Publisher Subscriber node
These are the basics nodes in ROS which used to share informations between nodes a publisher node publish the data using ROS Topics and Subscriber nodes will Subscribe to this topic to do this nodes specific task
In ROS we can write these nodes in c++ python lisp and a node can have many publishers and subscribers
Publisher nodes
These nodes in ROS which the user make only has funtionality of publishing required data to topics to which any number of subscriber can attach
Subscriber nodes
These nodes can subscribe to published topics from an another node and do its actions based on the input data
Publisher Subscriber nodes
These nodes can subscribe to a topic published topic from a node and also publish data based on conditions or refine the data and input to various nodes attached to it
All the examples from this this chapter can be done in c++ also but we use python for this course Since this is an introducry course plus learning curve for ROS is higher if many new to C++. ___ I hope Every one has Atom ide installed then
Open ATOM ide
File->Open Folder->select ROSworksop folder from UI appeard->click ok
after that you left side you have tree view
|--ROS_Worksop
|--devel
|--build
|--src
|--rosbasics
|--include
|--src
-- node1.py
-- node2.cpp
--CMakeLists.txt
--package.xml
Let’s create publisher/subscriber node
Create a file name ‘helloworldpub.py’ in src folder in rosbasics
Publisher node
helloworldpub.py
#!/usr/bin/python
import rospy #ROS module
from std_msgs.msg import String #ROSmsg module
def main():
rospy.init_node(name="Helloworld",anonymous=False) #init_node will start the node
pub=rospy.Publisher("mytopic",String,queue_size=0) #create publisher in name
r=rospy.Rate(hz=10) # Rate at which data publishes
msg=String()
msg.data="Helloworld"
while not rospy.is_shutdown():
pub.publish(msg)
r.sleep()
if __name__ == '__main__':
try:
main()
except rospy.ROSInterruptException:
pass
To make code excuatable
cd ROS_Worksop/src/rosbasics/src
#then
chmod +x helloworldpub.py
#then
cd ROSworksop
catkin_make
To run the node
- Open new termial (make sure roscore is running)
rosrun rosbasics helloworldpub.py - To see publsihed msg Open new terminal
rostopic echo /mytopic
Subscriber node
helloworldsub.py
#!/usr/bin/python
import rospy
from std_msgs.msg import String
def callback(msg):
print(msg.data)
def main():
rospy.init_node(name="mysub",anonymous=False)
sub=rospy.Subscriber("/mytopic",String,callback)
rospy.loginfo("subscriber init")
if __name__ == '__main__':
try:
main():
rospy.spin()
'''
rospy.spin() is same as
while not rospy.is_shutdown():
pass
'''
except rospy.ROSInterruptException:
pass
To make code excuatable
cd ROS_Worksop/src/rosbasics/src
#then
chmod +x helloworldsub.py
#then
cd ROSworksop
catkin_make
To run the node
- Open new termial (make sure roscore is running)
rosrun rosbasics helloworldsub.py
Subscribler Publishers in one node
Till now we have made is individual subscriber and publisher in a ROS node you can have many publishers and subscribers
As we metioned a ROS node may contain any number of combinations of
- publishers
- subscribers
- Services
- Service clients
- Service servers
- Actions
- Action clients
- Action servers
As the funtionality of the node increases its better to impliment oops(object oriented programming) to remove errors pubsub.py here we are going to create a node which control location turtle which has one subscriber and one publisher which
- publishes topic:-turtle1/cmd_vel
- subscribe:-turtle1/pose
pubsub.py
#!/usr/bin/python
import rospy
from geometry_msgs.msg import Twist
from turtlesim.msg import Pose
import math
class pubsub():
def __init__(self):
rospy.init_node("turtlecontroller",anonymous=False)
self.commad=Twist()
self.null=Twist()
self.pub=rospy.Publisher("turtle1/cmd_vel",Twist,queue_size=0)
self.sub=rospy.Subscriber("turtle1/pose",Pose,self.callback)
self.x =0
self.y = 0
self.pos=Pose()
r=rospy.Rate(hz=10)
self.flag=0
self.maxv=0.5
self.maxw=5.0
self.commad.linear.y=0
self.commad.linear.z=0
self.commad.angular.x=0
self.commad.angular.y=0
self.headx=0
self.heady=0
self.null.linear.x=0
self.null.linear.y=0
self.null.linear.z=0
self.null.angular.x=0
self.null.angular.y=0
self.null.angular.z=0
rospy.loginfo("commad node status")
def callback(self,msg):
self.pos=msg
def run(self):
while not rospy.is_shutdown():
if (self.flag==0 or self.flag==1):
self.pub.publish(self.null)
self.x,self.y=map(float,raw_input("input x,y :\t").split())
self.flag=2
else:
trgx=self.x-self.pos.x
trgy=self.y-self.pos.y
ttheta=self.pos.theta
self.headx=math.cos(ttheta)
self.heady=math.sin(ttheta)
sqrt=pow(trgx,2)+pow(trgy,2)
sqrt=pow(sqrt,0.5)
if sqrt<=0.03:
self.flag=1
self.pub.publish(self.null)
else:
u=(trgx*self.headx+trgy*self.heady)/sqrt
if u>1:
u=1
s=1-pow(u,2)
print(self.pos.x,self.pos.y,u,sqrt)
angle=math.atan2(pow(s,0.5),u)
v=min(self.maxv,sqrt)
if angle==0:
self.commad.linear.x=v
self.commad.angular.z=0
elif angle!=0:
self.commad.linear.x=v
self.commad.angular.z=(angle/math.pi)*-1.0* self.maxw
self.pub.publish(self.commad)
if __name__ == '__main__':
try:
mynode=pubsub()
mynode.run()
except rospy.ROSInterruptException:
pass
Important points in pub/sub nodes
- Topic name subsribing to
- ROSmsg struture which is use full to your rosnode
- Freqency of publisher node and subscriber
-
Suitable qeue size of subscribe node (mainly depends upon Freqency of publisher)