High performance two degrees of freedom attitude control for solar sails

The purpose of this paper is to present a high performance solar sail attitude controller which uses ballast masses moving inside the sail’s booms as actuators and to demonstrate its ability of performing time efficient reorientation maneuvers. The proposed controller consists of a combination of a feedforward and a feedback controller, which takes advantage of the feedforward’s fast response and the feedback’s ability of responding to unpredicted disturbances. The feedforward controller considers the attitude dynamics of the sailcraft as well as the disturbance torque due to the center of pressure offset to the center of mass of the sailcraft. Additional disturbance torques, like those coming from the environment or from asymmetry of the spacecraft structure, are then handled by the feedback controller. Simulation performance results are finally compared against results available in the literature.     

H-reversal trajectory: A new mission application for high-performance solar sails

The aim of this paper is to quantify the performance of a flat solar sail to perform a double angular momentum reversal maneuver and produce a new class of two-dimensional, non-Keplerian orbits in the ecliptic plane. For a given pair of orbital parameters, the orbital period and the perihelion distance, it is possible to find the minimum solar sail characteristic acceleration required to fulfil a double angular momentum reversal trajectory. This problem is addressed using an optimal formulation and is solved through an indirect approach. The new trajectories are symmetrical with respect to the sun-perihelion line and exhibit a bean-like shape. Two main difficulties must be properly taken into account. On one side the sail is required to perform a rapid reorientation maneuver when it approaches the perihelion. Suitable simulations have shown that such a maneuver is feasible. In the second place the new trajectories require the use of high performance solar sails. For example, assuming an orbital period equal to 5 years, the required solar sail characteristic acceleration is greater than 3.4 mm/s². Such a value, although beyond the currently available sail performance, is comparable to what is required by the original concept of H-reversal maneuvers introduced by Vulpetti in 1996.     

PowerMillTM高速铣床--真正的高速加工

IngersollPowerMillTM系列机床为卧式、具有高加速度和大功率的铣床,机床设计集中了所有先进机床的优点,具有极高的生产效率,能加工的零件涵盖了钛合金和铝合金零件     

Children's Use of Slope to Guide Navigation: Sex Differences Relate to Spontaneous Slope Perception

Recent findings show that human adults can use slope to guide spatial search, although men significantly outperform women. To examine the sex difference more closely, we tested school-age children in a similar paradigm. Over four trials, children (n = 110) were disoriented and asked to locate a hidden target when the floor of a square enclosure was flat (control condition), sloped (slope condition), or sloped with a “ball drop demonstration,” intended to make the slope more salient (ball drop condition). In the presence of the slope cue, children performed above chance, although boys significantly outperformed girls. Boys were also more likely to notice the slope, and spontaneous slope perception was key to using the slope cue.     

Modeling and identification of an electro-mechanical system: The LISA grabbing positioning and release mechanism case

In the frame of space missions, mechanisms often constitute critical systems whose functionality and performance need to be tested on ground before the mission launch. The LISA scientific space mission will detect gravitational waves by measuring the relative displacement of pairs of free-floating test masses set into geodesic motion onboard of three spacecrafts. Inside each satellite, the injection of the test masses from the caged configuration into the geodesic trajectory will be performed by the grabbing positioning and release mechanism. To provide a successful injection, the test masses must be dynamically released with a minimal residual velocity against adhesion with the holding device. A parameter that determines the test mass residual velocity is the quickness of the retraction of the holding device. The need arises then to characterize the dynamic response of the release mechanism in order to predict its behaviour in the in-flight conditions. Once a validated model of the mechanism is available, the compliance of the system to the tight requirement on the maximum allowed residual velocity of the test mass may be verified. Starting from an electro-mechanical model of the mechanism dynamics, this paper presents the results of the experimental identification of its relevant parameters.     

