Grasping Deficits and Adaptations in Adults with Stereo Vision Losses

PURPOSE. To examine the effects of permanent versus brief reductions in binocular stereo vision on reaching and grasping (prehension) skills. METHODS. The first experiment compared prehension proficiency in 20 normal and 20 adults with long-term stereo-deficiency (10 with coarse and 10 with undetectable disparity sensitivities) when using binocular vision or just the dominant or nondominant eye. The second experiment examined effects of temporarily mimicking similar stereoacuity losses in normal adults, by placing defocusing low- or high-plus lenses over one eye, compared with their control (neutral lens) binocular performance. Kinematic and error measures of prehension planning and execution were quantified from movements of the subjects’ preferred hand recorded while they reached, precision-grasped, and lifted cylindrical objects (two sizes, four locations) on 40 to 48 trials under each viewing condition. RESULTS. Performance was faster and more accurate with normal compared with reduced binocular vision and least accomplished under monocular conditions. Movement durations were extended (up to ∼100 ms) whenever normal stereo vision was permanently (ANOVA P < 0.05) or briefly (ANOVA P < 0.001) reduced, with a doubling of error rates in executing the grasp (ANOVA P < 0.001). Binocular deficits in reaching occurred during its end phase (prolonged final approach, more velocity corrections, poorer coordination with object contact) and generally increased with the existing loss of disparity sensitivity. Binocular grasping was more uniformly impaired by stereoacuity loss and influenced by its duration. Adults with long-term stereo-deficiency showed increased variability in digit placement at initial object contact, and they adapted by prolonging (by ∼25%) the time spent subsequently applying their grasp (ANOVA P < 0.001). Brief stereoreductions caused systematic shifts in initial digit placement and two to three times more postcontact adjustments in grip position (ANOVA P < 0.01). CONCLUSIONS. High-grade binocular stereo vision is essential for skilled precision grasping. Reduced disparity sensitivity results in inaccurate grasp-point selection and greater reliance on nonvisual (somesthetic) information from object contact to control grip stability

The Stability of Heavy Objects with Multiple Contacts

In both robot grasping and robot locomotion, we wish to hold objects stably in the presence of gravity. We present a derivation of second-order stability conditions for a supported heavy object, employing the tool of Stratified Morse theory. We then apply these general results to the case of objects in the plane

Incorporating contact area in soft finger grasp models

The ability to grasp is a significant function of many robotic hands, whether it be for fruit-picking or manufacturing tasks. Two major theories often used in conjunction to model grasping are Nguyen’s theorem and Coulomb’s friction law. Both theories model the contact between the fingers and the object in grasp as at a point. This runs true when the fingers and the object in grasp are made of hard materials. However, when the fingers or the object are soft, they deform around each other and result in a contact area between the objects rather than a contact point. This contact area means that soft-fingered grasps can apply an additional moment to balance any external moments, of which hard fingers cannot. To account for this, the soft-fingered grasp is often modelled as a point contact with a moment about the normal direction. However, in scenarios where there is no external moment both soft and hard fingers would exert a moment of 0 Nm and their grasps are modelled identically using Nguyen’s theorem and Coulomb’s law. Since soft-fingered grasping has been used extensively due to its superiority to hard-fingered grasping it does give rise to the question of whether the soft fingered grasp should be modelled identically to the hard fingered grasp in this case. This research expands the current grasp models to better showcase the differences between soft and hard finger grasps. The approach taken to achieve this was to incorporate the contact area of soft fingered grasping into the contact model so as to highlight the differences between soft and hard fingers when grasping an object. The research utilised this expansion in the contact model employing Nguyen’s theorem and Coulomb’s friction law. Nguyen’s theorem is a condition that must be met for a grasp to be in force closure, while Coulomb’s friction model limits the forces that can be applied by a finger onto an object to being within a friction cone at a point for no slipping to occur. Both Nguyen’s and Coulomb’s theories model the friction cone of the soft finger at the centre-point of the area of contact. A set-of-cones theory was proposed, where the area of contact is comprised of many friction cones corresponding to the points that make up the contact area instead of a single friction cone. An experiment was devised for a two-fingered symmetrical grasp of a cylinder, where the maximum angle of contact above the horizontal before slipping was investigated. The set-of-cones approach was made into a resultant friction cone model. The resultant cone model and the original centre-point cone model were used to predict the forces due to Coulomb’s theory at the maximum angle of contact. The predictions were compared to the data obtained from the experiment. It was found that the resultant cone contact model predicted the normal force applied at the maximum angle before slipping more accurately than the centre-point contact model for both soft finger materials being investigated in this research. When the resultant cone model was translated for use in the prediction of a force closure grasp by using Nguyen’s theorem, the range of positions where the object was grasped in force closure increased as compared to when using the centre-point contact model. If further verified, the resultant cone model would be used for soft fingers while the centre-point model would be used for hard fingers. This extends the modelling of soft finger contact so as to illustrate the differences in the stability of a grasp between hard and soft finger contact

