Step 5: Simultaneous Control of Multiple DYNAMIXELs

This tutorial shows you how to move multiple Dynamixels simultaneously using the Dynamixel Easy SDK.

NOTE: This tutorial explains how to write code using the Dynamixel Easy SDK. It assumes that you have already installed the SDK.

Make Python file

Create a new Python source file Open it with your editor.
```
$ touch tutorial_step5.py
```

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

from dynamixel_easy_sdk import *

def main():
    connector = Connector("/dev/ttyACM0", 57600)
    group_executor = connector.createGroupExecutor()
    motor1 = connector.createMotor(1)
    motor2 = connector.createMotor(2)

    motor1.disableTorque()
    motor2.disableTorque()
    motor1.setOperatingMode(OperatingMode.POSITION)
    motor2.setOperatingMode(OperatingMode.POSITION)
    motor1.enableTorque()
    motor2.enableTorque()

    target_position = 500
    group_executor.addCmd(motor1.stageSetGoalPosition(target_position))
    group_executor.addCmd(motor2.stageSetGoalPosition(target_position))
    group_executor.executeWrite()
    group_executor.clearStagedWriteCommands()

Import Library

Import dynamixel_easy_sdk.
```
from dynamixel_easy_sdk import *
```

Create Connector and Motor Object

Create a Connector object with port name and baud rate to manage the communication.
```
def main():
    connector = Connector("/dev/ttyACM0", 57600)
```
Create a GroupExecutor object using the createGroupExecutor method of the Connector class.
This object is used to execute multiple commands simultaneously.
```
    group_executor = connector.createGroupExecutor()
```
Create a Motor object for each Dynamixel servo you want to control, using the createMotor method of the Connector class.
```
    motor1 = connector.createMotor(1)
    motor2 = connector.createMotor(2)
```

Set Operating Mode to Position Control Mode

Use the methods provided by the Motor class to control the Dynamixel servo.

Set the operating mode to position control mode.

    motor1.disableTorque()
    motor2.disableTorque()
    motor1.setOperatingMode(OperatingMode.POSITION)
    motor2.setOperatingMode(OperatingMode.POSITION)
    motor1.enableTorque()
    motor2.enableTorque()

Move Motor to Goal Position

Add commands to set the goal position of each motor using the stageSetGoalPosition method of the Motor class.

    target_position = 500
    group_executor.addCmd(motor1.stageSetGoalPosition(target_position))
    group_executor.addCmd(motor2.stageSetGoalPosition(target_position))

Execute all the staged commands simultaneously using the execute method of the GroupExecutor class.
```
    group_executor.executeWrite()
```
This will send the commands to both motors at the same time, causing them to move to the specified goal position simultaneously.
This method decides the communication packet type automatically between Sync and Bulk based on the staged commands.
Clear the staged commands after execution using the clearStagedWriteCommands method of the GroupExecutor class.
```
    group_executor.clearStagedWriteCommands()
```

Error Handling

When an error occurs, DxlRuntimeError is raised.

You can catch this error using a try-except block.

try:
    group_executor.executeWrite()
except DxlRuntimeError as e:
    print(e)

DxlRuntimeError contains DxlError Enum that provides detailed information about the error.

try:
    group_executor.executeWrite()
except DxlRuntimeError as e:
    if e.dxl_error == DxlError.EASY_SDK_ADD_PARAM_FAIL:
        print("Failed to add param.")
    elif e.dxl_error == DxlError.EASY_SDK_COMMAND_IS_EMPTY:
        print("Command is empty.")

