Computing wrench-feasible paths for cable-driven hexapods

Motion paths of cable-driven hexapods must carefully be planned to ensure that the lengths and tensions of all cables remain within acceptable limits, for a given wrench applied to the platform. The cables cannot go slack -to keep the control of the robot- nor excessively tight -to prevent cable breakage- even in the presence of bounded perturbations of the wrench. This paper proposes a path planning method that accommodates such constraints simultaneously. Given two configurations of the robot, the method attempts to connect them through a path that, at any point, allows the cables to counteract any wrench lying in a predefined uncertainty region. The feasible C-space is placed in correspondence with a smooth manifold, which facilitates the definition of a continuation strategy to search this space systematically from one configuration, until the second configuration is found, or path non-existence is proved at the resolution of the search. The force Jacobian is full rank everywhere on the C-space, which implies that the computed paths will naturally avoid crossing the forward singularity locus of the robot. The adjustment of tension limits, moreover, allows to maintain a meaningful clearance relative to such locus. The approach is applicable to compute paths subject to geometric constraints on the platform pose, or to synthesize free-flying motions in the full six-dimensional C-space. Experiments are included that illustrate the performance of the method in a real prototype.Postprint (published version

Planning wrench-feasible motions for cable-driven hexapods

Motion paths of cable-driven hexapods must carefully be planned to ensure that the lengths and tensions of all cables remain within acceptable limits, for a given wrench applied to the platform. The cables cannot go slack-to keep the control of the robot-nor excessively tightto prevent cable breakage-even in the presence of bounded perturbations of the wrench. This paper proposes a path-planning method that accommodates such constraints simultaneously. Given two configurations of the robot, the method attempts to connect them through a path that, at any point, allows the cables to counteract any wrench lying in a predefined uncertainty region. The configuration space, or C-space for short, is placed in correspondence with a smooth manifold, which facilitates the definition of a continuation strategy to search this space systematically from one configuration, until the second configuration is found, or path nonexistence is proved by exhaustion of the search. The force Jacobian is full rank everywhere on the C-space, which implies that the computed paths will naturally avoid crossing the forward singularity locus of the robot. The adjustment of tension limits, moreover, allows to maintain a meaningful clearance relative to such locus. The approach is applicable to compute paths subject to geometric constraints on the platform pose or to synthesize free-flying motions in the full 6-D C-space. Experiments illustrate the performance of the method in a real prototype.Postprint (author's final draft

Planning singularity-free force-feasible paths on the Stewart platform

This paper provides a method for computing force-feasible paths on the Stewart platform. Given two configurations of the platform, the method attempts to connect them through a path that, at any point, allows the platform to counteract any external wrench lying inside a predefined six-dimensional region. In particular, the Jacobian matrix of the manipulator will be full rank along such path, so that the path will not traverse the forward singularity locus at any point. The path is computed by first characterizing the force-feasible C-space of the manipulator as the solution set of a system of equations, and then using a higher-dimensional continuation technique to explore this set systematically from one configuration, until the second configuration is found. Examples are included that demonstrate the performance of the method on illustrative situations.Preprin

6-D manipulation with aerial towed-cable system

We propose a new approach for the reliable 6-dimensional quasi-static manipulation of an aerial towed-cable system. The novelty of this approach lies in the combination of results deriving from the static analysis of cable-driven manipulators with the application of a cost-based motion-planning algorithm to solve manipulation queries. Using this approach, we can ensure that the produced paths are feasible and do not approach dangerous configurations that could provoque the malfunction of the system. The approach has been simulated on the FlyCrane, consisting of a platform attached to three flying robots using six fixed-length cables. The obtained results show the success and the suitability of the approach.Postprint (author’s final draft

On the numerical classification of the singularities of robot manipulators

This paper is concerned with the task to obtain a complete description of the singularity set of any given non-redundant manipulator, including the identification and the precise computation of each constituent singularity class. Configurations belonging to the same class are equivalent in terms of the various types of kinematic and static degeneracy that characterize mechanism singularity. The proposed approach is an extension of recent work on computing singularities using a numerical method based on linear relaxations. Classification is sought by means of a hierarchy of singularity tests, each formulated as a system of quadratic or linear equations, which yields sets of classes to which an identified singularity cannot belong. A planar manipulator exemplifies the process of classification, and illustrates how, while most singularities get completely classified, for some lower-dimensional subsets one can only identify a restricted list of possible singularity classes.Postprint (published version

A general method for the numerical computation of manipulator singularity sets

The analysis of singularities is central to the development and control of a manipulator. However, existing methods for singularity set computation still concentrate on specific classes of manipulators. The absence of general methods able to perform such computation on a large class of manipulators is problematic because it hinders the analysis of unconventional manipulators and the development of new robot topologies. The purpose of this paper is to provide such a method for nonredundant mechanisms with algebraic lower pairs and designated input and output speeds. We formulate systems of equations that describe the whole singularity set and each one of the singularity types independently, and show how to compute the configurations in each type using a numerical technique based on linear relaxations. The method can be used to analyze manipulators with arbitrary geometry, and it isolates the singularities with the desired accuracy. We illustrate the formulation of the conditions and their numerical solution with examples, and use 3-D projections to visualize the complex partitions of the configuration space induced by the singularities.Preprin

Geoarqueología y registro geológico del último milenio en el pozo de la antigua Fábrica de Tabacos (Gijón, N España)

During the 2008-2009 archaeological excavations carried out in the ancient Fábrica de Tabacos (Gijón), a Roman well structure was discovered, covered by a sedimentary infill with archaeological remains. This work deals with the geological and depositional characterization of the sedimentary infill. Two hand drill cores of 1.45 m and 1.65m long were studied, using stratigraphic, geochemical and geochronological techniques. The evolutionary model of the sedimentary infill includes 5 phases: 1) construction and functioning as a well (prior? to 5th-6th centuries); 2) abandonment of the well and sedimentation in a restricted palustrine-like environment (5th-7th centuries); 3) a phase dominated by dumping of organic-rich landfill material (7th century); 4) re-starting of palustrine-like environment with some landfill episodes (7th-8th centuries); and 5) a final phase as a dumping site (at least during the 8th century). The sedimentary infill is an outstanding geoarchaelogical archive that provides key information to improve the knowledge of the region from the 6th to the 8th centuries.Agradecemos a la empresa Terra Arqueos S.L. su aportación en la realización de los sondeos y al Ayuntamiento de Gijón la financiación de la investigación (proyectos FUO-EM-403-10 y FUO-EM-342-09)Peer Reviewe

Elaboració de recursos multimèdia per a l'ensenyament/aprenentatge en anglès en graus tecnològics

En aquest article es presenta el procés d'elaboració de recursos per a la impartició d'assignatures en anglès a l'ensenyament superior. Els principals resultats són les aplicacions Class-Talk i Multilingual Formulae, obertes a la comunitat educativa interessada en AICLE. Class-talk (http://www.upc.edu/slt/classtalk) conté la fraseologia docent d'us habitual. Multilingual Formulae (https://mformulae.epsem.upc.edu) conté la verbalització, oral i escrita, de fórmules i símbols matemàtics. Ambdós recursos s'adrecen a professorat i alumnat, d'origen català o internacional, involucrat en programes de millora de la competència lingüística; innovació docent al servei de la internacionalització de la universitat.Postprint (published version