Work in progress
The differencial drive robots have two parallel wheels motorized each one separately. If both the wheels are driven in the same direction and speed, the robot will go in a straight line. If both wheels are turned with equal speed in opposite directions, the robot will rotate about the central point of the axis. Otherwise, depending on the speed of rotation and its direction, the center of rotation may fall anywhere on the line defined by the two contact points of the tires. While the robot is traveling in a straight line, the center of rotation is an infinite distance from the robot. Since the direction of the robot is dependent on the rate and direction of rotation of the two driven wheels, these quantities should be sensed and controlled precisely.
This method allows the robot to turn in the same way a car does. It only needs one motor and a servo for steering.
All the multidirectional robots have individual traction on all wheels. We have indentify two types differentiated by their kind of wheel: You can see the way that they works in the following images: