Attawith Sudsang

On computing four-finger equilibrium and force-closure grasps of polyhedral objects

This article addresses the problem of computing stable grasps of three-dimensional polyhedral objects. We consider the case of a hand equipped with four hard fingers and assume point contact with friction. We prove new necessary and sufficient conditions for equilibrium and force closure, and present a geometric characterization of all possible types of four-finger equilibrium grasps. We then focus on concurrent grasps, for which the lines of action of the four contact forces all intersect in a point. In this case, the equilibrium conditions are linear in the unknown grasp parameters, which reduces the problem of computing the stable grasp regions in configuration space to the problem of constructing the eight-dimensional projec tion of an ll-dimensinnal polytope. We present two projection methods: the first one uses a simple Gaussian elimination ap proach, while the second one relies on a novel output-sensitive contour-tracking algorithm. Finally, we use linear optimization within the valid configuration space regions to compute the maximal object regions where fingers can be positioned inde pendently while ensuring force closure. We have implemented the proposed approach and present several examples.

Grasping novel objects with depth segmentation

We consider the task of grasping novel objects and cleaning fairly cluttered tables with many novel objects. Recent successful approaches employ machine learning algorithms to identify points on the scene that the robot should grasp. In this paper, we show that the task can be significantly simplified by using segmentation, especially with depth information. A supervised localization method is employed to select graspable segments. We also propose a shape completion and grasp planner method which takes partial 3D information and plans the most stable grasping strategy. Extensive experiments on our robot demonstrate the effectiveness of our approach.

Motion planning for disc-shaped robots pushing a polygonal object in the plane

This paper addresses the problem of using three disc-shaped robots to manipulate a polygonal object in the plane in the presence of obstacles. The proposed approach is based on the computation of maximal discs (dubbed maximum independent capture discs, or MICaDs) where the robots can move independently while preventing the object from escaping their grasp. It is shown that, in the absence of obstacles, it is always possible to bring a polygonal object from any configuration to any other one with robot motions constrained to lie in a set of overlapping MICaDs. This approach is generalized to the case where obstacles are present by decomposing the corresponding motion planning task into the construction of a collision-free path for a modified form of the object, and the execution of this path by a sequence of simultaneous and independent robot motions within overlapping MICaDs. The proposed algorithm is guaranteed to generate a valid plan, provided a collision-free path exists for the modified form of the object. It has been implemented and experiments with Nomadic Scout mobile robots are presented.

New techniques for computing four-finger force-closure grasps of polyhedral objects

It was shown in Ponce et al. (1993) that four-finger force-closure grasps fall into three categories: concurrent, pencil, and regulus grasps. The authors propose new techniques for computing these three types of grasps. The authors have implemented them and present examples.

A new approach to motion planning for disc-shaped robots manipulating a polygonal object in the plane

This paper addresses the problem of using three disc-shaped robots to manipulate a polygonal object in the plane in the presence of obstacles. The proposed approach is based on the characterization of the maximal discs (maximum independent capture discs, or MICaDS) where the robots can move independently while preventing the object from escaping their grasp. It is shown that, in the absence of obstacles, it is always possible to bring a polygonal object from any configuration to any other one with robot motions constrained to lie in a set of overlapping MICaDS. A strategy for computing these motions is used in conjunction with an exact motion planner to devise an algorithm guaranteed to find a motion plan avoiding collisions with obstacles as long as a collision-free path exists for the object grown by the diameter of the robots plus some arbitrary positive number /spl epsiv/.

Grasping and In-Hand Manipulation: Geometry and Algorithms

Abstract. This paper addresses the problem of grasping and manipulating three-dimensional objects with a reconfigurable gripper that consists of two parallel plates whose distance can be adjusted by a computer-controlled actuator. The bottom plate is a bare plane, and the top plate carries a rectangular grid of actuated pins that can translate in discrete increments under computer control. We propose to use this gripper to immobilize objects through frictionless contacts with three of the pins and the bottom plate, and to manipulate an object within a grasp by planning the sequence of pin configurations that will bring this object to a desired position and orientation. A detailed analysis of the problem geometry in configuration space is used to devise simple and efficient algorithms for grasp and manipulation planning. The proposed approach has been implemented and preliminary simulation experiments are discussed.

Two-finger caging of concave polygon

An object is captured by a set of fingers when there exists no trajectory to bring the object arbitrarily far from the fingers. Object concavity is a special geometric property that allows objects to be captured with only few fingers. In particular, certain concave objects may be captured by appropriately placing two fingers close to some pair of opposite concave sections. This paper addresses the problem of computing all configurations of the fingers that are farthest away from each other while still capable of capturing the object. We propose an O(n2lg n) algorithm for this task and present preliminary results showing efficiency of the algorithm

Fast computation of 4-fingered force-closure grasps from surface points

This paper addresses the problem of computing frictional 4-fingered force-closure grasps of three-dimensional objects. The proposed approach searches for force-closure grasps from a collection of sampled points on the objects surface. Unlike most other works, the approach is not limited to the objects with a certain class of shapes. It can be applied to an object in any shape since only the objects surface points and corresponding surface normal at the points are needed. The efficiency of the approach arises from a heuristic for search space pruning which is based on ability to efficiently locate regions in three dimensional space where friction cones intersect and a randomized test for checking force-closure condition. The proposed approach is implemented and preliminary results are presented.

Computing All Force-Closure Grasps of 2D Objects from Contact Point Set

Given a set of n contact points on the boundary of a 2D object together with their contact normals and frictional coefficient, we present an output sensitive algorithm for computing all combinations of three points from this set that achieve force-closure under the frictional contact assumption. The proposed algorithm runs in O(n2 lg2n + K) where K is the number of different solutions. Preliminary implementation is described along with experimental results showing efficiency of the algorithm

Two-Finger Caging of Nonconvex Polytopes

In general, any object can be restricted or caged within a bounded region if we evenly place a sufficient number of fingers around the object. This naive approach often leads to inefficient utilization of fingers because only two fingers are sufficient to cage most nonconvex objects. In this paper, we propose an algorithm that identifies all the caging sets, i.e., set of two-finger placements that cage a given polytope representing the object. Whether a finger placement could cage the object can be queried efficiently from a structure generated by the algorithm. We implemented and tested the algorithm in the case of 2-D and 3-D objects.