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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.

Test Method for Contact Safety Assessment of a Wearable Robot-Analysis of Load Caused by a Misalignment of the Knee JointYasuhiro Akiyama1 , Yoji Yamada1 , Koji Ito2 , Shiro Oda3 , Shogo Okamoto1 and Susumu Hara1
a wearable robot remains a necessity for a proper safetyassessment.
In addition, a study on a mechanism mimicking thecomplex motion of a human joint was also conducted [4].
Using the proposed mechanism, the study aimed to evaluate
the safety of a wearable robot. However, the researchers have
yet to measure the force applied to an actual human user.
Currently, development groups and robotics companies arein charge of safety on the user side, and a test method foran assessment of contact safety has yet to be established.
Therefore, in this study, a risk assessment of a wearable robot
was conducted. A lower–leg dummy was also developed forthe safety tests [5], [6].

Abstract— Wearable robots have finally reached a point at
which they can be put to more practical use. However, methods
for assessing their safety have yet to be properly established.
Furthermore, for ethical reasons, such assessments must be
conducted without exposing the human user to unnecessary
danger. Therefore, in this study, a lower–leg dummy for thesafety assessment of a wearable robot was developed to evaluate
the effects of a misaligned knee joint.

I. INTRODUCTION
A. Background
The importance of development of living assist robots
has increased, as improvements in the quality of life andproductivity of aging societies are becoming more vital.
Wearable power–assist robots (hereafter, ”wearable robots”)
are a kind of living assist robots that are fixed to the humanuser through a device such as a cuff, and provide power
assistance to the wearer.
However, compared to other types of human–assistingmachines, the degree of safety of wearable robots should
be of greater consideration, as they make direct contact withthe user. Therefore, before wearable robots can be used by
the general public, a proper safety assessment is needed.
ISO 13482 will become an industrial standard for service
robots. However, a proper concrete testing method and aquantitative target have yet to be adequately determined. For
ethical reasons, a real human user should not be used as atest bed. Therefore, the development of a test method that
does not use a real human user is required.
Some robot designs do consider contact safety. A redundant degree of freedom, back–drivability, and the useof proven safety braces are a few of the safety measuresused in such cases [1], [2]. However, studies in these areas
have mainly focused on the development of a particularmechanism or the robot itself. Therefore, safety analyses
such as risk assessments and a verification of safety functions
remain inadequate from the viewpoint of safety engineering.
Furthermore, while a study has been conducted on therelationship between the force applied to the skin and theresulting feeling of discomfort [3], with an emphasis placedon a safety evaluation, a prediction of the force applied by

B. Misalignment between the knee joints of a human user
and a wearable robot
First, hazard identification for contact safety was conducted through a risk assessment. In this study, rehabilitation
is assumed to be the main purpose of the wearable robot.
Therefore, a fall is excluded from this study, as a user
undergoing rehabilitation should be properly supported by abar or physical therapist. Examples of the identified hazards
are listed in table I. The table suggests that the user of awearable robot is at a risk of injury to the skin, muscle, andbones.
The present study focuses on a misalignment of the centerof rotation of the knee joints between a user and a wearable
robot. Although other kinds of hazards, including thoserelated to mechanical and electrical issues, are also importantto the safety of a wearable robot, they can be consideredbased on previous safety assessment methods. In addition,international standards for these types of hazards have beenpreviously established [7].
Fig. 1 shows the difference between the rotation mechanism in the knee joint of a human and a wearable robot.
A human knee joint consists of a femur and a tibia. The
condyle of the femur is shaped like an ellipse, while theupper side of the tibia is shaped like a plane. Therefore, thefemur slides on the tibia when the knee is bent. As a result of
this composite motion, the center of rotation moves with thebending angle [5], [8], [9], [10], [11], [12]. In contrast, theknee joint of a general wearable robot consists of a pin joint.
Therefore, the centers of rotation of the knee joints of boththe human and the wearable robot become misaligned withthe bending of the knee, even when their initial positions arecarefully aligned.
Currently, many wearable robots aimed at practical use
have a pin joint at the knee. Therefore, an evaluation of

