Human response in gazing at a moving figure

ELSEVIER

Nuclear Engineering and Design Nuclear Engineering and Design 165 (1996) 239 244

Human response in gazing at a moving figure Hajime T a k a d a ~, Yuji Sato b, Heki Shibata a ~Department of Mechanical Engineering and Material Science, Faculty of Engineering, Yokohama National UniversiO', Tokiwadai 156, Hodogaya, Yokohama 240, Japan bIshikawajima-Harima Heavy Industries, Shinakahara I, Isogo, Yokohama 235, Japan

Received 30 October 1995

Abstract

This paper deals with human body vibration in gazing at a moving figure. It is said that one gets 70% information by one's eyes among the five senses. We consider that this percentage becomes larger when one is gazing at a moving object. One acts reflexively in response to information. This can lead to human operating error in an emergency. When an automatic control breaks down in an atomic power plant and man has to operate manually, a severe accident may occur if man operates by mistake. In this paper, movements of man's centre of gravity were measured when he was gazing at a vibrating picture made by a personal computer in a darkroom. As a result, man's centre of gravity moves with his own resonance frequency.

I. Introduction

Some people w o r k u n d e r severe conditions such as a m o v i n g environment or handling a vibrating tool. A worker directly receives vibrations f r o m a vibrating tool or indirectely receives vibrations because objects at which he is gazing are vibrating. Such a bad labour environment is not only a problem for workers but also a worker m a y operate by mistake. This can lead to h u m a n operating error in an emergency, because m a n acts reflexively in response to information he gets. In an atomic power plant when an automatic control breaks d o w n because o f an earthquake and m a n has to operate manually, a severe accident m a y occur if m a n operates by mistake. M a n gets most informations t h r o u g h his eyes, but the information received by m a n ' s eyes m a y not be true. F o r example, when a m a n in a train is gazing at

another train starting, he thinks his train is starting, while another m a n thinks another train is going. It is very i m p o r t a n t what imagination he has. I f he thinks another train begins to start when his train is starting, he loses his balance. Even in the opposite case, if he thinks another train is going when his train begins to start, he can also lose his balance. There are a lot o f papers on b o d y vibrations in shaking o f the h u m a n body. There are also some papers (Griffin, 1975, 1976; Benson, 1978) on eye m o v e m e n t s or vision, but few papers on h u m a n b o d y vibration (Taguchi, 1981) in gazing at a m o v i n g object. In this paper we consider h o w m a n swings reflexively when he is gazing at a m o v i n g picture. M o v e m e n t s o f m a n ' s centre o f gravity (CG) were measured when he was standing on a force plate in a d a r k r o o m and gazing at a swinging picture made by a personal computer. As parameters, the period and

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H. Takada et al. / Nuclear Engineering and Design 165 (1996) 239-244

movement pattern of the vibrating picture were considered. The subject's own resonance frequency and the phase difference between the picture and his C G were calculated by a cross-relation function. This study makes the operation efficiency of and influences on a human body under a bad labour environment clear. The results obtained from experiments on a human body determine parameters of a human vibration model by which a good environment or a good man machine interface can be designed.

The locus of C G projected on the force plate is shown in Fig. 4. The feet position is also drawn. The C G swings not only right and left but also

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2. Experiments

The positions of m a n ' s C G were measured with four load transducers under a force plate in a d a r k r o o m as shown in Fig. 1. The phase difference between the C G position and the moving picture was also measured. The rectangular picture, which is 12 m m wide and 190 m m long, swings with the vertical position as centre in a 21 inch display. Two patterns (I and II) were tried as shown in Fig. 2. The picture swings in the frequency range 0.1 1 Hz. In pattern I the picture swings with its bottom as centre of rotation, while in pattern II it swings with the height of m a n ' s waist as centre of rotation. The subject's eyes are 30 cm away from the picture and both heights are the same. As the distance between feet, d = 0 and 15 cm were chosen. Each trial was continued for a period of 80 s. Six graduate students were examined to reduce differences between individuals.

