3D
仿真环境
sapien
urdf 文件
Articulated Robotics 博客 urdf 专题页
总体架构
采用 link （构件）加 joint （关节）的方式描述可动物体，采用树形结构。
区分构件的两种方式：
-
可动的每一个 part
-
不可动但特征性强的（e.g. camera）
几种常见的 joint：
- Revolute：限制旋转关节（最小/最大角度限制）。
- Continuous：无限制旋转关节 (e.g. a wheel)。
- Prismatic：直线滑动关节（最小/最大运动限制）。
- Fixed：父子构件刚连接（区分构件的第二种方式）。
基本语法结构
urdf 基于 XML（可扩展标记语言），只关心逻辑结构，有严谨的嵌套关系。
第一行声明这是 XML 语言并声明版本，头尾封装所有内容。
点击查看代码
<?xml version="1.0"?>
<robot name="my_robot">
...
all the rest of the tags
...
</robot>
Link tags
除了 link 的名字（name），其他都是可选的。
- Visual
- Geometry：几何图形及其参数，或是一个 mesh 模型（从外部文件导入）
- Origin：改变固有中心（几何图形的几何中心，mesh 文件导出时的原点）的位置，使 link 的位置符合关节逻辑
- Material
- Color：采用 RGBA 格式（RGBA 取值均为 0 ~ 1，A 是透明度（1是不透明））
- Texture：可以传贴图（e.g. .jpg）
- Collision
Geometry and Origin: 一般照搬 Visual 里的，但为了计算方便，会把 mesh 抽象成更简单的几何图形 - Inertial（惯性属性）
- Mass：link 的质量
- Origin：link 的质心，简单情况下一般为 link 的固有中心
- Inertia：惯性矩阵（惯性矩与惯性积）
visual 与 collision 的 tag 可以有很多个。sapien 中自动合成多个 collision 形成复合碰撞体。
Joint tags
- Name：关节名字（有时候不命名也没事，但命了一定没事）
- Type：关节类型 (最常见的 fixed, prismatic, revolute, continuous).
- Parent and child links：标明父子 links
- Origin：两个 links 的关节位置间的位移
对于 fixed 以外的 joint 类型，还需包括：
- Axis：关节运动（平移/旋转）的轴（axis）
- Limits：
- Upper and Lower position limits：距离（米）或角度（弧度制）
- Velocity limits：速度限制（m/s 或 rad/s）
- Effort limits：最大力或力矩限制（N 或 Nm）
Extra Tags
还有一些其他的 tag （这里考虑适用于 sapien 环境的）
material：定义颜色或贴图
mimic：模仿关节，用于 joint 标签内部
<mimic joint="joint_a" multiplier="1.0" offset="0.0" />
sapien 支持解析此标签。当你驱动 \(\text{joint_a}\) 时，被设置了 \(\text{mimic}\) 的从动关节会根据公式 \(\theta_{mimic} = \text{mult} * \theta_a + \text{offset}\) 自动同步。
transmission：传动系统，但在 sapien 中只判断那些是主动，哪些是从动。
对于 link 和 joint 的起名要符合命名规范，一般分别加 _link 与 _joint
ROS conventions for humanoid robots
ROS conventions for mobile platforms
xacro
拆分 urdf
拆分后的文件通过“包含”机制重新组合。可以将其理解为一种逻辑上的“复制粘贴”，将子文件的内容嵌入到主文件中。
<xacro:include filename="other_file.xacro" />
层级关系：
- 主文件：必须拥有完整的 robot 标签，并定义机器人名称 name 以及 xmlns:xacro 命名空间。
- 子文件（被包含文件）：同样需要 robot 标签和命名空间，但不需要指定 name。如果子文件指定了 name，它也可以作为一个独立的 URDF 文件使用。
为了区分文件功能，通常采用以下后缀规范：
- 顶层总装文件：通常以 .urdf.xacro 结尾。
- 子模块文件：后缀较为灵活，可以是 .xacro、.urdf.xacro 或其他自定义后缀。
变量定义与全局修改 (Properties)：
<xacro:property name="arm_radius" value="0.5" />`
...
<cylinder radius="${arm_radius}" length="7" />
宏定义与模块复用 (Macros)：可以定义一个“零件模板”，然后通过传入不同的参数来多次实例化。
<xacro:macro name="inertial_box" params="mass x y z *origin">
<inertial>
<xacro:insert_block name="origin"/>
<mass value="${mass}" />
<inertia ixx="${(1/12) * mass * (y*y+z*z)}" ixy="0.0" ixz="0.0"
iyy="${(1/12) * mass * (x*x+z*z)}" iyz="0.0"
izz="${(1/12) * mass * (x*x+y*y)}" />
</inertial>
</xacro:macro>
...then ...
