A ball drive, often linked to the Killough omnidirectional platform, is a robot drive system that can move in any direction on a flat floor without first turning its body. Instead of using standard wheels pointed forward, it uses angled omni wheels or rollers to push on a sphere or to drive the chassis through combined wheel forces. This matters because robots in tight spaces, game fields, labs, and service environments need smooth sideways, diagonal, and rotational motion.
The design turns wheel speeds into a controllable motion vector for the whole robot.
Key Facts
- Omnidirectional motion means the robot can control vx, vy, and omega independently on a flat surface.
- For a symmetric 3-wheel Killough platform, wheel speeds combine to create translation and rotation.
- Robot speed magnitude in the plane is v = sqrt(vx^2 + vy^2).
- Robot angular speed relates to heading change by omega = dtheta/dt.
- Ideal no-slip rolling gives v_contact = r omega_w, where r is wheel radius and omega_w is wheel angular speed.
- For three drive directions 120 degrees apart, balanced equal wheel speeds often produce mostly rotation, while different speeds produce translation.
Vocabulary
- Killough platform
- A three-wheel omnidirectional robot base that uses angled omni wheels to create motion in any direction.
- Omni wheel
- A wheel with small rollers around its rim that can drive along one direction while sliding freely sideways.
- Drive ball
- A sphere pushed by powered wheels so that its rotation produces motion of the robot or contact surface.
- Velocity vector
- A quantity that gives both the speed and direction of motion of the robot.
- Inverse kinematics
- The calculation that converts a desired robot motion into the individual wheel speeds needed to produce it.
Common Mistakes to Avoid
- Treating omni wheels like normal wheels is wrong because omni wheels can roll freely sideways due to their small rollers.
- Ignoring wheel angle is wrong because the force direction from each wheel depends on how the wheel or roller axis is mounted.
- Assuming the robot must face the direction it moves is wrong because a Killough or ball drive can translate sideways while keeping the same heading.
- Forgetting slip and normal force is wrong because real ball drives need enough wheel pressure and friction to transfer torque without skidding.
Practice Questions
- 1 A ball drive robot has vx = 0.60 m/s and vy = 0.80 m/s. What is the magnitude of its translational velocity?
- 2 An omni wheel of radius 0.050 m spins at 20 rad/s with no slip. What is the contact speed produced by the wheel?
- 3 A three-wheel Killough robot is commanded to move directly sideways while keeping its heading fixed. Explain why at least two wheel speeds must differ and why the robot does not need to rotate first.