A differential drive robot moves using two powered wheels mounted on the left and right sides of its chassis, usually with one or more caster wheels for balance. It is common in mobile robots because the mechanism is simple, robust, and easy to control with motors and encoders. By changing the speed of each wheel, the robot can drive straight, follow curves, spin in place, or stop.
This makes differential drive a core idea in robotics navigation and motion planning.
The key mechanism is that each wheel traces a different path when the robot turns. If the right wheel moves faster than the left wheel, the robot curves left because the right side travels farther in the same time. The turning motion can be described using the instantaneous center of rotation, which is the point around which the robot is rotating at that instant.
Engineers use these relationships to convert desired forward speed and turning rate into left and right wheel speeds.
Key Facts
- Forward speed of the robot center: v = (vR + vL) / 2
- Angular speed of the robot: omega = (vR - vL) / L
- Wheel speeds from desired motion: vR = v + omega L / 2 and vL = v - omega L / 2
- If vR = vL, the robot drives straight with omega = 0
- If vR = -vL, the robot spins in place about the midpoint between the wheels
- Turning radius of the robot center: R = v / omega, when omega is not 0
Vocabulary
- Differential drive
- A mobile robot drive system that steers by setting different speeds on two powered side wheels.
- Caster wheel
- A passive wheel that supports the robot while swiveling freely to follow the robot's motion.
- Wheelbase
- The distance L between the left and right wheel contact points in a differential drive robot.
- Instantaneous center of rotation
- The point in the plane that the robot is rotating around at a specific instant.
- Angular velocity
- The rate at which the robot changes its heading angle, usually measured in radians per second.
Common Mistakes to Avoid
- Treating the caster as a steering wheel is wrong because a differential drive robot steers by changing the powered wheel speeds, while the caster only supports the chassis.
- Using omega = (vL - vR) / L without checking the sign convention is wrong because the sign of angular velocity depends on whether positive rotation is defined as left turn or right turn.
- Forgetting that wheel speeds are linear speeds at the ground is wrong because motor angular speed must be converted using v = r omega_wheel before using differential drive equations.
- Assuming the robot can move sideways is wrong because an ideal differential drive has nonholonomic constraints and cannot slide laterally without wheel slip.
Practice Questions
- 1 A robot has wheelbase L = 0.40 m. Its right wheel speed is vR = 0.60 m/s and its left wheel speed is vL = 0.20 m/s. Find the forward speed v and angular speed omega.
- 2 A robot has wheelbase L = 0.50 m and should move forward at v = 0.80 m/s while turning with omega = 1.20 rad/s. Find vR and vL.
- 3 Two differential drive robots have the same forward speed, but Robot A has a larger wheelbase than Robot B. If both use the same difference between right and left wheel speeds, which robot turns more sharply and why?