1. Introduction
Giant Magneto-Impedance (GMI) is a large change of the AC impedance of some soft magnetic materials (such as amorphous wires or ribbons) when they are subjected to an external DC or low-frequency AC magnetic field [1]. Since its discovery in the early ninety [2,3], the GMI effect has continued to attract particular interest in the development of a new class of magnetic sensors for a wide palette of applications [4,5].
The simplified model of the phenomena attributes this GMI effect to the change in the skin (or penetration) depth of the high frequency current in the magnetic conductor upon application of external magnetic field. In the case of an amorphous magnetic wire, the skin depth d of a high frequency current of frequency f0 is given by (1)
(1)
where s is the conductivity of the magnetic material, µ0 the permeability of the vacuum and µr the relative transverse magnetic permeability of the wire. The strong dependence of the transverse permeability on the applied external magnetic field is in the origin of the change in the penetration depth and consequently in the AC impedance of the magnetic conductor. Measuring this impedance change gives then a measurement of applied magnetic field.
While intensive works and a large number of publications in this area focus mainly on materials and on the design of high sensitivity magnetic-field sensors [6,7], there are relatively few papers that have deeply investigated the application aspects of this type of magnetic sensors to indirectly measure other physical quantities such as electrical current, linear or angular position, force, strain, etc. [4]. Actually, all these quantities could be, roughly speaking, “converted” into a magnetic field, which in turn could be measured by a GMI sensor. In these kinds of applications, high sensitivity of the magnetic sensor could sometimes not be an issue.
The underling idea of the present work is to evaluate the potential of a GMI sensor for DC and low frequency AC electrical current measurement. The main applications in this area should concern power systems and automotive applications. Further works should also investigate linear and angular position sensors based on the GMI effect.
The principle of current measurement using a GMI sensor is relatively simple: a current produces a magnetic field which can be measured by a GMI sensor. A palette of articles has actually reported designs and performances of basic GMI current sensors utilizing both amorphous wires and ribbons [8-10]. A GMI current sensor was also patented [11]. These works have addressed the simple principle of GMI-based current sensors. Indeed, the sensor output was, as expected, non linear (or at best linear in a small dynamic range of the measured current) due to the intrinsic and strong non linear dependence of the impedance on the applied magnetic field. Moreover, the current direction could generally not be determined since amorphous materials exhibit symmetric dependence on positive and negative magnetic fields.
To develop linear GMI sensor, odd GMI characteristics should firstly be obtained. This could be done by a variety of methods such as the use of intrinsically asymmetric GMI materials [12] or the use of an external bias magnetic field. This external biasing could be achieved by a permanent magnet, a DC or an AC bias current [13- 16]. In such a way, the direction of the measured current could be determined, but the linearity of the sensor is only good in a limited range of current values. This linearity could secondly be improved using the concept of the negative feedback [14,17-19]. A GMI magnetometer setup that uses a feedback was investigated for high sensitivity magnetic field measurements [20].
For the application of current sensing, only Zhan et al. [18] have proposed a GMI sensor based on amorphous ribbons excited with a sharp pulse current. They used also field biasing and feedback. The sensor response was linear in a relatively small dynamic range of ±1.5 A. Only DC current measurements were performed.
In this paper, we present the full design and development of a linear current sensor using amorphous wire with both field biasing and strong negative feedback. Unlike the work presented in [18], we show that the optimization of the sensor allows achieving a very good linearity and a quite reasonable sensibility in a high dynamic range of ±40 A for application in power systems. In this context, low frequency AC current measurements have also been successfully performed using the developed sensor. The global performances of the sensor are shown to be very good in a large dynamic range of the measured DC and AC currents.
Circuits were fully designed and the operation principles of the sensor in both openand closedloops are analyzed to help the reader in easily reproducing the design with a minimum development time.
2. Sensor Design and Performances
2.1. Sensor Circuit and GMI Characteristics
A typical GMI sensor should be composed of a GMI conductor (wire in our case). A high frequency (f0) current with constant amplitude is supplied to the conductor. When an external magnetic field is applied, the change in the impedance gives a change of the amplitude of the voltage developed across the conductor. In other words; the applied magnetic field modulates the amplitude the voltage. A demodulation circuit allows then obtaining a voltage which is proportional to the impedance change.
These principles have been implemented in our sensor, together with field biasing and feedback. All the required sensor electronics was fully developed.
Figures 1 and 2 show a simplified block diagram and a photo of the developed sensor The GMI element is a Co-rich commercial wire (from Unitika Ltd.) of about 6.3 cm length and 30 µm diameter. This wire was curled to a toroidal core of a diameter d = 2 cm. A 25-turns bias coil and a 70-turns feedback coil are wound around the core (Figures 1 and 2). The measured current is Im.
The electronics is mainly composed of an oscillator based on an inverter trigger (74HC14), a peak detector and a high gain amplifier with zero adjust.
In open-loop operation, the oscillator generates a square wave form with 3 MHz frequency, 10 ns rise time and
Figure 1. GMI current sensor. The sensor diameter is d = 2 cm. Im is the measured current.
zero DC component. The voltage is converted into a constant square waveform current (iac) of 5 mA peak-topeak through the resistor R, and supplied to the GMI element. A peak detector, formed by the diode D and the filter Rp-Cp, provides a DC component proportional to the amplitude of the voltage of the GMI element and consequently to its impedance. This DC voltage is supplied to the instrumentation amplifier (AD620). The zero adjust of the amplifier is performed through a potentiometer. A zero voltage in the output of the amplifier should correspond to a zero external magnetic field. The amplifier output is measured using a digital voltmeter.
Figure 3 shows the GMI characteristics of the wire when excited under the previous conditions. This figure represents the GMI ratio which is defined as the relative change of the wire impedance (Z) with the applied magnetic field (H). This ratio is expressed by (2)