1,721,145 research outputs found
FEEDBACK CONTROL OF FLEXIBLE FOUR-BAR LINKAGES: A NUMERICAL AND EXPERIMENTAL INVESTIGATION
No full textA control scheme for a four-bar linkage with all the links flexible is proposed and tested both numerically and experimentally. The control strategy consists in selecting a reduced number of measurable variables through which performing position and vibration are controlled independently. The controlled variables are the crank angle and the link curvatures, which provide an adequate description of the temporal evolution of the mechanism position and vibrational phenomena. Position control is performed through a proportional integral and differential (PID)-like regulator while proportional controllers are employed to damp the fundamental components of the link oscillations. A force of gravity compensator is introduced to increase the control system performances and appropriate devices are proposed to avoid coupling effects among the controlled variables.
The control scheme is first tested and tuned in simulation, where the dynamic behaviour of the flexible linkage is reproduced through a fully coupled non-linear model based on the finite element theory. The performances of the control scheme are assessed by studying the step response of the closed-loop system. The numerical results attained prove that the proposed control scheme achieves efficient positioning and vibration suppression performances. The experimental validation of the control scheme is carried out on an instrumented prototype of the flexible four-bar linkage. Experimental recordings are in good agreement with the numerical results therefore confirming both the effectiveness of the control scheme and the accuracy of the dynamic model
Planning of dynamically feasible trajectories for translational, planar, and underconstrained cable-driven robots
Full text linkExtensively studied since the early nineties, cable-driven robots have attracted the growing
interest of the industrial and scientific community due to their desirable and peculiar attributes. In
particular, underconstrained and planar cable robots can find application in several fields, and specifically,
in the packaging industry. The planning of dynamically feasible trajectories (i.e., trajectories
along which cable slackness and excessive tensions are avoided) is particularly challenging when dealing
with such a topology of cable robots, which rely on gravity to maintain their cables in tension. This
paper, after stressing the current relevance of cable robots, presents an extension and a generalization
of a model-based method developed to translate typical cable tension bilateral bounds into intuitive
limits on the velocity and acceleration of the robot end effector along a prescribed path. Such a new
formulation of the method is based on a parametric expression of cable tensions. The computed kinematic
limits can then be incorporated into any trajectory planning algorithm. The method is developed
with reference to a hybrid multi-body cable robot topology which can be functionally advantageous but
worsen the problem of keeping feasible tensions in the cables both in static and dynamic conditions.
The definition of statically feasible workspace is also introduced to identify the positions where static
equilibrium can be maintained with feasible tensions. Finally, some aspects related to the practical
implementation of the method are discussed
UNDERCONSTRAINED PLANAR CABLE-DIRECT-DRIVEN ROBOTS: A TRAJECTORY PLANNING METHOD ENSURING POSITIVE AND BOUNDED CABLE TENSIONS
No full textA major open issue in the design and operation of cable-direct-driven-robots (CDDRs) is ensuring tensile cable forces for any admissible motion of the CDDR. Such a problem is particularly challenging when underconstrained and non-redundant CDDR configurations are considered. In this paper a new and general trajectory planning method is introduced, which has been specifically developed to ensure always positive and bounded cable tensions in underconstrained planar two-degree-of-freedom translational CDDRs. The proposed method translates the typical bilateral force constraints of the cables (i.e. positive and bounded tensions) into constraints on the velocity and acceleration of the CDDR end-effector along the path. Such constraints are computed making use of the robot dynamic model and are then incorporated in a suitable trajectory planning algorithm also yielding the minimum traversal time. The method is explained and validated numerically by applying it to a novel concept of underconstrained hybrid (serial/parallel) CDDR. The results achieved prove that the proposed method may a priori ensure positive and bounded cable tensions along any straight line and circular path
PERFORMANCE EVALUATION AND TRAJECTORY PLANNING FOR PLANAR CABLE ROBOTS
No full textCable robots (also called “cable direct driven robots” or “cable driven parallel robots”) are amongst the most promising robotic devices in the industrial and service field. As a consequence of their peculiar and desirable advantages over conventional robots, they have attracted the growing interest of the scientific and industrial community since the early nineties. In particular, depending on the application, they can be designed to have a very large workspace and a very high load capacity, or to generate very high speed motions. Additionally, their simple design makes them inexpensive, while their minimal moving mass usually makes them energy efficient. All these advantages are being promoting the deployment of cable-robots in several real-world applications.
