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.2020.2970630

IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JANUARY, 2020

1

Sheet-based gripper featuring passive pull-infunctionality for bin picking and for picking up
thin flexible objects
Kota Morino, Shiro Kikuchi, Shinichi Chikagawa, Masakazu Izumi,
and Tetsuyou Watanabe, Member, IEEE

Abstract— This paper presents a novel versatile parallel
gripper equipped with a sheet-embedded belt that enables passivepull-in operations. The belt is wrapped around each fingertip of
the gripper, and it passively moves with the opening and closing
motions of the gripper. The fingertip is made of plastic, and it isdeflected when a grasping force is applied to the fingertip. This
deflection enables the belt to roll up with the closing motion of thegripper, thereby passively pulling the grasped workpiece towards
the gripper. This pull-in mechanism is effective in performing
bin-picking tasks as well as in picking up thin, flexible objects one
at a time. The softness of the sheet-embedded belt surface
provides high adaptability to the shape of the object; therefore,
many kinds of workpieces can be picked up. The effectiveness ofthe proposed gripper is experimentally demonstrated in thisstudy.
Index Terms— Gripper, soft robotics, grasping, bin picking,flexible objects.

G

I. INTRODUCTION

RIPPERS are widely used as end effectors in industrial
robots and are commonly specialized to operate on asingle target or perform a single task. However, the demand forhigh-mix–low-volume production has been increasing recently[1]. If a gripper is specialized for a single target, it must bechanged based on the task at hand. Multiple grippers have to beprepared, and the time required to change the gripper reduces
the efficiency of work. This can increase the total cost incurred.
In this context, the development of versatile grippers that canhandle many kinds of workpieces is required.
Bin picking is among the manufacturing operations
considered difficult to automate [1]. This is because forgrasping, stacked workpieces are unstable in comparison with
Manuscript received: September 10, 2019; Accepted: January 10, 2020;
Date of current version December 12, 2019. This letter was recommended for
publication by Associate Editor and Editor J. Yi upon evaluation of thereviewers’ comments.
(Corresponding author: Tetsuyou Watanabe)
K. Morino is with the Graduated school of Frontier Science Initiative,
Kanazawa University.
T. Watanabe is with the Faculty of Frontier Engineering, Institute of
Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa,
9201192 Japan (e-mail: te-watanabe@ieee.org).
This letter has supplementary downloadable material available at
http://ieeexplore.ieee.org, provided by the authors.
Digital Object Identifier (DOI): see top of this page.

the workpieces that lay flat on the table. When grasping a target
workpiece from a stack of workpieces using a parallel gripper,
the workpieces surrounding the target interfere with thefingertip of the gripper; thus, the fingertip often fails to makecontact with the target workpiece at an appropriate position topick it up. Therefore, sophisticated strategies are required to
identify objects that can be picked up [2]–[7]. However, thissoftware-based solution has limitations; sometimes, the system
may fail to identify an object that can be picked up. Aninterference indicates that the contact area is limited and that apull-in operation (Fig. 1) is required. If the target workpiece ispulled towards the gripper, it can be grasped at an appropriate
position. A supporting surface such as a table cannot be usedduring a pull-in operation because bin picking is conducted in
an environment where the targets are stacked in bulk. The
easiest method of realizing the pull-in operation for bin picking
involves the use of suction grippers because the suction
operation corresponds to the pull-in operation. Only the suction
area is required for picking up an object; thus, suction grippers
can work even if the contact area is limited. However, forsuction, the surface of the target is required to be smooth. In
addition, if the target workpiece has a hole in it, the sufficientpressure required for suction cannot be applied to the target.
Hence, the types of workpieces for which suction can be usedby the gripper are limited. When suction is used on small or thin
flexible objects in an environment where the objects are stacked
in a bulk, the gripper can inadvertently affect multiple objects.
Multiple-object picking is not desirable during bin picking,which involves the picking up of objects one at a time. Flexible
objects can be damaged if the pressure is too high and thegripper is activated unexpectedly. A suction gripper is notsuitable in such cases. If there is a parallel gripper that cangrasp a target workpiece even if the contact area is small, binpicking and the corresponding grasp planning shall become
easier.

Workpiece is pulled toward the gripper

Fig. 1

Pull-in operation with the gripper developed in this study

Copyright (c) 2020 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.2020.2970630

IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JANUARY, 2020

Considering these problems, this study proposes a novel
versatile parallel gripper, equipped with a sheet-embedded belt,that enables the pull-in operation and effectively performs bin
picking. The features of the proposed gripper are summarized
as follows:
Passive workpiece pull-in mechanism: The fingertips ofthe proposed gripper are wrapped with a sheet-embedded beltthat moves passively with the opening and closing motions of
the grippers. The fingertips are made of plastic and deflect
when a grasping force is applied. This deflection enables the
belt to be rolled up with the closing motion of the gripper; thus,
the grasped workpiece is passively pulled towards the gripper.
Owing to this mechanism, the workpiece is pulled in with acontact area and a grasping position that are suitable for stablegripping; this is ensured even if only a part of the workpiece canbe reached during bin picking.
Picking of flexible thin objects: The passive workpiece
pull-in mechanism allows flexible thin objects such as paper ora cloth stacked in a bulk, to be picked up one at a time. This ispossible when the top of the object comes in contact with theedges of the fingertips and the former is pulled towards thegripper by the pull-in operation.
Picking up workpieces of various shapes: The softness ofthe sheet-embedded belt provides high adaptability to the shapeof the workpiece; thus, workpieces with different kinds ofshapes can be picked up.
Reduction of the interference from surrounding
workpieces: By adopting a parallel link mechanism for the
opening and closing operations, the fingers can be inserted
diagonally into the gap between a target and the surrounding
workpieces. Therefore, the grasping operation can be
performed with minimal obstruction due to the surrounding
workpieces or obstacles.
A. Related works
Many types of grippers have been developed; these include
prismatically shaped grippers [8], grippers with a food shaping
function [9], human-synergy-based underactuated robotic
hands [10], jamming grippers [11], grippers withvariable-friction surfaces [12], modular open-source robotic
hands [13], and suction-enabled robotic hands [14]. Recently,
soft materials have been utilized to improve the graspingstability by using elastic deformation to adapt to the shape ofthe target object, as surveyed in [15], [16]. The examples
include a gripper with a snake-like function [17], a robot handwith a squid- or starfish-like function [18], deformablefingertips filled with fluid [19], and an underactuated gripper
with deformable fingertips and ratchet mechanism at the joints[20]. The advantage of soft touch was utilized for grasping softand fragile objects [21], whereas the drawback of low load
capacity was improved by introducing rigid components into
the inside of the soft bodies [22]. Some grippers use a silicone
sheet in the area that makes contact with the workpiece. Furuta
et al. [23] developed a gripper with a high gripping power andversatility by attaching a silicone sheet to a two-link finger witha built-in torsion spring and passively changing the shape of the
sheet. However, a pull-in operation is not possible, and thesystem is wide, which is not ideal for operation in a bulk

