Overview

This branch (hydro-devel) is intended for ROS Hydro and above, and uses the Catkin buildsystem. It may also be compatible with ROS Groovy.

This ROS stack includes an Arduino library (called ROSArduinoBridge) and a collection of ROS packages for controlling an Arduino-based robot using standard ROS messages and services. The stack does not depend on ROS Serial.

Features of the stack include:

• Direct support for Ping sonar and Sharp infrared (GP2D12) sensors

• Can also read data from generic analog and digital sensors

• Can control digital outputs (e.g. turn a switch or LED on and off)

• Support for PWM servos

• Configurable base controller if using the required hardware

The stack includes a base controller for a differential drive robot that accepts ROS Twist messages and publishes odometry data back to the PC. The base controller requires the use of a motor controller and encoders for reading odometry data. The current version of the stack provides support for the following base controller hardware:

• Pololu VNH5019 dual motor controller shield (http://www.pololu.com/catalog/product/2502) or Pololu MC33926 dual motor shield (http://www.pololu.com/catalog/product/2503).

• Robogaia Mega Encoder shield (http://www.robogaia.com/two-axis-encoder-counter-mega-shield-version-2.html).

NOTE: The Robogaia Mega Encoder shield can only be used with an Arduino Mega.

• The library can be easily extended to include support for other motor controllers and encoder hardware or libraries.

Official ROS Documentation

A standard ROS-style version of this documentation can be found on the ROS wiki at:

http://www.ros.org/wiki/ros_arduino_bridge

System Requirements

Python Serial: To install the python-serial package under Ubuntu, use the command:

$ sudo apt-get install python-serial

On non-Ubuntu systems, use either:

$ sudo pip install --upgrade pyserial

or

$ sudo easy_install -U pyserial

The stack should work with any Arduino-compatible controller for reading sensors and controlling PWM servos. However, to use the base controller, you will need a supported motor controller and encoder hardware as described above. If you do not have this hardware, you can still try the package for reading sensors and controlling servos. See the NOTES section at the end of this document for instructions on how to do this.

To use the base controller you must also install the appropriate libraries for your motor controller and encoders. For the Pololu VNH5019 Dual Motor Shield, the library can be found at:

https://github.com/pololu/Dual-VNH5019-Motor-Shield

For the Pololu MC33926 Dual Motor Shield, the library can be found at:

https://github.com/pololu/dual-mc33926-motor-shield

The Robogaia Mega Encoder library can be found at:

http://www.robogaia.com/uploads/6/8/0/9/6809982/__megaencodercounter-1.3.tar.gz

These libraries should be installed in your standard Arduino sketchbook/libraries directory.

Finally, it is assumed you are using version 1.0 or greater of the Arduino IDE.

Preparing your Serial Port under Linux

Your Arduino will likely connect to your Linux computer as port /dev/ttyACM# or /dev/ttyUSB# where # is a number like 0, 1, 2, etc., depending on how many other devices are connected. The easiest way to make the determination is to unplug all other USB devices, plug in your Arduino, then run the command:

$ ls /dev/ttyACM*

or

$ ls /dev/ttyUSB*

Hopefully, one of these two commands will return the result you're looking for (e.g. /dev/ttyACM0) and the other will return the error "No such file or directory".

Next you need to make sure you have read/write access to the port. Assuming your Arduino is connected on /dev/ttyACM0, run the command:

$ ls -l /dev/ttyACM0

and you should see an output similar to the following:

crw-rw---- 1 root dialout 166, 0 2013-02-24 08:31 /dev/ttyACM0

Note that only root and the "dialout" group have read/write access. Therefore, you need to be a member of the dialout group. You only have to do this once and it should then work for all USB devices you plug in later on.

To add yourself to the dialout group, run the command:

$ sudo usermod -a -G dialout your_user_name

where your_user_name is your Linux login name. You will likely have to log out of your X-window session then log in again, or simply reboot your machine if you want to be sure.

When you log back in again, try the command:

$ groups

and you should see a list of groups you belong to including dialout.

