问题描述
在使用 moveit setup assistant 时，发现教程里没有关于 planning groups 中 chain 的描述，但是在查看其它的配置包时，发现有很多都在使用 chain。
理解
于是，通过查阅资料得到以下理解：
A serial chain is specified using the base link and the tip link. The tip link in a chain is the child link of the last joint in the chain. The base link in a chain is the parent link for the first joint in the chain.
即，chain 是一个Serial Chain（连续链），通过指定 base link 和 tip link 来指定 serial chain。tip link 是 last joint 中的 child link， base link 是 first joint 中的 parent link 。
用途
Joints, Links, Chains and Subgroups represent different ways that you can use to specify which parts of the robot (as specified in the URDF) are part of your current planning group.
Looking at the srdf wiki, you can find some more information on the different types. For instance, notice that when a joint is added, the child link is automatically also included.
A chain is just an easy way to add several joints and links to the srdf without having to manually select all the joints/links in between. It is not advisable to mix different methods in a single group.
Hope that clears things up a bit.
这是一个用来添加多个 joints 和 links 到 srdf 的便捷方式，不需要手动添加需要的 joints 或 links。