Bradley J. Buckham

Dynamics and control of a towed underwater vehicle system, part I: model development

Abstract Autonomous vehicles are being developed to replace the conventional, manned surface vehicles that tow mine hunting towed platforms. While a wide body of work exists that describes numerical models of towed systems, they usually include relatively simple models of the towed bodies and neglect the dynamics of the towing vehicle. For systems in which the mass of the towing vehicle is comparable to that of the towed vehicle, it becomes important to consider the dynamics of both vehicles. In this work, we describe the development of a numerical model that accurately captures the dynamics of these new mine hunting systems. We use a lumped mass approximation for the towcable and couple this model to non-linear numerical models of an autonomous surface vehicle and an actively controlled towfish. Within the dynamics models of the two vehicles, we include non-linear controllers to allow accurate maneuvering of the towed system.

Dynamics and control of towed underwater vehicle system, part II: model validation and turn maneuver optimization

This paper presents a validation of a three-dimensional dynamics model of a towed underwater vehicle system and discusses an application of the model to improve the performance of the system during a turn maneuver. The model was validated by comparing its results to experimental sea trial data, as well as to results from another independently developed simulation. The dynamics model was then imbedded in an optimization routine. This routine was used to vary turn radii in order to improve the U-turn performance. Significant improvements were obtained relative to a standard semicircular turn geometry.

Numerical optimization of a cage-mounted passive heave compensation system

Heave compensation systems are used to increase the safe operating sea-states of vertically tethered systems, as well as, decrease the tension in the tether and the motion of the underwater platform. However, the characteristics of the heave compensator must be carefully chosen or operating problems may be exacerbated. In this paper, a discrete representation of a deep-sea remotely operated vehicle system with a passive cage-mounted heave compensator is used within a sequential quadratic programming optimization routine to choose the stiffness and damping characteristics that minimize the tension in the tether and platform motion. Using the optimal values of the stiffness and damping, the rms cage motion and rms tension were significantly decreased over all operating depths and sea-states.

Finite Element Modeling of a Slewing Non-linear Flexible Beam for Active Vibration Control with Arrays of Sensors and Actuators

In this article, a new finite element model (FEM) of an Euler—Bernoulli beam, developed through an absolute nodal coordinate formulation (ANCF), is presented for simulation and analysis of the performance of surface-bonded piezoelectric actuators in suppressing non-linear transverse vibrations that are induced by very fast slewing. The elastic deformations experienced are an order of magnitude larger than cases considered to date, and the model employs a unique cubic spline approximation to the beam’s deformed elastic line that is in terms of node positions and curvatures. To ensure relevant commentary on the vibration suppression properties of the distributed piezoelectric actuators, a material damping model was introduced in the continuum equations to capture the non-linear damping of the very slender beam that is observed in experiments. Following the ANCF methodology, the constitutive damping moment is formulated in terms of the absolute nodal coordinates with care taken to ensure the calculation is singularity free. Galerkin’s method of weighted residuals is applied to discretize the revised equations of motion derived for the beam continuum. The FE beam model exploits a synergy between the twisted spline geometry and the lumped mass approximation to halve the size of the matrix equations that must be solved on each time step. However, this condensation of the matrix equations requires the use of interelement boundaries at the edges of the surface-bonded piezos. Using a single-link flexible manipulator as an example, a number of static and dynamic simulation examples that illustrate the validity of our FEM are presented, including comparisons to theoretical and other existing numerical solutions in literature. In addition, active vibration control examples are presented using proportional- and derivative-based hub motion and piezoelectric actuator controls in suppressing dramatic vibrations induced by fast slewing.

Robust Control of Underwater Vehicles with Fault-Tolerant Infinity-Norm Thruster Force Allocation

There are to objectives to this paper. First, a chattering-free sliding mode controller is proposed for the trajectory control of remotely operated vehicles (ROVs). Secondly, a new approach for thruster force allocation is proposed that is based on minimizing the linfin norm. With regards to the former, a new adaptive term is developed that eliminates the high frequency control action inherent in a conventional sliding-mode controller, and also removes the need for a priori knowledge of upper bounds on uncertainties in the dynamic parameters of the ROV. With regards to the latter, it is demonstrated that the linfin norm optimization can be cast as a linear problem that affords easy incorporation of the thruster saturation limits. Using numerical simulations, it is shown that the proposed linfin thruster allocation is capable of meeting the adaptive sliding mode controllers demands in the presence of thruster failures and is therefore fault tolerant. Finally, a recurrent neural network is designed in order to obtain a real time solution rate to the thruster allocation problem.

Comprehensive underwater vehicle-manipulator system teleoperation

In this work, a novel comprehensive scheme for the coordinated control of remotely operated vehicle-manipulator systems (ROVMs) is proposed. In the proposed scheme, instead of commanding the motion of the vehicle and the manipulator separately, a human pilot commands only the manipulators end-effector motion using a parallel- architectured six-degree-of-freedom (6-DOF) joystick. The generated reference motion is then converted into a set of desired ROV and manipulator joint motion by means of using a redundancy resolution scheme that provides the means to utilize redundant degrees of freedom to accomplish secondary objectives. The redundancy resolver uses the Gradient Projection Method combined with a Mamdani-based fuzzy determination of the hierarchy of the secondary objectives. The controller relies on a unified dynamic model of the system. The quasi-Lagrange method is used to derive the equations of motion in terms of the ROV body-fixed frame. For the control problem, a sliding-mode based controller is used that contains an adaptive term for the estimation of the upper bound on the lumped uncertainty vector. The hardware-in-the-loop simulation studies illustrate that detailed subsea tasks can be completed with a small, low-cost ROVM system using the proposed ROVM operation scheme.

