Why does the double pendulum implementation use 2 Joint Actuators? about drake-tutorial HOT 2 CLOSED

Double pendulum means two-link robot arm. I believe either is okay for this arm to be fully actuated or underactuated (first joint apply no torque). If you want the underactuated version, you could simply apply zero torque on the first joint (set kp ki kd on the first joint to be zero). Or you could just add actuator on the second joint. Hope this helps.

from drake-tutorial.

Yep good point. Fair enough :-) I think I've only ever seen the underactuated one but it's fair that they can both be actuated and you could have a fully actuated version

from drake-tutorial.