**3.1 Finger design**

The finger design is the most important part in this android robot hand. The presented hand has 5 fingers like human hand and total 5 D.O.F with 1 D.O.F each finger. Actually, human 86 The Future of Humanoid Robots – Research and Applications

The skin is the most important factor to make human like hand, because skin is the part which is shown intuitively. There are some considerations for an artificial skin. These are texture, color and details like wrinkle, so it needs not only technology but also arts. The silicon composite was selected as an artificial skin, because it is the closest material to human skin than any other materials and it is easy to handle. The artificial skin also used 3D data as a reference, but there are more steps to use. To make a silicon complex skin, the mold is needed, because the process of making silicon complex is like one of making plasters. The 3D data was used to make mock up model. To make exactly same model to original model, RP (Rapid prototype) was used to make mock up model. After making mock up, mold was made by using this mock up model, and a silicon complex skin can be made. The artificial skin which is made by silicon complex is very similar to human skin, but to raise the similarity, make up was taken in final step. Though the mechanical part was made by consideration of shape, there are gaps between parts and skin. This gaps can make wrinkles when the finger bended. To solve this problem, art clay was attached to gaps between mechanical parts and skin. Fig. 4 shows a silicon composite artificial skin for hand.

Fig. 4. A silicon composite artificial skin for an android robot hand.

The finger design is the most important part in this android robot hand. The presented hand has 5 fingers like human hand and total 5 D.O.F with 1 D.O.F each finger. Actually, human

**3. Hardware design** 

**3.1 Finger design** 

**2.2.3 Skin** 

finger has 3 D.O.F at proximal joint, middle joint and distal joint to bend or stretch and 1 D.O.F to spread, so there are 4 D.O.F in each finger basically. In case of thumb finger, there is no middle joint, so it has 3 D.O.F (proximal, distal and 1 D.O.F for abduction and adduction). There are at least 25 D.O.F in real hand, so it is very hard to realize by robotic hands, because there are not enough spaces to install actuators in hand. For this reason, most of humanoid or android hands have smaller number of D.O.F than one of human. Of course, there are some hands which have many D.O.F like one of human, but their size are bigger than human hands (H. Liu. et al., 2008) or they have other large spaces which have a lot of actuators instead (Yuichi Kurita. et al., 2009). For these reasons, the hands for humanoid robots or android robots should have less D.O.F than one of human hand. In this research, the 5 D.O.F hand is designed to express fundamental motion of human hand like straightening – bending of each finger. The best feature of this hand is that the each finger was made as a modular structure independently. Most of robotic hands have their actuators like motors in palm or forearm and their finger is connected to palm, but the finger of this hand has an actuator (DC motor), sensor and gears as its own components. The combination of these components becomes independent finger module. Why this finger was designed as a module is for easy maintenance. The android robot EveR 3 which this hand is applied to was designed for stage performance especially commercial performances not just using in the laboratory for researches, so reliability and maintenance is one of the most important factors. The modular structure can give fast and easy maintenance of hardware. For example, when the middle finger breaks down, replacing of the middle finger module is the only maintenance of all. It is fast, easy and low cost. Fig. 5 shows the disassembled hand by each module.

Fig. 5. The disassembled hand by each module (fingers and palm).

Design of 5 D.O.F Robot Hand with an Artificial Skin for an Android Robot 89

these inner links make subordinate relation of each phalange. The inner link 1 is connected to nut and its center of rotation is located under of proximal phalange and their centers of rotations are different, so when the inner link 1 is pulled by nut; the proximal phalange rotates by difference of their center of rotation. The shape of inner link 1 is like a boomerang and each tail is connected to nut and inner link 2, so when the inner link 1 is pulled, it starts rotate and it pulls inner link 2. The mechanism of rotation of middle phalange is same to one of proximal phalange. The inner link 2 is connected to middle phalange and the point is different to the center of rotation of the middle phalange, so when the inner link 2 is pulled by inner link 1, the middle phalange rotates. The inner link 3 is connected to proximal phalange and distal phalange. The connected point of inner link 3 in proximal phalange is different to the center of rotation of the middle phalange and the connected point in distal phalange is different to the center of rotation of distal phalange. When the inner link 3 is pulled by rotation

of the middle phalange, the distal phalange rotates as same to the middle phalange.

and little finger is the smallest size, so this hand wad followed that order.

Fig. 7. The mechanism of bending.

**3.3 Palm and wrist design** 

The angle of rotation of each joint is decided by the length and center of rotation of each inner links. The length of proximal phalange, middle phalange and distal phalange is fixed value which is based on the original human model, so the values of inner links (length, position of center of rotation) are design factors in this hand. This robotic hand was made to gesture but not grasp. The gesture which is purposed is natural fist, so the design factors of the inner links are decided when fully bended shape of hand becomes fist. The angles of proximal joint, middle joint and distal joint are 60 degree, 90degree and 45 degree when the shape of hand is fist. The structure of fingers is same except the thumb finger but the sizes are different. In case of human hand, the middle finger is the longest, the index finger and ring finger are similar,

The design of palm is simple as compared with the one of finger. The presented hand is just combination of finger modules, so the palm part is the connecter of finger modules. Even if the role is simple, there are some considerations in palm design, because it can decide the shape of hand by arrangement of finger modules. The most of robot fingers are arranged in same axis in front, top view but in case of human, they are different. The design of palm

The organization of a finger module consists of a geared DC motor as an actuator, a screwnut, a linear potentiometer and joint links. The joint links are composed of three phalanges as a proximal joint, middle and a distal like human finger. In case of a thumb finger, it is little different to other fingers. There are only a proximal and a distal phalange. These phalanges are frame of finger and these joints are connected inner links and the inner links make subordination of driving of three joints. Fig. 6 shows the composition of a finger module.

