Research On Force Servo Control And Reliable Drive Of Aircraft Electric Braking System

Aircraft electric braking system utilizes high-performance electronic equipment to control motor-driven brake actuators to achieve braking.Compared with traditional hydraulic braking,there is no oil leakage and fire risk.With well maintenance,high braking sensitivity,fast and safe braking performance,it has been a qualitative leap for aircraft braking systems and attracting attention from various countries.Aircraft electric braking system puts forward more stringent requirements on the servo control performance and the reliability of drive system.Firstly,the thesis discusses the force servo control technology of electric braking system,analyzes the open-loop characteristics of the electro-mechanical actuator(EMA),and the serious impact of friction on the braking performance.Then,with the stiffness curve and friction model of the actuator,the mathematical model of electromechanical actuation system is established.The model was verified by the Matlab/Simulink platform.Based on the model,the pressure-speed-current closed loop control with friction compensation was proposed.The closed-loop control method realizes the force servo control of EMA.Meanwhile,according to the requirements of high-precision brake force control for the actuation system with low duty-cycle,cascade Buck converter in front of full-bridge inverter,which decreases the dc-link voltage to improve the control accuracy of the braking force.Finally,by observing the brake force feedback signals with square wave,sine wave,anti-skid braking force curve,the experiment proves that the system has better steady-state accuracy and dynamic response.In the actual system,the connection cable between the electro-mechanically actuation controller(EMAC)and the EMA is up to 15 meters.While,the feedback channel is susceptible to electromagnetic interference(EMI),which leads to system malfunction.Aiming at the problem,the thesis analyzes the mechanism of EMI for the system,which concludes that the three-phase current of the motor and the output voltage of the EMAC are the main sources of EMI in the system and the coupling interference of the cable is the main propagation route.Through comprehensive treatment methods such as electromagnetic shielding,line stranding and grounding technologies,a reasonable EMAC and EMA connection cable was designed.For the Hall position sensor position feedback signal,anti-jamming capability was improved by converting transmission signal from voltage to current and isolating the control circuit with optocoupler.The experiment results show that the above comprehensive improvement method effectively suppresses the superimposed interference on the feedback signal of the brake loop and improves the system's anti-jamming capability.In addition,the good force servo control technology relies on the stable operation and reliable drive.Therefore,to high important and sensitive elements of the system,the thesis launches the research for reliable drive technology from the following two aspects:(1)The usage of a large number of electronic components in the electric braking system inevitably leads to an increasing probability of failure.To reduce the maintenance cost and maintenance period of the system and ensure the safety and reliability of the aircraft's brake assembly,the system must be built-in-tested(BIT)to locate faulty components in time.Therefore,the thesis proposes a power-on self-test method of electric braking system,which could detect faults from force sensors,current sensors,voltage sensors,hall position sensors,motor drive circuit consisting of three-phase windings and inverters,without adding any extra sensors or detection circuits and would not cause secondary damage to the system.For the motor drive circuit failure,the bus capacitor charge and discharge process are used to complete the detection.The detection process isolates the power supply to avoid the impact on the power supply.Meanwhile,limited energy stored on capacitors,which avoid secondary damage to the circuit.It is implemented only by software without changing the circuit topology,and the calculation amount is small.For the Buck based full-bridge inverter used for braking force control,a simple threshold calculation method was proposed.Both the simulation and the experiment verified the feasibility of the detection method and the correctness of the theoretical analysis.(2)As an important and prone to failure element,the reliability of power source can not be improved directly through redundancy design.Therefore,power source must be reconstructed after a failure occurs,to ensure reliable operation of the system.The thesis uses high-frequency transformer isolated bi-directional DC-DC converter(IBDC)to realize the fault reconstruction between power supply and control supply.A modified push-pull DC-DC converter with PWM Plus Phase-shift(PPS)control method is adopted to suppress the transformer leakage inductance current and achieve efficient and bidirectional power flow.Through the steady-state analysis,the converter's low-loss working region is obtained.With an improved state-space averaging method,the converter's small-signal model is established and the transfer function is obtained.Based on these,a double-closed-loop control method with output voltage and clamp voltage is proposed,which has been verified experimentally.According to the requirements of the system structure,with a simpler structure,an isolated bidirectional half-bridge DC-DC converter with PPS control is applied.For the coupling problem between the output voltage and the clamped voltage,a steady-state decoupling method is proposed,which eliminates the coupling and simplifies the design of the closed-loop controller.Finally,comparison of the PPS and Phase-shift(PS)control methods proves that the PPS decoupling control method is more suitable than the traditional PS control method for the reconstruction of power sources.