Quasi-Direct Drive (QDD)

Quasi-direct drive actuators sit between fully direct-drive motors and high-ratio geared motors. They use a low-ratio transmission (typically 1:6 to 1:9 planetary gearbox) to boost torque while preserving the back-drivability and low reflected inertia that make compliant control possible.

Why QDD for Legged Robots?

Traditional servo actuators use high-ratio gearboxes (50:1–100:1 harmonic or strain wave drives) that deliver high torque in a compact package but are:

Not back-drivable — external forces cannot move the joint, so impacts are transmitted fully to the structure
High reflected inertia — the motor inertia is multiplied by the gear ratio squared, making impedance control difficult
Fragile under impact — shock loads damage gearbox teeth

QDD actuators trade peak torque for physical compliance:

Property	High-ratio gear	QDD
Gear ratio	50:1 – 100:1	1:1 – 9:1
Back-drivability	No	Yes
Reflected inertia	High	Low
Impact tolerance	Low	High
Peak torque	Very high	Moderate
Control bandwidth	Low (series elasticity)	High

Impedance Control

Because QDD joints are back-drivable, you can implement impedance control in software rather than hardware:

$\tau = K_p · (\theta_{des} − \theta) + K_d · (\dot{\theta}_{des} − \dot{\theta}) + \tau_ff$

where $K_p$ and $K_d$ are virtual spring and damper gains set at runtime. This lets you tune joint stiffness from nearly free-swinging (for walking) to rigid (for manipulation) without changing the hardware.

MIT Cheetah Legacy

The QDD approach was popularised by the MIT Mini Cheetah (2018) and Cheetah 3 robots. Robstride actuators are directly inspired by the MIT design, using similar motor winding and encoder architecture with a CAN-based communication protocol derived from the MIT motor driver.

The Robstride 00 uses a 6:1 planetary gearbox. At the nominal 12 Nm stall torque, reflected inertia at the joint is approximately 0.003 kg·m², low enough for high-bandwidth torque control.

Key Design Principles

Low-ratio gearbox keeps reflected motor inertia small, so the joint “feels” light to external forces.

High-pole-count BLDC motor produces high torque at low RPM, partially compensating for the low gear ratio.

Integrated encoder (typically magnetic absolute) eliminates backlash error from a separate sensing stage.

Integrated driver — Robstride and similar actuators embed the motor driver PCB inside the housing, reducing cable runs to just power and CAN.