Identification of danger state for grasping delicate tofu with
fingertips containing viscoelastic fluid
Ryota Adachi, Yoshinori Fujihira, and Tetsuyou Watanabe, Member, IEEE

Abstract— In this study, we experimentally investigated theprocess leading to fracture in tofu grasping by deformablefingertips filled with a fluid. In our previous papers [1, 2], wedeveloped deformable fingertips using a rubber bag filled with aviscoelastic fluid, and presented a strategy for delicate tofu
grasping without any advance knowledge about fracture.
However, the predication point was close to fracture, and theprediction was then still a gamble. In order to realize fracture
prediction at an earlier stage, we examined the process leading
to fracture when pushing tofu by the deformable fingertips. The
stiffness of the fingertips can be controlled with the pressure of
the fluid inside the fingertips. The pushing force and fluidpressure were examined for different levels of stiffness of thefingertips. The main findings and contributions are as follows.
1) The convergence of the ratio of the contact force to fluidpressure gives an indication of dent occurrence. This
convergence could be seen when fingertip rubber bag was not
filled (low stiffness). 2) It was easier for a dent to occur when thefingertip rubber bag was not filled than when it was filled (highstiffness). 3) Changes in the rate of increase of the fluid pressure
as the tofu was pushed were repeatedly observed. We defined
this as a phase change and present a method for detecting suchchanges. The phase change points were detected by comparing
the fitting accuracies of different approximation models. 4) The
last and second to the last phase changes before fracture weredetected by detecting the first phase change (after theconvergence of the rate of the contact force to fluid pressure if
the fingertip bag was not completely filled). The detected points
can be regarded as alert points indicating a fracture risk that is
not close to the fracture point.

measured with a pressure sensor inside the fingertips. The
main benefits of this hand for delicate grasping are as follows.
1) A uniform contact pressure distribution is obtained thanks
to its viscoelasticity and incompressibility. 2) A rigid object
can be grasped by using the rigid layer of the fingertip. 3)
Fracture can be predicted before total fracture when graspinga fragile and soft object such as tofu. Fracture is in the pressure
or stress domain, and the fluid pressure profile has the potential
to show the fracture characteristic of a ductile material: theincreasing rate of stress decreases in the stress–strain diagram
just before breaking. In real situations, a decrease in theincreasing rate of fluid pressure was observed. We called thisthe initial break. We then formulated a strategy for graspingdelicate ductile objects (tofu) without any advance knowledge
about fracture. If the initial break was detected, the robot
gripper stopped closing and picked up the object. We
succeeded in grasping the object four out of five times.
However, there were still serious problems. As the name initial
break indicates, the detection point was very close to the total
fracture. The decrease in the increasing rate of fluid pressure
was sometimes unclear. Therefore, avoiding a fracture wasstill close to a gamble. In addition, local/partial fracture could
not be avoided. If the prediction could be done at an earlier
stage, safer delicate grasping could be realized. In addition, thecharacteristics of the developed viscoelastic fluid fingertips
were still unclear.
Outer rubbergel

I. INTRODUCTION
Everyday tasks must be performed by robots in order tosupport humans. Robotic hands [1–13] have essential roles as
end-effectors to complete such tasks. In order to substitute forthe hands of a human, robotic hands must be able to perform awide variety of functions to grasp and manipulate objects.
Most of the developed robotic hands have targeted rigid object
grasping [3-9], with only a few targeting delicate grasping [1,2, 10–13]. In the human environment, there are many fragileand soft objects such as foods and living tissues. If a robot
could deal with such fragile and soft objects, it is expected that
autonomous humanlike cooking and safe medical operations
could be realized by robots. One of the softest and most fragile
objects is (soft) tofu, which can be used as phantom objects
simulating brain tumors [14]. With this in mind, this study
targeted tofu grasping without breaking.
We previously presented [1, 2] a robotic gripper whose
fingertips (contact area) consisted of a rubber bag filled with aviscoelastic fluid or gel (Fig. 1). The fluid pressure can be
R. Adachi and F. Fujihira are with the Graduated school of Natural scienceand Technology, Kanazawa University, Kakuma-machi, Kanazawa, 9201192,
Japan (e-mail: {radachi,fuji3134}@stu.kanazawa-u.ac.jp).

