Abstract:
A method of calibrating a magnetoelastic force sensor includes the steps of mating a force transducer with a magnetometer, applying a force to the force transducer at each of a plurality of defined calibration points, recording output signals indicative of a magnetic field generated at each of the defined calibration points communicated to each of the at least two channels, and determining a correction factor for each of the at least two channels based on the recorded output signals for each of the defined calibration points.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     The application claims priority to U.S. Provisional Application No. 60/708,063, which was filed on Aug. 12, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention generally relates to a method of calibrating a magnetometer for a force sensor. More particularly, this invention relates to a method of calibrating a magnetometer for a force sensor for decreasing variability.  
         [0003]     A type of force sensor includes a transducer element that includes a magnetoelastic material containing two adjacent, oppositely circumferentially magnetically polarized axial regions, that each produces a magnetic field responsive to an applied force. This magnetic field is divergent in nature, for detection by a magnetometer circuit configured as a magnetic gradiometer. The generated magnetic field is then detected by the magnetometer that provides an output signal indicative of an applied force, and provides a minimal sensitivity to non-divergent extraneous magnetic fields, such as that of the Earth.  
         [0004]     A known magnetometer for application with such a force transducer includes independent magnetometer sections corresponding to an upper and lower axial section of the force transducer. The voltage difference of these two outputs provide the gradiometric senor output. Disadvantageously, although the individual magnetometer sections are produced to the same specifications, some differences occur and therefore can cause an asymmetrical sensitivity of the magnetic sense elements allowing non-zero sensitivity of the sensor to non-divergent magnetic fields. Such a phenomenon reduces the reliability and accuracy of the sensor. Further, hysteresis present within the magnetoelastic element may also prevent the transducer from returning to an original zero point after the application and subsequent removal of a force stimulus, also disrupting and reducing sensor accuracy.  
         [0005]     Accordingly, it is desirable to design and develop a method of calibrating a magnetometer that highly attenuates the sensitivity of the sensor to unwanted, extraneous magnetic fields.  
         [0006]     It is also desirable to design and develop a method of calibrating a force sensor magnetometer that corrects for hysteresis that may be present within the magnetoelastic sense element.  
       SUMMARY OF THE INVENTION  
       [0007]     An example method of calibrating a magnetoelastic force sensor according to this invention includes the step of mating a force transducer with a magnetometer, applying a force to the force transducer at each of a multiple of defined calibration points, recording output signals indicative of a magnetic field generated at each of the defined calibration points communicated to each of the at least two channels, and determining a correction factor for each of the at least two channels based on the recorded output signals for each of the defined calibration points.  
         [0008]     An example torque sensor assembly calibrated according to the method steps of this invention includes a torque transducer with a magnetoelastic ring. The magnetoelastic ring produces a divergent magnetic field responsive to the application of torque. A magnetometer assembly includes at least two sense elements disposed adjacent to the torque transducer.  
         [0009]     The method includes the initial step of mating the force transducer with the magnetometer. The magnetometer includes at least two channels that receive the signals indicative of the magnetic field generated by the force transducer responsive to application of force. A series of known forces are applied to the torque transducer and recorded as calibration points. The calibration points are indicative of a magnetic field generated by the magnetoelastic ring. The gain of each of the channels is then matched so that when they are summed there is no sensitivity to ambient magnetic fields. Calibration coefficients are then determined for each channel such that the ratio between the gain in the channels is equal to a ratio between differential voltages obtained with the torque transducer assembly pointing sequentially toward a north and south polar, non-divergent magnetic field.  
         [0010]     Subsequently coefficients used for compensation for system hysteresis are calculated based on measured hysteresis of the system measured as the shift in zero-force output of the system prior to and after application of a stimulus force.  
         [0011]     Temperature compensation of the sensor system is also provided by allowing these coefficients to be modified according to the measured value of an associated temperature sensor.  
         [0012]     Accordingly, the method according to this invention provides for improved accuracy of a force sensor assembly and magnetometer.  
         [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 an example torque transducer according to this invention.  
         [0015]      FIG. 2  is a graph illustrating and example relationship between an applied force and an output voltage for an example torque transducer.  
         [0016]      FIG. 3  is a schematic illustration of example method steps for calibrating a torque transducer according to this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]     Referring to  FIG. 1 , a torque sensor assembly  10  is schematically shown and includes a torque transducer  12  disposed about an axis  18 . The torque transducer  12  includes a shaft  14  with a magnetoelastic ring  16 . The magnetoelastic ring  16  produces a magnetic field  15  responsive to the application of torque on the shaft  14 . A magnetometer assembly  11  includes an inductor  21  disposed adjacent the torque transducer  12  that is magnetically saturated by a coil assembly. The coil assembly includes upper inner and outer coils  25 ,  27  and lower inner and outer coils  24 ,  26 . The inner coils  25 ,  24  are configured to generate a magnetic field equal and opposite to a magnetic field generated by the outer coils  26 ,  27 .  
