Universität Stuttgart
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Item Open Access A note on the predictive control of non‐holonomic systems and underactuated vehicles in the presence of drift(2023) Ebel, Henrik; Rosenfelder, Mario; Eberhard, PeterMotion planning and control of non‐holonomic systems is challenging. Only very recently, it has become clear how model predictive controllers for such systems can be generally furnished in the driftless case, where the key is to design a cost function conforming to the geometry arising from the non‐holonomic constraints. However, in some applications, one cannot neglect drift since the time needed to accelerate is non‐negligible, for example, when operating vehicles with high inertia or at high velocities. Therefore, this contribution extends our previous work on the class of driftless non‐holonomic systems to systems with simple kinds of actuator dynamics that allow to represent the boundedness of acceleration in the model. Moreover, we show in a prototypical example of a simple boat‐like vehicle model that a similar procedure can also work for systems that are not non‐holonomic but still under‐actuated. While the contribution is rather technical in nature, to the knowledge of the authors, it is the first time that MPC controllers with theoretical guarantees are proposed for these kinds of models. Moreover, we expect that the resulting controllers are directly of practical value since even the simpler driftless models are employed successfully in various approaches to motion planning.Item Open Access 3D FEM simulation of titanium alloy (Ti6Al4V) machining with harmonic endmill tools(2023) Kalu-Uka, Abraham; Ozoegwu, Chigbogu; Eberhard, PeterUsually, end milling operations have been carried out using conventional uniform helix tools with fixed helix angles. Thus, many studies have been conducted to study the effects of these tools on the thermomechanical properties of a milling process. Recently, there have been works that point to the benefits of using harmonic endmills. Harmonic endmills consist of cutting edge profiles that have continuously harmonically varying helix angles. The variation is described using a harmonic function of axial position (elevation) of points on the cutting edge. In this work, a 3D finite element simulation using ABAQUS, is carried out for the complex milling process of Titanium alloy Ti6Al4V. The envelope of the harmonic tool is first generated using a set of MATLAB codes and stored in a Standard Triangle Language (.stl) format. The machine tool is introduced into an FEM program which has been designed to provide for dynamic effects, thermo‐mechanical coupling, material damage law and the criterion for contact associated with the milling process. A Johnson‐Cook material constitutive equation which combines the effects of strain hardening, strain softening, and temperature softening is used. To account for the chip separation criterion, the Johnson Cook damage evolution equation is used. The milling process simulation for Ti6Al4V is then carried out. In the end, the stress distribution and the cutting forces are obtained.Item Open Access Broadband damping properties of particle dampers mounted to dynamic structures(2022) Schönle, A.; Gnanasambandham, C.; Eberhard, P.Background: The background of this work is the classification of the broadband properties of particle dampers (PDs). This broadband characteristic has experienced little systematic investigation in experiments. Objective: So the primary objective of this paper is to find a measure to quantify the broadband damping properties of PDs. Also the demonstration of applicability to technical structures is a desired goal and the experiments provide a sound basis. Methods: The methods for evaluating the performance of particle dampers and tuned mass dampers target the reduction of vibration amplitudes over the frequency range. The test bench consists of a mechanical frame structure with multiple eigenfreqencies up to 200 Hz harmonically excited with an electrodynamic shaker. From the differences in the dynamic behaviour the performance metric will be derived and evaluated. Results: As a result, a dynamic structure is set up as an effective test bench for different damper configurations. Differences of the tested concepts in regard to the dynamic behaviour over a wide frequency range are observed. From the experimental data a performance metric is deduced to quantify these differences. Conclusion: The conclusions drawn from this paper are, that PDs provide high damping over a wide frequency range. Furthermore, with a suitable performance metric this broadband damping properties can be quantified for the use in further development of PDs.Item Open Access Experimental research on the influence of modal nonlinearities of paintings under mechanical loads(2022) Gao, Yulong; Ziegler, Pascal; Heinemann, Carolin; Hartlieb, Eva; Eberhard, PeterIn the traditional transportation of paintings and the design of the packaging systems, paintings are usually assumed to behave like a linear system. In order to verify this hypothesis, in this contribution, by means of a hammer experiment and a sweep excitation experiment to simulate the shock and vibration during transportation, respectively, the modal nonlinearities of two real paintings and a dummy painting are experimentally studied. The experimental results show that paintings can be treated as a linear system only when being subjected to shock, but the modal nonlinearities of paintings cannot be ignored when being subjected to vibration. The general behaviour of the paintings modal nonlinearities is then summarised based on experimental results, and their consequences for painting transportation are discussed. First of all, the offset of the resonance frequency is the most important problem which will lead to failure of the original vibration