Performance of a complex manual control task during exposure to vertical whole-body vibration between 0·5 and 5·0 Hz

An experiment is described in which seated subjects performed first-order pursuit tracking with a simultaneous discrete task; performance with the discrete task was dependent on performance of the continuous task. Vertical, z-axis, whole-body sinusoidal vibration was presented at frequencies from 0·5 to 5·0Hz at an acceleration magnitude of 2·0 ms^?2 r.m.s. in three separate sessions. In the first session, inter-subject and intra-subject variability masked any disruption caused by the vibration. After further training, all vibration frequencies disrupted performance of the continuous task. Disruption was independent of vibration frequency below 3·15Hz and increased at 4·0 and 5·0Hz. A visual mechanism was assumed to account for the increased disruption at these higher frequencies. Mechanisms which may have been responsible for the disruption below 3·15 Hz are discussed. Effects of vibration on the discrete task were attributable to disruption in performance of the continuous task. The results illustrate the importance of adequately training subjects prior to investigating vibration effects.     

Effects of display vibration and whole-body vibration on visual performance

Fifteen subjects performed a numeral reading task during (a) vibration of the display, (b) vibration of the subject, (c) simultaneous vibration of both subject and display. Sinusoidal motion at eleven frequencies (0·5 to 5·0 Hz) was presented at five acceleration magnitudes (1·0 to 2·5ms^?2 r.m.s.). Measures of reading time and reading error showed that for all except the highest frequencies, vibration of the display resulted in the poorest performance. Simultaneous whole-body-and-display vibration produced least performance decrement. The effects of both the viewing conditions and the vibration frequency are discussed in relation to known characteristics of the visual system.     

Equivalent comfort contours for vertical vibration of steering wheels: effect of vibration magnitude, grip force, and hand position

Vehicle drivers receive tactile feedback from steering-wheel vibration that depends on the frequency and magnitude of the vibration. From an experiment with 12 subjects, equivalent comfort contours were determined for vertical vibration of the hands at two positions with three grip forces. The perceived intensity of the vibration was determined using the method of magnitude estimation over a range of frequencies (4-250 Hz) and magnitudes (0.1-1.58 ms⁻² r.m.s.). Absolute thresholds for vibration perception were also determined for the two hand positions over the same frequency range. The shapes of the comfort contours were strongly dependent on vibration magnitude and also influenced by grip force, indicating that the appropriate frequency weighting depends on vibration magnitude and grip force. There was only a small effect of hand position. The findings are explained by characteristics of the Pacinian and non-Pacinian tactile channels in the glabrous skin of the hand.     

Response of the seated human body to whole-body vertical vibration: biodynamic responses to sinusoidal and random vibration

The dependence of biodynamic responses of the seated human body on the frequency, magnitude and waveform of vertical vibration has been studied in 20 males and 20 females. With sinusoidal vibration (13 frequencies from 1 to 16 Hz) at five magnitudes (0.1–1.6 ms^? 2 r.m.s.) and with random vibration (1–16 Hz) at the same magnitudes, the apparent mass of the body was similar with random and sinusoidal vibration of the same overall magnitude. With increasing magnitude of vibration, the stiffness and damping of a model fitted to the apparent mass reduced and the resonance frequency decreased (from 6.5 to 4.5 Hz). Male and female subjects had similar apparent mass (after adjusting for subject weight) and a similar principal resonance frequency with both random and sinusoidal vibration. The change in biodynamic response with increasing vibration magnitude depends on the frequency of the vibration excitation, but is similar with sinusoidal and random excitation.     

Objective evaluation of eye strain using measurements of accommodative oscillation

Accommodative oscillations occurring during eye strain induced by two different types of experimental visual search task—visual display terminal (VDT) work and conventional hard-copy (paper) work— were objectively measured using infrared optometry. The low-frequency component (0 to 1-5 Hz) of the accommodative oscillation was found to be significantly increased after the VDT work, but not after the paper work. At other frequencies there were no significant differences between the two work types and no significant effect of the task itself. In a questionnaire survey the subjects using the VDT complained more than the subjects given paper work of ocular symptoms such as eye strain and blurred vision. These results suggest that measurement of changes in accommodative oscillation, particularly the low-frequency component (0 to 15 Hz), may provide an objective parameter of eye strain.     