environment. Grippers, with drivable belts on their fingertips,
have been developed to enhance the dexterity of in-handmanipulation [24]–[26]. However, the belts used in this study
are rigid, and adapting to the shape of the target object using
soft materials was not considered. Tincani et al. [24] presented
an underactuated robot hand with an active surface anddemonstrated how the active surface can enhance the dexterity.
Ma et al. [25] attached a drivable belt to one finger, whereas amulti-DOF (degree of freedom) finger was utilized as the otherfinger to perform primitive in-hand manipulations with asupporting surface (a table). The pull-in operation was included
as one of the primitives, and its importance was demonstrated.
Kakogawa et al. [26] developed a gripper that could passively
switch between grasping and pull-in operations with a single
actuator using a differential mechanism. The gripper could
grasp thin flexible objects, but they were required to be pushed
against a supporting surface, such as a table, with a force large
enough to enable the pull-in operation. Therefore, it is difficult
to use this approach for bin picking. Different from thesebelt-based structures, fin-ray structures [27] can perform thepull-in operation through the finger deflection activated by thecontact with objects on the middle area in the finger. However,the structure is not suitable for bin picking because bin picking
requires the grasping operation with small contact areas (onlyutilizing fingertips), whereas the activation of deflection
requires contact with the middle area of the finger. The
underactuated gripper [20] can also perform the pull-in
operation; however, a contact with the supporting surface isrequired to enable the pull-in. Furthermore, owing to its large
size, it cannot operate in a narrow space. Hence, the gripper isalso not suitable for bin picking. A detailed comparison ispresented in Table I. Considering these issues, this study
presents a passively moving surface that enables the pull-in
operation that can be used for bin picking. In addition, grasping
thin flexible objects without the help of a supporting surface isdemonstrated.
The passive pull-in mechanism has several advantages over
the active pull-in mechanism. The number of actuators is
minimized, and thus, the cost and size of the gripper are also
reduced. Consequently, the structure of the gripper is simplified,
and the fabrication is facilitated. The payload for a robotic arm
and the effort required to control the gripper are also reduced.
To the best of our knowledge, no study has developed a gripper
mechanism for bin-picking yet. Therefore, this study is aimed
at achieving this target.
Servo motor #1

Servo motor #2

Parallel link
Close

Belt end holder

Open

Constant load spring
X
Z
Y

Fig. 2FingerBelt + Silicone sheet
CAD model of the developed gripper

Copyright (c) 2020 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.

203 mm2

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.2020.2970630

MORINO et al.: SHEET-BASED GRIPPER FEATURING PASSIVE PULL-IN FUNCTIONALITY

3

Silicone sheet (Smooth-on Dragon Skin 30)

QbondedFig. 3118.3 mm

Flat belt (MISUMI HBLTWH10)

Structure of the sheet-embedded belt

II. SHEET-BASED GRIPPER WITH PASSIVE PULL-IN FUNCTION
A. Functional requirements
1. The gripper can perform a pull-in operation that
passively works without the help of a supporting
surface.
2. The finger is thin enough to be inserted in the gap
between the target and the surrounding objects.
3. Differently shaped workpieces (with a width of 90 mm
or less) and thin flexible objects can be grasped.
4. There is a minimal number of actuators.
As mentioned above, a pull-in operation that can be
performed without a supporting surface is important for bin
picking. In bin picking, the target and the surrounding
workpieces can interfere with each other; thus, the surrounding
pieces must be pushed aside when picking up objects. Inserting
the finger in a gap around the target is necessary. Here, we
focus on relatively small parts and enhance the versatility of bin
picking by setting a requirement (3). For a low cost, convenient
control and fabrication, and further applications, the structureof the gripper must be simple, and the number of actuators usedmust be minimized. Here, we assume that the actuators shall be
used only for the opening and closing motions.
B. Design of the gripper
Fig. 2 illustrates the CAD model of the developed gripper.
To reduce the transmission parts, two servo motors (Dynamixel
XM430-W350-R) that operate synchronously were installed toperform the opening and closing motions. The parallel linkmechanism was also adopted to ensure that the pull-in
operation is passively driven by these motions. The two fingersof the gripper are fixed to the mechanism; therefore, the fingers
diagonally approach the target and the surrounding workpieces.
For the pull-in operation, the sheet-embedded belt structure is
attached to the gripper so that the former can be wrappedaround the latter. The sheet-embedded belt establishes a contact
surface with the workpieces. Fig 3 illustrates the
sheet-embedded belt structure. It is constructed by bonding thesilicone sheet (Smooth-on Dragon Skin 30, thickness: 1 mm,
width: 10 mm) on a non-stretchable belt (Misumi
HBLTWH10). One end of the sheet-embedded belt was
attached to the belt end holder, whereas the other wasconnected to the constant load spring (Accurate CR-5: 5.88 N,
width: 8 mm) to control the belt tension constant irrespective of
the width of the gripper opening. The belt end holder was fixed
to the base of the gripper, whereas the constant load spring was
fixed to the joint. The guides were installed at the top andbottom of the finger (corresponding to the tip) so that the
sheet-embedded belt could move along the finger without
protruding from the finger area. The links and fingers were 3D
printed (Markforged Mark Two).

