C.H. Lewis

A review of the effects of vibration on visual acuity and continuous manual control, part I: Visual acuity

This is the first part of a review of the effects of vibration on vision and continuous manual control. In this part experimental research into the effects on human vision of both object vibration and whole-body vibration is summarized. Knowledge of the respective effects of vibration variables (principally amplitude, frequency and direction) and visual task variables (such as illumination, size and viewing distance) is discussed in separate sections. It is shown that while many factors are important and current understanding is far from complete, the literature does contain useful information. The conditions studied in those experiments concerned with the effects of whole-body vibration on vision are tabulated in the Appendix so as to provide a convenient guide to the literature.

Human factors consideration in clinical applications of virtual reality

Virtual reality environments have many potential applications in medicine, including surgical training, tele-operated robotic surgery, assessment and rehabilitation of behavioural and neurological disorders and diagnosis, therapy and rehabilitation of physical disabilities. Although there is much potential for the use of immersive virtual reality environments in clinical applications, there are problems which could limit their ultimate usability. Some users have experienced side-effects during and after exposure to virtual reality environments. The symptoms include ocular problems, disorientation and balance disturbances, and nausea. Susceptibility to side-effects can be affected by age, ethnicity, experience, gender and physical fitness, as well as the characteristics of the display, the virtual environment and the tasks. The characteristics of the virtual reality system have also been shown to affect the ability of users to perform tasks in a virtual environment. Many of these effects can be attributed to delays between the sampling of head and limb positions and the presentation of an appropriate image on the display. The introduction of patients to virtual reality environments, for assessment, therapy or rehabilitation, raises particular safety and ethical issues. Patients exposed to virtual reality environments for assessment and rehabilitation may have disabilities which increase their susceptibility to certain side-effects. Special precautions therefore need to be taken to ensure the safety and effectiveness of such virtual reality applications. These precautions include minimisation of possible side-effects at the design stage. Factors are identified which are likely to affect the incidence of side-effects during and after exposures, and which need to be understood in order to minimise undesirable consequences. There is also a need for the establishment of protocols for monitoring and controlling exposures of patients to virtual reality environments. Issues are identified which need to be included in such protocols.

A review of the effects of vibration on visual acuity and continuous manual control, part II: Continuous manual control

This second, and final, part of a review of the effects of vibration on human performance is concerned with continuous manual control, or tracking. As in the first part, which dealt with the effects of vibration on vision, the task and vibration variables which have been shown to affect the sensitivity of a task to vibration are discussed separately. Other sections are concerned with the measurement of tracking performance in vibration environments, general conclusions about the nature and mechanisms of the effects of vibration on tracking and the application of these conclusions in the form of predictive models. The procedures and results of most of the laboratory studies of vibration and tracking performance are separately summarized in tabular form in the Appendix as a convenient guide to the relevant literature.

Predicting the effects of vibration frequency and axis, and seating conditions on the reading of numeric displays

Ten subjects performed a numeral reading task under five levels of sinusoidal whole-body vibralion at various frequencies in each of the three translational axes. In the z-axis the vibration frequencies were spaced at half octaves between 2-8 and 63 Hz, and in the x and y-axes at half octaves between 2-8 and 32 Hz. All of the vibration conditions were presented with two sealing conditions: hard flat seat with fixed footrcst and a simulated helicopter seat with moving footrest. With both seats the effect of vibration level on reading accuracy was found to be significant at all but the highest frequencies of z-axis vibration. It was also significant for x-axis vibration with the simulated helicopter seat, but not for x-axis vibralion with the flat seat or for y-axis vibration with either seat. Results are presented in the form of equal performance contours.Measures of translational and rotational head motion were also made for each vibration axis and seat. Very little vibration was found to be transmitted t...

Predicting the effects of vertical vibration frequency, combinations of frequencies and viewing distance on the reading of numeric displays

This paper describes a series of experiments to determine the effects of vibration frequency, viewing distance and multiple frequency motions on the reading of numeric characters. Contours of vertical (z-axis) whole-body vibration levels resulting in equal degradation of the reading task were determined over the frequency range 2·8 Hz to 63 Hz. With the seating condition employed, the task was found to be most sensitive to vibration acceleration at a frequency of 11·2 Hz. A marked correlation was observed between reading error and reading speed. The effects of vibration on reading performance were found to be dependent on viewing distance for distance of less than 1·5 m, with the effect increasing as the viewing distance was decreased. The effect of 3·15 Hz vibration was found to increase more rapidly with reductions in viewing distance than that of 16 Hz vibration. The effects of 3·15 and 16 Hz vibration were independent of viewing distance greater than 1·5 m, indicating that the effects of rotational eye motion are dominant at these distances. Four methods were compared for predicting the effects of multiple frequency motion on reading performance given a knowledge of the effect of each component alone. The best predictions of reading error were obtained from the most severe weighted spectral component alone. Inspection of individual subjects data suggests that in many cases the effect of multiple frequency vibration on reading is even less than the effect of the largest sinusoidal component alone.

