Agile Spacecraft Attitude Control: An Incremental Nonlinear Dynamic Inversion Approach

This paper presents an agile and robust spacecraft attitude tracking controller using the recently reformulated incremental nonlinear dynamic inversion (INDI). INDI is a combined model- and sensor-based control approach that only requires a control effectiveness model and measurements of the state and some of its derivatives, making a reduced dependency on exact system dynamics knowledge. The reformulated INDI allows a non-cascaded dynamic inversion control in terms of Modified Rodrigues Parameters (MRPs) where scheduling of the time-varying control effectiveness is done analytically. This way, the controller is only sensitive to parametric uncertainty of the augmented spacecraft inertia and its wheelset alignment. Moreover, we draw some parallels to time-delay control (TDC) -more familiar in the robotics community- which have been shown to be equivalent to the incremental formulation of proportional-integralderivative (PID) control for second order nonlinear systems in controller canonical form. Simulation experiments for this particular problem demonstrate that INDI has similar nominal performance as TDC/PID control, but superior robust performance and stability

Design of a Control Allocation Solution for the Winged Reusable Launch Vehicle ReFEx

This paper presents a control allocation solution for the technology demonstrator mission ReFEx, which focuses on a vertical takeoff and horizontal landing strategy with autonomous navigation, online guidance, and controlled flight throughout the mission. The trajectory for the demonstration flight is aimed as one for a winged launch vehicle first stage: maintaining stability and control of the vehicle while reaching a predefined target. During the atmospheric phase the vehicle is stabilized by using an active aerodynamic control system which transforms inputs from the guidance and navigation systems into control commands for the individual actuators. In that sense, the control allocation subsystem translates commanded moments into commanded aerodynamic surface deflections. Due to the effect of modeling uncertainties, navigation errors, and underactuated regions, this subsystem needs to be robustified. The algorithm proposed in this paper addresses this challenge via a combination of the deflections required to trim the vehicle together with delta-deflections that aim at converging iteratively to the commanded moments. The combination of these two contributions is able to respond fast to state changes, compensate for modeling uncertainties and navigation errors, and provide a safe mode for the underactuated regions. The performance of the system is studied using a high-fidelity simulator

A Sampled-Data Form of Incremental Nonlinear Dynamic Inversion for Spacecraft Attitude Control

This paper presents a sampled–data form of the recently reformulated incremental nonlinear dynamic inversion (INDI) applied for robust spacecraft attitude control. INDI is a combined model– and sensor–based approach mostly applied for attitude control that only requires an accurate control effectiveness model and measurements of the state and some of its derivatives. This results in a reduced dependency on exact knowledge of system dynamics which is known as a major disadvantage of model–based nonlinear dynamic inversion controllers. However, most of the INDI derivations proposed in the literature assume a very high sampling rate of the system and its controller while also not explicitly considering the available sampling time of the digital control computer. Neglecting the sampling time and its effect in the controller derivations can lead to stability and performance issues of the resulting closed–loop nonlinear system. Therefore, our objective is to bridge this gap between continuous–time, highly sampled INDI formulations and their discrete, lowly sampled counterparts in the context of spacecraft attitude control where low sampling rates are common. Our sampled–data reformulation allows explicit consideration of the sampling time via an approximate sampled–data model in normal form widely known in the literature. The resulting sampled–data INDI control is still robust up to a certain sampling time since it remains only sensitive to parametric uncertainties. Simulation experiments for this particular problem demonstrate the bridge considered between INDI formulations which allows for low sampling control rates

Variable-Mass Dynamics Implementation in Multi-Physics Environment for Reusable Launcher Simulations

One of the driving challenges in launcher Guidance and Control (G&C) design is the strong coupling between different disciplines such as propulsion, aerodynamics, actuators, and structures, which all have a multi-physics nature. Enabling a more efficient and accurate modeling of these multi-disciplinary interactions through the use of a multi-physics modeling approach is believed to be beneficial in expediting the design process, reducing the conservatism in the assumptions used by G&C design and thereby improving the launcher performance. In particular, the accurate modeling of time-varying propellant mass dynamics is an essential component during preliminary design studies since it might have an impact on the dynamical simulations that in turn influence the assessment and evaluation of the launcher performance and trade-off designs across disciplines. This paper presents the implementation of variable-mass dynamics towards the preliminary design and development of a dedicated multi-physics simulator, termed R2M2 (Rapid Reusable Launcher Simulation via Multi-physics Modelling), for multi-actuated vertical take-off vertical landing (VTVL) launch vehicles. The paper first recapitulates previous investigations on variable mass dynamics from relevant literature, including specific descriptions of the dynamical effects during translational and rotational motion. It further addresses the implementation of these dynamical effects within a multi-physics modeling environment and shows the impact of variable-mass dynamical effects on the rotational motion. The model implementation and analysis of results is performed using simple yet representative models of different burn types of propellants or engines which are commonly found in launch vehicle configurations