LclosedLinitialR

Close

Lpull-in= Linitial - Lclosed

X

Y

P
Fig. 4Pull-in amount based on the closing motion of the gripper—This is
the foundation of the principle of the passive pull-in function.
X

O’

O
l1

θ

Φ

l2
L R
l7 A
l5
l6 l3 B
S Q

l4
P

Fig. 5Y
F
W

Nomenclature for the gripper

III. PASSIVE PULL-IN MECHANISM
A. Design
Fig. 4 demonstrates the principle of the pull-in mechanism.
Let P, Q, R, and S be the tip position of the finger, the point of
the belt end holder, the point of the top of the finger, and theposition of the constant load spring, respectively. The length
QR increases with the closing motion. The sheet-embedded belt
is pulled upwards relative to the finger by the change in length
𝐿𝑝𝑢𝑙𝑙−𝑖𝑛 ; thus, the belt is pulled in towards the base of thegripper. In this case, the constant load spring stretches by avalue 𝐿𝑝𝑢𝑙𝑙−𝑖𝑛 . In addition to this mechanism, fingertip
deflection plays an important role in realizing the pull-in
operation. If all the components of the gripper are rigid, the pull
in of the sheet-embedded belt stops when a rigid object comesin contact with the finger at the fingertip (point P). In contrast,
if the finger is elastic, the fingertip is deflected according to theforce exerted at the tip. Then, the sheet-embedded belt is pulled
in by an amount corresponding to the amount of the deflection.
In this study, by utilizing a plastic material to construct thegripper components, the proposed gripper performed fingertip
deflection and passive pull-in operation. In the sections that
follow, we shall discuss the extent to which a rigid object canbe pulled in.
B. Kinematics and statics
The basic formula for analyzing the pull-in amount is presented
here. The gripper structure is symmetric; only the left finger isconsidered here. Fig. 5 presents the nomenclature for thegripper. We considered a planar space where the origin O is
located at the driving joint of motor #1. Let 𝜃 be the rotational
angle of the motor and 𝑙𝑖 (i = 1, ⋯ 4) be the lengths of sections
OO’, OA, AB, and BP, respectively. The fingertip position(𝑃𝑥 𝑃𝑦 )T is given by𝑃𝑥
𝑙 cos 𝜃 − 𝑙3
[𝑃 ] = [ 2]𝑙2 sin𝜃 + 𝑙4𝑦Copyright (c) 2020 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.2020.2970630

4IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JANUARY, 2020

If 𝑙1 is the distance between the driving joints of the motors, thegripper opening width 𝑊 is obtained from (1) as follows:()

𝑊 = 2𝑃𝑥 + 𝑙1 = 2(𝑙2 cos 𝜃 − 𝑙3 ) + 𝑙1

If we solve (2) with respect to 𝜃 under the assumption that 0 ≤𝜃 ≤ 𝜋, we obtain the following equation:
𝑊 − 𝑙1 + 2𝑙3()𝜃 = cos−1 ()
2𝑙2
By differentiating (1) with respect to 𝜃, the result is as follows:
Δ𝑃𝑥−𝑙 sin 𝜃()[Δ𝑃 ] = [ 2] Δ𝜃
𝑙2 cos 𝜃𝑦From the principle of virtual work and (4), we obtain thefollowing:()𝜏 = −𝐹𝑥 𝑙2 sin 𝜃 + 𝐹𝑦 𝑙2 cos 𝜃

where 𝐹𝑥 and 𝐹𝑦 denote the forces applied to the fingertip (P) in
the x and y directions, respectively, and 𝜏 denotes thecorresponding motor torque. Rewriting (5), we obtain𝐹𝑥− sin 𝜃 𝜏[𝐹 ] = []()cos 𝜃 𝑙2𝑦Similarly, the position of the point R, (𝑅𝑥 𝑅𝑦 )T , is given by𝑅𝑥
𝑙 cos 𝜃 − 𝑙3
[𝑅 ] = [ 2]𝑙2 sin𝜃 − 𝑙5𝑦()

where 𝑙5 denotes the distance between B and R. The joint
torque corresponding to the force applied to R in the x and y
directions, 𝐹𝑅𝑥 and 𝐹𝑅𝑦 , respectively, is as follows:()𝜏𝑅 = −𝐹𝑅𝑥 𝑙2 sin 𝜃 + 𝐹𝑅𝑦 𝑙2 cos 𝜃

x

()

y
L

Q

O
θg

x

y
L

y
L

W0

W0 – 2δ

Each finger is deflected by δ

No deflection

Fig. 6Force gauge(IMADA, ZTS-50N)

Load
-.--N

Finger

Two-Placepinned Support

Deflection effectP

Automatic
positioning stage(IMADA, MX2-500N)
Y
X

Fig. 7
Experimental setup forinvestigating the relationship between
the load and deflection of a gripper
mechanism

10
9
8
7
6
5
4
3
2
1
0

ONYX
PLA
ONYX+Carbon fiber
ONYX (FEM analysis)
Approximation (ONYX)

