J. Hesselbach

Performance of Pseudo-Elastic Flexure Hinges in Parallel Robots for Micro-Assembly Tasks

This paper presents the application of pseudo-elastic flexure hinges in parallel robots for micro-assembly tasks. The material used for the flexure hinges is a shape memory alloy permitting angular deflections of ± 30°. Based on this flexibility sufficiently large workspaces of the robots can be achieved. Simulations show that kinematic deviations caused by flexure hinges are reducing the absolute positioning accuracy of compliant mechanisms. Likewise the simulations indicate that the kinematic behaviour of compliant mechanisms differs only slightly from their counterparts with conventional joints, which are analytically described by means of a rigid-body model. Experimental measurements with two compliant robots show the possibility to increase repeatability and resolution by using flexure hinges.

Self Management in a Control Architecture for Parallel Kinematic Robots

Maintainability, extendibility and reusability of components in the design of robot control architectures is a major challenge. Parallel kinematic robots feature a wide variety of structures and applications. They are subject to easy reconfiguration because of the passive structure limbs. This class of robots requires more extensive calculations in their control laws than serial manipulators. During complex motion tasks, such as the ones required in assembly sequences, the algorithmic load may also vary over time. However, no generic control approach exists in order to reduce the complexity of control design for these kind of robots. In this paper the authors introduce an architecture for handling and assembly applications featuring self-management techniques as an approach to tackle these problems. The existing architecture features a modular and layered design. Concepts of self-management and self-optimization applied to this architecture are outlined. These properties are realized by the integration of self-managers within crucial system components. The mechanisms are extended for a future distributed version of the architecture. Real-time properties are guaranteed by an online formal analysis that verifies planned adaptations before realizing them.Copyright

Singularity prediction for parallel robots for improvement of sensor-integrated assembly

The frequent occurrences of singularities inside a parallel robots workspace and in certain cases of sensor integration non-deterministic end-effector trajectories demand a powerful online singularity prediction and a control design that is capable of displacing motion algorithms. The theory of a power-inspired index of closeness to a singularity pose for parallel kinematic manipulators is illumined and presented in this paper, supported by comparison to Grassmann geometry and experimental results achieved on a HEXA robot. An experimentally validated strategy for jerk bounded braking the end-effector while approaching a singularity and an algorithm for departing it again are presented as well.

Size-Adapted Manipulation Robots for Microassembly

In order to assembly the microactuators which have been presented, unique microassembly techniques, robots, and peripheral sensors are needed. It has also been observed that an imbalance between the everdecreasing product sizes to the size of currently implemented manipulators exists. These things as well as the large initial investment costs of such manipulators, have been the inspiration for new microassembly technologies. Here special emphasis is placed on two examples, one showing a size-adapted robot and the other extending this concept to a miniaturized design. Different considerations to obtain a repeatability on the order of 1 µm or better are presented. In the presentation of the size-adapted design, a special emphasis is placed on the integration of a three-dimensional vision sensor and resulting sensor-guided assembly control concept. The second robot discussed is a highly miniaturized robot designed for desktop factories. Here the high level of performance is obtained using miniaturized, backlash-free microgears. Further insight into the highly accurate microgears and their dynamic effects on the robot are covered.

Parallel Robot Calibration by Working Mode Change

The positioning accuracy of robotic manipulators can be enhanced by identification and correction of the geometry parameters of the controller model in a way that it best matches the real physical robot. This procedure, denoted as kinematic calibration, is performed by analyzing the difference between conflicting information gained by the kinematic model and corresponding redundant measurement information. Most traditional robot calibration approaches require extra sensors or special constraint fixtures in order to obtain redundancy. This paper proposes a new calibration method that does not require any special calibration equipment, thus being very economical. The presented technique which is designed to be applied to parallel robots is based on a working mode change and incorporates special knowledge about serial singularities. Exemplarily the approach is verified by means of simulation studies on a 3-RRR-structure.

Assembly of hybrid microsystems using an assembly system with 3D optical sensor

For the assembly of hybrid microsystems a high accuracy in the range of a few micrometers is required. The combination of a parallel robot with an integrated 3D vision sensor uses positioning marks on the objects for object recognition. Within the assembly process, relative positioning accuracies in the submicrometer range have been obtained. Therefore, it is necessary to use a calibration strategy for matching the coordinate system of the 3D vision sensor with the coordinate system of the robot. The construction of the system, the calibration applied and the positioning accuracy achieved by the robot will be discussed.

The Increase of the Dynamic and Static Stiffness of a Grinding Machine

The dynamic stiffness of a grinding machine influences the process stability enormously. Among other things the stability of the grinding process is affected by influences like the specification of the grinding wheel, the condition of the workpiece and machine parameters. Unfavorable combinations of these lead to chatter vibrations of the machine and chatter marks on the workpiece. This paper presents the results of experimental and theoretical investigations of the vibration behavior of a grinding machine and the design of active modules. These modules will be implemented in the structure of the machine to minimize the vibrations and additionally increase its static stiffness of the machine.Copyright

A-priori Fisher information of nonlinear state space models for experiment design

This article presents advances in optimal experiment design, which are intended to improve the parameter identification of nonlinear state space models. Instead of using a sequence of samples from one or just a few coherent sequences, the idea of identifying nonlinear dynamic models at distinct points in the state space is considered. In this way, the placement of the experiment points is fully flexible with respect to the set of reachable points. Also, a method for model-based generation of prediction errors is proposed, which is used to compute an a-priori estimate of the sample covariance of the prediction error. This covariance matrix may be used to approximate the Fisher information matrix a-priori. The availability of the Fisher matrix a-priori is a prerequisite for experiment optimization with respect to covariance in the parameter estimates. This work is driven by the problem of parameter identification of hydraulic models. There are methods for hydraulic systems regarding the estimation of parameters from experimental data, but the choice of experiments has not been treated adequately yet. A hydraulic servo system actuating a stewart platform serves as an illustrative example to which the methods above are applied.

The Development of a Reconfigurable Parallel Robot with Binary Actuators

Performing activities autonomously and independent of human interaction during the production and handling process of goods is often achieved by robots. Since many of these handling tasks require a manipulator that reaches only a few points, a binary robot, such as the structure presented in this contribution, is suitable. These parallel robots with binary actuation can be designed at low cost and controlled by a simple programmable logic controller (PLC). Conformance of a predetermined positioning task can be achieved through the development of a synthesis approach. Furthermore, a calibration strategy based on additional components is introduced. These modules allow an adjustment of geometrical parameters to ensure positioning tasks with high accuracy.

Parallel Robot Calibration Utilizing Adaptronic Joints

Model based geometric calibration is well known to be an efficient way to enhance absolute accuracy of robotic systems. Generally its application requires redundant measurements, which are achieved by external metrology equipment in most traditional calibration techniques. However, these methods are usually time-consuming, expensive and inconvenient. Thus, so-called self-calibration methods have achieved attention from researchers, which either use internal sensors or rely on mechanical constraints instead. In this paper a new self-calibration technique is presented for parallel robots which is motivated by the idea of constrained calibration. The new approach utilizes a special machine component called the adaptronic swivel joint in order to achieve the required redundant information. Compared to similar approaches it offers several advantages. The new calibration scheme is described and verified in simulation studies using a R RRRR -structure as an example.Copyright