Non-contact Measurement And Evaluation Of Cylindricity Based On Area-array Sensing Technique

Cylindricity error is one of the important precision indexes of shaft parts. Accurate measurement and evaluation of cylindricity error can not only provide the basis for parts acceptance, but also guarantee the precision of parts processing and assembly. There exist some disadvantages in current cylindricity measurement methods, such as randomness, insufficient sampling and non-standard verification. Therefore, it needs urgently to research efficient and precision measurement method and evaluation technique which meet the requirement of GPS standard.Non-contact measurement of cylindricity error is carried out based on area-array sensing technique and multi-aperture connection technique. Moreover, standard evaluation is realized using GPS operator technique. A new measurement system and digital evaluation software are developed, and an efficient and feasible method is presented for cylindricity measurement & evaluation. We have paid more attention to the research of relative fields, and some results are shown as follows:1. Analysis of current cylindricity measurement methods and their problemsBased on the analysis of the standard sampling specification of ISO/GPS, the experimental measurements of cylindricity with the existing instrument are carried out. Then, the disadvantages and problems of current methods are discussed. Circumferential section method is used as the main type of sampling in current measurement program. In fact, it is an approximate measurement based on the theory of roundness measurement. Because of the point-contact sensing type of the instrument, too few numbers of the measuring section will cause under-sampling of axial information, and will lead to the instability and unreliability of the evaluation results. The concepts of sampling density and frequency are proposed based on the analysis of the measured results and ISO/TS12180-2. Computer simulation results show that the axial sampling frequency is the key factor to realize precise evaluation of cylindricity. Therefore, we need to find new technique for precision measurement of cylindricity.2. Novel method and new system for cylindricity measurementThe novel method is based on the theory of multi-aperture overlap scanning technique (MAOST), and digital grating projection technique is employed to capture the profile of part. Firstly each 3D profiles of part from different views can be measured, and then all profiles are transformed into the global coordinate by multi-aperture connection technique. The method has some advantages such as no-contact, high efficiency and whole-field information. Compared with current measurement method, the new method can significantly increase the sampling density and axial sampling frequency, and the abundant data of surface are obtained for cylindricity evaluation. A DLP projector is used to project digital fringe patterns onto the object; and a CCD camera is employed to capture the deformed fringe patterns. A multi-view measurement device is constructed based on digital grating projection technique, and attached software is developed. As a result, a complete system is established for cylindricity measurement.3. Novel strategy of system calibration and error-correction techniqueIn digital grating projection technique, system calibration is a critical step to carry out precision measurement. After comprehensive analysis of the geometry of measurement system, the accurate mathematical expressions of the mapping relationship of calibration are set up. The reference plane is measured, and the phase-to-depth and pixel to lateral coordinate mapping relationship are simultaneously calibrated. Then, the characteristic parameters of system calibration are determined. In the calibration process, an improved Harris corner extracting method is proposed, and characteristic corners of the calibration gauge in the image can be extracted with sub-pixel accuracy. Moreover, the neural networks are built to correct the error terms of the lateral coordinates of the reference plane by considering the aberration of CCD. Error analysis is the precondition to ensure the measuring precision. A novel nonlinearity correction method is proposed for fringe projection profilometry. In order to describe the nonlinearity of system, polynomial fitting is introduced instead of the single gamma parameter. Using the suggested method, the influence of nonlinear system is suppressed. Next, in order to improve the precision of measurement, the system error is analyzed by use of standard plane measuring method. The mathematic model of error correction is founded, and the system error can be compensated effectively.4. Multi-aperture connection method based on quaternionThe principle of multi-aperture overlap-scanning technique is to make the adjacent sub-apertures partially overlapped, and then their relative location and orientation can be obtained through the overlapped area. The sub-apertures can be transformed to a global coordinate system. Therefore, coordinate transformation is the key step to realize connection. In the computation process of the original connection algorithm, the traditional matrix method is used to realize coordinates transformation. In addition, connection model and coordinate transformation are established in cylindrical coordinates system and Cartesian coordinates system, respectively. In order to improve calculability of the connection algorithm for the precision requirement of cylindricity measurement, a new connection algorithm method using quaternion method is brought forward according to the original algorithm. The quaternion instead of the traditional matrix is used to realize coordinates transformation, and connection model and coordinate transformation are established in the same Cartesian coordinate system. On these bases, the error movement calculation and coordinates transformation are performed by the improved iterative algorithm. Then, the multi-aperture connection can be realized by means of the high precision error movement calculation.5. Cylindricity evaluation techniques of area-array sensor measurement based on GPS specificationBased on the GPS operator technology and the new measurement method, a complete verification operator of cylindricity is constructed, which includes partition, extraction, filtration, association and verification. Because the filtration is the critical role of operator, a weighted average filter with nonuniform sampling is proposed on basis of cubic-fit filtering algorithm. The filter can realize filtering and extract uniform data. Based on the standard and optimal operation process, the accuracy and reliability of cylindricity evaluation can be ensured. According to the characteristic of tested data, three new parameters for form errors evaluation are presented, including C-map, PV, RMS. New parameters are useful for analysis and improvement of the processing technology, and they can also supplement the cylindricity evaluation indexes. Moreover, according to the theory of measurement uncertainty and the uncertainty judging principle of GPS, the equation of uncertainty estimation for cylindricity measurement is deduced. The practical application shows that the uncertainty estimation of cylindricity can guarantee the integrality and validity of evaluation results.