Abstract:
A single-axis, fluxgate magnetometer apparatus comprises: an excitation subassembly comprising a toroidal core wound with a predetermined number of turns of an excitation coil; and a pick up subassembly comprising a coil form including a hollow chamber, and a predetermined number of turns of a pick up coil wound on the coil form about the hollow chamber, the excitation subassembly disposed in the hollow chamber of the coil form and secured in the hollow chamber at a desired position. A method of adjusting the single-axis, fluxgate magnetometer apparatus comprises the steps of: applying an excitation signal to the excitation coil while disposed in the hollow chamber; monitoring a signal waveform of the pick up coil responsive to the excited excitation coil; adjusting the position of the excitation subassembly in the hollow chamber to effect a desired signal waveform of the pick up coil; and securing the excitation subassembly in the hollow chamber at the position that provides the desired signal waveform.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to magnetometers, and more particularly to fluxgate magnetometer apparatus and method of adjusting same.  
         [0002]     In general, fluxgate magnetometers are used as a heading reference, or an attitude gyro drift reference in inertial navigation systems for aircraft, land vehicles, ships, underwater vehicles and the like. A fluxgate magnetometer usually comprises three fluxgate sensing elements Mx, My and Mz packaged mutually orthogonal to each other as shown in the diagram of  FIG. 1 . A magnetometer for use in a strapdown inertial navigation system (INS) has its axes aligned with the principle axes X, Y and Z of the aircraft or vehicle on which it is mounted. This configuration enables the attitude of such aircraft or vehicle to be determined with respect to the Earth&#39;s magnetic Field (EMF) vector T (see  FIG. 1 ).  
         [0003]     In the diagram of  FIG. 1 , a vectors H and η are representative respectively of the horizontal and vertical components of the EMF vector T, the vector g is representative of the force of gravity, the symbol I is representative of an angle of magnetic inclination between the vectors T and H, the Greek symbols ψ, ν, and γ are representative of the magnetic heading, the aircraft/magnetometer pitch and the aircraft/magnetometer roll, respectively. A magnetometer alone can not provide an unequivocal measurement of an aircraft attitude. Measurements made with a magnetometer for strapdown INS applications define an angle between the EMF vector T and a particular axis of the magnetometer. However, this magnetometer axis may lie anywhere on the surface of an imaginary cone of a semi-angle equal to that angle about the vector T. Accordingly, an additional measurement is desired to determine attitude with respect to another reference, such as the gravity vector g.  
         [0004]     Since attitude (pitch and roll) is known or measurable, the magnetic heading ψ may be calculated using the following expression: 
 
Ψ=arctan [(− Z  sin γ− Y  cos γ)/( X  cos ν− Z  cos γsin ν+ Y  sin γsin ν)];  (1) 
 
 where: X, Y and Z are the three axis strapdown magnetometer measurements (Mx, My, and Mz) which correspond to projections of vector T on the corresponding magnetometer axes. Equation (1) illustrates that to provide highly accurate heading information, the measurements of attitude (pitch and roll) and the magnetometer measurements of the projections of the EMF vector need to be highly accurate. 
 
         [0005]     Typically, a fluxgate magnetometer sensor comprises a sensor assembly and sensor signal conditioning electronics. The sensor assembly usually includes a toroidal core of high magnetic permeability  10  (see  FIG. 2   a ) which is wound with a wire coil  12  uniformly around the perimeter thereof for a predetermined number of turns (see  FIG. 2   b ). Wire coil  12  once wound about the core  10  becomes the excitation coil of the sensor assembly. Thereafter, a second or pick up coil  14  is would about the excitation coil in order to measure the EMF projection on one axis, such as the X-axis (see  FIG. 2   c ). The pick up coil  14  is wound transverse to the axis of measurement. If additional measurements are desired, say for the Y and Z axes, for example, then additional pick up coils may be wound about the excitation coil  12 . For example, for a Y-axis measurement, a pick up coil  16  is wound transverse to the Y-axis (see  FIG. 2   d ) and likewise, for a Z-axis measurement, a pick up coil  18  is wound transverse to the Z-axis (into the page), refer to  FIG. 2   e.    
