This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at
http://dx.doi.org/10.1109/LRA.2017.2662086
IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JANUARY, 2017

1

Variable-grasping-mode underactuated soft
gripper with environmental contact-based
operation
Toshihiro Nishimura1, Student Member, IEEE, Kaori Mizushima1, Yosuke Suzuki2,
Tokuo Tsuji2, Member, IEEE, and Tetsuyou Watanabe2, Member, IEEE
Abstract—A novel robotic gripper with soft surfaces andunderactuated joints was proposed. The soft surface wasfabricated from a deformable rubber bag filled withincompressible fluid and a microgripper inside the fluid. A ratchet
was installed at the underactuated joint so that the joint’s rotation
caused by contact with an environment, such as a supporting
surface, can be preserved, and the actions of scooping andenveloping an object are realized. With one actuator, the gripper
realized three modes i.e., parallel gripper, pinching, andenveloping. The range of graspable objects was wide and included
soft, rigid, deformable, fragile, small (boundary length less than 30mm), large (more than 80 mm long), thin (less than 0.5 mm) andheavy (more than 3 kg) objects.
Index Terms—Grasping, Grippers and Other End-Effectors,
Mechanism Design, Soft Material Robotics, Underactuated
RobotsS

I. INTRODUCTION

OFTNESS on surfaces or joints of robots is a key feature
for realizing versatile and dexterous grasping. Softness can
reduce impact at contact, while providing safe interaction withhumans and objects and allowing adaptation to object shape.
These benefits provide high robustness and the capability to
grasp a wide variety of objects to robotic hands. Based on theconcept of embedding softness on robotic hands, our groupdeveloped a fluid fingertip fabricated from a deformable rubber
bag filled with incompressible fluid [1]–[4]. The additional
benefits of the fluid fingertip because of its incompressibility
are as follows: 1) Contact pressure distribution is uniform. 2)
Surface stiffness automatically increases with contact
deformation (local passive compliance). The primary drawbackof the fluid fingertip is that the weight of graspable objects islow (the maximum graspable weight was less than 0.15 kg).
Therefore, a microgripper-embedded fluid fingertip was
proposed to increase the weight of graspable objects [4]. A
parallel gripper was developed to be used with themicrogripper-embedded fluid fingertip to ensure theManuscript received: September, 10, 2016; Revised December, 5, 2016;
Accepted January, 12, 2017.
This paper was recommended for publication by Editor Han Ding uponevaluation of the Associate Editor and Reviewers’ comments. This work werepartly supported by NEDO Strategic Advancement of Multi-Purpose UltraHuman Robot and Artificial Intelligence Technologies and JSPS KAKENHI
Grant Number 16H04298.

effectiveness of the fingertip. However, we did not consider
real tasks such as picking. As a result, although the gripper
succeeded grasping a wide variety of objects, it was not capableof picking up small or thin objects on a table. The nextchallenge is to develop a robotic hand with microgripperembedded fluid fingertip that can pick up small, thin, fragile,
heavy, deformable, soft, rigid, or large objects on a table.
Contact with an environment is part of the basic functionality
and working principle of a robotic hand. Here, the environment
refers to a supporting surface, other surrounding objects, etc.
Intentional contact with the environment rotates underactuatedjoints and causes the grasping mode to shift, for example, froma parallel gripper mode to an enveloping mode, as illustrated inFig. 1. This function provides several benefits such as reductionin the number of actuators, convenient fabrication, low cost,and convenient control. Note that another mechanism orfunction is required to preserve the modified (enveloping)grasping mode even after the robotic gripper leaves theenvironment.
This paper presents a novel underactuated robotic gripper
that considers the abovementioned factors. The main featuresof this novel gripper are as follows:

The gripper consists of two fingers, each of which has
an underactuated joint and a microgripper-embedded
fluid fingertip.



The range of graspable objects is wide and includes
soft, rigid, deformable, fragile, small, large, thin andheavy objects.

With one actuator, three grasping modes, i.e., parallel
gripper, pinching, and enveloping, are realized byutilizing contact with an environment.



Ratchet mechanisms are installed at underactuatedjoints to preserve the grasping mode even after thegripper leaves the environment.

