Big Blue · Alpha Competition Robot
KUdos VEX-U's 24" Over Under alpha robot with an over-the-frame intake, slip-gear catapult, and pushing wings.
Overview
Big Blue was my second KUdos VEX-U competition robot and my introduction to designing a complete robot in Onshape.
The robot served as the 24" robot for the VEX-U Over Under competition. Its primary job was to intake Triballs from the Match Load Zone, transfer them into the catapult, launch them across the field, and use its wings to push groups of Triballs offensively or defensively.
My Role
Lead Designer & Fabricator · Build Lead. I led Big Blue end to end as project lead, Lead Designer & Fabricator, and Build Lead.
- Ran the early game analysis and defined the robot's needs, wants, nice-to-haves, and design requirements
- Set the robot architecture and created the master sketches that controlled subsystem geometry and packaging
- Researched and prototyped the drivetrain, intake, catapult, and barrier-crossing sleds before committing to CAD
- Modeled the full robot in Onshape and established the team's collaborative cloud-CAD workflow across its alternating A-Section and B-Section groups
- Designed the non-parallel 4-bar intake, slip-gear catapult, pneumatic wings, and six-motor drivetrain
- Produced the BOM and manufactured parts across CNC routing, machining, and 3D printing
- Led assembly, integration, and pneumatic plumbing, then tuned the robot through testing
- Planned the autonomous and driver Skills routines
Outcome
I delivered a fully modeled, documented, fabricated, assembled, and competition-tested 24" robot. Big Blue gave KUdos VEX-U a working Match Load robot and helped the team qualify for VEX Worlds 2024. It was also where I established the team's Onshape-based collaborative design workflow, applying full-robot CAD, BOM management, CAM, CNC routing, and additive manufacturing within one project.
Competition Results
West Michigan Holiday VEXU Over/Under Tournament
MCCC/FIRST Solar Over Under VEXU Tournament
| Feature | Spec |
|---|---|
| Robot class | 24" VEX-U robot |
| Drivetrain | 6 motors at 360 rpm, omni wheels with center traction, ~17.5" x 17.5" footprint |
| Intake | Over-the-frame non-parallel 4-bar with 3 sprocket-driven rubber-band rollers |
| Catapult | 2 motors, 4:1 torque ratio, 24-tooth slip-gear indexing, rubber-band powered |
| Wings | Pneumatic cylinders |
| Estimated BOM cost | ~$1,100, including the brain but excluding hardware |
Problem & Goal
Over Under required KUdos VEX-U to operate two robots with different roles. Big Blue needed to handle rapid Match Loading while still being able to cross the field barriers and contribute to Goal scoring.
The goal was to build a reliable 24" alpha robot that could collect Triballs over the Match Load Bar, position each Triball consistently, launch it using a catapult, and deploy wings for offensive and defensive pushing.
Design Requirements
- Fit within the 24" x 24" x 24" VEX-U robot size limit
- Target a compact drivetrain footprint of approximately 17.5" x 17.5"
- Pick up Triballs over the Match Load Bar
- Transfer each Triball into a repeatable catapult position
- Launch Match Load Triballs across the field
- Cross the horizontal field barriers without damaging drivetrain components
- Deploy wings for offensive and defensive Triball pushing
- Protect the battery, pneumatic reservoir, brain, and radio
- Use sensor feedback for Match Load cycling
- Support fabrication with the team's 3D printers, CAM workflow, and CNC router
- Maintain a complete CAD model and BOM for purchasing and assembly
Process
- 01Analyzed the Over Under field, scoring rules, Match Load Zone, and role of the 24" robot
- 02Researched existing drivetrain, intake, catapult, and barrier-crossing concepts
- 03Prototyped the drivetrain, sleds, Triball holder, intake, and slip-gear catapult
- 04Increased the catapult reduction after testing showed that the first version needed more torque
- 05Modeled the full robot in Onshape using FeatureScripts, master sketches, configurations, and version control
- 06Used a parametric intake sketch to adjust linkage geometry around the Match Load Bar and Goal
- 07Completed the 24" robot CAD and custom-part designs in September 2023
- 08Created a full BOM and compared required parts against the team's existing inventory
- 09Generated CAM and CNC-routed the custom polycarbonate intake plates
- 10Manufactured aluminum components and 3D printed PETG and Onyx parts
- 11Assembled the drivetrain, intake, catapult, wings, electronics, and pneumatic system
- 12Tuned the intake, rubber-band tension, catapult position, optical sensor, and barrier-crossing sleds
- 13Tested the robot through skills practice and competition
Challenges
- Learned Onshape while managing the CAD of a complete competition robot
- Coordinated design work between team members in alternating academic sections
- Tuned the catapult reduction, rubber-band combinations, and Triball cup together to balance launch distance with a repeatable reset and a consistent hold
- Adjusted the intake after it jumped when contacting the floor
- Lengthened and revised the sleds after early versions did not cross the barrier consistently
- Modified the front C-channel and catapult structure to remove mechanical interference
- Worked through optical-sensor detections that did not always register when a Triball entered the catapult
Robot Subsystems
Big Blue packaged four subsystems onto its 24" chassis: the drivetrain, the over-the-frame intake, the slip-gear catapult, and the pneumatic wings.
