Proceedings of the 2012 IEEE
International Conference on Robotics and Biomimetics
December 11-14, 2012, Guangzhou, China

Basic Experiments on Collision of Sharp Mechanical Hazards
against Eye for Estimation of Injury Severity
Soichiro Ito, Yoji Yamada, Takamasa Hattori, Shogo Okamoto, and Susumu Hara
Abstract— This study deals with the avoidance/mitigation
possibilities in an estimation of severity of injury from amechanical hazard when a sharp-edged object approaches ahuman eye. We assume that the sharp end effector of a robot
approaches the eye of a human in a human-robot coexistencesystem. We conduct a static eye collision experiment anddynamic eye collision experiment taking into consideration
human avoidance/mitigation actions. Three conditions (collision position, collision angle, and eyelid state—open/closed)
are varied in the static collision experiment. Consequently,
it is confirmed that differences in severity result from thesechanges in the experimental conditions. In the dynamic collisionexperiment, Bell’s phenomenon, which is an upward rotationof the eyeball at the time of closing the eyelid, is taken into
consideration, and two conditions (collision position and angle)
are changed. Through these experiments, we find that Bell’s
phenomenon plays an important role in the risk estimation.
Although the avoidance/mitigation possibilities have not previously been taken into consideration in eye collision experiments,
we consider these to be indispensable to the risk estimation.

I. I NTRODUCTION
A. Background
When the eye is injured by a mechanical hazard, theseverity of injury is much higher than that for other partsof the human body. Haddadin et al. [1] indicated that the
injury severity of the soft skin of a swine leg traumatizedby the sharp tool of a robot was reduced by limiting thetorque of the robot. However, it is necessary to estimate therisk to the eye separately from other parts of the body when
considering human safety in the presence of a robot. The eye
may easily become seriously injured, especially by a sharpmechanical stimulus, even if the robot is allowed to exert
only a weak external force. A robot body can be designed
to eliminate mechanical hazards such as stabbing based on
the principle of an inherently safe design. However, it might
not be possible to apply the inherently safe design principle
to an end effector and gripped object. We targeted this casewith the goal of estimating the risk in terms of severity.
Specifically, we targeted an upper-body humanoid robot
for assembling machine parts and assumed a situation where
the robot’s sharp end effector impacts a worker’s eye. It
has been concluded that when a robot poses mechanicalhazards, a human has to take safety measures such as
wearing protective goggles for safe human-robot coexistence.
This work was supported by KAKENHI 21246041.
S. Ito, Y. Yamada, T. Hattori, S. Okamoto, and S. Hara are with the
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8603, Japan. ito.soichiro@a.nagoya-u.jp,

yamada-yoji@mech.nagoya-u.ac.jp,
hattori.takamasa@e.nagoya-u.jp,{okamoto-shogo, haras}@mech.nagoya-u.ac.jp

978-1-4673-2126-6/12/$31.00 © 2012 IEEE

ScleraVitreous
CorneaFig. 1.

Structure of eyeball

However, the severity of the injury to the eye depends onthe coexistence form, including the positional relationshipbetween the human and robot. That is, an eye collision may
not necessarily result in a serious injury such as blindness.
In a study on the eye injuries caused by a collision with anobject, Kennedy et al. [2] developed an artificial eyeball andorbit for collision experiments by referencing eye collisionexperiments with human cadavers. Weaver et al. [3] conducted simulations of eyeball collisions with various objects
such as a plastic bar and BB and investigated the stressapplied to the cornea and the pressure applied to the sclera.
See Fig. 1 for understanding of the structure of eyeball.
However, these studies did not take into consideration theinfluence of human avoidance/mitigation actions such as
closing the eyelid or changing of the collision angle.
B. Purpose
We conducted eye collision experiments by changing theapproach angle of a sharp end effector, as well as thecollision position.
The collision timing of the end effector could be divided
into three categories: an open eyelid, closing eyelid, andclosed eyelid. If an end effector moves too fast or a humandoes not notice its approach, it will collide when the eyelid
is open. If a human notices the approach of the end effector
and closes his/her eyelid quickly, the end effector will collide
during the closing of the eyelid or after it is closed. Moreover,eyelid movement and Bell’s phenomenon [4], which is an
upward rotation of the eyeball during the closing of theeyelid, should be taken into consideration when the end
effector collides during the closing of the eyelid. This isbecause the movement of the eyelid and eyeball may makea difference in the severity of the injury.
For the three collision timings mentioned above, we only
had to conduct a static collision experiment with the eyelid
open or closed, without taking into consideration eyeball1912

