2012 IEEE/RSJ International Conference onIntelligent Robots and Systems
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Psychological Experiments on Avoidance Action Characteristics
for Estimating Avoidability of Harm to Eyes from Robots
Takamasa Hattori, Yoji Yamada, Shuji Mori, Shogo Okamoto, and Susumu Hara
Abstract— In this study, psychological experiments are conducted to investigate harm-avoidance action characteristics
in humans in close contact with robotic devices. For theexperiments, a situation is created in which the sharp endeffector tip of a robot suddenly approaches the eyes of a facing
participant. The avoidance reaction time is defined as the timeinterval from the beginning of the end-effector motion to when
the participant begins movement to avoid harm. The results
suggest that the avoidance reaction time does not depend on
the type of work being performed but on the initial distance
between the human’s eyes and the approaching object. Using
this information, safe human-robot working distances can be
determined.

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
neglected or intuitively estimated because harm-avoidance
behavior is an unexplored human factor. Thus, it is difficult
to assess the optimal human-robot distance in a coexistence
space.
This 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

978-1-4673-1735-1/12/S31.00 ©2012 IEEE

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].1 This paper presents theexperimental results that contribute to reasonably estimating
avoidability.
II. M ETHOD FOR E STIMATING THE R EACTION T IMED ISTRIBUTION
We deal with the reaction time (RT) as a parameterthat represents the characteristics of avoidance actions. In
this study, probability distributions of the RT are estimatedfrom RT data obtained in the experiments. We use thekernel estimator [9] to estimate the RT distribution. This isexpressed as
)
(
n
1 ∑
t − Tiˆ,(1)
f (t) =
K
nh i=1hwhere t is the RT, n is the total number of RTs, h is the
bandwidth, Ti is the i-th RT, and K(t) is the kernel function.

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

Here, the standard normal probability density function isused for K(t):
( 2)
1
t
.
(2)
K(t) = √ exp −2
2π
The bandwidth is calculated by the method proposed by
Silverman [10], which is expressed as()
0.9
Q3 − Q1
h = 1/5 min s,
,
(3)
1.34
n
where s, Q1 , and Q3 are the standard deviation and the first
and third quartile of the RT data, respectively. Accordingly,
Q3 − Q1 is the interquartile range of the RT data.
III. P SYCHOLOGICAL E XPERIMENT I
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 the
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 experiments is shown in
Fig. 1.

1) Apparatus: An upper-body humanoid robot (HIRO,
Kawada Industries, Inc.), designed to operate collaboratively
with a human [1], [11], was used in the experiments. A
photograph of the robot is shown in Fig. 2. The participant
wearing protective glasses and the robot were separated bya working table. To minimize harming the participant, theoriginal end effectors for picking up and placing mechanicalparts were replaced with square pyramid-shaped flexible
polyurethane foam.
2) Participants: Eleven people, six males and five females
between the ages of 19 and 28, participated in the experiment. Every participant was healthy with good eyesight; noone reported suffering from 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
four tasks.
Task A Inserting two mechanical parts, i.e., rollers andretainers, between the bearing rings using tweezers.
Task B Removing the rollers between the bearing rings
using tweezers.
Task C Silently reading a document shown on a liquid
crystal display (LCD).
Task D Gazing at parts between the bearing rings.
Tasks A, B, and D were conducted with the bearing rings
shown in Fig. 3. Task A was performed in a manner similar

A. Experimental Overview
Four types of tasks were set that resembled work performed at production sites. The objective of the experiment
was to investigate whether human avoidance-action characteristics are influenced by the type of task.

Motion capture system

Participant

Robot

Bearing
End effector

Fig. 2.
Fig. 1.

Illustrative diagram of the psychological experiments

Upper-body humanoid robot

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

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y

200

330

Bearingor LCDOx270(a) Tasks A and D
Fig. 3.

(b) Task B

Bearing rings used for the tasks

End effector
Participant

Top view

100

to the way workers perform the task in actual production.
When the participant performed these tasks, the robot
initially idled without motion, and one end effector at the
tip of the robot arm suddenly approached the eyes of theparticipant.3 The movement of the participant and robot were
captured by a video camera, with the motion capture system
recording the participant’s head movement.

