Ingo Pietsch

How to reach the dynamic limits of parallel robots? An autonomous control approach

Based on closed kinematic chains, parallel robots obtain favorable dynamic properties as well as high stiffness. Hence, their application can significantly enlarge the productivity of automated production processes. A control concept for tapping the high potential concerning low cycle times and high path-tracking accuracy is presented. The proposed approach adapts autonomously to changing dynamic parameters as varying payload. The autonomous behavior is achieved by combining an adaptive control approach with an adaptive, time-optimal trajectory planning concept and an online-trajectory adaption mechanism. Extensive experimental results prove the performance of the proposed approach. Note to Practitioners -Many applications in the field of production automation (material handling, assembly, etc.) require high operating speeds and accelerations. During the past years, parallel robots proved to be an efficient and suitable supplement to serial robots. Unfortunately, the promising possibilities of parallel robots often cannot yield profit because their dynamic potential is still not fully exploited. The payload/robot mass ratio of parallel structures is even higher compared to serial robots, where the influence of the payload on the impedance of the robot is negligible. By use of direct drives the influence of a variable payload cannot be ignored. A modified adaptive control concept, which adapts autonomously to changing dynamic parameters-as varying payload due to diversity of assembly processes-guarantees high tracking accuracy and therefore better process quality as well as accurate estimates of changing dynamic parameters and therefore better process quality. In addition the productivity of the process can be enlarged, if the full drive power can be used at each point on the path. Thus, a new adaptive time-optimal trajectory planning algorithm is used to exploit the dynamic potential of the direct drives and consequently to shorten the cycle times. The aim of time-optimal trajectory planning, as it is commonly understood, is the determination of the maximum velocity profile along a given path that complies with all given dynamic and kinematic robot constraints like limited drive forces/torques, limited path and/or drive velocities and limited path jerk. Combining the adaptive control scheme and the adaptive, time-optimal trajectory planning algorithm with an online trajectory adaption mechanism, a control concept is realized, which autonomously adapts to changing dynamic robot behavior. Using this new approach, the advantages of parallel robots-as well as serial robots with direct drives-can better be utilized. This is a necessary prerequisite for a larger extension of PKMs for industrial applications.

Robust task-space control of hydraulic robots

This paper presents a model-based robust controller for hydraulically driven robot manipulators. The approach guarantees that the output error of the plant remains within a prescribed bound despite of model uncertainties. Manipulator dynamics and actuator dynamics including the dynamics of the valves are taken into account. The stability proof is based on the Lyapunov method and passivity arguments. It is shown that the stability proof is not restricted to the proposed control law of the hydraulic actuators but also holds for arbitrary hydraulic force control laws as long as certain requirements are fulfilled.

Detection and Avoidance of Singularities in Parallel Kinematic Machines

In this work a geometrical and a physically based method for the detection of singularities are presented, which provide information about the distance of a given position to singularities. The integration of this singularity detection into a robot controller is presented and validated by experimental results. Based on these results a path planning algorithm that avoids singularities is developed. It uses a particle based randomizing scheme for finding a path within the workspace to which a virtual potential field is applied. The HexaII demonstrator of the Collaborative Research Center 562 serves as a validation platform for this algorithm. The singularity avoidance path planner is integrated into the real-time context of the control application RCA562, from which experimental results are presented.

Passive joint-sensor applications for parallel robots

Beside a quantity of well-known positive characteristics parallel robots also obtain a couple of negative properties limiting their efficient use in certain application areas nowadays. Additional sensors integrated in the passive joints provide the possibility to overcome several of these limitations simultaneously thus leading towards an increasing overall system performance. In detail, different control functions, necessary for fast, precise and safe robot operation like calibration, workspace monitoring and position control can be improved. To tap the potential of the concept of passive joint-sensors, algorithms to evaluate the additional sensor information for certain control functions and an approach to determine the necessary sensor resolution for a class of parallel robots are developed. Different sensor configurations are investigated and compared due to computational efficiency and resolution demands. Based on these requirements, we report on the development of new inductive micro sensors for angle measurement capable for integration, e.g. into rotary joints of high-performance parallel robots. Basic design and process technologies of the single sensor components as well as first measuring results are described.

Hochgeschwindigkeitshandhabung und Montage

Kurzfassung Die Entwicklung leistungsstarker Roboter trägt dazu bei, ökonomische Fortschritte in Produktionssystemen zu erzielen. Neue Maschinenkonzepte, basierend auf hybriden kinematischen Strukturen, ermöglichen es, hohe Genauigkeit mit guten dynamischen Fähigkeiten zu kombinieren. Der Entwicklungsprozess eines Roborters für Handhabungs- und Montageaufgaben mit hohen Anforderungen an Genauigkeit und Geschwindigkeit wird von der Aufgabendefinition bis zum Aufbau eines Prototypen beschrieben. Die strukturellen Eigenschaften, die konstruktive Umsetzung und angepasste Steuerungsalgorithmen führen zu einem Roboter mit vorteilhaften Eigenschaften zu angemessenen Kosten.