Note: Here in this tutorial aspects like aerodynamics and vibration damping are not taken in account.
There are many kinds of techniques to move a robot from one place to other. But here I will discuss only some few techniques adequate for relatively smaller robots driven by dc motors.
1.Differential Drive system:
This is the simplest and the most common type of mechanism used in small robots.
It's easy to build, easy to control and can permit the robot to move in all required directions.
Two motors are connected, one to each of the two drive wheels at the right and at the left of the robot's base. Those two motors are responsible of driving the robot forward and reverse.
The turning of the robot relys on the velocity difference of the motors. Thats why it is called a 'differential'.
Figure on the right shows operation principle.
When the left and right motors are turning at the same speed, the robot moves forward or backward in a straight line.
In order to turn right, the right motor is slowed down and the robot steers to the right. The bigger the difference of speed between the two wheels, the tighter will be the steering curve.
2.Four wheel differential drive system:
The drive trains of tanks and bulldozers is similar to this one.
It requires relatively powerful motors, because during turning there is a lot of friction.
Similar kind of a differential mechanism is used as above.
The advantage of this system is that it is very easy to build and is most suitable for running on low friction and dusty surfaces where other drive systems are not effective.
3.Car-type drive system:
I think that the image is clear enough to understand this mechanism.
4. Divided chassis drive train:
This system relies on 4 independent motors, each one coupled to one of the four wheels, allowing the robot to move forward or backward with the
accumulated power of four motors or to turn right and left with approximately equivalent power, but with m minimum frictional losses.
The chassis is divided into two parts, hinged together, allowing it to change it's shape to turn in tight curves.
Usually, the two front motors are used only to drag the robot, while the two rear motors are used either to push it or to bend the chassis (like a toy train) to turn right or left by running one of the two motors faster than the other.