GB2293456A

GB2293456A - Measuring an external magnetic field

Info

Publication number: GB2293456A
Application number: GB9517891A
Authority: GB
Inventors: Ho Chul Kim; Chung Jun Sa
Original assignee: Korea Advanced Institute of Science and Technology KAIST
Current assignee: Korea Advanced Institute of Science and Technology KAIST
Priority date: 1994-09-03
Filing date: 1995-09-01
Publication date: 1996-03-27
Anticipated expiration: 2015-09-01
Also published as: KR960011451A; GB2293456B; GB9517891D0; KR960015562B1

Abstract

The magnitude and the direction of an external magnetic field are measured by applying 7 a current having a predetermined amplitude and period sufficient to saturate a magnetic core 6c of a sensor in the field to generate an induced voltage, generating 10, 11 first and second electric power valves for the induced voltage during first and second half periods of the generated current, and outputting a signal representative of the difference between the first and second power valves.

Description

IMPROVED METHOD AND APPARATUS FOR MEASURING AN EXTERNAL MAGNETIC FIELD BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to a method of measuring a magnetic field and, more particularly, to a method and apparatus for measuring the magnitude and the direction of an external magnetic field.

2. Description of the Related Art Among the various magnetometers, the fluxgate has a special position in applications such as in measuring the earth's magnetic field from space, nondestructive research, etc. since it directly measures the components of the DC or low frequency AC magnetic field vector in the ranges from 10-10 to 10-4T while having ruggedness, stability, and reliability. The fluxgate magnetometer uses the nonlinear magnetization of a ferromagnetic core (or cores) driven by a periodic current of a suitable wave shape in the driving coil. In the absence of an external field, the voltage induced in the pick-up coil is symmetrical and only odd harmonics of the fundamental frequency of the driving field are present. In the presence of an external field, however, the pick-up coil voltage becomes asymmetrical.This symmetry is sensitively related to the external field and can be detected by several techniques.

The fluxgate has three modes of operations; non-selective detection, all-even-harmonics detection and second-harmonic detection. The non-selective detection method has been developed for low-cost and low-power magnetometers which are exemplified by peak detection, phase delay, and auto-oscillation methods.

All-even-harmonics detection, of which the short-circuited method is an example, has been developed for reducing the instability in the 2nd harmonic detection method. Nevertheless, the second harmonic detection method is the most common type of method which makes use of the even harmonics, i.e., the 2nd harmonic signal of the driving field frequency. The single-core, two-core and ring-core types are examples of the core configuration of 2nd harmonic fluxgates. In the single-core type, a large signal is present at the driving frequency, which results in the reduction of the sensitivity of the 2nd harmonic signal by the odd harmonics caused by the transformer effect between the driving and pick-up coils.In the two-core or ring-core types, a large part of these odd harmonics is eliminated by reducing the mutual inductance between the driving and pick-up coils to near zero and reducing the transformer effect. Although the ring-core type has the best geometry for a low-noise magnetometer, it has a large demagnetizing factor which decreases the apparent permeability.

The demagnetizing factor, D, is approximated by D (d/L)2[2.Ollog(L/d) - 0.46] in the single core type for L > lOd, where d and L are the diameter and length of the rod, respectively, and in the ring-core type, D a d/L for d < L, where L and d are the diameter and thickness of the ring-core, respectively. In the two-core type, the demagnetization factor lies between the demagnetizing factor of the single-rod, D and 2D. Therefore, the demagnetizing factor of the single core type is very small compared to those of the ring-core and two-core types for the same size, and thus is more advantageous.

Hence, there has long been a need in the art for an improved apparatus and method for measuring the external magnetic field with a low demagnetizing factor and with low noise.

SUMMARY OF THE INVENTION The need in the art is addressed by the present invention which provides an improved fluxgate method suitable for a single core with a low demagnetizing factor and power consumption using a coupling property of even and odd harmonics in terms of the power difference between the two half periods of the magnetization.

In accordance with one aspect of the present invention, an apparatus for measuring an external magnetic field is provided which comprises a current source for generating a current having a predetermined amplitude and period, means responsive to the generated current for generating an induced voltage, means for generating first electric power of the induced voltage during the first half period of the generated current and second electric power of the induced voltage during the remaining half period of the generated current, and means for outputting a signal representative of the difference between the first electric power and the second electric power.

