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A Designing of Humanoid Robot Hands in Endoskeleton and Exoskeleton Styles
Ichiro Kawabuchi

KAWABUCHI Mechanical Engineering Laboratory, Inc.
Japan
1. Introduction

This PDF is an author’s copy.
Published in “Humanoid Robotics New Developments“
by Advanced Robotic Systems International in June 2007

For a serious scientific interest or rather an amusing desire to be the creator like Pygmalion,
human being has kept fascination to create something replicates ourselves as shown inlifelike statues and imaginative descriptions in fairy tales, long time from the ancient days.
At the present day, eventually, they are coming out as humanoid robots and their brilliant
futures are forecasted as follows. 1) Humanoid robot will take over boring recurrent jobs
and dangerous tasks where some everyday tools and environments designed and optimised
for human usage should be exploited without significant modifications. 2) Efforts ofdeveloping humanoid robot systems and components will lead some excellent inventions ofengineering, product and service. 3) Humanoid robot will be a research tool by itself forsimulation, implementation and examination of the human algorithm of motions,behaviours and cognitions with corporeality.
At the same time, I cannot help having some doubts about the future of the humanoid robot
as extension of present development style. Our biological constitution is evolved properly to
be made of bio-materials and actuated by muscles, and present humanoid robots, on thecontrary, are bounded to be designed within conventional mechanical and electric elements
prepared for industrial use such as electric motors, devices, metal and plastic parts. Suchelements are vastly different in characteristics from the biological ones and are low in somesignificant properties: power/weight ratio, minuteness, flexibility, robustness with selfrepairing, energy and economic efficiency and so on. So the “eternal differences” in function
and appearance will remain between the human and the humanoid robots.
I guess there are chiefly two considerable ways for developing a preferable humanoid robot
body. One is to promote in advance a development of artificial muscle that is exactly similar
to the biological one. It may be obvious that an ideal humanoid robot body will be
constructed by a scheme of putting a skeletal model core and attaching the perfect artificial
muscles on it (Weghel et al., 2004, e.g.). Another is to establish some practical and realistic
designing paradigms within extensional technology, in consideration of the limited
performance of mechanical and electric elements. In this way, it will be the key point thatmaking full use of flexible ideas of trimming and restructuring functions on a target. For
example, that is to make up an alternative function by integrating some simple methods,
when the target function is too complex to be a unitary package. Since it seems to take longtime until the complete artificial muscles will come out, I regard the latter way is a goodprospect to the near future rather than just a compromise.

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In searching the appropriate designing paradigms for humanoid robots, it may be just thestage of digging and gathering many diverse and unique studies, styles and philosophies ofthe designing. After examining their practicability, reasonability and inevitability throughthe eyes of many persons over the long time, it will work out the optimised designingparadigms. I believe the most outstanding ingenuities in mechanical design are brought out
in robot hands, as they are most complex mechanical systems on a humanoid robot body.
Fortunately, I have had some experiences of designing them in each endoskeleton andexoskeleton styles. To contribute on such evolutive process as a mechanical engineer, I bringup here some of my ideas of designing robot hands.

2. Humanoid Robot Hand in Endoskeleton Style
2.1 Basic Design Conditions
There are two opposite orientations in planning specifications of a robot hand. One isemphasizing similarity in motion and function to the human hand. Another is emphasizing
similarity in size and weight. A robot hand following the former orientation tends tobecome rather large against the human hand. Although attempts to make it small and lighthave been done by putting the actuators away into a forearm segment and actuating thefingers via links or wires (Jacobsen, 1984, e.g.), the whole system is still large and heavy asa load at an end of arm. A robot hand following the latter orientation tends to lack functions
as shown in general prosthetic hands. Most research robot hands, except some modern
prosthetic ones (Harada hand, e.g.), are designed following the former orientation to enlarge
their universality on study scenes. And it is rational estimation that the properties aimed in
the latter orientation will be resolved spontaneously as the industrial progress leads generalmechanical and electric elements high-performance. However, I have taken the latterorientation, because I believe it should be also constitutive approach to an ideal robot hand,moreover it is challenging for me to contrive novel designs of complex mechanisms. Then
I have set the basic design conditions as shown on Table 1.
1.
2.
3.

4.

5.

Outline should be similar to the human hand with 5 fingers.
Every motor should be embedded within the outline, and connections with outsidecontroller and power source are limited to some slim and limber electric cables.
Compactness and lightness should be equal to the average naked human hand.
Concretely, mass is at most 500g, longitudinal length from fingertip to prospective
center of wrist is at longest 200mm, and diameter of fingers is at most 20mm.
As far as the third condition is satisfied, number of motors and range of finger
movements should be enriched for realizing some practical functions like holding
bulk object, pinching thin card, and the most sign language. As key finger motions,
all fingers move widely from opening to clenching, and the thumb and each ofother fingers contact opposing on both fingertips.
As far as the fourth condition is satisfied, power of motors and robustness of
mechanisms should be augmented. The desirable output force at each fingertip isover 5N under a straight posture.

Table 1. The basic design conditions.

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2.2 Kinematical Arrangement of Joints
Fig. 1 shows the kinematical arrangement of joints in my robot hand. I identified each joint
with two numbers and describe it as symbol Jn,m, in which the first suffix means finger
number and the second one means joint number on each finger. The reason I avoid the
popular joint nomenclature with the anatomical term like MP (metacarpal phalangeal joint)and DIP (distal interphalangeal joint) is that some of original human joints are complex one
with multi degree of freedom (DOF) and hardly be substituted by a unitary joint mechanism.