Pressure sensor
Silicon layer
Gel layer

Fingertip (Detail view)

Figure 1. Structure of deformable fingertip and tofu grasping in [1, 2]

With this in mind, this study experimentally investigatedthe phenomenon when pushing tofu using the viscoelastic fluid
fingertips. The main findings and contributions are as follows.
1) The convergence of the ratio of the contact force to fluidpressure gave an indication of dent occurrence. This
convergence could be seen when the fingertips were notcompletely filled with the fluid.
2) It was easier for a dent to occur when the fingertip
rubber bag was not filled than when it was filled.

T. Watanabe is with the Institute of Science and Engineering, Kanazawa
University, Kakuma-machi, Kanazawa, 9201192 Japan (e-mail: tewatanabe@ieee.org).

3) A change in the increasing rate of the fluid pressure gave
an indication of an alternation of the material (tofu) related toa preliminary step/stage toward the yielding and fracture of the
tofu. This paper presents a method to detect/estimate thisindication: the phase change points. The phase change points
were detected by comparing the fitting accuracies of different
approximation models.
4) By detecting the first phase change when the fingertip
rubber bag was filled and the first phase change after theconvergence of the rate of the contact force to fluid pressure
when the fingertip rubber bag was not filled, the last andsecond to the last phase change points before fracture could be
detected. We called these points the danger area starting points,
and used them as a warning of the fracture risk. The pushingforce at the danger area starting points could be regarded as the
maximum grasping force free of the risk of fracture. We
investigated tofu grasping with the pushing forces and showedthat safe tofu grasping could be realized. Note that the danger
area starting points were not close to the initial break points
presented in our previous paper [1, 2].
The remaining parts of this paper are as follows. The
subsequent subsection describes the related works. In section
2, we introduce the developed fingertip system for delicate
grasping. In section 3, we describe the investigation of thephenomenon when pushing tofu using the viscoelastic fluid
fingertips. We also discuss the method for detecting the phasechange points. In section 4, we discuss danger area starting
points, and show the experimental results of tofu grasping withthe pushing force at the danger area starting points.
A. Related works
The efficiency of using gel for fingertips was discussed by
Simoga and Goldenberg [15]. They claimed that gel iseffective at reducing the contact impact and strain energy andfitting the object shape. We found other benefits from using
gel in our previous studies: a uniform contact pressure profile
and an automatic stiffness increase [1, 2]. The main drawbackof using gel for fingertips is the limitation of the maximum
applicable forces. Thus, we constructed a two-layer structure
where a rigid component was installed inside the gel for
grasping relatively heavy objects.
Kim and Song developed a gripper that included hybridvariable stiffness actuators that could control the contact
stiffness [10]. Brown et al. [11] developed a gripper thatutilized a jamming phenomenon based on the ideas presented
in [16] and [17]. Choi and Koc developed a gripper with
inflatable rubber pockets on the gripping sides [12]. Theirpneumatic inflation made it possible to grasp objects with awide variety of shapes. Pettersson et al. presented a gripper
with a gripping area covered by a magnetorheological (MR)fluid [13]. The area with the MR fluid was molded according
to the object shape, and the object was confined inside thespace formed by the molded MR fluid. In the above studies,
eggs or fruits were successfully grasped. However, the main
targets were not fracture control, and none of these studies
considered fragile and soft objects such as tofu.

II. VISCOELASTIC FLUID FINGERTIPS
A. Structure
In the previous studies [1, 2], we developed deformablefingertips with a two-layer structure. Its purpose was to makeit possible to handle both fragile objects using the fluid layer
and normal rigid objects using the rigid layer. Because thisstudy focuses on grasping fragile objects (tofu), we willconsider only the fluid layer here. For this purpose, weconstructed deformable fingertips using a rubber bag filledwith a viscoelastic fluid, as shown in Fig. 2. The radius of eachfingertip was 11 [mm]. The outer rubber material was made ofnitrile, and the fluid was oil (chainsaw oil, ISO VG 100(International Organization for Standard Viscosity Grade)). In
contrast to previous fingertips, a screw clamp was used to
improve the sealing performance. Thus, a fluid with a low
viscosity could be used. A pressure sensor (KEYENCE, AP12s) was connected using a tube. A piston was also connectedusing a tube to control the fluid pressure. According to Pascal'slaw, if a fluid is incompressible, the pressure is the same
everywhere, even when an external pressure/force is applied
to the container used to confine the fluid. Additionally, achange in the fluid volume indicates a change in the fluidpressure. Therefore, by controlling the piston, we could
control the fluid pressure. The main body was made of brass
to prevent corrode.
Because of the deformability of the fingertip, the fingertip
shape conformed to the object shape. As we showed in theprevious papers [1, 2], the contact pressure distribution was
uniform. These two features made it very effective forgrasping fragile objects. The fingertip stiffness increased withan increase in the deformation, as we also showed in theprevious papers [1, 2]. However, we did not previously
investigate the contact force. Here, we experimentallyinvestigated the behaviors of the contact force and fluidpressure at the same time.
Pressure control part
Brass
RubberSensor partconnected

