Contact Motion Research Articles

A surface enveloping-assisted approach on cutting edge calculation and machining process simulation for skiving

As a particular two degree of freedom generating method, skiving is effective to manufacture the periodically distributed internal and external profiles, and its cutting edge design is a foundational problem. In order to avoid the singularity in analytical methods, this paper studied a discrete surface assisted universal method to identify the cutting edge of skiving cutter and simulate the machining process by discrete cutting points. Firstly, the kinematic model of skiving was constructed in aspects of the parametrical modeling of the cutter and workpiece as well as the machining configuration. The principle of cutting edge identification was investigated based on the conjugation process of skiving in the following. Then, the entire procedure for cutting edge calculation was presented in detail. Therefrom, according to the profile generation during cutting action, the skiving spatial contacts were analyzed through reasonable coordinate frame transformation. At last, an external skiving for involute gear was taken, the cutting edges for two kinds of rake flank were calculated, and the machining error between the machined profile and desired profile was compared to proof the correctness of calculated cutting edges. Besides, the analysis of the spatial contact motions for these cases validated the effectiveness and the practicality of the proposed method.

Critical Velocity of Controllability of Sliding Friction by Normal Oscillations for an Arbitrary Linear Rheology

The application of ultrasonic vibrations is an established procedure in industry in order to significantly reduce and control sliding friction. One of the main characteristics of this phenomenon is that, beyond a certain critical sliding velocity, the friction is no longer controllable—although oscillations are still being externally applied. an a previous series of related studies, closed-form solutions of the critical velocity have been derived with respect to pure elastic and specific viscoelastic models. In the present paper we present a universal formula of the critical velocity which is valid for arbitrary linear rheology. The derivation relies on the same theoretical basis of the previous studies, where the reduction of friction is ascribed to a stick-slip motion of the contact. Therefore, all previous results represent limiting and special cases of this universal equation. In the second part of this paper we pursue the numerical analysis of the previous studies by investigating the reduction of friction for a viscoelastic Kelvin material for the first time.

Solid-state lithium battery chemistries achieving high cycle performance at room temperature by a new garnet-based composite electrolyte

LiBr has been incorporated into Li6.25La3Zr2Al0.25O12 garnet matrix to develop a new garnet-based composite electrolyte through high-energy ball milling followed by solid-state reaction. Of importance to note in the composite materials is joint occurrence of anion doping at Li6.25La3Zr2Al0.25O12 bulks and lithiated interface at Li6.25La3Zr2Al0.25O12 grain boundaries. Lattice distortion based on Br− substitution at partial O2− sites and high densification resulted from mergence effect of LiBr have been detected in the composite electrolytes. Li-ion conductivity and transport kinetics are thereby enhanced after LiBr incorporation. Also, LiBr addition into Li6.25La3Zr2Al0.25O12 causes the appearance of finely microporous feature for the composite materials without compromising pellet densification, which can favor interfacial contact and Li-ion motion between the composite electrolytes and electrode. A full cell with a Li metal anode, the newly developed solid electrolyte, tiny amount of liquid electrolyte, and a LiCoO2 cathode has been assembled in sequence. High cycle stability of the full cell can be derived from capacity retention of 91.8% and high discharge capacity of 111.6 mAh g−1 after 70 cycles at room temperature. Consequently, favorable contributions of LiBr addition towards solid-state cell system can be further ascertained.

Researches on Tie Rod Ends Lubricated by Grease with TiO2 and ZrO2 Nanoparticles

The nanoparticles of some materials can be used successfully to improve tribological properties through decreasing both wear and friction borne out of contact between the contact surfaces of elements in different devices, particularly vehicles. Nanoparticles of TiO2 and ZrO2 were chosen as additives to the lithium grease lubricating the contact surfaces in tie rod ends. The object of study was the steel ball – the component of the tie rod end – mating with the polymer insert and lubricated with the pure lithium grease or containing the addition of pure TiO2, pure ZrO2 nanoparticles, with a 1%wt. Studies on friction were carried out using the tester allowing cyclical rotational motion and different loading of contact. Wear was investigated by driving a car, whose tie rod ends were analysed, on a fixed ‘eight’-shape track and with a fixed velocity pattern. The aim of the study was to obtain the values and waveforms of friction moment and wear versus cycles, loading and composition of lubricating grease. The waveforms of friction coefficient were obtained using the FEM model of the analysed contact zone. Based on the obtained waveforms, recommendations for the composition of additives for lithium grease were made.

