Juergen Hesselbach

Pseudoelastic flexure hinges in robots for microassembly

The increasing tendency of products towards miniaturization makes the substitution of conventional hinges to flexure hinges necessary, since they can be manufactured almost arbitrarily small. On account of their multiple advantages like no backlash, no slip-stick-effects and no friction, their application is especially reasonable in high-precision robots for micro assembly. Particular pseudo-elastic shape memory alloys offer themselves as material for flexure hinges. Since flexible joints gain their mobility exclusively via the elastic deformation of matter, the attainable angle of rotation is strongly limited when using conventional metallic materials with approximately 0.4% maximal elastic strain. Using pseudo- elastic materials, with up to 15% elastic strain, this serious disadvantage of flexure hinges can be avoided. A further problem of flexible joints is their kinetic behavior since they do not behave exactly like conventional rotational joints. In order to examine the kinematics of the hinges an experimental set-up was developed whereby good compliance with theoretical computed values could be achieved. A three (+1) degree of freedom parallel robot with integrated flexure hinges is investigated showing its kinematic deviations to its rigid body model. The data of the kinematic model of the flexible joint can then be implemented into the control of this complaint mechanism in order to gain not only a higher repeatability but also a good absolute accuracy over the entire working space.

Centering electrostatic microgripper and magazines for microassembly tasks

Accurate handling of microparts is one of the major tasks for an automated microproduction. The development of centering electrostatic handling devices is described. Based on a planar design common microtechnical fabrication methods were used. Therefore the gripper electrodes can easily be miniaturized and the geometric form can be adapted to the shape of the objects to be handled. The optimization of the design of the gripper was done by using the Finite Element Method. This gave the possibility to improve the centering effect and the gripping forces without increasing the operating voltage. To enable the observation of the gripped parts with a camera, a transparent substrate was used (Pyrex-wafer). This facilitates the integration of the gripper into a sensor controlled microassembly station. Futhermore first successful tests of functional models are described.

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.

Simulation-based disassembly systems design

Recycling of Waste of Electrical and Electronic Equipment (WEEE) is a matter of actual concern, driven by economic, ecological and legislative reasons. Here, disassembly as the first step of the treatment process plays a key role. To achieve sustainable progress in WEEE disassembly, the key is not to limit analysis and planning to merely disassembly processes in a narrow sense, but to consider entire disassembly plants including additional aspects such as internal logistics, storage, sorting etc. as well. In this regard, the paper presents ways of designing, dimensioning, structuring and modeling different disassembly systems. Goal is to achieve efficient and economic disassembly systems that allow recycling processes complying with legal requirements. Moreover, advantages of applying simulation software tools that are widespread and successfully utilized in conventional industry sectors are addressed. They support systematic disassembly planning by means of simulation experiments including consecutive efficiency evaluation. Consequently, anticipatory recycling planning considering various scenarios is enabled and decisions about which types of disassembly systems evidence appropriateness for specific circumstances such as product spectrum, throughput, disassembly depth etc. is supported. Furthermore, integration of simulation based disassembly planning in a holistic concept with configuration of interfaces and data utilization including cost aspects is described.

Compliant parallel robot with 6 DOF

In this paper a patented parallel structure will be presented in which conventional bearings are replaced by flexure hinges made of pseudo-elastic shape memory alloy. The robot has six degrees of freedom and was developed for micro assembly tasks. Laboratory tests made with the robot using conventional bearings have shown that the repeatability was only a couple of 1/100 mm instead of the theoretical resolution of the platform of < 1 micrometers . Especially the slip-stick effects of the bearings decreased the positional accuracy. Because flexure hinges gain their mobility only by a deformation of matter, no backlash, friction and slip-stick-effects exist in flexure hinges. For this reason the repeatability of robots can be increased by using flexure hinges. Joints with different degrees of freedom had to be replaced in the structure. This has been done by a combination of flexure hinges with one rotational degree of freedom. FEM simulations for different designs of the hinges have been made to calculate the possible maximal angular deflections. The assumed maximal deflection of 20 degree(s) of the hinges restricts the workspace of the robot to 28x28 mm with no additional rotation of the working platform. The deviations between the kinematic behavior of the compliant parallel mechanism and its rigid body model can be simulated with the FEM.

Advanced technologies for microassembly

This paper discusses new technologies suited for application sin micro assembly. The discourse a new class of robots, based on closed kinematic chains and their advantages for applications in high precision applications with a focus on micro assembly. The discussion continues on design considerations for parallel robots focused on design methods concerning the kinematic aspects as well as for the design of the robot components. The paper presents two concepts for parallel robots focused on design methods concerning the kinematic aspects as well as for the design of the robot components. The paper presents two concepts for parallel robots with three and six degrees of freedom, covering the transformation algorithms including the kinematic characteristics of these robots. Finally the paper introduces micro gripper principles and specific design suggestions.

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.

Silicon microgripper for microassembly realized by photolithography and fast anisotropic silicon etching

In this paper we present a new silicon microgripper for microassembly realized by photolithography and fast anisotropic silicon etching. Technological and manufacturing problems of the silicon microgripper will be described. The optimized etch process results in a high silicon etch rate of up to 6.2 micrometers /min, a good selectivity silicon/photoresist of up to 100:1, a high anisotropy, a nearly vertical etch profile, and a smooth surface topography. Excellent profile control for trench etching with a depth of about 250 micrometers and an anisotropy of better than 0.98 at a mean etch rate of 4 micrometers /min was obtained. Higher etch rates of up to 6.2 micrometers /min have been achieved resulting, however, in lower anisotropy. The developed microgripper is driven by a differential-type shape memory alloy (SMA) actuator. SMA actuators exhibit the best power- to-volume ratio of all actuators, do not release any particles, and can perform various movements like bending, elongation or twisting. Heating can easily be achieved by direct electrical current. Therefore SMA actuators are well suited for microgripper applications.