Design, Integration and Testing of the Delfi-PQ Satellite Reaction Wheel

Abstract

A highly miniaturised reaction wheel was developed for Delft University of Technology’s PocketQube (PQ) mission as topic of this thesis project. Delfi-PQ, a picosatellite, aims to demonstrate a reliable core bus platform for PocketQubes with a form factor of 5x5x5 [cm] and one or more payload(s). PocketQubes hold the potential to reduce the cost of access to space and disrupt traditional space applications by providing cost effective global coverage. Three-axis stabilisation supports advanced capabilities such as Earth observation, high data rate transfers and propulsive manoeuvres. Precise attitude control requires reaction wheels with accurate speed control and low vibration levels. Power and volume requirements, however, drive a simple and compact design. Presently, no suitably miniaturised reaction wheel exists in the world. Therefore, a reaction wheel is developed specifically for the PocketQube mission. Due to a strict power requirement the research in this thesis focuses on characterising the power consumption of the reaction wheel.

Based on orbital disturbances and the operational context of the Delfi-PQ mission, requirements for the reaction wheel were derived. A suitable electric motor was selected and conclusions from a preliminary design were used to create a detailed design of a single reaction wheel. A simple flywheel is attached to a brushless electric motor and secured in a pressurised housing to protect the bearing lubrication from the vacuum of space. An in-depth model of the motor was created in Simulink and validated through functional testing of the motor. Model results were used to estimate control electronics power consumption. The housing of the reaction wheel was tested for its ability to seal off the motor and flywheel under atmospheric pressure. Then, the motor with flywheel was subjected to vibrations representative of the launch environment and functional testing of the reaction wheel including operational scenarios was performed. Furthermore, a micro-vibration test bench was developed in parallel to characterise the disturbances produced by the wheel. Based on conclusions, recommendations for improving a proposed design for a complete three-wheel assembly were derived.

With a mass of about 8 [g], average power consumption of around 15 [mW] and size of 20x20x12 [mm] the engineering model has been developed successfully. The reaction wheel can provide at least a torque of 2.7*10^-7 Nm over its full speed range and has a (one way) momentum storage of 1.1*10^-4 Nms. This is sufficient for attitude stabilisation and ground station tracking on a triple-unit PocketQube in a Low Earth Orbit with an altitude of above 360 [km]. The motor with flywheel uses about 35% more power than the motor alone. This is due to an imbalance of the flywheel which leads to more mechanical friction, although additional effects are present. Disturbances created by the wheel constrain Earth observation scenarios are speeds over 4000 [RPM]. A proposed three-wheel design was created that further pushes the boundaries of miniaturisation with dimensions of 31x31x22 [mm] including control electronics. Future developments should focus on developing control electronics, improving the vacuum seal of the housing and most importantly, the balance of the flywheel.