Custom messages

Topics messages: .msg

Service messages: .srv

Service message has two parts 

Request msg
---
Response msg

Use of msg primitives to define custom messages.

MSG:

  • std_msgs
  • sensor_msgs
  • geometry_msgs
  • actionlib_msgs
  • ...

SRV:

  • std_srvs
  • ...

MSG

We will create new topic message ledStatus composed of 

int64 ledNumber
string ledStatus
Create new package rpi_msgs 
cd ~/catkin_ws/src
catkin_create_pkg rpi_msgs rospy std_msgs
and build it 
roscd
cd ..
catkin_make
Go to the package and create new folder msg: 
roscd rpi_msgs
mkdir msg
cd msg

Create new file ledStatus.msg 

touch ledStatus.msg
code ledStatus.msg
and copy the following lines: 
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int64 ledNumber
string ledStatus

Open package.xml in rpi_msgs package 

code package.xml
and add the following lines
package.xml
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<build_depend>message_generation</build_depend>
<exec_depend>message_runtime</exec_depend>

Open CMakeLists.txt in rpi_msgs package 

code CMakeLists.txt
and add the following lines
CMakeLists.txt
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find_package(catkin REQUIRED COMPONENTS
  rospy
  std_msgs
  message_generation
)

CMakeLists.txt
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# Generate messages in the 'msg' folder
add_message_files(
  FILES
  ledStatus.msg
)
CMakeLists.txt
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# Generate added messages and services with any dependencies listed here
generate_messages(
  DEPENDENCIES
  std_msgs
)
CMakeLists.txt
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catkin_package(
  # INCLUDE_DIRS include
  # LIBRARIES my_robot_msgs
  CATKIN_DEPENDS rospy std_msgs message_runtime
  # DEPENDS system_lib
)

After that do catkin_make 

roscd
cd ..
catkin_make

After the build is finish you can check if the message rpi_msgs/ledStatus is available 

rosmsg list |grep rpi

SRV

We will create new service message safetyZone.srv composed of 

int16 zone
---
bool success
string message

Go to the package rpi_msgs 

roscd rpi_msgs
and create new folder srv: 
mkdir srv
cd msg

Create new file safetyZone.srv 

touch safetyZone.srv
code safetyZone.srv
and copy the following lines: 
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int16 zone
---
bool success
string message

Open CMakeLists.txt in rpi_msgs package 

code CMakeLists.txt
and add the following lines
CMakeLists.txt
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# Generate services in the 'srv' folder
add_service_files(
  FILES
  safetyZone.srv
)

After that do the catkin_make 

roscd
cd ..
catkin_make

After the build is finish you can check if the service message rpi_msgs/safetyZone is available 

rossrv list |grep rpi

Use of custom MSG and SRV

To use custom messages, you need to do some changes of the package.html and the CMakeLists.txt of the package where you want to use them (in our case rpi_feros)

package.html
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<depend>rpi_msgs</depend>
CMakeLists.txt
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find_package(catkin REQUIRED COMPONENTS
  rospy
  std_msgs
  rpi_msgs
)

To include new message in python code add 

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from rpi_msgs.msg import ledStatus, safetyZone

DEBUG

  • rosmsg list - list of available messages

  • rosmsg show <msg name> - show message details

  • rossrv list - list of available service messages

  • rossrv show <srv msg name> - show service message details

Exercise

Turn on LEDs regarding to distance \(d\) from safety sensor:

  • green: \(d\) > 0.4 m
  • yellow: 0.4 m > \(d\) > 0.2 m
  • red: \(d\) < 0.2 m

Turn on LEDs regarding to distance d.