Abstract:
A magnetic compass calibration system used in areas of magnetic disturbance. The invention allows the aircraft to remain in the hangar or hangar area while the aircraft&#39;s compass is compensated. The invention compensates for time variances in ambient magnetic field, as well as magnetic variances caused by hard and soft iron found in the hangar area, ramp area or aircraft.

Description:
FIELD OF THE INVENTION 
     This invention relates to magnetic compass compensation systems, and more specifically to aircraft magnetic compass compensation in areas of magnetic disturbance such as airport hangars and ramps. 
     DESCRIPTION OF THE PRIOR ART 
     A good description of prior art magnetic compass compensation systems can be found in U.S. Pat. No. 4,327,498 to Setter and Kesselring. U.S. Pat. No. 4,327,498 is hereby incorporated by reference. 
     As illustrated in FIG. 1, to avoid areas of magnetic disturbance, aircraft compasses are calibrated by moving the aircraft to a special area known as a compass rosette 2. The compass rosette 2 is located in an area away from sources of magnetic disturbance and in a magnetic field 5 which is as uniform as possible. The aircraft 1 is lined up with the compass rosette 2 and errors are checked in the compass system. For example, if the aircraft is pointing north according to the compass rosette, the difference from north is read from the aircraft&#39;s compass and is used to calibrate the compass. The compass compensation is done manually using potentiometers or with magnets located on top of the flux valve 4. The aircraft is rotated through the full 360° of the compass at each angle of 90° of the compass, namely, 0°, 90°, 180°, and 360°. Some systems allow the electrical rotation of the magnetic field to simulate the rotation of the plane. In both cases, the compass is constructed out of a magnetic flux valve 4 which measures the magnetic field 5 passing through the magnetic flux valve 4. The magnetic field 5 passing through the valve 4 is composed of the earth&#39;s magnetic field and any local fields. 
     One example of a flux valve is taught in U.S. Pat. No. 3,873,914 to Kesselring. U.S. Pat. No. 3,873,914 is hereby incorporated by reference. The magnetic flux valve is constructed using three coils offset 120° from each other. An AC signal is input to the coils and the resulting magnitude through the coil is sensed. The magnitude of the AC signal is proportional to the magnetic field at the point of the coil. 
     Aircraft compasses require calibration every six months to twelve months. The necessity to transport an aircraft away from its natural hangar environment, coupled with the frequency of calibration requirement, make compass calibration a real and significant burden on aircraft operators. 
     A magnetic compass must be compensated for either hard iron disturbances or soft iron disturbances. Hard iron disturbances traditionally include current in a wire, such as wire 21 shown in FIG. 2A, magnets 22 as shown in FIG. 2B and ferrous material 23 as shown in FIG. 2C. The current bearing wire 21 may be, for example, a wire carrying digital or analog signals or power. Another source of disturbance for a compass is caused by soft iron 25 as shown in FIG. 3, which would include some nickel alloys. Sources of hard iron and soft iron disturbances are often found in hangars, airframes and aircraft guidance systems, for example. The earth&#39;s magnetic field 14, as shown in FIG. 3A, can also change from time to time, making compass calibration difficult. 
     A third type of disturbance that effects aircraft compass calibration is called index error. Index error arises from misalignment of the flux valve coils with the earth&#39;s magnetic field. Also, the flux valve coils must be compensated for the magnetic effect that they have on each other. The measurement of the induced current in one coil will affect the measurement of induced current in another coil. 
     Prior workers in the field of compass compensation have proposed compensation systems that operate in severe magnetic field disturbances, such as illustrated by Setter and Kesselring in U.S. Pat. No. 4,327,498. Setter and Kesselring address the dynamic compensation of a magnetic compass system in a tank which is subject to varying magnetic fields as it moves. The Setter and Kesselring scheme achieves a small error goal of +3° by utilizing two flux valves subject to a similar magnetic field. One valve provides a reference with respect to the axis of the tank while the other valve measures the varying field. In regard to compensation of an aircraft compass system, the Setter and Kesselring scheme falls short. The planned for error of ±3° is almost 3000% more error than can be tolerated in an aircraft compass which may need less than ±0.1° of error. Even if the Setter and Kesselring system could be modified to achieve a smaller error signal, inherent design characteristics limit its applicability to ambient magnetic field environments where the reference and calibrated flux valves experience a similar, if not identical field, a field which is typically not available in an aircraft hangar or aircraft ramp. 
     The present invention, therefore, springs from a desire to provide a high degree of accuracy magnetic compass compensation system with less than ±1° of error, that can operate in a typical airport hangar/ramp environment and that can be used to calibrate a typical aircraft magnetic compass system. 
     SUMMARY OF THE INVENTION 
     It is one object of this invention to compensate a magnetic compass system. 
     It is a further object of the invention to compensate a magnetic compass system in a distressed magnetic field. 
     It is yet another object of the invention to compensate a magnetic compass system in a hangar or an airport ramp. 
     The invention provides a compass calibration and magnetic field measurement system wherein the system has a monitor flux valve, an aircraft flux valve, an aircraft flux valve turntable and a nonmagnetic stand. 