1 Department of Mechanical Science & Engineering, Nagoya University
Furocho, Nagoya, Aichi 464-8603, Japan akiyama-yasuhiro,

yamada-yoji, okamoto-shogo, haras atmech.nagoya-u.ac.jp
2 Toyota Industries Corporation Toyoda-cho 2-1, Kariya, Aichi 448-8671
Japan koji.ito.ac at mail.toyota-shokki.co.jp
3 Toyota Motor Corporation Toyota-Cho 1, Toyota, Aichi 471-8571, Japanoda.shiro at d.mbox.nagoya-u.ac.jp
978-1-4673-4605-4/12/$31.00 ©2012 IEEE

539

TABLE IE RGONOMIC HAZARD OF WEARABLE ROBOT

Ergonomic hazards

Fig. 1.

Hazard source
Misalignment at center of rotation of joint
Decrease in DOF of joint
Sudden robot motion
Constriction of fitting parts

Potential consequence result
Overload at human joint
Overload at human joint
Muscle damage
Internal bleeding, Abrasion of skin

The maximum burden should be determined from a medical and biomechanical viewpoint. Even though the applied
force is not harmful to a younger and healthier user, theelderly or people with certain diseases may run the risk of
injury. Therefore, the upper limit of applied force must be
carefully determined. On the other hand, the dummy shouldbe able to mimic the human motion of the joints and skin.
In this paper, only the knee joint is discussed. However, thedummies of other joints are considered now.

Difference of knee rotation mechanism

II. MEASUREMENT OF LOWER LEG MOTIONWITH BENDING OF THE KNEE

the risk posed by a misalignment of the center of the knee
is quite important. In addition, a misalignment will almost
certainly occur when the user wears a wearable robot, as thecenter of rotation of the knee is difficult to be seen externally.
These kinds of misalignments also create a problem for theuser. A misalignment of the knee joint will displace theposition between the user and the wearable robot, causing
an increase in the force applied to the user by the wearable
robot (cuff force).
A second kind of motion, rotation, also occurs in the lower
leg of a human user. A rotation motion occurs when the lower
leg is almost fully extended, and the amount of such rotation
can be up to several degrees. However, many wearable robots
do not consider this factor. Therefore, a rotation motion may
also cause a displacement of the cuff position and the cuff
force.
The most important difference between the change of thecenter of rotation and the rotation motion is the effect of themisalignment. A misalignment at the sagittal plane probably
causes a change in the relative position between the user andthe wearable robot. This means that the displacement andforce are not necessarily proportional to the misalignment.
Therefore, in this study, the effect of lower–leg movement
along the sagittal plane is analyzed prior to the rotationalmotion.

Many studies have suggested that the center of rotation in
the human knee joint moves with a bending of the knee [5],[8], [9], [10], [11], [12]. However, these studies have focused
only on the knee joint itself. In contrast, when operatinga wearable robot, the user receives force from the robot
through the cuff. Therefore, the motion of the user’s lower
leg at the cuff location needs to be analyzed to estimate thisforce. One effective way to measure the displacement at the
cuff is to conduct a physical experiment.
Fig. 2 shows an overview of such an experiment, whichaims to measure the displacement between a user and awearable robot at the cuff location. In this experiment, theuser’s thigh was fixed to a pole, which was fixed to theground with the thigh of the wearable robot at a natural
standing position. Only the lower thigh of the user wascontinuously bent.
In this sequence, the length between the center of rotationof the wearable robot and the user  s cuff location at the sagittal plane was measured. Motion capture equipment (Motion
Analysis, MAC 3D) was used to measure the position andlength during the experiment. The angle of the user  s knee
is defined as the angle between the vertical line and the line
connecting the marker at the center of the knee to the marker
at the user  s cuff location. The average measurement error
caused by the motion capture system itself is 0.07 mm.
Fig. 3 shows the results of ten flexion extensions. This
figure suggests that the length between the center of rotationof the knee joint and the cuff location of the user’s lower
thigh increases with the bending angle. The tissue between
the surface of the skin and the bone generally moves with abending of the knee. Therefore, the results in Fig. 3 include
the effect of the movement of the human skin and tissue,in addition to the effect of the movement of the center ofrotation. However, an actual wearable robot is not attachedto the human bone directly. Therefore, these experimental

