Research On Cantilever Based Fiber-Optic Ultrasonic Sensor

Partial discharge is an important cause of insulation breakdown in high-voltage electrical equipment.On-line monitoring of discharges and timely elimination of possible faults are of great significance for ensuring electricity consumption of living and production.This paper made an in-depth study on the application of optical fiber sensing technology in ultrasonic measurement of partial discharge.Common partial discharge monitoring methods and researchs of optical fiber sensing technology for ultrasonic monitoring were introduced.The effects of the size parameters of the cantilever beam ultrasonic sensor on the measurement sensitivity and natural frequency were discussed,and the effects of different diaphragm materials on the sensitivity were analyzed to clarify the material and size parameters of the cantilever beam.Finite element analysis software was used to simulate the physical characteristics of the sensor,which provided a theoretical basis for the design and manufacture of ultrasonic sensors.In order to improve the detection performance of optical fiber ultrasonic sensors,a cantilever ultrasonic sensor based on the principle of Fabry-Perot(F-P)interference was designed.A rectangular cantilever beam with one fixed side and three suspended sides was laser-carved on the stainless steel diaphragm,which was combined to the end face of the optical fiber to form an F-P interference structure.Under a certain sound pressure,the vibration displacement the cantilever beam's free end was more than two orders larger than the vibration displacement of the center of the ordinary diaphragm,with advantages of high sensitivity and large dynamic range..A set of fiber optic cantilever ultrasonic sensor demodulation system including DFB laser,coupler,sensor,data acquisition card,etc.was built,using an interference-intensity type demodulation scheme,combined with the principle of wavelength cyclic demodulation to stabilize the static operating point,The following experiments were carried out:(1)Under the low-frequency signal test of the demodulation system,the sound pressure response of the optical fiber cantilever beam acoustic wave sensor was 1.28 mV / Pa,and a 1572 nm / Pa sensitivity of the optical fiber cantilever beam acoustic wave sensor was calculated by voltage sensitivity.Under the same conditions,the signal-to-noise ratio is 2dB lower than that of the capacitive acoustic wave sensor.Under the high-frequency test of the demodulation system,a 40 kHz ultrasonic wave was input to the sensor with a center frequency of 40 kHz,and the sensor signal-to-noise ratio is about 70 dB under a certain sound pressure;(2)In the comparison experiment of suspension in water and stuck to the outer wall,the results showed that the sensitivity and signal-to-noise ratio of the method of suspension test in water were higher than the test method of the sensor stuck to the outer wall.(3)In the stability test of the optical fiber cantilever ultrasonic sensing demodulation system,by recording and processing the Q point output voltage and the voltage data input to the laser within a certain period of time,the experimental results showed that the relative standard deviations of the two indicators are within 5%,which inferred that the demodulation system has a certain stability.The above experiments showed that the optical fiber cantilever ultrasonic sensor is expected to be used in the monitoring of partial discharge ultrasonic waves in power equipment.