on the same test circuit consisting of typical road profiles ranging from a country .... only on professional test drivers from TÃV SÃD Auto CZ. A typical result is in ...
BULLETIN OF APPLIED MECHANICS 4(15), 99 -101 (2008)
Experimental Verification of Correlation between Objective and Subjective Evaluation of Passenger Car Vibration Comfort Valášek M., Pelikán, J., Úlehla, J., Vaculín, O., Steinbauer P. Abstract— T he paper deals with the exper imental investigation of the corr elation between obj ective and subj ective evaluation of vibration comfort in passenger car s. T he r ide comfor t plays an impor tant r ole in the vehicle design s well as in vehicle j udgment. Curr ent standar d appr oaches towar ds vibr ation comfor t evaluation more or less focus on onedimensional vibr ation evaluation of human body response. T his paper descr ibes the r esults of exper imental ver ification of a new approach for vibration comfort in passenger car s. This new approach measur es the influence of multiple vibr ation modes of a vehicle on the integral subj ective human vibr ation per ception. T he exper iments have ver ified that the cor r elation bet the obj ective and subj ective vibr ation comfort evaluation is almost 1:1. M oreover this paper descr ibes the r epeated exper iments that pr ove the concept in larger scale. I ndex Terms— Ride comfor t; vibr ation comfort assessment; vehicle vibr ations; human per ception; subj ective and obj ective measure; cor relation.
I NTRODUCTION
I.
T
he ride comfort plays an important role in the vehicle design as well as in vehicle judgment. The objective evaluation of vibration comfort is important for compu er simulation of human perception of vibration comfort because the methods of virtual product design with the aid of comp ter simulation made a significant progress during the last decade. Virtual prototypes of road vehicles comply very well with the reality in vehicle handling domain or in passive safet domains. However, there are still cases in the product assessment process, in which experiments with human test persons unavoidable, since objective measures have not been developed yet or simulation models are not enough advanced such as ride comfort. Ride comfort counts to the cases in which some development potential seems to be desirable despite so e already developed approaches for objective measures ex . ISO 2631 -1:1997 / VDI 2057, British Standard BS 6841:1987, or SAE -J 1013. Current standard a pproaches towards vibration comfort evaluation more or less focu on one -dimensional vibration evaluation of human body response [1], but the human body seems to be sensible to complex multidirectional vibrations. Then the fulfilment of such objective measures does not guarantee in all cases that the vehicle is really subjectively assessed as comfortable by its
potential users. The ultimate goal of the presented research is to develop objective measures for the assessment of the vibration comfort of vehicles. Such methodology has its potential in enabling the comfort evaluation also in free design stages of the vehicle even for the virtual prototypes and in acceleration the assessment of the existing vehicles. This paper deals with the comparison of subjective and objective assessment of vehicle vibration comfort. Such investigations are necessarily related to numerous field experiments, in which vehicle and/or occupant states are measured. This paper describes the results from such experiments and presents some important conclusions from the experimental results. The experiments were carried out repeatedly [3] and the comparison of their results is interesting. The experiments that take into account the multidimensional character of human perception of vibration comfort were based on the proposed approach [2]. This approach resulted i to very favorite comparison of objective and subjective evaluations. The objectives of this repeated comparison were (1) to verify that the method could be repeated with the same results (there are no missing details of this method) and (2) to obtain details that could be used for explanation and further develop ent of this method. For the comparison of the subjective and objective vehicle assessment a group of test persons evaluated a set of test vehicles from different OEMs. The subjective evaluatio was based on several ride comfort indicators. Furthermore, the group of the test persons was complemented with the professional test drivers. The objective information o the vehicle vibration level was captured with 16 accelerometers together with a special device evaluating comfort on t e seat according to the ISO 2631 -1. Both evaluations were performed on the same test circuit consisting of typical road profiles ranging from a country road to a motorway. The evaluation is based on decomposition of the vehicle vibration to several typical modes [2] that give the multidimensional excitation of human body for vibration perception.
II.PROPOSED SUBJECTIVE ASSESSMENT OF VIBRATION COMFORT The test circuit consisted of three sections: bumpy country road, main road and motorway. A set of ten passenger cars from different OEMs was objectively and subjectively evaluated for the ride comfort. The subjective
BULLETIN OF APPLIED MECHANICS 4(15), 99 -101 (2008) evaluation was based on a so-called reference vehicle. Other vehicles were assessed relatively to this reference case. A group of ten persons observed five ride comfort indicators on each car and assessed them with notes 1 -10. “1” represents the worst ride comfort, “10” generally the best vehi cle of all. The notes were set relatively to the reference vehicle as shown in Table 1. Furthermore, the group of the test persons was complemented with the professional test drivers from TÜV SÜD Auto CZ.
using transformation matrix derived from the vehicle geometry B and residual vector r (t ) as follows
m(t )− r (t ) = Ba(t ).