Contact angle hysteresis and motion behaviors of a water nano-droplet on suspended graphene under temperature gradient

In the present work, the effect of temperature gradient on the behavior of a water nano-droplet resting on a suspended graphene was studied based on a non-equilibrium molecular dynamics simulation. The acquired results indicate that the applied temperature gradient to the suspended graphene drives the water nano-droplet to the colder region. The droplet accelerates its motion toward the cold reservoir as the temperature gradient is increased. In addition to the translational motion of the nano-droplet, the vortical motion of the water molecules was also observed. Contact angle analysis was also utilized to describe the directional motion of the nano-droplet. The translational motion of the droplet leads to the estimation of contact angle hysteresis through advancing and receding contact angles while the rotational motion resulted in the advancing and receding fronts being switched with one another through the simulation. The average displacement vector of the water molecules shows that parts of the droplet seem to stagnate while other parts rotate around them. The reason behind this particular behavior was studied based on interaction energy contours between a water molecule and the suspended graphene. The obtained data indicate that the rotational motion is in agreement with the migration of the water molecules to low interaction energy regions in order to avoid high interaction energy areas.

Elastohydrodynamic lubrication and wear modelling of the knee joint replacements with surface topography

This numerical study predicted wear of lubricated total knee replacements with the existing of textured surface and the possibility of surface designs to reduce wear. In the first part, a wear model of metal-on-polyethylene total knee replacement was developed. The medial and lateral knee compartments was accounted for separately, with the contact force and motion during walking cycles applied. An adapted Archard wear formula was employed where the wear factor was an exponential function of the `Lambda ratio' (film thickness to the average roughness). Wear of the soft bearing surface (polyethylene insert) was simulated with regularly geometry update until a steady-state wear observed. In the second part, the effect of surface topography of the knee replacements was investigated. The surface texturing techniques have shown promising benefit to machine components in many areas of engineering practice. The texture parameters were designed using the Taguchi method for the geometry, size, and distribution of the micro dimples. It was observed that the lateral compartment may benefit from surface texturing if dimples were properly designed, while the texturing showed hardly advantageous effect on the medial surface in terms of lubrication enhancement and wear reduction. Some results were presented in the 6th World Tribology Conference.

Inferring Object Properties with a Tactile-Sensing Array Given Varying Joint Stiffness and Velocity

Whole-arm tactile sensing enables a robot to sense contact and infer contact properties across its entire arm. Within this paper, we demonstrate that using data-driven methods, a humanoid robot can infer mechanical properties of objects from contact with its forearm during a simple reaching motion. A key issue is the extent to which the performance of data-driven methods can generalize to robot actions that differ from those used during training. To investigate this, we developed an idealized physics-based lumped element model of a robot with a compliant joint making contact with an object. Using this physics-based model, we performed experiments with varied robot, object and environment parameters. We also collected data from a tactile-sensing forearm on a real robot as it made contact with various objects during a simple reaching motion with varied arm velocities and joint stiffnesses. The robot used 1-nearest-neighbor (1-NN) classifiers, hidden Markov models (HMMs), and long short-term memory (LSTM) networks to infer two object properties (hard versus soft and moved versus unmoved) based on features of time-varying tactile sensor data (maximum force, contact area, and contact motion). We found that, in contrast to 1-NN, the performance of LSTMs (with sufficient data availability) and multivariate HMMs successfully generalized to new robot motions with distinct velocities and joint stiffnesses. Compared to single features, using multiple features gave the best results for both experiments with physics-based models and a real-robot.

Transparency performance improvement for multi-master multi-slave teleoperation systems with external force estimation

Cooperative teleoperation combines two traditional areas of robotics, that is, teleoperation and collaborative manipulation. Cooperative telerobotic systems consist of multiple pairs of master and slave robotic manipulators operating in a shared environment. The most common control frameworks for nonlinear systems, that is, Proportional Derivative (PD) controllers, possess considerable deficiency in contact motion. In this paper, a novel control scheme is proposed for a nonlinear bilateral cooperative teleoperation system with time delay. In addition to position and velocity signals, force signals are employed in the control strategy. This modification significantly enhances the poor transparency when the slave robots are in collision with the environment. To cope with external forces measurement, a modified force estimation algorithm is proposed to estimate human and environment forces. The closed loop stability of the nonlinear cooperative teleoperation system with the proposed control scheme is investigated using the Lyapunov theory. The main achievement of this research is the stability of the closed loop cooperative teleoperation system in the presence of estimated operator and environmental forces. In addition, it is theoretically and experimentally proved that force reflection occurs and transparency is improved at the same time. Experimental results demonstrate the efficiency of the presented control strategy in free motion as well as when the slave robots are in contact with the environment.

