Relative Motion Research Articles

Current–Voltage–Friction Characteristics of Grease in Electromechanically Loaded Sliding Contacts

Abstract Electromechanically loaded contacts, which have relative motion between the contacting parts, experience severe damage compared to mechanically loaded contacts. The electromechanical environment occurs when different types of current flow through the bearings of traction motors due to the usage of electronic speed control devices. The current passage through the contact depends on the voltage potential developed across the contact. Grease is commonly used as a lubricant, and degradation and evaporation of lubricant due to the joule heating effect are concerns in electromechanical contacts. This study reports the current–voltage–friction characteristics of lithium mineral oil grease using a ball-on-disk configuration under combined electrical and mechanical loading. The characteristics indicated a transition of the lubricated contact from a non-conducting state to a conducting state with increased applied voltage. Two critical voltages are identified: one where the friction is observed to rise and the other where the current flow rapidly increases, leading to accelerated damage to the lubricant by inducing a significantly high temperature. The study helps in identifying permissible voltage levels for operating bearings safely from the perspective of grease lubricant using simplified ball-on-disk experiments.

Investigation of Excellence in Nd-YAG laser cutting of Al6061-T6 Thin sheet using GRA coupled with PCA

Abstract Cutting flying materials with minimum deviations and metallurgical damages is always preferred for airframe work in aircraft industries. In the present scenario, the laser cutting process has become the preferred choice for fine cutting of sheet materials with close tolerances. This paper reports the investigation of excellence in pulsed laser cutting of Al-alloy (Al6061-T6) sheet. Five quality characteristics for quantifying the dimensional accuracy (i.e., kerf width and deviation at the top and bottom side) and metallurgical damage (i.e., heat-affected zone) have been considered for the proposed work. These characteristics depend on the lamp current, laser parameters (pulse width and frequency), and relative motion between the sheetmetal and laser source. The experiments have been performed using the design of experiments technique. Further, the results of laser cutting have been optimized by using grey relational analysis (GRA) coupled with principal component analysis (PCA). The application of GRA-PCA is found capable of optimizing all five process responses at a time. The unevenness within the kerf is reduced by 22% and 32% at the top and bottom sides, respectively.

Shape design optimization of a flat endmill tool

AbstractAmong the many possible metal‐cutting processes, milling is regarded as one of the most widely applied processes to machine different engineering materials productively. During the milling process, there is relative motion between the endmill tool and the workpiece. Energy is required to drive the cutting force used by the tool in separating the chips from the workpiece. Several studies have focused on the effects of milling process parameters on the cutting force, for instance by varying the cutting parameters, machining properties and the extent of cooling/lubrication. Other studies have explored the influence of tool geometry and material properties on the cutting force. Results from these studies have brought about improvements in the design of milling machinery and tools. The current study presents a novel automated approach for tool geometry optimization which is fully coded in Matlab. An automated and parametrized endmill design procedure was created featuring input parameters such as rake angle, relief angle, clearance angle and helix angle, all of which have been shown, through numerous studies, to influence cutting forces. The parameters were here used to generate the shape profile for one cutting edge of a flat endmill. This cutting edge profile is next copied and rotated, as per the design's pitch angle, to form the complete set of cutting flutes for the flat endmill. Next, the surface defined by the completed profile is meshed using quadrilateral elements. The 3D endmill design is then formed by extruding, and twisting as per the helix angle, this quadrilateral mesh to form a solid hexahedral mesh. Finally, an automated iterative optimization process was defined, featuring the above parametrized design process coupled with Abaqus‐based finite element simulation of the milling process. The expected result is that the proposed optimization procedure will be able to identify the endmill design parameters which minimize the cutting force. At the completion of the optimization, a minimized maximum resultant cutting force value is obtained from using the newly optimized endmill. The resultant cutting force was reduced by 17.6%. It is anticipated that the automated design, meshing and simulation‐driven optimization approach is generalizable to other tool design variations.

