Underactuated Modular Finger with Pull-in Mechanism for a Robotic Gripper

Proceedings of the 2016 IEEE
International Conference on Robotics and Biomimetics
Qingdao, China, December 3-7, 2016
Underactuated Modular Finger with Pull-in Mechanism for a Robotic
Gripper
Atsushi Kakogawa1, Hiroyuki Nishimura2 , and Shugen Ma1
Abstract— This paper presents an underactuated modular
finger with a pull-in mechanism for a robotic gripper. Most
existing robotic grippers do not have an infinite rotating
mechanism. However, this movement has the potential for
industrial applications such as pulling in and pushing out an
object from the plane. Increasing functionality tends to lead
to more operational difficulties. Therefore, we have developed
a three-fingered underactuated gripper that not only grasps
but also pulls in the object. We have described the design of a
modular underactuated finger and a differential mechanism for
the same. Lastly, to test the grasping and underactuated objectpulling-in capabilities of the gripper, we conducted experiments
with objects of various shapes.
I. INTRODUCTION
End effectors are often used in industrial robotics to handle
objects. For more dexterous and versatile tasks, robotic hands
inspired by the human body are the future. However, this will
inevitably require more actuators, greater redundancy, and
complex control systems. Therefore, there has been research
into robotic grippers with fewer fingers, which are specially
designed for particular tasks. This is because routine tasks
are the norm in industrial robotics.
A gripper with two turntables [1] can rotate the object
that it is holding and even tighten or loosen a screw by
alternating the rotational direction of each turntable. A
gripper with two active rollers [2] can not only grasp but
also pull in a sheet-like object placed on a flat surface.
The finger mechanism is equipped with an omnidirectional
driving roller with two active rotational axes [3] to grasp and
pull in the object. Furthermore, rotating the object around
any axis and translating it along the fingers is possible.
A finger mechanism with two degrees of freedom and an
active surface [4], [5] can tightly grasp the object by pulling
it in, even when it pinches the object with its fingertips.
The fingers and additional gripping mechanism are each
independently controlled. Hence, various actuators are still
required and each movement is activated by using sensors.
In contrast, underactuated grippers with differential mechanisms have been reported that simplify the control and
require fewer sensors and actuators. A soft gripper [6] adopts
a novel mechanism known as an articulated differential.
This can grasp an object while passively adapting itself to
*This work was not supported by any organization.
1 Atsushi Kakogawa and Shugen Ma are with Department of Robotics,
Faculty of Science and Engineering, Ritsumeikan University, 1-1-1, Nojihigashi, Kusatsu, Shiga, Japan kakogawa@fc.ritsumei.ac.jp,
shugen@se.ritsumei.ac.jp
2 Hiroyuki Nishimura used to belong to Department of Robotics, Faculty
of Science and Engineering, Ritsumeikan University. He is currently with
Yushin Precision Equipment Co., Ltd., Kyoto, Japan
978-1-5090-4364-4/16/$31.00 ©2016 IEEE
Fig. 1. Developed robotic gripper with three underactuated finger modules.
the object’s shape. This theory has been applied to various
fingers to date, such as a differential gear chain [7]. Underactuated grippers with various types of differential mechanisms
(e.g., pulley, gear, linkage, bar) have been discussed [8], [9].
Recently, underactuated robotic fingers were invented that
can withstand impacts [10].
The rotational ranges of most gripper designs are finite
because opening and closing the fingers takes place over
a limited range. Also, the differential mechanisms that are
often used for underactuated fingers (e.g., linkages, pulleys)
tend to be limited to finite rotation. However, infinite rotational movement would be useful for grippers in certain
cases, e.g., when lifting a flat object from a flat surface.
Therefore, we previously developed a planar underactuated
gripper with two fingers that can pull in an object [11].
We used a planetary gear reducer to produce two types
of movement. In this paper, we extend our underactuated
gripper to one with three modular fingers as shown in Fig.
1. When this gripper grasps an object, it also automatically
pulls it in by using the belt that each finger is equipped
with. The three fingers of this robot (six degrees of freedom
in total) are driven by a single motor. Hence, grasping and
pulling in do not require any sensors or control systems for
activation.
