Multi-objective Control Design For Aero-engines Based On Switching Strategy

Aero-engine is a kind of highly complex and precise thermal machine.Multiple control objectives must be simultaneously considered during the control design procedure for an aero-engine.On one hand,the essential task of an aero-engine is to provide stable and reliable thrust quickly for the aircraft.On the other hand,since an aero-engine usually works in a broad flight envelope,during its working procedure,the aero-engine must be limited within various safety boundaries.At the same time,the aero-engines might suffer external disturbance and dynamic varying caused by uncertain flight condition varying.In addition,physical limitations of the actuators also require constraints for control input.All of the factors mentioned above limit the aero-engine to provide thrust for the aircraft stably,accurately and quickly.The switching control strategy is an effective solution to the multi-objective control problem for the aero-engines.However,due to the complexity of this problem,mechanism of the co-existence and interaction among the continuous dynamics,discrete dynamics and constraints,as well as their impacts on the system performance,are far from clear,so research on the systematic design methods of multi-objective switching control for aero-engines have been rarely found.This dissertation studies the multi-objective switching control design problem under cases such as safety constraints,uncertain flight condition varying,and control input constraints.The main contributions of this dissertation are as follows.1.To solve the contradictions between the two objectives of safety and tracking performance,the multi-objective control design method based on the command switching strategy is proposed.In the proposed design method,sub-controllers aiming at safety and tracking performance are designed respectively.Switching law guiding the switching among the sub-controllers is then designed to realize the tradeoff among the multiple objectives.The freedom degree of control design is enlarged,and the complexity and conservatism are reduced.At beginning,the command switching control strategy is proposed.Parameters to be designed,such as the reference command of the protection loop and the switching thresholds,are optimized to obtain the optimal transient tracking performance index.Then,the coordinated switching control strategy is proposed.The selecting method of the protection reference command is given.The coordinated law which is designed for sub-controllers to cooperate with each other is given.The number of parameters to be optimized is reduced.After that,the tracking-protection-recovery switching control strategy is proposed.The switching frequency and the conservatism of protection control are reduced.The switching law is designed based on Lyapunov function.Cooperation among sub-controllers is realized by using the integrator reset technique and the freedom degree of control design is enlarged.The switching law design based on the linear quadratic integrator performance index is discussed finally.2.When working in a broad region,the dynamic behavior of an aero-engine is influenced by the arbitrary varying external parameters such as the altitude and Mach number.In addition,the aero-engine often suffers external disturbance.In this dissertation,the H∞ fault-tolerant control problem for a class of switched linear-parameter-varying(LPV)systems with Markovian switching is studied based on the multiple parameter-dependent Lyapunov functions.At start,the completely known condition for transition rates is released.The solvability conditions of the H∞fault-tolerant controller are given under the condition of only partial transition rates are known.Then,the parameter-dependent controllers are designed to guarantee the stochastic stability of the closed-loop system and H∞ fault-tolerant performance index.3.The multi-objective switching control design problem for the aero-engines is investigated based on the equilibrium manifold expansion(EME)model,and the application limitation of the small deviation dynamic model is overcome.The EME model is a kind of nonlinear model which can reflect the nonlinear dynamic behavior of the aero-engines in a broader region with higher accuracy.In addition,the EME model usually has a simple form,and it is easy to identify and convenient for control design.In this dissertation,a switching controller is designed using the control Lyapunov method for the aero-engine to make it achieve the control objective of thrust acceleration with the safety constraints on the high pressure spool acceleration and the high pressure turbine outlet temperature.At beginning,aiming at objectives of acceleration constraint,temperature protection and set-point control,the corresponding sub-controllers are designed separately.Then,the concept of the safety region is proposed.The switching law is designed to realize the tradeoff between safety and tracking performance.Constructing the minimum phase nonlinear system is avoided,and freedom degree of pursuing tracking performance is enlarged.Through the verification on the nonlinear component level(NCL)model of a two-spool turbofan aero-engine,the proposed control design scheme is effective and robust。4.A nonlinear dynamic switching controller is designed based on the EME model,and the freedom degree of multi-objective control design for aero-engines is further enlarged.At start,nonlinear sub-controllers for the objectives of high pressure spool speed tracking and high pressure turbine temperature protection are designed respectively.Then,according to the monitoring to the tracking performance,the switching law is designed combined with the dynamic controller state reset technique,and the control discontinuity which is bad for the tracking performance is eliminated.The smooth switching among the control modes and the tradeoff between safety and tracking performance are well realized.After that,the sensor dynamic is taken into account.The design method of a locally monotonic tracking controller is utilized,and the difficulty that the control input cannot immediately prevent the protected output from violating the safety constraint is overcome.The conditions of globally monotonic tracking control are relaxed.5.Considering the actuator limitation and the output constraints of the aero-engine,the input and outputs constrained stabilization problem for switched nonlinear systems is studied based on the multiple Lyapunov functions method.At beginning,a nested switching controller is designed to deal with the input constraint by using an improved Backstepping method.After that,the input and output constraints are handled simultaneously.The barrier Lyapunov function method is used in each step of the improved Backstepping procedure to tackle the output constraint.Without assumption of the solvability of the subsystem stabilization problem,a proper dual design of subsystem controllers and switching law is implemented to guarantee the stabilization problem solvability of the switched nonlinear system.Finally,the results of the dissertation are summarized and further research topics are pointed out.