Infoscience

Thesis

Optimisation de la conception du robot parallèle Delta cube de très haute précision

In the high-precision industry, most operations require the use of robots able to accomplish highly accurate and repeatable motions. In order to meet the desired level of absolute accuracy, one has to limit or even suppress the effects of different sources of inaccuracy by means of an appropriate calibration. However, calibration techniques cannot be applied to compensate inaccuracies occuring along passive (non-actuated) degrees of freedom (e.g orientation errors on the end-effector of a 3 degrees-of-freedom (DOF) in translation robot). The main goal of this thesis is to evaluate and benchmark the effects of these different sources of inaccuracy. Moreover, a set of design rules are proposed in order to optimize flexure parallel robots regarding their absolute accuracy. The robots studied in this work are of "Delta Cube" type, having their kinematic chains composed of translational stages and space parallelograms (acting as universal joints). These robots have the following features: their 3 DOF are the (x, y, z) translations on the Euclidean space, the stroke of each DOF goes from ±1 mm to ± 4 mm, their repeatabilities are at the nanometre range (thanks to the absence of dry friction and mechanical play), their resolutions are within a few nanometers (only limited by the captors used), their small sizes go from 1,5 dm3 to 13 dm3. The analysis of the effects of the different sources of inaccuracy has been focused on the following points: the basic elements composing the robot structure (translational stage, space parallelogram). Studying the influence on the absolute accuracy caused by a variation of a given geometric dimension provides knowledge on how to optimize the overall robot dimensions regarding absolute accuracy; the assembly of the different basic elements on the kinematic chain (in particular their relative orientation) is also critical for the absolute accuracy of the robot since it may cause parasitic effects such as orientation errors in the end-effector; the type and location of the different captors, motors and the frame can also influence accuracy. The analysis of the influence of each source of inaccuracy (manufacturing tolerances, temperature variations, assembly defaults, effect of external loads) and their coupling has been mainly performed numerically by means of Finite-Element Models. Theoretical results have been verified by experimental data. Considering the simulation results for each basic structure as well as the comparison of different assembly variants, general design rules have been proposed in order to optimize a given flexure parallel robot regarding absolute accuracy and considering also the application the robot is made for. This work is a contribution to the design of high-precision flexure parallel robots regarding absolute accuracy. It is also an important tool for the engineer in order to make the calibration work easier.

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