Friction Control Research Articles

3D printing of micro/nano-hierarchical structures with various structural stiffness for controlling friction and deformation

Micro/nano-hierarchical structures are the building blocks of textured functional surfaces. Having control over the 3D shapes of these building blocks will lead to better control over their friction and deformation when in contact with other surfaces, paving the way for better-engineered surfaces. In this study, a high-resolution two-photon lithography additive manufacturing technique was utilized to produce micro/nano-hierarchical structures composed of nanohairs on top of micropillars. Varying the tapering angle of the micropillar and the length of the nanohairs enabled control over the effective stiffness of the micro/nano-hierarchical structures. Individual micro/nano-hierarchical structures were subjected to normal and tangential loading inside a scanning electron microscope, which enabled in-situ monitoring of the structural deformation in addition to their frictional response. This study revealed that changes in the structure stiffness by varying the tapering angle resulted in changes in the onset of sliding motion, friction force, and coupling between the deformation of the nanohair and the micropillar, thereby providing a new direction for friction and deformation control by tailoring structure stiffness through 3D printing.

Hybrid transmission of a multi-axle wheeled vehicle

The features of the formation of traction and dynamic characteristics of multi-axle wheeled transport vehicles are analyzed. The prospects for the use of electromechanical (hybrid) transmissions of various types as part of the chassis of transport and transport-traction wheeled multi-axle vehicles are analyzed. The principles for constructing kinematic diagrams of hybrid transmissions are proposed, allowing for complete control over changes in the traction forces of individual drive wheels when using the principle of parallel and parallel-series hybrids. Examples of kinematic diagrams of inter-axle and inter-wheel controlled power distribution mechanisms are considered, as well as principles for determining and implementing the values of their external parameters. Attention is paid to the issues of control technology for such mechanisms based on the principle of controlled slipping of the disc friction control element, as well as the prospect of localizing the production of components and assemblies of such transmissions at enterprises of the Russian Federation. Keywords transport vehicle, transmission, power distribution mechanism, mobility, controllability, motion stability

Rubber friction control method focusing on mechanical interlocking between surface roughness and internally dispersed hard particles

Rubber friction control method focusing on mechanical interlocking between surface roughness and internally dispersed hard particles

Seismic Performance of Irregular Building with different Variable sliding isolators and Semi active Dampers

The comparative performances of semiactive friction and stiffness dampers with different control laws in the base-isolated Irregular building subjected to bi-directionally acting strong earthquakes have been studied. The Irregular building is hybridly isolated with rubber bearings and friction pendulum system (FPS) or variable frequency pendulum isolator (VFPI) or variable curvature friction pendulum system (VCFPS). The shear type base-isolated Irregular building is modeled as three-dimensional linear elastic structure having three degrees-of-freedom at each floor level. Time domain dynamic analysis of the building has been carried out with the help of constant average acceleration Newmark-Beta method and non-linear isolation forces has been taken care by fourth-order Runge-Kutta method. The effects of variation of characteristic properties of semiactive dampers on their hysteresis loops and on the structural response of Irregular building is studied through parametric study. Comparative performances of different semiactive dampers with sliding isolation systems for seismic control of Irregular building have been observed through time history plots and peak response performance indices. It has been found that semiactive electromagnetic friction damper work efficiently with VFPI and VCFPS in comparison FPS for the Irregular building for near field earthquakes as it not only reduces base displacement at lower control force but also give lower base shear and story drift in base-isolated Irregular building. The control laws based on modulated homogeneous friction control semiactive friction dampers better than the predictive control law.

Hysteretic behaviour and structural control performance of a piezoelectric friction damper

This study proposes a novel piezoelectric friction damper (PFD). Experiments and numerical simulations are carried out to investigate the mechanical model of the PFD, and the effects of loading speed, displacement amplitude, voltage, friction plate material, piezoelectric stack actuator dimensions, and spring parameters on the hysteretic characteristics of the PFD. Results show that the PFD attains a stable mechanical performance within the design loading speed and displacement amplitude. The piezoelectric stack actuator functioning as a friction control device, can increase friction by 54% at 120 V. In damper design, an appropriate increase of the constraint stiffness of the damper and a piezoelectric stack actuator with the same stiffness as the damper constraint can achieve greater output force, and the preload of the spring should be greater than the friction to prevent residual displacement. The PFD has better seismic performance for large-span cable dome structures than the traditional friction damper. The vertical peak displacement and acceleration reduction ratio reached 45.1% and 67.7% with PFDs, respectively. The PFD with semi-active control is suitable for large-span spatial structures.

