Controller Design Method for Drag-Free Systems with Micro-Propulsion Constraints

§Precise control systems are one main challenge for future space missions in the field of geodesy and fundamental physics. The control system must minimize noise in control outputs that affect scientific measurements. Performance specifications for such influences are often given as spectral densities, i.e. H∞ control design methods are appropriate. The stringent performance requirements require low-noise sensors and actuation systems, therefore µ-Newton-Thrusters are used. These have limitations with respect to the maximal thrust and thrust slew rates that are often limiting a linear controller design. Normally these limitations are checked in simulations a posteriori and the controller design must be reiterated. In this paper a method is presented that directly considers these limitations in the controller design by mapping individual thruster limitations into individual control loop requirements. Moreover a controller optimization method is presented based on an initial H∞ design. The drag-free control loops of the LISA Pathfinder/ST7 mission serves as an application case.