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2012 IEEE RO-MAN: The 21st IEEE International Symposium on
Robot and Human Interactive Communication.
September 9-13, 2012. Paris, France.

Study on Avoidance Action Parameters against
a Sharp End-Effector Tip Approaching Human’s Eyes
Takamasa Hattori, Yoji Yamada, Shogo Okamoto, Shuji Mori, Shunsuke Yamada, and Susumu Hara
neglected or intuitively estimated because harm-avoidance
behavior is an unexplored human factor. The characteristics
of avoidance actions have to be investigated to assess the
optimal human-robot distance in a coexistence space.

Abstract— In this study, a psychological experiment is conducted to investigate harm-avoidance action characteristics
in humans in close contact with robotic devices. For theexperiment, a situation is created in which the sharp endeffector tip of a robot suddenly approaches the eyes of a facing
participant. We define three parameters that represent harmavoidance action characteristics: the avoidance reaction time,
maximum avoidance acceleration, and maximum avoidance
speed. The results suggest that the avoidance reaction timedepends on the initial distance between the human’s eyes
and the approaching object. The results show that there areindividual differences in all parameters studied. The results also
show that the avoidance reaction time is negatively correlatedwith the other two parameters, which have a strong positive
correlation with each other. We conclude that the avoidance
reaction time and maximum avoidance speed are considerable
parameters on avoidance actions.

B. Related Studies
Previous research into the safety of human-robot interactions has focused mainly on the end result of a harmfulinteraction. Oberer and Schraft [3] investigated injury indicesby simulating collisions of a robot with the head, chest,and pelvis of a dummy by using finite element models,and Haddadin et al. [4] followed up with head and chest
collision experiments. Park et al. [5] proposed a modelfor the collision between a human head and a robot andshowed that the optimal elastic modulus and thickness of therobot covering could be determined to prevent skin injuries.
However, harm-avoidance behavior, which a human is most
likely to engage in, was not taken into consideration in thesestudies.
Human reactions have been considered in some studies.
Ikeura et al. [6] measured the galvanic skin reflex of a subject
when a robot approached straight toward the subject’s face
to investigate which robot motions the human regarded as
threatening. Yamada et al. [7] measured the pupillary diameter of the subject when a robot endtip was accelerated toward
the subject’s face to identify robot motion conditions that
aroused human fear. However, statistical investigations of
the characteristics of avoidance actions against approachingrobot motion have not been conducted.
We conducted psychological experiments to investigate
human harm-avoidance action characteristics during a particular situation in which human’s eyes were threatened during
a human-robot interaction [8], [9].1 We here discuss parameters that represent harm-avoidance action characteristics,
based on the experimental results.

I. I NTRODUCTION
A. BackgroundWe previously proposed and developed a next-generation
cell production system in which a human and robot work
cooperatively in a practical study [1]. In that system, lightcurtains were placed between the human worker and therobot for safety reasons. However, there is a strong desire
to remove the light curtains at production sites because thereduction in available space tends to lead to a decrease inproductivity.
Light curtains can be removed if the coexistent robot
is basically designed as being inherently safe, yet there
is always the fundamental problem that a powerless robot
grasping a sharp-edged object will cause more harm to ahuman eye compared with another part of the body (e.g., ashoulder). Therefore, there should be a sufficient distance
between the human and robot to ensure a safe working
environment.
Humans, by nature, perform harm-avoidance behavior
when perceiving a threat. Hence, the possibility of avoiding
or limiting harm (hereafter referred to as “avoidability”),
which is one element of risk [2], should be taken into
consideration when the sufficient human-robot distance is assessed. Nonetheless, avoidability currently tends to be either

II. P SYCHOLOGICAL E XPERIMENT
We consider a general situation in which a gripper or
grasped object of a next-generation robot that is desired
to coexist with humans becomes a mechanical hazard. In
reference to a practical study [1], we assume a situation inwhich a sharp end-effector tip of a production site robot
suddenly attacks the eyes of a worker sitting opposite.
In the assumed situation, the worker essentially performs
harm-avoidance behavior based on visual and auditory information. Nevertheless, workers may not necessarily hear theThis work was supported by KAKENHI 21246041.
T. Hattori, Y. Yamada, S. Okamoto, and S. Hara arewith the Graduate School of Engineering, Nagoya University,
Furo-cho,Chikusa-ku,
Nagoya464-8603,
Japan

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

S. Mori is with the Faculty of Information Science and ElectricalEngineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 8190395, Japan mori@inf.kyushu-u.ac.jp
S. Yamada is with Mitsui Chemicals, Inc., Japan.

