There is temperature and pressure cross-sensitivity when using ordinary fiber to detect pressure. In order to solve this problem, a fiber Bragg grating pressure and temperature sensor based on double equal thickness and equal strength cantilever beam was proposed in this paper. Feasibility of the structure was verified by theoretical analysis and simulation. The first sensing element of the sensor is a cantilever beam with equal thickness and strength. It mainly consists of temperature-strain sensitization zone of bimetal and load-bearing zone of stress-strain optical fiber. The second sensing element consists of two fiber Bragg gratings with different grating spacing distributed on a single fiber along the axial direction. The distance between these two gratings are predetermined. Because the initial grating spacing of the two fiber Bragg gratings is different, the corresponding demodulated wavelength varies and possesses a certain wavelength difference as well. Hence, the measurement of two different parameters can be realized. The first Bragg grating is fixed on the bimetal temperature sensing region and cannot measure pressure as it does not vary with external pressure. The theoretical derivation of optical fiber sensing proves that the distance between the two peaks (the wavelength difference between the two peaks) of the second Bragg grating reflectance spectrum is only proportional to the pressure and independent of temperature variation. From this principle, the pressure is measured. The simulation results reveal that the proposed structure can realize two-parameter measurement of pressure and temperature. The fiber Bragg grating detection device has the advantages of low cost, stable and reliable operation.
In order to complete the high-speed wavelength demodulation of output signal of Fiber Bragg Grating (FBG) sensing unit, an edge filter wavelength demodulation system is established in this paper. This wavelength demodulation system uses a reference FBG as an edge filter. First, the mathematical model of the system is built based on demodulation principle. In the model, the spectrum of FBG is simplified to Gaussian distribution. The model describes the relationship between the output voltage and input wavelength differential. According to the research above, the hardware and software of this demodulation system are developed. Besides, a calibration method of the system is proposed. Finally, experiments are carried out. According to the experiment results, the sensitivity of the wavelength demodulation system is 6.3mV/pm. When the demodulation frequency is up to 5kHz, the wavelength resolution is 0.23pm. This system has many advantages, such as simple structure, low cost and high resolution in high speed wavelength demodulation
Regardless of many researches done in recent years, most wind turbines are still unable to reach their design lifetime . Failures in the gearbox, especially in the planetary stage, have been a major cause of reliability problems in the modern wind energy turbine system. The following paper proposes a fault diagnosis method based on the strain signal of the ring gear. First, the strain signal is collected from the side of the ring gear using FBG sensors in normal condition and faulty condition. Then the collected strain signal is processed and analyzed. In the time-domain analysis, traditional statistical indicators like Peak to Peak, Kurtosis, Crest factor and Peak value are adopted. The analysis results show the effectiveness of the proposed method for identifying tooth crack fault of the ring gear.
A large-range three-coil coaxial optical fiber displacement sensor for measuring the air gap of a direct-drive wind turbine is designed in this paper so as to overcome the problem that the traditional reflective optical fiber sensor has a small measuring range. The mathematical model of the modulation function of the three-coil coaxial large-range fiber displacement sensor is established by using the simplified geometric optical reflection spot model, so that the ratio compensation mechanism under different combinations is analyzed and compared, and the parameters affecting the characteristics of the sensor, including the fiber core radius R and the numerical aperture NA of the transmitting fiber, and the number of receiving fiber loops in the second, third and fourth layers of the sensor probe are analyzed by simulation. The results show: As the number of laps of the receiving fiber turns, the sensor range is significantly improved, and its sensitivity is improved, but the corresponding initial dead zone is also increased; the smaller the numerical aperture of the transmitting fiber, the larger the linear range of the output characteristic curve; the larger the radius of the fiber core, the larger the linear range of the output characteristic curve, but the corresponding initial dead zone becomes larger and the sensitivity is reduced, so that the final selected design parameters of the system are given by analysis. Finally, through the sensor characteristics experiment, the actual measurement range of the sensor probe is consistent with the theoretical simulation results, and its measuring range can reach 3.5mm- 8.5mm.