Delicate Grasping by Robotic Gripper with Incompressible Fluid-based
Deformable Fingertips
Ryoji Maruyama1 , Tetsuyou Watanabe2 , and Masahiro Uchida1 .
Abstract— This paper presents a gripper with fingertips
constructed from incompressible fluid covered by rubber withthe aim of grasping very fragile objects. Owing to the incompressibility, simply closing the gripper allows the fingertips toapproach the target object with low stiffness; after contact, thecontact stiffness increases with increasing fingertip deformation,and grasping with high stiffness can be realized. The adaptationof the fingertips to the object shape and a uniform contactpressure are further benefits of the proposed system. Fragile
and brittle objects can be grasped by controlling the contactpressure so that it does not exceed the fracture stress/pressure.
We found that the initial sign of fracture appears before total
fracture when soft and ductile objects are grasped. Basedon this phenomenon, we developed a strategy for grasping
ductile objects without any advance knowledge of fracture. The
proposed fingertips have a rigid layer inside the fluid to grasp
objects with normal rigidity. The effectiveness of the fingertips
was confirmed experimentally.

I. INTRODUCTION
Recently, much attention is being given to robots executing
everyday tasks in human society. Robotic hands have an
important role in completing such tasks. We focused ontasks related with fracture. Human environments have many
soft and fragile objects such as foods, glasses, and plastics.
This paper proposes a robot hand that can grasp such fragile
objects without breaking them.
A number of objects and tools are manufactured for use byhuman beings in daily life. Thus, robotic hands that resemble
human hands are advantageous in human society; many kindsof multi-fingered robotic hands have been developed [1]–[6]. Although multi-fingered hands are versatile, they are
not always suitable for equipping specialized functions—for
example, fracture control and dealing with complex shapes.
Grippers [7]–[11] are suitable for these purposes. Kim andSong developed a gripper with a variable contact stiffness
due to hybrid variable stiffness actuators that can grasp
an egg [8]. However, they did not consider adaptation toirregularly shaped objects. Hirose and Umetani [7] proposeda robotic hand whose fingers move like a snake to realize
a flexible grasp. Brown et al. [9] evaluated a universal
gripper constructed from an inflatable rubber bag containing
granular materials. By controlling the air pressure inside thebag and utilizing jamming, objects of any shape can be
grasped. The original idea was introduced by Schmidt [12]*This work was supported by The Mazda Foundation
1 R. Maruyama and M. Uchida are the Graduated school of Naturalscience and Technology, Kanazawa University, Kakuma-machi, Kanazawa,
920-1192, Japan. rmaru@stu.kanazawa-u.ac.jp
2 T. Watanabe is with the College of Science and Engineering, Kanazawa
University. te-watanabe at ieee.org