         [0006]     After the winding process is completed, several coats of varnish, for example, are applied to the sensor assembly to hold the windings in place. The excitation coil leads may be connected to an excitation circuit in the sensor electronics which may apply an excitation signal at a predetermined frequency f and waveform, which may be a square waveform at 4.5 kHz, for example. Leads of each of the pick up coils may be connected to a corresponding signal conditioning circuit which may produce a DC voltage output U that is proportional to the corresponding magnetometer axis measurement. Each output U may be defined by the following expression: 
 
 U = K ( U   2 f  cos φ+ U   q  sin φ);   (2) 
 
 where: U 2f  and U q  represent a second harmonic signal and its quadrature component, respectively, generated from the corresponding sensor pick up coil as a measure of the axis magnetic field, φrepresents a phase angle shift between the phases of signal U 2f  and the excitation signal, and K is a phase gain term of the sensor electronics. 
 
         [0007]     The sensor electronics operate to adjust the phase shift φ to substantially zero in order to provide the resultant magnetic axis measurement U with little or no quadrature component influence. However, due to a significant variation in ambient temperature on the magnetometer assembly, which may be from −55° C. to +85° C., for example, certain resistance and capacitance values of the assembly drift affecting the phase shift adjustment and causing an undesirable error influence of U q  sin φ on the magnetic field measurement U. To minimize the consequences of the temperature variation on the measurement, the quadrature component U q  should be kept as low as possible.  
         [0008]     The magnitude of the quadrature component is highly dependent on several mechanical design factors in the magnetometer assembly, like having the separate sections of pick up coil windings be identically and symmetrically distributed over the excitation coils, for example, which is not an easy task. As noted above, after the process of coil winding is completed, the entire sensor assembly is essentially encapsulated with varnish or other encapsulating material. This results in a completed sensor assembly with no mechanical adjustment capability. So, if the quadrature component of the measurement signal U is found to exceed an acceptable level, the resultant sensor assembly will be scrapped. This drawback is compounded for multi-axis sensing assemblies having two or more pick up coils wound over one core (see  FIGS. 2   d  and  2   e ).  
         [0009]     The present invention provides a fluxgate magnetometer sensor assembly design which overcomes the foregoing described drawbacks of the present design and allows for mechanical adjustment to minimize the undesirable quadrature component of the magnetic measurement which should improve the yield of sensor assemblies in the manufacturing process.  
       SUMMARY OF THE INVENTION  
       [0010]     In accordance with one aspect of the present invention, a single-axis, fluxgate magnetometer apparatus comprises: an excitation subassembly comprising a toroidal core wound with a predetermined number of turns of an excitation coil; and a pick up subassembly comprising a coil form including a hollow chamber, and a predetermined number of turns of a pick up coil wound on the coil form about the hollow chamber, the excitation subassembly disposed in the hollow chamber of the coil form and secured in the hollow chamber at a desired position.  
         [0011]     In accordance with another aspect of the present invention, a method of adjusting a single-axis, fluxgate magnetometer apparatus comprises the steps of: winding a toroidal core with a predetermined number of turns of an excitation coil to form an excitation subassembly; winding a coil form with a predetermined number of turns of a pick up coil around a hollow chamber thereof to form a pick up subassembly; disposing the excitation subassembly into the hollow chamber of the coil form; applying an excitation signal to the excitation coil while disposed in the hollow chamber; monitoring a signal waveform of the pick up coil responsive to the excited excitation coil; adjusting the position of the excitation subassembly in the hollow chamber to effect a desired signal waveform of the pick up coil; and securing the excitation subassembly in the hollow chamber at the position that provides the desired signal waveforn. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a graph illustrating three dimensional magnetometer sensing elements and attitude vectors of a moving vehicle with respect to the Earth&#39;s magnetic field vector.  
         [0013]      FIGS. 2   a  through  2   e  are illustrations of a sensor assembly currently being used for magnetometer applications.  
         [0014]      FIGS. 3   a  and  3   b  are side and profile views, respectively, of an exemplary pick up coil assembly suitable for use in an embodiment of the present invention.  
         [0015]      FIGS. 4   a  and  4   b  are side and profile views, respectively, of a single axis magnetometer sensor assembly suitable for embodying one aspect of the present invention.  
