Engineering reference by Robbie Dickson covering why bipedal robots destroy actuators and which mechanical solutions survive: QDD, roller screws, SEA, and back-drivability.
Key Takeaways
5,000 steps/hour means ~1 million load cycles per month; heel strike applies 2-3x body weight in under 100ms, faster than any sensor loop can respond.
Self-locking actuators (lead screws) fail immediately under repeated dynamic impact; back-drivable designs absorb shock mechanically without gearbox shear.
Reflected inertia scales with gear ratio squared: a 100:1 gearbox makes the rotor feel 10,000x heavier to the output, killing compliance on heel strike.
QDD (Quasi-Direct Drive) with 6:1 to 30:1 ratios is the industry convergence point; Tesla Optimus, Figure, Agility Digit, Unitree, and Boston Dynamics all use rotary actuators for major joints.
A 200g actuator overweight at the ankle compounds to 1.3kg system-level penalty via the mass penalty spiral across knee, hip, and battery sizing.