Fig. 1. The kinematical arrangement of joints.
The joint arrangement on the four fingers except the thumb would be common to mosthumanoid robot hand. At the same time, difference of thumb structures between several
competitive robot hands (Fig. 2) shows clearly the diverse design philosophies following
individual purposes and available technologies. The every thumb structure on Fig. 2 has
a complex joint that consists of a pair of revolute joints intersecting their axes at one point.
The complex joint is an influential technology and the Shadow hand, above all, has two ones
to realize the most similar action and appearance to those of human. On the other hand,
I avoided such joint with estimating its complexity will require large space that violates thebasic design conditions when it would be designed by my present technical capabilities.
The order of each direction of joint axis and fingertip is also an influential characteristic forrange and variety of the thumb motion. Especially in the NASA hand, to compensate
disadvantages by low of DOF, the order is decided advisedly with considering a reasonable
emulation of human thumb motion (Lovchik & Diftler, 1999). In my robot hand, the axis
direction of joint J1,3 is set for twisting the thumb tip that will conduce a stable pinching
mentioned after. The axis directions of two joints J1,1 and J1,2 are decided mainly because of
convenience to embed reducers with wide movable range.

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Fig. 2. Thumb structures of competitive robot hands.
2.3 Compression of Independent DOF
An “actuator assembly” that consists of a motor, an encoder and a reducer occupies
considerable space in robot hand rigidly. Therefore, to plan a compact robot hand, it isa reasonable way to compress the number of equipped assemblies. This means furthermore
compression of the independent DOF on the robot hand, so I pursued this idea boldly withkeeping a reasonable emulation to the human hand motion.
In the four fingers except the thumb, most robot hands are designed with the two distal
joints Jn,3 and Jn,4 coupled-driven by one motor, since those joints in the human hand often
move together. I furthered this idea of interlocking and made all three joints Jn,2, Jn,3 and Jn,4interlocked. In the same way, the relative opening-closing motion among the four fingers at
the three root joints J2,1, J4,1 and J5,1 is also interlocked. Moreover the joint J3,1 is eliminatedsince the human middle finger rarely moves at the joint. In the thumb, the two distal joints
J1,4 and J1,5 are interlocked and the joint J1,3 is fixed in a certain angle.
Based on the consideration above, the essential independent DOF was extracted into 8. The
member joints for each DOF are listed on Table 2(a) and named the “essential set”. Although
the essential set can realize almost all required finger motions like making a loop with the
thumb and the little finger, it lacks sophisticated dexterity for some delicate handlings like
pinching a thin card. At last, I have added some DOF on the essential set as listed on the
Table 2(b) and it is named the “latest set”.
In the following two sections I explain mechanisms corresponding to the essential set, and in
the later sections I introduce more complex mechanisms corresponding to the latest set.
1234MemberJ1,1
J1,2
J1,4+J1,5
J2,1+J4,1+J5,1

5
6
7
8

MemberJ2,2+J2,3+J2,4
J3,2+J3,3+J3,4
J4,2+J4,3+J4,4
J5,2+J5,3+J5,4

1
2
3
4
5

Member
J1,1
J1,2
J1,3
J1,4
J1,5

(a) The essential set (8DOF)
Table 2. The independent DOF set and member joints.

6
7
8
9
10

MemberJ2,1+J4,1+J5,1
J2,2 (differential)
J2,2+J2,3
J2,4
J3,2+J3,3

11
12
13
14
15

(b) The latest set (15DOF)

MemberJ3,4
J4,2+J4,3
J4,4
J5,2+J5,3
J5,4

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2.4 Global Finger Flexion Mechanism
As mentioned in the section 2.2 a complex joint in which two rotational axes intersect was
avoided in the thumb mechanism, however, the other four fingers retain such complex joint
at the root. The difficulties in developing such complex joint include how electric wirings
are passed through the joint. The delicate wirings should not be overstretched due to joint
rotation, so the wirings should be passed just or near the intersection point of two axes.
I introduced a scheme that no part of an actuator assembly is placed around the intersection
but enough empty space only for the wirings, at the same time the rotational power for thetwo axes is provided via wire or link mechanism from an off-site motors.
Fig. 3 shows the internal structure of a basic finger, all the four fingers have the samemechanism. A small DC motor with a built-in encoder (Faulhaber, model 1516SR or 1319SR)
is embedded in the largest finger segment lengthwise, like filling the segment volume.
A reducer that consists of a crown gear train and two-stage planetary gear train is built inthe joint Jn,3 and drives it. For optimum integration: a combination of larger and smaller
gears is effective in uniting higher reduction ratio and efficiency, axes of all gear trains arearranged coaxial to that of joint Jn,3, so that volume of the joint part admits possibly largest
diameter of gears without making the finger segment longer. In the actual reducer, thecrown gear which diameter is almost equal to that of finger segment engages with the motor
pinion and derives 10 times torque efficiently. In a general gear train with axes mutuallyorthogonal, the rigidity is low and the play is generated easily; so it is reasonable putting thecrown gear train at the first stage viewed from the motor, where the transmission torque is
low and the influence of play is negligible. Moreover the planetary gear train is constructed
as a unique mixture of planetary-type and star-type for most compactness, so this reducer
gets high reduction ratio 1/350 and the maximum joint torque is at least 0.5Nm.

Fig. 3. The internal structure of a basic finger.
It is very important feature of a robot hand that it has smooth back-drivability at each joint
for stable control of contacting force on a fingertip. I think it is more acceptable a slight playat gears as a consequence of high back-drivability than a large frictional resistance of gears
as a consequence of elimination of the play, thanks to the general robot hand is demanded toperform relatively small force and low absolute accuracy in finger motions. So the actual
fingers of my robot hand are manufactured with slight play to get most back-drivability;each finger can be moved passively by small force on the fingertip at most about 0.3N.

