Sensorless Admittance Control of a Manipulator Arm Using a Nonlinear Observer for Force and Velocity Estimation

This paper presents a nonlinear observer-based approach for estimating force and velocity in a joint-space admittance-controlled exoskeleton, designed to support safe and compliant physical human–robot interaction. The observer estimates external interaction forces and joint velocities using only joint position measurements, eliminating the need for external force or velocity sensors. Integrated into the ETS-MARSE upper-limb rehabilitation exoskeleton, the system generates compliant motion trajectories in response to user-applied forces. An experiment involving a human subject was conducted to validate the observer’s accuracy. The estimated forces and velocities were compared with reference sensor measurements. Results demonstrate that the observer provides reliable state estimates, enabling accurate tracking of motion and interaction forces with low error and high responsiveness. The system maintains compliant behavior, supporting natural, user-driven movement without compromising stability. This work highlights the potential of sensorless estimation in robotic rehabilitation and interaction-intensive control applications.