Regular Quaternion Equations of the Spatial Hill Problem in Kustaanheimo–Stiefel Variables and Quaternion Osculating Elements

Regular quaternion equations of the spatial Hill problem (a variant of the limited three-body problem (Sun, Earth, Moon (or another low-mass moving cosmic body under study)) are obtained, when the distance between two bodies with finite masses is considered very large, in four-dimensional Kustaanheimo-Stiefel variables (KS-variables) within the framework of the elliptical and circular spatial bounded three-body problem, as well as the regular quaternion equations of the planar Hill problem in two-dimensional Levi-Civita variables. In these equations, the variables are KS-variables or Levi-Civita variables and the energy of relative motion of the body under study, or a variable that converts for the circular Hill problem into a constant of motion of this body (the Jacobi integration constant), as well as the planetocentric distance of the Sun and real time associated with a new independent variable by the Sundman differential transformation of time or other more complex differential ratio. These equations are supplemented by the equation of the Earth’s orbit in polar coordinates and the equation for the true anomaly characterizing the Earth’s position in the orbit. The first integral of the obtained equations in KS-variables in the case of a circular problem is established. Another first partial integral in the general case is a bilinear relation connecting KS-variables and their first derivatives. Three new forms of regular equations of the spatial Hill problem in quaternion osculating elements (slowly changing quaternion variables) are proposed. The proposed regular quaternion equations have an oscillatory form or the form of equations with slowly changing variables, which makes it possible to effectively use analytical and numerical methods of oscillation theory and methods of nonlinear mechanics in the study of the Hill problem.

Evade Unknown Pursuer via Pursuit Strategy Identification and Model Reference Policy Adaptation (MRPA) Algorithm

The game of pursuit–evasion has always been a popular research subject in the field of Unmanned Aerial Vehicles (UAVs). Current evasion decision making based on reinforcement learning is generally trained only for specific pursuers, and it has limited performance for evading unknown pursuers and exhibits poor generalizability. To enhance the ability of an evasion policy learned by reinforcement learning (RL) to evade unknown pursuers, this paper proposes a pursuit UAV attitude estimation and pursuit strategy identification method and a Model Reference Policy Adaptation (MRPA) algorithm. Firstly, this paper constructs a Markov decision model for the pursuit–evasion game of UAVs that includes the pursuer’s attitude and trains an evasion policy for a specific pursuit strategy using the Soft Actor–Critic (SAC) algorithm. Secondly, this paper establishes a novel relative motion model of UAVs in pursuit–evasion games under the assumption that proportional guidance is used as the pursuit strategy, based on which the pursuit UAV attitude estimation and pursuit strategy identification algorithm is proposed to provide adequate information for decision making and policy adaptation. Furthermore, a Model Reference Policy Adaptation (MRPA) algorithm is presented to improve the generalizability of the evasion policy trained by RL in certain environments. Finally, various numerical simulations imply the precision of pursuit UAV attitude estimation and the accuracy of pursuit strategy identification. Also, the ablation experiment verifies that the MRPA algorithm can effectively enhance the performance of the evasion policy to deal with unknown pursuers.

Distributed Model Predictive Control Cooperative Guidance Law for Multiple UAVs

Aiming at the problem of multiple unmanned aerial vehicles (UAVs) cooperatively intercepting a maneuvering target, this paper proposes a cooperative guidance law with less energy consumption and a newly accurate time-to-go estimation algorithm in the two-dimensional (2D) plane. Firstly, based on the relative motion equations between UAVs and the target on the 2D plane, the line-of-sight (LOS) direction and the LOS normal direction models are established. Then, based on the distributed model predictive control (DMPC) theory, DMPC cooperative guidance laws are designed in two directions. This guidance law can ensure that all UAVs intercept the maneuvering target at the expected LOS angle at the same time and reduce the energy consumption during the guidance process. Then, a new time-to-go estimation algorithm is designed, which can reduce the time-to-go estimation error and improve the cooperative accuracy. Finally, the simulation results show that the DMPC cooperative guidance law reduces energy consumption by more than 50% compared to other guidance laws and the proposed time-to-go estimation algorithm improves the accuracy by 200% compared to traditional methods.

