Gazebo使用筆記（3） —— 控制器插件的編寫與配置

0. 準備

sudo apt-get install libgazebo9-dev

1. 修改SDF

在相應的<model>標籤下添加如下代碼：

<plugin name="velodyne_control" filename="libvelodyne_plugin.so"/>

或者在模型編輯器中的Plugin下Add也可以！

2. 編寫插件

gedit velodyne_plugin.cc

#ifndef _VELODYNE_PLUGIN_HH_
#define _VELODYNE_PLUGIN_HH_

#include <gazebo/gazebo.hh>
#include <gazebo/physics/physics.hh>

namespace gazebo
{
  /// \brief A plugin to control a Velodyne sensor.
  class VelodynePlugin : public ModelPlugin
  {
    /// \brief Constructor
    public: VelodynePlugin() {}

    /// \brief The load function is called by Gazebo when the plugin is
    /// inserted into simulation
    /// \param[in] _model A pointer to the model that this plugin is
    /// attached to.
    /// \param[in] _sdf A pointer to the plugin's SDF element.
	public: virtual void Load(physics::ModelPtr _model, sdf::ElementPtr _sdf)
	{
	  // Safety check
	  if (_model->GetJointCount() == 0)
	  {
	    std::cerr << "Invalid joint count, Velodyne plugin not loaded\n";
	    return;
	  }
	
	  // Store the model pointer for convenience.
	  this->model = _model;
	
	  // Get the first joint. We are making an assumption about the model
	  // having one joint that is the rotational joint.
	  this->joint = _model->GetJoints()[0];
	
	  // Setup a P-controller, with a gain of 0.1.
	  this->pid = common::PID(0.1, 0, 0);
	
	  // Apply the P-controller to the joint.
	  this->model->GetJointController()->SetVelocityPID(
	      this->joint->GetScopedName(), this->pid);
	
	  // Set the joint's target velocity. This target velocity is just
	  // for demonstration purposes.
	  this->model->GetJointController()->SetVelocityTarget(
	      this->joint->GetScopedName(), 10.0);
	}
	/// \brief Pointer to the model.
	private: physics::ModelPtr model;
	
	/// \brief Pointer to the joint.
	private: physics::JointPtr joint;
	
	/// \brief A PID controller for the joint.
	private: common::PID pid;

  };

  // Tell Gazebo about this plugin, so that Gazebo can call Load on this plugin.
  GZ_REGISTER_MODEL_PLUGIN(VelodynePlugin)
}
#endif

3. 執行

mkdir build && cd build
cmake ..
make

在build目錄下執行：

gazebo --verbose ../velodyne.world

4. 插件配置

修改插件，讀取自定義SDF參數

在velodyne.world中相應標籤下寫入：

<plugin name="velodyne_control" filename="libvelodyne_plugin.so">
  <velocity>25</velocity>
</plugin>

然後在插件Load函數中讀取此值，修改Load末尾，使用sdf::ElementPtr讀取<velocity>：

// Default to zero velocity
double velocity = 0;

// Check that the velocity element exists, then read the value
if (_sdf->HasElement("velocity"))
  velocity = _sdf->Get<double>("velocity");

// Set the joint's target velocity. This target velocity is just
// for demonstration purposes.
this->model->GetJointController()->SetVelocityTarget(
    this->joint->GetScopedName(), velocity);

編譯並運行：

cd ~/velodyne_plugin/build
cmake ../
make
gazebo --verbose ../velodyne.world

調整<velocity>SDF值，然後重新啓動仿真以查看效果。

創建API

法1：消息傳遞和函數
消息傳遞依賴於gazebo的傳輸機制，並且涉及創建一個命名主題，發佈者可以在該主題上發送雙精度值（double）。插件會收到這些消息，並適當設置速度。消息傳遞對於進程間通信很方便。

法2：創建一個新的公共函數來調整參數
新插件將從我們當前的插件繼承，子插件將由Gazebo而不是我們當前的插件實例化，並將通過調用我們的函數來控制速度。將gazebo連接到ROS時最常使用這種方法。

① 設置速度的函數

/// \brief Set the velocity of the Velodyne
/// \param[in] _vel New target velocity
public: void SetVelocity(const double &_vel)
{
  // Set the joint's target velocity.
  this->model->GetJointController()->SetVelocityTarget(
      this->joint->GetScopedName(), _vel);
}

② 設置消息結構

/// \brief A node used for transport
private: transport::NodePtr node;

/// \brief A subscriber to a named topic.
private: transport::SubscriberPtr sub;

③ Load末尾實例化 node和subscriber

// Create the node
this->node = transport::NodePtr(new transport::Node());
#if GAZEBO_MAJOR_VERSION < 8
this->node->Init(this->model->GetWorld()->GetName());
#else
this->node->Init(this->model->GetWorld()->Name());
#endif

// Create a topic name
std::string topicName = "~/" + this->model->GetName() + "/vel_cmd";

// Subscribe to the topic, and register a callback
this->sub = this->node->Subscribe(topicName,
   &VelodynePlugin::OnMsg, this);

④ 創建處理收到消息的回調函數

/// \brief Handle incoming message
/// \param[in] _msg Repurpose a vector3 message. This function will
/// only use the x component.
private: void OnMsg(ConstVector3dPtr &_msg)
{
  this->SetVelocity(_msg->x());
}

