CubeSats are widely used miniature satellites with dimensions of 10 cm x 10 cm x 11.35 cm and a maximum weight of 1.33 kg. They are used for vaious functions ranging from amateur radio to testing new technologies.
A great challenge faced by CubeSats is attitude (point direction) control. As soon as they are deployed into space they begin tumbling due to asymmetric deployment forces and weight distribution. For satellites that are required to point in a specific direction, some sort of attitude control system is necessary.
The goal of this group project was to design a robust attitude control system that would fit inside of a single CubeSat unit and could be attached to another CubeSat, resulting in a double-unit, or even triple-unit CubeSat with reliable attitude control.
Our final design comprised four momentum wheels with their axes of momentum arranged normal to the faces of a tetrahedron. The arrangement provides a single redundancy, allowing the attitude control system to continue functioning after the failure of any one wheel.
As a proof of concept, we demonstrated that our attitude control system would work on a single axis. We used a frictionless air-table to simulate space atmosphere in two dimensions. A small accelerometer was mounted on a floating disk to measure the angular acceleration caused by an external force. A momentum wheel driven by a small stepper motor would rotate to counteract angular displacement.
Parts for the air-table and prototype housing were designed in Adobe Illustrator and cut out of clear acrylic on a laser cutter. Other parts used to build our prototype include an Arduino microcontroller, a compact stepper driver, a small stepper motor, a momentum wheel, and a 3-axis accelerometer. The accelerometer is located directly under the stepper motor.
To demonstrate the accuracy of our attitude control system, we aligned an arrow on the edge of the floating disk with a stationary external reference point. An external force would then displace the prototype from its initial starting point. The momentum wheel would rotate to reorient the system back to its original position. We were able to achieve a pointing accuracy of < 3 degrees.
The project was a group effort for Brown University’s “Machine Design” course. I teamed up with three other engineers: Nicholas Ragosta, Gregory Alexander, and Parker Wells. I was responsible for component layout and sensor selection. I collaborated with Nick Ragosta on Arduino programming and with Gregory Alexander on prototype design.
Dimensions: 5cm x 5cm x 6cm
Components: Arduino Uno microcontroller, 3-axis accelerometer, steel momentum wheel, stepper motor, stepper driver, steel hardware, clear acrylic
Date: Spring 2012