Magnetometer with a ferromagnetic core and three windings. Fluxgate magnetometer

Recently, there have been no significant changes in the principles of magnetic field measurement. In the field of magnetic surveys, methods based on the phenomenon of magnetic resonance, optical orientation of atoms, etc. have been established. Flux-gate installations are used to determine the magnetic properties of rocks and observations in wells, and astatic magnetometers and rock generators are used to measure remanent magnetization. Let us dwell in more detail on such a device as a magnetometer.

Magnetometer- a device for measuring the characteristics of a magnetic field and the magnetic properties of substances (magnetic materials). Depending on the value being determined, instruments are distinguished for measuring: field strength (oerstedmeters), field direction (inclinators and declinators), field gradient (gradientometers), magnetic induction (teslameters), magnetic flux (Webermeters, or fluxmeters), coercive force (coercimeters) , magnetic permeability (mu-meters), magnetic susceptibility (kappa-meters), magnetic moment.

In a narrower sense, magnetometers are instruments for measuring the strength, direction and gradient of a magnetic field.

The most important parameter of a magnetometer is its sensitivity. At the same time, it is almost impossible to formalize this parameter and make it uniform for all magnetometers, and not only because magnetometers differ in the principle of operation, but also in the design of the converters and the function of signal processing. For magnetometers, sensitivity is usually denoted by the magnitude of the magnetic induction of the field that the device is capable of registering. Typically, sensitivity is measured in nanotesla (nT) 1nT = (1E-9) T.

The Earth's field is approximately 35000nT (35µT). This is an average value - in different parts of the globe it varies in the range of 35000nT (35µT) - 60000nT (60µT). Thus, the task of searching for ferromagnetic objects is to detect, against the background of the Earth’s natural field, an increase in the field caused by distortions from ferromagnetic objects.

There are several physical principles and types of magnetometric instruments based on them that make it possible to record minimal changes in the Earth's magnetic field or distortions introduced by ferromagnetic objects. Modern magnetometers have a sensitivity from 0.01nT to 1nT, depending on the principle of operation and the class of problems being solved.

There are magnetometers for measuring absolute values of field characteristics and relative changes in the field in space or time. The latter are called magnetic variometers. Magnetometers are also classified according to operating conditions and, finally, in accordance with the physical phenomena underlying their operation.

There are several types of magnetometers based on different principles of operation, such as: fluxgate, magnetoinductive, Hall effect, magnetoresistive, quantum (Proton).

Let us dwell in detail on fluxgate magnetic field converters, consider their operating principle, design and measurement technology.

The discovery of the properties of high magnetic permeability in iron-nickel alloys - permalloys led to the creation of fluxgate or flux-sensing magnetometers, the operation of which sensors is based on the effect of the reaction of the magnetic permeability of permalloy cores to the action of the Earth's constant magnetic field when powered by alternating current.

The fluxgate magnetic field transducer, or fluxgate, is designed to measure and indicate constant and slowly changing magnetic fields and their gradients. The action of a fluxgate is based on a change in the magnetic state of a ferromagnet under the influence of two magnetic fields of different frequencies. Depending on the magnitude of the applied voltage, the fluxgate can operate on the peak-type and second harmonic principles. Devices operating on the second harmonic principle have become more widely used(3).

Ferromagnetic probes are characterized by:

High sensitivity - the minimum change in the measured field element that the device is capable of registering when the power component changes; the sensitivity of the best devices is 1 nT, for an angular value - 01 sec;

Possibility of accurate (0.1%) calibration;

Low temperature coefficient, less than 0.01 nT/deg. Celsius in the temperature range from -20 to +50 degrees. Celsius;

Low level of own noise;

Small in size (10-20 cm) and weight (1-2 kg with a measuring unit);

Low power consumption(2).

In Fig. Figure 1 schematically shows some design options for fluxgates.

Rice. 1

In its simplest version, a fluxgate consists of a ferromagnetic core and two coils located on it: an excitation coil powered by alternating current and a measuring (signal) coil. The fluxgate core is made of materials with high magnetic permeability. An alternating voltage with a frequency of 1 to 300 kHz is supplied to the excitation coil from a special generator (depending on the level of parameters and purpose of the device). In the absence of a measured magnetic field, the core, under the influence of an alternating magnetic field H created by the current in the excitation coil, is remagnetized in a symmetrical cycle. A change in the magnetic field caused by the magnetization reversal of the core along a symmetrical curve induces an emf in the signal coil that varies according to a harmonic law. If at the same time a measured constant or slowly changing magnetic field Ho acts on the core, then the magnetization reversal curve changes its size and shape and becomes asymmetrical. In this case, the magnitude and harmonic composition of the EMF in the signal coil changes. In particular, even harmonic components of the EMF appear, the magnitude of which is proportional to the strength of the measured field and which are absent during a symmetric magnetization reversal cycle.

