Objective  The circular structured light based measurement system can generate circular structured light by reflecting a laser beam through a conical reflector, and utilizing laser triangulation and close-view photogrammetry algorithms to calculate three-dimensional coordinates. This kind of measurement system has been widely studied by researchers due to its advantages such as high flexibility, high accuracy and simple structure. The current circular structured light system has certain problems in measuring roundness. It is difficult for the laser, the camera and the deep hole to be measured to be parallel or coaxial, which leads to the inability to measure the accurate circular cross-section. A measurement system based on circular structured light is constructed, the mechanism of systematic error is analyzed and a method based on the circular structured light system to measure circularity is proposed, which provides some compensation for systematic errors.

  Methods  The mechanism that generates systematic errors in the measurement of roundness by the circular structured light system is analyzed (Fig.3). A high-precision electric linear slide is used to move the deep hole parts to be measured to complete the full-field measurement, and a high-resolution point cloud is obtained and fit the axis. Through the Rodrigues formula, the inner surface point cloud rigid body will be transformed to the point cloud axis and z-axis parallel to the location of the point cloud axis, searching for the z coordinate near the point as a cross-section of the roundness of the assessment point (Fig.4). The roundness evaluation is completed by the grid search algorithm (Fig.5).

  Results and Discussions  The compensation by the proposed method works well in roundness measurement experiments, in which the measurement uncertainty is 4.78 µm (Tab.3). For the circular structured light measurement system, a long rod can be assembled after the laser to increase the axial measurement range of the system. For the proposed circular structured light roundness measurement method, the resolution of the 3D point cloud can be improved by reducing the step length of the motorized linear slide to further improve the measurement accuracy.

  Conclusions  A three-dimensional measurement system of circular structured light is built, the error generated when the circular structured light system measures roundness is analyzed, and a method based on the circular structured light system to accurately measure roundness is proposed. It is verified that the use of the method to measure roundness has a good compensating effect through the measurement experiments.