Mechanical Engineering - Theses
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ItemStability improvement of periodic motion of helicopter rotor blades( 1996)Many machines and devices are designed to perform periodic motion (engines, rotors, vehicles, etc.). From an engineering point of view, the stability of this periodic motion is crucial. A lack of stability can lead to poor performance, damage or even destruction. Helicopter rotor blades are prone to various kinds of instability that can cause excessive vibration and lead to structural failure. For many systems, an accurate mathematical model cannot be produced because of the complexity of the system and poorly known or varying parameters. Such systems are called uncertain systems. The thesis presents and investigates a new control method for stabilising the periodic motion of uncertain systems - with particular application to helicopter rotor blades. The control method uses proportional displacement and velocity feedback with a time delay in the feedback path which is synchronised with the period of the motion being stabilised. This synchronisation is achieved in practice by using a phase-locked oscillator circuit. No knowledge of the dynamics of the system being controlled or the desired trajectory is required for this control method. So long as a signal having the required period can be fed to the phase-locked oscillator, the time-delayed feedback can be synchronised with the period. The system can be treated as a "black box" and the parameters of the control method can be adjusted in a trial-and-error fashion until the best performance is obtained. This is demonstrated in the experimental work described in the thesis. A further property of the control method is that it does not alter the steady-state motion of the uncontrolled system, but improves its stability. This is important for helicopters, where the steady-state motion of the rotor blades determines the flight path of the helicopter. As an initial investigation, the control method is applied to mathematical models of linear and non-linear one-degree-of-freedom systems. A computer program is developed to determine the stability margin by finding the dominant root of a characteristic equation with an infinite number of roots. The optimal values of the control parameters are found to vary in a clear pattern as a function of the period of the motion being stabilised. The control method is shown to enlarge greatly the region of asymptotic stability of the periodic steady-state motion of the non-linear system, and make it much less sensitive to disturbances. The effectiveness of the control method is verified experimentally using a multi-degree-of-freedom laboratory installation, employing appropriate hardware and software developed for this purpose. The existence of optimal values of the control parameters and their variation as a function of the period of the motion being stabilised is demonstrated. The pattern of the variation of the optimal values of the control parameters in the experiments is shown to agree with that found for the one-degree-of-freedom mathematical model, but it is more complex because of the additional degrees-of-freedom. The control method is shown to improve the stability of the equilibrium position of a helicopter rotor blade in hovering flight and the steady-state periodic motion of a helicopter rotor blade in forward flight, using a two-degree-of-freedom mathematical model, including the aerodynamic forces. This means that when the control method is applied, larger values of the chordwise centre of gravity offset and more flexible mechanical control linkages can be tolerated before the pitch-flap flutter instability occurs.
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ItemActive vibration control of rotor-bearing systems( 1995)This thesis is concerned with the active vibration control of rotating machinery utilizing an active journal bearing. The main objectives are to develop general mathematical model of rotor systems incorporating the active journal bearing, for the purposes of dynamic analysis and control synthesis, and to formulate the control concepts for the vibration control. The presence of suitable active bearings plays a very important part in the active vibration control of rotating machinery. A new active journal bearing has been proposed in the thesis in which a flexible sleeve and a pressure chamber are initially introduced to a conventional design of journal bearings. Fluid-induced self-excited vibration or instability is a serious problem in a rotor system supported upon oil-film journal bearings. In the case of a rotor system supported on conventional journal bearings only, there is no means to improve the system stability without stopping the operation of the machine. With the inclusion of the active journal bearing, the numerical analysis has illustrated that by simply applying an appropriate constant chamber pressure in the active journal bearing, the stability of the rotor system can be improved effectively. No sophisticated feedback control calculations and equipment are needed. The altering of the chamber pressure can be performed at any time while the machine is operating. The stability analysis has proved that it is always possible to increase the range of the configuration parameters as well as the range of the rotating speed within which the equilibrium position of the system is asymptotically stable. Outside these ranges of stability, great attenuation of the amplitude of the self-excited vibration can be achieved. An optimal open-loop vibration control algorithm has been presented for the control of forced vibration due to the mass imbalance in the rotor. The synchronous vibration is minimized in a least squares sense. Numerical simulations have shown that the proposed optimal open-loop control strategy is very effective in reducing the amplitude of the forced vibration. In contrast to conventional balancing techniques, the optimal open-loop strategy does not require an estimate of the unbalance distribution in the rotor, and more significantly, it can be implemented in the rotor system at any time without stopping the machine. The dynamic problems in a rotor system supported by fluid-film bearings are essentially non-linear. For a complex rotor system, it is a very difficult task to accurately model the real system due to inadequate measurements of some parameters. Some parameters may change with time or under different operating conditions. A fixed-parameter controller may not be able to handle such non-linearity, parameter uncertainty and variation with time. The multivariable self-tuning adaptive controller adopted in this thesis has shown to be able to cope with those difficulties. The numerical simulation has demonstrated that the proposed self-tuning controller is suitable for the forced vibration control of the rotor system incorporating the active journal bearing. The amplitude of the synchronous vibration has been significantly reduced. The self-tuning algorithm requires no pre-knowledge of the parameters of the rotor-bearing system and the unbalance distribution. The controller can adjust its parameters to adapt to the changes of the parameters of the rotor system. Since the proposed self-tuning controller is based on the input/output model, it is easy to implement in practical applications. The control algorithm is presented quite generally. It may be used as a general procedure in the applications of the active vibration control related with rotor-bearing systems. An experimental rig incorporating the proposed active journal bearing has been designed and constructed. Experimental investigations have been conducted on the rig to provide experimental support to the developed theory.