我们已经对转速进行了硬编码，但是每次修改参数就需要重新编译，不能实现动态地调节插件。因此，在本节中，我们将修改插件以读取自定义SDF参数，该参数是Velodyne的目标速度。   首先添加一个新标签<plugin>。新标签可以是任何东西，只要它是有效的XML。我们的插件将可以访问Load函数中的值。首先打开world文件编辑  

一、准备工作

gedit ~/velodyne_plugin/velodyne.world

  添加<plugin>,里面包含一个标签。  

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

  现在，让我们在插件的Load函数中读取此值。  

gedit ~/velodyne_plugin/velodyne_plugin.cc

  修改Load函数的末尾，使用 sdf::ElementPtr参数来读取<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);

  重新编译运行看下效果：  

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

二、创建一个API

  通过SDF调整目标速度非常方便，但支持动态调整会更好。此更改将需要添加其他程序可以用来更改速度值的API。 我们可以使用两种API类型：  

• 消息传递和函数。消息传递依赖于gazebo的传输机制，并且涉及创建一个命名主题，发布者可以在该主题上发送双精度值（double）。插件会收到这些消息，并适当设置速度。消息传递对于进程间通信很方便。

• 函数方法将创建一个新的公共函数来调整速度。为此，新插件将从我们当前的插件继承。子插件将由Gazebo而不是我们当前的插件实例化，并将通过调用我们的函数来控制速度。将gazebo连接到ROS时最常使用这种方法。

  由于我们插件的简单性，可以轻松同时实现两者。   1、打开velodyne_plugin.cc文件，添加以下函数：  

/// \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、现在，我们将设置传递消息的基础结构。先将node和 subscriber 添加到插件。  

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

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

  在Load函数末尾，实例化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);

  创建处理传入消息的回调函数：  

/// \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());
}

  向插件添加两个必要的头文件。  

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

  3、现在，该插件可以动态更改目标速度了。完整的插件应如下所示。  

#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

三、测试消息传递API

1、val.cc

  在您的工作空间中创建一个新的源文件。  

gedit ~/velodyne_plugin/vel.cc

  添加以下代码：  

#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
}

编辑cmake文件

  在工作区中的CMakeLists.txt文件中添加几行，以生成可执行的vel程序。  

# 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()

  编译运行之后，在新的终端中，进入文件所在目录并运行vel命令。确保设置数值，该数值被解释为目标速度值。  

cd ~/velodyne_plugin/build
./vel 2