Humanoid flexibility deformation can be efficiently estimated using only inertial measurement units and contact information

Most robots are today controlled as being entirely rigid. But often, as for HRP-2 robot, there are flexible parts, intended for example to absorb impacts. The deformation of this flexibility changes the configuration of the robot, particularly in orientation. Nevertheless, robots have usually inertial sensors (IMUs) to reconstruct their orientation based on gravity and inertial effects. Moreover, humanoids have usually to ensure a firm contact with the ground, which provides reliable information on the surrounding environment. We show in this study, how important it is to take into account these information to improve IMU-based position/orientation reconstruction. We use an extended Kaiman filter to rebuild the deformation, making the fusion between IMU and contact information, and without making any assumption on the dynamics of the flexibility. We show how, with this simple setting, we are able to compensate for perturbations and to stabilize the end-effector´s position/orientation in the world reference frame.