Finite element modelling of human-seat interactions: vertical in-line and fore-and-aft cross-axis apparent mass when sitting on a rigid seat without backrest and exposed to vertical vibration

Biodynamic models representing distributed human-seat interactions can assist seat design. This study sought to develop a finite element (FE) model representing the soft tissues of the body supported by seating and the vertical in-line apparent mass and the fore-and-aft cross-axis apparent mass of the seated human body during vertical vibration excitation. The model was developed with rigid parts representing the torso segments, skeletal structures (pelvis and femurs) and deformable parts representing the soft tissues of the buttocks and the thighs. The model had three vibration modes at frequencies less than 15 Hz and provided reasonable vertical in-line apparent mass and fore-and-aft cross-axis apparent mass. The model can be developed to represent dynamic interactions between the body and a seat over a seat surface (e.g. dynamic pressure distributions and variations in seat transmissibility over the seat surface).     

Modelling and simulation of a fore-and-aft driver''s seat suspension system with road excitation

This paper describes a simplified simulation of two configurations of the fore-and-aft seat suspension system. A fore-and-aft suspension system model was proposed based on the laboratory measurements of the seat vibration isolation performance. Friction was identified as an important parameter, so different approaches to simulating the suspension friction were investigated. Predicted seat vibration mitigation properties were compared with those measured in the laboratory in response to the recordings of the fore-and-aft vibration measured at the base of the driver's seat in an on-road tractor–trailer combination (articulated truck). Optimisation of the suspension elements parameters was then performed to identify the maximum attainable attenuation. A solution incorporating supercritical suspension damping predicted to give an improvement of the order of 10% in the x-direction mitigation properties as compared to a fixed (locked) horizontal suspension system.

Relevance to the industry

Simulations conducted in this study are of use to seat manufacturers in developing the fore-and-aft seat suspension systems with improved vibration mitigation properties and for predicting its dynamic performance. The optimisation study shows the attainable vibration mitigation limits for a horizontal suspension system.     

Whole-body vertical biodynamic response characteristics of the seated vehicle driver: Measurement and model development

The vertical driving-point mechanical impedance characteristics applicable to seated vehicle drivers are measured in the 0.625–10 Hz frequency range with excitation amplitudes ranging from 1.0 to 2.0 m s⁻² using a whole-body vehicular vibration simulator. The measurements are performed for seated subjects with feet supported and hands held in a driving position. Variations in the seated posture, backrest angle, and nature and amplitude of the vibration excitation are introduced within a prescribed range of likely conditions to illustrate their influence on the driving-point mechanical impedance of seated vehicle drivers. Within the 0.75–10 Hz frequency range and for excitation amplitudes maintained below 4 m s⁻², a four-degree-of-freedom linear driver model is proposed for which the parameters are estimated to satisfy both the measured driving-point mechanical impedance and the seat-to-head transmissibility characteristics defined from a synthesis of published data for subjects seated erect without backrest support. The parameter identification technique involves the solution of a multivariable optimization function comprising the sum of squared magnitude and phase errors associated with both the mechanical impedance and seat-to-head transmissibility target values, subject to limit constraints identified from the anthropometric and biomechanical data. The model response, however, is found to provide a closer agreement with the mechanical impedance target values than that with the seat-to-head transmissibility. From the model, the main body resonant frequencies computed on the basis of both biodynamic response functions are found to be within close bounds to that expected for the human body.

Relevance to industry

The development of an appropriate analytical seated vehicle driver model should provide means of estimating the forces and motions being transmitted within the body under specific vehicular vibration environments. Furthermore, its use in conjunction with a corresponding model for the vehicle seat should allow the prediction of the driver's vibration exposure levels and the seat's ability to attenuate the vibration in particular vehicles.     

