constant externalForce input troughout a trajectory

Hello,

I have a question regarding the externalForce function. When I specify a wrench for a given robot relative to the gripper, as in the example: fext5kg = externalForce(rbt, "ee", [-0.49050 0 0 0 -49.05], pickConfig), I am uncertain whether this force would remain constant throughout a trajectory.

Consider a scenario where the robot moves from a pickup point to a placement point. If the gripper's orientation changes, it suggests that the force and torque applied to the gripper will also change due to the angular shift of the payload relative to the gripper.

Please correct me if I am mistaken, but if my understanding is accurate, does it mean that I should update the external force for each waypoint? Should I consider the orientation of the payload and adjust its wrench accordingly? For instance, replacing 'pickConfig' with 'configurationWaypoints,' where I've stored my waypoints as joint configurations, could provide the force applied to the gripper and the robot for each configuration/orientation.

I am unsure about my reasoning. Does anyone have any insights or suggestions?

Answer 1

[Karsh Tharyani]

Hi @Denizhan AKINCI,

The external wrench provided to the externalForce function is defined in the "ee" (rigid body on the tree for your case) body's coordinate system when you also input the joint configuration to the function. Optionally, you could entirely omit passing the the joint configuration input to the function, but then, you would have to define the external wrench spatially transformed in the robot's base body's coordinate system.

This is clarified in great detail in this example here https://www.mathworks.com/help/robotics/ug/compute-joint-torques-to-balance-an-endpoint-force-and-moment.html

I hope this example helps you in formulating your problem and in defining the external wrench of your use case.

If you have follow-up questions or require further clarifications please do not hesitate to reach out MathWorks Technical Support.

Best,

Karsh

[Denizhan AKINCI]

Thank you for your answer but it's still unclear for me. So when the body (referred to as "ee" in this context) rotates around its own z-axis, any applied force will be subject to the same rotation ? This means that the direction of the force will change accordingly as the body rotates right ? I just want to double check.