Magnetic - Field - Measurement

FIGURE 48.7 In Schonstedt (a) and ring core (b) fluxgate sensors, the excitation field is at right angles to the signal winding axis. This configuration minimizes coupling between the excitation field and the signal winding.

0.1 nT to 1 mT range from dc to several kHz, make it a very versatile instrument. Geologists use them for exploration and geophysicists use them to study the geomagnetic field (about 20 ??T to 75 ?T on the Earth’s surface). Satellite engineers use them to determine and control the attitude of spacecraft, scientists use them in their research, and the military uses them in many applications, including mine detection, vehicle detection, and target recognition. Some airport security systems use them to detect weapons. The Fluxgate

The heart of the magnetometer is the fluxgate. It is the transducer that converts a magnetic field into an electric voltage. There are many different fluxgate configurations. Two of the more popular ones are shown in Figure 48.7. A very comprehensive explanation of the fluxgate principle and the different fluxgate configurations is given in [10].

The ring core fluxgate is constructed from a thin ribbon of easily saturable ferromagnetic material, such as 4-79 Permalloy wrapped around a bobbin to form a ring or toroid. As shown in Figure 48.8, an alternating current is applied through a coil that is wound about the toroid. This creates a magnetic field that circulates around the magnetic core. This magnetic field causes the flux in the ferrous material to periodically saturate first clockwise and then counterclockwise. A pick-up (signal) winding is wrapped around the outside of the toroid. While the ferrous material is between saturation extremes, it maintains an average permeability much greater than that of air. When the core is in saturation, the core permeability

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FIGURE 48.8 The excitation field of a fluxgate magnetometer alternately drives the core into positive or negative saturation, causing the core’s effective permeability to switch between 1 and a large value twice each cycle.

becomes equal to that of air. If there is no component of magnetic field along the axis of the signal winding, the flux change seen by the winding is zero. If, on the other hand, a field component is present along the signal winding axis, then each time the ferrous material goes from one saturation extreme to the other, the flux within the core will change from a low level to a high level. According to Faraday’s law, a changing flux will produce a voltage at the terminals of the signal winding that is proportional to the rate of change of flux. For dc and low-frequency magnetic fields, the signal winding voltage is:

d H

0 e

e t ?????

nA _______

dt

???????????d t?

?????e nA H?0

dt

(48.28)

where H = Component of the magnetic field being measured

n = Number of turns on the signal winding A = Cross-sectional area of the signal winding

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FIGURE 48.9 Typical circuit configuration for a field feedback fluxgate magnetometer. The sensor output is ac amplified, synchronously demodulated, and filtered. A magnetic field that nulls the ambient field at the sensor is produced by connecting the resistor Rf between the output and the signal winding.

As the core permeability alternates from a low value to a high value, it produces a voltage pulse at the signal winding output that has an amplitude proportional to the magnitude of the external magnetic field and a phase indicating the direction of the field. The frequency of the signal is twice the excitation frequency since the saturation-to-saturation transition occurs twice each excitation period.

The discussion about effective permeability in the induction coil section applies here as well. Consult [10, 13] for comprehensive discussions about fluxgate effective permeability and signal characteristics as they relate to excitation field level, excitation waveform, and winding geometry. Signal Conditioning

The signal from the fluxgate is an amplitude-modulated suppressed carrier signal that is synchronous with the second harmonic of the excitation signal. In a simple low-power magnetometer, this signal is converted to the base band using a synchronous demodulator, filtered, and presented as the final output. Example circuits are given in [11, 12]. The accuracy of magnetometers that use this open-loop architecture is limited by the linearity of the core’s magnetization curve and is about 5% for Earth’s field (60 ??T) applications. More precise and stable magnetometers use magnetic field feedback rather than the open-loop struc-ture described above. A simplified schematic of a typical second harmonic field feedback fluxgate mag-netometer is shown in Figure 48.9. The circuitry to the left of the fluxgate is called the excitation circuit. It consists of an oscillator tuned to twice the excitation frequency, a flip-flop that divides the oscillator frequency by two, and a power amplifier driven by the flip-flop and, in turn, provides the excitation current to the excitation winding.

The circuitry to the right of the fluxgate is called the signal channel circuit. It amplifies the output from the fluxgate signal winding, synchronously demodulates the ac signal using the oscillator signal as a reference, integrates and amplifies the base band output, and then feeds back the output through a resistor to the signal winding. The fed-back signal produces a magnetic field inside the sensor that opposes the external field. This keeps the field inside the sensor near zero and in a linear portion of the magnetization curve of the ferromagnetic core.

The flow diagram for the magnetometer is given in Figure 48.10. The external field Ha is opposed by the feedback field Hf, and the difference is converted into a voltage signal (Ks represents the transfer function from field to voltage). This signal is amplified (A), and the amplified signal is converted into a current If and then into the feedback field (Kc represents the transfer function from current to field). The overall transfer function for the magnetometer is:

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AK V0 ? ?? s

s Ha c K AK 1? Rf

(48.29)

FIGURE 48.10 Block diagram of a field feedback fluxgate magnetometer. Kc is the current-to-field constant for the coil. Ks is the field-to-voltage transduction constant for the sensor. The feedback field Hf opposes the ambient field Ha, thus keeping the net sensor field very small.

The amplifier gain is normally very high such that the second term in the denominator is much larger than one, and Equation 48.29 reduces to

V0 ??Rf

H K a

c

(48.30)

Under these circumstances, the transfer function becomes almost completely determined by the ratio of

Rf (the feedback resistor) to Kc (the current-to-field coil constant of the sensor winding). Both of these constants can be very well controlled. The consequence of this circuit topology is a highly stable and accurate magnetometer that is insensitive to circuit component variations with temperature or time. An accuracy of 1% over a temperature range of –80?C to 80?C is easily achievable. Accuracy and stability can be improved using a current feedback circuit, like the one described in [13], that compensates for the resistance of the signal winding or by using a separate feedback winding and a high-quality voltage-to-current converter instead of a simple feedback resistor.

The SQUID Magnetometer

Brian D. Josephson in 1962, while a graduate student at Cambridge University, predicted that supercon-ducting current could flow between two superconductors that are separated by a thin insulation layer. The magnitude of the superconductor (critical) current through this ―Josephson junction‖ is affected by the presence of a magnetic field and forms the basis for the SQUID magnetometer.

Figure 48.11 illustrates the general structure of a Josephson junction and the voltage–current (V–I) relationship. Two superconductors (e.g., niobium) are separated by a very thin insulating layer (e.g., aluminum oxide). The thickness of this layer is typically 1 nm. When the temperature of the junction is reduced to below 4.2 K (–269?C), a superconductor current will flow in the junction with 0 V across the junction. The magnitude of this current, called the critical current Ic, is a periodic function of the magnetic flux present in the junction. Its maximum magnitude occurs for flux values equal to n?0, where ?0 is one flux quantum (2 fW), and its minimum magnitude occurs for flux values equal to (n + 1/2)?0. The period is one flux quantum. This phenomenon is called the ―dc Josephson effect‖ and is only one of the ―Josephson effects.‖

Magnetometers based on the Superconducting Quantum Interference Device (SQUID) are currently the most sensitive instruments available for measuring magnetic field strength. SQUID magnetometers measure the change in the magnetic field from some arbitrary field level; they do not intrinsically measure the absolute value of the field. Biomedical research is one of the more important applications of SQUID magnetometers. SQUID magnetometers and gradiometers (measure spatial variation in the magnetic field) have the high sensitivities needed to measure the weak magnetic fields generated by the body [15].

? 1999 by CRC Press LLC