The Ball Boy is a mechanism I developed to grab/collect tennis balls. This is a simple prototype made using a tennis ball container, dc motor, a switch and a few rubber bands. 

The mechanism is intended to be mounted on a robotic arm, mounted on a movable platform, that can traverse a given area, collect balls that are randomly spread out and then deposit them at a specified location.
A closeup of the opening for the mechanism.
This design is simple in the sense that only two simple rubber bands have been used to make this work. As the mechanism (mounted on a robotic arm) pushes down on the tennis ball, the rubber bands stretch inwards, thereby enlarging the gap and allowing the ball to come in. As soon as the ball is inside the holding bin, the rubber bands stretch back into shape, keeping the balls inside from falling out.
A closeup of the motor mechanism for dropping the balls.
To drop the balls that were inside the holding bin, a locally sourced, low RPM DC geared motor was used. The motor had a flywheel attached to it which in turn was connected to the rubber bands at the opening using some string. When the motor would rotate clock-wise (from the front), the strings would pull up on the rubber bands thereby enlarging the gap and dropping the balls.
A closeup of the switch used as a stopping mechanism.
An SPDT (Single Pole Dual Throw) micro switch, commonly used in 3D printers for stopping the carriage, was used here as a 'safety' mechanism. As soon as the bolt on the flywheel would hit the switch, the current to the motor would be cutoff, thereby stopping the motor from ripping out the strings and the rubber bands.
The Picking Mechanism in Action
The Dropping Mechanism in Action
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