2020 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS)
October 25-29, 2020, Las Vegas, NV, USA (Virtual)

Designing A Dummy Skin by Evaluating Contacts between A Human Hand
and A Robot End Tip
Yumena Iki∗ , Yoji Yamada∗ , Yasuhiro Akiyama∗ , Shogo Okamoto∗ , and Jian Liu∗
Abstract— Many manufacturing industries have a high demand for the construction of collaborative operation systems
using industrial robots. Although there is a preexisting set
of safety verification data in ISO/TS 15066 for collaborative
operations, there is no established testing method for safetyvalidation. To establish a testing method, it is effective to
use a dummy that has mechanical properties similar to thoseof a human. However, there is no parametric study thatexists for designing a dummy that represents the static anddynamic mechanical properties and the nonlinearity of thestatic mechanical properties. In this study, static and dynamic
experiments were conducted to obtain the mechanical stiffnessof human subjects and the contact force transitions during
the dynamic contact between a robot system and a human.
Subsequently, the same experiment was conducted using the
proposed dummy. The biofidelity of the dummy was examinedby comparing the parameters of a viscoelastic model. This study
contributes to increasing the safety of collaborative operations
by developing a dummy that can be used for risk assessments
in collaborative operations.

I. I NTRODUCTION
Recently, there is a high demand for industrial robots thatcan be used to improve the productivity of collaborative
operations. In these situations, robot systems and humans
share the same workspace, resulting in various technicalquestions, including whether contact between them is acceptable. To address this problem, in the field of robotics,
Suita and Yamada et al. discussed a method to achievea safe human-robot coexistence by proposing that humansomatic pain must be the criteria for acceptability [1], [2],[3]. In relation with pain, Saito and Ikeda proposed the force
and deformation tolerance limits of the sense of pain forthe safety design of human-robot collaborative operations
from static measurement [4]. Povse et al. determined thecorrelation between the intensity of subject pain perceived
during human-robot collision and the impact energy density,
which is a function of the values of impact force, contact
surface area, and displacement [5]. These studies were basedon pain tolerance. Today, ISO/TS 15066 is an internationallyaccepted guideline for collaborative operation. It clarifies
appropriate procedures for the limit of speed values thatmaintain force and pressure values under the pain sensitivity
threshold. The biomechanical limit force and pressure values
for the pain onset levels for the transient and quasi-static
events were proposed in the worst-case scenario. These worst
cases indicate when a robot system may clamp or pin ahuman body [6]. However, as mentioned by Pilz, ISO/TS* All
authors are with Department of Mechanical Systems
Engineering,
NagoyaUniversity,
Nagoya,464-8603,
Japan

iki.yumena@g.mbox.nagoya-u.ac.jp
978-1-7281-6211-9/20/$31.00 ©2020 IEEE

15066 provides insufficient guidance on how the test must
be setup and the procedure to follow [7]. Therefore, it isnecessary to establish a testing method for safety validation.
To establish a testing method, it is effective to use adummy because there are various types of industrial robots
with differing types of contact, pressure and force that mustbe tracked on the human side in a human-robot systeminteraction. A dummy can be attributed with mechanicalproperties similar to those of a human and be equipped withsensors. While cases where human subjects are impacted
must be carefully considered from an ethical viewpoint, thedummy may be provided with an impact that corresponds to
its biomechanical limits to simulate this interaction.
There have been numerous studies that investigate humanmechanical properties in a dummy. The Code of Federal
Regulation provides the force-deflection curve when designing a dummy for highway traffic safety, which can also
be used for such a collaborative operation between robot
systems and humans that is necessary to consider a lower
level of impact, as mentioned by Haddadin et al. [8], [9].
Panasonic Corporation developed a full arm dummy for ahuman-inspired pain-sensing system by focusing on the painfree interaction between personal care robots and humans.
They verified the biofidelity of their human arm dummy
through the force and displacement curve [10]. These studies
express the human static mechanical properties of the humanskin-like force-displacement curve while also considering thenonlinearity of the static mechanical properties. However,they do not focus on the dynamic aspect of the mechanicalproperties of materials, although it is crucial to consider
dynamic parameters. The dynamic parameters are an indispensable consideration from the perspective of injury.
Desmoulin and Anderson determined contusion tolerance
according to absorbed energy [11]. Fujikawa and Sugiura
et al. proposed that the peak mean contact pressure and thetotal transferred energy in mechanics total energy supplied
to the skin of substitute animals should be considered as
the dominant parameters of bruise [12], [13]. All theseparameters are determined by the dynamic parameters. In
collaborative operations, contact between the robot systemand humans is commonly dynamic, and the peak values of
the transient contact force or pressure, according to ISO/TS
15066, depends on the dynamic mechanical properties suchas viscosity. GTE Industrieelektronik GmbH constructs ameasuring system considering not only spring constants
according to ISO/TS 15066 but also damping materials using
Deutsche Gesetzliche Unfallversicherung information [14].
Povse et al. developed a human lower arm dummy that can

