Abstract: A novel structure of differential A/B bi-rotary head was designed， which can be used for five-axis machine centers. The heads were driven by four servo motors. Pitching and tilting motions were achieved through controlling angle and velocity of the motors. The motors and reducers were fixed on the machine body. The structure of motion parts was more simplified than that of the typical A/B bi-rotary head. The backlash of the head was eliminated owing to the dual-motor master-slave control method being used， and the motion accuracy of the head was improved. Transmission characters， backlash eliminating method and structural characters of the head were analyzed， and the results provide an important theoretical basis for the design of A/B bi-rotary head.

Key words: A/B bi-rotary head, differential drive, backlash, anti-backlash control, bias torque