Lab Activities to Simulate in-Orbit Proximity Maneuvres

2013

Ground tests play a key role in verifying many critical aspects of a spacecraft mission, even within the current trend of high fidelity simulation tools. In fact numerical runs of complex manoeuvres still need some realistic indications of the expected system's behaviour to better define model assumptions. To this aim different test-beds have been recently built at the Guidance and Navigation Laboratory of Sapienza-Università di Roma. They share, for reasons of costs and complexity, the limitation of a bi-dimensional representation of the space gravity-less environment. Nevertheless, they allowed to better investigate complex dynamics' phenomena typical to space activities, and a number of interesting indications have been provided by these experimental tests. Among the different realizations there are robotic arms equipped with different end-effectors, a self-standing floating platform representing a spacecraft bus (also able to perform rendezvous or reorientation manoeuvres) and a scaled simulator for image-based navigated and controlled formation-flying. The paper is intended to present the main characteristics of these test-beds and to discuss their performance and the related research activities.

Ground simulation of spacecraft motion simulating all six degrees of freedom is a challenging problem due to several features of the natural dynamics in space that are difficult to reproduce on ground. Unlike terrestrial (aerial, land or underwater) vehicles, space vehicles have an overwhelmingly large percentage of their total energy in their translational motion. Dynamical coupling between the translational and rotational degrees of freedom can significantly affect the attitude motion of spacecraft. The attitude motion is particularly important for a spacecraft tasked to autonomously rendezvous and capture or dock with a target object in space. Here we present a ground simulator design for 6 DOF simulation of spacecraft engaged in autonomous rendezvous and proximity operation(ARPO) with an unaided target space object. These operations are very risky and difficult to carry out in space, since the target's motion is not well known in advance. Ground simulation using 6 DOF motion simulation capabilities can help reduce the risk of actual on-orbit ARPO missions. The novel design "Autonomous Rendezvous and Proximity Operation ground Simulator (ARPOS)" presented here mimics all the six DOFs of rigid spacecraft with high fidelity. ARPOS has the advantage of linear and spherical air bearings to reproduce the near frictionless environment of an actual spacecraft in space. Nomenclature b = position vector of the pursuer spacecraft in a geocentric inertial frame R = rotation matrix representing the attitude of the pursuer ν = translational (orbital) velocity of the pursuer in its body coordinate frame Ω = rotational (orbital) velocity of the pursuer in its body coordinate frame * 0 → superscript ( b 0 , R 0 , ν 0 , Ω 0 , b 0 g ) represents target object or target spacecraft a = (b 0 -b) relative inertial position vector of the target from the pursuer x = (R T a) relative position vector expressed in the pursuer's body frame Q = (R T R 0 ) attitude of the target resolved in the pursuer's body frame v = ν 0 -Q T (ν+Ω x x ) relative translational velocity of the target with respect to the pursuer in the target's body frame ω = (Ω 0 -Q T Ω) relative angular velocity of the target with respect to the pursuer in the target's body frame ω g = angular velocity of the spacecraft model mounted on the ground simulator with respect to the simulator base Ω g = angular velocity of the spacecraft model mounted on the ground simulator with respect to ground reference frame m b = mass of spacecraft model stage of the simulator J b = moment of inertia of spacecraft model stage of the simulator Π g = angular momentum of the spacecraft model stage in the body frame b g = position vectors of the centers of support of the two supported bodies in their corresponding simulator supports in a lab-fixed inertial frame x g = relative position between pursuer and target ARPOS expressed in inertial frame 2

AIAA Guidance, Navigation and Control Conference and Exhibit, 2008

Synchronized formation rotations are a common maneuver for planned precision formations. In such a rotation, attitudes remain synchronized with relative positions, as if the spacecraft were embedded in a virtual rigid body. Further, since synchronized rotations are needed for science data collection, this maneuver requires the highest precision control of formation positions and attitudes. A recently completed, major technology milestone for the Terrestrial Planet Finder Interferometer is the high-fidelity, ground demonstration of precision synchronized formation rotations. These demonstrations were performed in the Formation Control Testbed (FCT), which is a flight-like, multi-robot formation testbed. The FCT is briefly introduced, and then the synchronized rotation demonstration results are presented. An initial error budget consisting of formation simulations is used to show the connection between ground performance and TPF-I flight performance.

AIAA Guidance, Navigation, and Control (GNC) Conference, 2013

This paper presents a novel six degrees of freedom ground-based experimental testbed, designed for testing new guidance, navigation, and control algorithms for nano-satellites. The development of innovative guidance, navigation and control methodologies is a necessary step in the advance of autonomous spacecraft. The testbed allows for testing these algorithms in a one-g laboratory environment, increasing system reliability while reducing development costs. The system stands out among the existing experimental platforms because all degrees of freedom of motion are dynamically reproduced. The hardware and software components of the testbed are detailed in the paper, as well as the motion tracking system used to perform its navigation. A Lyapunov-based strategy for closed loop control is used in hardware-in-the loop experiments to successfully demonstrate the system's capabilities.

