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Yes effort controllers control torque in the case of rotary joints, or force in the case of linear joints. They are named that way since they are agnostic to the type of joint they're controlling.

Controllers are defined by the type of control input and the type of output they use to drive the joint. By the sounds of it you want a controller which accepts velocity commands going in, and produces torque (effort) commands going up. This will allow you to set an upper limit on the torque. You want an effort_controllers/joint_velocity_controller for this.

ROS uses SI units as standard, so yes an effort_controller connected to a rotary joint will output torque in Nm.

Hope this helps.