1
T. Nishimura and K. Mizushima are with the Graduated school of Naturalscience and Technology, Kanazawa University, Kakuma-machi, Kanazawa,
9201192 Japan (e-mail: to.nishimura@stu.kanazawa-u.ac.jp).
2
Y. Suzuki, T. Tsuji, and T. Watanabe are with the Faculty of MechanicalEngineering, Institute of Science and Engineering, Kanazawa University,
Kakuma-machi, Kanazawa, 9201192 Japan (e-mail: te-watanabe@ieee.org).
Digital Object Identifier (DOI): see top of this page.

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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at
http://dx.doi.org/10.1109/LRA.2017.2662086
2IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JANUARY, 2017

Figure 1. Shift from parallel gripper to enveloping mode by theunderactuated soft gripper utilizing contact with a supporting surface.
Scooping is included in the procedure.

The following subsection describes the study that produces
the abovementioned features. After the gripper design ispresented, the details of each grasping mode and control
methodologies to realize the three modes are described. Finally,
experimental validations are presented.
A. Related works
Several robotic hands have been developed [5]. Among these,
we focus on recently developed robotic hands with softness,
which are closely related to this study. Such soft robotic hands
can be divided broadly into two types. The first type has
softness at joints, e.g., snake-like gripper [6], FRH-4 hand [7],SDM hand [8][9], Pisa-IIT Hand [10][11], SoftHand Pro-D [12],ISR-SoftHand [13], UC-SoftHand [14], Velo Gripper [15], andadaptive prismatic gripper with passive rotational joints [16].
The second type has softness at surfaces, e.g., gel fingertips [17],ISR-SoftHand [13], UC-SoftHand [14], Kanazawa hand [18],deep reef sampling gripper [19], and magnetic fields based
gripper [20]. Similar to ISR-SoftHand [13] and UC-SoftHand[14], there are robotic hands that possess both types of softness.
Using pneumatic actuators facilitates combination of softness
types, for example, in ROBO hand [21][22], Starfish-like hand[23], lightweight underactuated pneumatic fingers [24], softelastomer quadrupedal robots [25], safe interaction gripper [26],inflatable rubber pockets based gripper [27], puncture resistant
soft gripper [28], modular soft robotic grippers [29], andjamming gripper [30][31]. Presence of both types of softness
facilitates grasping of several kinds of objects. Here, we
compared the two types of softness. The softness at surfaces has
superior capability of contact impact reduction and adaptation
to object shape, leading to grasping of a wide variety of objects,
particularly soft and fragile objects. On the other hand, softunderactuated joints are connected by rigid links and havesuperior capability of producing large grasping force, leading
to grasping of heavy objects. Thus, coexistence of soft and hardmaterial parts can produce a versatile robotic hand that cangrasp fragile and heavy objects. The development of the
microgripper-embedded fluid fingertips [4] was based on thisconcept.
Even if a robotic hand does not have sufficient DOFs
required for manipulating an object, it can complete themanipulation by making (intentional) contact with theenvironment and modifying the orientation of the object.
Contact with the environment can potentially extend themanipulation capability of robotic hands. Recent studies of
motion planning based on this concept are [32]–[35], where
motion planning is presented in more detail. Note that motion
planning is beyond the scope of this study. A pioneering studyof robot hand design that considers contact with theenvironment is the study of a gripper based on a remote centerof compliance [36]. Contact with the environment was
effectively utilized to realize grasping at the robotic hands
possessing softness [16], [21], [22]. However (based on our
knowledge), there is no study in which contact with theenvironment is intentionally utilized to modify the graspingmode and facilitate the realization of a versatile robotic hand.
This study will focus on this challenge.
II. GRIPPER DESIGN
A. Design requirements
The following are the design requirements of the gripper:
1. The gripper must grasp the following objects on atable: (a) soft and hard fragile objects, (b) deformableobjects, (c) objects with a boundary length less than 30
mm, (d) objects with a maximum width of 80 mm, (e)objects with a thickness less than 0.5 mm, and (f)objects with a weight more than 3 kg.
2.

The gripper must have a single actuator.

3.

Three grasping modes, i.e., (a) parallel gripper, (b)pinching, and (c) enveloping, are realized.