Electric connector assembly based on vision and impedance control using cable connector-feeding system

In this study, a method for the electric connector-mating process in a wiring harness assembly system is proposed. In connector-assembly tasks, grasping the electric connector is a crucial step. Therefore, a cable connector-feeding system is developed to align cable connectors so that they can be grasped straight and held firmly by the robot. After the grasping problems are solved, a tilt strategy with impedance control is used to overcome the position errors caused by the vision system. The location of the connector header (Female part of connector) is detected using visual servoing with markers. Then, impedance control is adopted to achieve compliant contact motion and minimize position/orientation errors. The performance of the proposed electric connector-assembly strategy is evaluated by a series of experiments using a six-degree of freedom industrial robot equipped with a force/torque sensor and an eye-in-hand camera.

Direct Formation of Lubricious and Wear-Protective Carbon Films from Phosphorus- and Sulfur-Free Oil-Soluble Additives

Extreme pressure (EP) lubricant additives form protective tribofilms at the site of contact using the heat and pressure of contact and relative motion. Common EP additives contain undesirable elements such as phosphorus and sulfur. A novel EP lubricant additive, which contains no phosphorus and sulfur, is presented for generating lubricious carbon films. The additive consists of a surface-active molecule with a metastable cycloalkane ring, which dissociates readily during tribological contact to form lubricious carbon films. Friction and wear performance of PAO4 with this additive under a range of loads and speeds were shown to be superior to that without the additive. Optical and scanning electron microscopy and Raman spectroscopy were used to analyze the tribofilms formed on post-test contact surfaces, providing direct evidence for the formation of carbon films. Quantitative kinetics for the carbon tribofilm formation was analyzed as a function of temperature and stress, from which the activation energy for carbon tribofilm formation was obtained.

Rotational Sliding Motion Generation for Humanoid Robot by Force Distribution in Each Contact Face

Recent studies have explored humanoid robots in contact with the environment in various ways. However, many of them assumed static rather than sliding contacts. Studies on humanoid shuffle motion planning have realized sliding motions, such as turning, but relied on quasi-static balance control. In this letter, we propose a dynamic balance control method for sliding contact motions. The proposed method consists of the distributed force contact constraint (D.F.C.C.), which describes rotational sliding contact constraints, and the slide friction control (S.F.C.), which controls humanoid dynamic balance based on the model predictive control by using the D.F.C.C. The D.F.C.C. segments a contact face into a grid of contact points and optimize the vertical component of the contact forces. This enables us to calculate the sliding friction forces at each contact point. The S.F.C. is the model predictive control for distributing contact forces to each contact face considering sliding frictional dynamics. The D.F.C.C. is simple and easy to apply to the S.F.C. In our online stabilizer, we control not only a ZMP, but also contact forces for realizing the contact force distributions planned in the S.F.C. Finally, we show our method's validity through the experiment using life-sized humanoid robot JAXON.

Periodic nonlinear waves resulting from the contact interaction of a crack

When two different inputs of distinct low and high frequencies are applied to a medium, the linear responses are composed of waves of two dominant frequencies. However, microcracks such as fatigue cracks generate nonlinear waves by modulating the characteristics of the incident waves. Although this phenomenon has been observed and used to detect microcracks, the underlying principles have not been thoroughly elucidated. The hysteresis properties were introduced to describe the nonlinear relationship between the stress and strain to explain these phenomena [Van Den Abeele et al., Res. Nondestruct. Eval. 12, 17 (2000) and Nazarov et al., Acoust. Phys. 49, 344 (2003)]. The generation of harmonics was explained by superimposing stress–strain relations that vary with crack width and excitation magnitude. As the crack depth increases, the ratio of magnitudes of the second harmonic to the first harmonic increases, but the increment becomes smaller [Kawashima et al., Ultrasonics 40, 611 (2002)]. Here, we show that the waves affected by the contact motion of the crack surfaces cultivate the nonlinearity in waveforms, resulting in high frequency off-band signals. With the hypothesis that the clapping of cracks might generate nonlinear components close to the high excitation frequency, we prove that the generation of the high frequency off-band peaks is directly affected by the clapping contact interaction of the crack surfaces. The amount of energy transmitted is closely related to the size of the crack width and the magnitudes of low and high frequency excitations.

