Hydraulic Underwater Research Articles

A computation effective singularity avoidance method for the underwater manipulator with a non-spherical wrist

In order to perform increasingly complex underwater tasks, we have designed a hydraulic underwater non-spherical wrist manipulator with a binocular vision system and a six-dimensional force sensor mounted behind the end-effector, which can improve the path accuracy and automation capabilities of the system. The system lays a foundation for the high precision and autonomous deep-sea operation and can improve its intelligence level. In this paper, we focus on solving the singularity avoidance problem of the hydraulic underwater manipulator for accurate control, and propose a computationally efficient singularity avoidance method to improve the path accuracy of the manipulator with the non-spherical wrist. Firstly, the singular configurations of the manipulator are analyzed. Secondly, the Jacobian is decomposed into sub-block matrices to obtain the singular factors according to the singular configurations. Thirdly, the singularities are avoided by the approximate damped reciprocal method. Theoretical analysis shows that the computation costs of the proposed method are only one-third to one-half of that of the traditional methods. The simulation results prove that the proposed method can largely improve the path accuracy of the manipulator with less computation costs, which shows that our method would have a certain significance in improving the precision and real-time response of deep-sea operations.

Model reference adaptive control for a hydraulic underwater manipulator

This work is focused on an underwater manipulator. To solve its nonlinear dynamics and hydrodynamics when it operates underwater, model reference adaptive control (MRAC) is applied. The output of the reference model represents the expected transient response. Adaptive controller parameters are adjusted on-line in real time according to the error between the reference model output and the system output and the outputs of the state filters, using an adaptive parameter regulator. The system control input is thus achieved such that the system output can track the reference model output. Simulation results demonstrate that the proposed control scheme has accurate tracking ability and can greatly improve the dynamic and static properties of the system. The MRAC for the underwater manipulator is also fairly robust to load disturbance and system parameter variation.

Integrate and Optimized Design of a Hydraulic Underwater Manipulator

Underwater manipulator as efficient tools plays important roles in ocean exploitation. Because of its specificity for underwater operations, the structure of an underwater manipulator should be designed compactly with light weight and small size. Structure of a 7-function underwater manipulator is designed here. Except forearm rotation and wrist rotation which use hydraulic motors, other functions are driven by hydraulic cylinders, whose parameters are determined by force analysis and limit positions. The hydraulic pipelines from the elbow joint to the end are passed through the forearm, and the hydraulic cylinder of the jaw is integrated into the wrist motor. Built-in pipelines are achieved in the designed structure and the whole weight and volume are greatly reduced by integrated and optimized design.

A comparison between robust and adaptive hybrid position/force control schemes for hydraulic underwater manipulators

Tele-operated hydrauilic underwater manipulators are commonly used to perform remote underwater intervention tasks such as weld inspection or mating of connectors Automation of these tasks to use tele-assitance requires a suitable hybrid position/force control scheme, to specify simultaneously the robot motion and contact forces. Classical linear control does not allow for the highly non-linear and time varying robot dynamics in this situation. Adequate control performance requires more advanced controllers. This paper presents and compares two different advanced hybrid control algorithms. The first is based on a modified Variable Structure Control (VSC-HF) with a virtual environment, and the second uses a multivariable self-tuning adaptive controller. A direct comparison of the two proposed control schemes is performed in simulation, using a model of the dynamics of a hydraulic underwater manipulator (a Slingsby TA9) in contact with a surface. These comparisons look at the performance of the controllers under a wide variety of operating conditions, including different environment stiffnesses, positions of the robot and dynamic parameters. Conclusions are drawn based on the relative performance of each controller and on the practicalities of the proposed schemes.

Hybrid position/force control of a hydraulic underwater manipulator

Current generation underwater remotely operated vehicles equipped with robotic manipulators are teleoperated and consequently place a large workload burden on the human operator. A greater degree of automation could improve the efficiency and accuracy with which underwater tasks are carried out. A hybrid position/force control scheme is proposed to control an industrial hydraulic underwater manipulator, modified to include a force/torque sensor, for tasks such as weld inspection. Modelling of the hydraulic actuation mechanism is performed and the resulting model is easily incorporated in the standard robot dynamic equations. An experimental facility using a PC-based digital signal processor is used to produce practical hybrid position/force results for a Slingsby TA9 hydraulic arm. Results are presented for the manipulator sliding across a planar surface in different parts of its work envelope. Good general agreement is obtained between the simulated and practical systems. A fixed gain control strategy is shown to work well, provided it is tuned for the arm configuration when sliding occurs.

Motion planning and contact control for a tele-assisted hydraulic underwater robot

Motion planning and contact control for a tele-assisted hydraulic underwater robot