CubeSat Attitude Control Testbed Design: Merritt 4-Coil per axis Helmholtz Cage and Spherical Air Bearing

Abstract

This paper describes the design, construction, and initial tests of a Merritt 4-coil per axis Helmholtz cage around a spherical air bearing for laboratory testing of a CubeSat’s attitude determination and control system (ADCS). While numerical modeling and simulations are important for ADCS design and sensitivity studies, it is important to perform hardware-in-the-loop testing, which results in higher levels of confidence of onorbit performance and lower risk. Attitude control systems measure the magnetic field with magnetometers, and actuate with torque rods or coils. These rods or coils use electromagnetic fields induced by the directional flow of current through the conducting coils to engage with the local magnetic field, and thus transfer momentum and spin the satellite. In order to test that that the attitude determination and control subsystem will perform as expected on orbit, it is important to test both the sensing capability of the magnetometer, the actuating capability of the torque rods, and the interface between the two. To this end, the MIT Space Systems Laboratory has constructed a three-axis Helmholtz cage using a Merritt 4-coil design to maximize uniform field volume around a spherical air bearing. This test bed allows for simulation of the Earth’s magnetic field at the orbital altitude of the satellite, with the combination of a spherical air bearing which supports three degree-of-freedom (3DOF) rotation of the test article. Initial tests have demonstrated the success of producing a uniform magnetic field, as well as a generating a field that follows an 11 th order model of Earth’s IGRF magnetic field, by controlling the amount of current to applied to coils of copper wire through a PC interface. In future work, we will use this facility to test attitude determination and control system for satellites being built in the MIT Space Systems Lab, including a CubeSat called MicroMAS, the Microsized Microwave Atmospheric Satellite.