Test and Verification of a CubeSat Attitude Determination and Control System in Variable Magnetic Fields

The Center for Space Research and Assurance (CSRA) at the Air Force Institute of Technology (AFIT) continues to explore CubeSat initiatives for solving many current space security issues. Regardless of the mission requirements, the success of the Cube- Sat on orbit frequently depends on the Attitude Determination and Control System (ADCS) functioning correctly. Previous research at AFIT has demonstrated single axis control on a spherical air bearing test bed incorporated within a Helmholtz cage utilizing artificially strong magnetic fields for better signal to noise ratios which are not experienced on orbit. This research explores the process of redesigning, testing, and programming a new 6U CubeSat ADCS to operate in representative magnetic fields using a three wheel reaction wheel array (RWA). A second external magnetometer is utilized while its effect on the quaternion estimate (QUEST) is characterized. The RWA is modularized and displaced from the ADCS μcontroller by the addition of a separate μcontroller on the RWA to handle Hall sensor interrupts allowing the control and estimation task to run uninterrupted. The displacement of the RWA from the primary ADCS μcontroller, which includes the primary magnetometer, minimizes electromagnetic disturbances caused by the RWA on the magnetometer. A quaternion error Proportional-Integral-Derivative (PID) control law is used to control the ADCS test bed while an external motion capture system captures its true orientation. This research effort shows that the quaternion estimate degrades as the magnetic field strength is reduced. The ambient Earth magnetic field increased the final angle error by 7.1° during a 90° rotation maneuver when compared to the maximum Helmholtz cage condition