Development of soft inchworm robot with friction control of feet using double-network gel

In this study, we investigate the development of an inchworm-inspired soft robot with active friction control of its feet. The focus is on its design, modeling, and locomotion principle. Friction control is achieved dynamically by using a unique material called double-network hydrogel (DN gel) to form the legs of the inchworm robot. DN gels are soft but tough materials, and they exhibit the unique characteristic of frictional difference on the surface under an applied voltage. This property is exploited to realize locomotion. A wide range of robot speeds can be realized through appropriate control of the voltage applied to its legs. Motion strategies with simultaneous control of actuation force and leg friction allow for greater flexibility in locomotion. The behavior of inchworm locomotion with change in the frictional difference between legs and actuator force was predicted using the developed mathematical model and was confirmed through experiments. Three different inchworm robot prototypes were developed and studied. The inchworm robot is characterized by low cost and inherent compliance, and it is easily disposable.

Temperature-dependent friction coefficient on flat graphite plane

Investigating the frictional properties of two-dimensional layered materials with dynamic evolution of contact quality advances understanding the mechanism underlying puckering effects, offering a new pathway for the rational control of friction. In this letter, friction force microscopy (FFM) in ultra-high vacuum (UHV) conditions has been used to probe the nanoscale friction on a freshly cleaved flat graphite surface (0001) as a function of normal force and surface temperature. It is found that friction force shows a nonmonotonic change with the decrease of temperature with the maximum friction appeared around Tmax=140±5 K. Besides, coefficients of friction (COFs) defined as the slope of friction-normal force curves remains nearly constant before the friction peak while surprisingly change into negative values with the further decrease of temperature. The loading–unloading results reveal that, the abnormal COFs behavior originates from the temperature induced contact quality evolution which leads to the ripple formation at cryogenic temperatures in the front of scanning tip. The results reported here provide a deep insight into the effects of temperature on the frictional behaviors of graphite, which give a new thrust in the field of friction on lamellar materials.

Active control of friction realized by vibrational excitation: Numerical simulation based on the Prandtl-Tomlinson model and molecular dynamics

Superlubricity and active friction control have been extensively researched in order to reduce the consumption of fossil energy, the failure of moving parts, and the waste of materials. The vibration-induced superlubricity (VIS) presents a promising solution for friction reduction since it does not require high-standard environment. However, the mechanism underlying the VIS remains unclear since the atomic-scale information in a buried interface is unavailable to experimental methods. In this paper, the mechanism of VIS was examined via numerical calculation based on the Prandtl—Tomlinson (PT) model and molecular dynamics (MD) simulations. The results revealed that the pushing effect of stick—slip is one of the direct sources of friction reduction ability under vibrational excitation, which was affected by the response amplitude, frequency, and the trace of the tip. Moreover, the proportion of this pushing effect could be modulated by changing the phase difference when applying coupled vibrational excitation in x- and z-axis. This results in a significant change in friction reduction ability with phase. By this way, active friction control from the stick—superlubricity can be achieved conveniently.

Exploiting surface textures dynamics for dry friction control

We study the dynamic behavior of a lattice of bristle-like elastic elements disposed at the interface between a rigid still substrate and a rigid sliding slab, in steady conditions. Due to normal and frictional interactions with the moving slab, complex bristles dynamics occur, which may eventually alter the overall frictional response of the structured interface. Indeed, up to three main mechanisms of friction control can be identified, depending on the specific bristles dynamics: the relative velocity-dependent modulation of local friction force; the misalignment between the local relative velocity and the slab velocity, due to the emergence of transverse vibration; the local friction coefficient variation due to the normal load acting on the bristle. Results show that, depending on the interface dynamic properties (i.e., bristles stiffness, normal load, slab velocity, etc.), a significant reduction of the friction force opposing the slab motion can be achieved, also involving self-excited bristle vibration. Since the present formulation is scale independent, this result may suggest possible mechanisms of friction control in different practical application fields, ranging from bio-inspired micro-structured interfaces to macro-scale features, such as brush seals in electric motors.

