Vojtech Petrucha

Automated system for the calibration of magnetometers

A completely nonmagnetic calibration platform has been developed and constructed at DTU Space (Technical University of Denmark). It is intended for on-site scalar calibration of high-precise fluxgate magnetometers. An enhanced version of the same platform is being built at the Czech Technical University. There are three axes of rotation in this design (compared to two axes in the previous version). The addition of the third axis allows us to calibrate more complex devices. An electronic compass based on a vector fluxgate magnetometer and micro electro mechanical systems (MEMS) accelerometer is one example. The new platform can also be used to evaluate the parameters of the compass in all possible variations in azimuth, pitch, and roll. The system is based on piezoelectric motors, which are placed on a platform made of aluminum, brass, plastic, and glass. Position sensing is accomplished through custom-made optical incremental sensors. The system is controlled by a microcontroller, which executes commands from a computer. The properties of the system as well as calibration and measurement results will be presented.

Calibration of a triaxial fluxgate magnetometer and accelerometer with an automated non-magnetic calibration system

A method, instrumentation used and results of calibration and testing of tri-axial magnetometers, accelerometers and also possibly gyroscopes are presented. The method is based on a scalar calibration technique with the use of an innovative computer controllable non-magnetic platform [1]. The speed, precision, comfort and repeatability of the measurement are superior to techniques which use hand-driven tools.

A Fluxgate Current Sensor With an Amphitheater Busbar

Large dc and ac electric currents are often measured by open-loop sensors without a magnetic yoke. A widely used configuration uses a differential magnetic sensor inserted into a hole in a flat busbar. The use of a differential sensor offers the advantage of partial suppression of fields coming from external currents. Hall sensors and AMR sensors are currently used in this application. In this paper, we present a current sensor of this type that uses novel integrated fluxgate sensors, which offer a greater range than magnetoresistors and better stability than Hall sensors. The frequency response of this type of current sensor is limited due to the eddy currents in the solid busbar. We present a novel amphitheater geometry of the hole in the busbar of the sensor, which reduces the frequency dependence from 15% error at 1 kHz to 9%.

Compact Digital Compass with PCB Fluxgate Sensors

The new compact digital compass with PCB fluxgate sensors and accelerometers will be introduced in this contribution. Competitive low-cost, low-accuracy compasses are dedicated for measurement in horizontal plane only. The main advantage of developed compass is that it is able to determine azimuth in every position. The compass module consists of a tri axial fluxgate magnetometer and tri axial MEMS accelerometer which is used as a tilt sensor. The excitation and evaluation electronics is build directly in the compass module. Compass module is driven by two microprocessors and is equipped with digital output. The dimensions and power consumption of the compass were decreased by using miniature PCB (Printed Circuit Board) fluxgate sensors, instead of traditional large ring core fluxgates. Improved calibration techniques have been used to decrease azimuth error of the compass. The using of accurate magnetic sensors leads to the possibility of using tri axial magnetometers for calculation of total Held intensity and use it as a vector magnetometer. The resulting device is small high accurate compass with digital output that is able to be used in underwater or underground exploration or measurement.

A Busbar Current Sensor With Frequency Compensation

DC/AC yokeless galvanically insulated electric current sensors are required for applications, e.g., in automotive and aerospace engineering, where size, weight, and/or price are strictly limited. A busbar current sensor with differential fluxgate in the hole has 1000 A range and 10 mA resolution. Using an asymmetric shape, we achieved a frequency error below ±3% up to 1 kHz, while keeping high temperature stability and low sensitivity to mechanical misalignments. The 2.5 mA/°C maximum dc drift is four times better than when using an AMR sensor and 1000 times better than when using a Hall sensor. The sensor linearity error is below 0.1%.

Localization of the Chelyabinsk Meteorite From Magnetic Field Survey and GPS Data

The Chelyabinsk meteorite fragment that landed in the Chebarkul lake in Russia on February 15, 2013 weighed over half a ton. We provide magnetic field maps that were obtained during underwater measurements above the fragment. The data acquisition process was multiple global position system referenced magnetic surveys 0.5-1 m above the top of the lake sediment layer at 10 m water depth. Gradiometric configuration of the survey using two triaxial fluxgate magnetometers helped to suppress local geological anomalies. The location of the ice crater and the underwater magnetic anomaly provided final meteorite landing coordinates, which were made available during meteorite recovery.

Compact fluxgate sensor with a vector compensation of a measured magnetic field

Compact tri-axial fluxgate sensor which combines new materials, traditional ring-core topology and vector field compensation is introduced. The core support material is BNP-2. This new material provides an extremely high thermal conductivity (92.6W/m.K), which helps to eliminate thermal gradients in the core. The core is made of Vitrovac 6025X amorphous metallic ribbon. Thermally stable glass filled laminate was used to manufacture the pick-up and vector coil compensation support. The vector compensation topology has several advantages - it eliminates the cross-field effect and brings down the non-orthogonality. The presented design should outperform common tri-axial fluxgate sensors while keeping the dimensions at acceptable level. Magnetometer construction and preliminary results are discussed.

Cross-Field Effect in a Triaxial AMR Magnetometer With Vector and Individual Compensation of a Measured Magnetic Field

Magnetic field sensors based on anisotropic magnetoresistance (AMR) are widely used in many scientific and industrial applications. The AMR sensor sensitivity is superior to Hall probes and size and power consumption is superior to fluxgates. However, the noise properties and the temperature stability of AMR sensors are typically worse than for fluxgates. These properties define the typical applications—less precise vectorial or gradient measurements of the magnetic field within less than ±1 mT range. AMR sensors are typically calibrated for sensitivity, offset, and orthogonality errors. However, there is another important source of error—sensitivity to the magnetic field applied in the perpendicular direction to the measurement axis. This so-called cross-field error is inherent to AMR sensors and can influence the measurements significantly. Flipping (set/reset pulses) and closed-loop operation of the sensor can reduce the cross-field error. In this paper, we present a novel approach using full vectorial compensation of the measured magnetic field resulting in a complete elimination of the cross-field effect. The vectorial compensation provided superior results over alternative approaches that were also evaluated.

Fluxgate magnetometer vector feedback homogeneity and its influence on sensor parameters

Precise fluxgate sensors are built with a vector feedback, which eliminates the cross-field effect and improves linearity. The sensor axes orthogonality should be then defined primarily by the orientation of the feedback coils while the sensitivities are defined by feedback coil constants. The influence of the homogeneity of the feedback field on calibration parameters of a vectorially compensated tri-axial fluxgate magnetometer is presented.

Study of Stress-Induced Anisotropy in METGLAS 2714

METGLAS 2714AZ material has been stress annealed without any applied magnetic field to obtain suitable magnetic properties for use as low-noise fluxgate sensor core. During the development of magnetic cores, we observed an unusual change of the anisotropy direction on Co-based amorphous alloy of METGLAS 2714 AZ. The anisotropy direction moves from its original direction along the ribbon axis to the transverse direction as expected but for short annealing times the process is not monotonous. This occurs when annealing is performed above the Curie temperature. We explain it as a mixture of different anisotropies, which are built and released during the annealing process, since the 2714AZ ribbon has manufacturer-induced anisotropy in ribbon axis. The effect of ribbon cooling on the ribbon properties is also shown as annealing in the radiation furnace was done in different ways.