experimental evaluation of dynamical

0 downloads 0 Views 114KB Size Report
Fortuna Z., Macukow B., Wąsowski J., Metody numeryczne, WNT, Warszawa 1993. 4. Graham B. B., Using an accelerometer sensor to measure human hand ...
EXPERIMENTAL EVALUATION OF DYNAMICAL MEASUREMENTS ACCURACY BASING ON THE LENGTH OF BREAKING DISTANCE OF A MOTOR CAR Marian Dudziak1, Andrzej Lewandowski2, Konrad J. Waluś1 Poznań University of Technology, Chair of Basics of Machine Design 2 Institute of Forensic Research 1

Summary: In the article there are shown the results of the experimental verification of dynamical parameters of car movement. The experimental verification was carried out by measurement of accelerations and angular velocity of car bodywork tilting using piezogyroscope and acceleration sensors. Besides for each test of intensive breaking there was statically measured the distance traveled by the car. There were carried out the statistical analysis and determined the measurement accuracy of designed appliance. Key words: acceleration sensor, piezo-gyroscope, vehicle motion dynamics, acceleration measurement accuracy

1. Introduction Measurements of vehicle dynamical characteristics are carried out for determining the values of acceleration, velocity and traveled distance obtained in any moment. Determining of these parameters, if it is possible, is carried out basing on road experiments (tests) using the adequate measuring apparatus. In cooperation with Forensic Investigation Institute in Cracow and Chair of Basic of Machine Design of Poznan University of Technology there was designed and manufactured the measuring instrument equipped with acceleration microchips, piezo-gyroscope and a photocell. Designed integrated module of sensors (IMS) takes current readings of passing longitudinal accelerations and of angular velocity of bodywork tilting. The rest of the car movement parameters, velocity and traveled distance are determined basing on measured values of accelerations. The designed measured instrument can be used for tests of tires, characteristics of suspension and of checking of the influence of type and condition of the road surface on changing process of movement parameters. IMS can be also utilized for estimation of car movement parameters in case of carry out accident event reconstruction necessity, what is important very much for contentious courts. The purpose of this elaboration is to present the method of dynamical measurements accuracy verification of longitudinal accelerations of motor car basing on the braking distance measurement. The assessment of the measuring sensors kit accuracy was done by the comparison of the motor car travel measured by wind-up measuring tape and the motor car travel calculated basing on the temporary longitudinal accelerations.

2. Characteristic of measuring system Acceleration sensors utilized in the designed measuring appliance have build-in inertial mass, which during its movement produces the change of capacitance and generates the output voltage proportional to its displacement. Created analog signal after amplification is given to analogue-digital card (A/D card) where is subjected to sampling, quantization and digitizing. Acceleration sensors are characterized by a very high sensitivity and that is why there is the necessity of separation of signals originated from earth gravitational field forces. It was calculated that size of gravitation component is dependent on car’s bodywork tilting angle, and e.g. for tilting angle 3 degrees is about 0.5 m/s2. The necessity of separation of longitudinal acceleration and acceleration originated from earth signals involves usage of piezo-gyroscope sensor for vehicle bodywork angle displacements measurement. Influence of vehicle bodywork tilting in vertical plane round the crosswise axis on results of temporarily accelerations is shown in figure 1. Piezo-gyroscope, serving for vehicle angle velocity measurements, generates the analogue voltage signal caused by piezoelectric crystal distortion. This signal after conversion into digital signal in A/D card is written on computer’s disc. Axis of piezogyroscope may be placed in any distance to tilting axis of vehicle bodywork, but both axes should be parallel to each other, what is connected with the sensor structure and its principle of operation. The measuring system besides acceleration sensors, piezo-gyroscope, photocell and A/D card consists of two specialist programs. First of them makes possible data raising and archive them on computer’s disk with frequency 1000 Hz, and the second one makes possible measured results graphical presentation and determining of mean values in given time intervals and also determining values of velocity and traveled distance. Analysis of so far laboratory measurements [7, 8] and road tests [6, 9] enabled for determining of the following advantages of the integrated module of sensors: - repeatability of measurements, - easy calibration, - easy data archivisation, - small dimensions, - possibility of non-invasive assembling on tested vehicle.