δ [mm]

P

where 𝑄x and 𝑄y denote the x and y coordinates of point Q. RQ
changes with the opening and closing motions. Therefore, thepull-in operation is activated by the change in distance RQ.
Here, we focus on RQ to derive the pull in amount. From (3),distance RQ can be obtained as a function of gripper width 𝑊.
𝑅𝑄 =√(𝑙2 cos 𝜃𝑛 − 𝑙3 − 𝑄x )2 + (𝑙2 sin 𝜃𝑛 − 𝑙5 − 𝑄𝑦 )

2

0

5

10

15
Fx [N]()𝜃𝑛 = cos−1 ((𝑊𝑜 − 𝑙1 + 2𝑙3 )/2𝑙2 )Here, we consider the effect of deflection in the finger to derive
the change in the sheet-embeeded belt distance RQ, as shown in
Fig. 6. We consider a case wherein the width of the grasped(rigid) object is 𝑊𝑜 and the deflection caused by the fingertip
load is 𝛿. If the gripper width is set to 𝑊𝑜 − 2𝛿, the practicalgripper width reaches 𝑊𝑜 . In this case, distance RQ
corresponds to the RQ at the controlled state in which thegripper width is 𝑊𝑜 − 2𝛿 and no load is applied to the fingers.
The distance can be expressed as follows:
𝑅𝑄 =
2

()

Hence, from (12) and (13), the pull-in length is given by
𝐿𝑝𝑢𝑙𝑙−𝑖𝑛 =22√(𝑙2 cos 𝜃𝑛 − 𝑙3 − 𝑄x )2 + (𝑙2 sin 𝜃𝑛 − 𝑙5 − 𝑄𝑦 )2√(𝑙2 cos 𝜃𝑔 − 𝑙3 − 𝑄x ) + (𝑙2 sin 𝜃𝑔 − 𝑙5 − 𝑄𝑦 )()𝜃𝑔 = cos −1 ((𝑊𝑜 − 2𝛿 − 𝑙1 + 2𝑙3 )/2𝑙2 ),𝜃𝑛 = cos−1 ((𝑊𝑜 − 𝑙1 + 2𝑙3 )/2𝑙2 )

Same

W0

Thus, these distances are constant even when the gripper isopen or closed. However, from (7), the distance RQ is given by
𝑅𝑄 =
()
2
√(𝑙2 cos 𝜃 − 𝑙3 − 𝑄x )2 + (𝑙2 sin 𝜃 − 𝑙5 − 𝑄𝑦 )

−

O
θg

R

A

()

𝜃𝑔 = cos −1 ((𝑊𝑜 − 2𝛿 − 𝑙1 + 2𝑙3 )/2𝑙2 )where 𝑙6 and 𝑙7 denote the distances between A and S along thex and y directions, respectively.
O𝑆𝑃√(𝑙 + 𝑙6 )2 + (𝑙4 + 𝑙7 )2
]=[ 3
]
𝑃𝑅
𝑙4 + 𝑙52𝑆𝑥
𝑙 cos 𝜃 + 𝑙6
[𝑆 ] = [ 2]𝑙2 sin𝜃 − 𝑙7
𝑦

θn

[

√(𝑙2 cos 𝜃𝑔 − 𝑙3 − 𝑄x ) + (𝑙2 sin 𝜃𝑔 − 𝑙5 − 𝑄𝑦 )The position of point S is given byx

C. Pull-in length
From (1), (7), and (9), the distances SP and PR can beobtained as follows:

20

25

30

Fig. 8Relationship between theload (𝐹𝑥 ) and deflection (𝛿)D. Relationship between the deflection and load at thefingertip
The relationship between the fingertip load and deflection 𝛿
is difficult to derive analytically. In addition, the material usedaffects the relationship; therefore, it must be selected carefully
to obtain the desired pull-in operation. Here, we derive therelationship experimentally and select the material mostsuitable for the pull-in operation. Fig. 7 illustrates theexperimental setup for the investigation. We prepared ahand-made jig that fixed the finger with the two pins in thesame way as the finger is fixed to the parallel link of the gripper.
The jig was attached to the base of the automatic positioning
stage (Imada, MX2-500N). A force gauge (Imada, ZTS-50N)
was attached to the driving part of the positioning stage. The
fingertip was pressed by the force gauge at a constant speed of

Copyright (c) 2020 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.2020.2970630

MORINO et al.: SHEET-BASED GRIPPER FEATURING PASSIVE PULL-IN FUNCTIONALITY()

where 𝐹x denotes the fingertip load and corresponds to 𝐹x in(5). The coefficient of determination was R2 = 0.9996 ,indicating that (15) approximates the relationship effectively.
FEM analysis was also conducted to study the relationshipbetween 𝐹x and 𝛿 when utilizing ONYX, based on the material
properties [28]. Fig. 11 includes the calculation results. The
tendency of the profile and the values were similar to those of
the experimental results, which validated the experimental
analysis.
E. Deviation of the pull-in amount
Based on the above analyses, we derive the pull-in amount.
From (14) and (15), the pull-in amount can be derived if thewidth of the object and the fingertip load are given. Forcesensors were not embedded in the finger; then, we estimated thefingertip load from the motor torque. It is possible to installsensors such as force sensors at the fingers for estimating thepull-in amount and facilitating bin picking. However, thesensors require wiring that can cause the disconnection of wires
owing to the opening and closing motions. Therefore, herein,
we do not consider installing any sensors on the fingers tofacilitate the maintenance and enhance durability. The motor
torque was set to 2 Nm by assuming that we controlled thecurrent value of the motor to the value corresponding to 2 Nm.
The constant load spring applies force on the fingertip.
Therefore, the effect of the constant load spring must be
included. From the geometrical relationship between points S,
P, and R, the applied forces in directions SP and PR arecounterbalanced. Additionally, these forces are applied fromthe initial state. Thus, we do not have to consider the deflections
by these forces; we only have to consider the force applied on
the finger at R in direction RQ. From (11), the force is given as
follows:(𝑙2 cos 𝜃𝑔 − 𝑙3 − 𝑄x )/𝑅𝑄
𝐹𝑅𝑐𝑥
[𝐹 ] = [] 𝐹𝑐
𝑅𝑐𝑦(𝑙2 sin 𝜃𝑔 − 𝑙5 − 𝑄𝑦 )/𝑅𝑄

