ESA GNC Conference Papers Repository
Model based active slosh damping experiment
This paper presents a model based experimental investigation to demonstrate the usefulness of an active damping strategy to manage fluid sloshing motion in spacecraft tanks. The active damping strategy is designed to reduce the degrading impact on stability, maneuvering and pointing performance of flight as well as to reduce fuel consumption via a force feedback strategy. Many problems have been encountered until now, such as instability of the closed loop system, excessive consumption in the attitude propellant or problems for engine re-ignition in upper stages and have been only addressed in a passive fashion via the design of baffles and membranes, which on their own have mass and constructive impacts. At GNC level, the destabilizing impact of the sloshing resonant mode is filtered via a notch filter (adaption to tank filling level) in the flight software. This is considered as a passive approach since there is no sensing, actuation and control action. Active management of propellant motion in launchers and satellites has the potential to increase performance on various levels. Examples are large angle slewing maneuvers of upper stages performed in such a way that the propellant stays close to the tank bottom or increasing the damping in powered flight phases of launchers.This paper demonstrates active slosh management using force feedback for the compensation of the slosh resonances. Force sensors between tank and the carrying structure provide information of the fluid motion via the reaction force. The control system is designed to generate an appropriate acceleration profile that leads to desired attenuation profiles in amplitude, frequency and time. The model based design approach allows approaching the development of GNC in a structured way (see ). At first, an analytical description of the slosh motion is derived. The next step is to set-up a simulation infrastructure based on physical modelling, in this case with FLOW3D for a computational fluid dynamic (CFD) representation of the propellant dynamics. It is of importance that the CFD must interact with the controller in closed loop (see ).In order to prove the concept of active damping and to show the mastering of the complete development cycle from theoretical design to CFD simulation to hardware-in-the-loop, a tank with 600 l water is placed on a Hexapod. The reaction force between tank and Hexapod attachment is measured with the purpose to increase the damping of the fluid after it has been excited into an oscillatory, very lightly damped motion. The Hexapod is then used to realize small accelerations to the tank, which quickly damp the oscillation.Two controller architectures are tested and compared. The first is a classical mu design with a model reduction applied to the resulting controller. The second one is a fixed structure controller based on the Mathworks' Robust Control Toolbox 'robust systune' feature, which implements the algorithm from . Designs are carried out for different weighting functions and the resulting controllers are tested on the Hexapod. Results of various tests are reported in the paper which prove that active damping can robustly be achieved within the time span of two to three harmonic cycles. The paper illustrates that is possible to actively influence sloshing via closed loop and that the algorithm described in  is very useful in designing robust feedback system. The CFD prediction is compared with the tests for various fill levels (up to 1.1 metric tons of water). It is further shown that a process can be established which automatically generates a LabView implementation from the Simulink design environment allowing an effective way of implementing complex algorithms into embedded systems.The activity has been performed under ESA's Future Launcher Preparatory Program (FLPP3) in the study Upper Stage Attitude Control Development Framework' (USACDF).