Drake Multibody

Multibody is the highlight of Drake. It represents a robot in a tree of bodies. MultibodyPlant exposes APIs that compute robot mathematics including the kinematics, the dynamics, jacobian, etc.

A good reading material to understand Multibody is Drake's documentation of Multibody. It show how MultibodyPlant is constructed and how it fit with other drake components.

Robot Modeling

Create Multibody

To create a Multibody robot, the best way is to parse robot description file such as URDF or SDF. Drake could read URDF or SDF using a parser. The parser uses tinyxml2 library to parse the XML file. It would iteratively parse each link, joint, transmission and generate a Multibody tree with the same structure described as in the XML file.

Another way to create the robot components is through MultibodyPlant API. For example, we could create bodies and connect bodies with joints. This normally works for simple models like inverted pendulum.

Be aware of some Drake features

Be careful that Drake does not support mesh file for geometry collision yet. In other words, if you need to simulate collision, you would need to create your own collision model, usually some simple geometry that wraps around your mesh file. (I found it more convenient to add collision model using Rviz, because it has a check box allows you to show the collision geometry, way easier to visualize and compare between mesh and the simplified collision geometry.)

If you migrate your robot from ROS, your URDF is probably using package to find the mesh file rather than using relative path. Drake support "package" concept used in ROS. However, you need to change the folder with URDF files into a package, which means you should provide a package.xml file and modify the BUILD.bazel file to make bazel recognize the package. In addition, in your code, the package.xml file path should be specified, so Drake could find the package as well. There is an example for that. (TODO: Add example here.)

Default behaviors Drake would do while parsing

One thing worth mentioning is that drake requires the model to have at least one transmission in the URDF, or it will fail to create input torque port for the plant. So if you do not have any transmission tag in the URDF, do create SimpleTransmission for each actuated joint.

One more thing, MultibodyPlant has a unique body called world_body. Every body created in Drake by default is a floating body, unless it is connected with some other bodies. Floating body is connected to the world_body with a floating joint. So if your robot should be connected to the ground, you would create new joints to connect the body and world. The floating joint will be overwritten.

Mobilizer

Drake uses Mobilizer to connect two links, it is a different concept from the traditional Joint.

Model Instance Index

ModelInstanceIndex is the ID of robots. The root diagram could have only one MultibodyPlant. So if you have more robots, they are grouped as ModelInstanceIndex within the same MultibodyPlant. Every element, like body, joint, etc, in the MultibodyPlant has a ModelInstanceIndex.

ModelInstanceIndex 0 is world. It has only one body, the world_body, with BodyIndex 0.

ModelInstanceIndex 1 includes bodies created by Drake API with no ModelInstanceIndex specified.

For each robot loaded from SDF/URDF, Drake will create a unique ModelInstanceIndex for that robot.

Kinematics

After the URDF file is imported, the MultibodyPlant is created. The plant is a class hosting a whole bunch of useful robotics function that we use to do motion planning and control.

Jacobian

Drake support compute all kinds of jacobian. To calculate the derivative with respect to something, AutoDiff is used to ease the way of calculating.

Dynamics

Drake support compute all kinds of dynamics algorithm, especially the ones in the book of Roy Featherstone 2008.