Segmentation of Three-dimensional Images with Parametric Active Surfaces and Topology Changes

In this paper, we introduce a novel parametric method for segmentation of three-dimensional images. We consider a piecewise constant version of the Mumford-Shah and the Chan-Vese functionals and perform a region-based segmentation of 3D image data. An evolution law is derived from energy minimization problems which push the surfaces to the boundaries of 3D objects in the image. We propose a parametric scheme which describes the evolution of parametric surfaces. An efficient finite element scheme is proposed for a numerical approximation of the evolution equations. Since standard parametric methods cannot handle topology changes automatically, an efficient method is presented to detect, identify and perform changes in the topology of the surfaces. One main focus of this paper are the algorithmic details to handle topology changes like splitting and merging of surfaces and change of the genus of a surface. Different artificial images are studied to demonstrate the ability to detect the different types of topology changes. Finally, the parametric method is applied to segmentation of medical 3D images

Segmentation and Restoration of Images on Surfaces by Parametric Active Contours with Topology Changes

In this article, a new method for segmentation and restoration of images on two-dimensional surfaces is given. Active contour models for image segmentation are extended to images on surfaces. The evolving curves on the surfaces are mathematically described using a parametric approach. For image restoration, a diffusion equation with Neumann boundary conditions is solved in a postprocessing step in the individual regions. Numerical schemes are presented which allow to efficiently compute segmentations and denoised versions of images on surfaces. Also topology changes of the evolving curves are detected and performed using a fast sub-routine. Finally, several experiments are presented where the developed methods are applied on different artificial and real images defined on different surfaces

A Flexible Image Processing Framework for Vision-based Navigation Using Monocular Image Sensors

On-Orbit Servicing (OOS) encompasses all operations related to servicing satellites and performing other work on-orbit, such as reduction of space debris. Servicing satellites includes repairs, refueling, attitude control and other tasks, which may be needed to put a failed satellite back into working condition. A servicing satellite requires accurate position and orientation (pose) information about the target spacecraft. A large quantity of different sensor families is available to accommodate this need. However, when it comes to minimizing mass, space and power required for a sensor system, mostly monocular imaging sensors perform very well. A disadvantage is- when comparing to LIDAR sensors- that costly computations are needed to process the data of the sensor. The method presented in this paper is addressing these problems by aiming to implement three different design principles; First: keep the computational burden as low as possible. Second: utilize different algorithms and choose among them, depending on the situation, to retrieve the most stable results. Third: Stay modular and flexible. The software is designed primarily for utilization in On-Orbit Servicing tasks, where- for example- a servicer spacecraft approaches an uncooperative client spacecraft, which can not aid in the process in any way as it is assumed to be completely passive. Image processing is used for navigating to the client spacecraft. In this specific scenario, it is vital to obtain accurate distance and bearing information until, in the last few meters, all six degrees of freedom are needed to be known. The smaller the distance between the spacecraft, the more accurate pose estimates are required. The algorithms used here are tested and optimized on a sophisticated Rendezvous and Docking Simulation facility (European Proximity Operations Simulator- EPOS 2.0) in its second-generation form located at the German Space Operations Center (GSOC) in Weßling, Germany. This particular simulation environment is real-time capable and provides an interface to test sensor system hardware in closed loop configuration. The results from these tests are summarized in the paper as well. Finally, an outlook on future work is given, with the intention of providing some long-term goals as the paper is presenting a snapshot of ongoing, by far not yet completed work. Moreover, it serves as an overview of additions which can improve the presented method further

Three-dimensional modeling and time-delay stability analysis for robotics docking simulation

Hardware-in-the-loop simulations of two interacting bodies are often accompanied by a time delay. The time delay, however small, may lead to instability in the hardware-in-the-loop system. The present work investigates the source of instability in a two spacecraft system model with a time-delayed contact force feedback. A generic compliance-device based contact force model is proposed with elastic, viscous, and Coulomb friction effects in three dimensions. A 3D nonlinear system model with time delay is simulated, and the effect of variations in contact force model parameters is studied. The system is then linearized about a nominal state to determine the stability regions in terms of parameters of the spring-dashpot contact force model by the pole placement method. Furthermore, the stability analysis is validated for the nonlinear system by energy observation for both the stable and unstable cases

Guidance, Navigation and Control for Autonomous Close-Range-Rendezvous

This article presents the Guidance, Navigation and Control system (Gnc-system) developed by GSOCs On Orbit Servicing (OOS)-group. It is used for research on autonomous rendezvous, optical sensors and operational concepts with the European Proximity Operations Simulator 2.0 (EPOS) facility and inside an End-to-End simulation. Its modular design intends to make it capable of deployment in space on a novel on-board computer with less effort. Major design decisions as well as plans for future development are introduced

