Answers

3.6 Project: precision parking

Control · Robot club · about 25 min

What this lesson is about

Round a box and into a bay facing east, on the worst robot in the course.

• The plan
• Leg one as a function
• Leg two: sideways, holding two things
• Leg three: turn and hold
• Where next

Questions 6 marks in all

1. [1 mark]In hold_line(0, 67), what does x = 0 mean?

   1. AThe line the robot started on
   2. BThe left edge of the mat
   3. CThe middle of the box
   4. DThe parking bay
   Answer: A. position() counts from the start, so 0 is the starting line, which passes to the left of the box.

2. [1 mark]In slide_right, which part of the drive is the speed?

   drive((target_y - y) * 10, 70, wrapped(0 - heading()) * 4)

   1. AThe sideways part, 70
   2. BThe forward part, holding y
   3. CThe rotation part, holding the heading
   4. DThere is no speed, it uses distance
   Answer: A. The robot is sliding right, so sideways does the travelling. Forward now holds y and rotation holds the heading.

3. [1 mark]hold_heading(90) is running and the robot faces 60. What rotation does max(-60, min(60, error * 3)) ask for?

   Answer: 60. The error is 30 and 30 times 3 is 90, but the clamp caps it at 60.

4. [1 mark]What does this program print?

   def wrapped(h):
       return (h + 180) % 360 - 180
   
   target = 90
   for heading in [350, 80, 120]:
       error = wrapped(target - heading)
       print(heading, max(-60, min(60, error * 3)))

   Answer:

   350 60
   80 30
   120 -60

   From 350 the shortest way to 90 is 100 degrees clockwise, which clamps to 60. From 80 it is 10, so 30. From 120 it is 30 the other way, which clamps to -60.

5. [1 mark]Put the parking program's main lines in order, after the functions have been defined.

   Number the lines 1 to 4 to put them in the right order.

   1. hold_line(0, 67)
   2. print("facing", round(heading(), 1))
   3. hold_heading(90)
   4. slide_right(67, 55)
   Answer:

   hold_line(0, 67)
   slide_right(67, 55)
   hold_heading(90)
   print("facing", round(heading(), 1))

   Up past the box, across the top to the bay, then turn to face east and report.

6. [1 mark]Why does hold_heading loop while abs(error) > 2 instead of until the error is exactly 0?

   1. AThe heading is never exactly on target, so it would never stop
   2. Babs() cannot give 0
   3. CAn error of 2 is the same as 0
   4. DPython cannot compare with 0
   Answer: A. Real sensors wobble by fractions of a degree. A small allowed error lets the loop finish.

The task: precision parking

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

def wrapped(h):
    return (h + 180) % 360 - 180

def hold_line(target_x, until_y):
    while position()[1] < until_y:
        # where am I? (cm from where I started)
        x, y = position()
        # forward, sideways, rotation: -100 to 100 each, until the next command
        drive(70, (target_x - x) * 10, wrapped(0 - heading()) * 4)
        # pause 0.1 s (the robot keeps doing what it was told)
        wait(0.1)
    # all motors off
    stop()

hold_line(0, 67)

from bugbot import *
connect()
def wrapped(h):
    return (h + 180) % 360 - 180
def hold_line(x_target, until_y):
    while position()[1] < until_y:
        x, y = position()
        drive(70, (x_target - x) * 10, -wrapped(heading()) * 4)
        wait(0.1)
    stop()

def hold_heading(target):
    error = wrapped(heading() - target)
    while abs(error) > 2:
        drive(0, 0, -max(-60, min(60, error * 3)))
        wait(0.05)
        error = wrapped(heading() - target)
    stop()

hold_line(0, 67)          # up the left of the box (x is relative: 0 means x = 20 on the mat)
while position()[0] < 55:  # strafe right along the top of the box, holding y and heading
    x, y = position()
    drive((67 - y) * 10, 70, -wrapped(heading()) * 4)
    wait(0.1)
stop()
hold_heading(90)
print('parked at', position(), 'facing', round(heading()))