DIfferential drive mechanism

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What is Differential drive? The term differential means difference between two entities, in the context of robotics it is just the speed difference between two motors.Based on this speed difference, a robot can be moved in any direction on a 2D layout. Why use differential Drive mechanism? Most of the robotic projects seen on the web or around you use this mechanism. The reason behind the popularity is the simplicity and efficiency of this mechanism. All you will require to make a robot is a pair of motors, a pair of wheels and a castor wheel. Just with these things we can make a robot that can move in all the directions!!! Isn't that cool.? How does Differential drive work? When two motors are connected to wheels in one line,opposite to each other(Just like a pair of wheels connected to a single shaft) the speed with which each motor rotates determines the direction of motion. When both the wheels rotate at the same speed the difference between the motors is Zero. This makes the robot move forward in a straight line.The robot can move in reverse direction if the direction of rotation of both the motors are reversed. This will again be in a straight line if the speed difference is zero.  Now changing the speed of any one motor will result in movement in a direction away from the straight line. For example, reducing the speed of the right motor will result in a speed difference and hence change in direction.The resultant force is such that the robot turns right. This direction change can be controlled to required angle by further reducing the speed of the motor.Slower is the right motor, sharper is the turn to right. This is exactly the same for Left turn.      As a conclusion, Slower right motor, sharper right turn. Slower left motor Sharper left turn. Below are some scenarios which explains working of differential drive mechanism.M1 and M2 are motors which drive wheels on left and right respectively.  Case 1: M1 and M2 rotating forward at the same speed. As there is no speed difference between the motors, the torque is produced in equal quantity on both the wheels, which makes the robot moves forward. To make the robot move in reverse direction, the direction of both the motors should be reversed and rotating as same speed. This makes the robot move in reverse direction. Case 2: M1 forward and M2 backward at the same speed.  Now the motors rotate at same speed but in opposite direction. This results in torque which is equal in magnitude  but opposite in direction. Since the wheels are displaced away from the center of the robot, there is a turning effect on the robot. This causes the robot to turn as if the center of the robot if fixed to the ground.The center of rotation will be the center of line where motors are mounted to the robot.  With M1 forward and M2 reverse, robot turns right. With M2 forward and M1 reverse, robot turns left. Case 3: M1 and  M2 forward with M2 slower than M1.  In this case the motor M1 rotates faster than M2, which causes a lesser torque on the right wheel and hence the robot starts turning towards right.The robot rotates with the wheel on the right as the center of rotation. Case 4: M1 and  M2 forward with M1 slower than M2. In this case the motor M2 rotates faster than M1, which causes a lesser torque on the Left wheel and hence the robot starts turning towards right.The robot rotates with the wheel on the left as the center of rotation. Note:  When the motors and wheels are mounted as seen above, make sure the weight of the robot is distributed on the side where castor wheel is fixed, else the chassis of the robot touches the ground.  As an alternative, fix castor wheel on the rear side of the robot also. Else shift the motors away from the center towards the rear side of the robot.