Note: Descriptions are shown in the official language in which they were submitted.
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Title: Design of Fault-Tolerant Dexterous Hand with Multi-fingers
1. Field of the Invention
The present invention relates to mechanical grippers, and more specifically to
fault-tolerant
underactuated grippers which also referred to as fault-tolerant underactuated
hands or
underactuated end effectors.
2. Background of the invention
There are several types of mechanical grippers.
Specific purpose mechanical grippers: these grippers are designed for specific
tasks. They
consists of simple mechanisms that are reliable and robust. They are easy to
fabricate and require
simple control schemes. However, they are just suitable for specific tasks and
for every new task
new gripper has to be redesigned. These mechanical grippers have only a few
degrees of freedom
and are widely used in industrial applications.
General purpose mechanical grippers: these grippers are more flexible and can
perform several
different tasks. However, they are difficult to fabricate and need complex
control schemes. At
the same time, these grippers include several actuators and can provide only
small grasping
forces. These types of mechanical grippers have several degrees of freedom.
Both grippers mentioned above possess shortcomings. Through the concept of
under-actuation,
the under-actuated grippers can control a large number of degrees of freedom
but with a reduced
number of actuators. These grippers have a flexible grasping function without
the complexity
associated with a large number of actuators, and can provide large grasping
forces as well.
Under-actuation can be achieved using tendons. The grasping forces provided by
these grippers
are limited and the tendons introduce friction and compliance.
Under-actuation can also be achieved with mechanisms, which allow larger
grasping forces.
In additional, there are situations when the grippers are damaged during the
work; so the grippers
should have fault-tolerant functionality to guarantee normal operations
especially for some long-
term unattended situations.
3. Description of the Prior Art
With the growing demand for robotic applications, especially in the areas of
severe and
challenging working conditions such as in deep water, space and nuclear, there
is a necessity for
a gripper to be versatile, flexible and general purpose to perform variety of
complex operational
tasks and grasp objects of any shapes.
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Through millions of years' of continuous evolution of human species, the human
hand is the
most flexible dexterity end-effecter. In most cases, the grippers are designed
to have some
features of human hand. Several researchers have focussed on the multi-
fingered dexterous robot
hands in order to improve the practicability of mechanical grippers.
Research on multi-fingered hands began in the 1970s; the typical example is
the Okada
dexterous hands from Japan Ell; it has three fingers and a palm, one of the
fingers has three
degree of freedoms (DOFs), and the other two fingers have four DOFs. Finger
joints are driven
by a motor via wire and pulley. Subject to tendon driven, Okada dexterous hand
can only execute
some simple repetitive operations.
In the 1980s, several multi-fingered dexterous robot hands are proposed.
Stanford University
successfully developed Stanford/JPL hand [2] that has three fingers and no
palm, and every finger
has three DOFs. 12 DC servo motors are used to actuate the gripper. JPL hand
has more
flexibility than Okada hand.
Utah/MIT hand is proposed by MIT and University of Utah [3]; it has four
unique four-DOF
fingers, can squeeze a bulb using the data glove.
Hitachi hand was developed in the year 1984;E41 it has three four DOF fingers
that is actuated by
shape memory alloy which provides high drive speed and high load capacity.
In 1990, University of Genoa in Italy developed DIST hand which has four DOF
fingers and 20
DC motors drive the hand work fluently [5].
Since 1985, University of Bologna in Italy has successfully developed
dexterous hands named
UB-I and UB-II [6].
In the year 1999, DLR-I and DLR-II hands were developed by German Aerospace
Center; they
consist of four-DOF fingers designed to work with more than 25 sensors
NASA dexterous hand is proposed in the late of 20th century by the United
States National
Aeronautics and Space Agency [8]. This hand is designed just like a human hand
with one
forehand, one wrist and five fingers. NASA hand can pick up some tools and
possesses dexterity
similar to human hand to some degrees.
HIT-1 hand is developed by Harbin Institute of Technology; it has four
fingers, each finger has
four DOFs 191.
Beijing University of Aeronautics and Astronautics has developed BH-1, BH-2,
BH-3 and BH-4
[10]
Most of the existing dexterous hands have 3-5 fingers, and each finger has 3-5
DOFs. Most of
the researchers focussed on geometry, kinematics, dynamics and structures of
the grippers and
related control algorithms and sensor systems.
