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1. Multifunctional Structures for Attitude Control

   https://doi.org/10.1115/SMASIS2019-5565  

   Vedant ; Allison, James T. ( September 2019 , American Society of Mechanical Engineers)

   The Engineering Systems Design Lab (ESDL) at the University of Illinois introduced Strain-Actuated Solar Arrays (SASAs) as a solution for precise satellite Attitude Control System (ACSs). SASA is designed to provide active mechanical vibration (jitter) cancellation, as well as small slew maneuver capabilities to hold a pose for short time periods. Current SASA implementations utilize piezoelectric distributed actuators to strain deployable structures, and the resulting momentum transfer rotates the spacecraft bus. A core disadvantage, however, is small strain and slew capability. Initial SASA systems could help improve pointing accuracy, but must be coupled with another ACS technology to produce large reorientations. A novel extension of the original SASA system is presented here that overcomes the small-displacement limitation, enabling use of SASA as a sole ACS for some missions, or in conjunction with other ACSs. This extension, known as Multifunctional Structures for Attitude Control (MSAC), can produce arbitrarily-large rotations, and has the potential to scale to large spacecraft. The system utilizes existing flexible deployable structures (such as solar arrays or radiators) as multifunctional devices. This multi-role use of solar panels extends their utility at a low mass penalty, while increasing reliability of the spacecraft ACS. 

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2. Impact of Strain-Actuated Attitude Control Systems for Variant Mission Classes

   Vedant, Alexander Ghosh ( October 2019 , 70th International Astronautical Congress)

   The University of Illinois, in collaboration with NASA Jet Propulsion Laboratory (JPL) and NASA Ames Research Center, has developed a novel Attitude Control System (ACS) called the Strain Actuated Solar Arrays (SASA), with sub-milli-arcsecond pointing capability. SASA uses strain-producing actuators to deform flexible deployable structures, and the resulting reaction forces rotate the satellite. This momentum transfer strategy is used for jitter reduction and small-angle slew maneuvers. The system is currently at a Technology Readiness Level of 4-5 and has an upcoming demonstration flight on the CAPSat CubeSat mission. An extension to the SASA concept, known as Multifunctional Structures for Attitude Control (MSAC), enables arbitrarily large-angle slew maneuvers in addition to jitter cancellation. MSAC can potentially replace reaction wheels and control moment gyroscopes for attitude control systems, thereby eliminating a key source of jitter noise. Both SASA and MSAC are more reliable because of fewer failure modes and lower failure rates as compared to conventional ACS, while having an overall smaller mass, volume, and power budget. The paper discusses the advantages of using SASA and MSAC for a wide range of spacecraft and variant mission classes. 

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3. Multifunctional Structures for Spacecraft Attitude Control

   Vedant, Vedant ; Patterson, Albert E. ; Allison, James T. ( February 2020 , 2020 AAS Guidance, Navigation, and Control Conference)

   A new attitude control system called Multifunctional Structures for Attitude Control (MSAC) is explored in this paper. This system utilizes deployable structures to provide fine pointing and large slewing capabilities for spacecraft. These deploy- able structures utilize distributed actuation, such as piezoelectric strain actuators, to control flexible structure vibration and motion. A related type of intelligent structure has been introduced recently for precision spacecraft attitude control, called Strain Actuated Solar Arrays (SASA). MSAC extends the capabilities of the SASA concept such that arbitrarily large angle slewing can be achieved at relatively fast rates, thereby providing a means to replace Reaction Wheel Assemblies and Control Moment Gyroscopes. MSAC utilizes actuators bonded to deployable panels, such as solar arrays or other structural appendages, and bends the panels to use inertial coupling for small-amplitude, high-precision attitude control and active damping. In addition to presenting the concept, we introduce the operational principles for MSAC and develop a lumped low-fidelity Hardware-in-the-Loop (HIL) prototype and testbed to explore them. Some preliminary experimental results obtained using this prototype provided valuable insight into the design and performance of this new class of attitude control systems. Based on these results and developed principles, we have developed useful lumped-parameter models to use in further system refinement. 

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4. Strain-Actuated Solar Arrays for Spacecraft Attitude Control Assisted by Viscoelastic Damping

   Yong Hoon Lee, Vedant ( May 2019 , 13th World Congress of Structural and Multidisciplinary Optimization)

   This article presents a utilization of viscoelastic damping to reduce control system complexity for strain-actuated solar array (SASA) based spacecraft attitude control systems (ACSs). SASA utilizes intelligent structures for attitude control, and is a promising next-generation spacecraft ACS technology with the potential to achieve unprecedented levels of pointing accuracy and jitter reduction during key scientific observation periods. The current state-of-the-art SASA implementation utilizes piecewise modeling of distributed piezoelectric (PZT) actuators, resulting in a monolithic structure with the potential for enhanced ACS reliability. PZT actuators can operate at high frequencies, which enables active vibration damping to achieve ultra-quiet operation for sensitive instruments. Relying on active damping alone, however, requires significant control system complexity, which has so far limited adoption of intelligent structures in spacecraft control systems. Here we seek to understand how to modify passive system design in strategic ways to reduce control system complexity while maintaining high performance. An integrated physical and control system design (codesign) optimization strategy is employed to ensure system-optimal performance, and to help understand design coupling between passive physical aspects of design and active control system design. In this study, we present the possibility of utilizing viscoelastic material distributed throughout the SASA substructure to provide tailored passive damping, intending to reduce control system complexity. At this early phase of study, the effect of temperature variation on material behavior is not considered; the study focuses instead on the design coupling between distributed material and control systems. The spatially-distributed design of both elastic and viscoelastic material in the SASA substructure is considered in an integrated manner. An approximate model is used that balances predictive accuracy and computational efficiency. This model approximates the distributed compliant SASA structure using a series of rigid links connected by generalized torsional springs and dampers. This multi-link pseudo-rigid-body dynamic model (PRBDM) with lumped viscoelastic damping models is derived, and is used in numerical co-design studies to quantify the tradeoffs and benefits of using distributed passive damping to reduce the complexity of SASA control systems. 

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5. Impact of Including Electronics Design on Design of Intelligent Structures: Applications to Multifunctional Structures for Attitude Control (MSAC)

   https://doi.org/10.1115/SMASIS2020-2331  

   Vedant, Vedant ; Allison, James T. ( September 2020 , 2020 ASME Smart Materials, Adaptive Structures and Intelligent Systems Conference)

   Multifunctional Structures for Attitude Control (MSAC) is a new spacecraft attitude control system that utilizes deployable panels as multifunctional intelligent structures to provide both fine pointing and large slew attitude control. Previous studies introduced MSAC design and operation concepts, simulation-based design studies, and Hardware-in-the-Loop (HIL) validation of a simplified prototype. In this article, we expand the scope of design studies to include individual compliant piezo-electric actuators and associated power electronics. This advance is a step toward high-fidelity MSAC system operation, and reveals new design insights for further performance enhancement. Actuators are designed using pseudo rigid body dynamic models (PRBDMs), and are validated for steady-state and step responses against Finite Element Analysis. The drive electronics model consists of a few distinct topologies that will be used to evaluate system performance for given mechanical and control system designs. Subsequently, a high-fidelity multiphysics multibody MSAC system model, based on the validated compliant actuators and drive electronics, is developed to support implementation of MSAC Control Co-design optimization studies. This model will be used to demonstrate the impact of including the power electronics design in the Optimal Control Co-Design domain. The different control trajectories are compared for slew rates and the vibrational jitter introduced to the satellite. The results from this work will be used to realize closed-loop control trajectories that have minimal jitter introduction while providing high slew rates. 

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