Assuming the final application is a shopping register task, we
considered a task of picking an item of daily use on a table as
the first step. The target items are those that can be found insupermarkets. Their attributes were selected such that they
include the items that the previously developed gripper [4] cangrasp and those that it cannot. Simplicity is also important for
grippers; and we used only one actuator.
As described above, a parallel gripper with microgripperembedded fluid fingertips [4] is a powerful tool for realizing
versatile grasping of fragile and heavy objects. We consideredutilizing the fingertips in the development of the gripper. The
primary issue is that the previous gripper is not capable ofhandling thin or small objects on a table. To pick up and grasp
such objects, the function of scooping is important.
Additionally, the enveloping mode is useful for grasping withsmall forces, as compared to the parallel gripper mode, because
in the enveloping mode, the number of contact is larger and thearea of contact is wide. The third requirement is based on thisanalysis.
B. Structure of microgripper-embedded fluid fingertip
Fig. 2 illustrates a schematic view of the simplified versionof the microgripper-embedded fluid fingertips [4]. The fluid
fingertip is fabricated from a deformable rubber bag filled withincompressible fluid [1]–[3]. A microgripper is installed inside
the fluid. The fingertip can have three “contact modes”, as
illustrated in Fig. 3. Contact mode 1 is adopted if the object
thickness is smaller than the space between the two arms of the
microgripper, while contact mode 2 is adopted if the object
thickness is larger than the space between the two arms. In bothFigure 2. Schematic view of microgripper-embedded fluid fingertip

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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at
http://dx.doi.org/10.1109/LRA.2017.2662086
NISHIMURA et al.: VARIABLE-GRASPING-MODE UNDERACTUATED SOFT GRIPPER WITH ENVIRONMENTAL CONTACT-BASED OPERATION(a) Contact mode 1:
The object is pressed
into the space between
the two arms.

(b) Contact mode 2:
The object is held by
or is in contact with thetwo arms.

(c) Contact mode 3:
The object is in contact
with the fluid part, but
not with the two arms.

3

Figure 5. Schematics of the structure of ratchet and flexible base belts

Figure 3. Three contact modes for the microgripper-embedded fluid fingertip

modes, the two arms prevent deformation of the outer rubber
and provide large grasping forces. If the weight of object is low,
contact with the microgripper is not required. In this case,contact mode 3 is adopted. Contact pressure is uniform andlocal passive compliance is obtained. Therefore, this mode isappropriate for grasping fragile and complex-shaped objects.
Note that the fluid part of the fingertips can reduce impact at
contact, absorb strain energy, and adapt to object shape. Thesebenefits are present in contact modes 1 and 2. In our previous
study [4], the two arms were able to rotate and their tips wereconnected by a flexible and nonexpendable sheet so that an
object pinched by the two arms can converge to the center ofthe sheet and high positioning ability can be obtained. Refer [4]for more details of the structure and functions.
C. Structure of underactuated soft gripper
The structure of the underactuated soft gripper is illustrated
in Figs. 4 and 5. The distance between the fingers is controlled
by a servomotor (Dynamixel XM430-W210) through a screw
feed mechanism. A microgripper-embedded fluid fingertip was
installed at every fingertip. The fingernail was embedded torealize scooping. Each finger incorporated one underactuatedjoint so that the parallel gripper and pinching modes are realized.
Contact with the environment is utilized to shift the grasping
mode. As illustrated in Fig. 4 (a), if the fingertip is pressedagainst a table or another object, the underactuated joint rotates.
To fix the rotation and realize the pinching and enveloping
grasps, a ratchet mechanism and a torsion spring were installedat the underactuated joint, as illustrated in Figs. 4 and 5. The
fixation of the ratchet is released by fully opening the gripper(Fig. 4 (b)). When the gripper is fully opened, the ratchet pawl
makes contact with the releasing pin, and the ratchet is released.
For the enveloping grasp, two separate flexible belts made ofsilicone were installed as illustrated in Fig. 5. When scooping
an object without the belt, there is a possibility of the object
moving up and the grasp becoming unstable (as illustrated in
the image on the top right). Because of the belts’ flexibility,
adaptation to object shape was obtained and a stable enveloping
grasp was realized. The belts were separated such that a tallobject is grasped conveniently in the parallel gripper mode.