Evaluating the vibration isolation of soft seat cushions using an active anthropodynamic dummy

Seat test standards require human subjects to be used for measuring the vibration isolation of vehicle seats. Anthropodynamic dummies, based on passive mass–spring– damper systems, have been developed for testing seats but their performance has been limited at low excitation magnitudes by non-linear phenomena, such as friction in the mechanical components that provide damping. The use of an electrodynamic actuator to generate damping forces, controlled by feedback from acceleration and force transducers, may help to overcome these limitations and provide additional benefits. The transmissibilities of five foam cushions have been measured using an actively controlled anthropodynamic dummy, in which damping and spring forces were supplied by an electrodynamic actuator. The dummy could be set up to approximate alternative single-degree-of-freedom and two-degree-of-freedom apparent mass models of the seated human body by varying motion feedback parameters. Cushion transmissibilities were also measured with nine human subjects, having an average seated weight similar to the dummy. At frequencies greater than 4 Hz, mean cushion transmissibilities measured with subjects were in closer agreement with the transmissibilities obtained with a two degree-of-freedom dummy than with a single degree-of-freedom dummy. However, at frequencies between 2 and 4 Hz, cushion transmissibilities obtained with the two-degree-of-freedom dummy showed consistently larger differences from mean transmissibilities with subjects than single-degree-of-freedom dummies, indicating a need for further development of human apparent mass models to account for the effects of magnitude and spectral content of the input motion. Vertical vibration isolation efficiencies (SEAT values) of the five foams were measured with four input motions, including three motions measured in a car. The SEAT values obtained using the active dummy were highly correlated with the median SEAT values obtained with the nine human subjects, with the two-degree-of-freedom apparent mass dummy giving the highest agreement.

Motions and crew responses on an offshore oil production and storage vessel

The motions of vessels may interfere with crew activities and well-being, but the relationships between motion and the experiences of crew are not well-established. Crew responses to motions of a floating production and storage offshore vessel at a fixed location in the North Sea were studied over a 5-month period to identify any changes in crew difficulties and symptoms associated with changes in vessel motion. Ship motions in all six axes (fore-aft, lateral, vertical, roll, pitch, and yaw) were recorded continuously over the 5-month period while 47 crew completed a total of 1704 daily diary entries, a participation rate of 66-78% of the crew complement. The dominant oscillations had frequencies of around 0.1 Hz, producing magnitudes of translational oscillation in accommodation areas of up to about 0.7 ms(-2)r.m.s., depending on the weather, and magnitudes up to three times greater in some other areas. The daily diaries gave ratings of difficulties with tasks, effort level, motion sickness, health symptoms, fatigue, and sleep. Problems most strongly associated with vessel motions were difficulties with physical tasks (balancing, moving and carrying), and sleep problems. Physical and mental tiredness, cognitive aspects of task performance, and stomach awareness and dizziness were also strongly associated with motion magnitude. There was a vomiting incidence of 3.1%, compared with a predicted mean vomiting incidence of 9.3% for a mixed population of unadapted adults. It is concluded that crew difficulties increase on days when vessel motions increase, with some activities and responses particularly influenced by vessel motions.

Susceptibility to seasickness

This paper explains part of the observed variability in passenger illness ratings aboard ships by gender, age and sickness history. Within the framework of a European project, 2840 questionnaires, gathered on several ships operating all over Europe, were analysed. Gender, age and sickness history all had a highly significant effect on seasickness. Furthermore, these effects could be characterized by two fixed parameters describing a general age effect, a third parameter dependent on sickness history and a fourth parameter dependent on gender. Female illness ratings peaked at an age of 11 years, 1.5 times as high as male ratings, which peaked at an age of 21 years. At higher ages, illness ratings decrease to only 20% of their maximum, reducing gender differences to zero. Passengers with a previous history of seasickness rated their illness about two times higher than those who had not felt sick before.

Virtual reality environments for psycho-neuro-physiological assessment and rehabilitation.

Virtual Reality Environments for Psychoneurophysiological Assessment and Rehabilitation-is an European Community funded project (Telematics for health-HC 1053 http:/(/)www.etho.be/ht_projects/vrepar/) whose aim is: to develop a PC based virtual reality system (PC-VRS) for the medical market that can be marketed at a price which is accessible to its possible end-users (hospitals, universities and research centres) and which would have the modular, connectability and interoperability characteristics that the existing systems lack; to develop three hardware/software modules for the application of the PC VRS in psychoneurophysiological assessment and rehabilitation. The chosen development areas are eating disorders (bulimia, anorexia and obesity), movement disorders (Parkinsons disease and torsion dystonia) and stroke disorders (unilateral neglect and hemiparesis). This paper presents the rationale of the different approaches and the methodology used.

The Transmission of Vibration to the Occupants of a Car Seat with a Suspended Back-Rest

A new type of car seat has been designed in which the back-rest is able to move freely in the vertical direction. The design allows the back-rest to follow the vertical vibration of the back of an occupant of the seat. This is intended to decrease any strain on the lumbar part of the spinal column caused by a different vertical motion at the seat squab and the seat back. The transmission of vertical (z axis) vibration from the base of this seat to the seat surface, to the back-rest and to the backs and heads of seat occupants has been measured. Measurements were made with the seat back fixed, to prevent vertical motion relative to the seat frame, and with the seat back free to move. Vibration transmissibilities from the seat base to the seat surface were characterized by a resonance at frequencies in the region of 4 Hz. The resonance frequencies were slightly higher with the fixed back-rest than with the moving back-rest. The transmissibilities at frequencies above 4 Hz were generally higher with the fixed back-rest. Measurements with no contact between the seat occupants and the seat back indicated that the dynamic properties of the human body and the seat cushion were less influenced by the moving seat back than by the fixed seat back. Differential motion between the seat surface and seat back was smaller with the moving seat back than with the fixed back.