Electricity: measuring and testing – Magnetic – Current through test material forms test magnetic field
Reexamination Certificate
1997-09-11
2002-04-02
Snow, Walter (Department: 2862)
Electricity: measuring and testing
Magnetic
Current through test material forms test magnetic field
C324S235000, C324S238000
Reexamination Certificate
active
06366085
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a measuring technique for measuring a magnetic field vector, particularly useful for mapping the distribution of electric currents in a sample in order to locate defects.
2. Description of the Background Art
Devices for measuring the distribution of a magnetic field are known and widely used. Such a device typically comprises either a single sensor, or probe, mounted for movement along the surface of a sample to be inspected, or a stationary arrangement comprising an array of such probes.
FIG. 1
illustrates the main principles of operation of the conventional magnetic sensor such as, for example, a Hall sensor. Hall sensors are based on the known Hall effect according to which a magnetic field applied to a semiconductor, along which an electric current flows, produces a voltage across the semiconductor in a direction perpendicular to the magnetic field and the current directions. A Hall sensor, generally designated
1
, typically has an active element
2
and two pairs of ohmic contacts
2
a
-
2
b
and
3
a
-
3
b
. An electric current I flows between the contacts
2
a
-
2
b
aligned in the direction x. This current I, the magnitude and direction of which are known from a calibration stage, in the presence of a perpendicular magnetic field, generates a respective Hall voltage V
y
in the contacts
3
a
and
3
b
aligned in a transverse direction y. As known, a Hall sensor is sensitive to that component of the magnetic field which is perpendicular to its surface. More specifically, the Hall voltage V
y
is responsive to the current flow I and to the strength of a magnetic field provided within the vicinity of the sensor
1
and directed perpendicular to the surface of the active element
2
. Thus, a component B
z
of the magnetic field B is measured. All these particulars are well known per se and, therefore, need not be described in more detail.
It is appreciated that in order to determine the gradient of the magnetic field B, within the vicinity of a conductive material, and thereby the actual magnitude and direction of the electric currents inside the conductive material, the magnetic field at different locations relative to the conductor should be determined. Although this information may be obtained by moving a single Hall sensor across the conductor, stationary arrangements of linear Hall sensor arrays have been developed by producing a row of such sensors aligned in a straight line.
FIG. 2
illustrates the geometrical arrangement of a device of this kind, generally designated
4
, comprising a pair of Hall sensors
6
and
8
. The device
4
is located within a magnetic field B which is either externally applied magnetic field or induced by an electric current. The sensors
6
and
8
are aligned in a line extending in a direction x. The magnetic field components Bz
1
and Bz
2
are measured independently by each of the sensors
6
and
8
by the way of direct measurement of Vy
1
and Vy
2
. Hence, the gradient of the magnetic field component B
z
along the direction x can be calculated as:
∂
B
z
∂
x
=
B
z
1
-
B
z
2
L
(
1
)
wherein L is the known distance between the sensors
6
and
8
.
A device of this kind is disclosed, for example, in the article “Automatic Devices for the Measurement of Flux Density Gradients”, H. W. Weber et al., Cryogenics, 1976. The device comprises eleven field probes made of InSb and mounted on a narrow gap between the two halves of a sample. Output voltages of the probes are recorded simultaneously in order to provide a complete description of the magnetic field distribution at eleven positions along the sample radius.
Turning back to
FIG. 2
, it is understood that the smaller the dimensions of each of the sensor and the distance L between the sensors
6
and
8
, the higher the resolution of the device
4
. It is often the case that the local value of a magnetic field should be measured rather than global, for example for inspecting high temperature superconductors.
A device of this kind is disclosed, for example, in the article “Local Magnetization Measurements in High Temperature Superconductors”, D. Majer et al. The device presents magnetization measurements in which a magnetic response to an externally applied field is investigated. The main purpose of the device is to provide local values of the magnetic induction B inside the sample. To this end, the device comprises arrays of substantially small Hall sensors each extending in a plane parallel to the sensors' surfaces and formed in a two dimensional electron gas (2DEG) material. These sensors have the advantage of a linear response to magnetic field, weak temperature dependence and high sensitivity. The advantage of 2DEG material is the ability to make several Hall sensors on the same device for measuring the magnetic induction across and outside the sample and giving a detailed local structure of the magnetic profile without limitation from sample's dimensions.
It is thus evident that according to the conventional approach as described above, the array of spaced parallel Hall sensors extends in the plane parallel to the sensors' surfaces. Each of the sensors measures the component B
z
of the magnetic field induction associated with respective location (x
i
;y
j
) on the sample. If the array extends in the direction x, as exemplified in
FIG. 2
, the spatial distribution of the perpendicular component B
z
(x) is mapped.
However, the distribution of the other two components B
x
(x;y) and B
y
(x;y) of the magnetic field B, i.e. components parallel to the sensors' surfaces, cannot be measured by the conventional device employing magnetic sensors of any known kind. This information is very important, for example, for mapping electric currents inside a conductive material in order to make a useful diagnostic tool for finding features like cracks in the conductive material.
A device for measuring magnetic properties has been developed and disclosed in the article “Three-Axis Cryogenic Hall Sensor”, J. Kvitkovic et al., Journal of Magnetism and Magnetic Materials, 1996. The device comprises three independent Hall sensors glued to a supporting ceramics and located at the corner edge thereof for detecting the spatial field profile within a small cube. The Hall sensors are arranged in such a manner that centers of their active areas are placed in three mutually perpendicular planes. The sensors are supplied by a single constant current source. A sample to be inspected is placed in an external magnetic field region. It is appreciated that such an arrangement of the device enables the magnetic field components to be measured along three directions x, y and z. However, the manufacturing and operation of the device are complicated requiring gluing processes and displacement of the device in order to obtain the map of a magnetic field vector.
It is often the case that a conductive structure has to be inspected without destroying the usefulness thereof. In other words, the contact to a conductive structure so as to directly connect it to a power source may be undesirable and/or impossible. Indeed, it turns out to be very difficult to place reliable electrical contacts on the surface of many conductive structures and in many cases a structure to be inspected in not accessible for attaching contacts. One of the conventional diagnostic techniques usually employed for inspecting such a conductive structure, the so-called ‘eddy current technique’, is based on the finding that an electric current flowing inside the structure is induced by an external alternating magnetic field. The standard way of measuring the magnetic field generated by eddy currents is based on the same process that generated the eddy currents, i.e. magnetic induction. A small coil, or array of small coils, is placed over the conductive structure and used for monitoring changes in the magnetic field patterns associated with the eddy currents.
However, the use of the magnetic induction method requires that the m
Abulafia Yosef
Majer Daniel
Shaulov Avner
Shtrikman Hadas
Wolfus Yehoshua
Bar-Ilan University
Snow Walter
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