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Pulley-block system as a differential mechanism: a Kinematics: Two degrees of freedom: Out of the three elements (two blocks and one pulley), any two can move independently at a time, while maintaining the string taut. Pulley displacement has to be the average of block displacements d 1 and d 2 . If one block stops moving, the other will move twice as fast as the pulley. b Statics: Starting from rest, if the blocks offer unequal resistances R 1 , R 2 to motion, the string tension T increases until the less resistant block starts moving. Pulling force is therefore, twice the lower resistance value. Assumptions: (i) Pulley axis friction is negligible. (ii) Rolling friction (no slip) between string and pulley is negligible compared to string tension T . (iii) String is very light to be considered massless. (iv) Pulley inertia force (either mass or acceleration) small compared to string tension T . Under these assumptions, string tension T remains uniform over its entire length, and the pulling force is twice the tension T . c Design: Configuration with minimum distance between pulley and a block: If d 1 , d 2 ≤ d, the minimum string length required is d + b + π b. d Design: Configuration with maximum displacement: Length l = d/2 + b. Hence, minimum space needed for the assembly = l + d = 3d/2 + b
Contexts in source publication
Context 1
... to [9], the basic elementary unit of this mechanism is shown in Fig. 6a, wherein blocks and strings represent fingers and artificial tendons, ...
Context 2
... unit has two degrees of freedom. Therefore, while moving the pulley, hindering the motion of one finger (block) does not prevent the motion of the other finger (block). At a given time instant, only the finger which offers lower resistance moves (Fig. 6b), when pulling force is just sufficient to cause uniform motion. In finger flexion, the tendon force (tendon tension) keeps increasing with flexion, in all fingers [10]. This means, as the finger with lower resistance flexes by some amount, its resistance increases, and can become higher than that of the other one. Then, the other ...
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This article presents a compact, portable fingertip-to-elbow hand exoskeleton (F-EL-EX) designed to assist in gross grasping activities involving hand opening and closing movements. The design mimics a biological tendon pulley system (TPS) for finger flexion, optimized for the maximum range of flexion while keeping bowstringing and maximum pulley stress under check. The exoskeleton finger integrates a jointless system of phalanges, designed with care to house the TPS while allowing unrestricted motion of the respective finger joints, each with variable centers of rotation. The exoskeleton is hybrid — fabricated with plastic, natural rubber, and metal, with individual or combination of materials used for different palm and forearm regions. Rigid components used for tendon routing help in modeling a relation between tendon excursion and flexion and provide high grasping force capabilities. The soft material on the palm region ensures retaining flexibility during grasping of objects with varied shapes and supports thumb carpometacarpal (CMC) adjustment. Compactness and portability are ensured through a sliding pulley based slack-tolerant differential mechanism (SPDM), driving all fingers with a single actuator and employing a separate actuator for the thumb. The experimental and functional results of the exoskeleton on a healthy subject demonstrate its adaptive, gross grasping abilities with everyday objects through power grasp, lateral pinch, and parallel extension. These findings encourage further exploration in clinical trials, especially for individuals with hand muscle weaknesses.
Exoskeletons and orthoses are wearable mobile systems providing mechanical benefits to users. Despite significant improvements in the last decades, the technology is not fully mature to be adopted for strenuous and non-programmed tasks. To accommodate this insufficiency, different aspects of this technology need to be analysed and improved. Numerous studies have tried to address some aspects of exoskeletons, e.g. mechanism design, intent prediction, and control scheme. However, most works have focused on a specific element of design or application without providing a comprehensive review framework. This study aims to analyse and survey the contributing aspects to this technology’s improvement and broad adoption. To address this, after introducing assistive devices and exoskeletons, the main design criteria will be investigated from both physical Human-Robot Interaction (HRI) perspectives. In order to establish an intelligent HRI strategy and enable intuitive control for users, cognitive HRI will be investigated after a brief introduction to various approaches to their control strategies. The study will be further developed by outlining several examples of known assistive devices in different categories. And some guidelines for exoskeleton selection and possible mitigation of current limitations will be discussed.
We present a kirigami-inspired design scheme for a robotic hand by 3D printable folds and cuts. The unique contribution is the printable flexible hand, which provides flexibility and maneuverability that is unavailable in rigid robotic systems. The integration of sensors in the robotic system enables force adjustment for robotic systems applicable in the future. The experimental results have shown that this design can perform everyday tasks through grasping and pinching different items. The fingers can bend from 40 to 100 degrees. Furthermore, the direct printable kirigami cuts and folds from soft elastic printable materials have significant potential for prosthetic devices. The printable kirigami design framework opens the possibility for future developments and modifications in numerous robotic applications.
The cable-driven mechanism is frequently used in many mechanical systems and usually requires one motor for each cable. If the system controls multiple cables, additional components in the motor system can increase the overall mass and volume, making the system less compact. Differential mechanisms had been proposed to pull multiple cables with a small number of motors but are limited because each cable cannot be controlled individually. In this letter, we proposed a switchable cable-driven (SCD) mechanism to control multiple cables individually using a single motor. An experimental testbed for the SCD mechanism was designed to control four output cables through a single input cable by integrating electrostatic clutches to the differential mechanism. The electrostatic clutch is an electrical brake system, and the differential mechanism is a mechanism to pull multiple cables simultaneously using movable pulleys. 3D semicircular electrode design was applied to maximize the friction force of the electrostatic clutch within a limited space, and controllable cables could be switched electrically without any interference between cables. We also verified the feasibility of the SCD mechanism through a simple experiment that demonstrated how the cable tension and stroke required for actual operation differ. When single or multiple brakes were activated in the testbed, the maximum input cable tension slightly increased compared to the no brake condition, but the input cable stroke considerably decreased with each increase in the number of brakes. Although this study had some limitations since it was the first to propose and verify the concept of the SCD mechanism, this novel mechanism can contribute to the miniaturization of the multi-cable-driven systems.
It is rare that existing prosthetic/orthotic designs are based on kinetostatics of a biological finger, especially its tendon- pulley system (TPS). Whether a biological TPS is optimal for use as a reference, say for design purposes, and if so in what sense, is also relatively unknown. We expect an optimal TPS to yield high range of flexion while operating with lower tendon tension, bowstringing, and pulley stresses. To gain insight into the TPS designs, we present a parametric study which is then used to determine optimal TPS configurations for the flexor mechanism. A compliant, flexure-based computational model is developed and simulated using the pseudo rigid body method, with various combinations of pulley/tendon attachment point locations, pulley heights, and widths. Results suggest that three distinct types of TPS configurations corresponding to single stiff pulley, or two stiff pulleys, or one stiff and one flexible-inextensible pulley per phalange can be optimal. For a TPS configuration similar to a biological one, the distal pulleys on the proximal and intermediate phalanges need to be like flexible-inextensible string loops that effectively model the behavior of joint and cruciate pulleys. We reckon that a biological flexor TPS may have evolved to maximize flexion range with minimum possible actuation tension, bowstringing and pulley stress. Our findings may be useful in not only developing efficient hand devices, but also in improving TPS reconstruction surgery procedures.