Relevant and challenging research issues are still open and have to be tackled when designing and operating a cable robot: indeed, such robots must always meet the stringent requirement that all cables are under tension during operation. Additionally, cable forces must be kept below some maximum permissible values related to the torque limits of the actuators or to the tensile force limits of the cables. Under such constraints, not only is motion planning and control for cable robots demanding, but also predicting cable robot performances within the workspace is not trivial, and cannot be done by just applying the performance indexes conceived for rigid-link parallel robots. The definition of workspace itself becomes much more elaborate.
The speech addresses both performance evaluation and trajectory planning for cable robots, and focuses explicitly on planar cable robots.
An example of underconstrained hybrid translational cable robot is introduced to show that a successful approach to prevent cable slackness and excessive tensions may consist in planning dynamically feasible trajectories by making use of a dynamic model of the cable-suspended robot to translate the cable tension bilateral bounds (i.e. positive and bounded tensile cable forces) into limits on the velocity and acceleration of the robot end-effector along the assigned path.
The fact that cables behave as unilateral actuators has a dramatic impact on performance evaluation too: suitable approaches to performance evaluation are discussed and appropriate performance indexes are presented
CONTROLLO SIMULTANEO DI VIBRAZIONI E MOTO RIGIDO IN MECCANISMI ARTICOLATI A MEMBRI DEFORMABILI
No full textIl presente lavoro approfondisce tematiche connesse alla modellazione ed al controllo di manipolatori leggeri nei quali si manifestano fenomeni vibratori indesiderati. In particolare, un originale schema per il controllo del moto e delle vibrazioni di sistemi articolati piani viene presentato e testato, numericamente e sperimentalmente, con riferimento ad un quadrilatero articolato piano con tutti i membri deformabili.
Lo schema è progettato per consentire un controllo il più possibile disaccoppiato del moto rigido, opportunamente definito, e dei fenomeni vibratori che si generano in un meccanismo a causa della deformabilità dei membri. A tal fine vengono utilizzati regolatori derivati da quelli standard della teoria dei controlli, che operano in parallelo su un sistema ridotto di variabili facilmente misurabili sperimentalmente. I fenomeni di accoppiamento dinamico tra le variabili controllate, che necessariamente si vengono a manifestare, sono attenuati adottando accorgimenti per la rielaborazione dei segnali. Inoltre, per rendere più efficace lo schema di controllo, gli effetti gravitazionali sono compensati separatamente sulla base di un modello a membri rigidi del meccanismo.
Per la messa a punto e la verifica delle prestazioni dello schema di controllo viene sviluppato un accurato simulatore dinamico. Esso riproduce il comportamento del sistema controllato utilizzando un modello dinamico ad elementi finiti in grado di evidenziare gli effetti di mutuo accoppiamento inerziale tra moto rigido e vibrazioni. Lo schema di controllo è implementato ed integrato con il simulatore del meccanismo utilizzando un software che semplifica significativamente anche le operazioni per la verifica sperimentale delle prestazioni del controllo.
I risultati ottenuti in simulazione, relativi alla risposta del sistema in catena chiusa ad un ingresso a gradino applicato a partire da una configurazione di equilibrio statico, dimostrano l’efficacia dello schema sintetizzato. Le prove sperimentali, condotte su un prototipo strumentato del meccanismo senza apportare alcuna modifica allo schema di controllo ed al valore dei parametri in esso presenti, hanno confermato la validità delle prestazioni dello schema e sono in buona corrispondenza con i risultati numerici a dimostrazione dell’accuratezza del modello dinamico
PLANNING OF DYNAMICALLY FEASIBLE TRAJECTORIES FOR TRANSLATIONAL AND PLANAR CABLE-SUSPENDED ROBOTS
No full textCable-robots are relatively simple robotic manipulators formed by attaching multiple cables to an end-effector. Cable-robots have several desirable advantages over conventional robots. Primarily, they can be designed to have a very large workspace, a very high load capacity, or to generate very high speed motions. Additionally, their simple design makes them inexpensive, modular, transportable and easily reconfigurable. Finally, their minimal moving mass makes them very energy efficient. All these advantages are being promoting the deployment of cable-robots in several real-world applications.
