Answers

6.3 Losing the line

Seeing more · Robot club · about 20 min

What this lesson is about

Gaps: carry on, then search with a state machine.

• Keep going first
• Then search
• Choosing the first direction

Questions 7 marks in all

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

   readings = [[160, 0], [], [], [170, 2], [], [], [], []]
   lost = 0
   for seen in readings:
       if seen:
           lost = 0
       else:
           lost += 1
   print(lost)

   Answer:

   4

   Every sighting resets the counter. After the last sighting come four empty readings, so lost is 4.

2. [1 mark]When the line disappears, why carry straight on for a while before searching?

   1. AMost gaps are short, so the line is usually just ahead
   2. BThe camera needs time to switch detector
   3. CSearching uses more battery
   4. DThe line always restarts straight ahead
   Answer: A. It is the cheapest fix and it works for most gaps. On this mat it does not, which is why the counter leads to a search.

3. [1 mark]What does this program print?

   for sweep in [0, 14, 15, 29, 30, 45]:
       print(sweep, 40 if (sweep // 15) % 2 == 0 else -40)

   Answer:

   0 40
   14 40
   15 -40
   29 -40
   30 40
   45 -40

   sweep // 15 counts blocks of 15 ticks. Even blocks slide right at 40, odd blocks slide left at -40.

4. [1 mark]The search slides from side to side instead of turning. Why?

   1. AThe robot keeps its heading, so it already points the right way when it finds the line
   2. BSliding is faster than turning
   3. CThe line detector cannot work while turning
   4. DTurning would make line() return an error
   Answer: A. Strafing searches without changing which way you face, so following can pick up straight away.

5. [1 mark]Which way should the search try first?

   1. AThe way the last angle said the line was heading
   2. BAlways right
   3. CAlways left
   4. DBackwards, to where the line was
   Answer: A. A line heading right when it vanished probably carries on to the right. Store the angle before it is gone.

6. [1 mark]The follower switches to search when lost > 8, adding 1 to lost on each tick with no line. On which tick without a line does it switch?

   Answer: 9. lost > 8 is first true when lost is 9. At ten ticks a second, that is just under one second.

7. [1 mark]The task starter does if not follow_step(60, 0.4): break. What happens at the gap?

   1. AThe robot stops at the first gap and never reaches the zone
   2. BIt searches for the line
   3. CIt follows the line through the gap
   4. DIt turns round
   Answer: A. Breaking the moment line() is empty is the follower that is stuck at the first gap. Count, then search.

The task: find the line again

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

def follow_step(speed=60, gain=0.4):
    # [cx, angle] of the line ahead, or [] if none
    seen = line()
    if not seen:
        return False
    cx, angle = seen
    # forward, sideways, rotation: -100 to 100 each, until the next command
    drive(speed, 0, (cx - 160) * gain)
    return True

# camera: the line detector
set_cv("line")
while position()[1] < 85:
    if not follow_step(60, 0.4):
        # leave the loop
        break
    # pause 0.1 s (the robot keeps doing what it was told)
    wait(0.1)
# all motors off
stop()

from bugbot import *
connect()
def follow_step(speed=60, gain=0.4):
    # one tick of line following: steer from where the line crosses the picture
    seen = line()
    if not seen:
        return False
    cx, angle = seen
    error = cx - 160                       # pixels left (-) or right (+) of centre
    drive(speed, 0, error * gain)
    return True
set_cv('line')
lost = 0
state = 'follow'
while position()[1] < 85:
    if state == 'follow':
        if follow_step(60, 0.4):
            lost = 0
        else:
            lost += 1
            forward(40)                    # keep going a little: gaps are usually short
            if lost > 8:
                state = 'search'
                sweep = 0
    elif state == 'search':
        if line():
            state = 'follow'
        else:
            sweep += 1
            drive(25, 40 if (sweep // 15) % 2 == 0 else -40, 0)   # creep forward while sweeping side to side
    wait(0.1)
stop()