Small Misalignment Angles Research Articles

Hybrid Mechanical and Electronic Beam Steering for Maximizing OAM Channel Capacity

Radio frequency-orbital angular momentum (RF-OAM) is a novel approach of multiplexing a set of orthogonal modes on the same frequency channel to achieve high spectrum efficiencies. Since OAM requires precise alignment of the transmit and the receive antennas, the electronic beam steering approach has been proposed for the uniform circular array (UCA)-based OAM communication system to circumvent large performance degradation induced by small antenna misalignment in practical environment. However, in the case of large-angle misalignment, the OAM channel capacity cannot be effectively compensated only by the electronic beam steering. To solve this problem, we propose a hybrid mechanical and electronic beam steering scheme, in which mechanical rotating devices controlled by pulse width modulation (PWM) signals as the execution unit are utilized to eliminate the large misalignment angle, while electronic beam steering is in charge of the remaining small misalignment angle caused by perturbations. Furthermore, due to the interferometry, the receive signal-to-noise ratios (SNRs) are not uniform at the elements of the receive UCA. Therefore, a rotatable UCA structure is proposed for the OAM receiver to maximize the channel capacity, in which the simulated annealing algorithm is adopted to obtain the optimal rotation angle at first, then the servo system performs mechanical rotation, at last the electronic beam steering is adjusted accordingly. Both mathematical analysis and simulation results validate that the proposed hybrid mechanical and electronic beam steering scheme can effectively eliminate the effect of diverse misalignment errors of any practical OAM channel and maximize the OAM channel capacity.

Self-organized quantum dots in marginally twisted MoSe2/WSe2 and MoS2/WS2 bilayers

Moiré superlattices in twistronic heterostructures are a powerful tool for materials engineering. In marginally twisted (small misalignment angle, θ) bilayers of nearly lattice-matched two-dimensional (2D) crystals moiré patterns take the form of domains of commensurate stacking, separated by a network of domain walls (NoDW) with strain hot spots at the NoDW nodes. Here, we show that, for type-II transition metal dichalcogenide bilayers MoX2/WX2 (X=S, Se), the hydrostatic strain component in these hot spots creates quantum dots for electrons and holes. We investigate the electron/hole states bound by such objects, discussing their manifestations via the intralayer intraband infrared transitions. The electron/hole confinement, which is strongest for θ < 0.5°, leads to a red-shift of their recombination line producing single-photon emitters (SPE) broadly tuneable around 1 eV by misalignment angle. These self-organized dots can form in bilayers with both aligned and inverted MoX2 and WX2 unit cells, emitting photons with different polarizations. We also find that the hot spots of strain reduce the intralayer MoX2 A-exciton energy, enabling selective population of the quantum dot states.

Evolution of stellar orbits around merging massive black hole binary

ABSTRACT We study the long-term orbital evolution of stars around a merging massive or supermassive black hole binary (BHB), taking into account the general relativistic effect induced by the black hole (BH) spin. When the BH spin is significant compared to and misaligned with the binary orbital angular momentum, the orbital axis ($\hat{\boldsymbol {l}}$) of the circumbinary star can undergo significant evolution during the binary orbital decay driven by gravitational radiation. Including the spin effect of the primary (more massive) BH, we find that starting from nearly coplanar orbital orientations, the orbital axes $\hat{\boldsymbol {l}}$ of circumbinary stars preferentially evolve towards the spin direction after the merger of the BHB, regardless of the initial BH spin orientation. Such alignment phenomenon, i.e. small final misalignment angle between $\hat{\boldsymbol {l}}$ and the spin axis of the remnant BH $\hat{\boldsymbol {S}}$, can be understood analytically using the principle of adiabatic invariance. For the BHBs with extremely mass ratio (m2/m1 ≲ 0.01), $\hat{\boldsymbol {l}}$ may experience more complicated evolution as adiabatic invariance breaks down, but the trend of alignment still works reasonably well when the initial binary spin–orbit angle is relatively small. Our result suggests that the correlation between the orientations of stellar orbits and the spin axis of the central BH could provide a potential signature of the merger history of the massive BH.

