# External Force/Wrench Feedback

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Having spent a considerable amount of time striving to discover a topic related to the external wrenches/contact forces, all my efforts have been to no avail. Is there any means of acquiring end-effector contact force feedback (3 moments and 3 forces) through the Intera SDK? How do the existing interaction/force control algorithms function without this feedback?

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Hi Emmanuel,

You can find the forces acting upon the end effector on the following topic:

/robot/limb/right/endpoint_state

The messages on this topic consist of intera_core_msgs/EndpointState . They contain Wrenches and Torques for the endpoint:

std_msgs/Header header
...
geometry_msgs/Pose pose
...
geometry_msgs/Twist twist
...
geometry_msgs/Wrench wrench
geometry_msgs/Vector3 force
float64 x
float64 y
float64 z
geometry_msgs/Vector3 torque
float64 x
float64 y
float64 z
bool valid

You can expect to see the force of gravity in the force value, but aside from that this message will show you the external forces you're looking for. Please let us know if you have any additional questions.

Best,

~ Ian

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Hi Ian,

Thank you for the enlightening response, and apologies for the belated acknowledgement.

I will be more attentive next time.

Best regards,

Emmanuel

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• 2 weeks later...

Hi Ian,

It has recently come to my attention that the following topic:

/robot/limb/right/endpoint_state

does not actually provide the external wrench values 'felt' by the end-effector, which would have been provided if a force/torque sensor were incorporated into the tip of the end-effector. It appears as though the forenamed topic actually publishes external force/moment values generated using some kind of disturbance observer. Otherwise, I would be led to assume that the mere motion of the system is inducing noisy signals into the wrench readings.

Is there any means of acquiring the actual wrenches at the end-effector, which would imply the generation of solely zero values during 'free motion', and non-zero values during environmental contact at the end-effector?

Kind regards,

Emmanuel

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Kind of related to this: What is in the field "endpoint_force_command" of the messages in the topic:

/robot/limb/right/interaction_controller/state

?

I expected to see directly my commands when I send a desired force to the interaction_controller, but that is not the case. Are they computed from \tau of the joints? Do you subtract gravity?

Thanks

--Roberto

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