Document that force-torque sensor should only be attached to joints, not links

[scpeters]

It has always been my understanding based on the usage in osrf/gazebo that a sensor of type force_torque may only be a //joint/sensor and not a //link/sensor. I believe it is not stated in the specification, and the test/sdf/sensors.sdf example even includes a //link/sensor of type force_torque. I noted this while reviewing the new sdf::ForceTorque DOM element, which does not enforce this condition, in keeping with the pattern currently set.

I would propose that we formalize this in SDFormat 1.8 and generate parsing errors if a //link/sensor has type force_torque.

[scpeters]

I discussed this with @azeey today, and the user story about force-torque sensors could use some improvement, as there are use cases in which a force-torque sensor can be attached to an end effector and used like a contact-wrench sensor rather. This could be considered attaching it to a link rather than a joint from a user perspective.

There's not quite enough time to sort this out for the libsdformat11 feature freeze, but something quick that we could do for libsdformat11 would be to add sensor accessor API's to the Joint DOM class, since those are currently missing. This would allow Joint sensors to be accessed from the DOM.

[traversaro]

I discussed this with @azeey today, and the user story about force-torque sensors could use some improvement, as there are use cases in which a force-torque sensor can be attached to an end effector and used like a contact-wrench sensor rather. This could be considered attaching it to a link rather than a joint from a user perspective.

Perhaps I am missing something, but that use case is not covered by contact sensors in SDF?

[scpeters]

I discussed this with @azeey today, and the user story about force-torque sensors could use some improvement, as there are use cases in which a force-torque sensor can be attached to an end effector and used like a contact-wrench sensor rather. This could be considered attaching it to a link rather than a joint from a user perspective.

Perhaps I am missing something, but that use case is not covered by contact sensors in SDF?

that was my first thought as well, but that is informed by knowledge of gazebo's implementation details, and @azeey pointed out that end users may not recognize that distinction. I think there is a documentation / narrative issue to be resolved for end users

[scpeters]

There's not quite enough time to sort this out for the libsdformat11 feature freeze, but something quick that we could do for libsdformat11 would be to add sensor accessor API's to the Joint DOM class, since those are currently missing. This would allow Joint sensors to be accessed from the DOM.

this was done, starting with #393

[scpeters]

I've started a prototype that warns if a ForceTorque sensor is attached to a link in 3953e7f