A stiffness-based quality measure for compliant grasps and fixtures

This paper presents a systematic approach to quantifying the effectiveness of compliant grasps and fixtures of an object. The approach is physically motivated and applies to the grasping of two- and three-dimensional objects by any number of fingers. The approach is based on a characterization of the frame-invariant features of a grasp or fixture stiffness matrix. In particular, we define a set of frame-invariant characteristic stiffness parameters, and provide physical and geometric interpretation for these parameters. Using a physically meaningful scheme to make the rotational and translational stiffness parameters comparable, we define a frame-invariant quality measure, which we call the stiffness quality measure. An example of a frictional grasp illustrates the effectiveness of the quality measure. We then consider the optimal grasping of frictionless polygonal objects by three and four fingers. Such frictionless grasps are useful in high-load fixturing applications, and their relative simplicity allows an efficient computation of the globally optimal finger arrangement. We compute the optimal finger arrangement in several examples, and use these examples to discuss properties that characterize the stiffness quality measure

Analysis and design of multi-arm robotic systems manipulating large objects.

by Ho Siu Yan.Thesis (M.Phil.)--Chinese University of Hong Kong, 1995.Includes bibliographical references (leaves 105-110).ACKNOWLEDGEMENT --- p.iABSTRACT --- p.iiNOMENCLATURE --- p.iiiTABLE OF CONTENTS --- p.vLIST OF FIGURES --- p.viiChapter 1 --- INTRODUCTION --- p.1Chapter 2 --- FORM-CLOSURE GRASP --- p.9Chapter 2.1 --- Condition for Form-closure Grasp --- p.9Chapter 2.2 --- Construction of Form-closure Grasp --- p.12Chapter 2.3 --- Configuration Stability of Form-closure Grasp --- p.28Chapter 2.4 --- Determination of Object Frame from a Form-closure Grasp --- p.33Chapter 3 --- DYNAMIC MODEL OF MULTI-ARM SYSTEMS HANDLING ONE OBJECT --- p.36Chapter 3.1 --- System Description --- p.36Chapter 3.2 --- Manipulator Dynamics --- p.37Chapter 3.3 --- Object Dynamics --- p.37Chapter 3.4 --- Contact Forces --- p.38Chapter 3.5 --- Kinematic Relations --- p.40Chapter 3.6 --- Overall System --- p.41Chapter 3.7 --- Constraint Space Matrices --- p.42Chapter 3.8 --- Motion Space Matrices --- p.48Chapter 3.9 --- General Joint Model --- p.54Chapter 4 --- FORWARD DYNAMICS OF MULTI-ARM SYSTEMS HANDLING ONE OBJECT --- p.65Chapter 4.1 --- Previous Works --- p.65Chapter 4.2 --- Modified Approach --- p.69Chapter 4.3 --- Constraint Violation Stabilization Method --- p.73Chapter 4.4 --- Computation Requirement of the Algorithm --- p.75Chapter 5 --- CONCLUSION --- p.78Chapter 5.1 --- Future Researches --- p.79APPENDICESChapter A --- PROOFS AND DISCUSSIONS RELATED TO CHAPTER TWO --- p.81Chapter B --- IMPLEMENTATION OF THE ALGORITHM FOR DETERMINING THE OBJECT FRAME FROM A FORM-CLOSURE GRASP --- p.95Chapter C --- EXPRESSING WRENCHES WITH ZERO-PITCH WRENCHES --- p.96Chapter D --- IMPLEMENTATION OF THE PROPOSED SIMULATION ALGORITHM --- p.98REFERENCES --- p.10

Dynamic whole-body motion generation under rigid contacts and other unilateral constraints

The most widely used technique for generating wholebody motions on a humanoid robot accounting for various tasks and constraints is inverse kinematics. Based on the task-function approach, this class of methods enables the coordination of robot movements to execute several tasks in parallel and account for the sensor feedback in real time, thanks to the low computation cost. To some extent, it also enables us to deal with some of the robot constraints (e.g., joint limits or visibility) and manage the quasi-static balance of the robot. In order to fully use the whole range of possible motions, this paper proposes extending the task-function approach to handle the full dynamics of the robot multibody along with any constraint written as equality or inequality of the state and control variables. The definition of multiple objectives is made possible by ordering them inside a strict hierarchy. Several models of contact with the environment can be implemented in the framework. We propose a reduced formulation of the multiple rigid planar contact that keeps a low computation cost. The efficiency of this approach is illustrated by presenting several multicontact dynamic motions in simulation and on the real HRP-2 robot

Intersegmental Coordination in the Kinematics of Prehension Movements of Macaques

The most popular model to explain how prehensile movements are organized assumes that they comprise two "components", the reaching component encoding information regarding the object's spatial location and the grasping component encoding information on the object's intrinsic properties such as size and shape. Comparative kinematic studies on grasping behavior in the humans and in macaques have been carried out to investigate the similarities and differences existing across the two species. Although these studies seem to favor the hypothesis that macaques and humans share a number of kinematic features it remains unclear how the reaching and grasping components are coordinated during prehension movements in free-ranging macaque monkeys. Twelve hours of video footage was filmed of the monkeys as they snatched food items from one another (i.e., snatching) or collect them in the absence of competitors (i.e., unconstrained). The video samples were analyzed frame-by-frame using digitization techniques developed to perform two-dimensional post-hoc kinematic analyses of the two types of actions. The results indicate that only for the snatching condition when the reaching variability increased there was an increase in the amplitude of maximum grip aperture. Besides, the start of a break-point along the deceleration phase of the velocity profile correlated with the time at which maximum grip aperture occurred. These findings suggest that macaques can spatially and temporally couple the reaching and the grasping components when there is pressure to act quickly. They offer a substantial contribution to the debate about the nature of how prehensile actions are programmed