  In this paper, we propose the "Kinetics Observer", a novel estimator
addressing the challenge of state estimation for legged robots using
proprioceptive sensors (encoders, IMU and force/torque sensors). Based on a
Multiplicative Extended Kalman Filter, the Kinetics Observer allows the
real-time simultaneous estimation of contact and perturbation forces, and of
the robot's kinematics, which are accurate enough to perform proprioceptive
odometry. Thanks to a visco-elastic model of the contacts linking their
kinematics to the ones of the centroid of the robot, the Kinetics Observer
ensures a tight coupling between the whole-body kinematics and dynamics of the
robot. This coupling entails a redundancy of the measurements that enhances the
robustness and the accuracy of the estimation. This estimator was tested on two
humanoid robots performing long distance walking on even terrain and
non-coplanar multi-contact locomotion.
