Modelling of Actuator Dynamics for Spacecraft Attitude Control

摘要

SPACECRAFT mission success is often highly dependent on thenperformance and robustness of the attitude control system,nwhich consists of different types of actuators, such as reaction controlnthrusters, reaction wheels, and magnetic actuators. Solutions tonspacecraft attitude control problems often rely on inherentnassumptions that the onboard actuators are able to deliver the exactntorque desired by the attitude controller at a specified time. Thenactuators are thus assumed to have no dynamics, or the actuatorndynamics are assumed to be fast enough to allow them to benneglected. Although this has shown to be a sufficient approach in thenpast, it is obvious that attitude actuators, as all electromechanicalndevices, have dynamics that might impact controller performance.nWith an increasing demand for high-precision attitude control fornpurposes such as formation control and optical intersatellite links, thennecessity of including actuator dynamics within the control solutionnis increasingly evident.nMathematical modelling of the complex and nonideal dynamicalnbehavior of actuators and its influence on spacecraft attitude controlnis a cumbersome task. This dynamical behavior has traditionallynbeen found through laboratory testing, and the subsequent controllerndesign has been influenced by these considerations. One example innthis direction is the requirement of reaction thruster response delaysnbeing less than the duration of the minimum activation pulse of thenactuator. There exist, however, theoretical results from the modellingnand analysis of actuator dynamics, such as in [1], in which the effectnof unmodelled fast actuator dynamics on the output feedbacknstabilization of feedback linearizable systems is studied. Similarly, inn[2,3], the robust stabilization of a class of nonlinear systems in thenpresence of unmodelled actuator and sensor dynamics isninvestigated. More applicable results for our purpose are found inn[4,5], in which general multiple-input/multiple-output (MIMO)nlinear actuator models for underwater vehicle thrusters withndynamics are presented. Because underwater vehicle thrusters arenessentially propellers connected to dc motors, analogous to, fornexample, reaction wheels for spacecraft attitude control, is it possiblento describe other actuators with the same model subject to minornchanges and tuning.nIn this paper, we substantiate a unified mathematical model ofnvarious attitude control actuators for space applications, in particular,nreaction thrusters, reaction wheels, and magnetic torquers. Thengeneral actuator dynamical model is based on the marine technologynwork of [5], appropriately fitted to the aforementioned actuatorncategories. To describe time delays in the response of actuators suchnas, for example, thrusters, an expansion of the general actuator modelnis suggested.