DYNAMIC AXLE LOAD OF AN AUTOMOTIVE VEHICLE WHEN DRIVEN ON A MOBILE MEASUREMENT PLATFORM

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1 Int. J. of Applied Mechanics and Engineering, 214, vol.19, No.3, pp DOI: /ijame DYNAMIC AXLE LOAD OF AN AUTOMOTIVE VEHICLE WHEN DRIVEN ON A MOBILE MEASUREMENT PLATFORM C. JAGIEŁOWICZ-RYZNAR Department of Applied Mechanics and Robotics Faculty of Mechanical Engineering and Aeronautics Rzeszow University of Technology al. Powstańców Warszawy 12, Rzeszów, POLAND cjr@prz.edu.pl An analysis of the dynamic axle load of an automotive vehicle (AV) when it is driven on a mobile measurement platform is presented in this paper. During the ride, the time characteristic of the dynamic force N(t), acting on the axle, was recorded. The effect of the vehicle axle mass on the maximum dynamic force value and the dynamic coefficient were studied. On this basis it was attempted to calculate the total vehicle s weight. Conclusions concerning the dynamic loads of the vehicle axles in relation to the reduced axle mass, were drawn. The optimal axle mass value, for which the dynamic coefficient reaches a minimum, was calculated. Key words: mechanic vibrations, dynamic load, automotive vehicle axle. 1. Introduction In the time of intensive development of the automotive industry, the maintenance and the development of road infrastructure are very big challenges. The road networ is increasingly exposed to surface destruction, as a result of excessive and improper operation, mainly by multi-tone and -axle automotive vehicles. One of the questions related to exploitation of public roads by heavy automotive vehicles is the measurement of the axle pressure force on the roadway. Vehicles in which the pressure force of a single axle does not exceed the limit values (Journal of Laws of the Republic of Poland, 212), taing into consideration also the type of road, were allowed to the traffic flow by the traffic law. The regulation of the Ministry of Infrastructure and Development (Traffic Law Act, 1997) enforces a limitation of axle pressure force from 8 or 1 tons on some road sections. To verify whether the limitation is observed, the mass of heaviest vehicles is controlled by measuring the pressure force of particular axles and the whole mass as well. There are two types of measurements. The first one is a static measurement. The vehicle is stopped, directed to the control place, where the proper measuring and other operations are conducted. These activities involve various inconveniences, such as a waste of time and difficulties in the traffic. The other one consists in using a stationary measuring device permanently embedded in the lane. This way of control is associated with a complete lac of mobility and limits the measurement possibilities on other sections of the road. The static measurement can be supplemented by a dynamic measurement using the fact that during motion individual parts of a vehicle (especially the body) vibrate vertically. For each vehicle, these vibrations are forced, inematically excited and due to road unevenness. Under ideal conditions, the vehicle does not vibrate or vibrations are very small. To induce vibrations of a car body (which will not be dangerous) it is suggested to use a mobile measuring platform (MMP), in the shape of an inclined plane with a small angle (Fig.1). It will be possible to set the MMP on any traffic lane in any road section. A patent application for the platform has been filed at the Patent Office (patent application No. P.42614). The inematically excited vibrations mainly depend on the vehicle weight, suspension stiffness, stiffness of the tire, damping in the shoc absorbers and tires.

2 586 C.Jagiełowicz-Ryznar Fig.1. Schema of the MMP with an automotive vehicle driven on it (Jagiełowicz-Ryznar, 213). Legend for Fig.1: 1- ramp plate 2- support plate 3- spring elements 4- slope edge of the ramp plate 5- projecting zone of the support plate 6- lower surface of the support plate 7- vibration probes 8- ramp plate plane 9- measuring probes 1-measuring probe 11,12,13 meters 14- programmed processor 15- monitor 16- horizontal speed sensor of an approaching vehicle 17-vehicle 2. Model Dynamic loads that act on the vehicle axle when the AV is driven on the MMP were analyzed. During the ride the time characteristics of the dynamic force acting on the axle were recorded. On the basis of the N(t) characteristics the maximum dynamic load of the vehicle axle, as well as that of the road under real conditions, i.e., during normal operation, can be determined. The recorded N(t) characteristics render it possible to determine some vehicle dynamic parameters such as speed, AV weight, shoc absorber damping, natural frequencies and other parameters by means of using an intelligent computing system. This wor neither resolves all the problems nor answers all the questions. It is merely an initial attempt to deal with the issue. Wor on this area ( with a model as well as parameter values) is continued and the results will be published. The dynamic model of the system under analysis is shown in Fig.2.

