Júlia Borràs

Singularity-Invariant Leg Rearrangements in Stewart–Gough Platforms

This work presents a necessary and sufficient condition to define a singularity-invariant leg rearrangement, based on an affine relation between the squared leg lengths before and after the rearrangement. This condition is then specified for four rigid components that can occur in Stewart– Gough platforms, leading to the characterization of singularity-invariant leg rearrangements on all of them.

Analyzing whole-body pose transitions in multi-contact motions

When executing whole-body motions, humans are able to use a large variety of support poses which not only utilize the feet, but also hands, knees and elbows to enhance stability. While there are many works analyzing the transitions involved in walking, very few works analyze human motion where more complex supports occur. In this work, we analyze complex support pose transitions in human motion involving locomotion and manipulation tasks (loco-manipulation). We have applied a method for the detection of human support contacts from motion capture data to a large-scale dataset of loco-manipulation motions involving multi-contact supports, providing a semantic representation of them. Our results provide a statistical analysis of the used support poses, their transitions and the time spent in each of them. In addition, our data partially validates our taxonomy of whole-body support poses presented in our previous work. We believe that this work extends our understanding of human motion for humanoids, with a long-term objective of developing methods for autonomous multi-contact motion planning.

New Geometric Approaches to the Analysis and Design of Stewart–Gough Platforms

In general, rearranging the legs of a Stewart-Gough platform, i.e., changing the locations of its leg attachments, modifies the platform singularity locus in a rather unexpected way. Nevertheless, some leg rearrangements have been recently found to leave singularities invariant. Identification of such rearrangements is useful not only for the kinematic analysis of the platforms, but also as a tool to redesign manipulators avoiding the implementation of multiple spherical joints, which are difficult to construct and have a small motion range. In this study, a summary of these singularity-invariant leg rearrangements is presented, and their practical implications are illustrated with several examples including well-known architectures.

A parallel robots framework to study precision grasping and dexterous manipulation

Dexterous, within-hand manipulation, in which an object generally held in the fingertips is manipulated by the fingers, shares many similarities to parallel robot configurations. This paper shows how to apply a mathematical framework commonly used for parallel robots to study the kinetostatic properties of hands manipulating objects using precision grasps, considering compliance and underactuation in the joints, without requiring the use of the grasp matrix. The proposed framework is suitable for any hand, but we focus on underactuated hands. We show how the natural redundancy present in fully-actuated hands can be eliminated using underactuation, leading to simplified non-redundant systems that are easier to control. We primarily focus our efforts on introducing and describing the theoretical framework, and follow this with an example application using a three-fingered underactuated hand. For this example, we define the feasible workspace as the subspace of the kinematic workspace for which the hand can accomplish a grasp, and we study how the compliance, rest angles, and joint coupling in the fingers can be designed to increase the size of this feasible workspace.

Extracting whole-body affordances from multimodal exploration

Humanoid robots that have to operate in cluttered and unstructured environments, such as man-made and natural disaster scenarios, require sophisticated sensorimotor capabilities. A crucial prerequisite for the successful execution of whole-body locomotion and manipulation tasks in such environments is the perception of the environment and the extraction of associated environmental affordances, i.e. the action possibilities of the robot in the environment, in order to generate whole-body locomotion and manipulation actions. We believe that such a coupling between perception and action could be a key to substantially increase the flexibility of humanoid robots. In this paper, we present an approach for the generation of whole-body locomotion and manipulation actions based on the affordances associated with environmental elements in the scene which are extracted via multimodal exploration. Based on the properties of detected environmental primitives and the estimated empty space in the scene, we propose methods to generate hypotheses for feasible whole-body actions while taking into account additional task constraints such as manipulability and balance. We combine visual and inertial sensing modalities by means of a novel depth model for generating segmented and categorized geometric primitives. A rule-based system is then incorporated to assign affordance hypotheses to these primitives. Finally, precomputed whole-body manipulability and stability maps are used for filtering affordances that are out of reach and for identifying the most promising locations for the action execution. We tested the developed methods in different scenes, unknown to the robot, demonstrating how reasonable the generated affordance hypotheses are.

Extraction of Whole-Body Affordances for Loco-Manipulation Tasks

Humanoid robots that have to operate in cluttered and unstructured environments, such as man-made and natural disaster scenarios, require sophisticated sensorimotor capabilities. A crucial prerequisite for the successful execution of whole-body locomotion and manipulation tasks in such environments is the perception of the environment and the extraction of associated environmental affordances, i.e., the action possibilities of the robot in the environment. We believe that such a coupling between perception and action could be a key to substantially increase the flexibility of humanoid robots. In this paper, we approach the affordance-based generation of whole-body actions for stable locomotion and manipulation. We incorporate a rule-based system to assign affordance hypotheses to visually perceived environmental primitives in the scene. These hypotheses are then filtered using extended reachability maps that carry stability information, for identifying reachable affordance hypotheses. We then formulate the hypotheses in terms of a constrained inverse kinematics problem in order to find whole-body configurations that utilize a chosen set of hypotheses. The proposed methods are implemented and tested in simulated environments based on RGB-D scans as well as on a real robotic platform.

