Force control with gazebo

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Hi there,

I have questions about force control of manipulator.
1. Which sensor is recommended to measure external force applied to fingertip,
force sensor, contact sensor or other one ?
2. How can I implement the recommended sensor ? Any good example or tutorial ?
3. Anybody who has done that before ?
Here, external force -> measured force between robot's fingertip and a rigid object like a table.

gazebo::physics::JointWrench seems to provide what I want, but it's placed at joint, not link.
Sensor which can be put on link is preferable.
Also, I know gazebo::physics::joint::GetForce provide joint torques.
However it's not what I want.

Thank you very much.