基于闭环理论的自由曲面测量与实时重构

Measurement and real-time reconstruction of freeform surface based on closed-loop theory

寻求更优的测量方法和更精确的重构模式一直是逆向工程中的研究重点。基此,对具有自由曲面特征的产品逆向,提出一种自适应测量、实时重构和在线评价的方法。依据几何特性,测量实物外形并用高次Bézier曲线拟合,微分推演探测点及矢量,指导三坐标测量机CMM(Coordinate Measuring Machining)自适应探测完整实物点云;借助非均匀B-spline架构曲面模型,基于扫描网格和实物特征从以上重构的曲面上抽取关键检测点,在线引导CMM确定上述检测点在实物上的准确位置并评价已构曲面误差,若误差高于阀值就将检测点加入更新曲面,逐步提高直至满足几何精度,集成测→构→评→添的全闭环逆向过程,建立精确的实物数字化模型。实例验证表明,该方法可利用曲面的几何特性,让测点分布随曲面曲率变化而呈疏密变化,以获得高质量的原始数据点云,实时模型重构、在线评价并及时添加检测点修正模型,可完整表述曲面的几何信息,可使重构模型精度达微米级。

Better method of measurement and more accurate model of reconstruction are focus in reverse engineering. A new method for freeform surface which includes an adaptive measurement, real-time reconstruction and online evaluation is presented in this paper. A Bézier curve fitting for measuring data according to geometric characteristics is developed.The coordinates and vectors of points can be calculated that can guide Coordinate Measuring Machining to complete measuring and get point cloud. A surface model can be fitted with the point cloud by using the non-uniform B-spline method.Some check points are added based on the surface models mesh. The location of these check points on the surface is reviewed with CMM and the model is evaluated. The surface model is updated to meet the accuracy. Measurement, reconstruction and evaluation are combined in the closed-loop reverse process that can bulid up an accurate model. The example shows that this method can obtain high efficiency data. The measuring points are distributed over the surface according to its geometric characteristics. Real-time reconstruction and online evaluation can timely update model. The accuracy of reconstructed model achieves micron order.