Asymmetric Friction Research Articles

Insect-Scale Biped Robots Based on Asymmetrical Friction Effect Induced by Magnetic Torque.

Multimodal and controllable locomotion in complex terrain is of great importance for practical applications of insect-scale robots. Robust locomotion plays a particularly critical role. In this study, a locomotion mechanism for magnetic robots based on asymmetrical friction effect induced by magnetic torque is revealed and defined. The defined mechanism overcomes the design constraints imposed by both robot and substrate structures, enabling the realization of multimodal locomotion on complex terrains. Drawing inspiration from human walking and running locomotion, a biped robot based on the mechanism is proposed, which not only exhibits rapid locomotion across substrates with varying friction coefficients but also achieves precise locomotion along patterned trajectories through programmed controlling. Furthermore, apart from its exceptional locomotive capabilities, the biped robot demonstrates remarkable robustness in terms of load-carrying and weight-bearing performance. The presented locomotion and mechanism herein introduce a novel concept for designing magnetic robots while offering extensive possibilities for practical applications in insect-scale robotics.

Investigating the feasibility of a novel asymmetrical friction damper

This paper presents a novel asymmetrical self-locking mechanism (ASLM) and compact asymmetrical friction dampers (AFDs) designed to enhance serviceability in vibration control for structures. The proposed ASLM is based on frictional surfaces that are engage to provide the necessary damping feature. Simplified and detailed theoretical models are developed and utilized for the parametric study of the proposed damper. A comprehensive parametric study demonstrates a near-linear relationship between the slope angle of the AFD and its corresponding damping force when the slope angle is less than 45 degrees. Additionally, the proposed damper is fabricated based on the parametric study. Computational and theoretical models are also validated experimentally, with energy dissipation errors below 10%. Moreover, the practical applicability and effectiveness of the proposed AFD are evaluated by implementing it on a benchmark structure. Through non-linear time history analysis, a significant reduction of 17.2 % in interstory drift and nearly perfect in residual interstory drift are observed compared to the control group. These results highlight the potential of the novel ASLM and AFD to enhance vibration control efficiency in diverse engineering applications.

Tribo-Electric Analysis of Multi-Cycle Wear in Asymmetric Laser-Structured Cu-Sn Connectors

One of the key objectives of the connector industry for the past years has been to reduce the insertion friction forces of detachable electrical connections, where the majority of approaches combine a particular connector design with the usage of lubricants. The necessity for a solution that uses less contact force to maintain a lasting electrically conductive connection without sacrificing the mechanical robustness of the connection during use is a significant challenge in this situation. The approach in this work is to create surfaces supporting asymmetric friction, meaning lower insertion than removal forces. By Direct Laser Interference Patterning (DLIP), asymmetric saw-tooth structures with different structural inclinations and periodicities were generated on Sn-coated Cu plates. It is determined that topographical interlocking is the primary operating mechanism providing the required anisotropy. Multiple insertion/removal cycles were simulated on the tailored surfaces to tribologically characterize them and to determine the evolution of the electrical contact resistance at the end of each insertion motion. The vast majority of the generated structures showed an improvement compared to the reference state, with lower insertion and higher removal forces while the electrical behavior is not significantly impacted.

The Role of Friction in the LISA-Pathfinder Release Mechanism Anomaly

Release mechanisms are crucial devices for the success of space missions. In LISA, the first gravitational waves observatory in space, the grabbing positioning and release mechanism (GPRM) is responsible for setting all the test masses (TMs) in the free-fall condition necessary for starting science observations. This mechanism is considered a key equipment for the success of the entire space mission. The GPRM was initially tested in LISA Pathfinder (LPF), a space mission flown between 2015 and 2017, and an anomaly was encountered in the TM velocity after the release, which also resulted in some failures. One of the suspected reasons of this behavior is that the trajectory of the release mechanism deviates from the nominally rectilinear one. We find that this anomaly is due to the presence of high and asymmetric friction between moving parts. In this paper, we characterize experimentally the deviation from the linear trajectory, using LPF qualification models of the mechanism available on ground, and we identify the mechanical features that determine the friction-driven spurious lateral motion via a numerical finite element model. The results are one of the basis to improve the design of the GPRM for LISA. Some risk-reduction strategies and design guidelines are also presented and discussed.

