11 is increased from small to big and decrease from big to small and the like the transmission case 42 also comprises a detection gear 425 engaged with the first gear 422 the optical sensor 52 mainly senses the rotational velocity and rotation direction of the detection gear 425 the optical sensor 52 is matched with the detection gear 425 so that the light beams of the optical sensor 52 irradiate towards the direction of the detection gear 425 the optical sensor 52 is electrically connected with the h bridge driving circuitry 55 through the control unit 51 the h bridge driving circuitry 55 controls the rotation direction of the motor 41 according to a signal of the pulse duration modulation module 54 the motor 41 drives each gear in the transmission case 42 to rotate meanwhile the detection gear 425 also rotates and is sensed by the optical sensor 52 during the rotating process moreover the signal sensed is fed back to the control unit 51 the rotation situation of the output gear 424 is detected through the detection gear 425 i e the rotational velocity and rotation angle of the output gear 424 can be obtained through the rotational.