Control of ROVs using a Model-free 2nd-Order Sliding Mode Approach

Remotely Operated Vehicles (ROVs) have had significant contributions in the inspection, maintenance and repair of underwater structures, related to the oil industry, especially in deep waters, not easily accessible to humans. Two important capabilities for industrial ROVs are: position tracking and the dynamic positioning or station-keeping (the vehicle's ability to maintain the same position with respect to the structure, at all times). It is important to remember that underwater environment is highly dynamic, presenting significant disturbances to the vehicle in the form of underwater currents, interaction with waves in shallow water applications, for instance. Additionally, the main difficulties associated with underwater control are the parametric uncertainties (as added mass, hydrodynamic coefficients, etc.). Sliding mode techniques effectively address these issues and are therefore viable choices for controlling underwater vehicles. On the other hand, these methods are known to be susceptible to chatter, which is a high frequency signal induced by control switches. In order to avoid this problem a High Order Sliding Mode Control (HOSMC) is proposed. The HOSMC principal characteristic is that it keeps the main advantages of the standard SMC, thus removing the chattering effects. The proposed controller exhibits very interesting features such as: i. a model-free controller because it does neither require the dynamics nor any knowledge of parameters, ii. It is a smooth, but robust control, based on second order sliding modes, that is, a chattering-free controller is attained. iii. The control system attains exponential position tracking and velocity, with no acceleration measurements. Simulation results reveal the effectiveness of the proposed controller on a nonlinear 6 degrees of freedom (DOF) ROV, wherein only 4 DOF (x, y, z, ψ) are actuated, the rest of them are considered intrinsically stable. The control system is tested under ocean currents, which abruptly change its direction. Matlab-Simulink, with Runge-Kutta ODE45 and variable step, was used for the simulations. Real parameters of the KAXAN ROV, currently under construction at CIDESI, Mexico, were taken into account for the simulations. In Figure 1 one can see a picture of KAXAN ROV. For performance comparison purposes, numerical simulations, under the same conditions, of a conventional PID and a model-based first order sliding mode control are carried out and discussed.