Stimulated Brillouin scattering in a sub-wavelength anisotropic waveguide with slightly-misaligned material and structural axes: misalignment-sensitive behaviors and underlying physics

The stimulated Brillouin scatterings (SBSs) in sub-wavelength rutile waveguides with slightly misaligned material and structural axes are numerically studied. The misalignment is introduced between the extraordinary material axis and longitudinal axis of the waveguide only. Four nanowire waveguides with different cross-sectional geometries are considered. They consist of a circular waveguide, two elliptical waveguides with different cross-sectional orientation angles, and a trapezoidal waveguide with a completely unsymmetrical cross-sectional shape. As previously reported, the resonant peaks emerge rapidly in response to the introduced small misalignment angle and can also be observed in the calculated Brillouin gain spectra of the considered waveguides. But these misalignment-sensitive resonant peaks further exhibit some extraordinary behaviors, which may not be intuitively understandable. For instance, despite a plausible absence of symmetry breaking, many misalignment-sensitive resonant peaks can still be observed in the forward SBS gain spectrum of the trapezoidal waveguide. Based on the symmetry properties of the considered waveguides, the physics underlying the observed extraordinary phenomena are revealed. The obtained results highlight the effectiveness of introducing symmetry breakings for activating/harnessing opto-mechanical couplings in photonic-phononic micro structures, which would enable us to gain some deeper insights into the sub-wavelength opto-mechanics in anisotropic media.

Frame-dragging effects in obliquely rotating magnetars

Magnetars are highly magnetized neutron stars. For a slowly rotating magnetar, the strong magnetic field deforms the star, making it axisymmetric with respect to the magnetic axis (the body symmetry axis). In magnetars, the rotation axis is tilted to the magnetic axis, and we have an oblique rotator. General relativistic treatment of the obliquely rotating magnetar gives rise to frame-dragging velocities both in the azimuthal and polar direction. Solving the Einstein equation up to first-order perturbation in rotation and second-order perturbation in the magnetic field, we calculate the geodesic of a particle near the star's surface. The polar frame-dragging velocity makes the particle orbit non-planar, and the particle moves both along the azimuthal and polar direction for a fixed radial distance. The extent of particle deviation from planar orbit depends on the magnetic field strength and the misalignment angle. We find that the continuous gravitational wave emitted from such obliquely rotating axisymmetric star is non zero, and for small misalignment angle, the gravitational wave amplitude depends more on the azimuthal frame-dragging velocity. In contrast, for a large misalignment angle, the polar frame-dragging velocity dominates. The energy loss from such a misaligned rotator depends more significantly on the polar frame-dragging velocity and therefore, can significantly affect the magnetosphere around a magnetar.

Deterministic Integration of hBN Emitter in Silicon Nitride Photonic Waveguide

AbstractHybrid integration provides an important avenue for incorporating atom‐like solid‐state single‐photon emitters into photonic platforms that possess no optically‐active transitions. Hexagonal boron nitride (hBN) is particularly interesting quantum emitter for hybrid integration, as it provides a route for room‐temperature quantum photonic technologies, coupled with its robustness and straightforward activation. Despite the recent progress of integrating hBN emitters in photonic waveguides, a deterministic, site‐controlled process remains elusive. Here, the integration of selected hBN emitter in silicon nitride waveguide is demonstrated. A small misalignment angle of 4° is shown between the emission‐dipole orientation and the waveguide propagation direction. The integrated emitter maintains high single‐photon purity despite subsequent encapsulation and nanofabrication steps, delivering quantum light with zero delay second order correlation function . The results provide an important step toward deterministic, large scale, quantum photonic circuits at room temperature using atom‐like single‐photon emitters.

Dynamic stiction without static friction: The role of friction vector rotation.

In the textbook formulation of dry friction laws, static and dynamic friction (stick and slip) are qualitatively different and sharply separated phenomena. However, accurate measurements of stick-slip motion generally show that static friction is not truly static but characterized by a slow creep that, upon increasing tangential load, smoothly accelerates into bulk sliding. Microscopic, contact-mechanical, and phenomenological models have been previously developed to account for this behavior. In the present work, we show that it may instead be a systemic property of the measurement apparatus. Using a mechanical model that exhibits the characteristics of typical setups of measuring friction forces-which usually have very high transverse stiffness-and assuming a small but nonzero misalignment angle in the contact plane, we observe some fairly counterintuitive behavior: Under increasing longitudinal loading, the system almost immediately starts sliding perpendicularly to the pulling direction. Then the friction force vector begins to rotate in the plane, gradually approaching the pulling direction. When the angle between the two becomes small, bulk sliding sets in quickly. Although the system is sliding the entire time, macroscopic stick-slip behavior is reproduced very well, as is the accelerated creep during the "stick" phase. The misalignment angle is identified as a key parameter governing the stick-to-slip transition. Numerical results and theoretical considerations also reveal the presence of high-frequency transverse oscillations during the "static" phase, which are also transmitted into the longitudinal direction by nonlinear processes. Stability analysis is carried out and suggests dynamic probing methods for the approaching moment of bulk slip and the possibility of suppressing stick-slip instabilities by changing the misalignment angle and other system parameters.

