# Integrated Magnetic Management of Stored Angular Momentum in Autonomous Attitude Control Systems
###### Abstract  
Autonomous spacecraft operations are at the front end of modern research interests, because they enable space missions that would not be viable only with ground control. The possibility to exploit onboard autonomy to deal with platform management and nominal housekeeping is thus beneficial to realize complex space missions, which could then rely on ground support only for the mission-critical phases. One routine operation that most spacecraft must perform is stored angular momentum management to maintain fully usable momentum exchange actuators. The execution of this activity may be scheduled, commanded from the ground, or automatically triggered when certain thresholds are reached. However, autonomous angular momentum management may interfere with other primary spacecraft operations if executed with a dedicated and separate system mode. This paper presents the magnetic management of stored angular momentum, integrated with the main attitude control system. The system design and implementation are intended for autonomous spacecraft, and it can be operated without significant ground support. The paper describes the system architecture and the attitude control laws integrated with the magnetic angular momentum management. Specifically, the capability of the autonomous system to keep the internal angular momentum far from the saturation and far from the zero-crossing levels is highlighted. The performance of an example attitude control system with four reaction wheels and three magnetic torquers is presented and discussed, with the simulation results at model-in-the-loop (MIL) level. integrated attitude control; stored angular momentum management; magnetic torquers; reaction wheels; autonomous spacecraft 202320231003390/aerospace100  
associated costs can be more focused and dedicated to mission-critical phases, more easily enabling the realization of complex space missions. The angular momentum stored in a spacecraft is mainly due to momentum exchange actuators, which exploit the conservation of angular momentum law to produce internal torque. Thus, momentum exchange actuators cannot exchange net angular momentum with the external environment, but they produce control torque by exchanging internal angular momentum with the spacecraft's main body. This can be achieved by putting a mass in rotation and generating reaction torques, as in reaction wheels or control moment gyros (CMGs). In both cases, there exists a maximum momentum capacity that the rotors can hold, and it is commonly related to the maximum angular rate that the electric motor driving the rotor can sustain for a given inertia of the actuator assembly. From the maximum momentum capacity, it is possible to compute for how long a certain torque can be borne before reaching the maximum rotation speed, which is called the saturation speed. In fact, the angular momentum stored in momentum exchange actuators evolves over time because of external torque and control actions. Secular terms, as well as asymmetric control torques, may lead to momentum accumulation or momentum build-up. When the saturation condition is reached, the actuator cannot provide any more torque in that direction, and its stored angular momentum is dumped out. This operation is denoted as stored momentum dumping or actuator unloading or desaturation, and it is performed exploiting devices that can exchange angular momentum with the surrounding environment (e.g., magnetic torquers and thrusters). These secondary actuators provide a net external torque that is capable of counteracting the internal reaction torque due to the momentum unloading (i.e., rotor braking). This paper proposes a method to autonomously manage the stored angular momentum in attitude control systems (ACSs) by integrating the momentum dumping functions with the main control loop.