Model-Free High-Order Sliding Mode Controller for Station-Keeping of an Autonomous Underwater Vehicle in Manipulation Task: Simulations and Experimental Validation

Abstract

The use of autonomous underwater vehicles (AUVs) has expanded in recent years to include inspection, maintenance, and repair missions. For these tasks, the vehicle must maintain its position while inspections or manipulations are performed. Some station-keeping controllers for AUVs can be found in the literature that exhibits robust performance against external disturbances. However, they are either model-based or require an observer to deal with the disturbances. Moreover, most of them have been evaluated only by numerical simulations. In this paper, the feasibility of a model-free high-order sliding mode controller for the station-keeping problem is validated. The proposed controller was evaluated through numerical simulations and experiments in a semi-Olympic swimming pool, introducing external disturbances that remained unknown to the controller. Results have shown robust performance in terms of the root mean square error (RMSE) of the vehicle position. The simulation resulted in the outstanding station-keeping of the BlueROV2 vehicle, as the tracking errors were kept to zero throughout the simulation, even in the presence of strong ocean currents. The experimental results demonstrated the robustness of the controller, which was able to maintain the RMSE in the range of 1-4 cm for the depth of the vehicle, outperforming related work, even when the disturbance was large enough to produce thruster saturation.

Keywords: AUV; SMC; finite-time; station-keeping.

Figure 1

Reference frames for underwater vehicles.

Figure 2

Thruster configuration for the BlueROV2. (A) Top view. (B) Front view.

Figure 3

BlueROV2 simulator. Simulink block diagram.

Figure 4

Experimental set-up, hardware configuration.

Figure 5

Additional thruster for external disturbance. (A) Front view diagram. (B) Implementation.

Figure 6

BlueROV2 software configuration.

Figure 7

Experimental set-up. (A) Semi-Olympic swimming pool. (B) Control station. (C) BlueROV2 deployment. (D) Station-keeping task execution.

Figure 8

Block diagram of the proposed controller.

Figure 9

Simulation results for finite-time trajectory tracking and station-keeping in the $x$, $y$, $z$ positions and $\psi$ orientation. External disturbances were introduced in the interval $10\mathrm{s}\le t\le 18\mathrm{s}$ as ocean currents $({\nu }_{oc})$.

Figure 10

Depth (left) and tracking error (right) results of the control test. No external disturbances were introduced.

Figure 11

Experimental results of the control test for depth station-keeping. Control signal ${\tau }_z$ (left) and thruster coefficients $u_5$,$u_6$ (right). No external disturbances were introduced.

Figure 12

Results of the open-loop experiments. Depth of the vehicle (upper left) with an external disturbance of $~50\mathrm{N}$ (upper right) introduced at $t=13\mathrm{s}$. Depth of the vehicle (lower left) with an external disturbance of $~-40\mathrm{N}$ (lower right) introduced at $t=13\mathrm{s}$.

Figure 13

Experimental results for depth station-keeping with an external disturbance of $~10\mathrm{N}$ introduced in the interval $13\mathrm{s}\le t\le 25\mathrm{s}$. Depth (left) and tracking error (right).

Figure 14

Experimental results for depth station-keeping with an external disturbance of $~10\mathrm{N}$ introduced in the interval $13\mathrm{s}\le t\le 25\mathrm{s}$. Control signal ${\tau }_z$ (left) and thruster coefficients $u_5$,$u_6$ (right).

Figure 15

Additional thruster coefficient of −0.25 in the interval $13\mathrm{s}\le t\le 25\mathrm{s}$.

Figure 16

Experimental results for depth station-keeping with an external disturbance of $~20\mathrm{N}$ introduced in the interval $13\mathrm{s}\le t\le 25\mathrm{s}$. Depth (left) and tracking error (right).

Figure 17

Experimental results for depth station-keeping with an external disturbance of $~20\mathrm{N}$ introduced in the interval $13\mathrm{s}\le t\le 25\mathrm{s}$. Control signal ${\tau }_z$ (left) and thruster coefficients $u_5$,$u_6$ (right).

Figure 18

Additional thruster coefficient of −0.50 in the interval $13\mathrm{s}\le t\le 25\mathrm{s}$.

Figure 19

Time-varying additional thruster coefficient in the interval $13\mathrm{s}\le t\le 25\mathrm{s}$. Minimum value −0.25 and maximum value 0.25.

Figure 20

Experimental results for depth station-keeping with a time-varying external disturbance introduced in the interval $13\mathrm{s}\le t\le 25\mathrm{s}$. Depth (left) and tracking error (right).

Figure 21

Experimental results for depth station-keeping with a time-varying external disturbance introduced in the interval $13\mathrm{s}\le t\le 25\mathrm{s}$. Control signal ${\tau }_z$ (left) and thruster coefficients $u_5$,$u_6$ (right).