Kinematics analysis of 6-DOF parallel micro-manipulators with offset u-joints : a case study

This paper analyses the kinematics of a special 6-DOF parallel micro-manipulator with offset RR-joint configuration. Kinematics equations are derived and numerical methodologies to solve the inverse and forward kinematics are presented. The inverse and forward kinematics of such robots compared with those of 6-UCU parallel robots are more complicated due to the existence of offsets between joints of RR-pairs. The characteristics of RR-pairs used in this manipulator are investigated and kinematics constraints of these offset U-joints are mathematically explained in order to find the best initial guesses for the numerical solution. Both inverse and forward kinematics of the case study 6-DOF parallel micro-manipulator are modelled and computational analyses are performed to numerically verify accuracy and effectiveness of the proposed methodologies

High performance control of a multiple-DOF motion platform for driver seat vibration test in laboratory

Dynamic testing plays an important part in the vehicle seat suspension study. However, a large amount of research work on vibration control of vehicle seat suspension to date has been limited to simulations because the use of a full-size vehicle to test the device is an expensive and dangerous task. In order to decrease the product development time and cost as well as to improve the design quality, in this research, a vibration generation platform is developed for simulating the road induced vehicle vibration in laboratory. Different from existing driving simulation platforms, this research focuses on the vehicle chassis vibration simulation and the control of motion platform to make sure the platform can more accurately generate the actual vehicle vibration movement. A seven degree-of-freedom (DOF) full-vehicle model with varying road inputs is used to simulate the real vehicle vibration. Moreover, because the output vibration data of the vehicle model is all about the absolute heave, pitch and roll velocities of the sprung mass, in order to simulate the vibration in all dimensions, a Stewart multiple-DOF motion platform is designed to generate the required vibration. As a result, the whole vibration simulator becomes a hardware-in-the-loop (HIL) system. The hardware consists of a computer used to calculate the required vibration signals, a Stewart platform used to generate the real movement, and a controller used to control the movement of the platform and implemented by a National Instruments (NI) CompactRIO board. The data, which is from the vehicle model, can be converted into the length of the six legs of the Stewart platform. Therefore, the platform can transfer into the same posture as the real vehicle chassis at that moment. The success of the developed platform is demonstrated by HIL experiments of actuators. As there are six actuators installed in the motion platform, the signals from six encoders are used as the feedback signals for the control of the length of the actuators, and advanced control strategies are developed to control the movement of the platform to make sure the platform can accurately generate the required motion even in heavy load situations. Theoretical study is conducted on how to generate the reasonable vibration signals suitable for vehicle seat vibration tests in different situations and how to develop advanced control strategies for accurate control of the motion platform. Both simulation and experimental studies are conducted to validate the proposed approaches

Appropriate Design of Parallel Manipulators

International audienceAlthough parallel structures have found a niche market in many applications such as machine tools, telescope positioning or food packaging, they are not as successful as expected. The main reason of this relative lack of success is that the study and hardware of parallel structures have clearly not reached the same level of completeness than the one of serial structures. Among the main issues that have to be addressed, the design problem is crucial. Indeed, the performances that can be expected from a parallel robot are heavily dependent upon the choice of the mechanical structure and even more from its dimensioning. In this chapter, we show that classical design methodologies are not appropriate for such closed-loop mechanism and examine what alternatives are possible

Sliding-Mode control for high-precision motion control systems

In many of today's mechanical systems, high precision motion has become a necessity. As performance requirements become more stringent, classical industrial controllers such as PID can no longer provide satisfactory results. Although many control approaches have been proposed in the literature, control problems related to plant parameter uncertainties, disturbances and high-order dynamics remain as big challenges for control engineers. Theory of Sliding Mode Control provides a systematic approach to controller design while allowing stability in the presence of parametric uncertainties and external disturbances. In this thesis a brief study of the concepts behind Sliding Mode Control will be shown. Description of Sliding Mode Control in discrete-time systems and the continuous Sliding Mode Control will be shown. The description will be supported with the design and robustness analysis of Sliding Mode Control for discrete-time systems. In this thesis a simplified methodology based on discrete-time Sliding Mode Control will be presented. The main issues that this thesis aims to solve are friction and internal nonlinearities. The thesis can be outlined as follows: -Implementation of discrete-time Sliding Mode Control to systems with nonlinearities and friction. Systems include; piezoelectric actuators that are known to suffer from nonlinear hysteresis behavior and ball-screw drives that suffer from high friction. Finally, the controller will be implemented on a 6-dof Stewart platform which is a system of higher complexity. -It will also be shown that performance can be enhanced with the aid of disturbance compensation based on a nominal plant disturbance observer