Design Synthesis of a Wave Energy Converter

A demonstration ocean wave energy project is planned for Hesquiaht Sound, British Columbia, Canada in 2010. This project is led by SyncWave Systems and involves the University of Victoria, Dynamic Systems Analysis Ltd. and Marinus Power. The design process for this demonstration wave energy converter (WEC) including site selection, linear dynamics, control system scheduling and non-linear dynamics modelling is presented. The WEC is a heaving point absorber that extracts energy through the relative motion between two axisymmetric bodies and utilizes an internally housed mechanical system with adjustable inertia characteristics for frequency response tuning. Site selection for the device was completed by using an established wave propagation model to translate off-shore sea conditions calculated from WAVEWATCHIII (WW3) into near-shore conditions. By analysis of the predicted near-shore conditions a suitable location with a frequently energetic sea state was chosen. The design process of the WEC consists of a linearized dynamics model to optimize the controller and a nonlinear dynamics model to analyze the mooring and hull components. The resulting unit effectively captures energy from the prevailing sea-states while ensuring adequate survival capability.Copyright

Development of a coordinated controller for underwater vehicle-manipulator systems

In this work, the motions of an underwater remotely operated vehicle (ROV) and a spatial manipulator are coordinated using a consolidated controller. The controller translates a single pilot command, the desired position and orientation of the end effector, into a coordinated set of ROV and manipulator joint motions that satisfy the pilot intent in addition to a series of secondary objectives. The controller relies on a unified dynamic model of the system. The quasi-Lagrange method is used to derive the equations of motion in terms of the ROV body-fixed frame. For the control problem, a novel sliding-mode based controller is proposed. The controller contains two layers of adaptivity. The first layer is for adjusting PID gains, whereas the second layer is for estimating the bound on a lumped uncertainty vector. The second level of adaptation is shown to relax the Lyapunov stability requirements leading to a more robust controller. To generate reference state values, a redundancy resolution technique is utilized that is based on the gradient projection method merged with a fuzzy determination of the hierarchy of the secondary objectives. The redundancy resolution method distributes the pilots end-effector command over the ROV and the manipulator in an optimal manner using the redundant degrees of freedom. The results illustrate that detailed subsea tasks can be completed with a small, low-cost ROVM system using the proposed unified control scheme.

A Fault-Tolerant Fuzzy-Logic Based Redundancy Resolution Method for Underwater Mobile Manipulators

In this work, a fault-tolerant redundancy resolution scheme is presented that allows a single 6-DOF command to be distributed over a small URVM system composed of an otherwise underactuated URV and serial manipulator. The URVM system admits an infinite number of joint-space solutions for each commanded end-effector state due to its inherent redundancy. The primary objective is realized using the right Moore-Penrose pseudoinverse solution. The secondary objectives are: avoiding manipulator joint limits, avoiding singularity and high joint velocity; keeping the end-effector in sight of the on-board camera minimizing the URV motion; and minimizing the drag-force resistance, or weathervaning. Each criterion is defined within the framework of the Gradient Projection Method. The hierarchy for the secondary tasks is established by a low-level artificial pilot that determines a weighting factor for each criterion based on if- then type fuzzy rules that reflect an expert human pilots knowledge. A Mamdani fuzzy inference system is used to interpret the fuzzy rules based on the sensory knowledge. The resulting weight schedule yields a self-motion (null-space motion) that emulates how a skilled operator would utilize the full capabilities of the URVM to achieve the secondary objectives. The proposed redundancy resolution scheme has a fault-tolerant property. When a joint failure occurs, the scheme automatically redistributes the end-effector velocity command taking into account the faulty joints. To demonstrate the efficacy of the proposed scheme, several numerical simulations are performed The results illustrate the validity of the proposed redundancy scheme.

Simulation of a High-Energy Finfish Aquaculture Site Using a Finite Element Net Model

Over half of the seafood in the world today is produced through aquaculture. Many finfish species, such as Atlantic salmon, are farmed in permeable net pens in the ocean. Traditionally, these sites have been located in regions protected from high energy ocean swell, current, and wind. However, in areas such as Nova Scotia, Canada, there is a declining number of such protected locations available and so aquaculturists are moving into exposed sites. To safely operate at these sites, it is necessary to engineer the pens to withstand the forces of the open ocean. To conclusively assess finfish aquaculture equipment and moorings in open ocean conditions, Dynamic Systems Analysis Ltd. (DSA) has developed a finite-element net model (FENM) to interface with the dynamics simulation software ProteusDS. The following paper presents the development of the FENM and demonstrates the capability of the FENM to model wave and current loadings by comparing FENM simulations with published results from tank tests. In addition, the results of simulations of a full scale finfish aquaculture site in hurricane conditions are presented. The conditions were collected with an acoustic Doppler current profiler during hurricane Earl on September 4, 2010. The mooring line tensions from the ProteusDS simulation are compared against tensions measured during the storm with a Submersible Tension Logger (STL).Copyright