Developing a simplified finite element model of a car seat with occupant for predicting vibration transmissibility in the vertical direction

The transmissibility of seat depends on the dynamics of both the seat and the human body, and shows how the amplification and attenuation of vibration varies with the frequency of vibration. A systematic methodology was developed for finite element (FE) modelling of the dynamic interaction between a seat and the human body and predicting the transmissibility of a seat. A seat model was developed to improve computational efficiency before models of the seat pan and backrest were calibrated separately using load–deflection and dynamic stiffness measurements, joined to form the complete seat model, and integrated with the model of a manikin for further calibration. The calibrated seat model was combined with a human body model to predict the transmissibility of the seat. By combining a calibrated seat model with a calibrated human body model, and defining appropriate contacts between the two models, the vibration transmissibility with a seat–occupant system can be predicted.     

Determination of reference values of apparent mass responses of seated occupants of different body masses under vertical vibration with and without a back support

The biodynamic response of human body seated without a back support and exposed to vertical whole-body vibration have been standardized in ISO 5982 and DIN 45676 in terms of driving-point mechanical impedance and apparent mass. A comparison of ranges defined in two standards, however, reveal considerable differences in both the magnitude and phase. Greater differences are more evident for the three body mass groups, which suggests the lack of adequate reference values of biodynamic responses of seated human subjects of different body masses. In this experimental study, the biodynamic responses of seated humans within three different body mass ranges are characterized under different magnitudes of vibration and three different sitting postures in an attempt to define reference values of apparent mass for applications in mechanical-equivalent model development and anthropodynamic manikin design. Laboratory measurements were performed with adult male subjects of total body mass in the vicinity of 55, 75 and 98 kg (nine subjects for each mass group) seated with and without an inclined back support and exposed to three different magnitudes of white-noise vertical vibration (0.5, 1.0 and 2.0 m/s² unweighted rms acceleration) in the frequency range between 0.5 and 20 Hz. The measured data were analyzed to derive the mean magnitude and phase responses for the three body masses, posture and excitation conditions. The mean magnitude responses of subjects within three mass groups were compared with idealized ranges defined in ISO 5982 and mean values described in DIN 45676 for no back support condition. The results revealed significant differences between the mean measured and standardized magnitudes, suggesting that the current standardized values do not describe the biodynamic responses of seated occupant of different masses even for the back not supported condition. The mean measured responses revealed most important effect of body mass, irrespective of the sitting posture. The reference values of apparent mass responses of seated body subject to vertical whole-body vibration are thus defined for three mass groups and different back support conditions that may be considered applicable for ranges of excitations considered. The responses of the body seated without a back support, also revealed notable influences of excitation magnitude, particularly on the primary peak frequencies.     

Comparisons of apparent mass responses of human subjects seated on rigid and elastic seats under vertical vibration

The apparent mass (AM) responses of human body seated on elastic seat, without and with a vertical back support, are measured using a seat pressure sensing mat under three levels of vertical vibration (0.25, 0.50 and 0.75 m/s² rms acceleration) in 0.50–20 Hz frequency range. The responses were also measured with a rigid seat using the pressure mat and a force plate in order to examine the validity of the pressure mat. The pressure mat resulted in considerably lower AM magnitudes compared to the force plate. A correction function was proposed and applied, which resulted in comparable AM from both measurement systems for the rigid seat. The correction function was subsequently applied to derive AM of subjects seated on elastic seat. The responses revealed lower peak magnitude and corresponding frequency compared to those measured with rigid seat, irrespective of back support and excitation considered.     

Seated body apparent mass response to vertical whole body vibration: Gender and anthropometric effects

The gender and anthropometric effects on apparent mass characteristics of the seated body exposed to vertical vibration are investigated through laboratory measurements. The study was conducted on 31 male and 27 female subjects, exposed to three levels of vertical vibration (0.25, 0.50 and 0.75 m/s² rms acceleration) in the 0.50 to 20 frequency range, while seated without a back support and against a vertical back support. The apparent mass responses were analyzed by grouping datasets in three ranges of mass-, build- and stature-related parameters for the male and female subjects. Comparisons of responses of male and female subjects with comparable anthropometric properties showed distinctly different biodynamic responses of the two genders. The primary resonance frequency of male subjects was significantly (p < 0.001) higher than the female subjects of comparable body mass but the peak magnitude was comparable for both the gender groups. The male subjects showed greater softening with increasing excitation magnitude compared to the female subjects, irrespective of the sitting condition. The male subjects showed significantly higher peak magnitude response than those of the female subjects for the same anthropometric properties, except for the total and lean body mass. The peak magnitude was linearly correlated with the body mass, body mass index, body fat and hip circumference (r² > 0.7), irrespective of the back support and excitation conditions for both the genders.Relevance to the industryThe apparent mass responses of the human body exposed to whole-body vibration form an essential basis for an understanding of mechanical-equivalent properties of the body, developments in frequency-weightings for assessment of exposure risks and anthropodynamic manikins for assessment of seats. The effects of gender and anthropometric parameters on the AM response are vital for seeking better seat designs, and anthropodynamic manikins for assessments of seating for male as well as female workers.     

