Adaptive Deflection Limiting Control for Slewing Flexible Space Structures

Paper number

IAC-05-C2.7.02

Author

Dr. Hirohisa Kojima, Tokyo Institute of Technology, Japan

Year

2005

Abstract

Space structures are required to be as light as possible to reduce launch cost, while to be large to contain as many mission devices as possible. It is almost impossible to ignore the flexibility of space structures during maneuvers because of light characteristics, and interaction between the attitude motion and flexible motion. To suppress vibrations of flexible space structures during maneuvering, many control methods have been proposed. Input shaping is one such control method, which generates to a command profile by convolving a sequence of impulse with any commands. There are various versions of the input shaping control technique, such as the Zero Vibration (ZV), the Zero Vibration and Derivative (ZVD), and the Specified-Insensitivity (SI) approach. These input shaping techniques can suppress residual vibrations as long as the system parameters are well known, or change of them is limited within prediction. However, if a large range of unknown or varying frequencies is included in systems, another approach is needed to make the control methods more robust. Tzes and Khorrami et. al. have proposed on-line adaptive schemes to update the input shaper parameters. Bodson has used a recursive least-squares technique to tune the input shaper parameters. Magee and Book have proposed a method in which the input shaper is modified as a function of the system configuration. Although these adaptive input shapers can eliminate or suppress residual vibrations in case of parameter uncertainties, they do not consider transient load on the flexible structure during maneuvers. If the resulting deflection of beams is greater than acceptable limits, it may damage flexible structures. In order to overcome this problem, Deflection Limiting(DL) Input Shapers have been proposed. Original Deflection Limiting Shapers, however, do not have functions to automatically tune the system parameters during maneuvers. Therefore, adaptive methods need to be added to Deflection Limit Shapers.
This paper proposes an adaptive deflection limiting input shaping control for slew maneuvering of a flexible space structure. Contrary to previous work that used a measurement of the beam’s tip acceleration to adjust the input shaper parameters and to eliminate the first modal vibration, here the bending moment at the root of the beam is measured and used to adjust the parameters. This measurement may be more easily obtained than the tip acceleration on a space structure. In addition, measuring and limiting the bending moment at the root is important to avoid damaging the flexible structure. The critical frequency, which is the first modal frequency, is estimated from the second modal frequency that is determined by the Empirical Transfer Function Estimate (ETFE) technique. Then, the command timing and amplitude of the second and third commands of input shaper are immediately tuned in accordance with the identified frequencies and measured magnitude of vibrations. Even if the tuned timing and amplitude of the second and third command inputs are not appropriate enough to suppress the bending-moment within the limitation, the rest command inputs can be tuned appropriately in accordance with the measured outputs during the first half of maneuver so that the bending-moment is suppressed under the limitation.
Experimental results show that the adaptive deflection limiting input shaping control can successfully suppress the bending-moment at the root of the beam during maneuver and eliminate the residual vibrations.

Abstract document

IAC-05-C2.7.02.pdf

Manuscript document

IAC-05-C2.7.02.pdf (🔒 authorized access only).

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