Direct measurement of dynamic structural deformations in order to improve dynamic models of PKM

In high speed machining, tool accelerations need to be increasingly high. Parallel kinematics machines present the highest ones, but combined with slimmer structures, the structural deformations cannot be still ignored. To solve this problem, some studies improve the dynamic model by taking into account the flexibility of the structure, but the identification of such model is still a difficulty. This paper proposes a method to obtain a direct measurement of the structural deformations under dynamic loading conditions due to the action of the actuators. The main difficulties are the measurement volume (1 m(3)), the deformation size (a few 0.01 mm) and the movement velocity (a few 10 m/min). The principle of the method is to merge position measurement by vision and acceleration measurement by accelerometers. Indeed these two kinds of material are complementary. Accelerometers have a high frequency of acquisition, but a double integration is necessary to obtain the deformation information. This involves an accuracy problem. On the contrary, vision systems take the position information directly but the acquisition frequency is too low to complete the task. The aim is to calculate the positions with the acceleration measurements and to adjust them with the fewest direct position measurements.