The author of this series of tutorials ： Little fish
official account ： Fish fragrance ROS
QQ Communication group ：139707339
Teaching video address ： Small fish B standing
Full document address ： Fish fragrance ROS Official website
Copyright notice ： Reprint and commercial use are prohibited unless permitted .

8.3 Start creating a mobile robot

Hello everyone , I'm little fish , In this lesson, let's work together to create a two wheel differential mobile robot fishbot, In the last section, we have installed radar on our robot , In this class, we will continue to improve our robot model .

Robots besides radar , It also needs to be IMU Acceleration sensors and wheels that can be driven , In Chapter 7, we introduced the robotics part , Two differential models have been introduced , So we also need to create two differential drive wheels and a support wheel .

So this little fish takes you to add the following parts and joints to the robot ：

1. IMU Sensor components and joints
2. Left wheel parts and joints
3. Right wheel part and joint
4. Support wheel parts and joints

1. add to IMU sensor ( Last assignment )

IMU Sensors and transparency and color modification are the last lesson , Homework , Xiaoyu will take you to finish it first

practice 1： Try changing the robot's body color to blue , Transparency is 50%(0.1 0.1 1.0 0.5)

practice 2： Try to URDF Add imu_link And use imu_joint Fix it above the center of the car body 2cm,imu The geometry used is box, Length, width and height are 2cm

1.1 Change the color

Transparency modification only needs to be done in base_link Add material

<link name="base_link">
    <visual>
    	<origin xyz="0 0 0.0" rpy="0 0 0"/>
    <geometry>
    	<cylinder length="0.12" radius="0.10"/>
    </geometry>
    <material name="blue">
    	<color rgba="0.1 0.1 1.0 0.5" /> 
    </material>
    </visual>
</link>

1.2 add to imu

  <link name="imu_link">
  	<visual>
      <origin xyz="0 0 0.0" rpy="0 0 0"/>
      <geometry>
		    <box size="0.02 0.02 0.02"/>
      </geometry>
    </visual>
  </link>

  <!-- imu joint -->
  <joint name="imu_joint" type="fixed">
      <parent link="base_link" />
      <child link="imu_link" />
      <origin xyz="0 0 0.02" />
  </joint>

2. Add right wheel

2.1 Add joints

The joint name is right_wheel_link, The little fish is doing ros2 The wheels used in the trolley are shown in the figure below ：

The width of the wheel is 4cm, The diameter is 6.4cm, The geometry is a cylinder , therefore geometry The configuration is as follows ：

<geometry>
	<cylinder length="0.04" radius="0.032"/>
</geometry>

It should be noted that , The default orientation of the cylinder is upward

We can origin Of rpy Change the rotation angle of the wheel , Let it go around x Axis rotate pi/2, therefore origin The configuration is

<origin xyz="0 0 0" rpy="1.57079 0 0"/>

Change color to black , You can get the following configuration ：

  <link name="right_wheel_link">
      <visual>
        <origin xyz="0 0 0" rpy="1.57079 0 0"/>
        <geometry>
          <cylinder length="0.04" radius="0.032"/>
        </geometry>
          <material name="black">
            <color rgba="0.0 0.0 0.0 0.5" /> 
          </material>
      </visual>
  </link>

2.2 add to joint

We fixed the center of the left wheel on the left rear of the robot

It should be noted that origin and axis Value settings

First look at origin

because base_link Is the height of 0.12, We

• z Express child relative parent Of z The relationship on the axis , Want to fix the wheel on the lower surface of the robot , therefore origin Of z Offset down 0.12/2=0.06m( The downward sign is negative )

• y Express child relative parent Of y The relationship on the axis ,base_link The radius of this is 0.10, So we let the wheel y Axial negative direction offset 0.10m( The left sign is negative )

• x Express child relative parent Of x The relationship on the axis , The backward offset is x The axis is offset backward , Let's use a similar value 0.02m( The backward sign is negative )

Look again axis

The wheel turns , Which axis should it rotate on ？ As can be seen from the above figure, it is around y The counterclockwise direction of the shaft , therefore axis The Settings for ：

