NO132883B - - Google Patents

Abstract

Device for compensation of disturbing magnetic fields in a means of transport.

Description

The invention relates to a device for compensating disturbing magnetic fields in a means of transport, in particular an aircraft, which is equipped with a magnetometer for measuring the field strength of an external magnetic field, e.g. the earth's magnetic field, comprising two magnetometer heads which are arranged in two places at a distance from each other.

In French patent no. 1,485,557, there is described a method for compensating or canceling parasitic magnetic fields, particularly in an aircraft which has a magnetometer and it is characterized by determining the difference in total magnetic field at two points in which the intensity of the external magnetic field is essentially the same while the intensities for the parasitic magnetic field are different and produce a quantity for control, in particular a current intensity which is essentially proportional to this difference and consequently to the parasitic magnetic field, and is produced under the control of the said quantity a compensating magnetic field which is directed in the opposite sense of the parasitic magnetic field and of the intensity which is essentially proportional to this size in order to cancel the said difference and the patent further describes a device for practicing this method and it is characterized in that it in combination includes devices for the production of an electric current whose intensity is essentially proportional to the difference in magnetic fields at two points in which the intensity of the external magnetic field is essentially the same, while the intensities of the parasitic magnetic field are different, coils of conducting wire and devices for feeding these coils with the said current, the arrangement of the said coils and the coefficient of proportionality between the said current and the difference being such that the said coils produce a compensating magnetic field which cancels the said difference.

This earlier device consequently includes coils which produce a magnetic field for compensation and which could affect the other devices on board the aircraft, which requires certain precautions to be taken.

In US Patent No.' 2.715-198, the magnetic disturbances that arise from the Foucault currents in an aircraft are compensated by knowing. proceeding from the indications of a special type of magnetometer and a differentiator as well as a coil produces a correcting magnetic field which is proportional and oppositely directed to the Foucault currents.

On the other hand, there is described in US patent no. 2,891,216 a compensation of a magnetic detection system

tem placed on board an aircraft comprising a disturbance source, the compensated system comprising a first magnetometer which is placed at a certain distance from the disturbance source and a second magnetometer which is placed at a distance from said source which is greater than said predetermined distance, as these two magneto-

meters electrically oppositely connected and a damping link is provided to compensate for the effects of the disturbances in the two magnetometers.'

The first and the second magnetometer according to the latter

nth patent is placed at two points in the plane where the total magnetic field on the one hand is H + hk, and on the other hand H + h, calling H the external magnetic field to be measured, h being the parasitic magnetic field at the the point where the other mangetometer is located

and K is a constant that is greater than 1, decreasing

ters the output of the first magnetometer in the ratio k, and one derives the difference between the output from the second magnetometer and the attenuated output from the first magnetometer so that a sig-

nal which is proportional to (H - h) - ~ (H + kh) = (1 - i) H.

Such a system has the disadvantage that it is not very accurate and in reality if you take a value for k that is

much above 1, in particular, equal to 2, as indicated in the example in patent

et, the two magnetometers are mutually distant from each other then they must be at distances proportional to 1 and X^k from the source of the magnetic disturbances given that the magnetic effect varies as the inverse of the root of the distance (as indicated in the aforementioned patent no. 2,891,216, column 1, line 64 66), while the two magnetometers are too far apart to be subjected to homothetic parasitic magnetic fields and the compensation cannot be

powerful for all positions in the plane.

If for k you take a small value very close to 1, e.g.

k = -j-gj ., the two magnetometers are sufficiently close to. to be subjected to magnetic fields. of perturbers which are essentially homothetic, but the system's precision is reduced as it only measures a small

; '" 20

fraction of the field H to be determined. For k = is the measured

1 IQ H

magnetic fields. namely (1 H equals (1 - 2o")H * ^TJ, and precision-

one is.divided by a.factor 20..

One is thus led to a compromise and one takes

10 H

a value for k of an order of magnitude of —^ and you measure from which you get a precision divided by 10.

The purpose of the present invention is to reduce the disadvantages mentioned above by allowing a precision that is at least equal to the precision according to the French patent no. 1,485,557, as an electronic compensation is realized without producing parasitic magnetic fields in the zone where the device is located in the aircraft.

