In need of a soft hand

Added: Tonny Dvorak - Date: 15.08.2021 13:05 - Views: 25127 - Clicks: 7655

To enable high forces with sufficiently high actuation speed at each fingertip, bundles of welded actuated SMA wires are used.

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In order to increase the compliance of each finger, flexible ts from superelastic SMA wires are inserted between each phalanx. The resulting system is a versatile hand prototype having intrinsically elastic fingers, which is capable to grasp several types of objects with a considerable force. The paper starts with the description of the finger hand de, along with practical considerations for the optimal placement of the superelastic SMA in the soft t.

The maximum achievable displacement of each finger phalanx is measured together with the phalanxes dynamic responsiveness at different power stimuli. Several force measurement are also realized at each finger phalanx. The versatility of the prototype is finally demonstrated by presenting several possible hand configurations while handling objects with different sizes and shapes.

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The de of a device able to reproduce all the functionalities of a human hand has been a challenge for many researchers since the late '. Up to date, the most In need of a soft hand prototypes in terms of force, speed, and versatility are deed with a rigid structure, and driven by electric motors. Despite the high performances achieved by these devices, they have encountered high rejection rates by users mainly due to the rigidity of the artificial hands, their heavy weight, and their noisy motion Kyberd et al.

Soft robotic hands represent a valuable alternative to this technology Dollar and Howe, The increased compliance of these devices confers an intrinsic robustness to manipulation and, at the same time, safe features which are highly desirable in human collaborative environments Cianchetti and Laschi, A first example of flexible and underactuated system is presented in Dollar and Howe In here, the authors introduce a two-finger gripper with reconfigurable ts, actuated by two DC motors.

In Deimel and Brocka three-fingered gripper made of silicone layers is presented. The structure of this prototype has no ts. The motion is directly related to the silicone deformation, induced by compressed air used for actuation. Inexpensive multi-fingered hands are introduced in Deimel and Brock, ; Homberg et al. The structure of these prototypes is realized thought an injection molding process, via a systematic and fast procedure.

The result is a hollow rubber finger, which can be deformed with compressed air. From the above discussion, it can be seen how most of the current soft hand prototypes are driven through pneumatic actuators. Even if pneumatic technology represents the most used and effective solution in soft robotics, it exhibits several limitations when used for hand prosthesis applications. The requirement of a compressed air source, together with its transportation systems e. Recently, a of researchers started using electric motors in combination with deformable finger ts to de soft prostheses.

The biggest advantage of this technology, compared to pneumatics, is represented by its compactness. One relevant example is exposed in Hussain et al. The actuation is provided by a DC motor, which transmits its motion to each t in the hand through tendons. In Catalano et al. We point out how, despite the high compliance and performances reached by those hand, their heavy weight and their noisy motion prevent them to be fully appreciated by amputee Trivedi et al.

Shape memory alloys SMA represent an alternative actuation technology which can overcome several limitations of the ly described solutions, in terms of noise, weight, and structure simplicity. These active transducers, typically consisting of Ni—Ti alloys, undergo a phase transformation when exposed to heat Lagoudas, When used in engineering applications, SMAs are typically shaped as thin wires. In this way, the heating can be effectively produced by sending an electric current through the wire. At the same time, the wire geometry can be easily integrate in small spaces.

Furthermore, if the electrical resistance of the SMA wire is measured during actuation, it can be used to reconstruct the wire length without using any additional sensor in the device Furst and Seelecke, Over the past years, several rigid hand structures driven by SMA wires have been developed Bundhoo et al.

However, so far, none of them can be considered advanced enough to support amputees in typical real life operations, mainly due to their limited performance in terms of dexterity and speed Kyberd et al. More recently, SMA technology has also been employed for deing soft robots as well as soft hand prototypes Verl et al. On the one hand, by analyzing the recent trends in literature, it can be noted that soft robotic hands represent a big improvement toward the development of a human-like hand device.