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Dexterous hands show good performance on gripping using multi-DOF mechanisms,
but these
mechanisms brings challenges in structure design and the arrangement of the
motors. How to
reduce complexity and number of actuators is an important consideration during
the dexterous
hand design. The general approach is to introduce underactuated mechanisms.
U.S. Pat. No. 5,052,736 [11] proposes modular dexterous hand. It is a modular
dexterous
grasping hand that comprises three rotatable fingers mounted on the mounting
plate of a Stewart
platform, which can rotate about parallel axes. This hand uses an L shape
finger for gripping an
object of any shapes in three points grasping mode. Its fingers are driven
independently by
motors mounted on the mounting plate. When the gripping is carrying out, the
gripped object is
pulled on the palm plate.
U.S. Pat. No. 5,108,140 [12] describes a reconfigurable end effecter, A highly
reconfigurable
end effecter employing three digits. Each digit has two pivoting degrees of
freedom and a
rotational degree of freedom. First of all, this hand is reconfigurable, and
possess a strong and
tendon-less digit actuation mechanism for pivoting DOF. A differential
transmission in
conjunction with a double parallelogram configuration provides a selective
enveloping or
parallel vise grip grasping action of the end effecter.
U.S. Pat. No. 4,955,918 [13] explains artificial dexterous hand for grasping
and manipulating
objects. This gripper possesses left and right thumbs with movable thumb base
separately, and its
center finger is interposed between the left and right thumbs. A pair of worm
gears is used as
transmission. Fingers are composed by series of four-bar linkages.
U.S. Pat. No. 5,280,981 [14] presents an end effecter with load-sensitive
digit actuation
mechanisms. Every digit (finger) with a reconfigurable end effecter can be
rotated and
manoeuvred by a single small motor. This gripper has strong adaptively: under
light load or no
load conditions, the finger can perform rapid positioning using a slip clutch
while in the case of
substantial loads( low speed, high force), the finger uses an oscillating
sprang clutch arrangement
for incrementing the drive shaft to grasp the object. The turret is locked in
the selected rotational
position by reenergizing a solenoid to allow finger position reconfiguration,
and high breakaway
friction prevents back drive of the drive shaft without the need for a braking
apparatus.
U.S. Pat. No. 4,957,320 [15] proposes methods and apparatus for mechanically
intelligent
grasping. This robotic end-effecter possesses a novel breakaway clutch, which
is combined with
mechanically linked finger joints to significantly reduce control complexity
while retaining the
ability to accomplish enveloping grasping. In another embodiment, a finger
using compliant
tendons to accomplish enveloping grasps is disclosed. A novel palm/finger
configuration which
further increases the versatility of the disclosed end effectors without
unduly increasing
complexity is also presented. Methods of manipulating an object are also
disclosed. An
important aspect of the present invention is the provision of compliance in
the joints of the
articulated members, which allows the members to "wrap" around an object.
U.S. Pat. No. 6,505,870[16] describes an actuation system for a highly
underactuated gripping
mechanism. This gripper possesses 10 DOFs with only two actuators; one of the
actuators is for
actuating the opening and closing of three fingers and it is more powerful for
grasping, while the
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other actuator controls orientation of two rotatable fingers with
synchronization and it is less
powerful. The powerful actuator actuates three fingers by a specially designed
one-input-three-
output differential mechanism. A Geneva mechanism is used for the orientation
transmission for
the rotational thumbs. At the same time, each finger of this gripper is
enabled to be self-locked in
its closing and opening action when the power is off.
U.S. Pat. No. 5,762,390 [In explains an underactuated mechanical finger with
three knuckles
and three DOFs. This finger is designed for a flexible and versatile
mechanical gripper which
uses only a limited number of actuators. This mechanical gripper has three
fingers. When
performing a grasping task, the fingers progressively envelope the object to
be grasped and
eventually reach a static equilibrium, and the mechanical gripper designed
Using these fingers
allows a stable grasping of a wide class of objects while specifying only two
coordinates for each
of the fingers.
U.S. Pat. No. 6,669,257 [18] presents a power switching mechanism for robotic
applications.
The two-DOF power input comprises a translation power input and a power shaft
rotation input.
The fingers are oriented in four predetermined positions, separated by thirty
degrees each (0, 30,
60, and 90) using a specially designed Geneva wheel, and hold self-locking.
U.S. Pat. No. 7,614,673 1191 proposes a conforming artificial mechanical
finger. It is a
prosthetic finger includes a crossed four-bar linkage system, which has a base
bar functioning as
a base frame, 2 cross bars and an interface bar that engages to grasp an
object.
