Shohei Kato and Minoru Ishida Nagoya Institute of Technology Japan

A lot of research on humanoid robots or biped robots has been conducted. This research focused on enabling robots to walk very smoothly, similar to the way humans walk, which is highly energy efficient. Motion control using a central pattern generator (CPG) has attracted much attention as an effective control mechanism for biped robots to achieve human-like walking (e.g., Kato & Itoh (2005); Kotosaka & Schaal (2000); Miyakoshi et al. (1998); Taga (1995a;b); Taga et al. (1991)). The CPG is modeled mathematically to a neural rhythm generator that exists at a relatively low level of the central nervous system, such as the spinal cord of animals. This motion control using the CPG has generated various motions: walking by Ishida et al. (2009a;b); Itoh et al. (2004); Nakamura et al. (2005); Taki et al. (2004), step by Miyakoshi et al. (1998), and drum motions by Kotosaka & Schaal (2000). Taga (1995a;b) proposed a neuro-musculo-skeletal model based on the CPG, and it enabled a biped robot to have a human gait in two dimensions. Among the researchers of highly energy-efficient walking, McGeer (1990) was the first to study passive dynamic walking (PDW). A PDW robot walks forward by placing the foot on the ground and riding on the supporting leg, which rolls forward as an inverted pendulum mounted on the supporting foot. At the same time, it places the swing foot forward by moving the swing leg in a pendular arc, so that it makes the foot strike a ground when the mechanism is in a configuration identical to that at the beginning of a step. If the dynamic characteristic of the robot and the environment (e.g., the slope and velocity when the walking begins) agree, then the PDW robot achieves highly energy-efficient walking without any actuator control. Sugimoto & Osuka (2004) proposed a control method for quasi passive dynamic walking (Quasi-PDW). Quasi-PDW means that the robot usually does PDW without any input torque, and the actuators of the robot are used for ensuring walking stability only when the walking begins or when a disturbance occurs. Haruna et al. (2001) researched a PDW robot with a torso. CPG-based motion control inputs some torque to all the joints of the robot’s lower limbs regardless of gait. When we think about human walking, we take into account the joints of the swing leg without any input torque. Active walking needs to be mixed with PDW for robot walking. Quasi-PDW is an example of a mixture of active walking and PDW. Quasi-PDW is applicable to the robot whose dynamic characteristic suits to PDW. However, there is a lot of robots that can not satisfy a dynamic characteristic for PDW. To achieve this mixture with the robots, we added the mechanism of PDW to an active walking robot. In this chapter, we describe a motion control method based on the mixture of CPG and PDW (Ishida et al. (2009a;b)), that is, the Shohei Kato and Minoru Ishida Nagoya Institute of Technology Japan Dynamic Joint Passivization for Bipedal Locomotion 11