Study of simultaneous shock and vibration control by a fore-and-aft suspension system of a driver’s seat

A single degree-of-freedom simulation model of a seat equipped with a non-linear fore-and-aft suspension system was developed. The performance of a typical fore-and-aft suspension system was analysed using typical vibration and shock acceleration signals measured in loaders while travelling on off-road terrain and while driving the bucket into a heap of soil. Damper modification was suggested to improve the suspension performance in the control of both vibration and shocks. Suspension system performance was analysed using the simulation approach for variations in both mass loading and excitation intensity. A discussion of results obtained and practical suggestions for improvement of the fore-and-aft driver’s seat suspension system concludes this contribution.

Relevance to industry

The simulations are of use to seat manufacturers in developing improved fore-and-aft seat suspension for wheel loaders. It is shown that a damper with an optimised progressive damping characteristic provides better shock mitigation than is available with either a seat without any suspension or a suspended commercial seat as presently available on the market.     

Fore-and-aft and dual-axis vibration of the seated human body: Nonlinearity,cross-axis coupling,and associations between resonances in the transmissibility and apparent mass

This study examined how the apparent mass and transmissibility of the human body depend on the magnitude of fore-and-aft vibration excitation and the presence of vertical vibration. Fore-and-aft and vertical acceleration at five locations along the spine, and pitch acceleration at the pelvis, were measured in 12 seated male subjects during fore-and-aft random vibration excitation (0.25–20 Hz) at three vibration magnitudes (0.25, 0.5 and 1.0 ms⁻² r.m.s.). With the greatest magnitude of fore-and-aft excitation, vertical vibration was added at 0.25, 0.5, or 1.0 ms⁻² r.m.s. Forces in the fore-and-aft and vertical directions on the seat surface were measured to calculate apparent masses. Transmissibilities and apparent masses during fore-and-aft excitation showed a principal resonance around 1 Hz and a secondary resonance around 2–3 Hz. Increasing the magnitude of fore-and-aft excitation, or adding vertical excitation, decreased the magnitudes of the resonances. At the primary resonance frequency, the dominant mode induced by fore-and-aft excitation involved bending of the lumbar spine and the lower thoracic spine with shear deformation of tissues at the ischial tuberosities. The relative contributions to this mode from each body segment (especially the pelvis and the lower thoracic spine) varied with vibration magnitude. The nonlinearities in the apparent mass and transmissibility during dual-axis excitation indicate coupling between the principal mode of the seated human body excited by fore-and-aft excitation and the cross-axis influence of vertical excitation.Relevance to industryUnderstanding movements of the body during exposure to whole-body vibration can assist the optimisation of seating dynamics and help to control the effects of the vibration on human comfort, performance, and health. This study suggests cross-axis nonlinearity in biodynamic responses to vibration should be considered when optimising vibration environments.     

Investigation of the transmission of fore and aft vibration through the human body

Understanding the behavior of human body under the influence of vibration is of great importance for the optimal motor vehicle system design. Therefore, great efforts are being done in order to discover as many information about the influence of vibration on human body as possible. So far the references show that the major scientific attention has been paid to vertical vibration, although intensive research has been performed lately on the other sorts of excitation. In this paper, the results of the investigation of behavior of human body, in seated position, under the influence of random fore and aft vibration are shown. The investigation is performed by the use of an electro-hydraulic simulator, on a group of 30 healthy male occupants.Experiments are performed in order to give results to improve human body modeling in driving conditions. Excitation amplitudes (1.75 and 2.25 m/s² rms) and seat backrest conditions (with and without inclination) were varied. Data results are analyzed by partial coherence and transfer functions. Analyses have been performed and results are given in detail.The results obtained have shown that the human body under the influence of random excitations behaves as a non-linear system and its response depends on spatial position.Obtained results give necessary data to define structure and parameters of human biodynamic model with respect to different excitation and seat backrest position.     