Modeling and Analysis of Inter-Panel Slipping for the Design of Rolled Gossamer Arrays

Abstract Many deployable satellite systems benefit from having low mass and high surface area, which has led to the proliferation of gossamer structures in space-based applications. Gossamer structures are characterized by lightweight, low-stiffness membranes, which can flex and roll to compactly stow. An effect of rolling a gossamer structure is that there is a tangential separation along adjacent panels as they roll, resulting in relative motion between panels. To aid designers in predicting and accommodating this motion, a method for modeling the slippage between adjacent panels that occurs while rolling is presented. This analytical slippage model and algorithm is a function of (1) the number of panels, (2) the thickness of each panel, (3) the length of each panel, and (4) the minimum bend radius of the material. It is shown that the thickness and length have a positive correlation with increased slippage, whereas the number of panels and minimum bend radius have a negative correlation with increased slippage. This model allows designers to predict both the magnitude of slippage that occurs where panels meet, as well as the relative range of slippage that occurs within the whole pattern. With these predictions, an appropriate strategy can be selected for accommodating this motion.

Studies of stationary features in jets: BL Lacertae II. Trajectory reversals and superluminal speeds on sub-parsec scales

High-resolution very long baseline interferometry observations have revealed a quasi-stationary component (QSC) in the relativistic jets of many blazars, which represents a standing recollimation shock. VLBA monitoring of the BL Lacertae jet at 15 GHz shows the QSC at a projected distance of about 0.26 mas from the radio core. We study the trajectory and kinematics of the QSC in BL Lacertae on sub-parsec scales using 15 GHz VLBA data of 164 observations over 20 years from the MOJAVE program and 2 cm VLBA Survey. To reconstruct the QSC's intrinsic trajectory, we used moving average and trajectory refinement procedures to smooth out the effects of core displacement and account for QSC positioning errors. We identified 22 QSC reversal patterns with a frequency of $ 1.5$ per year. Most reversals have an acute angle $<90 and a few have a loop-shaped or arc-shaped trajectory. Where observed, combinations of reversals show reversible and quasi-oscillatory motion. We propose a model in which a relativistic transverse wave passes through the QSC, generating a short-lived reverse motion, similar to the transverse motion of a seagull on a wave. According to the model, relativistic waves are generated upstream and the reverse motion of the QSC is governed by the amplitude, velocity, and tilt of the wave as it passes through. The apparent superluminal speeds of the QSC ($ 2\,c$) are then due to the relativistic speed of the jet's transverse wave ($<0.3\,c$ in the host galaxy rest frame) combined with the relativistic motion towards the observer. The measured superluminal speeds of the QSC indirectly indicate the presence of relativistic transverse waves, and the size of the QSC scattering on the sky is proportional to the maximum amplitude of the wave. We find that most of the transverse waves are twisted in space. In the active state of the jet, the directions of the twisting waves are random, similar to the behaviour of the wave in a high-pressure hose, while in the jet stable state, the wave makes quasi-oscillations with regular twisting. The study of QSC dynamics in BL Lac-type blazars is important for evaluating the physical characteristics of relativistic transverse jet waves. The latter have important implications for jet physics and open up possibilities for modelling the physical conditions and location in the jet necessary for the excitation of relativistic transverse waves.

Elastodynamics based Modelling of Acoustic Emission for Earlier Bearing Damage Detection

It is crucial for many applications to detect bearing damage as early as possible to allow for scheduling of maintenance with lead times that minimize operational disruption. State of the practice is the detection of spalling but damage initiates prior to spalling as subsurface and surface cracks. Such damage is much harder to detect and to model. This study proposes a unique application of the nanofrictional Prandtl-Tomlinson model to predict macroscopic acoustic emission (AE) signals that occur at cracked interfaces under relative motion. The study integrates large deformation modelling of structures with elastodynamic simulations to investigate early AE signals generated under different bearing rotational speeds. Experimental studies are carried out to measure acoustic vibrations from metal-metal surface friction using fiber optic sensors and compared to those predicted by the model. Broad agreement of results highlights the validity of this framework.

