Research On Key Technologies Of An Automated Microassembly System

Microassembly has become an important means that produces themicro-electro-mechanical system (MEMS) with complex three-dimensional geometricstructures and different materials. The microassembly only can be implemented byvision and force feedback control due to the restriction of the positioning accuracy ofthe micromanipulators. However, the assembly accuracy and efficiency of themicroassembly systems have been influenced by the vision and force feedback. Firstly,how to quickly get the legible multidimensional vision feedback information with highresolution is always the issue of the microassembly system. Secondly, it is of greatdifficulty to integrate the micro force sensor on the microgripper. Therefore, designingthe proper microscopic vision system of the microassembly systems, improving theaccuracy and efficiency of the autofocus, and establishing the effective micro-forcecontrol method are necessary to automatically, rapidly, and accurately implementmicroassembly tasks. These researches are of importance due to their academicsignificance and engineering application prospect.In order to solve the above problems, the Pareto optimization method of themicroscopic vision systems for tradeoff between the vision resolvability and theintersecting space of view of the binocular microscope vision system is proposedaccording to the vision resolvability model and the established space of view model inthis study; the bilateral prediction and intersection calculation autofocus method isproposed by dividing the focus measure curve into two independent focus measurecurves with the peak and regarding the intersection of the two independent focusmeasure curves as the best in-focus position; the relationship between the holding forceand the angle change tangent value of the gripping jaws of the microgripper isestablished and the microscopic visual servo control based holding force control methodis proposed as well.The major research works and the innovations in this dissertation can besummarized as follows:1. The automated microassembly system with single manipulator and five degreeof freedom is set up for providing the technical support and the experimental conditionswhen studying these key technologies about the automated microassembly systems. Thedisplacement and force features of the piezoelectric-driven microgripper are analyzed and the relationship between the holding force and the angle change tangent value of thegripping jaws of the piezoelectric-driven microgripper is established.2. The model of the intersecting space of view of the binocular microscope visionsystem is set up for calculating the intersecting volume of their space of views. Themicroscopic visual servo control based Pareto optimization method is proposed foroptimizing the topological relationship and the magnification by weighing the visionresolvability and the intersecting space of view of the binocular microscope visionsystems. The Pareto optimal curve (namely Pareto front) that represents the weighedrelationship between the vision resolvability and the intersecting space of view iscalculated by the multi-objective function established on the vision resolvability and theintersecting space of view and the "Pareto optimization" principle. The topologicalrelationships and the magnifications of the binocular microscope vision system can bedetermined by the Pareto front.3. The bilateral prediction and intersection calculation autofocus method isproposed for implementing the autofocus in the process of the microassembly. Theauto-focus method divides the focus measure curve into two independent focus measurecurves with the peak and regards the intersection of the two independent focus measurecurves as the best in-focus position. The theoretical analyses have shown that theauto-focus method cannot only effectively avoid the principle error caused by assumingthe symmetrical focus measure curve in the auto-focus methods based on curve fitting,but also eliminate the possible waver search near the peak position in the modified fastclimbing search method. The focusing accuracy and the focusing speed of the autofocusmethod have been tested and compared with those of the7-point hill-climbing searchmethod and the quadratic curve fitting method.4. The microscopic visual servo control based holding force control method isproposed according to the established relationship between the holding force and theangle change tangent value of the gripping jaw of the microgripper. The corner pointextraction method based on the cosine value of the contour point can accurately extractthe corner point of the contour of the part and the demarcation point between thedifferent parts in the fusion image if the threshold of the cosine value of the contourpoint can be properly set. The vision-based microscopic visual servo control method isexplored and the PID (Proportion Integration Differentiation) controller of thevision-based microscopic visual servo control method is designed. In the builtautomated microassembly system, the vision-based microscopic visual servo control method is tested by the controlling the part and microgripper positions and the gapbetween the gripping jaws, and the microscopic visual servo control based holding forcecontrol method is also by controlling the holding force when gripping the part.5. For implementing the automatic alignment and stack task of the tiny metalcylindrical cavities, the assembly sequence of the microassembly task is planned. Onthe built automated microassembly system, the automatic alignment and stack task ofthe tiny metal cylindrical cavities is tested.The researches in this dissertation would be beneficial to developing and designingthe automated microassembly systems.