Modal analysis of human body vibration model for Indian subjects under sitting posture

Need and importance of modelling in human body vibration research studies are well established. The study of biodynamic responses of human beings can be classified into experimental and analytical methods. In the past few decades, plenty of mathematical models have been developed based on the diverse field measurements to describe the biodynamic responses of human beings. In this paper, a complete study on lumped parameter model derived from 50th percentile anthropometric data for a seated 54- kg Indian male subject without backrest support under free un-damped conditions has been carried out considering human body segments to be of ellipsoidal shape. Conventional lumped parameter modelling considers the human body as several rigid masses interconnected by springs and dampers. In this study, concept of mass of interconnecting springs has been incorporated and eigenvalues thus obtained are found to be closer to the values reported in the literature. Results obtained clearly establish decoupling of vertical and fore-and-aft oscillations.     

Effects of elastic seats on seated body apparent mass responses to vertical whole body vibration

Apparent mass (AM) responses of the body seated with and without a back support on three different elastic seats (flat and contoured polyurethane foam (PUF) and air cushion) and a rigid seat were measured under three levels of vertical vibration (overall rms acceleration: 0.25, 0.50 and 0.75 m/s²) in the 0.5 to 20 Hz range. A pressure-sensing system was used to capture biodynamic force at the occupant-seat interface. The results revealed strong effects of visco-elastic and vibration transmissibility characteristics of seats on AM. The response magnitudes with the relatively stiff air seat were generally higher than those with the PUF seats except at low frequencies. The peak magnitude decreased when sitting condition was changed from no back support to a vertical support; the reduction however was more pronounced with the air seat. Further, a relatively higher frequency shift was evident with soft seat compared with stiff elastic seat with increasing excitation.     

Characterisation of the human-seat coupling in response to vibration

Characterising the coupling between the occupant and vehicle seat is necessary to understand the transmission of vehicle seat vibration to the human body. In this study, the vibration characteristics of the human body coupled with a vehicle seat were identified in frequencies up to 100 Hz. Transmissibilities of three volunteers seated on two different vehicle seats were measured under multi-axial random vibration excitation. The results revealed that the human-seat system vibration was dominated by the human body and foam below 10 Hz. Major coupling between the human body and the vehicle seat-structure was observed in the frequency range of 10–60 Hz. There was local coupling of the system dominated by local resonances of seat frame and seat surface above 60 Hz. Moreover, the transmissibility measured on the seat surface between the human and seat foam is suggested to be a good method of capturing human-seat system resonances rather than that measured on the human body in high frequencies above 10 Hz.Practitioner Summary: The coupling characteristics of the combined human body and vehicle seat system has not yet been fully understood in frequencies of 0.5–100 Hz. This study shows the human-seat system has distinctive dynamic coupling characteristics in three different frequency regions: below 10 Hz, 10–60 Hz, and above 60 Hz.     

Study of human–seat interface pressure distribution under vertical vibration

The influence of whole-body vertical vibration on the dynamic human–seat interface pressure is investigated using a flexible grid of pressure sensors. The ischium pressure and the overall pressure distribution at the human–seat interface are evaluated as functions of the magnitude and frequency of vibration excitation, and seated posture and height. The dynamic pressure at the seat surface is measured under sinusoidal vertical vibration of different magnitudes in the 1–10 Hz frequency range. Two methods based on ischium pressure and ischium force are proposed to study the influence of seat height, posture and characteristics of vibration. The results of the study reveal that the amplitude of dynamic pressure component increases with an increase in the excitation amplitude in almost entire frequency range considered in this study. The dynamic components of both the ischium pressure and the ischium force reveal peaks in the 4 to 5 Hz frequency band, the range of primary resonant frequency of the seated human body in the vertical mode. The mean values of the dynamic ischium pressure and the ischium force remain constant, irrespective of the excitation frequency and amplitude. The magnitudes of mean pressure and force at the human–seat interface, however, are dependent upon the seat height and the subject's posture. The inter-subject variability of the static ischium pressure and effective contact area are presented as functions of the subject weight and subject weight-to-height ratio. It was found that heavy subjects tend to induce low ischium pressure as a result of increased effective contact area.

