Magnetics Business & Technology - Summer 2014 - (Page 16)

FEATURE ARTICLE Handheld Three-Axis Magnetometers: Technology Overview By Philip Keller, Marketing & Product Management | Metrolab Technology SA A novel handheld three-axis Hall Magnetometer was introduced at the 2008 Magnetics Conference. In the meantime, this instrument has spawned a family of five, with a range from nanoteslas to 14T, suitable for a wide variety of applications. In this article, we review this family's technological underpinnings: sensors, electronics, firmware, software and calibration. A Family Resemblance: Slender but Powerful When introduced in 2008, the Three-axis Hall Magnetometer THM1176 surprised by its form factor as well as its functionality. Resembling a mass-market USB device rather than a "gaussmeter", it simultaneously measures all three components of the magnetic field vector, a feature generally reserved for high-end bench-top systems. This three-axis capability ensures that the vector magnitude B is always known, regardless of the probe's orientation. An optional palmtop computer provides a completely portable, battery-powered solution. Since then, additional probes have joined the family, and firmware and software has been perfected, but the basic hardware has remained the same (see Figure 1). The new probes, with their new 3-axis sensors, required developing appropriate calibration techniques. The following sections will examine the underlying technologies. Magnetometer on a Chip The sensor of the THM1176-HF (for High Field, see Figure 2) is essentially a magnetometer on a chip (Figure 3). The highest range is sufficient to measure all but the world's strongest magnets, and the resolution allows accurate measurements of the primary field of conventional magnet systems. The THM1176-HFC (High Field Compact) uses the same IC, packaged to provide a probe only 0.5 mm thick, well adapted for very small gaps, for example between a stator and rotor. The THM1176-MF (Medium Field) uses a very similar sensor, optimized for higher resolution with smaller ranges: 0.1/0.3/1.0/3.0 T, still amply sufficient for the field strengths encountered in most magnetic systems. Flux Concentrators and Flux Gates In contrast, the THM1176-LF (Low Field) uses a conventional arrangement of three individual sensors. Here the emphasis is on maximum sensitivity. A sensor architecture consisting of a microscopic soft-iron flux concentrator and a differential pair of Hall elements increases the sensitivity by approximately a factor of seven, providing µT resolution. The measurement range is limited to 8 mT; at that point the flux concentrator begins to saturate. Finally, the TFM1186 (Three-axis Fluxgate Magnetometer) uses a recent fluxgate sensor. With its three-axis sensor and low-power operation, this sensor fits right into the THM1176 family, pushing the resolution into the nT range, unachievable with Hall sensors. Figure 1. System Anatomy: the sensor, electronics and USB connector are little more than a cable with a few fat spots. The complexity resides in the firmware (embedded software) and host software. A calibration procedure unique to each sensor generates standardized calibration tables, written into flash memory in the electronics unit. Figure 2. Three-axis magnetometer family portrait (top to bottom): Fluxgate probe for earth-field perturbations; High-Field probe for very strong fields; Low-Field probe for fringe fields; MediumField probe for general-purpose measurements; and High-Field Compact probe for narrow gaps. Figure 3. Photograph of the sensor IC used in the THM1176-HF and -HFC probes. A combination of vertical and conventional planar Hall elements provides the core of the 3-axis system, with an active volume of around 100 µm diameter. The drive electronics (current sources and amplifiers) is integrated right on the same chip, as is a temperature sensor to enable drift compensation. On-chip switches that change the bias current and amplification provide four measurement ranges (0.1/0.5/3.0/20 T). Last but not least, the chip contains switches that implement spinning-current techniques, thus minimizing errors due to offset and Planar Hall Effect. (Photo © Senis GmbH) 16 Magnetics Business & Technology * Summer 2014 www.MagneticsMagazine.com http://www.MagneticsMagazine.com

Table of Contents for the Digital Edition of Magnetics Business & Technology - Summer 2014

Editor's Choice
Moving Beyond Helium in Magnetics Research
Advances in Ferrite-Integrated On-Chip Inductors Using Aerosol Deposition
A Fresh Look at Design of Buck and Boost Inductors for SMPS Converters
Research & Development
Handheld Three-Axis Magnetometers
Magnetics, Materials & Assemblies
Software & Design
Test & Measurement
Industry News
Marketplace / Advertising Index
Spontaneous Thoughts: A Different Paradigm

Magnetics Business & Technology - Summer 2014