()

where 𝐹𝑐 denotes the constant load of the spring. Then, from (8),the corresponding motor torque 𝜏𝑅𝐶 can be derived as follows:
𝜏𝑅𝐶 =
()
(𝐹𝑅𝐶𝑥 (𝑙3 + 𝑄x ) sin 𝜃𝑔 − 𝐹𝑅𝐶𝑦 (𝑙5 + 𝑄𝑦 ) cos 𝜃𝑔 )𝑙2 𝐹𝑐 /𝑅𝑄
We estimated that the motor torque corresponding to thegrasping force was 2 − 𝜏𝑅𝐶 . From (6), the corresponding 𝐹𝑥 isgiven by
𝐹𝑥 = −(2 − 𝜏𝑅𝐶 ) sin 𝜃 /𝑙2()From (13), (15), and (18), 𝜃g corresponding to 𝛿 can be derived
by solving the following equation with respect to 𝜃g :

5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0

Experimental data
Calculated values
0

Fig. 91020 30 40 50 60 70
Width of work piece [mm]8090Calculation and experimental results of the width of the grasped
workpiece vs the pull-in amountxyz automatic
positioning stage
X

Z

Fig. 10Y

Table

Gripper

Workpiece

10 mm

𝛿 = 0.0038𝐹𝑥2 − 0.191𝐹𝑥

𝑙2 cos 𝜃𝑔 + 0.0038(2 − 𝜏𝑅𝐶 )2 sin2 𝜃 /𝑙22
()
+0.191(2 − 𝜏𝑅𝐶 ) sin 𝜃 /𝑙2 = 𝑊𝑜 /2 − 𝑙1 /2 + 𝑙3
Given 𝑊𝑜 , by substituting 𝜃g (obtained by solving (19)) in (14),the pull-in amount can be derived. The results of the calculation
are presented in Fig. 9.
We also investigated the actual pull-in amount using theexperiment illustrated in Fig. 10. The workpieces we prepared
were 3D-printed rectangles (material: PLA) with dimensions
𝑊 × 20 (D) × 10 (H) mm where 𝑊 was 30, 40, ⋯, 90 mm;
the width was prepared in intervals of 10 mm. We measured thepull-in amount when each workpiece was picked up with thedeveloped gripper. The motor torque was maintained at 2 Nmby controlling the corresponding current value of the motor.
The experiment was conducted five times per workpiece. Fig. 9
summarizes the experimental and calculation results obtained.
The calculation results nearly agreed with the experimental
results. The estimation error of the load calculated indirectlyfrom the motor current and the softness of the sheet surface areconsidered the reasons behind the small difference between thecalculation and experimental results. It was theoretically andexperimentally shown that the finger deflections enable thepull-in operation.
Pull-in amount [mm]

10 mm/min. We prepared three fingers made from PLA,
ONYX, and ONYX+carbon fiber, respectively. Fig. 11
presents the results. As described above, a large deflection
offers a large pull-in length. In addition, an inflection in therelationship can indicate a change in the material property, andit may result in a fracture of the finger [21]. Thus ONYX wasselected. If ONYX is selected, the relationship between the loadand deflection is approximated as follows:5

Direction of
finger approach

Experimental setup to investigate the pull-in amount

IV. EXPRTIMENTAL PERFORMANCE EVALUATION
We conducted several experiments to evaluate theperformance of the developed gripper for bin picking.
A. Pull-in function
We investigated the efficacy of the pull-in mechanism
provided by the sheet-embedded belt by analyzing the amountof contact required for pick up. For comparison, we prepared
the gripper without the sheet-embedded belt (hereafter,“nominal gripper”). Its grasping structure was the same as thatof the gripper developed, and the sheet on the surface of the
sheet-embedded belt was bonded to the contact surface of thenominal gripper. The experimental setup was the same as theone used to investigate the pull-in amount (see Fig. 10). The
workpiece to be picked up was a 3D-printed rectangle(material: PLA) with a size of 30 (W) × 20 (D) × 10 (H) mm.
The experimental procedure is summarized in Fig. 11. First, theCopyright (c) 2020 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.2020.2970630

6IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JANUARY, 2020

gripper was maintained at the point where the centers of thegripper palms coincided with the geometrical center of theworkpiece and the fingertip height corresponded to the heightof the upper surface of the workpiece; the fingertip did notcome in contact with the object at all. We set the point of therequired contact amount as zero, e.g., 𝑑𝑦 = 0 mm. Second, thegripper was lowered at 0.5 mm intervals, and at each step, weinvestigated whether the workpiece could be picked up 10
times. We evaluated the success rate of the pick up at each step.
Fig. 12 presents the results for which the gripper with the pull in
operation required a contact amount of only 1 mm for grasping;
this was 2 mm less than that needed for the nominal gripper.
The results indicate that the proposed pull-in mechanism works
effectively for picking up workpieces when the allowablecontact area is limited.
B. Picking-up performances
We conducted pickup experiments with several kinds ofobjects in different environments to determine the effectivenessof the developed gripper. The experimental setup is presented
in Fig. 13; the developed gripper was attached to the tip of auniversal robot (UR5). The control was manual.
First, to determine the effectiveness of having asheet-embedded belt with a soft surface, we picked up several
kinds of objects on a table. Fig. 14 illustrates the successful
picking up of several objects, which indicates that thedeveloped gripper can grasp objects of various shapes owing to
the softness of the sheet.
0dyGripper

