Tesi di laurea magistrale
Limit Cycle Analysis for Spacecraft with Pulsed Thrusters
Corso di studi
relatore Mengali, Giovanni
- dual-input describing function
Data inizio appello
Throughout the last decades, attitude control systems with switching actuators<br>and discrete sensors have been used in satellites subjected to slowly varying<br>disturbances. Sun sensors are usually employed. Such sensors are discrete<br>and, typically, slower than actuators. Several types of on-off thrusters are<br>employed as actuators, such as hydrazine, cold-gas and pulse plasma thrusters.<br>These thrusters are typically affected by switching constraints. Due to these<br>constraints and the disturbances, the system shall operate in limit cycle<br>conditions. Two types of limit cycles can occur:<br>• Saturation limit cycles.<br>• Disturbance limit cycles.<br>Our purpose is the development of a controller design method which avoids<br>saturation limit cycles - that are very expensive in terms of fuel consumption -<br>and produces a disturbance limit cycle which meets amplitude and bandwidth<br>requirements. A reference scenario will be presented and simulations will be<br>performed to test potential outcomes.<br>The first part of the thesis will study the methods used to predict limit<br>cycles. Particular emphasis will be given to the classical describing function<br>theory. After that, we will develop the new dual-input describing function<br>theory which can deal with slowly varying disturbances. In order to address<br>strange behaviors the classical Tsypkin method will be presented and the hybrid<br>Tsypkin-dual-input describing function method, which takes into account<br>disturbances, will be applied to our case.<br>In the second part, we will focus on the design methods of the controller.<br>The Kharitonov approach, which is robust and uses the classical describing<br>function theory, will be studied in detail. In the end we will introduce the new<br>dual-input Kharitonov approach, developed by using the dual-input describing<br>function theory and capable of dealing with slowly varying disturbances.