最近在做一款2轮差速的机器人小车，在做gazebo仿真的时候，发现小车一直在缓慢的后退，一边后退一边缓慢拐弯。
环境：ros2 foxy gazebo-11
小车xacro模型代码
<?xml version="1.0"?>
<robot name="jtbot"
xmlns:xacro="http://ros.org/wiki/xacro">
<!-- =================================================================================== -->
<!-- | xacro文件转urdf文件方法： | -->
<!-- | 在当前文件夹打开终端，输入xacro jtbot_base.urdf.xacro > jtbot_base.urdf | -->
<!-- | 在当前文件夹生成jtbot_base.urdf纯urdf文件 | -->
<!-- =================================================================================== -->
<!-- 定义机器人常量 -->
<!-- 底盘 长 宽 高 -->
<xacro:property name="base_width" value="0.46"/>
<xacro:property name="base_length" value="0.46"/>
<xacro:property name="base_height" value="0.11"/>
<!-- 轮子半径 -->
<xacro:property name="wheel_radius" value="0.0775"/>
<!-- 轮子宽度 -->
<xacro:property name="wheel_width" value="0.06"/>
<!-- 轮子和底盘的间距 -->
<xacro:property name="wheel_ygap" value="0.01"/>
<!-- 轮子z轴偏移量 -->
<xacro:property name="wheel_zoff" value="0.055"/>
<!-- 轮子x轴偏移量 -->
<xacro:property name="wheel_xoff" value="0.15"/>
<!-- 支撑轮x轴偏移量 -->
<xacro:property name="caster_xoff" value="0.205"/>
<!-- 定义长方形惯性属性宏 -->
<xacro:macro name="box_inertia" params="m w h d">
<inertial>
<!-- <origin xyz="0 0 0" rpy="${pi/2} 0 ${pi/2}"/> -->
<mass value="${m}"/>
<inertia ixx="${(m/12) * (h*h + d*d)}" ixy="0.0" ixz="0.0" iyy="${(m/12) * (w*w + d*d)}" iyz="0.0" izz="${(m/12) * (w*w + h*h)}"/>
</inertial>
</xacro:macro>
<!-- 定义圆柱惯性属性宏 -->
<xacro:macro name="cylinder_inertia" params="m r h">
<inertial>
<!-- <origin xyz="0 0 0" rpy="${pi/2} 0 0" /> -->
<mass value="${m}"/>
<inertia ixx="${(m/12) * (3*r*r + h*h)}" ixy = "0" ixz = "0" iyy="${(m/12) * (3*r*r + h*h)}" iyz = "0" izz="${(m/2) * (r*r)}"/>
</inertial>
</xacro:macro>
<!-- 定义球体惯性属性宏 -->
<xacro:macro name="sphere_inertia" params="m r">
<inertial>
<mass value="${m}"/>
<inertia ixx="${(2/5) * m * (r*r)}" ixy="0.0" ixz="0.0" iyy="${(2/5) * m * (r*r)}" iyz="0.0" izz="${(2/5) * m * (r*r)}"/>
</inertial>
</xacro:macro>
<!-- 机器人底盘 -->
<link name="base_link">
<visual>
<geometry>
<box size="${base_length} ${base_width} ${base_height}"/>
</geometry>
<material name="Cyan">
<color rgba="0 1.0 1.0 1.0"/>
</material>
</visual>
<!-- 碰撞区域 -->
<collision>
<geometry>
<box size="${base_length} ${base_width} ${base_height}"/>
</geometry>
</collision>
<!-- 惯性特性 -->
<xacro:box_inertia m="0.2" w="${base_width}" d="${base_length}" h="${base_height}"/>
</link>
<!-- 机器人 Footprint -->
<link name="base_footprint"/>
<!-- 底盘关节 -->
<joint name="base_joint" type="fixed">
<parent link="base_link"/>
<child link="base_footprint"/>
<origin xyz="0.0 0.0 ${-(wheel_radius+wheel_zoff)}" rpy="0 0 0"/>