Reviving stochasticity: uncertainty in SMBH binary eccentricity is unavoidable

ABSTRACT We study supermassive black hole (SMBH) binary eccentricity of equal-mass galaxy mergers in N-body simulations with the ketju code, which combines the gadget-4 fast multipole gravity solver with accurate regularized integration and post-Newtonian corrections around SMBHs. In simulations with realistic, high-eccentricity galactic merger orbits, the hard binary eccentricity is found to be a non-linear function of the deflection angle in the SMBH orbit during the final, nearly radial close encounter between the SMBHs before they form a bound binary. This mapping between the deflection angle and the binary eccentricity has no apparent resolution dependence in our simulations spanning the resolution range of 1 × 105 to 8 × 106 particles per galaxy. The mapping is also captured using a simple model with an analytical potential, indicating that it is driven by the interplay between a smooth asymmetric stellar background potential and dynamical friction acting on the SMBHs. Due to the non-linearity of this mapping, in eccentric major merger configurations, small, parsec-scale variations in the merger orbit can result in binary eccentricities varying in nearly the full possible range between e = 0 and e = 1. In idealized simulations, such variations are caused by finite resolution effects, and convergence of the binary eccentricity can be achieved with increasing resolution. However, in real galaxies, other mechanisms such as nuclear gas and substructure that perturb the merger orbit are likely to be significant enough for the binary eccentricity to be effectively random. Our results indicate that the distribution of these effectively random eccentricities can be studied using even moderate resolution simulations.

Kinematic Behavior of an Untethered, Small-Scale Hydrogel-Based Soft Robot in Response to Magneto-Thermal Stimuli.

Fruit fly larvae, which exist widely in nature, achieve peristaltic motion via the contraction and elongation of their bodies and the asymmetric friction generated by the front and rear parts of their bodies when they are in contact with the ground. Herein, we report the development of an untethered, magnetic, temperature-sensitive hydrogel-based soft robot that mimics the asymmetric micro-patterns of fruit-fly-larvae gastropods and utilizes cyclic deformation to achieve directional peristaltic locomotion. Due to Néel relaxation losses of nanomagnetic Fe3O4 particles, the hydrogel-based soft robot is capable of converting changes in external alternating magnetic stimuli into contracting and expanding deformation responses which can be remotely controlled via a high-frequency alternating magnetic field (AMF) to realize periodic actuation. Furthermore, the Fe3O4 particles included in the hydrogel-based soft robot cause it to follow a gradient magnetic field in confined liquid environments and can be coupled with AMFs for the targeted release of water-soluble drugs or targeted magnetic hyperthermia therapy (MHT). We believe that such a controlled motion can enable highly targeted drug delivery, as well as vascular disease detection and thrombus removal tasks, without the use of invasive procedures.

A Multipole Magnetoactive Elastomer for Vibration-Driven Locomotion.

Smart materials such as magnetoactive elastomers (MAEs) combine elastic and magnetic properties that can be significantly changed in response to a magnetic field and therefore offer enormous potential for applications in both scientific research and engineering. When such an elastomer contains microsized hard magnetic particles, it can become an elastic magnet once magnetized in a strong magnetic field. This article studies a multipole MAE with the aim of utilizing it as an actuation element of vibration-driven locomotion robots. The elastomer beam has three magnetic poles overall with the same poles at the ends and possesses silicone bristles protruding from its underside. The quasi-static bending of the multipole elastomer in a uniform magnetic field is investigated experimentally. The theoretical model exploits the magnetic torque to describe the field-induced bending shapes. The unidirectional locomotion of the elastomeric bristle-bot is realized in two prototype designs using magnetic actuation of either an external or an integrated source of an alternating magnetic field. The motion principle is based on cyclic interplay of asymmetric friction and inertia forces caused by field-induced bending vibrations of the elastomer. The locomotion behavior of both prototypes shows a strong resonant dependency of the advancing speed on the frequency of applied magnetic actuation.

