Multibody System Dynamics最新文献

Finding the stable static equilibrium position of multibody systems is a well-known problem. Dynamic relaxation methods are frequently utilized in engineering, however, they often require a significant amount of time. Alternatively, most commercial software employs the Newton–Raphson iterative method to solve a set of nonlinear equations to find the equilibrium position directly, in which the time derivatives of any quantity are set to zero. Nevertheless, this approach is highly dependent on initial conditions and can only find one equilibrium position for a specific initial condition, no matter how many degrees of freedom a system has. A path-following method is implemented in this paper to find the equilibrium position of the multibody system by using the reduced multibody system transfer matrix method to evaluate the acceleration functions and its Jacobian matrix, where the notion of direct differentiation is applied. The solution curves for changing generalized accelerations are then tracked using the arc-length method to obtain candidate equilibrium states if they vanish and identify the stable static equilibrium position. To demonstrate the effectiveness of the proposed method, numerical examples are presented, which provide a detailed overview of the complete computational flow.

Frequency veering is a phenomenon that occurs during modal parameter changes and is closely related to the response characteristics of the system. First, by taking a system with simple DOFs as the research object, the variations in the modal damping ratio and mode shape in the process of frequency veering are analysed, and a criterion for identifying this phenomenon is preliminarily proposed. Then, to explore the modal changes in complex vehicle systems with multiple DOFs, an adaptive modal continuous tracking algorithm based on a local search algorithm is proposed that takes the Euclidean closeness between complex mode shapes as an index. Frequency veering is analysed with the established vehicle system dynamics model (Model I) and reproduced through the SIMPACK model (Model II) for multibody dynamics simulation. The perturbation method is used to analyse the mechanism by which the vehicle system eigenvectors are prone to mutations during frequency veering, and the abnormal changes in the mode shapes during this process are further verified. In addition, two quantitative indices for identifying frequency veering phenomena are proposed based on the modal assurance criterion and mode shape similarity. Finally, the mapping relationship between the frequency veering and vehicle system response is explored. The results indicate that before and after frequency veering, the mode shapes interchange, and in the frequency veering zone, the damping-hopping phenomenon occurs, resulting in a significant decrease in system stability. Corresponding to the phenomena of modal damping ratios and mode shapes, the motion morphology of the vehicle system is clearly observable. Moreover, the response at the DOFs of the car body and bogie are obviously enhanced; these responses are also manifested in the increasing vibrations of the car body and bogie and the deterioration of the vehicle ride quality.

Train derailments may have catastrophic consequences, and therefore suitable measures should be designed and installed at specific safety-relevant sites to mitigate their effects. Mitigation measures, such as guard rails and containment walls, aim at restraining the motion of the derailed vehicle using suitable derailment containment devices. However, the design and structural sizing of these devices is challenging as the quantification of the loads caused by the impact with the vehicle is complex.

The aim of this paper is to extend previous work from the same authors aimed at defining a non-linear multi-body model for the simulation in time-domain of the post-derailment behaviour of a railway vehicle and the impact on a derailment containment wall. The extension presented in this paper is concerned with the model of the interaction of the derailed vehicle with the sleepers and with the ballast. To this aim, an algorithm is introduced to manage the different possible contact conditions the wheels of the vehicle may undergo during the derailment process: contact with the rail, with the sleepers and with the ballast. Then, a model of the impact between the derailed wheels and the sleepers is introduced, and a terramechanic model defining the forces acting on the wheels sinking in the ballast is established. The effect of the accurate modelling of forces exchanged by the derailed wheels with the sleepers and the ballast is quantified for a relevant derailment scenario and shown to be highly relevant to the estimation of the impact loads applied to the containment structure.

In recent times, many industrial applications have demanded innovative energy-efficient solutions. One of the main causes of energy loss is due to friction between body surfaces in contact. A great amount of research has been aimed at understanding the friction mechanisms to allow for its reliable prediction during multibody simulation. In the 1950s and 1960s, many experimental studies were carried out, leading to the coefficient of friction formulas for lubricated surfaces under a combination of sliding and rolling relative motion. The formulas have been mainly derived by the mathematical fitting of results obtained from experimental measurements on rolling disks and different load, lubricating and kinematic conditions. The purpose of this paper is twofold: on the one hand, it reviews semi-empirical formulas for computing the friction coefficient in lubricated contact under various operating conditions; on the other hand, it implements and compares contact force models coupled with the metal-metal lubricated empirical friction formulas in a multibody dynamics simulation environment. Implementing empirical formulas is straightforward and computationally efficient, but one can evaluate the performance of these models in characterizing the dynamics of the lubricated joint. For this purpose, a multibody simulation of a Scotch yoke and a Whitworth quick return mechanisms with a nonideal prismatic joint are conducted. The existence of clearance causes the dynamic behavior of the system to be different from the ideal joint. The difference between each friction coefficient model is emphasized by simulation output and computation time.

