Abstract—Nowadays, an increasing number of industrial processes are expected to have robots interacting safely with humans and the environment. Compliance control of robotic systems strongly addresses these scenarios. This article develops a variable stiffness actuator (VSA) whose position and stiffness can be controlled independently. The method for adapting the stiffness in the VSA includes a member configured to transmit motion that is connected to a fluidic circuit, into which a biphasic control fluid circulates. Actuator’s stiffness is manipulated by varying pressure of control fluid into distribution lines. The control fluid used is composed of gas and liquid, which are separated from each other and in proportions with predefined ratio. An approach for the mathematical model is introduced and a model-based control method is implemented to track the desired position and stiffness. Results from force loaded and unloaded simulations and possible applications of the system are discussed.