The electrohydraulic elastic manipulator (EEM) includes an electro-hydraulic servo system (EHSS) and a variable stiffness mechanism (VSM). It is a complex system with the nonlinear elements, such as friction, parametric uncertainties, and modeling error as well as coupling torques in the elastic joint. This paper focuses on developing an equilibrium position controller for the EEM with the presence of the matched and unmatched uncertainties. The proposed controller is based on a backstepping scheme optimized using an adaptive sliding mode technique and an adaptive nonlinear PI structure. To estimate parameters of known functions in the system dynamics, the parametric learning laws are employed, while adaptive switching gain laws are used to adjust robust gains of discontinuous control terms. The robust switching gains increase or decrease as state control variables moves away or closes to the surfaces, respectively. Working together, these kinds of the adaptation laws can effectively deal with the problem of matched and unmatched uncertainties. Next, the Lyapunov approach is used to prove the robustness and stability of the whole system. The proposed method is practically implemented and compared with other controllers to demonstrate the effectiveness of the proposed method with the variant stiffness and the uncertainties.

This work gives the kinematic description and mathematical dynamic model of a three degrees of freedom (DOF) manipulator including hydraulic actuator dynamics, and then proposes a disturbance observer based robust control scheme for the position and torque tracking control subjected to external disturbances and parameter uncertainties. First of all, the kinematic of the manipulator system is built according to Denavit-Hartenberg (DH) notation. The joint space, actuator space, and the mathematical model of the manipulator including hydraulic actuator dynamics are then presented. Next, a robust control technique is designed for the fast and finite time tracking capability of the torque signals along the desired torque commands and a fast nonsingular terminal sliding mode control (FNTSMC) algorithm is developed to guarantee the fast convergence of the joint positions to their desired values. Moreover, two disturbance observer schemes are proposed to estimate and compensate the external disturbances and modeling errors in the manipulator system and hydraulic actuator system. Stability analysis of the cascade hydraulic manipulator system is analyzed and proved using backstepping technique and Lyapunov theory. Finally, numerical simulations are obtained to validate the effectiveness of the designed control algorithm.

The paper proposes an adaptive fuzzy position control for a 3-DOF hydraulic manipulator with large payload variation. The hydraulic manipulator uses electrohydraulic actuators as primary torque generators to enhance carrying payload of the manipulator. The proposed control combines backstepping sliding mode control, fuzzy logic system (FLS), and a nonlinear disturbance observer. The backstepping sliding mode control includes a sliding mode control for manipulator dynamics and a PI control for actuator dynamics. The fuzzy logic system is utilized to adjust the control gain and robust gain of the sliding mode control (SMC) based on the output of the nonlinear disturbance observer to compensate the payload. The Lyapunov approach and backstepping technique are used to prove the stability and robustness of the whole system. Some simulations are implemented, and the results are compared to other controllers to exhibit the effectiveness of the proposed control.

This paper is aimed at addressing the tracking control issue for an n-DOF manipulator regardless of unknown friction and unknown control direction. In order to handle the above issues, an adaptive sliding mode control (ASMC) is developed with a Nussbaum function. The sliding mode control (SMC) in the proposed control guarantees the tracking problem and fast responses for the manipulator. Additionally, there are adaptive laws for the robust gain in the SMC to deal with the unknown external disturbance and reduce the chattering effect in the system. In practice, the mistakes in the connection between actuators and drivers, named unknown control direction, cause serious damage to the manipulator. To overcome this issue, the Nussbaum function is multiplied by the ASMC law. A Lyapunov approach is investigated to analyze the stability and robustness of the whole system. Finally, several simulations are implemented on a 3-DOF manipulator and their results are compared with those of the existing controllers to validate the advantages of the proposed method.