The experimental tests presented here has been conducted at TH Ingolstadt, Faculty of Mechanical Engineering, Germany under the guidance of Professor J. Bienert, who is gratefully acknowledged.
The experimental campaign consist of impact (hammer) testing of an Audi TT car body, see a picture depicted below.
The car body was mounted free on four air cushions. Four references respectively impact points were excited with 14 accelerometers as responses. The data acquisition and processing of the FRF’s were made with a 24-channel data acquisition system. The FRF’s were stored in the four separate Universal File Format Files (Audi_TT_FRF_runX.unv) along with a test geometry in its own file (Audi_TT.unv).
Importing FRF’s into ARTeMIS Modal
Importing the FRF’s into ARTeMIS Modal starts with creating a new project using Universal File Format, which is illustrated in the picture below.
Once the files are uploaded, the geometry and FRF’s can be seen in the simplified Prepare Data task. The Data Organizer contains all four impact tests listed below each other, and the Geometry window visualize the positions and the directions of the response and reference channels. All the impact positions can be viewed by selecting the first item, All Test Setup, in the Data Organizer, illustrated below.
The 14 response accelerometers are seen in blue, and the impact positions, indicated by a hammer symbol, are colored green.
Estimating Modes using the RFP-Z method
The modal parameters of the car body are computed with the RFP-Z method. The work flow is as follows:
- Select a maximum model order to initialize the estimation of the system matrices of the highest dimension for the subsequent computation of a lower model orders.
- Select the desired frequency band.
- Select a specific range of the model orders by dragging the mouse down on the stabilization diagram, or simply click the Estimate Models button to estimate all the model orders.
- Stable modes are found by a study of the natural frequencies, damping ratios and mode shapes (MAC and maximum modal complexity).
- Global modes are estimated based on the stable modes in the diagram.
The figure below illustrates the analysis of the experimental test of the Audi TT in the frequency range from around 4 Hz up to 140 Hz. Maximum model order was set to 80, however only models up to 60 were estimated.
Validating Modes with Other Estimators
The modes obtained from the RFP-Z estimator are validated using the standard tools available in ARTeMIS Modal. By selecting the Validation Task, the modes can be compared with the other estimators as well as imported results from any other modal analysis tools. Below we are comparing the RFP-Z modes with the modes obtained with a simple peak-picking tool already available in ARTeMIS Modal. It is based on the peak picking on the Complex Mode Indicator Function (CMIF).