This paper is organized as follows. In Section II, we
discuss the mechanical design of the modular finger, the underactuated mechanism including the planetary gear reducer,
and a resistance regulation mechanism. In Section III, we
present the results of experimental verifications of our three-
556
Small resistance
Idler
Large resistance
Tensioner
Fig. 3.
Resistance regulation mechanism (RRM).
Belt
Base
Finger mechanism
Planetary gear reducer
Torque
distribution
gear
Resistance
regulation
mechanism
Ring gear
Worm gear
Gripping
Carrier
Resistance
regulation
Pulling-in
Ring gear
Worm gear
Gripping
Carrier
Resistance
regulation
Pulling-in
Ring gear
Worm gear
Gripping
Carrier
Resistance
regulation
Pulling-in
Sun gear
Idler
Driving pulley
Motor
Spur gear
Worm wheel
Worm gear
Planetary
gear reducer
Fig. 2.
Sun gear
Bevel gear
Spur gear
Carrier
Sun gear
Ring gear
Input gear
Sun gear
Cross-sectional view of an underactuated modular finger.
Fig. 4. Transmission of robotic gripper with three underactuated finger
modules.
fingered underactuated gripper. We present our conclusions
in Section IV.
II. MECHANICAL DESIGN
A. Underactuated modular finger
A cross-sectional view of one underactuated modular finger is shown in Fig. 2. It comprises a parallel link mechanism
to keep the pull-in belt in constant alignment. The belt itself
is made from silicone rubber sponge and has a semicircular
cross-section for grasping objects of various shapes.
An input gear connected to a motor drives the sun gear of
the planetary gear reducer. Because the carrier and the ring
gear of the planetary gear reducer are not fixed (it works as
a differential (underactuated) mechanism), the torque from
the motor is distributed to two outputs. The carrier output is
transmitted to the pull-in belt via several gear mechanisms,
while the ring-gear output is used for gripping the object.
When the gripper is pointing vertically downward, the fingers
tend to close on their own because of gravity. We use a worm
gear to constrain this movement because it cannot be driven
backward.
We favor the situation in which the fingers grasp the
object first before the pull-in belt starts moving. However,
to know which of the two outputs should come first depends
on how much resistance is applied to each finger and pull-in
belt. Therefore, a resistance regulation mechanism (RRM)
is installed on the driving pulley as shown in Fig. 7. It
comprises a coil spring that presses a free rubber roller onto
the driving pulley. The angle of the roller can be manually
set from 0◦ -90◦ by a rack and pinion. The resistance is least
when the angle is 0◦ because the roller turns freely with
the pulley. However, the frictional resistance increases with
the angle to which the roller is set, reaching a maximum at
90◦ . We can regulate the torque distribution by setting this
angle beforehand. If the object is solid, the grip force of the
fingers can be increased by setting a larger RRM angle. If
the object is soft and should not be deformed, the grip force
can be decreased by setting a smaller angle.
The transmission of a robotic gripper with three underactuated finger modules is illustrated in Fig. 4. A single motor
is used to drive the three fingers, and each finger is connected
in parallel through the torque distribution gear (the large
spur gear shown in Fig. 5). The input gear of each finger
is driven by this torque distribution gear. Subsequently, the
input gear drives the sun gears of each finger, and the carrier
is constrained by the RRM. The torque from the motor is
then transmitted to each ring gear, causing the three fingers
to close simultaneously. Once the gripper has the object
in its fingers, it cannot move any more. At that point, the
torque from the motor is transmitted to each carrier and the
belts start pulling in the object. By changing the rotational
direction of the motor, the gripper can release the object
because no resistance is applied from the outside in the radial
direction.
We can select the number of fingers, as shown in Fig. 6.
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Cylinder to
collect the
objects
Pinion
Torque
distribution
gear
Fig. 5.
Motor
Torque distribution gear.
Fig. 7.
Two fingered
Three fingered
Four fingered
Setup for measurement of rotational resistance.
Five fingered
1.8
Fig. 6.
Variation in the number of fingers.