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accurately replace humans from the viewpoint of impact
force and impact point speed [15]. However, their system
is short of quantitative parametric information.
Many studies model the static and dynamic mechanicalproperties by considering the viscoelasticity. Dinnar was thefirst to propose a viscoelastic model for human skin tissue[16]. He used two springs and two dashpots to represent
the epidermis, dermis, and subcutaneous tissue. There aremany viscoelastic models that represent human mechanicalproperties in combination with springs, dashpots, and otherelements for human skin [17], [18], [19] and human organs[20], [21]. However, their objective is usually for medical
applications, rather than modeling human mechanical properties of a dummy for collaborative operation.
Another method that considers the mechanical propertiesof humans is the finite element method in [22], [23]. However, it is difficult to apply this method to the developmentof a dummy, since a simple analytical model is required toconfirm the validity of the selected parameters.
No parametric study exists to design a dummy representing the static mechanical properties of humans and also
the dynamic mechanical properties and nonlinearity of thestatic mechanical properties from a holistic viewpoint. This
study develops such a dummy, which can be used for risk
assessment when introducing collaborative operations in the
manufacturing industry, thereby contributing to increasing
safe collaborative operations. First, we constructed an experimental apparatus to evaluate the static and dynamic contact
between a robot end-effector and a human in a clamping
situation, which is generally considered to be the worst-caseto measure the stiffness of human subjects and the transitionof the contact force during the impact. Subsequently, thesame experiment is conducted using the developed dummy.
The biofidelity of the dummy is examined by comparing theparameters of a series model each comprising a 3-element
Maxwell model. The parameters are optimized by a genetic
algorithm using MATLAB (with version 2019a) Simulink
Model.

Fig. 1: Experimental setup

Fig. 2: Movable standThe second was the force control system, which comprised
two force sensors; one mounted at the end-effector of therobot arm, and the other attached to the impactor. The
robot arm was programmed to stop the movement when themeasured value from either one of the force sensors wasobserved erroneous or when the force applied to the subject
was higher than the pre-set value. The value was set to 100
N in this experiment.
The contact surface of the impactor was 14 mm2 . A
chamfer plane of 2 mm was formed on each side of thecontact surface. The contact surface was chosen to be thesame as that in ISO/TS 15066 for safety validation.
B. Procedure

II. M ETHODS /E XPERIMENT
A. Setup
Fig. 1 illustrates the experimental setup assuming theclamping situations considered to be the worst-case scenario
for contact between a robot system and a human during
collaborative operation. The experiment was conducted withthe approval of the Ethics Committee of the Department ofEngineering, Nagoya University.
Two safety devices were established to safely perform thisexperiment.
The first was a movable stand with a release mechanism
fixing the hand of a subject, as depicted in Fig. 2. The standwas attached to a magnet with a slider. If a subject felt any
discomfort, or if the force applied to him/her was higher
than the attractive force of the magnet, they could avoid
contact immediately by activating the release mechanism.
The attractive force of the magnet was set to approximately
100 N.

