Friction Measurement on Road Surfaces 1

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1 PRÜFEN UND MESSEN TESTING AND MEASURING Rubber friction Friction measurement Road surface Friction coefficient field Friction measurements on real road surfaces are limited because of the necessity of complex measuring systems which are not available in the field. This investigation is important to improve vehicle dynamics and tire design. To perform this task a mobile measuring platform to measure the friction coefficient between a rubber testing wheel and the road surface was built. The main requirements of the system are autonomy (integrated energy source) and mobility. The measuring device is integrated into a 3-wheel robotic platform that is driven by two rear wheels and a non-driven but steered front wheel. The experimental results shall classify different street surfaces and give the influence of surface pressure and sliding velocity on the friction coefficient. Reibmessungen auf Straßenoberflächen Schlagworte: Gummireibung Reibwertmessung Straßenoberfläche Reibwertkennfeld Reibungsmessungen auf allgemeinen Straßenoberflächen sind bisher wegen teils komplexer Laboraufbauten nur eingeschränkt möglich. Die Untersuchung ist jedoch wichtig für den Fortschritt im Bereich Fahrzeugdynamik und Reifenentwicklung. Für die oben genannte Aufgabe ist eine mobile Messplattform entwikkelt worden, um Reibwerte zwischen einem Gummimessrad und der Straßenoberfläche zu messen. Die Hauptanforderungen an das System sind autonome Arbeitsweise und Mobilität. Die Messapparatur ist integriert in einen dreirädrigen Serviceroboter, der von zwei Hinterrädern angetrieben und einem nichtangetrieben Vorderrad gelenkt wird. Die experimentellen Ergebnisse sollen unterschiedliche Straßenoberflächen klassifizieren und sollen den Einfluss von Flächenpressung und Relativgeschwindigkeit auf den Reibwert zeigen. Friction Measurement on Road Surfaces 1 The measurement of friction coefficients between rubber compound and the road surface is limited because of the necessity for complex measuring systems, which are not available in the field. In this paper the design of a mobile measuring platform for measuring the friction coefficient between a rubber testing wheel and the road surface is presented. The integration of a measuring device into a mobile robot permits laboratory precision measurements on outdoor road surfaces. The measuring device is integrated into a 3-wheel robotic platform which is driven by two rear wheels and a non-driven, but steered front wheel. The platform provides batteries for power supply and an onboard computer (PowerPc) with the realtime operating system RTOS-UH. The system is operated by a WINDOWS notebook as user interface connected through TCP/IP protocol via radio LAN, which enables partially autonomous operation. To measure the friction coefficient a rubber testing wheel, which is adopted from the GROSCH rubber wear test rig [1, 2], is pressed onto the road surface by a predefined normal contact force in a closed loop force control. The rubber measuring wheel is driven providing constant macroscopic sliding velocity in the contact zone. By pressing the rotating rubber specimen onto the road surface, a friction force is generated. The friction coefficient is calculated as the ratio of the measured friction force and the actual normal contact force. Measurement robot design The measurement device is integrated into a 3-wheel robotic platform, which is designed on a modular basis. The platform design accounts for outdoor use aspects, e.g. the capsulation of the system against humidity and the required robustness. It is moved by two driven and one steered solid rubber wheels of the same size. The 235 kg system is capable of driving up a 1 Given at IRC 03, , Nürnberg 178 slope. It carries its own energy supply providing sufficient energy for a one day measuring operation. The control of the platform is done by the onboard computer which communicates through a CAN-Bus with the different modules (steering module, driving module, measurement device etc.). The operation of the measuring robot is done via a WINDOWS-notebook connected by radio LAN. See [3] for a more detailed description of the mobile platform design. A picture of the measuring device is given in Fig. 1. It is located at the back end of the robot and can be flapped out of the robot for inspection and set-up purposes. The operating sequence is as follows: A rotary driven measuring wheel, which is adopted from the GROSCH rubber wear test rig, is being pressed onto the road surface by closed loop force control. This way the normal contact force is kept constant with little disturbances resulting from frictional contacts at high speeds. The wheel is driven by a servomotor equipped with a right angled gear head. The measuring wheel is mounted directly onto the output shaft of the gear. This unit (servomotor, gear and measuring wheel) is connected to a vertical linear guide via a force/torque sensor. This sensor measures the normal contact force and the resulting friction force at controlled rotary speed of the measuring wheel. The slide itself is mounted into the aluminium profile skid. The remaining single-degree-of freedom system is operated by a servomotor driven ball screw spindle which exerts the normal contact force onto the linear guided parts. H. Blume, B. Heimann, M. Lindner, H. Volk, Hannover Corresponding author: Dipl.-Ing. H. Blume Hannover Center of Mechatronics Appelstraße 11, D Hannover, Germany Tel.: þ49(0) Fax: þ49(0) blume@mzh-uni-hannover.de KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 12/

