一. ros_control和Gazebo的数据流向
在Gazebo中模拟机器人的控制器是可以通过使用ros_control和一个简单的Gazebo插件适配器来完成。下面是仿真,硬件,控制器和传动之间关系的概览:
二. 使用
(1)向URDF中添加传动元素
为了使用ros_control,需要给URDF中添加一些其它元素,比如< transmission>,它是为了链接执行器和关节。而在< transmission>标签中一些重要子标签有:
- < joint name=" ">:name对应URDF中的joint的name
- < type>:传动的类型,目前只有"transmission_interface/SimpleTransmission" 实现
- < hardwareInterface>:此标签存在于< actuator>和< joint>标签,它会告诉gazebo_ros_control插件要加载的硬件接口是什么(position,velocity or effort interfaces),目前只实现effort interfaces。
一些可用的硬件接口:
- Joint Command Interface - Hardware interface to support commanding an array of joints. Note that these commands can have any semantic meaning as long as they each can be represented by a single double, they are not necessarily effort commands. To specify a meaning to this command, see the derived classes:
Effort Joint Interface - for commanding effort-based joints.
Velocity Joint Interface - for commanding velocity-based joints.
Position Joint Interface - for commanding position-based joints. - Joint State Interfaces - Hardware interface to support reading the state of an array of named joints, each of which has some position, velocity, and effort (force or torque).
- Actuator State Interfaces - Hardware interface to support reading the state of an array of named actuators, each of which has some position, velocity, and effort (force or torque).
- Actuator Command Interfaces:
Effort Actuator Interface
Velocity Actuator Interface
Position Actuator Interface - Force-torque sensor Interface
- IMU sensor Interface
(2)添加gazebo_ros_control插件
除了transmission标签,也需要向你的URDF中添加Gazebo插件,它会自动解析transmission标签,装载合适的硬件接口和controller manager。一般默认的gazebo_ros_control插件是很简单的,但是它也是可以通过插件架构在ros_control和gazebo之间来扩展自定义的机器人硬件接口。
一般添加到URDF的默认插件形如:
<gazebo>
<plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">
<robotNamespace>/MYROBOT</robotNamespace>
</plugin>
</gazebo>
这个 gazebo_ros_control的< plugin>标签有如下可选的子标签:
- < robotNamespace>: 插件实例所使用的ROS命名空间,默认为URDF中robot的name
- < controlPeriod>: 控制器的更新时钟,默认为gazebo的时钟
- < robotParam>: 参数服务器中robot_description的位置,默认为’/robot_description’
- < robotSimType>:所使用自定义机器人仿真接口的插件名字,默认为’DefaultRobotHWSim’
自定义gazebo_ros_control仿真插件:
这个gazebo_ros_controlGazebo插件为仿真更加复杂机理提供了基于插件的接口来实现自定义Gazebo和ros_control之间的接口。这些插件必须继承gazebo_ros_control::RobotHWSim,它实现一个模拟的ros_control的hardware_interface::RobotHW。RobotHWSim为对Gazebo仿真器的读取和控制关节属性提供API级别的访问。
相应的RobotHWSim子类在URDF模型中指定,并且在机器人模型装载时装载。例如,下面的XML会装载默认插件:
<gazebo>
<plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">
<robotNamespace>/MYROBOT</robotNamespace>
<robotSimType>gazebo_ros_control/DefaultRobotHWSim</robotSimType>
</plugin>
</gazebo>
三. 例子
(1)首先在rrbot.xacro中添加< transmission>标签,同时< hardwareInterface>必须包含在< joint>和< actuator>标签中,内容如下:
<transmission name="tran1">
<type>transmission_interface/SimpleTransmission</type>
<joint name="joint1">
<hardwareInterface>EffortJointInterface</hardwareInterface>
</joint>
<actuator name="motor1">
<hardwareInterface>EffortJointInterface</hardwareInterface>
<mechanicalReduction>1</mechanicalReduction>
</actuator>
</transmission>
<transmission name="tran2">
<type>transmission_interface/SimpleTransmission</type>
