Investigation Of Cascade Control System For Ramjet

The performance of the ramjet, which is the powerplant of the guided missile,determines the capability of the missile. Ramjet, an object with strong nonlinear,time-varying and complex thermodynamic characteristics, has been a hot area indomestic and foreign research. The shock position in the expansion section of theinlet is vital parameter, which can characterize the properties of the ramjet, and thereis a one-to-one correspondence between static-pressuer ratio and the shock position,when the flying conditions is determined. The shock position is susceptible to thedisturbance, in some severe cases, the shock may not be able to form, and in thesame time the unstart phenomenon leads to an abnormal operation of ramjet. Theunstart phenomenon that caused by many factors can be judged by thestatic-pressuer ratio. Although the disturbance can not be avoided, it can berestrained by selecting suitable control programme. Hence, cascade control systemis adopted in this paper, and operating boundaries are provided based on the analysisof the thermodynamic cycle of the ramjet. Furthermore, the dynamic model oframjet and its control system are builded, which is used to analysis the effect ofdisturbance attenuation with cascade control system.Firstly, the nonlinear ramjet model that is on the basis of fundamental equationsin aerodynamics was builded, the model mainly contains four parts: inlet, combustor,exhaust nozzle and fuel supply system, and then the thermodynamic cycle of theramjet was analysed. From the angle of cycle, the paper researched specific impulseand specific thrust of the ramjet, and defined the relations between specific impulse,specific thrust and cycle work. Based on the results, The regulation rules offuel-air-ratio and nozzle throat area during acceleration and cruise was obtained.Secondly the paper builded dynamic model of ramjet and its thrust-drag model:the air flow in the inlet and the nozzle approximate isentropic except that it throughthe shock, the combustor was assumed with uniform section, and volumetric effectsduring the burning process was considered. The aircraft model was mainly about itsbalance of the thrust and drag. As the dynamic pressure maintained constant duringthe flight, the drag was only related to the angle of attack. In the basis of thedynamic model, the paper analyzed response time, overshoot and so on.Thirdly, a study of control law is conducted. After comparing the classiccontrol law, cascade control system was adopted in consideration of therequirements that the ramjet faced. The ramjet was selected as inner-loop of thecascade control system, while the aircraft outer-loop, and controlled arguments of inner-loop and outer-loop were selected. Then a reasonable control system wasdesigned based on the actual demand.Finaly, the effect of disturbance suppression with the action of cascade controlsystem was studied. The disturbances affecting the terminal shock may ariseexternal to the ramjet engine, such as from the atmosphere upstream, or internal tothe engine, such as from within its combustor downstream. The disturbance modelwas established to analyze the shock position under the influence of disturbance,and the effect of disturbance suppression with the action of cascade control systemwas assessed.