2 Besides above mentioned data, the temperature of the measuring wheel surface behind the frictional contact and the street surface temperature are monitored by pyrometric sensors. The following process data is being observed during measurement at 1 khz control rate: vehicle speed, measuring wheel speed and torque, normal contact force, friction contact force, measuring wheel surface temperature, road surface temperature (behind contact). This data is stored during measurements in an ASCII-File and can be processed after measurements are completed. Operating sequence In order to qualify this measurement device as a reproducibly working and user friendly tool, it is equipped with an autonomous working operating sequence, which is capable of measuring a user defined friction coefficient field. The friction coefficient field shall contain a friction coefficient dependent on different combinations of constant normal contact forces and sliding velocities as parameters. The parameters must be specified in a list of consecutive measurement tracks (program). The process flow diagram is presented in Fig. 2: After hardware initialisation the program can be loaded. After issuing the start command, the autonomous operating sequence begins. For a measurement, the linear guided parts are driven out of the measuring device to establish ground contact. Once the ground contact is detected, force control is started and a vehicle speed is applied for moving measurements. These operations are considered Fig. 2. Operating sequence Fig. 1. Measurement unit pre-processing operations, upon which the measurement process (cycle) is entered. The measurement process consists of three different stages, while transition between these stages are triggered by time and temperature conditions. The three stage loop is passed once for every track in the program. The first stage (conditioning) accounts for constant starting conditions in every measurement loop. As through measurements with different parameters the temperature and surface condition of the specimen varies, it is necessary to prepare the specimen with a sequence for constant initial conditions: after cooling the measuring wheel (stage cooling), the wheel is abraded with a constant parameter set on the road surface, until the surface temperature exceeds a predefined bound (45 8C in Fig. 2). After exceeding the bound, the second stage is entered. During the first stage a process data protocol is recorded and stored in a file. The second stage is the actual measurement stage, where the parameters specified in the program tracks are processed. For every program track a maximum time and maximum wheel surface temperature is specified, which serve as break condition (55 8C and 5 s in Fig. 2). Depending on which condition applies first, measurement is stopped after exceeding the maximum time or maximum temperature. Here again process data are recorded. The measurement stage is followed by a cooling state. During cooling, the measuring wheel is driven with no sliding velocity between wheel and ground, while the vehicle is moving. This enables heat transfer to the ground. Cooling is ended by under running a temperature bound (38 8C in Fig. 2). After finishing the third stage the measurement process is either left or continued with the next track. After completion of the program, ground contact is released and the prior recorded data is processed. For each conditioning stage and each measurement stage, average friction coefficients and corresponding standard deviations are calculated and stored in matrix form. This data can be downloaded from the onboard computer and evaluated just after completion of the program. Fig. 3 shows a sample result file. Measurements In the following some measurement results shall be presented. Prior to using the measurement robot, reproducibility must be proven. In order to demonstrate the repeat accuracy, the above mentioned operating sequence is used to measure the friction coefficient of a representative specimen on a homogenous surface (glass). Running above described procedure, the program uses the same parameters for 678 KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 12/2003

3 several times. Fig. 4 shows the results of ten consecutive measurements with five different normal contact forces. On glass (Fig. 4a), a noticeable dependency of the friction coefficient on normal contact force can be observed. For the compound used the dependency of average surface pressure on normal contact force is measured statically as shown in Table 1, which shows a declining characteristic. The measurements show that the friction coefficient lowers with rising surface pres- Fig. 3. Sample result file Fig. 4. Reproducibility measurement a) on a glass surface and b) on safety walk Fig. 5. Friction coefficient field on a glass surface a) with linear velocity scale and b) with logarithmic velocity scale KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 12/

