ESA GNC Conference Papers Repository

Scheduling Schemes and Control Law Robustness in Atmospheric Flight of VEGA Launcher
Roux, C.R., Cruciani, I.
Presented at:
Tralee 2008
Full paper:

The article is aimed at presenting scheduling schemes and robustness for TVC control law of the VEGA launcher in atmospheric flight. Work is focused on scatterings modeling, scheduling schemes and robustness analysis. The control law is assumed to be designed in a classical framework. In atmospheric flight, parameters linked to trajectory (dynamic pressure, Mach number, velocity) are subject to important temporal variations. TVC control law, in charge of maintaining the launcher on the programmed trajectory in spite of atmospheric disturbances and propulsive scatterings, must evolve during the flight: a tuning is designed for different flight nodes assuming the system has frozen parameters. Then a non stationary control law is obtained by interpolating gains and filters between nodes values. Control law is designed to be sufficiently robust to variations of flight domain due to launcher and environment scatterings. In section 1 we introduce missions and payloads (PL) to be covered by VEGA, summarize the timeline in atmospheric flight and describe the flight domain the GNC has to cope with. In section 2 we remind the architecture of the GNC and the control law design in atmospheric flight in function of requirements. In section 3 we describe and classify the scattered parameters to be taken into account in control law design: parameters associated to flight parameters (dynamic pressure, thrust, Mach number, launcher massÂ…), to rotational motion, to elastic model and to actuator model. We distinguish between source parameters characterized by a statistical distribution or a variation range and derived parameters whose distribution or ranges are computed from source parameters. An important point is to use correlations between these variables (especially between thrust and dynamic pressure) in order to reduce the conservativeness of the design. In section 4 we present the scattered configurations as combination of scattered parameters and different approaches to generate them: Monte-Carlo, pure vertex analysis, vertex analysis with several correlated approaches. In section 5 we present different schemes for interpolating control law. The simplest but least robust variable is time. Other variables linked to trajectory and computed on-board - non gravitational velocity (VNG), relative velocity - improve the robustness of the control law. In tail off phase, in which thrust rapidly decreases, an adaptation versus longitudinal acceleration offers a good robustness. Furthermore, we propose an interpolation scheme based on both time and non gravitational velocity, which improves rigid margins by identifying in flight whether thrust is over or under-performing In section 6 we present resulting margins computed with various approaches.