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In classical robotic applications, robotic systems consist of servo motors, high-ratio reduction and rigid links; mechanical designers prefer to design robotic applications as stiff as possible to make robots manipulate with remarkable speed and precise position movements. However, these robotic applications can hardly interact with people and environments under safety constraints. It poses the very fundamental problem of ensuring safety to humans and protecting the robot. This paper presents an Active Variable Stiffness Elastic Actuator (AVSEA) that is designed and application for safe physical human robot interaction. The AVSEA consists of two DC-motors: one is used to control the position of the joint and the other is used to adjust the stiffness of the system. The stiffness is generated by a leaf spring. By changing the effective length of the leaf spring, AVSEA has the ability to minimize large impact forces due to shocks, to safely interact with the user and/or to become as stiff as possible to make precise position movements or trajectory tracking control easier. Experiment results are presented to show that AVSEA is capable of providing precise position movements while offering safe human-robot interaction.