Robots are traditionally used in factory environments, where they perform tasks with high precision and speed. This kind of robots is designed to have a high mechanical stiffness and with powerful motors, so that the required precision and speed can be achieved. However, such designs are inherently unsafe, and therefore these robots are separated from the factory workers. In order to have robots safely interact with humans, they need to meet new requirements in terms of safety and interaction. Actuators with variable stiffness are characterized by the property that their apparent output stiffness can be varied. This is generally realized by incorporating a number of elastic elements in the design. A number of internal degrees of freedom are then used to regulate how these elastic elements are perceived at the actuator output. A robot equipped with such actuators can thus adapt its impedance to levels suitable for a given task and environment. Furthermore, the elastic elements can be used to temporarily store energy, which can be beneficial for increasing the energy-efficiency of such robots. This dissertation explores the designs and applications of actuators with variable stiffness, with a focus on their energetic behavior. For this purpose a generic model has been developed, which enables the analysis for the power flows between the internal elastic elements and the environment. This analysis provides insight in the efficiency of the actuator by visualizing how externally supplied energy can be temporarily stored and reused. These insights are at the basis of two new control algorithms, which aim and minimizing the energy consumption of the actuator. Finally, the application of variable stiffness to bipedal locomotion is investigated. It is shown that the application of variable stiffness can have a positive influence on the robustness and efficiency of bipedal locomotion.
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