<axis xyz="0 1 0" />

  <joint name="right_wheel_joint" type="continuous">
      <parent link="base_link" />
      <child link="right_wheel_link" />
      <origin xyz="-0.02 -0.10 -0.06" />
      <axis xyz="0 1 0" />
  </joint>

3. Add left wheel

The left wheel is the mapping of the right wheel , I won't repeat

  <link name="left_wheel_link">
      <visual>
        <origin xyz="0 0 0" rpy="1.57079 0 0"/>
        <geometry>
          <cylinder length="0.04" radius="0.032"/>
        </geometry>
          <material name="black">
            <color rgba="0.0 0.0 0.0 0.5" /> 
          </material>
      </visual>
  </link>
    
  <joint name="left_wheel_joint" type="continuous">
      <parent link="base_link" />
      <child link="left_wheel_link" />
      <origin xyz="-0.02 0.10 -0.06" />
      <axis xyz="0 1 0" />
  </joint>

4. Add support wheel

The supporting wheel is fixed in front of the robot , Use a sphere , The radius uses 0.016m, The diameter of the ball is 0.032m Same radius as left and right wheels , Then offset down 0.016+0.06=0.076m, The downward value is negative , At the same time, move the support theory forward , Choose one 0.06m

The final results are as follows ：

<link name="caster_link">
    <visual>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry>
          <sphere radius="0.016"/>
      </geometry>
        <material name="black">
          <color rgba="0.0 0.0 0.0 0.5" /> 
        </material>
    </visual>
  </link>
    
  <joint name="caster_joint" type="fixed">
      <parent link="base_link" />
      <child link="caster_link" />
      <origin xyz="0.06 0.0 -0.076" />
  </joint>

Final URDF file ：https://raw.githubusercontent.com/fishros/fishbot/master/src/fishbot_description/urdf/fishbot_base.urdf

5. test run

5.1 Compile testing

colcon build
source install/setup.bash
ros2 launch fishbot_description display_rviz2.launch.py

5.2 final result

rviz Configuration of

final result

jointstate Two more sliders

Node relationship

Print joint_states topic of conversation

ros2 topic echo /joint_states

5.3 adopt joint_state_gui Change joints tf Middle joint angle

stay JointStatePublisher in , Drag the slider , Observe

1. rviz2 in tf Transformation of
2. joint_states Transformation of values in

You can see that as the progress bar drags , Values in the topic and rviz2 The joints of the robot rotate synchronously ,joint_states Topics can also be sent manually , In the next class, Xiaoyu takes you to send it manually joint_states To control the rotation of robot wheels

5.4 On how to land the wheel

Although the robot model is shown , But there's a problem I don't know if you find , That is in RVIZ The robot wheels in are under the ground .

The reason is that we fixed-frame The choice is base_link,base_link Your location is already left_wheel_link and right_wheel_link Just go up , So what should we do ？

It's very simple , We added a virtual link And joints , This joint is related to base_link Connected to a , Position is located in the base_link Down just to the lower surface of the wheel .

Come on , Let us to base_link Add a parent link-base_footprint, Newly added URDF The code is as follows ：

  <!-- Robot Footprint -->
  <link name="base_footprint"/>

  <joint name="base_joint" type="fixed">
    <parent link="base_footprint"/>
    <child link="base_link"/>
    <origin xyz="0.0 0.0 0.076" rpy="0 0 0"/>
  </joint>

Because it's a virtual joint , We don't have to deal with this link Describe the shape of ,joint Of origin Set to xyz="0.0 0.0 0.076" Represents the joint base_footprint Up 0.076 Namely base_link（ I don't think it's easy to understand. You can see the figure below ）.

Save the compilation and run the test again , Now the wheels are on the ground ~

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The authors introduce ：

I'm little fish , Senior players in Robotics , Now a one legged robot Algorithm Engineer in Shenzhen
Junior high school learning programming , High school began to contact robots , During the University, playing robot related competitions to achieve monthly income 2W+（ The prize money for the competition ）
Currently outputting Robot Learning Guide 、 Notes to the paper 、 Work experience , Welcome to pay attention to little fish , Exchange technology together , Learning robots