Mechanical Design
MPLabron consists of a drivetrain, a dunking mechanism, and subcomponents for sensing, status indication, and electronics mounting. The drivetrain uses two brushed DC motors with spider couplers, a bearing assembly, and large acrylic wheels. Sensors include two motor encoders, two ultrasound sensors, two touch bumpers, and an infrared beacon detector. The scoring system was designed to be modular: a low-risk 3-ball dunking mechanism was used for checkoff and was replaced with a more powerful 12-ball dunker for the tournament. Engineering drawings, as well as highlights of our decisions for various parts of the mechanical design, are below.

Engineering Drawings

Chassis and Motor Mounts

  • Two-wheel drive with two ball casters. This setup lets the robot pivot around its center and stay stable even with a tall dunking mechanism. The drive wheels are supported by ball bearings on both sides. Spider couplers connect the motors to the axles.
  • Multiple trays of electronics. There are 3 tiers of electronics – motors/batteries, motor control circuits, and strategy/scoring systems. The top two tiers are on removable trays that are held down by wingnuts. We were able to build the electronics from the bottom up and remove the upper trays when we needed to access something on the lower trays. Swapping the top tray also lets us quickly change our scoring mechanism, which was useful when switching from the 3-ball checkoff dunker to the 12-ball competition dunker.
  • Works with different wheel sizes. It is easy to remove the side panels, replace the wheels, and adjust the ball casters’ height to keep the robot level. MPLabron can work with wheels as small as 3.5” or as large as 6.5”. We started with large wheels to maximize speed but switched to smaller wheels to make aiming and driving easier.
  • Batteries as low as possible. The batteries are mounted to the underside of the robot. This makes them relatively easy to swap out. Also, since batteries are the heaviest component, MPLabron is very stable even at high speeds and with a tall dunker.
  • Custom encoder bracket. The bracket uses an inherited design to take advantage of existing motor features to secure the bracket in place. Bracket and magnet holder dimensions were modified to fit the provided encoder and motor geometries and to keep the magnet a fixed pre-determined distance from the encoder PCB . We added access slots in encoder bracket and the electronics panel allow for adjustments and debugging. 
  • Hinged limit switch bumpers. The entire front and back of the robot are hinged bumpers that toggle limit switches when pressed. The switches trigger reliably no matter where or at what angle the robot runs into a wall and are important for the robot to maintain alignment during games.
  • Recessed mounts for ultrasound sensors. The ultrasound sensors let us avoid running into walls at full speed, which we found often jarred screws loose. The sensors are recessed so as not to get smashed when the touch bumpers are triggered.
  • Cable pass-throughs and tie-downs. We added lots of holes to route cables or zip-tie them down to reduce wiring confusion.
Picture
Encoder Assembly
Picture
Magnet / PCB spacing

Indicators and Buttons

  • Arcade buttons for reloading and scoring. Large buttons are easy to press. Either can be pressed to start the game. During the game, one commands MPLabron to drive out of the reload zone and immediately return for further reloading, while the other tells him to drive all the way to the basket and score.
  • Servo-actuated eyebrows and LED eyes. MPLabron’s face haunts his opponents’ nightmares.

Scoring Mechanism

  • Gated ramp as the dunking mechanism. It’s simple, reliable, and easy to reload. The balls pick up a little bit of speed in case there’s a gap between the front of the robot and the edge of the basket
  • Dunking as final scoring mechanism. We planned the 3-ball dunker as a “minimum viable product” to complete checkoff and planned to experiment with better scoring devices as time permitted. We experimented with both a 3-pointer catapult and a 12-ball dunker as scoring mechanisms for the tournament. The 12-ball dunker proved higher-scoring and more reliable, so we went with that. 
  • Massively parallel dunking with a single servo. In scaling up the 3-ball dunker to the 12-ball dunker, we wanted to avoid adding extra actuators to the extent possible. We figured out how to have one gate release balls from up to 4 ramps at once. 4 channels turned out to be too wide, so the final design has 3 ramps that hold 4 balls each.
  • Integrated infrared sensor mount. The dunker also serves as a mount for the infrared sensor. The sensor is positioned as close to the center of the robot as the dunking ramps allow. It sits in a 3D-printed housing that narrows its field of view to give good alignment results.
  • Weight-saving cutouts. The dunking mechanism is pretty tall and sits high on the robot, so there was a risk that it would make MPLabron top-heavy. We added triangular cutouts wherever possible to make the dunker lighter. This seems to have worked, since he never tipped or wobbled even when he accelerated or decelerated quickly.
MPLabron with his 3-ball checkoff dunker.
MPLabron with his 12-ball tournament dunker.