Anthropomorphic Robot Hand

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Anthropomorphic Robot hand: Gifu Hand II

INTRODUCTION

IT IS HIGHLY expected that forthcoming humanoid robots will execute

various complicated tasks via communication with a human user. The humanoid

robots will be equipped with anthropomorphic multifingered hands very much like

the human hand. We call this a humanoid hand robot. Humanoid hand robots will

eventually supplant human labor in the execution of intricate and dangerous tasks

in areas such as manufacturing, space, the seabed, and so on. Further, the

anthropomorphic hand will be provided as a prosthetic application for handicapped

individuals.

Many multifingered robot hands (e.g., the StanfordJPL hand by Salisbury

et al. [1], the Utah/MIT hand by Jacobsen et al. [2], the JPL four-fingered hand by

Jau [3], and the Anthrobot hand by Kyriakopoulos et al. [4]) have been developed.

These robot hands are driven by actuators that are located in a place remote from

the robot hand frame and connected by tendon cables. The elasticity of the tendon

cable causes inaccurate joint angle control, and the long wiring of tendon cables

may obstruct the robot motion when the hand is attached to the tip of the robot

arm. Moreover, these hands have been problematic commercial products,

particularly in terms of maintenance, due to their mechanical complexity.


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To solve these problems, robot hands in which the actuators are built into

the hand (e.g., the Belgrade/USC hand by Venkataraman et al. [5], the Omni hand

by Rosheim [6], the NTU hand by Lin et al. [7], and the DLRs hand by Liu et al.

[8]) have been developed. However, these hands present a problem in that their

movement is unlike that of the human hand because the number of fingers and the

number of joints in the fingers are insufficient. Recently, many reports on the use

of the tactile sensor [9][13] have been presented, all of which attempted to realize

adequate object manipulation involving contact with the finger and palm. The

development of the hand, which combines a 6-axial force sensor attached at the

fingertip and a distributed tactile sensor mounted on the hand surface, has been

slight.

Our group developed the Gifu hand I [14], [15], a five-fingered hand driven

by built-in servomotors. We investigated the hands potential, basing the platform

of the study on dexterous grasping and manipulation of objects. Because it had a

nonnegligible backlash in the gear transmission, we redesigned the

anthropomorphic robot hand based on the finite element analysis to reduce the

backlash and enhance the output torque. We call this version the Gifu hand II.

In this paper, we present design concepts and mechanical specifications of

the Gifu hand II equipped with a distributed tactile sensor. The Gifu hand II has a

thumb and four fingers; the thumb has four joints with four-degrees-of-freedom

(DOF) and the finger has four joints with 3-DOF; and the two joint axes of the

thumb and the finger near the palm are orthogonal. Moreover, a developed

distributed tactile sensor with 624 sensing points can be attached to the hands

surface and it is equipped with a six-axes force sensor at each fingertip. The design

concepts and the specifications of the Gifu hand II, the basic characteristics of the

tactile sensor, and the pressure distributions at object grasping are described and

discussed herein. Our results show that the Gifu hand II has a high potential to

perform dexterous object manipulations like the human hand.

ROBOT HAND DESIGN


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An overview of the developed anthropomorphic right and left version of the

Gifu hand II is shown in Fig. 1, in which the right hand is equipped with force

sensors and tactile sensors. The right and left hands are designed symmetrically

and have a thumb and four fingers. The design mechanism of the thumb and finger

are shown in Fig. 2(a) and (b), respectively. The structure of the left hand is shown

in Fig. 3. Servomotors and joints are numbered from the palm to the fingertip. The

thumb has four joints with 4-DOF and the fingers have four joints with 3-DOF.

Movement of the first joint of the thumb and of the fingers allows adduction and

abduction, and that of the second joint to fourth joint allows anteflexion and

retroflexion. The main difference between the thumb and the fingers is that the

fourth joint of the fingers is actuated by the third servomotor through a planar four-

bar linkage mechanism. Thus, the Gifu hand II has 20 joints with 16-DOF. Each

servomotor (Maxon dc motor, made by Interelectric AG) has a magnetic encoder

with 16 pulses per revolution. Table I summarizes the characteristics of the Gifu

hand II.

A. Design Concept


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We conducted this study using the Gifu hand II as a platform for research

on dexterous grasping and manipulation of the multifingered hand. The hand is

designed to be compact, lightweight, and anthropomorphic in terms of geometry

and size, such that it performs grasping and manipulations like the human hand.

The design concept is as follows:

1) Size: It is desirable for the robot hand to resemble the human hand in size and

geometry for purposes of skillful manipulation. The robot hand was designed to be

similar to a relatively large human hand, and has a thumb and four fingers.

