tags: ROS Unity urdf

URDF Modeling in Unity

Create a robot

base_link

base_link is namely the base link of a robot. All links are represented as a relative transform to base_link. Generally it doesn't have Visuals or Collisions.

1. Select 3D Object > URDF Model (new) to create a new URDF model object.
2. In the Hierarchy, select base_link and open Inspector tab. Select Child Joint Type and click Add child link (with joint).

Let's add a new child link whose joint type is Fixed. You'll see a new child link named link by default is genetrated. Let's rename it to body_link.

body_link

body_link is a main frame of a robot.

Visuals

In the Hierarchy, select body_link > Visuals. You'll see Urdf Visual (Script) in Inspector tab. Select Type of visual and click Add visual

Let's add visual whose type of visual is Box. You'll see a box in the Scene.

Transform

Here we describe the points to note to transform.

Position

If you want to move the whole body object, ++be sure to select body_link++, not body_link > Visuals > unnamed though the name of the actual box you see is unnamed.

This is because the position of a link, Visuals and Collisions must be the same. Both Visuals and Collisions are child objects of the link so the positions will be different if you move only Visuals object.

Rotation & Scale

In contrast to Position, ++you must select link > Visuals > unnamed++ to change Rotation and Scale. This is because the values of Trasform of unnamed represents the relative transform to the link.

Collisions

Adding Collissions is quite simple if you transform the visual object correctly.

Select body_link > Visuals > unnamed and see Urdf Visual (Script) in Inspector tab. Just click Add collision to match visual to generate the collision object.

right/left_wheel_link

Open Inspector tab of body_link and select ++Continuous++ for Child Joint Type in Urdf Link (Script) and add a child link.

Let's add two children for right/left wheels.

Select right_wheel_link > Visuals and add visual whose type is Cylinder.

Change Transform to proper position.

Caster

In order to realize a caster on your robot, it is recommended to use a slippery box instead of a real caster because it is costly to implement a real caster with URDF model.