Room Temperature Strengthening and High-Temperature Superplasticity of Mg-Li-Al-Sr-Y Alloy Fabricated by Asymmetric Rolling and Friction Stir Processing

Magnesium-lithium alloy is the lightest alloy to date. To explore its room temperature strength and high-temperature ductility, a plate of a new fine-grained Mg-9.13Li-3.74Al-0.31Sr-0.11Y alloy was fabricated by asymmetric rolling, and the rolled plate was subjected to friction stir processing (FSP). The microstructure and mechanical properties at room and elevated temperatures were investigated by optical microscopy, X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and tensile tester. Grain refinement with an average grain size in the α-Mg phase of 1.65 μm and an average grain size in the β-Li phase of 4.24 μm was achieved in the water-cooled FSP alloy. For room temperature behavior, the ultimate tensile strength of 208 ± 4 MPa, yield strength of 193 ± 2 MPa, and elongation of 48.2% were obtained in the water-cooled FSP alloy. XRD and EDS analyses revealed that the present alloy consists of α-Mg and β-Li phases, Al2Y, Al4Sr, MgLi2Al, and AlLi intermetallic compounds. For high-temperature behavior, the maximum superplasticity or ductility of 416% was demonstrated in this fine-grained alloy with an average grain size of 10 μm at 573 K and 1.67 × 10-3 s-1. A power-law constitutive equation was established. The stress exponent was 2.29 (≈2) (strain rate sensitivity 0.44), and the deformation activation energy was 162.02 kJ/mol. This evidence confirmed that the dominant deformation mechanism at elevated temperatures is grain boundary and interphase boundary sliding controlled by lattice diffusion.

Frictional contact mechanics of asymmetric soft textures

Soft textures with asymmetric friction properties in shear against soft substrates have significant promise for many applications, particularly in the medical field. Here, we fabricate textures with directional friction properties and investigate the interaction between soft textures and soft substrates in shear. We experimentally determine that shear direction, texture stiffness, and presence of a lubricant affect coefficient of friction. Finite element models support experimental results and suggest underlying mechanisms. For soft, directional textures sheared against soft untextured substrates, unlubricated friction is primarily governed by adhesion and contact area while lubricated friction is primarily governed by a plowing effect of texture into the substrate.

Experimental study on low-damage performance of Sliding Hinge Joint

The Sliding Hinge Joint (SHJ) is a low damage beam-to-column joint primarily for moment resisting steel frames (MRSFs). It is intended to remain rigid during frequent earthquake events, while rotation occurs between the column and beam under design basis earthquake events. At the end of earthquake events, the joint is expected to seize up and become rigid again. However, residual strength and stiffness of the SHJ are considerably reduced even within design-level sliding. The objective of this study is to evaluate resilience and repairability of the SHJ by carrying out cyclic loading on a full-scale beam-to-column assembly considering both design-level sliding and post design-level sliding. Overall, the SHJ exhibited a predictable and stable hysteretic response. It was concluded that the SHJ experienced a 79.5% decrease in average moment resistance and a 59.4% decrease in elastic rotational stiffness with no repair actions taken. Resilience of the joint could be achieved by replacing and retightening the asymmetric friction connection (AFC) bolts to bring the joint back to an as-built condition. Moreover, a detailed finite element model was developed to further investigate the plastic strain distribution, frictional sliding and energy dissipation behaviours of the joint, and the low-damage performance was verified. Finally, a simplified analytical model based on the component method was proposed to predict the joint behaviour.

Encoder position feedback based indirect integral method for motor parameter identification subject to asymmetric friction

Servo motors have been widely used in the automation industry for many years. Its control performance will directly affect the quality of the produced products. For the design of the servo motor controller, accurate modeling and parameter estimation will be one of the key design steps. Although a considerable amount of literature has discussed the modeling and parameter identification of servo motors, most of them focus on symmetrical friction parameter models and assume that motor velocity information is available. Based on the practical experimental examination, the displacement movements of the servo motor driven by harmonic input appear to be a drift phenomenon, which concludes that the friction force should be asymmetric. Moreover, coarse encoder quantization error during the practical measurement is also a problem that causes noisy velocity and acceleration estimations. These measurement imperfections would lead to inaccuracy of parameter identification results. In order to solve these issues, this paper presents an asymmetric friction model and an indirect integral method (IIM). The asymmetric friction model is able to capture the nonlinear position drifting phenomenon. For the proposed IIM, the use of velocity information is avoided. Moreover, an optimization algorithm is developed to minimize the quantized output prediction. Compared with the direct difference method (DDM) and the filtered regression model (FRM) in the existing literature, the numerical simulations, as well as the experimental validations, reveal that the proposed IIM has better parameter estimation performance than both the DDM and the FRM.

