QDD Fundamentals
Quasi-Direct Drive Fundamentals
Most industrial robot actuators use high-ratio gearboxes (100:1 or higher). QDD takes a different position: keep the gear ratio low (6:1 to 9:1) and demand more from the motor.
Why Gear Ratio Matters
A gearbox with ratio N:1 transforms the motor's output as follows:
Output torque = Motor torque × N × η (η = efficiency, ~0.95)
Output speed = Motor speed / N
Reflected inertia = Motor inertia × N²
That last line is the key insight. With N = 100, the reflected motor inertia at the joint is 10,000× the motor's own inertia. The joint feels heavy and cannot respond quickly to external perturbations.
QDD Characteristics
With N = 8 (typical CatBot ratio):
| Property | High-ratio (N=100) | QDD (N=8) |
|---|---|---|
| Reflected inertia | 10,000 × J_m | 64 × J_m |
| Backdrivability | Very low | High |
| Torque sensing | Requires torque sensor | Infer from current |
| Efficiency | Lower (friction) | Higher |
| Peak torque | High (motor small) | Requires large motor |
Torque Transparency
Because the joint is backdrivable, motor current is a good proxy for joint torque:
τ_joint ≈ KT × I_phase × N × η
This enables implicit force control — by commanding current, you command force. No dedicated torque sensor required. This is how MIT Mini Cheetah and ETH ANYmal achieve compliant contact behavior.
CatBot Actuator Specification
Target requirements derived from CatBot's walking gait:
Peak joint torque: 15 N·m (hip), 20 N·m (knee)
Continuous torque: 8 N·m
Max joint speed: 15 rad/s
Gear ratio: 8:1 planetary
Output encoder: 14-bit absolute magnetic
Peak phase current: 30 A
Supply voltage: 48 V nominal
The motor must produce 15/8 ≈ 1.9 N·m peak at 30 A — requiring a motor with KT ≈ 0.063 N·m/A. This points to a motor in the 80–100mm diameter range.