</joint>
<!-- 创建轮子宏函数 -->
<xacro:macro name="wheel" params="prefix x_reflect y_reflect">
<link name="${prefix}_link">
<visual>
<origin xyz="0 0 0" rpy="${pi/2} 0 0"/>
<geometry>
<cylinder radius="${wheel_radius}" length="${wheel_width}"/>
</geometry>
<material name="Gray">
<color rgba="0.5 0.5 0.5 1.0"/>
</material>
</visual>
<!-- 碰撞区域 -->
<collision>
<origin xyz="0 0 0" rpy="${pi/2} 0 0"/>
<geometry>
<cylinder radius="${wheel_radius}" length="${wheel_width}"/>
</geometry>
</collision>
<!-- 惯性属性 -->
<xacro:cylinder_inertia m="0.2" r="${wheel_radius}" h="${wheel_width}"/>
</link>
<!-- 轮子关节 -->
<joint name="${prefix}_joint" type="continuous">
<parent link="base_link"/>
<child link="${prefix}_link"/>
<origin xyz="${x_reflect*wheel_xoff} ${y_reflect*(base_width/2+wheel_width/2+wheel_ygap)} ${-wheel_zoff}" rpy="0 0 0"/>
<axis xyz="0 1 0"/>
</joint>
</xacro:macro>
<!-- 根据上面的宏函数实例化左右轮 -->
<xacro:wheel prefix="left_wheel" x_reflect="1" y_reflect="1" />
<xacro:wheel prefix="right_wheel" x_reflect="1" y_reflect="-1" />
<!-- 差速驱动仿真插件 -->
<gazebo>
<plugin name='diff_drive' filename='libgazebo_ros_diff_drive.so'>
<ros>
<!-- 命名空间 -->
<!-- <namespace>/demo</namespace> -->
</ros>
<!-- 左右轮子 -->
<left_joint>left_wheel_joint</left_joint>
<right_joint>right_wheel_joint</right_joint>
<!-- 轮距 轮子直径 -->
<wheel_separation>${base_width+wheel_width}</wheel_separation>
<!-- <wheel_separation>0.52</wheel_separation> -->
<wheel_diameter>${wheel_radius*2}</wheel_diameter>
<!-- <wheel_diameter>0.155</wheel_diameter> -->
<!-- 最大扭矩 最大加速度 -->
<max_wheel_torque>20</max_wheel_torque>
<max_wheel_acceleration>1.0</max_wheel_acceleration>
<!-- 输出 -->
<!-- 是否发布里程计 -->
<publish_odom>true</publish_odom>
<!-- 是否发布里程计的tf开关 -->
<publish_odom_tf>true</publish_odom_tf>
<!-- 是否发布轮子的tf数据开关 -->
<publish_wheel_tf>true</publish_wheel_tf>
<!-- 里程计的framed ID，最终体现在话题和TF上 -->
<odometry_frame>odom</odometry_frame>
<!-- 机器人的基础frame的ID -->
<robot_base_frame>base_link</robot_base_frame>
</plugin>
</gazebo>
<!-- 支撑轮 -->
<link name="caster_link">
<visual>
<geometry>
<!-- <sphere radius="${(wheel_radius/2)}"/> -->
<sphere radius="${(wheel_radius)}"/>
</geometry>
<material name="Cyan">
<color rgba="0 1.0 1.0 1.0"/>
</material>
</visual>
<!-- 碰撞区域 -->
<collision>
<origin xyz="0 0 0" rpy="0 0 0"/>
<geometry>
<sphere radius="${(wheel_radius/2)}"/>
</geometry>
</collision>
<!-- 惯性属性 -->
<xacro:sphere_inertia m="0.2" r="${(wheel_radius/2)}"/>
</link>
<!-- 支撑轮gazebo颜色 -->
<gazebo reference="caster_link">
<material>Gazebo/Black</material>
</gazebo>
<!-- 支撑轮gazebo摩擦力 -->
<gazebo reference="caster_link">
<mu1 value="0"/>
<mu2 value="0"/>