Fast slew maneuvers for the High-Torque-Wheels BIROS spacecraft

The satellite platform BIROS is the second technology demonstrator of DLR’s ‘FireBIRD’ space mission aiming to provide infrared remote sensing for early fire detection. Among several mission goals and scientific experiments, to demonstrate a high agility attitude control system, the platform is actuated with an extra array of three orthogonal ‘HighTorque-Wheels.’ However, to enable agile reorientation, a challenge arises from the fact that time-optimal slew maneuvers are, in general, not of the Euler axis rotation type; especially whenever the actuators are constrained independently. Moreover, BIROS’ on-board computer can only accommodate rotational acceleration commands twice per second. The objective is therefore to find a methodology to design fast slew maneuvers while considering a highly dynamic plant commanded by piecewise-constant sampled-time control inputs. This is achieved by considering a comprehensive analytical nonlinear model for spacecraft equipped with reaction wheels and transcribing a time-optimal control problem formulation into a multi-criteria optimization problem. Solutions are found with a direct approach using the trajectory optimization package ‘trajOpt’ of DLR-SR’s optimization tool, Multi-Objective Parameter Synthesis (MOPS). Results based on numerical simulations are presented to illustrate this method

Fast slew maneuvers for the High-Torque-Wheels BIROS spacecraft

The satellite platform BIROS (Bi-spectral InfraRed Optical System) is the second technology demonstrator of the DLR R&D `FireBIRD' space mission aiming to provide infrared remote sensing for early fire detection. Among several mission goals and scientific experiments, to demonstrate a high-agility attitude control system, the platform is actuated with an extra array of three orthogonal `High-Torque-Wheels' (HTW). For agile reorientation, however, a challenge arises from the fact that time-optimal slew maneuvers are in general not of the Euler-axis rotation type, specially whenever the actuators are constrained independently. Moreover, BIROS' On-Board-Computer (OBC) can only accommodate rotational acceleration commands twice per second. Our objective is therefore to find a methodology to design fast slew maneuvers while considering a highly dynamic plant commanded by piecewise-constant sampled-time control inputs. We do this by considering a comprehensive analytical nonlinear model for spacecraft equipped with reaction wheels and transcribing a time-optimal control problem formulation into a multi-criteria optimization problem which is then solved with a direct approach in a sequential procedure using the trajectory optimization package `trajOpt' of DLR-SR's optimization tool MOPS `Multi-Objective Parameter Synthesis'. Our approach for efficient design of rest-to-rest fast slew maneuvers considers an attitude error whose magnitude is proportional to Euler-axis rotations between current and desired attitudes even for large initial attitude errors. Results based on numerical simulations are presented to illustrate our method

Modelica stage separation dynamics modeling for End-to-End launch vehicle trajectory simulations

Stage separation dynamics modeling is a critical capability of future launchers preparatory studies. The development of stage separation frameworks integrable in end-to-end launch vehicle trajectory simulations have been presented in the relevant literature but none profiting from the object-oriented and equation-based acausal modeling properties of MODELICA. The objective of this paper is therefore to present such an approach to this problematic. Based on the Constraint Force Equation (CFE) methodology, two case studies to evaluate the proposed approach are considered. Results demonstrate that the approach corresponds very well with the physics behind separation. In addition, we found easiness of implementation of the method within a single environment such as DYMOLA, demonstrating the benefits of an integrated approach

Launch vehicle multibody dynamics modeling framework for preliminary design studies

Launch vehicle dynamics modeling is quite challenging mainly because of the highly interconnected disciplines involved: propulsion, aerodynamics, structures, mechanisms, and GNC among others. Discipline experts perform their respective design often independently and with separate dedicated tools. Consequently, during launcher preliminary design studies, numerous iterations are required in order to keep mission objectives synchronized. These preliminary design efforts can potentially be reduced by using a multidisciplinary launch vehicle model integrated in one single tool. Because this allows to reduce the number of iterations and the associated costs, a launch vehicle multibody dynamics modeling framework is a key technology to aim for. Dedicated developments of multidisciplinary modeling tools for launch vehicle multibody dynamics have been presented in the relevant literature. However, none fully profits from an object-oriented, equation-based, and acausal modeling language like Modelica. As yet, such an approach is still missing. It is therefore the objective of this paper to introduce such an alternative approach employing this modeling framework. This framework enables object-oriented and physics-based modeling of subsystems and components related to most key analyses of a launcher system. These include among others: launcher configuration, staging and separation dynamics, end-to-end trajectories, performance, controllability and stability. Moreover, all this can be done within a single simulation environment. The paper gives an overview on the first building blocks leading to an integrated and multidisciplinary tool for launcher preliminary design studies. Particularly, its easiness of implementation is demonstrated along with the benefits of this approach