Fast line-following with Steer Bot

Python code and building instructions for the LEGO MINDSTORMS Robot Inventor Other Fan Creations (51515).

Fast line-following with Steer Bot

Super fast line following robot with Ackermann steering.


A robot with car like steering has been one of my favorite exercises in robot building. This project, in fact, is largely a variation of what I have previously done with the LEGO RCX, the NXT and to a lesser extent the EV3. But with the 51515 set I as able to take advantage of improved resolution of the color sensor as well as some cool features of Pybricks to make the main tracking code pretty simple.

The robot features Ackermann steering. This means that the steering linkage is more trapezoidal than parallel. The front wheels are only parallel when the steering is pointing straight ahead. When it turns the inside wheel turns sharper than the outside wheel so the wheels track more accurately since the inside wheel has a tighter turning radius. This is particularly important in order to achieve tight overall turning radius with a relatively wide robot.

The robot uses the Color Sensor mounted on an axle high and attached to the steering motor. This means that in order to track a line, actually just the edge of the line, the tracking code tries to keep the sensor on the edge of the line. If the sensor is on the edge, then as the robot drives it will follow the edge since the robot is now steering to where the edge is in front of the robot. The sensor is mounted high in order to give a gradual reading over the edge so that while near the edge the program can determine where the edge is relative to the sensor.

The driving is done through the super cool 5 gear differential that is included with the set. This is the first time I use this LEGO differential and I love it! The drive motor is actually gearing up with the intention of being able to go really fast. On my tight test track I was never able to make it go full speed without running off the line.


from pybricks.hubs import InventorHub
from pybricks.pupdevices import Motor, ColorSensor
from pybricks.parameters import Port, Button, Stop
from import wait

# Tuning parameters
SPEED_MAX = 1000

# Initialize the hub, motors, and sensor
hub = InventorHub()
steer_motor = Motor(Port.A)
drive_motor = Motor(Port.B)
sensor = ColorSensor(Port.C)

def wait_for_button(b):
    # Wait for press
    while b not in hub.buttons.pressed():
    # and release
    while b in hub.buttons.pressed():

# Use the color saturation value to track line
def get_light():
    return sensor.hsv().s

def calibrate():
    global a_steer_limit
    global l_min, l_max, sign_edge

    # Find the Right and Left hard limits
    a_right_limit = steer_motor.run_until_stalled(
        400, then=Stop.BRAKE, duty_limit=100
    a_left_limit = steer_motor.run_until_stalled(
        -400, then=Stop.BRAKE, duty_limit=100

    # Calculate the steering limit as average of two extremes then
    # reset angle to the negative limit since steering motor is now
    # at negative extreme
    a_steer_limit = (a_right_limit - a_left_limit) // 2

    # Scan from -30 to 30 to get min max of light sensor value
    steer_motor.run_target(1000, -30, then=Stop.BRAKE)
    l_min = 1024
    l_max = 0
    l_left = get_light()

    while steer_motor.angle() < 30:
        light = get_light()
        if light > l_max:
            l_max = light
        if light < l_min:
            l_min = light

    l_right = get_light()
    # sign_edge is positive 1 if left edge and -1 if right edge
    sign_edge = 1 if l_left < l_right else -1

    # Center the steering
    steer_motor.run_target(1000, 0, then=Stop.BRAKE, wait=False)

def track_speed_control():
    l_mid = (l_max + l_min + 1) // 2
    m = 20.0 / (l_max - l_mid)

    # Calculate a threshold to determine steering is not near the edge
    l_off_edge_thresh = (l_max - l_mid) * 0.7

    # Set max speed, acceleration, and max power for drive motor
    drive_motor.stop()  # must be stopped to set limits
    drive_motor.control.limits(1000, 2000, 100)

    while not any(hub.buttons.pressed()):
        # Get a new light value and subtract mid to get signed error
        # from edge
        light = sign_edge * (get_light() - l_mid)

        # Create a new target for the steering motor to move toward
        # the approximate position of the edge
        a = steer_motor.angle()
        t = a - m * light

        # Clamp the target angle to within +- a_steer_limit
        t = min(t, a_steer_limit)
        t = max(t, -a_steer_limit)

        # Now update target to move toward edge of line

        # Speed control
        if abs(light) < l_off_edge_thresh:
            # On edge of line
            if abs(t) < 25:
                # and going straight


    while not any(hub.buttons.pressed()):

while True:

This project was submitted by Gus Jansson.