Jan Vyhnanek

Dual-core fluxgate gradiometer with gradient feedback

A fluxgate magnetic gradiometer with two fluxgate sensors and gradient feedback loop is presented. The two feedback coils, gradient and homogeneous, are common to both fluxgate sensors. The signal from the two sensors acts as regulating input in the two feedback loops, improving stability of the gradiometer. The presented gradiometer overcomes the problems of state-of-the art gradiometers which do not allow to decrease the sensor spacing. Because the information about homogeneous field is also available at the gradiometer output, it is possible to astatize the gradiometer. The presented gradiometer has a gradient base of 20-mm with overall sensor head size of 10-cm only and its noise is less than 1.1 nT/√Hz @ 1 Hz.

CW metal detector based on AMR sensor array

We developed an eddy-current metal detectors with Anisotropic Magnetoresistors (AMRs) arranged in a 4×4 array. The magnetoresistive sensors in a gradiometric configuration are sensing the vertical component of the magnetic field - either originating from magnetized ferromagnetic bodies or induced by 1-kHz, alternating continuous-wave excitation. The AMRs are arranged so that the homogeneous Earths field and the large excitation field can be suppressed. The mine-detector was constructed as a standard portable device. The presented results show that the performance is limited by the noise of the selected magnetoresistive sensor. If the size of the sensor array was increased, the system could form an advanced mine-detector for quick, large-scale demining purposes.

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.

The Effect of Sensor Size on Axial Gradiometer Performance

In this paper, we examine the influence of sensor size, sensor spacing (gradiometric base), and distance from dipole source on the performance of a single-axis gradiometer positioned along the dipole axis. The case of a finite base gradiometer with ideal sensors is considered, then the influence of finite-size sensing elements is modeled, and finally, a comparison with experimental results obtained with two ring-core and race-track fluxgates and two anisotropic magnetoresistors (AMRs) is evaluated. Some of the effects found may be counterintuitive, and especially in close proximity of the dipole source, the gradient cannot be further modeled by simplified uniaxial approximation because of the active element size. Full Biot-Savart field model was considered in those cases.

Inductance position sensor for pneumatic cylinder

The position of the piston in pneumatic cylinder with aluminum wall can be measured by external inductance sensor without modifications of the aluminum piston and massive iron piston rod. For frequencies below 20 Hz the inductance is increasing with inserting rod due to the rod permeability. This mode has disadvantage of slow response to piston movement and also high temperature sensitivity. At the frequency of 45 Hz the inductance is position independent, as the permeability effect is compensated by the eddy current effect. At higher frequencies eddy current effects in the rod prevail, the inductance is decreasing with inserting rod. In this mode the sensitivity is smaller but the sensor response is fast and temperature stability is better. We show that FEM simulation of this sensor using measured material properties gives accurate results, which is important for the sensor optimization such as designing the winding geometry for the best linearity.

The effect of conductor permeability on electric current transducers

In this paper, experimental works and theoretical analysis are presented to analyze the influence of the conductor permeability on the precision of yokeless current sensors. The results of finite-element method (FEM) fit well the measured field values around the conductor. Finally we evaluate the difference in magnetic fields distribution around non-magnetic and magnetic conductor. The calculated values show that the permeability of the ferromagnetic conductor significally affects the reading of the electric current sensors even at DC.

Crossfield response of industrial magnetic sensors

Non-linear response to the magnetic field perpendicular to the sensing direction is unwanted property of all magnetic sensors which contain ferromagnetic material [1]. This so called crossfield response can cause serious error of compass, gradiometer or current sensor. In this paper we discuss crossfield resistance of AMR sensors and integrated fluxgate in the wider field range. This is important for applications such as electric current sensing and position sensing using magnetic field from coils or permanent magnets. Theoretical response is compared to the measured results. The crossfield response was measured by two sets of the Helmholtz coils, one generating field in the sensitive axis of the tested sensor, the other perpendicular. Ferromagnetic objects which could deform the magnetic field or cause nonlinearity were removed from the vicinity of the test setup. Each characteristics was measured several times to check repeatability; the data were not averaged, but they are shown in the same plot. The perpendicularity of the crossfield coil was adjusted to give minimum response. The residual linear response was corrected in the collected dataset.

Experimental Comparison of the Low-Frequency Noise of Small-Size Magnetic Sensors

Small-size ac magnetic-field sensors are used for nondestructive testing (NDT), magnetic particle detection, and other applications, which require high spatial resolution. Up to now, inductive coils dominated this area, as their sensitivity at kHz frequencies, is superior to other magnetic sensors. However, some applications, such as magnetic imaging through conducting sheath, require lower working frequencies, in extreme case units of Hz. We successfully replaced inductive coils by an AMR sensor in NDT application and for distance measurement. In this paper, we compare designs of miniature ac magnetic field sensors, their achievable frequency characteristics, dynamic range, and noise parameters.

AMR proximity sensor with inherent demodulation

Our novel position sensor is based on the combination of the eddy-current and permeability effects. The primary field is excited by a coil, but instead of induction coil, the sensing part uses anisotropic magnetoresistor (AMR), which also measures dc magnetic field. As the AMR is being flipped at the excitation frequency, the sensor is self-demodulated and the output is dc. The AMR sensitivity does not depend on frequency; therefore, this sensor can be used at ultralow frequencies, where coils fail as sensors. We show the response of our sensor to ferromagnetic and nonferromagnetic metals and possibilities to distinguish between them. We also show that our sensor can measure position through the conducting sheath.