2015 IEEE International Conference on Systems, Man, and Cybernetics

An Analysis of Recovery Motion of a Man Wearing
Physical Assistant Robot in Response to Collision
Yasuhiro Akiyama∗ , Ryota Kushida∗ , Yoji Yamada∗ and Shogo Okamoto∗∗ Department of Mechanical Science and Engineering

Graduate School of Engineering, Nagoya University, Nagoya, Japan
Abstract—Physical assistant robots are expected to be able toassist in our working and daily lives. However, safety problems
arise when robots are used in indoor environments with obstacles.
The focus of this study was on collision of the side of a robot
with an obstacle, and the recovery motion after the collision wasmeasured. A healthy young man participated in this experiment.
Two different types of recovery motions were observed. These
two motions differed in the degree of deceleration of the bodyrotation in the horizontal plane. The part of the robot and thetiming of the collision with the obstacle affected the recovery
motion strategy. A collision at the hip side in the middle swing
phase might be the most severe condition from which to recover,among those considered in this study. The results of this study
will be helpful in improving the safety of the use of physicalassistant robots.
Index Terms—Physical assistant robot, Safety, Recovery motion

I. I NTRODUCTION
In many countries, population aging has been causing
problems such as increasing demands for nursing care anddecreasing numbers of workers. Physical assistant robots,
which are attached to a human body and apply torque throughcuffs and shoes, are thought to be effective solutions to suchsocial problems because they can assist in the activities ofdaily living and rehabilitation of the wearer [1], [2]. Furthermore, physical assistant robots increase labor productivity bydecreasing workloads [3].
However, the use of physical assistant robots requires
greater attention to safety than the use of industrial robots
because of the close contact into which such robots come withusers. Among the anticipated hazards, a fall is a serious hazard
especially for the elderly [4]. However, the effect of a physicalassistant robot on the risk of a fall has rarely been analyzed,
except in a few studies [5]. Thus, ISO13482 [6], which is
the industrial standard that addresses the safety of physicalassistant robots, only addresses contact safety.
For a normal gait without physical assistant robot, fallingas a result of tripping has been analyzed previously in many
ways [7], [8], and the associated recovery motions have been
categorized [9]. This knowledge is helpful in evaluating therisk of a fall induced by wearing a physical assistant robot.
Furthermore, the environments in which physical assistantrobots are being used are becoming more complex as they are
increasingly coming to be used in daily living. For example,
doors are typical features of an indoor environment that theuser of a physical assistant robot encounters in a building orhouse. A collision between a robot and the casing of a door
978-1-4799-8697-2/15 $31.00 © 2015 IEEE
DOI 10.1109/SMC.2015.195

ϵĐŵ
ϭϬĐŵ
ϵϱĐŵ

Fig. 1.

MALO

Fig. 2.

Obstacle

was taken as the focus of this study because it is difﬁcult to
avoid such a collision in indoor environment.
The main objective in this study was measurement of therecovery motion of the user of the robot for the purposeof identifying the best recovery strategy and the factors that
determine which recovery motion should be made. Improving
the understanding of the recovery strategy that a robot should
apply in response to a collision will help in designing physicalassistant robots to avoid and mitigate falls.
II. M ETHOD
A. Apparatus
1) Physical assistant robot: The Motor-Actuated Lowerlimb Orthosis (MALO), developed in our lab, was used in thisstudy. MALO, which consists of a hip orthosis and a kneeankle-foot orthosis, is shown in Fig. 1. MALO has one degreeof freedom in the sagittal plane at the hip, knee, and ankle jointof each leg. The hip and knee joints of each leg are actuated by
DC motors (RE40, Maxon Motor, Switzerland) and the otherjoints move freely. MALO is attached to a wearer using a
corset, cuffs, and shoes. In this study, aluminum plates wereattached to the sides of the thigh links of the leg to serve as
surfaces that would come into contact with the obstacle.
The pattern of the assist torque generated by MALO wassynchronized with the user’s gait cycle [5]. In this study, thegait cycle was deﬁned as the time elapsed between consecutive
heel strikes of one leg, which is a typical deﬁnition. Torque
was applied to the hip joint during 5% to 20% of the gait
cycle to the extension direction and 55% to 70% to the ﬂexion
direction. Then, the knee joint was supported to the extension
direction during 80% to 90% and ﬂexion direction during 40%1089