Accurate proper motions of the protostellar binary system L 1551 IRS 5

Abstract We present an extensive astrometric study of the protostellar binary system L 1551 IRS 5, utilising nearly four decades of interferometric observations obtained between 1983 and 2022 with the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA). We focus on observations with sufficient angular resolution to separate the two protostars (L 1551 IRS 5 N and S) in the system and derive accurate absolute proper motions for the two sources, as well as the relative proper motion between them. The absolute proper motion is dominated by the solar motion with only a modest contribution from L 1551 IRS 5’s peculiar velocity, as expected for a young stellar object. The relative proper motions enable us to constrain the orbit and derive a total mass of 0.96 ± 0.17 M⊙ for the system. While the emission of both sources at wavelengths shorter than about 1.3 cm is compact, the emission at longer wavelengths (λ ≳ 2 cm) is often affected by a free-free contribution from nearby shock features. The results presented here demonstrate that, when appropriate care is taken to combine the observations, interferometric data collected with different facilities, at different frequencies, and with different gain calibrators can be combined to obtain accurate astrometry. Observations of L 1551 IRS 5 over the next several decades with the VLA, ALMA and eventually the ngVLA and SKA ought to improve its dynamical mass measurement down to an accuracy of a few percent. Similar observations of other young multiple systems have the unique potential to provide dynamical mass estimates for the youngest known stellar objects.

A Decision Model for Ship Overtaking in Straight Waterway Channels

Overtaking situations are commonly encountered in maritime navigation, and the overtaking process involves various risk factors that significantly contribute to collision incidents. It is crucial to conduct research on the maneuvering behaviors and decision-making processes associated with ship overtaking. This paper proposes a method based on the analysis of ship maneuvering performance to investigate overtaking behaviors in navigational channels. A relative motion model is established for both the overtaking and the overtaken vessels, and the inter-vessel distance is calculated, taking into account the psychological perceptions of the ship’s driver. A decision-making model for ship overtaking is presented to provide a safety protocol for overtaking maneuvers. Applying this method to overtaking data from the South Channel shows that it effectively characterizes both the permissible overtaking space and the driver’s overtaking desire. Additionally, it enables the prediction of optimal overtaking timing and strategies based on short-term trajectory forecasts. Thus, this method not only offers a safe overtaking plan for vessels but also provides auxiliary information for decision making in intelligent ship navigation.

Modulation of Deformation by Magmatic Tempo, Coast Mountains Batholith, British Columbia, Canada

AbstractThe tectonomagmatic evolution of the southern Coast Mountains batholith, British Columbia reveals the relation of magmatic tempo, deformation, and relative plate motions which inform models for growth of continental crust and convergent plate dynamics. Magmatism in the southern batholith is episodic, derived primarily from the mantle, and reflects lower‐plate dynamics (Cecil et al., 2018, https://doi.org/10.1029/2018gc007874, 2021, https://doi.org/10.1130/ges02361.1). New field, structural, geochronologic, and geobarometric results from the southern batholith document three episodes of deformation, each spatially and temporally coincident with a high magmatic flux event (HFE). Sinistral faulting (<117–103 Ma) and penetrative deformation (110–90 Ma) occurred only within two distinct areas affected by a HFE at 114–102 Ma. Following a magmatic lull, crustal shortening occurred after 90 Ma and before 72 Ma within the region affected by a second HFE (85–70 Ma). After 70 Ma and before 53 Ma, >100 km of dextral slip occurred on the Coast shear zone and minor shortening affected 64–62 Ma plutons, overlapping with the 64–61 Ma HFE. Thus, at the crustal level exposed in the southern batholith, the timing and location of deformation is linked to magmatic tempo and the style of deformation varies through time. These results suggest that magmatic HFE modulate the timing and location of deformation while relative plate motions during HFE influence the style of deformation. Periods of deformation in batholiths may thus record high‐flux magmatic events and coeval plate motion but do not necessarily signal changes in plate motions.

On space–time diversity in shock train self-excited oscillation mode during wide-range evolution in a scramjet isolator

