Existing Friction Models Research Articles

Learning-based adaption of robotic friction models

In the Fourth Industrial Revolution, wherein artificial intelligence and the automation of machines occupy a central role, the deployment of robots is indispensable. However, the manufacturing process using robots, especially in collaboration with humans, is highly intricate. In particular, modeling the friction torque in robotic joints is a longstanding problem due to the lack of a good mathematical description. This motivates the usage of data-driven methods in recent works. However, model-based and data-driven models often exhibit limitations in their ability to generalize beyond the specific dynamics they were trained on, as we demonstrate in this paper. To address this challenge, we introduce a novel approach based on residual learning, which aims to adapt an existing friction model to new dynamics using as little data as possible. We validate our approach by training a base neural network on a symmetric friction data set to learn an accurate relation between the velocity and the friction torque. Subsequently, to adapt to more complex asymmetric settings, we train a second network on a small dataset, focusing on predicting the residual of the initial network’s output. By combining the output of both networks in a suitable manner, our proposed estimator outperforms the conventional model-based approach, an extended LuGre model, and the base neural network significantly. Furthermore, we evaluate our method on trajectories involving external loads and still observe a substantial improvement, approximately 60%–70%, over the conventional approach. Our method does not rely on data with external load during training, eliminating the need for external torque sensors. This demonstrates the generalization capability of our approach, even with a small amount of data – less than a minute – enabling adaptation to diverse scenarios based on prior knowledge about friction in different settings.

Forces on a singing wineglass rim

The characterization of friction is a multiscale problem involving a balance between asperity contacts and fluid film forces, most commonly modeled with a velocity-dependent coefficient of friction. Periodic stick–slip phenomena are often associated with the friction force that causes mechanical vibration. This type of vibration can cause noise to radiate via structural modes of a system. Notable examples include journal bearings, brake squeal, turkey calls, and wineglasses. Singing wineglasses have been used for centuries as musical instruments, so musical acousticians have studied the structural vibrations and acoustic output. However, the tri-axial force applied to the rim has not been measured. A test rig was designed to measure the vertical, tangential, and radial dynamic forces applied to the spinning glass rim and simultaneously capture the radiated noise. The sound radiated from the wineglass was observed to depend on the relative velocity at the rim and applied vertical load, so a stability region for a clear sound was determined. Additionally, the strongest radiating modes and harmonics in the pressure were found in the concurrently measured nonlinear force. This work may be extended to confirm the force measurement by existing friction models, a water medium, and more complicated systems.

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.

Closed-Form Representations of Friction

The current developed friction models are basically based on the assumption of the normal forces exerted between the contact surfaces known in advance, less work has been done for closed-form (i.e., analytic) modeling in complex mechanical systems where the normal forces vary greatly over time. In this paper, the closed-form representations of friction forces in mechanical systems are newly derived in a way of dynamics. The Udwadia-Kalaba equation is first used to calculate the normal force exerted by the contact surface in mechanical system. The friction force is then calculated via existing friction models. Such closed-form expressions of friction forces contain both the magnitude and direction at any instant of time, even as the normal force is nonconstant. The novel representations offer an effective way for analytically expressing friction force in dynamical systems, which makes it possible for accurate simulation and control design of dynamical systems with non-negligible friction forces.

Modelado y control con compensación de fricción de un sistema pendubot

<p>El pendubot es un sistema mecánico subactuado de dos grados de libertad que comúnmente se utiliza como plataforma educacional y de investigación en las áreas de robótica y control. Se le considera como un sistema subactuado debido a que sólo cuenta con una entrada de control. Aunque se han aplicado distintas leyes de control para estabilizar el pendubot, la mayoría de ellas desprecian los efectos de fricción. En este artículo el análisis es diferente, ya que se considera un sistema pendubot en el que la articulación actuada tiene una cantidad de fricción significativa, que no se puede despreciar. En primer lugar, se revisan algunos modelos de fricción existentes en la literatura, incluyendo uno que fue propuesto por los autores recientemente. Después, se describe la implementación de algunos controladores de estabilización que compensan los efectos de la fricción. Por ultimo, se hace una comparación exhaustiva de los compensadores de fricción presentados para mejorar el desempeno de un sistema pendubot real.</p>

Friction behavior of Radial Shaft Sealing Ring subjected to unsteady motion

A friction model for evaluating friction force of a Radial Shaft Sealing Ring (RSSR) in contact with the rotating shaft subjected to unsteady motion is presented. The model takes both the magnitude and the acceleration sign-changes from positive to negative into account and removes the shortcoming of the existing friction models in the sliding domain. It is shown that acceleration has a pronounced influence on the hydrodynamic regime. The results signify that hysteresis behavior exists in the boundary, mixed lubrication and hydrodynamic regimes as well as the pre-sliding domain, making the conventional Stribeck friction model unsuitable for characterization of friction behavior. To account for the hysteresis effect in the hydrodynamic regime, the model incorporates the solution of the unsteady Navier-Stokes equation that accommodates the magnitude as well as the sign change in the acceleration (and, hence, the viscous friction). In addition to the hysteresis, the results show a frictional lag when the motion starts from stick to slip. Test results are presented at low speed, and unknown parameters of the friction model are determined using an experimental test rig.