Heat transfer of thermocapillary convection in a two-layered fluid system under the influence of magnetic field

Heat transfer of a two-layer fluid system has been of great importance in a variety of industrial applications. For example, the phenomena of immiscible fluids can be found in materials processing and heat exchangers. Typically in solidification from a melt, the convective motion is the dominant factor that affects the uniformity of material properties. In the layered flow, thermocapillary forces can come into an important play, which was first emphasized by a previous investigator in 1958. Under extraterrestrial environments without gravity, thermocapillary effects can be a more dominant factor, which alters material properties in processing. Control and optimization of heat transfer in an immiscible fluid system need complete understanding of the flow phenomena that can be induced by surface tension at a fluid interface. The present work is focused on understanding of the magnetic field effects on thermocapillary convection, in order to optimize material processing. That is, it involves the study of the complicated phenomena to alter the flow motion in crystal growth. In this effort, the Marangoni convection in a cavity with differentially heated sidewalls is investigated with and without the influence of a magnetic field. As a first step, numerical analyzes are performed, by thoroughly investigating influences of all pertinent physical parameters. Experiments are then conducted, with preliminary results, for comparison with the numerical analyzes.     

Performance analysis of an attitude control system for solar sails using sliding masses

This paper deals with the attitude control performance analysis of a square solar sail. Two sliding masses are moved inside and along mast lanyards for the control around the pitch and yaw axes. An optimal linear controller with a feedback and a feedforward part is used to control the attitude of the sail. Numerical simulations have been carried out to investigate the system’s ability of performing precise and near-time-optimal reorientation maneuvers as well as the controller’s sensitivity with respect to the sail parameters, as the center of pressure to the center of mass offset or the sail’s size. Our simulation results are finally shown and discussed.     

Object injection in geodesic conditions: In-flight and on-ground testing issues

In absence of forces, any object moves along the straightest possible path in space–time, called geodesic. The presence of a gravity field generates a space–time distortion that reflects on a curvature of the geodesics. Outer space yields a privileged environment to achieve high levels of geodesic purity, thanks to the absence of many non-gravitational force disturbances typical of the Earth, mainly due to the atmosphere, micro-seismic activity, stray electro-magnetic fields, etc. Many experiments in the field of Fundamental Physics, General Relativity and Earth Observation are performed through space missions, in which objects are either set in geodesic conditions or their deviation from a geodesic is measured. In both cases, stray non-gravitational forces acting on them must be reduced to a negligible level.     

In-flight testing of the injection of the LISA Pathfinder test mass into a geodesic

LISA Pathfinder is a technology demonstrator space mission, aimed at testing key technologies for detecting gravitational waves in space. The mission is the precursor of LISA, the first space gravitational waves observatory, whose launch is scheduled for 2034. The LISA Pathfinder scientific payload includes two gravitational reference sensors (GRSs), each one containing a test mass (TM), which is the sensing body of the experiment. A mission critical task is to set each TM into a pure geodesic motion, i.e. guaranteeing an extremely low acceleration noise in the sub-Hertz frequency bandwidth. The grabbing positioning and release mechanism (GPRM), responsible for the injection of the TM into a geodesic trajectory, was widely tested on ground, with the limitations imposed by the 1-g environment. The experiments showed that the mechanism, working in its nominal conditions, is capable of releasing the TM into free-fall fulfilling the very strict constraint imposed on the TM residual velocity, in order to allow its capture on behalf of the electrostatic actuation.However, the first in-flight releases produced unexpected residual velocity components, for both the TMs. Moreover, all the residual velocity components were greater than maximum value set by the requirements. The main suspect is that unexpected contacts took place between the TM and the surroundings bodies. As a consequence, ad hoc manual release procedures had to be adopted for the few following injections performed during the nominal mission. These procedures still resulted in non compliant TM states which were captured only after impacts. However, such procedures seem not practicable for LISA, both for the limited repeatability of the system and for the unmanageable time lag of the telemetry/telecommand signals (about 4400 s). For this reason, at the end of the mission, the GPRM was deeply tested in-flight, performing a large number of releases, according to different strategies. The tests were carried out in order to understand the unexpected dynamics and limit its effects on the final injection. Some risk mitigation maneuvers have been tested aimed at minimizing the vibration of the system at the release and improving the alignment between the mechanism and the TM. However, no overall optimal release strategy to be implemented in LISA could be found, because the two GPRMs behaved differently.     

Prediction of the LISA-Pathfinder release mechanism in-flight performance

In the LISA Pathfinder mission, the grabbing positioning and release mechanism will perform a critical phase, which is the injection of a test mass into a perfect free-fall condition (also called geodesic trajectory). A possible failure is the test mass remaining adhered to the mechanism or released with an excessive velocity, which would hinder the subsequent capacitive control to the desired geodesic status.