         [0016]      FIG. 5  is an isometric view of the single axis magnetometer assembly embodiment of  FIGS. 4   a  and  4   b  shown resting on its side.  
         [0017]      FIGS. 6 and 7  are exemplary pick up coil waveforms which may be viewed on a screen of an oscilloscope during adjustment of the single axis magnetometer embodiment of  FIG. 5 .  
         [0018]      FIG. 8  is an isometric view of the single axis magnetometer assembly embodiment of  FIGS. 4   a  and  4   b  shown resting top side up.  
         [0019]      FIG. 9  is an isometric view of the single axis magnetometer assembly embodiment of  FIGS. 4   a  and  4   b  showing a final assembly version.  
         [0020]      FIG. 10  is a circuit schematic of exemplary sensor conditioning electronics for use with the magnetometer sensor assembly embodiment of  FIG. 9 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     A single-axis, magnetometer sensor assembly in accordance with one aspect of the present invention includes two individual subassemblies. A first subassembly comprises an excitation coil  12  wound about a high magnetic permeability toroidal core  10  as shown in  FIG. 2   b . A predetermined number of coil turns are symmetrically distributed around the circumference of the core  10 . A second subassembly comprises a coil form  20  as shown in side and profile views in  FIGS. 3   a  and  3   b , respectively. In the present embodiment, the coil form  20  is a parallelepiped shell of special non-magnetic plastic material having a front side  22 , a back side  24  and a parallelepiped hollow chamber portion  26  which is shown in the view of  FIG. 3   b . The front  22  and back  24  sides are of a larger cross section than a center portion  28  (see  FIG. 3   a ) of the coil form shell  20  to form a raised lip around the periphery of the center portion  28  at both the front and back sides,  22  and  24 , respectively.  
         [0022]     A pick up coil  30  is wound a predetermined number of turns about the center portion  28 . The pick up coil windings  30  are held in place within the center portion  28  by the front side and back side lips. The front side  22  includes a rectangular window opening  32  to the chamber portion  26  and the back side  24  encloses the chamber portion  26 . The window  32  and chamber  26  are dimensioned to permit the excitation coil wound toroidal core subassembly to be slid through window  32  into the chamber  26  and fit snugly therein as shown in  FIGS. 4   a  and  4   b . An isometric view of an assembled sensor assembly embodiment is shown in  FIG. 5 .  
         [0023]     In the present embodiment, the front side  22  of the coil form shell  20  includes four electrical connection pin terminals a, b, c and d disposed thereon. Pins a and b are disposed respectively at the top corners of the front side  22 , and pins c and d are disposed respectively at the bottom comers of the front side  22  as shown in  FIG. 3   b . Leads  30   a  and  30   b  from the pick up coil  30  are brought from the center portion  28  through a notch  34  in the top of front side  22  and connected respectively to the terminal pins a and b, preferably by soldering (see  FIGS. 4   b  and  5 ). Leads  36  and  38  may provide external electrical connections to the pick up coil  30  via the terminal pins a and b, respectively (see  FIG. 5 ). For testing purposes, leads  12   a  and  12   b  from the excitation coil  12  may be left unconnected from the terminal pins c and d (see  FIG. 5 ).  
         [0024]     Once the excitation coil subassembly is slid into the chamber  26  as shown in  FIG. 5 , the sensor assembly is ready to be adjusted to minimize the quadrature component U q  by orienting the excitation coil subassembly within chamber  26  with respect to the pick up coil  30 . In the present embodiment, this may be accomplished by a rotation of the excitation coil subassembly while in chamber  26  as shown by the arrowed line  40  in  FIG. 4   a . The adjustment process commences with connecting the pick up coil  30  to an oscilloscope  42  via leads  36  and  38  (see  FIG. 4   b ) and connecting the excitation coil  12  to an oscillator circuit  44  via leads  12   a  and  12   b  (see  FIG. 4   a ). A resonant capacitor C res  may be connected across leads  36  and  38  in parallel with the pick up coil  30 .  
