Study On Multiple Sliding Mode Control And Nonlinear Compensation For EMA With Harmonic Gear Drive

Electro-Mechanical Actuator (EMA) system is an angular position trackingservo system with high precious and high bandwidth, and its static and dynamicperformances play a critical influence on tracking performance of aircraft. Frictionand backlash are two important factors that affect the EMA system performances,and their nonlinear characteristics can bring about some problems of tracking errorincrease, driving delay, dithering at a low speed or the limit cycle response, whichbadly restrict the improvement of the EMA system performances. Therefore, somemeasures must be taken to eliminate or reduce the adverse effects brought by thenonlinear factors. By generalizing the present studies in this field, a kind of EMAwith harmonic gear drive scheme was demonstrated in paper, and the in-depth studyon its key techniques such as multiple sliding mode control and nonlinearcompensation etc. were performed. It was proved that the proposed scheme isavailable by comparison the simulation and experimental results.Firstly, the genreal scheme of the EMA system was determined, including thetransmission scheme with harmonic gear and bevel gear, as well as the controlscheme with position loop and speed loop. Then, the parametric design of the EMAsystem was discussed, and the optimum gear ratio was determined. Also, theparameters design and selection of servo motor and harmonic drive trismission were introduced emphatically. In addition, the structure and its improved structure of theEMA system was designed. Among them, the dynamic simulation of the keystructural parts was implemented, and the anlysis results indicate that the designmeets the design requirements.To resolve the problems caused by friction and backlash, the methods of frictionand backlash identification and compensation were put forword. For the PI controlsystem with position loop and speed loop, the mathematical models based on LuGrefriction and hysteresis backlash were built; According to the identified nonlinearitymodels, the friction was compensated though feed-forward compensation, and thebacklash was done simultaneously though inverse model compensation as well. Theexperimental results show that the maximum position tracking error of the EMAsystem after compensation decreases from0.166°to0.096°, and the maximum speedtracking error decreases from2.723rpm to0.393rpm. It’s concluded that the frictionand backlash models can be accurately obtained by the proposed identificationmethods, and the tracking accuracy of the EMA system can be improved throughnonlinearity compensation on the basis of the proposed models.In order to improve further tracking precision, Multiple Sliding Mode Control(MSMC) strategy for the EMA system was studied. The mathematical model of theEMA multistage series systems was established, according to the model the MSMCcontroller was designed, and the RBF neural network was used to adaptive approachthe nonlinear factors, which overcame the dithering problem inevitably existed intraditional sliding mode control. By comparing the simulation results of the MSMCmethod and the method of PI control with compensation, it can be found that thenonlinear factors can be suppressed better by the MSMC method, and the EMAsystem performances can be further improved.Finally, a co-simulation between mechanism and control was carried out for theEMA system with rigid-flexible coupling based on RecurDyn and Matlab/Simulink.The control block diagram was established, and the PI controller parameters were setand optimized by using NCD module combined with optimization function. Then the simulation test for the EMA virtual prototype was executed, and the co-simulationresults satisfied the system requirements. Moreover, the semi-physical simulationplatform of the EMA system was built under the environment of Matlab/xPC. Theexperimental results reveal that, the maximum deflection angle of rudder can be upto±20°, the maximum angular velocity is greater than150°/s, the maximum outputhinge moment can reach15N·m, and the system bandwidth is not less than25Hz, thetracking error is not more than0.1°, which indicates that all the specifications of thedeveloped EMA have reached the general requirements of specifications.Studies show that the proposed scheme of EMA with harmonic gear drive isfeasible, and the thesis research results has a certain reference value to the furtherstudy and development of EMA with harmonic gear drive in the future work.