Nonlinear control of spacecraft formation flying with disturbance rejection and collision avoidance

A nonlinear controller for disturbances rejection and collision avoidance is proposed for spacecraft formation flying.The formation flying is described by a nonlinear model with the J_2 perturbation and atmospheric drag. Based on the theory of the state-dependent Riccati equation(SDRE), a finite time nonlinear control law is developed for the nonlinear dynamics involved in formation flying. Then, a compensative internal mode(IM) control law is added to eliminate disturbances.These two control laws compose a finite time nonlinear tracking controller with disturbances rejection. Moreover, taking safety requirements into account, the repulsive control law is incorporated in the composite controller to perform collision avoidance manoeuvres. A numerical simulation is presented to demonstrate the effectiveness of the proposed method.Compared to the conventional control method, the proposed method provides better performance in the presence of the obstacles and external disturbances.     

A technique for designing highly inclined spacecraft orbits using gravity-assist maneuvers

A highly accurate algorithm for synthesizing chains of cranking gravity-assist maneuvers (GAMs) has been constructed using formalization of the search for ballistic scenarios followed by the adaptive involvement of tens of millions of variants. Its use results in a significant change in the orbital inclination of a research spacecraft without a significant expenditure of propellant with a reasonable mission time.     

A simple method to detect audible echoes in room acoustical design

A simple method to detect audible echoes is proposed as an objective criterion for room acoustics. This method evaluates the perceptibility of sound reflections that are generated by an impulsive sound source and identifies from reflectograms harmful reflections perceived as echoes. Particularly with this method, the masking effect of reverberation is taken into consideration, which cannot be treated sufficiently by the existing objective criteria. The applicability to room acoustical design is verified by evaluating the impulse responses measured in real halls where audible echoes occurred. It is shown that the proposed method detects audible echoes at an accuracy of more than 90% and would be suitable for practical use.     

A simple instrument for relative microhardness measurement

The design of a simple inexpensive instrument which can be used along with a simple laboratory microscope to measure microhardness of crystals is described. The design is based on the fact that the lengths of arms of indentation dislocation rosette (idr) are related to the hardness. By controlled indentation and subsequent etching of two similar crystals, the microhardness of one can be estimated in terms of that of the other from measurements of the arms ofidr.     

A simple design method of negative refractive index metamaterials

We propose a very simple design method of negative refractive index (NRI) materials that can overcome some drawbacks of conventional resonant-type NRI materials. The proposed NRI materials consist of single or double metallic patterns printed on a dielectric substrate. Our metamaterials (MTMs) show two properties that are different from other types of MTMs in obtaining effective negative values of permittivity (ε) and permeability (μ) simultaneously; the geometrical outlines of the metallic patterns are not confined within any specific shape, and the metallic patterns are printed on only one side of the dielectric substrate. Therefore, they are very easy to design and fabricate using common printed circuit board (PCB) technology according to the appropriate application. Excellent agreement between the experiment and prediction data ensures the validity of our design approach.     

A modeling technique for active control design studies with application to spacecraft microvibrations

Microvibrations, at frequencies between 1 and 1000 Hz, generated by on board equipment, can propagate throughout a spacecraft structure and affect the performance of sensitive payloads. To investigate strategies to reduce these dynamic disturbances by means of active control systems, realistic yet simple structural models are necessary to represent the dynamics of the electromechanical system. In this paper a modeling technique which meets this requirement is presented, and the resulting mathematical model is used to develop some initial results on active control strategies. Attention is focused on a mass loaded panel subjected to point excitation sources, the objective being to minimize the displacement at an arbitrary output location. Piezoelectric patches acting as sensors and actuators are employed. The equations of motion are derived by using Lagrange's equation with vibration mode shapes as the Ritz functions. The number of sensors/actuators and their location is variable. The set of equations obtained is then transformed into state variables and some initial controller design studies are undertaken. These are based on standard linear systems optimal control theory where the resulting controller is implemented by a state observer. It is demonstrated that the proposed modeling technique is a feasible realistic basis for in-depth controller design/evaluation studies.     

