RigidBodyDynamics.jl attaching a new rigid body to the system


Is it possible to add an extra load to a mechanism created from an urdf? For example, add a sphere to the last joint of a system. Can RigidBodyDynamics.attach! be used for this?


Can RigidBodyDynamics.attach! be used for this?

Yep, that’s what it’s for. You could attach it using the Fixed joint type and then optionally call removed_fixed_tree_joints!(mechanism) to merge the rigid bodies connected by the fixed joint for efficiency.


How can I do this in a mechanism obtained from an urdf file?

robot = getmechanism("acrobot")
axis = SVector(0., 1., 0.) # joint axis
I_1 = 0.333 # moment of inertia about joint axis
c_1 = -0.15 # center of mass location with respect to joint axis
m_1 = 5. # mass
frame1 = CartesianFrame3D("new") # the reference frame in which the spatial inertia will be expressed
inertia1 = SpatialInertia(frame1, I_1 * axis * axis', m_1 * SVector(0, 0, c_1), m_1)
body = RigidBody(inertia1)
attach!(robot, ? , body, Fixed)


? in your code should be a body that’s already part of robot (e.g. last(bodies(robot))). The fourth argument should be a Joint, not the type of a JointType. It’s exactly the same as in cell 9 of the quickstart notebook. It doesn’t matter whether the Mechanism you’re attach!ing a new body to was created directly using API calls or using parse_urdf. See also the documentation for attach! if you hadn’t seen it yet, and feel free to open a documentation PR if anything is unclear there.


Thank you!