α

ax a’ x

a ’

az

az

Fig. 1. Influence of vehicle bodywork tilting on measurement results

3. Method of measurement results verification The designed integrated module of sensors serving for a measurement of dynamic characteristics of a vehicle required the determining of its accuracy of measurement results obtained during measuring tests. Experimental verification was done by carrying out tests of intensive motor car breaking. Evaluation of measurement accuracy of the sensors set was done by the comparison of traveled distance measured by a measuring tape and destined basing on temporary longitudinal accelerations. This method of verification is shown in the figure 2. The photocell included in the measurement system, non-invasive assembled on a vehicle, responds to reflective strips placed near the vehicle path. Passing of strips is registered by the photocell as a binary event, what is causing appearing in the signal the characteristic points enabling carrying out the measurement of traveled by a vehicle distance on interval L (Fig. 2). Interval L measured by measuring tape is compared to the traveled distance calculated basing on temporary values of acceleration. Carried out road tests archived on the computer disk enabled carrying out analysis of breaking process course, in a separate program, of traveled distance by a vehicle, by making double numerical integration of temporary values of acceleration. Obtained values of traveled by a vehicle distances are burdened with measuring system errors and with error originated by taken method of numerical integrating. Total error depends on type Newton-Cotes quadrature and on measure value in given point [2, 3]. Therefore it can be supposed that accuracy of accelerations measurements is higher than accuracy of traveled distance.

L Rys. 2. Verification method of traveled by a vehicle distance and its velocity

4. Measuring results and statistic analysis In the year 2005 there were carried out road tests of intensive breaking on a wet asphalted surface, constantly sprinkled with water. The test motor car was Ford Mondeo equipped with anti-lock braking system – ABS. During tests on the edge of the road there were spaced the reflective strips actuating the photocell and being the base referring to measurements using the measuring tape as well. There were carried out 10 tests of intensive km breaking starting from initial speed about 90 . All tests were archived on computer disk, h and obtained results are given in table 1. During carrying out of measurement tests the vehicle always started the process of breaking before the spaced reflective strips and at the

moment of passing them it had full deceleration possibilities to achieve for prevailing conditions. Table 1. Comparison of traveled by the vehicle distance measured with measuring tape and calculated using temporary values of the vehicle longitudinal accelerations Distance determining with IMS SIMS [m] 17,91 18,34 18,69 18,73 12,67 27,72 20,20 24,59 21,86 25,33

Measurement of the travel distance with measuring tape L [m] 18,00 18,50 18,60 18,50 12,65 27,45 20,05 24,55 21,90 25,40

Total breaking distance Sh [m] 41,51 40,26 42,23 39,06 36,66 46,79 42,86 41,09 42,97 42,75

Distances difference Δ = L – SIMS [m] 0,09 0,16 -0,09 -0,23 -0,02 -0,27 -0,15 -0,04 0,04 0,07

In table 1 in the first column there is shown the total breaking distance of motor car since moment of brake actuating. In the second column there is given the distance L (according to Fig. 2) between the reflecting strip and assembled on the vehicle photocell after stopping of the vehicle. In the third column there is given distance calculated basing on temporary values of accelerations, and in the fourth one there is presented the difference between measurements carried out with measuring tape and values obtained basing on numerical calculations. Besides determining of basic parameters of a vehicle movement such as: acceleration, velocity, and traveled distance, IMS cooperating with software enables examination of intermediate states courses of accelerations and car’s bodywork tilting angles. Registered exemplary deceleration course of a car during breaking is shown in the figure 3. 4

Acceleration [m/ss]

2 0 -2 -4 -6 -8 -1 0 -1 2 -1 4 13

1 3 ,5

14

1 4 ,5

15

1 5 ,5

16

1 6 ,5

T im e [s]

Fig. 3. Exemplary deceleration course during breaking process

17

Speed [m/s] Distance [m]

In deceleration course, given in figure 3, intensive breaking process lasted about 3 seconds, and value of Mean Fully Developed Deceleration (MFDD) for a whole breaking m process was 9,1 2 . Basing on obtained course there is also a possibility to determine of s transitory times of acceleration increasing and decreasing, and duration time of the whole breaking process as well. In the drawing 4 there are shown velocities courses and distances determined basing on acceleration course and current car’s bodywork tilting angle. 45 40 35 30 25 20 15 10 5 0

216 211 206 201 196 191 186 181 176 171

13

13,5

14

14,5

15

15,5

16

16,5

17

Time [s] Fig. 4. Exemplary velocity course and the traveled distance during breaking process