On the other hand, very few soft hand prototypes driven by SMA wires have been developed so far. We point out how, in contrast to the prototypes based on stiff ts, current deformable hands lack the gripping force needed to accomplish daily life operations Rodrigue et al. In order to overcome this issue, the present work introduces a new soft hand prototype actuated by SMA wires. A version of the three-fingered SMA actuated hand was presented in Simone et al. Despite showing remarkable performance, the hand developed in Simone et al. The novel hand prototype developed in this work permits to achieve higher versatility and higher forces with respect to the state-of-the-art prototypes Lee et al.

In order to obtain a lightweight and robust system, each wire is welded to a very small metal sheet, and then placed inside the structure along the phalanxes Scholtes et al. The use of bundles composed of many wires permits to achieve forces comparable to a human hand. In addition, by arranging the bundles in a protagonist—antagonist configuration and using optimized pulsed control scheme, higher actuation speed can be achieved in relation to standard SMA-spring mechanisms Fu et al.

In need of a soft hand described solution enables full opening-closing motion and a high versatility. In order to increase the overall structure compliance, flexible ts made of superelastic SMA wires are deed. This frictionless solid-state solution enables soft features in the prototype.

An easy-to-assemble structure, which does not require any complex handcraft work, is proposed and deed. In this way, the entire prototype manufacturing can be potentially integrated in a production chain. In order to highlight the prototype performances, several experiments are performed to evaluate the finger force, motion range, velocity, and grasping capabilities. We point out that an early version of this research has been presented in a conference paper Simone et al. More specifically, in Simone et al. The SMA bundles are realized through a winding process, generally used for manufacturing coils.

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Due to manufacturing problems and to the contact between each SMA wire in the bundles, the demonstrated motion of the prototype is extremely different from the human one Tubiana et al. This paper aims at improving and extending the work introduced in Simone et al. The remainder of the paper is organized as follows. Section De and Models describes the de of the full hand and its fingers. Section Fabrication provides information about the prototype fabrication procedure. In section and Discussion, several experiments are performed with the goal of evaluating the prototype performances in terms of force, motion, reactiveness, and grasping capabilities.

Finally, in section Conclusions, some concluding remarks and future research directions are outlined. The hand is the most complex and articulated part of the human body. It is composed of a 27 bones and 17 articulations, resulting in an overall of 23 degrees of freedom DOF Tubiana et al.

In order to de a bio-inspired prosthesis which is also functional and simple, the DOFs not strictly related to a grasping motion are neglected in this work Tubiana et al. The result is a modular 14 DOF prototype, depicted in Figure 1. It In need of a soft hand of a palm and five fingers, connected to the entire structure via screws. The structure de is dimensioned with respect to the average size of an adult human hand, as reported in Table 1and shaped according to the human hand morphology.

To ensure a lightweight structure, the hand and the fingers are left internally hollow. Each finger has a bio-inspired and modular structure. It is formed by three phalanxes and a stand, which represent the finger bone inside the palm, as shown in Figure 2. Figure 2. Finger structure. Each phalanx is depicted with a different color. In gray the central part, where the SMA wires are fixed.

The colored parts represent the external cover, which protects the SMA wires.

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Phalanxes with a hollow structure are deed, in order to increase the overall lightness. At the same time, to avoid undesired deformations, an arc shape pattern is used to de the finger cavities. Each finger has a modular structure composed of nine parts, i. In addition, the external parts also allow to constrain the finger bending and stretching motion.

Biologically, the human fingers are actuated by muscles located inside the arm and connected to each phalanx through tendons. Taking inspiration from the human biology, the SMA wires are collocated along the finger structure. When a SMA is actuated via an electric current, it contracts similarly to an artificial muscle or tendon, and a motion is induced in the finger phalanx. During the contraction, each SMA wire changes its electrical resistance according to the current length. Therefore, it is possible to relate its change in resistance to the finger motion and rotation Simone et al.

According to this behavior, the SMA wires act also as nerves.

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In general, a thick wire is able to exert high forces, but requires also a long cooling time. Thanks to a better surface per volume ratio, thinner wires require a shorter cooling time than thicker ones, but are not able to exert high forces. For these reasons, in order to achieve forces values in the same range of human ones Tubiana et al.

In need of a soft hand

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A Soft Five-Fingered Hand Actuated by Shape Memory Alloy Wires: De, Manufacturing, and Evaluation