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The interlocking mechanism among the three joints Jn,2, Jn,3 and Jn,4 consists of wire-pulleymechanisms with pulleys carved on the sidewall (Fig. 4(a)). Since they are thin enough thefinger segments afford some internal space for motor, sensor and electric component.
Although identical transmission can be constructed using a link mechanism, it tends tobecome larger due to restriction on facilitating smooth transmission in large rotational angle
near 90degree. Considering a reasonable emulation of the human motion, the transmission
ratios are set 7/10 from Jn,3 to Jn,2, and 5/7 from Jn,3 to Jn,4 respectively (same (b)).

(a) Wire-pulley mechanisms(b) Maximum flexion range

Fig. 4. Interlocking mechanism of the global finger flexion.
2.5 Abduction-Adduction Mechanism
Since the relative opening-closing motion among the four fingers except the thumb is called
abduction-adduction in the anatomical term, the interlocking mechanism among the three
joints J2,1, J4,1 and J5,1 was named after it. Introduction of this mechanism has mainly two
reasons as follows. The first is cutting back on the actuator assembly. The second isa practical idea that the ring and little fingers do not need such motion independently,
because they on the human hand rarely take part in a dexterous pinching function. So it canbe said that the target of this mechanism is only generating a relative opening-closing
motion between the index finger and the middle finger.
Rotational angle of those joints is small, so this mechanism is constructed as a linkmechanism (Fig. 5), and in quest of efficiency and compactness it is designed as below. The
joint J4,1 is selected as driven by a motor directly, so that the power transmission to otherjoints becomes the shortest route with high efficiency and low adverse influences like play.
To obtain a large gear ratio with possibly least stages of gear train, a large sector gear thatpasses over the palm longitudinal length is adopted and fixed on the joint J4,1 (Fig. 6(a)), andthe entire reduction ratio is implemented as 1/400. Thanks to the drive motor is located on
the edge of the palm, over half of the space in the palm can be free for electric components.
Even when each finger is bent to the maximum flexion as shown in the Fig. 4(b), theopening-closing motion can act independently thanks to the clearance between fingertip andpalm; the robot hand can perform V-sign actions, e.g. freely. An electric printed circuitboard (PCB) that contains a microcomputer for local processing, motor amplifiers, motor
current sensors and an interface unit communicating with an outside controller througha serial signal line, is embedded like filling almost all the space in the palm (Fig. 6(b)).

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Fig. 5. Connecting linkage for the abduction-adduction motion.

(a) Location of the reduction gears and the linkage(b) Embedded PCB

Fig. 6. Mechanism and electric equipment in the palm.
2.6 Introduction of Independent Fingertip Motion
Stably and softly pinching thin paper and handling small piece in complicated shape are thevital functions for a humanoid robot hand, when it could be recognised dexterous enough.
To realize theses delicate functions, the key feature is a fine force and motion control on thefingertips. The human finger has excellent feature of controlling them finely in any direction,and furthermore it can generate large force in case of necessity. On the contrary, usual small
actuator assembly that consists of motor and geared power train never has such large“dynamic range” in force control. In case of my robot finger, the main actuator assembly forthe global finger flexion should be designed to generate possibly larger force for moving
itself in sufficient speed against its mass and inertia, and moreover grasping a heavy object.
Then it is hard for the actuator to be expected to generate fine force on the fingertip.

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After considering above, I focused attention on the joint Jn,4 (J1,5 in case of the thumb), as it
could be free from generating the large force above; that means it could concentrate to
generate a minute and fine force suitable for delicate pinching. Then I introduced a special
independent DOF at the joint Jn,4 (J1,5) so that it can control force and motion on thefingertip independently and finely. It is surely a little strange assignment of DOF while
general robot hand has interlocking between the two distal joints Jn,3 and Jn,4 (J1,4 and J1,5),
however, I think that it maintains the basic design conditions as general harmony as
a humanoid robot hand; the human finger also has latent ability to move its terminal joint
independently.
In addition, I propose a new strategy of the handling that combines a powerful grasping anda delicate pinching in consideration of that the small additional actuator at the joint Jn,4 (J1,5)
cannot sustain large contacting force at the fingertip.
In power grasping (Fig. 7(a)), powerful grasping forces generated by the main actuator aretransmitted to an object through the surface of finger segments except the fingertip. At thesame time, the delicate force generated by the joint Jn,4 (J1,5) lets the fingertip compliant to
the object, and enlarge the effect of slip-free grasping. The movable range of the joint Jn,4
(J1,5) is broadened enough into the backside, so that the function of fingertip can work wellin wide range of its posture. The shape of surface around the terminal joint is formed to besuitable for the direct contacting against the object.
In delicate pinching (Fig. 7(b)), the contacting forces on the object are generated only by thejoint Jn,4 (J1,5). When larger pinching forces that the joint Jn,4 (J1,5) cannot sustain arenecessary, rotating the joint to the dead end of movable range where a mechanical stopper
sustains the large force generated by the main actuator; strong pinching is executed.
This strategy can be categorized as a general idea for a two-stage mechanism that compiles
separated rough and fine control methods for enlarging the total dynamic range.

(a) Power grasping(b) Delicate pinching

Fig. 7. New strategy of object handling with independent fingertip motion.