2) Number of Joints and Number of DOF: In a human hand, both the thumb and

fingers have four joints [6, p. 192]. It is difficult for humans to control the

outermost two joints of the fingers independently, because the fourth joint engages

with the third joint. However, humans can control the joint angles of the

thumb almost independently. The thumb is more dexterous and powerful than the

fingers. The independent joint needs an independent actuator in the robot hand.

This makes it hard to design a light weight hand. The finger can be modeled as a

link mechanismwith four joints and 3-DOF, and the thumb can be modeled as a

link mechanism with four joints and 4-DOF. Controlling the finger made with

coupled joints may be more difficult than controlling the finger made with all

independent joints, in terms of grasping and manipulation. However, the finger

made with coupled joints, which has more links than the finger made with

independent joints, can grasp and manipulate more objects of various shapes than

the finger made with fewer links. This is due to the fact that the area for grasping

in the finger made with coupled joints is larger than that of the grasping area of the

finger made with fewer links. Therefore, coupling will augment the dexterity of the

hand. The number of joints and number of DOF of the robot hand were designed to

mimic those of the human hand. The thumb is actuated by four servomotors and

the fingers actuated by three servomotors. The fourth joint of

the fingers are driven by the third servomotor through a planar four-bar linkage

mechanism. The first joint and the second joint of human finger cross almost


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orthogonally at one point. Hence, the hand was designed such that the first joint

and the second joint of each finger cross orthogonally at one point by means of an

asymmetrical differential gear. Moreover, the asymmetrical differential gear

enables the second joint axis to be placed near the surface of the palm, which make

an effect to resemble a finger motion of the human.

3) Opposability of the Thumb: The thumb of the human hand can move in

opposition to the fingers. Dexterity of the human hand in object manipulation is

caused by this opposability. The robot hand was designed such that it has an

opposable thumb.

4) Built-In Servomotor: For easy attachment to the robot arm, the robot hand was

designed such that all joints are driven by built-in dc servomotors with a rotary

encoder. To produce a high stiff hand, the transmission system was created by

using high stiff gears such as a satellite gear and a face gear instead of low stiff

gears such as a harmonic drive gear, and without using tendon cable.

5) Unit Design: Easy maintenance and easy manufacture of the robot hand are very

important, so each joint was designed as a module and each finger was designed as

a unit. Due to the unit design of the finger, hands having from two to five fingers

are easily made.

6) Force Sensor: Each finger of the robot handwas designed to be equipped with a

six-axes force sensor (nano sensor made by BL. AUTOTEC Company) for

compliant pinching.

7) Distributed Tactile Sensor: There are many sense organs in the human hand.

These permit the human hand to manipulate an object dexterously. It is expected

that more tactile sensors enable more dexterous manipulations. The robot hand was


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designed to be mounted with a developed distributed tactile sensor with 624

detecting points.

8) Wiring: Wiring is important in robotic mechanisms. All of the wires in the

motor, force sensor, and tactile sensor should not prevent object manipulation by

the robot hand.We designed all of the wires to be located along the back of each

finger and palm. The hard wire used in the commercialized force sensor was

changed to a soft wire, so as not to cause an external force to arise due to motion in

the hard wire.

B. Improvement of Hand Mechanism

The mechanism of the Gifu hand II is a dramatic improvement over the

Gifu hand I [14], in the following ways.

1) Reduction of Backlash: The Gifu hand I employed an asymmetrical differential

reduction gear consisting of bevel gears at the first and second joints of the thumb

and fingers. This asymmetrical differential gear enabled the second joint to be

placed near the surface of the palm and the axes of the first joint and the second

joint to be orthogonal. However, a backlash occurred because the axial force of the

bevel gear moved the shaft of the bevel gear. Moreover, abrasion of the shank

made by the aluminum material increased the backlash. To reduce the backlash in

the Gifu hand II, face gear, in which the axial force is not generated, was adopted


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instead of bevel gear, and the frame material was change to the titanium alloy,

which shows strength and excellent performance against abrasion. By these

modifications, the backlash of the first joint decreased from 8

to 1.0 . The backlashes of the other joints also decreased by nearly the same

amount.

2) Higher Output Torque and Higher Response: For higher output torque, a

higher-power motor and high-reduction ratio were adopted at the first and second

joints. As a result, output torques of the first joint and the second joint are 3.46

Nm. The output force at the fingertip of the thumb is 4.9 N, which is about eight

times that of the Gifu hand I. The frequency gain characteristics of the first joint of

the thumb in velocity feedback control is shown in Fig. 4. Its measurement was

made by setting the first joint axis vertical in order to ignore the effect of gravity.

