A Fiber Bragg Grating RMS Current Transducer Based on the Magnetostriction Effect Using a Terfenol-D Toroidal-Shaped Modulator

A new approach to the fabrication of robust and low-cost fiber Bragg gratings optical current transducers using a Terfenol-D magnetostrictive material is presented. Electro-erosion is used to manufacture a magnetic core in a toroidal shape, which results in a single piece robust mechanical design. By applying mechanical compression to the toroidal sensor and making its response very close to a quadratic function, it is possible to calculate the rms value of the current directly from the output of the sensor and eliminate the dc biasing magnetic field used in all previous techniques. A new electronic interrogation circuit technique, which allows for the measurement of ac signals and keeps the dc operation point of the fiber Bragg grating, is developed and successfully used in the prototype. Experimental results measured in the developed optical current transducer show that an error of ±0.6% is achieved for currents over the 320-900 A range. When tested over the temperature range of 25°C -45°C, a maximum error of ±2% is observed. The developed system presents a fast transient response, and needs only 34 ms to reach the steady state after a 150% amplitude step increase is applied to current being measured.