to 55%. The algorithm that synchronizes the robot’s motion to
the user’s gait cycle is a popular motion assistance method forphysical assistant robots [10], [11]. The algorithm assists inthe forward motion of the body mass during the stance phaseand assists to swing leg. The applied torque was 10 N·m forthe hip joints and 6.5 N·m for the knee joints. These torquelevels are approximately 25% of the average torque for humangait motion. In addition, MALO compensates for its kinetic
friction.
2) Experimental devices: A 7-meter straight walking lane
was used in this study. The ﬁrst 3 m and the last 1 m of the lane
were the acceleration and deceleration areas. The motion of thesubject in the middle area was recorded by a motion capturesystem (MAC 3D System, Motion Analysis Corporation, CA)
and video camera (SONY, Japan). In addition, mobile six-axis
force plates (M3D, Tech Gihan Co., Ltd., Japan) were attachedto each sole to measure the ground reaction force.
An obstacle, shown in Fig. 2, was used to cause collisions
with the subject. The height of the obstacle was adjustable
up to 1 m. A six-axis force sensor (DynPick, WACOH, Japan)
was attached to the contact surface. The obstacle was randomly
placed on the lateral side of the lane to strike the side of the
MALO. The subject wore goggles with the lower parts of thelenses covered to prevent anticipation and preparation.
For safety, the subject was connected to a gondola on theceiling using a safety harness to prevent him from falling.
Thus, the subject was supported by the harness when his torso
lowered to some degree. However, the harness did not disturb
the normal gait motion of the subject because of the allowance
provided. The subject also wore knee protectors and ankle
supporters.
B. Protocol
The experiment was performed with the permission of theinstitutional review board of Nagoya University. The subject
was a healthy twenty-three-year-old male whose height was
170.6 cm and whose weight was 62.0 kg. He was instructed
to walk at a constant speed and stride length along thewalking lane, to which the obstacle had been attached inadvance. Contact between the side of the physical assistantrobot and the obstacle occurred occasionally. Overview ofthese experimental devices were shown in Fig. 3.
Collisions occurred during the initial swing phase, deﬁned
as 60% to 75% of the gait cycle, or during the middle swing
phase, deﬁned as 75% to 85% of the gait cycle. The collision
timing was controlled by changing the position of the obstacle.
The obstacle collided with the upper thigh or lower part of thethigh of the MALO, depending on the height of the obstacle(approximately 55 or 80 cm). These collisions were producedon both the left and right sides of the subject to prevent asymmetric preliminary avoidance motion of the subject. Combining
these parameters, contact occurred at the hip side or kneeside of the thigh during the initial swing phase or at the hipside during the middle swing phase. These conditions wereselected on the basis of preliminary experiments because of
the seriousness of their effects on gait motion. In addition,KďƐƚĂĐůĞ

ϯͲϲŵ
Fig. 3.

Experimental overview
ɹɹ

TABLE IT IMING OF THE COLLISION AMONG GAIT CYCLERight side collision [%]Left side collision [%]Hip initial
74.9
69.9