     The invention provides a magnetic compass calibration and magnetic field measurement system that allows the aircraft, whose compass is to be calibrated, to remain in the hangar, hangar area or ramp. A monitor flux valve measures the magnetic environment some distance from the plane. In axis with the monitor is a turntable. The aircraft&#39;s flux valve is removed from the aircraft and placed on the turntable. The magnetic environment is measured while the aircraft&#39;s flux valve is on the turntable and the aircraft is removed from the measurement area. The aircraft is then returned to the measurement area, the flux valve is installed and the magnetic environment is measured. The monitor allows compensation for any variances in the ambient magnetic environment. To improve field measurement accuracy in distressed fields, the turntable and flux valve can be placed in a position approximating the installed aircraft flux valve&#39;s position. All flux valve measurements must be made with the flux valve reference axes parallel with each other. 
     Other objects, features and advantages of the invention will become apparent through the drawings, description of the preferred embodiment and claims herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To illustrate the invention, a preferred embodiment of this invention will be described hereinafter with reference to the accompanying drawings. The preferred embodiment concerns a magnetic compass compensation system utilizing a monitor and flux valve turntable. 
     FIG. 1 shows an aircraft undergoing calibration at a compass rosette. 
     FIG. 2 shows a block diagram of one example of a compass calibration system as provided by the invention. 
     FIG. 2A shows a schematic diagram of a magnetic disturbance generated by alternating current in a wire. 
     FIG. 2B shows a magnetic disturbance generated by a magnet. 
     FIG. 2C shows a magnetic disturbance generated by ferrous material. 
     FIG. 3A shows a magnetic interference generated by the magnetic field encompassing the earth. 
     FIG. 3B shows a magnetic disturbance generated by a soft iron. 
     FIG. 4 shows a vector diagram of the compass compensation system used to compensate an aircraft compass. 
     FIG. 5 shows a basic magnetic compass calibration compensation system and FIG. 6 shows a sector diagram of an example of a compass calibration system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 6, a vector diagram of one example of a compass calibration system as provided by the invention is shown. There are four potential flux valve measurement locations in the system. The system includes a compass calibration computer 45 which can accept information from a flux valve sensor 41 and a turntable location 42. Each location is used to with the monitor flux valve to relay data to the compass calibration computer 45 about the local magnetic field. The monitor flux valve 41 is used as the reference flux valve in the system. The turntable is used in conjunction with the monitor flux valve 41 to measure the relative change in magnetic flux between where the monitor is and where the turntable is. The first is the monitor flux valve location 91 and the second is the turntable location 92. The third position 93 is located closely approximate to the position of the aircraft&#39;s flux valve 44 when the aircraft 80 is in the measuring area. The measuring area could be a hangar or in a separate measurement building. The only requirement is that the monitor flux valve location 91, turntable flux valve location 92 and pseudo plane position flux valve location 93 be in the same general area. The inputs of both flux valves are sent to the compass calibration computer 45. One such compass calibration computer which may be used in the system is available as the Honeywell MC2000. 
     Referring now to FIG. 5, a diagram of the basic magnetic compass compensation system is shown. The basic system includes a monitor flux valve 41, a transit 73, and a turntable 72. In the basic magnetic flux measurement system, the monitor flux valve 41 is mounted with the transit 73. The ambient magnetic field is measured through the flux valve in these positions. 
     The axis of the monitor flux valve 41 can be aligned using the transit 73 to spy on the swing line 62 of the aircraft 80. When the aircraft&#39;s flux valve 44 is removed from the aircraft and placed on the turntable 72, the resulting magnetic field difference stored in the compass calibration computer 45, can be used to calibrate the aircraft&#39;s flux valve 44 in the aircraft. 
     Now referring to FIG. 6, a diagram of the calibration of an aircraft compass in a hangar area is shown. The compass calibration system is used to calibrate an aircraft flux valve 44. At monitor flux valve location 91 the magnetic field is indicated by the magnetic field vector He mon  74. The magnetic field vector at the turntable flux valve location 92 is indicated by He tt  75, and the magnetic field vector at the aircraft flux valve location 93 is indicated by He fv  76. The angle subtended from the axis 86 of the flux valve 44 in the aircraft location to the magnetic field at this location is indicated by theta (8). The angle subtended from the monitor magnetic field vector 74 and the axis 82 of the monitor flux valve is indicated by beta (β). The angle subtended from the axis 84 of the turntable flux valve 42 to the magnetic field at the turntable is indicated by phi (φ). All flux valve axes, 82, 84 and 86, must be made parallel to the swing line 62 of the aircraft 60. 
     The magnetic profile is determined either at the level where the flux valve will be located in the aircraft, a special nonmagnetic stand 52 (shown in FIG. 5) may be used to reach this area, or an area that approximates the aircraft compass 44 location when installed in the aircraft. The magnitude and direction of the magnetic field at the aircraft flux valve location 93, at the turntable location 92 and the monitor location 91 may now be determined using the monitor 41. The magnetic field at the aircraft flux valve location 93 can be described by: 
     