This is achieved according to the invention by devices (3,5) for generating a first voltage which, with a 'first proportionality coefficient, is proportional to the strength of the overall magnetic field at a first location located at a first distance from the Bary center of the disturbing magnetic fields,' devices (3, 8, 11 j 3, 3a, 31) to produce a second voltage which, with a second proportionality coefficient, is proportional to the difference between the strength of the total magnetic field at the first location and a second location at a second distance from the Barycentre, means (16) for multiplying the second voltage by a factor P which is dependent on the first distance, to produce a third voltage (e-^) which is proportional with the first coefficient of proportionality to the strength of the disturbing magnetic field at the first location , means (18) for forming the difference between the first and third voltages, and means (26,27) for determining the amplitude of one of the differences a generated fourth voltage that is proportional to the strength of the external magnetic field freed from the influence of the disturbing magnetic fields to be measured at the first location, whereby the first distance x, the distance A x between the first and second location, the first proportionality coefficient k^ , the second proportionality coefficient k2, the factor <p and the ratio./<0> between the strength of the disturbing magnetic fields at the second and the first site are interdependent in accordance with the formulas:

Further features of the invention appear from claims 2-4.

The invention will be explained in more detail below with reference to the drawings. Fig. 1 shows a block diagram of a gradient meter to perform the initial compensation of the magnetometer, namely determination of the gradient of the "disturbing" magnetic fields and the position of the Barycenter of these fields. Fig. 2 serves to explain the function of a core filter comprising the gradient meter from Fig. 1.

FjLg. 3 schematically shows the structure and effect of a

phase meter which also includes the gradient meter from fig. 1.

Fig. 4 schematically shows the device comprising the gradient meter from fig. 1 used to determine the position of the Barysenteret in the disturbing fields. Fig. 5 schematically shows a first embodiment of a compensated magnetometer according to the invention. Fig. 6 schematically shows the devices for the compensation according to the second application of the invention in that the magnetic disturbances are distributed by Foucault currents. Fig. 7 explains the effect produced by the devices shown in fig. 6.

Fig. 8 finally schematically shows another embodiment

for a magnetometer which is compensated according to the invention.

Before two preferred embodiments are described in detail

for a compensated magnetometer according to the invention, with reference to figures 1 and 3, a preliminary phase will be explained, the purpose of which is to study the Bary Center for the disturbing, field, rest-

ke is written from ferromagnetic masses, and determine the position of this. Bary center.

For this purpose, a magnetic gradient meter is used

the type described in French Patent No. 1,485,556 with two

■ probes or heads for measurement arranged at a certain distance (e.g. an order of magnitude of 1.50 meters), one from the other, with the gradient meter determining the distance (or gradient) between intensi-

the points for the total magnetic fields to which the two heads are subjected.

The determination of the Bary Centers for the disturbing mag-

netic fields are based on the fact that the variations in the external magnetic fields (which vary from the area flown by the aircraft) are indicated by a magnetic gradient of zero between the two heads, while on the other hand all magnetic disturbances produced by the aircraft are indicated by a magnetic gradient which is different from zero.

If the barycenter of ferromagnetic masses equivalent to the disturbance field of the aircraft is displaced during the flight to the interior of a volume of small dimensions compared to the distance between the heads of the magnetic gradient meter, the variations in the magnetic gradient determined by this gradient meter will have the same development and the same phase as the disturbances and this gives

a constant amplitude ratio (there exists a real nomo-

thetic state between these variations and the disturbances).

This allows, as will be explained below, to be compensated very precisely by means of the invention without deforming or dividing by any factor (such as the factor (1-^) in US Patent No. 2,891,216) the signal which is representative for the intensity of the external magnetic field that you want to measure.

It should be noted here that the precise determination of the previously mentioned Bary Center or Centers and the automatic self-compensation of the parasitic magnetic fields require measurement of the magnetic gradient with a very high precision. In reality, a disturbance of 1 ^ occurs on the probe or the head of the meter-

one with a distance of 1.50 meters and a field difference of 0.2^

and consequently precision in the measurement of an order of magnitude of OjOljf and a constant difference (ie the same value for the magnetic gradient) must be achieved for any position in the plane. However, the magnetic gradient meter according to the French patent no. 1,485,556 enables one to measure variations in magnetic field intensity between two points of an order of magnitude of 0.001 X.

which becomes 0.01^u G. These variations, once recorded, give the general development of the magnetic disturbances.

In fig. 1 shows very schematically the structure for

the magnetic gradient meter according to French patent no. 1,485,556. In its current version, it comprises two probes or heads L-^ and Lp

at a distance from each other of 1.50 meters (D' = 1.50 m) and are enclosed in a holder tube M of a rigid plastic material (to hold the relative position between the two heads L, and L^) and which is very sensitive to magnetic fields.

The head or probe, which is advantageously of the spin-coupling type, acts as a nuclear oscillator, i.e. it provides a voltage for the Larmor frequency of the nuclear spin it contains, a frequency that is exactly proportional to the intensity of the total magnetic field intercepted by the point N, of the probe L^. This voltage T^ is amplified in an amplifier P^ and its frequency is generally measured in a frequency meter Q which gives a voltage Tg which is proportional to said frequency. This voltage T2 is filtered in a filter R before it is recorded on the path I in a recording device V.

