Gero Pokar

Visual control and calibration of parallel robots for microassembly

The process of micro assembly requires high precision and accuracy for the positioning of micro parts. Therefore a demand exists for very precise and accurate handling devices with a specific focus on positioning devices. This paper presents an approach using robots based on closed kinematic chains, so called parallel robots, to achieve high precision in automated micro assembly. The discussion continues on a calibration process for parallel robot structures to increase the accuracy of the robot system. However obtaining an accuracy in the range of submicrometer requires an additional sensor controlled positioning process. Hence the paper presents an approach using visual control. That approach includes the application of area based matching techniques as well as photogrammetric calibration of the camera system to increase the accuracy within the image processing.

Assembly of a miniature linear actuator using vision feedback

The assembly of a miniature linear actuator is described. The actuator consists of a base plate including the stator, a runner and guides on both sides of the runner. Stator, runner and guides are micro machined, using silicon as base material. The overall dimensions of the complete actuator are approximately 9 mm x 3 mm. A parallel robot with 4-DOF and a resolution of less than 1 (mu) m is used to assemble the actuator. The robots high accuracy is reached as a result of the parallel structure of the robot in combination with linear piezo drives. In addition, an integrated vision system allows the exact positioning of the robot relative to a previously teached position. The accuracy of the vision system is about 0.25(mu) m. Communication between the robot and the vision system takes place over a high-speed RS-422 serial link. As a first assembly step the guides, which are 8 mm long and 700 (mu) m wide, have to be mounted onto the base plate, right and left to the runners track. A special focus is laid on the exact maintenance of distance and parallelism of the guides, which is assured by the vision system. If the gap between the guides is too wide, the runner can tilt above the z- axis, which causes the actuator not to work. The opposite, a too small gap, causes the runner to be stuck between the guides. The guides are handled by a SMA-actuated miniature gripper. To keep the guides in place they are fixed by droplets of glue, whic are dispensed by a micro dispenser.

Open architecture robot control based on Matlab/Simulink and a dSPACE real time system

The elements of an open architecture robot control system developed using Matlab/Simulink and a real time system are described. It offers the opportunity to control almost every robotic system (serial or parallel) with up to six axes while commercial robot controls are often designed for serial kinematic systems and can hardly be adapted to control robots with parallel structures. The described open architecture robot control programmed in Matlab/Simulink and ANSI-C is a modular system. To adapt the control to a new robotic structure it is necessary to add the transformation algorithms, position control algorithms, inputs and outputs and machine specific error states to the pre-programmed modules of the system. These modules are programmed by using Simulink elements extended by special functions of the real time system and so called S-Functions that are programmed in C-Code. In the control new functionalities can be implemented easily by adding new modules and connecting them with the present system. A pre-designed graphical user interface provides most of the input buttons and display information needed for a robot control. Graphical buttons or displays can be added and connected with the required signal from Matlab/Simulink by drag and drop. An application example of a parallel robot shows the functionalities of the control.

Parallel robot for micro assembly with integrated innovative optical 3D-sensor

Recent advances in the fields of MEMS and MOEMS often require precise assembly of very small parts with an accuracy of a few microns. In order to meet this demand, a new approach using a robot based on parallel mechanisms in combination with a novel 3D-vision system has been chosen. The planar parallel robot structure with 2 DOF provides a high resolution in the XY-plane. It carries two additional serial axes for linear and rotational movement in/about z direction. In order to achieve high precision as well as good dynamic capabilities, the drive concept for the parallel (main) axes incorporates air bearings in combination with a linear electric servo motors. High accuracy position feedback is provided by optical encoders with a resolution of 0.1 μm. To allow for visualization and visual control of assembly processes, a camera module fits into the hollow tool head. It consists of a miniature CCD camera and a light source. In addition a modular gripper support is integrated into the tool head. To increase the accuracy a control loop based on an optoelectronic sensor will be implemented. As a result of an in-depth analysis of different approaches a photogrammetric system using one single camera and special beam-splitting optics was chosen. A pattern of elliptical marks is applied to the surfaces of workpiece and gripper. Using a model-based recognition algorithm the image processing software identifies the gripper and the workpiece and determines their relative position. A deviation vector is calculated and fed into the robot control to guide the gripper.

3D sensor for the control of a microassembly robot

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.