Robot model

[blegat]

We would like to have examples showing how to

1. URDF -> RigidBodyDynamics.jl (with RigidBodyDynamics.parse_urdf)
2. RigidBodyDynamics.jl -> Symbolics.jl motion equations
3. URDF -> RigidBodyDynamics.jl -> Symbolics.jl motion equations. URDF has fixed values for masses, etc.. and these need to be replaced by symbolic variables. It would be nice to have a function that does that programatically.
4. RigidBodyDynamics.jl -> URDF (with RigidBodyDynamics.write_urdf)
5. RigidBodyDynamics.jl + general disturbances -> Symbolics.jl motion equations (externalwrenches vs external_torques)
6. RigidBodyDynamics.jl + semi-spherical foot (modeled as external disturbance) -> Symbolics.jl motion equations

We could add the following utilities in the code:

• Create a function that takes a RigidBodyDynamics body parametrized by numbers and returns one with Symbolics.jl symbol parameters (useful for 3.)
• Create a function that takes a RigidBodyDynamics body parametrized by Sumbolics.jl symbols and assignment of symbols to value and returns one with the values (useful for 4.)

These will be showcased in the following examples:

Double pendulum:

• Create body with RigidBodyDynamics
• Dynamical system of free robot (showcases 2.)
• Dynamical system with arbitrary disturbance (showcases 5.)
• Dynamical system with semi-spherical foot contact point (showcases 6.)

Our robot:

• Create body with RigidBodyDynamics
• Export to URDF (showcases 4.)
• Dynamical system of free robot (showcases 2.)
• Dynamical system with semi-spherical foot contact point (showcases 6.)

Atlas robot:

• Import Atlas robot from its URDF with RigidBodyDynamics.parse_urdf (done by AtlasRobot.jl) (showcases 1.)
• Dynamical system of free robot (showcases 3.)