Singularity Avoidance with Application to Online Trajectory Optimization for Serial Manipulators

This work proposes a novel singularity avoidance approach for real-time trajectory optimization based on known singular configurations. The focus of this work lies on analyzing kinematically singular configurations for three robots with different kinematic structures, i.e., the Comau Racer 7-1.4, the KUKA LBR iiwa R820, and the Franka Emika Panda, and exploiting these configurations in form of tailored potential functions for singularity avoidance. Monte Carlo simulations of the proposed method and the commonly used manipulability maximization approach are performed for comparison. The numerical results show that the average computing time can be reduced and shorter trajectories in both time and path length are obtained with the proposed approachComment: 8 pages, 2 figures, Accepted for publication at IFAC World Congress 202

Sampling-Based Trajectory (re)planning for Differentially Flat Systems: Application to a 3D Gantry Crane

In this paper, a sampling-based trajectory planning algorithm for a laboratory-scale 3D gantry crane in an environment with static obstacles and subject to bounds on the velocity and acceleration of the gantry crane system is presented. The focus is on developing a fast motion planning algorithm for differentially flat systems, where intermediate results can be stored and reused for further tasks, such as replanning. The proposed approach is based on the informed optimal rapidly exploring random tree algorithm (informed RRT*), which is utilized to build trajectory trees that are reused for replanning when the start and/or target states change. In contrast to state-of-the-art approaches, the proposed motion planning algorithm incorporates a linear quadratic minimum time (LQTM) local planner. Thus, dynamic properties such as time optimality and the smoothness of the trajectory are directly considered in the proposed algorithm. Moreover, by integrating the branch-and-bound method to perform the pruning process on the trajectory tree, the proposed algorithm can eliminate points in the tree that do not contribute to finding better solutions. This helps to curb memory consumption and reduce the computational complexity during motion (re)planning. Simulation results for a validated mathematical model of a 3D gantry crane show the feasibility of the proposed approach.Comment: Published at IFAC-PapersOnLine (13th IFAC Symposium on Robot Control

Real-time 6-DoF Pose Estimation by an Event-based Camera using Active LED Markers

Real-time applications for autonomous operations depend largely on fast and robust vision-based localization systems. Since image processing tasks require processing large amounts of data, the computational resources often limit the performance of other processes. To overcome this limitation, traditional marker-based localization systems are widely used since they are easy to integrate and achieve reliable accuracy. However, classical marker-based localization systems significantly depend on standard cameras with low frame rates, which often lack accuracy due to motion blur. In contrast, event-based cameras provide high temporal resolution and a high dynamic range, which can be utilized for fast localization tasks, even under challenging visual conditions. This paper proposes a simple but effective event-based pose estimation system using active LED markers (ALM) for fast and accurate pose estimation. The proposed algorithm is able to operate in real time with a latency below \SI{0.5}{\milli\second} while maintaining output rates of \SI{3}{\kilo \hertz}. Experimental results in static and dynamic scenarios are presented to demonstrate the performance of the proposed approach in terms of computational speed and absolute accuracy, using the OptiTrack system as the basis for measurement.Comment: 14 pages, 12 figures, this paper has been accepted to WACV 202

