Flexibility effects in controlled behaviour of space manipulators

Paper number

IAC-08.C2.3.1

Author

Ms. Chiara Toglia, University of Rome "La Sapienza", Italy

Coauthor

Prof. Paolo Gasbarri, Università di Roma "La Sapienza", Italy

Coauthor

Mr. Marco Sabatini, University of Rome "La Sapienza", Italy

Coauthor

Dr. Giovanni B. Palmerini, Universita' di Roma 'La Sapienza', Italy

Year

2008

Abstract

This paper aims to show the relevance of flexibility effects in the modelling of space manipulators dynamics, by investigating a set of possible realistic manoeuvres.

Correct modelling of space manipulators dynamics is needed in order to plan and safely execute an assigned manoeuvre such as the grasping of a satellite. Design, synthesis and implementation of the control laws should be exploited referring to a model that accurately reproduces the actual plant.

Space manipulators, generally represented as multi-body system, operate under different environmental conditions, including the dominant gravitational effect, its connected gravity gradient, and perturbing forces and torques such as the residual atmospheric drag if the considered mission is performed in a Low Earth Orbit. On account of different control laws options, different values of inertial forces can arise and consequently different amplitudes of the elastic displacements can be observed during the motion of the manipulator arms. 

Actually, the addition of the flexibility effects complicates the analytical model of a spacecraft and, while improving the accuracy of the model itself, it leads to approximation issues about the number of elastic degrees of freedom. Above all, flexibility causes a noticeable amount of computational work, which is very unwelcome in a near real-time operations perspective.

The choice of introducing the flexibility on the modelling of a space manipulator depends on the elastic properties and geometrical characteristics of the arms, and on the type of the manoeuvre the spacecraft must perform. In fact, during slow manoeuvres the inertial forces are small and the internal structural damping effects can be considered very relevant. In fact, during slow manoeuvres (quasi-steady) the inertial forces are small and the internal structural damping could reduce the amplitudes of oscillations in a reasonable time.
 
Unfortunately, there is also a large spectrum of operations for which a manipulator could undergo unsteady solicitations: grasping of a payload and docking of a spacecraft, lifting and moving an astronaut during extra vehicular activities are just some examples of impulsive or fast-varying manoeuvres.

Findings of this research will indicate the criteria for inclusion of flexibility as function of the manoeuvre. In particular, the increased complexity for guidance and control design required in such a case calls for the investigation of the possibility to split complex manoeuvres in a part accomplished neglecting flexibility and in a part including elastic displacements.

Abstract document

IAC-08.C2.3.1.pdf

Manuscript document

IAC-08.C2.3.1.pdf (🔒 authorized access only).

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