Automated MR Sensor Measurement J. Kubík, M. Vopálenský Department of Measurement, Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, 16627 Prague 6, Czech Republic Phone: +420-2-2435 3964, Fax: +420-2-3333 9929, E-mail:
[email protected]
Introduction The magnetoresistors are part of the ET (Emerging Technologies) nowadays. The serve in the wide range of applications and are quickly expanding due to the recent technological developments made by the leading manufacturers. There are three different types of magnetoresistors: • Anizotropic MRs (AMR), • Giant MRs (GMR), • Spin Dependent Tunneling MRs (SDT). The main reason for developing System for Automated MR Sensor Measurement is the demand to perform various measurements of MR sensors. The manual measurements are extremely time-consuming and with the increasing number of measurements it is more convenient to invest time into development of an automated system, which can perform the measurements very quickly, reliably and repeatedly.
Results The system was developed using the LabVIEW development system. LabVIEW provides excellent support for measurement applications and represents the most recent trend in the measurement applications development. The basis of the system consists of PC with Windows operating system, National Instruments LabVIEW development environment and GPIB controller card. We use the Helmholtz coils to generate the magnetic field. The measurement process is extremely simple and convenient (see Fig. 1). Tested MR sensor is placed in the centre of the Helmholtz coils according to its sensitivity axis. The sensor is supplied with the laboratory power supply. The user sets the range of testing current on the front panel of the virtual instrument (VI) and starts the measurement. The VI then controls all the measurement process via GPIB bus: sets the current to the Helmholtz coils and measures the current via voltage drop on the 0.1 Ohm resistor with one of the Agilent multimeters. Output voltage of the sensor is measured simultaneously by the second multimeter. The commutator controlled by the digital output of the power supply is used to switch the polarity of the current and thus the polarity of magnetic field generated by the Helmholtz coils. GPIB BUS
HP 34401A
PC with GPIB Card
HP 34401A
HP 6642 DC OUT Digital
Commutator
Helmholtz Coils
0.1 Ohm
DC 5V
MR Sensor
Fig. 1 System Layout
The characteristics of the NVE SDT sensor with various DC biasing
Output voltage [mV]
2000
No biasing Biasing 4.5 mA (2 V) Biasing 9 mA (4 V) Biasing 14 mA (6.2 V) Biasing 20 mA (10 V)
1500
1000 Field Intensity [A/m]
500
0 -500
-400
-300
-200
-100
0
100
200
300
400
500
-500
Fig. 2 SDT Sensor Measurement Results
Conclusions The described system is capable of testing MR magnetic sensors. Maximum generated current depends on the power supply and the Helmholtz coils used. The maximum current with HP 6624 power supply and our laboratory Helmholtz coils is 10A, which corresponds to the field of 3,845mT. The system is capable of generating this current and thus field in both polarities. The smallest current step is limited only by the power supply capabilities. The polarity is changed with the commutator. The tests can be performed in various options (bipolar/unipolar, hysteresis/one-way). The system contains software protection against voltage peaks caused by the attempt to change the current put through the Helmholtz coils. This protection does not allow to higher current step than defined in the Settings section of the program to appear on the output of the power supply. The results can be exported either to text file or to automatically generated measurement report, which is sent either to the printer or to the HTML file for direct web publishing of the results. At present time, the system is used in the student courses of Contactless Measurement on the Department of Measurement. M. Vopalensky and J. Saneistr used this measurement system for acquiring data for their diploma thesis. The results (see Fig. 2) of the measurements will be presented in two papers on the Eurosensors XVI Conference in Prague. The future perspective of the system is development of fully automated 3-channel microprocessor controlled commutator and 3D extension of the measurement system. This enhancement will be able to generate the magnetic field in any direction and to be able to compensate the Earth magnetic field during the measurement.
Acknowledgement This work was supported by Marie Curie Fellowship programme. The sensor prototype samples were supplied by NVE Corp. All the work was carried out under the supervision of Pavel Ripka.
References [1] P. Ripka (ed.): Magnetic sensors and magnetometers, Artech, 2001. [2] LabVIEW User Manual, National Instruments, July 2000.