The shock train self-excited oscillation can induce combustor instabilities and reduce engine margin. In a dual-mode scramjet, the shock train undergoes a complete evolution process, exhibiting structural changes closely tied to this inherent unsteadiness. This study aims to elucidate the space–time diversity in shock train self-excited oscillation mode and the underlying mechanisms during wide-range evolution. The experimental investigations were conducted at Ma = 1.95, capturing the complete evolution of the shock train. The results indicate the evolution can be categorized into three regimes based on structural characteristics. In regime I, the shock region gradually forms, followed by the occurrence of the mixing region in regime II. Regime III corresponds to inlet unstart. In regime II, isolator outlet pressure fluctuations exhibit higher frequency and lower amplitude compared to regime I, while the shock motion demonstrates lower frequency and higher amplitude. The shock train behaves in a large-scale, low-frequency (1.53 times the duct height, 10 Hz) unsteady motion in regime II, posing a potential threat to engine operation. Coherence and phase analysis reveal the disturbance source originates downstream. Proper orthogonal decomposition modal analysis shows two oscillation modes: low-frequency components correspond to shock motion, and high-frequency components correspond to pressure fluctuations across the entire pseudoshock. The propagating of downstream disturbance differs between the two regimes. In regime I, the shock train exhibits rigid-body motion synchronously. In regime II, the relative motion between each shock wave and the cumulative effect of pressure disturbance lead to frequency decay upstream, amplifying the shock train motion.

Sliding plane formalism for aeroacoustic and adjoint-based sensitivity calculations

This paper demonstrates a methodology for time-domain and time-accurate nonlinear, direct and adjoint simulations of unsteady flows and aeroacoustics for multi-component systems in relative motion. Here, the principal effort is directed towards mitigating the problem of distortion and contamination of the adjoint field at the moving interface, through a computationally lightweight, high-order sliding plane approach for which the adjoint equivalent is simple to obtain. This effort requires an attentive treatment of the interface conditions that surpasses the requirements of the more common forward (primary) problem. Sensitivity of a given quantity of interest from a time-varying flow with respect to a large number of parameters is then obtained through the adjoint operator, which is evaluated using nonlinear-adjoint looping. This technique is implemented using checkpointing and the PETSc TSAdjoint library and, after validation, applications including a rotor–stator interaction problem are presented.

Brittle Regime Slip Partitioned Damage and Deformation Mechanisms Along the Eastern Denali Fault Zone in Southwestern, Yukon

AbstractRare bedrock exposures of the eastern Denali fault zone in southwestern Yukon allow for the measurement, sampling, and analyses of brittle regime fault slip data and deformation mechanisms to explore relations to far field, oblique plate motions. Host rock lithologies and associated slip surfaces show episodic damage zone‐related deformation and calcite ± hematite ± chlorite related hydrothermal fluid flow. This regional scale network of asymmetric fault damage is spatially and kinematically linked to a discrete and narrow fault core. Fault network observations, orientations, slip data, and strain inversions document a slip partitioned strike‐slip fault system with locally and mutually overprinting strike‐, oblique‐, and dip‐slip components. Microstructural analyses reveal crystal plastic and co‐seismic brittle deformation mechanisms active in a narrow range of upper crustal temperature, pressure, fluid, and chemical conditions. The net damage related slip is not exclusively formed by a single kinematic system, but rather a fully partitioned, time integrated system likely operative for much of the fault's brittle regime evolution temporally constrained by previously published thermochronometric data. Although the fault slip data was collected from outcrop‐scale exposures at sites tens of kilometers apart, results show remarkable correlation between fault kinematics and plate motions along the ∼580 km long eastern Denali fault segment. End member, subhorizontal, northeast directed reverse and north directed dextral strike slip fault strain axes closely reflect relative plate motion interactions over at least the last 30 m.y. and act as a proxy for far‐field stresses compatible with the kinematics of the damage zone network.

Spectral and timing properties of the accreting millisecond X-ray pulsar IGR J17498−2921 during its 2023 outburst

We present a comprehensive study of the spectral properties of the accreting millisecond X-ray pulsar IGR J17498−2921 during its 2023 outburst. Similar to other accreting millisecond X-ray pulsars, the broadband spectral emission observed quasi-simultaneously by NICER and NuSTAR is well described by an absorbed Comptonized emission with an electron temperature of ∼17 keV plus a disk reflection component. The broadening of the disk reflection spectral features, such as a prominent iron emission line at 6.4–6.7 keV, is consistent with the relativistic motion of matter in a disk truncated at ∼21 Rg from the source, near the Keplerian corotation radius. From the high-cadence monitoring data obtained with NICER, we observed that the evolution of the photon index and the temperature of seed photons tracks variations in the X-ray flux. This is particularly evident close to a sudden ∼–0.25 cycle jump in the pulse phase, which occurs immediately following an X-ray flux flare and a drop in the pulse amplitude below the 3σ detection threshold. We also report on the non-detection of optical pulsations with TNG/SiFAP2 from the highly absorbed optical counterpart.