The matrix B is defined from rows for particular measurement mj at the point with coordinates [ l j ,aj] with respect to centre of mass (Fig. 2)
. Criteri a 1 2 3 4 5
A 5,00 5,00 5,00 5,00 5,00
B 4,00 3,50 4,00 5,00 5,00
Sub jective Vehicle Assesment C D E F G 6,00 4,00 4,00 4,50 3,50 4,00 4,50 3,00 4,00 5,50 6,00 5,50 4,00 4,00 4,00 6,00 5,00 3,00 5,00 4,40 4,50 4,50 4,00 3,50 4,00
(1)
z H 4,50 4,00 5,50 5,50 4,50
I 5,00 5,00 4,50 6,00 4,00
J 6,50 5,50 6,00 6,00 5,00
y
Table 1 Example of a vehicle assessment table (ride comfort indicators ).
mj[l j, aj, 0]
x Fig. 2 Position of the measurement point in the vehicle
[
mj = 1 − l j
aj
&z& &p& l j aj &r& && t
(2)
]
where z is heave, p is pitch, r is roll and t is torsion. It is supposed that the test passenger is sitting very near o the center of mass of the vehicle. The acceleration a(t )can be reconstructed from the measurements by solving the overdetermined system of linear algebraic equations (1)
Fig. 1 Location of basic accelerometers
III. PROPOSED OBJECTIVE ASSESSMENT OF VIBRATION COMFORT The objective information on the vehicle vibration level was captured with 10 basic accelerometers as indicated in Fig. 1. Moreover the vehicle was instrumented with 6 additional acceleration transducers together with a special device evaluating comfort on the seat according to the ISO 26 1 -1. In order to receive relevant data each vehicle passed the test circuit five times.
IV.
CORRELATION BETWEEN OBJECTIVE AND SUBJECTIVE EVALUATION
First the measured signals for the objective assessment were computationally processed and eva luated. Despite a majority of current comfort evaluation riteria is focused on vibrations only in one direction, an approach based on [2] was chosen. This approach takes basic vehicle vibration modes into the account. The evaluation is based on the decomposition of the vehicle vibration to four typical modes: heave, pitch, roll and carbody torsion. Global modal amplitudes a t are calculated from the measured data m t
()
()
a(t ) = (B T B) −1 B T m(t )
(3)
Further, the measured signals are evaluated in frequency domain. Power spectral density Φ ( f ) of the global modal amplitudes a(t ) is presented in Fig. 3.
Φ ( f ) = PSD (a(t )).
(4)
()
()
Using an evaluation function BM f from [2] ( BM f is an average of evaluation functions in Fig. 4), which defines the importance of frequencies the contribution of i -th modal amplitude K i (i.e. i = 1 for heave, 2 for pitch, 3 for roll and 4 for torsion) can be evaluated f2
() ()
K i = ∫ BM f Φ i f df 2
f2
2
.
(5)
The final contribution of all i observed modal amplitudes K i2 for the particular vehicle can be written as follows
BULLETIN OF APPLIED MECHANICS 4(15), 99 -101 (2008) 2 K objective = ∑ X i K i2
,
(6)
i
higher than four. In such a case e.g. least square method should be applied T T 2 [X i ]= ([K 2j ][ K 2j ]) [K 2j ][ K subjective ].
−1
where X i (i=1,…4) are unknown weighing coefficient to be found in order to match the subjective vehicle assessment as much as possible. 0.2
F=PSD(a(t))
2 The K objective can be then calculated according to the Equation
(6 ). In the following step, this result can be transforme to the
Heave Pitch Roll Torsion
0.15
(9)
notes as follows
(
Nobjective = 9 − 5 ln K objective
).
(10)
0.1
V.
0.05
0 0
10
20 Frequency [Hz]
30
40
Fig. 3 Decomposition of vehicle vibration.
EXPERIMENTAL RESULTS
The experiments were carried out according to the above described scheme. The test passengers (probands) went the car suite and evaluated their vibration comfort properties as the Tab. 1 shows. The statistical properties of proband’s dgment were quite consistent and a good agreement between the amateur and professional probands was achived (Fig. 5 ).
The subjective note N given by a test person can be recalculated to the K -domain as follows
10 9
)
8
(7)
.
In [2] another factor that describes the subjective consideration of the kind of testing road St was introduced. Its influence was to replace N with N/St in (7). In our experiments it was set St=1.
7 hodnocení
K subjective =
(
1 − N −9 e 5
6 5 4 3 2 1 ref
Evaluation function [s/m]
Seat Leg Hand Middle
A
B
C
D
E vozidla
F
G
H
I
ref2
Fig. 5 Statistical properties of subjective assessment
The objective measurements according to the above described scheme were provided. The computation of unknown coefficients Xi and the objective notes according to the equation (10) was carried out.
Frequency [Hz]
Fig. 4 Evaluation fuctions Bk(f) for different excitation sources In order to get the unknown weighing coefficients X i (i=1,… 4) one should solve a set of linear algebraic e ations for X i K subjective = ∑ X i K i 2
2
i
.
