3-D Free-form Shape Measuring System Using Unconstrained Range Sensor

Three-dimensional (3-D) free-form shape measurement, a challenging task pursued by computer vision, is mainly characterized with single view acquisition and multiple view registration. Most of the conventional scanning systems are less flexibility and difficult to realize engineering applications for employing sequential registration tactic. To develop portable scanning system and engineering registration method overcoming problems of error accumulation and propagation is the research direction. In this paper, one 3-D free-form shape measuring system using unconstrained range sensor is designed. A quasi-active stereo binocular visual sensor embedded within a scanning mechanism is used as the range sensor. Error compensation is performed by residual amendment according to camera calibration lattice. Artificial control points are designed and adhered on object and one camera is introduced to shot these control points from different positions and orientations. Then ray bundle adjustment (BA) method is used to calculate the space coordinates of all the control points, so as to set up a global control net work. Registration can be completed by mapping at least 3 control points observed by range sensor in single view acquisition into the global control network. In this system, no calibration for laser plane is required and the motion of range sensor is completely free. The overlapping of neighboring region is unessential for registration. Therefore, the working range of the system can be easily extended. The measuring precision mainly depends on the quality of global control network. The sequential distances of coding control points are observed by electronic theodolites and then compared with those obtained according to BA result. Experimental results show that relative distance error of control points is no more than 0.2%. The proposed measuring system is portable, provides good capacity for global error control, and contributes to the engineering application of 3-D free-form shape measurement.