Relevance to industry

Pressure distribution at the human–seat interface has been found to be an important factor affecting the seating comfort and work efficiency of various workers. The study of human–seat interface pressure distribution under vibration is specifically critical to the comfort, work efficiency and health of vehicle drivers, who are regularly exposed to vibration. The results reported in this paper will be useful to study dynamic response of the interface pressure and design vehicle seats.     

Transmission characteristics of suspension seats in multi-axis vibration environments

The multi-axis vibration transmission characteristics of selected suspension seats were investigated in the laboratory. Subjects were exposed to a flat acceleration spectrum and two low frequency signals extracted from multi-axis acceleration data recorded at the floor of a passenger locomotive. Triaxial accelerations were measured at the floor of the vibration table and at the interfaces between the subject and mounted seat (seat pan and seat back). The transmission ratios between the overall seat pan and seat back accelerations and floor accelerations provided an effective tool for evaluating the effects of measurement site, vibration direction, and posture among the selected seating systems. The results showed that the system transfer matrix, estimated using a multiple-input/single-output model, would be less than ideal for predicting low frequency operational seat vibration when using suspension seats. The Seat Effective Amplitude Transmissibility (SEAT), estimated for the tested locomotive seats, was used to predict the weighted seat pan accelerations and Vibration Total Values for assessing a 1-h operational exposure in accordance with ISO 2631-1: 1997.

Relevance to industry

Multi-axis SEAT values can be estimated for seating systems tested in the laboratory using representative operational exposures. These values can be applied to monitored vehicle floor accelerations to target potentially harmful vibration in accordance with ISO 2631-1: 1997, assuming the operational exposures have similar frequency and magnitude characteristics. The transmission at the seat back should be considered when substantial low frequency multi-axis vibration is present.     

针对机车低频晃车问题的频变阻尼减振器仿真分析研究

针对某型高速机车车体横向低频晃动问题,提出一种将传统抗蛇行减振器替换为频变阻尼减振器(FSD)的抑制方法.首先,应用Simpack建立该机车动力学模型,从车体振动频域分布对车体横向低频晃动特性进行分析,分析表明车体横向晃动频率在1-2Hz之间;然后,建立频率阀的CFD计算模型和FSD减振器数值简化模型;最后,对比应用传...     

Transmission of roll and pitch seat vibration to the head

A series of experiments has investigated the transmission of roll and pitch seat vibration to the heads of seated subjects. Head motion was measured in all six axes using a light-weight bite-bar while seated subjects were exposed to random motion at frequencies of up to 5 Hz at 1.0 rad.s ^?2 r.m.s. Subjects sat on a rigid flat seat in two body postures: ‘back-on’ (back in contact with backrest) and ‘back-off’ (no backrest contact). The influence of the position of the centre of rotation was also investigated.

Motion at the head occurred mostly in the lateral, roll and yaw axes during exposure to roll seat vibration and in the fore-and-aft, vertical and pitch axes during exposure to pitch seat vibration. A reduction in the magnitude of head motion occurred when the subjects sat in a 'back-off' posture compared with a 'back-on' posture. Varying the position of the centre of rotation along the lateral axis during roll seat vibration affected vertical and pitch head motion: least head motion occurred when the centre of rotation was in line with the subject's mid-sagittal plane. Varying the position of the centre of rotation along the vertical axis during roll seat vibration affected head motion in the mid-coronal plane: roll head motion decreased as the position of the centre of rotation was raised from below the seat surface to above the seat surface. Varying the centre of rotation (along the fore-and-aft and vertical axes) during pitch seat vibration altered head motion in the mid-sagittal plane. Head motion increased with increasing distance of the centre of rotation in front or behind the subject's ischial tuberosities and increased as the seat was raised from below the centre of rotation to above the centre of rotation.     

