Design of the passive joints of underactuated modular soft hands for fingertip trajectory tracking

In this letter, we propose a method to design tendon-driven underactuated hands whose fingertips can track a predefined trajectory, when actuated. We focus on passively compliant hands composed of deformable joints and rigid links. We first introduce a procedure to determine suitable joints stiffness and tendon routing, then a possible realization of a robotic underactuated finger is shown. The kinematic and kinetostatic analysis of a tendon-driven robotic finger is necessary to define the overall stiffness values of the finger joints. A structural analysis of the element constituting each passive joint allowed to define a relation between the stiffness and joint's main dimensional and material properties. We validated the proposed framework both in simulation and with experiments using the robotic Soft-SixthFinger as a case study. The Soft-SixthFinger is a wearable robot for grasping compensation in patients with a paretic hand. We demonstrated that different fingertip trajectories can be achieved when joint stiffness and tendon routing are properly designed. Moreover, we demonstrated that the device is able to grasp a wider set of objects when a specific finger flexion trajectory is designed. The proposed framework is general and can be applied to robotic hands with an arbitrary number of fingers and joints per finger. The modular approach furthermore allows the user to easily customize the hand according to specific tasks or trajectories

Structural Optimization of Adaptive Soft Fin Ray Fingers with Variable Stiffening Capability

Soft and adaptable grippers are desired for their ability to operate effectively in unstructured or dynamically changing environments, especially when interacting with delicate or deformable targets. However, utilizing soft bodies often comes at the expense of reduced carrying payload and limited performance in high-force applications. Hence, methods for achieving variable stiffness soft actuators are being investigated to broaden the applications of soft grippers. This paper investigates the structural optimization of adaptive soft fingers based on the Fin Ray® effect (Soft Fin Ray), featuring a passive stiffening mechanism that is enabled via layer jamming between deforming flexible ribs. A finite element model of the proposed Soft Fin Ray structure is developed and experimentally validated, with the aim of enhancing the layer jamming behavior for better grasping performance. The results showed that through structural optimization, initial contact forces before jamming can be minimized and final contact forces after jamming can be significantly enhanced, without downgrading the desired passive adaptation to objects. Thus, applications for Soft Fin Ray fingers can range from adaptive delicate grasping to high-force manipulation tasks

Replicating human hand synergies onto robotic hands: a review on software and hardware strategies

This review reports the principal solutions proposed in the literature to reduce the complexity of the control and of the design of robotic hands taking inspiration from the organization of the human brain. Several studies in neuroscience concerning the sensorimotor organization of the human hand proved that, despite the complexity of the hand, a few parameters can describe most of the variance in the patterns of configurations and movements. In other words, humans exploit a reduced set of parameters, known in the literature as synergies, to control their hands. In robotics, this dimensionality reduction can be achieved by coupling some of the degrees of freedom (DoFs) of the robotic hand, that results in a reduction of the needed inputs. Such coupling can be obtained at the software level, exploiting mapping algorithm to reproduce human hand organization, and at the hardware level, through either rigid or compliant physical couplings between the joints of the robotic hand. This paper reviews the main solutions proposed for both the approaches

Design and Analysis of a Body-Powered Underactuated Prosthetic Hand

As affordable and efficient 3-D printers became widely available, researchers are focusing on developing prosthetic hands that are reasonably priced and effective at the same time. By allowing anyone with a 3-D printer to build their body powered prosthetic hands, many people could build their own prosthetic hand. However, one of the major problems with the current designs is the user must bend and hold their wrist in an awkward position to grasp an object. The primary goal of this thesis is to present the design process and analysis of a mechanical operated, underactuated prosthetic hand with a novel ratcheting mechanism that locks the finger automatically at a desired position. The prosthetic hand is composed of the following components: a frame for the hand and forearm, ratcheting mechanism, finger mount, rack, pawl and stopper for ratchet, cable, springs, rigidly supporting finger and a compliant finger. The compliant finger was manufactured using shape deposition manufacturing. The joints of the finger were made using PMC 780, polyurethane material, and the finger pads were made of Polydimethylsiloxane(PDMS). To estimate how a compliant finger behaves on the actual system with the ratcheting mechanism and how much force is required to operate this finger, the preshaping analysis was conducted. The preshaping analysis data was verified by loading and unloading weights to the tendon cable and taking pictures of the finger each time the cable force was varied. Then, the pictures were processed using MATLAB image processing tools to calculate joint angles. Additionally, the contact force analysis was performed to determine the effects of the contact location and finger joint angles on the magnitude of contact force given the tension of the cable. Using the contact force analysis, it would be possible to estimate how much load the hand can hold. Finally, the hand was tested to hold various shapes of objects to prove how well it can grasp. Based on the experiment, the hand had a higher success rate of grasping objects that are lightweight (less than 500g) and cylindrical or circular shaped