完整代碼如下：

#ifndef _VELODYNE_PLUGIN_HH_
#define _VELODYNE_PLUGIN_HH_

#include <gazebo/gazebo.hh>
#include <gazebo/physics/physics.hh>
#include <gazebo/transport/transport.hh>
#include <gazebo/msgs/msgs.hh>

namespace gazebo
{
  /// \brief A plugin to control a Velodyne sensor.
  class VelodynePlugin : public ModelPlugin
  {
    /// \brief Constructor
    public: VelodynePlugin() {}

    /// \brief The load function is called by Gazebo when the plugin is
    /// inserted into simulation
    /// \param[in] _model A pointer to the model that this plugin is
    /// attached to.
    /// \param[in] _sdf A pointer to the plugin's SDF element.
    public: virtual void Load(physics::ModelPtr _model, sdf::ElementPtr _sdf)
    {
      // Safety check
      if (_model->GetJointCount() == 0)
      {
        std::cerr << "Invalid joint count, Velodyne plugin not loaded\n";
        return;
      }

      // Store the model pointer for convenience.
      this->model = _model;

      // Get the first joint. We are making an assumption about the model
      // having one joint that is the rotational joint.
      this->joint = _model->GetJoints()[0];

      // Setup a P-controller, with a gain of 0.1.
      this->pid = common::PID(0.1, 0, 0);

      // Apply the P-controller to the joint.
      this->model->GetJointController()->SetVelocityPID(
          this->joint->GetScopedName(), this->pid);

      // Default to zero velocity
      double velocity = 0;

      // Check that the velocity element exists, then read the value
      if (_sdf->HasElement("velocity"))
        velocity = _sdf->Get<double>("velocity");

      this->SetVelocity(velocity);

      // Create the node
      this->node = transport::NodePtr(new transport::Node());
      #if GAZEBO_MAJOR_VERSION < 8
      this->node->Init(this->model->GetWorld()->GetName());
      #else
      this->node->Init(this->model->GetWorld()->Name());
      #endif

      // Create a topic name
      std::string topicName = "~/" + this->model->GetName() + "/vel_cmd";

      // Subscribe to the topic, and register a callback
      this->sub = this->node->Subscribe(topicName,
         &VelodynePlugin::OnMsg, this);
    }

    /// \brief Set the velocity of the Velodyne
    /// \param[in] _vel New target velocity
    public: void SetVelocity(const double &_vel)
    {
      // Set the joint's target velocity.
      this->model->GetJointController()->SetVelocityTarget(
          this->joint->GetScopedName(), _vel);
    }

    /// \brief Handle incoming message
    /// \param[in] _msg Repurpose a vector3 message. This function will
    /// only use the x component.
    private: void OnMsg(ConstVector3dPtr &_msg)
    {
      this->SetVelocity(_msg->x());
    }

    /// \brief A node used for transport
    private: transport::NodePtr node;

    /// \brief A subscriber to a named topic.
    private: transport::SubscriberPtr sub;

    /// \brief Pointer to the model.
    private: physics::ModelPtr model;

    /// \brief Pointer to the joint.
    private: physics::JointPtr joint;

    /// \brief A PID controller for the joint.
    private: common::PID pid;
  };

  // Tell Gazebo about this plugin, so that Gazebo can call Load on this plugin.
  GZ_REGISTER_MODEL_PLUGIN(VelodynePlugin)
}
#endif

測試消息傳遞API

新建消息發佈者val.cc

#include <gazebo/gazebo_config.h>
#include <gazebo/transport/transport.hh>
#include <gazebo/msgs/msgs.hh>

// Gazebo's API has changed between major releases. These changes are
// accounted for with #if..#endif blocks in this file.
#if GAZEBO_MAJOR_VERSION < 6
#include <gazebo/gazebo.hh>
#else
#include <gazebo/gazebo_client.hh>
#endif

/////////////////////////////////////////////////
int main(int _argc, char **_argv)
{
  // Load gazebo as a client
#if GAZEBO_MAJOR_VERSION < 6
  gazebo::setupClient(_argc, _argv);
#else
  gazebo::client::setup(_argc, _argv);
#endif

  // Create our node for communication
  gazebo::transport::NodePtr node(new gazebo::transport::Node());
  node->Init();

  // Publish to the  velodyne topic
  gazebo::transport::PublisherPtr pub =
    node->Advertise<gazebo::msgs::Vector3d>("~/my_velodyne/vel_cmd");

  // Wait for a subscriber to connect to this publisher
  pub->WaitForConnection();

  // Create a a vector3 message
  gazebo::msgs::Vector3d msg;

  // Set the velocity in the x-component
#if GAZEBO_MAJOR_VERSION < 6
  gazebo::msgs::Set(&msg, gazebo::math::Vector3(std::atof(_argv[1]), 0, 0));
#else
  gazebo::msgs::Set(&msg, ignition::math::Vector3d(std::atof(_argv[1]), 0, 0));
#endif

  // Send the message
  pub->Publish(msg);

  // Make sure to shut everything down.
#if GAZEBO_MAJOR_VERSION < 6
  gazebo::shutdown();
#else
  gazebo::client::shutdown();
#endif
}

CMakeLists.txt末尾添加如下代碼：

# Build the stand-alone test program
add_executable(vel vel.cc)

if (${gazebo_VERSION_MAJOR} LESS 6)
  # These two
  include(FindBoost)
  find_package(Boost ${MIN_BOOST_VERSION} REQUIRED system filesystem regex)
  target_link_libraries(vel ${GAZEBO_LIBRARIES} ${Boost_LIBRARIES})
else()
  target_link_libraries(vel ${GAZEBO_LIBRARIES})
endif()

編譯並運行生成的速度發佈者可執行文件

./vel 1

---

參考文獻：

• http://gazebosim.org/tutorials?cat=guided_i&tut=guided_i5