Elasto-geometrical modeling and calibration of redundantly actuated PKMs

International audienceRedundantlyactuated parallel kinematic machines (PKMs) offer a number of advantages compared to classical non-redundant PKMs. Particularly, they show a better stiffness thanks to singularity avoidance and they have an improved repeatability due to a better behavior against backlashes. The main problem with the calibration of these machines is that the redundancy leads to some mechanical strains in their structure. This makes it difficult to identify the geometrical errors of their structure without taking into account the effects of the elastic deformations. The main originality of this work is to propose an efficient elasto-geometrical and calibration method that allows the identification of both the geometrical and stiffness parameters of redundantlyactuated parallel mechanisms with slender links. The first part of the paper explains the proposed method through its application on a simple redundant planar mechanism. The second part deals with its experimental application to the redundant Scissors Kinematics machine

Elasto-geometrical modeling and calibration of robot manipulators: Application to machining and forming applications

International audienceThis paper proposes an original elasto-geometrical calibration method to improve the static pose accuracy of industrial robots involved in machining, forming or assembly applications. Two approaches are presented respectively based on an analytical parametric modeling and a Takagi-Sugeno fuzzy inference system. These are described and then discussed. This allows to list the main drawbacks and advantages of each of them with respect to the task and the user requirements. The Fuzzy Logic model is used in a model-based compensation scheme to increase significantly the robot static pose accuracy in a context of incremental forming application. Experimental results show the efficiency of the Fuzzy Logic model while minimizing development and computational resources

Investigation of Parameter Estimation of a Car-Trailer System Using Condition Numbers

In this research we investigate condition numbers obtained from least squares estimation for a car-trailer system to characterize estimation performance. In this case, we can select better parameter estimation methods or well-posed measured data sets for the car-trailer system using condition numbers. We calculate condition numbers from several different linear model-based least squares methods which use four linear regression models and three least squares methods to estimate trailer parameters. We also consider three different observed data sets in ideal and non-ideal sensor scenarios for simulation tests

Étalonnage des machines-outils à cinq axes : configuration optimisée des artefacts et de la séquence de mesure de la méthode SAMBA en vue d'une estimation efficace des erreurs géométriques

RÉSUMÉ Les machines-outils à commande numérique (MOCN) sont assujetties à plusieurs sources d’erreurs, entre autres géométriques, thermiques et dynamiques qui peuvent contribuer à la dégradation de leurs performances. Une attention particulière est prêtée à l’usinage multi axes où le mouvement simultané des axes prismatiques et rotatifs engendre une erreur de positionnement et d’orientation de l’outil par rapport au point à usiner sur la pièce. Des moyens d’évaluations de ces erreurs et de leurs causes, à des fins de maintenance et de compensation, sont alors à développer en tenant compte des aspects économiques, techniques et humains. Il s’agit en particulier de minimiser les temps de mesures qui résultent en des arrêts de production et par conséquent des coûts indirects à éviter à l’entreprise. Le but de la présente thèse est d’améliorer la précision d’une machine-outil à cinq axes à travers l’optimisation d’une technique d’étalonnage existante. En vue de prédire au mieux le comportement de la machine, l’élaboration d’une routine d’inspection adéquate est nécessaire. Ceci comprend un positionnement optimal des éléments du dispositif de mesure, sous forme de billes de référence, ainsi qu’une planification judicieuse des poses de palpage dans l’espace de travail. Une approche analytique basée sur un algorithme d’échange pour la conception d’un plan D-optimal est adoptée pour générer des scénarios d’étalonnage en fonction des écarts géométriques à estimer, modélisés sous forme de polynôme, et du nombre d’inconnues définissant le modèle de la machine. L’évaluation de la pertinence des tests est effectuée à partir d’une étude comparative de critères appelés communément en robotique, indices d’observabilité, issus de l’analyse de la matrice jacobienne d’identification. La qualité prédictive des séquences de mesures générées par simulation est validée en deux étapes : la première consiste en des expériences de répétabilité des tests optimisés imbriqués, la deuxième est une analyse de l’incertitude sur les tests et les paramètres d’erreurs identifiés. Une validation par mesure directe d’une cale calibrée, montée sur la table de la machine, permet de confirmer les résultats qualitatifs fournis par l’indice d’observabilité et ceux quantitatifs déduits de l’estimation de l’incertitude. Les résultats montrent que les routines de vérification proposées sont capables de donner une description complète de la géométrie imparfaite de la machine en incluant les écarts de membrures et les écarts cinématiques. Une amélioration de 55.7% de la valeur de l’indice d’observabilité est constatée par rapport à celle de la stratégie de mesure utilisée présentement dans le laboratoire.----------ABSTRACT Numerically controlled machine tools are prone to potential geometric, thermal and dynamic errors that can have a negative impact on their performance. A careful attention is paid to multi-axis machining where the simultaneous movement of prismatic and rotary axes lead to a positioning and orientation deviation of the tool relative to the workpiece. Tools for assessing these errors and their causes, for maintenance and compensation purposes, are to be developed while taking into consideration economic, technical and human aspects. In particular, this involves minimizing the measurement duration which results in production downtimes and consequently indirect costs to be avoided by the company. This thesis aims to improve the accuracy of a five-axis machine tool through the optimization of an existing calibration technique. For a better prediction of the machine tool erroneous behavior, an adequate inspection routine is sought. This includes optimal positioning of the measuring device components, i.e. master balls, as well as a wise planning of the probing poses in the working volume. An analytical approach based on an exchange algorithm for a D-optimal design is carried out to generate calibration scenarios based on the estimated geometric errors, described as ordinary polynomials, and the number of unknowns predefined in the machine model. The evaluation of the optimized tests suitability relies on a comparison of criteria, commonly known in the robotics field as observability indices and are the outcome of the identification Jacobian matrix analysis. Simulation results are validated in two steps: the first one consists of a repeatability testing of nested optimized probing sequences while the second one is an analysis of the estimated uncertainty on the overall tests and the identified error parameters. Validation via a direct measuring of a calibrated gauge block, mounted on the machine workpiece, confirms the qualitative results provided by the observability index and the quantitative ones concluded from the uncertainty estimation. The outcome suggests that the proposed geometric model updating routines enable a comprehensive description of the machine tool behavior by including location errors and error motions. An improvement of 55.7% of the observability index value is depicted with respect to the currently used measurement strategy. The optimal calibration test duration varies between 30 minutes while probing one master ball for axes location errors identification and 2 hours and 18 minutes for the estimation of both axes location errors and error motions while measuring an artefact of three master balls

