Experimental Nonlinear Control for Flutter Suppression in a Nonlinear Aeroelastic System

Experimental implementation of input–output feedback linearization in controlling the dynamics of a nonlinear pitch–plunge aeroelastic system is presented. The control objective is to linearize the system dynamics and assign the poles of the pitch mode of the resulting linear system. The implementation 1) addresses experimentally the general case where feedback linearization-based control is applied using as the output a degree of freedom other than that where the physical nonlinearity is located, using a single trailing-edge control surface, to stabilize the entire system; 2) includes the unsteady effects of the airfoil’s aerodynamic behavior; 3) includes the embedding of a tuned numerical model of the aeroelastic system into the control scheme in real time; and 4) uses pole placement as the linear control objective, providing the user with flexibility in determining the nature of the controlled response. When implemented experimentally, the controller is capable of not only delaying the onset of limit-cycle oscillation but also successfully eliminating a previously established limit-cycle oscillation. The assignment of higher levels of damping results in notable reductions in limit-cycle oscillation decay times in the closed-loop response, indicating good controllability of the aeroelastic system and effectiveness of the pole-placement objective. The closed-loop response is further improved by incorporating adaptation so that assumed system parameters are updated with time. The use of an optimum adaptation parameter results in reduced response decay times

Experimental feedback linearisation of a non-smooth nonlinear system by the method of receptances

Input–output partial feedback linearisation is experimentally implemented on a non-smooth nonlinear system without the necessity of a conventional system matrix model for the first time. The experimental rig consists of three lumped masses connected and supported by springs with low damping. The input and output are at the first degree of freedom with a non-smooth clearance-type nonlinearity at the third degree of freedom. Feedback linearisation has the effect of separating the system into two parts: one linear and controllable and the other nonlinear and uncontrollable. When control is applied to the former, the latter must be shown to be stable if the complete system is to be stable with the desired dynamic behaviour

Feedback linearisation in systems with nonsmooth nonlinearities

This paper aims to elucidate the application of feedback linearization in systems having nonsmooth nonlinearities. With the aid of analytical expressions originating from classical feedback linearization theory, it is demonstrated that for a subset of nonsmooth systems, ubiquitous in the structural dynamics and vibrations community, the theory holds soundly. Numerical simulations on a three-degree-of-freedom aeroservoelastic system are carried out to illustrate the application of feedback linearization for a specific control objective, in the presence of dead-zone and piecewise linear structural nonlinearities in the plant. An in-depth study of the arising zero dynamics, based on a combination of analytical formulations and numerical simulations, reveals that asymptotically stable equilibria exist, paving the way for the application of feedback linearization. The latter is demonstrated successfully through pole placement on the linearized system

Feedback linearisation of nonlinear vibration problems: A new formulation by the method of receptances

New output feedback-linearisation theory is presented for the treatment of nonlinear vibration problems by a receptance-based approach. An important aspect is a new formulation for investigating the stability of the zero dynamics. The overall methodology possesses the usual benefits of the receptance method, namely that the system matrices (with associated assumptions and approximations) do not have to be known. In addition, it has the distinction of not requiring the form and parameter values of the nonlinearity when the input and output degrees of freedom are away from the nonlinearity itself. This represents a valuable advance over the conventional time-domain feedback linearisation approach

