A recursively feasible and convergent Sequential Convex Programming procedure to solve non-convex problems with linear equality constraints

A computationally efficient method to solve non-convex programming problems with linear equality constraints is presented. The proposed method is based on a recursively feasible and descending sequential convex programming procedure proven to converge to a locally optimal solution. Assuming that the first convex problem in the sequence is feasible, these properties are obtained by convexifying the non-convex cost and inequality constraints with inner-convex approximations. Additionally, a computationally efficient method is introduced to obtain inner-convex approximations based on Taylor series expansions. These Taylor-based inner-convex approximations provide the overall algorithm with a quadratic rate of convergence. The proposed method is capable of solving problems of practical interest in real-time. This is illustrated with a numerical simulation of an aerial vehicle trajectory optimization problem on commercial-of-the-shelf embedded computers

Simultaneous Capture and Detumble of a Resident Space Object by a Free-Flying Spacecraft-Manipulator System

The article of record as published may be found at https://doi.org/10.3389/frobt.2019.00014A maneuver to capture and detumble an orbiting space object using a chaser spacecraft equipped with a robotic manipulator is presented. In the proposed maneuver, the capture and detumble objectives are integrated into a unified set of terminal constraints. Terminal constraints on the end-effector’s position and velocity ensure a successful capture, and a terminal constraint on the chaser’s momenta ensures a post-capture chaser-target system with zero angular momentum. The manipulator motion required to achieve a smooth, impact-free grasp is gradually stopped after capture, equalizing the momenta across all bodies, rigidly connecting the two vehicles, and completing the detumble of the newly formed chaser-target system without further actuation. To guide this maneuver, an optimization-based approach that enforces the capture and detumble terminal constraints, avoids collisions, and satisfies actuation limits is used. The solution to the guidance problem is obtained by solving a collection of convex programming problems, making the proposed guidance approach suitable for onboard implementation and real-time use. This simultaneous capture and detumble maneuver is evaluated through numerical simulations and hardware-in-the-loop experiments. Videos of the numerically simulated and experimentally demonstrated maneuvers are included as Supplementary Material

Simultaneous Capture and Detumble of a Resident Space Object by a Free-Flying Spacecraft-Manipulator System

A maneuver to capture and detumble an orbiting space object using a chaser spacecraft equipped with a robotic manipulator is presented. In the proposed maneuver, the capture and detumble objectives are integrated into a unified set of terminal constraints. Terminal constraints on the end-effector's position and velocity ensure a successful capture, and a terminal constraint on the chaser's momenta ensures a post-capture chaser-target system with zero angular momentum. The manipulator motion required to achieve a smooth, impact-free grasp is gradually stopped after capture, equalizing the momenta across all bodies, rigidly connecting the two vehicles, and completing the detumble of the newly formed chaser-target system without further actuation. To guide this maneuver, an optimization-based approach that enforces the capture and detumble terminal constraints, avoids collisions, and satisfies actuation limits is used. The solution to the guidance problem is obtained by solving a collection of convex programming problems, making the proposed guidance approach suitable for onboard implementation and real-time use. This simultaneous capture and detumble maneuver is evaluated through numerical simulations and hardware-in-the-loop experiments. Videos of the numerically simulated and experimentally demonstrated maneuvers are included as Supplementary Material

Autonomous Capture of a Resident Space Object by a Spacecraft with a Robotic Manipulator: Analysis, Simulation and Experiments

AIAA/AAS Astrodynamics Specialist Conference, Long Beach, CA. The article of record may be found at:http://arc.aiaa.org | DOI: 10.2514/6.2016-5269This paper describes a set of laboratory-based experiments, which demonstrate the autonomous capture of a non-moving resident space object by a spacecraft equipped with a single robotic manipulator. An air bearing test bed is used to simulate weightlessness and frictionless maneuvering on a plane. The chaser is composed by a floating spacecraft simulator carrying a kinematically redundant four-link serial manipulator. The manipulator mass is similar to the mass of its base-spacecraft, resulting in an unusually large dynamic coupling. Emphasis is given to the guidance and control, demonstrating floating, flying and rotation-flying coordinated control strategies. A resolved-motion-rate controller regulates the manipulator joint velocities. The relative navigation problem, solved by the test bed metrology system, has been left outside the scope of this effort. The presented experiments increase the number of space robotics experimental evaluations conducted in dynamically representative environments

A tip-tilt hardware-in-the-loop air-bearing test bed with physical emulation of the relative orbital dynamics