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3. Experimental results and discussion

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3.1. CG m o v e m e n t s in gazing at a moving picture

C G movements in gazing at a vertical stationary bar in a d a r k r o o m for a period of 80 s were compared with those in gazing at one vibrating at f = 0.1 Hz. As shown in Fig. 3, the result indicates man can stand more stably in gazing at a vertical stationary bar than in gazing at a moving one. Also judging from this figure, man stands more unstably at f = 0.25 Hz or less in the latter case than in the former case.

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centre o f rotation• In this study we use pattern II. The stability o f standing position is also related to the distance between a subject's feet. Fig. 6 shows differences in C G m o v e m e n t with the distance between feet as parameter. Fig. 6(a) shows the C G m o v e m e n t s when a subject stands with the distance o f 0 cm between feet, while Fig. 6(b) shows those when he stands with the distance o f 15 cm. F r o m these results the a m o u n t o f movement in the former case is larger than that in the latter case, t h o u g h the data in the former case have a lot o f noise. Therefore in this study the distance o f 15 cm was used.

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H. Takada et al..; Nuclear Engineering and Design 165 (1996) 239 244

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own resonance frequencies in the range J = 0.1 0.3 Hz. The phase difference between C G swing and picture swings is shown in Fig. 9. The values plotted are the means o f three trials and the values c; are their deviations. These results indicate that the phase difference at the resonance frequency o f 0.25 Hz is smallest and its deviation there is smallest too. This means that m a n can stand most stably at his own resonance frequency and the results o f the examination are reproducible. The cross-relation function is calculated to determine for what time o f examination man swings with the same period. Fig. 10(a)

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3.2. Frequency response and phase difference The a m o u n t o f C G m o v e m e n t in terms o f vibration amplitude is related to the frequency at which the picture swings. F r o m Fig. 7(a) it looks as if m a n sometimes tunes with the same period as the m o v e m e n t o f the picture, but he does not always tune with it. The frequency response o f one subject is shown in Fig. 7(b), which indicates that he has a resonance frequency o f 0.25 Hz. In addition, frequency responses o f six subjects are shown in Fig. 8, indicating that they have their

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H. Takada et al. / Nuclear Engineering and Design 165 (1996) 239 244

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shows the calculation results. The frequency f = 0.1 Hz is the subjects's resonance frequency. This figure indicates that in the first period of examination he swings at the same frequency as the moving picture, but gradually a swing delay occurs. The results of another subject are shown in Fig. 10(b). The frequency f= 0.25 Hz is his resonance frequency. The figure indicates that in the first 1.5 periods he swings at the same frequency as the moving picture, but gradually a swing delay occurs, and in the last period he swings simulta-

neously again. Even if man swings with his resonance frequency, he does not continue to swing with that frequency, but he sometimes swings at the frequency of the moving picture and sometimes swings at another frequency.

4. Conclusions Movements of man's centre of gravity were measured when he was gazing at a vibrating pic-

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H. Takada et al. / Nuclear Engineering and Design 165 (1996) 239 244

ture in a d a r k r o o m . The conclusions are as follows. 1. M a n ' s C G m o v e m e n t s in gazing at a vertical s t a t i o n a r y b a r are smaller t h a n those in gazing at a m o v i n g one. 2. Even when m a n is gazing at a picture m o v i n g right a n d left, he swings not only right a n d left but also back a n d forth. 3. M a n swings with his waist as centre of rotation. 4. M a n sometimes tunes with the same frequency as the m o v i n g picture, but he does not always tune with it. Six subjects have their own resonance frequencies a n d they swing in the range

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5. The phase difference between C G a n d picture swings is smallest at the resonance frequency a n d its deviation there is smallest too. Therefore m a n

can stand most stably at his own resonance frequency a n d the results of the e x a m i n a t i o n are reproducible.

References A.J. Benson and G.R. Barnes, Vision during angular oscillation: the dynamic interaction of visual and vestibular mechanisms, Aviat., Space, Environ. Med., January (1978) 340- 345. M.J. Griffin, Levels of whole-body vibration affecting human vision, Aviat., Space, Environ. Med., August (1975) 1033 1040. M.J. Griffin, Eye motion during whole-body vertical vibration, Human Factors 18(6) (1976) 601-606. K. Taguchi, M. Kikukawa and T. Ishiyama, The effects of optokinetic stimulation on the head movement. Equilib. Res. 40 (2) (1981) 242 250 (in Japanese).