Image-based Visual Servoing of a Gough-Stewart Parallel Manipulator using Leg Observations

International audienceIn this paper, a tight coupling between computer vision and paral- lel robotics is exhibited through the projective line geometry. Indeed, contrary to the usual methodology where the robot is modeled indepen- dently from the control law which will be implemented, we take into ac- count, since the early modeling stage, that vision will be used for con- trol. Hence, kinematic modeling and projective geometry are fused into a control-devoted projective kinematic model. Thus, a novel vision-based kinematic modeling of a Gough-Stewart manipulator is proposed through the image projection of its cylindrical legs. Using this model, a visual ser- voing scheme is presented, where the image projection of the non-rigidly linked legs are servoed, rather than the end-effector pose

Minimal Representation for the Control of Gough-Stewart Platforms via Leg Observation Considering a Hidden Robot Model

International audienceThis paper presents new insights about the sensor-based control of Gough-Stewart (GS) platforms. Previous works have shown that it was possible to control the GS platform by observing its legs directions instead of using the encoders values or the measurement of the platform pose. It was demonstrated that observing only three legs directions was enough for the control but no physical explanations were given. Moreover, sometimes, the GS platform was not converging to the desired pose and the reasons of these divergences were not disclosed. This paper aims at answering to this two opened problems. It is shown that observing three leg directions involves controlling the displacement of a hidden robot whose models differs from those of the usual GS platform. This robot has assembly modes and singular configurations different from those of the GS platform. This involves that the legs to observe should be chosen carefully in order to avoid inaccuracy problems. In this sense, the accuracy analysis of the new robot is performed to show the importance of the leg selection. All these results are validated on a GS platform simulator created using ADAMS/Controls and interfaced with Matlab/Simulink

Visual Calibration, Identification and Control of 6-RSS Parallel Robots

Parallel robots present some outstanding advantages in high force-to-weight ratio, better stiffness and theoretical higher accuracy compared with serial manipulators. Hence parallel robots have been utilized increasingly in various applications. However, due to the manufacturing tolerances and defections in the robot structure, the positioning accuracy of parallel robots is basically equivalent with that of serial manipulators according to previous researches on the accuracy analysis of the Stewart Platform [1], which is difficult to meet the precision requirement of many potential applications. In addition, the existence of closed-chain mechanism yields difficulties in designing control system for practical applications, due to its highly coupled dynamics. Visual sensor is a good choice for providing non-contact measurement of the end-effector pose (position and orientation) with simplicity in operation and low cost compared to other measurement methods such as the coordinate measurement machine (CMM) [2] and the laser tracker [3]. In this research, a series of solutions including kinematic calibration, dynamic identification and visual servoing are proposed to improve the positioning and tracking performance of the parallel robot based on the visual sensor. The main contributions of this research include three parts. In the first part, a relative pose-based algorithm (RPBA) is proposed to solve the kinematic calibration problem of a six-revolute-spherical-spherical (6-RSS) parallel robot by using the optical CMM sensor. Based on the relative poses between the candidate and the initial configurations, a calibration algorithm is proposed to determine the optimal error parameters of the robot kinematic model and external parameters introduced by the optical sensor. The experimental results demonstrate that the proposal RPBA using optical CMM is an implementable and effective method for the parallel robot calibration. The second part focuses on the dynamic model identification of the 6-RSS parallel robots. A visual closed-loop output-error identification method based on an optical CMM sensor is proposed for the purpose of the advanced model-based visual servoing control design of parallel robots. By using an outer loop visual servoing controller to stabilize both the parallel robot and the simulated model, the visual closed-loop output-error identification method is developed and the model parameters are identified by using a nonlinear optimization technique. The effectiveness of the proposed identification algorithm is validated by experimental tests. In the last part, a dynamic sliding mode control (DSMC) scheme combined with the visual servoing method is proposed to improve the tracking performance of the 6-RSS parallel robot based on the optical CMM sensor. By employing a position-to-torque converter, the torque command generated by DSMC can be applied to the position controlled industrial robot. The stability of the proposed DSMC has been proved by using Lyapunov theorem. The real-time experiment tests on a 6-RSS parallel robot demonstrate that the developed DSMC scheme is robust to the modeling errors and uncertainties. Compared with the classical kinematic level controllers, the proposed DSMC exhibits the superiority in terms of tracking performance and robustness

Advanced Strategies for Robot Manipulators

Amongst the robotic systems, robot manipulators have proven themselves to be of increasing importance and are widely adopted to substitute for human in repetitive and/or hazardous tasks. Modern manipulators are designed complicatedly and need to do more precise, crucial and critical tasks. So, the simple traditional control methods cannot be efficient, and advanced control strategies with considering special constraints are needed to establish. In spite of the fact that groundbreaking researches have been carried out in this realm until now, there are still many novel aspects which have to be explored