and Perovskii [13]. However, they did not consider fracture
and the grasping of fragile and brittle objects. Choi and Koc[10] developed a gripper with inflatable rubber pockets on thegripping areas. By utilizing pneumatic inflation, they wereable to grasp objects of different shapes, including eggs,
without damaging them. However, the main target objects
were not fragile and brittle. Pettersson et al. [11] presented
an interesting parallel jaw gripper whose gripping area iscovered by magnetorheological (MR) fluid. If the gripper isclosed and contact occurs, the MR fluid is molded according
to the object shape. An electromagnetic force then changes
the viscoelasticity of the MR fluid, and the object is confined
inside the MR fluid. They were able to grasp fruits using thismethod.
The aim of this study was to develop a gripper that cangrasp more fragile and brittle objects than those considered
in conventional studies. This paper presents a novel robotic
gripper whose fingertips (gripping area) are constructed froma rubber bag filled with incompressible fluid; the paper alsopresents the grasping strategy for the gripper. Details on thecontributions of this paper are as follows.
Gripper with incompressible fluid-based deformableand controllable fingertips: Simoga and Goldenberg [14]showed that gel (incompressible fluid) is suitable for fingertips of robotic hands from the viewpoint of impact energy attenuation, surface conformability and strain energy
dissipation. The additional/other main benefits of the use ofincompressible fluid, which are not presented in [14] but
presented in this paper, are a uniform contact pressure profile
and automatic stiffness increase. In contrast to compressible
fluids like air, increasing the fingertip deformation indicatesan increase in fingertip stiffness (if controlling the volumeof the fluid to be constant). Both low-contact impact andhigh-stiffness grasping can be realized by simply closing thegripper. In addition, we attached a pressure sensor for the
fluid to control the contact pressure for fracture control.
A low maximum applicable force/pressure is a drawbackof the (presented) fingertips. It is hard to grasp normal objects
such as a bottle of water. In order to overcome this issue,
we constructed a two-layer structure with a rigid component
located inside the fluid. If large contact forces are exerted
and correspondingly large displacement happens, contact
between the rigid component and object occurs. In this case,large contact forces can be applied to the object via the rigid
component, and the object can be grasped.
Grasping strategy for delicate grasping: A fracture control
method for grasping fragile objects without breaking them ispresented. Fracture is not in the force domain but in the stressgelOuter rubber

Gel
Control unit
Motor
Slide screw

Pressure
sensorSilicon layer
Gel layer(a) Cross section of the fingertip partFig. 1.

Silicon(b) Manufactured fingertip

Fingertip structure

Fingertip

Fig. 2.

II. I NCOMPRESSIBLE FLUID - BASED DEFORMABLE
FINGERTIPS AND GRIPPER STRUCTURE

Fingertip and control unitΔd0.00 NF

Fingertip
Force gauge
Flat plate

Fig. 3.

Experimental set up for compression test

10
Gel layer

15 [kPa]
10 [kPa]
5 [kPa]987Force [N]or pressure domain. In conventional studies, fracture forceswere measured in advance, or only the results (whether or not
the grippers could grasp objects without breaking them) were
evaluated. Thus, evaluation was indirect, or fracture control
was not considered. In this study, we prevented fracture fromgrasping by controlling the contact pressure so that it isless than the fracture stress/pressure. We demonstrated theefficiency of the approach by having the gripper grasp brittle
objects—that is, potato chips. When the gripper grasped
ductile objects such as soft tofu, we found that signs offracture occurred before total fracture. We call this the initial
break. Based on the initial break, we developed a strategy
for grasping ductile objects without any advance information
on fractures.

Silicon layer

Initial pressure(when there is no contact)

6
5
4
3

Contact with the silicon layer

2

A. Incompressible fluid-based deformable fingertips and thetwo-layer system

100

The key to a delicate grasp is changing the shape of contact
areas according to the object shape and uniform contactpressure. Thus, we developed deformable fingertips filledwith an incompressible fluid (gel: Schick SWS-CG180). In
order to grasp conventional rigid objects, a silicon layer(made of Shinetsu Silicon KE-1308) was installed inside thegel. Fig. 1 shows the structure of the developed fingertip
and a manufactured prototype. Note that we call the contact/gripping area the “fingertip”.
Two pressure sensors were installed: one to measure thepressure of the gel and the other (located at the root of thesilicon layer) to measure the force applied to the siliconlayer. When a large grasping force is not required, only
the gel layer is used for grasping. If a large grasping force
is required, the silicon layer is mainly utilized with large
deformation of the fingertips. In order to achieve compact
and lightweight fingertips, the pressure control unit was
installed separately and connected to the fingertip by a tube,
as shown in Fig. 2. By shaving tubes for the connection,
we can control multiple fingertips with one control unit.
This approach was used in this study to reduce the numberof actuators. The pressure control is based on Pascal’s
law: if an arbitrary pressure anywhere is applied to theincompressible fluid in a confined container, the pressure isequally transmitted in all directions such that the pressure
anywhere is the same. By rotating the slide screw connectedwith the DC motor (Maxon Amax16 1.2 W), we controlled
the position of the piston made of silicon. If the piston moves,

1

2

Fig. 4.