1.6
1.44
1.4
B. Measurement of the rotational resistance of the belt
The rotational resistance of the pull-in belt depends on the
angle of the RRM. The resistance is least when the rotational
axes of the pulley and the RRM roller are aligned. It is largest
when the pulley and the roller are perpendicular to each other
because the roller slides rather than rotates. To measure the
rotational resistance of the belt, a brushed DC motor (Maxon
148867) and a DC stabilized power supply (Texio PW181.8AQ) are used, as shown in Fig. 7. The power supply
provides the current for the motor. The torque constant of
the motor used to measure the current is Kτ = 0.0302Nm/A.
Therefore, the rotational resistance of the belt (Resistance)
can be calculated using
Resistance = Kτ Im ,
(1)
where Im denotes the motor current.
Figure 8 shows the measured current of the motor. If
the pulley and roller of the RRM are aligned, the average
motor current is 1.06 A. When the pulley and roller are
perpendicular to each other, a current of 1.44 A is measured.
This current difference of 0.38 A equates to a selectable
RRM resistance of 0 ≤ Resistance ≤ 0.011Nm. When
gripping a brittle object like an egg, too much grip force
might destroy it. By reducing the RRM resistance, the pullin belt starts moving, before the fingers crush the object.
Depending on the softness of the object, RRM resistance is
adjustable in this range.
Motor current [A]
The whole gripper can be downsized if only two fingers are
used, even though the object might slide and fall. Having
more fingers allows a tighter grasp, albeit by a heavier
gripper. The upper limit on the number of fingers is when
they begin to interfere with each other.
A cylinder to collect the object is mounted on the palm
of the gripper (at its center). If the object is smaller than the
diameter of this cylinder, it can be stored inside the gripper.
0.38
1.2
1.06
1
0.8
Current (0°)
0.6
Average (0°)
Current (90°)
0.4
Average (90°)
0.2
0
0
5
10
15
Time [sec]
Fig. 8.
20
25
Measured motor current.
III. EXPERIMENT
We conducted experiments to test the grasping and
pulling-in capabilities of the three-fingered gripper. The
specifications of the gripper are listed in Table I. The pulley
and roller of the RRM were set perpendicular to each other
to maximize the grip force. Figure 9 shows the objects used
in the experiments. We fashioned a cube, a sphere, a cone, a
cylinder, and an ovoid from Styrofoam. We used a dishcloth
as the sheet-like object.
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TABLE I
S PECIFICATIONS OF THE ROBOTIC G RIPPER
Weight of each finger module [kg]
Weight of the base [kg]
Total weight [kg]
Maximum height [m]
Maximum width [m]
Graspable diameter [m]
0.9
2.1
4.8
0.495
0.381
Approx. 0.12
Cubic
Spherical
Cylindrical
Conical
Egg-like
Fig. 9.
1
2
3
4
Cloth
Graspable objects for the experiments.
1
2
3
Fig. 12.
Gripping and pulling in a cone.
1
2
3
4
4
Fig. 10.
Gripping and pulling in a cube.
A. Vertical gripping
The experimental outcomes are depicted in Figs. 10-14.
The results show that our proposed gripper could grasp and
pull in each of the variously shaped objects. As shown in
Figs. 12 and 13, if diameter of the object is small, the
gripper retracted it into the palm (cylinder). However, an
object (not the cylindrical one) would sometimes incline as
it was being pulled in. For example, as shown in Fig. 15,
the belts of the two front fingers started pulling in when
they touched the object. However, the rear finger was still
closing at that moment, hence it touched the object later than
the other fingers did. We conclude from this that the gripper
can grasp an object of any shape only if the three fingers
simultaneously touch the object.
1
Fig. 13.
Gripping and pulling in a cylinder.
1
2
3
4
2
Fig. 14.
3
Gripping and pulling in an ovoid.
4
Fig. 15.
Fig. 11.
Gripping and pulling in a sphere.
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Gripping and pulling in an inclined object.
1
2
1
2
3
4
3
4
Fig. 16.
Gripping and pulling in a cloth.
Fig. 17.
An unexpected result occurred in the experiment with
the cone. In order to pull in any conical object, the three
fingers must open gradually as the object’s cross-sectional
diameter increases. However, our finger modules cannot be
driven backward because of the worm gears. Thus, the fingers
opened slightly while pulling in the cone (see 2 and 3 in Fig.
