Electricity: measuring and testing – Of geophysical surface or subsurface in situ – By magnetic means
Reexamination Certificate
2001-04-24
2002-11-05
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
By magnetic means
C324S225000, C324S247000, C324S326000
Reexamination Certificate
active
06476610
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to magnetic sensors, and more particularly to a maneuverable magnetic anomaly sensing system and method that is rotationally invariant and that provides robust magnetic target detection and localization information.
BACKGROUND OF THE INVENTION
Magnetic sensor technologies are being developed to enhance the capabilities of highly-maneuverable autonomous underwater vehicles (AUVs) and other underwater systems. These underwater systems can include navigation, communications and “detection, localization and classification” (DLC) of underwater objects/obstructions or buried objects such as cables or mines. Magnetic sensors can detect and use the static and dynamic magnetic anomaly fields that emanate from magnetically polarizable underwater objects to localize/classify the objects. The low frequency magnetic anomaly signals are not affected by water turbulence and multi-path propagation effects that can limit the performance of other underwater sensing technologies.
Practical constraints on magnetic sensor operation aboard small underwater vehicles impose serious technical challenges principally due to the following factors. First, changes in vehicle orientation in the earth's background magnetic induction field B
E
of 50,000 nano-Tesla (nT) produces large non-target-related changes in the nT-level vector components measured by onboard vector magnetometers. As is the case for any mobile application of low frequency magnetic sensors, in order to obtain useful detection sensitivity and range, the adverse effects of sensor rotation in the earth's field must be eliminated or reduced to an acceptable level in the design of the field-sensing element or compensated by means of signal processing software. However, for small underwater vehicles, the adverse effects of sensor motion are exacerbated because their operation typically involves frequent large, nearly random changes in vehicle orientation due to water turbulence and the effects of uneven seafloor structure.
A second factor hampering magnetic sensor operation onboard small underwater vehicles is that the magnetic signatures of a vehicle's frame, drive motors and other vehicle subsystems may obscure the magnetic signature of a magnetic anomaly which can be indicative of a target object or a communication signal. Hence, in order to achieve effective sensitivity and range, reasonable efforts should be made to reduce the vehicle magnetic signature as much as is practical, to locate the magnetic sensors as far as practical from the magnetically “hot” sections of the vehicle, and to utilize sensor calibration and signal processing techniques that compensate for the non-target-related fields and gradients that are produced by the vehicle's self-magnetic signature.
In addition, the vehicle's limited size and power budget requires the sensing system to be compact, low power and easily portable. The system must combine hardware, software and an efficient target location methodology to optimize the vehicle's search capabilities. These technical problems are somewhat mitigated by the relatively reduced detection range requirements that are allowed by the operational paradigm of detection, localization and classification of magnetic objects using smart AUVs. In this paradigm, the AUV can adaptively modify its search patterns in response to the detected presence of objects, and, for example, autonomously maneuver (under guidance from an efficient magnetic sensor based homing algorithm) so as to maximize its probability of accurately localizing and classifying an object once it is detected.
A brief review is presented of some relevant physical and mathematical concepts that affect the design, development and use of magnetic sensing technologies. Throughout this disclosure, vector quantities are denoted by boldface letters and scalar quantities by normal type. For example, B
A
is a vector while B
A
is a scalar value or quantity. The methodologies and apparatus used for detection, localization and classification of magnetic “anomalies” that are produced by magnetic objects are based on and conditioned by:
1) The existence of vector magnetic induction fields B
A
that emanate from the objects' net magnetization (characterized as a vector “magnetic dipole moment” M) and,
2) The existence of the relatively large magnetic induction field of the earth B
E
that permeates all space around the planetary surface.
FIG. 1
presents a simplified qualitative representation of magnetic field lines of force. As designated by the arrowheads in
FIG. 1
, the (B
A
) field lines leave one end (i.e., the north pole) of a dipole moment M, and curve around and return to the other end (i.e., the south pole) of dipole moment M. Consequently, in the presence of a nearly constant background field such as B
E
, the B
A
field can, depending where the field values are measured, either add to or subtract from B
E
. At distances, r, from the object that are greater than about three times the object's largest dimension, the B
A
fields are described by the well-known magnetic dipole field equation of classical electromagnetic theory. Consequently, the B
A
fields that are produced by magnetic objects generate rapidly-varying, anomalous changes (i.e., amplitudes are reduced by 1/r
3
) in the slowly varying earth's background field B
E
.
As is well known in the art, the magnitude B
E
is generally much larger than B
A
(i.e., B
E
>>B
A
) except for field points that are measured very close to the dipole source M of the magnetic object's anomaly field B
A
. The earth's field and anomaly field vectors sum to create a total field B
T
=B
E
+B
A
. The problem of using B
A
to detect and localize magnetic objects, then, requires methods and apparatus than can detect and discriminate relatively small target signatures B
A
that are convolved with the relatively very large (yet also fairly constant) earth field B
E
.
There are, broadly speaking, two separate approaches to magnetic anomaly localization. One approach involves measurement of changes in the scalar field B
T
and the other approach involves measurement of changes in the vector (and/or tensor) components of B
T
. Both approaches have their advantages and limitations.
The scalar total field approach typically involves the use of magnetic field sensors or magnetometers (e.g., proton precession magnetometers, atomic vapor magnetometers, etc.) that detect and measure the scalar magnitude of the total field B
T
. An important advantage of this approach is the fact that true scalar quantities are “rotationally invariant”, that is, they do not change when the sensor coordinate system rotates. Therefore, scalar magnetometers are often used for mobile applications where the sensor platform can undergo large changes in orientation angle. However, total field magnetometers that can only respond to B
T
essentially only measure the components of B
A
that are parallel to B
E
(i.e., the scalar projection of B
A
on B
E
) and therefore cannot provide a complete set of the target localization information that is implicit in B
A
. In particular, although B
T
and the embedded anomaly field B
A
are rotational invariants, they are not by themselves “robust” quantities that allow efficient localization of magnetic anomalies. As used herein, the term “robust” will be applied to mathematical quantities or signals that always increase as a sensor approaches a target and always decrease as the sensor-target distance increases.
In essence then, true scalar magnetometers do develop rotationally invariant total field signals that contain some target signature information. However, the inherently limited target localization and classification capabilities of scalar total field data require inefficient use of platform mobility resources when trying to find magnetic objects. Specifically, in order to localize magnetic targets, scalar total field magnetometer based sensor systems must make many passes over the target area.
Price Brian L.
Wiegert Roy F.
Aurora Reena
Gilbert Harvey A.
Lefkowitz Edward
Peck Donald G.
The United States of America as represented by the Secretary of
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