C. W. Suggs

APPLICATION OF A DAMPED SPRING-MASS HUMAN VIBRATION SIMULATOR IN VIBRATION TESTING OF VEHICLE SEATS

Abstract There is a need for standardized methods for testing vehicle seats. Such methods would allow for the direct comparison of the merits of seats of diverse types and designs. At the present time standardized testing procedures are not possible because the dynamic characteristics of the human subjects occupying the seat during testing vary widely from man to man and affect the seat response. Testing with dead weight loading does not accurately portray the dynamic characteristics of the seat. A damped spring-mass system closely approximating to the dynamic characteristics of a seated man to vertical modes of vibration has been developed as the basis on which a standardized vehicle seat testing procedure can be built. Analysis of the problem by means of mechanical impedance techniques indicated that a two-degree-of-freedom system was sufficient to simulate the major dynamic characteristics of man in the frequencies below 10 Hz where seat vibration is most severe. Test procedures have been developed whi...

Hand-arm vibration part II: Vibrational responses of the human hand☆

Abstract When vibration is applied to the hand in the vertical (dorsal-to-ventral) and transverse direction, the hand arm system can be modeled by a three-mass model with each of the masses connected by a parallel spring and damper. For vibration input directed into the long axis of the forearm, the model requires an additional parallel spring and damper to connect the last mass to an infinite base. From absorbed power calculations it was determined that to minimize energy dissipation in the hand the vibration input should be in the vertical direction and the handgrip should be as tight as convenient for frequencies below 100 Hz. Above 100 Hz the direction of input should be in the transverse direction and the handgrip should be as loose as possible. The hand, in general, was found to be a highly damped system with the damping increasing with increasing handgrip and acceleration.

Hand-arm vibration part III: A distributed parameter dynamic model of the human hand-arm system

Abstract An anatomically analogous distributed parameter dynamic model of the human arm is proposed and quantitatively validated. Distributed mass and stiffness parameters have been obtained by representing each long bone of the arm as a flexural beam. A distributed damping parameter was introduced by allowing the beam stiffness to be a complex quantity. Hand properties were modelled as a lumped parameter damped spring-mass system. Mechanical driving point impedance techniques were used to verify the model. A dual beam model of the forearm was first proposed, and its frequency response was compared with impedance data collected on the forearm. After having established the validity of the forearm model, it was then extended to include the upper arm. The frequency response of the whole-arm model was then compared with impedance data on the whole arm collected by a previous investigator. It is concluded that the beam model of the human arm adequately represented its dynamic behavior as measured by mechanical driving point impedance techniques. The amount of information concerning the dynamic behavior of the arm yielded by the distributed parameter model is found to be vastly greater than that yielded by lumped parameter models.

Human reactions to whole-body transverse angular vibrations compared to linear vertical vibrations.

An electro hydraulic vibration device was used to study the effects of some forms of angular motion in the transverse mode and a combination of angular transverse and vertical motion, common in ground vehicles. Heart rate, tracking error, hip and shoulder acceleration, and subjective judgements of three subjects were recorded under different test conditions using three different seats. The excitation acceleration had a 0-25 and 0-50 g peak to peak linear component on the seat at frequencies of 1, 1-7, 2-5, and 40 Hz. Man is more affected and shows more degraded performance under the influence of transverse angular vibration than under vertical vibrations alone. Worst is a combination of transverse angular and vertical motion. Tractor seats good for vertical vibrations are not necessarily good for transverse vibrations

Development of a simulator for use in the measurement of chain saw vibration.

The measurement of vibration production of light, hand held power tools is important in the assessment of the potential for elicitation of vibration injury to the operators of these tools. The manner in which these measurements are made can greatly affect the results. The measured vibration production of the tool becomes a function of the operator holding the saw during the measurement and his physical characteristics. The objective of the work reported here was to develop a simulator of the operator which might be used in the vibration production analysis of chain saws. The operator was modelled in terms of the driving point mechanical impedance characteristics of humans for input to the hands. A simulator was developed based on the driving point impedance characterization of the operator and evaluated with chain saw vibration measurements.