(a) Parallel gripper(b) Pinching(c) Enveloping

Figure 6. Three variable grasping modes(a) Contact with the microgripper(b) Contact with the tip
embedded fluid fingertip
Figure 7. Two styles based on the contact area(a-1) Parallel gripper mode; grasp
at the fluid fingertip part(a-2) Parallel gripper mode; grasp
at the tip part(b) Pinching mode(c) Enveloping mode

Figure 8. Allowable size of objects in each grasping mode

Figure 9. Procedure for grasping a thin and rigid object utilizing
hyperextending motion with parallel gripper mode

III. GRASPING MODE
Using the design described in the previous section, the threegrasping modes illustrated in Fig. 6 are realized by utilizing
contact with the environment. Note that these are the primarymodes; however, a combination of two modes or a partiallyrealized mode (for example, only one finger is rotated) can bealso realized. The three grasping modes are described below.
A. Parallel gripper mode; Fig. 6 (a)There are two styles of contact (Fig. 7) depending on thecontact area. If the object is sufficiently tall for it to makecontact with the fluid fingertip area, it is grasped at the fluid
fingertip area. The features of this style are as follows:(a) Shift in grasping mode by contact with an (b) Releasing of pawl byenvironment such as a supporting surface
fully opening the gripper
Figure 4. Schematic view of function of the ratchet installed at anunderactuated joint to maintain grasping mode The functions of the fluid fingertip (described in section II B)
are fully utilized and a wide variety of objects are grasped.

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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at
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IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JANUARY, 20174

 The maximum width of graspable objects is 80 mm (Fig. 8(a-1)).
Otherwise, the object is grasped at the tips. The features ofthis style are as follows: Small and thin objects, whose size is larger than the space
shown in Fig. 8 (a-2), are pinched.
 Because of passive compliance of the finger, the fingertip is
hyperextended (Fig. 9) and the object is pinched.
B. Pinching mode; Fig 6 (b)The features of this mode are as follows: Scooping is realized and small or thin objects on a table are
picked up and grasped.
 Soft and fragile objects are grasped by utilizing the
microgripper-embedded fluid fingertip part.
 The ratchet has backlash; therefore, a control methodology
considering the backlash is required.
 The size of graspable objects is shown in Fig. 8 (b).

picking up an object. The basic strategy is to maintain the
distance between the nails of the fingertips constant when
picking up the object.
We consider the state in which the fingertip initially makes
contact with the supporting surface to be the reference state, andset the coordinate frame as illustrated in Fig. 10 (a). The
nomenclature is presented in Fig. 10.
Considering manufacturing and fabrication errors, theamount of backlash was experimentally evaluated. The gripper
was pressed against the table by controlling the pressing
distance, [mm]. There are three teeth on the ratchet and threelock-switching angles. We examined [mm] three times forevery angle, where the locking switches. Table I presents theresults. Because of manufacturing and fabrication errors, thevalues of [mm] for lock switching for the right and left
fingertips were different. Table II shows the range of [mm]where the gear-lock states of both fingertips are identical. The
range was derived by assuming that gear lock occurs within(Table I).
̅C. Enveloping mode; Fig 6 (c)This mode is adopted when attempting to pick up an object
in the pinching mode and the object makes contact with the basebelt (Fig. 5). The features of this mode are as follows: Scooping is realized.
 Geometrical constraints are applied through multiple andwidely spread contact points, and soft and fragile objects are
grasped with small contact forces.
 Gripper size limits the size of graspable objects (Fig. 8 (c)).
IV. PROCEDURES FOR GRASPING
In this section, we present the procedure for grasping objects
by employing the gripper developed in this study, following thegrasping modes illustrated in Fig. 6. When grasping an object
in the parallel gripper mode and the style illustrated in Fig. 7(a), the gripper is controlled such that the object can be grasped
at the microgripper-embedded fluid fingertip parts. When thestyle illustrated in Fig. 7 (b) is required, contact with a table isutilized (Fig. 9). First, the fingertips make contact with thetable; then, the gripper is closed. If the object makes contact
with the fingernail, the fingertip is hyperextended. For picking
up the object in this state, it is grasped at the area between the
nail and the fluid fingertip. It is necessary to note that it isdifficult to grasp objects at the exact nail tip because of
marginal friction.
If the pinching or enveloping grasping mode is required, anappropriate procedure has to be followed. First, the fingertips
make contact with the supporting surface and rotate their joints.
Then, the gripper is closed, and it scoops a target object. Finally,
the object is picked up. If the object makes contact with the basebelt illustrated in Fig. 5, the enveloping mode is adopted. Whenrotating the fingertips, the rotated joint angle is fixed by theratchet illustrated in Figs. 4 and 5. However, a recess in the gear
causes backlash in the ratchet, and the gripper is opened by thesame amount as that of the backlash during the picking.
Therefore, we regulate the closing distance of the gripper when(a) Reference state where the fingertip
initially makes contact with thesupporting surface(b) State when the gripper
moves downward by

Figure 10. Nomenclature for analysis of backlash at the ratchet
TABLE I.