978-1-4673-4605-4/12/$31.00 ©2012 IEEE

1 The present study was approved by a local ethics committee.

25motors installed in the robots while in the actual working environment. For this reason, we focus on the more hazardous
situation in which visual information only is provided to theworker.
A diagram of the psychological experiment is shown in
Fig. 1.
A. Experimental Overview
Three different initial positions of the robot’s end effectors
were set. The objective of the experiment was to investigate
whether human avoidance-action characteristics are influenced by the initial distance between the human’s eyes andthe end-effector tip.
1) Apparatus: An upper-body humanoid robot (HIRO,
Kawada Industries, Inc.), designed to operate collaboratively
with a human [1], [10], was used. A photograph of the robot
is shown in Fig. 2. The participant wearing protective glasses
and the robot were separated by a working table. To minimize
harming the participant, the original end effectors for picking
up and placing mechanical parts were replaced with squarepyramid-shaped flexible polyurethane foam.
2) Participants: Nine people, five males and four females
between the ages of 18 and 28, participated. Every participant
was healthy with good eyesight; no one reported sufferingfrom belonephobia.2
3) Experimental Setup: Each participant wore a cap on
which motion capture markers were attached and sat on
a stool in front of the robot. The participants wore noisecanceling earphones (NW-A845, Sony Corporation) to block
any external auditory information; they instead listened tosounds recorded in a factory. The participants were exposedto the working area of the robot and performed the following
task: they inserted two mechanical parts, i.e., rollers andretainers, between the bearing rings using tweezers. The task
was conducted with the bearing rings shown in Fig. 3 andUpper-body humanoid robot

Fig. 3.

Bearing rings used for the task

performed in a manner similar to the way workers performthe task in actual production. The participants were instructed
to keep their work productivity as high as possible.
When the participant performed the task with concentration, the robot initially idled without motion, and one end
effector at the tip of the robot arm suddenly approached theeyes of the participant.3 The movement of the participant
and robot were captured by a video camera, with the motion
capture system (Motion Analysis Corporation) recording theparticipant’s head movement.

Motion capture system

Participant

Fig. 2.

Robot

B. Experimental Conditions
The probability distribution of the foreperiod for a participant not expecting the timing of stimulus is often modeledusing an exponential distribution [11]. In this manner, statistically random foreperiods were used by taking the sum of
10 s and exponential random values with a mean of 15 s,
excluding those longer than 60 s.
Fig. 4 shows a schematic diagram of the locations anddistances for this experiment. The bearing rings were locatedat the position indicated by the bold cross. Three initial

Bearing
End effector

Fig. 1.

Illustrative diagram of the psychological experiment

2 For safety reasons, initial experiments had to be conducted with young
people in good health who seemed to have reasonably good reactions.

3 Every participant was informed in advance that the end effector would
approach his or her eyes.

26

330

TABLE I50

M OTION PARAMETERS OF THE ROBOT ’ S E ND -E FFECTOR T IP DURING
THE P SYCHOLOGICAL E XPERIMENT

170
Bearing

y

100

140

180

250

x
O
Pattern 3
Pattern 2

230

Pattern 1

Pattern

Maximum speed[mm/s]

Initial acceleration[mm/s2 ]

1
2
3

˜1330
˜1180
˜990

˜8900
˜7500
˜6800

C. Experimental Procedure

End effector

Each participant performed 60 trials. The end-effector
pattern and approaching end effector were both chosen to
be random for each trial.

Participant

Top view

1) Statistical Analysis on the Avoidance Reaction Time:In this study, the avoidance reaction time (RT) is definedas the time interval from the beginning of the end-effector
motion to the beginning of the participant’s head movement.
Statistical tests were carried out to investigate whether
the end-effector pattern had any effect on the avoidance RT.
Because conditions applicable to parametric tests were notmet, we used the Steel-Dwass test [12], [13], which is a
nonparametric test for investigating whether the difference
between two of more than three groups is significant. The
null hypothesis was set such that the avoidance RT for eachpair of patterns was identical. Table II summarizes the resultsof the Steel-Dwass tests based on the avoidance RT datafrom all participants. From this table, the difference between
patterns 1 and 2 is marginally significant. The difference
between patterns 1 and 3 is considered significant at asignificance level of α = 0.001, while that of patterns 2 and3 is considered significant at a significance level of α = 0.05.
These results suggest that a shorter initial distance between
the human’s eyes and the end-effector tip is associated witha shorter avoidance RT.
2) Investigation of Individual Differences in the Avoidance
RT: Steel-Dwass tests were carried out to investigate whether
the avoidance RT differed between participants, and the null
hypothesis was set such that the avoidance RT for eachpair of participants was identical. Tables III, IV, and V
summarize the results of the Steel-Dwass tests between theparticipants based on the avoidance RT data in patterns 1, 2,
and 3, respectively. Terms A–I denote the participants, and∗, ∗∗, and ∗∗∗ denote whether the difference is consideredsignificant at a significance level of α = 0.05, α = 0.01,

