Mark R. Cutkosky

On grasp choice, grasp models, and the design of hands for manufacturing tasks

Current analytical models of grasping and manipulation with robotic hands contain simplifications and assumptions that limit their application to manufacturing environments. To evaluate these models, a study was undertaken of the grasps used by machinists in a small batch manufacturing operation. Based on the study, a taxonomy of grasps was constructed. An expert system was also developed to clarify the issues involved in human grasp choice. Comparisons of the grasp taxonomy, the expert system, and grasp-quality measures derived from the analytic models reveal that the analytic measures are useful for describing grasps in manufacturing tasks despite the limitations in the models. In addition, the grasp taxonomy provides insights for the design of versatile robotic hands for manufacturing. >

PACT: an experiment in integrating concurrent engineering systems

The Palo Alto Collaborative Testbed (PACT), a concurrent engineering infrastructure that encompasses multiple sites, subsystems, and disciplines, is discussed. The PACT systems include NVisage, a distributed knowledge-based integration environment for design tools; DME (Device Modeling Environment), a model formulation and simulation environment; Next-Cut, a mechanical design and process planning system; and Designworld, a digital electronics design, simulation, assembly, and testing system. The motivations for PACT and the significance of the approach for concurrent engineering is discussed. Initial experiments in distributed simulation and incremental redesign are reviewed, and PACTs agent-based architecture and lessons learned from the PACT experiments are described.<<ETX>>

Frictional adhesion: a new angle on gecko attachment

SUMMARY Directional arrays of branched microscopic setae constitute a dry adhesive on the toes of pad-bearing geckos, natures supreme climbers. Geckos are easily and rapidly able to detach their toes as they climb. There are two known mechanisms of detachment: (1) on the microscale, the seta detaches when the shaft reaches a critical angle with the substrate, and (2) on the macroscale, geckos hyperextend their toes, apparently peeling like tape. This raises the question of how geckos prevent detachment while inverted on the ceiling, where body weight should cause toes to peel and setal angles to increase. Geckos use opposing feet and toes while inverted, possibly to maintain shear forces that prevent detachment of setae or peeling of toes. If detachment occurs by macroscale peeling of toes, the peel angle should monotonically decrease with applied force. In contrast, if adhesive force is limited by microscale detachment of setae at a critical angle, the toe detachment angle should be independent of applied force. We tested the hypothesis that adhesion is increased by shear force in isolated setal arrays and live gecko toes. We also tested the corollary hypotheses that (1) adhesion in toes and arrays is limited as on the microscale by a critical angle, or (2) on the macroscale by adhesive strength as predicted for adhesive tapes. We found that adhesion depended directly on shear force, and was independent of detachment angle. Therefore we reject the hypothesis that gecko toes peel like tape. The linear relation between adhesion and shear force is consistent with a critical angle of release in live gecko toes and isolated setal arrays, and also with our prior observations of single setae. We introduced a new model, frictional adhesion, for gecko pad attachment and compared it to existing models of adhesive contacts. In an analysis of clinging stability of a gecko on an inclined plane each adhesive model predicted a different force control strategy. The frictional adhesion model provides an explanation for the very low detachment forces observed in climbing geckos that does not depend on toe peeling.

Smooth Vertical Surface Climbing With Directional Adhesion

Stickybot is a bioinspired robot that climbs smooth vertical surfaces such as glass, plastic, and ceramic tile at 4 cm/s. The robot employs several design principles adapted from the gecko including a hierarchy of compliant structures, directional adhesion, and control of tangential contact forces to achieve control of adhesion. We describe the design and fabrication methods used to create underactuated, multimaterial structures that conform to surfaces over a range of length scales from centimeters to micrometers. At the finest scale, the undersides of Stickybots toes are covered with arrays of small, angled polymer stalks. Like the directional adhesive structures used by geckos, they readily adhere when pulled tangentially from the tips of the toes toward the ankles; when pulled in the opposite direction, they release. Working in combination with the compliant structures and directional adhesion is a force control strategy that balances forces among the feet and promotes smooth attachment and detachment of the toes.