3

6
4
5
Deformation [mm]

7

8

9

10

Force–deformation diagram

the entire volume of the fluid (gel) changes, which causesa change in fluid pressure. According to Pascal’s law, thefluid pressure corresponds to the contact pressure when thefingertip comes into contact with something.
B. Evaluation of fingertips
We experimentally investigated the variable range of pressure when the fingertips are not contacting anything. The
range was 0∼15 [kPa]. We then performed a compression
test. Fig. 3 shows the experimental setup for the test. We
fixed the fingertip and then pushed and deformed the fingertip surface with a flat plate attached to a force gauge(IMADA DS2-50N) to investigate the relationships of thefluid pressure, applied force, and deformation. Fig. 4 shows
the results. The markers show the experimental results, andthe curves are the regression curves. Assuming that thepresented fingertip is a hemisphere, we applied the model
presented by Xydas and Kao [15].
f = CΔdζ(1)where f is the applied force, Δd is the deformation, and C
and ζ are the parameters. Table I shows the parameters forthe regression curves. The rate of increase suddenly changed

TABLE I
PARAMETERS C AND ζ FOR THE REGRESSION CURVES IN F IG .4
Initialpressure
5 [kPa]10 [kPa]
15 [kPa]

When contactingwith gel layer
C
ζ
0.073
1.59
0.109
1.52
0.151
1.46

Encoder
Control unit

When contactingwith silicon layer
C
ζ
0.255
1.88
0.408
1.64
0.584
1.46

Motor
Parallel crankmechanism
Fingertip
Fingertip
Gripping area

Fig. 5.

around a deformation of 5 [mm]. This may have been due
to contact with the silicon layer. The data were then split
into two parts by making a boundary at the deformation of 5[mm]. The regression was applied to the two parts separately.
Fig. 4 shows that the model represented the relationship very
well. The stiffness can be regarded as the gradient at eachpoint of the regression curves. Thus, the contact stiffness canbe controlled by controlling both the piston of the control
unit and the fingertip deformation by contact. In addition,
if the deformation is large, the silicon layer can be used toobtain a large stiffness and apply a large force. This is usefulfor grasping rigid and relatively heavy objects.

C. Structure of gripper
With the aim of developing a simple structure and simple
control schema, we developed a gripper-type robotic hand.
Note that, with the proposed fingertips, we can construct a
multi-fingered hand. Fig. 5 shows the structure of the gripper.
Each presented fingertip is attached to every gripping area.
Our aim was to realize two grasping modes, as shown in Fig.
6. If the stiffness of the target object is higher than that of
the fingertip, the object penetrates the fingertip (Fig. 6(a));
if it is lower than the fingertip, the object is penetrated (Fig.
6(b)). In Fig. 6(a), the object is enveloped by the fingertips,
while the fingertips are enveloped by the object in Fig. 6(b).
In both cases, if each contact is approximated by a finite
number of point contacts with friction, there were at least
three points per contact area. In total, there were at leastsix contact points with friction. This number is sufficient for
constructing (passive) force closure [16], [17]. Thus, stablegrasp can be achieved.
The main body part has only one degree of freedom for
opening and closing the tip parts (gripping area). Table IIshows the specifications. We used a parallel crank mechanism such that the gripping surface of every fingertip part canalways move parallel to the corresponding opposed surface.
We used the Maxon Amax 32 (15 W) actuator to open andclose the tip parts. We measured the motor angle using an
encoder equipped with the motor and controlled the motor
angle corresponding to the distance between the fingertips.
Control of the closing of the gripper indicates control of
the fingertip deformation by contact with objects. Thus, thecontact stiffness can be controlled by opening and closing
the gripper.

Structure of gripper
ObjectFingertip

Fingertip(a) Harder object

Fig. 6.

