Recent advances in micro-vibration isolation

On-orbit service (OOS) of spacecraft: A review of engineering developments

This paper comprehensively reviews the state-of-the-art development in formation control of small satellites. Satellite formation flying, distributed satellite systems, and fractionated satellite formation are discussed first. Various formation control architectures and methods of small satellites are then introduced, including the leader-following method, the behavior-based method, the virtual structure method, the cyclic pursuit method, the artificial potential function method, the algebraic graph method, and the noncontact force method. Coordinative control of multiple small satellites is also reviewed, covering coordinative control of satellite formation, coordinative attitude control of satellite formation, and coordinative coupled attitude and orbit control of satellite formation. The achievements and development trends of the formation control of small satellites are considered and analyzed.

A Survey on Formation Control of Small Satellites

The use of propellant to maintain the relative orientation of multiple spacecraft in a sparse aperture telescope such as NASA’s Terrestrial Planet Finder (TPF) poses several issues. These include fuel depletion, optical contamination, plume impingement, thermal emission, and vibration excitation. An alternative is to eliminate the need for propellant, except for orbit transfer, and replace it with electromagnetic control. Relative separation, relative attitude, and inertial rotation of the array can all be controlled by creating electromagnetic dipoles on each spacecraft, in concert with reaction wheels, and varying their strengths and orientations. Whereas this does not require the existence of any naturally occurring magnetic fields, such as the Earth’s, such fields can be exploited. Optimized designs are discussed for a generic system and a feasible design is shown to exist for a five-spacecraft, 75-m baseline TPF interferometer.

Electromagnetic Formation Flight for Multisatellite Arrays

*A charged particle moving relative to a magnetic field accelerates in a direction perpendicular to its velocity and the magnetic field due to the Lorentz force. Although a negligible force for spacecraft potentials due to natural charging, this effect can be seen, for example, in the unusual celestial mechanics of dust particles in the rings of Saturn and Jupiter. This study evaluates the use of the Lorentz force as a means of orbit control for finite bodies, including small spacecraft. Although the Hamiltonian is constant in a frame that rotates with the earth, the co-rotational electrical field (associated with the rotating geomagnetic field) can do work, increasing or decreasing the orbit’s semimajor axis. A number of intriguing applications are offered, including earth escape, drag compensation, new stable satellite formations, inclination control, nodal precession control, new sunsynchronous orbits, and non-Keplerian orbits for polar satellites. A candidate spacecraft design is offered based on lessons learned from decades of research in spacecraft charging and plasma interactions. The performance of this design is demonstrated by simulation. Closed-form equations for actuator sizing are provided.

Prospects and Challenges for Lorentz-Augmented Orbits

In this paper, we consider the equations of motion of a two-spacecraft formation flying array that uses electromagnets as relative position actuators. The relative positions of the spacecraft are controlled by the forces generated between the electromagnets on the two spacecraft, and the attitudes of the spacecraft are controlled using reaction wheels. The nonlinear equations of motion for this system are linearized about a nominal operating trajectory, taken to be a steady-state spin maneuver used for deep-space interferometric observation. The linearized equations are analyzed for stability and controllability. Although the open-loop system proves to be unstable, a controllability analysis indicates that the system is fully controllable with the given suite of actuators, and is therefore stabilizable. An optimal linear feedback controller is then designed, and the closed-loop dynamics are simulated. The simulations demonstrate that the closed-loop system is indeed stable, and that linear control is a very promising technique for electromagnetic formation flight systems, despite the nonlinearity of the dynamics.

Electromagnetic formation flight dynamics including reaction wheel gyroscopic stiffening effects

A Coupled Disturbance Analysis Method Using Dynamic Mass Measurement Techniques

This paper presents a modeling methodology used to predict the performance of a flexible structure, such as a space telescope, in the presence of an on-board vibrational disturbance source, such as a reaction wheel assembly (RWA). Both decoupled and coupled analysis methods are presented. The decoupled method relies on blocked RWA disturbances, measured with the RWA hardmounted to a rigid surface. The coupled method corrects the blocked RWA disturbance boundary conditions using 'force filters' which depend on estimates of the interface accelerances of the RWA and spacecraft. Both methods were validated on the Micro-Precision Interferometer testbed at the Jet Propulsion Laboratory. Experimental results are encouraging, indicating that both methods provide sufficient accuracy compared to measured values; however, the coupled method provides the best results when the gyroscopic nature of the spinning RWA is captured in the RWA accelerance model. Additionally, the RWA disturbance cross spectral density terms are found to be influential.

Methodology for modeling the mechanical interaction between a reaction wheel and a flexible structure

This paper presents the modeling, testing, and validation methodologies developed to predict the optical performance of the Space Interferometry Mission (SIM) at the Jet Propulsion Laboratory (JPL). The modeling methodology combines structural, optical, and control system design within a common state space framework and incorporates reaction wheel assembly (RWA) disturbances to evaluate the end-to-end performance of the system requirements. The validation methodology uses the Micro-Precision Interferometer (MPI) testbed, which is a ground-based, representative hardware model of SIM. In this study, the integrated model of the MPI testbed was used to calculate the transfer functions from RWA input to optical performance output. The model-predicted transfer functions were compared with the MPI testbed measurements, and the accuracy of the integrated model was quantified using a metric that was based on output power of the transfer functions. The RWA disturbances were then propagated through the modeled and measured transfer functions to predict the optical performance of the MPI testbed. This method is called the "decoupled disturbance analysis" and relies on the "blocked" RWA disturbances, measured with the RWA hardmounted to a rigid surface. These predictions were compared with the actual (measured) optical performance of MPI, measured with the RWA mounted to MPI, to evaluate the accuracy of the decoupled disturbance analysis method. The results show that this method is not an accurate representation of the coupled boundary conditions that occurs when the RWA is mounted to the flexible MPI structure. In order to correct for the blocked RWA disturbance boundary conditions, the "coupled disturbance analysis" method was developed. This method uses "force filters" that depend on estimates of the interface accelerances of the RWA and the MPI structure to effectively transform the blocked RWA disturbance measurements into their corresponding "coupled" disturbances (the disturbances that would occur at the coupled RWA-MPI interface). Compared to the decoupled method, the coupled method more accurately predicts the system's performance. Additionally, the RWA cross-spectral density terms were found to be influential in matching the performance predictions to the measured optical performance of MPI.

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Laila Mireille Elias

Papers

Electromagnetic Formation Flight for Multisatellite Arrays

Electromagnetic formation flight dynamics including reaction wheel gyroscopic stiffening effects

A Coupled Disturbance Analysis Method Using Dynamic Mass Measurement Techniques

Methodology for modeling the mechanical interaction between a reaction wheel and a flexible structure

Predicting the Optical Performance of the Space Interferometry Mission Using a Modeling, Testing, and Validation Methodology