Dynamic modeling and system identification of internally actuated, small-sized continuum robots

Small-sized continuum robots (CRs) have shown promising applications in multiple fields, including minimally invasive surgeries. This paper contributes to the field by presenting dynamic modeling and system identification of internally actuated, small-sized CRs in which continuous interactions between the internal actuation mechanisms and the flexible backbones are considered. First, a dynamic model of a flexible backbone sheath is developed. Next, an equivalent discrete model is identified to simulate the dynamics of CRs with continuous interactions. Moreover, based on the existing friction models of passive tendon-sheath mechanisms, the friction model within the proposed discrete model is modified to better resemble the continuity of the interaction. A prismatic ablation catheter is then used as the benchmark for the experimental study, in which parameters for the handle, shaft, and the proposed discrete model for its bending section are considered unknown. The identified model estimates the catheter motion with a maximum error of 4.5 mm at the tip position, which represents 5.6% of the length of the bending section of our experimental catheter.

Friction Characteristics of a Cylinder Based on a Bridge-Type Pneumatic Energy-saving Circuit

A bridge-type energy-saving circuit is a new type of pneumatic system that uses four on-off valves to control the inlet and exhaust of two cylinder chambers. It saves energy through the open-and-close sequence of the four control valves. Cylinder friction is the key factor in accuracy and stability of the bridge-type pneumatic energysaving circuit. This paper focuses on research of the circuit’s friction characteristics. Based on friction theory and the classic Stribeck model, a composite dynamic friction model of a cylinder in a circuit system is established, and a cylinder friction test platform is constructed. The Nelder-Mead algorithm is used to identify static parameters of the model through the relationship between friction and velocity while the piston is moving. Friction model verification with error analysis is achieved by comparison with the traditional friction model. Experiments with the energy-saving circuit under certain conditions are carried out to illustrate the effectiveness of the composite dynamic friction model. Finally, compared with existing friction model, the validity of the model is proved to be applicable to different working conditions.

Translationalized Tire-Road Friction Formulation for Linear DC Machine Powered Mine Locomotives

Abstract A comprehensive friction model for a linear DC machine powered mine locomotive is presented. The friction model presented is an extension of an existing friction model based on slip. Hence, a piece-wise dry friction model is developed. The friction and linear DC machine dynamics are also simulated and presented. The friction curve shows that there are two friction regimes namely; static friction and kinetic friction. The results show that the locomotive tends to move slow when the friction force is incorporated and that there are more losses when the voltage is low. For a voltage of 1V, a velocity of 3.3 m/s is found for a friction-less locomotive while for a steady-state velocity of 2.1 m/s for a locomotive with friction losses.

Neural network based terminal sliding mode control for WMRs affected by an augmented ground friction with slippage effect

Wheeled mobile robots U+0028 WMRs U+0029 encounter unavoidable slippage especially on the low adhesion terrain such that the robots stability and accuracy are reduced greatly. To overcome this drawback, this article presents a neural network U+0028 NN U+0029 based terminal sliding mode control U+0028 TSMC U+0029 for WMRs where an augmented ground friction model is reported by which the uncertain friction can be estimated and compensated according to the required performance. In contrast to the existing friction models, the developed augmented ground friction model corresponds to actual fact because not only the effects associated with the mobile platform velocity but also the slippage related to the wheel slip rate are concerned simultaneously. Besides, the presented control approach can combine the merits of both TSMC and radial basis function U+0028 RBF U+0029 neural networks techniques, thereby providing numerous excellent performances for the closed-loop system, such as finite time convergence and faster friction estimation property. Simulation results validate the proposed friction model and robustness of controller; these research results will improve the autonomy and intelligence of WMRs, particularly when the mobile platform suffers from the sophisticated unstructured environment.

