Sensorless Normal Force Measurement and Constant-Force Tracking for Manipulators in Kelvin–Voigt Environments With Unknown Contact-Surface Positions

Abstract

Stable and accurate normal contact force measurement is critical for constant-force interaction between manipulators and their environment. End-effector force/torque sensors are costly and may reduce the payload capacity of the manipulator. A disturbance observer enables sensorless normal contact force measurement. Noise in joint measurements can degrade disturbance-observer outputs, and excessive collision force overshoot may damage the environmental surface whose exact position is unknown. First, to address the above issues, this article develops a multistage adaptive impedance control (MS-AIC) algorithm with enhanced robustness to measurement noise for constant-force tracking in a linear Kelvin–Voigt viscoelastic environment, and its stability is established using the Routh–Hurwitz criterion. In addition, the transient behavior during the free-space-to-contact transition and the propagation of generalized momentum observer (GMO) estimation error in the MS-AIC dynamics are analyzed. Second, four disturbance observers for sensorless normal contact force measurement are compared in terms of accuracy, noise robustness, time complexity, and parameter tuning. The stability of the sensorless constant-force tracking impedance control framework is validated in Kelvin–Voigt environments with unknown contact-surface positions. For the proposed MS-AIC algorithm, a systematic parameter sensitivity analysis is conducted, and practical tuning guidelines are provided. The simulation and experimental results demonstrate that the proposed algorithm improves robustness to measurement noise and significantly reduces force overshoot. The experimental results further indicate that improved friction-model identification leads to smaller force-tracking errors on unknown environment surfaces. The current validation mainly focuses on sensorless normal contact force measurement and constant-force tracking on stationary surfaces under tested low-speed linear-motion conditions, without explicit compensation for high-speed tangential-friction-induced coupling. These results provide theoretical and experimental guidance for sensorless normal-force-regulated surface-contact tasks.