Sensorless Ground Reaction Force Observation With Disturbance Compensation in Heavy-Legged Robots

This article proposes a framework to address the challenges of the uncalibrated cylinder-driven heavy-legged robot (HLR) in accurately observing the ground reaction force (GRF). The proposed framework eliminates the need for force/torque sensors mounted on end-effectors or joints. One key contribution of this article is the development of a combined model, referred to as an approximate PMSM model (APM), which incorporates permanent magnet synchronous motors (PMSMs), electric cylinders, and the HLR. This model establishes the relationship between the input phase currents and the movement of the HLR, and it captures the characteristics of GRF, the HLR nominal torque, and the overall disturbances. To enable GRF observation based on the measured currents, an improved sliding-mode observer with harmonic, nominal, and unmodeled compensation was used. Harmonic compensation enhanced real-time responses and accuracy. Additionally, a radial basis function neural network was used to compensate for the unmodeled portion, which includes friction in all drive components of the HLR. Subsequently, a modified form of the nonlinear disturbance observer compensation was introduced to account for the HLR nominal torque in the APM. Through experimental evaluation, the effectiveness of the proposed framework was validated for the GRF observation.