Table

Lowering(step: 0.5mm)
Workpiece(W = 30 mm)

Fig. 11

Procedure for investigating the pull-in function

dy [mm]dy [mm]

0.0
0.5
1.0
1.5
2.0
2.5
3.0

Contact
amount0

20 40 60 80
Success rate [%]0y

20 40 60 80
Success rate [%]100(a) With the pull-in function(b) Without the pull-in function
Fig. 12
Pick-up success rate
CasterAC adapter

Clamp

Toy part

Heatsink

Gear

IC chip2l pet bottle

UR5(universal Robots)
XY

GripperWork piece
Table
Fig. 13
Experimental setup
Fig. 14for investigating pick-up
performancePull-in(a) Heatsink
Fig. 15

Picking up several kinds of objects(b) Hexagonal bolt

Results of the bin-picking operation(a) Picking up a single piece of tissue (b) Picking up a single thin tea bag frompaper from a bundle
its boxFig. 16

Picking up a flexible sheet-type object

Fig. 17

0.0
0.5
1.0
1.5
2.0
2.5
3.0

100

Pull-inFlipping operation

Second, bin-picking operations were conducted as shown in
Fig. 15. The parts were able to be picked up one by one very
easily because the fingers could be inserted into the gaps
between the target and the surrounding workpieces easily, andvery little contact was required for the pick up owing to thepull-in operation.
Third, we attempted to pick up flexible thin objects. As
shown in Fig. 16(a), a single tissue paper was successfully
picked up from a bundle. First, we made the bottom of thefinger touch the top of the tissue paper. Then, by closing thegripper, the tissue paper was pulled towards the gripper owing
to the passive pull-in function; thus, only a single paper was
picked up. Fig. 16(b) illustrates how a tea bag was successfully
removed from its box. The fingers were first inserted into thegap between the target and the surrounding objects. The level ofinsertion was low, and thus, the contact amount was also low.
However, with the help of the pull-in operation, the subsequentclosing motion pulled the target tea bag towards the gripper andit succeeded in picking up the single target tea bag.
Fourth, a flipping operation was performed to check whether
the pull-in function can be utilized effectively for manipulation(Fig. 17). The flip operation is utilized by a human when theypick up a thin plate on a table. To perform the flipping
operation, conventional approaches [25], [29]–[31] utilized
multi-DOF fingers because the finger form was required tochange based on the way the object was rotated. In contrast, thepull-in operation can change the relative contact position on its

Copyright (c) 2020 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.2020.2970630

MORINO et al.: SHEET-BASED GRIPPER FEATURING PASSIVE PULL-IN FUNCTIONALITYTABLE I.

A COMPARISON OF THE DEVELOPED AND EXISTING GRIPPERS

Preferable
functions for bin
picking†Pull-inRequired contact
Grasping in
Requirement for Mechanism for
Flexibleamount for grasping
narrow
activating theactivating thesurface1
at fingertips
spaces2
pull-inpull-in(Fig. 14)
(Fig. 12)
(Figs. 15,16b)

Suction gripper

Yes

SuctionSuction

Small

No

Parallel-jaw gripper(supposing thinNofingertips)--

MediumFin-ray-based
gripper‡ [27]Yes

Load around the(intermediate)fin-ray area

Underactuated soft
gripper‡ [20]Yes

Underactuatedgripper with active Yes
surfaces [25]Underactuatedmodular finger withpull-in mechanism Yes[26]
Developed gripper Yes

7

Insertion in the
gap between
objects2,3(Figs. 15,16)

Picking up
Flipping
thin flexible Scooping3
operation
objects4(Fig. 17)
(Fig. 17)
(Fig. 16)

Easy

Difficult

Requiring no
Difficulthole

Difficult

No

Easy

Easy

Possible

Possible

DifficultFin-ray structure Large

Yes

Possible

Easy

Possible

Easy

DifficultSupporting surface

Ratchetmechanism

Small

Yes

Difficult

Possible

Possible

Easy

Possible

Motor

Motor

MediumNo

Possible

Possible

Possible

Possible

Easy

Load aroundfinger surface

Differential gear Medium

No

Difficult

Difficult

Requiringsupporting
surfaces

Difficult

DifficultLoad aroundfingertip

Finger deflection
and flexible
Smallsheet-embedded
belt

Yes

Easy

Easy

Easy4

Possible

Easy†: The number of the Figure demonstrating the corresponding functions is shown‡: These grippers have nails, although the developed gripper does not have them.

1: This function is associated with the high adaptability to the shape of the object, and therefore, many kinds of workpieces can be picked up.
2: Small thickness of the fingertips is effective in performing these functions.

3: Nails are effective in performing these functions.