A finite element modeling-based approach to predict vibrations transmitted through different body segments of the operator within the workspace of a small tractor

Agricultural tractor drivers are subjected to high levels of whole-body vibrations and hand arm vibrations during most part of the farm activities due to unevenness of field surface, uneasy posture, improper workplace design, moving parts of the tractor, and other unavoidable circumstances. The comfort level of the operator inside a dynamic tractor is dependent on the level of vibration generated inside the different human body segments. In the present study, a finite element modeling was proposed to predict vertical vibrations (Z-axis) and frequencies at the different body segments of the seated small tractor operator. The forces required for different controls of the tractor were measured to be used as input parameters in the finite element modeling. The maximum mean forces of the brake (172.8 N) and clutch (153.2 N) were used as the input parameters for the simulation study. The simulated results were validated with the field measured values of vertical accelerations at selected body segments of the operator. The simulation could successfully predict vertical vibrations at selected points of interest (i.e., foot, leg, thigh, lower arm, upper arm, back, and head) except the chest of the body, as the buttock of the operator model was fixed (degree of freedom is equal to zero) in the simulation. The obtained results were compared with the international standards ISO 2631-1 (1985/1997) and ISO 5349-1 (2001) to assess the vibration characteristics at the different body segments of the operator. The foot, leg, lower arm, and upper arm of the operator were subjected to vertical vibration frequencies from 10 to 200 Hz. Most of the resonance of vertical accelerations occurred in one-third octave bands of 20–80 Hz frequencies. The thigh, chest, back, and head of the operator were exposed to vibration frequencies below 40 Hz during field operation. At these parts of the body, the vertical acceleration resonated at lower frequencies, between 2 and 8 Hz.     

Effects of horizontal whole-body vibration on reading

Text from a newspaper was read by seated subjects (8 male, 8 female) during exposure to fore-and-aft and lateral whole-body vibration. With narrow-band random vibration at frequencies between 0.5 Hz and 10 Hz and with vibration magnitudes between 0.63 m s(-2) rms and 1.25 m s(-2) rms, reading speed was measured and subject ratings of reading speed were obtained. During exposure to fore-and-aft vibration, the subjects' ratings suggested that reading speed was significantly reduced at frequencies between 1.25 Hz and 6.3 Hz, with greater impairment at higher magnitudes of vibration. Maximum interference with reading was reported at 4 Hz. Measures of reading speed showed that subjects consistently overestimated their reduction in reading speed. Lateral vibration produced similar results, but the effect was less than that with fore-and-aft vibration.     

Vibrations transmitted from human hands to upper arm,shoulder, back,neck, and head

Some powered hand tools can generate significant vibration at frequencies below 25 Hz. It is not clear whether such vibration can be effectively transmitted to the upper arm, shoulder, neck, and head and cause adverse effects in these substructures. The objective of this study is to investigate the vibration transmission from the human hands to these substructures. Eight human subjects participated in the experiment, which was conducted on a 1-D vibration test system. Unlike many vibration transmission studies, both the right and left hand-arm systems were simultaneously exposed to the vibration to simulate a working posture in the experiment. A laser vibrometer and three accelerometers were used to measure the vibration transmitted to the substructures. The apparent mass at the palm of each hand was also measured to help in understanding the transmitted vibration and biodynamic response. This study found that the upper arm resonance frequency was 7–12 Hz, the shoulder resonance was 7–9 Hz, and the back and neck resonances were 6–7 Hz. The responses were affected by the hand-arm posture, applied hand force, and vibration magnitude. The transmissibility measured on the upper arm had a trend similar to that of the apparent mass measured at the palm in their major resonant frequency ranges. The implications of the results are discussed.Relevance to industryMusculoskeletal disorders (MSDs) of the shoulder and neck are important issues among many workers. Many of these workers use heavy-duty powered hand tools. The combined mechanical loads and vibration exposures are among the major factors contributing to the development of MSDs. The vibration characteristics of the body segments examined in this study can be used to help understand MSDs and to help develop more effective intervention methods.     