         He.sub.fv Sinθ±He.sub.fv Cosθ, 
    
     where He fv  is the horizontal field strength through the aircraft flux valve 44 and θ is the angle subtended from the magnetic field vector to the swing line 62 of the aircraft 80 at the aircraft flux valve location 93. 
     Similarly, the magnetic field at the turntable location 92 may be determined as: 
     
         He.sub.tt Sinθ+He.sub.tt Cosφ, 
    
     where He tt  is the horizontal field strength at the turntable 72 location, and φ is the angle subtended from the magnetic field vector to the aircraft swing line 62 at the turntable location. 
     The magnetic field at the monitor location 91 may also be determined in a similar manner as 
     
         He.sub.mon Sinβ+He.sub.mon Cosβ, 
    
     where He mon  is the horizontal field strength at the monitor 50 location, and β is the angle subtended from the magnetic field vector to the aircraft swing line 62 at the monitor location. 
     The difference in the magnetic field between the monitor location 91 and the aircraft flux valve location 93 is calculated as shown below: 
     
          delta.sub.-- Sin1=He.sub.fv Sinθ-He.sub.mon Sinβ, and 
    
     
          delta.sub.-- Cos1=He.sub.fv Cosθ-He.sub.mon Cosβ. 
    
     Also, the difference in the magnetic field between the turntable location 92 and the flux valve location 93 is calculated as: 
     
          delta.sub.-- Sin2=He.sub.fv Sinθ-He.sub.tt Sinφ, and 
    
     
          delta.sub.-- Cos2=He.sub.fv Cosθ-He.sub.tt Cosφ. 
    
