• This tutorial shows you how to read data from one Dynamixel and write the read value to another Dynamixel using the Dynamixel Easy SDK.

NOTE: It assumes that you have already installed and built the SDK.

Make cpp file

• Create a new C++ source file Open it with your editor.

  $ touch tutorial_step3.cpp
  

Check the Port Names

• Before running the code, check the port name of the connected Dynamixel.
• For General
  • OpenRB-150: ttyACM0: USB ACM device
  • U2D2: FTDI USB Serial Device converter now attached to ttyUSB0

    $ sudo dmesg | grep tty
    

Source Code Description

#include "dynamixel_easy_sdk/dynamixel_easy_sdk.hpp"

int main(){

  dynamixel::Connector connector("/dev/ttyACM0", 57600);
  std::unique_ptr<dynamixel::Motor> leader_motor = connector.createMotor(1);
  std::unique_ptr<dynamixel::Motor> follower_motor = connector.createMotor(2);
  int min_position = leader_motor->getMinPositionLimit().value() + 100;
  int max_position = leader_motor->getMaxPositionLimit().value() - 100;

  leader_motor->disableTorque();
  follower_motor->disableTorque();
  follower_motor->setOperatingMode(dynamixel::Motor::OperatingMode::POSITION);
  follower_motor->enableTorque();

  while (true) {
    int present_position = leader_motor->getPresentPosition().value();
    std::cout << "Leader Motor Present Position: " << present_position << std::endl;

    if (present_position < min_position) {
      leader_motor->enableTorque();
      leader_motor->setGoalPosition(min_position);
      while(true){
        present_position = leader_motor->getPresentPosition().value();
        if(present_position >= min_position) break;
      }
      leader_motor->disableTorque();
    } else if (present_position > max_position) {
      leader_motor->enableTorque();
      leader_motor->setGoalPosition(max_position);
      while(true){
        present_position = leader_motor->getPresentPosition().value();
        if(present_position <= max_position) break;
      }
      leader_motor->disableTorque();
    }
    follower_motor->setGoalPosition(present_position);
  }
}

Add Header Files

• Add dynamixel_easy_sdk/dynamixel_easy_sdk.hpp to the top of your CPP file. This class is included in the Dynamixel SDK package.

  #include "dynamixel_easy_sdk/dynamixel_easy_sdk.hpp"
  

Create Connector and Motor Object

• Create a Connector object with port name, baud rate, and protocol version to manage the communication.(Only protocol 2.0 is supported)

  int main(){
    dynamixel::Connector connector("/dev/ttyACM0", 57600);
  

• Create a Motor object for each Dynamixel servo you want to control, using the createMotor method of the Connector class.
• This method takes the motor ID as an argument and returns a unique pointer to a Motor instance. (shared_ptr is also available)
• Create motor objects while carefully distinguishing between the leader and follower IDs.

    std::unique_ptr<dynamixel::Motor> leader_motor = connector.createMotor(1);
    std::unique_ptr<dynamixel::Motor> follower_motor = connector.createMotor(2);
  

• This process throws a DxlRuntimeError if the object creation fails.

Set Position Limits

• Get the position limits of the leader motor using the getMinPositionLimit and getMaxPositionLimit methods.

    int min_position = leader_motor->getMinPositionLimit().value() + 100;
    int max_position = leader_motor->getMaxPositionLimit().value() - 100;
  

Set Operating Mode to Position Control Mode

• Use the methods provided by the Motor class to control the Dynamixel servo.
• Set the operating mode of follower motor to position control mode.

    leader_motor->disableTorque();
    follower_motor->disableTorque();
    follower_motor->setOperatingMode(dynamixel::Motor::OperatingMode::POSITION);
    follower_motor->enableTorque();
  

Leader and Follower Control Loop

• In a loop, read the present position of the leader motor using the getPresentPosition method.

    while (true) {
      int present_position = leader_motor->getPresentPosition().value();
      std::cout << "Leader Motor Present Position: " << present_position << std::endl;
  

• If the leader motor’s position exceeds the defined range, move it back within the range.

      if (present_position < min_position) {
        leader_motor->enableTorque();
        leader_motor->setGoalPosition(min_position);
        while(true){
          present_position = leader_motor->getPresentPosition().value();
          if(present_position >= min_position) break;
        }
        leader_motor->disableTorque();
      } else if (present_position > max_position) {
        leader_motor->enableTorque();
        leader_motor->setGoalPosition(max_position);
        while(true){
          present_position = leader_motor->getPresentPosition().value();
          if(present_position <= max_position) break;
        }
        leader_motor->disableTorque();
      }
  

• Set the goal position of the follower motor to the present position of the leader motor.

      follower_motor->setGoalPosition(present_position);
    }
  }
  

Error Handling

• To ensure your code is robust, every method that sends a command to the motor returns a Result object that encapsulates values and errors.
• This object lets you safely check for any communication or device errors before proceeding.
• You can check for communication errors and device(dynamixel) errors using the Result object.

• If you use value() when error occurred without checking for errors, it may throw an exception.

  Example

    auto result_uint32 = leader_motor->getMinPositionLimit();
    if (!result_uint32.isSuccess()) {
      std::cerr << dynamixel::getErrorMessage(result_uint32.error()) << std::endl;
      return 1;
    }
    int min_position = result_uint32.value();
  

Compile and Run

• You can compile and run the code using the following commands

  $ g++ tutorial_step3.cpp -o tutorial_step3 -l dxl_x64_cpp
  $ ./tutorial_step3