The procedure was divided into two parts.
The first part involved a static experiment. The robot arm
was moved from its original position, which was approximately 100 mm away from the contact part in the horizontal
direction, as depicted in Fig. 1. It was slowly shifted towards
the contact part. Three positions were determined by thestatic contact force, as illustrated in the next section.
The second part consisted of a dynamic experiment. The
robot arm was reverted to its original position after the static
experiment. It was subsequently moved to a position that wasdetermined in the static experiment to ensure the occurrenceof impact at a constant contact speed. Two modes of contact
speeds were commanded, as detailed in the subsequent
section.
In both experiments, the contact force was measured by
the force sensors and the position and speed of the robot arm
were measured by the robot system.

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C. Conditions
The positions determined in the static experiment assumed
the static contact force to be 5, 10, and 15 N. The maximum
static contact force was set to 15 N because subjects would
not experience pain below the range for static contact force.
However, if the subjects noted discomfort, the experiment
was stopped immediately. The contact speeds for the dynamic experiment were set at 10 and 250 mm/s, which
represent quasi-static and dynamic contacts, respectively. The
dynamic experiment was conducted thrice for each subject
and once for a dummy under each condition.

is considered that this phenomenon is caused by the different
characteristics of human multilayered soft tissue. In thisstudy, a series model of the simplest 3-element Maxwell
model was applied to represent human multilayered soft tissue. Each 3-element Maxwell model was considered as one
layer that can be expressed by one material. Fig. 5 depicts
the applied series model of 3-element Maxwell model. The
model determines that the stiffness is proportional to thesquare of its displacement expressed as αi xαi 2 and βi xβi 2
and the viscosity is constant expressed as ci experimentally(i = 1, 2; number of layers). α1 was set to be greater than
α2 for the static parameters.

D. Targets
Five male subjects participated in this study. The targets
were three parts (forefinger pad, palm, and thenar eminence)of the human body model, as shown in Fig. 3. These contact
points of the human body model were determined using
ISO/TS 15066. When considering the frequency of contact, it
is generally observed that contact is more likely to be madeat the back of human hands. It is hazardous, however, toconduct contact experiments at the back of the hands, andwe targeted the palm side of the hand as the initial focus of
this study.

Fig. 5: Series model of a 3-element Maxwell model

F. Methods

Fig. 3: Contact parts

The dummy developed in this study was composed ofurethane fabricated by the Tanac Corporation, as depicted
in Fig. 4. It simulates the palm of the subject. It consists of
two layers with a thickness of 5, and 10 mm.

The genetic algorithm technique with MATLAB (withversion 2019a) Simulink Model was used to optimize theparameters of the applied model for the dynamic experimentbased on the method described in [24]. It searches foran optimal solution that minimizes the error between thetransient contact force from the experiment and the output
from the Simulink Model for the speed input of the robot
arm from the experiment.
III. R ESULTS
A. Static response
Fig. 6 depicts the stiffness of the palm of five subjects(black lines) and the dummy skin (red line). The displacement corresponding to 10 and 15 N static contact force falls
in the individual difference ranging from 8.8-11.9 mm and10.2-13.1 mm, respectively. This is independent of the factthat the displacement corresponding to 5 N static contact
force is 0.4 mm outside the individual difference ranging
from 6.6-10.0 mm, meaning that it is a little stiffer inthe small displacement range. However, the stiffness of thedummy skin generally exhibits properties similar to those of
the subjects.

Fig. 4: Dummy skin

E. ModelMaxwell and Voigt models are the representative ones
for the viscoelastic behaviors of the materials. In general,
all viscoelastic behaviors of materials can be expressed by
combining these models.
From the preliminary experiment, it was found that when
the robot arm comes in contact a human at a high speed,
there are two types of time constants: rapid relaxation justafter the maximum contact force and slow relaxation later. It

B. Dynamic response
Fig. 7 illustrates the transient contact force of the palmof one subject (gray lines) observed thrice in the experimentand the optimized curve estimated by the genetic algorithm(red line) under each condition. The contact speed is 10
mm/s in the upper figures and 250 mm/s in the lower
figures. The displacement assumes that the static contact
force becomes 5, 10, and 15 N from the left figure. The