Modeling, Identification, and Compensation Control of Friction for a Novel Dual-Drive Hydrostatic Lead Screw Micro-Feed System

This paper investigates the transmission performance of a novel dual-drive hydrostatic lead screw micro-nano feed system (DDHLS) that can obtain extremely low speed. Firstly, the oil film liquid friction of hydrostatic transmission is modelled, and the calculation model of oil film dynamic friction is proposed based on the variable viscosity theory. Secondly, on this basis, combined with the LuGre friction model, a novel all-components refinement friction identification method (ACRFIM) for DDHLS was developed. The friction parameters of the feed drive components such as LM guide and hydrostatic lead screw can be identified independently using the proposed method, ensuring precise friction force modelling in all components. Then, an all-component adaptive friction compensation control algorithm (AACA) was designed by introducing the temperature and disturbance influence factors into the friction model and considering the influence of the dynamic friction of liquid. The experiments illustrate that the calculation accuracy of the oil film friction model based on the variable viscosity theory is substantially improved. DDHLS can effectively suppress the adverse effects of nonlinear friction, and the proposed AACA has an obvious compensation effect for the friction of the time-varying system.

Friction control of elastic materials on glass by means of textured surfaces

To investigate the friction behaviors of elastomer and polyacetal writing tips sliding on various textured glass surfaces, the influences of the pitch size and height of sub-millimeter to millimeter sized texture on friction were examined via reciprocating friction tests. The friction coefficients of each writing tip could be systematically varied by changing the pitch and height of the texture. These changes in friction were based on the relationship between the convex-concave shapes and the contact parts of the writing tip, and hence, influence the adhesive, abrasive, and deformation frictions. By inducing a surface texture with a pitch smaller than the contact area of the writing tip, the friction coefficient could be reduced effectively. By inducing a surface texture with a larger height, the friction coefficient of the elastomer could be increased due to deformation friction. These behaviors indicate the possibility of controlling the friction by changing the parameters such as the pitch and height of the textured glass surfaces.

Experimental study on flat plate skin friction control by porous media based on global fluorescent oil film measurement technology

Skin friction is a primary source of total aircraft drag. It is important, therefore, in science and engineering, to achieve drag reduction control in a boundary layer. In this paper, under the experimental conditions of Reθ = 5909 (x/c = 0.55) and with a zero-pressure gradient, the drag reduction control of a plate boundary layer in porous media is studied. The global skin friction of the plate is measured using fluorescent oil film test technology. The results show that, in contrast with the downstream frictional resistance coefficient of a flat plate that possesses a smooth surface, the coefficient for porous media reduced significantly. Also, the lower the pores per inch (PPI) of the porous media, the greater the drag reduction effect. Among the three porous media with different PPI, porous media with ten PPI has the best drag reduction effect. With increasing distance from the porous media, the drag reduction effect decreases gradually. Porous media significantly increase the slope of the logarithmic region of the velocity profile of the downstream turbulent boundary layer, the dimensionless wall velocity u+ moves upward, and the velocity pulsation in the logarithmic region increases so as to reduce skin friction.