Unimanual and Bimanual Control in a Compensatory Tracking Task∗

Abstract

This paper reports an experiment comparing the unimanual and bimanual control performance of human operators in a compensatory tracking task. Two variables of the control system dynamics, the target wave frequencies and the delayed visual feedback (DVF) of operator's hand control motion were also investigated. Target wave frequencies of 0.067 Hz, 0.167 Hz and 0.383 Hz, and DVF of 0.0, 0.2, 0.4, 0.8 and 1.5 s were used. The experiment was divided into one practice session run over four days and one experimental session of two days for each of the six subjects in the study. A computer system was designed to generate target wave patterns, to measure hand control motions, to regulate DVF conditions, and to record task data, all on a real-time basis. The results indicate that unimanual tracking was significantly superior to the bimanual tracking under DVF conditions and higher levels of wave frequency, but not under the zero DVF or low wave frequencies. Tracking performance in general degraded as a function of increased wave frequencies and of increased DVF. Motor learning was observed under three wave frequencies without DVF, but not when DVF conditions were introduced in a series of trials.     

Influence of reading format on reading activity under uniaxial whole body vibration

In a railway vehicle, the vibrations are transmitted to the passengers through the various interfaces such as floor, seat, backrest etc. These vibrations affect the passenger comfort as well as their performance to do any work such as reading, writing, typing etc. In the present work, effects of vibration magnitude, direction of vibration, postures and reading formats have been studied on the reading activity. Thirty healthy male subjects have performed reading task, one at a time. All subjects were exposed to uni-axial whole body vibration in 1–20 Hz frequency range at 0.5, 1 and 1.5 m/s² rms vibration magnitude. The experimental task involved reading a paragraph under the different 54 experimental conditions (three magnitude, three direction, two posture and three reading format). The task performance has been evaluated in terms of time taken by the subjects to read a given paragraph and also the subjective evaluation of perceived difficulty on Borg's CR 10 scale. Perceived difficulty and performance degradation in reading have been found to increase with the increase in vibration magnitude in each direction of vibration. The perceived difficulty and performance degradation in reading have been observed to be higher in the fore-&-aft direction in with-backrest posture. In vertical and lateral vibration, perceived difficulty and performance degradation have been higher in without-backrest posture compare to with-backrest posture. The perceived difficulty and performance degradation have been lower for the triple-column format.     

Motor performance patterns between unilateral mechanical assistance and bilateral muscle contraction

Motor performance patterns for mechanical assistance on unilateral force control can be affected by simultaneous muscle contraction. This study investigated how muscle activity and motor performance during the cooperation between dominant-arm force control and assistive force are affected by simultaneous non-dominant arm muscle contraction with inertial loading. Eleven participants (age: 24.1 ± 1.7 years) performed trajectory-tracking task based on visual feedback of real-time isometric force control. Their force for dominant-arm elbow flexion was released from reference magnitude of 47 N to magnitude of 23.5 N by providing mechanical assistance of a linear actuator. A 47 N of inertial loading on non-dominant arm elbow flexion was conditionally provided. For four time epochs of the experimental task, we measured responses of the assisted arm in terms of: (1) surface electromyography (EMG) amplitudes of biceps brachii and triceps brachii muscles, (2) peak perturbation, and (3) motor performance of force variability and target overshoot during manual force output. Simultaneous loading on unassisted arm did not affect peak perturbation of assisted arm. However, it caused lower force variability and overshoot ratio during the time epoch of force release and higher EMG amplitudes of biceps brachii muscle during the time epochs after mechanical assistance is provided, compared to the non-loaded condition. Our results indicate that simultaneous muscle contraction affects unilateral force control with mechanical assistance aimed at enhancing motor performance by creating extra agonist muscle activity. These findings can be utilized for improving the performance of human-robot cooperation during manual material handling in many industrial sites.     