3 Dynamic axle load of an automotive vehicle when driven 587 Fig.2. AV dynamic model. The system has got two degrees of freedom associated with masses: - the mass of body parts corresponding to the load of the axle under analysis, - the mass reduced to this axle. Forced vibrations are generated by the MMP when the AV is driven on it. (Fig.1) The dynamic equations describing the vibrations of the system have the form (1) m z z z z z, n n z n a n m z z z z z z z u u. z n a n og og og og (2.1) Initial conditions are zero. Vibrations coming from the road were not taen into consideration. The function u(t) describes the inematically excited vibrations of axles and the body (assuming that the AV V p speed is constant) and has the form (Jagiełowicz-Ryznar, 213) t p and value h L ut () Vp t; t ; tp. (2.2) L V Formulas (2.1) and (2.2) show that the force P () t has the shape of a trapezoidal impulse with time S w. w u og og h where Vu Vp, L w p P V t (2.3)

4 588 C.Jagiełowicz-Ryznar S w og L h og. (2.4) 2V p The dynamic force Nd () t acting on the AV axle is described by formula (2.5) N () t u z V V (2.5) d og og u where V dz. dt On the other hand, the dynamic coefficient n d Nd,max d. (2.6) g m m The free vibration frequency oi,, the coefficients of normal vibration modes i, and the orthogonal coefficient of the system are presented by formulas (2.7), (2.8) and (2.9) (Kalisi, 1966) oi, 2 z n og z z n og z n og z m m m m 4m m 2m m n, (2.7) for i =1, sign ( ); for i = 2, sign (+); 1 i mn 1 z 2 oi, ; i = 1, 2. (2.8) 3. Calculation results 1. (2.9) 1 2 Initially, 8 simulations were carried out for the following values of the axle mass (wheels) m 1(); 5; 1; 2; 5; 1; 25; [g] Other data: mn 1 [g]; z [N/m]; MMP dimensions: L=1[m]; h=2[cm]; [N/m]; og 6 Vp 2 [m/s] - speed of the vehicle; 4 a 2 1 [Ns/m]; og 2 [Ns/m]; The calculation results are shown in diagrams Nd () t and d () t (Figs: 3a, 3b, 3c, 3d, 3e, 3f, 3g, 3h) and Tabs 1 and 2. On the basis of graphs (3a-h), graphs for Nd,max lg m and d lg m in a comparatively large range m ; 25 were prepared (Fig.4). The simulations for m and m 1 were performed for computation purposes.

5 Dynamic axle load of an automotive vehicle when driven 589 a) 8 Model of the Model automotive zawieszenia vehicle samochodu suspension. The function Przebieg process dla mfor = m; N= d,max ; N dmax = = X:.9233 Y: 666 X:.4853 Y: X:.2913 Y: max = X:.9133 Y:.6182 X:.4853 Y:.53.2 (t) X:.2933 Y: b) 8 Model of Model the automotive zawieszenia vehicle samochodu suspension. The Przebieg function dla process m = for 1; Nm d,max =1 = ; N666.8 dmax = X:.9371 Y: 666 X:.4881 Y: X:.2929 Y:

6 59 C.Jagiełowicz-Ryznar.8 max = X:.947 Y:.6181 X:.4841 Y: (t) X:.2879 Y: Fig.3. Graph of the function Nd () t and d () t for a) m = [g], b) m =1 [g]. c) 8 Model of Model the automotive zawieszenia vehicle samochodu suspension. The function Przebieg dla process m = for 5; mn d,max = 5 = ; N d,max = X:.9171 Y: 679 X:.4868 Y: X:.2888 Y: max = X:.9359 Y:.6167 X:.4859 Y: (t) X:.2888 Y:

