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Grasping of a tumbling target satellite (DEOS/TECSAS)

The following text refers to the TECSAS project. In this section some preliminary methodology considerations are described.

We consider three modes of operation for the grasping of a tumbling satellite: tele-operated, autonomous and semi-autonomous. The tumbling motion of the target satellite is justified by a malfunctioning of its control system, for which a small constant rotation and nutation may result. Due to energy dissipation, a general tumbling motion will soon decay to a rotation about the major principal axes of inertia (flat spin), therefore a particular case of some importance should be that of pure rotation. The dynamic coupling between the chaser spacecraft and the manipulator is very pronounced, due to their mass ratio.

Following is a more detailed description of the planned operational strategies.

Autonomous mode

The autonomous grasping strategy involves the following tasks:

• Identification of the target motion parameters
• Planning of the grasping and stabilization sequence
• Execution of the grasping sequence with the aid of visual servo tracking
• Execution of the stabilization sequence

This mode strongly relies on visual data processing capabilities. The extent of the development of these capabilities depends on the amount of knowledge which is initially assumed about the target. This is either

• fully unknown
• the geometry is known, but not its motion parameters

Tele-operation mode

Due to the presence of the operator in the control loop, no processing of visual sensory data on the target is assumed necessary.

This operational mode includes:

• A real-time tele-operation controller of a free-floating robot accounting for singularity avoidance, collision avoidance, joint limits, joint velocity limits, base body (spacecraft) disturbance minimization
• An algorithm to derive the best initial configuration for grasping

The control algorithm is based on the inverse kinematics of a free-flying robot. The necessity to analyze singularities and collision avoidance for the particularly severe robot/spacecraft dynamic coupling is also to be recognized. A specification for the grasping task can be given, within which the above mentioned motion constraints are satisfied.

Semi-autonomous mode

In this operational mode visual sensory data processing aids the operator to select the optimal initial and/or grasping robot configurations. The autonomy also includes a supervised tracking of a planned grasping path and allows the operator a limited command of the end-effector to correct for small deviations and to accomplish the grasp completely.

In the case of a ‘fully unknown target’, some degree of semi-autonomy can be introduced if the operator is allowed to select features from the same target. Such task might arise from grasping of debris, including stranded satellites, etcetera.