         [0025]     Next, the excitation circuit  44  is powered and adjusted to generate an excitation signal for the excitation coil  12 . The excitation signal may be a square wave at approximately 4.5 kHz, for example. The oscilloscope  42  is powered to view the voltage potential of the resultant pick up signal on a screen thereof. In the present example, the resultant pick up voltage signal is of a sinusoidal waveform. During the adjustment process, the coil form  20  may be vertically disposed, with the front side  22  facing upward, and held in a stable or stationary position. With the sensor assembly is in this orientation, the excitation core subassembly inside of chamber  26  may be rotated while viewing the waveform displayed on the screen of the oscilloscope  42 . If the adjacent positive ( 50 ) and negative ( 52 ) peaks of the resultant pick up voltage waveform are not equal in amplitude as shown in the exemplary screen view of  FIG. 7 , then, the excitation coil subassembly is rotated to render the adjacent positive ( 50 ) and negative ( 52 ) peaks of the resultant pick up voltage waveform substantially equal as shown in the exemplary screen view of  FIG. 6 .  
         [0026]     Thereafter, without altering the position of the excitation coil subassembly, the coil form  20  may be disposed in a horizontal position with pins a and c at the bottom and pins b and d at the top as shown in  FIG. 5 . While maintaining this horizontal orientation, the entire sensor assembly may be rotated until the peak to peak amplitudes of the sinusoidal waveform appearing on the oscilloscope screen are at a minimum. At this point, the orientation of the front or terminal side  22  of the sensor assembly should be approximately 90° (i.e. perpendicular) to magnetic North. While maintaining the sensor assembly at this minimum peak to peak orientation, the excitation coil subassembly, which should be completely within chamber  26 , is rotated clockwise or counter-clockwise inside chamber  26  until the oscilloscope voltage waveform is reduced to the smallest obtainable peak to peak amplitude value. The resultant measured peak to peak value should not exceed a predetermined minimum peak to peak value.  
         [0027]     Thereafter, the maximum peak to peak value of the oscilloscope waveform may be determined by repositioning the sensor assembly to an angle of approximately 45° to the horizontal with the terminal or front side  22  directed upward, and while in this orientation, rotating the sensor assembly until the non-terminal or back side  24  of the sensor assembly points toward magnetic North. While observing the oscilloscope voltage waveform, the sensor assembly is moved slightly to the left and to the right and up and down to find the maximum peak to peak value which should be at least a predetermined maximum peak to peak value.  
         [0028]     Thereafter, the sensor assembly should be moved back to the minimum peak to peak value position noted above, and it should be verified that the minimum peak to peak voltage measured at this position has not changed substantially. The sensor assembly may be re-adjusted if the measured peak to peak voltage exceeds the predetermined minimum peak to peak value. Once the sensor assembly is adjusted to the point in which the output voltage of the pick up coil has adjacent positive and negative peak amplitudes that are approximately equal, and has minimum and maximum peak to peak values that are at acceptable levels, then the excitation coil subassembly is secured in position within the chamber  26  of the coil form subassembly  20 . This may be accomplished by applying an epoxy adhesive to certain tacking points where the excitation coil subassembly is juxtaposed with the sides of the chamber  26 . Two such tacking points are shown at  56  and  58  in  FIGS. 4   b  and  FIG. 8 .  
         [0029]     Once the epoxy adhesive is cured at the tacking points  56  and  58 , leads  12   a  and  12   b  from the excitation coil  12  may be cut to length and attached, preferably by soldering, to pins d and c, respectively, and wires  36  and  38  may be removed from pins a and b shown in  FIG. 9 . Thereafter, an epoxy material may be used to cover the excitation coil subassembly in the chamber  26  of the coil form  20  and left to cure. Then, the terminal side  22  and non-terminal side  24  of the coil form may be encapsulated with the epoxy material and left to cure one side at a time. The encapsulating epoxy material should not be permitted to extend beyond the edge dimensions of the coil form shell  20 .  
         [0030]     The resultant final sensor assembly embodiment is shown in  FIG. 9 . For the present example, the external A, B and C dimensions are approximately 0.495 in., 0.150 in. and 0.417 in, respectively, and the internal or chamber A, B and C dimensions are approximately 0.415 in., 0.120 in., and 0.400 in., respectively. The raised lip around the periphery of the center portion of the front side  22  and back side  24  is approximately 0.03 in.  
