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and
Machine Theory
Mechanism and Machine Theory 43 (2008) 1332–1348
ate/mechmt
Safe link mechanism based on nonlinear stiffness for
collision safety
Jung-Jun Park a, Byeong-Sang Kim a, Jae-Bok Song a,*, Hong-Seok Kim b
a Department of Mechanical Engineering, Korea University, 5, Anam-dong, Seongbuk-gu, Seoul 136-713, South Korea
b Department of Robot Technology Development, Korea Institute of Industrial Technology, 1271 Sa-1 dong, Sangnok-gu, Ansan,
Kyeonggi-do 425-791, South Korea
Received 3 November 2006; received in revised form 2 October 2007; accepted 14 October 2007
Available online 20 December 2007
Abstract
A safe robot arm can be achieved by either a passive or pliance system. A pliance -
posed of purely mechanical elements often provide faster and more reliable responses for dynamic collision than an active
one involving sensors and actuators. Since both positioning accuracy and collision safety are important, a robot arm
should exhibit very low stiffness when subjected to a collision force greater than the one causing human injury, but main-
tain very high stiffness otherwise. To implement these requirements, a novel safe link mechanism (SLM), which consists of
linear springs, a double-slider mechanism and shock-absorbing modules, is proposed in this research. The SLM has the
advantages of variable stiffness which can be achieved only by passive mechanical elements. Various experiments of static
and dynamic collisions showed the high stiffness of the SLM against an external force of less than the critical impact force,
but an abrupt drop in the stiffness when the external force exceeds the critical force, thus guaranteeing collision safety.
Furthermore, the critical impact force can be set to any value depending on the application.
Ó 2007 Elsevier Ltd. All rights reserved.
keywords: Safety mechanism; Nonlinear stiffness; pliance; Safe robot arm
1. Introduction
For industrial robots, safe human–ro