A position sensor based on magnetoimpedance R. Valenzuela,a) M. Vazquez,b) and A. Hernando Instituto de Magnetismo Aplicado, UCM, and Instituto de Ciencia de Materiales, CSIC, P.O. Box 155, 28230 Las Rozas, Madrid, Spain
A magnetic-field sensor based on the giant magnetoimpedance phenomenon is presented. It is shown that a low, negative magnetostriction CoFeBSi amorphous wire can be used to detect the presence or passage of moving pieces or vehicles, simply by pasting a small permanent magnet on the vehicles/pieces. The detection is observed as a decrease in the ac voltage on the wire’s ends. A system of such devices can be used to monitor and control a number of industrial processes. © 1996 American Institute of Physics. @S0021-8979~96!58208-X#
I. INTRODUCTION
Some ferromagnetic materials subjected to an ac electric current exhibit a strong decrease in their impedance in the presence of a dc magnetic field. This phenomenon is known as giant magnetoimpedance1– 4 ~GMI! and is receiving considerable attention because of its applications in magnetic sensors. GMI is a classical electrodynamics phenomenon1,2 which depends essentially on the interaction between the magnetic field created by the ac current and magnetic domains of the sample. GMI has been reported for ribbons and wires; however, it has been observed to be more efficient in low, negative magnetostriction wires, which spontaneously form5 an outer shell with circumferential domains, and still have a weak circular magnetic anisotropy. Since the magnetization direction in such domains has an alternate circumferential orientation, the interaction between magnetization and the magnetic field created by the ac current is strong. At high frequencies, where the impedance response becomes stronger, GMI shows a further complexity1,2 due to skin effect, which tends to concentrate the ac field on a small cross section near the surface of the wire. In this article, an application of GMI as a magnetic-field sensor, adapted to monitor the passage of moving pieces or vehicles typical in many industrial processes, is presented. The development of a system of sensors with a feedback loop to monitor and control the process is also discussed.
To produce an ac current along the wire ~in the range up to 20 mA rms!, a conventional signal generator ~Hameg HM 8030! was used. Voltage measurements were made with a Fluke model 45 multimeter. A dc magnetic field was applied by means of a pair of Helmholtz coils. In other experiments, a small, disk-shaped ~5 mm diameter, 2 mm thickness! NdFeB permanent magnet, axially magnetized was used to produce the dc magnetic field on the wire. During all measurements, the wire axis was perpendicularly oriented with respect to the earth’s magnetic field. III. BASIS OF THE SENSOR
The sensor is based on the dependence of total impedance Z on dc applied magnetic field. As is now well documented,1– 4 Z decreases steeply as H increases, see Fig. 1. This behavior can be explained on the basis of the magnetic domain structure5 of low, negative magnetostriction wires. It can be described as formed by an inner core with axial magnetization, and an outer shell composed of circumferential domains with alternate magnetization. Evidence showing deviations in magnetization directions in both axial and circumferential domains has been recently reported.8 As axial field increases, the inner core is first saturated in the field direction; on further increase, the spins in circumferential domains are deviated toward the axial field direction in a nearly pure rotational process.9 The circumferential
II. EXPERIMENTAL TECHNIQUES
We used amorphous wires of composition ~Co0.94Fe0.06!72.5B15Si12.5, of low, negative magnetostriction in the as-cast state, kindly supplied by Unitika Ltd., Japan, prepared by the in-rotating-water technology.6 A piece of approximately 9 cm was cut, and current and voltage leads were pasted on its ends with Ag paint, after cleaning with a soft acid solution. In order to improve its sensitivity to small, localized magnetic fields, the wire was carefully bent and placed inside a small, acrylic cylinder of 5 and 8 mm inner and outer diameter, respectively, and 4 cm in length. In this way, two sections of the middle part of the wire were exposed to small, localized magnetic fields. The highest sensitivity of wires has been observed7 to axially applied fields. a!
On sabbatical leave from the National University of Mexico. Corresponding author, electronic mail:
[email protected]
b!
J. Appl. Phys. 79 (8), 15 April 1996
FIG. 1. Voltage response of the CoFeBSi wire, as a function of dc axial field.
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© 1996 American Institute of Physics
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component of magnetization is thus decreased, as well as its interaction with the circular field. Therefore, a decrease in impedance response occurs. This process depends both on ac current frequency and amplitude. At low frequencies ~f ,500 Hz!, the decrease in impedance response associated with the presence of axial fields can be extremely small. For as-cast, CoFeBSi wires, a maximum sensitivity to axial fields is observed10 in the 10 kHz–1 MHz. For f .100 kHz, due to the skin-depth effect, field penetration depth becomes smaller than the actual dimensions of the wire. Then, an additional component of impedance appears, which depends on frequency and local permeability values in a complex way.1,2 Finally, the difference in response due to H becomes unnoticeable10 for f .20 MHz. In most reported results, the ac current amplitude is kept constant in spite of the impedance variations, by monitoring the current for each H value. Since we are interested in application conditions, particularly in the voltage response, we have measured the voltage decrease on the wire ends at an initial ~at H50! constant current amplitude of 10 mA ~rms!, as a function of magnetic field, Fig. 1. As the dc field increases, the impedance decreases, leading to an increase in current amplitude. A comparison of Fig. 1 with published Z vs H plots shows that variations of V are smaller than variations in Z for similar experimental conditions. This is due to the fact that Z depends also on the ac current amplitude i. As we have shown elsewhere11 circumferential magnetization processes occur in a way similar to any domain-wall magnetization process. Since the imaginary part of impedance is associated with inductance and, in turn, magnetic permeability is proportional to inductance,11 Z reflects these magnetization processes. Z exhibits, therefore, an increase for small i values ~start of wall propagation!; as i increases, Z goes through a maximum ~corresponding to the maximum value in circumferential permeability; about 5 mA for CoFeBSi wires! and then a hyperbolic decrease, as magnetization approaches the saturation value. As a summary, the most convenient conditions for a maximum sensitivity in GMI are: frequency work in the range 50–500 kHz; and ac current amplitude i in the 8 –15 mA range. IV. EXPERIMENTAL RESULTS AND DISCUSSION
In order to test the capability of the device, the wire ~bent inside the plastic cylinder, as discussed above! was submitted to an ac current of 10 mA ~rms!, at a frequency of 100 kHz. A small permanent magnet was then brought near the wire and the voltage response V was monitored, as a function of the distance between the wire and the permanent magnet ~see Fig. 2!. As can be observed, V decreases from 770 mV for large distances D to 605 mV, for physical contact. For practical purposes, a minimum distance of 2 cm between the sensor and the magnet can be considered, which leads to a threshold voltage of 680 mV. A decrease from 770 to any value V
