Analysis of simplified dynamic truck models for parameter evaluation
The ride comfort in heavy commercial trucks is an important property that requires detailed testing to investigate how different vehicle components affect the response to road input. Trucks come in many different configurations and a customer purchasing a new truck often has the choice to specify number of axles, drive line and cabin type, type of suspension and so on. This gives that there are many vehicle variations that have to be tested to ensure good ride comfort. Testing many combinations of, for example, dampers in different environments and road conditions might be time consuming. The work could be helped by pre-test computer simulations where the vehicle is simulated with the same conditions as in the tests. The simulation results could then be used to better understand how different components affect the vehicle response to a certain road input.
By using a MultiBody System (MBS) software, a full truck could be modeled and simulated to acquire accurate results. The simulation would however be computationally demanding and take long time. It also requires that the test engineer is familiar with the MBS software to be able to create the model and run the simulations. This thesis focuses on investigating if simplified dynamic truck models developed in Matlab could be an alternative to more complex models created in an MBS software.
Three different models are developed: a quarter car model, a 2D half truck model and a 3D truck model. The models are derived using the Lagrangian energy method and the dynamic response from a given road input is calculated numerically in Matlab. Different methods of solving the systems of differential equations are discussed and the implementation of the implicit Newmark -method is explained. To validate the truck models and solver, the models are replicated in the MBS software Adams View. The response of the Adams and Matlab models from an excitation on the wheels are compared to determine that the equations and solver are correctly derived and implemented. To test the models capabilities to predict the response in a real truck, tests on a road simulator are performed. A four-wheel Scania tractor is tested in a hydraulic road simulator rig. The road simulator excited the truck through the wheels with a sinus sweep from 0-20 Hz and the resulting accelerations in the tractor are measured. Three different setups of front axle dampers are tested to get a parameter variation to study with the models: a standard damper, harder damper and undamped front axle. The same tests are simulated in Matlab and the acceleration responses are compared to see how well the models predict the accelerations seen in the real truck.
The models in Matlab and Adams give the same results and are therefore reasoned to be mathematically correct. The Newmark -method is efficient and gives reasonable computing times. In the comparison with the road simulator test the models do not give the same results as measured on the truck. To be able to compare the results from the measurements and simulations, the tire stiffnesses have to be trimmed so that the correct eigenfrequency of the axles are found. The results with modified tire stiffnesses give better results but still with considerable deviations from the experimental results. The measurements on the truck show that the eigenfrequency of the front axle decrease when removing the front axle damper while the models show that the eigenfrequency increase. Also there are differences in the acceleration measured in the cabin and frame as the models do not predict many of the higher eigenfrequencies.
In conclusion it is discussed that the models have to be more complex to give useful information about the effects of variation of dampers on the axles. It is also discussed that using commercially available software to perform the same simulations might be a better alternative that gives the user more freedom to overlook and make changes to the model.
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