Load cell
Viscoelastic fluid
Tube
Pressure sensor

Figure 2. Structture of deformable fingertip constructed from rubber bag
filled with viscoelastic fluid

B. Behaviors of applied load and fluid pressure of fingertip
when pushing against flat plate
Fig. 3 shows the experimental setup for investigating the
behaviors of the fluid pressure and pushing force, anddetermining the stiffness of the fingertip. The fingertip was
attached to a handmade load cell (allowable load: 20 [N];resolution: 0.01[N]) fixed on an automatic positioning stage sothat the fingertip position and speed could be controlled. We
pushed the fingertip against a duralumin plate a slow speed of1.0 [mm/s] to minimize the influence of the speed. We
conducted the investigation with initial fluid pressure (thepressure when there is no contact) values of 1.5, 2.7, 4.0, and
6.0 [kPa]. The ascending filling rate order for the fluid was 1.5,
2.7, 4.0, and 6.0 [kPa].
Pulse signals

Motor driver
Automatic positioning stage

Data

AmplifiercircuitObject(Flat surface)Fig. 3, except for the object being contacted. Instead of the flat
surface, a fragile object, tofu (Topvalue, Silken tofu, size:
25 × 25 × 30 [mm3 ]), was used. The tofu and fingertip werein contact with each other at the initial state without any
contact pressure or contact force. Then, by moving theautomatic positioning stage, the tofu was pressed by thefingertip at a speed of 1 [mm/s] until the tofu completely broke,
as shown in Fig. 5. The experiment was conducted with initialinternal fluid pressures (pin) of 1.5, 2.7, 4.0, and 6.0 [kPa]. In
order to determine the repeatability, the experiments wereconducted three times for every condition. Note that pushingan object against a flat plate with one fingertip produces thesame physical conditions as pushing an object from both sides
with two fingertips.
Pushing distanced

Load cellPressure sensor

Amplifier filter
circuit

Analog-digitalconvertor

Figure 3. Schematic view of experimental setup when fingertip pushed
duralumin plate at speed of 1.0 [mm/s]

3

2
1
0
0

1

1.5 [kPa]2.7 [kPa]
4.0 [kPa]
6.0 [kPa]

10

Pressure [kPa]

Force [N]12

1.5 [kPa]2.7 [kPa]
4.0 [kPa]
6.0 [kPa]4

2 3 4 5 6 7 8
Pushing distance [mm]9Figure 5. Close-up of fingertip area in experimental setup for breaking
tofu

8
6
4
2

0

(a) Applied force

01

2 3 4 5 6 7
Pushing distance [mm]8(b) Fluid pressure

Figure 4. Results when fingertip pushed duralumin plate at speed of 1.0[mm/s]: (a) Pushing distance versus applied force, (b) Pushing distanceversus fluid pressure

Fig. 4 shows the results. Note that the pressure shown isthe differential pressure compared to the initial pressure.
Hereafter, we simply call this differential pressure “pressure”
except for special cases. The horizontal axis shows themoving/pushing distance (pushing amount) of the fingertip. It
can be seen that the order of the initial pressure corresponds tothe order of the fingertip stiffness. There is little difference inthe applied force between the cases when the internal fluidpressure (pin) is 4.0 and 6.0 [kPa], while there is little
difference in the fluid pressure between the cases when theinternal fluid pressure (pin) is 1.5 and 2.7 [kPa]. However, thebehavior of the force curves is complex/nonlinear when pin is
1.5 and 2.7 [kPa]. One reason is that the fingertip rubber bag
was not filled with fluid when there was no contact at 𝑝in =1.5 and 2.7 [kPa], while it was filled at 𝑝in = 4.0 and 6.0[kPa]. Therefore, from the force and pressure profiles whenpushing against a flat plate, we can determine whether thefingertip rubber bag is filled.
III. COMPRESSION TEST OF TOFU BY VISCOELASTIC FLUIDFINGERTIPS

