This mechanism is a differential gear train whose gears are all spur gears. It is basically a combination of two planetary gear trains on which the two sun gears are coaxial. The function of this mechanism is similar to the previous differential gear train. When the mechanism is driven and both wheels are releasing free, all spur gears are running; when the right wheel is held, the right sun gear will become fixed but the other gears are still rotatable; when the left wheel is held, the left sun gear will become fixed but the other gears are still rotatable. This mechanism is applicable for the situation that requires two controllable inputs.