Variable impedance actuation using biphasic media

Variable impedance actuators are actuators whose position and stiffness can be controlled independently. They are used in mechanical systems and robots when it is required to modify the stiffness/compliance of the joints during the functioning. A major application today is in robotic systems expected to interact safely with humans and the environment. This paper presents a concept of variable impedance actuator using a combination of fluid and gas to realize the adaptation of the position and stiffness. The actuator can be realized with the architecture of a common hydraulic piston. A mathematical model of the actuator is presented. The system inherent frequencies are analyzed with different sets of design parameters and initial conditions. The correlation between flows of the liquid present in the actuator and output motion are analyzed to obtain different values of the impedance for different initial conditions. A method of control of the actuator is proposed and the correlation between system parameters and output function is discussed.