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Robots are now requested to perform more and more complex tasks such as rescue activities in quite practical environments such as rough terrains. It is necessary for robots to have versatile joints that can flexibly adapt to such task environments and realize complex motions. It is desired to develop small and low cost actuators which can flexibly adapt to task environment. Flexibility in robot joints is especially necessary in the environment where the robots work around humans. The conventional joint mechanism makes the robots structurally complex, then heavy and large, and expensive. In addition it is difficult for them to control joint stiffness in order to handle soft objects or contact human body. Stiffness control by software servomechanism based on the reaction force from the object measured after contact has been actively investigated. But the time lag of servomechanism often results in undesirable response. On the other hand mechanical stiffness control causes no time lag because the stiffness is determined before the contact with work objects. Vertebrates’ joints easily realize joint angle control and stiffness control simultaneously by their antagonistic muscles. A wide variety of artificial muscles are being actively investigated (ex. Proc. 2nd Conf. on Artificial Muscles, 2004). McKibben artificial muscle is the most well-known pneumatic actuator and there are many applications including energy efficient low power joint (Linde, 1999). Another type of vertebrate-joint-like actuator developed by the author’s group is Strand-Muscle Actuator, StMA (Suzuki et al., 1997). The actuator can adapt to various tasks and environmental changes. The StMAs, having nonlinear elastic characteristics, realize joint angle/stiffness control, even for multi-DOF joints, by antagonistic actuator installation on the joint. In addition the actuator is expected to realize multi-DOF complex and flexible motions with simple mechanism. The joint angle/stiffness control by StMAs has been investigated, and legged walking robots as well as a multi-DOF human-shoulderlike joint, 5 fingered hands have already been developed. This chapter is to introduce the development and application of the StMAs. The StMA-based joints are suitable for complex and flexible motions in spite of their simple mechanism, and extendable joint mechanism will be possible with them. An StMAbased joint with redundant muscles realizes failure tolerant angle/stiffness control. The effectiveness has already been verified by legged walking robots, 5 fingered hands, and 3O pe n A cc es s D at ab as e w w w .ite ch on lin e. co m

Complex and Flexible Robot Motions by Strand-Muscle Actuators