Satellite Docking Simulator with Generic Contact Dynamics Capabilities

Satellite docking systems are critical for the servicing of satellites. Satellite servicing has relatively recently become a realistic space operation/mission. The paper addresses a project to develop a Simulink-based Satellite Docking Simulator (SDS) with generic Contact Dynamics (CD) capabilities. The emphasis on docking is not arbitrary, but is intended to meet immediate practical demands for the development of satellite docking systems and operations at MD Robotics. The docking phase is the most critical and complex phase of the entire servicing sequence, and without docking there is no servicing. Docking mechanisms are often quite complex, especially when built to dock with a satellite which has not been built with potential docking and servicing in mind, that is, without special docking/capturing interfaces. 1 Motivation and Background Satellite docking (and capture) systems are critical for the servicing of satellites. Satellite servicing has relatively recently become a realistic and promising space operation/mission. Satellite servicing includes several of the following operations: Rendezvous Docking (capturing) Inspection Towing (transporting) Refuelling Refurbishing (replacement of faulty or “used-up” modules/boxes) Un-docking (releasing) Because spacecraft servicing has been, until recently non-feasible or non-economical, spacecraft servicing technology has been neglected. Accordingly, spacecraft designs have featured self-contained systems without consideration for operational servicing. Consistent with this view, most spacecraft were designed and built without docking interfaces. If, through some mishap, a spacecraft was rendered non-operational, it was simply considered expended. Some very expensive and valuable satellites have been left stranded in the wrong orbit. Others are running out of fuel, although otherwise able to continue to function. The servicing of such satellites is more realistically called salvaging. A flurry of feasibility studies are being carried out on the feasibility of such salvaging operations (which, in fact had led to this project). The task of the designer of the docking system for a salvaging task is very tough. He/she has to work with whatever it is up there, and this excludes any special features which might have made his/her task easier. The docking simulator is almost a must tool for the docking system designer. As satellite servicing becomes accepted and a design requirement, many of the future satellites will be equipped with appropriate docking interfaces. The designer of docking systems will be faced with a slightly different challenges: reliable, cost-effective, docking (and re-supply) systems. Thus, the scope of the designers of docking systems increases from one of a kind, ad-hoc type of interfaces intended for salvaging operations, to docking systems for satellites and “caretaker” spacecraft which are meant for servicing and are produced in larger numbers. As in any space system (for which full and representative ground hardware testbeds are very expensive and often impossible to develop), simulations are mandatory for the development of systems and operations for satellite servicing. Simulations are also instrumental in concept studies during proposals and early development stages. Finally, simulations are useful during the operational phase of satellite servicing: improving the operational procedures, training ground operators, command and control, etc. Hence the need for a Satellite Servicing Simulator which will support a project throughout its lifecycle. The paper addresses a project to develop a Simulink-based Satellite Docking Simulator (SDS) with generic Contact Dynamics (CD) capabilities. The emphasis on docking is not arbitrary, but is intended to meet immediate practical demands for the development of complex docking systems and operations at MD Robotics. The docking phase is the most critical and complex phase of the entire servicing sequence, and without docking there is no servicing. Docking mechanisms are often quite complex, especially when they must interact with an interface which is not purpose-built for docking. The design of the docking system and the success of the docking operation itself is affected, among other things, by: The contact interface, which often has a complex 3D geometrical shape, The capture mechanism, which is often quite involved, Friction and stiction at the contacting surfaces The compliance (stiffness) and damping, in all axes, The positional (translation and rotation) misalignments and the relative velocities, in all axes The inertial properties of the docking satellites (including its distribution), The overall docking system includes complex driving mechanisms and control systems The docking operation being teleoperated from the ground, or fully autonomous 2 Context and Objectives of the Project To support industries in key technology areas, the Canadian Space Agency has a Strategic Technology Development Programme (STDP). The STDP covers three areas: Innovative Technologies and Concepts, Technologies for Future Canadian Space Missions and Technologies for the Enhancement of Industrial Competitiveness. The SDS project falls under the third category since it is the improvement and the packaging of contact dynamics tools developed by MD Robotics with some support of the Canadian Space Agency over the years. The SDS project is 15 months long. With completion at the end of year 2001. The work is being carried out mainly by MD Robotics. Anadyn Consultants from Victoria, BC, is a subcontractor for a small portion of the work. CSA is actively involved technically in the project, through reviews and through useful suggestions from their experts in Symofros (see below, Section 5), software and use of Simulink. The main objective of the project is to develop a Satellite Docking Simulator meeting the following toplevel requirements: a) Open-ended and versatile simulation environment b) Extensible to real-time simulation implementation without major effort c) Easy to use (by design engineers not simulation experts) and convenient for iterative design d) Generic contact dynamics e) 3D visualization for modelling and evaluation of results f) Two satellites, each consisting of a main and a contact body. The connection between bodies has between 0 dofs (rigid) and 6 dofs (with stiffness and damping). This relatively simple multibody system is replaceable by more complex multibody system dynamics. A secondary (but of significant effort) objective is the re-write of the critical CDT software (see below, Section 4) from Fortran 77+ into C and at the same time improvement of its architecture to make it more portable, maintainable and upgradeable. A final objective of the project is to modify the CDT so that implicit integration methods can be accommodated as well. The aim here is to obtain speedier simulation for the case of extremely stiff contact bodies. 3 Typical Use of the SDS In a typical scenario, the spacecraft servicing/docking systems engineer (here called, say, Howard) would use the SDS as described below. From drawings, sketches, CAD files, documents or discussions with a potential customer, Howard defines the two docking satellites/objects. These would include inertial properties of the satellites and docking objects, structural properties of the docking objects, 3D pictures of the docking satellites/objects, and detailed CD models of the contacting areas (surfaces, parameters). Howard then runs simulations of docking. He can look at plots of simulation data time histories (eg: loads, preand post-docking positions/orientations, velocities), but he prefers to look at post-simulation (or during simulation) 3D visualization of the results. To assess the influence of various designs and parameters on the overall performance of the docking system, Howard can alter, interactively: The initial relative position and orientation between the two docking objects and the relative velocity (initial conditions) The inertial properties of the satellites and structural properties of the docking system The friction, damping and stiffness properties of the contacting areas Redefine the geometry of the docking interface, to study the influence it has on the success/failure of the docking operation. Changing the geometry of the docking interface is a more delicate operation. For such changes, Howard often needs the help of an engineer familiar with modelling contact objects. To review and discuss special issues, Howard can easily generate simulation movies and plots for his peers or customers. By interactively running simulations, viewing the results and modifying design parameters, Howard eventually narrows down the design of his docking mechanism to a near optimal solution. Howard also uses the tool to help him derive or check system requirements or evaluating customer’s imposed system requirements. The open-ended architecture of the Simulink environment allows Howard to include, work with and improve the control systems associated with the docking devices/mechanisms/spacecraft.