**1. Introduction**

18 Magnetic Sensors – Principles and Applications

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224425.

Fluxgates are vectorial sensors of magnetic fields, commonly employed for high resolution measurements at low frequencies in applications where the sensor must operate at room temperature.

Fluxgates are usually classified in two categories: parallel and orthogonal fluxgates. In both cases, the working principle is based on a magnetic core periodically saturated in opposite directions by means of an excitation field. The measured field is superimposed to the excitation field and it alters the saturation process.

The basic structure of the orthogonal fluxgate and its difference to parallel fluxgates is illustrated in Fig. 1. The parallel fluxgate (Fig. 1-A) is composed, in its most common form, of a magnetic ring or racetrack core periodically saturated in both directions by the ac magnetic field Hex generated by the excitation coil. The output voltage is obtained with a pick-up coil wound around the core. Even harmonics arise in the output voltage when an external magnetic field Hdc is applied in the axial direction. The sensor is called a *parallel* fluxgate because the excitation field Hex and the measured field Hdc lay in the same direction. More details about parallel fluxgates can be found in (Ripka, 2001).

Orthogonal fluxgates are based on a similar principle, but they have a different structure, as shown in Fig. 1-B.

Fig. 1. Structure of parallel (A) and orthogonal (B) fluxgates.

Orthogonal Fluxgates 21

The axial field is assumed to be much lower than the saturation field HS, therefore during the part of the period when H<HS the core is not saturated. Under these conditions, when H increases, then both the angle and the amplitude of M also increase, while the component of M in the axial direction MZ does not change because HZ is constant. However, when H reaches the amplitude where the total field is Htot=HS, then the core gets saturated; if H further increases, then the amplitude of M does not increase anymore and the only effect of H is to rotate M along the circumference which describes the saturation state (Fig. 2c). Under this condition MZ is not constant anymore but it starts decreasing as the M reaches the saturated state (Fig. 2e). As a result, a variation of the magnetic flux occurs in

Since the excitation field is sinusoidal the saturation is reached twice per period (i.e. in both the positive and negative directions). This means that the induced voltage will contain even harmonics of the excitation frequency, wherein the second harmonic is generally extracted by means of a lock-in amplifier to obtain the output signal. The amplitude of the induced voltage depends on MZ, which in turn is determined by HZ. Finally, the amplitude of the

t t

Vind

e)

t

M

HS H

MZ

HZ

MZ

H

M

If the direction of HZ is reversed MZ becomes negative and the phase of the induced voltage is shifted by rad. This means that the orthogonal fluxgate is able to distinguish between positive and negative fields; usually, the real part of the second harmonic is used as an

the axial direction and a voltage is induced in the pick-up coil (Fig. 2d).

HZ

a) b) H

M

b) c)

Iex ≡ H

d)

Fig. 2. Working principle of orthogonal fluxgates.

even harmonics gives us a measurement of the axial field HZ.

The core is a cylinder of soft magnetic material, with a toroidal excitation coil wound around it. The excitation current flows through the toroidal coil generating excitation H in the circumferential direction. The core is periodically saturated in the circumferential direction by H in opposite polarities. Finally, the output voltage is obtained with a pick-up coil as it was in the parallel fluxgate. Once more, when the external field Hdc is applied in the axial direction, even harmonics arise in the output voltage. In this case, the sensor is called an *orthogonal* fluxgate because the measured field Hdc is orthogonal to the x-y plane where the excitation field H lays.

Orthogonal fluxgates have been originally proposed in (Alldredge, 1952), both with a cylindrical core and a wire-core. Later, Schonsted proposed an orthogonal fluxgate based on a magnetic wire wound in a helical shape around a conductive wire carrying an excitation current (Schonsted, 1959). Several years later, orthogonal fluxgates appeared again in (Gise & Yarbrough, 1975) where the authors proposed an orthogonal fluxgate with a core obtained by electroplating a Permalloy film on a 6.3 mm diameter glass cylinder after the deposition of a copper substrate. The sensor showed large hysteresis, and it was later improved in (Gise & Yarbrough, 1977) with a core composed of a 3.2 mm diameter copper cylinder and of an electroplated shell on it. An orthogonal fluxgate based on a composite wire, manufactured with a conductive core and electroplated magnetic thin film (about 1 µm thick), was also proposed in (Takeuchi, 1977) after which orthogonal fluxgates were almost forgotten.

From the early years, indeed, parallel fluxgates have been always preferred to orthogonal fluxgates because they usually offer better performances, especially lower noise. Thus, the mainstream of research and development focused on parallel fluxgates.

The development and improvement of techniques for the production of microwires obtained in the last decades (Vázquez et. al, 2011) have made it now possible to manufacture soft magnetic wires with an extremely narrow diameter (50-100 µm) and high permeability. Thanks to this, the principle of the orthogonal fluxgate has been rediscovered. For example, an orthogonal fluxgate sensor based on a glass covered Cobased alloy with a very narrow diameter was proposed in (Antonov et al., 2001) and a similar sensor with a Permalloy/copper wire was used as the core with a 20 µm diameter (Li et al., 2004).

Orthogonal fluxgates based on a microwire gained new popularity mainly due to the rising requests for miniaturized sensors of magnetic fields.
