1,342 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
No full textExtensively 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
Analysis and development of cable-driven robotic devices
No full textThe design of a mechanical system is often the result of an optimization process, aimed to obtain the best performances inside a given task space under fixed constraints.
Due to the unilateral actuation, cable-based devices possess specific features that make some of the tools commonly used in robotics completely unsuitable, thus requiring the definition of specific tools for analysis and design rules.
Even though several examples of geometrical, kinematic and dynamic performance indices have been introduced as analysis tools in the last years, only few authors proposed rigorous design methodologies. Traditional approaches seek to find the optimal set of design parameters by maximizing the robot capabilities inside a given reference workspace. However, since most of the properties of cable-based devices depend on both robot geometry (i.e., number and disposition of cable attachment points) and cable configuration (i.e., directions of cables in the end-effector reference frame), the capabilities of a given robot are extremely variable throughout the workspace. Thus, the structure of traditional devices often appears cumbersome if compared to the useful workspace they have been designed for.
The aim of this work is first to present a set of local, configuration dependent performance indices that properly characterize cable-based devices. Then, a new methodology is described to obtain effective, well-tailored designs. The formulated design paradigm takes advantage of the introduction of moving pulley-blocks, leading to the definition of a new class of cable devices defined as adaptive cable-based devices.
In the first half of this thesis, the new methodology is described in detail, and a numerical validation is performed by solving simple case-studies. In order to empirically validate the proposed methodology, the first prototype of Adaptive cable-based device has been designed and developed. The second half of this thesis deals with layout definition, mechanical design and control system design of the Sophia-3 prototype. Finally, results from the first experimental tests on the new device are reported.La progettazione di un sistema meccanico è spesso il risultato di un processo di ottimizzazione, il cui scopo è quello di massimizzare le performance all'interno di un dato workspace rispettando una serie di vincoli.
A causa dell'attuazione unilaterale, i dispositivi cable-driven possiedono caratteristiche specifiche, cosicch è gli strumenti di analisi tradizionalmente impiegati in robotica risultano inadeguati o poco efficaci. Di qui la necessità di definire indici di analisi e metodologie di progettazione specifici.
Sebbene siano stati sviluppati nel corso degli anni svariati indici di performance (geometrici, cinematici e dinamici), solo pochi autori hanno finora proposto metodi di progettazione rigorosi. Negli approcci tradizionali, lo scopo è quello di determinare il set di parametri di progetto che massimizza le performance del dispositivo all'interno di una data regione dello spazio di lavoro. Tuttavia, poichè molte delle proprietà dei dispositivi cable-based dipendono sia dalla geometria del manipolatore (numero di cavi, disposizione delle pulegge alla base, ecc..) sia dalla disposizione dei cavi (cioè dalla direzioni assunte dai cavi rispetto all'end-effector), le performance di un manipolatore risultano estremamente variabili all'interno dello spazio di lavoro. Di conseguenza, l'ingombro complessivo di un robot cable-driven risulta spesso decisamente maggiore rispetto allo spazio di lavoro per il quale è stato progettato.
Lo scopo di questo lavoro consiste dapprima nel presentare un set di indici di performance locali, in grado di caratterizzare le proprietà principali di un dispositivo. Successivamente, viene presentata una nuova metodologia di progettazione basata su questi indici, che permette di ottenere soluzioni progettuali più efficaci, cioè sviluppate su misura in base alle specifiche. La metodologia proposta si basa sull'introduzione di passacavi mobili, ed ha condotto alla definizione di una nuova classe di dispositivi cable-based denominati adattativi.
Al fine di validare numericamente la procedura, nella prima parte di questa tesi vengono presentati alcuni semplici esempi di progettazione. Allo scopo di dare una validazione empirica alla metodologia introdotta, è stato inoltre progettato e sviluppato il prototipo Sophia-3, primo esempio di sistema a cavi adattativo. Nella seconda parte di questa tesi viene presentato il prototipo, descrivendone il layout, la progettazione meccanica e l'architettura del controllo. Vengono inoltre presentati i primi risultati sperimentali dei test effettuati sul nuovo prototipo
Shaper-Based Filters for the compensation of the load cell response in dynamic mass measurement
No full textThis paper proposes a novel model-based signal filtering technique for dynamic mass measurement through load cells. Load cells are sensors with an underdamped oscillatory response which usually imposes a long settling time. Real-time filtering is therefore necessary to compensate for such a dynamics and to quickly retrieve the mass of the measurand (which is the steady state value of the load cell response) before the measured signal actually settles. This problem has a big impact on the throughput of industrial weighing machines. In this paper a novel solution to this problem is developed: a model-based filtering technique is proposed to ensure accurate, robust and rapid estimation of the mass of the measurand. The digital filters proposed are referred to as Shaper-Based Filters (SBFs) and are based on the convolution of the load cell output signal with a sequence of few impulses (typically, between 2 and 5). The amplitudes and the instants of application of such impulses are computed through the analytical development of the load cell step response, by imposing the admissible residual oscillation in the steady-state filtered signal and by requiring the desired sensitivity of the filter. The inclusion of robustness specifications tackles effectively the unavoidable uncertainty and variability in the load cell frequency and damping. The effectiveness of the proposed filters is proved experimentally through an industrial set up: the load-cell-instrumented weigh bucket of a multihead weighing machine for packaging. A performance comparison with other benchmark filters is provided and discussed too. (C) 2017 Elsevier Ltd. All rights reserved
Cable Robot Performance Evaluation by Wrench Exertion Capability
No full textAlthough cable driven robots are a type of parallel manipulators, the evaluation of their performances cannot be carried out using the performance indices already developed for parallel robots with rigid links. This is an obvious consequence of the peculiar features of flexible cables—a cable can only exert a tensile and limited force in the direction of the cable itself. A comprehensive performance evaluation can certainly be attained by computing the maximum force (or torque) that can be exerted by the cables on the moving platform along a specific (or any) direction within the whole workspace. This is the idea behind the index—called the Wrench Exertion Capability (WEC)—which can be employed to evaluate the performance of any cable robot topology and is characterized by an efficient and simple formulation based on linear programming. By significantly improving a preliminary computation method for the WEC, this paper proposes an ultimate formulation suitable for any cable robot topology. Several numerical investigations on planar and spatial cable robots are presented to give evidence of the WEC usefulness, comparisons with popular performance indices are also provided
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