Methods And Key Technologies Of Automatic Ultrasonic Thickness Measurement Of Large Thin-wall Parts

Large thin-wall parts are typical structural parts of core equipment in aerospace,advanced rail transportation and other engineering fields.The manufacturing precision of residual wall thickness is highly demanded.Accurate measurement of blank thickness and residual wall thickness after processing is a key link to guarantee the manufacture quality of parts.There are many disadvantages of the traditional gauge and handheld thickness gauge(ray,ultrasonic)in thickness measurement of large thin-wall parts,such as intensive labor,insufficient measurement efficiency and limited accuracy.Furthermore,the spacial coordinates of the measuring points are unknown,which can not accurately describe the thickness distribution of parts,and is unable to meet the demand of wall thickness measurement.Considering the advantages of ultrasonic thickness measurement method in measuring accuracy,measuring range,measuring reachability,solving the problem of "automatic ultrasonic measurement" is a feasible way to realize effective thickness measurement of large thin-wall parts.In this paper,aiming at meeting the requirement of accurate and efficient thickness measurement of large thin-wall parts,an automatic ultrasonic thickness scanning measurement method is proposed.And,key technical research work of accurate extraction of time difference of arrival(TDOA),automatic identification of ultrasonic incident angle,and shape adaptive adjustment of coupling space is carried out.The specific research contents are as follows:To measure the thickness of large thin-wall parts rapidly,an automatic non-contact ultrasonic thickness scanning measurement method is proposed.Sub-coordinate systems in the automatic measurement are defined.Relative position and posture relations between various coordinate systems are determined,and the kinematics model of the automatic ultrasonic thickness scanning measurement system is established.According to the kinematics analysis of the system,automatic ultrasonic thickness scanning measuring model of large scale workpiece is built.Main factors affecting the automatic thickness measuring precision of large thin-wall parts are analyzed,which lays a foundation for the subsequent research of key technologies.In order to satisfy the demand of real-time and accurate calculation of TDOA in dynamic continuous thickness measurement,a fast calculation method of TDOA is proposed based on echoes matching.According to the adaptive filtering theory,with Fourier analysis and time domain convolution processing of ultrasonic echo signal,a time-delay analysis model of ultrasonic echo is established and the theoretical basis for reducing the number of echoes in the TDOA calculation is determined.Based on the optimal similarity principle of adjacent echo waveformes,an echoes matching model is established for TDOA estimation.To realize a fast echoes matching process,an adaptive adjustment strategy of step size is proposed with consideration of the convergence speed and convergence stability,harmonizing the relationship between iteration step size and matching errors.An improved matching pursuit method for overlapping echoes separation has been developed.As a result,a new method for fast and accurate calculation of TDOA based on echo matching is formed out.A serial of samples with different thickness are used for simulation and experimental analysis,and the results show that compared with the existing calculating methods of TDOA,the measurement accuracy is improved and the calculation speed is increased by nearly 35%.Aiming at reducing the thickness measurement errors caused by ultrasonic incident deviation in thickness scanning measurement,an automatic identification method of ultrasonic incident deviation and error compensation is proposed.Based on the angle spectrum decomposition of sound source and an introduction of an equivalent sound source,with the combination of the equal-effect acoustic pressure propagation equation,the spatial sound pressure spectrum model is established considering the incident angle of ultrasonic pulse.By integrating the acoustic pressure of the sound source surface and receiving surface in frequency domain,the amplitudes of the surface reflected pulses at different incident angles are obtained by theoretical analysis.The sensitivity of echo energy to incident angle is found,and an incident angle identification method based on the calculation of first echo energy attenuation is proposed.Combining with the acoustic refraction law on the liquid-solid interface,the compensation model of thickness measurement errores caused by ultrasonic incident angle is established.The experimental results show that the ultrasonic thickness measurement errores can be reduced by up to 70%in the angle range of 0-2.5°,which effectively improves the accuracy of automatic ultrasonic thickness scanning measurement.In order to solve the control problem of coupling space between the ultrasonic measuring device and the measured workpiece surface in the process of non-contact ultrasonic thickness scanning measurement,an adaptive adjusting method of coupling space is proposed.The coupling space adjusting strategy based on the measurement of eddy current displacement sensor is developed.According to the measured displacement of eddy current sensor,the edge position of the workpiece can be identified to determine the measurement area of the ultrasonic sensor.A state discrimination model of coupling space is established and the safe adjusting position is estimated.Target position and posture of ultrasonic measuring device is calculated.Based on the developed CNC measuring platform,a serial of experiments are carried out to verify that the shape adaptive adjusting method can realize the accurate discrimination and smooth adjustment of coupling state,ensuring the coupling space is always stable within a reasonable range during the non-contact ultrasonic thickness scanning measurement.The synthetical application of the method and key technologies of automatic ultrasonic thickness measurement are validated by experiments conducted on samples with different shapes.The precision of thickness measurement system can reach 0.03mm,successfully meeting the requirements of wall thickness measurement of large thin-wall parts.Taking the wall plates of large liquid propellant tank as the application objects,automatic ultrasonic thickness measurement systems for large thin-wall parts are developed.The feasibility and effectiveness of the automatic ultrasonic thickness measurement system in engineering application are verified by the field experiments.