Hip middle
84.8
77.8

Knee initial
67.9
71.9

the order of the conditions was randomized between trials. In
total, six collision trials (two trials of each condition) and six
dummy trials, where no obstacles were used, were successfullyrecorded.
C. Data processing
The positions of the markers attached to the body of thesubject, were recorded at a frequency of 100 Hz and ﬁlteredat 6 Hz using a Butterworth ﬁlter. The marker positions wereused to calculate the attitude and position of the pelvis, torso,
and footmarks. The timing of the heel contact was detected
when the ground reaction force exceeded 10 N. The timing of
the collision was deﬁned as being when the force sensor of the
obstacle detected the contact. The data from the force sensor,
which performed at 1 kHz, of the obstacle was also used to
calculate the force impulse and peak force of the collision.
III. R ESULTS
The gait timings of the dummy cases were validated ﬁrst.
The subject walked at a speed of 4.6 km/h on average, with astandard deviation of 0.4 km/h. Each stride consisted of a 0.70(± 0.05) m step length and a 1.08 (± 0.03) s stride time. These
values suggest that the subject’s gait was normal for a typicalyoung person. The collision timing of each trial is listed inTable I. The initial swing phase started at 60% and ended at75%. The middle swing phase started at 75% and ended at85%. Thus, the collision timings successfully included within
the conditions considered.
Position of footprints are shown in Figs. 4 and 5. In thesegraphs, footprint was deﬁned as the line between the positionof toe marker and heel marker when the foot contacted to
the ground totally. Numbers of the steps were placed on thetoe side. The number of footprint started from the step justbefore the collision in Fig. 4. Then, following two steps could

1090

Right contact
Left Contact
Lateral direction [m]a black dot stands for the position of the obstacle

䢴
䢳 䢵

0
-0.2
-0.4
-0.6
-0.8

-0.2

0

0.8
0.6
0.4
0.2
0
-0.2

0.2

0.8

0.4
0.2

0.2

1.2

0.6

0.8

1

0.6
0.4
0.2

1

0

-0.2

1.4

0

0
-0.2
-0.4
-0.6

1.4

-0.8

䢳 䢴 䢵

0.8

0

䢳 䢴 䢵
0.4

0.6

0.6

1.2

0

0.4

䢳 䢴 䢵

0.8

-0.2

0

0.2

0.4

0.6

0.8

1

䢳 䢴 䢵
0.2

0.4

0.6

0.8

1

1.2

1.4

-0.2

0

0.2

0.4

0.6

0.8

䢳䢴 䢵

1

1.2

1.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0
-0.2
-0.4
-0.6
1.2

1.4

-0.8

Traveling direction [m]Lateral Direction [m]Fig. 4. Footprint of collision cases (left: collision with the hip during the initial swing phase, middle: collision with the hip during the middle swing phase,right: collision with the knee during the initial swing phase)䞊1Left foot step䞊Right foot step,ĞĂĚdƌĂǀĞůŝŶŐĚŝƌĞĐƚŝŽŶ

dƌĂǀĞůŝŶŐĚŝƌĞĐƚŝŽŶ

dŽƌƐŽ

0.8

dŽƌƐŽ

,ĞĂĚ

0.6
0.4

ɽ

0.2
0

-0.2

0

0.2 0.4 0.6 0.81

1.2 1.4 1.6 1.8

2WĞůǀŝƐ

2.2

ɽ














ɽ










ɽ




WĞůǀŝƐ

Traveling direction [m]Fig. 6. Deﬁnition of rotation angles of shoulder and pelvis (left: right sidecollision, right: left side collision)
Fig. 5.

