Antonin Platil

Temperature Drift of Offset and Sensitivity in Full-Bridge Magnetoresistive Sensors

A typical commercially available magnetoresistive sensor, and particularly an anisotropic magnetoresistive sensor, employs a full bridge of the Wheatstone type formed by two complementary magnetoresistive elements in each branch. This configuration provides linearized response and enlarged sensitivity compared to any other configuration made up of the same elements. Since in a large scale production it is practically impossible to adjust the zero-field resistances of all the four elements to an exactly identical value, there is always some zero-field offset present at the bridge output diagonal even when the sensor is placed in the zero magnetic field. The sensitivity of the sensor, i.e., the ratio of the output voltage change to the change of the measured field H, is associated with the sensitivity of the individual elements. The change of the output voltage is determined by the change of the resistance ΔR of the individual elements. Both the offset and the sensitivity of a full-bridge magnetoresistive sensor is dependent on the zero-field resistances Ri of the elements. However, as in most metallic material, the resistivity of a magnetoresistive element is influenced by temperature. Hence, both the offset and the sensitivity of a real magnetoresistive sensor is temperature dependent. It can be shown that, theoretically speaking, the offset is temperature independent when the bridge is supplied with a constant voltage (but the sensitivity in that case is temperature dependent), and the sensitivity is temperature independent when the bridge is supplied with a constant current (but the offset in that case is temperature dependent). This hypothesis has been verified on KMZ52 sensor (albeit in small temperature range-about 25°C-45°C).

Magnetic markers detection using PCB fluxgate array

We used an array of race-track fluxgate sensors, manufactured with printed circuit board (PCB) technology, forming a sensor head for detection of ferromagnetic and paramagnetic markers. The sensors were arranged perpendicularly to the measuring plane and we measured the difference of their output, giving us the horizontal gradient of normal component of the measured field. Due to the close match of the sensor’s parameters, subtraction of the fluxgate output signals could be done directly at the input of a lock-in amplifier, increasing the signal-to-noise ratio for small gradients. When moving the sensor head, we were able to map field gradients smaller than 6 nT/mm, which was verified while measuring the magnetic markers on a dollar bill, while suppressing the background field by a factor of 5. In a line-scanning mode, we scanned a marker formed by a 0.2 mm diameter Permalloy wire in a distance of up to 10 mm. With the help of perpendicular ac excitation at 30 Hz, we were able to detect a 0.1 ml Endorem i...

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.

Sampling measurements with digital hysteresisgraph

Digital hysteresisgraph can be used for the measurement of dynamic hysteresis loops up to 100 kHz. Digital feedback allows achievement of sinusoidal flux density by iterative modification of the excitation voltage waveform. This approach is used for the measurement of closed (toroidal) samples at higher frequencies.

Multiple Layer Scanning in Magnetopneumography

In magnetopneumography (MPG), we scan weak remanent magnetic fields (in the range of nT) of ferromagnetic dust deposited in the lungs of affected workers. This is carried out in an unshielded lab. Variations of the background field during the scanning period bias the inversion process, i.e., the estimation of the magnetic sources from the measured field. We present an innovative mathematical technique of finding a field equation uniquely describing the magnetic moments in the nodes of the measurement space. Simultaneously, it enables the tracking of the values of the disturbing gradient created by external distant sources during the time of scanning. Our method preserves the first-order gradients obtained in multiple layers (scan heights above the object) instead of losing data with the higher order gradients which are not effective here. Six coaxially aligned fluxgates were used for scanning the field maps. The obtained five first-order gradients for each node give us the parameters of the respective ldquoequivalentrdquo magnetic sources. Furthermore, we obtain a parameter describing the error originating mainly from the background field. The main limits of the presented method are: 1) the alignment of the probes; 2) their number being limited by a minimum distance; and 3) nonhomogeneity of the external field.

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.

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.