Cambridge University Researchers Microfabricate Fluxgate Magnetic Sensors

This content is excerpted from Sensor Technology Alert and Newsletter, a sensor intelligence service published by the Technical Insights unit of Frost & Sullivan.


Fluxgate mgnetic sensors represent an older, traditional magnetic field sensing technology that tends to be used for such applications as geological surveys, detecting submarines, and compass navigation systems. Conventional fluxgate magnetometers, which can measure the magnetude and direction of static magnetic fields that correspond to the range of the Earth's magnetic field (about 1 microgauss to 10 gauss) tend to be relatively bulky.

Conventional fluxgate magnetoemters are assembled from coils wound onto magnetic cores. A common type of traditional fluxgate magnetometer has two coils-a primary and a secondary coil-wrapped around a high-permeability ferromagnetic core. The magnetic induction of the core changes, due to the presence of an external magnetic field.

Dr. Paul Robertson and his team at the University of Cambridge have developed techniques to microfabricate fluxgate magnetic sensors, facilitating fabrication of small form factor fluxgate sensorssensors, whch are purportedly very sensitive, contained in a miniature package, and able to over a wide frequency range. Such features and capabilities puportedly can allow such sensors be used in a wider range of applications, such as measuring electric current in circuits and evaluating the magnetic properties of materials (both of which can be conducted in a non-destructive manner).

For example, the microfabricated fluxgate sensors being developed by Robertson's team could potentially enable lower cost magnetic microscopes, in which the magnetic sensor is scanned over the surface of an item and the measured magnetic field is displayed as an image on a computer screen. Commercially available magnetic microscopes tend to use highly cooled superconducting sensors. In contrast, the microfabricated fluxgate sensors under development operate at room temperature, contributing to a reduced system cost.

Potential uses for such an instrument include component surface defect detection and the development of ticketing, security cards, anti-counterfeiting devices, and security features on bank notes, which contain magnetic recording material.

The microfabricated fluxgate magnetometer technology is being developed with a local company for use in a new current probe for electronics engineers.

Dr. Robertson explained to Sensor Technology that the microfabricated fluxgate sensors are made using planar micro-fabrication techniques, similar to those used in IC manufacturer. However, the materials are quite different (not silicon based).

The microfabricated fluxgate sensors comprise a core of non-linear, high permeability material deposited by RF (radio frequency) sputtering onto a glass substrate and patterned via photo-lithography. The core is surrounded by a coil, either formed by planar techniques using copper and plastic insulators, or wound with a CNC micro manipulator, depending on the application. Single sensors are more compact, using a micro-wound coil, but a planar array can be more precisely produced on a flat substrate. The sensors are driven by a custom interface module to produce a linear output over a dynamic range of 5 orders of magnitude and a frequency range from dc to over 10 MHz. Aspects of the sensor and the interface system are covered by a granted patent.

Regarding promising applications for the microfabricated fluxgate sensor, Dr. Robertson noted that his team is looking at applications that require high bandwidth, high sensitivity, and good linearity in a compact device. In addition to magnetic microscopes for materials evaluation and non-destructive testing (including imaging surface currents in electronic devices and circuits), potential applications include position, vibration and orientation sensing, which are possible with reference to a fixed magnet, and the devices could make very good magnetic data read/write heads.

Moreover, the microfabricated fluxgate sensors could potentially be used in a turbine flow meter, which the fluxgate sensor would especially be beneficial for flow metering in high-pressure or high-temperature environments. Due to the micro fluxgate sensor's sensitivity to small magnetic fields, Dr. Robertson noted, the sensor could be placed farther away from the moving part of the flow meter, compared to competing magnetic sensing devices (such as Hall effect sensors) to better cope with high pressures. The microfabricated fluxgate sensor is well-suited for high-temperature environments because it has low cross-sensitivity to temperature.

Cambridge University is interested in licensing the microfabricated fluxgate sensor technology to exploit new application areas for the technology.

According to Frost & Sullivan's World Magnetic Sensor Components and Modules/Sub-Systems Markets report (published May 2004), global research for magnetic sensor components are projected to exceed $1.56 billion in 2010 (representing about 2,706,700 units). In 2010, the projected segmentation of the world revenues ofr magnetic sensor components by product type is: Hall effect sensors-77.0%; magnetoresistive sneosrs-22.1%; and SQUIDs (superconductivity quantum interference devices)-0.9%.

In 2010, global revenues for magnetic modules and sub-systems are projected to reach $17.2 million (representing 5,400,000 units). At that time, the disbution of the overall magnetic module and sub-system revenues is projected to be: magnetometers (that use an array of sensing elements)-39.7%; compass modules-35.8%; and linear position modules-24.5%.

Details: Dr. Paul Robertson, University of Cambridge, Department of Engineering, Trumpington Street, Cambridge CB2 1PZ, England. Phone: +44-1223 3 32683. E-mail: [email protected]