Oscar S. Alvarez-Salazar

Reducing Spacecraft Jitter During Satellite Reorientation Maneuvers via Solar Array Dynamics

utility of a number of spacecraft. In this article a new design and control strategy is presented that is based on advanced solar arrays that exploit their inertial properties and a balance of passive and active dynamics to minimize jitter and settling time, thereby boosting spacecraft scientic utility. This development will require the advancement of some promising optimization-based methods for simultaneous physical and control system design to determine the best possible synergy between the control system and passive structural system dynamics. The solar array will be designed specically to be compliant using pseudo-rigid-body dynamic modeling theory. Shooting will be utilized to solve the optimal control problem posed in the co-design formulation.

Space Interferometry Mission System testbed-3: external metrology inversion

The Space Interferometry Missions System testbed-3 has recently integrated its precision support structure and spacecraft bus, or backpack, on a pseudo free-free 0.5 Hz passive isolation system. The precision support structure holds a 3-baseline stellar interferometer instrument. The architecture of the instrument is based on the current SIM flight system design, and its main purpose is to demonstrate nanometer class fringe stabilization using the path length feed forward technique. This paper briefly describes the nanometer-class metrology system used in this testbed to estimate the length and orientation of the science baseline vector, which cannot be measured directly. The focus is on the mathematical inversion problem that results and its solution.

Space Interferometry Mission System testbed-3: architecture

The Space Interferometry Missions System testbed-3 has recently integrated its precision support structure and spacecraft backpack (bus) on a pseudo free-free 0.5 Hz passive isolation system. The precision support structure holds a 3-baseline stellar interferometer instrument. The architecture of the instrument is based on the current flight system design, and its main purpose is to demonstrate nanometer class fringe stabilization using the path length feed forward technique. This work describes the overall instrument architecture, brief theory of operation, and preliminary measurements.

Co-Design of Strain-Actuated Solar Arrays for Spacecraft Precision Pointing and Jitter Reduction

This work presents a novel spacecraft attitude control architecture using strain-actuated solar arrays that does not require the use of conventional attitude control hardware. A strain-actuated solar array enables attitude slewing maneuvers and precision pointing (image acquisition) stares, while simultaneously suppressing structural vibrations. Distributed piezoelectric actuators help achieve higher precision, higher bandwidth, and quieter operation than reaction wheels. To understand the design tradeoffs for this architecture, a framework for the integrated design of distributed structural geometry and distributed control is presented. The physical properties of the array are modeled and designed with respect to a piecewise linear distributed thickness profile. The distributed control is a voltage profile across the array modeled as a spatially continuous function. The dynamics of the system are modeled using a coupled ordinary differential equation–partial differential equation system using extended generalizations for hybrid coordinate systems. The combined physical and control system design, or co-design problem is investigated to understand the optimal performance of the system. Single-axis slew maneuvers of 7.2 milliradians or 1485 arcsec are achieved for a representative spacecraft model without increasing array mass or reducing array planform area. From additional tradeoff studies, a design criteria is revealed for the array structure and control strategy based on the optimal design tradeoff between large array inertia and fast structural dynamics. Moreover, the fundamental limits on strain-actuated solar arrays slew angle magnitude are demonstrated using an intuitive pseudorigid body dynamic model.

Co-design of strain-actuated solar arrays for precision pointing and jitter reduction

Many important spacecraft operations require precision pointing such as space astronomy and high-rate communications. Traditionally, reaction wheels have been used for this purpose but they have been considered unreliable for many missions. This work presents the use strain-actuated solar arrays (SASA) for precision pointing and jitter reduction. Piezoelectric actuators can achieve higher precision and bandwidth than reaction wheels, and they can also provide quiet operation for sensitive instruments. The representation of the array dynamics in the studies presented here is based on Euler-Bernoulli beam theory for high-fidelity simulations. This work also presents a methodology for the combined design of distributed structural geometry for the arrays and distributed control system design. The array geometry design allows for a distributed thickness profile, and the control design determines the distributed moment on the array. Fundamental limits on slew magnitude are found using pseudo-rigid body dynamic model (PRBDM) theory. A parametric study based on a representative spacecraft model demonstrates the validity of the proposed approach and illustrates optimal design trends.

Dim-star Tracking for Stellar Interferometry

This paper proposes a new dim-star angle tracking architecture applicable to multibaseline stellar interferometry missions like Space Interferometry Mission (SIM). The proposed architecture is being implemented on SIMs system test bed 3 (STB3) - a 3-baseline stellar interferometer test bed with similar instrument architecture to that of SIM. Preliminary implementation results, analysis and traceability to the flight system are discussed. The proposed dim-star tracking architecture consists of feeding angle tracking information from one of the guide interferometers back-end cameras to the fast steering mirror on the science interferometer. The information would allow the science interferometer to track its own dim-star, while not having the needed photon-rates to use its own camera as a sensor. One of SIMs requirements for STB3 is to show 20 dB of rejection of induced motion at any frequency between 0.1 and 1 Hz. This level of performance has been reached and is discussed in the results at the end of this paper

Combined feed-forward and feedback control for dim star fringe tracking on the sim test-bed 3

This paper will describe the STB3 setup, the pathlength control architecture and the data collected with the system and how they relate to SIM.