C. Suggested contact safety test of a wearable robot
The final aim of this study is the suggestion of a contact
safety test of a wearable robot. In our test, a wearable robot
is operated using a lower–leg dummy, and the forces applied
to the skin and bones of the dummy are measured andevaluated. The measurement should be conducted under two
types of conditions. The first type is the actual condition,such as walking, standing up, or sitting down, while thesecond type is the elemental condition, such as the flexion or
extension of the joints. The force applied to on the dummy
should be sufficiently small under such conditions.
540

Fig. 4. Geometry of lower leg model to simulate the misalignment ofrotation axis
Fig. 2. Flexion-extension of a human knee with the upper link of a robot
fixed by a cuff

Fig. 5.

Fig. 3.
cuff

Geometry of lower leg model in sagittal plane

the sagittal plane causes a displacement of the cuff positionin the longitudinal direction of the lower thigh with a bendingof the knee.
Equations (1)–(5) calculate the displacement between the
cuff positions of the wearable robot and the user at thesagittal plane of the wearable robot. A bending of the knee
shortens the length between the center of the knee joint andthe cuff location. Therefore, a misalignment of the rotation
axis drags the lower thigh of the user toward the directionof the knee region. As a result, the displacement of the cuff
position increases with a misalignment of the rotation axis.

Extension of distance between the human knee joint and the low

conditions are considered to be sufficiently similar to theactual conditions.
III. ANALYSIS OF EFFECT OF AN INITIALMISALIGNMENT
It is important to attach a wearable robot correctly, since
an incorrect attachment will endanger the wearer. However,
it is difficult to eliminate the misalignment at the knee joint
completely, as it is quite difficult to identify the center ofrotation of the joint externally. Therefore, a certain degreeof misalignment should be considered when attaching awearable robot. To predict the load placed on the user, anestimation of the displacement at the cuff location caused bya misalignment of the knee joint is useful. To estimate sucha displacement, the relationship between the misalignmentof the center of rotation and the displacement of the cuff
should be modeled.
In this study, a wearable robot fixed to the user throughthe cuff at the thigh and lower thigh is considered (Fig. 1).
In addition, the lower leg of the user is considered as fixedparallel to the wearable robot except when the axis of therevolution of the knee is misaligned. Three directions of
misalignment, up–down, forward–backward, and inner–outerrotations, are considered in this model. At the cuff locationof the lower thigh , the displacement in the longitudinal
direction is of particular focus.
First, the rotation of the axis of the knee revolution ismodeled. Fig. 4 shows the geometry of the lower legs of
the wearable robot and the user. β denotes the rotation
angle between the axis of revolution of the knee and thewearable robot. A misalignment of the axis of rotation at

Lcos γcos α
Lcos γsin α==

Lcos θ
Lsin θcos β

l(α)

=

Lcos γ

=

cos β
p
2
cos β cos α2 + sin α2

(1)
(2)

(3)

Next, misalignments of the up–down and forward–
backward directions along the sagittal plane are considered.
Fig. 5 shows the geometry of the wearable robot and thelower leg of the user at the sagittal plane. As the initial
misalignment, two parameters, dx and dy, are used, since theeffect of a misalignment differs according to the direction. A
displacement of the cuff position is calculated using (5). As
a result of this calculation, a misalignment in the backward
or upward direction increases the displacement toward thecentrifugal (plus) direction, while a forward or downward
misalignment has the opposite effect.
DE = AEcos α= (dytan α − dx)cos α= dysin α − dxcos α
541

(4)

Fig. 7.

