Where am I?

position(), heading(), velocity(), and resetting them.

2.1SensingRobot club15 min

A robot that does not know where it is cannot go anywhere on purpose. This module is about the robot's senses. It starts with the ones that answer "where am I and which way am I facing".

position()

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

print("start:", position())
# drive forward at 60 for 20 cm, then stop
forward(60, distance=20)
# slide right at 60 for 15 cm, then stop
right(60, distance=15)
print("now:", position())

Run this in the simulator

(x, y) in centimetres from where the program started: x to the right, y forward, as seen from the starting direction. The optical-flow sensor underneath measures it by watching the mat go by.

heading()

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

print("start:", round(heading()))
# turn 45 degrees clockwise, then stop
turn_right(30, angle=45)
print("after 45:", round(heading()))
# turn 135 degrees anticlockwise, then stop
turn_left(30, angle=135)
print("after -135:", round(heading()))

Run this in the simulator

Degrees clockwise from the starting direction, 0 to 360. -90 shows as 270. The compass inside is a 9-axis inertial sensor: gyroscope, accelerometer and magnetometer together.

velocity()

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

# drive forward at 80 (keeps going until the next command)
forward(80)
# pause 0.5 s (the robot keeps doing what it was told)
wait(0.5)
print("moving at", velocity(), "cm/s")
# pause 0.5 s (the robot keeps doing what it was told)
wait(0.5)
# all motors off
stop()
# pause 0.5 s (the robot keeps doing what it was told)
wait(0.5)
print("stopped:", velocity())

Run this in the simulator

(vx, vy): sideways and forward speed right now, in centimetres per second. Useful for knowing whether the robot has actually stopped.

Resetting

Sometimes "here" should be zero:

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

# drive forward at 60 for 20 cm, then stop
forward(60, distance=20)
# turn 90 degrees clockwise, then stop
turn_right(30, angle=90)
print("before reset:", position(), round(heading()))
# count from here as (0, 0)
reset_position()
# this way is now heading 0
reset_heading()
print("after reset:", position(), round(heading()))
# drive forward at 60 for 10 cm, then stop
forward(60, distance=10)
print("10 cm from the new zero:", position())

Run this in the simulator

reset_heading() makes the current facing 0. reset_position() makes the current spot (0, 0). Do this at the start of a task so the numbers mean what you think.

Estimates, not truth

Everything the robot tells you about itself is an estimate. Each sensor has some error, and errors add up:

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

# do this 4 times (i counts from 0)
for i in range(4):
    # drive forward at 60 for 30 cm, then stop
    forward(60, distance=30)
    # turn 90 degrees clockwise, then stop
    turn_right(30, angle=90)
print("should be home:", position(), "facing", round(heading()))

Run this in the simulator

A perfect robot would say (0, 0) facing 0. Yours is a few centimetres and degrees off. Real robots look at landmarks to correct that; the camera does it in Module 4.

Task: report from the corner

Get to the point 30 cm right and 30 cm forward of where you start, and print a line in the shape at (30.1, 29.8) facing 1.2 from position() and heading().

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

# drive forward at 60 for 30 cm, then stop
forward(60, distance=30)

Challenges

Print position() and velocity() every 0.5 s while driving.
Drive a triangle and print how far from home you end up: x*x + y*y under a square root.
Reset the heading after each turn of a square. Does the final error change?