Originating
accident

movement during the collision, because the eyeball is notmoving in these states. However, we needed to conduct a
dynamic collision experiment that took into consideration
eyeball movements in a collision where the eyelid was
closing, because of the occurrence of Bell’s phenomenon.
In this study, we conducted static and dynamic eye collisionexperiments as basic experiments, where a human head was
fixed under collision conditions that took into consideration
human avoidance/mitigation actions. We here discuss theinfluence of the collision conditions on the severity of the
injury.

Avoidance ofbody/head

Collision with eye
Severity（Collision conditions）

SA
EA

MC

－

EA・SA

Low

EA・FA・MCHigh

EA・FA・SC

FA
SC

Accident
chain

Study Objective

EA : End effector approaches
SA : Succeeds in avoiding, FA : Fails to avoid
MC : Mitigated condition, SC : Severe condition

Fig. 2.

Event tree for eye collision with end effector

C. Risk Assessment
One of the targets of our study was a rational risk
assessment in a human-robot coexistence system. In the fieldof safety of machinery, the risk elements in a risk assessment
are the severity of harm and the probability of occurrenceof harm, according to the international safety standard ISO
12100:2010 [5]. The probability of occurrence of harm isfurther divided into three elements: the exposure frequency,
occurrence probability of a hazardous event, and humanpossibilities for avoiding harm. These can be summarized
as follows.
• Severity of harm
• Probability of occurrence of harm
– Exposure frequency– Occurrence probability of a hazardous event– Human possibilities for avoiding harm
If all of these elements are estimated, we can conduct a
risk assessment. In this study, we investigated the severityof harm from the collision of an end effector approaching
the eye from various directions. The exposure frequency
and occurrence probability of a hazardous event can becalculated statistically under our assumed situation because
we deal with a repetitive task. In addition, Hattori et al. [6]and Sunada et al. [7], who are part of the author’s group,
have investigated the possibility of avoiding an approaching end effector by conducting psychological experiments
and proposed simulation of extrapolating robot movement.
Therefore, combining these findings will allow us to estimate
the risk under the assumed situation.
D. Study Objective
The event tree for an end effector colliding with a worker’s
eye is shown in Fig. 2. First, the end effector approaches
during “originating accident” (EA: End effector approaches).
The worker would try to avoid the end effector by throwing
himself/herself backward or turning his/her head. If theworker avoids the end effector or the end effector collides
with a different part of his/her body, his/her eye does not
get injured (EA·SA: Succeeds in avoiding). Otherwise, theend effector collides with the worker’s eye (EA·FA: Fails to
avoid), and he/she gets injured. If a body/head movementchanges the collision position or collision angle of the end
effector or the worker closes his/her eyelid before the endeffector collision, and the collision condition is moderated,

