FEMFAT LAB virtual iteration

Features overview:

• Identification of external loads of a dynamic system (typically a MBS) in such a way that internal measurements can be reproduced
• Ensure proper load flow in the model thanks to the use of internal measurement points which are close to investigated structures
• Optional computation of the displacement load of a model that cannot be measured
• Automated iteration process with MSC Adams®, Simpack®, Motionsolve®, Recurdyn® and VI-Grade®
• Powerful tool in simulation of passenger cars and trucks (full vehicles, suspensions and especially components)

Virtual iteration is based on determination of excitation of a model in the time domain using a dynamic simulation (multi-body simulation).

Using an iteration process with simulation (MBS) analogous to actual test bench testing can allow you to adjust external loadings placed on a structure in such a way that internal measurements, i.e. proper load flow, can be reproduced with the desired accuracy (solution of a non-linear inverse problem).  

This can be used to replace procedures such as the time-consuming measurement of structure forces or loads with wheel force transducers. In contrast, internal measurements are usually relatively simple (such as wheel hub or chassis accelerations, suspension travel, etc.).

The virtual iteration can be used in connection with the MBS software MSC/ADAMS® and SIMPACK® automatically.

The interface to ADAMS uses the .adm file (ADAMS solver database) for the simulation of the dynamic model. The input of the model is defined by splines and the output by requests.

The interface to SIMPACK works over the SIMPACK command line using SIMPACK script commands. The input is defined by input functions and the output by a formatted ASCII file (.csv file).

Higher life time prediction accuracy through virtual iteration.

Virtual iteration workflow:

1. Noise generator: Calculates a pink noise to determine the transfer function of the model
2. Simulation of the MBS model with the noise signals as input and a system response output.
3. Calculation of the transfer function. This is used as an approximation of the nonlinear MBS model and can be easily inverted.
4. One or more automated iteration steps are performed, i.e. calculation of the system input by applying the inverse transfer functions on the measurement, usually internal points.
5. The iteration is completed as soon as a defined accuracy has been reached. A relative damage comparison of both signals, which can be performed with FEMFAT LAB, is often used as a criterion for exactness. A comparison of the frequency spectra is also possible. A good correlation between measurement and simulation in time domain is done.

Measurements of different vehicle axes (load in the joints and steering, suspension travel) are performed without the use of wheel force transducers on a test track.

Parallel to this, an MBS model was created to determine the correct stresses and/or spectra of a sub frame or a knuckle for calculation of durability. In order to meet these needs, the external loads, usually the hub loads and torques need to be calculated using the outlined iteration process based on existing measurements (various loads and suspension travel). If these hub loads are determined using virtual iteration with a predefined exactness, various values can be calculated such as the aforementioned internal forces or modal results for components under vibrational stresses. These can subsequently be used as a basis for service life evaluation with FEMFAT MAX ).

A second important application area of the virtual iteration  is the calculation of vertical displacements used as input for a dynamical system. Absolute displacements cannot be measured but these are often necessary, specially for vertical direction of the load. Usually these global displacements can be determined from simple  internal measurements like acceleration or relative displacement signals. Such vertical displacements are typically used for

• Attachment parts, loads at frame or multiaxial simulation table
• full vehicle simulation, loads at wheel center
• generation of road profile, 4- poster simulation of full vehicle including vertical tire stiffness