Lesson 20 - 3-Axis Velocity Control

Learning Objective

• Publish custom 3-axis velocity vectors using bot.move(linear_x, linear_y, angular_z).
• Combine linear translation and angular rotation to create smooth curved trajectories.
• Understand how velocity vectors map to ROS 2 /cmd_vel (geometry_msgs/Twist) topics.

Introduction

Directional shortcuts like move_forward drive in straight lines. However, complex mobile robot motion (such as navigating smooth curves or executing spiral trajectories) requires specifying simultaneous linear translation and angular rotation.

This lesson covers driving with bot.move(linear_x, linear_y, angular_z).

Code

import time
from bonicbot_bridge import BonicBot
 
with BonicBot(host="192.168.29.52") as bot:
    print("1. Driving forward in a smooth arc to the left...")
    # linear_x = 0.3 m/s forward, angular_z = 0.4 rad/s counter-clockwise
    bot.move(linear_x=1, linear_y=0.0, angular_z=55)
    time.sleep(2.5)
 
    print("2. Driving backward while curving right...")
    # linear_x = -0.2 m/s backward, angular_z = -0.3 rad/s clockwise
    bot.move(linear_x=-1.5, linear_y=0.0, angular_z=-55)
    time.sleep(2.0)
 
    print("3. Pure in-place rotation...")
    bot.move(linear_x=0.0, linear_y=0.0, angular_z=360)
    time.sleep(1.5)
 
    print("4. Halting robot...")
    bot.stop()

[!NOTE] Before running the code, make sure to replace "192.168.29.52" with your BonicBot’s actual IP address.

Expected Output

🤖 Connected to BonicBot at 192.168.29.52:9090
1. Driving forward in a smooth arc to the left...
2. Driving backward while curving right...
3. Pure in-place rotation...
4. Halting robot...
🔌 Disconnected from BonicBot

🔧 Under the Hood

msg = {
    "linear": {"x": linear_x, "y": linear_y, "z": 0.0},
    "angular": {"x": 0.0, "y": 0.0, "z": angular_z}
}

Student Challenge

Challenge 1 — Python Only

Write a function generate_arc_cmd(radius_meters, linear_speed) that computes the required angular_z turn rate for a given turn radius radius_meters and linear_speed.

def generate_arc_cmd(radius_meters, linear_speed):
    # angular_speed = linear_speed / radius
    angular_z = linear_speed / radius_meters
    return {"linear_x": linear_speed, "angular_z": angular_z}
 
cmd = generate_arc_cmd(radius_meters=1.5, linear_speed=0.3)
print("Arc velocity vector:", cmd)

Challenge 2 — Robot

Drive the robot in an “S-curve” pattern: 2 seconds curving left (linear_x=0.3, angular_z=0.4), followed immediately by 2 seconds curving right (linear_x=0.3, angular_z=-0.4), then stop.

import time
from bonicbot_bridge import BonicBot
 
with BonicBot(host="192.168.0.188") as bot:
    print("Executing left curve...")
    bot.move(linear_x=0.3, angular_z=0.4)
    time.sleep(2.0)
 
    print("Executing right curve...")
    bot.move(linear_x=0.3, angular_z=-0.4)
    time.sleep(2.0)
 
    bot.stop()

Quick Reference

Reflection Questions