Control loop
============

The control loop is a class that initialize the controller manager (load, unload controller dynamically) and hardware interface.


.. code-block:: c

  #ifndef CONTROL_LOOP
  #define CONTROL_LOOP

  #include <chrono>

  // for shared pointer
  #include <memory>
  // for runtime_error
  #include <stdexcept>

  // ROS
  #include <ros/ros.h>

  // ROS control
  #include <controller_manager/controller_manager.h>

  // The hardware interface to dynamixels
  #include <dynamixel_control_hw/hardware_interface.hpp>

  namespace dynamixel {
      // To make use of steady_clock and duration_cast shorter
      using namespace std::chrono;

      template <class Protocol>
      class DynamixelLoop {
          using hw_int = typename dynamixel::DynamixelHardwareInterface<Protocol>;

      public:
          /** With this constructor a controller manager is create, then it loads rosparams

                @param nh ros node handle
                @param hardware_interface abstract Hardware Interface of your robot
            **/
          DynamixelLoop(
              ros::NodeHandle& nh,
              std::shared_ptr<hw_int> hardware_interface)
              : _nh(nh),
                _hardware_interface(hardware_interface)
          {
              // Create the controller manager
              _controller_manager.reset(new controller_manager::ControllerManager(_hardware_interface.get(), _nh));

              // Load rosparams
              int error = 0;
              ros::NodeHandle np("~");
              error += !np.getParam("loop_frequency", _loop_hz);
              error += !np.getParam("cycle_time_error_threshold", _cycle_time_error_threshold);
              if (error > 0) {
                  char error_message[] = "could not retrieve one of the required parameters\n\tdynamixel_hw/loop_hz or dynamixel_hw/cycle_time_error_threshold";
                  ROS_ERROR_STREAM(error_message);
                  throw std::runtime_error(error_message);
              }

              // Get current time for use with first update
              _last_time = steady_clock::now();

              // Start timer that will periodically call DynamixelLoop::update
              _desired_update_freq = ros::Duration(1 / _loop_hz);
              _non_realtime_loop = _nh.createTimer(_desired_update_freq, &DynamixelLoop::update, this);
          }

          /** Timed method that reads current hardware's state, runs the controller
              code once and sends the new commands to the hardware.

              Note: we do not use the TimerEvent time difference because it
                  does NOT guarantee that the time source is strictly
                  linearly increasing.
          **/

          /** Update function firstly get the hardware's state;  it compute the new command (via the controller manager),
              then it send the new command to hardware

                @param nh ros node handle
                @param hardware_interface abstract Hardware Interface of your robot
            **/
          void update(const ros::TimerEvent&)
          {
              // Get change in time
              _current_time = steady_clock::now();
              duration<double> time_span
                  = duration_cast<duration<double>>(_current_time - _last_time);
              _elapsed_time = ros::Duration(time_span.count());
              _last_time = _current_time;

              // Check cycle time for excess delay
              const double cycle_time_error = (_elapsed_time - _desired_update_freq).toSec();
              if (cycle_time_error > _cycle_time_error_threshold) {
                  ROS_WARN_STREAM("Cycle time exceeded error threshold by: "
                      << cycle_time_error - _cycle_time_error_threshold << "s, "
                      << "cycle time: " << _elapsed_time << "s, "
                      << "threshold: " << _cycle_time_error_threshold << "s");
              }

              // Input
              // get the hardware's state
              _hardware_interface->read(ros::Time::now(), _elapsed_time);

              // Control
              // let the controller compute the new command (via the controller manager)
              _controller_manager->update(ros::Time::now(), _elapsed_time);

              // Output
              // send the new command to hardware
              _hardware_interface->write(ros::Time::now(), _elapsed_time);
          }

      private:
          // Startup and shutdown of the internal node inside a roscpp program
          ros::NodeHandle _nh;

          // Settings
          ros::Duration _desired_update_freq;
          double _cycle_time_error_threshold;

          // Timing
          ros::Timer _non_realtime_loop;
          ros::Duration _elapsed_time;
          double _loop_hz;
          steady_clock::time_point _last_time;
          steady_clock::time_point _current_time;

          /** ROS Controller Manager and Runner

              This class advertises a ROS interface for loading, unloading, starting, and
              stopping ros_control-based controllers. It also serializes execution of all
              running controllers in \ref update.
          **/
          std::shared_ptr<controller_manager::ControllerManager> _controller_manager;

          // Abstract Hardware Interface for your robot
          std::shared_ptr<hw_int> _hardware_interface;
      };
  } // namespace dynamixel

  #endif