         [0031]      FIG. 10  is a circuit schematic of exemplary sensor electronics  60  suitable for operating the sensor assembly embodiment described herein above. Referring to  FIG. 10 , an oscillator circuit  62  is coupled to the excitation coil  12  via pins c (lead  12   b ) and d (lead  12   a ). Circuit nodes  64  and  66  are coupled to the pick up coil  30  via pins a (lead  30   a ) and b (lead  30   b ), respectively. Across nodes  64  and  66  is coupled a resonant capacitor C 1 . Node  64  is coupled to one input of an amplifier circuit A 1  though a series combination of capacitor C 2  and resistor R 1 . Node  62  is coupled to another input of amplifier A 1  which is connected to a common or ground potential. Coupled between the output and one input of A 1  is another resistor R 2 . The output of A 1  is coupled to one input of a phase sensitive detector (PSD) circuit  70  and an output of the excitation circuit  62  is coupled to another input thereof.  
         [0032]     One output of the PSD circuit  70  is coupled through a series connected pair of resistors R 4  and R 5  to one input of an amplifier circuit A 2 . Another output of the PSD circuit  70  is coupled through a series connected pair of resistors R 6  and R 7  to another input of A 2  which is coupled to ground potential through a capacitor C 4 . The connecting node  72  between resistors R 4  and R 5  is connected to one end of a potentiometer P 1 . The other end of PI is connected to the connecting node  74  between resistors R 6  and R 7  and the adjustment arm of P 1  is connected to a voltage supply which may be +5 VDC, for example, through a resistor R 3 . Coupled between the output and one input of A 2  is a capacitor C 3 . The output of A 2  is filtered by a series combination of resistor R 8  and capacitor C 5  to produce the desired axis measurement signal U. The output of A 2  is also fed back to the input node  64  through a resistor R 9 .  
         [0033]     In operation, the excitation circuit  62  generates an excitation signal, which may be a square wave at 4.5 kHz, for example, to drive the excitation coil  12  to create an AC excitation magnetic field in the core of the sensor assembly. The external (measured) magnetic field causes the pick up coil  30  to generate a voltage potential signal across nodes  64  and  66  at a second harmonic of the frequency of the excitation signal or 9 kHz. The resonant capacitor C 1  shapes the voltage signal into a sinusoidal waveform. If the sensor assembly is oriented properly with respect to the desired axis of EMF projection, the voltage signal from the pick up coil  30  will include the desired single axis measurement of the magnetometer. The amplifier circuit A 1  amplifies the voltage signal and provides it to the PSD circuit  70 . A second harmonic signal in phase with the excitation signal is generated by the excitation circuit  62  and provided to the PSD circuit  70 . The PSD circuit  70  produces a DC signal that is proportional to the product of the aforementioned two signals provided thereto. This DC signal drives the integrator circuit comprising amplifier A 2 .  
         [0034]     The amplifier A 2  provides a feedback signal to node  64  via resistor R 9  to null the second harmonic signal created by the magnetic field presence and reduce the DC signal output from the PSD circuit  70  to substantially zero. When the output of the PSD circuit  70  is at substantially zero, the output of the integrator circuit or A 2  is at a DC voltage potential that is representative of the magnetic field component applied to the pick up coil  30 . The potentiometer P 1  may be used to adjust the output of A 2  to be at a proper DC voltage potential for zero input at steady state conditions and room temperature, for example. The filter R 8 -C 5  provides further filtering of any voltage ripple that may be riding on the DC output signal of A 2 . The resultant DC voltage potential will be representative of the desired single axis magnetometer measurement U with the undesirable quadrature error component adjusted to a minimum. For multi-axis magnetometer applications, a sensor assembly (properly oriented) and sensor electronics combination may be used for each axis of the application.  
         [0035]     While the various aspects of the present invention have been described herein above in connection with one or more embodiments, it is understood that the various embodiments were presented by way of example with no intention of limiting the present invention in any way. Accordingly, the present invention should not be limited to by the embodiments presented above, but rather construed in breadth and broad scope in accordance with the recitation of the claims appended hereto.