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CHIPS Articles: Advances in Magnetometer Technology

Advances in Magnetometer Technology
U.S. Marines will no longer have to worry about what is hidden behind the next rock – they will know
By Tom LaPuzza - January-March 2010
A six-year collaboration by Space and Naval Warfare Systems Center Pacific (SSC Pacific) scientists and engineers, with colleagues in Sweden and Sicily, has put new force protection technology into the hands of U.S. Marines, who will be taking it to the battlefield sometime this year.

Drs. Adi Bulsara and Visarath In, serving as principal investigators, and several other SSC Pacific personnel, have been working with physicists and engineers from the University of Catania in Italy and the Swedish Defense Research Agency (FOI) in Stockholm, to harness the substantial potential of nonlinear dynamics for military and civilian applications.

These applications include battlefield sensors disguised as rocks that can communicate with each other and pass vital information to military planners via satellite links. Similar sensors can be placed on the seafloor and detect swimmers and divers passing in the water column a few meters away.

An add-on to the U.S. Marines' Tactical Remote Sensor System (TRSS) will allow a reconnaissance patrol to image armed individuals even through a wall, and can also be deployed in remote areas as an unattended ground sensor for persistent surveillance. Other sensors, the size of clothing buttons, can also be distributed randomly around a building to alert security personnel to the presence of intruders.

Technology Development

The basic idea of using the principles of nonlinear dynamics in developing a magnetometer with a simpler readout based on the idea of spike timing, which underpins the neural code, came during a 2003 discussion between Dr. Bulsara, his colleagues in Stockholm, and professor Luca Gammaitoni of the University di Perugia in Italy.

"We were chatting and jotting some thoughts on a chalkboard, when it hit us that the physics would allow us to develop a magnetometer that could sense minute changes in a magnetic field caused by Advances in Magnetometer Technology U.S. Marines will no longer have to worry about what is hidden behind the next rock – they will know objects made of ferrous metal, leading to a wide range of applications," Dr. Bulsara said.

"Of course, the basic fluxgate magnetometer had been around since World War II; however, the idea of modifying the readout to mimic the process whereby neurons are believed to code and process information in the nervous system was different. We realized quite rapidly that if certain physics constraints were met, then the idea afforded simplicity and elegance which are always desirable in new concepts. We persuaded FOI to do a quick experiment to test the idea (it worked), and [we] wrote a long article about it in Physical Review A in 2003," Dr. Bulsara said.

"Over the next six years, we rigorously proved the physics and began developing various pieces of hardware. It's a reflection on the dynamics of the group that one of the products of that early discussion and subsequent development, the single core magnetometer, is ready to go into the field within only six years from the initial ideas as an additional sensor of the TRSS that gives the Marines some remarkable capabilities, including the ability to 'see' moving ferrous material (e.g., rifles) through walls."

After Visarath In and Joe Neff arrived at the lab, research accelerated and theoretical work aimed at better understanding the physics of coupled nonlinear oscillators rapidly evolved into the "coupled-core magnetometer" which involves coupling an odd number of wound ferromagnetic cores cyclically to one another in a ring oscillator configuration. A magnetometer based on the unique physics of this configuration is far more sensitive than the single core magnetometer.

The coupled-core magnetometer is being refined, with a number of practical issues remaining to be addressed. However, it will likely render the single core magnetometer obsolete in a couple of years. The SSC Pacific group and their international collaborators are exploring other sensors and devices that employ the unique features of the coupled oscillator configuration.

TRSS

TRSS, developed mostly by other organizations, is a handheld device weighing only a few pounds, but it carries acoustic, infrared, seismic and magnetic sensors.

The SSC Pacific contribution is a magnetometer about 4 inches by 4 inches (shown on the next page) that will replace the magnetic sensor with a much more powerful one. It can detect extremely small changes in the ambient magnetic field, such that through a plaster and wood wall a handgun can be detected at a range of approximately 8 meters.

The technology requires that the object be moving. If a handgun were to be taken off a table, or an individual walked out of a room carrying a weapon, the device would detect it. Similar technology has been developed for placing magnetometers in objects that look like rocks. They could be placed, for example, in plain sight at a sentry location through which pedestrians pass. A Marine with a personal digital assistant could be positioned some distance away monitoring those passing through. An individual passing by with hidden ferrous metal objects (weapons) could be stopped for interrogation and search.

