In-Situ Monitoring Technology For Rim-Driven Thruster Motor Sheath Damage Based On Ultrasonic Guided Waves

The gap between the stator and rotor of Rim-driven thruster(RDT)motor allows water to flow to achieve motor cooling,and the stator and rotor of the motor are provided with a layer of waterproof sheath.Commonly used motor sheath materials include: stainless steel,titanium alloy,carbon fiber,reinforced resin,rubber or glass fiber reinforced plastic,etc.Among them,the eddy current loss of stainless steel sheath is large,the eddy current loss of titanium alloy is small but the manufacturing cost is high,so a large number of RDT motors use non-metal sheath.During the service of the RDT,the non-metallic sheath of the motor may be damaged such as cracks,hydrolysis and wear.With the expansion of the damage,faults such as motor water seepage and short circuit may eventually occur.There is a lack of identification methods for sheath status,so this paper studies an ultrasonic-based RDT motor sheath damage monitoring method to provide data conditions for motor system fault diagnosis.The specific research work is as follows:(1)We have analyzed the basic structure,material composition of the RDT motor sheath,and the characteristics of sheath damage.Comparing the advantages,disadvantages and application scope of different damage monitoring methods,a method on ultrasonic guided waves is determined.The difference between the propagation characteristics of ultrasonic guided waves in metallic materials and nonmetallic materials is analyzed through experiments.And the frequency range that ultrasonic guided waves need to be excited in different materials is determined.Using the principle that the ultrasonic guided wave signal will modulate at the damage,the damage modulation signal and frequency demodulation method are studied,and the damage monitoring scheme for the RDT non-metal motor sheath is designed.(2)The RDT motor sheath damage monitoring model based on nonlinear ultrasonic guided waves and the damage monitoring model based on ultrasonic helical guided waves are respectively established and simulated.The simulation results show that the nonlinear ultrasonic method can accurately complete the extraction of characteristic values,and meet the requirements of motor sheath damage location and effective identification.In the case of low SNR,the position of original signal can be analyzed by Short Time Wavelet Entropy(STWE)algorithm,which provides a theoretical basis for the identification of sheath damage by helical guided wave.(3)Building the ultrasonic guided wave damage identification test platform of RDT motor sheath,and conducted a nonlinear ultrasonic guided wave damage monitoring test.Using the same-side mixing excitation method,and spectrum analysis is used to complete the identification of damage.Using the method of delaying the grouping of the signal to realize the location of the damage,and combines the HibertHuang Trasformation(HHT)to extract the characteristic amplitude of the signal.Complete the identification of the depth of damage.The results show that the localization error of the damage by the nonlinear ultrasonic method is 5.3%,and the depth of the damage is inversely proportional to the characteristic amplitude of the damage signal.(4)Calculate the propagation path and helix angle of the helical guided wave in the pipeline,design the optimal arrangement scheme of the ultrasonic sensor,and carry out the ultrasonic helical guided wave damage monitoring test.The STWE algorithm is used to extract the entropy value of the ultrasonic signal,and the Time of Arrive(TOA)algorithm is used to complete the positioning of the sheath damage in threedimensional space.The results show that the ultrasonic guided wave method can locate the damage within 10%,and the damage depth is proportional to the energy value and entropy value of the damage signal.The in-situ monitoring of the non-metallic sheath of the RDT motor is completed by the method of ultrasonic guided waves,and the reliability of the method is verified by the combination of the simulation test and the actual test.Providing a monitoring method for the RDT motor sheath,which ensures that the motor sheath can be found and dealt with in time when the motor sheath is damaged during the operation of the RDT.