100 research outputs found
Advanced Anti-Windup Techniques for the Limitation of the Effects of the Actuator Saturation
In this thesis an industrial issue is analysed. The issue consist on the undesirable effect of actuator sturation. Two approaches are given to solve the issue: an accurate inertia identification algorithm based on the DFT coefficient; and advanced anti-windup compensators. The principle of the modern anti-windup (DLAW and MRAW, LMI-based design approach), and a systematic design design procedure for the observer-based anti-windup are given. Simulation and test results are also given.ope
Nonlinear constrained and saturated control of power electronics and electromechanical systems
Power electronic converters are extensively adopted for the solution of timely issues, such
as power quality improvement in industrial plants, energy management in hybrid electrical
systems, and control of electrical generators for renewables. Beside nonlinearity, this systems
are typically characterized by hard constraints on the control inputs, and sometimes
the state variables. In this respect, control laws able to handle input saturation are crucial
to formally characterize the systems stability and performance properties. From a practical
viewpoint, a proper saturation management allows to extend the systems transient
and steady-state operating ranges, improving their reliability and availability.
The main topic of this thesis concern saturated control methodologies, based on modern
approaches, applied to power electronics and electromechanical systems. The pursued
objective is to provide formal results under any saturation scenario, overcoming the
drawbacks of the classic solution commonly applied to cope with saturation of power converters,
and enhancing performance. For this purpose two main approaches are exploited
and extended to deal with power electronic applications: modern anti-windup strategies,
providing formal results and systematic design rules for the anti-windup compensator, devoted
to handle control saturation, and “one step” saturated feedback design techniques,
relying on a suitable characterization of the saturation nonlinearity and less conservative
extensions of standard absolute stability theory results.
The first part of the thesis is devoted to present and develop a novel general anti-windup
scheme, which is then specifically applied to a class of power converters adopted for power
quality enhancement in industrial plants. In the second part a polytopic differential inclusion
representation of saturation nonlinearity is presented and extended to deal with a
class of multiple input power converters, used to manage hybrid electrical energy sources.
The third part regards adaptive observers design for robust estimation of the parameters
required for high performance control of power systems
Control of systems subject to uncertainty and constraints
All practical control systems are subject to constraints, namely constraints aris¬ing from the actuator’s limited range and rate capacity (input constraints) or from imposed operational limits on plant variables (output constraints). A linear control system typically yields the desirable small signal performance. However, the presence of input constraints often causes undesirable large signal behavior and potential insta¬bility. An anti-windup control consists of a remedial solution that mitigates the effect of input constraints on the closed-loop without affecting the small signal behavior. Conversely, an override control addresses the control problem involving output con¬straints and also follows the idea that large signal control objectives do not alter small signal performance. Importantly, these two remedial control methodologies must in¬corporate model uncertainty into their design to be considered reliable in practice. In this dissertation, shared principles of design for the remedial compensation problem are identified which simplify the picture when analyzing, comparing and synthesiz¬ing for the variety of existing remedial schemes. Two performance objectives, each one related to a different type of remedial compensation, and a general structural representation associated with both remedial compensation problems will be consid¬ered. The effect of remedial control on the closed-loop will be evaluated in terms of two general frameworks which permit the unification and comparison of all known remedial compensation schemes. The difference systems describing the performance objectives will be further employed for comparison of remedial compensation schemes under uncertainty considerations and also for synthesis of compensators. On the ba¬sis of the difference systems and the general structure for remedial compensation, systematic remedial compensation synthesis algorithms for anti-windup and override compensation will be given and compared. Successful application of the proposed robust remedial control synthesis algorithms will be demonstrated via simulation
Application of robust control in unmanned vehicle flight control system design
The robust loop-shaping control methodology is applied in the flight control system
design of the Cranfield A3 Observer unmanned, unstable, catapult launched air vehicle.
Detailed linear models for the full operational flight envelope of the air vehicle are
developed. The nominal and worst-case models are determined using the v-gap metric.
The effect of neglecting subsystems such as actuators and/or computation delays on
modelling uncertainty is determined using the v-gap metric and shown to be significant.
Detailed designs for the longitudinal, lateral, and the combined full dynamics TDF
controllers were carried out. The Hanus command signal conditioning technique is also
implemented to overcome actuator saturation and windup. The robust control system is
then successfully evaluated in the high fidelity 6DOF non-linear simulation to assess its
capability of launch stabilization in extreme cross-wind conditions, control
effectiveness in climb, and navigation precision through the prescribed 3D flight path in
level cruise. Robust performance and stability of the single-point non-scheduled control
law is also demonstrated throughout the full operational flight envelope the air vehicle
is capable of and for all flight phases and beyond, to severe launch conditions, such as
33knots crosswind and exaggerated CG shifts.
