336 research outputs found
Intelligent Control Strategies for an Autonomous Underwater Vehicle
The dynamic characteristics of autonomous underwater vehicles (AUVs) present a control
problem that classical methods cannot often accommodate easily. Fundamentally, AUV dynamics
are highly non-linear, and the relative similarity between the linear and angular velocities about
each degree of freedom means that control schemes employed within other flight vehicles are not
always applicable. In such instances, intelligent control strategies offer a more sophisticated
approach to the design of the control algorithm. Neurofuzzy control is one such technique, which
fuses the beneficial properties of neural networks and fuzzy logic in a hybrid control architecture.
Such an approach is highly suited to development of an autopilot for an AUV.
Specifically, the adaptive network-based fuzzy inference system (ANFIS) is discussed in
Chapter 4 as an effective new approach for neurally tuning course-changing fuzzy autopilots.
However, the limitation of this technique is that it cannot be used for developing multivariable
fuzzy structures. Consequently, the co-active ANFIS (CANFIS) architecture is developed and
employed as a novel multi variable AUV autopilot within Chapter 5, whereby simultaneous control
of the AUV yaw and roll channels is achieved. Moreover, this structure is flexible in that it is
extended in Chapter 6 to perform on-line control of the AUV leading to a novel autopilot design
that can accommodate changing vehicle pay loads and environmental disturbances.
Whilst the typical ANFIS and CANFIS structures prove effective for AUV control system
design, the well known properties of radial basis function networks (RBFN) offer a more flexible
controller architecture. Chapter 7 presents a new approach to fuzzy modelling and employs both
ANFIS and CANFIS structures with non-linear consequent functions of composite Gaussian form.
This merger of CANFIS and a RBFN lends itself naturally to tuning with an extended form of the
hybrid learning rule, and provides a very effective approach to intelligent controller development.The Sea Systems and Platform Integration Sector,
Defence Evaluation and Research Agency, Winfrit
CONTROL TECHNIQUES APPLIED TO INTEGRATED SHIP MOTION CONTROL
Fins stabilisers are devices which are fitted to the hull of a ship and utilised to ameliorate its
rolling motions. They apply a regulated moment about the ship's axis of roll in order to
oppose the sea induced disturbances. Recognising their unsurpassed performance, the Royal
Navy, since the 1950's, equips all its vessels with fin stabilisers. It can be shown that the
rudders, in vessels of appropriate size, also have the potential to be harnessed as roll
stabilisers Rudder Roll Stabilisation (RRS) without degrading the ship's course-keeping.
Thus creating a more stable platform for the human operators and equipment.
The reported success of RRS imparted an impetus to the Royal Navy to initiate this study.
The objectives are to ascertain whether RRS is possible without rudder modifications and to
establish whether enhanced levels of stabilisation would accrue if the fins and RRS were
operated in congress. The advantages in this novel approach being: avoidance of redesign
and refit of rudders, three modes of operation (fins alone, RRS alone and combined RRS
and fins), reduced fin activity and by implication self-generated noise, and amenability to be
retrofitted by simple alteration of any existing ship's autopilot software.
The study initially examined the mathematical models of the ship dynamics, defining
deficiencies and evaluating sources of uncertainty. It was postulated that the dual purpose
of the rudder can be separated into non-interacting frequency channels for controller design
purposes. An integrated design methodology is adopted to the roll stabilisation problem.
Investigating the capabilities of the rudder servomechanism, a new scheme, the
Anti-Saturation Algorithm (ASA) was proposed which can eliminate slew rate saturation.
Application of the ASA is generic to any servomechanism.
The effects of lateral accelerations of the ship on human operators was examined. This
resulted in an unique contribution to the Lateral Force Estimator problem in terms of
generating time domain models and defining the limitations of the applicability of a control
design strategy.
Linear Quadratic Guassian and two types of classical controllers were constructed for the
RRS and fins. A novel application of linear robust control theory to the ship roll
stabilisation problem resulted in H . controllers whose performance was superior to the
other design methods. This required the development of weight functions and the
identification and quantification of possible sources of uncertainty. The structured singular
value utilised this information to give comparable measures of robustness.
The sea trials conducted represent the first experience of the integrated ship roll stabilisation
approach. Experimental results are detailed. These afforded an invaluable opportunity to
validate the software employed to predict ship motion. The data generated from the sea
trials concurs with the simulations data in predicting that enhanced levels of roll stabilisation
are possible without any modification to the rudder system. They also confirm that when the
RRS is acting in congress with the fin stabilisers the activity of both actuators diminishes
THE AUTOMATIC CONTROL OF LARGE SHIPS IN CONFINED WATERS
The design and evaluation of a control system, which can be
utilised for the automatic guidance of large ships in confined or
restricted waters, is investigated.
The vessel is assumed to be a multivariable system and it is
demonstrated that a non-linear, time-varying mathematical model
most accurately describes the motion of the hull, particularly in
tight manoeuvres.
A discrete optimal controller has been designed to control
simultaneously track, heading and forward velocity. The system is
most effective whilst operating under a dual-mode policy. It is
shown that feedback matrix adaption is necessary to deal with
changes in forward velocity and a form of gain scheduling is proposed.
Active disturbance control is employed to counteract effects of wind
and tide.