29th AAS/AIAA Space Flight Mechanics Meeting: Ka’anapali, Maui, Hawaii, U.S.A. Volume: Advances in the Astronautical Sciences (Vol. 168, pp. 3781–3799). Univelt Inc.A new hardware-in-the-loop (HIL) air bearing testbed that is capable of physically emulating the relative orbital dynamics is presented. Typically, air bearing testbeds consist of test vehicles operating on top of a planar and horizontally-leveled sur face. These test vehicles use air bearings to reduce the friction with the operating surface to negligible levels. The low friction, combined with the horizontally leveled surface, creates a low residual acceleration environment. These dynamics are representative of the environment that spacecraft experience during close proximity maneuvers. To extend the applicability of planar air bearing test beds to longer maneuvers or separations relative orbital dynamics need to be emulated. In this paper, using Hill-Clohessy-Wilshire dynamics, we emulated the relative orbital dynamics of a real spacecraft using a scaled Floating Spacecraft Simulator (FSS) on a dynamically inclined operating surface. The mathematical constructs of the tilt angles, screw height displacements and scaling parameters are developed via Euler’s rotation theorem, Buckingham’s Pi theorem and the similarity principle. The applicability of the new idea is demonstrated via a circumnavigation maneuver scenario of a spacecraft in a Low Earth Orbit (LEO). The simulation results show the viability and suitability of the new approach

Attitude Stabilization of Spacecraft in Very Low Earth Orbit by Center-Of-Mass Shifting

At very low orbital altitudes (≲450 km) the aerodynamic forces can become major attitude disturbances. Certain missions that would benefit from a very low operational altitude require stable attitudes. The use of internal shifting masses, actively shifting the location of the spacecraft center-of-mass, thus modulating, in direction and magnitude, the aerodynamic torques, is here proposed as a method to reject these aerodynamic disturbances. A reduced one degree-of-freedom model is first used to evaluate the disturbance rejection capabilities of the method with respect to multiple system parameters (shifting mass, shifting range, vehicle size, and altitude). This analysis shows that small shifting masses and limited shifting ranges suffice if the nominal center-of-mass is relatively close to the estimated center-of-pressure. These results are confirmed when the analysis is extended to a full three rotational degrees-of-freedom model. The use of a quaternion feedback controller to detumble a spacecraft operating at very low altitudes is also explored. The analysis and numerical simulations are conducted using a nonlinear dynamic model that includes the full effects of the shifting masses, a realistic atmospheric model, and uncertain spacecraft aerodynamic properties. Finally, a practical implementation on a 3U CubeSat using commercial-off-the-shelf components is briefly presented, demonstrating the implementation feasibility of the proposed method

Clinical, randomized, double blind clinical trial to study the effect of parenteral supplementation with fish oil emulsion in the nutritional support in esophagectomized patients

Introduction: Esophagectomy is a major surgery with a high degree of catabolic and post-surgical inflammatory response accompanied by high morbidity and significant mortality. Post-surgical nutritional support via enteral administration of omega-3 fatty acids has been seen to be effective although its bad tolerance. There are few clinical trials with parenteral omega-3 fatty acids in these patients. We propose to investigate the effect of combining a parenteral fish oil lipid emulsion with the standard enteral nutrition (EN) support. Materials and methods: Prospective, single-center, randomized, double-blind study in esophagectomized patients, and treated after surgery with parenteral lipid emulsions of omega-3 fatty acids or a mixture of omega-6 long-chain triglycerides/short-chain triglycerides 50%. These emulsions will be added to the standard nutritional support in continuous infusion until 5 days of treatment have been completed. Patients will be randomized 1:1:1 in Group A receiving 0.4 g/kg/d of fish-oil lipid emulsion and 0.4 g/kg/d of a lipid emulsion mixture of omega-6 long-chain fatty acids (LCT) plus medium-chain fatty acids (MCT) (total dose of 0.8 g/kg/d of lipid emulsion); Group B receiving 0.8 g/kg/d of fish oil lipid emulsion and Group C receiving 0.8 g/kg/d of LCT/MCT emulsion. The main objective is to determine whether 5 days administration of intravenous omega-3 fatty acid lipid emulsion is effective in normalizing interleukin-6 levels compared with LCT/MCT emulsions, and whether a 0.8 g/kg/d dose is more effective than 0.4 g/kg/d. Secondary outcomes include other inflammatory markers such as C-reactive protein, tumor necrosis factor alpha and interleukin-10, and parameters of morbidity, safety, nutrition and mortality. Samples will be collected at the time when surgery is indicated and on days 0, 1, 3, 5 and 21 to determine inflammatory, nutritional, hepatic and safety parameters. In addition, clinical follow-up will be continued throughout the hospital admision and up to 1 year after surgery. Discussion: Studies of omega-3 fatty acids administered parenterally in esophagectomized patients are scarce. This study proposes to investigate the effect of combining fish-oil lipid emulsions administered parenterally with EN support. Potential benefits include fast incorporation of lipids to the cellular membranes and to the inflammatory cascade, and the use of only 1 pharmaconutrient