Oberflächenbasierte Pfadfolgeregelung auf dreidimensionalen Freiformoberflächen

Zusammenfassung in deutscher SpracheAbweichender Titel nach Übersetzung der Verfasserin/des VerfassersToday, process automation solutions strongly rely on programmable and multi-functional robotic manipulators to perform repetitive and for the human operator partly tedious processing tasks for large-scale series productions. Many of these processes require the movement of a robot-mounted tool along a given geometric path defined in the Cartesian space. From a control engineering perspective, this task is called path following control and emerges in processes such as milling and cutting.The work at hand is concerned with the development of a novel surface-based path following control concept for the processing of 3D objects. In contrast to classical path following control, the presented control concept systematically takes into account the surface normal vector of an underlying freeform 3D surface and the path tangent vector of the surface-based path. For the interaction with the target 3D object, two distinct coordinate frames are introduced, i.e. the natural and the parallel contact frame. Using a coordinate transformation and feedback linearization, the nonlinear robotic system dynamics is transformed into a system with linear input/output behavior in new path-based coordinates. The underlying surface is systematically incorporated into this concept, which allows to formulate impedance control, admittance control, and force control in the new coordinates. Moreover, industrial processes on freeform 3D surfaces are described in terms of kinematic constraints, i.e. forbidden tool motions, and kinematic redundancies, i.e. free self-motions of the tool.The control concept is validated experimentally by a novel approach for the versatile application of (curved) pre-cut adhesive tapes on freeform 3D surfaces. The experimental design of the demonstrator results from a thorough concept study. In this process, an impedance-controlled draping roll traverses the target 3D object without turning around the surface normal vector to prevent wrinkles. Experimental results with a KUKA LBR iiwa 14 R820 demonstrate the high quality of the proposed approach.Surface-based path following control is a general and modular framework to describe and perform various industrial processes on freeform 3D surfaces with diverse kinematic and dynamic requirements, e.g., polishing, laser cutting, sewing, and spray painting.Heutige Prozessautomatisierungslösungen beruhen in hohem Maße auf programmierbaren und multifunktionalen Robotern, die die repetitiven und ermüdenden Verarbeitungsschritte in der Massenproduktion für den Menschen zum Teil übernehmen. Viele dieser Prozesse erfordern im Allgemeinen, dass der Roboter ein Werkzeug entlang eines geometrischen Pfades führt, welcher im kartesischen Raum definiert ist. Diese Aufgabe wird in der Regelungstechnik als Pfadfolgeregelung bezeichnet und tritt bei Prozessen wie z.B. Fräsen und Schneiden auf. Die vorliegende Arbeit widmet sich der Entwicklung eines neuen Konzeptes für die oberflächenbezogene Pfadfolgeregelung auf dreidimensionalen Werkstücken. Im Gegensatz zur klassischen Pfadfolgeregelung berücksichtigt das vorgestellte Regelungskonzept systematisch den Oberflächennormalenvektor der zugrundeliegenden dreidimensionalen Freiformoberfläche sowie den Pfadtangentenvektor des oberflächenbezogenen Pfades. Für die Interaktion mit der Oberfläche werden zwei unterschiedliche Koordinatensysteme für den Kontaktpunkt eingeführt, nämlich das natürliche und das parallele Koordinatensystem. Unter Verwendung einer Koordinatentransformation und einer Zustandsrückführung wird die Dynamik des nichtlinearen Robotersystems bezüglich eines nichtlinearen Pfades in ein System mit linearem Eingangs-/Ausgangsverhalten in den neuen pfadbezogenen Koordinaten transformiert. Diese berücksichtigen die zugrundeliegende dreidimensionale Freiformoberfläche, womit eine Impedanzregelung, Admittanzregelung und Kraftregelung in den neuen Koordinaten formuliert werden kann. Darüber hinaus werden industrielle Prozesse auf dreidimensionalen Freiformoberflächen in Bezug auf kinematische Einschränkungen, d.h. verbotene Werkzeugbewegungen, und kinematische Redundanzen, das sind freie Selbstbewegungen des Werkzeugs, beschrieben. Das Regelungskonzept wird experimentell anhand eines neuen Ansatzes für das flexible Aufkleben von zugeschnittenen (kurvigen) Klebebändern auf dreidimensionale Freiformoberflächen validiert. Der experimentelle Aufbau für einen Demonstrator geht aus einer ausführlichen Konzeptstudie hervor. Beim Aufklebeprozess wird eine impedanzgeregelte Drapierrolle über das dreidimensionale Objekt geführt, ohne diese um den Oberflächennormalenvektor zu drehen. Dies verhindert die Faltenbildung im aufgeklebten Streifen. Die experimentellen Ergebnisse mit einem KUKA LBR iiwa 14 R820 zeigen die hohe Qualität des vorgestellten Ansatzes. Die in dieser Arbeit vorgestellte oberflächenbezogene Pfadfolgeregelung ist ein allgemeines und modulares Konzept, mit dem verschiedene industrielle Prozesse auf dreidimensionalen Freiformoberflächen mit unterschiedlichen kinematischen und dynamischen Eigenschaften beschrieben und ausgeführt werden können, wie z.B. Polieren, Schweißen, Laserschneiden, Nähen und Spritzlackieren.10