Dynamics Modeling and Deployment Control Research of Insar Satellite System
|School||Harbin Institute of Technology|
|Keywords||tether satellite system lumped masses dynamics tension control optimization deployment steady-state keep|
The Tether System (TSS) was put forward last century. Since then, there weremany studies about it published. From the references before, we can know the tethersystem can be utilized for a wide range of space-based applications, such asmeasurement of the space electrical environment and propellant less orbitalmaneuvering by means of momentum transfer and electro-dynamic thrusting. Themodeling and deployment of tethered satellite system whose payload is InSAR devicesis studied in the paper. Moreover, the analysis of keeping state by numerical simulationis got in the end.Considering the requirement of the InSAR measurement, the baseline needs to bestable when the system works. As a result, the model should be more accurate than thesimple dynamics in which the tether is thought as a rigid rod. When the tether’s modelis flexible it is difficult to analyze the system’s dynamics. So some sensible hypothesesare made when the dynamics model of the tethered system is got by usingNewton-Euler method. An efficient mathematical model for flexible tethered systemsthen is derived, which treats the tether as composed of a system of lumped massesconnected via inelastic links. After getting the dynamic model, the deployment of thesystem is discussed to investigate the parameters’diversification.After the dynamics model is derived, the research of the stability of deployment ismade, from which the result that the liberating in-plane is persistent could beconcluded. Then two control laws, the tension control and velocity control, are testedand verified using the dynamics by numerical method. But the figures show the tethercould not be launched to the perpendicular. To make the tether vertical when thedeployment is end, a new tension control is deduced by linearize the system dynamics.The numerical simulation results show that the angle of the tether’s swing in-plane is alittle bigger than velocity control. However, its variation of velocity is much gentle.As mentioned before, the deployment under the three control laws are not ideal.So an optimization methodology for deployment is presented following the routinecontrol laws of deployment. However, the tether is modeled as an inelastic rod tosimplify optimal system trajectories. The direct collocation is used when optimizingthe deployment trajectory. The simulation results indicate that the optimizationmethodology for deployment is feasible for the simplified system. So it may beworkable for the lumped masses system too.In the last part, the angles variations during the keeping state of the tetheredsystem are simulated through numerical method. The figures show that the baseline change is small enough for the requirements of InSAR system and the swings cause byeccentricity, non-spherical perturbation and atmospheric drag are very tiny. Therefore,the system could be perturbed in the orbit which may lead to a swing of the tether. Tokeep the tether stable, a thrust control law is derived, which can make the vibration oftether in-plane or out-plane weaken quickly. The numerical results show it works well.