Design and modeling of a space docking mechanism for cooperative on-orbit servicing

This dissertation addresses the design procedure of a docking mechanism for space applications, in particular, on-orbit servicing of cooperative satellites. The mechanism was conceived to comply with the technical specifications of the STRONG mission. The objective of this mission is to deploy satellite platforms using a space tug with electric propulsion. This mission is part of the SAPERE project, which focuses on space exploration and access to space. A docking mechanism is used for recovering the misalignments left by the guidance, navigation, and control system of the servicer satellite when approaching the customer spacecraft. However, most importantly, the mechanism must safely dissipate the energy associated with the relative velocities between the spacecraft upon contact. Five concepts were considered as possible candidates for the docking mechanism: a system based on the Stewart-Gough platform with a position controller, a Stewart-Gough platform with impedance control, a central passive mechanism (probe-drogue), a central active mechanism, and a mechanism equipped with articulated arms. Several trade-off criteria were defined and applied to the concepts. The result of this trade study was the selection of the central passive mechanism as the most balanced solution. This mechanism is composed of a probe and a conical frustum equipped with a socket to capture the probe. It was further developed and tested using mathematical models of the docking maneuver. The results of the simulations showed that the passiveness of the system prevented the docking maneuver from being fully accomplished. Consequently, a second design iteration was performed. In this new iteration, the degrees of freedom of the mechanism were increased by adding two controlled linear axes in series with the degrees of freedom of the preliminary design. The electromechanical actuators and transmissions of this mechanism were selected following the guidelines of The ECSS standards. Also, in this case, numerical models were used to assess the functioning of the docking system. The results produced by these models demonstrated the suitability of the mechanism for completing the docking operation defined by the mission’s specifications. Furthermore, the results also showed the architecture and functioning of the mechanism to be possibly suitable for other cooperative docking operations between small and mid-sized satellites. In addition, the definition of the mechanical details as well as the control architecture led to the complete design of an engineering prototype for laboratory tests. In this regard, the laboratory tests were defined with the scope of verifying the different operating modes of the docking mechanism. The test rig was designed to be equipped with a serial manipulator connected to the female part of the mechanism through a force and torque module. The objective will be to simulate the relative motion between the docking halves using different techniques to generate the trajectory of the manipulator.