Thomas Evans

Experimental Evaluation of a Machine Vision Based Pose Estimation System for Autonomous Capture of Satellites with Interface Rings

This paper presents an experimental evaluation and description of a machine visionbased system for estimating the pose (position and orientation) of a replicated satellite featuring an interface ring of 1.22m diameter. Images of the satellite are obtained using a camera with a wide angle lens (125 deg) in order to have the entire ring in view. The sequences of images are then processed by a modified ellipse extractor. Next, an ellipse detector selects the extracted ellipse that best matches the ring on the image. The normal vector to the ring plane and the three coordinates of the ring center with respect to the camera are found analytically using the geometric parameters of the detected ellipse. The test setup features two 6DOF robotic arms. One arm holds a satellite mockup while the second arm is equipped with the camera system. Pose estimation using a laser tracker with active markers was used as the source of ground truth. Experiments have shown that the system is robust to camera/target relative position and motions, light conditions, and to some degree of occlusions. Visual servoing for tracking the mockup and approaching to grapple position was performed successfully.

An Analytical Model for Porous Polymer-Ceramic Capacitive Pressure Sensors

An analytical model for pressure sensors is constructed, predicting the capacitive response of a porous, polymer-ceramic composite under an applied pressure. Consisting of mechanical and dielectric counterparts, the iterative model is constructed in detail. The elastic modulus of the three-phase material is approximated by first considering only the polymer-ceramic composite mixture, and then incorporating porosity into the solid composite model. A new model has been developed for approximating the changing elastic modulus of porous polymers undergoing quasi-static compression, which induces the collapsing pores. Necessary material constants were obtained from experimental data published in literature. The permittivity of the paraelectric polymer matrix is modeled, accounting for piezodielectric effects imposed by external pressure and thermally induced stresses caused by substrate pinning. Similarly, the ferroelectric ceramic filler is modeled, considering changes in polarization caused by thermally induced phase transformations in the crystal structure. The final model is evaluated against experimental data, providing insight into composition and microstructure effects on the sensor response.

Testing of tactile sensors for space applications

There is a need to integrate tactile sensing into robotic manipulators performing tasks in space environments, including those used to repair satellites. Integration can be achieved by embedding specialized tactile sensors. Reliable and consistent signal interpretation can be obtained by ensuring that sensors with a suitable sensing mechanism are selected based on operational demands, and that materials used within the sensors do not change structurally under vacuum and expected applied pressures, and between temperatures of -80°C to +120°C. The sensors must be able to withstand space environmental conditions and remain adequately sensitive throughout their operating life. Additionally, it is necessary to integrate the sensors into the target system with minimum disturbance while remaining responsive to applied loads. Previous work has been completed to characterize sensors within the selected temperature and pressure ranges. The current work builds on this investigation by embedding these sensors in different geometries and testing the response measured among varying configurations. Embedding material selection was aided by using a dynamic mechanical analyzer (DMA) to determine stress/strain behavior for adhesives and compliant layers used to keep the sensors in place and distribute stresses evenly. Electromechanical characterization of the embedded sensor packages was conducted by using the DMA in tandem with an inductance-capacitance-resistance (LCR) meter. Methods for embedding the sensor packages were developed with the aid of finite element analysis and physical testing to account for specific geometrical constraints. Embedded sensor prototypes were tested within representative models of potential embedding locations to compare final embedded sensor performance.

Virtual simulator for testing a vision based pose estimation system for autonomous capture of satellites with interface rings

This paper describes the design and the performance of a virtual simulation environment to evaluate a machine vision based pose estimation system used for the general problem of satellite servicing. The vision system features a wide angle monocular camera to track the interface ring of a non-cooperative satellite using ellipse extraction. The effects of the camera parameters and of the relative camera-satellite position on the system accuracy are evaluated. Different parameters such as resolution, field of view angle, distortion, occlusions and errors in the intrinsic parameters are considered. The study reveals the importance of the availability of such a visual simulation environment for the purpose of mission planning.

Innovative Design of Lightweight on Board Hydrogen Storage Tank

The hydrogen economy envisioned in the future requires safe and efficient means of storing hydrogen fuel for either use onboard vehicles, delivery on mobile transportation systems or high-volume storage in stationary systems. The main emphasis of this work is placed on the high -pressure storing of gaseous hydrogen on-board vehicles. As a result of its very low density, hydrogen gas has to be stored under very high pressure, ranging from 350 to 700 bars for current systems, in order to achieve practical levels of energy density in terms of the amount of energy that can be stored in a tank of a given volume. This paper presents 3D finite element analysis performed for a composite cylindrical tank made of 6061-aluminum liner overwrapped with carbon fibers subjected to a burst internal pressure of 1610 bars. As the service pressure expected in these tanks is 700 bars, a factor of safety of 2.3 is kept the same for all designs. The results indicated that a stress reduction could be achieved by a geometry change only, which could increase the amount of pressure sustained inside the vessel and ultimately increase the amount of hydrogen stored per volume. Such reductions in the stresses will decrease the thickness dimension required to achieve a particular factor of safety in a direct comparison to a cylindrical design.Copyright

Innovative Structural and Joining Concepts for Lightweight Design of a Scaled Model of a Heavy Vehicle System

The research work presented in this paper aims at investigating the benefits and practicality of implementing lightweight composite materials and design concepts in heavy trailer systems. The overall purpose is to devise innovative, lightweight design and joining concepts for heavy vehicle structures. A 1 to 4 scaled model representation of a trailer was built to assess the feasibility of creating a trailer from fiber-reinforced polymer (FRP) composite materials and therefore using innovative bonding methods to join the design parts. This portion of work focused on using the process of designing and fabricating the model to have first hand knowledge and experience on possible bonding methods and the types of materials and structures that will optimize strength and weight saving characteristics. Current trailer configurations were studied and shown to have high weight sources in the flooring structure. The process of analyzing innovative bonding methods and the implementation of composite sandwich technology created an alternative lightweight structure to replace heavy flooring constituents of trailer hauls. Finite element modeling and experimental analysis reveal that sandwich technology with a core structure that adds flexural stiffness to the overall design and is aided by an FRP faceplate that geometrically matches the core structure provides solutions to decrease heavy vehicle system weight.Copyright