     Once the above delta values are determined and stored, the magnetic field at the flux valve location may be described in terms of the magnetic field at the turntable location 92 by: 
     
         He.sub.fv Sinθ=He.sub.tt Sinφ+delta.sub.-- Sin2, and 
    
     
         He.sub.fv Cosθ=He.sub.tt Cosφ+delta.sub.-- Cos2. 
    
     The magnetic field at the flux valve location 93 may also be described in terms of the magnetic field at the monitor location 91 by: 
     
         He.sub.fv Sinθ=He.sub.mon Sinβ+delta.sub.-- Sin1, and 
    
     
         He.sub.fv Cosθ=He.sub.mon Cosβ+delta.sub.-- Cos1. 
    
     As long as the delta values remain constant for a particular area, the magnetic field for one area can be determined in terms of the other. 
     The aircraft flux valve 44 characterization may now be performed at the turntable location 92 to emulate the field at the aircraft flux valve location 93. Also, any changes in the earth&#39;s magnetic field detected by the monitor flux valve 41 may be translated to the field at the aircraft flux valve 44 location and compensated. The aircraft swing line is made parallel with the turntable 72 reference axis 84 and the monitor flux valve 41 reference axis so that the reinstalled aircraft flux valve 44 will be in the proper orientation when calibrated. 
     To summarize the above, the magnitude and direction of the magnetic field are measured at each of the three locations, namely, the aircraft flux valve location 93, the turntable location 92, and the monitor location 91 by measuring the field at each location using the monitor flux valve. The delta values are then determined as previously described and the values stored in the compass calibration computer. 
     One alternative approach of the invention is to use only two positions, the aircraft flux valve position 93 and the monitor position 91. The primary reason for using the turntable position 91 is to allow the operator easy access to the aircraft flux valve 44 during characterization. 
     The aircraft flux valve 44 is now removed from the aircraft and placed on the turntable 92. The aircraft is removed from the area. The turntable 72 and tripod are located at the turntable location 92. The monitor flux valve 41 remains at the monitor location 91, pointing in the direction parallel to the swing line 62. 
     We are now ready to characterize the aircraft flux valve 44, by determining its sensitivity and crosstalk values. Sensitivity is a measure of the current in the flux valve coils that is required to cancel the ambient field. Crosstalk values are quadrature fields created in the flux valve when an electrical swing is performed by running currents through the flux valve legs. 
     The flux valve is first indexed so that the coil of the A leg is aligned with the magnetic field. This is accomplished by rotating the flux valve on the turntable at all the cardinal headings and noting the heading output from the flux valve. The deviations of the flux valve readings from the turntable indices are added and divided by four. This derivation is called the index. The flux valve readings and the index computation are done in the compass calibration computer. The flux valve is then rotated by the amount calculated by the compass calibration computer so that the index is zero. 
     A manual eight heading swing is next performed at 45 degree increments. The deviations at each heading are recorded by the compass calibration computer. Output voltages from the flux valve 10 are read and recorded at the cardinal headings. These values are used to determine the E1 and E2 voltages as follows: 
     
         E1= (voltage at 0 degrees+voltage at 180 degrees)/2 
    
     
         E2= (voltage at 90 degrees+voltage at 270 degrees)/2 
    
     These values are then used in a four heading electrical swing to determine the crosstalk values. The equations for the currents going to the flux valve legs are given below: 
     For a current servo interface: 
     
         A.sub.LEG 32 cos(30)E1 (cosψ -cos= )+E2 (sinψ -sin ∝ ) 
    
     
         B.sub.LEG =0 
    
     
         C.sub.LEG =E2 (sinψ -sin∝ ) 
    
     For a control transformer interface: 
     
         A.sub.LEG =cos(30)E1(cosψ -cos∝ ) 
    
     
         B.sub.LEG =0.5E2(sinψ -sin∝ ) 
    