The second head or probe is a nuclear filter which receives the voltage which is amplified in the amplifier with an angular or pulse frequency oJ^ = Jf H^,' calling % the gyromagnetic ratio of the nuclear spin and the intensity of the total magnetic field in N ^. The core filter L2 is a bandpass filter centered on the angular or pulse frequency uig = / Hg, calling Hg the intensity of the total magnetic field in Ng and assuming that the core filter includes the same core spins (the gyromagnetic ratio

) as the core oscillator L^ which constitutes the first head. The amplitude of the frequency curve, i.e. the voltage that comes out of Lg as func-

tion of iii2 is given by the upper curve in figure 2.

The core filter also has the purpose of phase-shifting T^ in relation to T. when Wg" is different from u^, as indicated by the lower curve in Fig. 2 which gives the variation in phase d <p between and 1^ as a function of u . When H1 = Hg, = u>2 the phase difference d <p introduced by the kernel filter L2 is zero, but as a gradient dH

of the magnetic field appears between the points N, and Ng, is- o)g different from and a phase difference d ( P is introduced by the filter". The variation in phase is very steep: it is j (d <f goes from - -jj- to

+ for a variation of dH of 5 Jf , which gives a very high sensitivity for the gradient meter.

The voltage coming out of the Core filter Lg is amplified in an amplifier Pg and the phase difference between the voltages T 1 and T, amplified in the amplifiers P^ and Pg is determined in a phase meter X (with two inputs X1 and Xg) which gives (at the output X^) a voltage IV which is proportional to d ( f , a voltage TV which is recorded on channel II of the recording device V in parallel with voltage

a T2, which after filtering using the filters R, is recorded on channel I in the same recording device.

In fig. 3 shows the structure and operation of the phase meter X from fig. 1. This phase meter 'comprises two channels Y± bg Y2 of which the channel in series comprises a phase inverter g, a'shaping unit h1 (of the Schmitt trigger type and a unit for differentiation and rectification j^ consisting of the capacitor' m^, diodes and p .^ and' the resistance r^.

The second channel Y2 comprises a shaping unit h2 (of the Schmitt trigger type) and a unit for differentiation and rectification j'2 with a capacitor m2, diodes n2 and p2 and resistance r2<

The signal from the two channels Y1 and Y2 is applied to the two inputs of a bistable circuit s of the Ecclés-Jordan type, one output signal is applied to an integrator v whose output forms the output of the phase meter X.

This .works as follows:

If one first assumes that d <p is zero, the two voltages (derived from Tn by amplification in P1 and inversion of the phase in g) and Tp<1> (derived from T„ by amplification in P„) become exactly in opposite phase

• • 2 2

as shown, and the same applies to the impulses T.. and T„ which are formed in fe1 and h2 respectively. The positive impulses T2 3 corresponding to the leading edge of the pulses T„<2> will thus come to stand exactly in the middle between the positive impulses 3 corresponding to the leading edge of the pulses T^. For this reason, the circuit s remains for equal periods in each state and consequently emits a signal T° having equal positive and negative waves, and the integrator v emits a zero voltage.

On the other hand, after dp ceases to be zero, the impulses H 3 will no longer place themselves exactly in the middle between two impulses T: 3r and the waves of T become asymmetrical about the zero line. The integrator v emits a positive or negative voltage (depending on the phase difference d <p and consequently the magnetic gradient), not zero, available in X^ and recorded on channel II of the recording device V.

As an example, it has been possible to obtain an integrated voltage Tj| — 5 volts for a phase shift of — •jj- for a gradient of —5t . As the electron noise in the unit is less than 1 millivolt, one can therefore accept 0.001/ .

Finally, the recording device registers V thanks to the precision of the phase meter X and the precision of the frequency meter Q as e.g. may be of the type described in Supplement 88,663 to French Patent No. 1,430,874 - simultaneously side by side the gradient of the magnetic field (actually the tail gradient of the aircraft) and the absolute value of the total magnetic field.

The unit as shown in fig. 1 therefore gives with regard to the magnetic disturbances in flight on the one hand magnetic disturbances in N-^ and on the other hand (AH2 - AH.^), the difference between AHg, the magnetic disturbance in'-Ng, and AH^

which is mentioned above'and this with great accuracy.-

In the following, with reference to fig. 4 shows that one can derive with good accuracy and thanks to a simple assembly the distance x in which the Bary center is located (the ferromagnetic masses) for the probe Lg, e.g. By making this determination in each main direction, one can assess the stability of the position of the Bary Center and for each type of aircraft or other carrier define the possibilities for self-compensation.