Performance evaluation of low-cost GPS/INS in-motion alignment model under ECEF frame

ABSTRACT Low-cost GPS/INS integration system is the ideal combination of navigation and positioning. However, the sensitivity of low-cost INS is not good enough for the initial alignment in statics base before navigation. Considering this problem, this paper presents an arbitrary misalignment angle error propagation model which does not rely on small misalignment angles assumption and two simplified versions. These models are presented in the ECEF frame approach and are suitable to implement the in-motion alignment with GPS aided. Another three error models based on quaternion, Rodrigues parameters and modified Rodrigues parameters were also proposed. Three experiments were designed to verify the accuracy and computational efficiency of these error models. The experiments’ results showed that the small misalignment angle model was applicable to the small misalignment angle for higher computational efficiency but without improvement of accuracy. And the large misalignment angle is the best error model for the initial alignment of the arbitrary misalignment angle.

A Model-Free Calibration Method of Inertial Navigation System and Doppler Sensors

The calibration of inertial navigation system (INS) and Doppler Velocity Log (DVL) is an essential task through which the positioning accuracy of the INS/DVL system can be improved significantly. Most existing calibration methods are dependent on the pre-established DVL error model and the hypothesis of small misalignment angles between DVL and INS. An integral calibration method without pre-established model is proposed, which utilizes Genetic Algorithm (GA) and Support Vector Regression (SVR) to construct regression predictor for integral calibration and consolidated compensation. GPS and pressure sensors are used to collect training data to construct the training target output. During underwater navigation, the output of GA-SVR predictor is regarded as the calibrated velocity to construct INS/DVL system. Simulation and semi-physical experiment results show that compared with other traditional calibration algorithms, the calibration accuracy of proposed algorithm is greatly improved. In addition, the limitations of other traditional algorithms can be overcome, which means proposed algorithm can perform smoothly when the misalignment angles are large.

Theoretical and experimental analysis of an unbalanced and cracked cardan shaft in the vicinity of the critical speed

This paper presents a theoretical and experimental analysis of a coupled lateral and torsional vibrations of two identical rotors interconnected by a flexible Hooke’s joint and modelled as a multibody system with a small misalignment angle. Using energy principle and a Lagrangian transformation, the governing equation of the propeller shaft system is established by considering a nonlinear elastic shaft time-dependent perturbation. To study the sensitivity of the crack for a rotating shaft, the model is enriched by considering the periodical feature of the time-varying stiffness deriving from the crack breathing model. The nonstationary response of a cracked rotor system in the presence of unbalance has been evaluated using orbit patterns and Fast Fourier Transform. The highly oscillated feature of the rotors system is theoretically obtained and experimentally analysed. The analysis demonstrated that the crack parameters in the input shaft tend to inhibit the occurrence of unstable oscillations in lateral deflection, orbit and frequency spectrum of the secondary response. It is also found that the passage of the cracked primary shaft near to an integral multiple of the critical speed leads to the phenomenon of sup-harmonic resonance. Subsequently, the experimental analysis conducted equally indicated that the quantitative relation between the faults and the performance of the transmission is impacted by the time-varying stiffness and is the main cause of the frequency-modulated feature in the Cardan shaft system. Finally, the experimental results were informative for the transient response exploration and comparable to the theoretical findings for validating the proposed twin-rotor model.

Internal loads and contact pressure distributions on the main shaft bearing in a modern gearless wind turbine

Abstract The double-row tapered roller bearing (TRB) widely used to support the main shaft in a modern gearless wind turbine is one of the main components and its faults can lead to the malfunctions and downtime of wind turbines. Over the past decades, some numerical approaches have been proposed for calculating the contact force and pressure distribution of double-row TRBs. Nevertheless, most of the existing studies did not take the angular misalignment between inner and outer rings and the frictional force between the rollers and raceways into account. This paper presents a comprehensive quasi-static model to investigate the internal load and contact pressure distribution in a double-row TRB by considering the angular misalignment, the combined external loads and frictional force. It is found that a small misalignment angle between inner and outer rings can result in a significant change in the magnitude and distribution of the contact force and pressure. The double-row TRB with crowned roller profile exhibits a substantial improvement in contact pressure distribution by eliminating the occurrence of pressure concentration. Moreover, the peak contact pressure can be significantly reduced on the roller with the crowned profile, even if in the case of misaligned bearing. Comparisons of the simulated contact loads and pressure distributions demonstrate the necessity of considering angular misalignment and frictional force in the modelling of large size and heavily loaded double-row TRB.

