6. Conclusion

The control performance of a humanoid robot depends on the definition of the reference trajectory. This must be in line with the humanoid robot's physical and technological capabilities. It need not be explicitly time-dependent, especially when the humanoid robot is under-actuated. In such cases, it is preferable for the reference trajectory to depend on a clock internal to the humanoid robot, which leads to the concept of virtual constraints. As a result, the control/reference trajectory duo is better able to take into account the force of gravity acting on the humanoid robot and the slight influence on stability of the torques at the pins. The lower power-to-weight ratio of electric actuators (with their mechanical transmission system) compared with human muscles, combined with the reduced number of humanoid ddl's, explains why humanoids currently have lower running performance than humans....