Application Research Of Sliding Mode Variable Structure Control In Airborne Electro-optical Platform

As a very important mission load of UAV,airborne photoelectric platform is widely used in aerial reconnaissance,target indication and location,strike calibration,battlefield damage assessment,aerial mapping and other fields.Its main function is to isolate the disturbance of the aircraft,so that the optical equipment’s line of sight has a stable spatial orientation and stable tracking of the designated target.With the increasing detection distance of the optical equipment in airborne photoelectric platform,the requirement for the performance index of the airborne photoelectric platform is getting higher.As an important part of the airborne photoelectric platform,the motion control performance of precision servo control system play a decisive role in improving the accuracy and performance.In engineering applications,traditional linear controllers such as proportional integral differential(PID)and lead-lag controllers remain to be the most widely used controllers.This kind of controller is relatively convenient and simple to use,and after years of development and improvement of the application on the airborne photoelectric platform has been relatively mature,it can also achieve good results.However,when the performance of the system needs to be further improved,such controllers have their fixed limitations.Firstly,the traditional linear controllers have limited ability to control the internal and external disturbances and non-linear effects such as carrier disturbance,friction,mass imbalance,air flow disturbance,output torque fluctuation,engine vibration,etc.The controller design based on accurate models does not fully consider the unmodeled motion.Secondly,in traditional applications,the control effect of single control for each frame on multiple input and multiple output(MIMO)systems with coupling among multiple degrees of freedom is not ideal.Aiming at the above problems,this paper takes a three-axis airborne photoelectric platform as the research object,and carries out the following theoretical and applied research.Firstly,the frame structure and characteristics of several airborne photoelectric stabilization platforms are analyzed.The mechanism of passive vibration isolation and active stability,as well as various disturbances affecting the performance of servo control system of airborne photoelectric platforms are analyzed comprehensively.Permanent magnet synchronous motor(PMSM)has attracted much attention in the control system of airborne photoelectric platform because of its high torque-inertia ratio,friction-free and high reliability.In this paper,the servo control system driven by permanent magnet synchronous motor is modeled and analyzed,and the system model in synchronous rotating coordinate system is obtained.The basic sliding mode controller is designed on the basis of the model.The anti-disturbance performance of the sliding mode controller is verified by numerical simulation.The problem of chattering caused by too large switching gain in the sliding mode controller is further analyzed,which must be suppressed in practical application.Secondly,a robust sliding mode control method based on rapid nonlinear tracking differentiator and disturbance observer is proposed for the disturbance complexity of airborne photoelectric platform system.Because of the high switching gain of sliding mode control law,it is easy to cause chattering,and in general,it is not easy to obtain signal differentiation without acceleration sensor in the actual system.Therefore,a rapid nonlinear tracking differentiator and a disturbance observer are introduced.The tracking differentiator can provide signal differentiation.Besides compensating the system with the disturbance observed by the observer,the disturbance observer can also effectively reduce the switching gain of the sliding mode control law and reduce the chattering effect of the system.Meanwhile,this paper improves the rapid nonlinear tracking differentiator to reduce the influence of input signal noise.The validity of the robust sliding mode control method based on rapid nonlinear tracking differentiator and disturbance observer is proved by the comparative experiments on a single-axis PMSM test platform.Thirdly,aiming at the problems of incomplete decoupling and unmodeled coupling between multiple degrees of freedom caused by the separate control of each frame in traditional applications,this paper establishes a relatively perfect three-degree-offreedom coupling model for the three-axis airborne photoelectric platform,which fully considers the dynamic coupling among the three frames of the research object,and gives its dynamic equation.On the basis of the established model,a MIMO fuzzy sliding mode control method for three-axis airborne photoelectric platform is proposed.The main feature of the method is that it can effectively reduce the chattering effect of sliding mode control while utilizing the coupling compensation of fuzzy logic.The experimental results verify the coupling compensation effect of the proposed method.Finally,according to the actual needs of a project,the design of servo control and driver of three-axis airborne photoelectric platform system is carried out,and the digital realization of SVPWM control of PMSM is introduced in detail.The experimental system was built on a five-axis swing table to verify and analyze the proposed control method.The experimental results show that the MIMO fuzzy sliding mode control method of the three-axis airborne photoelectric platform proposed in this paper is feasible.The improved algorithm in which the rapid nonlinear tracking differentiator is introduced has obvious advantages in improving the stability accuracy of the three-axis airborne photoelectric platform.