Validation Of Attitude/deformation Sensing Techniques For Space Flexible Manipulators
- Paper number
IAC-07-C2.3.01
- Author
Mrs. Chiara Toglia, University of Rome "La Sapienza", Italy
- Coauthor
Prof. Paolo Gasbarri, University of Rome "La Sapienza", Italy
- Coauthor
Dr. Giuliano Coppotelli, University of Rome "La Sapienza", Italy
- Coauthor
Dr. Giovanni B. Palmerini, University of Rome "La Sapienza", Italy
- Year
2007
- Abstract
The development of new materials, sensors and actuators are promising tools for the design of a smaller size new space manipulator family with respect to the ones currently used for space shuttle missions. The study of the motion of a free-flying manipulator is rather more complex than a similar system working on the Earth. In fact, as the base of the manipulator floats in space, any displacement of its links produces a motion of the base which consequently affects the motion of the end-effector. Because of this, control laws must be studied and applied to control the overall movement of these systems. Moreover the interaction between the rigid controlled motion and the elastic vibrations of the flexible parts of the manipulators, which in turn affects the choice and the synthesis of the control laws, must be considered. The dynamic of the space-structure is characterized by a low damping ratios, therefore artificial devices should be introduced in order to control the vibrations. To correctly define the vibration-control laws, the actual in-orbit elastic characteristics of the structure (that could significantly be different from the results obtained from ground tests) have to be estimated. In such a frame, this paper proposes two suitable approaches to sense and identify both rigid attitude motion and flexible dynamics of a manipulator. A first technique, based on the use of accelerometer sensors, will be applied in order to identify modal parameters during the motion of the flexible link. The modal parameters, i.e., natural frequencies, damping ratios, and mode shapes, are estimated considering only the responses of the structure to its natural loading environment. The developed technique is based on the well-known frequency domain Output-Only method called Frequency Domain Decomposition (FDD). Since the input level is not measured, the modal scale factors or generalized masses, and therefore the whole modal model, are not available: these participation factors could be derived using sensitivity-based methods. Then a technique for accurate attitude motion determination, based on the use of a Global Navigation Satellite Systems (GNSS) signals, will be used. Two antennas are placed at each flexible link ends, in order to obtain the observable phase of the GNSS signal with a double difference approach. This method ensures a very high accuracy to identify the attitude of the arms. Using more than two antennas, placed along one arm, the link can be considered as divided into virtual smaller segments. For each segment the corresponding antennas measure the local attitude. Due to flexibility, each segment is characterized by a different value of attitude angle, so the elastic behaviour of the arm can be reconstructed. The developed algorithms used for both the attitude estimation and the output-only modal analysis are validated by experimental activities carried out on an in-house test bed representing a two flexible arm manipulator. This mechanical system has a planar motion (to limit gravity effect) guaranteed by two stepper motors, controlled by means of a computer in order to follow assigned trajectories. The flexible arms are made by two tubular carbon fibre reinforced beams. The actuators are placed at the joints and control the planar motion of both the arms.
- Abstract document
- Manuscript document
IAC-07-C2.3.01.pdf (🔒 authorized access only).
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