Modal Test Of A SUV’s BIW And Analysis Of Results

The NVH (Noise、Vibration、Harshness) of the vehicle depend largely on themodal parameter of Body-In-White (BIW), Modal test is a kind of vibration testaimed to ascertain the modal parameter of the linear vibration system. Through themodal test we can get the structural dynamics characteristics of BIW to guide theresearch and design of new automotive products. At the same time, the experimentalmodal analysis of the BIW plays a very important role in improving the safety andreliability of the vehicle.Firstly, this paper does a systemic research on the theory of modal analysis. Thendesign and builds the test bench, selects the test equipment, determines the testmethod and process, and solves the problem of the selection of exciting point, and theinstallation of vibration exciter and other issues in the process of experiment.Secondly, this study, taking the BIW of a SUV as the experimental subjects arethe modal test is carried out in time domain and frequency domain respectively. In themodal test of the BIW incented by time domain, by making use of the principle thatsingle point excitation in different areas and Sampling on multi-point in threedirections in the process of excitation and sample-taking. The BIW is incented by asoft-rubber-headed hammer in three different positions but with fixed angle andexciting point with relatively large stiffness. This incentive way has such advantagesas short testing time and easy to be operated. But at the same time it is worthmentioning that in the test the force is controlled by man. Whether it is too big or toosmall, errors will be caused. In terms of the sample-taking, there are223measuringpoints set in the BIW. Experimental data will be collected by severalthree-dimensional acceleration sensors and NI data collecting instruments. Theprocedures of the test based on frequency domain are analogous to that of the testbased on time domain. However its incentive way is different. Thefrequency-domain-based test uses vibration excitation, in which the excitation signalis a digital step sine sweep one.Finally, written Matlab program based on SVM method (The State Variablemodal parameter identification method) for dealing with the time-domain test data,and identified the modal parameter of the BIW of this SUV. By comparing the modalparameters collected from SVM method with those identified by STD method, ITDmethod and P-P method, it is proved that the maximum deviation of the naturalfrequency is within0.5%. The frequency-domain signal acquired by vibration exciterwill be used to identify the modal paramet by frequency-domain-based method. And then the results got from these two different methods will be compared. Theexperimental results show that the body of this SUV is the mode of combinedvibration in the region of low frequency. Its local modal is not ideal but the overallvibration mode is relatively good as to the low-frequency dynamic characteristic. Theanalysis of the test results in this paper is of great guiding significance with respect tothe performance improvement of this SUV’s BIW.