A novel design of shape-memory alloy-based soft robotic gripper with variable stiffness

Mingfang Liu,Zhirui Zhao,Lina Hao,Wei Zhang

doi:10.1177/1729881420907813

Mingfang Liu, Zhirui Zhao + Show 2 more

Open Access

https://doi.org/10.1177/1729881420907813

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Abstract

Soft robotic grippers with compliance have great superiority in grabbing objects with irregular shape or fragility compared with traditional rigid grippers. The main limitations of such systems are small grasping force resulted from properties of soft actuators and lacking variable stiffness of soft robotic grippers, which prevent them from a larger wide range of applications. This article proposes a shape-memory alloy (SMA)-based soft gripper with variable stiffness composed of three robotic fingers for grasping compliantly at low stiffness and holding robustly at high stiffness. Each robotic finger mainly consisted of stiff parts and two variable stiffness joints is installed on the base with a specific angle. The paraffin as a variable stiffness material in the joint can be heated or cooled to change the stiffness of the robotic fingers. Results of experiments have shown that a single robotic finger can approximately achieve 18-fold stiffness enhancement. Each finger with two joints can actively achieve multiple postures by both changing the corresponding stiffness of joints and actuating the SMA wire. Based on these principles, the gripper can be applied to grasp objects with different shapes and a large range of weights, and the maximum grasping force of the gripper is increased to about 10 times using the variable stiffness joints. The final experiment is conducted to validate variable stiffness of the proposed soft grippers grasping an object.

Highlights

Soft robots composed of soft structures with the potential to exhibit unprecedented adaptation, sensitivity, and agility can achieve safe interactions between human and robot compared with traditional rigid robots.[1]
austenite start temperature (As) soft pneumatic gripper was successfully applied to collect multiple irregular shaped seafood animals of different sizes and stiffness at the bottom of the natural oceanic environment[2]; a soft pneumatic gripper can recognize the sizes of objects and sort the objects with different sizes[3]; a soft gripper actuated by ionic polymer– metal composite can manipulate micro object less than 1 mm in diameter and macro-sized object within 10 mm diameter[4]; a shape-memory alloy (SMA)-based soft gripper can actively achieve multiple postures to grasp objects with
We propose a novel soft robotic gripper actuated by SMA wire with variable stiffness mechanism embedded in the robotic finger as the robotic finger joint composed of paraffin, elastomeric membrane, and resistance heating wire

Summary

Introduction

Soft robots composed of soft structures with the potential to exhibit unprecedented adaptation, sensitivity, and agility can achieve safe interactions between human and robot compared with traditional rigid robots.[1]. Deformable shapes and varying shapes with a large range of weight.[5] Usually, soft robotic grippers are actuated by different actuation techniques, for example, pneumatic actuation,[3,6,7,8] cable-driven actuation,[9,10] electroactive polymer (EAP, an abbreviation for electroactive polymers, which can change its shape or size in response to electrical stimulation.) actuation,[11,12,13] and SMA actuation.[14,15,16,17,18,19] Among these methods, pneumatic actuation is unsuitable for applications because the needs for air pumps and valves to produce the compressed gas for power increase the weight and the volume of robotic systems inevitably. Due to the merits of low noise, driving voltages, and small size, SMA wires are conveniently integrated with the robotic systems without auxiliary structure and large space for benefiting to optimize resources and costs

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