z

280

110

155

200

D. Experimental Results

Ox

Side viewFig. 4. Schematic diagram of the interactive human-robot locations during
the psychological experiment (unit: mm)positions were set for the end effectors: pattern 1 started
at the farthest position, while pattern 3 was the nearest.
The initial distance between the participant’s eyes and an
end-effector tip was set to be approximately 470, 370,
and 270 mm for patterns 1, 2, and 3, respectively. Eachparticipant was asked to confirm that the end-effector tips
were in his or her peripheral vision when the tips werelocated at a viewing angle of approximately 30◦ for all
patterns with the task position in the center of the visual field.
In a trial, the end-effector tip arrived at a point approximately
50 mm forward of the participant’s initial eye position at theend of the robot motion.
Table I lists the motion parameters of the robot’s endeffector tip for this experiment. The maximum speed andinitial acceleration of the end-effector tip were set foreach pattern, as shown in the table, based on the runaway
assumption.4 The approaching paths of both end effectors
were elliptical and symmetrical, and the approaching motion
profile was the same for each pattern.

TABLE IIS TEEL -DWASS T ESTS FOR THE AVOIDANCE RT BETWEEN THE
PATTERNS

4 We set the robot to output the same highest possible speed percentage
for each pattern. The maximum speed and initial acceleration inevitably
decreased in the order pattern 1, 2, 3 due to differences in the travel distance.

27

Patterns

Range of two-sided p-value

1&2
1&3
2&3

0.05 < p < 0.10
p < 0.0001
0.025 < p < 0.05

TABLE III

TABLE VIS TEEL -DWASS T ESTS FOR THE AVOIDANCE RT BETWEEN THE
PARTICIPANTS IN PATTERN 1S TEEL -DWASS T ESTS FOR THE M AXIMUM AVOIDANCE ACCELERATIONBETWEEN THE PATTERNS

A
B
C
D
E
F
G
H

B

C

D

E

F

G

H

I

Patterns

Range of two-sided p-value

∗∗

∗∗∗
+

∗∗∗
n.s.
∗∗

∗
n.s.
∗∗
n.s.

∗
∗∗∗
∗∗∗
∗∗∗
∗∗∗

∗∗∗
∗∗∗
∗
∗∗∗
∗∗∗
∗∗∗

n.s.
n.s.
∗
n.s.
n.s.
∗∗∗
∗∗∗

∗∗
n.s.
∗∗
n.s.
n.s.
∗∗∗
∗∗∗
n.s.

1&2
1&3
2&3

p > 0.20
p > 0.20
p > 0.20

appear to depend on the initial distance between the human’seyes and the end-effector tip.
4) Investigation of Individual Differences in the Maximum
Avoidance Acceleration: Steel-Dwass tests were carried out
to investigate whether the maximum avoidance acceleration
was different between participants, and the null hypothesis
was set such that the maximum avoidance acceleration foreach pair of participants was identical. Table VII summarizes
the results of the Steel-Dwass tests between the participants
based on the maximum avoidance acceleration data.
This table shows that the maximum avoidance accelerationdiffers between participants.
5) Statistical Analysis on the Maximum Avoidance Speed:
In this study, the maximum avoidance speed is defined as the
maximum speed of movement at the participant’s glabellar
position.
Steel-Dwass tests were carried out to investigate whether
the end-effector pattern had any effect on the maximum
avoidance speed, and the null hypothesis was set such thatthe maximum avoidance speed for each pair of patterns was
identical. Table VIII summarizes the results of the SteelDwass tests based on the maximum avoidance speed datafrom all participants. From this table, the difference between
any combination is considered insignificant at a significancelevel of α = 0.05.
Therefore, the maximum avoidance speed does not appear

TABLE IVS TEEL -DWASS T ESTS FOR THE AVOIDANCE RT BETWEEN THE
PARTICIPANTS IN PATTERN 2

A
B
C
D
E
F
G
H

B

C

D

E

F

G

H

I

+

∗∗∗
∗∗

∗∗
n.s.
∗∗

+
n.s.
∗∗∗
n.s.

