How to understand builder.joint_target_mode

[galou]

I understand the type POSITION as a closed-loop control on position and VELOCITY as a closed-loop control on velocity. But what is a POSITION_VELOCITY control?

[jcarius-nv]

POSITION_VELOCITY means both target arrays are active: the position target contributes a stiffness term and the velocity target contributes a damping/velocity-tracking term. Conceptually:

force = kp * (target_pos - q) + kd * (target_vel - qd)

For SolverMuJoCo, this maps to two MuJoCo actuators: one driven by Control.joint_target_pos and one driven by Control.joint_target_vel. POSITION uses only joint_target_pos; if joint_target_kd is nonzero it still adds damping, but the velocity target is effectively zero rather than read from Control.joint_target_vel.

So if you want ordinary position servo behavior with damping, POSITION is enough. Use POSITION_VELOCITY when you want a nonzero velocity target to be part of the control input.