality of magnetically saturatable elements  44 . The magnetically saturatable elements  44  comprise a plurality of thin axially orientated strips of magnetically saturated material such as wire that are equal angularly distributed about the circumference of the first inner coil  42 . The first inner coil  42  and the first outer coil  40  include an equal number of windings that are comprised of a common grade or size of wire. The first inner coil  42  is wound in a first direction such that it generates a magnetic field having a first orientation. The first outer coil  40  includes an equal number of windings and is wound in a direction opposite that of the direction of the first inner coil  42  such that it generates a magnetic field that is equal and opposite to the magnetic field generated by the first inner coil assembly  42 .  
         [0018]     A second coil assembly  33  includes a second inner coil  38  and a second outer coil  36  disposed and spaced apart an axial distance from the first inner and outer coils  40 ,  42 . The second inner and outer coil assemblies  38 ,  36  are also of substantially equal and opposite configurations to generate magnetic fields that are opposed yet equal. The second inner and outer coils  36 ,  38  include a second plurality of equal angularly magnetically saturated elements  46 . As appreciated, this configuration provides for identical first and second coil assemblies to detect changes or magnetic fields generated in a first portion  20  and a second portion  22  of the torque transducer  12 .  
         [0019]     The first inner and outer coils  40 ,  42  and the second inner and outer coils  36 ,  28  generate outputs that are differentially summed to subtract the common mode effects caused by exposure to the earth&#39;s magnetic field. It is desirable for the magnetic fields generated by the magnetically saturatable elements and the outputs generated by the first and second coil assemblies  31 ,  33  to be equal and opposite such that they cancel out external magnetic fields such as those produced by the earth&#39;s magnetic field effects.  
         [0020]     However, manufacturing tolerances and other inconsistencies can cause undesirable asymmetric magnetic saturations which can errantly be determined and read as application of torque to the torque transducer  12 . It is therefore desirable to include a system or a means of independently adjusting the first coil assembly  31  and the second coil assembly  33  to account for relative inconsistencies such that each axial region  30 ,  32  of the torque transducer  12  are applied equally such that opposing magnetic fields cancel out any external magnetic fields in the absence of the application of torque.  
         [0021]     Referring to  FIG. 2 , the first and second coil assemblies  31 ,  33  are driven by a magnetometer circuit  50 . The magnetometer circuit  50  includes a first circuit  52  and a second circuit  54  that are independent of each other. The first circuit  52  is attached to drive and receive signals from the first coil assembly  31  and the second circuit  54  is attached to communicate and drive the second coil assembly  33 .  
         [0022]     The first circuit  52  includes a Schmidt trigger RC clock oscillator  51  that provides a time-based reference for both the first and second circuits  50 , 52 . The frequency from the clock oscillator  51  is divided by a d-flip-flop  56  to provide a pair of complementary square wave outputs at half the reference clock frequency. This frequency from the d-flip-flop  56  is fed to input flip-flops  58  and  60 . These divide the single frequency an additional time to yield quadrature outputs from the two respective flip-flops  58 , 60 . The complementary square wave output is then used to drive MOSFETS  62  and  64 . These provide low impedance complementary voltage square waves with which to drive the first coil assembly  30 .  
         [0023]     Terminals  66 , 68 , 70 , 72  correspond to the start of the windings for the first coil assembly  31 . Resistors  74  and  77  are placed in series with the terminals  70  and  72  to provide for monitoring of the current flow through the plurality of magnetically saturatable members  44 . A large value capacitor  78  connects the opposite terminals  66  and  68  of the outer and inner coils  40 , 42  to provide a low impedance path for alternating current flow of the excitation voltage.  
         [0024]     The polarity of the inner coil  40  and the outer coil  42  is such that the magnetic field resulting from the alternating current excitation is concentrated into the plurality of magnetically saturatable members  44 . This results in a periodic magnetic saturation at positive and negative current feeds. The saturation of the magnetically saturatable members  44  results in a change in the inductance that affects the impedance. The saturation of the magnetically saturatable members  44  further results in a distortion of voltage present at the common node of the inductor and the sense resistor  74 .  
         [0025]     Magnetic fields present at the magnetically saturatable members  44  are superimposed on the alternating magnetic field generated by the coil assembly  31 . This creates an asymmetry in the duration and saturation of the saturatable members  44  between the positive and negative half cycles of the citation waveform. While the normal waveform at the resistor  74  will be completely comprised solely of odd order harmonics of the excitation waveform, asymmetric saturation due to the non-zero magnetic field will result in the presence of an even order harmonics. These even order harmonics are detected through the use of a switching demodulator and operated at a frequency double that of the excitation frequency. The output of the demodulator will contain a DC term proportional to the even order harmonic content of the sample resistor voltage. This is then integrated in an op-amp  86  and this output buffered through a second op amp  88  through a desired gain.  
         [0026]     The two op-amps  86 ,  88  possess complimentary outputs centered around 2.5 volts. These outputs connect through current limiting resistors  75  and  76  to the terminals  66 ,  68  opposite the driving signals. The current through the resistors  75 ,  76  and through their respective windings serves to create a steady state magnetic field that exactly opposes the infinite field that results in the flux imbalance in the fluxgate conductor. The voltage present at the op-amp output  86  being proportional to the current necessary to balance the infinite magnetic field is therefore also proportional to the torque applied to the transducer  12 . The even order components resulting from the saturation imbalance are a small fraction of the sample signal that is comprised primarily of odd order harmonic components.  
         [0027]     Accordingly, this provides an input current through the op-amp  88  of a large amplitude and odd order harmonic. In this invention, the circuit is provided and comprised of capacitors  90  and  92  along with the resistor  77  which in conjunction with the circuit comprised of resistor  74 , the inductance of the fluxgate winding and the series resistance of the fluxgate coil form a bridge circuit where the fluxgate sample voltage waveform is largely duplicated at the common node of capacitor  90  and resistor  77 . Because these components are linear, odd order components of the drive wave form and if the values of capacitor  90  and resistor  77  are properly selected can be made to largely resemble that of the fluxgate voltage. By employing a differential integrator circuit, the effects of the odd order components present at the fluxgate are largely eliminated.  
         [0028]     The second circuit  54  is a duplicate of the first circuit  52  so that the ability is provided to measure the magnetic field and torque on the second half of the torque transducer  12 . The gain of the two circuits  52 ,  54  may be adjusted independently so that the transducer is given the ability to reject the effect of common mode signals such those generated and created by the earth&#39;s magnetic field.  
         [0029]     Accordingly, the configuration and application of a first circuit and a second circuit according to this invention provides for the adjustment of gain along different portions of a torque transducer such that the effects of external magnetic fields can be eliminated. Further, the adjustment of independent circuits along the torque transducer provides for the accommodation of inconsistent manufacturing assembly of magnetometer components.  
         [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.