**3.3 Palm and wrist design**

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

Design of 5 D.O.F Robot Hand with an Artificial Skin for an Android Robot 91

Fig. 9. The wrist design and controllers (hand controller, forearm controller).

The skin part, especially an artificial skin, is very important factor of hand for an android robot, because it is the only shown part optically in outside. The skin design is basically similar to one of hand, but there are more steps than hand design. There are three steps to make skin part. These are mock up, mold and material making. Firstly, the mock up is made by original 3D model. This mock up is used to make mold for skin, so it is needed to be like original model as possible. To realize original model exactly, the mold was made by RP (Rapid prototype) and the exact same model to original can be made. Secondly, the mold was used to cast skin and this is made by mock up. This process is similar to make a plaster caster. The mold was made by silicon, but this is not tough to make skin several times. The CNC based metal mold was replaced to the silicon mold and this mold was tough to use several times, but it was expensive to make. Thirdly, the artificial skin is made by mock-up and mold. There are some materials as an artificial skin like urethane, latex, rubber and silicon. The silicon complex was selected the material of artificial skin, because it has the closet texture to human skin and it is easy to handle. In addition, the characteristic of silicon can be changed easily by the mixture ratio of an emulsion. This mixture ratio between silicon and an emulsion decides the durableness of the skin. It is very important the durableness of material to make an artificial skin, because if the durableness of the skin is hard, it can be stiff resistance to move but soft, it tears easily. Through many times of experiments, the optimal mixture ratio between silicon and an emulsion can be obtained. The pigments were used to make human like color and they were made by blending of some colors and added with an emulsion. After completion of skin, the wrinkles are added to skin by make-up to raise reality. Even if this artificial skin of hand can realize the human hand and mechanical parts are designed by 3D data from human, there are some gaps between skin and mechanical parts. The kind of art clay was used to fill these gaps. The art clay is easy to handle and build shape. From these processes, the artificial skin for robot

**3.4 Skin design** 

considered this difference, so the height and distance of each fingers is adjusted by arrangement of attached points. Fig. 8 shows arrangement of finger modules. The one more consideration of palm design is the attachment of thumb finger. The human thumb finger has 3 D.O.F especially including abduction-adduction and some of humanoid robot has same D.O.F. In case of robot hand which is designed for grasping, the D.O.F for abductionadduction motion is very important, but in this research, there is only 1 D.O.F by out of space. Because the only 1 D.O.F is for bending, the attachment to palm is important to decide the shape of hand. The attachment position is considered natural shape of hand when the hand is fully straightened and fist. The attachment angle is decided 81 degree in top view and 15 degree in front views. This arrangement is shown in Fig. 8. The angle and position are decided by one part and the shape can be changed easily by replacement of this part, thumb connecter. The palm part consists of a palm connecter which connects 4 finger modules, thumb connecter and wrist connecter, so it is very simple and it is easy to change the shape.

The wrist design is based on 3D model like hand design. The human wrist has 2 D.O.F but the wrist of this robot has 1 D.O.F by out of space. The wrist design is included to forearm design and this forearm has wrist joint, forearm yaw joint and controllers for hand and forearm, so there are space problems. To solve this problems, smallest harmonic drive (CSF-5-100-2XH, Harmonic drive), BLBC motors (RE20Flat 5W, Maxon) and self-developed controller was used for getting over narrow space problem. The controller for hand used DSP and it has 6 analog input ports for sensors (Linear potentiometer) and it can control 6 DC motors. Though this hand has 5 D.O.F, this controller has 6 ports for abductionadduction motion of thumb finger in future works. The BLBC controller was also selfdeveloped to satisfy small spaces. It can control 2 BLDC motors and there are 4 ports for encoders, proximity sensors and it used DSP, too. The proximity sensor (GL-6H, Sunx) was used for initializing. From these efforts, slim size wrist design which can match to reference model can be made. Fig. 9 shows the wrist design and controllers.

Fig. 8. The shape of palm, thumb joint and attachment position.

Fig. 9. The wrist design and controllers (hand controller, forearm controller).