The robust TDF control law is finally compared with the classical PMC law where the
actual number of variables to be manipulated manually in the design process are shown
to be much less, due to the scheduling process elimination, although the size of the final
controller was much higher. The robust control law performance superiority is
demonstrated in the non-linear simulation for the full flight envelope and in extreme
flight conditions
Design and application of advanced disturbance rejection control for small fixed-wing UAVs
Small Unmanned Aerial Vehicles (UAVs) have seen continual growth in both research and
commercial applications. Attractive features such as their small size, light weight and
low cost are a strong driver of this growth. However, these factors also bring about some
drawbacks. The light weight and small size means that small UAVs are far more susceptible
to performance degradation from factors such as wind gusts. Due to the generally low
cost, available sensors are somewhat limited in both quality and available measurements.
For example, it is very unlikely that angle of attack is sensed by a small UAV. These
aircraft are usually constructed by the end user, so a tangible amount of variation will
exist between different aircraft of the same type. Depending on application, additional
variation between flights from factors such as battery placement or additional sensors may
exist. This makes the application of optimal model based control methods difficult.
Research literature on the topic of small UAV control is very rich in regard to high
level control, such as path planning in wind. A common assumption in such literature
is the existence of a low level control method which is able to track demanded aircraft
attitudes to complete a task. Design of such controllers in the presence of significant wind
or modelling errors (factors collectively addressed as lumped disturbances herein) is rarely
considered.
Disturbance Observer Based Control (DOBC) is a means of improving the robustness
of a baseline feedback control scheme in the presence of lumped disturbances. The method
allows for the rejection of the influence of unmeasurable disturbances much more quickly
than traditional integral control, while also enabling recovery of nominal feedback con-
trol performance. The separation principle of DOBC allows for the design of a nominal
feedback controller, which does not need to be robust against disturbances. A DOBC
augmentation can then be applied to ensure this nominal performance is maintained even
in the presence of disturbances. This method offers highly attractive properties for control
design, and has seen a large rise in popularity in recent years.
Current literature on this subject is very often conducted purely in simulation. Ad-
ditionally, very advanced versions of DOBC control are now being researched. To make
the method attractive to small UAV operators, it would be beneficial if a simple DOBC
design could be used to realise the benefits of this method, as it would be more accessible
and applicable by many.
This thesis investigates the application of a linear state space disturbance observer to
low level flight control of a small UAV, along with developments of the method needed
to achieve good performance in flight testing. Had this work been conducted purely in
simulation, it is likely many of the difficulties encountered would not have been addressed
or discovered.
This thesis presents four main contributions. An anti-windup method has been devel-
oped which is able to alleviate the effect of control saturation on the disturbance observer
dynamics. An observer is designed which explicitly considers actuator dynamics. This
development was shown to enable faster observer estimation dynamics, yielding better
disturbance rejection performance. During initial flight testing, a significant aeroelastic
oscillation mode was discovered. This issue was studied in detail theoretically, with a pro-
posed solution developed and applied. The solution was able to fully alleviate the effect in
flight. Finally, design and development of an over-actuated DOBC method is presented.
A method for design of DOBC for over actuated systems was developed and studied. The
majority of results in this thesis are demonstrated with flight test data
Gradient projection anti-windup scheme
Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 205-217).It is a well-recognized fact that control saturation affects virtually all practical control systems. It leads to controller windup, which degrades/limits the system's closed-loop performance, and may cause catastrophic failures if it induces instability. Anti-windup compensation is one of two main approaches to mitigate the effects of windup, and is conceptually and practically attractive. For the idealized case of constrained linear time invariant (LTI) plants driven by LTI controllers, numerous anti-windup schemes exist. However, most practical control systems are inherently nonlinear, and anti-windup compensation for nonlinear systems remains largely an open problem. To this end, we propose the gradient projection anti-windup (GPAW) scheme, which is an extension of the conditional integration method to multi-input-multi-output (MIMO) nonlinear systems, using Rosen's gradient projection method for nonlinear programming. It achieves controller state-output consistency by projecting the controller state onto the unsaturated region induced by the control saturation constraints. The GPAW-compensated controller is a hybrid controller defined by the online solution to either a combinatorial optimization subproblem, a convex quadratic program, or a projection onto a convex polyhedral cone problem. We show that the GPAW-compensated system is obtained by modifying the uncompensated system with a passive operator. Qualitative weaknesses of some existing anti-windup results are established, which motivated a new paradigm to address the anti-windup problem. It is shown that for a constrained first order LTI plant driven by a first order LTI controller, GPAW compensation can only maintain/enlarge its region of attraction (ROA). In this new paradigm, we derived some ROA comparison and stability results for MIMO nonlinear as well as MIMO LTI systems. The thesis is not that the GPAW scheme solves a centuries-old open problem of immense practical importance, but rather, that it provides a potential path to a solution. We invite the reader to join us in this quest at the confluence of nonlinear systems, hybrid systems, projected dynamical systems, differential equations with discontinuous right-hand sides, combinatorial optimization, convex analysis and optimization, and passive systems.by Chun Sang Justin Teo.Sc.D
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