Switching PD-based sliding mode control for hovering of a tilting-thruster underwater robot.

Sangrok Jin,Jongwon Kim,Jeongae Bak,TaeWon Seo,Hwa Soo Kim,Jun Ma

doi:10.1371/journal.pone.0194427

Sangrok Jin, Jongwon Kim + Show 4 more

Open Access

https://doi.org/10.1371/journal.pone.0194427

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Abstract

This paper presents a switching PD-based sliding mode control (PD-SMC) method for the 6-degree-of-freedom (DOF) hovering motion of the underwater robot with tilting thrusters. Four thrusters of robot can be tilted simultaneously in the horizontal and vertical directions, and the 6-DOF motion is achieved by switching between two thruster configurations. Therefore, the tilting speed of thruster becomes the most essential parameter to determine the stability of hovering motion. Even though the previous PD control ensures stable hovering motion within a certain ranges of tilting speed, a PD-SMC is suggested in this paper by combining PD control with sliding mode control in order to achieve acceptable hovering performance even at the much lower tilting speeds. Also, the sign function in the sliding mode control is replaced by a sigmoid function to reduce undesired chattering. Simulations show that while PD control is effective only for tilting duration of 600 ms, the PD-based sliding mode control can guarantee the stable hovering motion of underwater robot even for the tilting duration of up to 1500 ms. Extensive experimental results confirm the hovering performance of the proposed PD-SMC method is much superior to that of PD method for much larger tilting durations.

Highlights

As the robot technologies have been developed, the missions for underwater robots have rapidly expanded from monitoring or inspection to practical underwater tasks that are similar to those of a human diver
The proposed PD-based sliding mode control (PD-SMC) is designed by combining the sliding mode control with the PD control, which can lead to more acceptable performance even for much larger tilting duration in comparison with the previous PD control
The contributions of this paper are summarized as follow: first, a switching PD-based sliding mode control is successfully designed for tilting thrust mechanism of TTURT

Summary

Introduction

As the robot technologies have been developed, the missions for underwater robots have rapidly expanded from monitoring or inspection to practical underwater tasks that are similar to those of a human diver. We simplified the control algorithm, and a selective switching PD controller was designed for 6-DOF hovering control [5] The proposed PD-SMC is designed by combining the sliding mode control with the PD control, which can lead to more acceptable performance even for much larger tilting duration in comparison with the previous PD control. The contributions of this paper are summarized as follow: first, a switching PD-based sliding mode control is successfully designed for tilting thrust mechanism of TTURT. Note that the TTURT’s thrusters are tilted at four vertices of the robot body so that the thrust vectoring matrix B has nonlinear terms with a sinusoidal function for the tilting angles [4]. Since the all thrusters are synchronously tilted, the thrust vectoring matrix is given by 2 cposffiffia cposffiffia cposffiffia cposffiffia 3

À Lsina

DZ þ KD À ΛðCZ þ DZ À

Experiments

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