Representative footprint of normal gait motion

be shown in the ﬁgure. At the same time, the position of the
obstacle was plotted by a black dot.
A representative pattern from the dummy trials is shown in
Fig. 5. As Fig. 5 shows, the step length during normal walking
was approximately 0.7 m, which corresponded to the average
step length mentioned before.
However, footprints of collision cases differed from thatof normal cases as shown in Fig. 4. The step length just after
tripping (step no. 2) became smaller than that for a normal step.
In addition, the direction of the foot differed from that during
normal walking, especially during the second step (step no.
3): the subject opened his leg in the coronal plane. Thus, thesubject immediately lowered the swing leg when the collisionoccurred. He then spread the opposite leg. The same tendencies
were observed in every trial in response to a collision.
It is important to consider the rotation of the body inanalyzing the recovery motion of the subject. Geometry ofrotation angles are shown in Fig. 6. The direction of theshoulder was deﬁned using line that connected the markers
of left and right shoulders. Then, the angle of the torso in thehorizontal plane was deﬁned as the lines between the directionof shoulder and transverse direction. The angle of the pelvis
was deﬁned as the same manner as the torso using markersof the posterior superior iliac spine. The rotation angles ofthe pelvis and torso in the horizontal plane at the time ofcontact and during the succeeding two steps are compared inFig. 7. The rotation angle becomes zero when the body is alongwith the walking lane. To analyze the trials with collisions on
different sides at the same time, rotations toward the side atwhich obstacle was placed were assigned positive values. The
results suggest that the difference in the motions of the tripping
legs was not apparent.
According to Fig. 7, the rotation angle differed drasticallydepending on the collision timing and part. The recovery
motion differed between trials, with continuous or decelerated
rotation occurring after the ﬁrst step in hip initial collisioncases. However, both the shoulder and pelvis rotated continuously in all hip middle collision cases. In contrast, theincreases in these angles decelerated after the ﬁrst step in allof the knee initial collision cases. In all of the trials, although

1091

250

WĞůǀŝƐ͗ZŝŐŚƚĐŽŶƚĂĐƚ͗>ĞĨƚĐŽŶƚĂĐƚ

ϴϬ
ϲϬ
ϰϬ
ϮϬ

䠖䠖

Right contact
Left contact

150
100
50

Ϭ

&ŝƌƐƚ
ƐƚĞƉ

^ĞĐŽŶĚ ,ŝƚƐƚĞƉ ƚŝŵŝŶŐ

&ŝƌƐƚ
ƐƚĞƉ

^ĞĐŽŶĚ ,ŝƚƐƚĞƉ ƚŝŵŝŶŐ

&ŝƌƐƚ
ƐƚĞƉ

^ĞĐŽŶĚ
ƐƚĞƉ

Fig. 7. Shoulder and pelvis rotational angle (left: collision with the hip during
the initial swing phase, middle: collision with the hip during the middle swing
phase, right: collision with the knee during the middle swing phase)͗ZŝŐŚƚĐŽŶƚĂĐƚ͗>ĞĨƚĐŽŶƚĂĐƚ
ϴϬ
ϲϬ
ϰϬ
ϮϬ
Ϭ
ͲϮϬ

Hit
timingFirst
step

Second Hit
step timing

First
step

Second Hit
step timing

First
step

Secondstep

Fig. 8. Foot rotational angle (left: collision with the hip during the initialswing phase, middle: collision with the hip during the middle swing phase,right: collision with the knee during the middle swing phase)

there were some differences between the pelvis and shoulder
angles, the largest difference between pelvis rotation angle andshoulder rotation angle, which meant the twist of upper body,was approximately 30 deg.
The angle of the foot in the horizontal plane, shown in
Fig. 8, was the same as the angle of the footprint shown in
Fig. 4. Rotations from the traveling direction toward the side on
which the obstacle was placed were assigned positive values,
as with the body rotation angles.
Although the angle of the foot increased just after contact inall cases, it decreased during the second step in some cases. In
addition, the increase in the rotation angle decelerated even in
the other cases. These trends depended on the condition of thecontact timing and the contact part. The former pattern was
mainly observed in the knee initial collision cases, whereas
the latter pattern was mainly observed in the hip middle cases.
Both patterns were observed in the hip initial cases. These
characteristics corresponded to the rotation of the body shownin Fig. 7.
The peak force acting on the obstacle in the traveling
direction and the impulse are shown in Fig. 9. These graphs
suggest that the peak force in the knee initial cases wereapproximately double those in the hip cases. However, themagnitudes of the impulses did not differ drastically.
IV. D ISCUSSION
As is the common tendency, the subject rotated his wholebody toward the side on which the obstacle was placed during

