为串联机械臂写一个ROS控制器

上一篇博客中，实现了gazebo中控制器的应用，但是由于PID调的不好，控制效果实在是不好意思贴出图来，作为一个控制理论与控制工程毕业的人，我怎么忍得了。今天就试着来编一编控制器。当然是参考了一些教程。12

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ROS中有一些标准的控制器，比如joint state, position, trajectory，这些在gazebo仿真中也用到了一部分。

以下是控制器的工作流程

加载控制器后，第一个执行的函数就是初始化int()，这并不是启动控制器，初始化函数的参数包括机器人的状态参数和Nodehandle。初始化函数只执行一次。

启动控制器函数starting()，也只执行一次，用于控制器的启动。

update()函数保证了控制器的活跃状态。以1000Hz的频率执行代码，意味着一微秒就要计算完成一个执行周期。

最后stop()停止控制器命令也只执行一次。

建立一个控制器的过程需要以下几个步骤：

• 新建一个带有必要依赖的ROS包；
• 编写控制器C++代码；
• 将C++类注册或者发布为一个插件；
• 在XML文件中定义插件；
• 更新package.xml来发布插件；
• 编写CMakeLists.txt
• 编译代码
• 为控制器参数编写配置文件
• 在gazebo中进行仿真
• 使用controller manager加载控制器。

创建控制器功能包

$ catkin_create_pkg my_controller_pkg roscpp pluginlib controller_interface hardware_interface

建立一个控制器的头文件

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

namespace my_controller_pkg{

class MyControllerClass: public controller_interface::Controller
{
private:
  hardware_interface::JointHandle joint_;
  double init_pos_;
  ros::Subscriber sub_command_;

public:
  /**
   * \brief Store position and velocity command in struct to allow easier realtime buffer usage
   */
  struct Commands
  {
    double position_; // Last commanded position
    double velocity_; // Last commanded velocity
    bool has_velocity_; // false if no velocity command has been specified
  };
  realtime_tools::RealtimeBuffer command_;
  Commands command_struct_; // pre-allocated memory that is re-used to set the realtime buffer
  virtual bool init(hardware_interface::EffortJointInterface *robot, ros::NodeHandle &n);
  virtual void starting();
  virtual void update(const ros::Time& time, const ros::Duration& period);
  virtual void stopping();
    void setCommandCB(const std_msgs::Float64ConstPtr& msg);
  /*!
   * \brief Give set position of the joint for next update: revolute (angle) and prismatic (position)
   *
   * \param command
   */
  void setCommand(double pos_target);

  /*!
   * \brief Give set position of the joint for next update: revolute (angle) and prismatic (position)
   *        Also supports a target velocity
   *
   * \param pos_target - position setpoint
   * \param vel_target - velocity setpoint
   */
  void setCommand(double pos_target, double vel_target);
};
} 

建立代码文件

#include "my_controller_pkg/my_controller_file.h"
#include <pluginlib/class_list_macros.h>

namespace my_controller_pkg {

/// Controller initialization in non-realtime
bool MyControllerClass::init(hardware_interface::EffortJointInterface *robot,
                            ros::NodeHandle &n)
{
  std::string joint_name;
  if (!n.getParam("joint_name", joint_name))
  {
    ROS_ERROR("No joint given in namespace: '%s')",
              n.getNamespace().c_str());
    return false;
  }

  joint_ = robot->getHandle(joint_name);
//  if (!joint_)
//  {
//    ROS_ERROR("MyController could not find joint named '%s'",
//              joint_name.c_str());
//    return false;
//  }
  // Start command subscriber
  sub_command_ = n.subscribe<std_msgs::Float64>("command", 1, &MyControllerClass::setCommandCB, this);
  return true;
}


/// Controller startup in realtime
void MyControllerClass::starting()
{
  init_pos_ = joint_.getPosition();
}