Particle filter-based relative rolling estimation algorithm for non-cooperative infrared spacecraft

The issue of feature point mismatching among infrared image sequence would bring big challenge to estimating the relative motion of non-cooperative spacecraft for it couldn’t provide the prior knowledge about its geometric structure and motion pattern. The paper introduces particle filter to precisely match the feature points within a desired region predicted by a kinetic equation, and presents a least square estimation-based algorithm to measure the relative rolling motion of non-cooperative spacecraft. The state transition equation and the measurement update equation of non-cooperative spacecraft are represented by establishing its kinetic equations, and then the relative pose measurement is converted to the maximum posteriori probability estimation via assuming the uncertainties about geometric structure and motion pattern as random and time-varying variables. These uncertainties would be interpreted and even solved through continuously measuring the image feature points of the rotating non-cooperative infrared spacecraft. Subsequently, the feature point is matched within a predicted region among sequence infrared image using particle filter algorithm to overcome the position estimation noise caused by the uncertainties of geometric structure and motion pattern. Finally, the position parameters including rotation motion are estimated by means of solving the minimum error of feature point mismatching using least square estimate theory. Both simulated and real infrared image sequences are induced in the experiment to evaluate the performance of the relative rolling estimation, and the experimental data show that the rolling motion estimated by the proposed algorithm is more robust to the feature extraction noise and various rotation speed. Meanwhile, the relative rolling estimation error would increase dramatically with distance and rotation speed increasing.     

A new method for determination of satellite orbits by transfer

The original idea of a new method for determination of satellite orbits by transfer is from Two-Way Satellite Time and Frequency Transfer (TWSTFT). The original method is called “determination of satellite orbit by transfer”. The method is not only for determination of satellite orbit but also for the time transfer with high accuracy and precision. The advantage is that the accuracy and the precision for determination of satellite orbit are very high and the new method is favorable for various applications. The combination of various signals disseminated and received forms various modes of satellite orbit determinations. If receivers at stations receive the own station-disseminated signals via a satellite transponder, it forms an orbit determination mode called “receiving the own station-disseminated signals mode”. If receivers at all stations receive the signals disseminated from the master station via satellite transponders, it forms an orbit determination mode called “receiving the master station-disseminated signals mode”. If all of receivers at stations receive all stations-disseminated signals via satellite transponders, it forms an orbit determination mode called “receiving all stations-disseminated signals mode”. Also there are other combinations of signals for satellite orbit determination. For different orbit determination modes with different signal combinations, their rigorous formulae of processing are hereby presented in this paper. The accurate and the precise satellite orbit determination for both of the modes, “receiving the own station-disseminated signals mode” and “receiving the master station-disseminated signals mode” is attempted. It shows that the accuracy and precision for both of modes are nearly the same, the ranging accuracy is better than 1 cm, and the observation residuals of satellite orbit determination are better than 9 cm in the observation duration of 1 day. Supported by the National Basic Research and Development Program of China (Grant No. 2007CB815503100453001)     

A simple scheme for precise relative frequency stabilization of lasers

We present a simple scheme for tuneable relative frequency stabilization of lasers. A highly sensitive and accurate frequency-to-voltage converter is used to derive an error signal from the beat note between two lasers. We analyze in detail detector noise and drift, modulation detection bandwidth, and cross-talk from power modulation. The results indicate that sub-kHz relative linewidth and a locking point drift on the order of 100 Hz for times scales of 1 h are achievable. The scheme can, therefore, be applied to situations where up to now only optical PLLs could provide sufficient accuracy and precision. To demonstrate its potential for high-resolution, high-precision spectroscopy we lock a diode laser to a fs-frequency comb and find a relative linear drift of 314 Hz during a 2.8 h period. PACS 42.62.Fi; 42.62.Eh     

A simple method for clock recovery

All optical bit clock recovery is one of the key technologies for all optical 3R recovery. In this paper, a simple method applying the combination of Fabry–Perot (F–P) filter and semiconductor optical amplifier (SOA) for clock recovery is proposed. The effect of the F–P filter finesse on the clock recovery and the reduction in the amplitude fluctuation of the clock pulse by the SOA are discussed theoretically. With this technology 10 Gbit/s clock recovery with equal amplitude and wavelength transparency was realized experimentally.     

A simple method for NMR photography

The application of a multi-frequency weak pulse to a liquid crystal can excite narrow 1H NMR peaks at the applied frequencies. By using two-level amplitude coding, namely setting the amplitudes of some of the harmonics to zero, this method can be used to store up to 1024 bits of binary information in the liquid crystal molecules. When the information is retrieved in the form of a spectrum, which is plotted as an array of 32 or 16 segments, the stack of spectral segments reproduces a 2D input pattern quite well. This technique is called "NMR photography." The original method was a pseudo-2D technique that applies the 1024-frequency pulse in the first step, and reads the signal row-by-row in the second step. The present improvement involves subtracting two spectra obtained with the same 1024-frequency pulse but with different durations, so that the method becomes a 1D technique, with a tremendous saving of experimental time. Several examples are given to illustrate the results.     