We investigated the phenomena when breaking tofu. The
experimental setup was the same as the previous one shown in(a) Pushing distance versus applied force, where dotted lines show fracture
points(b) Pushing distance versus differential fluid pressure, where dotted lines
show fracture points and circles show detected danger area starting points
Figure 6. Results of breaking tofu by pushing it at speed of 1.0 [mm/s]

Fig. 6 shows the results. One case for every condition (rigidline) and the mean value with the standard deviation (point anderror bar) are shown for easy visualization. Fig. 6 (a) shows
the applied force profile. There is no significant difference ifthese values are separated into two cases: a small initialpressure (at pin = 1.5 or 2.7 [kPa] (not-filled fingertip)) andlarge initial pressure (at pin = 4.0 or 6.0 [kPa] (filled fingertip)).
Fig. 6 (b) shows the fluid pressure profile. Note that thispressure is the differential pressure, and a relatively larger
differential pressure was obtained when pin was smaller. The
dotted line is the point where the tofu broke. The broken pointswere determined to be the points just before which the fluidpressure had decreased for 0.1 [sec]. We focused on thedecrease in pressure, because the fracture is in the pressure or
stress domain and there was no clear difference of the decrease
between using force and pressure. Note also that a relatively
flat area can be seen just before the tofu broke (although it is
not easy to see especially at pin = 4.0 [kPa]). This is the initial
break area discussed in our previous papers [1, 2]. The purposeof this study was to detect the danger area for breaking before
this area. The pressure profile is nonlinear, and both an area
where the increasing rate is rapid and an area where theincreasing rate is slow can be seen. Thus, the profile is divided
into several phases.

Method: Two (simple) and three (complex) dimensional
polynomial functions were prepared for approximation models.
The regression started from the case when the pushing distance
was 0.1 (the number of data points was 10). Every time newdata became available, regression was performed, and the root
mean squared error (RMSE) was calculated for every model.
The RMSE difference was computed. The local minimum andmaximum points and the initial rise point were taken as thephase change points.
Let RMSEpoly2 and RMSEpoly3 be the RMSE values for two(simple) and three (complex) dimensional polynomial
functions. Then, the RMSE difference can be written as
follows:∆RMSE = RMSEpoly2 − RMSEpoly3(a) Pushing distance versus ∆RMSE when initial pressure was low (pin = 1.5
and 2.7 [kPa]), where circles show danger area starting points, squares show
convergence points, diamonds show other phase change points, and Xmarks show fracture points

Fig. 7 shows the calculated ∆RMSE for each case. Notethat to make it easy to see, one case for every condition isshown, but the qualitatively same profiles (same number ofwaves) were obtained for every condition. The local minimum
and maximum points and the initial rise point are the phasechange points (circles and diamond shapes in Fig. 7). It can beseen that the phases, which are not easy to detect in Fig. 6 (b),can be clearly distinguished. According to the results and fromobserving the movies, the following phases were defined.

Phase 0Phase 1
(Second time)

Phase 1
(First time)

Phase 2
(Second time)

Phase 2
(First time)

Phase 1(Third time)

Tofu breakingpin (Initial pressure)=1.5 [kPa](b) Pushing distance versus ∆RMSE when initial pressure was high (pin =4.0 and 6.0 [kPa]), where circles show danger area starting points, diamonds
show other phase change points, and X-marks show fracture points
Figure 7. Results for phase change detection

A. Phase change detection
A change in phase appeared with an increasing rate of fluidpressure (see Fig. 6 (b)). However, this change was not easy tosee. Therefore, a method to make the change clearer wasneeded. This paper presents a method based on the differencein the fitting accuracies of different approximation models.
When the phase changed, the arrangement of data alsochanged. If new data deviated from the original data, a more
complex model would be required for good fitting accuracy.
In this case, the difference in the fitting accuracies of complex
and simple models would be large. If new data decreased theeffect of the deviation/variation of the original data, a simpler
model would be sufficient to describe the data. In this case, thedifference in the fitting accuracies of complex and simple
models would be small. Namely, the difference in the fitting
accuracies of complex and simple models could be used to
predict the phase change. Based on this concept, the following
method was developed.