Fast and Accurate Relative Motion Tracking for Dual Industrial Robots

Fast and Accurate Relative Motion Tracking for Dual Industrial Robots

Effects of argon plasma treatment on carbon fiber surface characteric and its reinforcing polyimide composites interfacial properties at room and elevated temperatures

In high temperature conditions, this can cause considerable changes in the mechanical properties of the composite. In order to determine the structural and mechanical property changes that occur in composite materials at elevated temperatures, to elucidate the damage mechanisms at elevated temperatures, and to improve the stability and durability of the materials. This paper studies how plasma treatment time affects the surface polarity, roughness, wettability and mechanical properties of carbon fibers. The results showed that the best wettability was attained after 10 min of plasma treatment, and new oxygen-containing functional groups (COO- and -C=O) developed on the fiber surface. The fundamental explanation is that during plasma surface treatment, the CH group in the bisphenol A part chain segment present in the epoxy resin sizing agent's composition is oxidized, forming an organic oxide layer. In this paper, the plasma modification technology was utilized to improve the interfacial compatibility of carbon fiber and Polyimide (PI) resin, and the interlaminar shear strength reached 103.98 MPa, up 10.49 %, and the strength retention rate was 84.3 % at 300 °C. In this paper, the plasma modification technique was employed to increase the interfacial compatibility of carbon fiber and PI resin. It was found that the Inductively Coupled Plasma (ICP) treated carbon fiber surfaces underwent physical and chemical changes that effectively enhanced the interfacial compatibility with the resin. However, the chemical groups and physically etched regions on the surface of the plasma-modified fibers are able to impede the relative motion of the resin to a certain extent, thus improving the interfacial strength.

Coupling analysis between wave resonance in the moonpool and heave motion of the twin hulls with mooring effect

Fluid resonance in the moonpool formed by two identical rectangular hulls in water waves is investigated by employing the Computational Fluid Dynamics (CFD) package OpenFOAMⓇ. The influence of vertical stiffness on the behavior of moonpool resonance coupling with the heave motion response is presented. Numerical simulations show that the free surface oscillation in the moonpool exhibits a two-peak variation with the incident wave frequency, defined as the first and second peak frequencies. A local Keulegan–Carpenter (KC) number is introduced for describing the influence of fluid viscosity and flow rotation on the fluid resonance and heave motion resonance. At the first peak frequency, the free surface oscillation and heave motion response show an in-phase relationship, where increase in the vertical stiffness can increase the relative motion between them. This finally leads to an increase in the KC number, indicating the increased effect of energy dissipation with increase in the vertical stiffness. At the second peak frequency with an out-of-phase relationship between the free surface oscillation and heave motion response, the variation of the KC number is not sensitive to the vertical stiffness. Correspondingly, the influence of energy dissipation is not strongly dependent on the vertical stiffness.

Multi-body dynamic assessment of vessel-floater accessibility for floating wind

Abstract The floating wind industry faces several challenges in the road map to full maturity of commercial-scale floating projects. Accessing the floating offshore wind turbines poses larger risks due to the added degrees of freedom of the platform, with respect to bottom-fixed technology. When a vessel approaches, the platform’s motion leads to the introduction of multi-body dynamic effects. This work deals with the modeling of the vessel-platform dynamics for the VolturnUS-S 15MW platform and IEA15MW Reference Wind Turbine and a generic Crew Transfer Vessel (CTV) by means of performing a fully coupled assessment. The hydrodynamic properties are computed by solving the multi-body radiation-difraction problem. The inertial and hydrodynamic properties are included in an OrcaFlex model to perform a time-domain analysis under several sea states. A P-controller represents the helmsman pushing the vessel against the access point. The contact forces are modeled using a linear fender system to represent slip-and-stick events during the CTV’s push-on maneuver. The resulting relative motions are analyzed, and industry-typical limits are considered to quantify accessibility levels. The results show that peak wave period and wave heading play a major role in the accessibility scores.