EXPERIMENTAL RESULTS OF PRESSING DISTANCE Z [MM]
WHERE LOCKING OF THE RATCHET GEAR SWITCHED.

Gear lock
Mean ( ̅ [mm])Standard
deviation ([mm] )TABLE II.

Right fingertip
1st
2nd
3rd
6.43
10.3
15.2

Left fingertip
1st
2nd
3rd
5.93
10.9
14.9

0.18

0.21

0.32

0.05

0.21

0.33

RANGE OF PRESSING DISTANCE Z [MM] WHERE THE GEARLOCK STATES OF BOTH FINGERTIPS ARE IDENTICAL.

Gear-lock state
1st gear lock2nd gear lock3rd gear lockRange of z [mm] where the gear-lock statesof both fingertips are identical
6.61–9.98
11.1–14.6
15.3 –TABLE III.

PRESSING DISTANCE Z [MM] IN THE CASE WHERE ROTATIONDUE TO BACKLASH OF THE RATCHET STOPS WHEN MOVING UP THE GRIPPER.
Initial state of theratchet before thegripper moves
upward
1st gear lock2nd gear lock3rd gear lock

Mean z [mm]Standard deviation
[mm]

4.46
7.87
11.9

0.12
0.16
0.32

Because of backlash of the ratchet, the joint angle returns
when the gripper leaves the supporting surface, and the gripper
opens. There is a possibility of the object falling down becauseof the opening motion. We investigated the return angle of theratchet. Initially, we set [mm] such that it is within any of theranges listed in Table II (both gear-lock states were identical).

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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at
http://dx.doi.org/10.1109/LRA.2017.2662086
NISHIMURA et al.: VARIABLE-GRASPING-MODE UNDERACTUATED SOFT GRIPPER WITH ENVIRONMENTAL CONTACT-BASED OPERATION

Then, we moved the gripper upward and measured [mm]when the return rotation of the joint angle stopped. Table III
presents the result.
By controlling the speeds of lifting and closing , weresolved the issue of opening of the gripper because of backlashof the ratchet when lifting the gripper (in the pinching orenveloping mode). Consider the case in which the gripper
moves downward by , as illustrated in Fig. 10 (b). We obtain(1)
By differentiating (1) with respect to time, we obtain
tan

(2)

where
tan
(2) indicates the process by which the fingertips open
because of lifting of the gripper; it is the relationship between
the speeds of opening of the fingertips ( ) and lifting of thegripper ( ). Let
be the speed of closing the gripper. Then,by controllingsuch thattantan (), the distance between the tips is maintained constantduring lifting of the gripper, irrespective of joint rotation due tobacklash of the ratchet. Note that tan is a function of .
Moreover, in the developed gripper,
44.2 mm,46.5
mm,
14.3 mm, and17.9°.
Fig. 11 illustrates a summary of the control schema forgrasping an object with the pinching or enveloping mode. First,
the lowering distance determined by object size determines thenumber of gear-lock, and [mm] is controlled such that it iswithin any of the ranges listed in Table II (both gear-lock states
are identical).
Next, we select the value of [mm] from Table III according
to the gear-lock states, and set it to be[mm], which is the
distance above which it is necessary to control the closing
distance of the gripper. Then, the gripper moves upward withspeed
tan while being closed at speed, until[mm] becomes equal to[mm]. When [mm] becomesequal to[mm], we stop closing the gripper and move it
upward to the desired state. Here, we set
1.2 mm/s. Fig.
12 provides an overview of the actual application of thisprocedure. The blue line implies that the closing distance wasmaintained constant.