Fingertip(b) Softer object

Grasping mode based on object stiffness
TABLE IIS PECIFICATIONS OF GRIPPER

Maximum height of gripper
Maximum width of gripper
Maximum gripping area (length)
WeightMaximum joint velocity
Maximum joint torqueMaximum grasping force

300 [mm]
200 [mm]
90 [mm]
1.0 (weight for motors: 0.4) [kg]75.3 [rpm]1.43 [Nm]11.44 [N]III. I NVESTIGATION OF UNIFORMITY OF CONTACTPRESSURE

In conventional fingertips made of silicon, the contactpressure is maximum at the contact center and decreases inthe direction of the periphery [15]. In contrast, the contactpressure of the proposed fingertips is expected to be uniform.
In order to confirm this characteristic, we investigated thecontact pressure distribution. For comparison, we used two
types of fingertips: gel layer and silicon fingertips (Fig. 7). To
avoid unexpected contact with the silicon layer, the siliconlayer was not included in the gel layer fingertip. The siliconfingertip was a hemisphere made of silicon; the size andshape were fitted to be the same as those of the developed
fingertip.
Fig. 8 shows the experimental setup. We measured thecontact pressure profile using pressure-sensitive paper withPrescale mat (FUJIFILM). In the test, each fingertip pushed
the Prescale mat covered with the pressure-sensitive paper
from above. The pushing force was kept constant andcontrolled by a force gauge. When the pressure-sensitive
paper was pressed, it became red. The intensity of the redcolor corresponded to the magnitude of the applied pressure.
The Prescale mat was constructed from dot-patterned cones;
around the cones, the pressure became concentrated for easy
visualization of the pressure profile. Due to the dot patternof the cones, the obtained red color pattern was also a dot
pattern (4 × 4), as shown in Fig. 9. The other conditions forthis experiment are shown in Table III.

TABLE III
E XPERIMENTAL CONDITIONS FOR INVESTIGATION OF CONTACTPRESSURE PROFILE

Load weight
10 [N](a) Silicon fingertip

Fig. 7.

(b) gel layer fingetip

Used fingertips

Pressuresensitive paper

Fingertip

Temperature
24 [◦ C]

Humidity
64 [%]TABLE IVPARAMETERS FOR REGRESSION CURVED SURFACE
Silicon fingertip
Gel layer fingertip

Force gauge

Loading time
2 [min]α0.14-0.010

β
0.15
0.011

γ
-0.0170.0039

A
2.2
2.2

B
2.7
3.8

M
0.65
0.27

can be generated with small contact pressure.

Prescale mat

IV. G RASPING STRATEGY
Fig. 8.

Investigation of contact pressure profile
X

X
1
Y

2

3

Y

2

3

4

1
2

2
3

3

4

4

Silicon fingertip

Gel layer fingertip

Contact pressure profile for silicon and gel layer fingertips

Pressure[MPa]Fig. 9.

1

4

1

Y

X
Pressure[MPa]Silicon fingertip

YX

Gel layer fingertip

Fig. 10.

Contact pressure profile for silicon and gel layer fingertips

We derived the magnitude of the pressure from the intensity of each dot in the obtained patterns and plotted thepressure profile, as shown in Fig. 10. We also showed theregression curved surface using the following model:
22

p = M − α(x − A) − β(y − B) + γ(x − A)(y − B)(2)where p is the pressure and the values of the parameters
are as shown in Table IV. When comparing the results, thesilicon fingertip showed high pressure in the area aroundthe center, while the gel layer fingertip had an almost uniform pressure profile. With regard to the maximum pressure
value, the maximum pressure for the gel layer fingertip was
small compared with that for the silicon fingertip. Thus, thedeveloped fingertip can be used to grasp objects without
producing high local pressures, and (large) grasping forces