<joint name="joint2">
<hardwareInterface>EffortJointInterface</hardwareInterface>
</joint>
<actuator name="motor2">
<hardwareInterface>EffortJointInterface</hardwareInterface>
<mechanicalReduction>1</mechanicalReduction>
</actuator>
</transmission>
(2)在rrbot.gazebo中添加ros_control插件
内容如下:
<!-- ros_control plugin -->
<gazebo>
<plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">
<robotNamespace>/rrbot</robotNamespace>
<robotSimType>gazebo_ros_control/DefaultRobotHWSim</robotSimType>
</plugin>
</gazebo>
(3)创建.yaml配置文件
cd ~/catkin_ws/src
catkin_create_pkg rrbot_control controller_manager joint_state_controller robot_state_publisher
cd rrbot_control
mkdir config
mkdir launch
在config文件下创建一个文件rrbot_control.yaml,内容如下:
rrbot:
joint_state_controller:
type: joint_state_controller/JointStateController
publish_rate: 50
joint1_position_controller:
type: effort_controllers/JointPositionController
joint: joint1
pid: {p: 100.0, i: 0.01, d: 10.0}
joint2_position_controller:
type: effort_controllers/JointPositionController
joint: joint2
pid: {p: 100.0, i: 0.01, d: 10.0}
保存在yaml的PID增益和控制器设置可以通过launch文件加载到参数服务器中,因此在launch文件夹创建一个rrbot_control.launch文件,内容如下:
<launch>
<!-- Load joint controller configurations from YAML file to parameter server -->
<rosparam file="$(find rrbot_control)/config/rrbot_control.yaml" command="load"/>
<!-- load the controllers -->
<node name="controller_spawner" pkg="controller_manager" type="spawner" respawn="false"
output="screen" ns="/rrbot" args="joint1_position_controller joint2_position_controller joint_state_controller"/>
<!-- convert joint states to TF transforms for rviz, etc -->
<node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher"
respawn="false" output="screen">
<remap from="/joint_states" to="/rrbot/joint_states" />
</node>
</launch>
< rosparam>加载控制器配置文件到参数服务器中;节点controller_spawner通过运行一个python脚本为rrbot启动两个关节位置控制器,这个python脚本会向ros_control控制器管理者(controller manager)请求服务调用,这个服务调用会告诉controller manager你想要的是哪个控制器。同时,这个节点也加载了第三个控制器joint_state_controller,用来发布所有关节的关节状态,相应话题为“/joint_states“。
最后是启动robot_state_publisher节点,接收来自joint_state_controller的话题是"/joint_states",发布的话题是“/tf"。
(4)使用roslaunch启动控制器
首先启动rrbot仿真:
roslaunch rrbot_gazebo rrbot_world.launch
然后给两个关节加载控制器:
roslaunch rrbot_control rrbot_control.launch
rostopic pub -1 /rrbot/joint1_position_controller/command std_msgs/Float64 "data: 1.5"
rostopic pub -1 /rrbot/joint2_position_controller/command std_msgs/Float64 "data: 1.0"
rosrun rqt_gui rqt_gui
在RQT的菜单栏中“Plugins”中点击"Topics->Message Publisher"插件,然后从Topic的下拉框中选择你想要给控制器发布的话题,比如/rrbot/joint1_position_controller/command,设置结果如下(其中i是RQT的时间变量):
此时Gazebo中的rrbot开始做周期运动:
在RQT的菜单栏中Plugins->Visualization点击“Plot”,订阅话题“/rrbot/joint1_position_controller/command/data”可以显示发给控制器/rrbot/joint1_position_controller的数据,添加订阅另一个话题“/rrbot/joint1_position_controller/state/process_value”可以显示实际关节运动的状态值,效果如下图:
通过调节PID参数可以减小关节跟踪误差,而调节PID参数可以通过Plugins->Configuration中的Dynamic Reconfigure插件,如下图:
(4)利用Riz显示
rosrun rviz rviz
刚打开rviz时会报错,如下图:
解决方法:将Global Options的Fixed Frame修改为world,就可以在rviz中显示Gazebo里的模型运动状态。
节点图如下:
参考:
http://gazebosim.org/tutorials/?tut=ros_control
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