T.T. Nguyen, Q.T. Nguyen, A. Amthor, and C. Ament (Germany)
Laser Tracker, Identification and Tracking Control, Optical Measurement, Automation and Manufacturing
This paper presents model-based tracking control of a multi laser tracker system (MLTS). The MLTS is built of four laser trackers and is used as a high precision feed back sensor in order to provide contactless measurement of the position of the positioning stage. A single laser tracker consists of a homodyne laser interferometer as well as a galvanometer scanner, and tracks the retroreflector which is mounted on the Tool Center Point (TCP) of a positioning stage. First a mathematical relation is derived, which describes the beam path from the laser tracker system to the target. Then the dynamical behaviour of the galvanometer scan ner is modeled by an ordinary differential equation second order and the model parameters are identified via exten sive experimental studies. Based on these results, a closed loop tracking controller of the galvanometer scanner is presented. This model-based control consists of a PID controller in combination with disturbance compensation. Within the presented study, the parameters of the PID controller were varied in order to achieve a stable and fast behaviour of the laser tracker. A deeper analysis showed that the parameters depend on the absolute distance be tween the target and the laser tracker. The required dis tance is computed using the MLTS because one tracker can measure only a relative distance.
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