the severity will be low (EA·FA·MC: Mitigated condition).
Otherwise, the collision condition is severe and the severity
would be high (EA·FA·SC: Severe condition). An exampleof high severity will be a case of severe vision loss thatmakes it difficult to return to work, while some other injury
would be a slight one. The severity is classified in detailin Section II-D. In Fig. 2, while “avoidance of body/head”
indicates whether the worker can avoid a collision between
the end effector and his/her eye, “collision conditions” affectthe severity. Our study objective is to determine how changes
in the collision conditions at the eye affects the severity.
II. S TATIC E YE C OLLISION E XPERIMENT
A. Experiment Objective
In this experiment, we investigated the severity by varying three conditions: eyelid state—open/closed, collision
position, and collision angle. It was important to compare collisions when the eyelid was open and closed todetermine whether it could protect the eyeball from theend effector and mitigate the severity. Furthermore, human
avoidance/mitigation actions such as turning the face couldchange the collision position or collision angle and affect theseverity.
B. Experimental Equipment
We used a dummy eye composed of a porcine eyeball,
artificial eyelid, and mechanisms to fix the eyeball and eyelid,
and to express the elasticity of the soft tissue around theeyeball. The appearance of this dummy eye is shown in
Figs. 3 and 4, and its specifications are listed in Table I.
A porcine eyeball has often been used as a substitute for ahuman eyeball because its structure is similar to that of ahuman’s and its availability is comparatively easier [8]. Two
springs expressing the elasticity of the soft tissue were placed
behind the eyeball. Schutte et al. [9] showed that the Young’s
modulus of soft tissue is 300 Pa. Therefore, we supposed thatthe spring constant corresponding to this characteristic was
19.9 N/mm.
The impactor was an end effector made of aluminum(YH75), which had the shape of tweezer. Its appearance
and dimensions are shown in Figs. 5 and 6, respectively.
The shape was the same as that developed for graspingmechanical parts at production sites. This end effector was

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TABLE I
S PECIFICATION OF D UMMY E YE
· Porcine eyeball· Elliptical ball
· Diameters: 25 mm, 24 mm, 22 mm· Silicon series rubber material· Thickness: 1 mm
· Stab strength: 22.3–31.2 N· Tensile strength: 0–4.9 N/mm2· 1 DOF in the direction of optical axis
· Spring constant: 19.9 N/mm

Eyeball

EyelidSofttissueFig. 3.

Fig. 4.

Dummy eye without eyelid

Fig. 5.

Dummy eye with eyelid

End effector

Fig. 7.

attached to an upper-body humanoid robot (HIRO, Kawada
Industries, Inc.) [10], [11] and operated.
C. Experimental Conditions
The robot and dummy eye were set up to face each other,as shown in Fig. 7. C-clamps were used to mount the dummy
eye on a stand. Ideally, human avoidance/mitigation actions
by body/head movements should be considered, instead of
having the eye clamped in place. However, we did nothave detailed knowledge of such human avoidance/mitigation
actions. Therefore, we clamped the eye in a fixed positionin this experiment as a worst-case scenario where the humanbody and head do not move. The robot’s right arm wasoperated in a straight line, and the end effector was madeto collide with the dummy eye. The end effector was madeto stop about 10 mm past the surface of the eyeball whenit collided from the normal direction. If the collision depth
was greater than 10 mm, the end effector would puncture theeyeball and the vitreous would escape, which would result ina high severity of injury. A depth of 10 mm made it possible
to discuss the injury condition. The collision speed of the end

Eye collision experiments

effector was about 280 mm/s. This is the speed of the end
effector at the eyeball surface when the robot’s right arm
operates at the maximum speed possible. This speed was
set to investigate the most hazardous case in the assumed
situation.
While the robot’s operation condition was fixed, the threecollision conditions were varied for the dummy eye: eyelidstate, collision position, and collision angle. There were twoeyelid states (open/closed), two collision positions, and threecollision angles. These are shown in Fig. 8. When the eyelid
was open, the end effector collided directly with the eyeball,and when the eyelid was closed, the artificial eyelid was
set in front of the eyeball. The collision positions were thecornea and sclera near the cornea. The sclera was set 45◦
from the optical axis on the same horizontal surface as theaxis. We chose this angle to place the impact to the sclera
between the cornea and the corner of the eye. We expectedthat the severity of injury to the cornea would be different
from that to the sclera because their anatomies are different.
The collision angles were the normal direction of the eyeball,along with 30◦ and 45◦ from the normal direction. The
collisions at 30◦ and 45◦ from the normal direction wereset as being representative of a human avoidance/mitigation

1.8

8

1.5
15

°

30°

15° 3
0°

°

30°

15° 3
0°

15

Cornea22

EyelidSclera45°45°(a) Open eyelid(b) Closed eyelid

Unit [mm]Fig. 6.