ʹͲ
ͳ͸
ͳʹ
ͺ
Ͷ
Ͳ

0

ͲϮϬ

,ŝƚ
ƚŝŵŝŶŐ

dǁŝƐƚĂŶŐůĞ΀ĚĞŐ΁

200

䡡

䠖

Contact impulse [N s]dǁŝƐƚĂŶŐůĞ΀ĚĞŐ΁^ŚŽƵůĚĞƌ

Contact force [N]䠖

Hip_initial

Hip_mid Knee_initial̴

̴ ̴

Fig. 9. Force and impulse at contact (left: peak contact force, right: impulseof contact force)the recovery motion. However, the recovery motions of thetrials could be categorized as following one of two strategies, deep rotation or short rotation. Sometimes, the subject
continued to rotate his body and foot even during the second
step. This deep rotation could be observed in the hip middle
collision case and some of the hip initial cases. In contrast, ashort rotation accompanied a rapid decrease in foot rotation.
This motion was observed in the knee initial contact cases andsome of the hip initial cases.
The most signiﬁcant difference between these two strategies
appeared during the second step. In the case of short rotation,
the foot angles decreased at the second step and the bodyrotation slowed down, which suggests that the subject tried
to recover the direction of his body. In contrast, although thesame tendency (the foot angle becoming smaller than the bodyangle) was observed during the second step in the deep rotation
cases, the foot angle did not decrease. However, even in the
deep rotation cases, deceleration of the foot rotation could bea sign of deceleration or stopping of body rotation. For both
strategies, even during rotation of the body, the difference
between the pelvis rotation angle and the shoulder rotation
angle remained approximately 30 deg, as mentioned previously. This limitation of the torso twist angle was probably
due to how MALO is attached to the torso. Because MALO is
attached to the torso of the subject using a corset for effective
torque translation, it limits twisting of the torso to some degree.
Thus, the rigidity of the corset probably affected the shoulder
rotation.
Although the number of trials was not sufﬁcient to evaluate
the recovery strategy statistically, the results suggest thatthe selection or development of the recovery motion clearly
depended on the timing and body part of the contact. In the hip
middle collision case, the deep rotation recovery was observed,
whereas the short rotation recovery was observed in the knee
initial collision case. In addition, both patterns appeared in the
hip initial collision case. From the perspective of kinetics, thecontact position affected the distribution of the reaction force
and moment. When the obstacle struck the lateral part of therobot, the swing leg was pushed back and the body rotated in
the horizontal plane. The rotation moment might have becomelarger when the hip side collided with the obstacle because thehip side is closer to the pelvis, so the contact force affected the
body directly. On the other hand, a contact force at the kneeside was probably transformed into an extension moment of1092

