18 ATZ worldwide

1 Introduction

Research activities in the field of drive trainengineering require a variety of investiga-tions, and in particular the integrated con-sideration of the complete system, sub-sys-tems, parts and elements. The aim of re-search work conducted at the Institute ofMachine Design and Automotive Engineer-ing at the University of Karlsruhe, Germany,is to carry out drive train engineering as aclosed process using modern design metho-dology.

Due to customer demand for ever safer,more economical and more comfortable ve-hicles, the entire vehicle system becomesmore and more complex and the interactionof different sub-systems is constantly in-creasing.

Customer demands and the competitive sit-uation in the market require continuouslynew and faster product development. Toachieve this, the integrated consideration ofthe product development process is one pre-condition. An integrated development envi-ronment in addition to real driving situa-

tions in road traffic includes experimentsand simulations. Only in this way can thedevelopment process be managed efficient-ly with a simultaneous reduction in costsand increased customer acceptance. The realdriving situations are characterized by indi-vidual drivers and stochastic environmentalinfluences. Ultimately, they reflect the cus-tomer’s driving impression and thus influ-ence the decision to buy a certain vehicle.The tests include standard driving tests inthe vehicle, an evaluation of comfort on theacoustic test bench, a driver classification atthe comfort evaluation centre as well as testbench examinations of the drive train, itscomponents and elements, including thephysical effects involved. Furthermore, allthese tests are accompanied by suitable nu-merical simulation methods in order toachieve a development process that is as op-timal as possible, Figure 1.

In this article, part of the integrated develop-ment environment of the Institute of Ma-chine Design and Automotive Engineeringat the University of Karlsruhe, Germany, isintroduced, using clutch chatter in an auto-motive drive train as an example. The chat-ter phenomenon is examined both in the ve-hicle and on the test bench.

2 The Universal Drive Train Test Bench

A large clamping plate is available for fixingthe mechanical parts on the drive train testbench, Figure 2. This allows all current drive

train configurations to be set up and exam-ined under conditions which are as close toreality [1] as possible. The test samples –complete drive trains or their single compo-nents – are connected to the drive unit in amanner that is rigid to torsion, in order topermit torsional vibration excitation over awide range of frequencies.

The drive unit is made up of two dynamicsynchronous electric motors which can becombined to form one drive motor by meansof a pick-off gear. This drive unit allows en-gine speed ranges of current internal com-bustion engines, including their irregularityof rotation in a frequency range up to 250 Hz,to be achieved. In this way, different enginecharacteristics, numbers of cylinders andtypes of combustion can be flexibly simulat-ed.

Two asynchronous electric motors are uti-lized as retarders to cover the whole spec-trum of speed and torque of the drivingwheels of a mid-size vehicle. To actuate theclutch, a disengagement device is availablewhich allows the total disengagement dis-tance to be covered in less than 100 ms.

The test bench control system has a modularhierarchical composition. The user first de-fines the configuration of the test bench andsets the limiting values for speed and torquein the preparation stage. The test sequencecan be defined by freely configurable testsoftware.

During the test, the driving resistances arecalculated online, corresponding to the para-meters of the vehicle and the driving cycleprofile. The characteristic torque oscillationis read out of a map during operation and ac-tuated in the drive unit. The simulation ofthe engine and of the vehicle parameters al-lows a simple variation of the boundary con-ditions. Control of the test sequence, com-munication with the user, test bench controland data logging are handled by differentcomputers. Due to the good accessibility ofthe test sample and because dangers andconstraints of real driving tests do not exist,it is easier to measure engine speed, torque

Development DriveTrain

The Institute of Machine Design and Automotive Engineering at theUniversity of Karlsruhe, Germany, has established an integrated devel-opment environment for drive trains.The possibilities offered by theintegrated approach are shown by the example of an investigation ofclutch chatter.The practicality of the applied strategy has been provenon the basis of corresponding results from measurements in the car,on the test-bench and in a numerical simulation environment.

By Albert Albers, Arne Krüger,Ralph Lux and Marc Albrecht

Drive Train Examinations Based on theExample of Clutch Chatter – Integrated Drive Train Development

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Prüfen von Antriebssträngen amBeispiel des Kupplungsrupfens –Ganzheitliche Antriebsstrang-entwicklung

You will nd the gures mentioned inthis article in the German issue ofATZ 1/2001 beginning on page 44.

19ATZ Automobiltechnische Zeitschrift 103 (2001) 1

and temperature on the test bench. Further-more, the universal drive train test benchmakes available a testing environmentwhich allows specific environmental condi-tions, such as temperatures, to be definedand controlled.

3 The Test Vehicle

The test vehicle available at the institute isan upper mid-size saloon car. Therefore, us-ing both the test bench and the test vehicle,identical drive trains exist for comparativeand complementary studies. This vehicle isused for carrying out tests on the subjectivecomfort impression in order to find a correla-tion between objectively measured valuesand the subjective evaluation by the drivers.Characteristic objective measures are, for ex-ample,:– torque at the prop shaft– speed at the flywheel– speed at the gearbox– speeds at the rear wheels– temperatures at the gearbox and the dif-

ferential gear– accelerations at the seat, steering wheel,

pedals, gearshift, etc.– accelerations of characteristic parts of the

driver’s body [2].