Theoretical and Experimental Study of the Friction Behavior of Halogen-Free Ionic Liquids in Elastohydrodynamic Regime

Ionic Liquids have emerged as effective lubricants and additives to lubricants, in the last decade. Halogen-free ionic liquids have recently been considered as more environmentally stable than their halogenated counterparts, which tend to form highly toxic and corrosive acids when exposed to moisture. Most of the studies using ionic liquids as lubricants or additives of lubricants have been done experimentally. Due to the complex nature of the lubrication mechanism of these ordered fluids, the development of a theoretical model that predicts the ionic liquid lubrication ability is currently one of the biggest challenges in tribology. In this study, a suitable and existing friction model to describe lubricating ability of ionic liquids in the elastohydrodynamic lubrication regime is identified and compared to experimental results. Two phosphonium-based, halogen-free ionic liquids are studied as additives to a Polyalphaolefin base oil in steel–steel contacts using a ball-on-flat reciprocating tribometer. Experimental conditions (speed, load and roughness) are selected to ensure that operations are carried out in the elastohydrodynamic regime. Wear volume was also calculated for tests at high speed. A good agreement was found between the model and the experimental results when [THTDP][Phos] was used as an additive to the base oil, but some divergence was noticed when [THTDP][DCN] was added, particularly at the highest speed studied. A significant decrease in the steel disks wear volume is observed when 2.5 wt. % of the two ionic liquids were added to the base lubricant.

A new analytical tool-chip friction model in dry cutting

To be much closer to the cutting process and the actual cutting phenomenon, a new tool-chip friction model proposed in this paper takes account of the thickness of the material transfer layer of the chip which is ignored by most existing friction models. The material transfer layer, in which the chip material nearly stagnates on the tool rake face during machining, is located between the tool-chip contacting interface and the cutting interface in the secondary shear zone. Furthermore, the proposed model also contains the sticking, the transition, and the sliding friction regions along the tool-chip contacting surface and comprehensively depicts every region’s physical performance during dry machining. The global and the local friction characteristics between the tool and the chip are described based on coupled thermomechanical analytical method. Numerical results solved by the proposed model are compared with the existing experimental and simulated data in available literature. The results show that the proposed model was in good agreement with the experiments. The model provides an analytical method to predict friction characteristics conveniently and efficiently on the tool rake face during dry machining.

Friction of sea ice on sea ice

This paper presents the results from field tests on the friction of sea ice on sea ice performed in the Barents Sea and fjords at Spitsbergen. The effects of the sliding velocity (6mm/s to 105mm/s), air temperatures (−2°C to −20°C), normal load (300N to 2000N), presence of sea water in the interface, and ice grain orientation with respect to the sliding direction on the friction coefficient were investigated. The effect of the hold time on the static friction coefficient was also studied. The roughness of the ice surface is an important parameter that determines the value of the friction coefficient. Repeated sliding over the same track led to surface polishing and decreased the kinetic friction coefficient from 0.48 to 0.05. The studies showed that the friction coefficient is independent of the velocity when sliding occurs between natural ice surfaces. As the contacting surfaces became smoother, the kinetic friction coefficient started to depend on the velocity, as predicted by existing ice friction models. Both very high (~0.5) and low (~0.05) kinetic friction coefficients were obtained in the tests performed at high (−2°C) and low (−20°C) air temperatures. The presence of sea water in the sliding interface had very little effect on the static and kinetic friction coefficients. The static friction coefficient logarithmically increased with the hold time from ~0.6 at 5s to 1.26 at 960s. The results are discussed, and the dependences are compared with existing friction models.

Friction in Double Action Extrusion

A novel extrusion testing method, double action extrusion (DAE), to highlight the effect of friction at the die bearing in aluminum extrusion was developed. It was found that the lengths of the extrudates and extrusion force were indeed sensitive to the die bearing length and thus to the friction. FEM simulations of DAE were carried out to evaluate the shear and Coulomb friction models over a wide range of friction factors/coefficients from 0.2 to 1. The full sticking friction appeared to represent the interfacial contact between hot aluminum and die the best. The friction factor values in the shear friction model over a range of 0.3 to 0.6 commonly used to describe the contact at the billet-die interface in FEM simulation appeared to be too low. The comparison between the experimental and simulation results indicated that the shear friction model at m = 1 predicted the extrusion force the best, while the Coulomb friction model at µ = 1 predicted the extrudate lengths the best. Of the existing friction models and friction factors/coefficients, it is recommended to use the shear friction model at m = 1 to describe the friction at the billet-die interface in FEM simulation.

