We have designed and constructed a magnetic gradiometer for underwater mine detection, location and tracking. The United States Naval Surface Warfare Center (NSWC PC) in Panama City, FL has conducted sea tests of the system using an unmanned underwater vehicle (UUV). The Real-Time Tracking Gradiometer (RTG) measures the magnetic field gradients caused by the presence of a mine in the Earth's magnetic field. These magnetic gradients can then be used to detect and locate a target with the UUV in motion. Such a platform can also be used for other applications, including the detection and tracking of vessels and divers for homeland (e.g., port) security and the detection of underwater pipelines. Data acquired by the RTG in sea tests is presented in this paper.
Magnetic sensors configured as a tensor magnetic gradiometer not only detect magnetic targets, but also determine their location and their magnetic moment. Magnetic moment information can be used to characterize and classify objects. Unexploded ordnance (UXO) and thus many types of improvised explosive device (IED) contain steel, and thus can be detected magnetically. Suitable unmanned aerial vehicle (UAV) platforms, both gliders and powered craft, can enable coverage of a search area much more rapidly than surveys using, for instance, total-field magnetometers. We present data from gradiometer passes over different shells using a gradiometer mounted on a moving cart. We also provide detection range and speed estimates for aerial detection by a UAV.
Conventional concealed weapons detection portals, deployed worlwide at airports, governent buildings, courthouses, and other security critical facilities, are challenged by today's need for stringent and effective entry point screening. Modern threats, like exotic lightweight handguns, are becoming increasingly difficult to detect. Conventional portals do little, if anything, to assist security personnel in resolving the true nature of a potential threat. Systems that rely on blind operator intervention for alarm resolution invite too much human error so security is ultimately compromised. These systems do not instill confidence in the operator or the general public and thus do not present the necessary deterrent to those with sinister intent. Increased detection and improved discrimination do not adequately advance the utility of these security tools. High precision alarm object location information presented to the operator will result in the rapid resolution of the potential threat. Providing this information to the person under scrutiny via a static or streaming video image can enable self-divesting of the offending item without the need for security personnel intervention. Robust detection, effective discrimination, and precise object location information will result in superior entry point screening operations.
KEYWORDS: Sensors, Magnetic sensors, Detection and tracking algorithms, Magnetic tracking, Magnetism, Target detection, Data communications, Calibration, Weapons, Data conversion
Magnetic sensors are unaffected by rolling terrain, by vegetation and
by weather phenomena, so provide a consistent and predictable performance. Most magnetic tracking algorithms work best for a limited span of closest approach distances between the target and any sensor array node and have specific requirements in terms of the relative node placement. Unlike demonstration fields, where the nodes are carefully manually placed, proposed field deployment methods cannot reliably achieve specific spacings. We propose a novel algorithm that is insensitive to variations in spacing, thereby eliminating the limit on closest approach distances, so that the
sensor array can locally adapt the algorithm to the array configuration. This local adaptation also enables scalability for whole-array tracking.
KEYWORDS: Sensors, Target detection, Magnetism, Magnetic tracking, Magnetic sensors, Electromagnetism, Signal detection, Signal processing, Inspection, Submerged target detection
Passive magnetic measurements may be used to determine the azimuth, bearing and range to the target in real time using suitable signal processing without illuminating the target. Collision and even close approach must be avoided for classes of targets such as proximity fuzed mines. This paper describes a simple safety algorithm that monitors data from such a tensor magnetometer system and overrides the normal autopilot to ensure a minimum radius is always maintained
from all magnetic targets encountered.
Magnetic sensors are able to detect, track and characterize
targets without line of sight, vegetation or weather constraints.
This paper discusses performance models, experimental data
and the capabilities of a specific magnetic sensor design.
These factors are combined to infer the operational parameters
of an effective future magnetic sensor array.
KEYWORDS: Magnetism, Sensors, Magnetic sensors, Unmanned aerial vehicles, Transducers, Interference (communication), Signal detection, Magnetic tracking, Data modeling, Computer simulations
Tensor magnetic gradiometry reports accurate direction and approximate range to an UXO using a single point measurement. Instead of using a sparse grid of points, a UAV can fly a series of adjacent tracks and acquire the data needed to locate all items. Some recent modeling suggests that normal UAV flight speeds can achieve an area coverage rate of 15 acres/minute. This contrasts sharply with total field magnetic techniques that require a much denser grid of sensor tracks. Synthetic data from equivalent surface-based measurements and simulations of real time processing substantiate the models.