Installation of the ros_arduino_bridge Stack

$ cd ~/catkin_workspace/src
$ git clone https://github.com/hbrobotics/ros_arduino_bridge.git
$ cd ~/catkin_workspace
$ catkin_make

The provided Arduino library is called ROSArduinoBridge and is located in the ros_arduino_firmware package. This sketch is specific to the hardware requirements above but it can also be used with other Arduino-type boards (e.g. Uno) by turning off the base controller as described in the NOTES section at the end of this document.

To install the ROSArduinoBridge library, follow these steps:

$ cd SKETCHBOOK_PATH

where SKETCHBOOK_PATH is the path to your Arduino sketchbook directory.

$ cp -rp `rospack find ros_arduino_firmware`/src/libraries/ROSArduinoBridge ROSArduinoBridge

This last command copies the ROSArduinoBridge sketch files into your sketchbook folder. The next section describes how to configure, compile and upload this sketch.

Loading the ROSArduinoBridge Sketch

• If you are using the base controller, make sure you have already installed the appropriate motor controller and encoder libraries into your Arduino sketchbook/librariesfolder.

• Launch the Arduino 1.0 IDE and load the ROSArduinoBridge sketch. You should be able to find it by going to:

  File->Sketchbook->ROSArduinoBridge

NOTE: If you don't have the required base controller hardware but still want to try the code, see the notes at the end of the file.

Choose one of the supported motor controllers by uncommenting its #define statement and commenting out any others. By default, the Pololu VNH5019 driver is chosen.

Choose a supported encoder library by by uncommenting its #define statement and commenting out any others. At the moment, only the Robogaia Mega Encoder shield is supported and it is chosen by default.

If you want to control PWM servos attached to your controller, change the two lines that look like this:

//#define USE_SERVOS
#undef USE_SERVOS

to this:

#define USE_SERVOS
//#undef USE_SERVOS

You must then edit the include file servos.h and change the N_SERVOS parameter as well as the pin numbers for the servos you have attached.

• Compile and upload the sketch to your Arduino.

Firmware Commands

The ROSArduinoLibrary accepts single-letter commands over the serial port for polling sensors, controlling servos, driving the robot, and reading encoders. These commands can be sent to the Arduino over any serial interface, including the Serial Monitor in the Arduino IDE.

NOTE: Before trying these commands, set the Serial Monitor baudrate to 57600 and the line terminator to "Carriage return" or "Both NL & CR" using the two pulldown menus on the lower right of the Serial Monitor window.

The list of commands can be found in the file commands.h. The current list includes:

#define ANALOG_READ    'a'
#define GET_BAUDRATE   'b'
#define PIN_MODE       'c'
#define DIGITAL_READ   'd'
#define READ_ENCODERS  'e'
#define MOTOR_SPEEDS   'm'
#define PING           'p'
#define RESET_ENCODERS 'r'
#define SERVO_WRITE    's'
#define SERVO_READ     't'
#define UPDATE_PID     'u'
#define DIGITAL_WRITE  'w'
#define ANALOG_WRITE   'x'

For example, to get the analog reading on pin 3, use the command:

a 3

To change the mode of digital pin 3 to OUTPUT, send the command:

c 3 1

To get the current encoder counts:

e

To move the robot forward at 20 encoder ticks per second:

m 20 20

Testing your Wiring Connections

On a differential drive robot, the motors are connected to the motor controller terminals with opposite polarities to each other. Similarly, the A/B leads from the encoders are connected in the reverse sense to each other. However, you still need to make sure that (a) the wheels move forward when given a positive motor speed and (b) that the encoder counts increase when the wheels move forward.

After placing your robot on blocks, you can use the Serial Monitor in the Arduino IDE to test both requirements. Use the 'm' command to activate the motors, the 'e' command to get the encoder counts, and the 'r' command to reset the encoders to 0. Remember that at the firmware level, motor speeds are given in encoder ticks per second so that for an encoder resolution of, say 4000 counts per wheel revolution, a command such as 'm 20 20' should move the wheels fairly slowly. (The wheels will only move for 2 seconds which is the default setting for the AUTO_STOP_INTERVAL.) Also remember that the first argument is the left motor speed and the second argument is the right motor speed. Similarly, when using the 'e' command, the first number returned is the left encoder count and the second number is the right encoder count.