A major requirement that has to be met in cable-robots is ensuring that during operation all cables are under tension, and that such a tension is below the maximum permissible value related to the torque limits of the actuators or to tensile force limits of the cables. Assuring feasible tensions in all cables is particularly difficult in underconstrained or cable-suspended robots which use an external force, typically gravity, to maintain their cables in tension. A successful approach to prevent cable slackness and excessive tensions may consist in planning dynamically feasible trajectories by making use of a dynamic model of the cable-suspended robot to translate the cable tension bilateral bounds (i.e. positive and bounded tensile cable forces) into limits on the velocity and acceleration of the robot end-effector along the assigned path.
The lecture starts with an introduction to cable-robots, providing some essential definitions and showing successful examples of application of these robots. Subsequently the main open research issues in cable robotics are presented. Then, proceeding from general to particular, the focus is posed on an hybrid, translational and planar cable-suspended robot, proposed as a representative example of cable-suspended robot for which the planning of dynamically feasible trajectories is particularly challenging. The dynamic model of the studied robot is then presented as well as the robot workspace. Afterwards, the attention is focused on the model-based trajectory planning method developed to ensure dynamically feasible trajectories. It is proved that the method leads to kinematic limits that can be incorporated into any trajectory planning algorithm. What is more important, the low computational complexity of the method proposed makes it suitable for implementation in real-time systems. Finally, the validity of the method is proved by experimental results obtained by referring to two paths of industrial interest
Dynamic compensation of load cell response in high speed weighing devices through Shaper-Based Filters
No full textDESIGN AND CALIBRATION OF AN OPTO-MECHANICAL APPLIANCE FOR 3D NON-CONTACT ORTHOPEDIC MEASUREMENTS - PART I: MATHEMATICAL MODEL AND LABORATORY PROTOTYPE
No full textAn opto-mechanical system performing fast and accurate non-contact foot measurements is proposed. By processing the measured data it is possible to get three-dimensional foot models and to design custom-made shoes. This work is subdivided into two parts: in Part I, after introducing the system operating method and describing the vision technology employed, a mathematical model of the system is developed accounting for the main internal parameters of the camera. In order to reduce scanning time and the time necessary to create the 3D foot model, the measured data have been arranged in a particular structure and a suitable acquisition procedure has been developed. A laboratory prototype has been built to test the performances of the system: a detailed analysis of its geometry and of its components is provided as well as a concise description of the simple user interface developed to run and control the system
Experimental Validation of a Trajectory Planning Approach Avoiding Cable Slackness and Excessive Tension in Underconstrained Translational Planar Cable-Driven Robots
No full textThe objective of this paper is providing the first experimental evidence of the effectiveness of an off-line trajectory planning approach developed to ensure positive and bounded cable tensions in under constrained planar two-degree-of-freedom translational cable robots. The hybrid (serial/parallel) topology of the investigated robot is general enough to ensure wide applicability of the proposed trajectory planning method, which translates the usual bilateral tensile cable force constraints into kinematic constraints on the velocity and acceleration of the robot tool center point along the desired path. Kinematic constraints are computed making use of the robot dynamic model and can then be incorporated in any trajectory planning algorithm. In this work a smooth trajectory planning algorithm based on quintic polynomials is adopted. The experimental setup is presented and the results obtained by applying the method to two sample paths are discusse
SIMULTANEOUS RIGID-BODY MOTION AND VIBRATION CONTROL OF A FLEXIBLE FOUR-BAR LINKAGE.
No full textA model and a control scheme for a flexible four-bar linkage is described. The discrete finite element model of the mechanism accounts for geometric and inertial nonlinearities. A reduced number of measured variables are selected to control both rigid-body motion and vibration separately. Rigid-body motion control is performed by means of a PID-like regulator while proportional controllers are employed to damp link oscillations. Appropriate devices are proposed to avoid coupling effects among variables. Numerical results demonstrate the effectiveness of the control scheme
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