         [0018]     A controller  36  energizes the coils  24 ,  25 ,  26 ,  27  with an alternating current to generate an alternating magnetic field. The alternating magnetic field causes a magnetic saturation of the inductor  21 . When a torque is applied to the torque transducer  12 , the generated magnetic field  15  is superimposed on to the inductor  21 . The superimposition of the magnetic field  15  causes an asymmetry in magnetic fields between upper coils  25 ,  27  and the lower coils  24 ,  26 . The asymmetry is detected as a voltage signal across nodes  28 ,  30 . The voltage signals  32 ,  34  are then utilized to determine a magnitude of applied torque.  
         [0019]     Accurate operation of the torque transducer assembly  10  depends on the alignment and calibration of the coil assembly with the torque transducer  12 . A method according to this invention provides for the accurate calibration of the torque transducer to the coil assembly and the controller  36 . This is accomplished by mating the torque transducer  12  with the controller  36  and then determining a series of calibration coefficients.  
         [0020]     Referring to  FIG. 2 , calibration of the torque transducer  12  provides for the accommodation of hysteresis in the sensor assembly.  FIG. 2  is a graph representing a relationship  48  between an applied force  58 , and a voltage output  56 . The application of a force in a first direction provides a relationship between force and output indicated by line  50 . The release of force from a high point results in another relationship indicated at  52 . A gap  54  between the relationship for the application of force  50  and the release of force  52  can cause undesirable inaccuracies. However, this gap  54  can be calibrated and accommodated by the method according to this invention.  
         [0021]     Referring to  FIG. 3 , the method includes the initial step of mating the force transducer  12  with the magnetometer  11  as indicated at  60 . The magnetometer  11  includes at least two channels  33 ,  35  that receive the signals  32 ,  34  indicative of the magnetic field  15  generated by the force transducer  12  responsive to application of force. A first known force  20  is applied to the torque transducer  12  in a first direction as is indicated at  62 . This provides a calibration point. In this example the first force comprises a full-scale positive torque applied to the torque transducer  12 . The first force  20  is then released and the torque transducer  12  allowed to move back to a zero position as indicated at  64 . A voltage output is recorded at this zero-force point as another calibration point. A second force  22  is applied to the torque transducer  12  in a second direction opposite to the first direction and another calibration point is recorded as is indicated at  66  and  68 . In this example, the second force  22  is a full force in a negative torque direction. The calibration points are voltage values that are indicative of a magnetic field generated by the magnetoelastic ring  16 . The calibration points also reveal any difference that may be present between the actual applied force value  20 ,  22  and the actual reading obtained from the torque transducer.  
         [0022]     The gain of each of the channels  33 ,  35  can then be matched so that when they are summed there is no sensitivity to ambient magnetic fields. Output values are obtained with the torque transducer assembly  10  facing a magnetic north pole  40  and a magnetic south pole  42  ( FIG. 1 ) as indicated at  70  and  72 . Accordingly, calibration points are required for pointing the torque transducer assembly  10  toward magnetic north  40  and taking a calibration point. Further, the torque transducer  12  is then pointed in a direction indicative of magnetic south  42  and another calibration point determined. The calibration points are determined as an output value for each of the outputs  32  and  34  from each of the nodes  28 ,  30 .  
         [0023]     A correction factor or bias is then determined as indicated at  74  such that the ratio between the gain in the channels  33  and  35  is equal to a ratio between differential voltages obtained with the torque transducer assembly  12  pointing toward the north  40  and south  42 . That is a gain for each of the two channels  28 , 30  is set such that a ratio between the first channel  28  and the second channel  30  is equal to a ratio between an output value with the torque transducer assembly  10  facing north and an output value with the torque transducer facing toward the south magnetic pole  42 .  
         [0024]     The method also includes the step of determining a hysteresis value based on the calibration values obtained from the first and second forces  20 ,  22  as is indicated at  76 . This is accomplished by determining a span between output values  32 ,  34  for each calibration point received by each of the two channels  33 ,  35 . Utilizing the span, a calibration coefficient or correction value is determined as a percentage of the span. The determination of the hysteresis correction value includes combining the span with a backlash value indicative of a difference between a hysteresis-containing signal and a desired output value.  
         [0025]     The hysteresis correction values are determined using known mathematical compensation techniques such as Prandt-Ishlinskyi Operators. As appreciated, the specific mathematical techniques for determining the hysteresis correction factors are application specific and tailored to the specific torque transducer assembly  10 .  
         [0026]     The method also includes determination of a temperature coefficient of the sensor system. The determination of temperature coefficient provides a correction factor to accommodate operation at varying temperatures and the effects that such temperature changes have on output voltages to the channels  33  and  35 . Temperature compensation values are determined by obtaining temperature values at known time intervals along with voltage values. A thermal correction factor is then determined utilizing known relationships between temperature, resistance and voltage and applied to the outputs  32 ,  34 .  
         [0027]     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.