Computing and controlling compliance of a robotic hand

The authors express the compliance of the grasp of a robotic hand as a function of grasp geometry, contact conditions between the fingers and the grasped object, and mechanical properties of the fingers. It is argued that the effects of structural compliance and small changes in the grasp geometry should be included in the computation. Factors are then examined that can lead a grasp to become unstable, independently of whether it satisfies force closure. Finally, the authors examine the reverse problem of how to specify servo gains at the joints of a robotic hand so as to achieve, as nearly as possible, a desired overall grasp compliance. It is shown that coupling between the joints of different fingers is useful in this context. >

An overview of dexterous manipulation

Presents an overview of research in dexterous manipulation. We first define robotic dexterous manipulation in comparison to traditional robotics and human manipulation. Next, kinematics, contact types and forces are used to formulate the dexterous manipulation problem. Dexterous motion planning is described, which includes grasp planning and quality measures. We look at mid- and low-level control frameworks, and then compare manipulation versus exploration. Finally, we list accomplishments in the different areas of dexterous manipulation research, and highlight important areas for future work.

Fast and Robust

Robots to date lack the robustness and performance of even the simplest animals when operating in unstructured environments. This observation has prompted an interest in biomimetic robots that take design inspiration from biology. However, even biomimetic designs are compromised by the complexity and fragility that result from using traditional engineering materials and manufacturing methods. We argue that biomimetic design must be combined with structures that mimic the way biological structures are composed, with embedded actuators and sensors and spatially-varied materials. This proposition is made possible by a layered-manufacturing technology called shape deposition manufacturing (SDM). We present a family of hexapedal robots whose functional biomimetic design is made possible by SDMs unique capabilities and whose fast (over four body-lengths per second) and robust (traversal over hip-height obstacles) performance begins to compare to that seen in nature. We describe the design and fabrication of the robots and we present the results of experiments that focus on their performance and locomotion dynamics.

Estimating friction using incipient slip sensing during a manipulation task

A scheme is presented by which a manipulator can use dynamic tactile sensing to detect when it is about to lose hold of a grasped object. By detecting localized slips on the gripping surface which precede gross slip, the controller can modify the grasp force to prevent the object from slipping. Also, by monitoring normal and tangential forces at the contact when these incipient slip signals occur, the controller obtains an accurate estimate of the friction coefficient which can then be used during the manipulation task. Accurate knowledge of the friction coefficient is essential when grasping fragile objects or manipulating with sliding.<<ETX>>

A traction stress sensor array for use in high-resolution robotic tactile imaging

A high resolution large-area array capable of resolving the three independent components of a 2D triaxial contact stress profile has been developed. The array, composed of 4096 (64/spl times/64) individual stress sensing elements, was constructed with a fully CMOS-compatible fabrication process, allowing integration of the sensing structures with digital control circuitry. The individual array elements have been shown to demonstrate linear responses to both applied normal stress (1.59 mV/kPa, 0-35 kPa) and applied shear stress (0.32 mV/kPa, 0-60 kPa). A spatial resolution comparable to the spacing of the papillary ridges of the human dermis (/spl ap/300 /spl mu/m) has been achieved within the 1.92/spl times/1.92 cm active sensing area of the array. Descriptions of the sensor structure, the required signal conditioning, and the array architecture are presented in this paper. The results of electrical and mechanical characterization studies are also outlined.

Human grasp choice and robotic grasp analysis

In studying grasping and manipulation we find two very different approaches to the subject: knowledge-based approaches based primarily on empirical studies of human grasping and manipulation, and analytical approaches based primarily on physical models of the manipulation process. This chapter begins with a review of studies of human grasping, in particular our development of a grasp taxonomy and an expert system for predicting human grasp choice. These studies show how object geometry and task requirements (as well as hand capabilities and tactile sensing) combine to dictate grasp choice. We then consider analytic models of grasping and manipulation with robotic hands. To keep the mathematics tractable, these models require numerous simplifications which restrict their generality. Despite their differences, the two approaches can be correlated. This provides insight into why people grasp and manipulate objects as they do, and suggests different approaches for robotic grasp and manipulation planning. The results also bear upon such issues such as object representation and hand design.