Artificial Muscles for Humanoid Robots

1. The question of actuator choice for humanoid robots It is important to recall that humanoid robot technology derives from the technology of industrial robots. It is obvious that the developments of bipedal robots such as the integration of robot-upper limbs to com plex anthropomorphic structures have benefited from progress in mechanical structures, sensors and actuators used in industrial robot-arms. A direct link is sometimes made between the technology of redundant robot-arms and humanoid robots as underlined in some technical documents of the Japanese AIST where it clearly appears that the HRP2 humanoid robot upper limb is directly derived from the Mitsubshi PA10 7R industrial robot-arm. Due to its high number of degrees of freedom in comparison to i ndustrial robots, a humanoid robot requires great compactness of all actuator and sensor components. This is why we believe that the harmonic drive technology associated with direct current electric motor technology has played a non-negligible part in humanoid robot development. The DC actuator offers the great advantage of being a straightforwar d technology, associated with simple and well-known physical models, its integration into mobile robots benefits from new developments in embedded batteries. However, its low maximum-torque-onmass and maximum-torque-on- volume ratios are a serious drawback for its use in direct drive apparatuses. On the other hand, the ability of electric motors to generate very high velocities in comparison with moderate jointed velocities needed by industrial robot-arms and more by jointed anthropomorphic limbs, gives the possibility of using high ratio speed reducers to amplify motor-torque. Moreover, the choice of a high ratio speed reducer offers the advantage of masking inertial perturbations such as external torque perturbations. The technical achievement of such ratios induces specific mechanical difficulties due to the bulkiness of successive gears; harmonic drive technology – represented for example by Harmonic Drive AG – resolves this problem in a very elegant manner: the harmonic drive and the actuator fit together without excessive increase in mass and volume in comparison with the actuator alone. It can be considered that most of today’s humanoid robots are actuated by DC motors with harmonic drives (this actuation mode is mentioned, for example, by Honda from its first paper about the P2 robot onwards (Hirai et al., 1998) and then in the official ASIMO web site, as well as in papers concerning other Japanese and European humanoid robots). But if this technology simplifies actuator mechanical integration and leads to the use of simple joint linear control, despite the highly non-linear character of robot dynamics, it is well-known that the use of a speed reducer multiplies the