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How to get topic data to process then publish

asked 2021-02-23 09:48:10 -0500

Draaka gravatar image

Hi all, I'll start off with saying I'm very new to ROS and only just figuring out python too. I've followed all the tutorials and scoured forums but can't find a solution to my issue. I have a ROS package with node 1 publishing message data to a topic, then node 2 subscribes to the topic, processes the data then publishes it again. However I'm not receiving actual values from my topic through the subscriber callback even though they are being published to the topic(confirmed with rostopic echo).

Node 2-

#!/usr/bin/env python

import rospy
import numpy as np
from AR_week4_test.srv import compute_cubic_traj
from AR_week4_test.msg import cubic_traj_param
from AR_week4_test.msg import cubic_traj_coeffs

param = None
coeffs = None

def Computecubics(param):
                pub_coeffs = cubic_traj_coeffs()
                pub_param = cubic_traj_param()

                m_list1 =pub_coeffs
                x = np.array(m_list1)

                m_list2 = [[1,(pub_param.t0),(pub_param.t0**2),(pub_param.t0**3)],
                        [0, 1, (2*pub_param.t0), (3*pub_param.t0**2)],
                        [1, (, (**2), (**3)],
                        [0, 1, (2*, (3***2)]]
                a = np.array(m_list2)
                m_list3 = [[pub_param.p0],
                b = np.array(m_list3)

                inv_a = np.linalg.inv(a)

                x = np.linalg.inv(A).dot(b)


    if __name__ == '__main__':

                rospy.init_node('cubic_traj_listener', anonymous=True)

                rate = rospy.Rate(0.05) #20 seconds


                rospy.ServiceProxy('computing_cubics', compute_cubic_traj)

                rospy.Subscriber("cubic_traj_params", cubic_traj_param, Computecubics, queue_size=10)

                param = rospy.Publisher("cubic_traj_newparams", cubic_traj_param, queue_size=10)



        except rospy.ROSInterruptException:

Node 1 -

#!/usr/bin/env python

import rospy
import numpy as np
import random as ra
from std_msgs.msg import Float64MultiArray
from std_msgs.msg import Float32
from AR_week4_test.msg import cubic_traj_param
pos = None
class pointsgen():
        def __init__(self):
                pub = rospy.Publisher('cubic_traj_params', cubic_traj_param, queue_size=10)
                rospy.init_node('points_generator', anonymous=True)

                global pos
                while not rospy.is_shutdown():  
                        pos = cubic_traj_param()
                        pos.p0 = ra.uniform(10,-10)
               = ra.uniform(10,-10)
                        pos.v0 = ra.uniform(10,-10)
                        pos.vf = ra.uniform(10,-10)
                        pos.t0 = 0
                        dt = ra.uniform(5,10)

                        rate = rospy.Rate(0.05)  # Publish at 0.05hz or 20 times every second

if  __name__ == "__main__" :
                ne = pointsgen()
        except rospy.ROSInterruptException: pass

Currently the error I get is a linAlg error due to the matrix m_list2 being filled with zero values

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1 Answer

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answered 2021-02-25 16:11:15 -0500

ejalaa12 gravatar image

Your Computecubics method takes param as input, but you're using pub_param in the function code. Replace pub_param with param, and you're good to go !

Also here are some advice:

  • try to follow the pep8 and Ros code style for python. Also use CamelCase names forros msg and srv
  • in node 1: create your publisher after the rospy.init
  • in node2:
    • you're creating a ServiceProxy but not saving it into a variable. You'll need it if you want to use it later
    • you are saving your publisher in the param variable instead of the coeffs. try using more self-explanatory variable names
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Asked: 2021-02-23 09:48:10 -0500

Seen: 514 times

Last updated: Feb 25 '21