n.s.
∗∗∗
∗∗∗
∗∗∗
∗∗∗

∗∗∗
∗∗∗
n.s.
∗∗∗
∗∗∗
∗∗∗

n.s.
n.s.
∗∗∗
n.s.
n.s.
∗∗∗
∗∗∗

∗∗
n.s.
∗
n.s.
n.s.
∗∗∗
∗∗∗
∗

and α = 0.001, respectively. Symbol + denotes that thedifference is marginally significant, and n.s. denotes that thedifference is not considered significant.
These tables show that the avoidance RT differs between
participants.
3) Statistical Analysis on the Maximum Avoidance Acceleration: In this study, the maximum avoidance acceleration
is defined as the maximum acceleration of movement at theparticipant’s glabellar position.
Steel-Dwass tests were carried out to investigate whether
the end-effector pattern had any effect on the maximum
avoidance acceleration, and the null hypothesis was set suchthat the maximum avoidance acceleration for each pair ofpatterns was identical. Table VI summarizes the results of theSteel-Dwass tests based on the maximum avoidance acceleration data from all participants. From this table, the difference
between any combination is considered insignificant at asignificance level of α = 0.05.
Therefore, the maximum avoidance acceleration does not

TABLE VIIS TEEL -DWASS T ESTS FOR THE M AXIMUM AVOIDANCE ACCELERATIONBETWEEN THE PARTICIPANTS

A
B
C
D
E
F
G
H

TABLE VS TEEL -DWASS T ESTS FOR THE AVOIDANCE RT BETWEEN THE
PARTICIPANTS IN PATTERN 3

A
B
C
D
E
F
G
H

B

C

D

E

F

G

H

I

∗∗∗

∗∗∗
n.s.

∗∗∗
n.s.
n.s.

∗
n.s.
∗∗
+

+
∗∗∗
∗∗∗
∗∗∗
∗∗∗

∗∗∗
∗∗∗
∗
∗∗∗
∗∗∗
∗∗∗

∗
n.s.
∗
n.s.
n.s.
∗∗∗
∗∗∗

∗∗∗
n.s.
n.s.
n.s.
∗
∗∗∗
∗∗∗
n.s.

B

C

D

E

F

G

H

I

∗∗∗

∗∗∗
∗∗∗

∗∗∗
∗∗∗
∗∗∗

∗∗∗
n.s.
∗∗∗
∗∗∗

∗
∗∗∗
∗∗∗
∗∗∗
∗∗∗

∗∗∗
∗∗∗
∗∗∗
n.s.
∗∗∗
∗∗∗

n.s.
∗∗∗
∗∗∗
∗∗∗
∗∗∗
n.s.
∗∗∗

∗∗∗
∗∗∗
∗∗∗
n.s.
∗∗∗
∗∗∗
n.s.
∗∗∗

TABLE VIIIS TEEL -DWASS T ESTS FOR THE M AXIMUM AVOIDANCE S PEEDBETWEEN THE PATTERNS

28

Patterns

Range of two-sided p-value

1&2
1&3
2&3

p > 0.20
0.10 < p < 0.20
p > 0.20

Maximum avoidance acceleration [mm/s2]

to depend on the initial distance between the human’s eyes
and the end-effector tip.
6) Investigation of Individual Differences in the Maximum Avoidance Speed: Steel-Dwass tests were carried out
to investigate whether the maximum avoidance speed was
different between participants, and the null hypothesis was
set such that the maximum avoidance speed for each pair ofparticipants was identical. Table IX summarizes the resultsof the Steel-Dwass tests between the participants based on
the maximum avoidance speed data.
This table shows that the maximum avoidance speed
differs between participants.
7) Relationships between the Avoidance Action Parameters: We focus on the relationships between the avoidance
action parameters: the avoidance RT, maximum avoidance
acceleration, and maximum avoidance speed.
Fig. 5 shows the relationship between the avoidance RT
and maximum avoidance acceleration based on the data fromall participants. The graph shows that there is a negative correlation between the two, with a coefficient of approximately−0.59.
Fig. 6 shows the relationship between the avoidance RT
and maximum avoidance speed based on the data from allparticipants. The graph shows that there is a negative correlation between the two, with a coefficient of approximately−0.60.
Fig. 7 shows the relationship between the maximum
avoidance acceleration and maximum avoidance speed basedon the data from all participants. The graph shows that there
is a strong positive correlation between the two, with acoefficient of approximately 0.94. This positive correlation
was confirmed for all participants.

