This page presents the design process we went through to develop the physical design for the robot and to see it through to construction. There were three major components to the physical design of the subsystem. These included the drive and navigation system, the sorting mechanism, and the shooting mechanism. For each of these compononets, in the following sections we first describe our initial design concept, discuss some of the revisions we ended up making as the design progressed and as we ran into new challenges and performance requirements, and finally we give a detailed description of the final design and show the finished robot.
The following images show a rough sketch of our initial design concept, a CAD representation of our final design, and the finished robot. The CAD model was created using Autodesk Inventor, and the files can be downloaded from the github repository. The structural components for the final construction of the robot were cut from 1/4" cedar plywood directly from the CAD files on a laser cutter, and other components were hand-machined from aluminum and sheet metal stock.
Our initial concept for the drive and navigation system was to have two drive wheels with third wheel that would allow the robot to drive forward and backwad and to rotate. We planned to use ultrasonic rangefinders to allow the robot to navigate to the center of the arena. In order to decrease the risk of moving away from the center of the arena once we got there while trying to aim the robot, we planned to have the sorting and shooting mechanisms mounted on a turret that would be able to rotate relative to the base of the robot.
We quickly realized that this system was more complex than it needed to be, and made revisions to simplify. The drive system was reduced to having two drive wheels powered by stepper motors, with a post at the front and back of the robot to provide stability. Stepper motors were chosen to allow the robot to acurrately move in a straight line or to rotate in place to a desired angle. The stepper motors were attached to the base using a custom-machined mounting bracket, and the rubber wheels were mounted to the motor shafts using custom-machined wheel hubs.
The turret concept was quickly eliminated due to the added complexity, instead relying on the stepper motors to provide the rotation without causing the robot to move out of position. After testing the ultrasonic rangefinders, we found that the measurements they provided were too unreliable, and so they were replaced with three limit switches on the front of the robot to use for navigation purposes. Three switches were used to allow the robot to tell the difference between hitting a flat wall and hitting a corner.
Our initial concept for the sorting mechanism remained largely unchanged for the final design. The balls are loaded in random order into a carousel on top of the robot. This carousel rotates (driven by a stepper motor), and a color detection system indexes the color of each ball in the carousel. An opto-interrupt is used in conjunction with slots around the perimeter of the carousel to allow the robot to keep track of the position of the carousel. When a certain color of ball is requested to be shot, the carousel is rotated into the appropriate position and the shooting mechanism takes over.
The initial concept for the ping-pong ball shooter was to have two spinning horizontal wheels mounted slightly apart from each other so that the ping-pong ball would get grabbed by the wheels as it passed through and be shot out. This concept was used in the final design, although the method of delivering the ball to the wheels was designed later. To make the mechanism as simple as possible, we chose to use a simple chute that the ball would roll down from its position in the carousel and into the wheels. The balls are dropped into this chute by opening a small trap door that is rotated out of the way of the ball by a servo motor.