External dimensions of end effector

Fig. 8.

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Experimental conditions for static collision experimentTABLE IIC LASSIFIED S EVERITY AND T HEIR I NJURY C ONDITIONS
Severity
432
1

Injury condition
The eyeball was punctured and the vitreous leaked out
The cornea surface was injured
The sclera surface was injured
No injury was able to be confirmed with the naked eye(a) Punctured eyeballaction such as turning the face. We set these two angles in
addition to the normal direction to confirm the influence ofthe magnitude of the collision angle on the severity.
D. Experimental Results
We classified the injury conditions and their severity into
four categories, as shown in Table II, in imitation of aninternational standard for safety of machinery [12] . It is
hard to consider the influence on eyesight in the case of theseverity “1”. The severity “2” and “3” may cause impaired
vision. The severity “3” is more severe than the severity “2”in terms of visual function. The severity “4” may cause
severely impaired vision. In the worst case, this presents
the possibility of losing sight. Based on the above, theexperimental results of the collisions with the cornea andsclera are presented in Tables III and IV, respectively.
In the case of a collision with the cornea from the normal
direction, the end effector punctured the eyeball in boththe open and closed state of the eyelid. Fig. 9(a) shows aphotograph of an eyeball that underwent a collision withthe eyelid open and was punctured. We confirmed that thecornea was punctured by the end effector. Fig. 9(b) showsa photograph of the punctured eyelid after a collision in theclosed state of the eyelid. We confirmed that the eyelid was
split where the end effector collided. In the case of a collision
with the cornea from an angle, we confirmed a scratch on
the surface, but the end effector did not puncture the eyeball,
except in the collision at 45◦ from the normal direction withthe closed eyelid.
While we could confirm a scratch on the surface of theeyeball in the case of a collision with the sclera from thenormal direction, we could not confirm injury in the othercases of collision with the sclera.
E. Discussion
First, we discuss the influence of the collision position. In
the case of a collision with the cornea, the eyeball was often
TABLE IIIR ESULTS OF S TATIC C OLLISION E XPERIMENT WITH C ORNEA

Open eyelid
Closed eyelid

0◦
4
4

30◦
3
3

45◦
3
1

TABLE IVR ESULTS OF S TATIC C OLLISION E XPERIMENT WITH S CLERAOpen eyelid
Closed eyelid

0◦
2
1

30◦
1
1

45◦
1
1

Fig. 9.

(b) Punctured eyelid
Injured eye

punctured or scratched. In contrast, the severity of injury inthe case of a collision with the sclera was lower. Therefore,
the severity in the case of a collision with the cornea wasestimated to be comparatively high, and in some cases, there
was a possibility of blindness.
Second, we discuss the influence of the collision angle.
In the case of a collision with the cornea from the normal
direction, the eyeball was first dented and then puncturedby the end effector. On the other hand, in the case of acollision with the cornea from an angle other than the normal
direction, even if the eyeball might have been dented, it did
not extend to be punctured. With an increase in the angle
from the normal direction, the end effector began to slide
across the eyeball and eyelid, or the eyeball began to rotateand divert the end effector. Therefore, if a human exercises
an avoidance/mitigation action such as turning his/her faceto avoid a collision from the normal direction, the severity is
estimated to be lower during the collision of an end effector
with human eye.
Third, we discuss the difference in severity of injury between collisions with an open eyelid and closed eyelid. In thecase of a collision with the cornea from the normal direction,
the eyeball was punctured in both cases (open/closed eyelid).
We confirmed that in some cases, the eyelid was injured but
the eyeball was not injured with a closed eyelid, while theeyeball surface was injured with an open eyelid in the case ofa collision from a direction other than the normal direction.
Therefore, it is estimated that closing the eyelid would reduce
the severity of injury, except for a collision from the normal
direction.
In addition, we confirmed the repeatability of this experiment under one collision condition in which the result wasobserved to be the severity “3”. The condition was a collision
with the cornea at 30◦ from the normal direction with theopen eyelid. The collision experiments were repeated 10
times under the condition. As a result, the severity was “3”in all trials. Although only one condition was dealt with,the high repeatability of this experiment was confirmed.
Since the individual differences in the porcine eyeballs weused were small, it is expected that results of repetition
experiments will not change greatly, irrespective of collisionconditions. Therefore, it is considered that the boundary
value of the collision angle at which an eyeball does not
get punctured is up to 30◦ from the normal direction.
However, these results cannot necessarily be applied to1915