Development of a Robotic Positioning and Tracking System for a Research Laboratory

Measurement of residual stress using neutron or synchrotron diffraction relies on the accurate alignment of the sample in relation to the gauge volume of the instrument. Automatic sample alignment can be achieved using kinematic models of the positioning system provided the relevant kinematic parameters are known, or can be determined, to a suitable accuracy. The main problem addressed in this thesis is improving the repeatability and accuracy of the sample positioning for the strain scanning, through the use of techniques from robotic calibration theory to generate kinematic models of both off-the-shelf and custom-built positioning systems. The approach is illustrated using a positioning system in use on the ENGIN-X instrument at the UK’s ISIS pulsed neutron source comprising a traditional XYZΩ table augmented with a triple axis manipulator. Accuracies better than 100microns were achieved for this compound system. Although discussed here in terms of sample positioning systems these methods are entirely applicable to other moving instrument components such as beam shaping jaws and detectors. Several factors could lead to inaccurate positioning on a neutron or synchrotron diffractometer. It is therefore essential to validate the accuracy of positioning especially during experiments which require a high level of accuracy. In this thesis, a stereo camera system is developed to monitor the sample and other moving parts of the diffractometer. The camera metrology system is designed to measure the positions of retroreflective markers attached to any object that is being monitored. A fully automated camera calibration procedure is developed with an emphasis on accuracy. The potential accuracy of this system is demonstrated and problems that limit accuracy are discussed. It is anticipated that the camera system would be used to correct the positioning system when the error is minimal or notify the user of the error when it is significant

Industrial Robotics

This book covers a wide range of topics relating to advanced industrial robotics, sensors and automation technologies. Although being highly technical and complex in nature, the papers presented in this book represent some of the latest cutting edge technologies and advancements in industrial robotics technology. This book covers topics such as networking, properties of manipulators, forward and inverse robot arm kinematics, motion path-planning, machine vision and many other practical topics too numerous to list here. The authors and editor of this book wish to inspire people, especially young ones, to get involved with robotic and mechatronic engineering technology and to develop new and exciting practical applications, perhaps using the ideas and concepts presented herein

Modeling and experimental validation of a parallel microrobot for biomanipulation

The main purpose of this project is the development of a commercial micropositioner's (SmarPod 115.25, SmarAct GmbH) geometrical model. SmarPod is characterized by parallel kinematics and is employed for precise and accurate sample's positioning under SEM microscope, being vacuum-compatible, for various applications. Geometrical modeling represents the preliminar step to fully understand, and possibly improve, robot's closed loop behaviour in terms of task's quality precision, when enterprises does not provide sufficient documentation. The robotic system, in fact, represents in this case a "black box" from which it's possible to extract information. This step is essential in order to improve, consequently, the reliability of bio-microsystem manipulation and characterization. Disposing of a detailed microrobot's model becomes essential to deal with the typical lack of sensing at microscale, as it allows a 3D precise and adequate reconstruction, realized through proper softwares, of the manipulation set-up. The roles of Virtual Reality (VR) and of simulations, carried out, in this case, in Blender environment, are asserted as well as an essential helping tool in mycrosystem's task planning. Blender is a professional free and open-source 3D computer graphics software and it is proven to be a basic instrument to validate microrobot's model, even to simplify it in case of complex system's geometries

Vision-based trajectory control of unsensored robots to increase functionality, without robot hardware modication

In nuclear decommissioning operations, very rugged remote manipulators are used, which lack proprioceptive joint angle sensors. Hence these machines are simply tele-operated, where a human operator controls each joint of the robot individually using a teach pendant or a set of switches. Moreover, decommissioning tasks often involve forceful interactions between the environment and powerful tools at the robot's end-effector. Such interactions can result in complex dynamics, large torques at the robot's joints, and can also lead to erratic movements of a mobile manipulator's base frame with respect to the task space. This Thesis seeks to address these problems by, firstly, showing how the configuration of such robots can be tracked in real-time by a vision system and fed back into a trajectory control scheme. Secondly, the Thesis investigates the dynamics of robot-environment contacts, and proposes several control schemes for detecting, coping with, and also exploiting such contacts. Several contributions are advanced in this Thesis. Specifically a control framework is presented which exploits the constraints arising at contact points to effectively reduce commanded torques to perform tasks; methods are advanced to estimate the constraints arising from contacts in a number of situations, using only kinematic quantities; a framework is proposed to estimate the configuration of a manipulator using a single monocular camera; and finally, a general control framework is described which uses all of the above contributions to servo a manipulator. The results of a number of experiments are presented which demonstrate the feasibility of the proposed methods