The means for generating an induced voltage of the present invention comprises a magnetic core, a driving coil to which the generated current is applied, and a pick-up coil where said driving coil and said pick-up coil are coaxially wound around said magnetic core.

In accordance with another aspect of the present invention, a method for measuring the external magnetic field is provided which comprises the steps of applying a current having a predetermined amplitude and period to a sensor to generate an induced voltage, generating first electric power of the induced voltage during the first half period of the applied current and second electric power of the induced voltage during the remaining half period of the applied current, and outputting a signal representative of the difference between the first electric power and the second electric power.

In accordance with the present invention, the driving coil (6a in Fig. 2 to be mentioned hereinbelow) and the pick-up coil (6b in Fig. 2 to be mentioned hereinbelow) are coaxially wound around the single magnetic core to form a sensor used in magnetizing with a magnetic field having a fixed frequency in the direction parallel to the external magnetic field, thereby eliminating the necessity of using a second harmonics tuned filter having a narrow passband while enjoying the full advantages attainable from the single magnetic core. The present invention provides an improved magnetic field measuring apparatus which simplifies the overall structure, and thus facilitates inexpensive and ready manufacturing. Additionally, the magnetic field measuring apparatus, according to the present invention, enables measuring with high sensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS The aforementioned aspects and other features of the invention are explained in the following description, with reference to the accompanying drawings, wherein: Fig. 1 illustrates a block diagram of the external magnetic field measuring apparatus in accordance with the teachings of the present invention; Fig. 2 illustrates one embodiment of an apparatus in accordance with the present invention; Fig. 3 is a graph showing the variations of the power difference as a function of the DC external magnetic field.

Fig. 4 is an enlarged view of a linear region of the variation of the power difference in Fig. 3; Fig. S is a graph showing the driving frequency dependence of the proportionality constant at various driving amplitudes; and Fig. 6 is a graph which shows power differences varying as a function of the D.C. external magnetic field when the induced voltage is doubled.

DETAILED DESCRIPTION OF THE INVENTION In an ideal magnetization process, the induction field, B, is expressed as an odd power series in the H field on account of the symmetrical property of the magnetization curve.

where al, a3, and aS are the coefficients related to the magnetic properties of the ferromagnetic core material. The field strength of the ferromagnetic core subjected to a DC external field superposed on an AC driving field is H = Hacsin(#t) + Hexa, where Hacsin(#t) and Hext are the AC driving and DC external field, respectively. The induced voltage is given by the time derivative of the magnetic flux, V = NA dB(H)/dt, where N and A are the number of turns in the pick-up coil and the cross sectional area of the core, respectively. Thus, the induced voltage is given by a harmonic series of driving frequency .

Considering the phase shift of the harmonics, #2n, due to the hysteresis effect, the induced voltage is:

where n is an integer (n = 1, 2, 3,...), and A2ssI and A2n represent the odd and even harmonic amplitudes, respectively, which are complicated functions of both Hac and Hex.

Under the condition that Hxt < Hac and Hac is constant, the amplitudes of harmonics can be simplified as A1 # NA# [a1Hac - 3/4#a3Hac3 + 5/8 asHac5 1 A2 # NAuHext[3a3Hac2 -5a5Hac4 + 105/16#a7Hac6 1 A2n-1 # #f2n-1 (Hac) = constant (odd harmonics amplitude) (3) A2n # #f2n(Hac)Hext Hex (even harmonic amplitude) (4) where fn(HaC) are functions of the driving field amplitude, which are constant if the driving field amplitude and the coefficients related to the magnetic property of the material, a1, ..., a2n-1, are constant.

The power of the induced voltage in the first and second half-period of the magnetization, P1 and P2, are given respectively as:

where T is the period of Hacsin(t). Substituting Eq. (2) into Eqs.(5) and (6), we obtain P1 and P2, respectively, as:

r = A2n-1#A2m-1cos((2n-1)#t + #2n-1)cos((2m-1)#t + f2m 1) + A2n-1#A2mcos((2n-1)#t + #2n-1)sin(2m#t + f2m) + A2n#A2m-1sin(2n#t + #2n)cos((2m-1)#t + #2m-1) + A2n#A2msin(2n#t + #2n)sin(2m#t + f2m) (9) where n and m are integer (n, m = 1, 2, 3, ...).