In fig. 4 there are the two heads L, and L„ from fig. 1 in that head Lg is closest to the source S 0 of the disturbances (the center of gravity of the magnetic disturbances) which gives an intensity AH-^ for the magnetic disturbance in N.^ where the head L^ is placed and an intensity AHg for magnetic disturbance in Ng where the head Lg is placed at a distance Ax (actually 1.50 meters) from L^

The compensation system shown in fig. 4 comprises: a source for compensation voltage V , a voltage divider Z with a slider Z^ which distributes the voltage V® between the resistors with values R^ and Rg and two windings. and Wg flowing through respectively

of the currents 1^ and lg which are proportional to R^, respectively Rg, these windings W^-, Wg producing respective compensation magnetic fields AH'^ and. AH'g which are proportional respectively with I^and Por a complete compensation one has

The condition A H 2 = p as mentioned above is realized by the following three AH^ conditions: - the heads L^ and Lg have a high precision, of an order of magnitude of at least 0.01 if , and regardless of the position of the heads in the magnetometers .in relation to the direction of the field, - the distance Ax is completely known and slightly ahead of x (—> 10),

A x ".,.■''.'..,' '

•-, the regulation, of the currents 1^ and lg ensures the compensation. When. the three conditions are met, one can calculate x by to start from known Ax and of p determined by the skyyer Z. By setting m as the disturbance mass in S one has

Ax

By setting X = — one has

X

One can therefore calculate x by starting from Ax and p.

In practice, the regulation of the currents 1^ and lg in the compensators W1 and W2 is carried out during the specific movements of the aircraft at each end or corner (e.g. a yaw movement with an amplitude _+ 10° in periods of 6" seconds ).One affects 1^,

I2 and V° to obtain simultaneously AP^ - AH^ = 0 and (AH2 - AH'^) -

(AH.^ - AH^) =0. At this moment, the relationship between resistance-,

one Rg/R-^ (which can be read on the divider Z which can be a potentiometer graduated for ten steps, I^/^-^ = p

The measurement of the distance "x from the center of gravity to be described with reference to Figs. 1 - 4" may allow compensating for magnetic disturbances arising from the aircraft according to the method and device described in the French patent no.

•1,485,557., It is sufficient to apply to the compensation coils 16 and 17 in this patent the voltage that represents the difference

(AH2 - AH-j^). Nevertheless, the great precision with which, by means of the method just described, the measurement of the gradient of the magnetic field in the vicinity of the detector probe is achieved

L-^ allows an electronic compensation directly by drawing a voltage aom proportional to aH, generated by applying main - the layout according to the invention as shown below in detail with reference to fig. 5.

As the explanations regarding the study of the first phases in the case of disturbing fields have been completed, we shall now proceed to deal with the devices according to the invention for compensation respectively of parasitic fields that arise from ferromagnetic masses in the plane (fig. 5) and Foucault currents (fig. .6 and 7).

The great sensitivity for the measurement of the gradient allows the two heads (core oscillator and core filter) in the device according to the invention and corresponding to the heads L.. and Lg in fig. 4 can approach a small distance between themselves of e.g. 1 meter.

One can then directly write the classical equation

AH^ due to H varying as —^ (the symbol ^

represents the derivative with respect to x). When Ax is small, the difference is (AH0 - AH,), essentially x . When the dx obtained signal phase is strong, it is sufficient to realize the condition AH, . d^AHi) = o to completely cancel the influence of the in-plane magnetic masses at the level of the head or probe L r d(AH }

It is therefore sufficient to multiply —g-^

(ie AHg - AH^) with a constant factor $ and utélate the value of the measurement carried out with the head L, to cancel the disturbances induced by the magnetic masses in Nn da AH, , which

d(AH ) one wants to compensate is exactly equal to x. •

3 dx

An arrangement for performing the aforementioned operations is shown

on fig. 5. It includes in combination:

- A magnetometer head 3 (which, for example, consists of a core oscillator of the type described in the French patent no. 1,485,556) is arranged. at a first point 1 at a first distance dl^dl = x+ Ax) 'from the Barycenter 4 of the disturbing magnetic., field (the arrow m shows the disturbing.magnetic masses) is suitable to induce a first emf with a frequency ■f ^ which is proportional to the intensity HQ + AH^) that the: total magnetic field (calling HQ the external magnetic field seam to be measured and AH^ the disturbing field in point .1) in the first point 1.