<kp value="1000000.0" />
<kd value="10.0" />
</gazebo>
<!-- 支撑轮关节 -->
<joint name="caster_joint" type="fixed">
<parent link="base_link"/>
<child link="caster_link"/>
<origin xyz="${-caster_xoff} 0.0 ${-(base_height+wheel_radius)/2}" rpy="0 0 0"/>
</joint>
<!-- imu -->
<link name="imu_link">
<visual>
<geometry>
<box size="0.06 0.03 0.03"/>
</geometry>
</visual>
<!-- 碰撞区域 -->
<collision>
<geometry>
<box size="0.06 0.03 0.03"/>
</geometry>
</collision>
<!-- 惯性属性 -->
<xacro:box_inertia m="0.1" w="0.1" d="0.1" h="0.1"/>
</link>
<!-- imu关节 -->
<joint name="imu_joint" type="fixed">
<parent link="base_link"/>
<child link="imu_link"/>
<origin xyz="-0.05 0 -0.055"/>
</joint>
<!-- imu仿真插件 -->
<gazebo reference="imu_link">
<sensor name="imu_sensor" type="imu">
<plugin filename="libgazebo_ros_imu_sensor.so" name="imu_plugin">
<ros>
<!-- 命名空间 -->
<!-- <namespace>/demo</namespace> -->
<remapping>~/out:=imu</remapping>
</ros>
<!-- 初始方位_参考 -->
<initial_orientation_as_reference>true</initial_orientation_as_reference>
</plugin>
<always_on>true</always_on>
<!-- 更新频率 -->
<update_rate>100</update_rate>
<visualize>true</visualize>
<imu>
<!-- 角速度 -->
<angular_velocity>
<x>
<noise type="gaussian">
<mean>0.0</mean>
<stddev>2e-4</stddev>
<bias_mean>0.0000075</bias_mean>
<bias_stddev>0.0000008</bias_stddev>
</noise>
</x>
<y>
<noise type="gaussian">
<mean>0.0</mean>
<stddev>2e-4</stddev>
<bias_mean>0.0000075</bias_mean>
<bias_stddev>0.0000008</bias_stddev>
</noise>
</y>
<z>
<noise type="gaussian">
<mean>0.0</mean>
<stddev>2e-4</stddev>
<bias_mean>0.0000075</bias_mean>
<bias_stddev>0.0000008</bias_stddev>
</noise>
</z>
</angular_velocity>
<!-- 线性加速度 -->
<linear_acceleration>
<x>
<noise type="gaussian">
<mean>0.0</mean>
<stddev>1.7e-2</stddev>
<bias_mean>0.1</bias_mean>
<bias_stddev>0.001</bias_stddev>
</noise>
</x>
<y>
<noise type="gaussian">
<mean>0.0</mean>
<stddev>1.7e-2</stddev>
<bias_mean>0.1</bias_mean>
<bias_stddev>0.001</bias_stddev>
</noise>
</y>
<z>
<noise type="gaussian">
<mean>0.0</mean>
<stddev>1.7e-2</stddev>
<bias_mean>0.1</bias_mean>
<bias_stddev>0.001</bias_stddev>
</noise>
</z>
</linear_acceleration>
</imu>
</sensor>
</gazebo>
<!-- 雷达 -->
<link name="laser">
<visual>
<origin xyz="0 0 0" rpy="0 0 0"/>
<geometry>
<cylinder radius="0.04" length="0.04"/>
</geometry>
</visual>
<!-- 惯性属性 -->
<!-- <inertial>
<origin xyz="0 0 0" rpy="0 0 0"/>
<mass value="0.125"/>
<inertia ixx="0.001" ixy="0" ixz="0" iyy="0.001" iyz="0" izz="0.001" />
</inertial> -->
<!-- 碰撞区域 -->
<!-- <collision>
<origin xyz="0 0 0" rpy="0 0 0"/>
<geometry>
<cylinder radius="0.04" length="0.04"/>
</geometry>
</collision> -->