Difference thresholds for the perception of whole-body vertical vibration: dependence on the frequency and magnitude of vibration

When seeking to reduce vibration in transport it is useful to know how much reduction is needed for the improvement to be noticeable. This experimental study investigated whether relative difference thresholds for the perception of whole-body vertical vibration by seated persons depend on the frequency or magnitude of vibration. Relative difference thresholds for sinusoidal seat vibration were determined for 12 males at three vibration magnitudes and eight frequencies (2.5, 5, 10, 20, 40, 80, 160, 315 Hz) using the three-down-one-up method in conjunction with a two-interval-forced-choice procedure. The median relative difference thresholds were in the range 9.5% to 20.3%. There appeared to be a frequency-dependence at the lowest vibration magnitude, such that higher frequencies had higher difference thresholds. The relative difference thresholds depended on the vibration magnitude only at 2.5 and 315 Hz. The influence of both vibration frequency and vibration magnitude on the measured difference thresholds suggests that vision (at 2.5 Hz) and hearing (at 315 Hz) contributed to the perception of changes in vibration magnitude.     

Effects of vertical vibration on passenger activities: writing and drinking.

Two laboratory studies have investigated how handwriting ability and holding a cup of liquid depend on the characteristics of whole-body vertical vibration. The effects of vibration magnitude (0.16 to 2.5 ms-2 r.m.s.), vibration frequency (0.5 to 10 Hz), and vibration duration (2 cycles to 10 s) on handwriting were studied with 20 subjects. Subjects were asked to copy letters of the alphabet by writing on a hand-held surface. Writing speed decreased and subjective ratings of writing difficulty increased with increasing vibration magnitude, particularly in the frequency range 4 to 8 Hz. Writing difficulty also increased with increasing duration of vibration. A 10 s exposure to 5 Hz vibration at 2.0 ms-2 r.m.s. resulted in subjective estimates corresponding to 'extremely difficult'. The effects of vibration magnitude (0.63 to 1.6 ms-2 r.m.s.), vibration frequency (0.5 to 10 Hz), and vibration duration (2 cycles to 10 s) on the spilling of liquid from a hand-held cup were also investigated in a group of 20 subjects. The probability of spilling the liquid, the quantity of liquid spilt, and subject's estimates of the probability of spillage were determined for all conditions. Greatest interference with the task occurred at 4 Hz, with the lowest vibration magnitude (0.63 ms-2 r.m.s.) causing measured and estimated spillage probabilities of approximately 85%. The interference was much less at other frequencies, with 0.63 ms-2 r.m.s. causing less than 10% measured probability of spillage below 3 Hz and above 5 Hz. The estimated probability of spillage was generally greater than the observed probability of spillage when the spillage probability was low, but less than the observed probability when the spillage probability was high. Increasing the duration of vibration increased the probability of spillage, and also increased the volume of liquid spilt.     

Power hand tool vibration effects on grip exertions

Operation of vibrating power hand tools can result in excessive grip force, which may increase the risk of cumulative trauma disorders in the upper extremities. An experiment was performed to study grip force exerted by 14 subjects operating a simulated hand tool vibrating at 9.8 m/s² and 49 m/s² acceleration magnitudes, at 40 Hz and 160 Hz frequencies, with vibration delivered in three orthogonal directions, and with 1.5kg and 3.0kg load weights. Average grip force increased from 25.3 N without vibration to 32.1 N (27%) for vibration at 40 Hz, and to 27.1N (7%) for vibration at 160 Hz. Average grip force also increased from 27.4 N at 9.8 m/s² acceleration to 31.8 N (16%) at 49m/s². Significant interactions between acceleration x frequency, and frequency x direction were also found. The largest average grip force increase was from 25.3N without vibration to 35.8N (42%) for 40 Hz and 49 m/s² vibration. The magnitude of this increase was of the same order as for a two-fold increase in load weight, where average grip force increased from 22.5N to 35.0N (56%). A second experiment studied hand flexor and extensor muscle responses using electromyography for five subjects holding a handle vibrating at 8 m/s² using ISO weighted acceleration, with frequencies of 20 Hz, 40 Hz, 80 Hz and 160 Hz, and grip forces of 5%, 10% and 15% of maximum voluntary contraction. Muscle responses were greatest at frequencies where grip force was affected, indicating that the tonic vibration reflex was the likely cause of increased grip exertions.     