Low Frequency Eddy Current (EC) probes are capable of measurement from 5 MHz down to DC through the use of Magnetoresistive (MR) sensors. Choosing components with appropriate electrical specifications allows them to be matched to the power and impedance characteristics of standard computer connectors. This permits direct attachment of the probe to inexpensive computers, thereby eliminating external power supplies, amplifiers and modulators that have heretofore precluded very low system purchase prices. Such price reduction is key to increased market penetration in General Aviation maintenance and consequent reduction in recurring costs. This paper examines our computer software CANDETECT, which implements this approach and permits effective probe operation. Results are presented to show the intrinsic sensitivity of the software and demonstrate its practical performance when seeking cracks in the underside of a thick aluminum multilayer structure. The majority of the General Aviation light aircraft fleet uses rivets and screws to attach sheet aluminum skin to the airframe, resulting in similar multilayer lap joints.
Blades, knives, handguns and vehicles are similar targets for magnetic tracking purposes, and their differences can be described parametrically. These parameters can be used for classification, and to model the performance of a sensor node being used for a proposed application. The scaling laws that relate these parameters to actual performance will be reviewed and applied to a real world example. This computed noise limit will be contrasted with the practical limit observed in those measurements.
KEYWORDS: Sensors, Magnetic sensors, Magnetism, Magnetic tracking, Target detection, Surveillance, Digital electronics, Acoustics, Infrared technology, Algorithm development
The development of magnetic sensors for the detection, localization, and classification of time-critical targets is of great importance in monitoring, surveillance, intelligence, and security applications. This is particularly true for the military where precision targeting of armed enemy troops, tracked and wheeled vehicles requires timely updates of their movements. To address this need, Quantum Magnetics (QM) is developing small, low power, low cost magnetic sensor modules, and high performance digital electronics that can be used to passively detect magnetic anomalies in the battlefield generated by the presence and movement of armed troops and military vehicles. The focus is on Magneto-Resistive (MR) sensors that can be fabricated by microelectronics techniques. These sensors represent a mature technology and are widely available commercially. They operate at room temperature with high sensitivity and have a broad bandwidth. The long-term vision is to integrate these modules into a network of battlefield microsensors that include a variety of other sensing technologies (acoustic, seismic, IR, etc.). We will discuss MR system design considerations and results obtained in recent field tests. A highly sensitive magnetic sensor module would also find numerous applications in security operations and surveillance of perimeters and borders, landmine/UXO detection, and detection of concealed weapons.
Quantum Magnetics (QM) has developed a sensing array using small and lightweight magnetoresistive (MR) sensors. These sensors, which operate at room temperature with high sensitivity and wide bandwidth, provide new operational performance capabilities. The wide bandwidth makes them ideal for both passive and active detection techniques. Using a DSP-based electronics developed by QM, we have been able to operate these sensors with an unprecedented noise performance at low frequencies. Recent tests using an MR room temperature gradiometer show that its resolution equals that of a fluxgate room-temperature gradiometer we have previously developed. These results represent an important development for both attended and unattended ground sensor applications since MR sensors cost about ten times less than fluxgate sensors.
KEYWORDS: Sensors, Magnetism, Magnetic sensors, Digital signal processing, Signal detection, Unattended ground sensors, Target detection, Calibration, Unmanned aerial vehicles, Analog electronics
Under funding from the Air Force Research Laboratory at Hanscom Air Force Base, Quantum Magnetics has been developing a room temperature, multi-axis magnetic gradiometer for the detection and characterization of underground structures. The gradiometer uses small, inexpensive, but highly sensitive magnetoresistive sensors operating with a flat frequency response from DC to several MHz and employs an innovative sensor configuration; called the three-sensor gradiometer (TSG) invented at IBM. The TSG affords unprecedented dynamic range that enables detection of signals near the sensor noise floor even when the system is in motion in the earth's field. The wideband response and high sensitivity of these sensors make them ideal for both passive and active detection techniques. A single sensor can detect perturbations in the earth's magnetic field from ferrous materials used in the construction of the structure, emissions at power and mechanical frequencies from equipment within the structure, and eddy currents in metallic materials within the structure induced by externally applied probe signals. These complimentary data sets can be combined in a sensor fusion scheme to minimize sensor clutter and discriminate against false targets. The sensing technology under development supports deployment by way of unattended ground sensors as well low-flying unmanned aerial vehicles.