Controlling high-speed dry friction through the geometry of micro-patterned asperities

Advances in manufacturing technologies have provided means to create surface textures with regular patterns of uniform asperities, leading to the potential for improved control of friction. In order to design surface topologies that induce desirable tribological effects, an understanding of the influences of the geometric features of asperities on measures of frictional resistance is required. Dynamic elastic-plastic finite element modeling methods, which included material damage and failure, were used to study the interactions of directly modeled 100 micron rib-like asperities on two deformable aluminum blocks. The relationships between the mechanics of the deformation and failure of five unique asperity geometries, the coefficients of static and kinetic friction, and the system energy stored and dissipated were studied under dry, high-load rate conditions, where motion was initiated in under 1 ms and acceleration approached 100 kG. Influences of the geometric features of the asperities were explored using semi-circular, triangular, and square-shaped cross-sectional profiles and evaluated for complex geometries created by combinations of these basic shapes. Static coefficients of friction were found to vary more than two-fold with asperity geometry based on the contact area normal direction. The study found that it was also possible to maintain the static friction coefficient but more than triple the force to initiation motion simply by changing the asperity shape. While kinetic friction coefficients were less influenced by asperity shape for the high-speed conditions studied, the geometric characteristics directed the way an asperity deformed under load and the extent of the material failure during sliding. A more than four-fold variation in energy stored within the system and over an order of magnitude variation in energy dissipated by the system was found for the geometries examined. This study demonstrates the importance of understanding the mechanical behavior of the asperity when designing surface textures to tailor dry, high-speed friction.

Tuning super-lubricity via molecular adsorption

The state of vanishing friction known as super-lubricity is highly desirable to reduce parasitic energy loss. However, without efficient means to turn on and off the super-lubricity, it would be difficult to realize its full potential in engineering applications. Here we report that the nanoscale super-lubricity between amorphous silica tip and graphite basal plane can be tuned by physisorbing various molecules from the environment. In particular, the super-low fiction can be further reduced by about 54% in n-pentanol vapor, but increases by around 25 and 45 times in water and phenol vapor respectively. The vapor molecules influence the friction by adsorbing onto the silica surface and being entrained into the sliding interface. Based on the structures of adsorbed molecules, it could be deduced that adsorbates with high conformational entropy can suppress the interfacial commensuration and enhance the super-lubricity, while the ones that can facilitate commensurate interactions with the graphite surface increase friction. This finding provides the basic principle enabling on-demand control of friction into and out of the super-lubricious state of solid interface.

Reducing Migration of Knee Exoskeletons With Dynamic Waist Strap

Downward migration of knee exoskeletons under external forces is one major concern against their normal operations. It may be reduced by increasing the friction between exoskeletons and the human thigh. However, the effectiveness of friction control remains questionable, as the natural inverted-cone shape of the human thigh will aggravate downward migration, and the overwhelming strapping intensity will degrade the activation level of muscles. In this paper, we propose a new suspension system called the Dynamic Waist Strap. Theoretical analysis and experimental validations on multiple subjects are conducted to show its advantages over the three mainstream suspension systems commonly used for knee exoskeletons in terms of the reduction of migration, interaction torque, and maximum dip angle. This study highlights the importance of the suspension system when attaching a knee exoskeleton to the human and introduces a new dynamic interaction interface to improve the coupling from a knee exoskeleton to an individual.

Theoretical and experimental study of motion suppression and friction reduction of rotor systems with active hybrid fluid-film bearings

Active bearings (including fluid film bearing) are efficient in reducing vibration and improving dynamic performances of rotary machinery. The control systems have significant influence in the working processes of rotor-bearing systems, including hydrodynamic and tribological performance. The aim of the study is to investigate the feasibility of the joint controlling of the rotor vibration and simultaneously minimizing friction losses in active hybrid bearing (AHB) – rotor systems. The theoretical and experimental results show the rotor vibrations are reduced at the energy-efficient operating modes under complex loads. The theoretical study is performed by the verified simulation models of rigid rotors with thrust and journal AHB. The artificial neural networks are proposed to calculate the bearing forces to provide a balance of accuracy and computational costs. The controller for the thrust AHB is based on the PI law, while the journal AHB controller implements an adaptive nonlinear proportional law with integrating properties. Both controllers perform well in reducing the vibration magnitudes for the systems under the periodic and constant load forces. Such rotor motion control is also able to reduce power losses caused by the viscous friction in AHB. The theoretical study shows the relations between the position of the rotor with AHBs and the magnitude of the viscou forces of the lubricant film. The minimum friction areas could be used to choose the setpoints for AHBs’ controllers for joint rotor motion and friction control.