Experimental study of precast self-centering concrete shear walls with external friction dampers

Self-centering walls perform relatively well in reducing structural damage and residual deformation owing to the joint rocking mechanism. Nevertheless, drawbacks of the self-centering walls were limited robustness and redundancy, thus sufficient dissipating capacity is essential for improving stiffness, reducing seismic response, and controlling structural collapse. Among all these dampers used in self-centering systems in previous studies, slip-friction dampers are highly preferred because of their simplicity, perfect rigid-plastic hysteresis characteristic, large stroke, and stable dissipating capacity. After the earthquake ceases, friction force can be eliminated by loosening the tightening bolts to help facilitate post-earthquake recovery, and it is convenient to be replaced or repaired. In the present study, experiments of asymmetric friction dampers with brass-steel interfaces were conducted. To obtain the optimal characteristics for stable dissipating, different configurations of slip-friction dampers were designed and tested. The friction dampers were further installed vertically at the base rocking joint of the self-centering shear walls, and the performance of the walls was studied under horizontal quasi-static cyclic loading. Application of the friction dampers significantly increases the energy dissipating capacity and the post-yield stiffness. The energy dissipating ratio improved up to 81.2% compared to rocking walls with no additional damping. Damage was limited to the brass shims in friction dampers, while wall specimens were able to exhibit up to 3.0% horizontal drift without cracks, degradation, and significant concrete damage at the boundary elements. An average maximum residual drift of both positive and negative directions observed at the end of the test was 0.415% at 3.0% horizontal drift, that major structural realignment is not required, and repair of the structure is practically feasible.

Seismic performance of a low-damage rocking column base joint along weak axis

This paper presents an experimental and analytical study on seismic performance of a low-damage rocking column base joint equipped with an asymmetric friction connection (AFC). Cyclic loading tests were conducted along weak axis of the column, including INITIAL, AFTERSHOCK and REPAIR cases. Compared with the INITIAL case, the joint exhibited a 2.5% decrease in average moment resistance and a 24.9% decrease in rotational stiffness in the AFTERSHOCK case without any repair taken. By re-installing and tightening the AFC bolts, the moment resistance was completely restored and the rotational stiffness could reach 84.1% of the INITIAL case, achieving resilience and reparability of the joint. Internal forces of the column were effectively limited by frictional sliding of the AFC, and the maximum local equivalent plastic strain at the column base was only 1.0% at a drift ratio of 3%. Axial force, as the main source of restoring forces, provides a favorable tendency for self-centering of the joint. In addition, a finite element (FE) model was developed to quantitatively evaluate damage and sliding behaviors of the joint, and the low-damage characteristics were verified. Finally, a simplified analytical model was proposed to guide the design of this low-damage joint.

Responses of Coastal Upwelling to Tidally Induced Bottom Friction Dynamics and Plume-Modulated Geostrophy: A Process-Oriented Modeling Study

Abstract We used a high-resolution cross-shelf two-dimensional numerical model to investigate the response of coastal wind-driven upwelling circulation to barotropic tidal forcing and lateral buoyant discharge over a broad continental shelf. We found that the tidally amplified asymmetric friction effect arising from the interaction between tidal and subtidal currents modulated the upwelling structure across the shelf. The interaction weakened the water outcropping (upwelling) in the inner shelf due to tidally amplified mixing, but enhanced cross-shore velocity offshore due to tidally induced asymmetric friction effect and nonlinear advection. The enhanced mixing changed the density in the bottom boundary layer and subsequently in the upwelling front, which eventually weakened the geostrophic alongshore flow. The mass and stratification inputs of the lateral buoyant discharge weakened or even reversed geostrophic dynamics for alongshore and upslope transports. The reversed cross-shore density and elevation gradient induced by the buoyant influx weakened the alongshore current and the associated bottom friction effect. The upslope cross-shore transport was reduced due to weakened alongshore flow and the associated bottom Ekman transport. The mass of buoyant influx compensated for the wind-driven offshore transport in the upper layer. The upwelling could be reversed to downwelling when the transport of lateral influx exceeded the wind-driven offshore transport. The responses of upwelling circulation to tidal and lateral buoyancy forcing highlighted in this process-oriented study are fundamental for interpreting more complex wind-driven shelf circulation.