KEYWORDS: Magnetism, Sensors, Magnetic sensors, Computing systems, Magnetometers, Electronics, Signal processing, Digital signal processing, Head, Signal detection
In Phase II of a Small Business Innovation Research contract funded through the Office of Secretary of Defense, Quantum Magnetics has developed a fieldable room-temperature gradiometer (RTG). The RTG affords unprecedented dynamic range that enables detection of signals near fluxgate sensor noise (approximately 5 pT/Hz1/2 at 1 Hz while the system is in motion in the earth's field (approximately 5 X 107 pT). To achieve this sensitivity when the RTG is integrated with an autonomous underwater vehicle (AUV), the magnetic interference generated by magnetic sources onboard the AUV must be removed. Direct magnetic feedback or signal processing using both ancillary sensors and a priori information about the interfering sources can be used to recover the baseline sensitivity of the RTG.
We have developed a low frequency electromagnetic technique using sensitive room temperature magnetoresistive (MR) sensors for a variety of nondestructive evaluation (NDE) applications. These applications include the NDE of medical implants and aircraft structures, the detection of cracks and corrosion in metals, the detection of ferromagnetic foreign objects in the eye and the brain, and the noninvasive determination of iron content in the liver. Our technique consists of applying a low frequency ac magnetic field to the sample and detecting the sample response. The low excitation frequency enables us to probe deep into metal structures; the sensitivity of the MR sensor allows us to detect weak responses from the sample without applying too large an excitation field, particularly in the case of human tissue. The MR sensors are small and relatively inexpensive compared to other sensitive magnetic field sensors such as fluxgates and superconducting quantum interference devices or SQUIDs; hence the resulting NDE instrument will be compact and cost-efficient, enabling its commercialization for practical applications. In this paper, we focus primarily on NDE of orthopedic implants.
KEYWORDS: Magnetism, Sensors, Electronics, Navigation systems, Magnetic sensors, Prototyping, Global Positioning System, Naval mines, Data acquisition, Algorithm development
In Phase II of a Small Business Innovation Research (SBIR) contract funded through the Office of Secretary of Defense (OSD), Quantum Magnetics is developing a fieldable room- temperature gradiometer (RTG). The RTG uses an innovative sensor configuration, called the three-sensor gradiometer (TSG), invented at IBM. The TSG affords unprecedented dynamic range that enables detection of signals near fluxgate sensor noise while the system is in motion int eh earth's field. Sensor enhancements undertaken in this Phase II program include: incorporation of ancillary sensors to enable gradiometer balancing in the presence of ambient field gradients; improved feedback linearity and use of a wideband reference fluxgate sensor to reduce motion noise; and improved filter matching between channels. Operational developments in progress include reduction of the sensor electronics package for man-portability and implementation of real-time operating and target localizing software. The Phase II system will be used for land-based operations to locate unexploded ordnance, and the sensor is being integrated with differential global position satellite navigation to locate targets in geographical coordinates. Separately funded programs will adapt the RTG for use by divers or AUVs in finding naval mines and obstacles.
Ultrasonic inspection methods are often used to detect defects in ferrous and non-ferrous metal casting processes. The finite sensor recovery time following emission of the ultrasonic pulse leads to a 'shadow zone', from the surface to a depth of about 1 cm, that cannot be probed by ultrasonics. Conventional eddy current methods, operating at frequencies of several kHz or higher, can only detect surface-breaking flaws. Giant Magneto-REsistive and Anisotropic Magneto-Resistive sensors offer their full performance independently of frequency from 100kHz down to DC. This allows eddy current-based technique to prove ferrous materials deeper than heretofore, and thus to provide coverage through the shadow zone. The work presented here demonstrate the performance and sensitivity that can be achieved. The separation of eddy current and ferromagnetic signals allows the structure to be probed and the material properties to be imaged. We have detected artificial defects in ferrous steel samples to a depth of 1 cm as well as surface flaws smaller than those found by conventional eddy current methods. We also show results of probing a cast and rolled steel sample with real defects, where a band of presumed subsurface ferromagnetic inclusions masks the signal from tight surface cracks.
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