An inertial navigation system, together with an optimal controller
and filter, is installed on-board a car ferry model. Free-sailing
tests show that the performance characteristics of the system are in
accordance with theoretical predictions.
The feasibility of implementation on a full-size vessel is
considered.University College, Londo
An adaptive autopilot design for an uninhabited surface vehicle
An adaptive autopilot design for an uninhabited surface vehicle
Andy SK Annamalai
The work described herein concerns the development of an innovative approach to the
design of autopilot for uninhabited surface vehicles. In order to fulfil the requirements of
autonomous missions, uninhabited surface vehicles must be able to operate with a minimum
of external intervention. Existing strategies are limited by their dependence on a fixed
model of the vessel. Thus, any change in plant dynamics has a non-trivial, deleterious effect
on performance. This thesis presents an approach based on an adaptive model predictive
control that is capable of retaining full functionality even in the face of sudden changes in
dynamics.
In the first part of this work recent developments in the field of uninhabited surface vehicles
and trends in marine control are discussed. Historical developments and different strategies
for model predictive control as applicable to surface vehicles are also explored. This thesis
also presents innovative work done to improve the hardware on existing Springer
uninhabited surface vehicle to serve as an effective test and research platform. Advanced
controllers such as a model predictive controller are reliant on the accuracy of the model to
accomplish the missions successfully. Hence, different techniques to obtain the model of
Springer are investigated. Data obtained from experiments at Roadford Reservoir, United
Kingdom are utilised to derive a generalised model of Springer by employing an innovative
hybrid modelling technique that incorporates the different forward speeds and variable
payload on-board the vehicle. Waypoint line of sight guidance provides the reference
trajectory essential to complete missions successfully.
The performances of traditional autopilots such as proportional integral and derivative
controllers when applied to Springer are analysed. Autopilots based on modern controllers
such as linear quadratic Gaussian and its innovative variants are integrated with the
navigation and guidance systems on-board Springer. The modified linear quadratic
Gaussian is obtained by combining various state estimators based on the Interval Kalman
filter and the weighted Interval Kalman filter.
Change in system dynamics is a challenge faced by uninhabited surface vehicles that result
in erroneous autopilot behaviour. To overcome this challenge different adaptive algorithms
are analysed and an innovative, adaptive autopilot based on model predictive control is
designed. The acronym ‘aMPC’ is coined to refer to adaptive model predictive control that
is obtained by combining the advances made to weighted least squares during this research
and is used in conjunction with model predictive control. Successful experimentation is
undertaken to validate the performance and autonomous mission capabilities of the adaptive
autopilot despite change in system dynamics.EPSRC (Engineering and Physical Sciences Research Council
A Predictive Fuzzy-Neural Autopilot for the Guidance of Small Motorised Marine Craft
This thesis investigates the design and evaluation of a control system, that is able to adapt
quickly to changes in environment and steering characteristics. This type of controller is
particularly suited for applications with wide-ranging working conditions such as those experienced
by small motorised craft.
A small motorised craft is assumed to be highly agile and prone to disturbances, being
thrown off-course very easily when travelling at high speed 'but rather heavy and sluggish
at low speeds. Unlike large vessels, the steering characteristics of the craft will change
tremendously with a change in forward speed. Any new design of autopilot needs to be to
compensate for these changes in dynamic characteristics to maintain near optimal levels of
performance.
This study identities the problems that need to be overcome and the variables involved.
A self-organising fuzzy logic controller is developed and tested in simulation. This type of
controller learns on-line but has certain performance limitations.
The major original contribution of this research investigation is the development of an
improved self-adaptive and predictive control concept, the Predictive Self-organising Fuzzy
Logic Controller (PSoFLC). The novel feature of the control algorithm is that is uses a
neural network as a predictive simulator of the boat's future response and this network is
then incorporated into the control loop to improve the course changing, as well as course
keeping capabilities of the autopilot investigated.
The autopilot is tested in simulation to validate the working principle of the concept and
to demonstrate the self-tuning of the control parameters. Further work is required to establish
the suitability of the proposed novel concept to other control
Variable structure techniques in control system design
During the last twenty years, control theorists belonging almost exclusively to the USSR, have laid down the foundations of variable-structure systems (commonly abbreviated to vsS). As the name implies, such systems are allowed to change their structure through time in accordance with some preassigned algorithm. The theory has demonstrated that some significant advantages could be gained by adopting that approach in the, design of automatic control systems, amongst which are good transient responses and insensitivity to parametric variations and to external disturbances. The VS controller is slightly more complex than a fixed structure design based on standard methods such as state feedback or frequency response techniques, but is a great deal less complex than some adaptive designs. It also lends itself to a straightforward microcomputer implementation.
While the theoretical aspect of VSS has been well explored, its general applicability to engineering problems is yet to be established. There are still unanswered questions as to the suitability of the method for practical systems, which invariably
contain a certain amount of noise, uncertainties and nonlinearities. The work described in this thesis concentrates on that particular aspect and is, in brief, an investigation of VSS as an engineering design procedure. The theory of VSS is reviewed and the principles are then applied to a number of engineering examples. The performance of the systems are assessed from digital simulation runs, hybrid computation and the microcomputer control of a DC motor
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