The importance of ADCP alignment with GPS in moving-boat streamflow measurements

This paper addresses the importance of the alignment of an acoustic Doppler current profiler (ADCP) with a global positioning system (GPS) in moving-boat streamflow measurements. It presents a mathematical analysis of the discharge bias induced by a misalignment angle. A small misalignment angle may cause a significant bias in a transect discharge. The bias consists of non-directional and direction components. The directional bias is proportional to the ratio between boat velocity and water velocity. In a normal condition of ADCP streamflow measurements, however, the directional bias in transect discharges can be approximately cancelled in the average discharge of reciprocal transects, even if heading-dependent errors are involved in the misalignment. This paper also presents a trial-and-error method for estimating the misalignment angle. We analyzed the transect discharge data obtained from a field measurement on the Yangtze River at the Huangling Temple hydrology station located about 5 km downstream from the Three Gorges Dam to gain insights on the effect of misalignment. Results of this case study suggest that the data for transect discharges must be processed to remove directional bias prior to the Type A evaluation of the random uncertainty of the measured discharge. Otherwise, the estimated Type A uncertainty would be false and misleading.

Fine Alignment Algorithm for UUVs With a Large Misalignment Angle

In order to overcome the problem that the conventional algorithms cannot be applied in the polar region, a polar initial alignment algorithm for Unmanned Underwater Vehicles (UUVs) with a large misalignment angle is proposed in this paper. The error model, the dynamics model and the observation model of the polar UUV initial alignment algorithm is established under the grid frame. This initial alignment algorithm is designed for the temporary anchoring UUV. According to the complex environment in the polar region, the large misalignment angle is considered instead of the small misalignment angle in other algorithms. An unscented Kalman filter is used to realize the data fusion. Based on the characteristics of the dynamics and the observation model, the unscented Kalman filter is simplified. The errors of the velocity are chosen as the observations. The misalignment angle of UUVs is estimated by the simplified unscented Kalman filter. The results demonstrate that the polar UUV initial alignment algorithm can be effective in improving the initial alignment accuracy, especially when large misalignment angles occur.

Borosilicate nozzles manufactured by reproducible fire shaping

Borosilicate glass nozzles are used by researchers in many fields. They are mainly manufactured by two methods: (i) heating and drawing (or pulling), and (ii) fire shaping (or fire polishing). The latter produces the nozzle by heating one capillary end with a flame, and nozzles are typically much shorter than those obtained by pulling, what may be necessary for some applications. In this work, we built an experimental setup and we evaluated the performance and reproducibility of two different approaches to fire shaping. The first approach heats the tip of a horizontal capillary on the side of the flame. The resulting nozzles are very short and have very good symmetry at the neck. However, there is a small but significant misalignment angle between the nozzle neck and the capillary axes that may be unacceptable for its use in some experimental setups. The second fire shaping approach heats a vertical capillary at the top of the flame. The resulting nozzles show very good symmetry and a very low, and acceptable, misalignment angle between the nozzle neck and the capillary axes. As the phenomena involved in the process are complex, the diameter variability for the same experimental conditions (heating position and time) is high. Results show that using the nozzle shortening to control the neck diameter may be an alternative to produce nozzles of a desired size. However, the shape reproducibility is very good, as nozzles of the same diameter have the same shape, even when they have been produced in different conditions.

Molecular dynamics study on friction of polycrystalline graphene

Graphene, as a one-atom thick 2-dimensional material, is an ideal solid lubricant for small length scale devices such as NEMS/MEMS (nano/micro-electro-mechanical systems) and is often synthesized using chemical vapor deposition (CVD). While CVD-grown graphene consists of a large number of randomly-oriented grains, the effects of this polycrystalline structure on graphene friction still remain far from completely understood. In this study, we investigate the tribological properties of the multigrain structure against a pristine (defect-free single-crystal) graphene surface using molecular dynamics (MD) simulations. The MD simulations presented here test the friction of multiple such configurations created by a novel method mimicking the natural growth of grains in two dimensions. The simulation results reveal that most multigrain configurations exhibit persistent negligible friction without transiting to high friction states. However, there also exist several configurations having relatively large friction forces compared with those from perfectly-aligned single-crystal graphene interactions. A systematic analysis of the independent effects of the grain orientation and grain boundary explains these observations. In particular, it is found that there exists a critical range in the misalignment angle of grain (0.3–1.5°, to the pristine surface) where the shear stress (friction force per unit area) is two or three times larger than the perfectly-aligned grain. Thus, configurations which contain grains with a small misalignment angle in this critical range exhibit large friction forces, despite their substantially smaller grain area. Moreover, for those configurations where all grains have misalignment angles greater than the critical angle, grain boundaries act as the primary source of friction.