Figure 12. Example of application of the control schema illustrated in Fig. 11(grasping an object with pinching mode)

V. EXPERIMENTAL VALIDATION
To evaluate the capability of the developed robotic gripper,
several experimental tests were conducted.
A. Picking up and grasping an object on a table
We conducted tests of grasping a wide variety of objects ona table, and evaluated the efficacy of each grasping mode. The
closing distance, the grasping mode, and the lowering distancewere predefined manually while the control schema shown in
Fig. 11 was adapted in an automatic manner. When graspingfragile objects such as tofu, the closing distance was manually
determined based on the methods presented in [1]–[3]. The
objects were randomly placed on the table manually, such thatthe center of the objects was around the depicted circle with 5
mm radius. Then, the object position had cumulative deviation
resulting from manual placement and the radius of the circle.
Fig. 13 illustrates the representative examples of the grasping
test. Table IV shows all results and success rates (see also
attachment video for the full results).

(a-1) Parallel gripper mode; grasp at the fluid fingertip part(a-2) Parallel gripper mode; grasp at the tip part(b) Pinching mode(c) Enveloping mode
Figure 11. Control schema for grasping an object with pinching or enveloping
mode

5

Figure 13. Representative results of grasping test

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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
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http://dx.doi.org/10.1109/LRA.2017.2662086
IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JANUARY, 2017

6

TABLE IV. GRASPING TEST RESULTS WITH SUCCESS RATE (RED VALUES CORRESPOND TO THE WIDTH OF OBJECT IN THE OPENING DIRECTION OF GRIPPER
Success
rate

Parallel gripper mode

Pinching mode

Enveloping mode

Dove bar1
Bolt (M3)
TomatoTennis ball2
Dice2
Outlet plug covers1(68 × 187 × 30)
(94 ×
× 35)
( × 16 × 16)(∅ . × 13)(∅70)(∅64.7)
Soft tofu
Washer (M3)
Glass bottle
Thin plate
Cups1
Dog squeaky toy (Duck)1(50 × 97)(∅158)
( × 26 × 30)(∅ × 0.5)(∅ × 135)(72 ×
× 1.6)
Soft tofu
Baseball2
Battery
PET sheet
Bath toy1
Dog squeaky toy (Frog)1(30 × 116)
( × 26 × 30)(∅ )(∅ . × 50.5) (100 ×× 1) ( × 133 × 71)
Spark plug1
Spam can2
Daifuku
Toy model
BananaBall dog toys1(67 × 123)
(96 ×
× 22)
( × 97 × 82)
(∅50)
(60 ×
× 30)
(∅ × 150)
Screwdriver
PET bottle (2L)Business card
Washer (M6)
Onion
Plastic strawberry2(43.8 × 55)( × 13 × 130)
(cap:∅ )(91 ×
× 0.2)(∅11.5 × 1.6)
(70 × ∅55)
Battery
Plier
Hummer
Balloon
Rope2
Packaging bag (large)(250 × 60 × 150)
(8 × 2000)(70 × 200 × 40)(∅14.5 × 50.5)( × 135 × 15)(∅ × 250)
Screwdriver
Racquetball2
Tablet pill box
Glue1
Cord
Packaging bag (small)(47 × 78 × 7)
3/3
(8 × 1000)(50 × 150 × 30)(∅55.3)( × 13 × 130) (29 × 63 × 146)
Paper cup with
Ball point pen
2
Pen
Carrot
Plastic strawberry
Binder clip
Packaged breadliquidrefill(43.8 × 55)(20 × 37 × 9)
(65 × 180 × 30)(∅12 × 139)(∅45 × 180)
(25 × 134)
( × 187 × 30)
Small spray
Toy model
Japanese coin
Chocolate
Small spray
Electric bulb
Dental treats1(210 × 54 × 160)(∅40 × 150)
(60 ×
× 30)
(∅22 × 1)(∅27)(∅40 × 150)
(base:∅35 × 40)
Large Spray
Large book1
Small book1
Safety glasses1
Bottle brush1(113 × 170 × 8)
(115 × 55 × 290) (125 × 198 × )(65 × 50 × 190)(∅ × 150)
Paprika
Color pen set1
Juice box
PET bottle (500mL)(118 × 120 × 20)(70 × 140)
(70 × 70 × 190)(∅55 × 85)
Can of juice
PET sheetSelf-stick notes1(100 × 70 × 0.45)(51 × 116 × 39)(∅66 × 122)
Large SprayTomatoCrayon1(145 × 35 × 126)(∅ × 150)(∅70)
Index cards1
Pringles can2
1/3
( × 128 × 20)(∅ × 250)
1
1
1
Color pens1
Large book
Small book
Bottle brush
0/33(113 × 170 × 8)
(115 × 55 × 290)
(125 × 198 × 12)(118 × 120 × 20)
1: Amazon picking challenge objects,
2: YCB objects,
3: By changing the pose, these objects were grasped with a success rate of 3/3
Pencil cup(Thickness:4)
Board eraser(55 × 132 × 35)
Golf ball2(∅42.7)
Tennis ball1(∅63 × 108 × 190)
Tomato can2(∅66 × 101)
Washer(M6)(∅11.5 × 1.6)
Screwdriver set1
(77 × 137)