Analyzing dexterous hands using a parallel robots framework

Dexterous, within-hand manipulation, in which an object held in the fingertips is manipulated by the fingers, shares many similarities with parallel robots. However, their mathematical formulations appear to be substantially different. This paper introduces a formulation typical from parallel manipulators to model the kinetostatics of a hand-plus-object system, including the fingertip forces formulation to describe a feasible grasp. The framework also includes compliance in the joint and considers pulling cable transmission mechanisms to model underactuated hands. The resultant static equilibrium equations are equivalent to the typical grasping formulation, but the involved matrices are different, allowing the interpretation of the resulting Jacobian matrix in terms of wrenches exerted by the joints.We primarily focus our efforts on describing in detail the theoretical framework, and follow this with an example application using a three-fingered underactuated hand. We show how the natural redundancy present in fully-actuated hands can be eliminated using underactuation, leading to simplified non-redundant systems that are easier to control. For the studied hand, we show how to use the framework to analyze the design parameters involved in the underactuation and their relationship with the resultant feasible workspace where the object can be manipulated.

On

Any set of two legs in a Gough-Stewart platform sharing an attachment is defined as a Delta component. This component links a point in the platform (base) to a line in the base (platform). Thus, if the two legs, which are involved in a Delta component, are rearranged without altering the location of the line and the point in their base and platform local reference frames, the singularity locus of the Gough-Stewart platform remains the same, provided that no architectural singularities are introduced. Such leg rearrangements are defined as Delta-transforms, and they can be applied sequentially and simultaneously. Although it may seem counterintuitive at first glance, the rearrangement of legs using simultaneous Delta-transforms does not necessarily lead to leg configurations containing a Delta component. As a consequence, the application of Delta-transforms reveals itself as a simple, yet powerful, technique for the kinematic analysis of large families of Gough-Stewart platforms. It is also shown that these transforms shed new light on the characterization of architectural singularities and their associated self-motions.Any set of two legs in a Gough-Stewart platform sharing an attachment is defined as a Delta component. This component links a point in the platform (base) to a line in the base (platform). Thus, if the two legs, which are involved in a Delta component, are rearranged without altering the location of the line and the point in their base and platform local reference frames, the singularity locus of the Gough-Stewart platform remains the same, provided that no architectural singularities are introduced. Such leg rearrangements are defined as Delta-transforms, and they can be applied sequentially and simultaneously. Although it may seem counterintuitive at first glance, the rearrangement of legs using simultaneous Delta-transforms does not necessarily lead to leg configurations containing a Delta component. As a consequence, the application of Delta-transforms reveals itself as a simple, yet powerful, technique for the kinematic analysis of large families of Gough-Stewart platforms. It is also shown that these transforms shed new light on the characterization of architectural singularities and their associated self-motions.

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Exploiting interaction with the environment is a promising and powerful way to enhance stability of humanoid robots and robustness while executing locomotion and manipulation tasks. Recently some works have started to show advances in this direction considering humanoid locomotion with multi-contacts, but to be able to fully develop such abilities in a more autonomous way, we need to first understand and classify the variety of possible poses a humanoid robot can achieve to balance. To this end, we propose the adaptation of a successful idea widely used in the field of robot grasping to the field of humanoid balance with multi-contacts: a whole-body pose taxonomy classifying the set of whole-body robot configurations that use the environment to enhance stability. We have revised criteria of classification used to develop grasping taxonomies, focusing on structuring and simplifying the large number of possible poses the human body can adopt. We propose a taxonomy with 46 poses, containing three main categories, considering number and type of supports as well as possible transitions between poses. The taxonomy induces a classification of motion primitives based on the pose used for support, and a set of rules to store and generate new motions. We present preliminary results that apply known segmentation techniques to motion data from the KIT whole-body motion database. Using motion capture data with multi-contacts, we can identify support poses providing a segmentation that can distinguish between locomotion and manipulation parts of an action.

-Transforms

Many robotic hands use compliant joints because they provide several advantages when interacting with objects in unknown environments, but they also modify the relation between external and internal forces and vary the reachable workspace. This work proposes a detailed study of how compliant joints modify the statics of hands, from the point of view of parallel manipulators. The chosen mathematical framework clarifies the role of joint compliance and its effect on the manipulator performance. This framework is then used in an example application to quantify the reduction/increase of torque exerted by the active joints due to the influence of the passive compliant ones for a three fingered hand.