A simple design rule for 1st order form-closure of underactuated hands

The property of form-closure of a grasp, as generally defined in the literature, is based on the assumption that contact points between the hand and the object are fixed in space. However, this assumption is false when considering a grasp exerted by an underactuated hand, since in this case, it is not possible to control the position of each phalanx independently. In spite of researchers' interest in studying form-closure, none of the available published work on this subject takes into consideration the particular kinematics of underactuated hands. Actually, there are few available tools to qualify or quantify the stability of a grasp exerted by an underactuated hand, thus the design of underactuated hands mostly results from an intuitive approach. This paper aims to reduce this gap. <br><br> A classification of underactuated hands is proposed, based on the expression of contact forces. This highlights the influence of non-backdrivable mechanisms introduced in the transmission of the closing motion of the hand on the stability of the grasp. The way to extend the original definition of form-closure to underactuated grasps is illustrated. A more general definition is formulated, which checks the stability of the set "object + hand". Using this new definition, a simple rule is proposed for designing a hand capable of achieving 1st order form-closed grasps. <br><br> <i>This paper was presented at the IFToMM/ASME International Workshop on Underactuated Grasping (UG2010), 19 August 2010, Montréal, Canada.</i&gt

Master of Science

thesisAutonomous and teleoperated flying robots capable of perch-and-stare are desirable for reconnaissance missions. Current solutions for perch-and-stare applications utilize various methods to enable aircraft to land on a limited set of surfaces that are typically horizontal or vertical planes. Motivated by the fact that songbirds are able to sleep in trees, without requiring active muscle control to stay perched, the research presented here details a concept that allows for passive perching of rotorcraft on a variety of surfaces. This thesis presents two prototype iterations, where perching is accomplished through the integration of two components: a compliant, underactuated gripping foot and a collapsing leg mechanism that converts aircraft weight into tendon tension in order to passively actuate the foot. This thesis presents the design process and analysis of the mechanisms. Additionally, stability tests were performed on the second prototype, attached to a quadrotor, that detail the versatility of the system and ability of the system to support external moments. The results show promise that it is possible to passively perch a rotorcraft on multiple surfaces and support reasonable environmental disturbances

Design of an affordable anthropomorphic mechanical prosthetic hand

Includes bibliographical references.This dissertation outlines the conceptualisation, design, manufacture, assembly and experimental testing of an affordable anthropomorphic mechanical hand prosthesis. In many countries, upper-limb amputees lack access to prosthetic hand devices. Furthermore, currently available mechanical devices require a large amount of effort to actuate; fatiguing and frustrating patients who have no other alternative but to use them. Consequently, a need has arisen to provide a mechanical device that is affordable enough to be accessible to low and middle-income patients, is functional enough to allow users to easily perform their Activities of Daily Living (ADLs), and is aesthetically appealing enough to ensure that patients feel comfortable and confident when wearing it. Concept solutions of several mechanisms were identified and evaluated from which the final design was selected. Analytical force analysis was used to generate a mathematical model to analyse the response of each dynamic member in the hand. A linear relationship between the input-force and applied grasp-forces of the hand was identified. Finite Element Analysis (FEA) used to investigate the lateral and hyperextensive loading limits of the phalanges, generated results that corresponded well to the experimental outcomes. Amongst the utilised actuation mechanisms (levers, pulleys, tendon-wires, bearings and springs), the tendon-wires were of concern due to their repetitive tensile loading and relative movement with the phalanges. Tensile testing of various tendon-wires and endurance testing of the phalangeal tendon-channels, yielded a combination which surpassed the infinite life requirement of 1,200,000 loading cycles; with carbon-nylon contact wearing at the lowest rate as confirmed by gravimetric tests in accordance with ASTM F2025 (2000). Manufacture of the hand used rapid prototyping in combination with traditional machining methods and standard components, enabling a fully-assembled cost of R 11,628.37; below the required R 18,000 limit. Various power and precision grasping configurations were achieved and the contact forces satisfactorily maintained, using the hand’s built-in locking mechanism. Feedback gathered from the prosthetist and patients suggested making slight alterations to the hand’s aesthetics and to address minor functional challenges, such as the control of the closing trajectory for precision grasps