Elasto-plastic contact of materials containing double-layered inhomogeneities

Double-layered inhomogeneities consist of enclosed, or layered, inhomogeneities of different material properties, embedded in a matrix material. They are found in engineering materials used for components under contact and relative motion. Currently, most theoretical investigations on the double-layered inhomogeneities are limited to elastic or plane-strain problems. This work proposes a novel model, based on the numerical equivalent inclusion method, for the elasto-plastic contact of materials with double-layered inhomogeneities. The current analysis is focused on such inhomogeneities as a stiff core enclosed by a compliant outer layer. A group of in-depth parametric studies is performed to reveal the effects of this type of double-layered inhomogeneities on the contact plasticity of the matrix material. The results indicate that the plastic strain distribution in the matrix material is related to the Young's modulus and geometric eccentricity of the inner and outer inhomogeneities, as well as the location and shape of the inhomogeneities. For the cases of individual double-layered inhomogeneity embedded at different locations, the maximum equivalent plastic strain in the matrix appears in the vicinity of the double-layered inhomogeneity, wherever is the closest to the location of the theoretical maximum elastic stress from the homogeneous solution. For the cases of multiple double-layered inhomogeneities, the overlap of the plastic strain concentration regions amplifies the disturbance caused by these inhomogeneities, and the amplification effect is related to the inhomogeneity layout. If the stiff core is completely encircled in the outer inhomogeneity layer, plastic strains would initiate from the outer layer and then permeate to the matrix material.

Effects of high frequency drive speed modulation on rotor with continuous stator contact

In many mechanical systems, rotating structures experience continuous rotor-stator contact and torsion motions are dominant in the response. Drill strings used in the oil and gas industry represent one example of rotating structures. Torsion vibrations can be deleterious to the components and operations of the drilling system. As a novel approach to mitigate undesired vibrations, the effects of adding a sinusoidal input to the rotation speed of a drill string are studied. The drill string is modeled as an extended Jeffcott rotor with sinusoidal drive speed modulation. After constructing a three degree-of-freedom model to capture lateral and torsion motions, the equations of motion are reduced to a single differential equation governing torsion vibrations during continuous stator contact. An approximate solution has been obtained by applying the Method of Direct Partition of Motions to obtain an analytical approximation for the solution of this governing equation of motion. The results show that for a rotor undergoing either forward whirling or backward whirling, the addition of sinusoidal excitation to the drive speed can cause an increase in the equivalent torsional stiffness, smooth the discontinuous friction force at contact, and reduce regions of negative slope in the variation of friction coefficient with respect to the contact surface relative speed. Experiments with a laboratory scale drill-string apparatus have also been conducted and the experimental results show good agreement with the numerical results obtained from the reduced-order models. These findings suggest that the developed reduced-order models can be useful for studies of rotor dynamics in situations with continuous rotor-stator contact. Furthermore, the results obtained suggest that the considered drive-speed modulation scheme can be useful for attenuating drill-string vibrations.

Comprehensive 3D-elastohydrodynamic simulation of hermetic compressor crank drive

Mechanical, electrical and thermodynamic losses form the major loss mechanisms of hermetic compressors for refrigeration application. The present work deals with the investigation of the mechanical losses of a hermetic compressor crank drive. Focus is on 3d-elastohydrodynamic (EHD) modelling of the journal bearings, piston-liner contact and piston secondary motion in combination with multi-body and structural dynamics of the crank drive elements. A detailed description of the model development within the commercial software AVL EXCITE Power Unit is given in the work. The model is used to create a comprehensive analysis of the mechanical losses of a hermetic compressor. Further on, a parametric study concerning oil viscosity and compressor speed is carried out which shows the possibilities of the usage of the model in the development process of hermetic compressors for refrigeration application. Additionally, the usage of the results in an overall thermal network for the determination of the thermal compressor behaviour is discussed.

Adaptive control of bilateral teleoperation system with compensatory neural-fuzzy controllers

This paper presents two adaptive neural-fuzzy controllers equipped with compensatory fuzzy control in order to adjust membership functions, and as well to optimize the adaptive reasoning by using a compensatory learning algorithm. To the first controller is applied compensatory neural-fuzzy inference (CNFI) and to the second compensatory adaptive neural fuzzy inference system (CANFIS). Each controller is incorporated into a two channel bilateral teleoperation architecture involving force-position scheme, which combines the position control of the slave system with force reflection on the master. An analysis of stability and transparency based on a passivity framework is carried out. The resulting controllers are implemented on a one degree of freedom teleoperation system actuated by DC motors. The experimental results obtained show a fairly high accuracy in terms of position and force tracking, under free space motion and hard contact motion, what highlights the effectiveness of the proposed controllers.

