urdf 学习笔记一

学习写urdf有几个地方需要注意

1. 一定要记住它使用的是右手坐标系。

x正方向朝左， y 正方向向内， z轴正方向朝上

2. 构建树结构， 即写link和joint

3. 每个link的参考坐标系都在它的底部，并与关节的参考坐标系正交，为了添加尺寸，需要指定偏移从一个link到它的关节的子link, 这通过添加origin到每个节点解决。

这么说origin表示的是关节相对于父关节的距离和旋转， xyz和rpy

用ros wiki上的r2d2实例学习

1.初始版本

<?xml version="1.0"?>
<robot name="origins">
<link name="base_link">
<visual>
<geometry>
<cylinder length="0.6" radius="0.2"/>
</geometry>
</visual>
</link>

<link name="right_leg">
<visual>
<geometry>
<box size="0.6 .2 .1"/>
</geometry>
</visual>
</link>

<joint name="base_to_right_leg" type="fixed">
<parent link="base_link"/>
<child link="right_leg"/>
</joint>

</robot>

2. 加入关节origin

因为basekink没有改变，所以这里省略baselink...

<link name="right_leg">
<visual>
<geometry>
<box size="0.6 .2 .1"/>
</geometry>
</visual>
</link>

<joint name="base_to_right_leg" type="fixed">
<parent link="base_link"/>
<child link="right_leg"/>
<origin xyz="0.22 0 .25"/>（左移和上移， x轴左为正， z为上， y 垂直平面， 右手坐标系）
</joint>

3. 加入link origin y轴 旋转

因为basekink没有改变，所以这里省略baselink...

<link name="right_leg">
<visual>
<geometry>
<box size="0.6 .2 .1"/>
</geometry>
<origin rpy="0 1.57075 0"/> （y轴旋转90度）
　　</visual>
</link>

<joint name="base_to_right_leg" type="fixed">
<parent link="base_link"/>
<child link="right_leg"/>
<origin xyz="0.22 0 .25"/>
</joint>

4. 加入link origin xyz偏移

因为basekink没有改变，所以这里省略baselink...

<link name="right_leg">
<visual>
<geometry>
<box size="0.6 .2 .1"/>
</geometry>
<origin rpy="0 1.57075 0" xyz = “0 0 -0.3 ”/> （y轴旋转90度, 往下0.3）
</visual>
</link>

<joint name="base_to_right_leg" type="fixed">
<parent link="base_link"/>
<child link="right_leg"/>
<origin xyz="0.22 0 .25"/>
</joint>

5. 给模型加入材质（穿上一件衣服）

<?xml version="1.0"?>
<robot name="origins">
<link name="base_link">
<visual>
<geometry>
<cylinder length="0.6" radius="0.2"/>
</geometry>
<material name = "blue">
<color rgba = "0 0 .8 1"/>
</material>
</visual>
</link>

<link name="right_leg">
<visual>
<geometry>
<box size="0.6 .2 .1"/>
</geometry>
<material name = "white">
<color rgba = "1 1 1 1"/>
</material>
<origin rpy = "0 1.57 0" xyz = "0 0 -0.3"/>
</visual>
</link>

加上左臂

<?xml version="1.0"?>
<robot name="origins">
<link name="base_link">
<visual>
<geometry>
<cylinder length="0.6" radius="0.2"/>
</geometry>
<material name = "blue">
<color rgba = "0 0 .8 1"/>
</material>
</visual>
</link>

<link name="right_leg">
<visual>
<geometry>
<box size="0.6 .2 .1"/>
</geometry>
<material name = "white">
<color rgba = "1 1 1 1"/>
</material>
<origin rpy = "0 1.57 0" xyz = "0 0 -0.3"/>
</visual>
</link>

<joint name="base_to_right_leg" type="fixed">
<parent link="base_link"/>
<child link="right_leg"/>
<origin xyz="0.22 0 .25"/>
</joint>

<link name="left_leg">
<visual>
<geometry>
<box size="0.6 .2 .1"/>
</geometry>
<material name = "white">
<color rgba = "1 1 1 1"/>
</material>
<origin rpy = "0 1.57 0" xyz = "0 0 -0.3"/>
</visual>
</link>

<joint name="base_to_left_leg" type="fixed">
<parent link="base_link"/>
<child link="left_leg"/>
<origin xyz="-0.22 0 .25"/> 仅仅这里不同， 关节右移所以是负
</joint>

</robot>

6. 添加头，sphere几何体

注意每定义个link都要为它添加一个关节，否则check_urdf时报错

Error: Failed to find root link: Two root links found: [base_link] and [head]

没有给头link添加关节导致的错误。

<link name = "head">
<visual>
<geometry>
<sphere radius = "0.2"/>
</geometry>
<material name = "white">
<color rgba = "1 1 1 1"/>
</material>
</visual>
</link>
<joint name = "head_swivel" type = "fixed">
<parent link = "base_link"/>
<child link = "head"/>
<origin xyz = "0 0 0.3"/>
</joint>

8. 添加车轮和车轮底座

<link name = "right_wheel_base"> //右车轮底座
<visual>
<geometry>
<box size = "0.1 .4 .1"/>
</geometry>
<material name = "yellow">
<color rgba = "1 1 0 1"/>
</material>
</visual>
</link>

<joint name = "right_wheel_base_joint" type = "fixed">
<parent link = "right_leg"/>
<child link = "right_wheel_base"/>
<origin xyz = "0 0 -0.65"/>
</joint>

<link name = "right_front_wheel"> // 右前轮
<visual>
<geometry>
<cylinder length = "0.08" radius = "0.04"/>
</geometry>
<material name = "black">
<color rgba = " 0 0 0 1"/>
</material>
</visual>
</link>

<joint name = "right_front_wheel_joint" type = "fixed">
<parent link = "right_wheel_base"/>
<child link = "right_front_wheel"/>
<origin xyz = "0 .14 -.05" rpy = "1.5 0 1.5"/> // 旋转了车轮
</joint>

<link name = "right_back_wheel"> // 右后轮
<visual>
<geometry>
<cylinder length = "0.08" radius = "0.04"/>
</geometry>
<material name = "black">
<color rgba = " 0 0 0 1"/>
</material>
</visual>
</link>

<joint name = "right_back_wheel_joint" type = "fixed">
<parent link = "right_wheel_base"/>
<child link = "right_back_wheel"/>
<origin xyz = "0 -.14 -.05" rpy = "1.5 0 1.5"/> // y轴为负，往后移
</joint>

左轮也是一样设置，只需把right 改成left即可, 最后效果如下