On Adjustable Stiffness Artiicial Tendons in Bipedal Walking Energetics

Inspired by locomotion in nature, researchers have developed the passive dynamic walking machine principle and applied it to the legged robotics (Coleman & Ruina, 1998; Collins et al., 2001; Garcia, 1999; McGeer, 1990; Wisse, 2004; Wisse & Frankenhuyzen, 2006). The passive dynamic walking machines provide human-like locomotion in legged robots that is more efficient than the precisely joint-angle-controlled robots. On the other hand, tuning the parameters of the passive dynamic walking robots are tricky, time consuming and requires much experimentation. In addition, passive dynamic walking robots still have considerable energy loss through rapid changes in the velocity direction of the center of mass of the robot during collision of the foot with the ground. Precisely joint-angle-controlled bipedal walking robots also undergo significant energy loss caused by rigid collision of the foot with the ground in addition to their common energy dissipation in robot’s power systems. Collision of the foot with the ground during bipedal walking is inevitable which is one of the major sources of the energy loss. Establishing a new technique to reduce this energy loss is a challenging problem which we desire to address in this work by developing the idea of using the adjustable stiffness elastic elements in robot’s structure. We believe that the adjustment of the elasticity as a control strategy can significantly improve the energetics of locomotion in bipedal walking robots by reducing the energy loss during the collision phase, which starts with an impact of the heel-strike followed by continuous motion and ends by a second impact at the foot-touch-down. This work , as a first step in this research area, constructively demonstrates the idea through two main efforts. The first effort is to develop the conceptual designs of the adjustable stiffness artificial tendons (ASAT) to show that the idea can be implemented in practice. The second effort is to study the effects of adjustable stiffness elasticity on reducing the energy loss by adding the model of each ASAT into the robot dynamics. This introductory section reviews the research on legged locomotion which indicates the importance of elasticity in mechanics of locomotion in nature. In human walking, part of the kinetic and potential energy from the body is transiently stored as elastic strain energy during the collision phase and is released later during the rebound phase by elastic recoil (Kuo et al., 2005). This phenomenon greatly reduces the work required from the muscles and lowers the metabolic cost of locomotion (Alexander & Bennett, 1989; Cavagna et al., 1977). The mechanics of elastic recoil were also studied for running and it was found that, the forward kinetic energy of the body’s center of mass is in phase with fluctuations in gravitational potential energy (Cavagna et al., 1964). It was also found that, humans and animals most likely store 9