7 Dynamic axle load of an automotive vehicle when driven 591 d) 8 Model of the automotive vehicle suspension. Model zawieszenia samochodu The function Przebieg process dla for m m = 1; = 1 N ; N d,max d,max = = X:.9159 Y: 693 X:.4863 Y: X:.2954 Y: max = X:.9246 Y:.615 X:.4845 Y:.5.2 (t) X:.2894 Y: Fig.3. Graph of the function Nd () t and d () t for c) m = 5 [g], d) m =1 [g]. e) 8 Model of the automotive vehicle suspension. The function Model process zawieszenia for m = 2 samochodu ; N d,max = Przebieg dla m = 2; N d,max = X:.891 Y: 6118 X:.4888 Y: X:.2961 Y:

8 592 C.Jagiełowicz-Ryznar.8 max = X:.9164 Y:.6117 X:.4862 Y: (t) X:.2894 Y: f) 8 Model of the automotive vehicle suspension. The function Model process zawieszenia for m samochodu Przebieg dla m = 5; = 5 ; N N d,max = d,max = X:.136 Y: 2983 X:.9179 Y: 6456 f 1 =2,5215 [Hz] f 1 =2,54 [Hz] X:.4912 Y: f 2 =25,1 [Hz] f 2 =24,6 [Hz] -4 X:.2915 Y: max = X:.8989 Y:.626 X:.4883 Y: (t) X:.2943 Y: Fig.3. Graph of the function Nd () t and d () t for e) m = 2 [g], f) m =5 [g].

9 Dynamic axle load of an automotive vehicle when driven 593 g) 8 Model of the automotive vehicle suspension. The function Model process zawieszenia for m samochodu Przebieg dla m = 1; = 1 ; N N d,max = d,max = X:.1996 Y: 4119 X:.7432 Y: 786 X:.4898 Y: X:.2825 Y: max =.66.6 X:.7339 Y: X:.498 Y: (t) X:.287 Y: h) 1 Model of the automotive vehicle suspension. Model zawieszenia samochodu The function Przebieg process dla mfor = m25; = 25 N ; N d,max = d,max = X:.2938 Y: 6386 X:.194 Y: 8676 X:.4764 Y: X:.1874 Y: X:.2534 X: Y: Y:

10 594 C.Jagiełowicz-Ryznar.8 X:.194 Y:.775 max =.71.6 X:.2938 Y: X:.1854 Y:.1428 X:.4734 Y:.4617 (t) X:.2514 X: Y: Y: Fig.3. Graph of the function Nd () t and d () t for g) m = 1 [g], h) m =25 [g]. Fig.4. Graph of the function: N d,max and d depending on Table 1. The calculation results of frequency and coefficients of normal vibration modes as a function of m g % m m n f Hz o1, fo2, Hz 1 2 (1) > e e e e e e-3.75 m. m.

11 Dynamic axle load of an automotive vehicle when driven 595 Table 2. The summary of of <; 25>. N d,max and d values depending on the weight of the axle (wheel) in the range m [g] N dmax [N] η d (1x1-3 ) The calculations show that for the value m 1[ g], which is about 1% of the body weight of the AV being analyzed, the maximum dynamic force and the dynamic coefficient have an approximately constant value d 62,. When the value of m is about 2 g (approximately 2% of the body weight), the dynamic coefficient reaches a minimum d,min 61.. To verify this result more accurately, additional calculations were performed in the range m 1 35 [g], changing the value of m each time by 25 [g]. The detailed calculation results for N d,max and the coefficient d in the range of: m 1; 35 with the accuracy of 25 [g] of the reduced axle mass (the accuracy of the calculations.25%) are tabulated in Tab.3 and shown in the graphs (Fig.5). Table 3. The values N d,max and d depending on axle weight (wheel) in the range <1; 35>. m [g] N dmax [N] η d (1x1-3 ) m [g] N dmax [N] η d (1x1-3 ) Fig.5. Graph of the function: N d,max and d depending on m in the range <1; 35>. The calculations show that the force N d,max comes to a local minimum for m 325 [g] with the accuracy 25 [g], while the coefficient d has 2 local minima: for m 1 () 2[g] equal to d1 (). 6117, and m2 ( ) 325 [g] equal to d( 2). 65, [g], when d( 2) d( 1). After exceeding