U.S. Pat. No. 8,231,158 [201 explains a robust compliant adaptive gripper and
method of
manufacturing. It is a multi-fingered under-actuated mechanical gripper driven
by a single
actuator, which can grasp objects spanning a wide range of size and shape.
U.S. Pat. No. 5,046,375 [21] describes a compact cable transmission with cable
differential. In
this transmission, a pretensioned cable transmits power along the first link
to a rotating output
joint. A reducer is mounted at a point removed from the actuator, and
preferably as close as
possible to the output joint. An axially split spool wraps the outgoing and
returning cable
sections to provide pretensioning of the cable in the entire cable circuit at
one point. The links
and pulleys of the transmission are long and narrow. This high aspect ratio
provides a compact
configuration that is conductive to whole arm manipulations where any exterior
surface of the
links can engage an object. A low inertia, low friction brushless D.C. motor
operating under the
control of a pulse width modulated controller responsive to the current drawn
by the motor, in
combination with the cable transmission of this invention, provides excellent
force control.
Dexterous humanoid robot hands have good prospects; they can perform several
operations
especially in harsh working conditions or some unattended conditions. There is
an urgent need to
design a new type of dexterous hands.
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Summary of the Invention
This invention provides a flexible and versatile mechanical underactuated
gripper named UNG
Gripper which uses only a limited number of actuators, and this UNG gripper is
fault-tolerant by
design. This UNG gripper has five fingers with three-knuckle, a movable palm,
and a Hook ramp
simulating the function of thumb-index web. Two rotatable fingers can adjust
the gripper
gesture to guarantee robust grasp for various shapes. For fault-tolerant
functionality, two
rotatable fingers of the five fingers have functional alternative in the case
of partial gripper finger
failure.
Unlike traditional underactuated grippers, this invention involves two types
of mechanisms, one
for the hand layer and another for the finger layer. In the hand layer, a
specially designed
planetary gear train (one-input-six-output) with a single actuator is used to
drive six
subassemblies (five fingers and a palm). In the finger layer, the
underactuated three-knuckle
fingers use two series connected four-bar linkage mechanisms.
Furthermore, a movable palm is designed to synchronize with the motion of five
fingers with its
minimal influence on position and attitude of the gripping object compared to
the existing design
wherein the palm is generally fixed. Also, a hook ramp simulating the function
of thumb-index
web can provide a reference to the planning of the manipulator on which the
gripper is installed.
This approach can results in simpler planning similar to how a human being
grasps an object.
In comparison to existing grippers, the UNG gripper can provide fault-tolerant
functionality to
work even under failure of one or two fingers, especially during long-term
unattended situation
in which the subsystem failure cannot be fixed in time.
The UNG gripper is an universal gripper, i.e, it can be used to grasp any
types (shape and size) of
objects.
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Brief Description of the Drawings
Objects, features and advantages of the invention will emerge from the
following description
given by way of non-limiting example with reference to the appended drawings
and in which,
Fig. 1 shows general assembly drawing of the gripper.
Fig. 2 shows two rotational fingers which can perform a gesture to guarantee a
robust grasp.
Fig. 3 shows fault-tolerant functionality of the gripper.
Fig. 4 shows a specially designed single-input six-output planetary gear train
for the
underactuated 16-DOF mechanism.
Fig. 5 shows an underactuated three-knuckle finger with two four-bar linkage
mechanisms.
Fig. 6 shows five fingers and removable palm which can perform grasping.
Fig. 7 shows a thumb-index web liked hook which provides a reference to the
planning of the
manipulator on which the gripper is installed.
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Detailed Description of the Invention
This patent is about an under-actuated gripper with fault-tolerant
functionality. The UNG gripper
is first briefly described (see Fig. 1).
The gripper has five three-knuckle fingers (1.1, 1.2, 1.3, 1.4 and 1.5) and a
movable palm (1.6).
Three of these five fingers are fixed in position; they are named Fixed Thumb
(1.5), Fixed Power
Finger 1st (1.1) and Fixed Power Finger 2nd (1.2). The other two fingers are
movable; they are
called Rotational Thumb 1st (1.3) and Rotational Thumb 2nd (1.4). These two
rotational fingers
driven by two Geneva mechanisms (1.9 and 1.10) can rotate around their
respective axis to have
a required gesture for grasping irregularly shaped objects.
