Design and Functional Validation of a Mechanism for Dual-Spinning CubeSats
Session
Pre-Conference: CubeSat Developers' Workshop
Abstract
The mission of the Micro-sized Microwave Atmospheric Satellite (MicroMAS) project, a 3U CubeSat in development at MIT to deliver useful atmospheric profile data using a low-cost satellite, stretches the limit of what has been previously accomplished using the CubeSat platform. The payload, a multispectral passive microwave radiometer being developed by MIT Lincoln Laboratory, occupies one-third of the total vehicle volume as a standard 1U CubeSat structure. All other spacecraft functions are integrated within the remaining 2U bus structure under development in the MIT Space Systems Laboratory (SSL). In order to effectively collect data with the radiometer sensor, the spacecraft must simultaneously sweep the radiometer field of view perpendicular to the groundtrack while maintaining sub-degree pointing accuracy fixed in the local-vertical, local-horizontal (LVLH) frame. Preliminary design analyses determined that precessing the angular momentum of an entire spinning CubeSat as it progresses through its orbit would require torque in excess of the capabilities of available commercial, off-the-shelf (COTS) magnetorquers or reaction wheel sets. This attitude control problem led to a zero-momentum, dual-spinner design in which a purpose-designed spinner assembly would join the rotating payload module to the rest of the satellite, which remains fixed in the LVLH frame. The design for the MicroMAS spinner assembly incorporates a brushless DC zero-cogging motor, an optical encoder disk and sensor, a shielded bearing, and a slipring enclosed in an aluminum housing. Considering the lack of flight heritage of a dual-spinning CubeSat and the criticality of the spinner assembly to the mission, a rigorous series of staged functional tests are currently being implemented. The first stage of structural tests involved the in-house design and 3D-print fabrication of an initial engineering model, used to prove the concepts of proper assembly and function. Next, a second model was machined and assembled with a test motor to help identify areas for improvement regarding the alignment of parts to ensure proper rotation and better encoder functionality. After incorporating these design changes, a third-stage engineering model is currently being machined for testing during the spring. Future stages involve integrating and testing the engineering model with a flight-hardware motor prior to fabricating, integrating, and testing the final stage of assembly into the MicroMAS flight model. Thermal tests are also following a staged approach. Spinner assembly components, such as the motor controller and encoder hardware, are currently being tested individually in a thermal-vacuum chamber in the first stage of testing prior to assembly. The next stages of thermal-vacuum tests involve characterizing heat transfer between a fully-integrated spinner assembly and a mock payload module, and validating calculations on the effects of thermal expansion within the spinner assembly. All functional testing on the spinner assembly is scheduled to be completed before payload integration this summer.
Presentation Slides
Design and Functional Validation of a Mechanism for Dual-Spinning CubeSats
The mission of the Micro-sized Microwave Atmospheric Satellite (MicroMAS) project, a 3U CubeSat in development at MIT to deliver useful atmospheric profile data using a low-cost satellite, stretches the limit of what has been previously accomplished using the CubeSat platform. The payload, a multispectral passive microwave radiometer being developed by MIT Lincoln Laboratory, occupies one-third of the total vehicle volume as a standard 1U CubeSat structure. All other spacecraft functions are integrated within the remaining 2U bus structure under development in the MIT Space Systems Laboratory (SSL). In order to effectively collect data with the radiometer sensor, the spacecraft must simultaneously sweep the radiometer field of view perpendicular to the groundtrack while maintaining sub-degree pointing accuracy fixed in the local-vertical, local-horizontal (LVLH) frame. Preliminary design analyses determined that precessing the angular momentum of an entire spinning CubeSat as it progresses through its orbit would require torque in excess of the capabilities of available commercial, off-the-shelf (COTS) magnetorquers or reaction wheel sets. This attitude control problem led to a zero-momentum, dual-spinner design in which a purpose-designed spinner assembly would join the rotating payload module to the rest of the satellite, which remains fixed in the LVLH frame. The design for the MicroMAS spinner assembly incorporates a brushless DC zero-cogging motor, an optical encoder disk and sensor, a shielded bearing, and a slipring enclosed in an aluminum housing. Considering the lack of flight heritage of a dual-spinning CubeSat and the criticality of the spinner assembly to the mission, a rigorous series of staged functional tests are currently being implemented. The first stage of structural tests involved the in-house design and 3D-print fabrication of an initial engineering model, used to prove the concepts of proper assembly and function. Next, a second model was machined and assembled with a test motor to help identify areas for improvement regarding the alignment of parts to ensure proper rotation and better encoder functionality. After incorporating these design changes, a third-stage engineering model is currently being machined for testing during the spring. Future stages involve integrating and testing the engineering model with a flight-hardware motor prior to fabricating, integrating, and testing the final stage of assembly into the MicroMAS flight model. Thermal tests are also following a staged approach. Spinner assembly components, such as the motor controller and encoder hardware, are currently being tested individually in a thermal-vacuum chamber in the first stage of testing prior to assembly. The next stages of thermal-vacuum tests involve characterizing heat transfer between a fully-integrated spinner assembly and a mock payload module, and validating calculations on the effects of thermal expansion within the spinner assembly. All functional testing on the spinner assembly is scheduled to be completed before payload integration this summer.
