Characterizing Limit Cycles in the Cassini Thrust Vector Control System

The Cassini spacecraft dynamics telemetry during long main engine burns has indicated the presence of stable limit cycles between 0.03 and 0.05 Hz frequencies. Even though these limit cycles have been observed in telemetry, they have not hindered the spacecraft performance to any degree of concern....

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Bibliographic Details
Published in:Journal of guidance, control, and dynamics control, and dynamics, 2013-09, Vol.36 (5), p.1490-1500
Main Authors: Rizvi, Farheen, Weitl, Raquel M
Format: Article
Language:English
Subjects:
Actuators
Aerospace engines
Burns
Cassini mission
Control systems design
Controllers
Copyright
Dynamical systems
Error analysis
Fourier transforms
Mathematical models
Moon
Nonlinear dynamics
Nonlinearity
Orbit insertion
Orbital maneuvers
Saturn
Space missions
Spacecraft
Spacecraft performance
Telemetry
Thrust vector control
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Summary:The Cassini spacecraft dynamics telemetry during long main engine burns has indicated the presence of stable limit cycles between 0.03 and 0.05 Hz frequencies. Even though these limit cycles have been observed in telemetry, they have not hindered the spacecraft performance to any degree of concern. The performance of the main engine thrust vector control system has been excellent throughout the prime and extended missions. However, it is important to understand these stable limit cycles from a controller design viewpoint. It is proposed that the observed limit cycles in the dynamics telemetry appear from a stable interaction between the unmodeled nonlinear elements and the linear main engine thrust vector control system. This paper focuses on one such nonlinearity that emerges from the gear backlash in the engine gimbal actuator system. The gear backlash nonlinearity is modeled via a describing function, and the interaction between the nonlinear element model and the overall dynamics of the spacecraft is studied. With this interaction, the stable limit cycle is reproduced analytically and in simulation. The results are compared with the actual telemetry from long burns like Saturn orbit insertion and main engine orbit trim maneuvers. The predicted stable limit cycle frequency and amplitude show good agreement within 6% error between the analytical, simulation, and telemetry results. This work claims that the stable limit cycles occur due to the stable interaction between the unmodeled nonlinear elements and linear main engine thrust vector control system.
ISSN:0731-5090
1533-3884
DOI:10.2514/1.57295