the hip joint in the sagittal plane. Thus, it was probably more
difﬁcult to stop the rotation of the body in the horizontal planein the hip contact case. The timing of the contact also affected
the ability of the subject to stop his body from rotating. During
the initial phase of the swing motion, the center of mass of thesubject remained close to the stance leg. However, it gradually
moved forward and increased in speed when the swing motion
entered the middle phase. Thus, the contact in the initial phasemight be easily compensated for by strengthening the muscle
force of the stance leg. As a result, a collision at the hip sidein the middle swing phase might be the most severe condition
from which to recover, among those considered in this study. In
contrast, it is probably relatively easy to overcome a collisionat the knee side in the initial swing phase. The hip initial caselies between these two other cases.
Because of the marginal numbers of trials, there was no time
for the subject to get accustomed to the disturbance during thisexperiment. However, increased number of trials are required
to obtain general trend. Thus, randomization of the disturbed
conditions becomes more important to prevent the learningof the subject. However, even using such countermeasures, it
probably will be not easy to increase the number of trials
for each subject. This limitation becomes a problem wheninspecting the effect of various assist algorithm to the recovery
motion. Limited number of trials increases the importanceof the simulation. The fundamental strategy of the recovery
motion obtained by the experiment will help to assume therecovery strategy of the wearer.
V. C ONCLUSIONSACKNOWLEDGMENT
This work was supported by JSPS KAKENHI Grant Number 26750121.
R EFERENCES[1] K. Yasuhara, K. Shimada, T. Koyama, T. Ido, K. Kikuchi, and Y. Endo,“Walking assist devices with stride management system,” Honda R&D
Technical Review, vol. 21, no. 2, pp. 54–62, 2009.
[2] K. Suzuki, G. Mito, H. Kawamoto, Y. Hasegawa, and Y. Sankai,“Intention-based walking support for paraplegia patients with robot suithal,” Advanced Robotics, vol. 21, pp. 1441–1469, 2007.
[3] H. Kobayashi, T. Aida, and T. Hashimoto, “Muscle suit development andfactory application,” International Journal of Automation Technology,
vol. 3, no. 6, pp. 709–715, 2009.
[4] J. S. Brach, S. Perera, S. Studenski, M. Katz, C. Hall, and J. Verghese,“Meaningful change in measures of gait variability in older adults,” Gait
& Posture, vol. 31, pp. 175–179, 2010.
[5] Y. Akiyama, I. Higo, Y. Yamada, and S. Okamoto, “An analysis ofrecovering motion of human to prevent fall in responce to abnormality
with a physical assistant robot,” in IEEE International Conference on
Robotics and Biomimetics, 2014, pp. 1493–1498.
[6] ISO, “Robots and robotic devices – safety requirements for personal carerobots,” International Organization for Standardization, Tech. Rep. ISO
13482:2014, 2014.
[7] T.-Y. Wang, T. Bhatt, F. Yang, and Y.-C. Pai, “Adaptive control reduces
trip-induced forward gait instability among young adults,” Journal of
Biomechanics, vol. 45, pp. 1169–1175, 2012.
[8] M. Pijnappels, M. F. Bobbert, and J. H. van Dieën, “Changes in walking
pattern caused by the possibility of a tripping reaction,” Gait and Posture,
vol. 14, pp. 11–18, 2001.
[9] J. J. Eng, D. A. Winter, and A. E. Patla, “Strategies for recovery from atrip in early and late swing during human walking,” Experimental Brain
Research, vol. 102, no. 2, pp. 339–349, 1994.
[10] G. Taga, Y. Yamaguehi, and H. Shimizu, “Self-organized control of
bipedal locomotion by neural oscillators in unpredictable environment,”
Biological Cybernetics, no. 65, pp. 147–159, 1991.
[11] L. Wang, S. Wang, E. H. F. van Asseldonk, and H. van der Kooij,“Actively controlled lateral gait assistance in a lower limb exoskeleton,”in IEEE/RSJ International Conference on Intelligent Robots and Systems,
2013, pp. 965–970.

The goal of this study was to detect the strategy used bya human wearing a physical assistant robot to recover from acollision with an obstacle. In the experiments conducted, theobstacle struck the side of the robot, an occurrence that was
assumed to be similar to the wearer failing to pass through adoorway.
In all cases, the subject rotated his body toward the side
on which the obstacle was placed after contact with the
obstacle. However, the subject’s motions in the trials differedconsiderably and could be separated into two patterns: deep
rotation and short rotation. Deep rotation occurred when the
obstacle hit the upper thigh part of the wearer in the middle
swing phase. In such cases, the rotation of the body did notstop even at the second step after the collision occurred. In
contrast, short rotation occurred when the obstacle hit the lower
part of the thigh of the wearer in the initial swing phase.
The subject successfully stopped the rotation of his body anddecreased the rotation angle. In addition, an analysis from akinetic perspective suggests that the moment in the horizontal
plane differed depending on the part of the robot struck
and the motion phase in which the collision occurred. The
experimental results are consistent with the kinetic analysis
and suggest that the factors that determine the recovery motion
taken were successfully detected.

1093