Journal of Guidance, Control, and Dynamics

AIAA Modeling and Simulation Technologies Conference and Exhibit, 2008

Ground testing of multi-spacecraft proximity operations with hardware in-the-loop is currently an expensive and challenging process. We present our approach to this problem, applicable to proximity operations of small spacecraft. We are developing a novel autonomous mobile robotic system to emulate full 6 degree of freedom relative motion at high fidelity. An omni-directional robotic base provides unlimited 3-DOF planar motion with moderate precision, while a micron-class hexapod on top provides high precision, limited 6-DOF motion. This multi-vehicle robotic system is designed to accommodate multiple untethered vehicles simultaneously, allowing for the real-time emulation of relative motion for a large variety of multi-spacecraft proximity operations. Compared with other facilities with similar goals, this approach will allow greater freedom of motion at a target operating cost much lower than existing facilities. We believe these capabilities will be invaluable to the growing number of small and micro satellite programs. Nomenclature ASOC = Active Split Offset Castor DOF = Degree of Freedom PID = Proportional-Integral-Derivative RMV = Relative Motion Vehicle

2009

ABSTRACT The importance of simulators as testbeds for proposed space robotic systems is unquestionable in our days. In this paper, the NTUA approach on both software and hardware space robots simulators, is presented. The software simulator is fully parameterized in order to be capable to simulate any system that consists of a base and a number of serial appendages. The hardware simulator emulates 2D motion in zero gravity, and consists of a two-manipulator space robot moving on top of a granite table.

The increasing application of robots in hazardous space environments and in support of extra vehicular activities has drawn attention to problems related with robust and safe application of free flyer space robots. As a key point in addressing these topics CISAS has since 1996 worked on the development and construction of free floating robots to be used in laboratory tests under micro gravity conditions. The free floater robot architecture is composed by two main parts: the floating base, that is free to move and rotate on a plane, and an anthropomorphic manipulator with three degrees of freedom, connected to the base. The two dimensional micro gravity condition is achieved by means of low pressure air bearings, that are located under the robot and allow it to float over a low roughness, high planarity granite table. Friction reduction is so effective that residual disturbing accelerations in the order of 10 -5 g have been measured. Control of the capture operation is possible through the elaboration of object location and configuration from a visual system (CCD camera, frame grabber and elaboration software) and manipulator sensors (encoders). An extensive campaign of tests has been conducted in order to prove the ability of the robot to grab with the manipulator a free floating object passing over the plane starting from a random position. To improve the already satisfying results of the first prototypes of the free floater robot, a set of actions has been promoted. Last technological developments are related to increase overall efficiency through components improvement, speed up elaboration time and reduce overall weight thus increasing zero gravity autonomy. The ongoing optimisation activity is related to the following new aspects : CPU calculation through a PC 104 plus axes control board, resulting in a position measurement and estimator with higher frequency rate, multi-tasking real time software on board for faster data acquisition, elaboration and command processing. Also, one of the main drawbacks of the former configuration, concerning the use of Led on both robot arm and target, will be mitigated by means of last generation frame grabber in conjunction with higher definition cameras. In the future a second camera will be installed on board in parallel with an on board PC 104 frame grabber in order to address vision problems related to on board tracking of moving targets, estimation of tumbling trajectories, docking and rendezvous optimisation in the robot fixed frame. CISAS is also working on extension of the 2D robot technologies to a three dimensional testbed dedicated to parabolic flight tests. This will allow to optimise base control strategies without thruster firings for a free flying 3D robot with comparable mass and inertia of base and arm.

2012

For human spaceflight missions rendezvous and docking (RvD) of two spacecrafts is state of the art today. For future satellite missions this close operation scenario has become more and more interesting during the last years. These comprise so called on-orbit servicing missions (OLEV, DEOS) as well as exploration missions (Mars Sample Return). One of the critical issues of such missions is to ensure a safe and reliable rendezvous and docking process. Since the RvD process is known to be the most risky part, these operations must be carefully analyzed, simulated and verified before the mission can be launched. Required by the new type of satellite missions DLR set up a completely new and more advanced RvD simulation facility in 2010. The new facility called EPOS 2.0 has full test and verification capabilities for on-orbit servicing missions as well as other RvD scenarios. The facility is based on two large industrial robots to deliver the 6-DOF motion in a representative maneuvering space for typical rendezvous and docking operations. The test bed allows simulation of the last critical phase (ranging from 25m to 0m) of the final approach process including the contact dynamic simulation of the docking process. For the last two years the facility has been continuously extended for different RvD applications which are described hereafter.

2001

2 112 6.20 The torque profile of joint 2 (experimental slow moving link 2 with the gravitational effect) 113 6.21 The orientation profile of link I (experimental fa st moving link 2 with the gravitational effect) 113 6.22 The torque profile of joint 1 (experimental fa st moving link 2 with the gravitational effect) 113 6.23 The orientation profile of link 1 (experimental fa st moving link 1 without the gravitational effect) 114 6.24 The angular velocity profile of link 1 (experimental fast moving link 1 without the gravitational effect) 115 6.25 The torque profile of joint 1 (experimental fa st moving link 1 without the gravitational effect) 115 6.26 The orientation profile of link 2 (experimental fa st moving link 1 without the gravitational effect)