How to connect cmd_vel to motor_command in ROS

Question

I have a motor driver for a BLDC wheel. I have successfully made the motor driver and ROS connections and when I send motor_command as in the "user manual" of the motor drivers, my wheels are working.

I found a package to make Differential Drive. Differential Driver Package link

I can use the "twist_to_motors.py" file to be able to drive in this package. But how can I provide a bridge between my motor driver and this file?

my motor command : rostopic pub -r 10 /bldc_driver_node/Command chip_bldc_driver/Command "motor_command: 100"

if i run this command my wheel is spin. But cmd_vel and i want use teleop, how to bridge?

this my motor driver package link : motor-driver-package

Answer 1

Based on the twist_to_motors.py documentation, it sounds like you have to map the following two published topics to your motor command subscribers:

• lwheel_vtarget - (std_msgs/Float32)

  The target velocity for the left wheel (in M/s)

• rwheel_vtarget - (std_msgs/Float32)

  The target velocity for the right wheel (in M/s)

One way I can think of to do this is to implement a bridge node. The bridge node would subscribe to the lwheel_vtarget and rwheel_vtarget topics, and in the callback functions you would add the logic to convert from m/s to your motor command and publish the motor command.

Here's a sketch implementation in Python (I haven't tested it):

import rospy
from chip_bldc_driver.msg import Command
from std_msgs.msg import Float64


def main():
    rospy.init_node('bridge_motor_command')

    left_pub = rospy.Publisher('left/motor_command', Command, queue_size=10)
    right_pub = rospy.Publisher('right/motor_command', Command, queue_size=10)

    def convert(msg):
        command = Command()
        # TODO: change this line to properly convert from m/s to motor command
        command.motor_command = msg.data * 1000

    def left_callback(msg):
        nonlocal left_pub
        left_pub.publish(convert(msg))

    def right_callback(msg):
        nonlocal right_pub
        right_pub.publish(convert(msg))

    left_sub = rospy.Subscriber("lwheel_vtarget", Float64, left_callback)
    right_sub = rospy.Subscriber("rwheel_vtarget", Float64, right_callback)

    rospy.spin()

The publisher topic names and queue sizes can be changed to fit your needs. Similar code could be implemented in C++, if that is your preferred language to use.