Pavel Ripka

Review of fluxgate sensors

Abstract Since the 1930s, fluxgate sensors have been used for measuring d.c. magnetic fields up to 1 mT with a maximum resolution of 10 pT. In the sensor core the flux is gated by the excitation field. The preferable sensor geometry is a ring-core; both crystalline and amorphous ferromagnetic materials can be used for the core. Although a lot of fluxgate magnetometer types have appeared, the classical type with detection of the second harmonics by a phase-sensitive detector is the most popular. Fluxgate sensors are reliable and rugged and their applications range from space research to submarine detection.

Advances in fluxgate sensors

This paper reviews recent achievements in the technology and design of fluxgate sensors and magnetometers. The major recent trends were decreasing of the sensor size, power consumption and price, and, on the other hand, increasing of the precision in the large range of the measured fields. The potential frequency range was increased up to units of kHz. Present fluxgate sensors have a resolution comparable with high-temperature superconducting quantum interference devices (SQUIDs), while their precision is the best of vectorial field sensors.

Advances in Magnetic Field Sensors

The most important milestone in the field of magnetic sensors was when AMR sensors started to replace Hall sensors in many applications where the greater sensitivity of AMRs was an advantage. GMR and SDT sensors finally found applications. We also review the development of miniaturization of fluxgate sensors and refer briefly to SQUIDs, resonant sensors, GMIs, and magnetomechanical sensors.

Electric current sensors: a review

The review makes a brief overview of traditional methods of measurement of electric current and shows in more detail relatively new types of current sensors. These include Hall sensors with field concentrators, AMR current sensors, magneto-optical and superconducting current sensors. The influence of the magnetic core properties on the error of the current transformer shows why nanocrystalline materials are so advantageous for this application. Built-in CMOS current sensors are important tools for monitoring the health of integrated circuits. Of special industrial value are current clamps which can be installed without breaking the measured conductor. Parameters of current sensors are also discussed, including geometrical selectivity. This parameter specific for current sensors means the ability to suppress the influence of currents external to the sensor (including the position of the return conductor) and also suppress the influence on the position of the measured conductor with respect to the current.

New directions in fluxgate sensors

Although fluxgates may have a resolution of 50 pT and an absolute precision of 1 nT, their accuracy is often degraded by crossfield response, non-linearities, hysteresis and perming effects. The trends are miniaturization, lower power consumption and production cost, non-linear tuning and digital processing. New core shapes and signal-processing methods have been suggested.

Giant magnetoimpedance sensors

Magnetoinductance and giant magnetoimpedance (GMI) sensors have greatly benefited from the development of amorphous wires. These soft ferromagnetic substances exhibit exquisite sensitivity (in the nT range) and wide bandwidth (MHz) in thin film structures. Combining these properties with surface wave technology produces passive, wireless sensors.

AC-driven AMR and GMR magnetoresistors

Anisotropic and giant magnetoresistive (AMR and GMR, respectively) sensors are attractive for industrial applications, as they are more sensitive and stable than Hall sensors. Their performance can be improved by AC excitation: flipping for AMR and AC biasing for GMR. AC excitation lowers the hysteresis, reduces the offset and, in some cases, also decreases sensor noise. The sensitivity to perpendicular fields is reduced in case of AMR sensors. AC-driven magnetoresistors are competitive with miniature fluxgate sensors and they are suitable for precise applications such as compasses.

Magnetic sensors for industrial and field applications

Abstract Vector-type magnetic-field sensors suitable for use in a heavy industrial environment and for field measurements include Hall sensors, magnetoresistive sensors and fluxgates. The basic properties of these sensors are discussed. Hysteresis and offset of the permalloy anisotropic magnetoresistance sensors can be lowered by periodically flipping the sensor characteristics. While low-noise fluxgate sensors may work in the sub-nanotesla range and miniature fluxgates may still have nanotesta (nT) resolution, the typical measuring range of permalloy anisotropic magnetoresistors starts at tens of nT and the resolution of ordinary Hall sensors is of the order of 10 μT.

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.

Precise DC current sensors

Three novel DC current sensors are described. Portable 40 A DC current clamps have resolution of 1 mA and high immunity to unclamped currents and external fields. AC/DC comparator with amorphous core may serve for precise measurement of DC currents with AC component up to 1 kHz. Magnetoresistive sensor field has limited accuracy but potential bandwidth from DC to 1 MHz.