Finally, you can use the 'r' and 'e' commands to verify the expected encoder counts by rotating the wheels by hand roughly one full turn and checking the reported counts.

Configuring the ros_arduino_python Node

Now that your Arduino is running the required sketch, you can configure the ROS side of things on your PC. You define your robot's dimensions, PID parameters, and sensor configuration by editing the YAML file in the directory ros_arduino_python/config. So first move into that directory:

$ roscd ros_arduino_python/config

Now copy the provided config file to one you can modify:

$ cp arduino_params.yaml my_arduino_params.yaml

Bring up your copy of the params file (my_arduino_params.yaml) in your favorite text editor. It should start off looking like this:

port: /dev/ttyUSB0
baud: 57600
timeout: 0.1

rate: 50
sensorstate_rate: 10

use_base_controller: False
base_controller_rate: 10

# === Robot drivetrain parameters
#wheel_diameter: 0.146
#wheel_track: 0.2969
#encoder_resolution: 8384 # from Pololu for 131:1 motors
#gear_reduction: 1.0
#motors_reversed: True

# === PID parameters
#Kp: 20
#Kd: 12
#Ki: 0
#Ko: 50
#accel_limit: 1.0

# === Sensor definitions.  Examples only - edit for your robot.
#     Sensor type can be one of the follow (case sensitive!):
#	  * Ping
#	  * GP2D12
#	  * Analog
#	  * Digital
#	  * PololuMotorCurrent
#	  * PhidgetsVoltage
#	  * PhidgetsCurrent (20 Amp, DC)

sensors: {
  #motor_current_left:   {pin: 0, type: PololuMotorCurrent, rate: 5},
  #motor_current_right:  {pin: 1, type: PololuMotorCurrent, rate: 5},
  #ir_front_center:      {pin: 2, type: GP2D12, rate: 10},
  #sonar_front_center:   {pin: 5, type: Ping, rate: 10},
  arduino_led:          {pin: 13, type: Digital, rate: 5, direction: output}
}

NOTE: Do not use tabs in your .yaml file or the parser will barf it back out when it tries to load it. Always use spaces instead. ALSO: When defining your sensor parameters, the last sensor in the list does not get a comma (,) at the end of the line but all the rest must have a comma.

Let's now look at each section of this file.

Port Settings

The port will likely be either /dev/ttyACM0 or /dev/ttyUSB0. Set accordingly.

The MegaRobogaiaPololu Arudino sketch connects at 57600 baud by default.

Polling Rates

The main rate parameter (50 Hz by default) determines how fast the outside ROS loop runs. The default should suffice in most cases. In any event, it should be at least as fast as your fastest sensor rate (defined below).

The sensorstate_rate determines how often to publish an aggregated list of all sensor readings. Each sensor also publishes on its own topic and rate.

The use_base_controller parameter is set to False by default. Set it to True to use base control (assuming you have the required hardware.) You will also have to set the PID paramters that follow.

The base_controller_rate determines how often to publish odometry readings.

Defining Sensors

The sensors parameter defines a dictionary of sensor names and sensor parameters. (You can name each sensor whatever you like but remember that the name for a sensor will also become the topic name for that sensor.)

The four most important parameters are pin, type, rate and direction. The rate defines how many times per second you want to poll that sensor. For example, a voltage sensor might only be polled once a second (or even once every 2 seconds: rate=0.5), whereas a sonar sensor might be polled at 20 times per second. The type must be one of those listed (case sensitive!). The default direction is input so to define an output pin, set the direction explicitly to output. In the example above, the Arduino LED (pin 13) will be turned on and off at a rate of 2 times per second.

Setting Drivetrain and PID Parameters

To use the base controller, you will have to uncomment and set the robot drivetrain and PID parameters. The sample drivetrain parameters are for 6" drive wheels that are 11.5" apart. Note that ROS uses meters for distance so convert accordingly. The sample encoder resolution (ticks per revolution) is from the specs for the Pololu 131:1 motor. Set the appropriate number for your motor/encoder combination. Set the motors_reversed to True if you find your wheels are turning backward, otherwise set to False.

