Overview

Our bot's name was inspired by our team's roll of purple electrical tape that people in SPDL always asked to borrow due to the shortage of tape in lab. Purple Tape is an sturdy bot who's always on Santa's Naughty list. Purple Tape would rather find coal on Christmas morning than anything else.

Purple Tape is an autonomous robot designed for the 2026 ME 218B final showcase. Photo credit: Joshua Phelps

The Game: Tilt-A-World

For the 2026 218B final project, teams were tasked with creating an Autonomous Coal-Loading Robot that could navigate the coal factory (game board), collect coal (yellow balls), and strategically deposit it onto large seesaws. The game was a head-to-head competition between two robots to move coal into their seesaw buckets, and the final score was a count of how many seesaws are tilted towards each side.

Each robot was a fully autonomous, untethered system built specifically for the competition using only SPDL-provided motors and battery power. Robots were required to automatically determine which side of the arena they start on and use electromechanical indicators to display both their starting region and when a round is in progress. Systems were required to operate using at least two PIC32 microcontrollers communicating via SPI.

Project Design Requirements

Robots must be autonomous (no tele-operation) and untethered (battery power only).
Robots must automatically determine which side of the arena it has been placed on and use an electro-mechanical indicator to indicate which side.
At the beginning of a game round, robots must fit within a cube measuring 320 mm per side and can expand to 480 mm by 480 mm in length and width and 570 mm in height after the game starts.
Each robot shall contain a network of processors consisting of, at a minimum, one PIC32 SPI leader and one PIC32 SPI follower device.
Each team is limited to a total expenditure of $200.00/team for all materials and parts used in the construction of your project.
Each robot must provide an indication of when it thinks that a game round is in progress that is clearly visible from 7 meters away.

Core Mechanisms

Orientation and Line Following

IR phototransistor signals conditioned with an op-amp and hysteretic comparator were used for digital beacon detection and characterization on the Follower PIC32. Purple Tape has five additional line-following IR sensors on its underside to drive along the tape and detect intersections as it drives.

Coal Intake

Purple Tape uses a servo to actuate an L-shaped arm behind a ball and then drives backward, using its momentum to pull the ball towards the flywheel. The spinning flywheel uses friction to pull balls onto the conveyor belt.

Coal Transport and Storage

The friction conveyor belt, made of flexible TPU, pushes coal up the ramp at a 67.7° incline. Coal is deposited into the container for storage. The container has a sloped floor so that coal will naturally roll out when the door is opened.

Coal Dispensing

Once Purple Tape arrives at a bucket, the arm will bring the bucket down from its starting height to the ground. The robot will then drive towards the bucket and open its container door, dispensing coal into the bucket to keep it on the ground.

Overall Strategy

At the start of a round, Purple Tape rotates until its IR phototransistor detects either the left or right beacon, which tells the robot which side of the field it is on, and it updates its indicator servo accordingly. At this point, the robot knows its configuration, so it rotates 90° counterclockwise and then continues to turn in that direction until its front line-following sensors detect the black tape, after which it begins line following.

At the central junction of the field, the robot rotates 225° clockwise and begins moving across the board diagonally until its line-following sensors detect the third tape path. Once it reaches this last line, it rotates to orient itself with the coal dispenser and drives toward it until a limit switch on the front of the robot tells it that it has reached the dispenser.

Purple Tape acquires coal from the dispenser using an L-shaped servo-actuated PLA arm and a DC motor-driven TPU flywheel, driving back and forth so that the momentum helps push balls onto the friction conveyor ramp that brings the balls to their storage container at the top of the robot.

Once Purple Tape has acquired four pieces of coal, it drives backwards to the left-most bucket and pulls it to the ground with a servo-actuated arm. Once the bucket is on the ground, it opens the container door and the balls roll down the sloped container floor into the bucket, where the weight of the coal keeps the bucket on the ground.

At this point, the robot will have deposited the three pieces of coal from its container, but one piece will remain on the conveyor belt. Purple Tape closes its container door and drives the conveyor belt for a few seconds to move this last piece of coal to the container. Then, the robot backs up, angles itself, and drives to the last bucket to lower it and deposit this final piece of coal.

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