4: In this function, “easy” implies that the gripper is able to pick up the object from a bundle one by one.

fingers with the closing operation, thereby making it possible toflip the object using only closing motion of the developed
gripper. First, one finger came in contact with the surface of one
side of the target box while the other finger came in contact
with the top of the target box. The gripper was initially tilted forthe contact. The tilted gripper was then rotated while being
lowered with the closing operation, which straightened thegripper and lowered the relative contact position. Using thisstrategy, the flip operation was conducted successfully.
V. DISCUSSION
We compared the proposed gripper with other existing
grippers. Table I presents the comparison results, listing thefunctionalities featured by the developed gripper and otherexisting grippers that are suitable for bin picking or for picking
up thin flexible objects. It can be inferred that the utilization of
sheet-embedded belt and finger deflection provides a simple
structured gripper equipped with the highest number offunctions suitable for bin picking and for picking up thin
flexible objects.
Next, we discuss the durability of the developed gripper. The
sheet-embedded belt must be replaced after every few months.
The main reason for the replacement is not the breaking of the
sheet due to overload or overstress but the deterioration due tothe chemical reaction with substances in the air or adhered bycontact. The sheet (Die B tear strength = 18.91 N/mm) [32] andthe belt (allowable tension = 40 N) [28] break primarily due totension. The opening and closing motions exert tension on thebelt but not on the sheet. The main component of the belt ispolystyrene, which has high durability and a long service life.
When grasping workpieces, the contact between the finger andthe sheet-embedded belt also prevents the large stretching of
the sheet and the belt. Therefore, the effect of contact with theworkpiece on the stretching of the sheet is relatively low incomparison with the effect of deterioration. When an
experiment was conducted for the extreme cases, a cut occurredin the sheet with a width of 10mm when a force of 4.5 N ormore was applied on it with a cutter.
The finger is constructed by ONYX, which is a nylon mixed
with chopped carbon fiber, and thus, it has high durability and along service life. The FEM analysis based on the material
properties [28] such as tensile strength (36 MPa) provides
allowable loads of 75 N (𝐹x ) and 111 N (𝐹y ) in the x and y
directions, respectively. These values are lower than the
maximum fingertip force that can be generated by the motor(51 N). Hence, the maximum fingertip force can be applied, and
it is large enough to grasp heavy objects, such as a 2 l PET
bottle, without breaking. The fatigue or breaking issues must be
considered if the deflection of the components is utilized for thegrasping operation. But, if the component materials are selected
appropriately, the methodology of usage of deflection works
effectively.
VI. CONCLUSION
This paper presents a novel and versatile parallel gripper forbin picking and for picking up thin flexible objects. The
developed gripper is equipped with a sheet-embedded belt thatenables the target workpiece to be pulled towards the gripper
even when the contact area is limited. When picking up a
workpiece, the grasping force deflects the fingertip, thedeflection rolls up the sheet-embedded belt, and the grasped
workpiece is pulled towards the gripper. The pull-in operation

Copyright (c) 2020 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.2020.2970630

8IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JANUARY, 2020

was performed passively with the opening and closing motions.
Owing to the pull-in function, workpieces were able to bepicked up one at a time in conditions where the targets werestacked in bulk. Flexible thin objects such as tissue paper could
be grasped when the bottom of the gripper fingertips touched
the top of a thin object and pulled it towards the gripper with aclosing motion. The sheet on the sheet-embedded belt is softenough to enable a wide variety of workpieces to be picked up.
The effectiveness of the pull-in function and the gripper itself
were demonstrated through several experiments.
However, the developed gripper also has several limitations.
The range of the width within which the gripper can performthe pull-in operation was 30–90 mm, and workpieces withwidths beyond this range are difficult for the gripper to grasp. A
2 l PET bottle could be grasped; however, picking up objects
heavier than the PET bottle was not attempted. The grasping
forces were not optimized; however, they must be determinedaccording to the targets. The optimization of the finger shape
was also not considered. The difference in the arrangements of
the left and right finger components (for example, thedifference in the bulge of the sheet-embedded belt) could tilt
the grasped workpiece. A larger gripper is required to pick up
larger objects such as car parts. The scaling-up of the main
design concept is possible. However, for realizing the scale-up,
we must resolve several issues: feasibility of material selection,
development of fabrication methods instead of 3D printing, andso on. The installation of sensors such as tactile sensors can beeffective in applying the gripper to more complex assembly
tasks. These issues are beyond the scope of this study and may
thus be a part of our future work.
REFERENCES[1]“Assembly Challenge in World Robot Summit.” [Online]. Available:
https://worldrobotsummit.org/en/wrs2020/challenge/industrial/assembly.
html.
[2] V. Vonásek, A. Vick, and J. Krüger, “Distributed Motion Planning for
Industrial Random Bin Picking,” Procedia CIRP, vol. 76, pp. 121–126,
2018.
[3] Y. Domae, H. Okuda, Y. Taguchi, K. Sumi, and T. Hirai, “Fastgraspability evaluation on single depth maps for bin picking with generalgrippers,” in 2014 IEEE International Conference on Robotics andAutomation (ICRA), 2014, pp. 1997–2004.
[4] D. Buchholz, D. Kubus, I. Weidauer, A. Scholz, and F. M. Wahl,“Combining visual and inertial features for efficient grasping andbin-picking,” in 2014 IEEE International Conference on Robotics andAutomation (ICRA), 2014, pp. 875–882.
[5] H.-Y. Kuo, H.-R. Su, S.-H. Lai, and C.-C. Wu, “3D object detection andpose estimation from depth image for robotic bin picking,” in 2014 IEEE
International Conference on Automation Science and Engineering(CASE), 2014, pp. 1264–1269.
[6] J. Mahler and K. Goldberg, “Learning Deep Policies for Robot Bin
Picking by Simulating Robust Grasping Sequences,” Proc. 1st Annu.
Conf. Robot Learn., vol. 78, no. CoRL, pp. 515–524, 2017.
[7] J. Shi and G. S. Koonjul, “Real-Time Grasping Planning for Robotic
Bin-Picking and Kitting Applications,” IEEE Trans. Autom. Sci. Eng., vol.
14, no. 2, pp. 809–819, Apr. 2017.
[8] S. B. Backus and A. M. Dollar, “An Adaptive Three-Fingered Prismatic
Gripper With Passive Rotational Joints,” IEEE Robot. Autom. Lett., vol. 1,
no. 2, pp. 668–675, Jul. 2016.
[9] G. Endo and N. Otomo, “Development of a Food Handling Gripper
Considering an Appetizing Presentation *,” in IEEE International
Conference on Robotics and Automation, 2016, pp. 4901–4906.
[10] M. G. Catalano, G. Grioli, E. Farnioli, A. Serio, C. Piazza, and A. Bicchi,“Adaptive synergies for the design and control of the Pisa/IIT SoftHand,”
Int. J. Rob. Res., vol. 33, no. 5, pp. 768–782, Apr. 2014.