Ensemble Deep Learning Ultimate Tensile Strength Classification Model for Weld Seam of Asymmetric Friction Stir Welding

Friction stir welding is a material processing technique used to combine dissimilar and similar materials. Ultimate tensile strength (UTS) is one of the most common objectives of welding, especially friction stir welding (FSW). Typically, destructive testing is utilized to measure the UTS of a welded seam. Testing for the UTS of a weld seam typically involves cutting the specimen and utilizing a machine capable of testing for UTS. In this study, an ensemble deep learning model was developed to classify the UTS of the FSW weld seam. Consequently, the model could classify the quality of the weld seam in relation to its UTS using only an image of the weld seam. Five distinct convolutional neural networks (CNNs) were employed to form the heterogeneous ensemble deep learning model in the proposed model. In addition, image segmentation, image augmentation, and an efficient decision fusion approach were implemented in the proposed model. To test the model, 1664 pictures of weld seams were created and tested using the model. The weld seam UTS quality was divided into three categories: below 70% (low quality), 70–85% (moderate quality), and above 85% (high quality) of the base material. AA5083 and AA5061 were the base materials used for this study. The computational results demonstrate that the accuracy of the suggested model is 96.23%, which is 0.35% to 8.91% greater than the accuracy of the literature’s most advanced CNN model.

Motion control of hydraulic actuators with nonlinear friction compensation: Applied to variable valve systems of diesel engine

Friction force plays an important role in motion control systems. Research shows that friction force is strongly nonlinear in most applications. And the nonlinear friction force has great influence on control performance, especially in motion control systems of hydraulic actuators. To improve control performance, many model-based controllers have been developed. In the past years, LuGre model has been widely used in most hydraulic actuators. However, for systems with one-way seals whose friction force is asymmetric between instroke and outstroke stage, LuGre model is no longer suitable. The paper developed a friction force test bench and studied the transient friction force of a hydraulic actuator which includes one-way seals. Experiment results show that friction force at instroke stage is bigger than that of outstroke stage for the variable valve actuator. Based on the experiment results, a bristle direction hypothesis is proposed. The bristle of the friction surface is thought to have an initial direction. For actuators with two-way seals or without seals, the bristle is thought to be perpendicular to the friction surface. And for actuators with one-way seals, the angle between bristles and friction surface is not 90° anymore. To apply the bristle direction hypothesis to controllers, a bristle direction coefficient is introduced to LuGre model to explain asymmetric friction behaviors. The simulation results have good agreements with experiment results. Based on the improved LuGre model, a backstepping controller is developed to compensate the nonlinear friction force. The friction force and system matched uncertainty are estimated by an extended state observer. Finally, some comparative experiments are implemented to verify the advantages of the improved LuGre model. The experiment results show that the improved LuGre model can help to reduce the engine valve lift tracking errors. At outstroke stage (engine valve close), the maximum absolute errors can be reduced by 40.6% to 65.0%, and the mean absolute errors can be reduced by 21.2% to 59.4%.

Effect of partial asymmetric friction on sliding over fractures

Asymmetric friction refers to different friction forces resisting sliding in opposite directions. This property is found to arise from material anisotropy that can form coupling between normal and shear stress. It is plausible that such material anisotropy can emerge as a result of fractures of asperities or micro-cracks that precedes shear fractures. This study considers a model with multiple blocks where some blocks have asymmetric friction, while others have conventional symmetric friction. This assembly is driven by an external surface that connects to each block through leaf springs in the spirit of the Burrige-Knoppof model. The driving force is considered to undergo harmonic oscillations. It is found that asymmetric friction can induce intermittent regions of tension and compression that can form fractures normal (tension) or parallel (compression) to the sliding surface. Asymmetric friction produces velocity spectra with a frequency falloff of 1/f2, where f is the frequency; these are observed in sliding over shear fracture produced by strain localisation in compression. Opposite to this, symmetric friction leads to the falloff of 1/f observed in sliding over pre-existing fractures or faults. This distinguishes sliding over pre-existing faults from sliding over fractures formed in compression. This model can shed light on the mechanisms of fault sliding and production of geological fractures observed near the faults, as well as to complement the micromechanics-based asperity models in the emerging fields such as surface morphing.

