of human grasping procedures. That is, the output force of a robot hand may not be necessarily big for fast motion, and the robot hand can move slow if a large grasping force is required. Therefore, the high speed motion and large actuating force can be achieved, depending on the operational modes, even though the motor has low power. To implement this actuation principle, a new type of power transmission, referred to dualmode twisting actuation, was devised by combining the twisted string actuation method with a passive clutch mechanism.Fig. 2 shows the structure of the dual-mode twisting mechanism, which consistsof two couplings(TC1and TC2), a shaft between TC1 and TC2, two strings, and a brake. Note that the strings come through the holes of TC2, and the friction on TC2 is imposed by the brake.
The operation proceduresfor contraction process is illustrated in Fig. 3. When the motor starts to rotate, the strings are twisted on the shaft between TC1 and TC2 because the brake prevents the rotation of TC2. This operation mode allows high contraction speed of the strings, which is called Mode I. If the strings are fully twisted and a sufficiently large tension force is applied by the resistance(i.e. contacting an object), the torque exerted on TC2 exceeds the friction by the brake and TC2 starts to rotate together with TC1. The rotation of TC2 generates a large contracting force by twisting the strings themselves. This mode is called Mode II.
According to the analysis of Shin et al. (2012), the major design parameters of the dual-mode twisting actuation are the radii of the shaft and string. The radius of the shaft dominantly affects the contraction speed in Mode I. The large radius of the shaft make the contraction speed fast in Mode I. And the radius of the strings mainly determines the contraction force in Mode II. In other words, the small radius of the strings returns the large contraction force in Mode II. Consequently, designing the large ratio of the radii of the shaft and strings is desirable for high speed motion and large actuating force. However, there exist the limitations of the ratio for practical use. The radius of the shaft can not be arbitrary large because the contraction force in Mode I would be very small if the shaft radius is too large. It implies that a small external load can disturb the operation in Mode I, and Mode II would be activated before contacting an object. Moreover, the small radius of the strings leads to the short life expectancy of the strings because the stress induced