     
         C.sub.LEG =0.5E2(sinψ -sin∝ ) 
    
     where ψ is the electrical swing angle, and ∝ is the position of the flux valve on the turntable, which is set to 0 degrees. 
     The reason for two sets of equations for a CT and current servo is as follows: A CT or &#34;control transformer&#34; is used in many existing compass systems as an interface in a control loop in the aircraft compass system. In this type of system, swinging currents are fed to all three legs of the flux valve, which is the normal case. 
     In a current servo system however, direct current is fed to only two legs of the flux valve. See, for example, U.S. Pat. No. 3,678,598 to Donald A. Baker, et al., entitled &#34;Compass System and Components Therefor Having Automatic Field Cancellation&#34;, which describes compensation in a current servo system. U.S. Pat. No. 3,628,593 is incorporated herein by reference. In such a system, currents are always flowing in the two coils in order to null the output of the flux valve. When an electrical swing is performed, the currents coming from the calibrator go to the same two legs used in the feedback of the current servo. This scheme is more accurate than feeding currents to all three legs. The swinging currents will precisely cancel the currents in the feedback path of the current servo, eliminating stray fields which would be difficult to quantify. 
     After the electrical swing is performed, the crosstalk values are determined by: ##EQU1## where EL is the electrical swing error at the respective heading and MAN is the heading error at the respective heading during the manual swing. These values are then used in the swing equations as follows: 
     for a current servo interface: 
     
         A.sub.LEG =cos(30)·[E1(cosψ -cosα )+Δ270·E2(sinψ -sinα ) ] 
    
     
         B.sub.LEG =0 
    
     
         C.sub.LEG =E2(sinψ -sinα )-Δ180·[cos(30)·E1(cosψ -cosα ) 
    
     for a control transformer interface: 
     
         A.sub.LEG =cos(30)·[E1(cosψ -cosα )+Δ270·E2(sinψ -sinα )] 
    
     
         B.sub.LEG =0.5·E2(sinψ -sinα)-Δ180· cos(30)·E1(cosψ -cosα) 
    
     
         C.sub.LEG =-B.sub.LEG 
    
     A second electrical swing is performed at ∝=0, for ψ values of 90 and 270 degrees. From this swing the value of EW is determined as: ##EQU2## where EL and MAN are the electrical and manual swing errors respectively. The EW value is used to correct the E value used in the swing equations. 
     
         E1C=E1(1-tan(EW)) 
    
     where E1C is the corrected value of E1. 
     Another two heading swing is performed using the corrected value of E1, this time for a ψ of 90 degrees, and ψ values of 0 and 180 degrees. From this swing, the value of NS is determined as: ##EQU3## where EL and MAN are the electrical and manual swing errors respectively. This value of NS is used to correct the E2 value used in the swing equations. 
     
         E2C=E2(1+tan(NS)) 
    
     where E2C is the corrected value of E2. Using the corrected values of E1 and E2, a four heading electrical swing is performed with ψ =90 degrees and ψ=the intercardinal headings of 45, 135, 225, and 315 degrees. The resulting data is used to determine an ICARD value as: 
     
         ICARD=[(EL.sub.45 -MAN.sub.45)-(EL.sub.135 -MAN.sub.135)+ 
    
     
         (EL.sub.225 -MAN.sub.225)-(EL.sub.315 -MAN.sub.315)]/4 
    
     where EL and MAN are the electrical and manual swing errors at the respective headings. 
     This value of ICARD is used to remove the intercardinal two cycle errors from the electrical swing currents as: ##EQU4## where E2S is a value used to determine the swing current. A value of E1S could also be similarly obtained after correcting for cardinal two cycle errors, however it has been shown that these errors are very small. The value of E1C is therefore used as a good approximation for E1S. 
     The final set of swing equations now appear as follows: 
     for a current servo interface: 
     
         A.sub.LEG = cos(30)·E1C(cosψ -cosα)+(Δ270+0.5)(E2S·sinψ-E2C·sin.alpha. ) 
    
     
         B.sub.LEG =0 
    
     
         C.sub.LEG =E2S·sinψ-E2C·sinα-Δ180[cos(30)·E1C(cosψ -cosα)+0.5(E2S·sinψ-E2C·sinα) 
    
     for a control transformer: 
     
