A novel shape memory alloy actuated soft gripper imitated hand behavior

Abstract

The limited length shrinkage of shape memory alloy (SMA) wire seriously limits the motion range of SMA-based gripper. In this paper, a new soft finger without silicone gel was designed based on pre bent SMA wire, and the finger was back to its original shape by heating SMA wire, rather than relying only on heat exchange with the environment. Through imitating palm movement, a structure with adjustable spacing between fingers was made using SMA spring and rigid spring. The hook structure design at the fingertip can form self-locking to further improve the load capacity of gripper. Through the long thin rod model, the relationship of the initial pre bent angle on the bending angle and output force of the finger was analyzed. The stress-strain model of SMA spring was established for the selection of rigid spring. Three grasping modes were proposed to adapt to the weight of the objects. Through the test of the gripper, it was proved that the gripper had large bending amplitude, bending force, and response rate. The design provides a new idea for the lightweight design and convenient design of soft gripper based on SMA.

Key words： shape memory alloy (SMA); pre bent; wire; gripper; grasping mode; lightweight

Nomenclature

Abbreviations
DE	Dielectric elastomer
HASEL	Hydraulically amplified self-healing electrostatic
IPMC	Ionic polymer metal composite
PLA	Polylactic acid
PZT	Piezoelectric materials
SMA	Shape memory alloy
SMP	Shape memory polymer
Variables
a	Pre bent angle of the SMA wire
Af	Temperature at the finish of austenite transformation
As	Temperature at the start of austenite transformation
D	Outer diameter of SMA spring
E	Young’s modulus
E(ρ)	The second type of complete elliptic integral
F	Force of SMA spring
Fcr	Critical load
Fe	Fingertip force
I	Moment of inertia
K(ρ)	The first type of complete elliptic integral
kAB	Slope of line \overrightarrow{AB}
kCA	Slope of line \overrightarrow{CA}
L	Length of the straight SMA wire
l	Length of the pre bent SMA wire
M	Torque of the elastic rod
M1	Torque of the rigid skeleton
M2	Torque of the finger
Msma1	Torque of the pre bent SMA wire
Msma2	Torque of the straight SMA wire
r	Wire diameter of SMA spring
s	Arc length
T	Temperature
xa	x axis displacement generated by the free end of the SMA wire
xe	x axis displacement of the finger
y	Displacement of the elastic rod along the y axis
ya	y axis displacement generated by the free end of the SMA wire
ye	y axis displacement of the finger
ρ1	Bending curvature
θ	Bending angle of the pre bent SMA wire
θ1	Bending angle of the finger

Acknowledgements

The authors gratefully acknowledge the reviewers’ comments. This work was supported in part by the National Natural Science Foundation of China (Grant No. U1813221) and the National Key R&D Program of China (Grant No. 2019YFB1311200).

Electronic Supplementary Materials

The supplementary materials can be found in the online version of this article at https://doi.org/10.1007/s11465-022-0700-8 and are accessible to authorized users.

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