The main aim of this work is to allow the automated but nevertheless flexible sensor guided micro assembly with a new
3D vision sensor. The practical application of the sensor guided assembly of micro systems is realised with a special
micro assembly system, which contains a parallel robot, an innovative 3D vision sensor, micro grippers and workpiece
fixtures. The 3D vision sensor, which is arranged directly above the assembly place and moved synchronized to the
micro gripper, makes the uninterrupted observation of the workpiece and the determination of its position possible, in
this way allowing the sensor guided assembly of micro systems and a detailed analysis of the principles in the micro
assembly process. The basic principle of this photogrammetric measurement concept is given through the fiducial
marks. The circular fiducial marks will be realised within the photolithographic production of the components. Using
these fiducial marks it is possible to measure the relative position of the components during assembly without reference
to their exterior shape.
New developments in production technology increasingly focus on hybrid microsystems. Especially for systems with movable components, the process step of assembly is mandatory. In general, the accuracy of positioning of the parts has to be better than 1 μm. This makes specialized and automated production equipment necessary, which can lead to a conflict with the aim of flexibility of the range of products. Design for manufacturing is a well known remedy. Assembly aids are common practice today. These features of the workpieces bear no functionality for the end product but considerably ease certain process steps. By standardization of assembly aids generalized production equipment free from product-specific features could be developed.
In our contribution, we demonstrate the photogrammetric determination of the positions of workpieces without reference to their exterior shape, using circular fiducial marks of 150 μm in diameter. The surface properties of the workpieces, however, still have an influence on image formation. As an example, the marks may be hidden by local specular reflections. A solution to this problem is to add an exclusive optical property to the fiducial marks to get an image with high contrast against the surface of the workpiece. In biology and medicine samples are stained with fluorescing dyes to enhance the contrast in optical microscopy. In fluorochromes, light of a characteristic wavelength is emitted after the absorption of light with a shorter wavelength.
In our experiments we added a fluorochrome to a common photoresist and coated the surface of the workpiece with a thin layer thereof. Using photolithography as a patterning technique we generated fiducial marks with structures down to 25 μm. These marks can be identified by their characteristic emission wavelength under short-wavelength illumination. Only the fiducial marks remain visible in the images and processing these images is straightforward.
The generation of fluorescing patterns by photolithography opens new possibilities for testing and process control in many fields of microtechnology.
At the Technical University Braunschweig a robot for the assembly of micro structures with an assembly positioning uncertainty below 1 μm is under development. In order to reach the demanded accuracy and robustness, an optical sensor is used. First a 2D-Sensor was tested. To reach the required cycle time a substantial reduction of the response time of this 2D-sensor was necessary. By the application of optimised 2D-algorithms the image processing takes place in video real time now. The extensive software update required an examination of the repeatability results of the 2D-sensor (standard deviation of 0.1 μm in both axes). An overview of the reproducibility and the dynamic cooperation with the parallel robot are presented. A prototype of a 3D-sensor for the robot was built. The reached reproducibility is presented. The 3D-sensor integrated into the robot allows the execution of assembly tasks with a positioning uncertainty better than 1 μm. For the assembly tasks the accurate position of the structures must be determined, which is marked by a defined pattern of circular marks. By photogrammetric measurement of the marks and a pattern recognition the spatial position of the structure is determined. The measurement volume is 7,5 x 11 x 6 mm3. To show the efficiency of the 3D-sensor a first test of its cooperation with the robot is described.