Effects of stretch reflex on back muscle response during sinusoidal whole body vibration in sitting posture: A model study

This study seeks to examine human vibration response using a musculoskeletal model that appropriately considers stretch reflex. The stretch reflex is modeled with a feedback control approach, and integrated into a generic musculoskeletal model to study the active muscle forces during seated whole body vibration. The model is used to investigate the effects of stretch reflex gain, vibration frequency and vibration magnitude on transmissibility from the seat to upper body and lower body and on muscle activations.The overall model is validated by comparison with thoracic and lumbar muscle activities measured in human participants during whole body vibration. The simulation results were consistent with the experimental results that the peak transmissibility occurred at resonance frequency of 5–6 Hz, and were in line with other experimental studies that found a primary resonance of 4–6 Hz. Furthermore, the peak normalized Electromyography (EMG) level accorded with the activation level for both thoracic and lumbar regions. What's more, an increase of primary resonance frequency was observed with increasing gains of stretch reflex. In contrary, the peak seat transmissibility of the upper body and lower body had a significant reduction.The major contribution of this model is that the proposed stretch reflex model provides a useful method to consider muscle active response in whole body vibration simulation. This may be used in future studies to better understand how stretch reflex affects spinal loading in a variety of conditions.     

Assessments of two dynamic manikins for laboratory testing of seats under whole-body vibration

The aptness of two anthropodynamic manikins for assessing vibration isolation effectiveness of suspension seats is evaluated through laboratory measurements. The evaluations were performed using five different suspension seats exposed to idealized white noise (0.5–20 Hz) and target vehicle excitations along the vertical axis using a whole-body vehicular vibration simulator. The measurements were performed to derive acceleration transmissibility and seat effective amplitude transmissibility (SEAT) characteristics of seats loaded with: human subjects of body masses in the vicinity of 55, 75 and 98 kg; manikins configured to same masses; and equivalent rigid masses. The dynamic responses of the manikins were also measured under different magnitudes of white-noise excitations and expressed in terms of apparent mass. The relative applicability of the manikins for selected seats was evaluated by comparing the measures with those obtained for the seat–human and seat–mass systems. The comparisons suggested that the SEAT measures attained with manikins are comparable with those obtained with equivalent rigid mass, irrespective of the body mass, for the low natural frequency seats (2 Hz) considered in the study. Both the manikins and the equivalent rigid masses, however, provided an overestimate of isolation effectiveness of seats, when compared to those with human subjects. The manikins resulted in better estimates of SEAT values for high natural frequency seats than the rigid mass. The dynamic responses of manikins were also compared with the ranges of standardized values reported in ISO-5982 and DIN-45676. The results revealed considerable differences between the biodynamic responses of manikins and the standardized ranges.     

Equivalent comfort contours for vertical seat vibration: effect of vibration magnitude and backrest inclination

This study determined how backrest inclination and the frequency and magnitude of vertical seat vibration influence vibration discomfort. Subjects experienced vertical seat vibration at frequencies in the range 2.5-25 Hz at vibration magnitudes in the range 0.016-2.0 ms(-2) r.m.s. Equivalent comfort contours were determined with five backrest conditions: no backrest, and with a stationary backrest inclined at 0° (upright), 30°, 60° and 90°. Within all conditions, the frequency of greatest sensitivity to acceleration decreased with increasing vibration magnitude. Compared to an upright backrest, around the main resonance of the body, the vibration magnitudes required to cause similar discomfort were 100% greater with 60° and 90° backrest inclinations and 50% greater with a 30° backrest inclination. It is concluded that no single frequency weighting provides an accurate prediction of the discomfort caused by vertical seat vibration at all magnitudes and with all backrest conditions. PRACTITIONER SUMMARY: Vertical seat vibration is a main cause of vibration discomfort for drivers and passengers of road vehicles. A frequency weighting has been standardised for the evaluation of vertical seat vibration when sitting upright but it was not known whether this weighting is suitable for the reclined sitting postures often adopted during travel.