Ing. Vojtěch Petrucha, Ph.D.

In this paper, we present research, development, calibration and characterization of a novel concept of a digitally compensated, low-noise magnetometer based on anisotropic magnetoresistance sensors that is suitable for space applications. The main idea of the design was to reduce the number of precise analog components while using the digital signal processing power available in a modern microcontroller. Our most recent effort targeted lowering power consumption, enhancement of radiation hardness and overall improvement of the parameters. The principle of operation is presented in detail, along with a detailed description of the instrumentation used to characterize the real instrument, including its noise, linearity, and temperature stability in the range of -20 to +70 °C. The results of total ionizing dose (TID) testing at a gamma-ray irradiation facility are discussed at the complete magnetometer and part levels. This is an extended version of a paper presented at I2MTC 2020 that contains the results of a second radiation test done with a slightly modified design. The instrument worked well throughout the entire irradiation session (TID of 1.05 kGy over 72 hours), and the stability of main parameters was very good (50 pT/Gy offset and 1 ppm/Gy sensitivity stability).

Fluxgate sensors with a flat race-track core can provide significant advantages in terms of noise and manufacturing complexity. We present a comparison of parameters of several such sensors of a similar construction but with different dimensions that might be useful for a wide spectrum of applications, including state-of-the-art geomagnetic measurements. The effect of core demagnetization is discussed, and optical Kerr effect microscopy is used to present the results of the thermomagnetic treatment of these cores, which significantly improves overall sensor noise.

In this paper, we present research and development regarding a novel concept of a digitally compensated, low-noise magnetometer based on anisotropic magnetoresistance sensors that is suitable for space applications. The central idea of the design is to reduce the number of precise analog components while using the digital signal processing power available in a modern microcontroller. Our most recent effort targeted lowering power consumption and overall improvement of the parameters. Only commercial off-the-shelf components are used in the concept reported here, keeping costs manageable even for low-budget CubeSat missions. The principle of operation is presented in detail, along with a full characterization of a real instrument, including its noise, linearity, and temperature stability. The results of Total Ionizing Dose testing at a gamma-ray irradiation facility are discussed at the complete magnetometer and part levels; they suggest the instrument's good potential.

In this paper, we present an advanced fully digital solution of a fluxgate magnetometer with both demodulation and compensation carried out by a low-cost field programmable gate array (FPGA). For feedback operation, we avoid using a costly precise digital-to-analog converter, instead employing an FPGA to generate a hybrid pulse width modulation sigma-delta signal. Even with only a few additional components to process such signals, we were able to achieve excellent linearity, noise, and stability, as supported by measurements. On the front-end side only one pick-up signal preamplifier is necessary, greatly reducing the number of analog circuits needed. This can be an advantage in radiation-hazard sites like space missions, as there are fewer radiation-susceptible parts that can degrade. We provide a short description of the entire setup—electronics, fluxgate sensor construction, and final power budget—and a parameter summary in the conclusion.

This paper considers the possibilities of using digital feedback for precise anisotropic magneto-resistance (AMR) magnetometers using commercial off-the-shelf (COTS) components. This paper presents the full characterization of a real instrument, including its noise, linearity, stability, and power consumption.

This paper describes a novel method for calibrating dc-precise magnetometers in the low field range (± 100 μT), which gives acceptable results even in laboratory conditions with significant magnetic interference. By introducing a closely mounted reference magnetometer and a specific calibration procedure, it is possible to compensate for the external magnetic field disturbances caused, e.g., by the local transportation operated with dc power supplies.

