Available at: https://digitalcommons.calpoly.edu/theses/2858
Date of Award
6-2024
Degree Name
MS in Aerospace Engineering
Department/Program
Aerospace Engineering
College
College of Engineering
Advisor
Eric Mehiel
Advisor Department
Aerospace Engineering
Advisor College
College of Engineering
Abstract
The goal of this thesis is to investigate automatic mass balancing methods for spacecraft attitude dynamics simulators, create a hardware design for a mass balancing system, and assemble the hardware on the Cal Poly Spacecraft Attitude Dynamics Simulator (SADS). Spacecraft attitude dynamics simulators replicate the torque-free environment of space with ground-based hardware. The SADS is mounted on a spherical air bearing, and includes a pyramid of four reaction wheels. The air bearing allows frictionless, unbounded rotation about the vertical axis, and 30 degrees about the horizontal axes. The torque-free configuration of the SADS can be used to test spacecraft attitude control software and hardware. For spacecraft attitude simulators, it is essential to accurately align the center of mass with the center of rotation. Any alignment error will cause a gravitational disturbance torque that quickly saturates actuators. It is necessary to use an automatic mass balancing system if the system's hardware is regularly adjusted, since manual mass balancing methods become prohibitively time consuming. Once balanced, it is also useful if the attitude simulator can measure its inertia tensor for use in control software. To avoid displacing the center of mass, a symmetrical six-sliding-mass balancing system was developed for the SADS, driven by geared DC motors. Several mass balancing algorithms were designed that do not require additional actuators. These algorithms enable mass balancing prior to the integration of reaction wheels, and are intended to eventually be part of a two-stage balancing approach. In the controlled stage, a continuous time controller corrects the gravitational disturbance torque. A moment distribution law for the sliding masses is then used with an FSFB or adaptive controller to align the center of mass and rotation. In the system identification stage, angles and angular rates are measured during phased torque sine sweeps so that the inertia tensor and center of mass position can be estimated. After making a correction to the sliding mass positions, the identification and correction process is repeated until the system is balanced. To be considered 'balanced,' any remaining disturbance torques must be negligible on the time scale of a control system simulation. The horizontal axis positional errors for the center of mass should be on the order of one tenth of a millimeter, and it is generally acceptable for the vertical axis error to be approximately an order of magnitude greater. Since a vertical center of mass offset is generally aligned with the structure of the air bearing, its corresponding gravity disturbance torque is reduced. Since the mass properties of the SADS must be measured regardless, they are a convenient metric for the effectiveness of the balancing system.