Dynamic interaction between satellites during final phase of the rendezvous maneuver

Title: Dynamic interaction between satellites during final phase of the rendezvous maneuver
Consignee: ASTRIUM Space Transportation (Bremen)
Project Manager: K. Seweryn
Participants: T. Rybus (SRC)

In recent years, more and more seriously the possibility of carrying out unmanned servicing missions that allow repairing located in orbit satellites is considered. Despite the increased reliability of equipment and despite the fact that the commercial satellites market is nowadays dominated by large specialized firms, which widely use standardized components, failures of launched satellites are still happening. Occurrences of serious malfunction to the system during operation (such as the attitude control system malfunction) result in a significant shortening of the lifespan of the satellite. Sometimes a malfunction occurs immediately after launching of an object into orbit (such as a failure to upon up the solar panels), or when satellite launch vehicle leaves it on the wrong (e.g., too low) orbit. Then the satellite cannot commence its normal operation. Therefore, the satellite operators try to assess whether servicing (repair) of malfunctioned satellites is economically viable, and at the same time the space industry and research institutes analyze the technical capacity to carry out such missions.

A key element of the servicing mission is to capture malfunctioned satellites by the servicing satellite, which allows carrying out the planned repair (e.g., assuming the control over the spatial orientation of a faulty satellite). In the analyzed scenarios of satellites’ servicing it is considered to employ for this purpose a manipulator, i.e., a mechanical arm. This type of solution enables capturing malfunctioning satellites, even in cases where there is no control over the satellite and its orientation is not constant (following the failure of the satellite attitude control system, it may begin to rotate around its center of gravity at high velocity), and then enables precisely position captured satellites in order to connect them to a servicing satellite (docking). The manipulator can also be used to repair a malfunctioned satellite (e.g., to replace components or to unlock a jammed unfolding mechanism of solar panels).

Carried out at the Space Research Centre PAS implementation project "Dynamic interaction between satellites during final phase of the rendezvous maneuver" is associated with work on the technology demonstrator of satellite servicing. The aim of the project was to develop simulation software that allows the comprehensive analysis of the dynamics of satellites equipped with a manipulator. Simulations conducted with the use of this software provide data which are required in designing the satellite attitude control system.

Issues associated with control of satellite manipulators and satellites equipped with such manipulators are extremely complex due to interactions between different elements of the system. Manipulator movements cause reaction forces and torques acting on its base, which causes changes in the orientation of the satellite, which in the considered scenario are compensated by the satellite attitude control system. The capture of the malfunctioned satellite may require performing very fast movements of the manipulator (if the satellite in question rotates at high velocity around its center of gravity), and afterwards a need to move a manipulator, whose end includes a large mass (the captured satellite), is required. Due to the satellite-manipulator interactions, only performing full simulations of the considered maneuvers allows estimating reaction forces and torques induced by movements of the manipulator, which is why there a demand emerged to create such simulation software.

Developed as a part of the project software module is based on trajectory planning and computation of driving torques for manipulator’s joints (module is based on the results presented in the doctoral dissertation Ph.D. eng Karol Seweryn "Dynamika manewru zbliżenia satelitów i ich połączenia za pomocą manipulatora o więzach nieholonomicznych ") and on the model satellite-manipulator system created using Matlab Simulink. This approach allows taking into account various additional forces (e.g., friction in the manipulator’s joints) while solving forward dynamics (i.e., simulating system’s behavior). Moreover, it is possible to easily verify the carried out simulation as the system performance simulation executed in Simulink is not based on equations introduced into the trajectory planning module, but is completely independent.

The created software allows for the analysis of the different stages of orbital rendezvous maneuver:
• Unfolding of the manipulator.
• Performing complex sequences of manipulator’s movements (e.g., in order to observe a malfunctioned satellite by a camera positioned at the manipulator’s end effector).
• Catching a malfunctioned satellite, which can rotate around a fixed axis or tumble (this is the case, e.g., in the event of failure of attitude control system).
• Navigating the manipulator after the capture of a malfunctioned satellite and the positioning of this satellite to perform docking with the servicing satellite.

The software allows carrying out simulations for various parameters of the system (e.g., various angular velocities of malfunctioned satellite, different mutual position of both satellites). Analysis of the various phases of the rendezvous maneuver allows the selection of proper orbital parameters of the orientation of the satellite control system.

The software made it possible to find answers to many questions relevant to the design of the servicing satellite attitude control system:
• How much do the movements of the manipulator during each phase of the orbital rendezvous maneuver disturb the orientation of the servicing satellite?
• How should the trajectory be planned so that these disturbances were minimal?
• Is the standard attitude control system able to maintain a constant orientation of the satellite during manipulator’s motion?
• To what extent does the feed-forward control (enabling to predict interference from manipulator before the commencement of the rendezvous maneuver and taking them into account in design of the control system) improves the attitude control system of the satellite?

Dynamic interaction between satellites during final phase of the rendezvous maneuver
Dynamic interaction between satellites during final phase of the rendezvous maneuver