10000

8000

6000

4000

2000

0
0

Maximum avoidance speed [mm/s]

Even though the types of end-effector patterns were limited, we could determine the avoidance action characteristics
under experimental conditions.
The velocity and acceleration of an approaching object
are likely to be important factors for the avoidance action
characteristics. Differences in the maximum speed and initialacceleration of the robot’s end-effector tip would affect the
avoidance RT in the psychological experiment. A higher
maximum speed and initial acceleration were expected toresult in a shorter avoidance RT; however, the experiment

600

400

200

C

D

E

F

G

H

I

∗∗∗
∗∗∗
∗∗∗

∗∗
∗∗∗
∗∗∗
∗∗∗

∗
∗∗∗
∗∗∗
∗∗∗
∗∗∗

∗∗∗
∗∗∗
∗∗∗
n.s.
∗∗∗
∗∗∗

n.s.
∗∗∗
∗∗∗
∗∗∗
∗∗∗
n.s.
∗∗∗

∗∗∗
∗∗∗
∗∗∗
∗∗∗
∗∗∗
∗∗∗
∗
∗∗∗

100

200 300 400 500 600
Avoidance reaction time [ms]

700

800

Relationship between the avoidance RT and maximum avoidance

showed the opposite. We concluded that the avoidance RT
was, instead, mainly influenced by the initial distance between the participant’s eyes and the end-effector tip.
It is possible that the participants prepared themselves
for the approaching end effector in the experiment. Thisis based on the idea that avoidance actions are associatedwith cognitive processes. In contrast, we consider avoidance
actions to be reflexive processes, but we cannot rule out any
cognitive influences at this point.
The maximum avoidance acceleration and maximum
avoidance speed have a strong positive correlation, and thedata of the latter can be manipulated more easily than thoseof the former. On the basis of these facts, we conclude
that the avoidance RT and maximum avoidance speed areconsiderable parameters on avoidance actions. From theviewpoint of estimating avoidability, we should focus on
people with a lower ability to avoid harm. Therefore, wesuggest that it is important to investigate people whose data

BETWEEN THE PARTICIPANTS

∗∗∗
∗∗∗

800

800

Fig. 6.
speed

TABLE IXS TEEL -DWASS T ESTS FOR THE M AXIMUM AVOIDANCE S PEED

B

700

1000

0
0

∗∗∗

200 300 400 500 600
Avoidance reaction time [ms]Fig. 5. Relationship between the avoidance RT and maximum avoidance
acceleration

III. D ISCUSSION

A
B
C
D
E
F
G
H

100

29

Maximum avoidance speed [mm/s]

1000

maximum avoidance speed. The results also showed thatthe avoidance RT is negatively correlated with the maximum
avoidance acceleration and maximum avoidance speed, whilethat the latter two have a strong positive correlation.
We believe that avoidance actions are reflex processes;
however, the influence of cognitive processes cannot be ruledout. The relationship between avoidance actions and thevelocity and acceleration of an approaching object has not yet
been clarified. These are areas for further study, and further
investigations into avoidance action characteristics should be
pursued with a focus on people who have a lower ability to
avoid harm.

800

600

400

200

0
0

V. ACKNOWLEDGMENT
2000
4000
6000
8000
Maximum avoidance acceleration [mm/s2]

10000

We are indebted to Professor Kazuo Koga of Kyoto Notre
Dame University for his insightful advice. We are also
grateful to graduate students Soichiro Ito and Koji Sunadaat Nagoya University for their cooperation.

Fig. 7. Relationship between the maximum avoidance acceleration andmaximum avoidance speed

R EFERENCES
has a tendency of being distributed in the lower-right area of
the graph shown in Fig. 6. In reference to the relationshipbetween the avoidance RT and maximum avoidance speed
for each participant shown in Fig. 8, participant G should be
investigated.

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IV. C ONCLUSIONS AND F UTURE W ORK
We have conducted a psychological experiment to investigate human avoidance-action characteristics under ascenario in which a sharp end-effector tip of a robot suddenly
approached the eyes of a participant sitting in front of therobot. The experimental results suggested that neither the
maximum avoidance acceleration nor maximum avoidance
speed depended on the initial distance between the human’seyes and the approaching object, but the avoidance RT does.
The results showed that there are individual differences in
the avoidance RT, maximum avoidance acceleration, andMaximum avoidance speed [mm/s]

1000

Participant A
Participant B
Participant C
Participant D
Participant E
Participant F
Participant G
Participant H
Participant I

800

600

400

200

0
0

100

200 300 400 500 600
Avoidance reaction time [ms]

700

800

Fig. 8. Relationship between the avoidance RT and maximum avoidance
speed for each participant30