چكيده به لاتين
Abstract:
The control system has a significant role in the performance development and safety of the gas turbine engines. A conventional method used for fuel control of turbofan engines is Min-Max algorithm. According to this switching algorithm, the requested thrust command from the pilot should be fulfilled and the structural and operational limits must be kept. Two important issues in Min-Max algorithm design, are ensuring engine limit protection and stability analysis of this switching structure. Few studies have been published in these areas.
In this dissertation, limit protection issue and the stability analysis of Min-Max controller have been addressed. First, according to the analysis of the conventional Min-Max controller response, a new strategy has been presented to design of Min-Max regulators to improve limit protection. For this objective, a nonlinear thermodynamic model of a two-spool turbofan engine in 30000 lbf thrust class, has been linearized about an operating point and a traditional Min-Max algorithm containing linear compensators has been designed. Then, by using the analysis results of system response during limit violation, in one step, the regulators of various loops have been designed overshoot-free and in another step, the constraints of some outputs influenced by engine acceleration/deceleration have been considered in control deisgn of other loops. Then, the regulators of these outputs have been removed from the Min-Max structure. For this purpose, state feedback method and a set of linear matrix inequalities (LMIs) were used and the regulator of each loop was designed according to a convex optimization problem. Thus, in addition to improvement of limit protection, computational burden has also been decreased due to loop reduction.
On the other hand, due to the switching nature of Min-Max algorithm, the stability of single loops does not necessarily ensure the overall system stability. So for the first time, a methodology has been presented for the stability analysis of the closed-loop system involving Min-Max algorithm. For this objective, Min-Max structure has been transformed into a canonical form and using the Multivariable Circle Criterion, the necessary conditions for stability assurance of the closed-loop system have been extracted. Then, using the provided methodology, the asymptotic stability of the designed Min-Max controller containing state feedback regulators has been guaranteed. In simulation section, the performance of the designed controller to improve the limit protection has been evaluated by linear and nonlinear themodynamic model and the results have been compared with the traditional Min-Max method and the novel Min-Max/SMC approach. Based on the simulation results, the new strategy has an efficient performance in fulfillment of thrust command, reducing the possibility of limit violation in transient regime and reducing the computational burden and is a useful technique to improve the performance of the Min-Max algorithm for fuel control of turbofan engines.
Keywords: Turbofan engine, Min-Max algorithm, Limit protection, Linear Matrix Inequalities (LMIs), Asymptotic stability