<xacro:inertial_box mass="12" x="2" y="3" z="4">
<origin xyz="0 2 4" rpy="0 0 0"/>
</xacro:inertial_box>
... will expand to ...
<inertial>
<origin xyz="0 2 4" rpy="0 0 0"/>
<mass value="12" />
<inertia ixx="25" ixy="0.0" ixz="0.0"
iyy="20" iyz="0.0"
izz="13" />
</inertial>
数学运算 (Math Expressions)：URDF 只能填死数字，而 Xacro 允许在 ${} 中写数学表达式。
<cylinder length="${4*arm_radius + pi}">
Example
File:
点击查看代码
<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="robot">
<!-- This is an example of a URDF. -->
<!-- As we move through the file, new things to note will be pointed out. -->
<!-- It's not meant an example of GOOD design, but an example of some of the various features of URDF/xacro. -->
<!-- This will include all the contents of example_include.xacro first. Go check it out! -->
<xacro:include filename="example_include.xacro" />
<!-- This first link called "world" is empty -->
<link name="world"></link>
<!-- A simple fixed joint from our empty world link, to our base. -->
<!-- The base origin is offset from the world origin. -->
<joint name="base_joint" type="fixed">
<origin xyz="1.5 1.0 0" rpy="0 0 0"/>
<parent link="world"/>
<child link="base_link"/>
</joint>
<!-- base_link is a large rectangular plate. Some things to note: -->
<!-- - We set the visual origin Z to half the box height, so that the link origin sits at the bottom of the box -->
<!-- - We set the collision to be identical to the visual -->
<!-- - We specified the colour manually (but still need to enter a name) -->
<!-- - We specified all the inertial parameters manually -->
<link name="base_link">
<visual>
<origin xyz="0 0 0.05" rpy="0 0 0"/>
<geometry>
<box size="2.5 1.5 0.1" />
</geometry>
<material name="green">
<color rgba="0.2 1 0.2 1"/>
</material>
</visual>
<collision>
<origin xyz="0 0 0.05" rpy="0 0 0"/>
<geometry>
<box size="2.5 1.5 0.1" />
</geometry>
</collision>
<inertial>
<origin xyz="0 0 0.05" rpy="0 0 0"/>
<mass value="12" />
<inertia ixx="2.26" ixy="0.0" ixz="0.0" iyy="6.26" iyz="0.0" izz="8.5" />
</inertial>
</link>
<!-- slider_joint lets slider_link move back and forth along the top of the base in one dimension. -->
<!-- - Origin is set to one of the top edges of the base_link box, so that our link skims across the top -->
<!-- - It moves along the X axis -->
<!-- - We need to specify limits for the motion -->
<joint name="slider_joint" type="prismatic">
<origin xyz="-1.25 0 0.1" rpy="0 0 0"/>
<parent link="base_link"/>
<child link="slider_link"/>
<axis xyz="1 0 0"/>
<limit lower="0" upper="2" velocity="100" effort="100"/>
</joint>
<!-- slider_link is the box skimming across the top of the base. Its parameters are similar to the base_link, however: -->
<!-- - Instead of explicitly describing a colour, it uses the named material "blue". It knows about "blue" that material was included in example_include.xacro. -->
<!-- - Instead of explicitly describing the inertia, we use a macro that was defined in the example_include.xacro -->
<link name="slider_link">
<visual>
<origin xyz="0 0 0.075" rpy="0 0 0"/>
<geometry>
<box size="0.5 0.25 0.15" />
</geometry>
<material name="blue" />
</visual>
<collision>
<origin xyz="0 0 0.075" rpy="0 0 0"/>
<geometry>
<box size="0.5 0.25 0.15" />
</geometry>
</collision>
<xacro:inertial_box mass="0.5" x="0.5" y="0.25" z="0.15">
<origin xyz="0 0 0.075" rpy="0 0 0"/>
</xacro:inertial_box>
</link>
<!-- arm_joint describes the rotation of the arm and is centred around the top corner of the slider box. -->
<!-- - The axis of rotation is -1 in Y, so that positive is "up" -->
<!-- - The upper limit uses xacro's mathematical features -->
<joint name="arm_joint" type="revolute">
<origin xyz="0.25 0 0.15" rpy="0 0 0"/>
<parent link="slider_link"/>
<child link="arm_link"/>
<axis xyz="0 -1 0"/>
<limit lower="0" upper="${pi/2}" velocity="100" effort="100"/>
</joint>
<!-- arm_link describes the arm -->
<!-- - We use the "property" feature to define the arm length and radius and use them multiple times -->