The velocity signal of the joint was obtained by the digital differential with regard

to the position signal. The velocity feedback gain was set such that a proportional

and differential (PD) control with a desired angle of 15 generates no vibration; the

results of this are shown later. The bandwidth is 8.6 Hz. The other joints were also

measured similarly, and the results are shown in Table I. The minimum bandwidth

of the robot hand is 7.5 Hz, which exceeds the responsibility of the human finger,

the bandwidth of which is, at most, 5.5 Hz. This means that the robot hand can

move more quickly than the human hand.


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3) Higher Stiffness: By stress analysis using a finite element analysis system, the

regulation of the safety factor of the mechanism was attempted. On the condition

that the largest force acting on the fingertip is 12 N, the force affecting each part

impacted and the safety factor of each part were calculated. After reviewing the

part or parts that had too low or too high a safety factor, the thickness and form of

the relevant parts were revised. Table II shows examples of the safety factor

analysis. We assumed that a standard safety factor was 10. In the Gifu hand I, the

safety factors of some parts were less 10 or over 50. As an example, results of the

finite element analysis of the No. 1 gearhead housing in Gifu hand I are shown in

Fig. 5. Fig. 5(a) and(b) are an equivalent stress distribution diagram and

deformation figure, respectively. Redesign reduced the maximum stress of thegear-head housing from 120 to 26 Mpa. The maximum deformation was also

reduced from 0.0075 to 0.004 mm. The Gifu hand II was greatly improved in terms

of the safety factor and deemed to have good balance.


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DISTRIBUTED TACTILE SENSOR

The distributed tactile sensor for the Gifu hand II was developed with the

cooperation of Nitta Corporation. The shape of the tactile sensor is shown in Fig. 6.

Tactile sensors are distributed on the surface of the fingers and palm. The thumb

and the fingers of Gifu hand II each consist of four links and four joints, in which

the base link is mounted in the palm, and other three links are equipped with the

tactile sensors. The base link and the other links of the fingers of Gifu hand II

correspond to the metacarpal bone and three phalanges of a human hand.

In a human hand, the metacarpal bone is located in the palm, and is barely mobile.

However, in Gifu hand II, the base link and the other three links of the thumb

correspond to the carpal bone, metacarpal bone, and two phalanges of a human

thumb, with the difference being that the metacarpal bone is mounted in the palm

and can move independently, and the carpal bone is immobile. Hence, three parts

of the finger and thumb are equipped with the tactile sensors. The tactile sensor has

a grid pattern electrode and uses conductive ink in which the electric resistance

changes in proportion to the pressure on the top and bottom of a thin film.

Characteristics of the tactile sensor are shown in Table III. Numbers detecting


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points on the palm, the thumb, and the finger are 312, 72, and 60, respectively, and

the total number of measurement points is 624. Electrode width is 2 mm, column

pitch is 4 mm, and row pitch is 6 mm. Maximum load is about 74 kN/m ,

resolution of measurement is 8 bits, and sampling cycle is 10 ms/frame. A response

characteristic of the tactile sensor is more than 1 kHz. Detected data on the sensor

sheet is transported to a PC through a special interface board. This tactile sensor

has nonlinear characteristics such as hysteresis, creep, and dispersion in

measurement sensitivity. The hysteresis characteristic curve and creep

characteristic curve at three arbitrarily selected detecting points are shown in Fig. 7

and Fig. 8, respectively. Vertical axes in these figures are output of the tactile

sensor through an analog-to-digital (A/D) converter. These figures also show the

dispersion in measurement sensitivity. Signal processing taking such nonlinear

characteristics into account is required.

Fig. 9 shows an overview of the 6-DOF robot arm (VS6354B, made by

DENSO Company) and the Gifu hand II that is equipped with the six-axes force

sensor at each fingertip and the developed tactile sensor. The Gifu hand II can be

easily attached to another industrial robot by changing a base plate of the hand.

Fig. 10 shows computer graphics of output patterns of the tactile sensor, measured

while Gifu hand II is holding a soft spherical object 95 mm in diameter, and a hard

cylindrical object 66 mm in diameter. The height of the pole

listed in Fig. 10 refers to the output level. Some outputs may include noise because

of an inadequacy of attachment between the tactile sensor and the hand frame. The

extended output patterns of the tactile sensor are shown in Fig. 11. The vertical line

in Fig. 11 is the output of the tactile sensor of which the unit is millinewtons. The

points on the horizontal plane are the measurement points of the tactile sensor,

which are opened to show the measurement data of the force. It is clear that the

output pattern depends on the shape of the object. When the human hand grasps the

same objects using the globe attached to the tactile sensor, the output patterns of

the tactile sensor are those shown in Fig. 12. In this case, a part of the tactile sensor

was cut to fit the size of the human hand. The output pattern by the human hand


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differs from that by the robot hand in part because the human hand is covered with

soft derma. However, these results show that the Gifu hand with the distributed

tactile sensor has a high potential for dexterous grasping and manipulation.


