This wheel controller has been developed for the 3WD Mecanum Wheel Mobile Arduino Robotics Car 10011 from NEXUS ROBOT
For teleoperation of the Nexus wheelbase a (wireless) game pad like the Logitech F710 is recommended. The base controller runs on a PC or a Raspberry Pi 4B with ROS Melodic (-desktop) installed. To install ROS Melodic on a Rasberry Pi, it is recommended to start with Ubuntu MATE 18.04.
| Logitech F710 | ROS Melodic |
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Clone this package in your workspace_name/src directory.
This project has been tested with ROS Melodic. The project uses an external C++ library. Clone the required PID_Library in nexus_base_ros/lib directory:
git clone https://github.com/MartinStokroos/PID_Controller.git
The following ROS packages must be installed:
sudo apt-get install ros-melodic-rosserial-arduino
sudo apt-get install ros-melodic-rosserial
sudo apt-get install ros-melodic-joy
Run catkin_makefrom the root of the workspace.
It is possible to compile the Arduino firmware with the catkin make proces without making use of the Arduino IDE. However, now it is done with the Arduino IDE.
The Arduino IDE should be installed and the Arduino/libraries directory must exist in your home folder.
Generate the ros_lib with the header files of the custom message types used in this project.
- cd into
Arduino/libraries - type:
rosrun rosserial_arduino make_libraries.py .
In the newly made ros_lib , edit ros.h and reduce the number of buffers and downsize the buffer sizes to save some memory space:
#elif defined(__AVR_ATmega328P__)
//typedef NodeHandle_<ArduinoHardware, 25, 25, 280, 280> NodeHandle;
typedef NodeHandle_<ArduinoHardware, 8, 8, 128, 128> NodeHandle;
- download and include the digitalWriteFast Arduino library from: digitalwritefast
- clone and install the PinChangeInt library:
https://github.com/MartinStokroos/PinChangeInt
Program the Nexus_Omni4WD_Rosserial.ino sketch from the firmware folder into the Arduino Duemilanove-328 based controller board from the 10011 platform.
NOTE: The Sonars cannot be used simultaneously with the serial interface and must be disconnected permanently!
The picture below shows the rosgraph output from rqt_graph:
- Topics
The nexus_base_controller node runs on the platform computer and listens to the cmd_vel topic and uses the x-, y and angular velocity inputs. The cmd_vel is from the geometry_msg Twist type. The nexus_base_controller node publishes odometry data based on the wheel encoder data, with the topic name sensor_odom.
nexus_base is a rosserial type node running on the wheelbase controller board. The code for this node can be found in the firmware directory. nexus_base listens to the cmd_motor topic and publishes the wheel_vel topic at a constant rate of 20Hz (default). The data structure of the cmd_motor topic is a 4-dim. array of Int16's. The usable range is from -255 to 255 and it represents the duty-cycle of the PWM signals to the motors. Negative values reverses the direction of rotation. The wheel_vel topic data format is also a 4-dim. array of Int16's.
wheel_vel is the raw wheel speed and that is the number of encoder increments/decrements per sample interval (0.05s default). Three seconds after receiving a speed (x,y and angular) of '0' the motors are set idle to save power by stopping the PWM.
/joy is the standard message topic from the joystick node. The teleop_joy node does the unit scaling and publishes the cmd_vel topic. It also contains the service clients of the ROS services EmergencyStopEnable and ArmingEnable.
- Services
Node nexus_base runs two ROS service servers. The first service is called EmergencyStopEnable and enables the emergency stop and the second service is called ArmingEnable and this service is to rearm the system.
- Block diagram of the Nexus base controller
The blockdiagram shows the internal structure of the nexus_base_controller node.
The tele-operation demo can be launched after connecting a game pad (tested with Logitech F710) and the wheel base USB-interface with the platform computer (small formfactor PC or Raspberry Pi 3B). Check if the game pad is present by typing:
ls -l /dev/input/js0
Check if the wheel base USB interface is present:
ls -l /dev/ttyUSB0
Launch:
rosrun nexus_base_ros nexus_teleop_joy
The left joystick handle steers the angular velocity when moved from left to right. The right joystick controls the x- and y-speed. The red button enables the emergency stop. The green button is to rearm the robot after an emergency stop.
Some diagnostics:
$ rostopic list
/cmd_motor
/cmd_vel
/diagnostics
/joy
/joy/set_feedback
/rosout
/rosout_agg
/sensor_odom
/wheel_vel
$ rosservice list
/arming_enable
/base_controller/get_loggers
/base_controller/set_logger_level
/emergency_stop_enable
/joystick/get_loggers
/joystick/set_logger_level
/nexus_base/get_loggers
/nexus_base/set_logger_level
/rosout/get_loggers
/rosout/set_logger_level
/teleop_joy/get_loggers
/teleop_joy/set_logger_level
$ rostopic hz /wheel_vel
subscribed to [/wheel_vel]
average rate: 20.010
min: 0.047s max: 0.064s std dev: 0.00505s window: 40
The script to bringup the robot when booting the PC/RPi can be generated with the robot_upstart package.
sudo apt-get install ros-melodic-robot-upstart
Create the install script(s) (run from catkin workspace):
rosrun robot_upstart install nexus_base_ros/launch/nexus_teleop_joy.launch
sudo systemctl daemon-reload && sudo systemctl start nexus
One last thing to do, is giving perimission for using the serial port at boot.
cd /etc/udev/rules.d
sudo touch local.rules
edit local.rules
ACTION=="add", KERNEL=="dialout", MODE="0666"
ACTION=="add", KERNEL=="js0", MODE="0666" //to be confirmed if really needed.
ACTION=="add", KERNEL=="ttyUSB0", MODE="0666"
The script can be enabled/disabled by:
sudo systemctl nexus start
sudo system ctl nexus stop
If needed, check the upstart log with:
sudo journalctl -u nexus
Uninstalling the script can be done with:
rosrun robot_upstart uninstall nexus
When building the package for the first time, the following error may appear:
fatal error: nexus_base_ros/Encoders.h: No such file or directory
#include "nexus_base_ros/Encoders.h"
^~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
The reason for this could be that there is something wrong in the sequence of instructions in the CMakeList.txt file. It could also be that the build of some dependencies are not finished before linking because the make proces is threaded. For the moment the workaround is:
cd build/
make -j4 nexus_base_ros
cd ..
catkin_make
ROS service calls do not work with rosserial 0.8.0. ROS Melodic comes with rosserial version 0.8.0. Rosserial 0.7.7 works. Workaround: Download rosserial 0.7.7 . Unzip and copy the following module from the rosserial-0.7.7 package into the catkin src directory and rebuild the project:
rosserial_python
Sometimes the wheels do not respond for a short moment when the robot is rearmed after an emergency stop.