Stejně jako původní vydání knihy Elektrická měření – přístroje a metody z roku 2003 i toto shrnuje informace týkající se oblasti elektrických a magnetických měření na úrovni absolventa inženýrského studia elektrotechnických oborů. Vzhledem k prudkému rozvoji elektroniky a číslicové techniky však byla náplň většiny kapitol značně modernizována popř. rozšířena a část věnovaná analogovým přístrojům naopak významně zredukována. V publikaci jsou vysvětleny základní principy používané v současné době pro měření elektrických popř. magnetických veličin a principy nejčastěji používaných metod pro měření ostatních fyzikálních veličin s mezipřevodem na elektrický signál. Jsou popsány též běžně používané měřicí přístroje a principy jejich činnosti, s čímž samozřejmě souvisí i jejich vlastnosti. Vzhledem k tomu, že jejich základním blokem jsou analogově-číslicové popř. číslicově-analogové převodníky, je samostatná kapitola věnována i digitalizaci signálu. Problematika určování nejistot měření je klíčovým parametrem při řízení a kontrole kvality výroby. Nejistotám měření je proto věnována nejen celá 1. kapitola, ale odhad nejistot je také zmíněn i v kapitolách věnovaným metodám měření aktivních elektrických veličina parametrů pasivních součástek.

Possibilities of achieving high dynamic range for fluxgate magnetometer are discussed. In order to measure small variations in a large bias, a variometric arrangement can be used instead. ADC then digitizes only small range of the magnetometer as most of measured magnetic field is removed by a precise compensation system.

The article presents testing of a miniature fluxgate sensor developed and manufactured by Texas Instruments as well as an application of the sensor in a compact magnetic field probe with USB interface and LabView based software. Several basic properties of the sensor were evaluated and compared with the datasheet specifications. Offset stability measurements have indicated some possible problems for applications that require very high DC and low-frequency precision.

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%.

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 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 <±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 our paper, we present a novel approach of 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.

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%.

A new construction of magnetometer with commercially available AMR (anisotropic magnetoresistive) sensors intended for vehicle detection experiments is presented. For further experiments and acquisition of representative data, a new design of precise multi-channel magnetometer was developed. The design supports two models of commercial AMR sensors: the proven and reliable, but obsolete Honeywell HMC1021-series sensors and newly available Sensitec AFF755B sensors.

A low-cost dual-axes fluxgate sensor with a flat ring-shaped core is presented. The amorphous magnetic core material was field annealed prior to sensor assembly. Perpendicular field anisotropy created by the annealing procedure around the circular core improved the sensor noise properties in the two perpendicular sensitive directions of the dual-axes fluxgate sensor. Results were evaluated in terms of BH-loop measurement, domain observation, and fluxgate sensor noise measurement.

We present a low-noise, high-stability observatory magnetometer with race-track sensors, as developed by the Czech Technical University in Prague for National Observatory of Athens. As opposed to the standard instruments, we used our novel race-track fluxgate sensors with planar oval core which were cut by state-of-the art pico-second UV-laser.

Using full-tensor gradiometers has various advantages over simplified uniaxial gradiometers or even scalar measurements. The full-tensor information is used mainly in geophysical prospecting and in security/military applications. In our new design we combined 10 small (20-mm) fluxgate sensors operating in feedback compensated mode into one compact head in the form of 46-mm cube.

The Chelyabinsk meteorite fragment that landed in the Chebarkul lake in Russia on 15 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 GPS referenced magnetic surveys 0.5 m – 1 m above the top of the lake sediment layer at 10 m water depth. Gradiometric configuration of the survey using two tri-axial 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.

Hall sensors with an analog output were used as rotational speed sensors and provided an angular position reference for other diagnostics measurements during a cryogenic Propellant Electric Pump (PEP) test campaign. Frictionless foil bearing, which is a very important technology that needs to be well characterized during the tests, was tested in the PEP. Hall sensor outputs helped to process signals from eddy current radial and axial shaft displacement sensors and piezoelectric vibration sensors. The Hall sensor signal was also used as the main motor speed feedback signal for the test-bench control system.