Doctor of Philosophy

dissertationFingernail imaging is a method of sensing finger force using the color patterns on the nail and surrounding skin. These patterns form as the underlying tissue is compressed and blood pools in the surrounding vessels. Photos of the finger and surrounding skin may be correlated to the magnitude and direction of force on the fingerpad. An automated calibration routine is developed to improve the data-collection process. This includes a novel hybrid force/position controller that manages the interaction between the fingerpad and a flat surface, implemented on a Magnetic Levitation Haptic Device. The kinematic and dynamics parameters of the system are characterized in order to appropriately design a nonlinear compensator. The controller settles within 0.13 s with less than 30% overshoot. A new registration A new registration technique, based on Active Appearance Models, is presented. Since this method accounts for the variation inherent in the finger, it reduces registration and force prediction errors while removing the need to tune registration parameters or reject unregistered images. Modifications to the standard model are also investigated. The number of landmark points is reduced to 25 points with no loss of accuracy, while the use of the green channel is found to have no significant effect on either registration or force prediction accuracy. Several force prediction models are characterized, and the EigenNail Magnitude Model, a Principal Component Regression model on the gray-level intensity, is shown to fit the data most accurately. The mean force prediction error using this prediction and modeling method is 0.55 N. White LEDs and green LEDs are shown to have no statistically significant effect on registration or force prediction. Finally, two different calibration grid designs are compared and found to have no significant effect. Together, these improvements prepare the way for fingernail imaging to be used in less controlled situations. With a wider range of calibration data and a more robust registration method, a larger range of force data may be predicted. Potential applications for this technology include human-computer interaction and measuring finger interaction forces during grasping experiments

Topics in Machining with Industrial Robot Manipulators and Optimal Motion Control

Two main topics are considered in this thesis: Machining with industrial robot manipulators and optimal motion control of robots and vehicles. The motivation for research on the first subject is the need for flexible and accurate production processes employing industrial robots as their main component. The challenge to overcome here is to achieve high-accuracy machining solutions, in spite of the strong process forces required for the task. Because of the process forces, the nonlinear dynamics of the manipulator, such as the joint compliance and backlash, may significantly degrade the achieved machining accuracy of the manufactured part. In this thesis, a macro/micro-manipulator configuration is considered to the purpose of increasing the milling accuracy. In particular, a model-based control architecture is developed for control of the macro/micro-manipulator setup. The considered approach is validated by experimental results from extensive milling experiments in aluminium and steel. Related to the problem of high-accuracy milling is the topic of robot modeling. To this purpose, two different approaches are considered; modeling of the quasi-static joint dynamics and dynamic compliance modeling. The first problem is approached by an identification method for determining the joint stiffness and backlash. The second problem is approached by using gray-box identification based on subspace-identification methods. Both identification algorithms are evaluated experimentally. Finally, online state estimation is considered as a means to determine the workspace position and orientation of the robot tool. Kalman Filters and Rao-Blackwellized Particle Filters are employed to the purpose of sensor fusion of internal robot measurements and measurements from an inertial measurement unit for estimation of the desired states. The approaches considered are fully implemented and evaluated on experimental data. The second part of the thesis discusses optimal motion control applied to robot manipulators and road vehicles. A control architecture for online control of a robot manipulator in high-performance path tracking is developed, and the architecture is evaluated in extensive simulations. The main characteristic of the control strategy is that it combines coordinated feedback control along both the tangential and transversal directions of the path; this separation is achieved in the framework of natural coordinates. One motivation for research on optimal control of road vehicles in time-critical maneuvers is the desire to develop improved vehicle-safety systems. In this thesis, a method for solving optimal maneuvering problems using nonlinear optimization is discussed. More specifically, vehicle and tire modeling and the optimization formulations required to get useful solutions to these problems are investigated. The considered method is evaluated on different combinations of chassis and tire models, in maneuvers under different road conditions, and for investigation of optimal maneuvers in systems for electronic stability control. The obtained optimization results in simulations are evaluated and compared