The probability distribution of the foreperiod for a participant not expecting the timing of stimulus is often modeledusing an exponential distribution [12]. In this manner, statistically random foreperiods were used by taking the sum of
10 s and exponential random values with a mean of 20 s,
excluding those longer than 90 s in the experiment.
Fig. 4 shows a schematic diagram of the locations anddistances for psychological experiment I. The device for thetask, the bearing rings or the LCD, was located at the positionindicated by the bold cross. The initial distance between
the participant’s eyes and an end-effector tip was set tobe approximately 400 mm. Each participant was asked toconfirm that the end-effector tips were in his or her peripheral
vision when the tips were located at a viewing angle ofapproximately 40◦ with the task position in the center ofthe visual field. In a trial, the end-effector tip arrived in thevicinity of the participant’s initial eye position at the end of
the robot motion.
On the assumption that the robot fell into a runaway
state, the maximum speed of the end-effector tip was set
to approximately 960 mm/s. The initial acceleration was set
to approximately 4400 mm/s2 . The approaching paths ofboth end effectors were elliptical and symmetrical, and theapproaching motion profile was the same in each trial.
C. Experimental Procedure
Each participant performed 24 trials: six trials for eachtask. Tasks were assigned in a random order to prevent anyorder effects. The approaching end effector was also chosen
to be random for each trial.
D. Experimental Results
1) Avoidance RT Distributions for All Types of Tasks:
In this study, the avoidance RT is defined as the time
3 Every participant was informed in advance that the end effector would
approach his or her eyes.

z280

B. Experimental Conditions

O

x

Side viewFig. 4. Schematic diagram of the interactive human-robot locations during
psychological experiment I (unit: mm)

interval from the beginning of the end-effector motion to
the beginning of the participant’s head movement.
Fig. 5 shows the avoidance RT distributions estimated withrespect to each task based on the data from all participants.
The graph shows that the positions of the leading edges aswell as the positions of the distribution peaks do not differ
notably from each other.
2) Statistical Analysis: Statistical tests were carried out toinvestigate whether the task had any effect on the avoidance
RT. Because conditions applicable to parametric tests were
not met, we used the Steel-Dwass test [13], [14], which isa 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 tasks was identical. Table I summarizes the results of
the Steel-Dwass tests based on the avoidance RT data fromall participants. From this table, the difference between any
combination is considered insignificant at a significance levelof α = 0.05.
Therefore, the avoidance RT does not appear to dependon the type of work.
3) Effect of Participant Posture on the Avoidance RT: We
consider the possibility that participant posture at the moment
the end effector began to move affected the avoidance RT.

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0.01

0.008
Probability

1) Participants: Nine people, five males and four females
between the ages of 18 and 28, participated in the experiment.
2) Experimental Setup: Each participant was exposed to
the working area of the robot and performed task A as set
in psychological experiment I.

Task A: Insert parts
Task B: Remove parts
Task C: Read a document
Task D: Gaze at parts

0.006

B. Experimental Conditions
0.004

0.002

0
0

100

200 300 400 500 600
Avoidance reaction time [ms]

700

800

Fig. 5. Probability distributions of the avoidance RT for all types of tasks
TABLE IS TEEL -DWASS T ESTS FOR THE AVOIDANCE RT BETWEEN THE TASKS
Tasks

Range of two-sided p-value

A&B
A&C
A&D
B&C
B&D
C&D

p > 0.20
0.10 < p < 0.20
p > 0.20
p > 0.20
p > 0.20
p > 0.20

Hence, we defined the x-coordinate of the participant’s eyes
at this moment as xe1 and carried out further analysis. Here,
we regard the position of the eyes to be that of the glabella.
The median of the xe1 data from all participants was denotedby x̃e1 , and all trials were then divided into trials F in which
xe1 < x̃e1 and trials B in which xe1 > x̃e1 . Namely, typeF trials are those in which the participant sat forward.
For this test we used the Mann-Whitney U test [15], which
is a nonparametric significant difference test for two groups.
The null hypothesis was set such that the avoidance RT for
F and B was identical. The result of the Mann-Whitney U
test for the avoidance RT between F and B was a two-sided
p-value of 0.0002. From this, the difference between F and
B is considered to be significant at a significance level of
α = 0.001.
This result suggests that the avoidance RT is shorter when
the participant sits forward.

