4.2.1 Adjustable-speed amplitude. In order to correctly denote the variable velocity of the differential gear train of the hybrid mechanism, the concept of adjustable-speed amplitude is introduced. As shown in Figure 1, adjustable-speed amplitude is the ratio (or percentage) of absolute value of up and down adjustable amount to the base speed. In the differential gear train of the hybrid mechanism, the adjustable-speed amplitude equals the ratio value of axis 0 output speed of operating a single servomotor at the specific speed to axis 0 output speed of operating a single conventional motor[13].