Paul Fulmek

Magneto-optical rotational speed sensor

Abstract A new passive and wireless magneto-optical rotational velocity sensor is described. It is based on measuring the intensity oscillations of linearly polarized light transmitted through an orthoferrite plate and an analyzer. The plate is subjected to the action of the magnetic field of a disc whose angular velocity has to be measured. Thanks to the high velocity and smooth reproducible character of the domain wall motion in the orthoferrite plate, one can continuously measure angular velocities in a very wide range—from ultralow to the MHz range.

Flipping field and stability in anisotropic magnetoresistive sensors

Switched-capacitor flipping circuits developed for Philips KMZ51 and KMZ52 anisotropic magnetoresistance (AMR) sensors give up to 2.8 A/1 kHz current peaks. Such unusually high current deeply saturates the sensor and thus removes hysteresis, reduces noise, and increases the resistance against field shocks. These necessary strong flipping fields are predicted by the energetic model (EM), applied to the magnetization reversal in thin films. The EM parameters have been correlated to microscopic variables, revealing the field dependence of the speed of magnetization reversal. This is responsible for the value of the critical switching field (and therefore for the stability of the AMR sensor) in the easy axis directions, depending on the saturating field amplitude.

Magnetization reversal in an energetic hysteresis model

Abstract In the energetic model the magnetic state of the material is represented by the minima of the total energy function. The model describes hysteretic curves from the demagnetized state up to saturation. At an arbitrary point of magnetization reversal, however, the distribution of the distances between domain walls and pinning centers has to be considered with respect to the magnetic history, which is important for the calculation of minor hysteresis loops.

Capacitive sensor for relative angle measurement

Based on a capacitive angle/angular speed sensor a sensor measuring the relative angle between two rotating shafts has been developed. Two rotatable grounded electrodes are placed between two sensor plates. The relative angle between the two rotors and the absolute position of the rotor blades are calculated from measurements of the capacitive coupling between different transmitting stator segments. A prototype of this sensor has been developed with a range of the relative angle of /spl plusmn/7.5/spl deg/ with a resolution of 0.1.

Magneto-optical current sensor by domain wall motion in orthoferrites

A brief review of the latest developments on optical current measurements based on the Faraday effect is presented. In all existing magneto-optical current transformers, the main measured parameter is the polarization state of the light transmitted by a sensor element. A new transformer is described by means of new results concerning the domain wall excitation. The measured parameter is the geometrical position of the boundary between domains with opposite magnetizations. In a wide frequency band, including DC currents, the measurement results are a linear function of the measured current. They are not affected by temperature changes and mechanical factors.

The effect of mechanical stress on the magnetization curves of Ni- and FeSi-single crystals at strong fields

The spatial distribution of the total energy is used to determine the magnetic behavior of ferromagnetic single crystals. For a single ferromagnetic domain there exists only a homogeneous magnetization the direction of which must correspond to a minimum energy direction. Magnetization curves are calculated by observing the stable positions of the magnetization under the application of strong magnetic fields. These magnetization curves show significant variations under different states of mechanical stress for FeSi and Ni crystals. >

Automatic Antenna Tuning Unit to Improve RFID System Performance

Systems for radio-frequency identification (RFID) realize a bidirectional wireless communication between a reader device and cheap passive tags (e.g., an RFID chip with an antenna printed on badges for access-control systems) in the near field of an antenna. This paper deals with the automatic and fast tuning of the resonant reader antennas in the widely used 13.56-MHz frequency band, resulting in a performance improvement and an increased read/write range. A novel RFID reader system is proposed with a tuning transformer for the automatic adjustment of the resonant circuit center frequency. This is achieved by shifting the magnetic operating point (i.e., the biasing direct-current (dc) magnetization) of a Ni-Zn ferrite transformer by means of an additional dc winding. The operation principle and the performance potential of this device are demonstrated by measurements on a prototypical RFID system.

A tuning transformer for the automatic adjustment of resonant loop antennas in RFID systems

Radio frequency identification (RFID) systems allow contactless communication between a small portable tag and a reader device. In this paper, we address the problem of performance degradations due to the detuning of resonant reader antennas in the widely used 13.56 MHz frequency band. A novel design of a tuning transformer for the adjustment of the center frequency of resonant loop antennas is proposed. The ferrite core of the transformer is premagnetized using an additional DC winding. The HF and DC windings are decoupled by an iron core. Besides the tuning, the transformer may also be used for impedance matching and for balancing an unbalanced signal. The function of the device was proven by network measurements. Key Words: Loop antenna, resonance tuning, RFID, tunable inductance.

Magnetic field sensor by orthoferrites

Abstract Among all ferromagnets orthoferrites possess the highest velocities of domain wall motion. Dynamic properties of a magnetic field meter based on domain wall dynamics in yttrium orthoferrite are reported. It is shown that at low driving magnetic fields and at frequencies up to the MHz band domain wall dynamics can be adequately described by linear equations. In a wide range of frequencies the results of measurements do not depend on the temperature of the crystal.

Virtual rotor grounding of capacitive angular position sensors

This paper presents a new approach for the design of capacitive angular position and angular speed sensors. The idea is to obtain nonconductive grounding of the rotable part of the sensor. This approach allows the use of conductive rotor blades fixed to a nonconductive shaft. A simple prototype has been developed to show the theory of operation. Model calculations and experimental results show that virtual grounding allows to neglect the rotor-ground resistance in a wide range.