Posture-based motion planning: Applications to grasping.

Abstract

This article describes a model of motion planning instantiated for grasping. According to the model, one of the most important aspects of motion planning is establishing a constraint hierarchy--a set of prioritized requirements defining the task to be performed. For grasping, constraints include avoiding collisions with to-be-grasped objects and minimizing movement-related effort. These and other constraints are combined with instance retrieval (recall of stored postures) and instance generation (generation of new postures and movements to them) to simulate flexible prehension. Dynamic deadline setting is used to regulate termination of instance generation, and performance of more than one movement at a time with a single effector is used to permit obstacle avoidance. Old and new data are accounted for with the model.

Getting a grip: different actions and visual guidance of the thumb and finger in precision grasping

“…How the underlying visuomotor transformations are implemented, however, remain matters of widely differing opinion (e.g., see Smeets and Brenner 1999;commentaries by Various Authors 1999). Some argue that the thumb and finger have essentially equivalent functions in grip formation and are visually guided together (Jeannerod 1981;Hoff and Arbib 1993;Paulignan et al 1997;Rosenbaum et al 2001;Meulenbroek et al 2002) or independently (Smeets and Brenner 1999, while others suggest that only the thumb is selected for active guidance (Wing and Fraser, 1983;Haggard and Wing 1997;Galea et al 2001). The data obtained from our two experiments are mutually consistent with this latter suggestion.…”

“…The act is traditionally conceived as comprising two temporally coordinated components (Jeannerod 1981): transporting the hand -or more specifically, the wrist -towards the object's location (reach component); and opening the thumb and finger to a maximal aperture near the object before closing them together in a 'pincering' action onto optimal contact points for grip stability (grasp component). According to this 'two-visuomotor-channel' (2VMC) model (Jeannerod 1981) -and variants of it (Hoff and Arbib 1993;Paulignan et al 1997;Meulenbroek et al 2001;Rosenbaum et al 2001; Mon-Williams and Tresilian 2001) -visual computation of the position, size and geometry of the goal object is transformed into motor commands that cause the opposing digits to move relative to each other as functional grasping unit to achieve this.…”

Effect of speed manipulation on the control of aperture closure during reach-to-grasp movements

“…A digit model proposed by Smeets and Brenner (1999) is based on an idea that the thumb and index finger are independently controlled based on the minimum-jerk model (Flash and Hogan 1985) and move independently to final positions of an object being grasped perpendicularly to its surface. A model by Rosenbaum et al (2001) that finds an arm-and-hand goal posture, subsequently finds a trajectory in arm-hand posture space so that a collisions with an object being grasped would be avoided. While the Hoff and Arbib model specifies aperture closure initiation, the models by Smeets and Brenner and Rosenbaum et al achieve the aperture closure through a constraint of minimum jerk or movement cost reduction.…”

Grasping reveals visual misjudgements of shape

“…If we want to lift a vertically placed cylinder with a precision grip (only thumb and index finger), the grip axis should ideally pass through the cylinder's vertical axis above the centre of gravity. This leaves an infinite number of suitable grasp positions, which are further limited by constraints such as the relative comfort of the grasp (Rosenbaum et al 2001). If the cylinder's circumference is elliptical rather than circular, suitable grasp positions are also limited by the fact that the grip axis will not be perpendicular to the surface for most grip orientations.…”

“…The preceding arguments relate to systematic errors, but uncertainty about the cylinder's shape could also influence the grip orientation, assuming that subjects take the comfort of their grip orientation into account (Rosenbaum et al 2001;Elsinger and Rosenbaum 2003), because the more uncertain the physically optimal grip orientation is, the more the statistically optimal grip orientation will be biased towards the comfortable grip orientation (Ernst and Banks 2002;Knill 2005). To understand what is physically optimal we need to look at the physical constraints in more detail.…”