# Ignition Gazebo

## API Reference

7.0.0~pre1
Detachable Joints

The DetachableJoint system allows two models to start off rigidly attached and then detach during simulation by publishing to a topic. The system internally uses a fixed joint between two links, each belonging to two separate models. Because the system uses joints to connect models, the resulting kinematic topology has to be a tree, i.e., kinematic loops are not currently supported. This affects the choice of the parent link, and therefore, the parent model, which is the model that contains the DetachableJoint system.

For example, detachable_joint.sdf demonstrates a four wheel vehicle that holds three objects that are later detached from the vehicle. As seen in this example, the parent model is the vehicle. The kinematic topology is the following.

world---vehicle_blue---B1
\
---B2
\
---B3

If the objects were each parent models, instead, there would be multiple kinematic loops, as shown below.

world---B1---vehicle_blue
\ / /
---B2 /
\ /
---B3

Due to a limitation in the implementation of this system, if detached models need to collide with a parent model or other detached models that have the same parent, the parent model needs to have <self_collide> set to true. However, due to an issue in DART, the default physics engine, it is important that none of the parent or child models be in collision in their initial (attached) state.

The system has the following parameters:

• <parent_link>: Name of the link in the model containing this system that will be used as the parent link in the detachable joint.
• <child_model>: The name of the model containing the child link in the detachable joint.
• <child_model_link>: Name of the link in the <child_model> that will be used as the child link in the detachable joint.
• topic (optional): Topic name to be used for detaching connections. If empty, a default topic will be created with a pattern /model/<model_name>/detachable_joint/detach.