Asymmetric time‐varying BLF‐based model‐free hybrid force/position control for SEA‐based 2‐DOF manipulator

Summary In this work, an asymmetric time‐varying barrier Lyapunov function‐based model‐free hybrid force/position controller (ABLF‐MFC) is proposed for the series elastic actuator‐based 2‐DOF manipulator. Inspired by large surface machining, many scenarios require hybrid force/position control, and the simple position control can no longer meet the above requirements. Therefore, ABLF‐MFC with a dual‐loop structure is established, which are force sub‐control loop and position sub‐control loop. Based on the idea of admittance control, the force sub‐control loop generates a reference trajectory according to the desired interaction force and trajectory. Then, for the position sub‐control loop, to simplify the controller design process, an ultra‐local model (ULM) is introduced, which can approximate the original system. Since the ULM has an unknown term, time‐delay estimation (TDE) is applied to estimate it, which can also reduce the dependence on accurate model parameters. The position sub‐control loop is designed by an asymmetric time‐varying barrier Lyapunov function, an integral‐type Lyapunov function and an adaptive compensation term, so the tracking error can be kept within the preset boundary while ensuring the convergence, and the TDE estimation error can be compensated. Rigorous mathematical proofs and simulation results verify the effectiveness of ABLF‐MFC.