Abstract:
A circuit for a fluxgate magnetometer includes a plurality of digital function blocks that are programmed as functions within a microcontroller. The microcontroller includes and implements the functions of an analog circuit to lower the cost and complexity of a digital fluxgate magnetometer circuit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention is generally related to a fluxgate magnetometer for detecting a magnetic field. More particularly, this invention is directed toward a digital magnetometer implemented with a microcontroller.  
         [0002]     Non-contact torque sensors utilize a magnetoelastic material applied to a torque transducer. Application of torque to the torque transducer generates a magnetic field. The generated magnetic field is detected and converted to a usable electric signal by a magnetometer. Current magnetometers utilize a fluxgate circuit to detect and convert the generated magnetic field into a usable electric signal proportional to the applied torque.  
         [0003]     A fluxgate circuit includes a non-linear magnetic element that is contained within a coil of wire. An alternating electrical current is applied to the coil to generate an alternating magnetic field. The alternating magnetic field magnetically saturates the magnetic element twice each excitation period. An external magnetic field as is generated by the torque transducer is superimposed onto the magnetic field produced by the coil of wire causing an asymmetric saturation of the magnetic element. The asymmetry of the saturation in turn causes a voltage waveform across the coil at a frequency twice that of the excitation frequency. The amplitude and phase of this signal is used as a feedback signal to the coil. The feedback provides a stable, linear sensor response desirable for many sensing applications.  
         [0004]     Disadvantageously, conventional fluxgate circuits are comprised of several analog components that require delicate assembly and take up valuable space. Some fluxgate circuits have digitally integrated some features, however, an analog switch is retained for performing heterodyning functions, and also includes an analog buffer device to provide current feedback.  
         [0005]     Accordingly, it is desirable to develop and design a magnetometer that does not utilize analog components and that is implemented entirely on an integrated circuit microcontroller.  
       SUMMARY OF THE INVENTION  
       [0006]     An example fluxgate magnetometer circuit according to this invention includes a microcontroller providing the function of a comparator and a gate device to provide both a feedback signal to the various fluxgates utilized to detect the magnetic field and an output utilized for communicating information indicative of the sensor value.  
         [0007]     An example fluxgate magnetometer circuit according to this invention includes an integrated circuit that provides the various functions otherwise provided by way of separate analog components. A microcontroller provides the function of a voltage comparator that receives a signal from a common node from various fluxgates in magnetic communication with the coil assembly of the sensor. The voltage comparator provides a signal that is combined with a clock signal to a flip-flop gate circuit. The flip-flop gate circuit provides an output that is communicated through a digital filter and to a feedback loop. The feedback loop provides current to the fluxgates inducing them with a magnetic field such that the long-term average magnetic field within them is zero. The output from the digital filter is also provided as a function programmed into the microcontroller. The digital filter provides a conversion to a modulated signal that is used and encoded as is required for a system that utilizes output provided by the sensor.  
         [0008]     Because each of these functions otherwise requiring specific analog devices in prior art functions is comprised in a single microcontroller, the cost and complexity of the magnetometer circuit is reduced. Accordingly, the fluxgate magnetometer circuit according to this invention provides reduced size and elimination of analog components, and also allows the example circuit to be easily adapted to application specific requirements.  
         [0009]     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  
       [0010]      FIG. 1  is a schematic illustration of an example sensor and magnetometer assembly according to this invention.  
         [0011]      FIG. 2  is another schematic representation of an example sensor and magnetometer assembly according to this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]     Referring to  FIG. 1 , a torque sensor assembly  10  includes a sensor  11  comprising a torque transducer  12 . The torque transducer  12  includes a shaft  14  that supports a ring  17 . The ring  17  is comprised of a magnetoelastic material. The magnetoelastic material generates a magnetic field in response to torque  15  applied to the shaft  14 . A magnetometer circuit  24  detects the magnetic field generated by the magnetoelastic ring  17 .  
         [0013]     The magnetometer circuit  24  is shown schematically in this  FIG. 1  and includes a coil assembly  18  that surrounds the torque transducer  12  and is excited by an alternating current. The alternating current generates a corresponding alternating magnetic field. The alternating magnetic field in turn creates a magnetic saturation of a plurality of fluxgates  22 . The fluxgates  22  comprise magnetically saturatable inductors. These inductors are embedded within the coil  18  such that alternating current within the coil  18  will alternately saturate each of the inductors.  
         [0014]     In this example, there are two pairs of inductors  22 , a first fluxgate pair  23  and a second fluxgate pair  25 . Each of the fluxgate pairs are each coupled at a separate node  50 ,  52 . The first fluxgate pair  23  is coupled at the first node  50 . The second pair  25  is coupled at the second node  52 .  
         [0015]     A drive signal exerted on the coil  18  at 90°, 180° and 270° is provided. This drive signal is a periodic square-wave digital logic signal of amplitude and frequency determined and provided to saturate the fluxgates  22 . The fluxgates  22  are saturated in a negative and a positive extreme depending on the bias of the alternating current. As appreciated, the alternating current includes negative and positive peaks. At each of these peaks the fluxgates  22  are magnetically saturated.  
         [0016]     Generation of a magnetic field by the magnetoelastic ring  17  creates an imbalance in the magnetic field produced by the coil  18 . Any imbalance in the magnetic field between adjacent fluxgates  22  creates an asymmetry or imbalance at the time at which saturation occurs within each of the fluxgate pairs  23 ,  25 . It is this imbalance that results in a signal at each of the nodes  50 ,  52  between the fluxgates  22 . The signal at each of the common nodes  50 ,  52  is at a frequency that is twice that of the coil drive frequency utilized to excite the coil  18 .  