Fig. 6. The composition of a finger module.

The power of the used DC motor is 6mNm and it uses 5V (GM-12F, Motor Bank). The dimension of motor is 36x10x12mm (including gear head) and its weight is 10g. The power of motor is not enough to drive the finger with skin, a gear head whose ratio is 1/50 was attached to the motor. The screw-nut is used to drive links of finger for saving spaces. If the motor is placed to rotation of links horizontally, it is hard to arrange the motors, because all joints of proximal phalanges are placed in same axis. The advantage of this type to arrange motors is easy to design and disadvantage is using of space ineffectively. To place motor vertically to rotational axis of links, worm gear, bevel gear or screw-nut can be considered which connect motor to joint. Why the screw-nut is selected is that it is easy to make small size and a linear potentiometer can be attached to nut. The nut is connected to a linear potentiometer and an inner link 1. The linear potentiometer (RDC1047, ALPS) is used to feedback control of motor and initialize. The motor is small, so it is hard to attach an encoder to it, so the linear potentiometer is used by calculating of gear ratio and lead of screw-nut, the RPM of motor can be obtained. The linear potentiometer uses 5V and its linearity is +/- 5%. The most of parts are made by aluminum (6061alloy) for light weight, a screw-nut is made by brass for low friction and inner links are made by steel (sus304).

### **3.2 Mechanism of motion**

The motion of the finger is occurred by rotation of inner link 1 and the one of left links are subordinated by kinematic relation. Fig. 7 describes the motion of each links when the finger bended and straightened. The motion of finger can be described by following. The proximal phalange is connected to housing of actuating parts and middle phalange, distal phalange are connected in serial order. Three inner links are connected among three phalanges and 88 The Future of Humanoid Robots – Research and Applications

The organization of a finger module consists of a geared DC motor as an actuator, a screwnut, a linear potentiometer and joint links. The joint links are composed of three phalanges as a proximal joint, middle and a distal like human finger. In case of a thumb finger, it is little different to other fingers. There are only a proximal and a distal phalange. These phalanges are frame of finger and these joints are connected inner links and the inner links make subordination of driving of three joints. Fig. 6 shows the composition of a finger

The power of the used DC motor is 6mNm and it uses 5V (GM-12F, Motor Bank). The dimension of motor is 36x10x12mm (including gear head) and its weight is 10g. The power of motor is not enough to drive the finger with skin, a gear head whose ratio is 1/50 was attached to the motor. The screw-nut is used to drive links of finger for saving spaces. If the motor is placed to rotation of links horizontally, it is hard to arrange the motors, because all joints of proximal phalanges are placed in same axis. The advantage of this type to arrange motors is easy to design and disadvantage is using of space ineffectively. To place motor vertically to rotational axis of links, worm gear, bevel gear or screw-nut can be considered which connect motor to joint. Why the screw-nut is selected is that it is easy to make small size and a linear potentiometer can be attached to nut. The nut is connected to a linear potentiometer and an inner link 1. The linear potentiometer (RDC1047, ALPS) is used to feedback control of motor and initialize. The motor is small, so it is hard to attach an encoder to it, so the linear potentiometer is used by calculating of gear ratio and lead of screw-nut, the RPM of motor can be obtained. The linear potentiometer uses 5V and its linearity is +/- 5%. The most of parts are made by aluminum (6061alloy) for light weight, a screw-nut is made by brass for low friction and inner links are made by steel (sus304).

The motion of the finger is occurred by rotation of inner link 1 and the one of left links are subordinated by kinematic relation. Fig. 7 describes the motion of each links when the finger bended and straightened. The motion of finger can be described by following. The proximal phalange is connected to housing of actuating parts and middle phalange, distal phalange are connected in serial order. Three inner links are connected among three phalanges and

module.

Fig. 6. The composition of a finger module.

**3.2 Mechanism of motion** 

these inner links make subordinate relation of each phalange. The inner link 1 is connected to nut and its center of rotation is located under of proximal phalange and their centers of rotations are different, so when the inner link 1 is pulled by nut; the proximal phalange rotates by difference of their center of rotation. The shape of inner link 1 is like a boomerang and each tail is connected to nut and inner link 2, so when the inner link 1 is pulled, it starts rotate and it pulls inner link 2. The mechanism of rotation of middle phalange is same to one of proximal phalange. The inner link 2 is connected to middle phalange and the point is different to the center of rotation of the middle phalange, so when the inner link 2 is pulled by inner link 1, the middle phalange rotates. The inner link 3 is connected to proximal phalange and distal phalange. The connected point of inner link 3 in proximal phalange is different to the center of rotation of the middle phalange and the connected point in distal phalange is different to the center of rotation of distal phalange. When the inner link 3 is pulled by rotation of the middle phalange, the distal phalange rotates as same to the middle phalange.

The angle of rotation of each joint is decided by the length and center of rotation of each inner links. The length of proximal phalange, middle phalange and distal phalange is fixed value which is based on the original human model, so the values of inner links (length, position of center of rotation) are design factors in this hand. This robotic hand was made to gesture but not grasp. The gesture which is purposed is natural fist, so the design factors of the inner links are decided when fully bended shape of hand becomes fist. The angles of proximal joint, middle joint and distal joint are 60 degree, 90degree and 45 degree when the shape of hand is fist. The structure of fingers is same except the thumb finger but the sizes are different. In case of human hand, the middle finger is the longest, the index finger and ring finger are similar, and little finger is the smallest size, so this hand wad followed that order.

Fig. 7. The mechanism of bending.