         A.sub.LEG =cos(30)·E1C(cosψ -cosα )+Δ270(E2S·sinψ·E2C ·sinα ) 
    
     
         B.sub.LEG =0.5(E2S·sinψ-E2C·sinα)-Δ180·cos(30)·E1C(cosψ -cosα ) 
    
     
         C.sub.LEG =-B.sub.LEG 
    
     The values of E1C, E2C, Δ180, Δ270, and E2S are the characterization values for the particular flux valve being swung. If a particular aircraft has two flux valves, the above is repeated for the second valve. Before these values are used, they must be adjusted for the existing magnetic field conditions. 
     The delta values for the flux valve in terms of the turntable readings are given as: 
     
         He.sub.fv sinθ=He.sub.tt sinφ+Δsin2 
    
     
         He.sub.fv cosθ=He.sub.tt cosφ+Δcos2 
    
     Values of E1C, E2C, and E2S modified for the flux valve location as follows: 
     The total field He fv  76 is: 
     
         He.sub.fv =[(He.sub.fv sinθ ).sup.2 +(He.sub.fv cosθ).sup.2 ].sup.1/2 
    
     and similarly the total field He tt  75 is: 
     
         He.sub.tt =[(He.sub.tt sinθ ).sup.2 +(He.sub.tt cosθ ).sup.2 ].sup.1/2 
    
     The values of E1C, E2C and E2S are multiplied by the factor ##EQU5## to accommodate the field at the flux valve location. ##EQU6## 
     Also, the swing bearing at the flux valve location is determined as: ##EQU7## Before continuing, the field at the monitor 41 is read and the characterization values at the flux valve are modified as follows: 
     
         He.sub.fv &#39;sinθ=He.sub.MON sinβ+delta-sin1 
    
     
         He.sub.fv &#39;sinθ=He.sub.MON sinβ+delta-cos1 
    
     The total field is: 
     
         He.sub.fv &#39;=[(He.sub.fv &#39;sinθ ).sup.2 +(He.sub.fv &#39;cosθ ).sup.2 [.sup.1/2 
    
     The values of E1C&#39;, E2C&#39; and E2S&#39; are &#34;normalized&#34; to a standard field (i.e., about 0.18 Oersted) as follows: ##EQU8## where K is a constant representing a standard field. 
     If the direction of the magnetic field has changed, the new swing line bearing is determined. ##EQU9## 
     The new swing line bearing is then displayed, and the operator rotates the flux valve to this bearing. An optical transfer adapter is attached to the flux valve. This transfer adapter contains a telescope with which a reference target bearing can be established. The aircraft flux valve 44 and optical alignment adapter are then removed from the turntable 72. 
     The aircraft 80 is now brought into position over the swing line 62. Any misalignment of the aircraft 80 with the swing line 62 is measured and entered into the compass calibration computer 45. The computer 45 calculates the misalignment angle and compensates for this misalignment by adding an angle to the value of the swing line bearing ∝ in the swing equations. The optical alignment adapter 73 is also adjusted for this misalignment angle. 
     The aircraft flux valve 44, with the transfer adapter 73, is mounted in the aircraft 80 and the reference target sighted through the telescope 73. The aircraft flux valve 44 is secured and the telescope removed. 
     The compass calibration computer 45 is connected to the aircraft flux valve 44 and the compass system. The compass system output heading may be read out of the compass calibration computer 45, or via another readout in the aircraft 80, provided the readout is sufficiently accurate. 
     Before beginning the electrical swing and compensating the aircraft 80 compass system, magnetic field measurements are again made at the monitor 41 and the characterization values and swing line bearing modified as follows: ##EQU10## 
     These values are now used in the swing equations to perform the electrical swing. The aircraft compass system may now be compensated using well known techniques. 
     This invention has been described herein in considerable detail in order to comply with the Pat. No. Statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, can be accomplished without departing from the scope of the invention itself.