update model after loading URDF

[hxrkpib]

There is a case that I want to add a payload to a quadruped robot. The payload is dynamically loaded
to body link(random mass/com/inertia).Before that, I import a model by loading URDF.
Now I have two choice:

1. update body link mass/com/inertia (treat body and payload as one link)
2. add a new link(fixed-joint) to body's child

but i don't find related API to achieve them.

[hxrkpib]

if I choice 2. add a new link(fixed-joint) to body's child, the new joint/link should can be deleted.

[jcarpent]

You can use appendBodyToJoint in the Model API

[hxrkpib]

You can use appendBodyToJoint in the Model API

thanks for your reply! If I use appendBodyToJoint, the payload body should can be deleted for update the payload body inertia.
in other words, the body should be add-delete-add... cycle like this to update payload body inertia(the payload mass/position maybe changed).

for example, when the robot is running, add a backpack to the body, and then remove it.

[jcarpent]

We should add the feature to remove inertia from a given joint inertia.
In the meantime, you can mimic this effect by saving the inertia of a given joint const pinocchio::Inertia saved_inertia = model.inertias[joint_id] and using the method appendBodyToJoint. When you want to remove its effect, you need to restore the saved value as model.inertias[joint_id] = saved_inertia. Is it fine?

[jcarpent]

@hxrkpib I will close the issue and transfer it as Q&A in the discussions.

[Gregwar]

Hello,

I have similar question so I ask it in this thread since it is related

If we update inertias entries in the model (e.g update a mass on the fly), do we have to run some specific computations to update related quantities (as a random example, I see there is a subtreeMasses in the data), or is it always done by relevant algorithms (like computing the mass matrix) ?