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C. Optimized parameters

20
Subject
Dummy skin

Force (N)

15
10
5
0

0

5

10

15

20

Displacement (mm)Fig. 6: Stiffness of the palm of five subjects (black lines) and thedummy skin (red line)

contact force increases nonlinearly after the contact starts,
attains the maximum value when the robot arm reaches the
maximum displacement, and after that, the contact force
relaxation occurs. The vibratory signal variation observed in
the phase of the contact force relaxation is due to the pulseof the subject.
Fig. 8 illustrates the transient contact force of the dummyskin (black line) and the optimized curve estimated by
the genetic algorithm (red line) under each condition. The
contact speed is 10 mm/s in the upper figures and 250 mm/s
in the lower figures. The displacement assumes that the static
contact force becomes 5, 10, and 15 N from the left figure,respectively. The transient contact force of the dummy skingenerally shows properties similar to those of the subjects
when comparison is made between Fig. 7 and Fig. 8.
Fig. 9 illustrates the maximum dynamic contact force ofthe palm of five subjects and the dummy skin under eachcondition. The bar graphs are those of the subjects, and thepoints in red circles are those of the dummy. The horizontal
axis represents each condition (f is the static contact force
and s is the contact speed.). Table I displays the mean
(standard deviation) values of the subjects and the dummy.
The maximum contact force becomes large when the contact
speed becomes high under the same static contact force
owing to viscosity. When comparing only the maximum
dynamic contact force, the values of the dummy are includedin the range of individual differences under all conditions.
TABLE I: The mean (standard deviation) values of the maximum
dynamic contact force (N) of the subjects and thedummy

Conditionf (N) s (mm/s)
5
10
5
250
10
10
10
250
15
10
15
250

Subject

Dummy

8.2 ± 0.5
9.0 ± 1.0
14.8 ± 1.6
17.0 ± 1.8
21.7 ± 2.5
27.3 ± 2.6

8.3
10.1
15.0
17.7
20.9
26.1

Fig. 10 depicts the optimized parameters of the palm of
five subjects (bar graphs) and the dummy (colored circles).
The bar graph of each parameter contains all conditions
regardless of static contact force and contact speed because
no difference is observed depending on these conditions.
Table II shows the mean (standard deviation) values of eachparameters for the subjects’ palms and the dummy. The
differences between α1 and α2 , β1 and β2 , and c1 and c2 are
distinct, showing the different characteristics of multilayered
soft tissue.
TABLE II: The mean (standard deviation) values of the parametersof the subjects and the dummyParameter
α1 (N/mm3 )α2 (N/mm3 )β1 (N/mm3 )β2 (N/mm3 )c1 (N/(mm/s))
c2 (N/(mm/s))

Subject
0.29 ± 0.27
0.04 ± 0.03
0.68 ± 0.52
0.13 ± 0.15
3.24± 2.67
1.93± 2.39

Dummy
0.34 ± 0.19
0.06 ± 0.02
0.45 ± 0.23
0.64 ± 0.35
5.10 ± 2.53
0.40 ± 0.28

IV. D ISCUSSION
A. Static response
As mentioned in the previous section, the stiffness of thedummy skin is harder than those of the subjects in smaller
ranges of displacement, as shown in Fig. 6. Two phases
appear in the stiffness of the subjects: the slow curve insmall displacement and the rapid curve in large displacement.
When the displacement is small, the contact force does
not increase rapidly because the dermis, which consists ofwater and collagen, moves. However, when the displacement
reaches the limit where the dermis can no longer move easily,
soft tissue, such as the epidermis, dermis, and subcutaneous
tissue, is clamped by the hard tissue, such as the bone,
and the contact force increases rapidly as a result. Thisphenomenon is pertinent only to human soft tissue.
B. Dynamic response
When focusing on the phase until arriving at the maximum
dynamic contact force of the transient contact force (Fig. 11
and 12), the viscosity of the dummy is observed to be larger
than that of the subjects; in particular, a convex shape can beseen under the condition that the contact speed is 250 mm/s
for the dummy. It is said that the peak mean contact pressure
and the total transferred energy are the dominant parameters
for injury in [13]. The total transferred energy was obtainedby eq.1.
Z
1 δ1
U=
F dδ
(1)
A 0
where A is the contact area, F is the applied contact force,δ is the displacement, and δ1 is the maximum displacement.
The dummy we developed is quite useful from the viewpoint of safety validation for injury because the dummy