The data are recorded by means of a high-speed data recording system called TORna-dO, developed and produced by the compa-ny Atlas Fahrzeugtechnik GmbH [3]. Themeasurement data are stored on a PC. Thecomfort impression can be influenced for ex-ample by gear noise, by chatter or by buck-ing vibrations. During these tests, the dri-ver’s sensitivity threshold must first of all beconsidered. The test vehicle is used to exam-ine driver collectives. This article examinesone typical and tangible comfort featurewhich depends only on the drive train –clutch chatter.

4 Clutch Chatter in General

By definition, clutch chatter refers to thosevibrations which occur during the slippingphase of the clutch in the drive train of a ve-hicle and which are generated in the clutchregion. Chatter results when a slippingclutch generates periodically changingtorques which are in the range of the naturalfrequency of the drive train when it is dy-namically separated by the clutch. The firstnatural frequency of vehicle drive trains un-der those circumstances are in a range be-tween 8 and 12 Hz. Due to their high efficien-cy, modern drive trains have a low damping

effect, which means that chatter oscillationseasily occur. Chatter can be caused either byself-excited or by forced oscillations. Self-ex-cited oscillations are caused by a decline inthe coefficient of friction of the clutch plateover the slipping speed, and can assumeconsiderable dimensions depending on thedegree of drive train damping.

Forced chatter is caused by external sources.It can be caused, for instance, by axial vibra-tion of the crankshaft or by misalignment atthe pressure plate in combination with an-gle offsets, some of which are even withinpermissible tolerance ranges [4, 5]. Accord-ing to the combination of these geometricaldefects, chatter is dependent on engine, dif-ferential or gear speed.

The rotational oscillations in the drive traincaused by clutch chatter are transformedinto longitudinal oscillations by the wheelsand are transmitted to the passengers by theoperating controls (pedals, steering wheel,etc.) and the seats. Chatter is perceived asuncomfortable vibrations and oscillations,which may also be accompanied by noise.The dynamic transmission behaviour of thevehicle may lead to a pronounced amplifica-tion of the vibration. In the following, a de-scription is given of how the clutch chatterphenomenon is examined at the institute,both in the vehicle, on the test bench and insimulation.

5 Clutch Chatter in the Vehicle

Clutch chatter in the vehicle can be deter-mined by measuring the speed, torque andacceleration or by the subjective evaluationof the driver. The advantage of measuringphysical variables is that they are indepen-dent of the driver, whereas subjective mea-surement reflects the human sensitivity tovibration and noise. Hence, the subjectiveevaluation is more customer-relevant. Be-cause an approximate correlation can bemade between a subjective evaluation anda measurement of acceleration, conclu-sions for the subjective evaluation can bedrawn from the acceleration signals. Thiscorrelation in the area of comfort evalua-tion is the object of research at the Instituteof Machine Design and Automotive Engi-neering.

Figure 3 shows a driving off process at idlingspeed. The engine speed measured at the fly-wheel, with the 3rd order torque oscillationtypical of a six-cylinder engine, is plotted.The speed of the clutch plate measured atthe gear shaft is also shown. It shows signif-

icant chatter vibrations during the drivingoff process in the region between 22.3 and26.5 seconds. The longitudinal accelerationcaptured at the seat rail (shown in the lowerpart of the diagram) shows vibrations withchatter frequency and an acceleration of 0.4m/s2. These vibrations are sensed by the dri-ver because his threshold of sensitivity forlongitudinal vibrations is about 0.2m/s2.

Due to a better transparency and the inte-grated examination method, the drive traintogether with the whole vehicle is repre-sented as a simulation model. By this means,influences can be better determined,changes can be made and their effects can bemore easily evaluated. Figure 4 shows thesimulation model of the test vehicle. Thecomplete drive train including the connect-ed masses of the vehicle is shown next to asimple engine model.

6 Clutch Chatter on the Test Bench

On the test bench, clutch chatter can be gen-erated in the same manner as in the car.Thanks to the ease of assembly and its acces-sibility, the test bench makes it possible toadjust geometric deviations and inaccura-cies in assembly or to use different combina-tions of clutch linings with characteristicgradients of the coefficient of friction. Thisallows the different chatter phenomena tobe stimulated. Because the clutch used onthe test bench was new, it showed only alow tendency to chatter. For this reason, wa-ter was sprayed onto the clutch disk to gen-erate self-excited chatter. Figure 5 shows thespeeds of the drive unit with the typicalspeed oscillation as well as the speed of theclutch disk. Chatter vibrations can be detect-ed in the speed signal of the clutch disk. Dueto the type of excitation in combination withthe new clutch disk, a different curve ap-pears for the vibrations.

