L. Vita

Comfort analysis of car occupants: comparison between multibody and finite element models

This paper deals with numerical models developed in order to simulate the vibrational and postural comfort of car occupants. The proposed models have been based on different mathematical approaches: the first is a multibody dynamics model and the second is a finite elements model. Both models have been validated by means of static and dynamic experimental tests on vehicles using appropriate test rigs. The authors focus on the advantages and disadvantages of each model in order to have useful information about which approach has to be used to predict objectively the comfort of driver and passengers.

Multibody dynamics in advanced education

The use of multibody dynamics software in industrial firms is steadily increasing. Engineering curricula often do not include mandatory courses where multibody dynamics is taught. This paper discuss some relevant teaching issues in this specific cultural area. Noteworthy examples of the great variety of kinematic and dynamic formulations, available for teaching a basic course in multibody dynamics, are illustrated. The experience of the first author, when introducing a basic multibody dynamics course at an Italian university, is also reported.

A Dual Number Approach to the Kinematic Analysis of Spatial Linkages With Dimensional and Geometric Tolerances

Design robustness is somewhat connected to tolerances. In fact, the lower is the sensitivity of the kinematic function to the deviations of manufacturing process, the higher is the robustness of the design. In this investigation is described a tolerance analysis method based on dual vectors kinematic modeling of spatial linkages and on Monte Carlo simulation of the random variables. In the present analysis the hypothesis of rigid bodies is valid and only kinematic variables are considered in output. The method is applied to a Cardan joint modelled as an RCCC linkage with main dimensions considered as stochastic variables with Gaussian distribution. Dual vectors are well known in kinematic analysis and synthesis of spatial mechanisms. When compared with traditional vectorial methods, dual vectors show an enhanced capability to model misalignments among kinematic pairs axes. Although this is not the first time that dual vectors are used for the kinematic and dynamic analysis of spatial mechanisms with manufacturing errors, the present use of dual vectors to model joint clearances seems somewhat novel.© 2004 ASME

Using a Virtual Dummy to Simulate Vibration Dose Value for Different Car Occupants

The comfort assessment for car occupants is not an easy task to achieve. It is a matter of personal perceiving, thus a solution which may seem comfortable to one could be not to another. Moreover it is a field which requires knowledge both from science and medicine. The vibrational comfort analysis is the topic of this paper. There are different international standards to define the vibrational comfort through computed indices. In particular the British Standard BS 6841 (1) introduces the Vibration Dose Value (VDV). The main idea which lies behind the VDV is to estimate the whole body vibration. In fact the standard suggests the main locations for acceleration measurement (feet, seat and back) and their direction axes. The overall number of acquisitions is twelve. A numerical code for the vibrational comfort analysis of car occupants, estimating the VDV for each of them, has been developed. The code, named DAViD, is based on a multibody approach (2). It has been validated by means of experimental tests (3,4). In this paper the influence of the anthropometric features (height and weight) and the inclination of the backrest on the VDV has been investigated. The code is an useful tool for predicting at the seat design level the vibrational discomfort.

Mechanical Efficiency Analysis of a Cardan Joint

This investigation concerns with the mechanical efficiency of Cardan joints. The model includes also the effects due to manufacturing and mounting errors and the influence of rotation speed on the efficiency. The joint has been modeled as an RCCC spatial linkage and the full dynamic analysis performed by means of dual vectors algebra.Copyright

Kinematic and dynamic analysis of a sit-ski to improve vibrational comfort

This paper reports both procedures for the kinematic design and the results of dynamic analyses of three models of sit-ski in order to evaluate their vibrational comfort. The model described in the US Patent US 6,019,380 has been assumed as a reference in the following kinematic and dynamic analyses of the new models proposed. The two main features to be considered while designing a sit-ski are the manoeuvrability and the range of vision of the driver. These two features appear to be important considering that the sit-ski is generally used by disabled people (e.g. paraplegic). Thus the kinematic design phase should account for the fulfillment of these requirements. On the basis of these, three new models have been proposed and herein named: Flex, modified Flex and eight-bar linkage (Smarrini 2003). Once the kinematic synthesis and analysis have been completed the dynamic performance of each model is investigated. For this purpose two kinds of slope profiles are simulated in order to reproduce the stresses which may likely occur during a downhill race. In particular the position and the acceleration of the seat have been monitored during the simulations. This is important in order to test the range of vision and handling requirements. The seat acceleration is also important to evaluate the vibrational comfort of the sit-ski according to the British standard BS 6841 (Griffin 1990). The dynamic simulation is first executed by means of the multibody dynamics code NumDyn3D, then the vibrational comfort analysis is performed using in-house developed software (Vita 2001, Valentini and Vita 2003).

MODELLING OF HYDRODYNAMIC JOURNAL BEARING IN SPATIAL MULTIBODY SYSTEMS

The model of a three dimensional journal bearing with hydrodynamic lubrication is herein presented. This model is suitable for embodiment into the equations of spatial multibody systems. Both rotational and squeeze effects together with tilting effect have been taken into account. Moreover a simplified model of friction has been also reported. The proposed methodology has been applied to an example concerning an unbalanced rotor supported by two journal bearings.

Strategies for the numerical integration of DAE systems in multibody dynamics

The number of multibody dynamics courses offered in the university is increasing. Often the instructor has the necessity to go through the steps of an algorithm by working out a simple example. This gives the student a better understand of the basic theory. This paper provides a tutorial on the numerical integration of differential‐algebraic equations (DAE) arising from the dynamic modeling of multibody mechanical systems. In particular, some algorithms based on the orthogonalization of the Jacobian matrix are herein discussed. All the computational steps involved are explained in detail and by working out a simple example. It is also reported a brief description and an application of the multibody code NumDyn3D which uses the Singular Value Decomposition (SVD) approach.

A DYNAMIC SIMULATION OF CAM ACTUATED ROBOTIZED GEARBOX

In this paper a simulation of the cam actuator of a robotized gearbox is presented. The actuator is a barrel cam which moves a pin according to a prescribed motion law. The model of contact, between cam and follower, is based on a finite elements approach of the theory of beam on continuous elastic foundation, modified by the authors to include the effect of shear on the deformation and dynamic effects. The foundation modulus is computed using Hertz theory taking also into account the change of principal curvature values during the relative motion between cam and follower. This model has been embodied in a complete gear shifting simulation in order to compare the effects on contact forces, wear actions and shifting time of different cam profiles.

An integrated tool for design, shape modelling and performance analysis of 3D cam

In this paper, an easy procedure to integrate three dimensional cam profile synthesis and Computer Aided Design is presented. The aim of the proposed method is to combine an accurate kinematic analysis with shape modelling using CAD programmes. In many cases, the manufacturing process and cutting approximation cannot be neglected and affect the final drawing. For specific needs, as spatial cams, an appropriate and more accurate synthesis is required. On the other hand, the only geometrical design misses all the advantages of modelling, for example the correct calculation of mass properties, interferences, and the possibility to perform dynamical and stress analysis. The authors develop an integrated tool which interacts with both CAD software and algebraic solver in order to draw an accurate cam model and to compute performances as regards errors on position, acceleration, jerk and jounce. This methodology can help the designers in the definition of digital mock-up.