Design of MMIC oscillators using GaAs 0.2 μm PHEMT technology

We propose a feedback type oscillator and two negative resistance oscillators.These microwave oscillators have been designed in the S band frequency.A relatively symmetric resonator is used in the feedback type oscillator.The first negative resistance oscillator uses a simple lumped element resonator which is substituted by a microstrip resonator in the second oscillator to improve results.The negative resistance oscillator produces 4.207 dBm and 7.124 dBm output power with the lumped element resonator and microstrip resonator respectively,and the feedback type oscillator produces ?10.707 dBm output power.The feedback type oscillator operates at 3 GHz with phase noise levels at-83.30 dBc/Hz and-103.3 dBc/Hz at 100 kHz and 1 MHz offset frequencies respectively.The phase noise levels of the negative resistance oscillator with the lumped element resonator are-94.64 dBc/Hz and-116 dBc/Hz at 100 kHz and 1 MHz offset frequencies respectively,at an oscillation frequency of 3.053 GHz.With the microstrip resonator the phase noise levels are-99.49 dBc/Hz and-119.641 dBc/Hz at 100 kHz and 1 MHz offset frequencies respectively,at an oscillation frequency of 3.072 GHz.The results showed that both the output power and the phase noise of the negative resistance oscillators were better than those of the feedback type oscillator.     

Frequency weighting for the evaluation of steering wheel rotational vibration

The human perception of rotational hand–arm vibration has been investigated by means of a test rig consisting of a rigid frame, an electrodynamic shaker unit, a rigid steering wheel, a shaft assembly, bearings and an automobile seat. Fifteen subjects were tested while seated in a driving posture. Four equal sensation tests and one annoyance threshold test were performed using sinusoidal excitation at 18 frequencies in the range from 3 to 315 Hz. In order to guarantee the generality of the equal sensation data, the four tests were defined to permit checks of the possible influence of three factors: reference signal amplitude, psychophysical test procedure and temporary threshold shift caused by the test exposure. All equal sensation tests used a reference sinusoid of 63 Hz at either 1.0 or 1.5 m/s² r.m.s. in amplitude. The four equal sensation curves were similar in shape and suggested a decrease in human sensitivity to hand–arm rotational vibration with increasing frequency. The slopes of the equal sensation curves changed at transition points of approximately 6.3 and 63 Hz. A frequency weighting, called W_s, was developed for the purpose of evaluating steering wheel rotational vibration. The proposed W_s has a slope of 0 dB per octave over the frequency range from 3 to 6.3 Hz, a slope of −6 dB per octave from 6.3 to 50 Hz, a slope of 0 dB per octave from 50 to 160 Hz and a slope of −10 dB per octave from 160 to 315 Hz. W_s provides a possible alternative to the existing W_h frequency weighting defined in International Standards Organisation 5349-1 (2001) and British Standards Institution BS 6842 (1987).

Relevance to industry

For the manufacturers of tyres, steering systems and other vehicular components the proposed W_s frequency weighting provides a more accurate representation of human perception of steering wheel rotational vibration than the W_h weighting of ISO 5349-1 and BS6842.     