         [0017]     The signal present at each of the nodes  50 ,  52  is a voltage waveform that provides an input to the microcontroller  16 . The microcontroller  16  is programmed to provide various functions as is shown schematically in  FIG. 1 . The voltage signals from the nodes  50 ,  52  are fed to a voltage comparator  28 . The microcontroller  16  provides and operates the functions of the voltage comparator  28 . The resulting signal from the voltage comparator  28  is a square wave that is twice the frequency of the drive signal in either a positive or negative phase. The positive and negative phases are determined depending on the direction of the magnetic field generated by the torque transducer  12 . The signal output from the voltage comparator  28  is combined with a clock signal  33 . The clock signal  33  is twice that of the drive signal frequency provided for driving and exciting the coil  18 . Combination occurs at the gate  35 . Output of gate  35  is logic 0 or 1 depending on the direction of the magnetic field generated by the torque transducer  12 . This provides a detection for the direction of the magnetic field generated by the torque transducer  12  and experienced by each of the fluxgate pairs  23 ,  25 .  
         [0018]     The signal from the gate  35  is then input into a flip-flop gate  34 . The flip-flop gate  35  is clocked at four times the drive frequency with the clock edges nominally corresponding to 45°, 135°, and 310° relative to the fluxgate drive signal. As appreciated and discussed here and above the fluxgate drive signal is an alternating current such that an alternating magnetic field is produced that magnetically saturates each of the fluxgates  22 . The output of the flip-flop gate  34  is split into two.  
         [0019]     A first portion of the output is sent to a feedback loop generally indicated at  36 . The feedback loop  36  provides a current to the flux common nodes of each of the respected flux gate pairs  23 ,  25 . The current input by the feedback induces a magnetic field within each of the fluxgates such that an average magnetic field within each of the fluxgates is close to zero. In other words, the feedback provided by the feedback loop  36  is a current that produces a magnetic field equal to that magnetic field that is generated equal and opposite to that magnetic field that is generated by the torque transducer  12 . The equal and opposite magnetic field is an accurate representation of the actual torque provided with the magnetic field generated by the magnetoelastic ring  17 . Accordingly, the output of the flip-flop gate  54  is essentially the magnetic field that is seen by each of the fluxgates  22  responsive to application of torque to the torque transducer  12 .  
         [0020]     The second portion is passed to a digital filter  46 . The digital filter  46  receives the same signal from the flip-flop gate  34  that is input to the feedback loop  36 . The digital filter  46  is integrated within the microcontroller  16 . The digital filter converts the signal from the flip-flop gate  34  into the form and recording that is required by a system to utilize data gathered from the sensor. The signal output from the digital filter  46  may be in the form of impulse modulated frequency or other type of signal modifications that are utilized to accurately generate data representative of physical parameters measured by the sensor assembly.  
         [0021]     As appreciated, the various functions described with reference to the comparator  28 , the exclusive or logic gate  35 , the flip-flop gate  34  and the digital filter are all integrated as software in the microcontroller  16 .  
         [0022]     Referring to  FIG. 2 , another torque sensor assembly  52  according to this invention is shown. The torque sensor assembly  52  according to this invention includes a first fluxgate  56  and a second fluxgate  58  that are coupled at a common node  60 . The fluxgates  56 ,  58  are also disposed in a parallel fashion with a first resistor  64  and a second resistor  62 . The output from the node  60  is fed to a first input of the comparator  28 . A second output is produced and fed through the first resistor  64  into the second input of the comparator  28 .  
         [0023]     This is an alternate simpler configuration utilizing only a first fluxgate  56  and a second fluxgate  58 . The fluxgates  56  and  58  are driven by a squared current signal that is applied to the coil  18 . This square wave signal provides for the alternate magnetic saturation of the fluxgates  56 ,  58 . A reference circuit is provided that consists of two resistors connected in series across the two complementary drive amplifiers. The voltage comparator  28  compares the voltage signals of the common node  60  of the common node  63  of the resistors  64 ,  62 . The comparator output signal is then combined with a signal twice the frequency of the fluxgate drive in an exclusive gate as indicated at  35 . The signal  33  is the clock signal and is twice the frequency of the fluxgate drive signal.  
         [0024]     The output from the exclusive logic gate  35  is input into the flip-flop gate  34  that is clocked at four times the drive frequency with the clock edges nominally corresponding with 45°, 135°, 225°, and 315° with respect to the fluxgate drive signal. The output of the flip-flop gate  34  is then digitally buffered within the digital filter  46 .  
         [0025]     As in the previous example embodiment, the signal from the flip-flop  34  is sent to the digital filter  46  and is also provided to a feedback circuit  36 . The feedback circuit or loop  36  provides for additional current to the fluxgates to induce with additional current that provides a zeroing of the magnetic field. The current provides for an equal and opposite magnetic field generation with respect to the magnetic field generated by the magnetoelastic ring. Accordingly, this example utilizes only a single pair of fluxgates in combination with a pair or resistors  64 ,  62  simplify the circuit and components of the microcontroller  55 .  
         [0026]     Accordingly, this invention utilizes a single low cost microcontroller that stands alone and does away with the use and requirement of using analog and digital converters. Further, the microcontroller utilized in this invention can be reprogrammed for each application for which a magnetometer is required. Therefore, the magnetometer circuit of this invention is capable of being utilized for many applications and for measuring magnetic fields for differently configured torque sensors.  
         [0027]     Further, as no analog circuit components are used in the power consumption required by the microcontroller is significantly reduced and the micro-controller can also be programmed to add additional signal processing as is required to fully utilize the capabilities of the microcontroller that are otherwise not available in analog magnetometer circuits.  
         [0028]     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.