How to publish odom (nav_msgs/Odometry) from MD49 encoders outputs?

I am using MD49 Motor Drive with its motors

https://www.robot-electronics.co.uk/htm/md49tech.htm

http://wiki.ros.org/md49_base_controller

How to subscribe (encoder_l and encoder_r) from md49_base_controller package and publish (vx , vy ,and vth ) as a form odom (nav_msgs/Odometry) ?

There are two problem:

1-The first is that the encoders outputs are not correct "the package needs to be modified.

2-The second is the I want to create a package that subscribe the right and left wheels encoder counts (encoder_l and encoder_r) and publish (vx , vy ,and vth) as a form odom (nav_msgs/Odometry) to be compatable wth imu MPU9250

http://wiki.ros.org/robot_pose_ekf

The proposed package is:

1- We have to convert (encoder_l and encoder_r) to (RPM_l and RPM_r) as follow:

if (speed_l>128){newposition1 = encoder_l;}
else if  (speed_l<128){ newposition1 = 0xFFFFFFFF-encoder_l;}
else if  (speed_l==128) {newposition1=0;}

newtime1 = millis();
RPM_l = ((newposition1-oldposition1)*1000*60)/((newtime1-oldtime1)*980);
oldposition1 = newposition1;
oldtime1 = newtime1;
delay(250);

if (speed_r>128){ newposition2 = encoder_r;}
else if  (speed_r<128){ newposition2 = 0xFFFFFFFF-encoder_r;}
else if   (speed_r==128) { newposition2=0;}
newtime2 = millis();
RPM_r = ((newposition2-oldposition2)*1000*60)/((newtime2-oldtime2)*980);
oldposition2 = newposition2;
oldtime2= newtime2;
delay(250);

2- We have to convert (RPM_l and RPM_r) to (vx, vy, and vth) as follow:

vx=(r/2)*RPM_l*math.cos(th)+(r/2)*RPM_r*math.cos(th);
vx=(r/2)*RPM_l*math.sin(th)+(r/2)*RPM_r*math.sin(th);
vth=(r/B)*omega_l-(r/B)*omega_r;

Hint: r and B are wheel radius and vehicle width respectively.

3- The odom (nav_msgs/Odometry) package is:

#!/usr/bin/env python

import math
from math import sin, cos, pi

import rospy
import tf
from nav_msgs.msg import Odometry
from geometry_msgs.msg import Point, Pose, Quaternion, Twist, Vector3
from md49_messages.msg import md49_encoders

rospy.init_node('odometry_publisher')

odom_pub = rospy.Publisher("odom", Odometry, queue_size=50)
odom_broadcaster = tf.TransformBroadcaster()


x = 0.0
y = 0.0
th = 0.0

vx =0.1
vy = -0.1
vth = 0.1

current_time = rospy.Time.now()
last_time = rospy.Time.now()

r = rospy.Rate(1.0)
while not rospy.is_shutdown():
    current_time = rospy.Time.now()


    # compute odometry in a typical way given the velocities of the robot
    dt = (current_time - last_time).to_sec()
    delta_x = (vx * cos(th) - vy * sin(th)) * dt
    delta_y = (vx * sin(th) + vy * cos(th)) * dt
    delta_th = vth * dt

    x += delta_x
    y += delta_y
    th += delta_th

    # since all odometry is 6DOF we'll need a quaternion created from yaw
    odom_quat = tf.transformations.quaternion_from_euler(0, 0, th)

    # first, we'll publish the transform over tf
    odom_broadcaster.sendTransform(
        (x, y, 0.),
        odom_quat,
        current_time,
        "base_link",
        "odom"
    )

    # next, we'll publish the odometry message over ROS
    odom = Odometry()
    odom.header.stamp = current_time
    odom.header.frame_id = "odom"

    # set the position
    odom.pose.pose = Pose(Point(x, y, 0.), Quaternion(*odom_quat))

    # set the velocity
    odom.child_frame_id = "base_link"
    odom.twist.twist = Twist(Vector3(vx, vy, 0), Vector3(0, 0, vth))

    # publish the message
    odom_pub.publish(odom)

    last_time = current_time
    r.sleep()

First off, you need to import nav_msgs/Odometry as the following:

from nav_msgs.msg import Odometry

You must have a function that performs those conversions and then in rospy.Subscriber import those variables, like this:

def example(data):
    data.vx=<conversion>
    data.vth=<conversion>

def listener():
    rospy.Subscriber('*topic*', Odometry, example)
    rospy.spin()

if __name__ == 'main':
    listener()

I think this would work