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EXPERIMENTS

We constructed a PC-based robot hand control system as shown in Fig. 13.

Four 4ch counter boards, two 8ch digital- to-analog (D/A) boards, four 16ch A/Dboards, and a timer board are connected to the PCI bus. Signals of D/A are inputted

to servomotors through a 16ch linear amplifier. The operating system of the PC is

Windows98. A tactile sensor is connected to another PC for measurement and

display, and the maximum sampling cycle of detecting the 624 points is 100 Hz.

The force resolutions coupled with the A/D converter are 32, 32, and 98 mN along

the x,y, and z axes, and the moment resolutions are 0.20, 0.20, and 0.26 m N m at

about the x,y, and z axes, respectively. The Coulomb frictions of the first joint to

the fourth joint of the thumb are 3.01, 0.77, 1.30, and 0.46 m N m, respectively. As

shown in Table I, these Coulomb frictions are at most 1.5% of the output torque of

the thumb. at most 1.5% of the output torque of the thumb.

Step response of the joints of the thumb is shown in Fig. 14. Each joint

angle moved from 0 rad to the desired joint angle at 0.26 rad in at least 0.15 s. The

rising times of the first joint and the second joint are larger than those of the third

joint and fourth joint due to saturation of servomotor input, which is caused by

high gear reduction ratio. The results show that the robot finger moved more

quickly than the human finger, the response time of which is 5.5 Hz, at most.


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We examined the characteristics of force control during a contact task by

using the thumb as shown in Fig. 15. Each joint position error and contact force are

presented in Fig. 16. The initial position of the tip of the thumb was 105 mm from

a constraint surface and a desired joint position was given by a 5 polynomial in

time, which interpolates between the initial joint position and the terminal joint

position within 0.5 s. A desired contact force in the anti-normal direction of the

constraint surface was given by a step with 0.49 N after contact. In the noncontact

state, proportional, integral, and differential (PID) control was adopted as a joint

position control, and in the contact state a position and force hybrid control [16]

was adopted. The sampling cycle was 2 ms. This figure shows that the contact

force converges to the desired value with a rapid response.


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CONCLUSION

Anthropomorphic robot hands have been actively developed during past

two decades. However, development of a five-fingered hand with built-in

actuators, and equipped with a force sensor at the fingertips and a tactile sensor on

the fingers and palm has only demonstrated in the NTU hand [7], in which the

bandwidth of the first joint of the thumb is 0.15 Hz, and the number of detecting

points of the tactile sensor is 18. The Gifu hand II has a much higher number of

sensors and higher response than any previously developed robot hand, and can

move more quickly than the human hand. We consider that the Gifu hand II is

useful as a research tool for dexterous robot manipulation using force sense and

tactile sense. We have presented herein the design concept and the mechanism

features of the Gifu hand II, which is designed to be used as a standard

anthropomorphic robot hand. The Gifu hand II is actuated by built-in servomotors,

distributed tactile sensors can be attached to its surface and it is equipped with a

six-axes force sensor at each fingertip. The mobile space and geometrical size of

the fingers are similar to those of the fingers on the human hand. We intend to use

the Gifu hand II for future study of dexterous manipulation by the robot arm.


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REFERENCES

IEEE/ASMS TRANSACTIONS ON MECHATRONICS, VOL.7.NO.3,

SEP 2002

MECHANISM DESIGN OF ANTHROPOMORPHIC ROBOT

HAND:GIFU HAND 1J.ROBOT.MECHATRON.,VOL 11,1999

IEEE ROBOTICS AND AUTOMATION,1995

WWW.ANDROIDWORLD.COM

WWW.HUMANOID.COM


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ABSTRACT

This paper presents an anthropomorphic robot hand called the Gifu hand II,

which has a thumb and four fingers, all the joints of which are driven by

servomotors built into the fingers and the palm. The thumb has four joints with

four-degrees-of-freedom (DOF); the other fingers have four joints with 3-DOF;

and two axes of the joints near the palm cross orthogonally at one point, as is the

case in the human hand. The Gifu hand II can be equipped with six-axes force

sensor at each fingertip and a developed distributed tactile sensor with 624

detecting points on its surface. The design concepts and the specifications of the

Gifu hand II, the basic characteristics of the tactile sensor, and the pressure

distributions at the time of object grasping are described and discussed herein. Our

results demonstrate that the Gifu hand II has a high potential to perform dexterous

object manipulations like the human hand.


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CONTENTS

INTRODUCTION

ROBOT HAND DESIGN

DISTRIBUTED TACTILE SENSOR

EXPERIMENTS

CONCLUSION

REFERENCES


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ACKNOWLEDGEMENT

Anthropomorphic Robot Hand

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