</link>
<!-- 雷达关节 -->
<joint name="laser_joint" type="fixed">
<parent link="base_link"/>
<child link="laser"/>
<origin xyz="0.16 0 0.078" rpy="0 0 0"/>
</joint>
<gazebo reference="laser">
<sensor name="laser" type="ray">
<always_on>true</always_on>
<visualize>false</visualize>
<update_rate>5</update_rate>
<ray>
<scan>
<horizontal>
<samples>360</samples>
<resolution>1.000000</resolution>
<min_angle>0.000000</min_angle>
<max_angle>6.280000</max_angle>
</horizontal>
</scan>
<range>
<min>0.120000</min>
<max>3.5</max>
<resolution>0.015000</resolution>
</range>
<noise>
<type>gaussian</type>
<mean>0.0</mean>
<stddev>0.01</stddev>
</noise>
</ray>
<plugin name="scan" filename="libgazebo_ros_ray_sensor.so">
<ros>
<remapping>~/out:=scan</remapping>
</ros>
<output_type>sensor_msgs/LaserScan</output_type>
<frame_name>laser</frame_name>
</plugin>
</sensor>
</gazebo>
<!-- 相机 -->
<link name="camera_link">
<visual>
<origin xyz="0 0 0" rpy="0 0 0"/>
<geometry>
<box size="0.015 0.130 0.022"/>
</geometry>
</visual>
<!-- 碰撞区域 -->
<!-- <collision>
<origin xyz="0 0 0" rpy="0 0 0"/>
<geometry>
<box size="0.015 0.130 0.022"/>
</geometry>
</collision> -->
<!-- 惯性属性 -->
<!-- <inertial>
<origin xyz="0 0 0" rpy="0 0 0"/>
<mass value="0.035"/>
<inertia ixx="0.001" ixy="0" ixz="0" iyy="0.001" iyz="0" izz="0.001" />
</inertial> -->
</link>
<!-- 相机关节 -->
<joint name="camera_joint" type="fixed">
<parent link="base_link"/>
<child link="camera_link"/>
<origin xyz="0.16 0 0.11" rpy="0 0 0"/>
</joint>
<!-- 深度相机 -->
<!-- <link name="camera_depth_frame"/>
<joint name="camera_depth_joint" type="fixed">
<origin xyz="0 0 0" rpy="0 0 0"/>
<parent link="camera_link"/>
<child link="camera_depth_frame"/>
</joint> -->
<!-- 相机仿真 -->
<!-- <gazebo reference="camera_depth_link">
<sensor name="depth_camera" type="depth">
<visualize>true</visualize>
<update_rate>30.0</update_rate>
<camera name="camera">
<horizontal_fov>1.047198</horizontal_fov>
<image>
<width>640</width>
<height>480</height>
<format>R8G8B8</format>
</image>
<clip>
<near>0.05</near>
<far>3</far>
</clip>
</camera>
<plugin name="depth_camera_controller" filename="libgazebo_ros_camera.so">
<baseline>0.2</baseline>
<alwaysOn>true</alwaysOn>
<updateRate>0.0</updateRate>
<frame_name>camera_depth_frame</frame_name>
<pointCloudCutoff>0.5</pointCloudCutoff>
<pointCloudCutoffMax>3.0</pointCloudCutoffMax>
<distortionK1>0</distortionK1>
<distortionK2>0</distortionK2>
<distortionK3>0</distortionK3>
<distortionT1>0</distortionT1>
<distortionT2>0</distortionT2>
<CxPrime>0</CxPrime>
<Cx>0</Cx>
<Cy>0</Cy>
<focalLength>0</focalLength>
<hackBaseline>0</hackBaseline>
</plugin>