On the universal test bench described here,longitudinal accelerations cannot be mea-sured, since the bench is set up withoutwheels or a vehicle. However, it is possible tocalculate the resulting longitudinal accelera-tions at the seat rail using the transmissionbehaviour determined in the actual vehicle.For this, the data recorded on the test benchis used as input data for the simulation mod-el, which is then validated on the test vehi-cle. The present stage of development doesnot provide a feedback to the driver of the vi-brations generated on the test bench. Futuredevelopments will incorporate the driverand his evaluation ability.

DevelopmentDriveTrain

20 ATZ worldwide

7 Simulation for Calculating the Longitudinal Acceleration

A quarter-vehicle model is used to calculatethe accelerations at the seat rail [6]. In Figure6, the rear part of the drive train beginningat the clutch disk, with the gearbox, the propshaft and the axle drive up to the wheels, issimulated as a system of rotating masses.The module for the wheel transmission isfollowed by the translatory wheel stiffness,the axle suspension, the sub-frame and theremaining vehicle mass, in which the accel-erations are recorded. The torque, which ismeasured at the prop shaft of the vehicle, isintroduced into the simulation model at thesame place.

It is also possible to achieve an ideal self-ex-citation by pre-defining the characteristic ofthe coefficient of friction, the clutch actua-tion and the engine speed control (notshown) [7]. Measurements from the vehicleare used to verify the quality of the model.The torque at the prop shaft and the longitu-dinal acceleration at the seat rail are mea-sured in the vehicle. Thus, the accelerationscalculated in the simulation model, based onthe torque in the prop shaft, can be com-pared to the measured ones. As can be seenin Figure 7, the simulated accelerations andthose measured at the seat rail show a goodcorrespondence.

Therefore, using this simulation model, theresulting longitudinal accelerations for thevehicle being examined can be simulated forthe drive train oscillations measured on thetest bench. This clearly demonstrates that

drive train engineering cannot be carried outon single components but must consider thecomplete system. When it comes to clutchchatter, the characteristic transmission be-haviour of the vehicle from the driving axlethrough the vehicle body to the seat is whatmainly decides whether the vibrations gen-erated in the clutch are perceived as disturb-ing by the driver. Using the calculated longi-tudinal accelerations, initial predictions canbe made about their influence on the subjec-tive evaluation. This is illustrated in Figure 8for the chatter vibrations created on the testbench.

8 Conclusions

The examination method presented in thispaper clearly proves that an efficient prod-uct development process is only possiblewith an integrated testing concept. If theyare considered separately, examinations onthe car, on the test bench and in the simula-tion do not lead to the synergy effects of theholistic method.

Driving tests consider the driver in the eval-uation of the vehicle, but, due to the com-plexity of the total system and the variety ofpossible influencing parameters, they can beused only to a limited extent in defining andsolving problems. This is where the simula-tion environment is most useful, enablingdifferent variants of solutions to be testedand evaluated quickly and easily. Takinginto account the judgment of the driver al-ready at this early stage provides informa-tion about customer acceptance. The varia-tions showing the maximum potential in

the simulation are then tested in real condi-tions on the test bench under reproducibleconditions, and a further selection of solu-tions can then be made. In the vehicle, onlythe already implemented and tested vari-ants need to be adapted and checked. Thisapproach ensures that one of the best solu-tions is found. As a result, time and develop-ment costs can be reduced with a simultane-ous increase in customer satisfaction.

References[1] Albers, A.; Lux, R.; Burger, W.: Neuartiger, uni-

versell einsetzbarer Antriebsbaugruppen-prüfs-tand. VDI-Berichte Nr. 1470, Düsseldorf: VDI-Verlag 1999, S. 143-161

[2] Scholl, P.: Untersuchung des Zusammenhangsvon messbaren physikalischen Größen am oderim Fahrzeug und subjektivem Empfinden derInsassen bei Fahrzeuglängsbe-schleunigungen,welche durch plötzliche Motorleistungszugabeoder -wegnahme verursacht werden. Diplomar-beit Institut für Maschinenkonstruktionslehreund Kraftfahr-zeugbau Nr. 542, Universität Karl-sruhe (TH), 1990

[3] AFT Atlas Fahrzeugtechnik GmbH (Hrsg.): Tor-nado The Automotive High Speed Dataloggerby A F T. User Manual, Werdohl, AtlasFahrzeugtechnik GmbH 1998

[4] Albers, A., Herbst, D.: Kupplungsrupfen – Ur-sachen, Modellbildung und Gegenmaßnahmen.VDI-Berichte Nr. 1416, Düsseldorf : VDI-Verlag1998

[5] Albers, A.: Selbsteinstellende Kupplung (SAC)und Zweimassenschwungrad (ZMS) zurVerbesserung des Antriebsstrangkomforts. VDI-Berichte Nr. 1175, Düsseldorf : VDI-Verlag 1995

[6] Albers, A.; Lux, R.; Krüger, A.: Dynamiksimula-tion komplettiert Prüfstands- und Fahrver-suche. Simulation im Maschinenbau 24./25. Feb-ruar 2000, Dresden, S. 349-366

[7] Pfeiffer, F.: Das Phänomen der selbsterregtenSchwingungen. VDI-Berichte Nr. 957, Düssel-dorf : VDI-Verlag 1992

Development DriveTrain