Farid Al-Bender

An integrated friction model structure with improved presliding behavior for accurate friction compensation

Presents a dynamical friction model structure which allows accurate modeling both in the sliding and the presliding regimes. Transition between these two regimes is accomplished without a switching function. The model incorporates a hysteresis function with nonlocal memory and arbitrary transition curves. These last aspects prove essential for modeling presliding friction that is encountered in real physical situations. The model as a whole can also handle the Stribeck effect and stick-slip behavior as has been demonstrated by validation on a KUKA IR 361 robot. In this sense, this model can be considered as more complete in comparison with others found in the literature. The general friction model allows modeling of individual friction systems through the identification of a set of parameters that determine the complete behavior of the system. In this way, the model structure has been used to identify the friction behavior of a linear slide as well as that of the above mentioned KUKA robot. The results of the latter identification have been consequently used for feedforward friction compensation to obtain the most accurate tracking.

The generalized Maxwell-slip model: a novel model for friction Simulation and compensation

A novel, multistate friction model is presented, which is obtained from the Maxwell-slip model by replacing the usual Coulomb law at slip by a rate-state law. The form of the latter state equations is arrived at by comparison with a recently developed generic friction model as well as limiting behavior cases. The model is particularly suitable for quick simulation and control purposes, being both easy to implement and of high fidelity. This communication situates the model, by outlining its development background and structure, and highlights its basic characteristics.

Modification of the Leuven integrated friction model structure

This note presents a modification of the integrated friction model structure proposed by Swevers et al. (2000), called the Leuven model. The Leuven model structure allows accurate modeling both in the presliding and the sliding regimes without the use of a switching function. The model incorporates a hysteresis function with nonlocal memory and arbitrary transition curves. This note presents two modifications of the Leuven model. A first modification overcomes a recently detected shortcoming of the original Leuven model: a discontinuity in the friction force which occurs during certain transitions in presliding. A second modification, using the general Maxwell slip model to implement the hysteresis force, eliminates the problem of stack overflow, which can occur with the implementation of the hysteresis force.

A novel generic model at asperity level for dry friction force dynamics

This paper presents a theoretical model for (dry, low-velocity, wear-less) friction force dynamics based on asperity interaction considerations subject to the phenomenological mechanisms of creep/relaxation, adhesion and (elasto-plastic) deformation in their most generalized forms. The model simulates the interaction of a large population of idealized, randomly distributed asperities with arbitrarily chosen geometrical and elastic properties. Creep and adhesion are simulated by an expedient local coefficient of friction that increases with time of contact, while deformation effects are accounted for by rate-independent hysteresis losses occurring in the bulk of the material of an asperity that is breaking loose. An energy method is adopted to calculate the instantaneous, local friction force leading to better insight into the problem as well as higher numerical efficiency. The results obtained by this model show both qualitative and quantitative agreement with the known types and facets of friction force dynamic behaviour; in particular, pre-sliding quasi time-independent frictional hysteresis in the displacement, velocity weakening, slider “lift-up” effect and frictional lag, in addition to the influence of the various process parameters, all in a single formulation, such as no extant friction model could show before. Moreover, the model is still open for and capable of further refinement and elaboration so as to incorporate local inertia and viscous effects and thus to be extended to include velocity strengthening and lubricated rough contacts.

Characterization of friction force dynamics

Friction modeling has been steadily gaining in interest over the last two decades. Despite persistent and painstaking efforts, however, no satisfactory, comprehensive, practicable friction model that captures all of the experimentally observed aspects of friction force dynamics in one formulation is available. Friction comprises multiscale processes requiring multiscale theories. This article presents an example of comprehensive model building, which, starting from the generic mechanisms behind friction, leads to the construction of a model that explains observed macroscopic friction behavior. Effective physics-based models at multiple scales can facilitate future work on the inter-relationships among models by furthering the understanding of emergent, collective frictional properties. Moreover, predictive physical models facilitate the derivation of simple models for control purposes. An example is the generalized Maxwell-slip (GMS) model, which is discussed in this article. Considering friction as a mechanical system, a close examination of the sliding process reveals two friction regimes, namely, the presliding regime and the gross sliding regime. In the presliding regime the adhesive forces owing to asperity contacts are dominant, and thus the friction force is primarily a function of displacement rather than velocity. The reason for this behavior is that the asperity junctions deform elastoplastically, thus behaving as nonlinear hysteretic springs.

