Design considerations in the development of microcantilever based non contact current sensor.

摘要

A microcantilever based non contact current sensor was developed. This development included the evaluation of a readout method, design of the cantilever sensor element.; Three readout methods were evaluated: resistive readout, optical beam deflection (OBD), and dual fiber reflective readout. Of the three, the resistive readout was the most difficult to operate, because the readout required special microcantilevers made from boron doped silicon. These cantilevers have a higher stiffness than the silicon nitride cantilevers used with the optical readouts increasing the resonant frequency. Also, due to the way the cantilevers are fabricated it is difficult to modify them to reduce the stiffness or add a magnetic coating. The most sensitive of the readout is the OBD readout which uses a laser focused on the end of the cantilever and reflected into position sensitive photodetector. While this method is sensitive and reliable, it requires a good deal of optics that increase the costs. The method that shows the greatest potential is the dual fiber probe. While the dual fiber is not as sensitive as OBD, it has the advantage that it can use inexpensive light sources (LED) and photodetectors (photodiode).; The cantilever sensor element is the key to the current detector. Some basic things about the cantilever geometry were found. First, the resonant frequency is critical factor in the design, since for frequencies near the resonant frequency a mechanical amplification is gained from the cantilever. Because the resonant frequency is directly proportional to the stiffness of the structure, the selection of substrate material is critical. Second, it was found that increasing the thickness of the magnetic material coating improved the response of the cantilever. Third, it was found that with some of the readout techniques increasing the surface area improved the response of the cantilever. This is thought to be for one of two reasons: (1) more of the coating material interacts with the magnetic field, (2) the greater surface area increases the amount of reflected light from the optical readouts.; Finally, it was found using the dual fiber readout that amplitude of the response of a cantilever sensor is directly proportional to the current and indirectly proportional to the radial separation distance. It was also shown that a 1 A current could be detected at a radial distance of 5 mm. A calibration technique was developed taking both effects into account.