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2.7 Independent Terminal Joint Mechanism
The mechanical feature necessary for the new handling strategy above is a smoothcompliance of fingertip to an object, in concrete terms a following motion to keep a soft
contacting force. Although a similar feature could be produced with adding a spring on theway of power transmission to the fingertip, the spring occupies not negligible space in thesmall finger segment. Another way introducing a force sensor on the fingertip is also hard inacquiring appropriate sensor and tuning up a force feedback controller robust. So the mostpractical way to realize the compliance is exploiting a simple open-loop torque control
method of the motor only by observing its current.
In this case, it requires a high efficient reducer with low frictional resistance and play ofgears. I adopted a gear train that uses only high precision spur gears with expectation of
their high efficiency. The motor length is restricted to the width of the finger segment,
because the motor must be laid widthwise to be parallel to the joint axis. I searched on the
web a possibly higher performance motor that fits into the limited length, and found a small
DC motor (Sanyo Precision, model KS-2717) in production for small electric appliances.
Since the output torque of the motor is very small 0.5Nmm in maximum, the reduction ratio
is needed to be possibly higher. To realize high gear ratio avoiding deterioration ofefficiency and inflation of volume, a two-stage thin gear train is built in against a sidewallwith combining possibly largest and smallest gears for maximizing the gear ratio (Fig. 8);
the gear module is 0.16 and the number of smallest gear’s teeth is only nine. Consequently,
the gear ratio is 3/250, and the maximum fingertip force is almost 2N.
A thin potentiometer is introduced to measure the angular displacement and put against
another sidewall. By this alignment of the reducer and the potentiometer on the bothsidewalls, the finger segment retains enough empty space inside.

Fig. 8. Actuator assembly for the independent terminal joint.
To confirm the smooth compliance of a fingertip, a primary experiment was conducted.
Concretely I examined active spring effect of the joint Jn,4 generated as follows. The joint Jn,4
was always position-controlled to keep its angular displacement θ n,4 at a certain target. The
original target was set 0, that meant the terminal segment and middle segment were in line.

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When the motor current of the joint Jn,4 exceeded a certain threshold, the target was shifted
gradually to a temporary target by adding/subtracting a small variation proportional to thedifference between the threshold and the present motor currents, so that the excess of motor
current got reduced. When the motor current fell below the threshold and besides the
temporary target was different from the original one, the temporary target was restored
gradually to the original one by adding/subtracting a small variation proportional to thedifference between the latest temporary target and the original one. In consequence ofrepeating this control in high frequency, the active spring effect was created stably.
In the experiment, the external pressure for examining the compliance was provided by theother finger part. Concretely, the finger part between the joints Jn,2 to Jn,4 was moved
repeating a slight sine wave motion by a position-control of the main motor, and keptpressing the fingertip against a fixed object. Contacting force between the fingertip and theobject was measured using a film-like force sensor (Nitta, FlexiForce) placed on the object;
as a matter of course the observed force was not used in the motor control.
Fig. 9 shows the result of experiment with transitions of the contacting force and the angulardisplacement θ n,4. The threshold of motor current was set in two values as examples thatprovide the limits of contacting force as about 0.9N and 1.4N respectively. Effectiveness ofthe smooth compliance of fingertip is confirmed by analysing each transition as follows.
When the contacting force just exceeded the limit, the joint Jn,4 started to rotated to release
the excess, so that the contacting force was stuck at the limit. The joint Jn,4 started to returnto the original position when the contacting force fell below the limit. These movements are
just the characteristics of desirable compliance. And stable constancy of the maximum
contacting force at the limit reveals high efficiency of force transmission through the reducer,
and prove adequacy of the estimating way of contacting force from the motor current.
This smooth compliance cannot be produced by a simple spring mechanism because it
cannot change the output force freely, then this practical function with minimum additionalparts is one of the distinctive features on my robot hand.
1.5

0.2
Limit

Contacting Force [N]

0.1
0.5
0

0

2

4

6

8

10

1.5

0
0.2

1
Limit

0.1

Angular Displacement [rad]

1

0.5
0

0

2

4

6

8

10

0

Time [s]Fig.9. Experimental result of the contacting force and the angular displacement of a fingertip.

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2.8 Thumb Twisting Mechanism
To fully maximize the capability of the smooth compliance generated by the independent
terminal joint mechanism, the direction of fingertip force should be oriented rightly in thenormal line of contacting surface on an object. This manner furthermore increases thestability of pinching in pressing the surface by entire flat pad on the fingertip. In order torealize this manner concretely, either or both of cooperating fingers in a pinching need totwist its fingertip along its longitudinal axis in a considerable angular displacement. Then
I gave the thumb the distinctive joint J1,3 for this twisting function.
Fig. 10(a) shows 5 segments in the thumb; each segment is identified with the number
starting from the root, and described as symbol S1,n. In the primitive robot hand with theessential 8DOF, the two segments S1,2 and S1,3 are integrated by fixing the twist angle.
Avoiding deterioration of the original mechanical harmony, the actuator assembly for thejoint J1,3 was designed additionally. To minimize the joint J1,3 volume, cylindrical case of the
motor for the joint J1,4 that is already embedded over segments S1,2 and S1,3 is exploited as
the axis of J1,3. Unlike the terminal joint mechanism, this twist function requires relatively
large motor for moving large mass of the finger structure, so a bulge is added on thesegment S1,2 to mount a motor same as that for the global finger flexion (Fig. 10(b)).
Although this bulge is a little unbecoming, I accept it as a reasonable way for preventing
stretch of the thumb length. The movable range of twisting is 45degree.
By the way, the two joints J1,1 and J1,2 have the same actuator assembly for the global finger
flexion shown in the Fig. 3, and each has adequate movable range of over 120degree and60degree respectively.

(a) Segments in the thumbFig. 10. Thumb mechanism with twisting function.