A unified model for transfer alignment at random misalignment angles based on second-order EKF

In the transfer alignment process of inertial navigation systems (INSs), the conventional linear error model based on the small misalignment angle assumption cannot be applied to large misalignment situations. Furthermore, the nonlinear model based on the large misalignment angle suffers from redundant computation with nonlinear filters. This paper presents a unified model for transfer alignment suitable for arbitrary misalignment angles. The alignment problem is transformed into an estimation of the relative attitude between the master INS (MINS) and the slave INS (SINS), by decomposing the attitude matrix of the latter. Based on the Rodriguez parameters, a unified alignment model in the inertial frame with the linear state-space equation and a second order nonlinear measurement equation are established, without making any assumptions about the misalignment angles. Furthermore, we employ the Taylor series expansions on the second-order nonlinear measurement equation to implement the second-order extended Kalman filter (EKF2). Monte-Carlo simulations demonstrate that the initial alignment can be fulfilled within 10 s, with higher accuracy and much smaller computational cost compared with the traditional unscented Kalman filter (UKF) at large misalignment angles.

Applied Quaternion Optimization Method in Transfer Alignment for Airborne AHRS Under Large Misalignment Angle

The airborne attitude and heading reference system (AHRS) is an inertial system that provides the aircraft with navigation parameters in real time. A new coarse transfer alignment method is proposed for low-accuracy AHRS with a large misalignment angle between AHRS and master inertial navigation system (MINS). The new coarse alignment method is derived from the quaternion-optimization-based in-flight alignment (IFA) method. By resolving the alignment problem in body frame, the proposed method makes full use of the navigation information of MINS and transforms the alignment problem into a quaternion optimization problem. Its accuracy is improved by presenting a two-step iterative algorithm. Compared with the quaternion-optimization-based IFA method, the proposed method is more robust and accurate by introducing the attitude information of MINS. The simulation and experiment results show that the proposed method is applicable to both large and small misalignment angles, and it serves as a nice coarse alignment that permits the implementation of linear error model in fine alignment process.

A novel gyrocompass alignment method under large azimuth misalignment angle

Conventional gyrocompass alignment methods are based on relatively small azimuth misalignment angles. However, a marine strapdown inertial navigation system may face large azimuth misalignment angle caused by a failed coarse alignment algorithm. This paper provides a novel gyrocompass alignment method to solve the problem. Effects of system parameters are analyzed and the proper scenario of parameter switch based on the classic control theories is derived. Test results show that compared with the conventional methods, our method can accomplish the initial alignment quickly and accurately under large azimuth misalignment angle.

Time-varying gyrocompass alignment for fiber-optic-gyro inertial navigation system with large misalignment angle

Conventional strapdown gyrocompass alignment methods are based on the assumption that the fiber-optic-gyro inertial navigation system has a small azimuth misalignment angle. A large azimuth misalignment angle would lead to an extension of the alignment duration. A time-varying gyrocompass alignment method to solve this problem is provided. An appropriate parameter setting is given for the gyrocompass alignment with a large misalignment angle. Also, a proper protocol for a parametric switch is derived. Simulation and trail results show that the proposed method has better alignment performance than conventional ones, as the system has large misalignment angles.

Cluster X-ray line at $$3.5~\mathrm{keV}$$ 3.5 keV from axion-like dark matter

The recently reported X-ray line signal at $E_\gamma \simeq 3.5\, {\rm keV}$ from a stacked spectrum of various galaxy clusters and the Andromeda galaxy may be originating from a decaying dark matter particle of the mass $2 E_\gamma$. A light axion-like scalar is suggested as a natural candidate for dark matter and its production mechanisms are closely examined. We show that the right amount of axion relic density with the preferred parameters, $m_a \simeq 7 \,{\rm keV}$ and $f_a \simeq 4\times 10^{14}\, {\rm GeV}$, can be naturally obtainable from the decay of inflaton. If the axions were produced from the saxion decay, it could not have constituted the total relic density due to the bound from structure formation. Nonetheless, the saxion decay is an interesting possibility, because the $3.5\, {\rm keV}$ line and dark radiation can be addressed simultaneously, being consistent with the Planck data. Small misalignment angles of the axion, ranging between $\theta_a\sim 10^{-4} -10^{-1}$ depending on the reheating temperature, can also be the source of axion production. The model with axion misalignment can satisfy the constraints for structure formation and iso-curvature perturbation.