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

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.

Actuation Technologies for Soft Robot Grippers and Manipulators: A Review

“…Shape memory alloys (SMA) (Table 4) have also been used in soft grippers for actuation purposes owing to a number of merits including low noise, high force to weight ratio, small size [31], and the ease of their usage (Fig. 5).…”

Position and Force Control of a Twisted and Coiled Polymeric Actuator

“…1. In the control system, X ref is the desired contraction deformation of TCP, F ref is the desired contraction force of TCP, and T ref is the corresponding TCP temperature estimated by (3) or (4). The output is the actual contraction deformation or force of the TCP.…”

“…G P T is the transfer function described in (2), whose subscript denote the input and output variables. X(T) is the relationship between TCP temperature and strain described in (3), while F(T) is the relationship between TCP temperature and the force response F described in (4). For the control process, firstly, the desired temperature of the TCP actuator was tranfered from desired strain or force.…”

Shape-Adaptive Universal Soft Parallel Gripper for Delicate Grasping Using a Stiffness-Variable Composite Structure

“…performance improvement. A stiffness transition using low melting point alloy has also been used in robot grippers [14][15][16]. However, the slow response time owing to the heating and cooling of the alloy to change the stiffness is a shortcoming of such an approach.…”