Prediction of the Ultimate Tensile Strength (UTS) of Asymmetric Friction Stir Welding Using Ensemble Machine Learning Methods

This research aims to develop ensemble machine-learning methods for forecasting the ultimate tensile strength (UTS) of friction stir welding (FSW). The substance utilized in the experiment was a mixture of aluminum alloys AA5083 and AA5061. An ensemble machine learning model was created to predict the UTS of the friction stir-welded seam, utilizing 11 FSW parameters as input factors and the UTS as a response variable. The proposed approach used the Gaussian process regression (GPR) and the support vector machine (SVM) model of machine learning to build the ensemble machine learning model. In addition, an efficient technique using a differential evolution algorithm to optimize the weight for the decision fusion was incorporated into the proposed model. The effectiveness of the model was evaluated using three datasets. The first and second datasets were divided into two groups, with 80% for the training dataset and 20% for the testing dataset, while the third dataset comprised the test data to validate the model’s accuracy. The computational results indicated that the proposed model provides more accurate forecasts than existing methods, such as random forest, gradient boosting, ADA boosting, and the original SVM and GPR, by 30.67, 49.18, 16.50, 48.87, and 49.33 %, respectively. In terms of prediction accuracy, the suggested technique for decision fusion surpasses unweighted average ensemble learning (UWE) by 10.32%.

An Impact Inertial Piezoelectric Actuator Designed by Means of the Asymmetric Friction

With simple structure and control, impact inertial piezoelectric actuators are one promising type for precision positioning with large working stroke and high resolution. However, their applications still face some challenges, especially the incompatibility between speed and resolution as well as the motion instability due to the overturning moment. Therefore, innovative design of impact inertial actuators is urgently required. In this article, by designing an asymmetric compliant mechanism with a thick end and a thin end to generate asymmetric friction between the driving feet and U-groove, a novel impact inertial piezoelectric actuator was proposed. Its structure design, working principle, and output performances were addressed in detail. The results indicated that the thick end and thin end had quite different stepping characteristics, and the ideal stepping characteristic with quite small backward motion was achieved at the thick end. The displacement-time curves confirmed the motion stability of the actuator for bidirectional motions, and the small difference in forward and reverse speeds was explored. This actuator achieved the maximum speed of 7311 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m/s, the resolution of 221 nm, vertical and horizontal loading capacities being over 20 N and 1.4 N, respectively. Performance comparison with some previous impact inertial actuators further demonstrated the advancement.

Self-Sustained Snapping Drives Autonomous Dancing and Motion in Free-Standing Wavy Rings.

Harnessing snapping, an instability phenomenon observed in nature (e.g., Venus flytraps), for autonomy has attracted growing interest in autonomous soft robots. However, achieving self-sustained snapping and snapping-driven autonomous motions in soft robots remains largely unexplored. Here, harnessing bistable, ribbon ring-like structures for realizing self-sustained snapping in a library of soft liquid-crystal elastomer wavy rings under constant thermal and photothermal actuation are reported. The self-sustained snapping induces continuous ring flipping that drives autonomous dancing or crawling motions on the ground and underwater. The 3D, free-standing wavy rings employ either a highly symmetric or symmetry-broken twisted shape with tunable geometric asymmetries. It is found that the former favors periodic self-dancing motion in place due to isotropic friction, while the latter shows a directional crawling motion along the predefined axis of symmetry during fabrication due to asymmetric friction. It shows that the crawling speed can be tuned by the geometric asymmetries with a peak speed achieved at the highest geometric asymmetry. Lastly, it is shown that the autonomous crawling ring can also adapt its body shape to pass through a confined space that is over 30% narrower than its body size.