Phase 0*Phase 1
(Second time)

Phase 1
(First time)

Phase 2
(Second time)

Phase 2
(First time)

Phase 1(Third time)

Tofu breakingpin (Initial pressure)=2.7 [kPa]

Phase 1
(First time)

Phase 2
(First time)

Phase 1
(Second time)pin (Initial pressure)=4.0 [kPa]

Tofu breaking

Phase 1

Phase 2

Tofu breakingpin (Initial pressure)=6.0 [kPa]

Figure 8. Photos of each phase when pushing tofu, with the red coloredphases corresponding to danger areas (*Note that Phase 0 at pin = 2.7 [kPa] wasderived manually because it was not detected by the phase change point analysis)

Phase 0: The fingertip (internal fluid) pressed against
the tofu because the rubber bag of the fingertip was not
filled. This phase is basically the same as phase 2. The
difference is when it occurs (before phase 1) and the tofu does
not stretch.
Phase 1: The fingertip dented the tofu
Phase 2: The denting by the fingertip decreased due tothe increase in the tofu stiffness. The fingertip swelled while
the tofu stretched.
Fig. 8 shows a photo at every phase. Note that Phase 0 was
not detected by the method at pin = 2.7 [kPa] because its
period/term was short. It was observed that phase 1 and phase2 were alternately repeated, especially when pin was low (pin =1.5 or 2.7 [kPa]). The phase change is considered to be theresult of the change in relative stiffness between the tofu andthe fingertip. If the fingertip was stiffer than the tofu, thefingertip dents the tofu (Phase 1), whereas if the tofu wasstiffer than the fingertip, denting by the fingertip decreased(Phase 2). A dent/yielding could occur in the change from
Phase 2 to Phase 1, because at Phase 2, the tofu stores a large
strain energy preventing the penetration of fingertip, whereas
the phase change indicates that the stiffness of the fingertip
overcomes that of the tofu. A change from Phase 1 to Phase 2
indicates that the tofu becomes hard. This alternation alsomeans that a large strain energy is stored and the risk of
breaking increases. Therefore, the key for preventing the tofu
from breaking might be to detect the point where the last or
second to the last phase change occurs before breaking. The
detection point is defined as the starting point for the danger
area. When pin is high (pin = 4.0 or 6.0 [kPa]), the first phasechange corresponds to the last or second to the last phasechange. Therefore, it is sufficient to detect the first phasechange. However, this is not true when pin is low (pin = 1.5 or
2.7 [kPa]). In this case, which phase change point to consider
becomes the issue. In order to resolve this issue, we focusedon the rate of the applied force to the fluid pressure, as shownin Fig. 9. In order to avoid dividing by zero, the following rate
was used:𝑓𝑝+𝜖0

convergence point, the rate of increase for the applied force
was the same as that for the fluid pressure. This meant theapplied force was directly transferred to the fingertip fluid. It
was believed that a kind of assimilation between the tofu andfingertip could have occurred. Based on the phase change
point analysis shown in Fig. 7 (a), multiple phase changes wereconsidered to have occurred before the convergence.
Especially the change from Phase 2 to Phase 1 is importantbecause it could cause a dent/yielding. When the fingertip
rubber bag was not filled, the rubber bag could have wrinkles
in this situation. Because of capillarity, a high-pressure area
could occur at a wrinkle, which could cause a partial dent in
the tofu. It could proceed especially at a phase change from
Phase 2 to Phase 1. In practice, a small dent of tofu could beseen around this convergence point, as shown in Fig. 10 (10mm). However, the dent was small and far from the complete
fracture. It can be said that at the convergence point, the tofuchanged and a dent could be seen, but the complete fracture
could be avoided if the application of the force stopped. The
phase at the convergence point was safe from the viewpoint ofcomplete fracture. Inversely, there was at least one phasechange after the convergence with a high possibility for
fracture. Therefore, we devised a way to detect the first phasechange point after the convergence, and defined this phasechange point as the starting point for the danger area.
It is also interesting to note that a smaller initial pressure
resulted in a clearer/larger dent. One reason might be the large
number of phase changes. If a small dent is not desired, a larger
initial pressure (the fingertip rubber bag is filled) should be
used for grasping.

