Horst Peter Wölfel

Determination of vibration-related spinal loads by numerical simulation

OBJECTIVE Dynamic spinal loads due to human whole body vibrations are extremely difficult to determine experimentally. However, they can be predicted by numerical simulation. This paper presents an approach for the prediction of dynamic spinal loads caused by whole body vibrations, as well as some basic considerations concerning the process of numerical simulation. BACKGROUND Long-term whole body vibrations have been found to cause health risks for the lumbar spine. As an increasing percentage of the population is exposed to whole body vibrations at work, more and more people have to face the risk of whole body vibrations-related injury. Knowledge about the actual loads in the lumbar spine is essential when spinal loads are to be compared with spinal strength in order to assess the possible health risks caused by whole body vibrations. METHODS Since an extrapolation of results to unknown data such as spinal loads can only be done using anatomical models of the human body, a simplified finite-element model is presented which is adaptable to body height, body mass, and posture of any specific subject under investigation. The model has been built by reducing a very detailed, nonlinear finite-element model of seated man in its complexity (number of degrees of freedom). Furthermore, the simplified model has been linearised to avoid nonlinear solution procedures. RESULTS The model has been verified for vertical and horizontal excitation at the seat. Model results have been compared to measurements on subjects. Individual exposure-effect relationships may be predicted by this model, due to the adaptability to a specific subject. Additionally, a new phenomenological method of eliminating the influence of local skin-accelerometer vibrations on vibration measurements on the skin surface is discussed. This method may provide data about bone acceleration that can be used in the process of model verification. CONCLUSIONS Integral loading measures, such as spinal loads, may be predicted with simplified finite-element models. Quantitative judgements of these loads may be performed for individual conditions. Linearised models may be used for limited ranges of excitation intensities. Energy dissipation should be modeled by discrete dashpot elements instead of proportional damping. RELEVANCE In order to assess the risk of an injury to the lumbar spine due to whole body vibrations, spinal loads have to be compared with spinal strength. This paper presents the development and verification of a simplified finite-element model of the human body which is based on human anatomy and therefore well-suited to occupational/clinical biomechanics for the prediction of spinal loads.

Active vibration control of a high speed rotor using PZT-patches on the shaft surface

This paper describes the development of a structural model of a high speed rotor for the examination of active vibration control in rotor dynamics. Suppression of lateral bending vibrations of the elastic shaft is realized by means of surface-bonded piezoceramic actuator patches on the shaft surface. Models for actuator implementation are derived. Simulations demonstrate the effectiveness of this approach. To validate the simulation, a rotor test-rig was built. The characteristics of the system model with implemented actuators are compared to experimental tests. Both results show good agreement.

Design of an active vibration dummy of sitting man

OBJECTIVE The determination of vibration transmission through vehicle seats today is still performed with small groups of test subjects. This method suffers from several severe disadvantages, in particular poor repeatability and objectivity due to varying test conditions and limited group sizes. Replacing test persons with a vibration dummy helps to solve this problem. DESIGN The active vibration dummy simulates the dynamic behaviour of sitting man expressed in terms of the driving point impedance for arbitrary body masses and excitation signals. METHODS The dummy is realized as a mechatronic system basing on a single degree of freedom setup. A real-time control loop of mass accelerations (and thus acting forces) fits the active dummy to the desired driving point impedance data set. Model and controller parameters are determined by a parameter-identification technique giving meaningful results for arbitrary impedance data sets. RESULTS The prototype shows excellent agreement with the target data under laboratory conditions. Body mass and excitation level can be varied over the full range of car seat test requirements. RELEVANCE The determination of vibration transmission through vehicle seats should be possible without human experiments. An active vibration dummy with adjustable vibration behaviour expressed by the vertical driving point impedance covering the entire scope of car seat tests (masses/excitation intensities) is presented. With the dummy, improved seat test procedures could be established, leading to design improvements and therefore to prevention of whole-body vibration injuries.

Biodynamische Modellierung des Menschen : Anwendung ingenieurwissenschaftlicher Methoden auf das biologische System Mensch

Das Ziel biodynamischer Modelle ist die Simulation des menschlichen Schwingungsverhaltens. Die experimentellen und numerischen Menschmodelle liefern mechanische Grosen, die die Grundlage fur die Beurteilung von Ganzkorperschwingungen und deren Auswirkungen auf Gesundheit und Komfortempfinden des Menschen darstellen. Es werden zwei Ansatze, der phanomenologische und der anatomiebasierte, vorgestellt. Die unterschiedlichen Vorgehensweisen und Verwendungszwecke werden anhand eines Hardware-Schwingungsdummys und eines Software-Dummys, einem dynamischen Finite-Elemente-Modell, veranschaulicht.