DYNAMIC FRICTION COMPACT MODEL : A NEW MULTICONTACT TRIBOMETER

Transcription

1 : 1 st Euro-Mediterranean Conference Apr 2013 on Structural Dynamics and Vibroacoustics Marrakech (Morocco) ABSTRACT DYNAMIC FRICTION COMPACT MODEL : A NEW MULTICONTACT TRIBOMETER J.L.DION, G.CHEVALLIER, O.PENAS, F.RENAUD 1Lab or Company Name LISMMA- EA 2336 SUPMECA - 3 rue Fernand Hainaut Saint Ouen Cedex France jean-luc.dion@supmeca.fr This paper presents an original friction measurement setup. It is designed to uncouple normal and tangential load. Friction measures acquired with this setup present a high signal to noise ratio. This friction experimental device allows characterizing various types of surfaces and several levels of excitation from static strain to dynamical motion including micro-sliding and macrosliding. A measurement example is presented for a contact pair of Aluminum alloy starting from sticking limit until complete sliding. Finally, in order to use the results in vibration reduced models, a method is proposed to identify the compact Lugre Model from these measurements.

2 1 DESCRIPTION Many previous works use or have developed tribometers ([1-11], [16-21]) The proposed test bench allow to uncouple normal and tangential loads over a wide frequency bandwidth to ensure the good quality of measurements and test conditions, as previously exposed. In order to verify this assumption and to identify state and rate models, normal force, tangential force, relative displacement and sliding velocity have to be measured over a large frequency range and with a great accuracy. Our tribometer has been so mounted on a MTS Elastomer Test System 830 which allows to obtain at 200 Hz a [1-100] µm displaceme ent range inducing a [ ] mm/s velocity range. To decouple normal and tangential load, the test setup is composed of three orthogonal symmetry-planes with four contact areas (Figure 1 and Figure 3). Each symmetry-plane allows an equal repartition of normal forces (planes P1 & P2) and tangential forces (plane P3). The complete experimental set, composed of samples in the tribometer, is iso-static. The base sample (Figure 1) is clamped on the reference frame of the hydraulic machine but the mobile sample is free in rotation around the jack displacement axis and avoids hyperstatism in the mechanism. For the same reason, the normal force (N) is performed with a flexible screw. The axis of this screw is placed along the intersection of planes P1 and P2. Moreover, the two application points of the normal force applied by the screw are symmetric on both sides of P3. These technical choices allow to assume a satisfying symmetry for three orthogonal planes, both for geometric properties of the mechanical setup and for load distributions. Figure 1: Principle of contact areas and normal and tangential forces The three orthogonal symmetry-planes (P1, P2 and P3) allow decoupling between normal and tangential forces and an equal repartition of these forces on each sample. 2

3 23-25 Apr 2013,, Marrakech (Morocco) During the test, with tangential motion, motion static normal load is assumed to remain static. However the normal load is always measured and controlled with high frequency sampling during the test. This assumption is often not verified on other tribometers. A particular attention has been paid in the the design of this tribometer in order to avoid coupling between normal and tangential load. For all the tests, hydraulic jack movements are displacement controlled with the feed-back feed signal of the internal displacement transducer (LVDT). Measurements are performed with 3 force sensors (Fn, Ft, Ft2), 1 LVDT displacement sensor (X), 3 Foucault displacement sensors (xm, xh, xb) and 2 accelerometers (Ae, Af). Most of them are used in order to verify assumptions, ions, only 3 sensors are necessary for dynamic friction measurement : 2 force sensors (Ft, Fn) n) and 1 displacement LVDT (X). Figure 2:: Schema and partial picture of the tribometer. tribomet IDENTIFIC 3 COMPACT MODEL SELECTION AND PARAMETRIC IDENTIFICATION Many compact models have already been developed in previous works ([12]]-[15], [22-29]). The LuGre model [14] can be considered as an evolution of Dahl s model with a velocity dependant friction factor introduced by the g function, often chosen ch en as a Gaussian function. function The LuGre model was completed in 2002 [12] and it includes elastoplastic behavior that occurs before the slipping limit has been reached. Ft = ( K 0 z + C0 zɺ + C1 xɺ ) Fn xɺ dz = xɺ K 0 z dt g ( xɺ ) g ( xɺ ) = µ d + ( µ s µ d ) e 3 (1) xɺ Vs α

4 The proposed identification method is robust and has been applied for a very large number of tests with several frequencies and normal loads. The quality of fitting between simulation and measurements in Figure 3 is similar for most of the tests; some are better while only a few (less than 5%) are worse. Figure 3: Comparison between measurement and simulation obtained with the proposed identification method 2 CONCLUSION This work was performed in the field of structural vibrations. In a complex mechanism, the level of vibration strongly depends on the dissipation in the connected parts. This work presents a new test bench designed for improving the accuracy of measurements of non linear dissipative behaviors of frictional interfaces. This test bench is used for dynamic friction studies. Magnitudes of displacement have been tested from 10-5 m to 10-2 m while those of velocity have been studied from 10-7 m/s to 10 m/s. The tribometer design allows a very good independence between normal and tangential forces. Measurement techniques and signal processing methods highlight excellent accuracy with both direct measurements and parametric identification. As an illustration and because it is very well-formulated for vibrations problems, the LuGre Model has been identified from experimental results. An accurate method for parametric identification has been performed and a rheological description of the LuGre model has been presented. The prospects for improvement of this test means are to enable the measurements of the dissipative behavior on the scale of roughness. Thus the next test bench should make it possible to obtain measurements of displacements under 10-7 m. 4

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