Figure 9. Pushing distance versus ratio of applied force to fluid pressure,with squares showing convergence points and X-marks showing fracture
points

5 mm

10 mm

15 mm

pin (Initial pressure) = 1.5 [kPa]

5 mm

10 mm

15 mm

pin (Initial pressure) = 2.7 [kPa]

where f denotes the applied force, p denotes the differential
fluid pressure, and 𝜖0 (= 0.001) denotes a small constant.
In order to see the state of yielding, we also observed theappearance of the contact surface of the tofu with pushingdistances (d) of 5, 10, and 15 [mm], as shown in Fig. 10.

5 mm

When pin was low (pin = 1.5 or 2.7 [kPa]), the rate
converged to a certain value (see square marks). At the

Figure 10. Photos of contact surfaces after pushing tofu when pushingdistances were 5, 10, and 15 [mm]

10 mm

15 mm

pin (Initial pressure) = 4.0 [kPa]

5 mm

10 mm

15 mm

pin (Initial pressure) = 6.0 [kPa]IV. DANGER AREA STARTING POINT FOR AVOIDINGFRACTURE AND DELICATE TOFU GRASPING

Based on the definition mentioned above, the danger area
starting points were detected as follows.
Step 1) If fingertip rubber bag is filled, go to step 3, letting𝑑s = 0. Otherwise go to step 2.
Step 2) Detect the convergence point for the rate of applied
force to fluid pressure. Let 𝑑s be the pushing distance at
the convergence point.
Step 3) Detect the first phase change point, counting from𝑑s .
The obtained danger area starting points (circle marks) are
shown in Fig. 7. These danger area starting points are also
shown in Fig. 6 (b). It can be seen that the danger area starting
points are the last or second to the last phase change points
before breaking if we ignore the phase changes very close to
the breaking points, and are not very close to nor far away fromthe breaking points. Thus, it can be said that the points can beused as warnings of the potential for fracture.
On the other hand, the danger area starting points can be
regarded as grasping points. The danger area starting point
indicates that increasing the pushing distance/force from thispoint presents a risk of fracture, whereas fracture can be
avoided if the pushing distance (applied force) is not increased
any further. In other words, the pushing distance/force at thedanger area starting point is the guaranteed maximum(grasping) force that can avoid fracture. In addition, a phasechange from Phase 1 to Phase 2 indicated an alteration in the
tofu (it became hard), and there have been reports that a hardercontact area can produce larger frictional forces whencontacting soft materials [18, 19]. Therefore, easier grasping
can be expected compared to previous phases. In this regard, asimple grasping strategy is to grasp an object with the force atthe danger area starting point. This is preferable if the grasping
succeeds, because the grasping force has the guaranteedmaximum value. If the grasping fails, there is no other choice
but gradually increasing the grasping force (so that breaking
does not occur). With this in mind, we attempted to grasp tofu
using the pushing amount (force) at the danger area starting
point. The nominal method is to calculate ∆RMSE step by stepand detect the danger area starting point. However, here, inorder to verify the repeatability of the methodologies, the meanof the pushing distances (forces) at the danger area starting
points obtained in the experiments described in the previous
section was calculated and used (Table 1).
The experimental setup was the same as the one shown in
Fig. 5, except the table for the tofu was removed. First, by
moving the automatic positioning stage, we pressed on the tofu
with the fingertip using the mean pushing distance/force at thedanger area starting points. Subsequently, the table for tofu
was removed to determine whether grasping could be realized.
Three trials were conducted for every condition (𝑝in).
Fig. 11 shows the results. The grasping succeeded under
all the conditions and in all the trials, which showed thevalidity of our approach. It can be said that the danger area
starting points could work as grasping points for delicate tofu
grasping, although to be precise, whether grasping could be
performed also depended on the weight of the object. Whenthe fingertip rubber bag was not filled (pin = 1.5 and 2.7 [kPa]),
the grasping points came after the convergence of the rate of
the applied force to fluid pressure, and the occurrence of dents
could not be avoided. On the other hand, when the fingertip
rubber bag was filled (pin = 4.0 and 6.0 [kPa]), only a small
dent could be seen.
TABLE I.