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Humanoid Robots, New Developments

2.9 Independent Root Joint Mechanism
In the four fingers except the thumb, since the both joints Jn,2 and Jn,3 need no little power inthe global finger flexion, the idea of interlocking these two joints and actuating them by one
relatively large motor has adequate rationality, as far as the finger has no more capacity toaccept two motors for actuating them independently. However, in some cases, theindependent motion of each joint is required to realize some slight motion like adjusting thecontacting place of a fingertip on an object. In order to demonstrate my technical capability
to realize such complex requirement additionally, an actuator assembly was introduced at
the joint J2,2 particularly.
As a matter of course there is no capacity to accept a large motor, the additional motor isselected as the same small one driving the terminal joint. As the global finger flexion shouldbe generated by the existing mechanism, the additional small actuator assembly should bedesigned to generate a differential motion as being overlapped on the global finger flexion.
Well, the pulley on the joint J2,2 is existing as a basement of the global finger flexion and its
shape is round and coaxial to the axis of joint J2,2, so it is convenient for realizing thedifferential motion by rotating the pulley around the axis.
Fig. 11 shows the actuator assembly to rotate the pulley. To sustain the large torque aroundthe joint J2,2 for the global finger flexion, it needs possibly larger reduction ratio. Therefore
a worm gear train, that generally has large gear ratio, is introduced, so that the entire
reduction ratio gets 1/1000. Although a worm gear train has no back-drivability, it is also anadvantage in this case because that gear train can support any large torque in case ofnecessity. The movable range of the pulley is +15 to -15degree that makes useful adjusting
motion at the fingertip in 10mm order.

(a) Worm gear mechanism to drive the pulley(b) Actual embedded situation

Fig. 11. Differential mechanism for the independent root joint motion.

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2.10 Smart Wiring for Bypassing Reducer
The quality of a robot system is evaluated from many kinds of dimension including
neatness of the electric wiring, since its weight and volume can bring recognizable
deterioration in the performance of high-speed motion and indisputably deteriorate theappearance. The lack of space for containing the wiring is the most common cause of thisproblem because expelling the wiring outside makes its weight and volume to increase. In
my robot hand, as mentioned in the section 2.4, the discussion about the designing root joint
structure of each finger was started by consideration of this problem. And more problem isoutstanding around the joint filled with the large reducer of ratio 1/350 meaning J1,1, J1,2,
J1,4, J2,3, J3,3, J4,3 and J5,3. Recognizing the significance of this problem, a unique andpractical design of wiring is introduced.
The role of the wiring is electric connection between the motor and sensor for the terminal
joint and the main PCB in the palm, and a thin flexible PCB with 3.5mm width makes it.
When the wiring is led as going around the reducer’s circular outline, the change of shortest
path length due to the finger flexion is remarkable, and then the method to retract andextract the corresponding length of wiring becomes the practical problem. My robot hand,
fortunately, has enough margin space in the finger segments, and it can be formed an empty
space where the wiring can adapt to the change of path length with changing the curving
line by itself as shown in Fig. 12.
By the way, this wiring style cannot be adopted on the two thumb root joints J1,1 and J1,2
because of lack of the internal space, and then the wirings through these joints are forced togo outside in a wide circle unbecomingly. This problem will be solved in the next
development step waiting for an investment opportunity.

(a) Change of wiring path due to the finger flexion(b) Flexible PCB

Fig. 12. Design of the wiring around the joint that contains the large reducer.

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2.11 Overall view of the Humanoid Robot Hand
As a conclusion of all previous considerations the latest model of my robot hand is built up
as shown in Fig. 13; it has 15DOF as defined on the Table 2(b) while it satisfies the basic
design conditions on the Table 1. The total mass including the internal electric equipment
except the long cable connecting outside controllers is just 500g. The connections to outside
systems are only φ 2.4 signal cable and φ 4.5 power cable. Some dimensions of details likethe length of each finger segment are referred to my hand.

Fig. 13 Overall profile of the latest model.

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To confirm dexterity of the robot hand, some experiments of representative and practicalhandling motions were conducted; this paper displays two handling types: pinching abusiness card and holding a pen (Fig. 14). The key evaluation items in these experiments
were the two distinctive functions: the smooth compliance on a fingertip and the twisting of
the thumb. All the fingertip forces were generated by the simple open-loop torque control
method explained in the section 2.7 without force sensors.
By the way, the smart wiring style explained in the section 2.10 is installed only to the latest
model, and the robot hand used in the experiments did not have it unfortunately.

(a) Pinching a business card(b) Holding a pen

Fig. 14 The representative and practical handling motions.
In the experiment of pinching a business card, the robot hand performed switching several
times two couples of pinching fingers: the thumb and the index finder/the thumb and the
middle finger (Fig. 15). In the junction phase when all the three fingers contacted on the card,
the thumb slid its fingertip under the card from a position opposing a fingertip to anotherposition opposing another fingertip. In the experiment of holding a pen, the robot hand
moved the pen nib up and down and sled the thumb fingertip along the side of the pen (Fig.
16). In both experiments, the objects: card and pen were held stably, and these achievements
prove the contacting force appropriate in both strength and direction could be generated ateach fingertip.

Fig. 15 Cyclical steps in the experiment of pinching a business card.

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Humanoid Robots, New Developments

Fig. 16 Cyclical steps in the experiment of holding a pen.
At the SIGGRAPH 2006, I got an opportunity to join into a participating party of the“Hoshino K. laboratory in the university of Tsukuba” which introduced my humanoid robot
hand for the first time. The robot hand was demonstrated on a humanoid robot arm that is
actuated by pneumatic power, and has 7DOF wide movable range, slender structure anddimensions like an endoskeleton of a human arm (Fig. 17). While its power is low and thepractical payload at the wrist joint is about 1kg, it could move the robot hand smoothly.
The conclusive advantage of the robot hand is that many complex functions are condensed
in the humanlike size, weight and appearance, and realize the sophisticated dexterity. As
the robot hand has rich suitability for delicate robot arms, after more sophistication, it will
be developed to a good prosthetic hand in the near future.

Fig. 17 Demonstration in the international exhibition SIGGRAPH 2006 in Boston.