Advanced passive and active methods for vibration control in rotating machines

Effective control of vibration in rotating machinery is a major concern in many industries and research institutions. With the ever-increasing drive for higher operating speeds, the need for developing vibration mitigation methods that cater for the arduous operating conditions that consequently arise is paramount. Phenomena that may have been insignificant in relatively low-speed rotating machines begin to gain importance with increasing speed, an example of which is the gyroscopic effect. This thesis is aimed at enriching the knowledge base on rotordynamic vibration control. Independent Modal Control (IMC) is addressed, within the context of rotating machinery. A study is performed on various actuation technologies that may be used to implement active vibration control. The well-known problem of balancing rotating machinery is also considered. The first-order modal filters based on Structure Preserving Transformations (SPTs) are capable of decoupling a rotor dynamic system into individual modes of vibration, such that IMC may be performed. Unlike traditional control schemes, the method based on first-order modal filters does not require the imposition of highly restrictive conditions on the system (classical damping). As a result, gyroscopic effects - which are substantial in high-speed rotating machinery - and non-classical damping may be fully accounted for in the modal domain. The main problem pertaining to this method arises from the fact that the response of the controlled system is linked with the stability of the modal filters. As such, if the filters are unstable, the controlled response is eventually overcome by noise. This thesis explores the spectral properties of the modal filter with a view to understanding the factors that affect its stability; some interesting findings on the filter eigenvalues are presented. Furthermore, the question as to whether filter stability is an essential requirement is addressed. The relationship between the rotordynamic system and the modal filter is also investigated. An illustration of the techniques developed in relation to IMC using first-order modal filters is presented in the form of a FE simulation on a realistic aero-engine model. The implementation of active vibration control in a dynamic system is realised through the application of control forces by actuators. In the case of rotating machines, these would normally be located at the bearings. Actuation may be achieved from a variety of technologies such as electromagnetic, piezoelectric, magnetostrictive, ultrasonic etc. This thesis conducts a study on some popular actuation technologies, with the aim of finding an effective alternative to the ubiquitous squeeze film damper. The merits and drawbacks of the various technologies are compared. Also, some novel design concepts are proposed, and (in some cases) their viability demonstrated through calculations. It is well-known that rotor unbalance is usually the main source of vibration in rotating machines. Thus, improvements in procedures for balancing such machines are continuously being sought. With increasing in-service operating speeds and ever more stringent standards, traditional balancing methods progressively become inadequate. One of the reasons for this is the inability of balancing tests to capture the contribution of patterns of unbalance that excite higher modes of vibration, as the tests speeds are usually lower than in-service speeds. This thesis proposes a robust balancing approach that utilises additional information on rotor unbalance, in the form of a covariance matrix, to improve the balancing procedure. The method is illustrated in a FE model of a rotating machine, and is shown to be superior to the traditional method.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Active vibration control using piezoelectric actuators employing practical components

Unwanted vibrations are a common occurrence within structures and systems, and often pose a threat to their integrity or functionality. This research aims to seek a solution to attenuate the vibrations experienced within a link of a system using active vibration control with piezoelectric patches as actuators, whilst avoiding the use of large and expensive equipment which would contravene with the common objective of maintaining the smallest mass possible of the system. Previous research has employed large and expensive equipment as the controller, with sensors often only being able to measure the vibrations of the structure along one axis; this research aims to address these issues. The choice of utilizing the small, lightweight, and low-cost Raspberry Pi 3 combined with petite, mountable sensors and actuators was made based upon the greater practicality that the controller, sensors, and actuators exhibit, allowing for their use in a wide variety of applications. An analytical model of the structure was created based on Euler–Bernoulli beam theory and validated through the modal parameters and the frequency response obtained from a finite element model and experimental data. A controller was then designed and applied to the analytical model to attenuate the vibrations along the link, and then the same design was implemented within the Raspberry Pi 3, and experimental studies were carried out. The introduction and effectiveness of a purposeful time delay within the controller was explored within the experimental and analytical studies, with the intention of counteracting unfavorable results produced by the control system. The results of the experiment proved the control design to be effective for a range of frequencies that included the first natural frequency of the link, and validated the analytical model including the control design

Active Control for Nonlinear Aeroelastic Systems

The dynamic responses to aeroelastic gusts/manoeuvres play an important partacross much of the design and development of an aircraft and have an impact upon structuraldesign, aerodynamic characteristics, weight, flight control system design, control surfacedesign, and performance. They determine the most extreme stress levels, fatigue damage, anddamage tolerance for a particular design. To this end, there has been great interest in efficientloads alleviation in aircraft structures.The ultimate aim of this project is to study the dynamic behaviour of a nonlinear aeroelasticsystem due to gust loads, and to investigate the use of linear/nonlinear active control with theaim of mitigating vibrations in the system, and reducing the aerodynamic loads that would, inthe real world, be introduced into the airframe. The present paper discusses the authors’existing research which will serve as the foundation for the aforementioned aim of the project.Implementation of techniques such as partial feedback linearisation, the combination in realtime of simulation-based and experimentally measured data during control – which weenvisage will play a vital role in the current project – are addressed, including results ofclosed-loop control of a pitch-plunge aerofoil. As the present research is ongoing – and anysignificant results are yet to be generated – this paper will present briefly the current statusand next steps planned for the project