## 3 Analysis of Key Technologies
### Extraction Method of the Leading and Trailing Edges Based on A Priori Planning  
The high-precision extraction of the leading and trailing edges is the primary and key issue to be solved of blade twist measurement. The leading and trailing edges of the blade have characteristics of high geometric accuracy, ultra-thin shape and large curvature fluctuation, which cause great difficulties with the machining and measurement of blade edges [19; 20]. Influenced by the probe accuracy and system scanning resolution, the existing measuring method of blade edges has disadvantages of large coordinate error, serious fluctuation or even loss of measured data.  
In order to solve the technical problem of the blade edge measurement, a new measuring method for leading and trailing edges based on a priori planning is proposed in this study. The blade edges are measured in three steps using the method proposed in this study. Firstly, the test profiles are scanned by the probe system in the absence of the theoretical model of measured blade [21; 22], and a set of a priori coordinates of \(P_{si}(x_{si},y_{si},z_{si})\) for planning are obtained, \(i=1,2,\ldots,N\). Then, the number and location of measurement points are optimized based on the feature recognition and sampling strategy analysis for the a priori coordinates collected [23; 24]. Finally, according to the optimization results, the probe system is controlled to collect data at the planned positions, and the precise coordinates of \(P_{mi}(x_{mi},y_{mi},z_{mi})\) of each measuring point are obtained.  
The curves of the leading and trailing edges measured by priori planning technology are shown in Figure 7. It can be concluded from Figure 7 that the sampling strategy is optimized automatically without the theoretical model of the measured blade, and the edge features of the blade are extracted effectively by planning measuring points densely at the edge parts with larger curvature variation. Based on the least squares fitting for the precision coordinates collected of the leading and trailing edges, parameters of center coordinates and radius of the blade edge can be solved quickly and accurately. The high-precision measurement of the leading and trailing edges based on the technologies of a priori planning and least squares fitting is a key issue and innovation, which overcomes the influence of the error factors such as the movement accuracy of the machine tool and provides the authentic  
Figure 6: Registration diagram of aeroengine axis.  
measurement data for the extraction of the profile chord and subsequently the calculation of twist angular position.  
The adaptive optimization of sampling strategy is achieved through form-free and a priori planning measurement of the measured blade in this method. In addition, coordinates of \(P_{mi}(x_{mi},y_{mi},z_{mi})\) are collected at the planned positions, where the depth of field is close to zero, thus the inclination error is effectively reduced and the measurement accuracy is improved greatly [25; 26]. The research results show that the measuring method based on a priori planning has the characteristics of being high precision and form-free. The comprehensive measurement accuracy reaches the level of 10 \(\upmu\)m, which can meet the measuring requirements of the leading and trailing edges of ultra-thin blade.