Effect of Nanoparticles on Spontaneous Imbibition of Water into Ultraconfined Reservoir Capillary by Molecular Dynamics Simulation

Imbibition is one of the key phenomena underlying processes such as oil recovery and others. In this paper, the influence of nanoparticles on spontaneous water imbibition into ultraconfined channels is investigated by molecular dynamics simulation. By combining the dynamic process of imbibition, the water contact angle in the capillary and the relationship of displacement (l) and time (t), a competitive mechanism of nanoparticle effects on spontaneous imbibition is proposed. The results indicate that the addition of nanoparticles decreases the displacement of fluids into the capillary dramatically, and the relationship between displacement and time can be described by l(t) ~ t1/2. Based on the analysis of the dynamic contact angle and motion behavior of nanoparticles, for water containing hydrophobic nanoparticles, the displacement decreases with the decrease of hydrophobicity, and the properties of fluids, such as viscosity and surface tension, play a major role. While for hydrophilic nanoparticles, the displacement of fluids increases slightly with the increase of hydrophilicity in the water-wet capillary and simulation time, which can be ascribed to disjoining pressure induced by “sticking nanoparticles”. This study provides new insights into the complex interactions between nanoparticles and other components in nanofluids in the spontaneous imbibition, which is crucially important to enhanced oil recovery.

接触動作による作業空間知覚に関するハプティック・ディスプレイの基礎的研究( ハプティックインタフェース)

接触動作による作業空間知覚に関するハプティック・ディスプレイの基礎的研究( ハプティックインタフェース)

Film formation and friction in grease lubricated rolling-sliding non-conformal contacts

This study investigates the film formation and friction in grease lubricated, rolling-sliding, non-conformal contacts over a range of entrainment speeds, surface roughnesses and contact temperatures. The effects of grease composition are assessed by employing custom made, additive free, lithium and diurea thickened greases, whose composition is systematically varied so that the effects of the thickener and the base oil can be isolated. All film thickness and friction measurements were conducted under fully-flooded conditions. It was found that at low speeds all tested greases are able to form thicker films than the corresponding base oils. The thickness and behaviour of these films is determined by the thickener type and is independent of the base oil viscosity and the test temperature. At higher speeds, the film thickness is governed by the base oil properties alone and can be predicted by the EHD theory. At low speeds, films with diurea greases grow with time under constant speed and residual films persist under load after contact motion ceases. The real lambda ratio, based on measured grease film thickness, was shown to correlate well with contact friction. The transition from the thickener dominated behaviour to that dominated by the base oil occurs at a relatively constant film thickness, regardless of the base oil viscosity and test temperatures, rather than at a given entrainment speed. Based on the presented evidence, it is here proposed that the mechanism of formation of grease films at low speeds, is analogous to that reported to operate in EHL contacts lubricated with colloidal dispersions, namely the mechanical entrapment and deposition of thickener fibres, and that, rather than the widely quoted ‘transition speed’, it is the ratio of the thickener fibre size to prevailing film thickness that determines the range of conditions under which the film enhancement due to the action of thickener is present.

Whole body motion control of humanoid robots using bilateral control

This paper deals with the whole body motion control problem of humanoid robots by using automatic and bilateral control methods. Advanced motion controllers are proposed so that human daily life activities, such as walking on an uneven terrain and contacting an unknown object, can be performed. It is shown that the uncertainty of environmental impedance variation is one of the main challenging issues in the design of the automatic control systems, i.e. a humanoid robot may easily lose its stability when it contacts an unstructured environment. A novel bilateral control method is proposed for humanoid robots so as to overcome the environmental uncertainty problem. It is shown that more dexterous and versatile tasks can be easily performed by transforming the skills of humans to humanoid robots in the proposed bilateral control system. A disturbance observer is used not only to achieve the robustness of the motion control system but also to perform sensorless contact motion control tasks. The performance of sensorless contact force estimation is significantly improved by deriving the exact dynamic model of humanoid robots. A floating point base exact dynamic model is derived by using the extended Newton-Euler algorithm. A new robot simulator is designed by using the proposed dynamic model and the Virtual Reality toolbox of MATLAB. Simulation results are given to show the validity of the proposals.