- A frequency meter 5 (e.g. of the type described

et in the French supplement no. 88,663) connected to the output 6 of the said magnetometer head 1 - preferably through an amplifier 7 - to derive from the said first emf with the frequency f^ a first voltage e.^ which is proportional to the said frequency and consequently with the said intensity (HQ + "AH^) of the total magnetic felt.

- A core filter 8 (preferably of the type described in the aforementioned French patent no. 1,485,556), placed in another point 2 at a different distance d2 (d2 = x) from the aforementioned center of gravity 4 and which has its input 9 connected to the output 6 as said head 1 - preferably through an amplifier 7 - in order to obtain the first emf with the frequency f^ from there and for at its output 10 to

emit another emf of frequency f2 which, with the said first emf of frequency f^, produces a phase difference proportional to the difference (AH2 - AH1) between the intensities (HQ + AH2) and (HQ + AH^)

for the total magnetic field in this second point 2 and the first point 1 (calling AHg the disturbance field in point 2).

- A phase meter 11 (e.g. of the type described

in the aforementioned French patent no. 1,485,556 and shown in fig. 3 in the drawing) with two inputs 12,13 which are connected, the first 12 with the output 6 of the said magnetometer head 1 - preferably through the said amplifier 7 - to receive the said first emf with / frequency f1 and the probe 13 at the output 10 of the core filter 8 - preferably through an amplifier 14 - to receive the said second emf with the frequency f2-, and an output 15 which gives another voltage e2 which is proportional to the said difference (AH2 - AH^) between the intensities for the total magnetic fields.

An amplifier 16 with adjustable outlet if input

17 is connected to the output 15 of the said phase meter 11 in order to receive said second voltage e2 and derive a third voltage e^ from it. - A subtraction unit 18 with two inputs 19520 connected, the first 19 with the output 21 of the said frequency meter 5 to.. receive the said first voltage e-^ and the second 20, with the output 22 of the said amplifier 16 to receive said third voltage e^ and an output 24 which gives a fourth voltage e^ which is proportional to the difference between said first voltage applied to the first input 19 and said third voltage e^ applied to the second input 20 , and - Devices for measuring the aforementioned fourth voltage ejj which is proportional to the intensity HQ in the external magnetic field to be measured in the aforementioned first point 1 for an appropriate regulation of the output from the amplifier 16, as well as specific positions of the aforementioned magnetometer head 3 and the said core filter 8 mainly the plane's axis 25 or other carrier when this is possible, the said devices comprising e.g. a filter 26 which filters the desired components from e^ and is followed by a recording device 27.

The effect of this device is as follows: The core oscillator or mangetometer head 3 produces an emf with frequency f^ with frequency fj which is proportional to méd (Hq + AH^). This emf with the frequency f^ which is amplified in the amplifier 7 is fed to the frequency meter 5 to produce a voltage e^ with an amplitude a^ = k^ (Hq + AH^), where k^ is a constant, - to the core filter 8 which provides an emf with frequency f2 whose phase shift d <p in relation to the emf with frequency f^ emitted by the oscillator 3 is proportional to (AH2 - AH^), - to the phase meter 11 which likewise receives emf with' frequency fg emitted by filter 8 and amplified in amplifier 14 and gives a voltage e^ with an amplitude a2 = k2 (AH2 - AH^)', the idea being that k2 is a constant which can be equal to k^.

The voltage e2 is multiplied in the amplifier 16 by a factor ^which is a function of the distance d^ = x, so that a voltage e^ with an amplitude a^ = J^(x) k2 (AH2 - AH^) is obtained. Mån' chooses ^?(x) so that one can realize the condition k2 ^/k-^ = x/3, where a^ = k]_ I (AH2 - AH-^).

It should be noted that if k2 k^ is <<>f x/3.

The subtraction unit 18 thus receives a voltage e^ with' an amplitude a-^ .= k^ (Hq + AH^j in its input 19 "bg a voltage e-j with an amplitude a? = kx | (AHg - AH^) k-L g d ( AH^)- k_^. AH^

dx

(as explained above with reference to Fig. 4) at its entrance

20 and it consequently emits, at its output 24, a voltage e^ with an amplitude a^ = k1 (HQ + AH-j^) k-j^ A^ = ^Hq . One thus sees that the amplitude a^ for the voltage e^ is exactly proportional to the field Hq that one wishes to measure. It should be noted that the coefficient k, is not at all a fractional coefficient like the coefficient (1 - j1^) in the cited U.S. Patent No.' 2,891,216, which reduces the accuracy, but is simply the proportionality coefficient which is a result of the amplifier 7 and the frequency meter 5 which gives an emf with the frequency f^ which is not divided by any reduction factor.

