Interfaces: 3-DoF versus 6-DoF

Should rotational Degrees of Freedom (DoF) be included in the interface description, and is a Virtual Point Transformation (VPT) beneficial when this is not the case?

To answer this question, we conducted a project with the German automotive supplier ZF from Friedrichshafen. The Electromagnetic Roll Control (ERC) was characterized by Blocked Forces with the interface being modeled based on three different approaches:

  • 6-DoF interface with VPT
  • 3-DoF interface with VPT
  • 3-DoF interface without VPT

The first approach, which is generally recommended as it is the most complete, uses the Virtual Point Transformation to calculate Blocked Forces for 6 DoFs at each interface. The second approach also uses the Virtual Point Transformation but only includes the 3 translational DoFs per interface. The impacts were transformed to the Virtual Point described using 6-DoF forces and moments (case 1) as well as 3-DoF forces (case 2). In both cases, all sensors were retained in the FRF matrix to help identify the forces. The third approach also uses 3-DoF Blocked Forces to describe each interface, but this time without using the Virtual Point Transformation.

We applied all three sets of Blocked Forces obtained in the original vehicle to the FRFs of the modified vehicle and compared these predictions with a validation measurement at the seat rail:

Main takeaways:

  • The Virtual Point Transformation dramatically improves the quality of results, even when only 3 DoFs are considered. This is likely because we average away the measurement inaccuracies inherent to impact testing.
  • Rotational DoFs become essential for getting accurate results at higher frequencies (>600 Hz for this project). The Virtual Point Transformation enables us to accurately characterize the rotational DoFs for better results at higher frequencies. Note that the frequency at which rotations become important will vary for other components.

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