SDF based double pendulum example for Drake

[Michel Hidalgo]

Description

As part of the integration of sdformat with Drake, we should write a very simple usage example by means of loading a double pendulum from an SDF file into a RigidBodyTree so as to simulate and visualize it using Drake's codebase (see drake#6135).

What has to be done

1. Load a double pendulum from an SDF (i.e. using Gazebo standard model) into a RigidBodyTree.
2. Simulate it using a RigidBodyPlant and visualize it using a DrakeVisualizer.
3. Write a comprehensive explanatory README, with instructions on how to build and run it.

How to test it

Mainly by visual inspection. No new code to unit test.

[Andrés Fortier]

Just as a very small comment, consider that the double pendulum is linked in Drake's repo https://github.com/RobotLocomotion/drake/blob/master/drake/multibody/benchmarks/acrobot/double_pendulum.urdf. Check if it is the same as the one in Gazebo examples.

[Michel Hidalgo]

@andres_fortier thanks for the FYI, though I used Gazebo's (that lacks inertial data, which I had to add, but it's much more complete). I just pushed the example code to a branch on our drake fork. Since drake_visualizer has been broken for a week or so, I had to checkout an old commit to record this:

How do we want to proceed?

• We could add comprehensive documentation and comments.
• We could add test coverage for the minimal parser that we have.

[Michel Hidalgo]

PR sent upstream, with added documentation and tests.

[Michel Hidalgo]

PR finally merged. I think we can close this issue now.

[Andrés Fortier]

:+1: