Research On Positioning Method Of Pixel Sensor Chip In The Assembly Of ALICE ITS

ALICE (A Large Ion Collider Experiment) is a heavy-ion physics experiment at the Large Hadron Collider (LHC) at CERN, and CERN decided to upgrade several detectors in the ALICE before 2019.The ITS (Inner Tracking System) is one of the sub-detectors of ALICE, it will use a new pixel sensor chip. For ITS upgrades, the positioning of the pixel sensor chip is an important part. The common methods of positioning in the industry is optical positioning,but it has low productivity and poor uniformity, and High-precision positioning platform is always expensive. For that reason, this paper presents a positioning method of non-contact, high precision and high efficiency based on scene-type image processing technology.This positioning method uses a high-precision camera to extract the scenes of chip during assembling as template images, and marks the object chip manually. Using filtering, contrast enhancement and image sharpening to reduce noises and enhance image quality. Using canny algorithm to get all the contour points of objects in the image quickly, and using boundary tracking algorithm to connect discrete contour points into a closed contour, calculating the arc length and curvature of each pixel on the contour as the characteristics of the contour to complete the matching and positioning of target object. The control strategy of the platform is based on visual servo, completing the closed-loop control of the platform and the positioning of chip during assembling according to the scene image sequence.This paper describes the traditional positioning methods and presents a new positioning methods based on the arc length and curvature, the image processing algorithms involved were also reviewed, including image preprocessing, identification, matching and positioning. The paper described the designing of the positioning platform in details, including hardware modules, features and hardware selection. The paper also introduced the control strategy and software of the platform at the same time. Finally, the paper presents the test results of the positioning platform, the results show that image processing technology can complete the recognition and positioning accurately, the platform can complete a series of positioning of the chip during assembling by following the sequence of scene pictures.