Rendezvous Simulation for On-Orbit Servicing Missions Using Advanced Robotic Technology

Increasing complexity and costs of satellite missions promote the idea of extending the operational lifetime or improving functionalities/performance of a satellite in orbit instead of simply replacing it by a new one. Further, satellites in orbit can severely be affected by aging or degradation of their components and systems as well as by consumption of available resources. These problems may be solved by satellite on-orbit servicing (OOS) missions. One of the critical issues of such a mission is to ensure a safe and reliable Rendezvous and Docking (RvD) operation performed autonomously in space. Due to the high risk associated with an RvD operation, it must be carefully analyzed, simulated and verified in detail before the real space mission can be launched. This paper describes a ground-based hardware-in-the-loop RvD simulation facility. Designed and built on 2-decade experience of RvD experiment and testing, this unique, high-fidelity simulation facility is capable of physically simulating the final approach within 25-meter range and the docking/capture process of an on-orbital servicing mission. Additionally this paper presents first results of hardware in the loop simulations for a rendezvous process to a non-cooperative target

Hardware-in-the-Loop Rendezvous Simulation Involving an Autonomous Guidance, Navigation and Control System

The rendezvous process is a key technology in multi-spacecraft missions like on-orbit servicing missions. An active spacecraft (chaser) approaches a passive spacecraft (target) in its orbit by performing controlled orbit and attitude maneuvers. The paper presents an autonomous guidance, navigation and control system for rendezvous using a monocular camera as vision-based sensor for relative navigation. Image processing algorithms and navigation filters are employed to get accurate information about the relative position and attitude between the two spacecrafts. The rendezvous sensor and the entire GNC system is tested and verified at DLR's robotic-based test bed European Proximity Operations Simulator 2.0

Hardware-in-the-loop Rendezvous Simulation using a Vision Based Sensor

One of the critical issues of a satellite On-Orbit Servicing (OOS) mission is to ensure a safe and reliable Rendezvous and Docking (RvD) process. This most risky part of the mission must be carefully analyzed, simulated and verified before the mission can be launched. This paper focuses on the utilization of the new RvD simulation facility called EPOS 2.0 (European Proximity Operations Simulator) to establish a hardware-in-the-loop (HIL) simulation of a close-range rendezvous process. As navigation sensor a monocular camera is used to measure the relative position and orientation of a mock-up of a Geo-stationary target satellite. A new developed image processing algorithm tracks the outer edges of the satellite body under different illumination conditions. The complex software functionality for relative guidance, navigation and control (GNC) and for the satellite dynamics is developed under Matlab/Simulink environment and auto-coded with Real Time Workshop

Visual Navigation For On-Orbit Servicing Missions

Increasing complexity and costs of satellite missions promote the idea of looking for opportunities to extend the operational lifetime or to improve the performance of a satellite instead of simply replacing it by a new one. Satellites in orbit can severely be affected by ageing, limited fuel source, or degradation of their hardware components. Also the disposal of spacecraft after the end of lifetime will play a more and more important role in the future, especially, if the involved orbits are of strategic importance. Therefore, satellite on-orbit servicing (OOS) has increasingly caught the interests of both satellite developers and users. One of the critical issues of a satellite on-orbit servicing mission is to ensure a safe and reliable Rendezvous and Docking (RvD) process. DLR is developing new navigation algorithms using standard camera systems and advanced 3D sensor systems like PMD (Photonic Mixing Device). Furthermore DLR has built a new and more advanced RvD simulation facility called EPOS 2.0 (European Proximity Operations Simulator). The facility uses robotic manipulators to generate the relative motion between two satellites and allows full RvD test and simulation capabilities for OOS missions up to a range of 25m

Rendezvous Involving a Non-Cooperative, Tumbling Target - Estimation of Moments of Inertia and Center of Mass of an Unknown Target

Safe approach and docking to a non-cooperative, tumbling target satellite is one of the main critical issues in on-orbit servicing missions. Knowledge of the inertia properties of the target spacecraft is a prerequisite for many rendezvous and docking aspects. In this paper we propose a method to estimate the center of mass and moments of inertia using optical sensor data. For that, kinematic equations of motion and the conservation of the angular momentum are employed to estimate the unknown quantities with least squares methods. Observability and limitations are discussed and results gained from computer simulations involving different test cases are presented