Series Elastic and Quasi-Direct-Drive Actuators

A stiff, high-ratio gearbox makes a robot strong and precise, but a collision hits hard. Two designs soften that. A series elastic actuator puts a spring between the motor and the load, which also lets the robot measure force from how much the spring stretches. A quasi-direct-drive actuator uses a big motor with a low gear ratio, so the joint gives way when pushed.

Actuator Spring stiffness (series elastic) Approach speed

Why impacts hurt

Through a gearbox of ratio $N$, the motor's own rotor inertia $J_m$ feels $N^2$ times larger at the output. That reflected inertia hits the wall along with the link:

$$J_{\text{reflected}} = N^2 J_m \qquad F_{\text{peak}} \approx v\,\sqrt{k_{\text{eff}}\, m_{\text{eff}}}$$

A series spring of stiffness $k$ limits $k_{\text{eff}}$ and also gives a force sensor for free:

$$F = k\,(x_{\text{motor}} - x_{\text{load}})$$

What to notice

The stiff high-ratio actuator gives a sharp, tall force spike, because the big reflected motor inertia slams into the wall through a stiff gear train. The series elastic actuator spreads the impact out, and a softer spring makes the peak lower. The quasi-direct-drive actuator has a small ratio, so its reflected inertia is tiny and it also backs off quickly when it feels the wall.

Series elastic actuators are common in robots that work near people. Quasi-direct-drive is the choice for many legged robots, which hit the ground with every step.