| Legged robots have the potential to extend our reach to terrains that challenge the traversal capabilities of traditional wheeled platforms. To realize this potential, diverse legged robot designs have been proposed. However, combining mobility with energy efficiency is still a challenging task due to the inherently dissipative nature of legged locomotion.;Biological systems demonstrate the great potential of utilizing compliant elements in legged locomotion. During running, part of the mechanical energy is recovered by the elastic deformation of muscles and tendons and returned to the system when it is needed. In addition, compliance also alleviates the requirement for powerful actuators. Introducing compliance into legged robots, however, is not a straightforward task.;With the objective to close the gap between mobility and efficiency, this thesis explores the applications of both active and passive compliant elements in the design and control of running robots. The thesis begins with reduced-order running model with massless springy legs before delving into higher-dimensional models that constitute the more faithful representation of robotic systems. Using time-reversal symmetries of the underlying dynamics of these reduced-order models, this thesis states analytic conclusions on the stability of periodic running gaits, which can be used to facilitate controller design. Next, a detailed model with segmented leg and inelastic impact is adopted to study the periodic bounding of quadrupedal robot HyQ. Mimicking the reduced-order models, stable periodic bounding gaits emerge as the interaction results between the robot and its environment.;Inspired by the complementary benefits of passive and active compliance in energy efficiency and control authority, respectively, we propose in this thesis a novel actuation concept: the switchable parallel elastic actuator (Sw-PEA). This concept relies on adding compliance in parallel with the actuator to reduce both the energy consumption as well as the torque requirement related to running robots. In addition, a mechanical switch is used to disengage the spring when it is not needed to facilitate control of joint movement. The effectiveness of the concept is demonstrated experimentally by monopedal robot SPEAR which is actuated by a Sw-PEA. |
