Skip to main content

The Actuator Revolution: From Hydraulics to QDD Electric

Duration: 50 min · Level: Intermediate · Module: 2. Actuator Architecture · Focus: actuators, QDD, motors, compliance

Learning objectives

By the end of this lesson you will be able to explain and apply:

  • MIT Mini Cheetah (2018) proved QDD motors at robot-scale
  • QDD motors use low gear ratios (3:1 to 9:1)
  • High gear ratio motors (100:1+) are strong but non-backdrivable
  • Torque density comparison
  • Key players

Why this matters​

Boston Dynamics' hydraulic Atlas was an engineering marvel but hydraulics are noisy, leak-prone, and require bulky pumps.

Overview​

Boston Dynamics' hydraulic Atlas was an engineering marvel but hydraulics are noisy, leak-prone, and require bulky pumps. The shift to quasi-direct drive (QDD) electric motors — pioneered by MIT's Biomimetic Robotics Lab — unlocked a new generation of robots that are quieter, cleaner, and inherently backdrivable.

Key concepts​

Key idea

MIT Mini Cheetah (2018) proved QDD motors at robot-scale: 6:1 planetary gear ratio, 12 motors, backdrivable, 14kg total weight

  • QDD motors use low gear ratios (3:1 to 9:1) — the motor spins slowly but is directly coupled to the joint; backdriving requires only small forces, making contact inherently safe
  • High gear ratio motors (100:1+) are strong but non-backdrivable — a force applied to the output cannot spin the motor; this is dangerous in contact situations
  • Torque density comparison: QDD ~10-20 Nm/kg, SEA ~5-15 Nm/kg, hydraulic ~100+ Nm/kg — hydraulic still wins on raw torque but at 10x cost and complexity
  • Key players: T-Motor (China) supplies QDD actuators to Unitree, many startups; Moog and Parker supply to industrial robots; BD designs in-house
  • Peak vs. continuous torque: QDD motors can briefly produce 3-5x their continuous torque rating for impact absorption and dynamic maneuvers

Check your understanding​

Try to recall each answer before expanding it.

Q1. What do you know about MIT Mini Cheetah (2018) proved QDD motors at robot-scale?

6:1 planetary gear ratio, 12 motors, backdrivable, 14kg total weight

Q2. What do you know about QDD motors use low gear ratios (3:1 to 9:1)?

the motor spins slowly but is directly coupled to the joint; backdriving requires only small forces, making contact inherently safe

Q3. What do you know about High gear ratio motors (100:1+) are strong but non-backdrivable?

a force applied to the output cannot spin the motor; this is dangerous in contact situations

Q4. What do you know about Torque density comparison?

QDD ~10-20 Nm/kg, SEA ~5-15 Nm/kg, hydraulic ~100+ Nm/kg — hydraulic still wins on raw torque but at 10x cost and complexity

Q5. What do you know about Key players?

T-Motor (China) supplies QDD actuators to Unitree, many startups; Moog and Parker supply to industrial robots; BD designs in-house

References​

  • MIT Cheetah 3: Design and Control of a Robust, Dynamic Quadruped Robot — Bledt et al. (2018). IROS 2018
  • Quasi-Direct Drive for Low-Cost Compliant Robotic Manipulation — Bhatt et al. (2021). ICRA 2021

Next: 2.2 Series Elastic Actuators — Compliance by Design →

Part of Module 2: Actuator Architecture.