Angle of upward rotation [degrees]

25
20
15
10

Fig. 11.

5

TABLE V00

Fig. 10.

Collision positions in dynamic collision experiment

50

100
150
Time [ms]

200

250

R ESULTS OF DYNAMIC C OLLISION E XPERIMENT

CorneaUpper part of eyeballLower part of eyeballUpward rotation of eyeball driven by motorother cases. Tweezers are one of the sharpest and most usefultools. It is estimated that the severity would have been lower
if the impactor was a rounded tool.
III. DYNAMIC E YE C OLLISION E XPERIMENT
A. Experiment Objective
It has been reported that Bell’s phenomenon [4], in which
the eyeball rotates upward, occurs when a human closes
his/her eyelid. Seo et al. [13] investigated Bell’s phenomenonusing the search coil method. In some people, an upward
rotation of their eyeball is associated with a slight outward or
inward rotation. For simplicity, in this experiment, we took
into consideration a simple upward rotation with a closed
eyelid. Specifically, the end effector was made to collide withthe dummy eye at different collision positions and angles
during the upward rotation of the eyeball, with the eyelid
closed, and the severity was investigated. The objective of
this experiment was to investigate how the upward rotationof the eyeball expressed by closing the eyelid, which is anaction to mitigate harm, affects the severity.
B. Experimental Equipment
The experimental devices were primarily the same as thoseused in the static collision experiment. The difference was
the mechanism of eyeball rotation. The eyeball was rotatedusing motors (RE16, Maxon Motor AG) connected via gears,
pulleys, and belts.
C. Experimental Conditions
The operation conditions of the robot were the same as in
the static collision experiment. The dummy eye was operated
to mimic human eyeball movement, unlike the static collisionexperiment described in II. Specifically, the initial angle ofthe eyeball was considered as the primary position, andthe eyeball began to rotate upward when the end effector
reached the eyeball surface. The eyeball rotated by 20◦ in
200 ms, which was the speed measured by Seo et al. [13].
The eyeball’s upward rotation pattern is shown in Fig. 10.
The eyelid of the dummy eye was closed, and threecollision positions and three collision angles were set. The
collision positions were the cornea in the initial state before