Vector feedback is a concept which can significantly improve linearity and stability of a magnetic field sensor. The feedback coils effectively cancel the measured magnetic field in the inner volume of the triaxial sensor. Thus, in case of fluxgates, it suppresses one possible source of nonlinearity— cross-field sensitivity error. The triaxial sensor axes orthogonality should be primarily defined 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 and the influence of the sensor inner layout on calibration parameters of a vectorially compensated triaxial fluxgate magnetometer are presented.

Precise fluxgate sensors are built with a vector feedback, which eliminates the cross-field effect and improves linearity. The influence of the homogeneity of the feedback field on calibration parameters of a vectorially compensated tri-axial fluxgate magnetometer is presented.

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.

The temperature dependence of the sensitivity and orthogonality of a tri-axial vector magnetometer is measured with the use of a dedicated thermostatic system and a non-magnetic positioning platform. The dependency is obtained from the results of repeated scalar calibrations for different temperatures.

Compact tri-axial fluxgate sensor which combines new materials, traditional ring-core topology and vector field compensation is introduced. Magnetometer construction and preliminary results are discussed.

Tester for a space micro-accelerometer was developed for pre-flight tests of the micro-accelerometer electronics.

Automated non-magnetic calibration platform with three axes of freedom is presented. Main applications are scalar calibration of vector magnetometers and accelerometers and testing of their assemblies (e.g. an electronic compass).

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

This paper deals with measurement setup created for educational purposes - demonstration of speed measurement using correlation. Circumferential speed of rotating wheel is determined by measuring physical properties (reflectance, magnetic field, thermal emissivity, etc.) of various metallic pieces on the surface. Two sensors separated by a known distance measure essentially the same signal and correlation is used to determine a time delay. The setup is used during laboratory lessons of "Sensors and Transducers" and the primary motivation in its development was the poor technical condition of the existing device that had to be replaced. Several sensor modules were developed in order to demonstrate function of the same general principle (correlation of signals) independently of type of the specific measured property. The new setup brings mechanical reliability and also makes possible to measure more than one physical property of the same object.

Short introduction to scalar calibration of magnetometers is presented as well as an instrument delevoped at the CTU for this type of calibration.

Micro-accelerometer MAC04 has been developed in order to measure very low accelerations such as those caused to satellites by atmospheric drag and other non-gravitational forces. The instrument uses a cubic proof-mass inside a small cavity. In an open loop the change of capacitance between the cube and 12 electrodes on the inner cavity surface is a measure of the applied acceleration. It is very difficult to ground test and calibrate such a device due to gravity. The tester simulates the change of capacitances (base capacitance 13,5pF, changes in a range of +-1.5pF). Complete closed loop system is presented.

A complete system for scalar calibration of magnetometers is being introduced in this article. The main part of the system is an absolutely non-magnetic mechanical platform which is driven by piezoelectric motors. Together with the electronic control unit and software equipment it forms a unique system for on-site scalar calibration of spacegrade magnetometers. The parameters of the calibration system and experiences obtained during the construction phase are presented.

Satellite based navigation systems (GPS) are widely used for ground, air and marine navigation. In case of its malfunction or satellite signal inaccessibility some back-up navigation system is needed. Electronic compass can provide this function. Compass module described in this paper is designed for precise navigation purposes. The compass module is equiped with electronic tilt error compensation and includes all in one package - electronics with digital output, sensors. Typical application of this compass is e.g. underground drilling. Critical parameter in this application is heading accuracy. Heading error of 1 degree can cause displacement of 1.8meter in target area (length of tunnel 100m). That's not acceptable in urban conglomeration and therefore more accurate heading sensing device must be used.

The fully digital compass with PCB fluxgate sensors and accelerometers is presented. Calibration techniques and final accuracy of the compass are discussed.

The new compact compass with PCB fluxgate sensors and accelerometers will be introduced in this contribution.

The fully digital compass with PCB fluxgate sensors and accelerometers is presented. Calibration techniques and final accuracy of the compass are discussed.

Kompaktní kompas s PCB fluxgate senzory