Optimization of vehicle accelerator-brake pedal foot travel time

This study was directed towards reducing the lag time between stimulus and incidence of braking. The effect of the relative vertical heights of the brake and accelerator pedals on foot travel time was the subject of the first part of the investigation. In the second part, two new pedal designs in which the accelerator was mounted directly on the brake pedal were evaluated. A significant reduction in foot travel time of approximately 12.5% was realised by locating the accelerator pedal 25-50 mm (1-2 in) higher than the brake pedal. Mounting of the accelerator pedal adjacent to or directly on the brake pedal allowed reductions in braking lag time of 46% to 74%.

Dynamic properties of the human head

Abstract Driving point mechanical impedance measurements were used to determine the dynamic response of the human head to sinusoidal vibration in the frequency range between 30 Hz and 5000 Hz at excitation levels of 0·98 m/s 2 and 3·4 m/s2. Because of the low excitation levels, the weight of the head was sufficient to couple the head to the vibration source. At 20 Hz the impedance magnitude was about 790 N-s/m but increased at approximately 6dB/octave to a peak near 3500 N-s/m at 70–90 Hz. Between 100 Hz and 2000 Hz impedance decreased by about two orders of magnitude while the apparent mass decreased by three orders of magnitude indicating good vibration isolation at higher frequencies. The impedance response contains the information for modelling the head as a dynamic system. The response of the head to vibration can be simulated by a two degree-of-freedom, mass-excited system consisting of a series connection of a small driving mass, a damper, a spring and damper in parallel and a large final mass. Parameter values, derived by computer techniques, suggest that the large mass represents the total head, the small mass the tissue in contact with the vibration input and the spring the skull stiffness.

Resistance of wood chips and sawdust to airflow.

ABSTRACT AIRFLOW resistance through variable height columns of wood chips and sawdust was evaluated by means of the pressure drop across an orifice plate. Input pressure to the bottom of the column was controlled by means of a sliding gate valve or damper on the supply fan air intake. Flow per unit of cross section plotted against input pressure per unit of bed depth yielded the expected straight line response on a log-log plot. The response for chips was similar in both actual value and slope to the flow characteristics of similar size products such as bean pods. The flow through sawdust was similar to the flow through fescue seed. Coefficients for the classical airflow equation were evaluated from the data.

Mechanical impedance techniques for evaluating the dynamic characteristics of biological materials

Driving point mechanical impedance, which is the complex ratio of force to velocity at the driving point, is a useful tool for characterizing the dynamic response of a non-rigid distributed system. Being a complex quantity it has a magnitude and a phase angle for each frequency of excitation. The necessary apparatus includes a shaker, force and acceleration transducers and means for recording the transducer outputs. To simplify analysis of results the shaker should have a sinusoidal output which is variable over the frequency range of interest. Although vibrational intensity is usually detected as acceleration and integrated to get velocity it could be sensed directly. In order to eliminate manual treatment of the transducer outputs to get the magnitude and phase angle there are instruments available which compute these values automatically as excitation frequency is scanned. Observing the dynamic characterization of biological system by means of mechanical impedance techniques is particularly appropriate because the parameters of these systems are usually distributed. The method has the advantage of not requiring that transducers be attached to the material being tested; that is, the technique is an input-output measurement. The method is analogous to impedance evaluation of electrical circuits in alternating current theory. The method has been successfully used in the mathematical modelling and subsequent construction of a dynamic stimulator describing the responses of a seated subject to low frequency whole-body vibration. Work is underway on the medium to high frequency responses of the hand-arm system. Potential applications range from evaluation of structures to testing of fruits, vegetables, eggs and other animal products as well as intact plant systems.

A Proposed Phenomenological Model to Characterize Mechanical Properties of Flue-cured Tobacco Leaves

It was determined that the visco-elastic behavior of four flue-cured tobacco leaves could be modeled by the equation; sI = EIe + CIe1+b + sd; where sI = stress index (N/mm), EI = elastic index coefficient (N/mm), cI = non-linear elastic index parameter (N/mm), b = elastic exponent, m = damping coefficient dependent on strain rate (N/mm) and d = damping exponent. The relationship between m and strain rate was determined to be m = Ge. b where G = viscous constant (Nsb/mm), b = constant, and e. = strain rate (L/s). In all cases, the coefficient of determination (R2) was above 95% thereby indicating that this phenomenological model can be used to describe the non-linear loading curve of these tobacco leaves. Results indicated the non-linear elastic parameter (cI) influenced the response of the tissue more than the linear elastic coefficient (EI). Viscous behavior was non-linear and resembled the characteristics of pseudoplastic flow.