Design of a Soft Composite Finger with Adjustable Joint Stiffness

Abstract

This research presents the design of a soft composite finger with tunable joint stiffness. The composite finger, made from two different types of silicone, has hybrid actuation principle combining tendon and pneumatic actuation schemes. Tendons control the finger shape in a prescribed direction to demonstrate discrete bending behavior due to different material moduli, similar to the human finger's discrete bending. Whereas, pneumatic actuation changes the stiffness of joints using air chambers. The feasibility of adjustable stiffness joints is proven using both the parallel spring-damper model and experiments, demonstrating the stiffening effect when pressurized. A set of experiments were also conducted on fingers with four different chamber designs to see the effect of chamber shape on stiffening and the discrete bending capability of the finger. These stiffened fingers lead to firm grasp as they constrain the object better and apply higher grasping force. The gripper made up of soft composite fingers can grasp objects of various sizes, shapes and in different orientations.

A review of soft manipulator research, applications, and opportunities

“…The stiffening methods: (a) dual‐stiffness origami gripper (Mintchev et al, 2018), (b) fiber jamming transition as a stiffening mechanism (Brancadoro et al, 2020), (c) variable stiffness continuum finger (Al Abeach et al, 2017), (d) bistable hybrid soft grippers (Tang et al, 2020), (e) soft composite finger with tunable joint stiffness (Shahid et al, 2019), (f) the snake‐like robot with variable stiffness actuators (D. Zhang et al, 2020), (g) leverage mechanism for stiffness adjustment (shape memory alloy, SMA) (C. Yang et al, 2020), (h) a variable‐stiffness tendril‐like soft robot(Must et al, 2019), (i) stiffness‐varying structure embedded with SMP (W. Wang et al, 2020), (j) shape memory actuators of stiffness modulation (W. Wang & Ahn, 2017) [Color figure can be viewed at wileyonlinelibrary.com]…”

“…Yin et al (Tang et al, 2020) presented a design of soft gripper with a wide range of tunable stiffness by using hybrid actuation of tendon‐spring and air pressure, as shown in Figure 14d. Similarly, Shahid et al (2019) developed a soft composite finger with tunable joint stiffness by using hybrid actuation of the cable‐tension and the air pressure, as shown in Figure 14e.…”

Flexible soft Pneumatic Bionic Hand Based on Multi-Jointed Structure

“…Single-chamber actuators have a more standard circular shape under pressure, relying on their own material compliance to passively adapt to the objects to be contacted [14,15], but cannot rely on the cooperation between multiple joints to realize sophisticated gestures and actively adapt to the surface of the object like a real human hand. Composite finger with two soft joints, and springs in the chamber, controlled by a mixture of tendon and pneumatic driven, capable of grasping objects stably under high acceleration conditions [16]. The fingers have two air chambers that are connected and supplied with air pressure from a single source, bending or extending at the same time, and are not fundamentally different from single-chamber actuators from the view of precision of control.…”

The bending control of the soft pneumatic fìnger

“…And this structure of the soft gripper embedded a series of chambers that can hold fragile objects like a human hand [5] . Other researchers design a soft composite finger with adjustable joint stiffness [6] . A soft robotic gripper combined pneumatic actuator and geckoinspired adhesives can not only realize the grasping function of existing soft gripper properties but also generate new capabilities [7] .…”