Richard Huberjohn's Distance-Measuring Desk Robot Is Ready for a Game of Catch
This surprisingly static robot "catches" balls in a net on its back, then measures the distance to the player to lob them back again.
Maker Richard Huberjohn has designed a robotic tennis ball launcher that can catch, too, after a fashion — tracking the ball and player with a combination of a low-cost HC-SR04 ultrasonic distance sensor and a simple limit switch.
"I designed and built this robotic tennis ball launcher, intended to automatically catch and throw a projectile using data from sensors," Huberjohn explains of the little desktop ball-thrower. "This project was tailored to build experience with design and electronic integration of sensors, and to showcase how sensors can be used in real applications. A large consideration for this project was using materials I already had on hand to reduce costs and time investment. So many design decisions are influenced by making a component I own work for this specific use case."
The robot itself is a fairly compact device, sitting on a desk — and while it can, indeed, play catch, it does so in a relatively passive fashion: a net mounted at the top-rear serves as a target for the player to hit with the ball, which then drops down inside the robot itself. When the ball's in place, detected by a limit switch, a rubber-coated drum spins up to fling the ball back at the player.
Where the ultrasonic sensor comes in, then, is in the throwing rather than catching stage: the sensor is used to measure the distance from the robot to the player and control the speed of the drum. If the player is close, the drum spins slowly to gently lob the ball; if the player is further away, the drum speeds up to give the ball more force to cover the distance.
"There were a few hurdles with [3D] printing and power draw from the motor," Huberjohn admits. "So if I were to redo this project, I would experiment with using a separate ESC [Electronic Speed Controller] for the motor, with better specs than the motor shield. Another improvement I could imagine is utilizing a time-of-flight sensor for distance measurement and something like a beam break sensor for reliable object detection. However those sound like upgrades for a future project."
The project is documented in full, including 3D print files, a bill of materials, and the source code under an unspecified license, on Instructables.