Make cpp file

Create a new C++ source file Open it with your editor.
```
$ touch tutorial_step5.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::GroupExecutor> group_executor = connector.createGroupExecutor();
  std::unique_ptr<dynamixel::Motor> motor1 = connector.createMotor(1);
  std::unique_ptr<dynamixel::Motor> motor2 = connector.createMotor(2);

  motor1->disableTorque();
  motor2->disableTorque();
  motor1->setOperatingMode(dynamixel::Motor::OperatingMode::POSITION);
  motor2->setOperatingMode(dynamixel::Motor::OperatingMode::POSITION);
  motor1->enableTorque();
  motor2->enableTorque();

  int target_position = 500;
  group_executor->addCmd(motor1->stageSetGoalPosition(target_position));
  group_executor->addCmd(motor2->stageSetGoalPosition(target_position));
  group_executor->executeWrite();
  group_executor->clearStagedWriteCommands();
}

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 GroupExecutor object using the createGroupExecutor method of the Connector class.

This object is used to execute multiple commands simultaneously.

  std::unique_ptr<dynamixel::GroupExecutor> group_executor = connector.createGroupExecutor();

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)

  std::unique_ptr<dynamixel::Motor> motor1 = connector.createMotor(1);
  std::unique_ptr<dynamixel::Motor> motor2 = connector.createMotor(2);

This process throws a DxlRuntimeError if the object creation fails.

Set Operating Mode to Position Control Mode

Use the methods provided by the Motor class to control the Dynamixel servo.

Set the operating mode to position control mode.

  motor1->disableTorque();
  motor2->disableTorque();
  motor1->setOperatingMode(dynamixel::Motor::OperatingMode::POSITION);
  motor2->setOperatingMode(dynamixel::Motor::OperatingMode::POSITION);
  motor1->enableTorque();
  motor2->enableTorque();

Move Motor to Goal Position

Add commands to set the goal position of each motor using the stageSetGoalPosition method of the Motor class.

  int target_position = 500;
  group_executor->addCmd(motor1->stageSetGoalPosition(target_position));
  group_executor->addCmd(motor2->stageSetGoalPosition(target_position));

Execute all the staged commands simultaneously using the execute method of the GroupExecutor class.
```
  group_executor->executeWrite();
```
This will send the commands to both motors at the same time, causing them to move to the specified goal position simultaneously.
This method decides the communication packet type automatically between Sync and Bulk based on the staged commands.
Clear the staged commands after execution using the clearStagedWriteCommands method of the GroupExecutor class.
```
  group_executor->clearStagedWriteCommands();
```

Error Handling

To ensure your code is robust, every method that sends a command to the motor returns a Result object that encapsulates 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.

Example

  auto result_void = group_executor->executeWrite(); // type of 'result_void' variable is Result<void, DxlError>
  if (!result_void.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_void.error()) << std::endl;
    return 1;
  }

stage functions return Result<void, Error> type.

You can either pass this value directly to the addCmd() function, or perform error checking first and then pass the resulting command value. Example

  auto result_cmd = motor1->stageSetGoalPosition(target_position); // type of 'result_cmd' variable is Result<stagedCommand, DxlError>
  if (!result_cmd.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_cmd.error()) << std::endl;
    return 1;
  }
  group_executor->addCmd(result_cmd.value());

Compile and Run

You can compile and run the code using the following commands

$ g++ tutorial_step5.cpp -o tutorial_step5 -l dxl_x64_cpp
$ ./tutorial_step5

Full Source Code With Error Handling

#include "dynamixel_easy_sdk/dynamixel_easy_sdk.hpp"

int main(){

  dynamixel::Connector connector("/dev/ttyACM0", 57600);
  std::unique_ptr<dynamixel::GroupExecutor> group_executor = connector.createGroupExecutor();
  std::unique_ptr<dynamixel::Motor> motor1 = connector.createMotor(1);
  std::unique_ptr<dynamixel::Motor> motor2 = connector.createMotor(2);

  auto result_void = motor1->disableTorque();
  if (!result_void.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_void.error()) << std::endl;
    return 1;
  }

  result_void = motor2->disableTorque();
  if (!result_void.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_void.error()) << std::endl;
    return 1;
  }

  result_void = motor1->setOperatingMode(dynamixel::Motor::OperatingMode::POSITION);
  if (!result_void.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_void.error()) << std::endl;
    return 1;
  }

  result_void = motor2->setOperatingMode(dynamixel::Motor::OperatingMode::POSITION);
  if (!result_void.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_void.error()) << std::endl;
    return 1;
  }

  result_void = motor1->enableTorque();
  if (!result_void.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_void.error()) << std::endl;
    return 1;
  }
  result_void = motor2->enableTorque();
  if (!result_void.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_void.error()) << std::endl;
    return 1;
  }

  int target_position = 500;
  auto result_cmd = motor1->stageSetGoalPosition(target_position);
  if (!result_cmd.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_cmd.error()) << std::endl;
    return 1;
  }
  group_executor->addCmd(result_cmd.value());

  result_cmd = motor2->stageSetGoalPosition(target_position);
  if (!result_cmd.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_cmd.error()) << std::endl;
    return 1;
  }
  group_executor->addCmd(result_cmd.value());

  result_void = group_executor->executeWrite();
  if (!result_void.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_void.error()) << std::endl;
    return 1;
  }

  group_executor->clearStagedWriteCommands();
}