A New Approach to Identify Rigid Body Rotations When Measuring the Body Distortion of a Full Vehicle

The Opening Distortion Fingerprint (ODF) is a method to evaluate the overall body stiffness of a vehicle using the Multi Stethoscope (MSS). Using the MSS, the time history of distortion in diagonals located at all body openings and cross sections of a vehicle are captured. Evaluation of the ODF is applicable in both test and Multibody Dynamics (MBD) simulation. It has been found that the Evaluation-line (E-line) method, which is used for the distortion calculation, gives valid results in the absence of large rotations originating from rigid body modes that occur at low frequencies. The E-line method measures the relative displacement between two points in a local coordinate system. One straightforward way of identifying the frequency at which the large rotations start to make the E-line results invalid, is to compare the distortion in the local z-component (pointing in the direction of the diagonal) of the E-line method with the total distance (i.e. the total deformation of the diagonal). The total distance can be calculated and requested in an MBD simulation. However, in a dynamic full vehicle test, the total distance cannot be measured easily in the same way, and therefore the method is currently only applicable to simulated data. With a simplified beam example, this article highlights the difference between the acceleration measurements for MBD simulation and experimental data. Then, a new method to identify the threshold frequency to enter into the large rotation’s regime is presented. The new method is based on the evaluation of the same raw data that is used by the E-line method. Raw data is in this context the recorded acceleration in global coordinates in all MSS points. This new approach enables an accurate identification of the frequency range for valid test data.