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Documentation

Mechanical Design

Calvin's physical structure — the chassis that holds everything together and the drive system that keeps it moving.

01 Chassis

Structure

Calvin's chassis is a rectangular box frame built with 20x20mm aluminum extrusions. There is a fixed, solid 10mm axle attached to the bottom of the frame for the free-spinning wheels. 3D printed attachment brackets and panels comprise the non-structural parts that hold the electrical and mechanical hardware in place. The layout keeps the center of mass as high as possible to make balancing easier.

  • Two-wheeled self-balancing form factor
  • Vertical electronics stack — Jetson on bottom, GIGA R1 mid, battery high
  • Motor mounts integrated into the side panels
  • Removable front, side, top, and bottom panels

Key Dimensions

Frame 240mm W x 190mm H x 150mm D
Wheels xxxmm Diameter, xxmm wide
Weight No idea. About 5-7 pounds total weight?

Electronics Layout

Instinctus
Teensy 4.1
Mounted to front of frame out of the way of the motors and Jetson.
Instinctus
ToF sensors
Mounted low on the frame, facing forward and rear.
Instinctus
Balance IMU
Mounted just above the axle, as close to to the center of the axle as possible.
Instinctus
ODrive S1 Motor Drivers
Mounted on bracket at top of the chassis frame
Cogitator
Seeed Studio ReSpeaker 4 Mic Array
Mounted above the chassis frame, with clear view of sound.
Cogitator
Jetson Orin Nano
Mounted low, just above axle.
Cogitator
Visaton FR 58 Speaker & Amplifier
Mounted above the chassis frame to raise center of gravity.
Cogitator
OAK-D Pro W Camera
Mounted on vibration isolators above chassis frame.
Cogitator
S3s LiPo Battery
Mounted above the chassis frame to raise center of gravity.

02 Drive System

Motors

ODrive D5312S brushless outrunner motors. Connected to wheels by 2GT belts with 8:1 reduction.

  • 53 mm stator diameter
  • ~170 KV rating
  • Peak torque ~1.5 N·m
  • Belt drive
  • One per side, mirrored mount

Wheels

Wheels mount directly to pulleys. The wheel assembly rotates on bearings around the fixed axle. Tires are 3D printed Varioshore TPU printed at 210 C for optimal friction coefficient.

  • Free rotation on solid axle
  • 3D Printed Varioshore with grid infill for compliance
  • V-tread design

Motor Controllers

Each motor is driven by a dedicated ODrive S1 single-axis brushless controller. The ODrives accept high-level velocity setpoints from the Teensy 4.1 over CAN bus.

  • ODrive S1 × 2
  • PID tuning for smoothness and control
  • Velocity control mode
  • CAN bus · 250 kbps
  • Up to 56 V / 40 A per controller

Drive System Design Note

The ODrive S1s are probably overkill. Preliminary calculations predicted a motor current of about 4-5 amps, which is just outside the smaller ODrive Micro's capability, so I went with the more robust S1 drivers to be safe.

03 Power

Battery

Calvin runs on a 4S 14.8V LiPo RC battery pack.

Chemistry LiPo
Nominal voltage 3S · 14.8 V
Capacity 106.56 Wh (7.2 Ahr)

Power Distribution

The battery feeds the ODrive S1 controllers and Jetson directly at pack voltage. A buck converter will provide power to the 3.3/5 V boards.

  • Motor rail Battery voltage direct to both ODrive S1 controllers
  • 5 V logic Buck converter step-down for Teensy 4.1 and peripheral sensors