Welcome to ROSMASTER R2 repository

8 Robot URDF model

According to different models, you only need to set the purchased model in [.bashrc], X1(ordinary four-wheel drive) X3(Mike wheel) X3plus(Mike wheel mechanical arm) R2(Ackerman differential) and so on. Section takes X3 as an example:

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#Raspberry Pi 5 master needs to enter docker first, please perform this step
#If running the script into docker fails, please refer to ROS/07, Docker tutorial
~/run_docker.sh

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sudo vim .bashrc 

Find the [ROBOT_TYPE] parameter and modify the corresponding model

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export ROBOT_TYPE=X3   # ROBOT_TYPE: X1 X3 X3plus R2 X7

8.1 URDF overview

Function package reference path: ~/yahboomcar_ws/src/yahboomcar_description

8.1.1 Introduction

URDF, the full name of Unified Robot Description Format, translated into Chinese as Unified Robot Description Format, is a robot model file described in xml format, similar to DH parameters.

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<?xml  version="1.0" encoding="utf-8"?> 
< robot  name = "yahboomcar" > 
​
</ robot > 

The first line is required for xml, which describes the version information of xml.

The second line describes the current robot name; all information about the current robot is contained in the [robot] tag.

8.1.2 Components

1) link, connecting rod, can be imagined as a human arm.

2) joint, joint, can be imagined as a human elbow.

The relationship between link and joint: two links are connected by joints.

8.1.3 links

1 Introduction

In the URDF descriptive language, links are used to describe physical properties.

• describe the visual display, <visual>Label.
• describe collision properties, <collision>Label.
• describe physical inertia, <inertial>Labels are not commonly used.

Links can also describe the link size(size)\color(color)\shape(shape)\inertial matrix(inertial matrix)\collision properties(collision properties) etc. Each Link will become a coordinate system.

2) sample code: ~/yahboomcar_ws/src/yahboomcar_description/urdf/yahboomcar_X3.urdf

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    < link  name = "front_left_wheel" > 
        < inertial > 
            < origin  xyz = "2.3728E-06 -9.4228E-07 0.00064068"  rpy = "0 0 0" /> 
            < mass  value = "0.051543" /> 
            < inertia  1.4597E = -05"  ixy = "-4.7945E-10"  ixz = "-2.4786E-10" 
                     iyy = "1.4598E-05"  iyz = "1.7972E-09"  izz = "2.4267E-05" /> 
        </ inertial > 
        < visual > 
            < origin  xyz = "0 0 0"  rpy = "0 0 0" /> 
            < geometry > 
                < mesh  filename = "package://yahboomcar_description/meshes/mecanum/front_left_wheel.STL" /> 
            </ geometry > 
            < material  name = "" > 
                < color  rgba = "0.7 0.7 0.7 1" /> 
            </ material > 
        </ visual > 
        < collision > 
            < origin  xyz = "0 0 0"  rpy = "0 0 0" /> 
            < geometry > 
                < mesh  filename = "package://yahboomcar_description/meshes/mecanum/front_left_wheel.STL" /> 
            </ geometry > 
        </ collision > 
    </ link > 

3) label introduction

• origin

  Describes the pose information; xyzThe attribute describes the coordinate position in the environment, rpyAttributes describe their own posture.

• mess

  Describes the quality of the link.

• inertia

  The inertial reference frame, due to the symmetry of the rotational inertia matrix, only needs 6 upper triangular elements ixx, ixy, ixz, iyy, iyz, izz as attributes.

• geometry

  The label describes the shape; meshThe main function of the attribute is to load the texture file, filenameThe file address of the attribute texture path. The label also includes other label descriptions:

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  <box size="1 2 3"/>     #box box, describe the length, width and height of the box through the size attribute.       
  <cylinder length="1.6" radius="0.5"/>    #cylinder is cylindrical, the height of the cylinder is described by the `length` property, and the radius of the cylinder is described by the `radius` property. 
  <sphere radius="1"/>     #sphere is spherical, and the radius of the sphere is described by the `radius` attribute. 
  

• material

  The label describes the material; nameAttributes are required, can be empty, and can be repeated. Through the [color] tag in rgbaAttributes to describe red, green, blue, and transparency, separated by spaces. The range of colors is [0-1].

8.1.4 joints

1 Introduction

Describe the relationship between two joints, motion position and velocity limits, kinematic and dynamic properties.

Joint Type:

• fixed: fixed joints. Movement is not allowed and acts as a connection.
• continuous: Rotate the joint. It can be rotated continuously, and there is no limit to the rotation angle.
• revolute: Rotate the joint. Similar to continuous, there is a limit to the rotation angle.
• prismatic: sliding joints. Move along a certain axis, there is a position limit.
• floating: floating joints. With six degrees of freedom, 3T3R.
• planar: Planar joints. Allows translation or rotation above the plane orthogonal.

2) sample code

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< joint  name = "front_right_joint"  type = "continuous" > 
        < origin  xyz = "0.08 -0.0845 -0.0389"  rpy = "-1.5703 0 3.14159" /> 
        < parent  link = "base_link" /> 
        < child  link = "front_right_wheel" /> 
        < axis  xyz = "0 0 1"  rpy = "0 0 0" /> 
        < limit  effort = "100"  velocity = "1" /> 
    </ joint > 

In the [joint] tag, the name attribute is required, which describes the name of the joint and is unique.

In the type attribute of the [joint] tag, fill in the corresponding six joint types.

3) label introduction

• origin

  subtab, referring to the rotation joint in parentThe relative position of the coordinate system.

• parent,child

  The parent and child sub-labels represent two links to be connected; parent is the reference, and child rotates around the praent.

• axis

  The child label indicates which axis(xyz) the corresponding link of the child rotates around and the amount of rotation around the fixed axis.

• limit

  The child tag is mainly to limit the child. lowerproperties and upperThe property limits the radian range of rotation, effortThe property limits the force range during rotation. (positive and negative value, the unit is cattle or N) velocityThe property limits the speed at which it turns, in meters/second or m/s.

• mimic

  Describes the relationship of this joint to existing joints.

• safety_controller

  Describe the safety controller parameters. Protect the movement of the robot joints.

8.2 URDF visualization

8.2.1. Start

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roslaunch yahboomcar_description display.launch 

8.2.2. Sample pictures

The red axis is the X axis; the green axis is the Y axis; the blue axis is the Z axis; the coordinate system formed by the three axes is called the base coordinate system.Adjusting the [joint_state_publisher_gui] component can control the rotation of the wheel.