B. Feasibility analysis and test
Here, the maximum weight of graspable objects wasinvestigated. Fig. 14 illustrates the model for the analysis.
When adopting the parallel gripper mode, the stopper illustrated
in Fig. 14 (a) is utilized to generate large grasping forces. The
tip area is meant for grasping thin and small objects. Therefore,
the case in which the microgripper-embedded fluid fingertip
part is utilized is the problem. When grasping heavy objects
with the fluid fingertip part, the microgipper is utilized. Then,
we analyzed the most critical case illustrated in Fig. 14 (a),where an external force is exerted at the tip of the lower arm in
the vertical direction. The lower arm is modeled by a cantileverbeam. Let be the allowable load exerted at the tip of thelower arm. Then, is expressed by
6(3)

is the flexural strength of the material (ABS plus
where
plastic (Stratasys) was utilized;= 35 MPa), = 13 mm,= 5 mm, = 13 mm are the dimensions of the arm, andis the section modulus. Then,
150 N (15 kgf). Moreover,it can be observed that the gripper can grasp an object with aweight of more than 3 kg (design requirement 1 (f)) if the motor
has the required power capacity and there is adequate amountof friction.
Next, we consider the allowable load in the pinching andenveloping modes. In these cases, the amount of load that theratchet can withstand is the most important factor. Therefore,
we consider the model illustrated in Fig. 14 (b), assuming thatthe maximum stress occurs at the gear tooth. In this case, theallowable torque at the joint is expressed by
6(4)

12 mm,
1.57 mm,
2 mm are thewheredimensions of the tooth and
8 mm. Therefore,
0.69 Nm. If the operating point of the load is the tip of thefingertip illustrated in Fig. 14 (b), then the allowable load is 22
N (2.2 kgf). If both fingertips are utilized, then the total
allowable load (weight) is 44 N (4.4 kgf). Moreover, it can beobserved that the gripper can grasp an object with a weight of
more than 3 kg in these modes. To confirm the validity of thisanalysis, we conducted FEM analysis, where the derived
0.69 Nm) was applied. Fig. 15 presents
allowable torque (the results for the setting in which the model type was an
isotropic elastic medium, Young’s modulus was 2.4 GPa, andPoisson’s ratio was 0.39. The calculated stress was the highest
at the tooth, and its magnitude was 34 MPa, which is. The result
approximately equal to the flexural strength,
validated the above analysis.
Additionally, a feasibility test was conducted to determine
whether the maximum weight of graspable objects is more than

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This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at
http://dx.doi.org/10.1109/LRA.2017.2662086
NISHIMURA et al.: VARIABLE-GRASPING-MODE UNDERACTUATED SOFT GRIPPER WITH ENVIRONMENTAL CONTACT-BASED OPERATION

3 kg in every mode. We investigated the parallel gripper modeand the case in which the ratchet gear at the joint was locked
once. The gripper grasped a thin plate (universal substrate), andthe plate was pulled by a force gauge (IMADA DS2-500N)
until the magnitude of the load measured by the force gauge
exceeded 30 N (3 kgf). Fig. 16 presents the results where thegripper restrained the applied load in both cases. Moreover, we
can conclude that the gripper is capable of grasping an object
whose weight is equal to or less than 3 kg with any mode
illustrated in Fig. 6.