The 1st and 4th terms in Eqs. (7) and (8) are even functions, while the 2nd and 3rd terms are odd. Therefore, the power difference, AP, due to the presence of the external is described in terms of the 2nd and 3rd terms and expressed as a product of the even and odd harmonics as follows:: AP = P1 - P2 (10)

where 2n-i,2in iS a coupling constant between the odd and even harmonics and is expressed as a function of the phase of the harmonics with respect to the reference time for building the average of Eq.(5),

The coupling constant changes sign as the direction of the external field is reversed since the phase of the even harmonics 2m changes by 1800 on reversal while that of the odd harmonics #2n-1 does not change. Thus, the coupling constant includes information on the direction of the external field.

The amplitude of the odd harmonics determined by Hac is constant as in Eq.(3) while the amplitude of the even harmonics is proportional to the external field, Heft, as in Eq. (4).

Substituting Eqs.(3) and (4) into Eq.(10), the power difference, AP, in Eq.(ll) can be rewritten as:

where K iS the proportionality constant, defined as the ratio of P to H,,, which represent the sensitivity of the fluxgate magnetometer according to the present invention.

Thus, the external magnetic field is measured from the power difference obtained by squaring the induced voltage at the pickup coil over two half periods. In accordance with the present invention, the large odd harmonic signals play an important role in increasing the sensitivity while in second-harmonic detection method, the signal is reduced by the odd harmonics due to transformer effect. Moreover, the use of the multiple-frequency all harmonic signals makes the circuitry simpler than that for single-frequency 2nd-harmonic signal equipment.

The magnetic field measuring apparatus in accordance with the present invention will now be described with reference to Fig. 1. The apparatus, according to the invention, generally includes a current source 1, means 2 for generating an induced voltage, means 4 for generating first and second electric power, and means 5 for outputting a power difference. The current source 1 generates a current having a predetermined amplitude and period. Means 2 for generating an induced voltage generates, in response to the current from the current source 1, an induced voltage which is applied to means 4 for generating first and second electric power.Means 4 for generating first and second electric power generates first electric power of the induced voltage over the first half period of the current from the current source 1 and second electric power of the induced voltage over the remaining half period of the current from the current source 1. In a preferred embodiment of the present invention, the induced voltage may be amplified by an amplifier 3 to enhance the sensitivity of measurement. Means 5 for outputting a power difference outputs a signal representative of the power difference between the first electric power and the second electric power.

Further details of the present invention will be given with reference to Fig. 2 which illustrates one embodiment of an apparatus according to the present invention and in connection with which the principle of measuring the external magnetic field in accordance with the present invention will be explained. The AC driving signal having a constant amplitude and period output from a signal generator 8, a,sin(wt), is converted into a current by a voltage-to-current converter 7. The converted current is then applied to means 2 for generating an induced voltage which comprises a sensor 6 which may be exposed to the external magnetic field. The sensor 6 includes a sensor core 6c, a driving coil 6a to which the current from the current source 1 is applied, and a pick-up coil 6b. The driving coil 6a and the pick-up coil 6b are coaxially wound around the sensor core 6c.

Along with a digital oscilloscope 10, a personal computer 11 may be used to implement the means 4 for generating first and second electric power and the means 5 for outputting a power difference.

The induced voltage is digitized at a high sampling rate, using the digital oscilloscope 10, to be sent to the personal computer 11 where P1, P2 and the power difference P are computed according to each of Equations (5), (6), and (10).

The reference time for building the average of Eq.(5) can be arbitrarily taken by using the AC driving signal as the time reference. However, the coupling constant varies with the phase of the harmonics with respect to the reference time as in Eq.(12), and is largest when the phase of the first harmonic signal, 1 is zero. To make 1 near zero, the reference time was shifted to the time position of the positive peak of the induced voltage at the external field H - 0.

Fig. 3 is a graph which depicts the variation of the power difference AP as a function of the DC external magnetic field Hext without using the amplifier. For the measurement in Fig. 3, the sensor core material of 2605SC metallic glass in two ribbons with the dimensions of 15cm x lcm x 30m were sandwiched between two slide glasses each having a thickness of lmm. The driving and pick-up coils were coaxially wound with 350 turns on two slide glasses. The external magnetic field was superposed on the AC driving signal having a constant amplitude, Hac = 3 Oe, with an increasing step of 0.03 Oe from -1.5 Oe to 1.5 Oe, at a driving frequency U = 300Hz.The proportionality constant K was 0.6 V2/Oe in the linear range of + 0.2 Oe. Fig. 4 is an enlarged view of a linear region of the variation of the power difference in Fig. 3. In Fig. 4, the offset of P at HeXt = 0 arises from the earth's magnetic field. The sign of the power difference changes from positive to negative depending on the polarity of the external field. The result confirms that the external DC magnetic field and the field direction '+' or '-' can be measured by the power difference in accordance with Eq.(13).

