Control loop

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

#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