Contribution to improving the accuracy of serial robots

The goal of the present study is to improve the accuracy of six-revolute industrial robots using calibration methods. These methods identify the values of the calibrated robot model to improve the correspondence between the real robot and the mathematical model used in its controller. The calibrated robot model adds error parameters to the nominal model, which correspond to the geometric errors of the robot as well as the stiffness behavior of the robot. The developed methods focus on using low cost measurement equipment. For instance, the first work makes a comparison between a robot calibration performed using a laser tracker and a stereo camera (MMT optique) separately. The accuracy performance is validated using a telescoping ballbar for each of the two methods. While the calibration result is the same for both methods, the price of a laser tracker is more than twice the price of a stereo camera. The method is tested using an ABB IRB120 robot, a Faro ION laser tracker, and a Creaform CTrack stereo camera to calibrate the robot. A Renishaw QC20-W ballbar is used to validate the accuracy. A novel measurement system to measure a set of poses is described in the second work. The device is an extension of a known approach using an hexapod (a Stewart-Gough platform). One fixture is attached to the robot base and the other to the robot end-effector, each having three magnetic cups. By taking six ballbar measurements at a time, it is possible to measure 144 poses of the triangular fixture attached to the robot end-effector with respect to the base fixture. The position accuracy of the device is 3.2 times the accuracy of the QC20-W ballbar: ± 0.003 mm. An absolute robot calibration using this novel 6D measurement system is performed in the third work of this thesis. The robot is calibrated in 61 configurations and the absolute position accuracy of the robot after calibration is validated with a Faro laser tracker in about 10,000 robot configurations. The mean distance error is improved from 1.062 mm to 0.400 mm in 50 million pairs of measurements throughout the complete robot workspace. To allow a comparison, the robot is also calibrated using the laser tracker and the robot accuracy validated in the same 10,000 robot configurations

Online pose correction of an industrial robot using an optical coordinate measure machine system

In this article, a dynamic pose correction scheme is proposed to enhance the pose accuracy of industrial robots. The dynamic pose correction scheme uses the dynamic pose measurements as feedback to accurately guide the robot end-effector to the desired pose. The pose is measured online with an optical coordinate measure machine, that is, C-Track 780 from Creaform. A root mean square method is proposed to filter the noise from the pose measurements. The dynamic pose correction scheme adopts proportional-integral-derivaitve controller and generates commands to the FANUC robot controller. The developed dynamic pose correction scheme has been tested on two industrial robots, FANUC LR Mate 200iC and FANUC M20iA. The experimental results on both robots demonstrate that the robots can reach the desired pose with an accuracy of ±0.050 mm for position and ±0.050° for orientation. As a result, the developed pose correction can make the industrial robots meet higher accuracy requirement in the applications such as riveting, drilling, and spot welding

Engineering for a changing world: 60th Ilmenau Scientific Colloquium, Technische Universität Ilmenau, September 04-08, 2023 : programme

In 2023, the Ilmenau Scientific Colloquium is once more organised by the Department of Mechanical Engineering. The title of this year’s conference “Engineering for a Changing World” refers to limited natural resources of our planet, to massive changes in cooperation between continents, countries, institutions and people – enabled by the increased implementation of information technology as the probably most dominant driver in many fields. The Colloquium, supplemented by workshops, is characterised but not limited to the following topics: – Precision engineering and measurement technology Nanofabrication – Industry 4.0 and digitalisation in mechanical engineering – Mechatronics, biomechatronics and mechanism technology – Systems engineering – Productive teaming - Human-machine collaboration in the production environment The topics are oriented on key strategic aspects of research and teaching in Mechanical Engineering at our university

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