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3. Master Hand in Exoskeleton Style
3.1 Introduction of Circuitous Joint
As a dream-inspiring usage, the dexterous humanoid robot hand will be employed intoa super “magic hand” with which an operator can manipulate objects freely from far away
and get feedback of handling force and tactile sensations. Such intuitive master-slave control
method of a humanoid robot with feedback of multi-modal perceptions is widely known as
the Telexistence/Telepresence, however, developments of adequate master controllers forthem have been rare in comparison with slave humanoid robots. I guess one of major
reasons is a difficult restriction in mechanical design that any mechanism cannot interfere
operator’s body. To solve this problem an idea of exoskeleton is brought up by associationof a suit of armour that can follow wide movable range of human body with covering it.
The most popular and practical master hand in exoskeleton style is the CyberGrasp, andmost conventional master hands in exoskeleton style have the similar structure to it. They
are designed to be lighter and slenderer with less material, so they have no core structure
and cannot sustain their form as a hand without parasitism on operator’s hand. This means
they gives some constriction feeling to the operator and the slight force sensation in the
feedback is masked. Then I have tried to design an ideal exoskeleton that fulfils every oflightness, slenderness and self-sustainability in its form.
In designing such exoskeleton, the main theme is focused on joint mechanisms. The mostpractical joint is a revolute one that consists of an axis and bearings, and general ways toplace it corresponding to an operator’s joint are in parallel on backside or in coaxial beside.
However, the former tends to deteriorate the movable range of operator’s joint (Fig. 18(a))
and the latter cannot find an existing space between operator’s fingers. Therefore I propose
a novel joint mechanism named “circuitous joint” that has a virtual axis coincided with theaxis of operator’s skeleton while the all mechanism exists on backside of operator’s finger.
Technically this virtual axis is the instantaneous center of relative rotation of two segments.
Fig. 18(b) shows the principle of the circuitous joint that realizes the virtual axis bystretching displacement s of two segments in proportion to the joint angular displacement θ.

Fig. 18 Behaviour of two types of revolute joint in following operator’s finger.

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Humanoid Robots, New Developments

3.2 Fundamental Mechanism of the Circuitous Joint
In order to realize the principle of the circuitous joint mentioned above, rack and gearwheelmechanism was adopted in consideration of high rigidity of structure, certainty of motion,and facility of manufacturing. Fig. 19 shows the fundamental mechanism prepared fora principle study. A gearwheel is rotated on a rack by relative rotation of two segments, andshifting of its axis provides stretching of a segment that has the rack (Fig. 20). Since the twosegments should make same stretching displacement together, two sets of the mechanism
are combined in opposite direction. The gearwheel is formed to be sector gear by removing
unnecessary part. We may note, in passing, this mechanism is an “over-constrained”
mechanism, so it can keep its behaviour even without the actual axis.

Fig. 19 The fundamental mechanism as a unit of the circuitous joint.

Fig. 20 Mixed motion of rotating and stretching of two segments.

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3.3 Kinematical Design of the Optimal Circuitous Joint
To make the virtual axis coincide exactly to the axis of operator’s skeleton, the relationshipbetween the angular displacement θ and the stretching displacement s must be non-linear.
This means the rectilinear rack and the circular gearwheels should not be adopted, however,they can get practical use with optimal specifications calculated as follows.

Fig. 21 Kinematical symbols in the circuitous joint.
Fig. 21 shows definition of kinematical symbols of parts and parameters; for example, point
V is the virtual axis. The specifications that provide the shape of rack and sector gear are
only the pitch circle radius r of the sector gear and the standard offset p between the centerlines of the Segment A and the Bone A. Since the standard offset p is decided 10mm due toconvenience of practical design of mechanism, only the radius r is an object to be optimised.
The point V moves on the Y-axis by change of θ and its behaviour is divided into three types
according to the size of r (Fig. 22). Considering its nearest trajectory to the point C, thepreferable range of r is presumed as 0.5p ≤ r ≤ (2/π )p.

Fig. 22 Motion of the virtual axis V on the Y-axis by change of θ.
The evaluation item for the optimisation was set a deviation d defined by next formula thatmeans deformation of kinematical relationship between two datum points A and B as
shown in the Fig. 21, and the optimal radius r should minimise it.

d = u 2 + ( p − v ) 2 {where u = −rθ cosθ + p sinθ − rθ , v = p cosθ + rθ sinθ }(1)

Fig. 23 shows curves of the deviation d vs. θ in several settings of the radius r. The radius r is
set within the presumed range. To generalise the optimisation each parameter is dealt as
dimensionless number by dividing with the offset p. Screening many curves and seeking
a curve which peak of d during a movable range of θ is minimum among them, the optimal r420

Humanoid Robots, New Developments

is found as the value that makes the sought curve. For example, when the movable range is
0 ≤ θ ≤ π/2 the optimal radius r is 0.593p and the peak deviation d is 0.095p, and when the
movable range is 0 ≤ θ ≤ π/3 the optimal radius r is 0.537p and the peak deviation d is 0.029p.
As the offset p is set 10mm, the peak of d is below acceptable 1mm; therefore, the mechanismwith rectilinear rack and circular gearwheels has practicability enough.

d/p

0.2

r / p = 0.5
0.537
0.593

0.1

2/π

0.095
0.029

π/3

0
Fig. 23 Variation of curves of the deviation d.

θ

π/2

3.4 Driving Method of the Circuitous Joint
To design the joint mechanism light and slender, a method to drive it from away via a wirerope is introduced. The wire rope is set along two segments veering by a pulley on the
sector gear’s axis, and one end is fixed on a segment and another end is retracted/extracted
by a winding drum set at a stationary root (Fig. 24(a)). Since the wire rope can generate only
pulling force that rotates the joint in straightening direction, a spring is added to generate
pushing force that rotates it in bending direction (same (b)). This driving method has further
conveniences to be applied to a tandem connection model (same (c)). A wire rope to a distal
joint from the root can be extended easily through other joints. Its tensile force sharesaccessorily a part of driving force of other joints they are nearer to the root and needstronger driving force. Moreover, a coupled-driving method of plural joints can be realized
only by winding their wire ropes together with one drum. The rate of each rotation can be
assigned separately by independent radii on the drum.

