This paper reports on efforts to control a tethered formation flight spacecraft array for NASA's SPECS mission using the SPHERES test-bed developed by the MIT Space Systems Laboratory. Specifically, advances in methodology and experimental results realized since the 2005 SPIE paper are emphasized. These include a new test-bed setup with a reaction wheel assembly, a novel relative attitude measurement system using force torque sensors, and modeling of non-ideal tethers to account for tether vibration modes. The nonlinear equations of motion of multi-vehicle tethered spacecraft with elastic flexible tethers are derived from Lagrange's equations. The controllability analysis indicates that both array resizing and spin-up are fully controllable by the reaction wheels and the tether motor, thereby saving thruster fuel consumption. Based upon this analysis, linear and nonlinear controllers have been successfully implemented on the tethered SPHERES testbed, and tested at the NASA MSFC's flat floor facility using two and three SPHERES configurations.
The high cost associated with spaceflight research often compels experimenters to scale back their research goals significantly purely for budgetary reasons; among experiment systems, control and data collection electronics are a major contributor to total project cost. ESF-X was developed as an architecture demonstration in response to this need: it is a highly capable, radiation-protected experiment support computer, designed to be configurable on demand to each investigator's particular experiment needs, and operational in LEO for missions lasting up to several years (e.g., ISS EXPRESS) without scheduled service or maintenance. ESF-X can accommodate up to 255 data channels (I/O, A/D, D/A, etc.), allocated per customer request, with data rates up to 40kHz. Additionally, ESF-X can be programmed using the graphical block-diagram based programming languages Simulink and MATLAB. This represents a major cost saving opportunity for future investigators, who can now obtain a customized, space-qualified experiment controller at steeply reduced cost compared to 'new' design, and without the performance compromises associated with using preexisting 'generic' systems. This paper documents the functional benchtop prototype, which utilizes a combination of COTS and space-qualified components, along with unit-gravity-specific provisions appropriate to laboratory environment evaluation of the ESF-X design concept and its physical implementation.