Date of Award

January 2014

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

William Semke

Abstract

A control scheme is proposed for a satellite orbit controller around a small, irregularly shaped near-Earth object (NEO) combining classical control theory and orbital mechanics into a continuous hybrid control system that achieves and maintains a circular orbit in a perturbed environment. NEOs are asteroids and comets that approach Earth's orbit around the Sun. They are currently being studied for resource allocation and threat mitigation, while providing unique opportunities for control systems. The NEO environment consists of a weak and complex gravity field, as well as other perturbations such as solar radiation pressure (SRP) and third-body gravitational disturbances. This project focuses on the gravity field of the NEO and characterizes orbital stability within the NEO's gravity field. A three-term Proportional, Integral, and Derivative (PID) controller is utilized in order to achieve and maintain a circular orbit in close-proximity to the NEO 25143 Itokawa. The proposed control scheme merges a simple controller with orbital mechanics to maximize the effectiveness and efficiency of the thrusters. It uses the PID controller to thrust in the radial direction in order to maintain the proper orbital radius, which is found to be an effective method of correcting perturbed orbits in the NEO environment. This is followed by a change in the orbital velocity of the spacecraft in order to match the specific mechanical energy for the desired circular orbit, which is typically the most efficient method of correcting perturbed orbits. Systems Tool Kit (STK) is used to run the simulation and a MATLAB-STK interface was developed that allows for sophisticated orbit control development. Using the STK simulation software allows for the ability to test multiple orbit parameters for stability. This was applied in studying the interaction between the complex gravity model and its effect on the satellite using a harmonic excitation analysis. It was found that when the ratio of the excitation frequency to the natural frequency (ω/ωn) is greater than seven, the orbit is stable. This thesis provides methods for simulating and predicting satellite orbit control as well as providing guidelines for regions of stability for NEO missions.

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