The Study Of Hovering Control For The Lunar Module Based On Variable Structure Control

The hovering phase is a significant step of the soft landing for the lunar module.Obstacle avoidance is the primary mission at this stage.A three-dimensional image of the lunar surface is created to determine whether it is suitable for landing.The lunar module needs to maneuver to choose an appropriate landing position in the horizontal direction.Six degrees of freedom should be controlled during the hovering period for the lunar module including positions and velocities.The successful of hovering is important for the implementation of safe landing and subsequent moon mission,although the hovering step do not exist in each lunar exploration program until now.Referring to the hovering of quadrotor,the hovering control for the lunar module is studied combining with the flight environment of lunar module and working characteristics of power plants in this dissertation.The dynamics model of six degrees of freedom for the lunar module is established based on three assumptions by using Newton-Euler equations.The state-space equations of the lunar module and the block diagram of the hovering control system is given according to the dynamics model.A new program based on full-actuated and under-actuated subsystems of the hovering control is proposed referring to the control of quadrotor.The controller is designed for full-actuated and under-actuated control systems respectively according to the dynamics model and the block diagram of control system.The variable structure controller is designed for the full-actuated system.And the variables(altitude and yaw)are controlled by the thrust engine and attitude control engines directly in the vertical direction.The combined control scheme of variable structure control and thrust vector control is given for the under-actuated system.The linear displacements are controlled by attitudes in the horizontal direction which are seen as virtual inputs according to dynamics equations.And attitude angles are controlled by moments and desired angles which are calculated by dynamics model.The actual thrust and moment is given according to the working principle of power plants.The simulations are implemented for these two systems above by variable structure control and proportion-integration-differentiation control.It indicates that results of variable structure controller are better than the outcomes obtained by proportion-integration-differentiation controller according to simulation.The process of descending and approach are steady without numerical wave and steady error.The controllers of the full-actuated and under-actuated systems are combined with each other.And the simulation of hovering of six degrees of freedom for lunar module is also carried out,which are compared with the results of Apollo-11 lunar module,Chang'E-3 probe and references.It demonstrates that the variable structure control scheme can achieve the goals of the hovering control and meet requirements.