Fig. 6. Displacement at the cuff position related to the combination of
misalignments

D(α)

=
=

l(α) − (l(0) − dx) + DE
cos β
− 1)
L( p
2
cos β cos α2 + sin α2+dysin α + dx(1 − cos α)

Overview of the lower leg dummy(5)

Finally, a combination of misalignments in different directions is considered. Equation (3) also indicates how misalignments of the up–down, forward–backward, and inner–outer rotations independently affect the displacement of thecuff position at the sagittal plane . Therefore, in the caseof a combinatorial misalignment, the total displacement at
the cuff location can be calculated as the linear sum of thedisplacement of each misalignment. The displacements ofvarious parameters are shown in Fig. 6. Case 1 indicates a
condition in which dx = −10 mm, dy = 15 mm, and β =0 deg; case 2 corresponds to dx = −10 mm, dy = 15 mm,
and β = 10 deg; and finally, case 3 is for dx = −5 mm, dy =15 mm, and β = 0 deg. The figure shows that the tendency of
a force applied to the cuff (cuff force), such as the maximum
force and its angle, changes based on these parameters.
In this section, a displacement between a wearable robot
and its user caused by an initial misalignment of their centersof rotation is analyzed using a simple model. The resultsof this simulation suggest that an independent displacement
is caused by each misalignment. In addition, the trend of
displacement caused by each misalignment is calculated.

Fig. 8.

Overview the the experiment condition

and its diameter was designed using the database in [13]to correspond to the average adult human. In addition, an
artificial skin was applied to the surface of the dummy.
A wearable robot was then attached to the dummy, as
shown in Fig. 8. A force sensor was attached to the lower–
thigh cuff of the wearable robot, and the sensor measured
the force applied to the dummy by the wearable robot toevaluate the burden placed on the skin. In further research,
the force applied to the high cuff will be measured to evaluate
the burden placed on the knee joint. The compliance of the
fixed area in the longitudinal direction was 0.2 mm/N. This
compliance was measured using a tension and compression
testing machine. This parameter does not completely matchthat of a human user owing to the underdevelopment of theartificial skin. However, the same tendency was measured
experimentally using a human lower leg at the prospective
degree of displacement and cuff force.

IV. ANALYSIS OF FORCE OF A DUMMY USER  S
LOWER THIGH CUFF

A. Effects of the initial misalignment at the center of theknee

An experiment using a dummy was conducted to evaluate
the cuff force. An overview of the dummy used is shown in
Fig. 7. This dummy consists of a thigh, lower thigh , and aknee joint. Although the knee of the dummy has a pin joint,
the lower thigh has an extension mechanism. Therefore, thetrajectory of the motion of the cuff position of the humanuser can be mimicked using this mechanism.
The extension rate is based on the experimental data shownin Fig. 3. In addition, to realize the different parameters, therelative position between the dummy and the wearable robot
varied through the use of adjustable parts. Furthermore, an
encoder was attached at the knee joint to measure its angle
and to control the extension mechanism of the lower thigh.
The surface of the dummy has a simple cylindrical shape,

In a bending experiment, a wearable robot was attachedto the dummy. The parameters of the misalignment were
initially set as follows: β = 0, l = 150 mm, dx = 0, anddy = 15 mm. The parameters of the initial misalignment
were then changed to evaluate their effects. In these experiments, the extension mechanism of the lower thigh was
stopped to exclude the effect of the movement of the centerof rotation.
Fig. 9– 12 compare the results of the force measurements
and calculations of the displacement at the lower thigh cuff.
These figures show that the force of the lower thigh cuff andthe displacement of the cuff position have similar tendencies,
which suggests that a displacement caused by an initial542

Fig. 9.

The lower thigh cuff force (dy = 15 mm)

Fig. 10.

The lower thigh cuff force (dy = −15 mm)

Fig. 11.