The voltage e^ represents the intensity of the earth's magnetic field Hq (to which it is strictly proportional) and its disturbances independent of the aircraft's movements and the magnetic masses therein (with the exception of what will be said below for Foueault currents).

The band filter 26 allows a recording on the recording device 27 of the irregularities whose frequency corresponds to the frequency of the searched irregularities and to eliminate as much as possible the natural disturbances and gradients (horizontal and vertical) of the earth's magnetic field.

As described, the compensation of magnetic fields emanating from an aircraft or other carrier will sometimes be incomplete, as the aircraft's movements in the earth's magnetic field, in the conducting surfaces of the aircraft, especially in the fuselage, induce Foucault currents which distribute parasitic magnetic fields which is stronger the greater the airspeed.

Fortunately, the symmetry of the aircraft structure, or other carrier, allows to compensate for these phenomena in the same way as for the permanent or induced parasitic magnetic fields. Only the position of the Bary Center for the magnetic forces arising from the Foucault currents differs from the position of the Bary Center for the permanent or induced magnetic fields. Consequently, in order to achieve a single regulation of the compensation (as previously indicated) it is necessary to allow these two Barycenters to coincide and, consequently, change the direction of the gradient of the magnetic field induced by the Foucault currents. In the following, it will be explained how one can proceed in this regard (fig. 6 and 7). According to another device according to the invention, for compensation of the magnetic disturbances distributed by the Foucault currents in a plane or other carrier (from the axis 25, fig. 6), - in particular by means of coils 28 which are fed by voltage collected in a set of generating coils 29 with trirectangular axes and fixed in the plane, the voltage of which is adjustable in amplitude thanks to an adjustable resistance 30 - in the vicinity of the aforementioned second point 2 (where the filter 8 is placed while the oscillator . 3 is placed at point 1), a magnetic field correction Ah^ is made in the opposite phase to the disturbance field Ahg. which is written from the Bary center for the Foucault currents, the value of this field correction Ah^ being regulated so that it coincides with the aforementioned Bary center for the magnetic disturbances which is induced by the Foucault currents with the Bary center for the ferromagnetic masses (the Bary center for the Foucault currents is moved from its real position Br into virtual position Bv by the te correction field Ah^).

In a very precise way, one can consider the field for magnetic fields which are induced by movements of the fuselage and which appear in the north-south and south-north doves. The real Barysenter Br gives on the head 3 and the filter 8 variations in fields which are respectively Ah-^ and Ahg. The coils 2 8 produce a field - Ah^

(opposite phase with Ahg). The gradient that results ( Ahg - Ah^)

- Ah-j^ for Ah1 constant gives a Barycenter image in relation to head 3 which seems to be more removed than (Ah2 - Ah^) and approaches Ah1): The Barycenter moves towards infinity when (Ahg - Ah^) moves towards Ah^ (fig. 7). By using the adjustable resistor 30 to regulate the intensity of the current passing through the coils 28 and consequently the intensity of the field Ah^, the Center of Bary of the Foucault currents can be shifted to allow them to coincide with the Center of Bary of the ferromagnetic masses (the center of gravity of The Foucaul currents go from their real position Br to their virtual position Bv which is the position of the Bary center for the ferromagnetic masses). In fig. 7, curves 31 and 32 show the gradient that emerges from the virtual Center of Bary and for the real Center of Bary for the Foucault currents.

This possibility is written from the formula indicated

in the foregoing

if one allows too constant Ah^ lar

Ah^ vary, vilp' varies and a visible variation "± x" thereby occurs (in Fig. 7, in addition to the value x, the value x' is indicated which corresponds to the real center of gravity Br.).

The measurement of the distance between the centers of bary of the Foucault currents and the ferromagnetic masses caused by the Foucault currents, as the measured distance of the centers of gravity of the Foucault currents and of the ferromagnetic masts which produce the Foucault currents gives a resultant of disturbances which are closer to the points. 1 and 2 ann for them. ferromagnetic masses, since h^ must be negative which during the regulation of the resistance 30 gives a clear displacement of the Bary center caused by the Foucault currents from Br to Bv as shown in

follow 7.

It should be noted that the coils 28 can optionally be fed not only by the coils which are subjected to the magnetic field as generators for eddy currents, but by all other devices that can. produce a current comparable to these Foucault currents, e.g. a calculator'.

Finally, in fig. 8 shows another embodiment of a magnetometer which is compensated according to the invention, as this device in fig. 8 is a variant of the device according to fig. 5 (fig. 8 differs from fig. 5 exclusively in the part which

ner framed by dashed lines in this fig. 8).

