Abstract:
A fluxgate magnetometer drive circuit includes a fluxgate inductor that is driven through magnetic saturation by altering voltage pulses. Following each drive pulse, the spurred magnetic energy in the fluxgate is allowed to dissipate by connecting a fluxgate across a fixed reverse voltage. The time for the fluxgate current to decay to zero is measured and is indicative of a magnetic field induced in the torque transducer.

Description:
BACKGROUND OF THE INVENTION 
   This invention is generally related to a fluxgate magnetometer for detecting an external magnetic field. More particularly, this invention is directed toward a circuit for driving and controlling a flux gate magnetometer device. 
   Non-contact torque sensors utilize a magnetoelastic material affixed to a torque transducer. Application of torque to the torque transducer generates a magnetic field that is detected and converted to a usable electric signal by a magnetometer. Current magnetometers utilize fluxgates to detect and convert the generated magnetic field into a usable electric signal proportional to the applied torque. 
   Typically, a fluxgate magnetometer operates by observing magnetically induced changes of impedance of a fluxgate inductor. The fluxgate inductor is driven to magnetic saturation by an alternating current while observing an output voltage. 
   Disadvantageously, the resulting output voltage is not necessarily a direct liner relationship with the magnetic field produced by the torque sensor. Further, temperature fluctuations can cause undesirable fluctuations in signals indicative of an applied force. 
   Accordingly, it is desirable to develop and design a fluxgate magnetometer circuit that provide a stable, proportional, and temperature independent method and device for measuring changes in magnetic field. 
   SUMMARY OF THE INVENTION 
   An example magnetometer according to this invention includes at least one magnetically saturatable inductor that is driven by a circuit that energizes the conductor such that it becomes magnetically saturated and once magnetically saturated removes any drive current to allow the inductor to drain. 
   The drive circuit according to this invention magnetically saturates the inductor and once the inductor is magnetically saturated, removes the drive current to allow the drain of current from each of the inductors. The duration in which the inductor drains current is then measured and is used to determine the amplitude and magnitude of force applied to the torque transducer. 
   The magnetometer circuit according to this invention is used along with a torque sensor including and utilizing a magnetoelastic material. The magnetoelastic material produces a magnetic field responsive to an applied force or torque. The magnetic field produced by the magnetoelastic material impinges on the saturatable inductor. The impinging magnet field causes a change in the duration of time in which current drains from each inductor. Measuring the change in the duration of time in which the inductor drains current provides an accurate stable and temperature independent measurement of the impinging magnetic field. With this accurate and stable measurement of the magnetic field, an accurate and substantially linear relationship can be determined to provide accurate, reliable force measurements. 
   Accordingly, the flux gate magnetometer circuit according to this invention provides a stable proportional and temperature independent method and device for measuring changes in a magnetic field and thus the force applied to an accompanying force transducer. 
   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 
       FIG. 1  is a schematic illustration of a force-sensing device according to this invention. 
       FIG. 2  is a schematic representation of current outputs during operation of the torque of the force sensor according to this invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a torque sensor assembly  10  is schematically shown and includes a torque transducer  12  disposed within a coil  14 . The torque transducer  12  includes a ring of magnetoelastic material  18 . The ring  18  of magnetoelastic material generates a magnetic field schematically shown at  24  in response to the application of torque schematically indicated at  22  about axis  20 . The magnetic field  24  created by the application of torque  22  impinges upon fluxgate inductors  16 . The fluxgate inductors  16  are in turn driven by a drive circuit  26 . The drive circuit  26  provides for energization of the coil  14  to create an alternating magnetic field that magnetically saturates each of the inductors  16 . 
   A voltage source provided by the circuit  26  is connected across the inductor  16 . The connection of the voltage source connected across the inductors  16  is a proportional applied voltage that is inversely proportional to the inductance of the inductors  16 . At a point at which a current induced within the inductors  16  is sufficiently saturated to the core, the permeability of each of the fluxgate inductors  16  drops towards unity. This drop consequently provides the drop in the inductance of the inductors  16  as well. At the point of magnetic saturation, current ramps up increasingly due to the reduced impedance at the inductor  16 . 
   Once the inductor  16  is magnetically saturated the circuit  26  reverses current to the conductor  16 . This provides for the rapid drop in current from the inductor  16 . The saturated core and dropping current result in an abrupt increase in an initial non-saturated current value. Then the inductor  16  current ramps down linearly as the stored energy in the inductor  16  is returned to the voltage source. Because the saturation of the inductor  16  is affected by the magnetic field  24  produced by the torque transducer  12 , a time between the initial high current value and the reduction to zero current is used to measure the magnitude and direction of the magnetic field  24 , and thus the magnitude and direction of the applied force  22 . 
   The circuit  26  drives the inductor  16  by way complementary N and P channel MOSFETS  28 ,  30 . The circuit includes a first P channel MOSFET  28  and a second N channel MOSFET  30 . Each of the fluxgate inductor  16  includes a P channel MOSFET  28  and an N channel MOSFET  30 . Also disposed parallel to the inductor  16  are capacitor  34  and resistor  32 . 
   During operation the P channel MOSFET  28  is driven to an on state while the N channel MOSFET  30  is also driven to an on state. The on states of both of the MOSFETs  28 ,  30  places a desired voltage across each of the inductors  16 . After the inductor  16  is completely and magnetically saturated both the N channel MOSFET  30  and the P channel MOSFET  28  are driven to an off position. While the N channel MOSFET is switched off and the P channel device MOSFET  28  is switched on, the stored energy in the fluxgate results in a current flow through the body diode of the N channel MOSFET  30 . The P channel MOSFET  28  also experiences such a current flow. This places a negative voltage across each of the inductors  16 . This negative current flow causes a current to ramp toward a zero value. As the inductor  16  current crosses a zero value and begins to flow in reverse, the body diode of the N channel MOSFET  30  recovers and includes a high impedance. At this instance, the voltage of the drain to the end channel MOSFET  30  begin to rise towards the desired voltage. The signal is then used to toggle a flip-flop device that controls the drive of the N channel MOSFET to turn it on. This, in essence, turns on both the N channel MOSFET  30  and the P channel MOSFET  28  to renew the cycle of saturation and draining of each of the fluxgate inductors  16 . 
   Referring to  FIG. 2 , this alternating and periodic wave form is illustrated schematically by a current value emitted by the P channel MOSFET as illustrated at  40  in view of the current values driven by the N channel MOSFET at  42 . These are also shown in reference to current at the inductors  16  as indicated at  44  and the current drain voltage that is output from the MOSFETs as is indicated at  46 . 
   The presence of the magnetic field  24  results in an asymmetrical saturation of each of the inductors  16 . This asymmetrical saturation causes a different pulse within the N channel MOSFET  30 . By low pass filtering and amplifying to the differential voltage between these drive signals, a signal related to the incident magnetic field  24  at the fluxgate inductor  16  is formed. If the supply voltage is used as a reference for an analog to digital converter processing, the magnetometer output voltage, the system is immune from errors resulting from a precision or drift in the supply voltage. 
   Accordingly, a drive circuit for a magnetometer according to this invention provides for the accurate measurement of a magnetic field that is independent, temperature stable and substantially linear providing for an increased accuracy in the measurement of an applied force. 
   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.