(a) Path of the wire rope(b) Pushing springFig.24 Driving method of the circuitous joint.

(c) Tandem connection

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421

rp : Radius of the pulley (constant)k : Spring constant of the compression spring (constant)Fs : Spring force generated by the spring (intermediate variable)
Fs’ : Spring force generated by the spring when θ = 0 (constant)w : Retracting/extracting displacement of the wire rope (input variable)
F : Pulling force of the wire rope (input variable)θ : Joint angular displacement (output variable)τ : Joint torque (output variable)
Fig. 25 Statical symbols in the circuitous joint.
The definition of statical symbols is shown in Fig. 25, and the formulas for inverse statics
calculating the input (manipulated) variables: w and F, from the output (controlled)
variables: θ and τ are derived as follows.
w = (2 r + r p)θ

F=

2

1
2k ⋅ r
2 Fs′ ⋅ rτ−θ+
2r + r p2( 2 r + r p)
2r + r p(2)(3)

As these formulas show simple and linear relationship between the input and output
valuables, this driving method promises further advantage that the algorithm of controllingboth position and force is fairly simple. When the spring effect is negligible, as the second
and third terms on the right side of formula (3) are eliminated, we would be able to control
the output torque τ by using only the motor torque as the controlled variable.
3.5 Master Finger Mechanism (MAF)
Fig. 26 shows the practical master finger mechanism (MAF hereafter) corresponding toa middle finger of my hand and my humanoid robot hand, and proves the mechanism canfollow them in wide movable range from opening to clenching. MAF is constructed withthree discrete joint units, so that they are connected adapting to various pitch of operator’sfinger joints (Fig. 27). To make MAF narrow and short enough, each unit is designed
possibly thin and aligned with partly overlapping. In this instance, all joints are coupleddriven by one relatively large motor (Faulhaber, model 1724SR).
As shown in Fig. 28, the actual rack is placed in opposite side viewed from the axis incomparison with the previous illustrations. The reason is to dissolve the interference
between the mechanism and operator’s finger that has came up in the previous
arrangements. Inverse gear is added to correct the stretching direction of each segment andcarried on a slider to keep the position at midpoint of the rack and the sector gear.

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Humanoid Robots, New Developments

Fig. 26 Master finger mechanism (MAF) following various finger flexions.

Fig. 27 Adjustable tandem connection of three joint units.

Fig. 28 Internal mechanism of the joint unit.

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3.6 Master-Slave Control with Encounter-Type Force Feedback
As an ideal scheme of force display to the operator, the “encounter-type” has been proposed(McNeely, 1993, Tachi et al., 1994); that means a small object held up by a robot arm isapproached and pressed to a part of operator’s body where tactile sensation is necessary as
occasion demands. Its chief advantages are making the operator to discriminate clearly thetwo phases “non-contact” and “contact”, and free from constriction feeling during the noncontact phase. As it is suitable for the feature of my desired master hand, MAF introduced afunction of non-contact following to the operator’s finger.
Since the present MAF has only 1DOF, the target motion of operator’s finger is reduced tosame 1DOF, and a gap between both fingertips of MAF and the operator is set as thecontrolled variable during the non-contact phase. Concretely, a sensor at fingertip of MAF
measures the gap, and MAF is position-controlled to keep the gap at the desired value 2mm.
Fig. 29 shows the fingertip assembly that contains a micro optical displacement sensor(Sanyo Electric, SPI-315-34), technically that detects motion of a swing reflector moved by
the operator’s nail in slight force, and the gap is presumed from the motion.
During the contact phase, on the other hand, MAF should generate a desired contacting
force against the operator’s fingertips at the contact tip of the fingertip assembly. So a filmlike force sensor (Nitta, FlexiForce) on the contact tip measures the contacting force, and
MAF is force-controlled by changing the motor torque of winding the rope in proportion to
the difference between the measured and desired contacting forces.
An experimental master-slave system between MAF and a slave humanoid robot finger(SLF hereafter) was constructed as follows. SLF is always position-controlled to realize thesame motion of MAF. The two phases of contact/on-contact on controlling MAF areswitched according to detecting existence/non-existence of the contacting force on SLF.
A film-like force sensor on the surface of SLF’s fingertip measures the contacting force, andthe desired contacting force that MAF should generate is given as equal to that of SLF.

Fig. 29 Fingertip assembly for the master finger mechanism (MAF).
In order to confirm practicability of the master-slave system, an experiment was conducted.
Fig. 30 shows the coupled motion of MAF and SLF in the non-contact phase; MAF was
following the operator’s finger with keeping a small gap at the fingertips. MAF and SLF
could follow the operator’s finger exactly as high as a less drastic speed. Since MAF had
only 1DOF, SLF was prepared as the 1DOF mechanism interlocked all three joints. Moreover,
the operator should also make his/her finger motion interlocking the three joints roughly
similar to the behaviour of MAF. Though, I could forget an uncomfortable feeling by the
fixed behaviour after familiarization, and enjoyed this experience.

