External force observer for medium-sized humanoid robots

In this paper, we introduce a method to estimate the magnitude of an external force applied on a humanoid robot. The approach does not require using force/torque sensors but instead uses measurements from commonly available force-sensing resistors (FSR) inserted under the feet of the humanoid robot. This approach is particularly interesting for affordable medium-sized humanoid robots such as Nao and Darwin-OP. The main idea is to use a simplified dynamic model of a linear inverted pendulum model (LIPM) subjected to an external force, and the information from the robot inertial measurement unit (IMU) and FSR sensors. The proposed method was validated on a Nao humanoid robot to estimate the external force applied in the sagittal plane through two experimental scenarios, and the results pointed out the efficiency of the proposed observer.