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Fig. 7: Transient contact force of the palm of one subject (grey lines) and the optimized curve (red line) under each condition

Fig. 8: Transient contact force of the dummy skin (black line) and the optimized curve (red line) under each conditionshows the same maximum dynamic contact force as that of
the subjects (Fig. 9), while the total transferred energy (Fig.
11 and 12) is estimated to be higher than that of the subject,making criteria safer than the subject.

Maximum dynamic contact force (N)3025Subject
Dummy skin

C. Optimized parameters
20
15
10
5
0
f:5,s:10 f:5,s:250 f:10,s:10f:10,s:250f:15,s:10f:15,s:250

Condition

Fig. 9: Maximum dynamic contact force of the palm of five subjects (bar graphs) and the dummy (red circles) under eachcondition (f : static force, s: contact speed)

To examine the biofidelity of the dummy, Mann Whitney’s
U test, which is a non-parametric test, and Welch’s t-test,
which is one of the parametric tests, was conducted on the
results of Fig. 10. Table III depicts the results of Mann
Whitney’s U test and Welch’s t-test. The asterisks ** and*** indicate that the coefficients show statistical differenceof significance at 5 and 1 percent level, respectively. The
significant difference cannot be seen except for α2 by Mann
Whitney’s U test and β2 by both tests. The difference in α2
is caused by the difference in stiffness, as mentioned above.
The difference in β2 is caused by that of the viscoelasticity inthe experiment where the impact profile arrives at the maximum dynamic contact force. As mentioned above, we canconclude that the biofidelity of the dummy is considerably

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Fig. 10: Optimized parameters of the palm of five subjects (bar graphs) and the dummy (colored circles) (not to scale between eachparameter)

Fig. 11: Transient contact force of the palm of one subject until it arrives at the maximum dynamic contact force under each condition

Fig. 12: Transient contact force of the dummy skin until it arrives at the maximum dynamic contact force under each condition

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high.
TABLE III: The results of Mann Whitney’s U test and Welch’s
t-testparameter
α1
α2
β1
β2
c1
c2

Mann Whitney’s U test
0.18
**
0.46
***
0.18
0.16

Welch’s t-test
0.65
0.09
0.31
***
0.13
0.13
nsubject = 30
ndummy = 6

V. C ONCLUSION AND FUTURE WORKIn this study, we developed a dummy skin to mimic a human hand palm with considerably high biofidelity. The experimental setup was constructed assuming clamping situations
that represented the worst-case scenarios for contact between
robot systems and humans in collaborative operation. Staticand dynamic contact experiments were conducted to measure
the stiffness and the transition of the contact force withthe permission of the ethical committee. The biofidelity ofthe dummy was discussed based on the stiffness, maximum
dynamic contact force, and the parameters of the viscoelastic
model. The parameters were optimized using the genetic
algorithm with MATLAB (with version 2019a) Simulink
Model. The dummy is required for safety validation tests thatinvolve injury from the viewpoint of the maximum dynamic
contact force and the transferred energy.
In the future, we plan to change the target from the palmof the hand to the back of the hand. Another challenge is individuality - there is a wide variety of physical characteristics
that are applicable for humans. By appropriately determining
the mechanical properties, we can reproduce a worst-casescenario using the dummy.
ACKNOWLEDGMENT
This paper was financially supported by ”Knowledge
Hub Aichi” research project: R7 and conducted through”Construction of Risk Assessment System for Service Robot
Applications: Establishment of A V&V Testing Method.”
We would like to thank the Tanac Corporation for useful
discussion and assistance with many samples.
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