The proportionality constant K is proportional to w2 as in Eq. (14) in the ideal magnetization process without magnetization loss. However, the eddy current loss in the core increases significantly as , resulting in a markedly reduced proportionality constant K as the driving frequency increases.

Then, the proportionality constant K can be rewritten as: K = (J2/Q (o) (15) where Q(U) is the loss factor which cannot be well expressed with an analytic function. We assume that Q(U) varies as ess#, so Q # as ss - > x and rewrite Eq.(14) as: # α ##e-ss# where P is a fitting parameter.

Fig. 5 is a graph which shows the driving frequency dependence of K at driving amplitude, 3, 6, 12, and 15 Oe. The symbol marks are the measured results, and the solid lines are the fitted results of Eq.(16). From Fig. 5, it can be readily seen that the sensitivity increases as a function #1.3 of the driving frequency. Since the magnetization loss is compensated by the increase of the driving amplitude, the fitting parameter ss decreases with the driving amplitude.

Fig. 6 is a graph which depicts power differences AP as the D.C. external magnetic field varies when the induced voltage is doubled using the amplifier 3, with the driving frequency and amplitude being 1 KHz and 3 Oe, respectively. It is apparent from Fig. 6 that when the induced voltage is doubled, the high proportionality constant of 5.2 V2/Oe is obtained which is four times 1.3 V2/Oe, the proportionality constant attained without doubling the induced voltage.

As described above, the driving coil 6a and the pick-up coil 6b are coaxially wound around the single magnetic core 6c to form the sensor 6 to be magnetized with a magnetic field having a constant period in a direction parallel to the external magnetic field, such that a necessity for using the second harmonics tuned filter having a narrow passband may be eliminated and full advantages attainable from the single magnetic core may be obtained according to the present invention. According to the present invention, the improved magnetic field measuring apparatus may be produced, allowing for an overall simplified structure to facilitate inexpensive and easy manufacturing. The magnetic field measuring apparatus according to the present invention also allows measuring with high sensitivity.

The present invention has been described with reference to a particular embodiment in connection with a particular application. Those having ordinary skill in the art and access to the teachings of the present invention will recognize additional modifications and applications within the scope thereof. For example, the invention is not limited to the specific structure of the sensor, which is described herein only for illustrative purposes. Further, it will be appreciated by those skilled in the art that the means 4 for generating first and second electric power and the means 5 for outputting a power difference can be implemented by any hardware arrangements adapted for use in the present invention other than those described herein.

It is therefore intended by the appended claims to cover any and all such applications, modifications, and embodiments within the scope of the present invention.

Claims

WHAT IS CLAIMED IS:

1. Apparatus for measuring an external magnetic field comprising: a current source for generating a current having a predetermined amplitude and period; means responsive to the generated current for generating an induced voltage; means for generating first electric power of the induced voltage during the first half period of the generated current and second electric power of the induced voltage during the remaining half period of the generated current; and means for outputting a signal representative of the difference between the first electric power and the second electric power.

2. The apparatus of Claim 1 wherein said means for generating an induced voltage comprises a magnetic core, a driving coil to which the generated current is applied, and a pick-up coil, said driving coil and said pick-up coil being coaxially wound around said magnetic core irrespective of the order.

3. The apparatus of Claim 1 further comprising an amplifier for amplifying the induced voltage.

4. A method for measuring an external magnetic field comprising: applying a current having a predetermined amplitude and period to a sensor to generate an induced voltage; generating first electric power of the induced voltage during the first half period of the applied current and second electric power of the induced voltage during the remaining half period of the generated current; and outputting a signal representative of the difference between the first electric power and the second electric power.

5. The method of Claim 4 wherein said sensor comprises a magnetic core, a driving coil to which the current is applied, and a pick-up coil, said driving coil and said pick-up coil being coaxially wound around said magnetic core irrespective of the order.

6. The method of Claim 4 wherein the induced voltage generated in said applying step is amplified prior to said generating step.