A Review of Key Technologies for Friction Nonlinearity in an Electro-Hydraulic Servo System

In a high-precision servo system, the nonlinear friction link is the key factor affecting the system performance. Reasonable solving of the friction link in servo systems has become a focus of current research. This paper summarizes the friction nonlinearity that affects the control performance of servo systems. First, the characteristics of friction are summarized, and the advantages and disadvantages of typical friction models in recent years are analyzed. Subsequently, existing friction model parameter identification methods are introduced and evaluated. On this basis, the development level of the friction nonlinear control strategy is analyzed from three aspects: friction model-based control, friction model-free control, and compound control. Finally, the objective advantages and disadvantages of the existing technology are summarized, and the future development direction of the friction model and selection reference for the nonlinear friction control strategy are comprehensively discussed.

Study of the Running-In Period in the Twin-Disc Wear Test Using Steel From a Class C Forged Railway Wheel

Abstract The railway wheels are one of the most critical components of the train and, any problem that may arise in the wheel, can generate numerous consequences. The ability to manage friction at the wheel–rail interface currently represents one of the greatest challenges and one of the most powerful tools in railway engineering. To define the efficiency of a product for friction control, retentivity tests are performed mainly by the suppliers. However, there is no strict standardization for the tribological test performance, and some critical issues like the influence of running-in period and the necessary superficial conditions of the rolling track to apply the friction control products are neglected. Thus, the objective of this work is to conduct dry twin-disc wear tests with different number of cycles to understand the running-in period and verify the tribosystem behavior. Starting from a rolling track with a polished surface, it was noted that the roughness increased rapidly and stabilized around 1500 cycles. The coefficient of traction (COT) increased at the beginning of the tests reaching the maximum at 750 cycles, reducing the value after that, and stabilizing at a value close to 0.4 with 5000 cycles. Thus, for twin-disc tests using these conditions, it is suggesting the application of friction control products from 5000 cycles because the COT and roughness would be stabilized.

Spatiotemporally Programmable Surfaces via Viscoelastic Shell Snapping

Many species can dynamically alter their skin textures to enhance their motility and survivability. Despite the enormous efforts on designing bio‐inspired materials with tunable surface textures, developing spatiotemporally programmable and reconfigurable textural morphing without complex control remains challenging. Herein, a design strategy is proposed to achieve surfaces with such properties. The surfaces comprise an array of unit cells with broadly tailored temporal responses. By arranging the unit cells differently, the surfaces can exhibit various spatiotemporal responses, which can be easily reconfigured by disassembling and rearranging the unit cells. Specifically, viscoelastic shells as the unit cells is adopted, which can be pneumatically actuated to a concave state, and recover the initial convex state sometime after the load is removed. It is shown computationally and experimentally that the recovery time can be widely tuned by the geometry and material viscoelasticity of the shells. By assembling such shells with different recovery times, surfaces with pre‐programmed spatiotemporal textural morphing under simple pneumatic actuation is built, and temporal evolution of patterns, such as digit numbers and emoji, and spatiotemporal control of friction are demonstrated. This work opens up new avenues in designing spatiotemporal morphing surfaces that could be employed for programming mechanical, optical, and electrical properties. A preprint version of the article can be found at: https://www.authorea.com/doi/full/10.22541/au.164020946.62560710.

Stabilization Control of Underactuated Spring-Coupled Three-Link Horizontal Manipulator Based on Energy Absorption Idea

A spring-coupled three-link horizontal manipulator (STHM) is an underactuated mechanical system that possesses two control inputs and three degrees of freedom (DOF). This paper discusses the stabilization control problem for this multi-DOF underactuated system. By using an energy-absorbing idea, we design two types of virtual friction controllers: PsD controller and PD controller. Additionally, the stability of the control system is analyzed based on Lyapunov theory and LaSalle’s invariance principle. The design of the stabilizing controller in this paper makes good use of the physical characteristics of the STHM system. The design process of the whole control system is simple. Numerical examples demonstrate the validity and superiority of our developed control strategy.