[11] E. Brown et al., “Universal robotic gripper based on the jamming of
granular material,” Proc. Natl. Acad. Sci., vol. 107, no. 44, pp. 18809–
18814, Nov. 2010.
[12] A. J. Spiers, B. Calli, and A. M. Dollar, “Variable-Friction Finger
Surfaces to Enable Within-Hand Manipulation via Gripping and Sliding,”
IEEE Robot. Autom. Lett., vol. 3, no. 4, pp. 4116–4123, Oct. 2018.
[13] R. R. Ma, L. U. Odhner, and A. M. Dollar, “A modular, open-source 3D
printed underactuated hand,” in 2013 IEEE International Conference on
Robotics and Automation, 2013, pp. 2737–2743.
[14] K. Yamaguchi, Y. Hirata, and K. Kosuge, “Development of robot handwith suction mechanism for robust and dexterous grasping,” in 2013
IEEE/RSJ International Conference on Intelligent Robots and Systems,
2013, pp. 5500–5505.
[15] J. Shintake, V. Cacucciolo, D. Floreano, and H. Shea, “Soft Robotic
Grippers,” Adv. Mater., vol. 30, no. 29, 2018.
[16] T. Watanabe, K. Yamazaki, and Y. Yokokohji, “Survey of robotic
manipulation studies intending practical applications in real environments-object recognition, soft robot hand, and challenge program andbenchmarking-,” Adv. Robot., vol. 31, no. 19–20, pp. 1114–1132, Oct.
2017.
[17] S. Hirose and Y. Umetani, “Development of soft gripper for the versatilerobot hand,” Mech. Mach. Theory, vol. 13, no. 3, pp. 351–359, Jan. 1978.
[18] R. F. Shepherd et al., “Multigait soft robot,” Proc. Natl. Acad. Sci. U. S.
A., 2011.
[19] R. Maruyama, T. Watanabe, and M. Uchida, “Delicate grasping by
robotic gripper with incompressible fluid-based deformable fingertips,”
in 2013 IEEE/RSJ International Conference on Intelligent Robots andSystems, 2013, pp. 5469–5474.
[20] T. Nishimura, K. Mizushima, Y. Suzuki, T. Tsuji, and T. Watanabe,“Variable-Grasping-Mode
UnderactuatedSoft
GripperWith
Environmental Contact-Based Operation,” IEEE Robot. Autom. Lett., vol.
2, no. 2, pp. 1164–1171, Apr. 2017.
[21] T. Nishimura, Y. Suzuki, T. Tsuji, and T. Watanabe, “Fluid Pressure
Monitoring-Based Strategy for Delicate Grasping of Fragile Objects by A
Robotic Hand with Fluid Fingertips,” Sensors, vol. 19, no. 4, p. 782, Feb.
2019.
[22] T. Nishimura, Y. Fujihira, and T. Watanabe, “Microgripper-Embedded
Fluid Fingertip-Enhancing Positioning and Holding Abilities for
Versatile Grasping,” J. Mech. Robot., vol. 9, no. 6, p. 061017, Dec. 2017.
[23] Y. Furata, T. Tsuji, Y. Suzuki, T. Watanabe, M. Hikizu, and H. Seki,“Adaptive gripper with soft sheets for a uniformly distributed grasping
force,” in 2017 IEEE International Conference on Robotics andBiomimetics, ROBIO 2017, 2018, vol. 2018-Janua, pp. 373–378.
[24] V. Tincani et al., “Velvet fingers: A dexterous gripper with active
surfaces,” in IEEE International Conference on Intelligent Robots andSystems, 2012, pp. 1257–1263.
[25] R. R. Ma and A. M. Dollar, “In-Hand Manipulation Primitives for a
Minimal, Underactuated Gripper With Active Surfaces,” in Volume 5A:
40th Mechanisms and Robotics Conference, 2016, p. V05AT07A072.
[26] A. Kakogawa, H. Nishimura, and S. Ma, “Underactuated modular finger
with pull-in mechanism for a robotic gripper,” in 2016 IEEE International
Conference on Robotics and Biomimetics, ROBIO 2016, 2016, pp. 556–
561.
[27] Festo, “BionicTripod with FinGripper Energy-efficient movement andform-fitgripping,”
2009.
[Online].
Available:
https://www.festo.com/rep/en_corp/assets/pdf/Tripod_en.pdf.
[28] Markforged, “Plastic Matrix,” p. 1, 2019.
[29] L. U. Odhner, R. R. Ma, and A. M. Dollar, “Open-loop precision graspingwith underactuated hands inspired by a human manipulation strategy,”IEEE Trans. Autom. Sci. Eng., vol. 10, no. 3, pp. 625–633, 2013.
[30] C. Eppner and O. Brock, “Planning grasp strategies That Exploit
Environmental Constraints,” in 2015 IEEE International Conference on
Robotics and Automation (ICRA), 2015, pp. 4947–4952.
[31] C. Eppner, R. Deimel, J. Álvarez-Ruiz, M. Maertens, and O. Brock,“Exploitation of environmental constraints in human and robotic
grasping,” Int. J. Rob. Res., vol. 34, no. 7, pp. 1021–1038, Jun. 2015.
[32] Smooth-On, “Dragon Skin ® Series,” Smooth-On, 2015.

Copyright (c) 2020 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.