Magnetic attitude control verified on CubeDyna

Abstract

Magnetic-only attitude control has no moving parts and draws little power, but it is underactuated: the available torque is always orthogonal to the local magnetic field. Controllability is recovered along the orbit, where the field direction rotates, and this makes the real behaviour hard to predict from simulation alone.

The thesis implements a magnetic control law and verifies it on CubeDyna, the air bearing platform inside the Helmholtz cage, where the field along the orbit is actually reproduced.

Objectives

1. Implement a magnetic control law for initial detumbling and for pointing.
2. Verify it in simulation on a low orbit, with the dominant disturbance torques.
3. Repeat it in hardware in the loop on CubeDyna, with the field generated by the cage.
4. Analyse the differences between simulation and testbed and trace their causes.

Methodology

1. Dynamic model with the magnetic field from a geomagnetic model and orbit propagation.
2. Control synthesis and stability check on the orbit averaged system.
3. Attitude estimation from magnetometer and sun sensors.
4. Platform balancing, to reduce the residual gravity torque, and measurement of the residual.
5. Testbed campaigns with the field commanded along the orbit, and true attitude from optical tracking.

Expected contributions

1. A magnetic control law verified in hardware, not only simulated.
2. Quantification of the gap between simulation and testbed, with the main causes.
3. A documented balancing procedure, reusable in later campaigns.

Tools

MATLAB and Simulink, the CubeDyna air bearing platform, Helmholtz cage, the lab magnetometer and sun sensors.