The magnetometer which is compensated and shown in fig. 8 comprises in combination: - A first magnetometer head 3 of the same type as the magnetometer head 3 in fig. 5 and arranged in a first point 1 at a distance d^ from the Bary center 4 for the disturbing magnetic fields, as this head is suitable to produce in its output a first emf with the frequency f^ which is proportional to the intensity' (Hq + AH^)

from the total magnetic field in this point 1.

- A frequency meter 5 of the same type as the frequency meter

5 in fig. 5 connected at the output 6 in the head of the magnetometer 1 - preferably through an amplifier 7 - and emits as a result of emf with frequency f, a first voltage e.. which is proportional to the frequency of the electromotive force with frequency f^ and consequently with ( HQ + A^).

- Another magnetometer head 3a which is identical to the magnetometer head 3 arranged in another point 2 at a different distance dg (dg = x) from the Bary center 4 and suitable for producing a different emf at a frequency f^ which is proportional with the intensity (Hq +

AHg) from the total magnetic field in this second point 2.

- A difference frequency meter 31 with two inputs 32 and

33 which is connected the first 32 to the output 6' of the said magnetometer head 1 preferably through an amplifier 7 in order to receive the first emf with the frequency f1 and the second 33 to the output 6a from the second magnetometer head 3a preferably through an amplifier 7a to receive the other emf with the frequency f^ and an output 35 which gives another voltage eg which is proportional to the difference (AHg -

AH-j^) between the intensities of the total magnetic fields in points 2 and 1.

- An amplifier 16 with adjustable gain, whose input 17 is connected to the output 35 of the aforementioned difference frequency meter 31 in order to receive a second voltage e2 and derive a third voltage e^.

- A subtraction unit 18 with two inputs 19 and 20

is connected respectively to the output 21 of the frequency meter 5 and to the output 22 of the amplifier 16 to receive respective e^ and e-^,

as this unit 18 on the output 24 gives a fourth voltage e^ which is proportional to the difference between e^ and e^, and

- devices for measuring said fourth voltage e^

which is proportional to the intensity HQ of the external magnetic field to be measured in the aforementioned point 1 for an appropriate regula-

ing of the amplification factor from the amplifier 16 and certain styles

lings for the aforementioned magnetometer heads 3 and 3a, the aforementioned devices e.g. comprises a filter followed by a recording device 27.

The effect of the magnetometer on fig. 8 is the following:

The core oscillators or magnetometer heads 3 and 3a which

gives respectively an emf with a frequency f-^ which is proportional to (Hq + AH1) and an emf with a frequency f^ which is proportional to (Hq + AHg).- These emfs with frequencies f1 and f 3 are amplified in the amplifiers 7 or 7a.

The difference frequency meter 31 determines the difference

between the frequencies for f^ and f^ and emits at the output 35 a voltage

ning e2 with an amplitude a., kg (AHg - AH.^), where kg is a constant which can be equal to / while the frequency converter 5 measures the frequency f' of the electromotive force and gives "Religibly a voltage e1 with an amplitude a-j^ = k x (HQ + AH., ), Where- k, is a constant.

In the case of the design shown on rig. 8 is the voltage

eg multiplied in the amplifier 1.6 by a factor • -- r which is a function of the distance d1 = x, 3like that 0.1 voltage e--^ with an amplitude a^ = cp( x) k, is obtained. (AHg - AH-L). One chooses <p( x) so that one can realize conditional! -sen kg ' f/ k^ :;/3, then a^ = k^ ^ (AHg - AH1).

One should note that if k? k-^ becomes <P - x/3.

As with the embodiment shown in fig. 5, the sub-traction unit 18 receives a voltage e^ with an am; in i'; .ide a, = k^ - (Hq + AH-^) on the input 19 and a voltage 3^ with e^ amplitude

a3 f (AHg aHx) kx f d( AH^ >L ( as explained in the fore-dx

going with reference to fig. 4) on s ri i-mgang 20 and it thus emits at its output 24 a signal e^ with an amplitude a^ = k1 (HQ .+ A^) k± A<H>2<=> k^ H0.

One thus sees that the signal e^ me-in the amplitude a^ is exactly proportional to the field HQ which it is desirable to measure.

The device according to fig. 8 shows the same advantages as the device in fig. 5, with the further possibility of working in real time, with one reaction which is faster, given that it does not include a core filter (in which the spins are subjected to magnetic fields in point 2, while this filter at its input receives an emf whose frequency is proportional to the magnetic field at point 1).,

As a result, no matter what progress is made,

method used, always a compensated magnetometer whose effect is evident from the foregoing to a sufficient extent that it would be unnecessary to emphasize the advantages especially in relation to already existing compensated magnetometers, as the present invention in particular has the following advantages:

First of all, the accuracy is very high.