Modeling of dry sliding friction dynamics: from heuristic models to physically motivated models and back.

After giving an overview of the different approaches found in the literature to model dry friction force dynamics, this paper presents a generic friction model based on physical mechanisms involved in the interaction of a large population of surface asperities and discusses the resulting macroscopic friction behavior. The latter includes the hysteretic characteristic of friction in the presliding regime, the velocity weakening and strengthening in gross-sliding regime, the frictional lag and the stick-slip behavior. Out of the generic model, which is shown to be a good, but rather computationally intensive, simulation tool, a simpler heuristic model, which we call the generalized Maxwell-slip friction, is deduced. This model is appropriate for quick simulation and control purposes being easy to implement and to identify. Both of the generic and heuristic model structures are compared, through simulations, with each other and with experimental data.

Application of dimensional analysis to selective laser melting

Purpose – This paper aims to present a possible complete set of dimensionless parameters to describe the process of selective laser melting (SLM). This makes it possible to compare the similarity between different experiments, a sine‐qua‐non for a correct comparison of the results.Design/methodology/approach – The paper describes the application of dimensional analysis to SLM.Findings – Although the idea of dimensionless numbers is far from new, it has apparently never been applied rigorously to rapid prototyping and rapid manufacturing technologies. The technique is important, since it reduces the number of factors and makes it possible to compare results of different research groups. Furthermore, some more fundamental insights about the process can be gained.Originality/value – This work is a first step towards a manageable system to control very difficult processes.

Identification of pre-sliding friction dynamics

The hysteretic nonlinear dependence of pre-sliding friction force on displacement is modeled using different physics-based and black-box approaches including various Maxwell-slip models, NARX models, neural networks, nonparametric (local) models and dynamical networks. The efficiency and accuracy of these identification methods is compared for an experimental time series where the observed friction force is predicted from the measured displacement. All models, although varying in their degree of accuracy, show good prediction capability of pre-sliding friction. Finally, we show that even better results can be achieved by using an ensemble of the best models for prediction.

FRICTION IDENTIFICATION AND COMPENSATION IN A DC MOTOR

Abstract Friction modeling and identification is a prerequisite for the accurate control of electromechanical systems. This paper considers the identification and control of friction in a high load torque DC motor to the end of achieving accurate tracking. Model-based friction compensation in the feedforward part of the controller is considered. For this purpose, friction model structures ranging from the simple Coulomb model through the recently developed Generalized Maxwell Slip (CMS) model are employed. The performance of those models is compared and contrasted in regard both to identification and to compensation. It turns out that the performance depends on the prevailing range of speeds and displacements, but that in all cases, the CMS model scores the best.

Modelling a pneumatic servo positioning system with friction

Treats the problem of modelling and identification of the various elements of a pneumatic servo positioning system with the aim of constructing a complete and effective model that can be used for simulation and accurate control of such systems. Particular attention is paid to two important elements that manifest a strong nonlinear behaviour, viz, air flow and friction. In the first instance, an empirical model connecting the pneumatic valves driving voltage, the pressures upstream and downstream, and the flow is hypothesised based on the nozzle formula. With this model, the flow function is then systematically identified. As regards friction, the Leuven model structure is used as basis for identification. Here, the two basic friction regimes, viz. presliding, with its hysteresis behaviour, and gross sliding are well exposed and their essential parameters identified.