Fig. 12.

misalignment of the knee applies force to the human user.
However, the effect of a twisting motion, as shown in Fig. 12,
causes a particular problem. In this case, the increase in theforce is relatively larger than that in the displacement, as
compared to the case shown in Fig. 9, in which a stirring
motion is observed at the cuff; this effect is shown in Fig. 12
as a shift in the force at 0, deg. These results suggest theexistence of a more complex mechanism in the case of atwisting motion.
The cuff force can be simulated through a simple multiplication of the compliance and displacement at the cuff
position. However, the magnitude of the measured force does
not correspond to the force calculated using the complianceor displacement. The calculated forces are several times
higher than the measured values. One assumed reason forthis is the twisting movement of the fixed area of the thigh.
In the calculation, the human thigh is fixed parallel to thewearable robot. However, the fixed area of the thigh rotatedduring the experiment. Therefore, the effect of the lower–
thigh displacement was possibly absorbed into the rotation
motion of the thigh.

The lower thigh cuff force (dx = 10 mm, dy = 15 mm)

The lower thigh cuff force (dy = 15 mm, and twist)

experimental conditions were set to correspond to the non–extension cases mentioned below. Fig. 13 shows the results ofa bending experiment using a dummy attached to the lower–
thigh extension. A comparison of Figs. 9 and 13 shows theeffect of the lower–leg extension.
These figures indicate that a lower–thigh extension increases the force of the lower–thigh cuff by approximately
20mathchar‘ − 50 deg. The lower–thigh extension pushes
the dummy’s lower thigh toward the foot, thereby applying
force to the cuff. However, the force is reduced by thetwisting movement of the thigh, which is the same as thatin the non–extension cases.
Furthermore, the same tendency can be seen in the othercases shown in Figs. 14– 16. An increase in the force
of around 20mathchar‘ − 50 deg commonly occurred. In

B. Effect of an ergonomic misalignment at the center of theknee joint
A misalignment of the center of the knee caused by amovement of the center of rotation is mimicked by thelower–leg dummy through the use of a lower–thigh extension
mechanism. In this section, the effect of the misalignmentof the center of rotation is evaluated using the dummy.
To determine the effect of an ergonomic misalignment, theFig. 13.
15 mm)

543

The lower thigh cuff force with lower thigh extension (dy =a mathematical model. Finally, a lower–leg dummy that can
mimic the displacement at the cuff attached to the user’s
lower leg was introduced.
An experiment using the lower–leg dummy was conducted
to evaluate the effect of a misalignment. As a result of theexperiment, the force of the lower–thigh cuff showed thesame tendency as in the simulation. Although the magnitude
did not correspond to the simulation results, the results
suggest that a twisting movement at the thigh cuff reduces
the amount of force. Therefore, the motion and force at thethigh cuff should be considered. As a result, the effect of amisalignment of the initial setting was measured to a certain
degree, which suggests the importance of the movement of
the center of rotation when evaluating the force of the lower–
thigh cuff.

Fig. 14. The lower thigh cuff force with lower thigh extension (dy =−15 mm)ACKNOWLEDGMENT
This study was conducted as a part of the ”Practical
Applications of Service Robot Project,” which is directedby the New Energy and Industrial Technology Development
Organization (NEDO). The authors appreciate their discussion with T. Ohnishi and the members of the consortium (K.
Kawakami, K. Isogai, K. Tominaga, Y. Hirano, M. Ishihara,
S. Ito, and K. Homma).
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addition, in some of the cases, the range of force increased
relative to a non–extension case. This result suggests that the
movement of the center of rotation causes an increase in theforce of the lower–thigh cuff under certain conditions.
V. CONCLUSIONSThis study focused on the risks posed by the use of awearable robot, such as the initial misalignment of the centerof the knee and the movement of the center of rotation
caused by the ergonomic mechanism. First, the displacement
between the user and the wearable robot at the cuff position
caused by the ergonomic mechanism was measured using a
motion capture system. Second, the displacement at the cuff
caused by the misalignment of the knee was analyzed using

Fig. 16. The lower thigh cuff force with lower thigh extension (dy =15 mm, and twist)544