(8)
The number of equations (8) corresponds to the number of subjective assessments, i.e. test persons. It is an overdetermined set of linear algebraic equations if the number of test persons is
The relationship between subjectively assessed notes and notes calculated using the presented approach is shown in Fig. 5. Fig. 6 is plotted for vehicles denoted A to J. The dotted line of regression indicates the optimal situation in which the objective assessment would be identical with the subjective one. The same experiment has been repeated however based only on professional test drivers from TÜV SÜD Auto CZ. A typical result is in Fig. 7. The relationship between objective and subjective evaluation is a little worse than in th first experiment. The classical definition of vibration comfort based on one dimensional norm from ISO 2631 with subjective assessm t is in Fig. 8 and demonstrates not good correlation if ly one dimension of vibration excitation is taken into accoun .
BULLETIN OF APPLIED MECHANICS 4(15), 99 -101 (2008)
7
6
Objective note
dimension. The carried out experiments have proven that multiple dimensions are important for passenger vibration comfort feeling, at least heave, pitch, roll, torsion.
A B C D E F G H I J
5
4
3 3
4
5 Subjective note
6
7
Fig. 6 Relationship of objective and subjective assessment – subjective and objective notes N (first experiment, year 2007 ) An interesting issue is the stability of the values of the coefficients Xi among the experiments. Their values are varying but their ratios are rather constant. For the experiments carried out in the year 2007 it is X=1e -3[0.046, 0.085, -0.018, 0.24], the ir ratio XP=[0.54, 1.0, -0.21, 2.82]. For the experiments carried out in the year 2008 it is X=1e -3[0.008, 0.302, -0.087, 0.92], the ir ratio XP=[0.028, 1.0, -0.29, 3.03]. However, the more important are the components of the objective evaluation from (4), i.e.
X i K i . It is for the year
X i K i =[2.0, 1.3, 1.0, 0.9] and for the year 2008
2007
X i K i =[1.0, 2.5, 2.3, 1.6] and their ratios for the year 2007 [1.5, 1.0, 0.8, 0.7] and for the year 2008 [0.4, 1.0, 0.9, 0.6].
Fig. 8 Relationship of ISO 2631 and subjective assessment VI.
CONCLUSION
This paper describes the experiments for the verificat n of the proposed correlation between objective and subjective evaluation of vibration comfort in passenger vehicles. The experimental results support the correlation to be 1:1. If this is true then the proposed objective assessment of vibration comfort can be easily carried out by simulations and c be an important step forward towards virtual design of new v icles. The objective measure does not need to be absolute. It is sufficient if it is just relative because during the virtual design it is important only to compare two design variants.
ACKNOW LEDGMENT
8 7.5
The authors highly appreciate the Vehicle test team of TÜV SÜD Auto CZ as well as the colleagues from CTU in Prague. The authors are thankful to the following companies from the Czech Republic for lending vehicles and data acquisition equipment for the experiments performed in the Fall 2007: ASCOT MB, s.r.o., Ford Motor Company, s.r.o., Honda Ceská republika s.r.o., HYUNDAI Motor CZ s.r.o., NISSAN Sales Central & Eastern Europe, Renault Ceská republika, a.s., ŠKODA AUTO a.s., Toyota Motor Czech spol. s r.o., TECHLAB s.r.o., Spectris Praha spol. s.r.o., Kistler r.o. Last but not least the authors acknowledge the kind financial support of the Ministry of Education, Youth and Sports of the Czech Republic by the grant 1M0568.
7
Objective note
6.5 6 5.5 5 4.5 4 3.5 3
3
3.5
4
4.5
5 5.5 6 Subjec tive note
6.5
7
7.5
8
Fig. 7 Relationship of objective and subjective assessment – subjective and objective notes N (second experiment, year 2008 ) It is clear that these investigations require more repetitions and further considerations. However, it seems that the importances of particular acceleration components of vehicle vibration modes are approximately equal. It also a great difference to standard consideration of passenger vibration comfort where it is usually taken into account just the heave
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Griffin, M . J.: “Discomfo rt from feeling vehicle vibration”, Int. J. of Vehicle System Dynamics 45(7 - 8), pp. 679 – 698, 200 7.
[2]
Kosfelder, M.: “Die Objektivierung des Schwingungskomforts von Kraftfahrzeugen auf der Grundlage globaler Bewegungsformen”, In: 14 . Aachener Ko lloquium Fahrzeug - und Mo torentechnik 2005, pp. 1 6 31 – 1644, 200 5.
[3]
Vaculín, O., Valasek, M., Svoboda J., P elikan, J., Ulehla, J., Steinbauer, P.: Objective and Subjective Evaluation o f Vehicle Vibrations, In: Proc. of FISITA 20 08, Munich 20 08, F200 8 - 03 - 040, pp. 1 - 10.