Drivetrain
- Used a six-motor 360 rpm drivetrain with omni wheels and center traction wheels
- Added sleds and gear guards under the drivetrain to cross the horizontal barriers and protect exposed components
- Tested 55-degree and 60-degree sled profiles at approximately 1.75" tall during prototyping
Non-Parallel 4-Bar Intake
- Used an over-the-frame non-parallel 4-bar to reach across the Match Load Bar and pull Triballs into the robot, with the robot angled about 17 degrees from the Match Load Bar toward the Goal
- Drove three sprocket rollers wrapped in rubber bands, mounted between 1/4" polycarbonate side plates, after the prototype showed reliable Triball contact
- Used tapped 1/4" 6061 aluminum rods as crossbars with rubber tubing around the outside
- CNC-routed the side plates from 1/4" polycarbonate after printed prototypes proved too fragile
- Replaced individual shaft collars with long 3D printed spacers, since repeated intake motion loosened the collars and let the rollers shift
- Added adjustable printed hardstops to control intake extension
- Controlled the critical intake spacing from a parametric master sketch, so the linkage geometry could change without remodeling the subsystem
Slip-Gear Catapult
- Powered the catapult with two motors through a 4:1 torque ratio and rubber-band energy
- Used 24-tooth slip gears to reset the catapult after each launch
- Added a ratchet to prevent the rubber bands from back-driving the catapult
- Used an adjustable hardstop to set the resting position and launch angle
- Designed a custom Triball cup to hold the game piece in a repeatable launch position
- Added an optical sensor to detect when a Triball entered the catapult
Wings
- Used pneumatically actuated wings to push groups of Triballs and to protect the battery, pneumatic reservoir, brain, and radio
Onshape Workflow
Big Blue was the first complete robot I modeled in Onshape. I used the project to learn and apply several tools that improved the team's design process.
- Used cloud CAD so multiple team members could view and edit the robot remotely
- Used version control to preserve design milestones and review changes
- Used master sketches to control subsystem geometry and robot packaging
- Used FeatureScripts to speed up repeated modeling tasks
- Used parametric dimensions to update mechanism geometry without rebuilding the CAD
- Modeled the full robot before fabrication to identify packaging and interference problems
- Connected the robot CAD to a structured BOM for inventory and purchasing
Skills Plans
The autonomous (programming) and driver Skills routines shared the same structure, differing mainly in how many Triballs the wings pushed into the Goal:
- Match Loaded 23 Triballs for 46 points
- Crossed through the red alley for 2 points
- Deployed the wings and pushed Triballs into the Goal: at least 4 for 12 points in autonomous, at least 7 for 21 points in driver control
- Parked for 5 points if time remained
What I Learned
- How to model and manage a complete robot in Onshape, using cloud CAD, FeatureScripts, master sketches, and version control to support collaborative design across remote team members
- How to design and tune a non-parallel 4-bar linkage
- How to design a slip-gear catapult around torque, geometry, and elastic power
- How to manage a robot project from research and prototyping through fabrication and competition
- How to use 3D printing for rapid prototypes and CNC-routed polycarbonate for final competition parts
- How a complete BOM supports budgeting, purchasing, inventory, and assembly
