Step 6: Simultaneous Reading of Multiple DYNAMIXELs

This tutorial shows you how to read data from 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_step6.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)

    group_executor.addCmd(motor1.stageGetPresentPosition())
    group_executor.addCmd(motor2.stageGetPresentPosition())
    present_positions = group_executor.executeRead()
    group_executor.clearStagedReadCommands()

    motor1_position = present_positions[0]
    motor2_position = present_positions[1]
    print("Motor1 Present Position:", motor1_position)
    print("Motor2 Present Position:", motor2_position)

if __name__ == "__main__":
    main()

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)
```

Read Data from Multiple Motors Simultaneously

Add commands to read the present position of each motor using the stageGetPresentPosition method of the Motor class.

    group_executor.addCmd(motor1.stageGetPresentPosition())
    group_executor.addCmd(motor2.stageGetPresentPosition())

Execute all the staged commands simultaneously using the executeRead method of the GroupExecutor class.
```
    present_positions = group_executor.executeRead()
```
Clear the staged read commands after execution using the clearStagedReadCommands method of the GroupExecutor class.
```
    group_executor.clearStagedReadCommands()
```
This will send the commands to both motors at the same time and read the present position of each motor.

The return type of this method is List[int].

    motor1_position = present_positions[0]
    motor2_position = present_positions[1]
    print("Motor1 Present Position:", motor1_position)
    print("Motor2 Present Position:", motor2_position)

This method decides the communication packet type automatically between Sync and Bulk based on the staged commands.

Error Handling

When an error occurs, DxlRuntimeError is raised.

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

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

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

try:
    group_executor.ReadWrite()
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.")

If only certain values among those read by group control have errors, those values are displayed as None.

Make cpp file

Create a new C++ source file Open it with your editor.
```
$ touch tutorial_step6.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);

  group_executor->addCmd(motor1->stageGetPresentPosition());
  group_executor->addCmd(motor2->stageGetPresentPosition());
  auto result = group_executor->executeRead();
  group_executor->clearStagedReadCommands();

  int motor1_position = result.value()[0].value();
  int motor2_position = result.value()[1].value();
  std::cout << "Motor1 Present Position: " << motor1_position << std::endl;
  std::cout << "Motor2 Present Position: " << motor2_position << std::endl;
}

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.

Read Data from Multiple Motors Simultaneously

Add commands to read the present position of each motor using the stageGetPresentPosition method of the Motor class.

  group_executor->addCmd(motor1->stageGetPresentPosition());
  group_executor->addCmd(motor2->stageGetPresentPosition());

Execute all the staged commands simultaneously using the executeRead method of the GroupExecutor class.
```
  auto result = group_executor->executeRead();
```
Clear the staged read commands after execution using the clearStagedReadCommands method of the GroupExecutor class.
```
  group_executor->clearStagedReadCommands();
```
This will send the commands to both motors at the same time and read the present position of each motor.
The return type of this method is Result<std::vector<Result<int32_t, Error>>, Error>.

You can get the actual position values using the value() method of the Result object.

  int motor1_position = result.value()[0].value();
  int motor2_position = result.value()[1].value();
  std::cout << "Motor1 Present Position: " << motor1_position << std::endl;
  std::cout << "Motor2 Present Position: " << motor2_position << std::endl;

This method decides the communication packet type automatically between Sync and Bulk based on the staged commands.

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.
executeRead() method returns Result<std::vector<Result<int32_t, Error», Error> type. So you need to check for errors twice.

You can check for communication errors and device(dynamixel) errors using the Result object.

Example

  auto result = group_executor->executeRead(); // type of 'result_void' variable is Result<void, DxlError>
  if (!result_void.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_void.error()) << std::endl;
    return 1;
  }
  std::vector<int> positions;
  for (const auto& result : result.value()) {
    if (!result.isSuccess()) {
      std::cerr << dynamixel::getErrorMessage(result.error()) << std::endl;
      return 1;
    }
    positions.push_back(result.value());
  }

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->stageGetPresentPosition(); // 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_step6.cpp -o tutorial_step6 -l dxl_x64_cpp
$ ./tutorial_step6

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_cmd = motor1->stageGetPresentPosition();
  if (!result_cmd.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result_cmd.error()) << std::endl;
    return 1;
  }
  group_executor->addCmd(result_cmd.value());

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

  auto result = group_executor->executeRead();
  if (!result.isSuccess()) {
    std::cerr << dynamixel::getErrorMessage(result.error()) << std::endl;
    return 1;
  }
  group_executor->clearStagedReadCommands();

  std::vector<int> positions;
  for (const auto& result : result.value()) {
    if (!result.isSuccess()) {
      std::cerr << dynamixel::getErrorMessage(result.error()) << std::endl;
      return 1;
    }
    positions.push_back(result.value());
  }
  std::cout << "Motor1 Present Position: " << positions[0] << std::endl;
  std::cout << "Motor2 Present Position: " << positions[1] << std::endl;
}