Nikolaos I. Xiros

Control Theory and Applications

In this chapter, a presentation of control theory and engineering as applied to ocean engineering is given. The chapter starts with the fundamentals of systems science and theory, i. e., descriptions of systems in state space and for linear, time-invariant ones in the frequency domain using the Laplace transform as well as with ordinary differential equations with respect to time. Then stability, controllability, and observability with an emphasis to linear, time-invariant systems are presented. Bode plots for sinusoidal steady-state analysis as well as the root locus technique for proportional gain feedback design are presented. For single-input, single-output systems, PID control is introduced as both a pole placement problem as well as in the framework of conventional Ziegler–Nichols methods. Pole placement design with the addition of Luenberger observers is presented for linear, time-invariant systems with any time of inputs and outputs. A brief presentation of digital controller implementations is given. Applications from ocean engineering include control of autonomous underwater vehicles and autopilots for surface vessels.

Power Take-Off Control of In-Stream Hydrokinetic Turbines

In-stream hydrokinetic electricity production, electricity generation from moving currents without the use of dams, has significant potential for increasing electric power production. This project evaluates a control system designed to regulate rotor rate (rpm) to improve power production from in-stream hydrokinetic turbines. The control algorithm is evaluated using both a numerical model of a rigidly mounted tidal turbine and a numerical model of a moored ocean current turbine system. These two system models are each coupled to an induction electric machine model. Based on the turbine torque-speed characteristic, as well as the asynchronous machine features, a Look-Up-Table (LUT) is used to generate the frequency of the sinusoidal voltages of the three phases to be supplied to the machine. However, to compensate for disturbances and perturbations of the power-plant a PI controller is generating a correction term for electrical frequency superimposed to the output of the LUT. A first round of simulations using the numerical models was performed in order to evaluate the developed algorithms.Copyright

Ocean Wave Energy Conversion Concepts

The purpose of this chapter is to present the basic concepts of ocean wave energy conversion as an extension to the supporting subjects presented earlier in this handbook so that they can be useful to scientists, engineers and inventors. Although new research regarding all wave energy conversion aspects has been unveiling continuously over the past few decades and is expected to remain so in the foreseeable future, they are based on only a few fundamental wave energy conversion techniques. These methods are described and their uses and performance are illustrated. Finally, an account of several recent developments and advances in ocean wave energy research is presented as an appendix with an emphasis to commercialization of different technologies in conjunction with their financial viability, techno-economic, and environmental impact.

Nonlinear Identification and Input-Output Representation of the Modal Dynamics of Marine Slender Structures

The present work treats the problem of the dynamic behavior of a vertical slender structure subject to combined axial and transverse motions. The solution method is based on a Galerkin-type semi-analytical formulation. The responses to sinusoidal monochromatic excitation are assessed with respect to the significance of each mode and their spectral content. As a result, a reduced, yet nonlinear, lumped model for each one of the significant modes of the structure is generated. The parameters of these fixed-structure models can be determined systematically by two methods relying on the spectral analysis of the numerically calculated modal responses of the structure. The resulting models constitute an explicit input-output relationship between the imposed motions and the modes of the structure, useful for stability analysis, design and control.

Nonlinear Control Modeling for Arrays of Coupled Mechatronic Transducers

A typical mechatronic oscillator involving a tuned resistor-inductor-capacitor circuit driven by a voltage source and coupled to a mass-spring-damper mechanical subsystem is analyzed in order to develop a nonlinear control model. The analysis is approached using the Volterrra/Wiener framework of nonlinear systems combined with the Hilbert Transform. The former is needed since the coupling between the electrical and mechanical parts is lost, should standard linearization is adopted. The latter is needed since a very important characteristic of the system, due to the presence of the capacitance, is frequency selectivity. To use this very frequency selectivity in an application, involving more than one transducer units, modulation of the carrier signal, amplitude modulation in this case, must be implemented. Carrier modulation enables multiple units, structurally identical otherwise but for the capacitance value adjusting the tuning frequency setting, in the same application. Both a low-pass and a band-pass model are developed and subsequently simulation runs are performed. Since the modulation method employed is not of the suppressed-carrier type, the equilibrium point is of dynamic rather than static nature, corresponding to initial values for current and displacement. The model is enhanced by superimposing an external disturbance in form of a baseband force acting on the mass payload of the mechanical subsystem. Then, the response of the system to monochromatic and polychromatic excitations is investigated; making sure among others none of the constraints is violated. The transducer configuration investigated here can be employed as an energy harvesting device in cases where vibrational or oscillatory motion of a mass is involved, e.g. ocean wave energy concepts, Vortex-Induced Vibrations etc., especially if no physical contact between the driving process and the driven circuit is feasible or practical.Copyright

Model predictive control for moored ocean current turbines

We investigate the feasibility of model predictive control (MPC) in ocean current turbine (OCT) control. A prediction model is obtained from a nonlinear model of a moored OCT via linearization. Three control inputs are assumed, the electromechanical torque applied to the rotor shaft and two blade pitch cyclic controls. The control problem contains is highly constrained, which prompts consideration of MPC. For MPC evaluation, comparisons with another constrained control technique, output variance constrained control, are included, as well as simulations of the linear and nonlinear OCT dynamics when the MPC strategy is applied.

