A special pattern of thermoacoustic oscillation has been observed in a Rijke burner. The acoustic pressure histories show obvious amplitude modulation at very low frequencies, namely beating instability. Accompanied with the amplitude modulation, the porous-plug stabilized flame also shows low frequency pulsations. This beating can be caused by the interaction of flame dynamics at different time scales. Accordingly, dynamic models of a ceramics stabilized flat flame are developed at two distinctive time scales and embedded in an acoustic network system. Simulation results show that low frequency flame temperature pulsation can lead to obvious amplitude modulation. Furthermore, aiming to suppress the beating instability with active control, the effects of amplitude modulation on thermoacoustic oscillation control have been studied. A phase shift controller and a phase lag compensator are tested, and the simulation results illustrate the challenge that may be encountered in the control of beating instabilities.
In order to achieve a robust control performance of the gasifier which has dynamic characteristics of multi-variable coupling, large inertia and multi-disturbance, an optimization method for decentralized PID/PI controller parameters based on probabilistic robustness is developed. First, the control structure and target of the Shell gasifier is analyzed and a crude model is introduced. Model uncertainties and other detailed industrial requirements could be considered at the same time in the method. The probability of satisfaction with the dynamic performance is computed statistically, and then it is presented as the objective function to optimize the controller parameters based on genetic algorithm. The Monte Carlo experiment was applied to test the robustness of the control system. In comparison with the tuning methods based on internal model control (IMC) and the optimization algorithm under the nominal model, simulation results show the method could exploit the potentialities of PID/PI controllers in a maximal probability.
Lean Premixed Pre-vaporized technology aiming at reducing nitrogen oxide emission has an inherent drawback of causing combustion instability. Modelbased active controller design to suppress the unstable combustion needs the open-loop transfer function of the object which is difficult to acquire and often with inaccuracies in many ways. Desired dynamic equation based PID (DDE-PID) is a model free control strategy which requires little knowledge of the object process, and also has the advantage of limited number of parameter variables to be tuned and simplicity of tuning method. A certain DDE-PID controller is designed and a set of parameter tuning method is given and tested on a open loop transfer function obtained from real experimental data fitting. With nonlinear features like input amplitude limiting and output limiting to resemble the actual system, the simulation results are compared with two model based control strategies which has been tested in actual system and proved to be feasible. The result shows DDE-PID could achieve similar control effect with structural simplicity and tuning easiness.
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