Calibration Aspects of Multiple Line-scan Vision System Application for Planar Objects Inspection

The System Set-up Time it is one of the characteristics of an Industrial Vision System, besides the accuracy performances and response time. Minimizing the set-up time while keeping the performances in accuracy and in response time is one of the goals of any advanced Vision System. Starting from the purpose and the required performances of the proposed Industrial Vision System, in the paper is presented a calibration method developed for a multiple line-scan camera Vision System (in particular for a dual line-scan camera system). The calibration method presented is based on analyzing the image of a calibration tool exposed to the Vision System. There are presented the type of dimensional distortions identified from the experimental results. The second part of the paper presents the calibration method. The Industrial Vision System described in the paper is designed for silhouette inspection of planar objects located on a moving scene (transport conveyor), in a robotic handling application (it is a pure 2D Vision System, the volumetric characteristics of the analyzed objects being not relevant for the application). However the height of the object is varying in time (from one set of objects to another). Due to the fact the distance between the cameras and the objects is changing, the measuring results are affected. The proposed calibration method allows the Vision System to self adjust the calibration parameters for a known change in height of the objects, without affecting the accuracy system performances. In the final section of the paper are presented some practical aspects of the proposed calibration method, and the balance between the off-line and the online required computational efforts from the Vision System. 1 MULTI LINE-SCAN CAMERA VISION SYSTEMS CHARACTERISTICS The class of the Artificial Vision Systems dedicated for analyzing objects located on a moving scenes (conveyor) presents some specific characteristics relative to the Artificial Vision Systems dedicated for static scenes. These characteristics are identified also on the Image Calibration Process (Borangiu, et al., 1994)., (Haralick and Shapiro, 1992). Figure 1 presents the model of the image obtained from a dual line-scan camera Vision System. For this class of the Artificial Vision Systems we could identify as relevant for the calibration process the following characteristics: The system is using line-scan cameras for the image acquisition. The system is a dual-camera. The obtained image has significant distortions on (and only on) the image sensors direction. There is an overlapped image area between the two cameras. The end of the acquisition line of the 1 camera is overlapping the beginning of the acquisition line of the 2 camera. This overlapping area is significant in dimension and is a constant parameter resulted during the artificial vision system installation process. There is a lengthwise conveyor distance between the acquisition lines of the two cameras. This distance is also a constant parameter and its value is fixed during the system installation process.