We present strategies for grasping objects using the developed gripper with incompressible fluid-based deformablefingertips; we experimentally investigated the validity of thepresented approach. We considered grasping and picking up
objects on a table using the developed gripper.
Since the contact between the fingertip and an object is nota point but an area, a power grasping style is used [18], [19].
In a power grasp, larger grasping internal forces (contact
forces which do not affect object motion and whose total
sum is zero) mean that a larger external wrench (gravitational
force) can be balanced. In addition, a larger contact areameans that (large) external wrench (gravitational force) can
be balanced in wider directions. However, a large contact
force/pressure can damage the grasped object, and it should
be avoided for delicate grasping. Since the developed fingertip is very elastic, a large contact area can be easily obtained.
Therefore, the key to the success of delicate grasping issetting and controlling the contact pressure (and the corresponding grasping internal (contact) forces). Because fracture
occurs due to not force but stress, contact pressure control
is essential for delicate grasping.
In the developed gripper, directly controllable factors are
the angles of the motor for opening and closing the gripper
and for controlling the piston positions inside the control
unit to control the fingertip fluid pressure: these are denoted
here as qg and qp , respectively. The fluid areas for both
fingertips are connected, and their pressures are controlled
by one control unit. The fluid pressure inside the fingertip
when there is no contact (pf i ) is expressed by:
pf i = pf = pf (qp ).

(3)

pf is the pressure when contact occurs and should be low
to reduce the contact impact. The fluid pressure inside theith fingertip when there is contact (contact pressure; pci ) isrepresented by
pci = pc = pc (qp , qg ).

(4)

If qp is constant, pc (qp , qg ) = pc (qg ) can be regarded tomonotonously increase as the gripper is closed by the controlof qg . We can then take the following simple strategy forcontrol.
1) Let qp be its desired value; for fixing pf , keep qp constant.

Fingertip pressure [kPa]65

initial break

3
2
1
0
0

1

0.5

Fig. 13.

1.5

2Time [s]

2.5

3

3.5

4

Initial break when grasping soft tofu

Grasping of potato chipFingertip pressure
Desired pressure0

1Fig. 12.

2

3Time [s]

4

5

1

6

2
Fig. 14.

Pressure profiles when grasping potato chip2) Close the gripper (by the control of qg ) until pc (qg )reaches its desired value.
3) Move the gripper upward to pick up the target object.
We remark that we can increase pc by increasing qp when
there is contact, but we do not take the way in order to get
relatively larger contact area and make the control strategy
easy. The role of the control of qp is the control of initialpressure (the pressure when there is no contact) such that thegripper can always contact objects with desired stiffness.

Fingertip pressure [kPa]

Fingertip pressure [kPa]1

2
Fig. 11.

8
7
6
5
4
3
2
1
0

break

Fingertip pressure

4

4
3.5
3
2.5
2
1.5
1
0.5
0

Grasping of soft tofu

Fingertip pressure

0

5

10
Time [sec]15

20

Detection of initial break

A. Grasping fragile and brittle object (potato chip)
We investigated the efficiency of the developed gripper
when grasping a potato chip (1.89 [g]) to represent fragile
and brittle objects. When brittle objects are grasped, fracturesuddenly occurs when the stress applied to the object isover a certain value (fracture stress). In order to determine
the fracture stress, we conducted an experiment of closing
the gripper and breaking the potato chip. We set the initial
fluid pressure to pf = 3 [kPa]. Let pcf be the contactpressure (pc ) when fracture occurs. We set the desired contactpressure (pcd ) such that pcd = s(pcf − pf ) + pf . Note thatthe controllable part is pcf − pf ; we then multiplied it by
the safety ratio s and derived pcd . We selected s = 0.5 sothat we could obtain a sufficiently large contact pressure tocompensate for the object weight while avoiding fracture.
We grasped potato chips using the control method. Fig.
11 is a photo of a potato chip being grasped. Fig. 12 shows
the results for the fluid pressure transition inside the fingertip(pc ). These figures show that the pressure can be controlled
to its desired value, and the potato chip was successfully
grasped. We repeated the same experiment five times; inevery case, we succeeded at grasping the potato chip without
breaking it.
In general, it is hard to grasp potato chips with the posture
shown in Fig. 11, and there were individual differencesin shapes and weights. We used the same desired contact

Fig. 15.

