Senior design project quadruped built under $1000 and 20 lbs using rewound $18 BLDC motors and capstan drives.
Key Takeaways
Actuator cost dropped from ~$250 each (CARA 1.0) to ~$80 each by swapping to TYI 5008 motors ($18) and MKS XDrive Mini FOC controllers ($41).
TYI motors were rewound from 335 KV to 90 KV (22 AWG delta to 24 AWG star, 40 turns/slot) to achieve 1.274 Nm stall torque vs. 0.421 Nm stock.
XDrive Mini CAN bus communication failed with all ODrive open-source firmware; stable operation required a community-patched 0.5.1 firmware with encoder and current feedback added.
Capstan drives use 1mm Mastrant-M Dyneema DM20 rope, 9.6:1 reduction, 12 Nm peak torque, 120 deg ROM; rope pre-stretching reduces tensioning need at assembly.
Full build guide is paid (Patreon); BOM is free.
Hacker News Comment Review
Commenters noted that 12 motors at $18 each is a genuine cost breakthrough, and TYI’s “custom KV” ordering option could eliminate rewinding at volume.
Motor thermals are a flagged risk: stalled motors under static load generate heat with no graceful failure path, a known issue from robots like Nao.
RL gait training and max speed potential are open questions; commenters pointed to Stanford Pupper v3 (kit ~$2,350-$3,000) as a documented comparable with an RL pipeline.
Notable Comments
@zzzeek: “this guy is going to get a high paying job at some defense contractor somewhere and we’ll never see anything he does again.”
