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1. Attitude​ ​Control​ ​System​ ​Complexity​ ​Reduction​ ​via​ ​Tailored​ ​Viscoelastic​ ​Damping Co-Design

   Chendi Lin, Daniel R ( February 2018 , 2018 AAS Guidance & Control Conference)

   Intelligent structures utilize distributed actuation, such as piezoelectric strain actuators, to control flexible structure vibration and motion. A new type of intelligent structure has been introduced recently for precision spacecraft attitude control. It utilizes lead zirconate titanate (PZT) piezoelectric actuators bonded to solar arrays (SAs), and bends SAs to use inertial coupling for small-amplitude, highprecision attitude control and active damping. Integrated physical and control system design studies have been performed to investigate performance capabilities and to generate design insights for this new class of attitude control system. Both distributed- and lumped-parameter models have been developed for these design studies. While PZTs can operate at high frequency, relying on active damping alone to manage all vibration requires high-performance control hardware. In this article we investigate the potential value of introducing tailored distributed viscoelastic materials within SAs as a strategy to manage higher-frequency vibration passively, reducing spillover and complementing active control. A case study based on a pseudo-rigid body dynamic model (PRBDM) and linear viscoelasticity is presented. The tradeoffs between control system complexity, passive damping behavior, and overall dynamic performance are quantified. 

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2. 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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3. 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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4. Multifunctional Structures for Attitude Control

   Vedant, James T ( September 2019 , Proceedings of the ASME Conference on Smart Materials Adaptive Structures and Intelligent Systems)

   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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5. 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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