Motivation:
With the rapid growth in space-based communications a new generation of spacecraft has emerged with higher power capabilities, larger solar arrays, and larger deployable payloads. Spacecraft that provide land-mobile communications services may have payload reflectors up to 15 meters in diameter, and solar arrays up to 20 meters long. These flexible appendages, characterized by closely spaced and lightly damped vibration modes, may account for more than 99% of the total spacecraft rotational inertia. With dynamics dominated by these flexible structures, maintaining the required attitude pointing accuracy can be difficult, especially when periodic disturbances are present. For example, during orbit adjust maneuvers, if harmonics of the thruster pulsing frequency are near system flexible mode frequencies, the resulting vibrations can be large. In these situations it is necessary for the control system to provide adequate disturbance rejection, so that the presence of the disturbances does not interfere with payload operation. Other types of spacecraft, such as those used for remote sensing, can experience similar difficulties. These spacecraft may include imaging payloads with rotating or scanning instruments that have high accuracy pointing and jitter requirements. Again, control systems are needed that can suppress spacecraft vibrations, especially when instrument frequencies coincide with the system lightly damped flexible modes.
Research Goals:
Our research is directed towards the development of a method that can be applied to solving real spacecraft disturbance-rejection control problems. The requirements are indeed demanding -- the method must be applicable to systems with lightly damped and closely spaced flexible modes, where both the system dynamics and disturbance frequencies are unknown. The method must provide control when both the system dynamics and frequencies vary with time, and when the dynamics are multi-input, multi-output, non-collocated, and non-minimum phase. Operation must be possible in an environment where disturbances are always present and cannot be directly measured. The method must be able to recognize when disturbance cancellation is possible and when it is not possible. It must provide information that can be used to determine which disturbance frequency components to cancel and which to ignore. For space applications this need for selective disturbance cancellation is dictated by the necessity to limit the weight of all components, and the power they consume. Therefore, it is desirable to use small, light, and less capable actuators. In addition to theoretical treatments, all developed identification and control methods are thoroughly tested with simulations and experiments to confirm that they will really work in practice.
What is Clear-Box Adaptive Control?
In contrast to the results in the literature that address the disturbance-rejection problem strictly from a control system synthesis standpoint, this research addresses the problem from a system identification perspective. The research first focuses on system identification in the presence of unknown harmonic disturbance inputs, and then uses the identification results to solve the related disturbance-rejection control problem. Identification makes the control system intelligent because it brings out information that is normally hidden in other adaptive control methods. As far as the system identification problem is concerned, the goal is to extract as much information about the system and disturbances as possible, starting with as little information and making as few assumptions as possible. As far as the control problem is concerned, an important objective is to develop a method that can intelligently select a subset of the identified frequencies for cancellation depending on the disturbance environment, system dynamics, required performance, and available control resource. In short, identification makes this control approach a "clear-box" one by drawing out hidden information that is normally unused in a typical black-box adaptive control strategy.
To maximize the utility of the methods and the likelihood of practical implementation, no initial knowledge of the system dynamics is assumed. In addition, the disturbance frequencies are assumed to be unknown, as well as their waveforms and where they enter the system. No direct measurements of the disturbances or disturbance-correlated reference signals are assumed to be available. The only information available for identification are measurements of the control inputs, and the disturbance-corrupted system responses. We only make assumptions on the upper bounds on the effective order of the system and the number of disturbance frequencies, and sufficient amount of (disturbance-corrupted) input-output data is available from which all necessary information is extracted.
Recent Highlights:
Please click on the following item for additional details:
Goodzeit, N.E. and Phan, M.Q., "A Clear-Box Adaptive System for Flexible Spacecraft Identification and Disturbance Rejection," Journal of Vibration and Control, in press.
Goodzeit, N.E. and Phan, M.Q., "System and Disturbance Identification for Feedforward-Feedback Control Applications," Journal of Guidance, Control, and Dynamics, accepted for publication. An earlier version of this paper appeared as "System and Periodic Disturbance Identification for Feedforward-Feedback Control of Flexible Spacecraft," Proceedings of the 35th AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, January 1997.
Goodzeit, N.E. and Phan, M.Q., "Exact System Identification in the Presence of Completely Unknown Periodic Disturbances," Journal of Guidance, Control, and Dynamics, accepted for publication. Also, Department of Mechanical and Aerospace Engineering Technical Report No. 2096, Princeton University, 1997.
Phan, M.Q., Goodzeit, N.E., and Juang, J.N., "Identification of Input-Output Model and Periodic Disturbance From Disturbance-Corrupted Data," Journal of Vibration and Acoustics, submitted. A more detailed version of this paper appeared as "Identification of Systems and Periodic Disturbances," Proceedings of the 1997 ASME Design Engineering Technical Conferences (Paper DETC97/VIB-4256), Sacramento, CA, September 1997.
Goodzeit, N.E., Phan, M.Q., and Frueh, J.A., "Experiments in Identification and Adaptive Disturbance Rejection with a Flexible Lightly Damped Structure," (to appear).
For additional references on this topic, please refer to List of Publications.