Closed-loop Thrust Magnitude Control System for Nano- and Pico-Satellite Applications

Abstract

The growing needs of nano- and pico-satellite missions require several enhancements in micro-propulsion capabilities to enable the satellites to perform an increasing variety of orbital maneuvers. Among them, the possibility to accurately control the thrust would open up new scenarios for nano- and pico-satellites applicability to include, for example, missions such as space debris removal and orbit transfer.

This thesis presents the design and the implementation of a closed-loop control system for thrust magnitude regulation in micro-resistojets. This is achieved by controlling the propellant mass flow in the micro-valve of the feeding system which is designed to make extensive use of off-the-shelf components. The Vaporizing Liquid Micro-resistojet (VLM) is one of the micro-thrusters under development at TU Delft and, for this reason, it is selected for performing the tests. The outcome of this work is meant to give insights into the design and performance level of future technologies for thrust magnitude control, which will be designed and manufactured at TU Delft. In order to develop appropriate controllers, a non-linear state-space model of the micro-valve system is developed analytically. The model includes different domains, such as electro-magnetic, fluid and mechanical, in order to bring together the complex dynamic behavior of the actuator. The performance of the analytical model is compared to a more sophisticated multi-domain analysis performed with finite element method (FEM) and computational fluid dynamics (CFD). Two controllers, namely PID (proportional-integral-derivative) and SMC (sliding mode control), are designed and tested using the models developed.
The ON/OFF micro-valves are controlled with PWM (pulse width modulation) by tuning the operating duty cycle. A hybrid sliding mode control scheme is developed based on the insights gave by the experiments, which enables the thrust magnitude control of the comprehensive system.
Finally, the closed-loop control system is implemented in the preliminary hardware design of the micropropulsion system. The experimental set-up comprises the propellant tank, the micro-valve, the micro-thruster, the feeding channel, pressure and temperature sensors and the processing microcontroller. The tests are focused on the performance of the controller, and the fine tuning of its parameters, and also in the validation of the design approach.

The controlled micro-propulsion system delivers the commanded input with a response time of ~1.5 s and a resolution of 50 mbar in the chamber pressure. As a first implementation of a control system for micro-propulsion at TU Delft, the results are promising and certainly pave the way for future developments on this research field.