This gripper is an under-actuated gripper with 16 DOFs including five three
DOF three-knuckle
fingers (1.1, 1.2, 1.3, 1.4 and 1.5) and one DOF movable palm (1.6). There are
two stages of
under-actuation: the first stage for five fingers with one movable palm and
the second for the
three-knuckle finger.
For the three-knuckle finger, two four-bar mechanisms are connected serially.
It is a traditional
approach for the linkage finger. The prominent feature of this invention is
the under-actuated
five fingers and one movable palm. A specially designed single-input-six-
output planetary gear
train (1.8) with a single actuator is used to drive five fingers and a palm.
The key point of this
gear train focuses on uniform distribution of the torque being transferred.
A grasping object can have a variety of shapes and sizes. In the case of
irregular shaped objects,
a three-finger gripper may fail to grasp them because of its inherent
characteristics of limited
gesture. To circumvent this major limitation, the present invention provides a
gripper having a
varied gesture.
In this invention, the gripper comprises of two rotational fingers (2.4 and
2.5) and three fixed
fingers (2.1, 2.2 and 2.3). With these two rotational fingers, the gripper can
have the best gesture
for a robust and power grasping of a wide class of objects. Referring to Fig.
1, two Geneva
mechanisms (1.9 and 1.10) are designed to achieve a variety of gripper
gestures for different
shapes of an object. Each rotational finger has five positions; so twenty
gestures of the gripper
can be possible according to the shape of the object. Fig. 2.a ¨ Fig. 2.d show
some examples of
gestures.
In this invention, the two rotational fingers play an important; they are the
key factors for fault-
tolerant functionality of the proposed gripper. The proposed gripper can work
even when some
of the fingers are damaged especially during its long-term unattended
operations in which the
subsystem failure cannot be fixed in time.
Fig. 3 shows fault-tolerant functionality of the gripper. Fig. 3.a shows the
case when the power
finger (3.3) is damaged wherein its function is substituted by the rotational
finger (3.5).
Simultaneously, the rotational finger (3.4) can be adjusted to provide the
required gesture
corresponding to the shapes and sizes of the gripped object. Fig. 3.b shows
the case when both
the power fingers (3.2 and 3.3) are damaged wherein their functions are
substituted by the two
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rotational fingers (3.4 and 3.5), respectively, to provide the required
gripping ability with the
three-finger gripper. Fig. 3.c shows the case when the fixed thumb is damaged.
Fig .3.d shows
the distribution of working positions of the fingers.
In the present invention, the gripper can be repaired easily within a short
period. This feature of
easy and fast to repair is due to the fact that this invention uses the same
structure for all five
fingers and the main fingers can be replaced with the rotational fingers.
Unlike traditional underactuated grippers, this invention involves two types
of mechanisms, one
for the hand layer and another for the finger layer. In the hand layer, a
specially designed
planetary gear train (one-input-six-output) (Fig. 4) with a single actuator is
used to drive six
subassemblies (five fingers and a palm). This train is consisted of five
differential gear trains
connected in series; each differential gear train is one-input-two-outputs.
There are two types of
these trains; one has an output ratio of 1:1 (three differential trains) while
the other has an output
ratio of 1:2 (two differential trains). These differentials in combination
provide a uniform driving
torque to five fingers and the movable palm. Motor (4.1) drives the second
(count from right to
left) differential gear trains 4.3. The outputs of 4.3 drive the third
differential gear trains 4.4 and
fourth differential gear trains 4.5, respectively. One of the outputs of 4.4
drives the first
differential gear trains (4.2), and the other drives the Fixed Power Finger
1st (1.10). And one of
the outputs of 4.5 drives the fifth differential gear trains (4.6), and the
other drives the Fixed
Thumb (4.9). Finally, 4.2 drives the Rotational Thumb 2nd (4.11) and the Fixed
Power Finger 2nd
(4.12), respectively, and 4.6 drives the Rotational Thumb 1st (4.8) and Palm
(4.7), respectively.
In the existing under-actuated grippers, the driving method in general
includes tendon driven,
rope pulling, differential gear trains, specially designed linkages, and
artificial muscles. This
invention uses special linkage fingers for their simple structure and low
cost; these fingers can
provide a large gripping force. In the finger layer, the underactuated three-
knuckle fingers use
two series connected four-bar linkage mechanisms. Furthermore, a set of
adjustable linkage is
used to extend the range of applications. Referring to Fig. 5, Spirit Knuckle
(5.1), Reality
Knuckle (5.2) and Substance Knuckle (5.3) form a three-knuckle finger.