/// Controller update loop in realtime
void MyControllerClass::update(const ros::Time& time, const ros::Duration& period)
{
  command_struct_ = *(command_.readFromRT());
  double command_position = command_struct_.position_;

  double desired_pos = init_pos_ + 0.5 * sin(ros::Time::now().toSec());
  double current_pos = joint_.getPosition();
    double commanded_effort = -1000 * (current_pos - command_position);
  //joint_.commanded_effort_ = -10 * (current_pos - desired_pos);
  joint_.setCommand(commanded_effort);

}


/// Controller stopping in realtime
void MyControllerClass::stopping()
{}

void MyControllerClass::setCommandCB(const std_msgs::Float64ConstPtr& msg)
{
  setCommand(msg->data);
}
// Set the joint position command
void MyControllerClass::setCommand(double pos_command)
{
  command_struct_.position_ = pos_command;
  command_struct_.has_velocity_ = false; // Flag to ignore the velocity command since our setCommand method did not include it

  // the writeFromNonRT can be used in RT, if you have the guarantee that
  //  * no non-rt thread is calling the same function (we're not subscribing to ros callbacks)
  //  * there is only one single rt thread
  command_.writeFromNonRT(command_struct_);
}

// Set the joint position command with a velocity command as well
void MyControllerClass::setCommand(double pos_command, double vel_command)
{
  command_struct_.position_ = pos_command;
  command_struct_.velocity_ = vel_command;
  command_struct_.has_velocity_ = true;

  command_.writeFromNonRT(command_struct_);
}

} // namespace


// Register controller to pluginlib
PLUGINLIB_EXPORT_CLASS(my_controller_pkg::MyControllerClass, 
                         controller_interface::ControllerBase);

其中hardware_interface::JointHandle可以参考：http://docs.ros.org/jade/api/hardware_interface/html/c++/classhardware__interface_1_1JointHandle-members.html

代码解释

等我以后插在代码注释里

建立插件描述文件

<library path="lib/libmy_controller_lib">
  <class name="my_controller_pkg/MyControllerClass" 
         type="my_controller_pkg::MyControllerClass"           
         base_class_type="controller_interface::ControllerBase" />
    <description>
      My test.
    description>
library>

更新package.xml文件

<export>
    
    <controller_interface plugin="${prefix}/controller_plugins.xml" />
export>

更新CMakeLists.txt文件

## my_controller_file library
add_library(my_controller_lib src/my_controller_file.cpp)
target_link_libraries(my_controller_lib ${catkin_LIBRARIES})

建立控制器

使用catkin_make命令编译。然后用下列命令可以查看控制器是否编译成了一个插件

rospack plugins --attrib=plugin controller_interface

写控制器的配置文件

在上一文章的yaml基础上修改，让新编的控制器控制joint1

myfirstrobot:
  # Publish all joint states -----------------------------------
  joint_state_controller:
    type: joint_state_controller/JointStateController
    publish_rate: 50  

  # Position Controllers ---------------------------------------
  joint2_position_controller:
    type: effort_controllers/JointPositionController
    joint: joint2
    pid: {p: 120.0, i: 10.01, d: 20.0}

  # Position Controllers ---------------------------------------
  joint1_position_controller:
    type: my_controller_pkg/MyControllerClass
    joint_name: joint1

其他跟上一博客中的内容一致。

在gazebo中应用

运行roslaunch myurdf gazebo.launch，会启动gazebo和rviz，运行以下命令，改变joint1的目标值

rostopic pub -1 /myfirstrobot/joint1_position_controller/command std_msgs/Float64 "data: 1"
rostopic pub -1 /myfirstrobot/joint1_position_controller/command std_msgs/Float64 "data: 0"

效果如下：

毕竟是个比例控制，不要有强迫症。
…………

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当然作为一个控制专业毕业的，怎么能受得了，这次能看出超调吗？

扰动也不惧啊

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1. Mastering ROS for Robotics Programming. ↩
2. https://github.com/ros-controls/ros_controllers ↩