0◦
3
3
1

30◦
3
3
—

45◦
1
1
—

the eyeball’s upward rotation, the upper part of the eyeball,and the lower part of the eyeball. These collision positions
are shown in Fig. 11. The upper and lower parts of theeyeball were about 7 mm above and below the cornea center,
respectively. These parameters were set considering that achange in the collision position by eyeball rotation could
affect the injury condition. For example, a collision with theupper part of the eyeball could affect the cornea because of
the eyeball’s upward rotation, while the end effector would
collide with the sclera if the eyeball did not move. The
collision angles were the normal direction of the eyeball,along with 30◦ and 45◦ from the normal direction.
D. Experimental Results
The experimental results are presented in Table V. For
the numbers in the table, see Table II. The symbol “—”
represents the case that no experiment was conducted.
In the case of a collision with the cornea and upper partof the eyeball, the eyeball was scratched, with the exceptionof 45◦ from the normal direction. No injury was able to be
confirmed in the case of collision with the cornea at an angleof 45◦ from the normal direction and collision with the lower
part of the eyeball.
E. Discussion
First, we discuss the effect of the collision angle. As can benoted from Table V, the severity decreased with an increasein the angle from the normal direction of the eyeball.
Second, we discuss the effect of the collision position.
In the case of a collision with the cornea, only the surfaceof the eyeball was injured. The deviation from the centerof the cornea as a result of the eyeball rotation produced
this outcome. The collision condition with the upper partof the eyelid was similar to the collision condition withthe sclera from an angle in the static collision experiment,
the exception being eyeball rotation that occurred in theformer. While no injury was able to be confirmed in the static
collision experiment, injury to the cornea was confirmed in
the dynamic collision experiment. A slight scratch was also
confirmed. The injury condition is shown in Fig. 12. It canbe said that the upward rotation of the eyeball influenced the1916

Fig. 12.

Slight scratch

injury condition. We believe that no injury was confirmed in
the case of a collision with the lower part of the eyeballbecause the end effector collided with the sclera even whenthe eyeball rotated upward.
This experiment involved collisions with a closed eyelid.
It is considered that the eyeball was not punctured because
it was protected by the eyelid. Thus, we found that while
a closed eyelid mitigated the severity to some extent, theeyeball movement associated with closing the eyelid also
influenced the severity. It is estimated that even if a human
could close his/her eyelid before an end-effector collision,
the severity would be higher than that of a collision with thesclera if the eyeball rotated in association with this closing
and the cornea then returned to a position behind the collisionarea of the eyelid. While closing of the eyelid against acollision with an object is one of the mitigation actions,the eyeball movement associated with this action would notnecessarily mitigate the severity. Therefore, the position of
the cornea at the collision is important in an investigation of
the severity of injury to an eye.
However, the above discussion might not necessarily beapplicable to another end effector with different shape.
IV. C ONCLUSIONSIn this study, we investigated the impact of a robot’s sharp
end effector on a human eye in a human-robot coexistencesystem. We conducted basic eye collision experiments undervarious collision conditions, taking into consideration human
avoidance/mitigation actions. And we examined how differences in the collision conditions influenced the severity of
injury. Three collision conditions (collision angle, collision
position, and eyelid state—open/closed) were varied in astatic eye collision experiment. The results showed that
all of the conditions had some influences on the severity.
Specifically, the eyelid served as a safeguard. In regard to
the collision position, the severity of injury to the sclera wasestimated to be lower than that to the cornea. In regard to
the collision angle, the severity decreased with an increasein the angle from the normal direction of the eyeball. This
was a very interesting result. If the probability that a human
can avoid the end effector is the same against approach fromany direction, the risk of collision from the side is estimatedto be lower than that from the front. In this instance, it isdesirable to locate the robot so that its motion range is besidethe human. In the dynamic eye collision experiment, thecollision position and collision angle were varied taking into
consideration Bell’s phenomenon, in which an eyeball rotates
upward in association with a closing eyelid. As a result,

we found that eyeball movement was an important factor
influencing the severity. Although avoidance/mitigation actions have not been taken into consideration in previous eye
collision experiments, we consider these to be indispensable
to the risk estimation.
We need to increase the number of collision conditions
and trials to statistically assess the probability of occurrenceof injury corresponding to the severity under each collisioncondition.
V. ACKNOWLEDGMENT
We are indebted to Associate Professor Yasuki Ito of
Nagoya University for his insightful advice. We are alsoindebted to Tsuyoshi Mizuno of Mizuno Technical Institutefor his assistance in designing and manufacturing the experimental equipment. We are grateful to graduate student Koji
Sunada at Nagoya University for his cooperation.
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