相对法测量表面张力系数

从细管缓慢流出的液体,并不是呈现连续的液流,而是一些继续的液滴。通过对不同液体液滴大小的比较,可测得表面张力系数。     

A simple method to adjust cooling rates for supercooling point determination

A simple method to obtain predetermined constant cooling rates for insect supercooling point (SCP) determination is described. A transient heat transfer equation was used to design polystyrene cubes of different sizes to yield constant rates of cooling at their centers when held at a constant surrounding temperature. Cubes of 0.29 x 0.29 x 0.29 m and 0.19 x 0.19 x 0.19 m were found to produce cooling rates of about -0.5 and -1 degree C per min, respectively, from 0 to -40 degree C. The observed temperature variations at the geometrical center of the cubes were similar to those predicted by the equation. Temperature plots showed a nearly constant rate of cooling. Supercooling points of Tribolium castaneum (Herbst) at different stages of development were recorded using polystyrene cubes. These SCPs compared favorably with published values. This method of obtaining cooling rates is economical, flexible, and allows for multiple simultaneous SCP measurements.     

A simple method for timing an XFEL source to high-power lasers

We propose a technique for timing an X-ray free-electron laser (XFEL) to a high-power conventional laser with femtosecond accuracy, yielding the relative jitter between pump and X-ray probe, and allowing sorting of experimental results over a certain time window. The same electron bunch is used to produce both an XFEL pulse and an ultrashort optical pulse by means of an optical radiator downstream of the X-ray undulator. Being produced by the same electron bunch, these pulses are perfectly synchronized. Cross-correlation techniques will allow to determine relative jitter between the optical pulse (and, thus, the XFEL pulse) and a pulse from an external pump-laser. Technical realization of the proposed timing scheme uses an optical-replica synthesizer setup to be installed after the final bunch-compression stage of the XFEL for electron bunch diagnostics purposes. A number of critical issues are quantitatively analyzed.     

正弦光栅的制作

介绍了一种制作正弦光栅的方法。与常用的马赫－曾德方法相比，该方法具有使用元件少，逃跑和调节方便，所获得的光栅对比度高等优点。     

A simple method to determine magnetic field gradients

The position of a straight domain wall in a magnetic field gradient in a narrow bar of magnetically uniaxial material is connected with the magnitude of the field gradient. The corresponding relationship is derived. It allows one to determine the field gradient. from a visual measurement of the coordinate of a domain wall and the known parameters of the platelet.     

A simple scheme for the design of solvent-suppression pulses

Excitation sculpting was first introduced as a way to efficiently suppress solvent signals. It requires a pulse sequence that acts as a null pulse at the solvent-resonance frequency and as an inversion pulse everywhere else. In this article, it is shown that such a goal can be achieved starting with "top-hat" inversion shaped pulses such as I-BURP-2 or gaussian cascade G3. The result is a Globally Antisymmetric Selective Pulse, or GASP. Numerical optimization was used to extend the performance of such pulses. Multifrequency signal suppression was shown to be possible through application of successive excitation sculpting modules.     

A spacecraft shielding model for calculating exposures to radiation

A spacecraft shielding model for calculating exposure to radiation is proposed. This model is based on the concept of the randomly inhomogeneous character of the material thickness distribution both in separate equipment units and the spacecraft as a whole. This concept has made it possible to obtain the shielding function of an arbitrary point in a spacecraft necessary to calculate the radiation exposure using its mean density, geometry and the ratio of the shielding function dispersion to its mean. This is constant for the spacecraft. This conclusion was verified for several Soviet and U.S.A. spacecraft, including the Space Shuttle.     

一种简单推导匀速转动电偶极子电磁辐射的方法

电偶极子在绕其中心转动时,正负电荷作加速运动,因此转动的电偶极子会辐射电磁波.利用运动电荷的磁感应强度公式,推导出匀速转动的刚性电偶极子在近场区产生变化磁场的表达式;结合电磁场建立的推迟效应,推导了非相对论情形时、匀速转动电偶极子在远场区的电磁辐射公式,并对比了匀速转动电偶极子辐射与谐振电偶极子辐射的异同.     

A simple method to calculate dispersion coefficients for isolated and condensed-phase species

(Molecular Physics, 1998, 95, 549–570)