4 Tab. 1. Dependency of average surface pressure on normal contact force contact force surface pressure 30 N 0,194 N/mm 2 60 N 0,271 N/mm 2 90 N 0,343 N/mm N 0,415 N/mm N 0,465 N/mm 2 sure, which is being expected from literature. Experimental results prove good repeat accuracy on a homogeneous glass surface, while repeat accuracy is smaller on safety walk, a floor covering for slippery steps and floors. Here the dependency of the friction coefficient on surface pressure is smaller but still visible. The less accurate reproducibility might be caused by high abrasion rates and the cohesion based part of the friction coefficient, respectively. Rubber particles in the contact area due to the high abrasion rate might as well influence the measurement on safety walk. Furthermore, friction coefficient fields are measured on three different surfaces: the first one is glass (shown in Fig. 5). The second one is safety walk (Fig. 6). Finally there is a measurement on a wet road surface, shown in Fig. 7. As the dependency of friction on relative velocity and excitation frequency respectively is more evident, friction coefficient fields are shown, both with linear and logarithmic velocity scale. Comparing the absolute values of the friction coefficient between glass and the road surface, safety walk respectively, it comes out that dependency of friction on surface pressure is higher on glass. In the friction coefficient field measured on a wet road surface (Fig. 7) no dependency on the normal contact force is detectable. It should be noted, that the range between maximum and minimum normal force is smaller than in the other measurements. Fig. 6. Friction coefficient field on safety walk a) with linear velocity scale and b) with logarithmic velocity scale Fig. 7. Friction coefficient field on a wet road surface a) with linear velocity scale and b) with logarithmic velocity scale 680 KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 12/2003

5 As expected from theory, a friction coefficient maximum in respect to the relative velocity is being observed. This effect is due to the hysteresis friction with varying excitation frequencies. On the wet road surface there is a friction coefficient maximum at v rel 0.1 m/s. Due to the water layer adhesion can be neglected. Theory does also predict a hysteresis based maximum at v rel 0.1 m/s in [5] where temperature influence is accounted for by calculating a rising contact temperature with rising sliding velocity. The hysteresis influence on the safety walk measurement (Fig. 6) is different. The dominating roughness wave length is smaller, which leads to a friction coefficient maximum at a smaller sliding velocity of v rel 0.01 m/s in this case. On the glass surface a bigger influence of adhesion is assumed. The friction coefficient maximum is measured at v rel m/s, while [6] predicts maxima at v rel 0.02 m/s. It should be noted that during measurements on glass, parameter combinations can be found which lead to stick-slip effects. This kind of measurements cannot be analysed in comparison to regular measurements, because of the different physical phenomena. The road surface and the safety walk surface did not show any unstable regions in the parameter field. Conclusion and future work In this paper the design and operating mode of a new device for friction measurement on road surfaces is presented. The partly autonomous working measurement device is capable of measuring friction coefficient fields on wet and dry surfaces. Because of the very recent completion of the device, only few measurements can be shown here. The measurements represent friction phenomena described in literature, which emphasises the plausibility of the results shown. The major aim of the measurements shown is to prove the (repeat) accuracy of measurement principle and device. In future, the wet surface measurements capabilities will be analysed more precisely. After completion of preliminary testing, the device will be used as a tool to investigate varying rubber recipes and varying locations (road surfaces) respectively. Creating a database of parameter dependent friction coefficients as well as characterisation of rubber compound and road surfaces is the major motivation for future operation of this device. References [1] Grosch, K. A., Journal: Kautschuk Gummi Kunststoffe, Band 49, Heft 6, 1996, pp [2] Grosch, K. A., Journal: Kautschuk Gummi Kunststoffe, Band 50, Heft 12, 1997, pp [3] Blume, H.; Heimann, B.; Lindner, M., Proceedings of the International Colloquium on Autonomous and Mobile Systems, Magdeburg, 2002, pp. 65. [4] Bachmann, T., Literaturrecherche zum Reibwert zwischen Reifen und Fahrbahn Fortschritt-Berichte VDI, Reihe 12, Nr. 286, VDI Verlag, Darmstadt, [5] Persson, B. N. J., Fortbildungsseminar Elastomerreibung und Kontaktmechanik, Deutsches Institut für Kautschuktechnologie, Hannover, [6] Persson, B. N. J., Sliding Friction Physical Principles and Applications, Second Edition, Springer, Berlin, The authors Dipl.-Ing. H. Blume is mechanical engineer in the field of mechatronics and works as a scientific assistant at the Hannover Center of Mechatronics, Germany. Prof. Dr.-Ing. habil. B. Heimann is professor in the department of mechanical engineering at the university of hannover and executive head of the Hannover Center of Mechatronics, Germany. Dipl.-Ing. M. Lindner is mechanical engineer in the field of elastomer friction and works as a scientific assistant at the Institute of Mechanics in Hannover, Germany. Dipl.-Phys. H. Volk is head of the department for strategic technology at Continental AG in Hannover, Germany. KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 12/