Pressure profiles when grasping soft tofu

pressure pcd in all cases. Nonetheless, we succeeded ingrasping the potato chips in every case.
B. Grasping fragile and ductile object (soft tofu)
Fig. 13 shows the result when grasping and breaking soft
tofu. In contrast to grasping brittle objects, fracture does notoccur suddenly but gradually. The important point is aroundthe red-circled area in Fig. 13. A flat line can be seen before
the final fracture. Around the flat area, there is no change inappearance. We call this the initial break. If we can detect
the initial break and control the contact pressure pc such that
pc is lower than the pressure at the initial break, grasping
the soft tofu without knowing the fracture stress/pressurein advance should be possible. Note that the grasping force
corresponding to the contact pressure at the initial break is
considered to be the maximum grasping force that can beapplied without breaking the object. We thus considered thefollowing approach to grasping soft tofu without any advanceknowledge about fracture.
1) Closing the gripper until the initial break (flat area) is
detected.

Fig. 16.

Grasping of heavy object

2) If the initial break is detected, we stop closing thegripper and control the contact pressure (pc ) to its
desired value.
We detected the initial break by investigating whether or not
the following condition held:|pc (t) − pc (t − tb )| ≤ (tb = 0, · · · , T )(5)where t is time, pc (t) is the contact pressure at time t, T =0.1 [s] is the parameter related to how long the flat area
should continue at the initial break, and  is a small positive
constant. We supposed that the initial break should have asufficiently long flat line. If the flat area is short (for example,
see the several flat regions before the initial break in Fig. 15),
we do not regard it as the initial break. Note that breaking
was allowed a few times (2 in this case) to take noise into
consideration. We set the desired contact pressure pcd suchthat pcd = pcI , where pcI is the contact pressure pc at theinitial break.
Fig. 14 shows the photo of soft tofu being grasped. Fig. 15
shows the results for the transition of the contact pressure(pc ). We succeeded at grasping soft tofu without breaking
it and without any advance knowledge about fracture. We
repeated the same experiment five times. We succeeded at
grasping the soft tofu without breaking it four times. The
one failure was due to a failure to detect the initial break.
C. Grasping heavy object
To confirm the effectiveness of the two-layer structure, we
conducted an experiment of grasping heavy objects using thedeveloped gripper. The target object was a bottle of juicewith a weight of 535 [g]. Fig. 16 shows the result, which
demonstrated the effectiveness of the silicon layer and thegrasping success.
V. C ONCLUSION
This paper presented a novel gripper with fingertips constructed from a rubber bag filled with gel, an incompressible
fluid. A silicon layer was also installed inside the gel so that
not only fragile objects (special objects) but also relatively
heavy, normal, and rigid objects can be grasped. Owing to
the incompressibility of the fluid, simply closing the gripper
allowed it to approach the target object with low stiffness;after contact, the contact stiffness increased with fingertip
deformation, and grasping with high stiffness was realized.

The fingertips have uniform contact pressure and can provide
a contact force with a small maximum contact pressure.
We presented strategies for grasping fragile objects withthe gripper. We were able to grasp potato chips (brittle
object) by controlling the contact pressure such that thepressure did not exceed the fracture stress/pressure. The same
strategy was valid for grasping soft tofu (ductile object).
We also demonstrated another method. When grasping soft
tofu, we found that invisible local fracture occurred before
total fracture. Based on this characteristic, we presented astrategy for grasping soft tofu (ductile object) without any
advance knowledge of fracture. Finally, we confirmed theeffectiveness of the silicon layer for grasping normal heavy
objects with the proposed fingertips.
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