Estimation of contact forces and floating base kinematics of a humanoid robot using only Inertial Measurement Units

Abstract

A humanoid robot is underactuated and only relies on contacts with environment to move in the space. The ability to measure contact forces and torques enables then to predict the robot dynamics including balance. In classical cases, a humanoid robot is considered as a multi-body system with rigid limbs and joints and interactions with the environment are modeled as stiff contacts. Forces and torques at contacts are generally estimated with sensors which are expensive and sensitive to calibration errors. However, a robot is not perfectly rigid and contacts may have flexibilities. Therefore, external forces create geometric deformations of the body or its environment. These deformations may modify the robot dynamics and produce unwanted and unbalanced motions. Nonetheless, if we have a model of contact stiffness and are able to reconstruct reliably the geometric deformation, we can reconstruct forces and torques at contact. This study aims at estimating contact forces and torques and to observe the body kinematics of the robot with only an Inertial Measurements Unit (IMU). We show that we are able to reconstruct efficiently the position of the Center of Pressure (CoP) of the robot with only the IMU and proprioceptive data from the robot.

Model-Based External Force/Moment Estimation for Humanoid Robots with no Torque Measurement

“…This is due to the large part of redundancy which lies in these measurements. Indeed, we have shown that the IMU alone is able to reconstruct the center of pressure of contact forces relatively accurately when coupled with our model [18]. The idea here is to take profit from this redundancy in order to rebuild an additional variable, the overall external force and moment which are not directly sensed.…”

“…Indeed, the orientation and the velocity of the floating base are also subject to external forces, especially in the case of an unpredicted contact with the environment [18], [3]. In fact, several advanced methods for reconstructing the floating base kinematics actually use the data of external forces to reconstruct reliably the state of the robot [20], [2].…”

From multi-modal tactile signals to a compliant control

“…When there is an external tactile perturbation (user applies forces to the arm), e.g. t = [5,7]s (1a), the robot reacts to the tactile event and changes its position (1b), as soon as the external perturbation is removed, e.g. t = 8s the robot returns to its original position smoothly.…”

“…The interaction is repeated several times (t = [11,26]s), generating changes reflected in the joint position of the robot. We provide a video 7 to illustrate these behaviors in our robot TOMM using the proposed approach.…”

“…wrist or ankles, or using Inertial Measurement Units (IMU) [7] based on models of the contact stiffness. Another interesting methodology is to use proprioceptive sensors to estimate the external contacts based on the internal generalized forces, this method is known as momentum-based residual [8], [9].…”