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When reading serial data between Arduino and a node, it hangs. How can I properly send and read data?

asked 2021-07-22 00:38:03 -0500

rasa911216 gravatar image

updated 2021-07-22 13:01:39 -0500

Hi. I'm using ROS2 foxy with no libraries installed. It runs on an Ubuntu 21.04 x64 machine with kernel 5.8.0-59-generic.

I created a project with the goal of controlling an Arduino using python serial in a workspace with the following structure:

- workspace
    - src
        - arduino_comm
             - serial_server.py
             - serial_client.py
        - motor_interface
        - other_interface

the motor interface provides the action that is used by the arduino_comm package to send serial data. I have an old Arduino Nano that I've had for like 7 years ago that uses the old legacy bootloader. Still operational but sadly seems incompatible with the rosserial and I've checked the Micro-Ros packages and they also seem incompatible with my old arduino. Also I wanted to understand the code I'm working with at its bare basics before adding more complexity with an external library.

So I created the following code.

Arduino code

//Variable to store the bot command.
String botCommand;

void setup() {
  //Initialize Serial port.
  Serial.begin(9600);
  //Initialize BUILTIN LED to light up for feedback.
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  if(Serial.available()){
    botCommand = Serial.readString();
    if(botCommand == "forward"){
      //Light up the led.
      digitalWrite(LED_BUILTIN, HIGH);
      delay(10);
    }
    Serial.println(botCommand);
  }
  //Shut down the LED.
  digitalWrite(LED_BUILTIN, LOW);
  delay(10);
}

The Server and Client use the following action, which is based on the tutorial

##################################################
#Action that sends robot towards a specific spatial position.
##################################################
#Goal of achieving a specific position.
int32 botgoalposition
---
#Resulting position that we're aiming to attain.
int32[] botendposition
---
#Feedback message about current position.
int32[] botcurrposition

The arduino_comm serial_server.py dedicated to sending commands to the Arduino to follow.

import time
#Import the Serial library to send commands to Arduino.
import serial

#Elements required to generate an ActionServer.
import rclpy
from rclpy.action import ActionServer

#Library required to generate a ROS2 Node.
from rclpy.node import Node

##################################################
#                   ACTIONS
##################################################
#Action to calculate Fibonacci code. FOR DEBUGGING.
from code_interfaces.action import Fibonacci

#Action to send commands to the Arduino until it reaches the desired coordinates.
from bot_move.action import BotMove

#Class that encapsulates the Action Server.
class SerialCommServer(Node):
    def __init__(self):
        #Declare the Node with the name "serial server".
        super().__init__('serial_server')
        #Declare it'll be an ActionServer that publishes to the topic 'botmove'.
        self._action_server = ActionServer(
                self,
                BotMove,
                #Action name.
                'botmove',
                #Callback for executing accepted goals.
                self.executeCallback
                )

    #When it has been initialized, it'll start executing the following callback.
    def executeCallback(self, goalHandle):
        #Set the logger to indicate we're starting to execute our goal.
        self.get_logger().info('Executing goal...')

        #Get feedback to see if we're close to our goal.
        feedbackMsg = BotMove.Feedback()
        feedbackMsg.botcurrposition = [0,1]

        goalHandle.publish_feedback(feedbackMsg)

        #Send a small Hello to Arduino.
        arduino = serial.Serial(port="/dev/ttyUSB0")
        for i in range(1, 10):
            arduino.write(bytes(str("forward"), 'utf-8'))
            feedbackMsg.botcurrposition.append(i)
            self.get_logger().info('Feedback: {0}'.format(i))

            #Read back response from the Arduino.
            #TODO: THIS SECTION HANGS, NEED TO FIGURE HOW TO FIX IT.
#            response = arduino.readline()
#            print("Response: " + str(response ...
(more)
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Comments

1

Can you please, upload your action file also.

Ranjit Kathiriya gravatar image Ranjit Kathiriya  ( 2021-07-22 09:50:00 -0500 )edit

Done! Added the action, its based on the tutorial so I haven't modified much because I'm scared of screwing up the data type, I assume I'll have to change it to an array of only 2 or 3 elements representing the x,y,z coordinates I want the robot to take.

But first gotta solve the serial issue.

rasa911216 gravatar image rasa911216  ( 2021-07-22 13:02:30 -0500 )edit

1 Answer

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answered 2021-07-28 14:51:43 -0500

rasa911216 gravatar image

For the sake of an answer to newcomers struggling with ROS, here is the answer I was given at: https://robotics.stackexchange.com/a/...

found what I believe is a relevant answer here, and I think it's relevant because of the following lines in your serial_server.py file:

When it has been initialized, it'll start executing the following callback.

def executeCallback(self, goalHandle):

    # <other code>

    #Send a small Hello to Arduino.
    arduino = serial.Serial(port="/dev/ttyUSB0")

The way I read this makes it look like you're starting a serial connection repeatedly. The post I linked states,

Establishing a serial connection to an arduino causes it to reset, so it was never online at the moment the pi was sending data, and so never replied.

There's a longer explanation about the behavior over at Arduino.SE that says, in part,

The Arduino uses the RTS (Request To Send) (and I think DTR (Data Terminal Ready)) signals to auto-reset.

The answer there also links here for instructions on disabling the auto-reset-on-serial-connection feature. Links can rot, so in the interest of future visitors I'll quote a subset of the page here, describing how to use a pull-up resistor on the reset pin to disable the auto-reset:

The simple way that doesn't require any permanent modifying of your hardware or software configuration changes: Stick a 120 ohm resistor in the headers between 5v and reset (you can find these on the isp connector too). 120 is hard to find so just combine resistors. Don't go below 110 ohms or above 124 ohms, and don't do this with an isp programmer attached. You can just pull out the resistor when you want auto-reset back.

In summary, it looks like you're restarting your serial connection in every execute callback. Establishing the serial connection is resetting your arduino, and then it's resetting while you perform the serial write so it doesn't realize you're expecting a response when you get to the serial read.

You can avoid this by either starting the serial connection and storing the handle to the connection as a member variable or taking it as a callback parameter (preferred) or by disabling the auto-restart function.

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Asked: 2021-07-22 00:38:03 -0500

Seen: 267 times

Last updated: Jul 28 '21