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Explanation:
Shoulder, elbow, and wrist (pitch) use mechanically identical modular motor modules, each built around a Harmonic Drive 17-100-118879-11 (100:1) reducer, differing only in the T-Motor brushless outrunner selected to meet torque requirements (200 Nm shoulder, 120 Nm elbow, 50 Nm wrist). These motors are drone-class brushless motors adapted for robotic joint actuation, interfaced via an in-house machined lathed coupler connecting the motor shaft to the harmonic input.
Each module integrates an ODrive motor controller and custom fuse boards, housed in FDM-printed ABS motor enclosures (Bambu P1S) enabling rapid motor replacement and iteration. The harmonic drive housings and matching structural housings were contract manufactured in 6061 aluminum, while the 6061 aluminum tube links (Industrial Metal Supply) feature radial mounting holes cut on a 4-axis laser cutter for precise alignment and modular assembly. Harmonic drives were sourced second-hand and integrated into fully custom joint modules, forming a scalable modular architecture rated for a 5 kg payload with 2.5 factor of safety.
Verification/FEA
Verification began with a spreadsheet-based worst-case loading model. I built a mass breakdown of all major components, treated each as a point mass, assigned a radius from the base, and summed torque contributions (τ=w*r) across the chain, including the 5 kg payload torque. This was evaluated at the worst-case configuration (fully extended) to capture maximum motor torque demand and peak structural loading. The final design basis uses a 2.5 factor of safety (older PDF value of 2 is outdated).
From the same model, we also extracted Z-direction shear and bending moments (including symmetric loading cases), then used these combined load conditions as inputs to tube and module FEA. FEA showed minimal deflection and a high structural safety margin, and directly informed a design change: the bottom tube wall was increased from 1/16" to 1/8" after an early iteration showed ~1 mm deflection that we judged too high for future accuracy and robustness.
Yonder - 3.0 ARM Load Calculations.pdf
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