Friction from nano to macroforce scales analyzed by single and multiple-asperity contact approaches

Understanding friction and wear at different force and length scales is an interesting aspect in tribology. The variety of processes that make friction such a complex phenomenon includes mechanical, chemical and atomic interactions, each operating at their own time, length, and force scale. Extending local tribological information obtained from a single-asperity contact study (e.g. by lateral force microscopy LFM experiments) to contacts with multiple-asperity interactions is a challenging task. In this work, we analyze the localized friction recorded during a single-asperity LFM experiment, and compare it with that of multiple-asperity sliding contacts. Such a detailed study is relevant for miniature contacts which prevail in microelectromechanical systems of devices. In this work, the friction loops (i.e., tangential force versus displacement curves) obtained from nano to macroforce and different length scales were used to understand the scale effects of friction. Experiments were performed in reciprocating sliding conditions with a ball-on-flat configuration on diamond-like carbon and titanium nitride coatings. With decreasing contact size and stiffness of the measuring equipment, the effect of roughness is strikingly evident. The effect of surface roughness is clearly visible on the recorded friction loops as fluctuations, which were later attributed to geometrical and adhesive components. Based on our observations, a simple contact model was proposed to describe the nature of localized friction in single and multiple-asperity configurations during a sliding pass. The existing friction model for smooth and chemically interacting surfaces was extended to rough surfaces and it was found that most of the existing experimental claims could be explained as special cases of the proposed model.

Identification of Friction Coefficient for Constrained Robotic Tasks

Correct modeling of friction forces during constrained robotic operations is critical to high-fidelity contact dynamics simulation. Such simulations are particularly important for the development, mission planning and operations analysis of space robotic systems. Most existing friction models employ the coefficient of friction to capture the relationship between the friction force and the normal load. Hence, accurate identification of this parameter is prerequisite to accurate simulation. This issue is particularly important for space robotic operations since friction characteristics of materials are very different in space. In this manuscript, the problem of identification of the coefficient of friction is investigated experimentally and numerically. The motivating application being space manipulator systems, our principal objective is to develop a practical off-line identification algorithm, requiring minimum number of measurements from sensors available on space robots. To this end, a strategy is proposed to determine the coefficient of friction by using only the measured end-effector forces. The key idea behind the method is that during one-point contact, these forces represent the contact force and hence, can be directly used to calculate the coefficient of friction. The proposed approach is tested with the experimental data from peg insertion experiments conducted on a planar robotics test-bed with a specially designed contact interface. The algorithm is generalized to arbitrary complex geometries and applied to identify the coefficient of friction for a simulated battery drop test.

An Investigation of New Tribological Variables in Friction Modeling of Bulk Metal Forming

The present “realistic” friction model is examined by a series of upsetting tests. Specimens made of different aluminum alloys and prepared by various processes display diverse results. Severe surface roughening takes place in some cases using A6061T4 bar manufactured by hot extrusion. It results in a lubrication condition which is very different from the theoretical prediction. From hardness tests and microstructure observations, it is found that there is no obvious correlation between the hardness and the roughening process for A6061T4 alloy. The severe roughening is attributed to the distortion of the coarse grains recrystallized during the preprocess, that is, the hot extrusion. This is rare in sheet metal forming where most workpieces are prepared from cold rolled sheet metals. The orange peel appearance not only changes the lubrication mechanics, but also makes the surface finish of the product unacceptable. Since the existing friction model fails to predict such phenomena, surface roughening and all its related factors must be considered as new, important tribological variables in the friction modeling of bulk metal forming.

Boundary friction model in punch friction test

A boundary friction model which includes both plowing and adhesion stress has been developed to simulate the stretching of sheet over a cylindrical pin boundary regime. The equilibrium equation defining the local frictional shear stress on the workpiece has been combined with the existing friction model to determine fractional contact area and friction in the contact region. Numerical methods have been used to solve the governing equation with the measured initial condition obtained from the experiments. The punch friction test which simulates the stretching of sheet over a punch corner radius in a typical draw die is used to compare the theoretical friciton modeling. The measured friciton coefficients increase with lower stretching speed and decreases if lubricant is appled at the interface between workpiece and cylindrical pin. The theoretical prediction shows the same trend of increasing friction coefficient with increase in strain in the contact region.

Finite Element Modeling of Plane-Strain Strip Drawing With Interface Friction

Numerical results of plane-strain strip drawing are presented emphasizing the interfacial friction conditions between workmaterial and drawing die. The results are based on a large-strain elasto-plastic finite element code with a special boundary condition capability to treat interface friction. Two different modeling approaches are developed. In the interface velocity model, finite element solutions are based on the velocity boundary conditions along the die-work interface measured through transparent sapphire dies. In the interface traction model, a technique called “General Linear Boundary Condition,” is developed to incorporate many different existing friction models. Predicted values of drawing force, die separating force, global friction, and residual stresses from both models are favorably compared with the corresponding experimental measurements.