In the experiment, statistically random foreperiods wereused by taking the sum of 10 s and exponential random
values with a mean of 15 s, excluding those longer than
60 s.
Fig. 6 shows a schematic diagram of the locations anddistances for psychological experiment II. The bearing rings
were located at the position indicated by the bold cross.
Three initial positions were set for the end effectors: pattern
1 started at the farthest position, while pattern 3 was thenearest. 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 II 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 for eachpattern, as shown in the table, based on the runaway assumption.4
C. Experimental Procedure
Each participant performed 60 trials, and the end-effector
pattern and approaching end effector were both chosen to berandom for each trial.
D. Experimental Results
1) Avoidance RT Distributions for All Types of Patterns:
Fig. 7 shows the avoidance RT distributions estimated withrespect to each end-effector pattern based on the data fromall participants. The graph shows that each distribution peak
position is different. There is a tendency also for a shorter
TABLE IIM OTION PARAMETERS OF THE ROBOT ’ S E ND -E FFECTOR T IP DURING

IV. P SYCHOLOGICAL E XPERIMENT IIP SYCHOLOGICAL E XPERIMENT II

A. Experimental Overview
The results of psychological experiment I suggest that the
avoidance action characteristics are contingent on the initial
distance between the human’s eyes and the robot’s endeffector tip. To investigate this dependence, psychologicalexperiment II was conducted with three different initial
positions of the end effectors. The experimental setup andconditions are similar to those in psychological experiment
I except as described below.

Pattern

Maximum speed[mm/s]

Initial acceleration[mm/s2 ]

1
2
3

˜1330
˜1180
˜990

˜8900
˜7500
˜6800

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.

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330

50

0.01

Pattern 1: Far
Pattern 2: Middle
Pattern 3: Near

250

Probability

170
Bearing

y

100

140

180

0.008

x

O
Pattern 3
Pattern 2

230

0.006

0.004

Pattern 1

0.002

End effector
Participant

Top view

00100

200 300 400 500 600
Avoidance reaction time [ms]

700

800

110

These results suggest that a shorter initial distance between
the human’s eyes and the end-effector tip is associated witha shorter avoidance RT.

280

155

200

Fig. 7. Probability distributions of the avoidance RT for all types of patternsz

V. D ISCUSSIONO

xSide viewFig. 6. Schematic diagram of the interactive human-robot locations during
psychological experiment II (unit: mm)initial distance between the human’s eyes and the endeffector tip to result in a shorter avoidance RT.
2) Statistical Analysis: Steel-Dwass tests were carriedout to investigate whether the end-effector pattern had any
effect on the avoidance RT, and the null hypothesis was set
such that the avoidance RT for each pair of patterns was
identical. Table III summarizes the results of the Steel-Dwass
tests based on the avoidance RT data from 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 a significance level of
α = 0.001, while that of patterns 2 and 3 is consideredsignificant at a significance level of α = 0.05.
TABLE IIIS TEEL -DWASS T ESTS FOR THE AVOIDANCE RT BETWEEN THE
PATTERNS
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

Even though the types of tasks and 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 psychological experiment II. A higher maximum speed and initial acceleration were expected to resultin a shorter avoidance RT; however, the experiment 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 forthe approaching end effector in the present experiments. Thisis based on the idea that avoidance actions are associatedwith cognitive processes. In contrast, we consider avoidance actions to be reflexive processes with reference to theresults of psychological experiment I, but we cannot ruleout any cognitive influences at this point. The avoidance
RT distributions, which statistically represent the avoidance
action characteristics, can, however, be regarded as statistical
distributions based on various psychological states. Hence,these findings can be used to reasonably estimate avoidability. For a simple example, based on the fact that thelongest avoidance RT is approximately 550 ms as shownin Figs. 5 and 7, we can assess whether sufficient distance isprovided in the coexistence space for preventing a humanrobot collision. Thus, the findings can contribute to managing
human-robot coexistence spaces by taking avoidability into
consideration.

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VI. C ONCLUSIONS AND F UTURE W ORK
We have conducted psychological experiments to investigate human avoidance-action characteristics under a scenario in which a sharp end-effector tip of a robot suddenly
approached the eyes of a participant sitting in front of therobot. While the results of psychological experiment I did
not suggest that the avoidance RT depended on the type ofwork, they did suggest that the participant posture affected
the avoidance RT. The results of psychological experiment
II suggested that the avoidance action characteristics dependon the initial distance between the human’s eyes and theapproaching object.
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.
VII. ACKNOWLEDGMENT
We are indebted to Professor Kazuo Koga of Kyoto Notre
Dame University and Professor Kazunori Hase of Tokyo
Metropolitan University for their insightful advice. We arealso grateful to graduate students Soichiro Ito and Koji
Sunada at Nagoya University for their cooperation.
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