Abstract-Exploiting variable impedance for dynamic tasks such as walking is both challenging and topical -research progress in this area impacts not only autonomous, bipedal mobility but also prosthetics and exoskeletons. In this work, we present the design, construction and preliminary testing of a planar bipedal robot with joints capable of physically varying both their stiffness and damping independently -the first of its kind. A wide variety of candidate variable stiffness and damping actuator designs are investigated. Informed by human biophysics and locomotion studies, we design an appropriate (heterogenous) impedance modulation mechanism that fits the necessary torque and stiffness range and rate requirements at each joint while ensuring the right form factor. In addition to hip, knee and ankle, the constructed robot is also equipped with a three part compliant foot modelled on human morphology. We describe in detail the hardware construction and the communication and control interfaces. We also present a full physics based dynamic simulation which matches the hardware closely. Finally, we test impedance modulation response characteristics and a basic walking gait realised through a simple movement controller, both in simulation and on the real hardware.

Variable stiffness and variable damping can play an important role in robot movement, particularly for legged robots such as bipedal walkers. Variable impedance also introduces new control problems, since there are more degrees of freedom to control, and the resulting robot has more complex dynamics. In this paper, we introduce novel design and fabrication methodologies that are capable of producing cost effective hardware prototypes suitable for investigating the efficacy of impedance modulation. We present two variable impedance bipedal platforms produced using a combination of waterjet cutting and 3D printing, and a novel fused deposition modeling (FDM) 3D printing based method for producing hybrid plastic/metal parts. We evaluate walking trajectories at different speeds and stiffness levels.

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