12 596 C.Jagiełowicz-Ryznar the value m 5[g], there is a substantial, approximately linear growth both of the force N d,max and the coefficient d. (Fig.5). When the value is m 25[ g] (25% of weight of m z ) d,max 71. which is a growth of about 17.4%, relative to the minimum value (.65). The static force exerted by the MMP and acting on the axle can be determined by Nd,max N. (3.1) sta d On the basis of calculation results (Tab.1), d was determined for the range m 5; 5, which is from.5% to 5% of the mass m n. The average dynamic pressure force on the axle in this range is Nd,max, śr 627 [N]. The accuracy of the force measurement can be assumed to be of 5 [N]. Upon substitution to Eq.(3.1), Nsta, śr 132 [N] is obtained. The maximum error (for the values assumed above) is Nsta, śr 161 [N], which represents approximately 1.6%. The actual static Nsta, rz approx pressure force for m 275 [g] and mn 1 [g] is [N], which gives an error of Nsta 132 [N], approx., 5% in relation to the value determined by the MMP. 4. Conclusions The wheels mass (with axles) does not have much effect on the 1st (fundamental) AV s frequency (Tab.1). The wheels mass has a significant effect on the 2nd frequency (Tab.1). For a given AV (body mass) there is a certain value of the axles mass, for which the dynamic coefficient d reaches a minimum (Fig.4). For larger masses (above 5%), the dynamic force acting on the axles increases approximately linearly (and so does the dynamic coefficient) (Fig.3). The dynamic axle load is generated mainly by the 1st frequency (particularly, when the axle mass is small) (Figs 3a-3h). The second self-resonant frequency is revealed clearly with heavier axle mass (above 2 [g], i.e., about 2% of the body mass. Vibrations having this frequency disappear rapidly due to the damping action of the shoc absorber. Nomenclature f 1,, f 2, the first and the second frequency of free vibrations of the system g gravitational acceleration h platform height z stiffness of the suspension wheel tires stiffness og L platform length m axle mass m axle mass corresponding to d1 (), d2 ( ) coefficients m n body mass N() t total axle load () t dynamic axle load m 1 (), ( 2) Nd N maximum dynamic axle load d,max N sta static load N real static load sta, rz P w vibration forcing function

13 Dynamic axle load of an automotive vehicle when driven 597 S w value of impulse of the exciting force ut () lifting displacement u lifting speed in vertical direction V axle speed in vertical direction V vehicle speed p V u lifting speed z vertical axle displacement z n vertical body displacement z axle speed in vertical direction z n body speed z axle acceleration in vertical direction z n body acceleration a shoc absorber damping coefficient wheel tires damping coefficient og d dynamic coefficient maximum dynamic coefficient d,max d1 (), d2 ( ) References dynamic coefficients for the local minimum 1 and 2 orthogonal coefficient 1, 2 coefficients of the first and the second normal vibration modes i, main frequencies of the system Journal of Laws of the Republic of Poland (212): No. 161 of 25 th Sept.. Jagiełowicz-Ryznar C. (213): Dynamic analysis of the vehicle in the movement on the measurement platform. XII Symposium Environmental effect of vibrations Cracow Wiśnicz. Kalisi S. (1966): Vibrations and waves in solids. IPPT PAN, Warsaw. Mitsche M. (1989): Motor vehicle dynamics. Vibrations; WKiŁ, vol.2., Warsaw. Traffic Law Act of 2 th June, Received: May 12, 214 Revised: June 22, 214