Embedded Files
My group designed and built an autonomous robot capable of drawing a 24-inch diameter circle using a differential-drive propulsion system. Our main goals going into this project was to design with simplicity, optimized size, and reliable motion, while balancing performance metrics such as drawing time, circularity, mass, and build quality. Through multiple design iterations, CAD refinement, and experimental testing, the final robot achieved perfect circularity with consistent and repeatable performance.
Scroll down to see all about it!
Best Circle Drawn RunOur Best Run!
Problem Statement & Constraints
The robot was required to autonomously draw a precise 24-inch circle on paper while meeting strict constraints:
Minimize overall size to maximize performance score
Maintain predictable and repeatable motion
Balance speed, weight, and build quality
Use simple, robust mechanical solutions
Operate reliably without external positioning systems
Our Design Strategy
Minimize complexity – reduce failure points and simplify troubleshooting
Optimize compactness – minimize dimensions in x, y, and z
Ensure controlled propulsion – prioritize predictable motion over over-engineered solutions
These principles guided every design decision, from propulsion selection to electronics layout.
Initial Cad Model
Final Cad Model
Design Refinements Based on Initial Design CAD Review
After reviewing the initial CAD layout and anticipated system behavior, several targeted design refinements were made to address predicted risks and improve performance and simplicity.
TPU Wheel Treads: PLA wheels were expected to slip on paper, which is especially problematic for torque-differential drive. TPU tire inserts with grooves were added to increase friction and improve circularity.
Caster Limiter: The rear caster was originally unconstrained. A simple mechanical limiter was added to restrict its rotation and prevent orientations that could resist motion.
Motor Speed Tuning (Arduino): Differential-drive kinematics provided an initial speed ratio estimate, but final motor values were tuned experimentally in code to reliably achieve a 24-inch circle.
Battery Relocation & Wheel Diameter Increase: Both battery packs were moved to the bottom chassis to reduce width and lower the center of gravity. Wheel diameter was increased to maintain ground clearance.
Ultrasonic Sensor Removal: A revised start strategy eliminated the need for positional sensing. Removing the sensor reduced mass, wiring, and system complexity.
Wire Slot Redesign: Three small wire slots were consolidated into one larger slot to simplify wiring, assembly, and troubleshooting.
Addition of TPU on the Wheels
Addition of Caster Limiter
Battery Pack Location moved to bottom of chassis
Standoffs
Manufacturing & Assembly
Most structural components were 3D printed, while standard fasteners and off-the-shelf electronics were used where possible. The design incorporated:
Heat-set threaded inserts for repeatable assembly
Standoffs to maintain chassis alignment and rigidity
Heat Inserted Hole
Testing & Results
The robot was tested through multiple trial runs to tune motor speeds and evaluate consistency. Final performance metrics:
Circularity: 1
Time: 21.37 s
Mass: 522 g
Maximum Dimension: 6.375 in
Testing revealed motor overheating during repeated runs, highlighting real-world limitations not captured in theoretical models. Despite this, the robot maintained stable motion and completed full circles with minimal slip.
Exploded Drawing
GD&T Drawing for one component of our Drawing
Reflection & Proposed Improvements
Reduce chassis thickness to lower mass
Replace Arduino Uno with an Arduino Nano for size and weight reduction
Centralize the pen mechanism to balance drawing friction
Replace the caster with a lighter omnidirectional support
Introduce a passive free wheel to reduce motor count
These changes would improve speed, balance, and manufacturability while preserving design simplicity.
Skills & Tools Used
Design & CAD
Onshape: For our Robot Model
Inventor: For Exploded and GD&T Drawing
Mechanical Engineering
Differential-drive kinematics and motion planning
3D printing
Tolerancing, fits, and fastener selection
Electronics & Controls
Arduino (Uno): motor speed control, iterative tuning, and timing-based logic
PWM-based motor control for differential drive
Manufacturing & Assembly
3D printing of structural components
Heat-set threaded inserts
Mechanical assembly using standoffs and standard fasteners
Soldering of electrical connections for limit switch and motors
Testing & Analysis
Experimental tuning and validation
Comparison of theoretical predictions vs real-world behavior
Root-cause reasoning based on friction, torque, and system constraints
Page updated
Google Sites
Report abuse