Design and experimental study of a passive power-source-free stiffness-self-adjustable mechanism

doi:10.1007/s11465-020-0604-4

Front. Mech. Eng.

2021, Vol. 16

Issue (1) : 32-45 https://doi.org/10.1007/s11465-020-0604-4

RESEARCH ARTICLE

Design and experimental study of a passive power-source-free stiffness-self-adjustable mechanism

Yuwang LIU¹(

), Dongqi WANG¹, Shangkui YANG¹, Jinguo LIU¹, Guangbo HAO^1,²(

)

¹. State Key Laboratory of Robotics, Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016, China; Institutes for Robotics and Intelligent Manufacturing, Chinese Academy of Sciences, Shenyang 110169, China
². School of Engineering-Electrical and Electronic Engineering, University College Cork, Cork, Ireland

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Abstract

Passive variable stiffness joints have unique advantages over active variable stiffness joints and are currently eliciting increased attention. Existing passive variable stiffness joints rely mainly on sensors and special control algorithms, resulting in a bandwidth-limited response speed of the joint. We propose a new passive power-source-free stiffness-self-adjustable mechanism that can be used as the elbow joint of a robot arm. The new mechanism does not require special stiffness regulating motors or sensors and can realize large-range self-adaptive adjustment of stiffness in a purely mechanical manner. The variable stiffness mechanism can automatically adjust joint stiffness in accordance with the magnitude of the payload, and this adjustment is a successful imitation of the stiffness adjustment characteristics of the human elbow. The response speed is high because sensors and control algorithms are not needed. The variable stiffness principle is explained, and the design of the variable stiffness mechanism is analyzed. A prototype is fabricated, and the associated hardware is set up to validate the analytical stiffness model and design experimentally.

Keywords variable stiffness mechanism stiffness self-regulation bionic robot modeling

Corresponding Author(s): Yuwang LIU,Guangbo HAO

Just Accepted Date: 05 November 2020 Online First Date: 02 December 2020 Issue Date: 11 March 2021

Cite this article:

Yuwang LIU,Dongqi WANG,Shangkui YANG, et al. Design and experimental study of a passive power-source-free stiffness-self-adjustable mechanism[J]. Front. Mech. Eng., 2021, 16(1): 32-45.

URL:

https://academic.hep.com.cn/fme/EN/10.1007/s11465-020-0604-4
https://academic.hep.com.cn/fme/EN/Y2021/V16/I1/32

Tab.1 Typical passive variable stiffness joints

Fig.1 Passive power-source-free stiffness-self-adjustable mechanism for elbow joint.

Fig.2 Illustration of the principle of the stiffness-self-adjustable module.

Fig.3 Schematic of the regulation module of the external force point.

Fig.4 3D model of the variable stiffness mechanism. (a) Stiffness-self-adjustable module using the lever principle; (b) regulation module of the external force action point. 1?Point O; 2?Pillar; 3?Point A; 4?Synchronous pulley; 5?Synchronous belt; 6?Lever; 7?Limit mechanism; 8?Torsion spring; 9?Connection cover (point C in principle diagram); 10?Slider (point B in principle diagram); 11?Lower cover; 12?Connection cover; 13?Driving rod; 14?Connecting rod 1; 15?Driven rod; 16?Connecting rod 2; 17?Slider; 18?Guide; 19?Stretch spring.

Fig.5 Explosion diagram of the stiffness-self-adjustable mechanism.

Constraint	Expression	Constraint function
The transmission angle is not less than 60° [³³]	${∠ K B D ≥ 60 ° ∠ D E H ≥ 60 °$	${g 1 (x) = φ 3 − 240 ° g 2 (x) = arcsin l 0 − x 4 sin φ 2 x 5 − 4 π 3$
Existence condition of the crank of the four-linkage mechanism [³³]	${l 4 + l 0 ≤ l 2 + l 3 l 3 + l 4 ≤ l 0 + l 2 l 2 + l 4 ≤ l 0 + l 3 l 5 − l 6 < l 0 l 3 ≥ l 4 l 2 ≥ l 4$	${g 3 (x) = l 0 + x 3 − x 1 − x 2 g 4 (x) = x 2 + x 3 − x 1 − l 0 g 5 (x) = x 1 + x 3 − l 0 − x 2 g 6 (x) = x 4 − x 5 − l 0 g 7 (x) = x 3 − x 2 g 8 (x) = x 3 − x 1$
Range of rod size^a)	${0 mm < l 2 ≤ 56 mm 60 mm ≤ l 3 ≤ 120 mm 24 mm ≤ l 4 ≤ 50 mm 35 mm ≤ l 5 ≤ 50 mm 35 mm ≤ l 6 ≤ 40 mm$	${g 9 (x) = x 1 − 56 g 10 (x) = x 2 − 120 g 11 (x) = x 3 − 50 g 12 (x) = x 4 − 50 g 13 (x) = x 5 − 40 g 14 (x) = 60 − x 2 g 15 (x) = 24 − x 3 g 16 (x) = 35 − x 4 g 17 (x) = 35 − x 5$

Tab.2 Derivation of the constraint functions of the double-stage four-linkage mechanism

Tab.3 Comparison of the bar length before and after optimization

Fig.6 Schematic of the stiffness-self-adjustable module. (a) Stiffness-self-adjustable module model and (b) stiffness-self-adjustable module schematic.

Fig.7 Stiffness adjustment range of the stiffness-self-adjustable module.

Fig.8 Variation in joint stiffness with different stiffness coefficients.

Fig.9 External force at the end of the arm with different spring stretching.

Fig.10 Experimental platform of the prototype.

Fig.11 Validation of the stiffness model of the stiffness-self-adjustable module through experiments.

Fig.12 Step response for the position of the variable stiffness mechanism at loads of 0 and 1 kg.

Fig.13 Step response for the stiffness of the variable stiffness mechanism.

Fig.14 Status of the variable stiffness mechanism under three situations: External force of (a) 0 N, (b) 11.29 N, and (c) 64.13 N.

Fig.15 Interval of stiffness change after correction.

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