Design d'un manipulateur robotique à architecture anthropomorphique

RÉSUMÉ Les robots à architecture anthropomorphique, c’est-à-dire inspirée des mécanismes du corps humain, sont des outils qui pourraient être d’une grande utilité dans le domaine de la robotique de réadaptation et d’assistance étant donné l’aspect intuitif, relativement à leur fonctionnement et leur comportement, que leur apporte leur forme familière. Toutefois, les robots anthropomorphiques actuellement existants sont généralement développés dans le cadre d’applications particulières et ne sont par conséquent pas appropriés pour un transfert vers le domaine de la robotique d’assistance et de réadaptation. Par conséquent, il n’existe pas de plateforme de développement robotique permettant le développement d’applications mettant à profit au maximum une architecture biomimétique pour des usagers sans formation en robotique comme les cliniciens en réadaptation et les bénéficiaires de robots d’assistance. Dans ce contexte, l’objectif de ce mémoire est de concevoir une plateforme robotique à architecture anthropomorphique suivant le plus fidèlement possible l’architecture du bras humain pour maximiser l’apport intuitif de la forme familière de l’appareil pour son fonctionnement et son interaction avec son environnement. Particulièrement, un effort est appliqué pour inclure dans le mécanisme la structure à deux os de l’avant-bras afin de rendre possible une reproduction réaliste du mouvement de pronation-supination du bras, le mouvement le plus complexe du membre supérieur puisqu'il implique une boucle cinématique fermée et de nombreux joints passifs. En premier lieu, un modèle biomécanique du membre supérieur a été choisi pour répondre à la problématique, puis ses caractéristiques principales ont été spécifiées. La chaîne cinématique du modèle biomécanique a par la suite été traduite en chaîne cinématique pour un robot, en s’assurant que chacun des sous-mécanismes reproduise bien les caractéristiques soulignées du modèle. Ces mécanismes ont permis de faire les plans entier du manipulateur robotique à l’aide d’un logiciel de dessin assisté par ordinateur. Afin de choisir les actionneurs nécessaires au fonctionnement du robot, une simulation de suivi de trajectoire avec contrôle par couple pré-calculé a été élaborée. Les modèles cinématiques et dynamiques du bras, étant nécessaires à la simulation, ont été développés. Enfin, un prototype de la plateforme a été conçu par impression 3D. En comparant les plages angulaires accessibles aux différents joints du mécanisme à celles des joints du modèle biomécanique, il a été déterminé que le bras pourrait reproduire toute la gamme des mouvements humains pour laquelle la position de l’épaule est près de la position de repos.----------ABSTRACT Bio-inspired robots, more specifically, anthropomorphic robots, would be very suitable tools for assistance and rehabilitation robotics because they are, by design, very familiar and intuitive-looking both visually and in their behavior, which is ideal in a field where most users, such as patients and clinicians, do not possess an advanced knowledge of robotics. However, previously developed anthropomorphic robots are not well-suited for a direct use in such applications since they were generally designed for other industrial applications which makes them cumbersome and complex to use. It is not ideal for use in assistance robotics. Moreover, none of them actually correctly mimics the two-bones structure of the arm-forearm complex of the human arm and thus the designs are not biofidelic. Therefore, there is currently no robotic development platform dedicated to anthropomorphic robotics for the field of rehabilitation and assistance. Given this context, the objective of this study is to design the first robotic development platform with an anthropomorphic architecture which reproduces as best as possible the biomechanics of the human upper-limb for intuitive control and interaction. Specifically, the mechanism of the robotic arm will include the two-bones structure of the forearm for an accurate replication of the motion of pronation-supination, which is the most intricate motion of the arm. First, a biomechanical model of the upper-limb was chosen to fit the objective of the project and its key characteristics were analysed. This model was then translated into a robotic kinematic chain whose sub-mechanisms mimic the highlighted characteristics of the biomechanical model. The designed mechanisms were then included in a complete computer-assisted drawing of the robot. The choice of actuators on the robot was made based on the results of a path-following by computed-torque simulation of the robot, which required the kinematic and dynamic models of the robot to be developed. Finally, a physical prototype of the robot was built using rapid prototyping. Comparing the range of motion of each of its actuators, it appeared that the robot arm should be able to imitate accurately any human motion of the arm for which the shoulder is close to its resting position. Applications and research in rehabilitation and assistance robotics where accurate reproduction of the human motion of the arm is necessary. For example, determining the mathematical optimization criterion for humanlike path-planning can now be performed with the designed robot