(a) Parallel gripper mode(b) Pinching or enveloping mode

Figure 14. Models for feasibility analysis

7balloon, a PET bottle, and a juice box, were grasped byadopting the parallel gripper mode. A representative example
of small objects with a boundary length of less than 30 mm is a
M3 washer (width 7 mm). The parallel gripper mode could not
be adopted because of the space under the fluid fingertips, as
illustrated in Fig. 8 (a-2). A width of 8 mm is required to grasp
an object with the parallel gripper mode. For such an object, thescooping function is effective, and the pinching mode wasselected. Fig. 8 illustrates the allowable size of objects for every
grasping mode, namely, the graspable size of the developed
gripper. This is the primary limitation, and the grasping failed
frequently when the object size was close to the size limitation(see Table IV). The other limitations are that the grasping modecannot change on a supporting surface where the fingernails
cannot slide (another method should be adapted), or we cannot
obtain a sufficiently large reaction force (2.5 N) to rotate thepassive joints. Fig. 17 presents the recommended procedure forselecting the grasping mode. This is a recommendation because,
for example, it is possible that an object (such as the tomato
illustrated in Fig. 13 (c)) that can be grasped with theenveloping mode, can be also grasped with the parallel gripper
mode. The enveloping mode can reduce grasping force, and werecommend this mode.

Figure 15. FEM analysis to determine the point where maximum stress occurs
at the ratchet and the value of the maximum stress

Figure 17. Recommended procedure for selecting grasping mode
Figure 16. Results for feasibility test to evaluate whether the maximum
weight of graspable objects is more than 3 kg

C. Discussion
As observed from Figs. 1, 9, 12, and 13, the developed
gripper is capable of grasping a wide variety of objects. First,
we determine whether the design requirements are satisfied. We
observe that the design requirements 1(f), 2, and 3 (section II.A)
were satisfied. Then, the design requirements 1 (a)–(e) areevaluated. Examples of soft and fragile objects that were
grasped are a paper cup with liquid, soft tofu, and daifuku(Japanese sweet). The fluid fingertips were effective forgrasping these objects. Examples of hard and fragile objects
that were grasped are a toy model, a glass bottle, an electric bulb,
and a thin plate. If the object has complex shape, the fluid
fingertip adapts to the shape. If the object is thin, the functionsof scooping and hyperextending are carried out effectively.
Next, deformable objects, such as packaging bags and cords,
were grasped by adopting the enveloping mode. This mode canreduce the required force for grasping. Therefore, the mode
functioned effectively while grasping deformable objects. Thin
(thickness less than 0.5 mm) and deformable objects, such as abusiness card made of paper and a PET sheet, were grasped at
the tips. Either the scooping or the hyperextending function was
utilized. Long objects of a length more than 80 mm, such as a

Figure 18. Examples of manipulation by the developed gripper, based on
manual motion planning (Inserting a thin and fragile object into a paper box,
utilizing desk edge)

It is necessary to note that a combination of the graspingmodes can be adopted. For example, a juice box (the second
column of the second row of Fig. 13 (a-1)) was grasped with a
combination of the parallel gripper and enveloping modes. If
motion planning and environments other than a flat area of atable are utilized, the range of the grasping modes can beextended. Fig. 18 illustrates an example of such combinations;however, manual planning was adopted. The examples
illustrate the possibility of application of the developed gripper
in dexterous manipulation.
VI. CONCLUSION
This paper presented a novel robotic gripper that can change
grasping modes by utilizing contact with an environment. The

Copyright (c) 2017 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.

This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available at
http://dx.doi.org/10.1109/LRA.2017.2662086
IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JANUARY, 20178changeable grasping modes are the parallel gripper, pinching,and enveloping modes. Microgripper-embedded fluid
fingertips [4] were used to realize versatile grasping. The
fingertip has a passive joint with a ratchet and a torsion spring.
The ratchet has the function of fixing the joint angle, which is
passively rotated through contact with the environment. The
fixation of the ratchet is released by fully opening the gripper.
By incorporating these mechanisms, the developed gripper
functions with one actuator and has passive compliance at thesurface and the joint. Utilizing the three grasping modes, thegripper realized the action of picking up a wide variety ofobjects on a table. The range of graspable objects is wide andincludes soft, rigid, deformable, fragile, small (boundary length
less than 30 mm), large (more than 80 mm long), thin (less than
0.5 mm), and heavy (more than 3 kg) objects. A few of our
future works focus on grasping and motion planning based on
sensor data or optimal design of size and material based on
actual tasks. Identifying and automizing manually determinedfactors are the keys for automatic grasping and manipulation.
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