Non-Contact Magnetic Field Sensor

Microscale magnetic sensors with high sensitivity, quick response, and high temperature stability are needed for recent developments such as magnetic recording technology, highly accurate rotary encoding for intelligent robot control, and non-destructive testing and sensing in various industrial, automotive, and environmental measurement tasks. For such applications, anisotropic magnetoresistance (AMR) elements have been widely used as micromagnetic heads and micromagnetic sensors [7]. However, the AMR element 3 is predicted to become obsolete for small size field sensors due to its low signal/noise ratio (with an AMR ratio of 2 - 4% [8] for a field of 2 - 4mT, about 1%/mT). Giant magnetoresistance (GMR) [9] is a promising phenomenon for the development of a new micromagnetic sensor due to its higher GMR ratio of 4% for about 400T (about 10%/mT) as a spin valve sensor. However, more sensitive microelements will be required in the near future for high-resolution magnetic measurements [10]. The impedance of a high-permeability element sensitively changes with an external (quasistatical) field due to the skin effect in a high-frequency current application. This GMI effect [11, 12] shows an extremely high impedance ratio of more than 100%/mT in zero-magnetostrictive amorphous wires. In addition, the magneto-voltage of a GMI element in a self oscillation circuit such as a Colpitts oscillator shows a high changing ratio of the element voltage amplitude, 5 . . . 6 times larger than the GMI ratio. Another important feature of the GMI element is its independence of the demagnetizing field with respect to the external field. This feature allows the construction of a microscale sensor without decreasing the field-detection sensitivity due to the circumferential magnetization with the element current. Quick response is also an advantageous point of the GMI effect due to the high-frequency current magnetization which works as a carrier of the amplitude or frequency modulation.

Many technical applications need wireless sensing, because a cable connection between the sensor and the measurement system cannot be established. Surface acoustic wave (SAW) elements in combination with a magnetic field sensor and an interrogation unit enable wireless installation as well as completely passive operation and are maintenance free. They are small, robust and can withstand extreme conditions. SAW elements are manufactured using one lithography process only (Al sputtering and etching). The measurement system consists of a SAW element combined with a giant magnetoimpedance (GMI) sensor (wire or thin film), a transceiver and an analyzing unit. A radio frequency burst is transmitted from the interrogation unit and received by the antenna of the SAW element. The interdigital transducer of the SAW converts the incoming burst to a surface acoustic wave. It propagates towards the reflectors of the SAW. The reflectors (one terminated with the GMI sensor) are placed in distinct displacements and reflect the incoming wave towards the antenna. The interrogation unit receives this signal (a pulse train) and gains the magnetic field depending information about the reflectors. We used the 434 MHz band of the possible industrial, scientific and medical radio bands because the devices are more easily to manufacture and the components are readily available. Due to the GMI effect the impedance of a thin wire or film changes in dependence of an applied magnetic field. The technological issues of interconnecting a thin amorphous GMI wire with electronic circuits have been solved by laser-microwelding. Furthermore, thin film trilayer structures have been sucessfully produced and tested for magnetic GMI sensor applications. The main result of the project could be an innovative nondestructive evaluation method for monitoring ferromagnetic constructions: The passive sensor chip is placed close to ferrous parts, e.g. re-inforcement within concrete and it will be wirelessly requested by a RF burst. Thus the change of the magnetic stray field due to degradation by oxidizing or stress fatigue could be detected by periodical control during the whole lifetime of the construction. A collapse of buildings, bridges, etc., could be avoided by early warning.