The influence of seat backrest angle on human performance during whole-body vibration

This study investigated the effects of reclined backrest angles on cognitive and psycho-motor tasks during exposure to vertical whole-body vibration. Twenty participants were each exposed to three test stimuli of vertical vibration: 2-8 Hz; 8-14 Hz and 14-20 Hz, plus a stationary control condition whilst seated on a vibration platform at five backrest angles: 0° (recumbent, supine) to 90° (upright). The vibration magnitude was 2.0 ms(-2) root-mean-square. The participants were seated at one of the backrest angles and exposed to each of the three vibration stimuli while performing a tracking and choice reaction time tasks; then they completed the NASA-TLX workload scales. Apart from 22.5° seat backrest angle for the tracking task, backrest angle did not adversely affect the performance during vibration. However, participants required increased effort to maintain performance during vibration relative to the stationary condition. These results suggest that undertaking tasks in an environment with vibration could increase workload and risk earlier onset of fatigue. PRACTITIONER SUMMARY: Current vibration standards provide guidance for assessing exposures for seated, standing and recumbent positions, but not for semi-recumbent postures. This paper reports new experimental data systematically investigating the effect of backrest angle on human performance. It demonstrates how workload is elevated with whole-body vibration, without getting affected by backrest angle.     

The effects of vibration-reducing gloves on finger vibration

Vibration-reducing (VR) gloves have been used to reduce the hand-transmitted vibration exposures from machines and powered hand tools but their effectiveness remains unclear, especially for finger protection. The objectives of this study are to determine whether VR gloves can attenuate the vibration transmitted to the fingers and to enhance the understanding of the mechanisms of how these gloves work. Seven adult male subjects participated in the experiment. The fixed factors evaluated include hand force (four levels), glove condition (gel-filled, air bladder, no gloves), and location of the finger vibration measurement. A 3-D laser vibrometer was used to measure the vibrations on the fingers with and without wearing a glove on a 3-D hand-arm vibration test system. This study finds that the effect of VR gloves on the finger vibration depends on not only the gloves but also their influence on the distribution of the finger contact stiffness and the grip effort. As a result, the gloves increase the vibration in the fingertip area but marginally reduce the vibration in the proximal area at some frequencies below 100 Hz. On average, the gloves reduce the vibration of the entire fingers by less than 3% at frequencies below 80 Hz but increase at frequencies from 80 to 400 Hz. At higher frequencies, the gel-filled glove is more effective at reducing the finger vibration than the air bladder-filled glove. The implications of these findings are discussed.     

Visual perception of direct-view and collimated displays under vibration

An experiment is reported in which the effect of whole body vibration on visual acuity was investigated. Varying contrast and frequencies of 2–14 Hz were used, with vibration in the horizontal and vertical axes, as well as in both axes simultaneously. Resolution performance was measured on both collimated and uncollimated visual displays. Both contrast and vibration affected visual acuity, but in an additive and orthogonal fashion. Significant amelioration of the degrading effects of vibration was obtained by collimating the display.     

On human response to prolonged repeated whole-body vibration

The experiment was aimed at investigating human response to different doses of whole-body vibration (WBV), at checking adaptation to repeated exposures, at further evaluating the frequency weighting, and at examining the effect of a distinct interruption of prolonged exposure. Eight male seated subjects were exposed for 3 h to sinusoidal WBV in the z-axis with the frequencies 4 Hz and 8 Hz, at a constant acceleration level of 1·0ms^-2 rms,each frequency being repeated 4 times on consecutive days. Transmissibility, impedance, bioelectrical activity of trunk muscles, postural sway, performance in vigilance tasks, and the subjectively assessed psychological state, efforts, and stress experienced in performing the tasks were investigated. The transmissibility decreased during exposure time at 4 Hz and increased at 8 Hz when a controlled posture was maintained. The power-spectral density distribution and amplitude of postural sway were affected by WBV, depending on both duration and frequency. Performance data and rating data exhibited decrements and adverse effects, being greater beyond the ‘fatigue-decreased proficiency’ boundary (FDPB); adaptation and habituation were more pronounced at the FDPB dose. Generally, there were no cumulative effects. A pause for 20min did not essentially affect the reactions investigated.