The effort_controllers/JointPositionController take the Position as the target and apply the effort to the joint using the PID values you have to specify.

The effort_controllers/JointVelocityController take the Velocity as the target and apply the effort to the joint using the PID values you have to specify.

The effort_controllers/JointEffortController directly set the Effort to the joint, no PID is used.

For the controllers that apply the effort to the joints, you can chose among those three. You do not need a sensor, unless that is something you want to model (perhaps you have one specific sensor and want to simulate how it works with your robot, otherwise you can just use the ros controllers). ROS Controllers know about the state of the joint. You can also find the states in the /joint_states topic.