Tracking control of an underwater manipulator using active disturbance rejection

An active disturbance rejection controller for a 2-DOF underwater manipulator is proposed in this study. The manipulator system has the characteristics of non-linearity, strong coupling and uncertainties within its model. Its operation is often disturbed by unknown water currents, so the anti-disturbance control of underwater manipulators has always been a challenge. The proposed controller in this study basically does not rely on the precise mathematical model of the object, and the model even can be decoupled. This method can eliminate the influence of model errors, time-varying parameters and external interference on the control effect. First, the entire manipulator treats different joints as several subsystems. For each joint subsystem, hydrodynamic forces, coupling terms between joints and unknown environmental disturbances are regarded as total disturbances. Subsequently, an extended state observer was designed to estimate and compensate for the total interference. In order to improve the disturbance observation effect of the extended state observer, the inertia matrix of the system is used to decouple the static part. Finally, the effectiveness of the proposed method is verified by both simulation and experiments. From the comparisons, it is confirmed that the quality of our controller in the presence of a certain inertial matrix error is better than traditional PD and continuous sliding mode control in terms of accuracy, dynamic characteristics as well as robustness.