The PID parameters are trickier to set. You can start with the sample values but be sure to place your robot on blocks before sending it your first Twist command.

Launching the ros_arduino_python Node

Take a look at the launch file arduino.launch in the ros_arduino_python/launch directory. As you can see, it points to a config file called my_arduino_params.yaml. If you named your config file something different, change the name in the launch file.

With your Arduino connected and running the MegaRobogaiaPololu sketch, launch the ros_arduino_python node with your parameters:

$ roslaunch ros_arduino_python arduino.launch

You should see something like the following output:

process[arduino-1]: started with pid [6098]
Connecting to Arduino on port /dev/ttyUSB0 ...
Connected at 57600
Arduino is ready.
[INFO] [WallTime: 1355498525.954491] Connected to Arduino on port /dev/ttyUSB0 at 57600 baud
[INFO] [WallTime: 1355498525.966825] motor_current_right {'rate': 5, 'type': 'PololuMotorCurrent', 'pin': 1}
[INFO]
etc

If you have any Ping sonar sensors on your robot and you defined them in your config file, they should start flashing to indicate you have made the connection.

Viewing Sensor Data

To see the aggregated sensor data, echo the sensor state topic:

$ rostopic echo /arduino/sensor_state

To see the data on any particular sensor, echo its topic name:

$ rostopic echo /arduino/sensor/sensor_name

For example, if you have a sensor called ir_front_center, you can see its data using:

$ rostopic echo /arduino/sensor/ir_front_center

You can also graph the range data using rxplot:

$ rxplot -p 60 /arduino/sensor/ir_front_center/range

Sending Twist Commands and Viewing Odometry Data

Place your robot on blocks, then try publishing a Twist command:

$ rostopic pub -1 /cmd_vel geometry_msgs/Twist '{ angular: {z: 0.5} }'

The wheels should turn in a direction consistent with a counter-clockwise rotation (right wheel forward, left wheel backward). If they turn in the opposite direction, set the motors_reversed parameter in your config file to the opposite of its current setting, then kill and restart the arduino.launch file.

Stop the robot with the command:

$ rostopic pub -1 /cmd_vel geometry_msgs/Twist '{}'

To view odometry data:

$ rostopic echo /odom

or

$ rxplot -p 60 /odom/pose/pose/position/x:y, /odom/twist/twist/linear/x, /odom/twist/twist/angular/z

ROS Services

The ros_arduino_python package also defines a few ROS services as follows:

digital_set_direction - set the direction of a digital pin

$ rosservice call /arduino/digital_set_direction pin direction

where pin is the pin number and direction is 0 for input and 1 for output.

digital_write - send a LOW (0) or HIGH (1) signal to a digital pin

$ rosservice call /arduino/digital_write pin value

where pin is the pin number and value is 0 for LOW and 1 for HIGH.

servo_write - set the position of a servo

$ rosservice call /arduino/servo_write id pos

where id is the index of the servo as defined in the Arduino sketch (servos.h) and pos is the position in degrees (0 - 180).

servo_read - read the position of a servo

$ rosservice call /arduino/servo_read id

where id is the index of the servo as defined in the Arduino sketch (servos.h)

NOTES

If you do not have the hardware required to run the base controller, follow the instructions below so that you can still use your Arduino-compatible controller to read sensors and control PWM servos.

First, you need to edit the ROSArduinoBridge sketch. At the top of the file, change the two lines that look like this:

#define USE_BASE
//#undef USE_BASE

to this:

//#define USE_BASE
#undef USE_BASE

NOTE: You also need to comment out the line that looks like this in the file encoder_driver.ino:

#include "MegaEncoderCounter.h"

so it looks like this:

//#include "MegaEncoderCounter.h"

Compile the changes and upload to your controller.

Next, edit your my_arduino_params.yaml file and make sure the use_base_controller parameter is set to False. That's all there is to it.