Spacecraft Flight in the Atmosphere

Spacecraft that orbit in Low Earth Orbit travel through a tenuous atmosphere and hence experience aerodynamic forces that can become quite significant, specially at low altitudes. The presence of these forces can become a major design driver for missions that fly at very low altitudes. Unfortunately, spacecraft aerodynamics are not well understood. In this dissertation, a CubeSat mission is proposed which will study rarefied-gas aerodynamics, with the objective of determining the effect of surface composition, surface finishing and flow incidence angle on the drag and lift coefficients with an error of less than 5% using a novel method. The CubeSat, has been named ΔDsat, because this study, will be performed using differential measurements of drag and lift coefficients in order to eliminate any measurement bias. ΔDsat carries 4 deployable fins that can rotate independently and expose different surface types to the flow at different incident angles. In addition, in the dissertation four methods to exploit the aerodynamic forces for the missions advantage are proposed and described in detail. The first one is aerostability, which by shaping the spacecraft appropriately, the resulting aerodynamic torques stabilise the attitude spacecraft with respect to the flow. The second method uses aerodynamic drag and lift to change de inclination of a decaying spacecraft in order to maintain the Sun-synchronous aspect of an orbit whilst decaying. The required lift to drag ratio is in the order of 1.0-1.6, which is not currently achievable (it is theoretically possible), but it could be achieved if drag compensating propulsion is used (thus becoming a fuel saving strategy). The third method controls the atmospheric re¬entry interface (the location of the burn-up) by modulating the drag, hence controlling the decay profile. When applied to ΔDsat an error of less than 200 km 3cr on the re-entry location is achieved. Finally, aerostable spacecraft can be used to perform in-situ measurements of the atmospheric winds, by observing their attitude evolution. The aerostable ΔDsat CubeSat would be capable of determining the cross-track winds with an error of less 4 m/s 3cr

Autonomous optical navigation for orbits around Earth-Moon collinear libration points

The analysis of optical navigation in an Earth–Moon libration point orbit is examined. Missions to libration points have been winning momentum during the last decades. Its unique characteristics make it suitable for a number of operational and scientific goals. Literature aimed to study dynamics, guidance and control of unstable orbits around collinear libration points is vast. In particular, several papers deal with the optimisation of the Δv budget associated to the station-keeping of these orbits. One of the results obtained in literature establishes the critical character of the Moon–Earth system in this aspect. The reason for this behaviour is twofold: high Δv cost and short optimal manoeuvre spacing. Optical autonomous navigation can address the issue of allowing a more flexible manoeuvre design. This technology has been selected to overcome similar difficulties in other critical scenarios. This paper analyses in detail this solution. A whole GNC system is defined to meet the requirements imposed by the unstable dynamic environment. Finally, a real simulation of a spacecraft following a halo orbit of the L2 Moon–Earth system is carried out to assess the actual capabilities of the optical navigation in this scenario.Peer Reviewe

Near-optimal real-time spacecraft guidance and control using harmonic potential functions and a modified RRT

To be published in the proceedings of the 27th AAS/AIAA Spaceflight Mechanics Meeting, San Antonio, TX, Feb. 6-9, 2017A primary requirement for any rendezvous and proximity (RPO) guidance algo- rithm is to ensure mission safety through the generation of collision-free trajec- tories in a fuel-efficient manner. This work presents a real-time hybrid guidance method which fuses the flexibility and robustness of Harmonic Potential Functions with the asymptotically-optimal Rapidly-expanding Random Tree Star method. The proposed method allows to plan trajectories on cluttered environments while producing near-fuel-optimal trajectories. To quantify and validate the performance of this method an experimental campaign is performed utilizing the Naval Post- graduate School POSEIDYN test bed. Lastly, implementation considerations and experimental results are discussed