Christopher Lomanno, Air Force Institute of Technology; Robert Bettinger, Air Force Institute of Technology; Paul Gindre, École de l’air
Keywords: Attitude control, modular systems, CubeSats
Abstract:
The viability of using multiple small commercial-off-the-shelf (COTS) attitude determination and control systems (ADCS) is investigated to improve the performance of a 6U (10 x 20 x 30 cm) CubeSat. For CubeSats, a single bus unit (or “U”) is defined by a (10 x 10 x 10) cm volume. As one of the most complex subsystems for all classes of satellite, attitude control subsystems require a specialized design in order to satisfy the control requirements for a given space mission and its constituent payload(s). The complexity of the attitude control subsystems usually leads spacecraft developers to purchase a COTS unit in order to reduce risk during both the pre-launch development and post-launch spacecraft operation. CubeSat-sized attitude control subsystems are typically manufactured to measure 0.5U (10 x 10 x 5 cm) and output a maximum of 900 mN·m of torque. Due to the non-custom nature of COTS attitude control units, however, CubeSat developers using large busses of a volume 6U or larger routinely utilize undersized reaction or momentum wheels for spacecraft control which, in turn, decreases the operating potential of CubeSat payloads.
For the present research, the attitude control subsystem units are constructed with each unit featuring a unique (x,y,z) spatial offset from the CubeSat center-of-gravity, and a control law utilizing a quaternion feedback regulator. The physical properties of the reaction wheels comprising each unit are representative of publically-available values for the Blue Canyon Technologies’ XACT system, a contemporary attitude control subsystem unit employed by numerous CubeSat missions. Fixing each unit inside the 6U chassis is accomplished by constructing a motor-to-body torque distribution matrix using the wheel location properties and defining a generic pyramidal array assembly with four reaction wheels.
In a typical CubeSat, a COTS attitude control subsystem unit is a calibrated “black box” that minimally interfaces with the rest of the CubeSat bus subsystems. This research replicates that behavior by implementing two independent units to act on the same CubeSat structure with a minimum of shared information between each unit. The only characteristic shared between the individual units is their relative torque contribution on the CubeSat system. A switch is devised to facilitate changing the control from using either one of the attitude control subsystem units, or to using both simultaneously. Simulations will be conducted to illustrate these different control states for a variety of different attitude control subsystem locations and orientations throughout a 6U CubeSat chassis. These locations will consist first of a corner case in which both units are in a single corner U of the chassis. For the second case, the two units will operate in a single middle U. For the third and final case, the two units will operate in opposing corners of the 6U CubeSat chassis.
Overall, the present research seeks to develop new method by which to control CubeSats through the use of multiple COTS attitude control subsystem units. The use of multiple units will provide a viable control alternative to designing a custom, mission-dependent attitude control solution, or settling for a sub-optimal control solution with the use of only a single attitude control subsystem unit.
Date of Conference: September 15-18, 2020
Track: Space-Based Assets