Rails experience contact with a range of wheel profiles that pummel their surface at different points and with different intensities. This work compares two methods for evaluating pummeling analyzes for the wheel-rail interaction: simplified quasi-static model and multibody dynamics simulations. The first is solved with the GIRAFFE program and simulates the interaction of a single wheelset with the rail in a quasi-static approach. In the second, the full dynamics of a railway wagon on a track layout are evaluated using the multibody dynamics simulation programs SIMPACK^® and VAMPIRE^®. The proposal for a quasi-static model is to reduce the time and computational effort to perform a pummeling analysis and quickly evaluate thousands of cases of wheel-rail contact. Track parameters and vehicle loads of a heavy haul railway are considered for the simulations. The results showed that the quasi-static model has a good correlation with the dynamic models on tangent track sections. For the curved sections, differences were observed in the distribution of pressures due to the absence of creep forces in the quasi-static model. The comparison between the models also showed slightly different results due to the different calculation of contact in each approach. The quasi-static approach reduced the time consuming by at least 73.4% over the multibody approach. Notwithstanding, the proposed model shows to be promising in replacing complete dynamic analysis for time-consuming tasks such as pummeling.

The modeling and simulation of coupled neuromusculoskeletal-exoskeletal systems play a crucial role in human biomechanical analysis, as well as in the design and control of exoskeletons. This study incorporates the integration of exoskeleton models into a reflex-based gait model, emphasizing human-exoskeleton interaction. Specifically, we introduce an optimization-based dynamic simulation framework that integrates a neuromusculoskeletal feedback loop, multibody dynamics, human-exoskeleton interaction, and foot-ground contact. The framework advances in human-exoskeleton interaction and muscle reflex model refinement. Without relying on experimental measurements or empirical data, our framework employs a stepwise optimization process to determine muscle reflex parameters, taking into account multidimensional criteria. This allows the framework to generate a full range of kinematic and biomechanical signals, including muscle activations, muscle forces, joint torques, etc., which are typically challenging to measure experimentally. To evaluate the validity of the framework, we compare the simulated results with experimental data obtained from a healthy subject wearing an exoskeleton while walking at different speeds (0.9, 1.0, and 1.1 m/s) and terrains (flat and uphill). The results demonstrate that our framework can capture the qualitative differences in muscle activity associated with different functions, as well as the evolutionary patterns of muscle activity and kinematic signals with respect to varying walking conditions, with the Pearson correlation coefficient R > 0.7. Simulations of the human walking with the exoskeleton in both passive mode and assisting mode at a peak torque of 20 N⋅m are further conducted to investigate the effect of exoskeleton assistance on human biomechanics. The simulation framework we propose has the potential to facilitate gait analysis and performance evaluation of coupled human-exoskeleton systems, as well as enable efficient and cost-effective testing of novel exoskeleton designs and control strategies.

Exoskeleton robots have a wide range of applications in industrial field as well as for patients with locomotor disability. Among them, the flexible exoskeleton, known as “exosuit”, has attracted great interest from researchers. They are usually made up of flexible components such as cables and pieces of fabric. Since there are no rigid frames and links in the exosuits, they are much lighter and have less misalignment problems than the rigid exoskeletons. However, excessive pressure exerted by cables on soft tissues and skeleton of the human will lead to discomfort or even injuries. In this paper, a cable transmission system is incorporated into the exosuit system for gravitational compensation. The human body is assumed to be upright in the cable-driving wearable robot modeling. Then, a multi-criteria optimization approach, based on swarm intelligence, has been developed and adopted for reducing the uncomfortable forces applied on the user. Furthermore, the energy consumption is also taken into account in the design phase. Numerical simulation results demonstrate that the proposed exosuit design results in a reduction of more than 50% and 34% in the forces exerted on human body with loads of 0.5 kg and 5 kg, respectively. The energy loss was also reduced by up to 63% and 21% in these two cases.

Regularized static friction models have been used successfully for many years. However, they are unable to maintain static friction in detail. For this reason, dynamic friction models have been developed and published in the literature. However, commercial multibody simulation packages such as Adams, RecurDyn, and Simpack have developed their own specific stick-slip models instead of adopting one of the public domain approaches. This article introduces the fundamentals of these commercial models and their behavior from a practical point of view. The stick-slip models were applied to a simple test model and a more sophisticated model of a festoon cable system using their standard parameters.

In this paper, a new high-performance and memory-efficient contact and road model is developed. Specifically, the road is modeled as a rectangular structured grid of deformable springs in the vertical direction, thus enabling fast execution. The new road model stands out due to its ability to handle large road scenarios by allocating computer memory dynamically for each spring, resulting in efficient memory utilization. Furthermore, each spring represents a small road patch that entails various information, such as the soil elevation, the soil properties, and the soil compaction, allowing for complicated simulations incorporating spatially varying soil properties and phenomena related to the multi-pass effect. In addition, using the new contact model, complex terrain geometries are handled in a computationally efficient way by approximating locally the irregular road profile with a suitable equivalent plane. For this, two different strategies are proposed, namely the radial basis function (RBF) interpolation method and the 3D enveloping contact model. Finally, the proposed techniques are implemented in Altair MotionSolve, a comprehensive multi-body simulation software for complex mechanical systems. In particular, a single-wheel test bed is initially examined followed by a four-wheeled rover model and the next-generation NATO reference mobility model (NG-NRMM). In all cases, the proposed model is validated by using available experimental data. Lastly, a case involving both wheeled and tracked vehicles is also examined by using a shared road model.

Current engineering design trends, such as lightweight machines and human–machine interaction, often lead to underactuated systems. Output trajectory tracking of such systems is a challenging control problem. Here, we use a two-design-degree of freedom control approach by combining funnel feedback control with feedforward control based on servo-constraints. We present experimental results to verify the approach and demonstrate that the addition of a feedforward controller mitigates drawbacks of the funnel controller. We also present new experimental results for the real-time implementation of a feedforward controller based on servo-constraints on a minimum phase system.