MEAN VALUE OF DERIVED PUSHING DISTANCE AT DANGER
AREA STARTING POINTS

Initial pressure 𝑝in [kPa]

1.5

2.7

4.0

6.0

Mean pushing distance [mm]

12.6

12.4

8.8

9.1

Grasping tofu

Tofu’s surface
after grasping

Grasping tofu

Tofu’s surface
after graspingpin (Initial pressure) = 1.5 [kPa]pin (Initial pressure) = 2.7 [kPa]

Grasping tofu

Grasping tofu

Tofu’s surface
after graspingpin (Initial pressure) = 4.0 [kPa]

Tofu’s surface
after graspingpin (Initial pressure) = 6.0 [kPa]

Figure 11. Results when grasping tofu using mean pushing distance/force atdanger area starting points and Tofu’s surface after grasping

V. CONCLUSION
In this study, we experimentally investigated the process
leading to fracture in tofu grasping using a deformablefingertip fabricated with a rubber bag filled with a fluid. In ourprevious papers [1, 2], we developed deformable fingertips
using a rubber bag filled with a viscoelastic fluid, andpresented a strategy for delicate tofu grasping without any
advance knowledge about fracture. A decrease in theincreasing rate of fluid pressure was observed just before thecomplete facture. By detecting this decrease, we succeeded ingrasping tofu without breaking. However, the detection point
was close to the complete fracture. It is definitely preferable
for fracture prediction to be done at an earlier stage. Aiming at
the realization of early fracture prediction, we investigated thephenomena that occurred when a deformable fingertip
fabricated with a rubber bag filled with a fluid was pushed
against tofu. The main findings and contributions are as
follows (see also Table 2 for the major differences between thecases using fingertips with low and high initial pressures).


The convergence of the ratio of the contact force
to fluid pressure caused the occurrence of dents inthe tofu. The convergence occurred only when thefingertip rubber bag was not filled (pin = 1.5 or 2.7[kPa]).

A higher initial pressure for the fluid in thefingertip rubber bag made it less likely for dents
to occur.

Phase changes were observed. There were mainly
two phases: the fingertip dented the tofu and thefingertip denting decreased when the fingertip
swelled. These two phases were alternately repeateduntil breakage occurred.

Phase change detection method: The phase change
points were detected by comparing the fitting
accuracies of different approximation models.



Danger area starting point detection: By detecting
the first phase change when the fingertip rubber bag
was filled and the first phase change after theconvergence of the rate of the applied force to fluidpressure when the fingertip rubber bag was not filled,
the last or second to the last phase change point
before fracture could be detected. If the applied force
was increased from that point, a risk of breaking wasconsidered to occur. We called these points thedanger area starting points, and used them as
warnings of fracture risk. The danger area starting
points were not close to the initial break points
presented in our previous papers [1, 2].



Tofu delicate grasping without breaking: We
showed that tofu grasping using the (pushing) force
at the danger area starting points succeeded under all
conditions of initial fingertip fluid pressure. Thismeans the danger area starting points could work as
grasping points for delicate grasping without
breaking.

There appears to be no merit in using fingertips with a low
filling rate of the fluid. However, if much softer objects aretargeted, convergence may not occur and breaking may easily
occur with filled fingertips. In that case, the strategy for filled
fingertips can be applied to grasping with non-filled fingertips.
In summary, this study provided multiple fundamental
methodologies for dealing with soft and fragile objects. Whichmethod should be taken depends on the materials of object andfingertips. More detailed investigation for this issue is beyondof scope of this paper and might be topics for future works.
REFERENCES
[1]

[2]
[3]
[4]
[5]

[6]

[7]

