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For classical robotic applications, robotic systems consist of servo motors, high-ratio reduction and rigid links; mechanical designers prefer to designing 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-passive variable stiffness elastic actuator (APVSEA) which is designed for safety robot systems. The APVSEA 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 two antagonistically nonlinear springs. By changing the preload length of the two antagonistically nonlinear springs, APVSEA has the ability to minimize large impact forces due to shocks, to safely interact with the user and/or become as stiff as possible to make precise position movements or trajectory tracking control easier. Experiment results are presented to show that APVSEA is capable of providing precise position movements while offering safe human-robot interaction.