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Humanoid Robots, New Developments

Fig. 30 Circumstance of the experimental master-slave control.
Contact Phaseθ1+θ2+θ3π0Contacting Force [N]Tip Gap [mm]

4
2
0
6
4

Master (MAF)
Slave (SLF)

2
0

5

10

15

20

Time [s]Fig. 31 Experimental result of transferring the contacting force.
Fig. 31 shows an experimental result; θ 1+θ 2+θ 3 means the sum of three joint angulardisplacements on MAF. The two vital features are shown: prompt switching of contact/noncontact phases, and transferring the contacting force from SLF to MAF. The contacting force
at the fingertip of SLF was given by an assistant pushing on it; for example, the two contact
phases at the time 7s and 10s were caused by assistant’s tapping. While the algorithm
switching the phases was a primitive bang-bang control, an oscillation iterating contact/
non-contact did not occur. I guess the season: since the gap between the fingertips is kept
small during the non-contact phase, the impact at the encounter that will lead the oscillation
is not so serious, moreover the human fingertip has effective damping to absorb it.
As shown by the curves after the time was 13s, the operator’s finger could be moved by theassistant’s force; the master-slave (bilateral) control with force feedback was verified.
In conclusion of this experiment, MSF has enough performance as a principal part of themaster hand for the Telexistence/Telepresence.

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3.7 Overall view of the Master Hand
Since it comes to the end of width of this paper, I describe briefly the overall view of themaster hand. By the way, the nomenclature of each joint is same as shown in the Fig. 1.
I gave four fingers to the master hand (Fig. 32); the little finger was omitted due to its little
worth in general activities. The three finger mechanisms are same as shown in the Fig. 26,
and the second and fourth finger have the abduction-adduction motion with active joints at
J2,1 and J4,1. The each joint is position-controlled to follow lateral motion of the operator’sfinger detected at fingertip with similar sensor mechanism as shown in the Fig. 29; however,
the additional sensor put beside the fingertip is omitted in the Fig. 32.
In the thumb mechanism, the distal three segments are constructed with two circuitous
joints at J1,4 and J1.5. At the same time, elated ingenuity is exercised to design the joint
mechanism corresponding to the carpo-metacarpal (CM) joint of operator; to make the two
joint axes J1,1 and J1.2 intersected in an empty space for containing the CM joint, a slider
mechanism is introduced where a motor-driven carriage runs on a sector rail in a wide circle.
While the two joint axes J1,3 and J1.4 for the MP joint are not intersected, the order of eachdirection of joint axis and fingertip is identical to that of the Shadow hand (Fig. 2).
In the non-contact phase, the thumb mechanism is position-controlled to follow theoperator’s thumb opposing on both fingertips; each independent DOF has individual sensor
similar to the previous one. As the mechanism does not touch the operator’s thumb, slightdeviation of the controlling is negligible. In the contact phase, only the joints J1,4 and J1.5 areswitched its control mode to the force-control. More sophisticated control algorithm for thisthumb mechanism is under study in the “Tachi S. laboratory in the university of Tokyo”
where I started developing this master hand as a researcher in 2001.

Fig. 32 Whole picture of the master hand.

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Humanoid Robots, New Developments

4. Conclusion
To contribute on the evaluative process of searching the appropriate designing paradigms asa mechanical engineer, I bring up in this paper some of my ideas about the robot handdesign concretely. While the designs of my robot hands may seem to be filled with eccentric,
vagarious and serendipitous ideas for some people, I believe they are practical outcomes offlexible ingenuity in mechanical designing, so that they can take on pre-programmed but
robust actuating roles for helping the programmable but limited actuators, and realize
higher total balance in mechatronics. At the same time, for examining their practicability,
reasonability and inevitability through the eyes of many persons, it will need to establish
a standard definition and evaluation items in kinematics, dynamics, control algorithms andso on, that can subsume almost all humanoid robots. Concretely, a standard formats would
be prepared to sort and identify any robot system by filling it. The Fig. 1 and 2 show mysmall trial of comprehensive comparison under a standard definition in the robot handkinematics. And I hope the worldwide collaboration, so that it will promote developmentsof many sophisticated mechanical and electric elements that are easy to be used by manyengineers like me who want any help to concentrate on his/her special fields.

5. Reference
CyberGrasp: Immersion Co.
http://www.immersion.com/3d/products/cyber_grasp.php
DLR Hand: German Aerospace Center
http://www.dlr.de/rm/en/desktopdefault.aspx/tabid-398/
Gifu Hand: Dainichi Co., Ltd.
http://www.kk-dainichi.co.jp/e/gifuhand.html
Harada Hand: Harada Electric Industry Inc.
http://www.h-e-i.co.jp/Products/e_m_g/ph_sh_2_004.html
Jacobsen, S.C. et al. (1984). The UTAH/M.I.T Dextrous Hand: Works in Progress, Int. J. of
Robotics Research, Vol.3, No.4 (1984), pp.21-50
Lovchik, C.S. & Diftler, M.A.(1999). The Robonaut Hand: A Dexterous Robot Hand For
Space, Proc. of IEEE Int. Conf. on Robots & Automation, Detroit, MI, May 1999
McNeely, W.A. (1993). Robotic Graphics: A New Approach to Force Feedback for Virtual
Reality, Proc. of IEEE Virtual Reality Annual Int. Symp., pp.336–341, Seattle, Sep 1993
NASA Hand: National Aeronautics and Space Administration
http://robonaut.jsc.nasa.gov/hands.htm
Shadow Hand: Shadow Robot Company Ltd.
http://www.shadowrobot.com/hand/Tachi, S. et al. (1994). A Construction Method of Virtual Haptic Space, Proc. of the 4th Int.
Conf. on Artificial Reality and Telexistence (ICAT '94), pp. 131-138, Tokyo, Jul 1994
Teleistence: Tachi, S. et al., Tele-existence (I): Design and evaluation of a visual display withsensation of presence, Proc. of the RoManSy ‘84, pp. 245-254, Udine, Italy, Jun 1984
Telepresence: Minsky, M., TELEPRESENCE, OMNI, pp. 44-52, Jun 1980
Weghel, M.V. et al. (2004). The ACT Hand : Design of the Skeletal Structure, Proc. of IEEE Int.
Conf. on Robots & Automation, pp. 3375-3379, New Orleans, LA, Apr 2004