R. Maruyama, T. Watanabe, and M. Uchida, “Delicate grasping by
robotic gripper with incompressible fluid-based deformable fingertips,''in Intelligent Robots and Systems (IROS), 2013 IEEE/RSJ
International Conference on, pp. 5469-5474, 2013.
T. Watanabe, R. Maruyama, and M. Uchida, “Delicate food graspingby robotic gripper with viscoelastic fluid-based deformable fingertips,''
Transaction on Control and Mechanical Systems, vol. 3, no. 3, 2014.
C. Lovchik and M. Diftler, “The robonaut hand: A dex-terous robot
hand for space,'' Proceedings of the IEEE International Conference on
Automation and Robotics, vol. 2, pp. 907-912, 1999.
N. Fukaya, S. Toyama, T. Asfour, and R. Dillmann, ``Design of thetuat/karlsruhe humanoid hand,'' Proceeding of IEEE/RSJ International
Conference on Intelligent Robots and Systems, pp. 1754-1759, 2000.
J. Butterfass, M. Grebenstein, H. Liu, and G. Hirzinger, “Dlr-hand ii:
next generation of a dextrous robot hand,'' Proceeding of the IEEE
International Conference on Robotics and Automation, pp. 109-114,
2001.
J. Ueda, Y. Ishida, M. Kondo, and T. Ogasawara, “Development of thenaist-hand with vision-based tactile fingertip sensor,'' Proceeding of theIEEE International Conference on Robotics and Automation, pp. 23322337, 2005.
W. U. S. L. (http://twendyone.com), “Twendy-one.''[8]

D. ALLEN, S. LEFRANÇOIS, and J. JOBIN, “A gripper having a twodegree of freedom underactuated mechanical finger for encompassing
and pinch grasping,'' May 30 2013, wO Patent App.
PCT/CA2012/050,844.
[Online].
Available:\url{http://www.google.com/patents/WO2013075245A1?cl=en}[9] S. Hirose and Y. Umetani, “Development of soft gripper for theversatile robot hand,'' Mechanism and Machine Theory, vol. 13, pp.
351-359, 1978.
[10] B.-S. Kim and J.-B. Song, “Object grasping using a 1 dof variable
stiffness gripper actuated by a hybrid variable stiffness actuator,''Proceeding of the IEEE International Conference on Robotics andAutomation, pp. 4620-4625, 2011.
[11] E. Brown, N. Rodenberg, J. Amend, A. Mozeika, E. Steltz, M. R. Zakin,
H. Lipson, and H. M. Jaeger, “Universal robotic gripper based on thejamming of granular material,'' Proceedings of the National Academyof Sciences, vol. 107, no. 44, pp. 18\,809-18\,814, 2010.
[12] H. Choi and M. Koc, “Design and feasibility tests of a flexible gripper
based on inflatable rubber pockets,'' International Journal of Machine
Tools and Manufacture, vol. 46, no. 12-13, pp. 1350-1361, 2006.
[13] A. Pettersson, S. Davis, J. Gray, T. Dodd, and T. Ohlsson, “Design of amagnetorheological robot gripper for handling of delicate food productswith varying shapes,'' Journal of Food Engineering, vol. 98, no. 3, pp.
332-338, 2010.
[14] T. Yoneyama, T. Watanabe, H. Kagawa, J. Hamada, Y. Hayashi and M.
Nakada, “Force detecting gripper and force feedback system forneurosurgery applications,” International Journal of Computer Assisted
Radiology and Surgery, vol. 8, no.5, pp. 819-829 , 2013[15] K. B. Shimoga and A. A. Goldenberg, “Soft robotic fingertips part i: A
comparison of construction materials,'' The International journal ofrobotics research, vol. 15, no. 4, pp. 320-334, 1996.
[16] I. Schmidt, “Flexible molding jaws for grippers,'' Industrial Robot, vol.
5, pp. 24-26, 1978.
[17] A. Perovskii, “Universal grippers for industrial robots,'' Russ. Eng. J.,
vol.~60, pp. 3--4, 1980.
[18] T. Watanabe and Y. Fujihira, “Experimental investigation of effect offingertip stiffness on friction while grasping an object," Proceeding of
the IEEE International Conference on Robotics and Automation, pp.
889 - 894, 2014[19] Y. Fujihira, K. Harada, T. Tsuji, and T. Watanabe, “Experimental
Investigation of Effect of Fingertip Stiffness on Resistible Force in
Grasping," Proceeding of the IEEE International Conference on
Robotics and Automation, pp. 4334-4340, 2015.
TABLE II.

MAJOR DIFFERENCES BETWEEN CASES USING FINGERTIPS
WITH LOW AND HIGH INITIAL PRESSURES

Initial pressure of fingertip 𝑝in [kPa]

High (4.0, 6.0)

Low (1.5, 2.7)

Stiffness of fingertip

High

Low

Slow

Fast

Dent risk

Low

High

Number of phase changes

LowfrequencyHighfrequency

Prediction of danger area

Easy

Complex

Convergence speed of𝑓𝑝+𝜖0