Research Papers

Novel Design and Three-Dimensional Printing of Variable Stiffness Robotic Grippers

[+] Author and Article Information
Yang Yang

Department of Mechanical Engineering,
The University of Hong Kong,
Hong Kong 999077, China
e-mail: meyang@hku.hk

Yonghua Chen

Department of Mechanical Engineering,
The University of Hong Kong,
Hong Kong 999077, China
e-mail: yhchen@hku.hk

Ying Wei

Department of Mechanical Engineering,
The University of Hong Kong,
Hong Kong 999077, China
e-mail: u3001913@hku.hk

Yingtian Li

Department of Mechanical Engineering,
The University of Hong Kong,
Hong Kong 999077, China
e-mail: liyingtiantj@gmail.com

1Corresponding author.

Manuscript received September 30, 2015; final manuscript received May 9, 2016; published online September 8, 2016. Assoc. Editor: Satyandra K. Gupta.

J. Mechanisms Robotics 8(6), 061010 (Sep 08, 2016) (15 pages) Paper No: JMR-15-1286; doi: 10.1115/1.4033728 History: Received September 30, 2015; Revised May 09, 2016

In this paper, a novel robotic gripper design with variable stiffness is proposed and fabricated using a modified additive manufacturing (hereafter called 3D printing) process. The gripper is composed of two identical robotic fingers and each finger has three rotational degrees-of-freedom as inspired by human fingers. The finger design is composed of two materials: acrylonitrile butadiene styrene (ABS) for the bone segments and shape-memory polymer (SMP) for the finger joints. When the SMP joints are exposed to thermal energy and heated to above their glass transition temperature (Tg), the finger joints exhibit very small stiffness, thus allow easy bending by an external force. When there is no bending force, the finger will restore to its original shape thanks to SMP's shape recovering stress. The finger design is actuated by a pneumatics soft actuator. Fabrication of the proposed robotic finger is made possible by a modified 3D printing process. An analytical model is developed to represent the relationship between the soft actuator's air pressure and the finger's deflection angle. Furthermore, analytical modeling of the finger stiffness modulation is presented. Several experiments are conducted to validate the analytical models.

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Fig. 1

The variable stiffness finger design: (a) the human index finger, (b) schematic showing human finger's internal structure, and (c) components of the designed finger

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Fig. 2

Interlock features of the finger joint

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Fig. 3

Change of elastic modulus in shape-memory polymers

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Fig. 4

Schematic diagram of the finger design's motion: (a) initial position, (b) bending motion, and (c) restored position

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Fig. 5

Shape change of the finger design

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Fig. 6

Change of the maximum deflection angle Θmax: (a) variation of Θmax with different maximum extension length of L, (b) variation of Θmax with the change of l0, and (c) variation of Θmax with the change of D

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Fig. 7

Three-dimensional printing process of gripper

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Fig. 8

Manufactured gripper

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Fig. 9

Soft actuator design: (a) side view and (b) cross-sectional view (dimension unit: mm)

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Fig. 10

Schematic of soft actuator working principle

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Fig. 11

Three-dimensional printed mold and manufactured soft actuator: (a) top-half mold, (b) bottom-half mold, (c) long rod with different diameters as the core, and (d) fabricated soft actuator

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Fig. 12

Pressure (p)–deflection (θ) relationship of soft actuator: (a) initial position, p = 0 kPa, (b) p = 100 kPa, (c) p = 125 kPa, (d) p = 150 kPa, (e) p = 175 kPa, and (f) p = 200 kPa

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Fig. 13

Approximate line showing relationship between SMP's elastic modulus and temperature

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Fig. 14

Bending analysis of a module subjected to pressure p

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Fig. 15

Inflation of soft actuator (cross section): (a) primary shape and (b) inflated shape

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Fig. 16

The contact area of SMP and silicone rubber

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Fig. 17

Simplified model for analyzing crease SMP part extension

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Fig. 18

Finger stiffness modulation: (a) deflection of the finger under applied force Fs, (b) torque applied to joint i, (c) approximation of the joint using a cantilever beam, and (d) cross section of approximated cantilever beam

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Fig. 19

Finger stiffness variation with temperature

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Fig. 20

Schematic diagram for bending angle testing of the finger

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Fig. 21

The actual experimental setup

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Fig. 22

Pressure (p) versus deflection angle (Θ) for the finger

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Fig. 23

Schematic showing experimental principle of stiffness test setup

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Fig. 24

Experimental setup for measuring finger stiffness

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Fig. 25

Comparison of force–deflection at different temperatures (p = 0)

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Fig. 26

Comparison of force–deflection at different temperatures (p = 125 kPa)

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Fig. 27

Comparison of force–deflection relationship, with different air pressures p

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Fig. 28

Comparison of finger stiffness between theoretical modeling and experimental measurement

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Fig. 29

Grasping experiments: (a) primary position of the gripper, (b) grasp a bottle with air on and T > Tg, (c) bottle drop when air off, (d) grasp a bottle with air on and T > Tg, and (e) cool down below Tg and off the air, bottle is still hold tight




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