Compensation is achieved automatically for all parasitic magnetic fields.

The compensation produces neither disturbing magnetic fields

in the zone where the other devices in the aircraft or other carrier are located, such as in the cited French patent no. 1,485,557 and also no reduction of the signal proportionally, with the intensity of the magnetic field to be measured, such as the occurs according to US Patent No. 2,891,216.

Claims (4)

1. Device for compensation of disturbing magnetic fields in a means of transport, in particular an aircraft, which is equipped with a magnetometer for measuring the field strength of an external magnetic field, e.g. the earth's magnetic field, comprising two magnetometer heads which are arranged at two locations at a distance from each other, characterized by directions (3,5) for producing a first voltage which, with a first proportionality coefficient, is proportional to the strength of the overall magnetic field on a first location located, at a first distance, from the Barycenter of the disturbing magnetic fields, arrangements (3, 8, 11; 3,3a,31) for producing a second voltage which, with a second proportionality coefficient, is proportional to the difference between the strength of the combined magnetic fields at the first location and at a second location at a second distance from the center of bary, devices (16) for multiplying the second voltage by a factor <p which is dependent on the first distance, for producing a third voltage ( e^) which with the first proportionality coefficient is proportional to the strength of the disturbing magnetic field at the first location, means (18) for: forming the difference between the first and three th voltage, and devices (26,27) to determine the amplitude of a fourth voltage formed by the difference which is proportional to the strength of the external magnetic field freed from the influence of the disturbing magnetic fields to be measured at the first location, whereby the first distance x, the distance Ax between the first and second place, the first proportionality coefficient k^, the second proportionality coefficient k2 , the factor ( fi and the ratio^/0 between ■ the strength ■■of- the disturbing magnetic fields on the second and the first place are-interdependent according to the formulas: 2. Device according to claim 1, characterized by a magnetometer head (3) arranged on the first path (1) at a first distance d.^ from the center of gravity (4) for the noise fields, which in its output (6) supplies an EMF with a frequency f^ which is proportional to the strength of the overall magnetic field (Hq + AH^) at the first location, a frequency meter (5) connected to the output of the first magnetometer head (3) which derives a voltage e from the received EMF ^ which is proportional -to the frequency f^ and thus to the strength of the overall magnetic field, at the second location (2) i., a second distance dg from the center of bary arranged second magnetometer head (8,3a) which, together with the first magnetometer head ( 3) forms a magnetic gradient meter (3,8, 11; 3, 3a, 31) which delivers a voltage eg that is proportional to the difference between the strength of the combined magnetic fields at the second and first location, an amplifier (16) with adjustable amplification factor, whose input (17) is connected to the output (15; 35) of the gradient meter, and which amplifies the second voltage eg to a third voltage e^, a subtraction unit (18) with two inputs (19 > 20) of which the first (19.) is connected to the output (21) from the frequency meter ( 5) and the second (20) is connected to the output (22) of the amplifier (16), and to an output (24) which delivers a fourth voltage e^ which is proportional to the difference between the first e^ and the third voltage e-j, and devices (26,27) for measuring the fourth voltage e^ as for a suitable setting of the amplifier's (16) gain and a specific position of the first and second magnetometer head (3 resp.8,3a) , is proportional to the strength Hq of the external magnetic field. which should. is measured approximately on the longitudinal axis (25) of the aircraft or another carrier. 3. Device according to claim 2, characterized in that the second magnetometer head (8) is mainly formed by a core filter located at the second location (2) whose input (9) is connected to the output (6) from the first magnetometer head (3 ) to receive from it an EMF with a frequency f1 and from its output (10) to deliver a second EMF with a frequency fg which, in relation to the first EMF, has a phase shift that is proportional to the difference between the strength of the combined magnetic fields in the second place (Hq + AHg) and in the first place (HQ". + ÅH^), and that a phase meter (11) with two inputs is arranged, of which the first (12) is connected to the output of the first magnetometer head (3) and the other (13) are connected to the output of the core filter (8), and the voltage e2 appears in the output (15) of the phase meter (11). 4. Device according to claim 2, characterized in that the second magnetometer head (3a), like the first (3), delivers a second EMF at its output (6a) with a frequency f^ which is proportional to the strength of the magnetic field at the other location (HQ + AH2), that a differential frequency meter (31) is arranged with two inputs (32,33), of which one (32) is connected to the output (6) from the first magnetometer head (3) and the other (33) is connected to the output from the second magnetometer head (3a), and that the second voltage e2 appears in the output (35) from the differential frequency meter.