Tracking Control for n-Link Flexible-Joint Robots with Output Constraints and Disturbances: An FAS Approach

This paper proposes a fully actuated control method to address the tracking control problem of n-link flexible-joint robots (FJRs) with output constraints and external disturbances. Firstly, a disturbance observer is constructed to estimate and compensate for unknown disturbances in real time. Subsequently, the fully actuated system (FAS) model of the FJR system is obtained by using the variable elimination method. Meanwhile, a constraint function is introduced to transform the constrained system into an unconstrained new system. Theoretical analysis shows that as long as the initial value of the tracking error is within the constraint interval and the constraint function remains bounded, the tracking error will always satisfy the predefined constraint. On this basis, leveraging the FAS characteristics, a tracking controller with bandwidth directly serving as the gain is designed, and the convergence of the closed-loop system depends on the magnitude of the disturbance estimation error. Compared with the traditional backstepping and dynamic surface control (DSC) methods, the developed approach not only avoids the repeated differentiation of virtual controller, but also achieves a globally convergent result. Finally, the effectiveness of the proposed control algorithm is verified by the simulation of a two-link FJR.