An Adaptive Fuzzy Logic Algorithm for the Thrust Control of a Small Turbojet Engine

ABSTRACT This paper proposes a Fuzzy technique for the thrust control of small-scale turbo-jet engines, as an effective alternative to conventional PID techniques. Fuzzy rules have been preliminarly chosen and tuned so as to achieve rapidity and stability of response, as well as absence of overshoots, by simulating the transient operation of the Pegasus MK3 small-scale turbo-jet. Three experimental tests with large increases or decreases of set thrust have been carried out on the same engine: excellent results in terms of response speed, stability and absence of overshoots have been achieved. The proposed thrust control technique has general validity and can be applied to any small-scale turbojet engine, as well as to microturbines for electricity production, provided that thrust being substituted with the net mechanical power. INTRODUCTION Recently, the development of unmanned aerial vehicles (UAVs) has increased the interest for small turbojet engines [1,2,3,4] derived from turbocharger rotor components. A small-scale turbojet engine can also be employed as gas generator core for small ramjet engines, powering supersonic UAVs. For both applications, i.e., for small portable power generation systems and for mini or micro UAVs, the potentially very high power density of the gas turbine allows a strong reduction in battery, and thus of the overall system weight [5,6,7,8,9,10,11]. Such a rapid development makes it crucial to develop a fast and reliable thrust control system for these small-scale turbojet engines. Most of the automatic controls employed in industrial application are based on PID (Proportional, Integral, Derivative) algorithms, which are the most common and studied controllers. In addition to PID controllers, research is studying alternatives such as control systems based on Fuzzy logic: several authors report theoretical studies of Fuzzy controllers and applications to real systems [12-22]. Fuzzy logic is much closer to human reasoning than conventional algorithms: it is mainly based on the employment of degrees of partial truth, which allows to study a physical phenomenon thoroughly. For these reasons Fuzzy logic fits very well to nonlinear systems and to systems whose mathematical model is not known [12,15]. As for the conventional controllers, the controllers based on Fuzzy logic may have a proportional action (P), a proportional-integral action (PI) or a proportional-integral-derivative action (PID). This paper describes the application of a PI Fuzzy controller for the automatic control of the axial thrust of a turbojet engine, namely “Pegasus MK3”. The paper will first propose a brief description of the Pegasus small-scale turbojet engine and of the experimental rig employed to test the turbojet engine. Then, characteristics of the closed-loop control are described and parameters of the Fuzzy controller are reported in detail. Some experimental tests will be finally presented to validate the proposed thrust control technique.