Parallel Manipulators

In recent years, parallel kinematics mechanisms have attracted a lot of attention from the academic and industrial communities due to potential applications not only as robot manipulators but also as machine tools. Generally, the criteria used to compare the performance of traditional serial robots and parallel robots are the workspace, the ratio between the payload and the robot mass, accuracy, and dynamic behaviour. In addition to the reduced coupling effect between joints, parallel robots bring the benefits of much higher payload-robot mass ratios, superior accuracy and greater stiffness; qualities which lead to better dynamic performance. The main drawback with parallel robots is the relatively small workspace. A great deal of research on parallel robots has been carried out worldwide, and a large number of parallel mechanism systems have been built for various applications, such as remote handling, machine tools, medical robots, simulators, micro-robots, and humanoid robots. This book opens a window to exceptional research and development work on parallel mechanisms contributed by authors from around the world. Through this window the reader can get a good view of current parallel robot research and applications

A redundantly actuated 2-DOF 3RRR PKM with flexure joints: less is more

The development of a planar manipulator with flexure joints and redundant actuation has been considered before, showing that the redundancy can be exploited to increase the support stiffness and to reduce static actuator loads. In this previous design the manipulator’s workspace has been defined to encompass all kinematically accessible end effector positions. In the current paper we reconsider the design philosophy. It will be shown that limiting the workspace ("less") ultimately results in a better performance in a larger area ("more").The dynamic performance of the manipulator is evaluated with a flexible multibody model. The links can safely be considered as rigid parts, but the model has to account for the nonlinear stiffness behaviour of the flexure joints undergoing relatively large deflections. The nonlinear spatial flexible beam elements implemented in the spacar software result in numerically efficient models that have proven to be well-suited for design optimisation. With such a flexible multibody model, the geometry of the manipulator is optimised to maximise the workspace area while assuring a minimal parasitic natural frequency and limiting the local stresses throughout the full workspace. Furthermore, the simulations show that preloading of the flexure joints results in reduced actuator torques that are needed to counteract the finite joint compliance for stationary positioning anywhere except for the equilibrium position of the end effector.The optimised design has been build and validated experimentally. A control system that handles the actuator redundancy by minimising the 2-norm of the driving torques has been synthesised. It is demonstrated that the setup’s behaviour is similar to the model and that in particular the preloading significantly lowers the required actuator torques

A compliant and redundantly actuated 3-DOF 4RRR PKM: First step to full planar motion

The development and optimisation of compliant (or flexure-based) manipulators with redundant actuation have been considered before, showing that the redundancy can be exploited to increase the support stiffness and reduce actuator loads. However, so far only 2-DOF manipulators have been considered which enable translational motion in two directions. In this paper a third degree of freedom, the in-plane rotation of the end effector, is added. The goal is to design and evaluate a first prototype capable of full planar motion. The dynamic performance of the manipulator is analysed with a flexible multibody model. The links are assumed to be rigid. The SPACAR software is used as its flexible beam element can describe the non-linear behaviour of the flexure joints well at rather large deflections and accounts for constraint warping. In this prototype the end effector range of motion is limited such that the joint rotations do not exceed ±30 deg. Butterfly and cartwheel flexure joints can handle the specifications without violating stress constraints. In the final design the lowest natural frequencies are 3.6 Hz for both translations and 7.9 Hz for the rotation. The first parasitic frequency is expected at 76 Hz, which is sufficiently high. This prototype has been manufactured with 3D printing. The lowest translational frequencies appear to be somewhat higher than expected, which could arise from stiffness added by the flexible coupling between actuator and upper arm. A higher resonance frequency is found near 80 Hz, which agrees well with the expected first relevant parasitic mode

Static force capabilities and dynamic capabilities of parallel mechanisms equipped with safety clutches

Cette thèse étudie les forces potentielles des mécanismes parallèles plans à deux degrés de liberté équipés d'embrayages de sécurité (limiteur de couple). Les forces potentielles sont étudiées sur la base des matrices jacobienne. La force maximale qui peut être appliquée à l'effecteur en fonction des limiteurs de couple ainsi que la force maximale isotrope sont déterminées. Le rapport entre ces deux forces est appelé l'efficacité de la force et peut être considéré ; comme un indice de performance. Enfin, les résultats numériques proposés donnent un aperçu sur la conception de robots coopératifs reposant sur des architectures parallèles. En isolant chaque lien, les modèles dynamiques approximatifs sont obtenus à partir de l'approche Newton-Euler et des équations de Lagrange pour du tripteron et du quadrupteron. La plage de l'accélération de l'effecteur et de la force externe autorisée peut être trouvée pour une plage donnée de forces d'actionnement.This thesis investigates the force capabilities of two-degree-of-freedom planar parallel mechanisms that are equipped with safety clutches (torque limiters). The force capabilities are studied based on the Jacobian matrices. The maximum force that can be applied at the end-effector for given torque limits (safety index) is determined together with the maximum isotropic force that can be produced. The ratio between these two forces, referred to as the force effectiveness, can be considered as a performance index. Finally, some numerical results are proposed which can provide insight into the design of cooperation robots based on parallel architectures. Considering each link and slider system as a single body, approximate dynamic models are derived based on the Newton-Euler approach and Lagrange equations for the tripteron and the quadrupteron. The acceleration range or the external force range of the end-effector are determined and given as a safety consideration with the dynamic models