Svein I. Sagatun

DESIGN OF A DYNAMIC POSITIONING SYSTEM USING MODEL-BASED CONTROL

Abstract A dynamic positioning (DP) system includes different control functions for the automatic positioning and guidance of marine vessels by means of thruster and propeller actions. This paper describes the control functions which provide station-keeping and tracking. The DP controller is a model-based control design, where a new modified LQG feedback controller and a model reference feedforward controller are applied. A reference model calculates appropriate reference trajectories. Since it is not desirable, nor even possible, to counteract the wave-frequency movement caused by first-order wave loads, the control action of the propulsion system should be produced by the low-frequency part of the vessel movement caused by current, wind and second-order mean and slowly varying wave loads. A Kalman-filter-based state estimator and a Luenberger observer are used to compute the low-frequency feedback and feedforward control signals. Full-scale experiments with a multipurpose supply ship demonstrate the performance of the proposed controller.

Adaptive control of nonlinear systems: A case study of underwater robotic systems

The problem of controlling underwater mobile robots in 6 degrees of freedom (DOF) is addressed. Underwater mobile robots where the number of thrusters and control surfaces exceeds the number of controllable DOF are considered in detail. Unlike robotic manipulators underwater mobile robots should include a velocity dependent thruster configuration matrix B(q), which modifies the standard manipulator equation to: Mq + C(q)q + g(x) = B(q)u where x = J(x)q. Uncertainties in the thruster configuration matrix due to unmodeled nonlinearities and partly known thruster characteristics are modeled as multiplicative input uncertainty. This article proposes two methods to compensate for the model uncertainties: (1) an adaptive passivity-based control scheme and (2) deriving a hybrid (adaptive and sliding) controller. The hybrid controller combines the adaptive scheme where M, C, and g are estimated on-line with a switching term added to the controller to compensate for uncertainties in the input matrix B. Global stability is ensured by applying Barbalats Lyapunov-like lemma. The hybrid controller is simulated for the horizontal motion of the Norwegian Experimental Remotely Operated Vehicle (NEROV).

Adaptive control of nonlinear underwater robotic systems

The problem of controlling underwater mobile robots in six degrees of freedom (DOF) is addressed. Uncertainties in the input matrix due to partly known nonlinear thruster characteristics are modeled as multiplicative input uncertainty. Two methods to compensate for the model uncertainties are proposed: (1) an adaptive passivity-based control scheme, and (2) deriving a hybrid (adaptive and sliding) controller. The hybrid controller consists of a switching term which compensates for uncertainties in the input matrix and an online parameter estimation algorithm. Global stability is ensured by applying Barbalats Lyapunov-like lemma. The hybrid controller is simulated for the horizontal motion of the Norwegian Experimental Remotely Operated Vehicle (NEROV).<<ETX>>

Wave synchronizing crane control during water entry in offshore moonpool operations - experimental results

A new strategy for active control in heavy-lift offshore crane operations is suggested by introducing a new concept referred to as wave synchronization. Wave synchronization reduces the hydrodynamic forces by minimizing variations in the relative vertical velocity between payload and water using a wave-amplitude measurement. Wave synchronization is combined with conventional active heave compensation to obtain accurate control. Experimental results using a scale model of a semi-submerged vessel with a moonpool shows that wave synchronization leads to significant improvements in performance. Depending on the sea state and payload, the results indicate that the reduction in the standard deviation of the wire tension may be up to 50%.

Active control of underwater installation

An augmented impedance control scheme is proposed together with passive heave compensation to achieve higher operability on offshore underwater installation. The dynamics of the controller, the crane, the water surface in the moonpool, and a detailed mathematical model of the hydrodynamic loads and load effects on the object hitting the water surface and proceeding through the splash zone are included. Stability of the controller is proven, and simulation results with relevant full-scale parameters are included.

Wave synchronizing crane control during water entry in offshore moonpool operations

A new strategy for active control in heavy-lift offshore crane operations is suggested, by introducing a new concept referred to as wave synchronization. Wave synchronization reduces the hydrodynamic forces by minimization of variations in the relative vertical velocity between payload and water using a wave amplitude measurement. Wave synchronization is combined with conventional heave compensation to obtain accurate control. Experimental results using a scale model of a semi-submerged vessel with a crane and moonpool shows that wave synchronization leads to significant improvements in performance. Depending on the sea state and payload, the results indicate that the reduction in the standard deviation of the wire tension may be up to 50%.

Inertance Control of Underwater Installations

Abstract This article presents active control of offshore underwater installations using a modified impedance control scheme. The impedance control is augmented with acceleration feedback resulting in a concept named inertance control. The control scheme is used to tune the response of the handled object exposed to hydrodynamic loads. The proposed method is general and stability is proven. A detailed mathematical model of the MHS and loads and load effects on the handled object hitting the water surface and proceeding through the splash zone is included together with an illustrative simulation.

Boundary Control of a Transversely Vibrating Beam Via Lyapunov Method

Abstract This paper discusses the boundary stabilization of a beam in free transversal vibration. We consider a nonlinear partial differential equation (PDE) and based on this model construct two linear control laws to stabilize the system. The first control law guarantees globally convergent of states, while the second control law results into an exponentially stable closed loop. The latter control law is formed by feedbacking slope and velocity of beam’s boundary displacement. Numerical simulations are performed to test each of the control laws. The outcome of simulations are compared and discussed. The novelty of this article is that globally convergece and exponentially stabilization of transversely vibration in a beam is achieved, via boundary control without resorting to truncation of model.

Adaptive wave synchronization for lowering of crane load

Abstract An adaptive observer for wave synchronization of an object through the wave zone is proposed in this article. The observer is based on measurements of the vessel accelerations in heave, pitch and roll, the winch motor position and the wire force. These measurements are normally available in active heave compensation systems. Comparison between wave synchronization and conventional heave compensation shows large improvement with respect to hydrodynamic forces on the object during wave synchronization. This is especially the case for large sea states.

Optimal and Adaptive Control of Underwater Vehicles

This article contains a continuous-time optimal and adaptive control scheme for underwater vehicles moving in six degrees of freedom. The control scheme is an extension of the algorithm of [4] and a modification of the algorithm found in [6]. The algorithm is optimal in the sense that it minimizes the state errors and the forces which contribute to the vehicles kinetic energy that is spend to correct these errors. The performance measure does also contain a term which penalizes the quadratic tracking errors proportional to the rate of energy which dissipates from the system due to damping.