Infoscience

Thesis

Préhenseurs, conditions et stratégies pour une micromanipulation de précision

Currently microsystems consist of more and more functionalities in even smaller volumes. Components size has then to be reduced as well, coming through micro- and then nanoscale. The industrial equipments dedicated to their fabrication and assembly have comparatively a huge size, but one observes now a new and welcome trend for reducing their overall dimensions. However the latest robotic technologies allow achieving nanometric resolution and precision of positioning. Microcomponents assembly is still yet badly controlled and many surprising effects can occur without understanding of the real phenomenon. In fact, below one millimeter size objects are becoming insensitive to gravity comparing to surface forces. The manipulation of such components needs new and innovative ways to control these forces and so on to allow the release. In this study pick and place operations have been analyzed with an adimensional parameter Γ. It represents the ratio of the forces that act at the "object – gripper" and "object – target surface" interface. The adhesion effect is then taken into account in a comparative manner. The advantage of such procedure is due to the large amount of uncontrolled or unknown parameters that are needed to evaluate the adhesive forces. The behaviors of the micro object during pick and place operations are then studied on the base of Γ. Characteristics allowing a reliable transfer can be extracted with focus on the ones that generate fewer disturbances on the position. Generalization of the models allows finally to study different manipulation principles. The conception of microgrippers needs to consider the adhesion effects. Thus a methodology is proposed. Its main point concerns the importance of a strong interaction between the designers of the different elements that are the component, the receiver and the gripper. In such a way optimal choices for the surfaces and principles of manipulation can be done. After having defined the main trends theoretically, the difficult evaluation of the adhesion effect in real conditions causes the need to check rapidly by experiments the feasibility of the pick&place; operations. A micromanipulation setup has been developed in order to make comparative tests between the different gripper principles. Gripper characterization means in particular the measurement of the positioning errors induced during the placing step. Several gripper families were conceived during this study: microtweezers, inertial gripper based on adhesion, capillary grippers that use the condensation/evaporation of the relative humidity, electrostatic grippers as well as vacuum gripper. Experiments were conducted with polystyrene spheres of ∅ 50µm. A high sensitivity to the alignment between finger tip or gripper tip and micro object was observed. It concerns in fact mainly the need to limit the force applied on the component during manipulation. The gripper withdrawal direction has also showed to be of the most importance to control the operations and their reliability. Finally robotic assembly of MEMS components was realized to get a 3D structure. The methodology was experimented on this application. The importance of a close interaction of the different designers has been demonstrated. We proposed here an approach as well as the models allowing studying the main trends with taking into account the adhesion forces. The whole arrangement of the contact areas as well as each force present during the operations is included into the models. The interfaces characteristics can so be analyzed. This allows defining the optimal strategy. This study is a tool for the designer of microassembly equipment. With the proposed methodology we hope or rather give the opportunity to stimulate integration or combination of innovative micromanipulation principles.

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