Substance Link (5.7),
Substance Knuckle (5.3), R&S Link (5.6) and Reality Link (5.5) form the near
four-bar linkage,
and Reality Link (5.5), Reality Knuckle (5.2), S&R Link (5.4) and Spirit
Knuckle (5.1) form far
four-bar linkage. The finger is driven by Driver Rocker (5.8). The motion of
the output of
differential gear trains is transferred by Driver Screw Rod (5.9) to 5.8;
then, 5.8 actuates the near
four-bar linkage, and thus, the motion is transferred to 5.5, and then to the
far four-bar linkage.
When the three knuckles contact the surface of an object, the movement of the
finger stops and
the gripping force is exerted on the object until successful grasp of the
object is achieved.
Mostly in the traditional design, a palm is always designed to be fixed as a
fixed frame. At the
beginning of the grasping action, an object (to be grasped) is required to be
at a close distance
from the palm; next, the object moving towards to the palm is pushed by the
fingers and its
position and attitude are changed inevitably requiring location recalibration
and subsequent
trajectory re-planning. Unlike the fixed palm used in the traditional design,
a movable palm
named Palm (1.6) is designed to synchronize with the motion of five fingers
with its minimal
influence on position and attitude of the gripping object. As shown in Fig. 6,
when griping is
carrying out, finger 6.2 and finger 6.3 installed on the frame 6.5 move
towards object 6.1. When
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the finger knuckle contacts the object, it applies the grasping force on
object 6.1 to move towards
the frame. The position and attitude of the object get changed and the
friction occurs between
the knuckle and object. In this invention, a movable palm (6.4) as well as
five fingers are
proposed to provide a grasping force to object 6.1. When an appropriate
damping ratio for finger
joints is applied, there is no influence to the original position and attitude
of the gripped object;
recalibration and re-planning can be omitted and furthermore, the off-line
calculations can
replace the on-line calculations leading to reduced cost of hardware and
software implementation.
Another consideration of this invention is the bionics based multi finger
gripping. A thumb-index
web liked hook named Hook Slope (7.1) is designed to function as the initial
position reference
of the manipulator on which the gripper is installed.
References
[1] Okada T. Computer control of multi jointed finger system for precise
object handing. IEEE
Transactions on Systems, Man and Cybernetics. 1982, 12 (3): 289- 299.
[2] Mason M T, Salisbury J K. Robot hands and the mechanics of
manipulation. M IT Press,
Cambridge, USA, 1985: 3- 93.
[3] Jacobsen S C, Wood J E, Knutti D F, et al. The U TAH/MIT Dextrous Hand:
Work in Progress.
The International Journal of Robotics Research. 1984, 3 (4):21- 50.
[4] Nakano Y, Fujie M, Hosada Y. Hitachi's robot hand. Robotics Age, 1984, 6
(7): 18- 20.
[5] Caffaz A, Cannata G. The design and development of the D IST 2 hand
dextrous gripper.
Proceedings of the IEEE International Conference on Robotics and Automation.
Leuven,
Belgium. 1998: 2075-2080.
[6] Melchiorri C, Vassura G. Implementation of Whole-Hand Manipulation
Capability in the UB
Hand System Design. Advanced Robotics, 1995, 9(5): 547-560.
[7] Hirzinger G, Fischer M, B runner B, et.al. Advances in robotics: the DLR
experience [J].The
International Journal of Robotics Research. 1999, 18 (1): 1064- 1087.
[8] Loychik C S, MD ifler M A. The robonaut hand: a dexterous robotic hand for
space
[J].Proceedings for the IEEE International Conference on Robotics and Automat
ion. Detroit,
Michigan. 1999: 907-912.
[9] Dinsdale J. Short course on electronics and control for mechanical
engineers: high performance
servo actuators and drives. Cranfield, Co 11. Manuf. Cranfield Inst.
Technology, 1986.
[10] Archer J R, Blenkinsop P T. Actuation for industrial robots. Proceedings
of the Institution of
Mechanical Engineers, Part B: Journal of Engineering Manufacture May 1986 vol.
200, No. 2,
85-89.
[11] US Patent: US005052736
[12] US Patent: US005108140
[13] US Patent: US004955918
[14] US Patent: US005280981
[15] US Patent: US004957320
[16] US Patent: US006505870
[17] US Patent: US005762390
[18] US Patent: US006669257
[19] US Patent: US007614673
[20] US Patent: US008231158
[21] US Patent: US005046375
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