**2.1 Basic design**

This section describes the basic design of developed robot hand. Fig. 2 (left) shows "Universal Robot Hand II". The height between the lower limit of the palm and the upper limit of the middle finger is 290mm. The width of the robot hand opened up between the thumb and the little finger is 416mm. The size of this robot hand is a little larger than human hand. Thus, this size is enough to imitate human hand workings. The weight of the robot thumb is 0.262kg, the weight of the every other finger is 0.250kg, and the total weight of the robot hand without the pedestal is 1.323kg.

Fig. 2. Universal robot hand II and configuration of DOFs

This robot hand has 16 DOFs. Thumb has four DOFs (the IP, the MP, the CM1 and the CM2 joints), and the other fingers have three DOFs (the PIP, the MP1 and the MP2 joint). Every DIP joint is interlocked with the PIP joint. These DOFs and the movable directions of joints are shown in Fig. 2 (right).

This robot hand has the multi-axis force/torque sensors in every fingertip and tactile sensors on every finger pad. The multi-axis force/torque sensor is able to measure the force and torque at fingertip. This sensor in every fingertip is as shown in Fig.3 (upper right). Tactile sensor is able to measure the pressure distribution on the finger pad. This sensor on every finger pad is as shown in Fig. 3 (lower right).

98 The Future of Humanoid Robots – Research and Applications

In this paper, a new robot hand is developed on resulting knowledge for advancing our study. Universal Robot Hand II has actuators, transmission gears, reduction gears, and Torque Limiter Mechanisms in the fingers. Using the Torque Limiter Mechanisms, the fingers can sustain overload not by the gears but by the structure. This is the imitative behavior of a human finger. This paper describes that new small robot hand mechanism has five fingers at the first. At the second, this Torque Limiter Mechanism is introduced. At the third, the effects of Torque Limiter Mechanism are verified in experiments. At the last,

This section describes the basic design of developed robot hand. Fig. 2 (left) shows "Universal Robot Hand II". The height between the lower limit of the palm and the upper limit of the middle finger is 290mm. The width of the robot hand opened up between the thumb and the little finger is 416mm. The size of this robot hand is a little larger than human hand. Thus, this size is enough to imitate human hand workings. The weight of the robot thumb is 0.262kg, the weight of the every other finger is 0.250kg, and the total weight of the

This robot hand has 16 DOFs. Thumb has four DOFs (the IP, the MP, the CM1 and the CM2 joints), and the other fingers have three DOFs (the PIP, the MP1 and the MP2 joint). Every DIP joint is interlocked with the PIP joint. These DOFs and the movable directions of joints

This robot hand has the multi-axis force/torque sensors in every fingertip and tactile sensors on every finger pad. The multi-axis force/torque sensor is able to measure the force and torque at fingertip. This sensor in every fingertip is as shown in Fig.3 (upper right). Tactile sensor is able to measure the pressure distribution on the finger pad. This sensor on

results of these experiments are summarized and concluded.

**2. Specifications of developed robot hand** 

robot hand without the pedestal is 1.323kg.

Fig. 2. Universal robot hand II and configuration of DOFs

every finger pad is as shown in Fig. 3 (lower right).

are shown in Fig. 2 (right).

**2.1 Basic design** 

Fig. 3. Multi-axis force/torque sensor (BL AUTOTEC, LTD.) and array-type tactile sensor

The overview of the control system for this robot hand is shown in Fig. 4. This control computer gets the pulse from the encoders in every motor, the value from multi-axis force/torque sensors in every fingertip and the pressure distribution from the array-type tactile sensors on every finger pad. The fingers are controlled through driver circuits according to these data.

Fig. 4. Control system for universal robot hand

Development of Multi-Fingered Universal Robot Hand withTorque Limiter Mechanism 101

Torque Limiter Mechanism is constructed by a fixed plate, a rotating plate and rollers held

 sin

*P* is the pressure by the adjustment nut. Every 20 joints of this robot hand have this

is the coefficient of the friction between rollers, plates. *r* is the radius of rollers, and

(1)

degrees. The

between these plates as shown in Fig. 6. These rollers are tilted on an angle of

Fig. 6. Inner structure of finger joint with torque limiter mechanism

Fig. 7. Cross-section view and side view of torque limiter mechanism

**3. Torque limiter mechanism** 

skidding torque *T* is expressed in (1).

mechanism as shown in Fig. 7.

*T rP*

**3.1 Mechanism** 

where, 

## **2.2 Basic performance**

It is shown that the basic performance of the developed robot hand. The movable range of joints is as shown in Table 1. 0 [deg.] is extended position and the flexion direction is the plus direction. This movable range of robot hand is similar or over the human's one.


Table 1. Movable range of each joint

The step responses of every finger are shown in Fig. 5. From this figure, DIP & PIP operates slower than the other joints. As the PIP joint is operated with the DIP joint, the load of DIP & PIP is about twice larger than the other joints' one. Thus, DIP & PIP operates with about half the angular velocity. Therefore this Universal Robot Hand II has enough response velocity for our future study.

Fig. 5. Result of step response experiments

### **2.3 Superior function**

Typically, the robot finger is classified into a hard finger and an elastic finger. In the hard finger, the rotation of the actuator responds plainly to the angle of the joint. In the elastic finger, the fingertip can be moved with elastic members depending on the external force. However, a human finger acts as both a hard finger and an elastic finger depending on a situation. Thus, Torque Limiter Mechanism is fitted into the joint of this Universal Robot Hand II. With this mechanism, driving mechanism in joints is started to skid from setup skidding torque. By implementation with this mechanism, the driving mechanism can be protected against overload, and the robot hand may grasp objects flexibly.