12). This was because backlash in the worm gears and the
reaction force from the conical object allowed the fingers
to open. Had the bottom face of the cone been wider, the
gripper might not have been able to hold it so well.
Figure 16 shows the results of gripping and pulling in a
cloth. This kind of movement is one of the most difficult
tasks for existing robotic grippers. However, our developed
gripper successfully pulled in a cloth. The gripper starts
pulling in the cloth when the robotic fingertips touch it and
stop moving forward. If the base is fixed on the end of a
robotic arm that is not compliant, the fingers cannot push
upward by themselves. Therefore, if the gripper is too close
to the contact surface, the pulling-in belt starts to rotate while
the fingers are opening, which works as a shock absorber.
However, when pulling in a sheet-like object placed on the
contact surface, the distance between the surface and the
fingers is important. If the starting position of grasping is
determined by measuring this distance, the gripper can pull
in the sheet-like object properly. Alternatively, if the robotic
arm is compliant, the gripper can push itself up passively
until the fingers close completely.
We also conducted an experiment on gripping and pulling
in a 2-ℓ plastic bottle filled with water. Generally, the gripper
succeeded in grasping and pulling in the bottle without
crushing it, as shown in Fig. 17. However, the outcome
depended on where the fingers first touched the bottle. If
they did so as shown in Fig. 17 (when approximately 30%
of the finger length touches the object), then the procedure
would be successful. However, if they first touched the bottle
higher than the location shown in Fig. 17 (panels 1 and 2),
then the bottle would only be held by the fingertips. Thus,
the bottle would slip and fall because the contact area is too
small.
Gripping and pulling in a 2-ℓ plastic bottle of water
1
2
3
4
Fig. 18.
Horizontal gripping and pulling in a cloth.
B. Horizontal gripping
To test the grasping and pulling-in capabilities in a horizontal direction, we conducted additional experiments of
horizontally positioned three-fingered gripper. In such case,
lower fingers are more difficult to close than upper fingers
due to effect of gravity. Currently, lower fingers cannot close
from downward to upward. In other words, lower fingers
torque cannot exceed the gravity, while the RRM is slipping
on the driving pulley. Therefore, the object is put on a stand
in order to prevent the fall. Figure 18 shows the results of
horizontal gripping and pulling in a cloth. By using only one
upper finger, the gripper can pull in the cloth.
If configuration of a cylinder mounted on the palm of the
gripper is changeable (for example, Fig. 19), it would be
possible to grip the object without the stand in a horizontal
direction. In this case, upper two fingers are not affected by
gravity, then the gripper can grasp and pull in the object. It
works as the gripper previously we developed in [11].
A video of the experiments is uploaded on
https://www.youtube.com/watch?v=UDCjklzbjVs.
IV. CONCLUSIONS
We have presented an underactuated modular finger with
a pulling-in mechanism for a robotic gripper, along with
validation of its capabilities. The main features of the robotic
gripper are:
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Fig. 19.
Configuration of fingers.
A differential mechanism that provides two outputs for
grasping and retracting through a single actuator
• Elimination of sensor systems for detecting contact with
the object
• A grip force that can be varied by using a resistance
regulation mechanism
•
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From the experimental results, it was confirmed that our
proposed gripper could grasp and pull in variously shaped
objects.
However, further work is required. Firstly, because of the
worm gears, the grasping is relatively slow. One possible solution to this problem is a pneumatic differential mechanism.
A branched tube for pumping air is a differential mechanism
in itself. If the worm gears are replaced by an air cylinder,
the grasping could be faster and the differential mechanism
could be smaller and lighter.
We have so far considered only one posture of the gripper,
namely the fingers closing from top to bottom. However,
greater dexterity is required for actual industrial tasks. For
example, when moving an object from a floor to a wall, the
gripper becomes horizontal. To adapt to this situation, we
need to design the output ratio of the differential mechanism
properly. If the effect of gravity is ignored, the pulling-in
belt might be driven without the fingers grasping the object.
ACKNOWLEDGMENT
We gratefully acknowledge the work of Hiroyuki
Nishimura (a past member of our laboratory) for manufacturing and assembling the experiments.
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