Raúl Suárez

Grasp quality measures: review and performance

The correct grasp of objects is a key aspect for the right fulfillment of a given task. Obtaining a good grasp requires algorithms to automatically determine proper contact points on the object as well as proper hand configurations, especially when dexterous manipulation is desired, and the quantification of a good grasp requires the definition of suitable grasp quality measures. This article reviews the quality measures proposed in the literature to evaluate grasp quality. The quality measures are classified into two groups according to the main aspect they evaluate: location of contact points on the object and hand configuration. The approaches that combine different measures from the two previous groups to obtain a global quality measure are also reviewed, as well as some measures related to human hand studies and grasp performance. Several examples are presented to illustrate and compare the performance of the reviewed measures.

Computation of Independent Contact Regions for Grasping 3-D Objects

Precision grasp synthesis has received a lot of attention in past few last years. However, real mechanical hands can hardly assure that the fingers will precisely touch the object at the computed contact points. The concept of independent contact regions (ICRs) was introduced to provide robustness to finger positioning errors during an object grasping: A finger contact anywhere inside each of these regions assures a force-closure grasp, despite the exact contact position. This paper presents an efficient algorithm to compute ICRs with any number of frictionless or frictional contacts on the surface of any 3-D object. The proposed approach generates the independent regions by growing them around the contact points of a given starting grasp. A two-phase approach is provided to find a locally optimal force-closure grasp that serves as the starting grasp, considering as grasp quality measure the largest perturbation wrench that the grasp can resist, independently of the perturbation direction. The proposed method can also be applied to compute ICRs when several contacts are fixed beforehand. The approach has been implemented, and application examples are included to illustrate its performance.

Synthesizing grasp configurations with specified contact regions

In this paper we present a new method to solve the configuration problem on robotic hands: determining how a hand should be configured so as to grasp a given object in a specific way, characterized by a number of hand—object contacts to be satisfied. In contrast to previous algorithms given for the same purpose, the method presented here allows such contacts to be specified between free-form regions on the hand and object surfaces, and always returns a solution whenever one exists. The method is based on formulating the problem as a system of polynomial equations of special form, and then exploiting this form to isolate the solutions, using a numerical technique based on linear relaxations. The approach is general, in the sense that it can be applied to any grasping mechanism involving lower-pair joints, and it can accommodate as many hand—object contacts as required. Experiments are included that illustrate the performance of the method in the particular case of the Schunk Anthropomorphic hand.

Analysis and classification of multiple robot coordination methods

The use of multiple robots sharing a workspace can increase the productivity and the versatility of complex applications, making the existence of cells with several robots more common. On the other hand, when the robots are used to perform independent tasks in a shared workspace each one becomes a mobile obstacle for the other. Several methods have been proposed to deal with the problem of robot coordination in order to avoid collisions in these situations. The paper analyzes these methods identifying the basic tools used in each one and trying to unify the nomenclature. Illustrative works are listed and classified. The classification of the different approaches can be useful for future developments in the field.

Fast and Flexible Determination of Force-Closure Independent Regions to Grasp Polygonal Objects

Force-closure independent regions are parts of the object edges such that a grasp with a finger in each region ensures a force-closure grasp. These regions are useful to provide some robustness to the grasp in the presence of uncertainty as well as in grasp planning. Most of the approaches to the computation of these regions for N fingers work on the contact space, implying a N-dimensional problem. This paper presents a new approach to determine independent regions on polygonal objects considering N friction or frictionless contacts. The approach works on the object space, implying that it is always a two-dimensional problem and, since it is not necessary to compute all the force-closure space, it becomes a very fast approach. Besides, the approach is also flexible since constraints on the fingers placement can be easily introduced. Some graphical examples are included in the paper showing the simplicity of the methodology.

Geometrical approach for grasp synthesis on discretized 3d objects

Grasp synthesis on real 3D objects is a critical problem in grasp and manipulation planning. This paper presents a geometrical approach to compute force closure (FC) grasps, with or without friction and with any number of fingers. The objects surface is discretized in a cloud of points, so the algorithm is applicable to objects of any arbitrary shape. One or more FC grasps are obtained with a geometrical approach, which embeds the FC test in the algorithm to simplify achieving the force-closure property. This initial FC grasp may be improved with a complementary optimization algorithm. The grasp quality is measured considering the largest perturbation wrench that the grasp can resist with independence of the perturbation direction. The efficiency of both algorithms is illustrated through numerical examples.

Local search heuristics for the assembly line balancing problem with incompatibilities between tasks

This paper deals with the assembly line balancing problem considering incompatibilities between the tasks with the aim of: first minimizing the number of workstations, and then minimizing the cycle time for the minimum number of workstations. In order to solve the problem we propose the use of a greedy randomized adaptive search procedure obtained from the application of some classic heuristics based on priority rules, and a genetic algorithm that searches for the solution in the heuristic space. A computational experience is included to illustrate the performance of the proposed approach.

Efficient Determination of Four-Point Form-Closure Optimal Constraints of Polygonal Objects

This paper proposes a new and more efficient solution to the problem of determining optimal form-closure constraints of polygonal objects using four contacts. New grasp parameters are determined based only on the directions of the applied forces, which are then used to determine the optimal grasp. Given a set of contact edges, using an analytical procedure a solution that is either the optimal one or is very close to it is obtained (only in this second case an iterative procedure is needed to find a root of a nonlinear equation). This procedure is used for an efficient determination of the optimal grasp on the whole object. The algorithms have been implemented and numerical examples are shown.

Determining independent grasp regions on 2D discrete objects

This paper deals with the problem of determining independent grasps regions on the object boundary such that a four frictionless grasp with a contact point in each region assures a form-closure grasp independently of the exact position of the contact point. These regions are useful to provide some robustness to the grasp in front of finger positioning errors as well as in the design of fixtures. Given a discrete description of a 2D object, the methodology takes into account the uncertainty in the object description and it determines the independent regions without using hard iterative search procedures. The procedure has been implemented and an example of the proposed methodology is included in the paper.

Study of coordinated motions of the human hand for robotic applications

This paper presents an acquisition method that comprehensively looks for the mimic configurations of the human hand. The data obtained through this process is further analyzed, transformed, and then used to synthesize a reduced configuration space of a robot anthropomorphic hand. The method rely on a dimensionality reduction technique that provides a new basis of the full configuration space, from which one can select a subset of the vectors forming that basis, and finally obtaining a simpler configuration subspace. These vectors are called Principal Motion Directions, and represent the coordinated motions captured by a sensorized glove on a human hand and transferred to the robot hand. The characteristics and limitations of the subspace are discussed, as well as its application in several scenarios within robotics such as the motion planning of robot hands, where the subspace has been successfully implemented and executed.