OK. I see you have a pendulum and a cart-pole model as examples; something like that is very easy to set up. RigidBodyDynamics can load URDFs, so you could just take the cart-pole URDF from bullet and load it as follows:
urdf = download("https://raw.githubusercontent.com/bulletphysics/bullet3/0e1dce41eab75fd210ec73a52adbf249710c8edf/data/cartpole.urdf", "cartpole.urdf")
cartpole = parse_urdf(Float64, urdf)
which results in
Vertex: world (root)
Vertex: slideBar, Edge: slideBar_to_world
Vertex: cart, Edge: slider_to_cart
Vertex: pole, Edge: cart_to_pole
No non-tree joints.
Then, to evaluate the dynamics, you can do:
state = MechanismState(cartpole)
fixedjoint, slidingjoint, pinjoint = joints(cartpole) # unpack joints
configuration(state, slidingjoint) .= 0.1
configuration(state, pinjoint) .= 0.2
velocity(state) .= 0
result = DynamicsResult(cartpole)
after which the joint accelerations can be retrieved from
And you can follow the RigidBodySim quickstart guide to simulate. If you need gradients, check out this notebook.
Note that there are some visualization changes coming soon in RigidBodySim (https://github.com/JuliaRobotics/RigidBodySim.jl/pull/64): fully switching to MeshCat.jl for visualization, making RigidBodySim quicker to install and available on Windows, among other things. Reducing load time and dropping more dependencies is also on my list.