Modeling, System Identification and Linearization of Underwater Turbine Power Plant Dynamics

A suite of nonlinear dynamical simulations of in-stream hydrokinetic devices has been developed and this paper discussed the linearization of these models for control system development. One of these numerical simulations represents a small 3 meter rotor diameter, 35 kW turbine with fixed pitch blades, and the other a 20 meter, 700 kW turbine with variable pitch blades. Each turbine simulation can be operated to represent a bottom mounted tidal turbine or a moored ocean current turbine. These nonlinear dynamical models can serve as stepping stones toward control system design using linear or nonlinear, time or frequency-domain methodologies. A common step further toward controller synthesis is to obtain linearized models of the system dynamics. Towards this end, two linearization techniques are presented. The first is based straightforward analytical and numerical linearization of the full nonlinear state-space equations of the plant; this method has been applied for the underwater flight dynamics of the 700 kW plant. The second is a phenomenological system identification approach consisting of data analysis performed on time series obtained through simulations; it has been used to model the system of systems in the case of the 35 kW plant. In the first approach, the linearized model is valid for specific operating conditions around equilibrium values of the state variables. In the second approach, the plant dynamical model is used as a black-box in order to obtain the simulated response of the system to a variety of test input signals, like e.g. sinusoids of relatively small amplitudes and various frequencies superimposed to steady-state offsets; in effect, a phenomenological model is derived describing the plant dynamics. The outcomes of both approaches are assessed and several conclusions are drawn from the analysis.Copyright

Simulation of Electric Propulsion Drive With Surface Piercing Propeller for Planing-Hull Watercraft

A simulation model for planing-hull watercraft propulsion consisting of a 3ph induction motor as prime mover, direct shaft drive, surface piercing propeller as thruster and speed controller was developed, tested, and operated demonstrating the ability to analyze the entire system numerically. The modularity of the model allows for the user to easily substitute different or more advanced modules, or add additional ones to obtain greater level of detail or simulate more complex interactions. Trends indicate an interest in all electric ship design as well as the use of surface piercing propellers for small craft. All-electric drive plants offer distinct advantages due to their flexibility in arrangements, ability to eliminate reduction gears in many cases, and wide range of available sizes. Of particular interest is the ability to apply electric drive to small craft with planing-hull, which achieve significantly higher velocities and where arrangements and maneuverability are of high concern. The accurate control of speed combined with the ability to output fairly constant torque across a wide range of speeds allows application of non-conventional thrusters such as surface piercing propellers to small craft. Due to the availability of towing tank data from a surface piercing propeller, a methodology to translate measured performance data for non-conventional thrusters as a computer simulation block needed to be developed. The derived modeling block of the surface piercing propeller, which took the form of a neural net, was integrated with an AC induction motor prime mover module, detailed dynamic propulsion shaft module and Proportional-Integral (PI) control module. The modules were finally augmented with a block implementing Savitsky’s method for planing-hull modeling. Simulations were conducted using full-scale real-world conditions for a hull Bayliner 170 Bowrider boat the hull of which can be adjusted to accommodate for a 50HP electric motor and a surface-piercing propeller. The runs conducted demonstrated the model functionality and level of detail.Copyright

System Identification Using Neural Nets for Dynamic Modeling of a Surface Marine Vehicle

The scale model of a surface marine vehicle with electric propulsion by a dc motor and waterjet is built. A mathematical model capable to adequately describe the motion of the vehicle under a variety of conditions is developed by fusing basic principles with data series obtained through a series of field experiments. The aim is to minimize the number and cost of sensors needed in this end, without unacceptably compromising accuracy, by employing knowledge of vehicle dynamics in order to form a customized gray-box modeling approach. A set of nonlinear differential equations, used to depict the behavior of the marine vehicle at hand are derived. This dynamic model will form the basis for applying physicomimetic approaches to control and navigation of a standalone or swarm of similar vehicles. In the physicomimetic controller synthesis approach, the control problem is tackled by the concept of virtual forces acting on the vehicle and in result generating motion patterns that are desired in a certain application, e.g. avoid obstacles and collisions. To achieve physicomimetic control one needs to effectively cancel the actual dynamics or physics to which a vehicle’s motion complies with and then impose the desired dynamics through virtual forces. In the present work, as first step, a series of open loop experiments allow us developing the actual dynamics of vehicle motion.© 2014 ASME

Modeling and Control of the Electrical Actuation System of an Active Hydromagnetic Journal Bearing (AHJB)

The architecture of the electrical actuation module driving a magnetic-hydraulic bearing system is presented. The bearing is intended to be scaled for use in applications of all sizes in industries like shipboard for support of the engine-propeller shaft or in power-plants for the shaft through which the prime mover, e.g. steam or gas turbine, is driving the electric generator. The benefits of this new bearing is first and foremost its superb performance in terms of low down to practically no friction losses since there is no direct contact between the supporting bearing surface and the rotating shaft supported. Other benefits include the potential of active, inline, real-time balancing and alignment. To implement such concept of a magnetic-hydraulic bearing, the following tasks need to be carried out. First, identification of mechanical, electrodynamical and circuit properties of the bearing’s electromagnets in the system is necessary. Toward such identification, a series of experiments needed to be carried out. To be able to carry out these experiments, a specific power electronic converter is developed to drive each electromagnet. The power electronic drive is a quad MOSFET circuit based on full-bridge converter topology and outfitted with appropriate sensory instrumentation to collect and record measurements of all the physical variables of interest. Special care has been taken to compensate for magnetic hysteresis of the electromagnets, mitigate any induction heating effects and maintain operation within the material’s linear region i.e. without significant saturation occurring. The use of a power transistor bridge allows rapid changes to be applied on the electromagnet’s load force which could compensate disturbance or misalignment developed on the shaft supported. The data series from these experiments are useful for formulating a possibly nonlinear model of the electromagnetical and electromechanical processes involved in the bearing’s operation. Such a model can then be employed to help design a digital microcontroller system which could effectively drive the power electronics and electromagnets to perform their required tasks as part of the bearing. Besides, the model could also be used for the synthesis of the nonlinear, sampled-data (discrete-time) control law which will be programmed on the microcontroller system board.Copyright