isolation measures. Further, the decrease in the resonance peak amplitude will increase the probability of the eigenmode being excited. Besides, it is also necessary to attenuate the harmonic vibrations of paintings. Lastly, the different modal characteristics obtained by a sweep with increasing and decreasing frequency make the analysis of different excitation schemes more complicated. Therefore, the identification of the paintings modal nonlinearities is necessary and important.Item Open Access Application of data-driven surrogate models for active human model response prediction and restraint system optimization(2023) Hay, Julian; Schories, Lars; Bayerschen, Eric; Wimmer, Peter; Zehbe, Oliver; Kirschbichler, Stefan; Fehr, JörgSurrogate models are a must-have in a scenario-based safety simulation framework to design optimally integrated safety systems for new mobility solutions. The objective of this study is the development of surrogate models for active human model responses under consideration of multiple sampling strategies. A Gaussian process regression is chosen for predicting injury values based on the collision scenario, the occupant's seating position after a pre-crash movement and selected restraint system parameters. The trained models are validated and assessed for each sampling method and the best-performing surrogate model is selected for restraint system parameter optimization.Item Open Access Towards learning human-seat interactions for optimally controlled multibody models to generate realistic occupant motion(2023) Fahse, Niklas; Harant, Monika; Roller, Michael; Kempter, Fabian; Obentheuer, Marius; Linn, Joachim; Fehr, JörgItem Open Access Reversible inter-particle bonding in SPH for improved simulation of friction stir welding(2022) Shishova, Elizaveta; Panzer, Florian; Werz, Martin; Eberhard, PeterFriction stir welding (FSW) is a complex joining process which is governed by multiple intertwined physical phenomena. Besides friction, inelastic heat generation, and heat conduction, it involves high plastic deformations, resulting in a need for a numerical method being able to handle all these. Such a scheme is smoothed particle hydrodynamics (SPH), which is a mesh-free computational technique. Absence of a fixed mesh results in the ability of the method to deal with another challenge of friction stir welding, a coalescence of initially separate workpieces into one due to bonding mechanisms. The background of this phenomenon is a transition from contact between two pieces to one continuum due to enormous changes in several material condition, such as temperature, pressure, strain, and strain rate. This work deals with a new development related to bonding, which will provide deeper understanding about the physical weld formation during FSW. The SPH framework must be extended to consider this bonding mechanism. This involves the bonding criterion definition, the interaction type change, and the SPH-SPH contact formulation. Then, the implementation is tested for two different examples, a compression test and FSW.Item Open Access Improving the accuracy of musculotendon models for the simulation of active lengthening(2023) Millard, Matthew; Kempter, Fabian; Stutzig, Norman; Siebert, Tobias; Fehr, JörgVehicle accidents can cause neck injuries which are costly for individuals and society. Safety systems could be designed to reduce the risk of neck injury if it were possible to accurately simulate the tissue-level injuries that later lead to chronic pain. During a crash, reflexes cause the muscles of the neck to be actively lengthened. Although the muscles of the neck are often only mildly injured, the forces developed by the neck’s musculature affect the tissues that are more severely injured. In this work, we compare the forces developed by MAT_156, LS-DYNA’s Hill-type model, and the newly proposed VEXAT muscle model during active lengthening. The results show that Hill-type muscle models underestimate forces developed during active lengthening, while the VEXAT model can more faithfully reproduce experimental measurements.Item Open Access Finite element simulations of motorcyclist interaction with a novel passive safety concept for motorcycles(2021) Maier, Steffen; Doléac, Laurent; Hertneck, Holger; Stahlschmidt, Sebastian; Fehr, JörgItem Open Access On shift selection for Krylov subspace based model order reduction : an iterative greedy approach combined with singular value decomposition(2023) Frie, Lennart; Eberhard, PeterMechanical systems are often modeled with the multibody system method or the finite element method and numerically described with systems of differential equations. Increasing demands on detail and the resulting high complexity of these systems make the use of model order reduction inevitable. Frequently, moment matching based on Krylov subspaces is used for the reduction. There, the transfer functions of the full system and of the reduced system are matched at distinct frequency shifts. The selection of these shifts, however, is not trivial. In this contribution we suggest an algorithm that evaluates an increasing number of shifts iteratively until a reduced model that approximates the full model in a subspace with very low approximation error is found. Thereafter, the projection matrix that spans this subspace is decomposed with singular value decomposition and only most important directions are retained. In this way, small reduced models with good approximation properties that do not exceed a predefined error bound can be found or low-error models for a given reduced order can be generated. The evaluation of more shifts than necessary and further reduction by means of singular value decomposition is the novelty of this contribution. In this paper, this novel approach is extensively studied and, furthermore, applied to the numerical example of an industrial helicopter model.