Fluxgates are divided into:

single-element rod (a)

differential with open core (b)

differential with a closed (ring) core (c).

A differential fluxgate (Fig. b, c), as a rule, consists of two cores with windings that are connected in such a way that the odd harmonic components are practically compensated. This simplifies the measuring equipment and increases the sensitivity of the fluxgate. Fluxgate probes are characterized by very high sensitivity to magnetic fields. They are capable of recording magnetic fields with strengths up to 10-4-10-5 A/m (~10-10-10-11 T).

Modern fluxgate designs are compact. The volume of the fluxgate with which domestic G73 magnetometers are equipped is less than 1 cm 3, and the three-component fluxgate for the G74 magnetometer fits into a cube with a side of 15 mm

As an example in Fig. Figure 2 shows the design and dimensions of a miniature fluxgate rod.

Rice. 2

The design of the fluxgate is quite simple and does not require any special explanation. Its core is made of permalloy. It has a cross-section that varies along its length, decreasing by approximately 10 times in the central part of the core, on which the measuring winding and the excitation winding are wound. This design provides, with a relatively short length (30 mm), high magnetic permeability (1.5x105) and a low value of the saturation field strength in the central part of the core, which leads to an increase in the phase and time sensitivity of the fluxgate. Due to this, the shape of the output pulses in the measuring winding of the fluxgate is also improved, which makes it possible to reduce the errors in the time-pulse signal generation circuit. The measurement range of fluxgate converters of a standard design is ±50… ±100 A/m (±0.06… ±0.126 mT). The magnetic noise density in the frequency band up to 0.1 Hz for fluxgates with rod cores is 30 - 40 μA/ m (m x Hz1/2) depending on the excitation field, decreasing as the latter increases. In the frequency band up to 0.5 Hz, the noise density is 3 - 3.5 times higher. An experimental study of ring fluxgates revealed that their noise level is an order of magnitude lower than that of fluxgates with rod cores(3).

Electromagnetic phenomena in a transformer with a steel core are similar to those in an air transformer, but the magnetic flux that penetrates both windings is closed not through the air, but through the steel core (Fig. 15.31).

When a transformer is loaded, there are three magnetic fluxes: F– main in the core, F σ 1 – scattering, connected only to the primary winding, F σ 2 – scattering, associated only with the secondary winding.

The main magnetic flux induces an emf in the primary and secondary windings. respectively e 1 and e 2. Magnetic flux leakage F σ 1 and F σ 2 is induced in the primary and secondary windings of the emf. respectively e σ 1 and e σ 1 .

Voltage u 1 , applied to the primary winding is balanced by the voltage drop across the active resistance of the winding and electromotive forces e σ 1 and e σ 1, i.e.

Let us first consider an ideal transformer in which r 1 = 0; x σ 1 = 0; r 2 = 0; x σ 2 = 0; w 1 = w 2 .

At idling such a transformer does not differ from an ordinary ideal coil and can be represented by an equivalent circuit (Fig. 15.33).

r m

Let's construct a vector diagram of an ideal transformer at no-load (Fig. 15.34).

Magnetizing force at idle

Let us now draw up an equivalent circuit for an ideal transformer under its load (Fig. 15.35).

If a load with resistance is connected to the terminals of the secondary winding Z n, then a current will pass through it , which, in turn, will tend to reduce the magnetic flux , and this will lead to a decrease in emf. , as a result of which the current will increase to such a value at which the magnetic flux will acquire its original value and equation (15.35) will be fulfilled.

Thus, the appearance of current in the secondary circuit leads to an increase in the current in the primary circuit. In a loaded transformer, the magnetic flux in the core is equal to the magnetic flux at no-load, i.e. Always F= const. Under load magnetic flux is created under the influence of magnetizing forces of the primary and secondary windings:

Let's construct a vector diagram of an ideal transformer under load (Fig. 15.36).

Let's transform the equivalent circuit of an ideal transformer, for which we will get rid of inductive coupling. If you connect the same terminals of the transformer windings to each other, the operating mode of the transformer will not change.

Let us first consider inductively coupled elements that now have a common point. The coupling coefficient of two elements in in this case is equal to unity, since the entire magnetic flux is completely linked to the turns of the primary and secondary windings, i.e.

therefore, given that w 1 = w 2, we find:

Let us now replace part of the circuit with inductively coupled elements with a common point (Fig. 15.37 A) to an equivalent circuit without inductive coupling (Fig. 15.37 b).

;

Taking into account what was found, the circuit takes the form shown in Fig. 15.37 V, and the equivalent circuit of an ideal transformer is the type shown in Fig. 15.38.

If we now take into account the active and inductive leakage resistance of both windings, then for a transformer in which w 1 = w 2, we get the equivalent circuit shown in Fig. 15.39.

Let us write down the equations of the primary and secondary circuits of the circuit:

;

Let's construct a vector diagram of the circuit (Fig. 15.40).