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

Dynamic modelling of hexarot parallel mechanisms for design and development

In this research, the kinematics, dynamics, and general closed-form dynamic formulation of the centrifugal high-G hexarot-based manipulators have been developed through the different mathematical modeling techniques. The vibrations of the mechanism have also been investigated

Function based control for motion control systems

Motion control systems are gaining importance as more and more sophisticated developments arise in technology. Technological improvements enhance incorporation of different research areas into the same framework while trying to make systems function in unstructured environments renders the design of control systems increasingly complex. Since motion systems are complex, they have complex forward or inverse kinematics, or interactions with other systems. In this study, motion of the systems is decomposed into the tasks, so called “functions”. Independent controllers are designed for these functions in the function space. It is proven that motion systems will be controlled in the original space if function based control outputs are superposed. Applicability of this method is demonstrated on bilateral systems and parallel mechanisms. Bilateral systems application proved that function based control can be used in controlling systems with interactions while establishing desired functional relation between them. Moreover, investigation of a pantograph and a three-legged manipulator, which come from the parallel mechanisms family and have nonlinear and coupled system dynamics, showed that creating an appropriate reference configuration to realize the task of motion control helps decouple system dynamics. Satisfactory simulation results show that functional control can be implemented and its characteristics promise successful future designs for motion control systems

Contribution à l’amélioration de la précision absolue des robots parallèles

Le but de la présente étude est de contribuer à l’amélioration de la précision absolue des robots parallèles, en ayant recours aux méthodes d’étalonnage géométrique. Ces méthodes consistent à identifier les valeurs des paramètres géométriques du robot, en vue d’améliorer la correspondance entre le robot réel et le modèle mathématique utilisé par son contrôleur. En plus de la compensation des erreurs géométriques, les opérations d’étalonnage proposées permettent d’identifier précisément le référentiel de base de chaque robot étudié. Les méthodes développées sont appliquées à deux robots parallèles à moins de six degrés de liberté (ddl) : une table de positionnement précis à trois ddl (PreXYT) et un robot plan cinqbarres (DexTAR) à deux ddl. Pour le premier robot, l’étalonnage est effectué en utilisant d’abord une méthode d’identification directe. Le deuxième travail destinée à améliorer la précision absolue du PreXYT résulte de la méthode géométrique directe d’étalonnage. En ce qui concerne le robot DexTAR, sa précision est améliorée en utilisant une approche d’autoétalonnage qui exploite les modes de fonctionnement et les modes d’assemblage, pour réduire le nombre de positions d’étalonnage. Cette approche est particulièrement intéressante pour sa simplicité : à chaque position d’étalonnage une sphère de précision est installée en permanence pour servir de cible de mesure. Les positions de ces billes, placées sur une plateforme amovible, n’est mesurée qu’une seule fois, en utilisant une machine de mesure tridimensionnel (MMT). Après la réinstallation de la plateforme sur la base du robot, l’étalonnage peut se faire n’importe quand en n’utilisant que les informations provenant des encodeurs des actionneurs. Les données d’étalonnage et de validation des résultats sont récoltées en utilisant deux appareils mesurant par palpage. Le premier appareil est un bras articulé de mesure de coordonnées, de la compagnie FARO Technologies ; le second est une MMT de la compagnie Mitutoyo. Les incertitudes de mesures de ces machines sont respectivement ±18 μm et ±2,7 μm (niveau de confiance de 95%). Sachant que la qualité de l’étalonnage est inversement proportionnelle aux incertitudes de mesures, l’utilisation d’instruments précis avec des modèles géométriques d’étalonnage quasi-complet nous a permis d’atteindre ces résultats : les erreurs maximales en position et en orientation ont été réduites respectivement à 0,044 mm et 0,009° pour le PreXYT, à l’intérieur d’un cercle de 170 mm de diamètre. Pour le robot DexTAR, l’erreur maximale de position a été réduite à 0,080 mm dans l’ensemble de son espace de travail, soit une zone d’environ 600 mm × 600 mm. Améliorer la précision des robots au-delà de ces valeurs, en utilisant juste les approches géométriques, pourrait s’avérer peu probable. En ce sens, l’ajout de la modélisation et la compensation des erreurs non géométriques serait utile pour obtenir des résultats meilleurs