In designed measuring system sensors give analog signal on output, which in analoguedigital card is subjected to quantization, sampling, digitizing with given measuring frequency. This signal is burdened with an error, arising from a measuring path and errors of analogue signal transformation into digital signal. Carried out statistical analysis consisted in determination of standard deviation for an individual measurement of vehicle traveled distance. Measurements of this distance carried out with measuring tape were treated as model measurement which is not burdened with any error. For results obtained from measuring tests represented in table 1 (on wet asphalted road surface) standard deviation for a single measurement of obtained results was σS = 0,15 [m], and after applying of corrections according to t-Student distribution for 10 measurements and for confidence level 95 %, error of determination of traveled distance was σS = 0,34 [m]. Carried out tests have shown that difference between measured travel distance and travel distance calculated basing on temporary vehicle accelerations hasn’t excided 1.5 % in any case, and majority of results was burdened with relative error less than 1 %. For presentation of influence of car bodywork tilting angle on indications of singleaxis acceleration sensor there should be noticed that the differences between calculated traveled distances (without signals separation from earth gravitational acceleration) and measured traveled distances has pointed that obtained values always exceeded 4 %.

5. Conclusion Vehicle acceleration measurements carried out with integrated sensors module enable for determination of traveled distance with uncertainty less than 1.5 %. Acceleration measurements, because of transformation being subjected during process of conversion from analog form into digital form and according to processing method, are burdened with errors originated from these transformations. Basing on above it is considered that accelerations measurements are carried out with higher accuracy than determined travel distance.Experimental verification of integrated sensors module together with its software has demonstrated its usability for testing of dynamical characteristics of vehicle movement. The designed instrument can be applied in case of road accidents reconstruction processes, in case of testing of tires and testing of car suspension and also in cases of researches of influence of type and state of road surface on motion parameters changing.

6. Literature 1.

Dąbrowski Z., Dziurdź J., Klekot G., Radkowski St., Laboratorium podstaw pomiarów wielkości dynamicznych, Warszawa 2005. 2. Fichtenholz G. M., Rachunek różniczkowy i całkowy, Tom 2, Wydawnictwo Naukowe PWN, Warszawa 1997 3. Fortuna Z., Macukow B., Wąsowski J., Metody numeryczne, WNT, Warszawa 1993. 4. Graham B. B., Using an accelerometer sensor to measure human hand motion, Massachusetts Institute of Technology, 2000. 5. Hagel R., Zakrzewski J., Miernictwo dynamiczne, WNT, 1984. 6. Lewandowski A., Waluś K. J., Dynamiczny pomiar przemieszczeń nadwozia samochodu osobowego, XIV Konferencja nt. „Metody i środki projektowania wspomaganego komputerowo”, Politechnika Warszawska 2003, str. 275-280. 7. Lewandowski A., Waluś K. J., Wykorzystanie czujników przyspieszeń do wyznaczania parametrów ruchu pojazdu (wyniki badań laboratoryjnych), W: FRICTION 2002, Wyd. Politechniki Warszawskiej, Warszawa, 2003, str. 205-210. 8. Waluś K. J., Problematyka wyznaczania parametrów ruchu obiektu na podstawie jego przyspieszenia, Zeszyty Naukowe Politechniki Poznańskiej nr 58, str. 97-102. 9. Waluś K. J., Wyznaczanie parametrów ruchu samochodu osobowego podczas hamowania z wykorzystaniem czujników przyspieszeń, Maintenance and Reliability, (Niezawodność i Eksploatacja), Nr 2(22)/2004, str. 69-73. 10. Katalog WWW Analog Devices.

DOŚWIADCZALNA OCENA DOKŁADNOŚCI POMIARÓW DYNAMICZNYCH NA PODSTAWIE POMIARU DŁUGOŚCI DROGI HAMOWANIA SAMOCHODU OSOBOWEGO Streszczenie: W artykule przedstawiono wyniki doświadczalnej weryfikacji pomiarów dynamicznych parametrów ruchu samochodu. Weryfikację doświadczalną zrealizowano dokonując pomiarów przyspieszeń oraz prędkości kątowej przechyłu bryły nadwozia za pomocą piezożyroskopu i czujników przyspieszeń. Ponadto dla każdej próby intensywnego hamowania mierzono statycznie przebytą przez pojazd drogę. Wykonano analizę statystyczną i określono dokładność pomiarową zaprojektowanego urządzenia. Słowa kluczowe: czujniki przyspieszeń, piezożyroskop, dynamika ruchu pojazdu, dokładność pomiarów przyspieszeń

Author(s): Marian Dudziak Prof. Dr. Eng. Politechnika Poznańska, Katedra Podstaw Konstrukcji Maszyn Andrzej Lewandowski Ph. D. Eng. Instytut Ekspertyz Sądowych w Krakowie Konrad J. Waluś M. Sc. Eng. Politechnika Poznańska, Katedra Podstaw Konstrukcji Maszyn