Similarly, a network of such magnetometers disguised as rocks could be placed strategically along paths through mountain passes to alert security forces to the passing of heavily armed individuals.

"We could send a Tomahawk (missile) through a mountain pass dropping sensors at predetermined time separations," Dr. Bulsara said. "They are designed always to land right-side up."

The sensors can, if necessary, carry GPS receivers to provide critical position data, and radio frequency communications to "talk" to other sensors in the area or to transmit collected information to a satellite. In another application, a "rock" containing a magnetometer could be programmed to transmit a command to a nearby camera to shoot still photos or videos of a passing individual armed with ferrous metal, thus providing security personnel with images of subjects of interest.

Critical to the successful operation of the technology was the realization that rather than using changes in power, a time-do-main description that underpins the neural code could be used.

"We're talking at a basic level about the firing of neurons, wherein a membrane voltage crosses a threshold and generates a spike, which means a neuron has fired," Dr. Bulsara said. "As an example, if we're monitoring an individual and someone sticks him with a pin, then the sensory neurons fire more rapidly leading to a change in the statistics of the interspike intervals. Changes in measurable quantities like the mean firing rate can be correlated with the stimulus that led to these changes. The so-called neural code is widely believed to be based on the timing between spikes.

"Sensors based on this operating principle require simplified readout circuitry: a clock and a counter for keeping a running arithmetic mean of the interspike intervals suffice." (In this case, the intervals between the crossings of the upper and lower core magnetization thresholds by the internal magnetization parameter.)

This allowed the group to eliminate time-honored signal processing techniques, such as Fast Fourier Transforms (FFT), and merely calculate time differences so the readout became event-based. The standard time unit employed is one-tenth of a second.

The core of the current single core magnetometer is an exotic material about as thick as a human hair, with very favorable magnetic properties. The hardware is hand assembled on-site at SSC Pacific at a cost of about $400 per unit, compared to a price tag of $6,000 or so for commercially available magnetometers that are used in geophysics or other military surveillance applications.

"Make it small, make it light, make it cheap"

The refinement of the technology required removing original designs from shielding against the Earth's magnetic core and then determining sources of interference, reducing false alarms and optimizing the thresholds to ensure the signal wasn't missed by being buried in noise. Then there was the need to make it small, make it light, make it cheap.

In a planned competition among eight magnetometers, the SSC Pacific model was first in all categories except maintenance, since the developers' basic approach was: "It costs $100, if it breaks, throw it away, and we'll send you a new one."

With the competition settled, the SSC Pacific group was funded by the Office of Naval Research for three years to build hardware for the Marines to put into the field.

Planned improvements underway include completion of the coupled-core magnetometer and development of ultra-low power electronics, since its power requirements are significant.

"We have demonstrated the coupled-core magnetometer successfully in a sea test," Dr. Bulsara said. "Once we get the power requirement to a manageable level, our current single core model could be obsolete. In the meantime, the single core magnetometer has a measured in-the-field (i.e., unshielded) resolution of 0.5 to 1nT (nano tesla – unit of magnetic flux density), making it possibly the best room-temperature magnetometer available today."

SSC Pacific personnel involved in the effort are Drs. Joe Neff, Brian Meadows and Visarath In; and Andy Kho, Chris Obra and Greg Anderson. Their collaborators are professors Bruno Ando and Salvatore Baglio of the University of Catania, Italy; Drs. John Robinson, Peter Krylstedt, Peter Sigray and Bjorn Lundqvist of the Swedish Defense Research Agency in Stockholm; and Dr. Antonio Palacios of San Diego State University.

Tom LaPuzza is with the public affairs office of SSC Pacific. For more information about SSC Pacific, go to the SPAWAR Web site at www.spawar.navy.mil.

TAGS:
The magnetometer to be placed on the U.S. Marines’ Tactical Remote Sensor System and the "rock" under which it would be hidden to gather information without being noticeable to those passing by.
The magnetometer to be placed on the U.S. Marines’ Tactical Remote Sensor System and the "rock" under which it would be hidden to gather information without being noticeable to those passing by.

The 4-inch by 4-inch magnetometer to be placed on the Marines' Tactical Remote Sensor System
The 4-inch by 4-inch magnetometer to be placed on the Marines' Tactical Remote Sensor System
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