<!-- - The visual/collision origin is set to halfway along the length (similar to the box), but also with a rotation (again using the mathematical features). -->
<!-- This is because the cylinder extends along the Z axis by default, but we want it to be along the X axis (when the joint is at 0) -->
<xacro:property name="arm_length" value="1" />
<xacro:property name="arm_radius" value="0.1" />
<link name="arm_link">
<visual>
<origin xyz="${arm_length/2} 0 0" rpy="0 ${pi/2} 0"/>
<geometry>
<cylinder length="${arm_length}" radius="${arm_radius}" />
</geometry>
<material name="orange" />
</visual>
<collision>
<origin xyz="${arm_length/2} 0 0" rpy="0 ${pi/2} 0"/>
<geometry>
<cylinder length="${arm_length}" radius="${arm_radius}" />
</geometry>
</collision>
<xacro:inertial_cylinder mass="1.0" length="${arm_length}" radius="${arm_radius}">
<origin xyz="${arm_length/2} 0 0" rpy="0 ${pi/2} 0"/>
</xacro:inertial_cylinder>
</link>
<!-- camera_joint describes where the camera is relative to the arm -->
<!-- - Even though the camera isn't moving relative to the arm, it will probably be helpful to have its own link/frame rather than just adding more visuals to the arm -->
<!-- - For this example, the camera_link origin will be at the centre of the camera's "sensor" -->
<joint name="camera_joint" type="fixed">
<origin xyz="${arm_length} 0 ${arm_radius + 0.075}" rpy="0 0 0"/>
<parent link="arm_link"/>
<child link="camera_link"/>
</joint>
<!-- camera_link describes the camera at the end of the arm -->
<!-- - It has multiple visual elements, which ultimately get combined together -->
<!-- - Even if we specify different materials, RViz will just colour them all the same as the first -->
<!-- - Although we could also specify multiple collision geometries, instead we just use a single box that encompasses the whole camera -->
<link name="camera_link">
<visual>
<origin xyz="-0.03 0 0" rpy="0 0 0"/>
<geometry>
<box size="0.06 0.15 0.15" />
</geometry>
<material name="white" />
</visual>
<visual>
<origin xyz="0.03 0 0" rpy="0 ${pi/2} 0"/>
<geometry>
<cylinder length="0.06" radius="0.04" />
</geometry>
<material name="blue" />
</visual>
<collision>
<origin xyz="0.0 0 0" rpy="0 0 0"/>
<geometry>
<box size="0.12 0.15 0.15" />
</geometry>
</collision>
<xacro:inertial_box mass="0.1" x="0.12" y="0.15" z="0.15">
<origin xyz="0.0 0 0" rpy="0 0 0"/>
</xacro:inertial_box>
</link>
<xacro:include filename="example_gazebo.xacro" />
</robot>
Include File：
点击查看代码
<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro">
<!-- This file is not a robot in and of itself, it just contains some useful tags that could be included in any robot -->
<!-- Specify some colours -->
<material name="white">
<color rgba="1 1 1 1"/>
</material>
<material name="orange">
<color rgba="1 0.3 0.1 1"/>
</material>
<material name="blue">
<color rgba="0.2 0.2 1 1"/>
</material>
<!-- Specify some standard inertial calculations https://en.wikipedia.org/wiki/List_of_moments_of_inertia -->
<!-- These make use of xacro's mathematical functionality -->
<xacro:macro name="inertial_sphere" params="mass radius *origin">
<inertial>
<xacro:insert_block name="origin"/>
<mass value="${mass}" />
<inertia ixx="${(2/5) * mass * (radius*radius)}" ixy="0.0" ixz="0.0" iyy="${(2/5) * mass * (radius*radius)}" iyz="0.0" izz="${(2/5) * mass * (radius*radius)}" />
</inertial>
</xacro:macro>
<xacro:macro name="inertial_box" params="mass x y z *origin">
<inertial>
<xacro:insert_block name="origin"/>
<mass value="${mass}" />
<inertia ixx="${(1/12) * mass * (y*y+z*z)}" ixy="0.0" ixz="0.0" iyy="${(1/12) * mass * (x*x+z*z)}" iyz="0.0" izz="${(1/12) * mass * (x*x+y*y)}" />
</inertial>
</xacro:macro>
<xacro:macro name="inertial_cylinder" params="mass length radius *origin">
<inertial>
<xacro:insert_block name="origin"/>
<mass value="${mass}" />
<inertia ixx="${(1/12) * mass * (3*radius*radius + length*length)}" ixy="0.0" ixz="0.0" iyy="${(1/12) * mass * (3*radius*radius + length*length)}" iyz="0.0" izz="${(1/2) * mass * (radius*radius)}" />
</inertial>
</xacro:macro>
</robot>
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