Redundant Inertial Measurement Unit Reconfiguration and Trajectory Replanning of Launch Vehicle

The launch vehicle control system is composed of navigation, guidance, attitude control, system integration and other subsystems. It is the brain and nerve center of launch vehicles. The technical level of control system directly determines the success or failure of launch mission, and also plays a vital role in maximizing the carrying capacity. Smart control is the core capability of future rockets, mainly reflected in state monitoring, fault detection and handling, risk reduction, capability assessment, and mission planning. The launch vehicle can handle faults autonomously, and this is an effective way to improve the reliability of launch vehicles.

The design of control system is based on the dynamics equations and motion equation of the controlled object. The launch vehicle is taken as the controlled object in this section. First, the coordinate system was defined, the force acting on the launch vehicle was analyzed, the motion equation was established, and then its dynamics was analyzed to provide the basis for trajectory re-planning. The zero error, installation error and scale factor error were considered in establishing of the combined error model and the digital pulse output model of redundant strapdown IMUs. The noise model of single-meter gyro was analyzed. The information reliability model is established in view of common outliers in the information. Finally, the number of reconfigurable units and the minimum included angle are proposed as intuitive reference indicators for evaluating the performance of system after failure.

The fundamental problems for both trajectory planning and optimal guidance control are solutions to the optimal control problem. This section puts forward the optimal control problem, the methods for solving the extreme value problem, and introduces the methods for calculating the optimal control problem in detail, including indirect methods, direct methods and intelligent optimization algorithms. The nonlinear planning method and convex optimization method are introduced. The problems encountered in solution process are described and corresponding schemes are put forward.

The fault detection method of redundant strapdown IMU is closely related to its redundant configuration. This section introduces the starting point of scheme of redundant IMU configuration and typical redundant configurations. On this basis, the fault detection methods of redundant IMUs are analyzed.

It is required for smart rockets that the redundant strapdown IMUs can be self-reconfigured after failure. After one sensor fails, the control system shall be able to select the remaining sensors for redundant reconfiguration through data fusion.

The launch vehicle will deviate from its intended trajectory during flight due to flight deviations and malfunctions, and the launch vehicle’s flight trajectory needs to be re-planned when the flight state is abnormal. Two approaches are available for launch vehicle trajectory re-planning under the condition of satisfying constraints and residual capabilities: off-line re-planning and on-line re-planning. For off-line planning, the optimal parking and standby orbits are designed in advance considering the residual orbit entry and control capabilities of various failures. The optimal target orbit matching the current capability is chosen based on the current flight state and fault level under the condition of satisfying constraints and residual capabilities. Finally, the optimal guidance strategy is selected to complete the flight mission. For on-line planning, it is required to assess the launch vehicle’s residual orbit entry and control capabilities in real time, to achieve autonomous rapid planning using optimal trajectory planning control technology, to achieve dynamic processing of constraints during flight and relaxation of some constraints to ensure that on-line planning problems are solved and converged for on-line solving of optimal problem with the least fuel consumption or the largest radius of degraded orbit.

The safety and reliability of the rocket will be greatly improved via trajectory re-planning design in case of propulsion system failure, which is a key technology for design and development of new generation launch vehicles. This section takes the two-stage launch vehicle as an example, and different trajectory re-planning schemes are designed according to the characteristics of launch vehicle in different flight stages, to adapt to the trajectory planning and command reconstruction under different fault conditions during flight.