</sensor>
</gazebo> -->
</robot>gazebo仿真launch启动程序
# 此launch文件是机器人仿真程序，包含 gazebo启动，机器人仿真生成，机器人模型状态发布
import os
from launch import LaunchDescription
from launch.actions import ExecuteProcess
from launch_ros.actions import Node
from launch_ros.substitutions import FindPackageShare
from launch.substitutions import LaunchConfiguration
import xacro
def generate_launch_description():
robot_name_in_model = 'jtbot' #机器人模型名字
package_name = 'jtbot_description' #模型包名
remappings = [('/tf', 'tf'),
('/tf_static', 'tf_static')]
ld = LaunchDescription()
use_sim_time = LaunchConfiguration('use_sim_time', default='true')
pkg_share = FindPackageShare(package=package_name).find(package_name)
gazebo_world_path = os.path.join(pkg_share, 'world/jt.world') #世界仿真文件路径
default_rviz_config_path = os.path.join(pkg_share, 'rviz/mrviz2.rviz') #rviz配置文件路径
urdf_xacro_file = os.path.join(pkg_share, 'urdf/jtbot_base.urdf.xacro') #xacro模型文件路径
#解析xacro模型文件
doc = xacro.parse(open(urdf_xacro_file))
xacro.process_doc(doc)
params = {'robot_description': doc.toxml()}
# 开启ros Gazebo server
start_gazebo_cmd = ExecuteProcess(
cmd=['gazebo',
'--verbose',
gazebo_world_path,
#初始化gazebo-ros
'-s', 'libgazebo_ros_init.so',
#gazebo产卵程序启动
'-s', 'libgazebo_ros_factory.so',
],
output='screen')
# gazebo内生成机器人
spawn_entity_cmd = Node(
package='gazebo_ros',
executable='spawn_entity.py',
arguments=['-entity', robot_name_in_model, '-topic', 'robot_description'],
output='screen',
parameters=[{'use_sim_time': use_sim_time}]
)
# Start Robot State publisher
start_robot_state_publisher_cmd = Node(
package='robot_state_publisher',
executable='robot_state_publisher',
output='screen',
remappings=remappings,
parameters=[params,{'use_sim_time': use_sim_time}]
)
## Launch RViz
# start_rviz_cmd = Node(
# package='rviz2',
# executable='rviz2',
# name='rviz2',
# output='screen',
# arguments=['-d', default_rviz_config_path]
# )
ld.add_action(start_gazebo_cmd)
ld.add_action(spawn_entity_cmd)
ld.add_action(start_robot_state_publisher_cmd)
# ld.add_action(start_rviz_cmd)
return ld仿真效果：
小车实际重量在20公斤左右。
经测试，小车缓慢移动只和urdf文件惯性参数有关，就是下面这段代码：
<!-- 惯性特性，m后面是base_link重量 -->
<xacro:box_inertia m="0.2" w="${base_width}" d="${base_length}" h="${base_height}"/>
当我们把m=20，这里是国际单位公斤。以真实重量测试仿真小车。2-3秒钟后已经离开了出生的中心，并且后他的速度肉眼可见。
当我们把m=5，后退的速度明显减慢。当我们把base_link重量设到1公斤以下，小车不再移动。
我也找了小鱼讲课的代码，把base_link重量加到10，也会发生移动，说明这基本是普遍现象，只要把重量设小一点就不会影响实验效果了。
总结
gazebo仿真是理想状态下仿真，在现实情况下车轴是有摩擦力的，所以真实的小车是不会动的，要想不动，应该也要给车轴加上摩擦力，但是我给支撑轮加上了摩擦力，效果也不是很明显，这也许是gazebo的bug，不过这超出了我们仿真的范围了 。
注意事项
xacro写的模型文件简化了代码复用，但不能在gazeo和rviz2下直接运行，需要转换成urdf格式，转换方法看我xazro模型的备注，如果不转换按我的launch文件在启动前转换也能正常启动。
