Answers

6.6 Bumps and stalls

Seeing more · Robot club · about 20 min

What this lesson is about

bumped(), stalls from velocity, back off and go round.

• bumped()
• Stalls
• Back off, go round
• Wall or robot?

Questions 7 marks in all

1. [1 mark]The robot drives into a 2 cm tall box, but distance() never saw it. Why?

   1. AThe depth sensor's level rows look straight over the top of it
   2. BThe box is the same colour as the mat
   3. Cdistance() only works when the robot is still
   4. DThe camera was switched to the line detector
   Answer: A. Low things are invisible to a sensor looking level. The first the robot knows is the bump.

2. [1 mark]Which sensor does bumped() use?

   1. AThe accelerometer in the IMU, which feels the jolt
   2. BThe camera
   3. CThe distance sensor
   4. DThe optical-flow sensor
   Answer: A. A collision is a sudden jolt, and the accelerometer feels it. bumped() is true for a moment afterwards.

3. [1 mark]How does the stall check know the robot is stuck?

   1. AIt compares the real speed from velocity() with the speed it asked for
   2. BIt waits for bumped()
   3. CIt checks whether distance() is under 5
   4. DIt counts how long the program has run
   Answer: A. Told to drive at 60 and moving at almost nothing means something is in the way.

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

   speeds = [0, 10, 25, 30, 29, 28, 1, 0]
   for tick, vy in enumerate(speeds):
       if tick > 5 and vy < 2:
           print("stalled at tick", tick)
           break

   Answer:

   stalled at tick 6

   Tick 0 is slow too, but tick > 5 ignores the start while the robot gets up to speed. The first real stall is tick 6.

5. [1 mark]Which of these can a stall check catch that bumped() might miss?

   Tick every answer that is true.

   1. AA slow push into a wall
   2. BDriving off the edge of a table onto nothing
   3. CA tag 30 cm ahead
   4. DAnother robot's LED changing colour
   Answer: A, B. A slow collision gives no jolt, and a robot hanging off an edge feels nothing. In both, the flow sensor sees no motion.

6. [1 mark]Put the states of the bump and back behaviour in the order they run after a bump.

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

   1. go: drive forward until `bumped()`
   2. back: drive backward for a few ticks
   3. side: slide right for a few more ticks
   4. go: carry on forward
   Answer:

   go: drive forward until `bumped()`
   back: drive backward for a few ticks
   side: slide right for a few more ticks
   go: carry on forward

   A bump starts a small behaviour that runs to completion, like the avoid layer in lesson 5.4.

7. [1 mark]After a bump, a tag is right in front of you. What is the polite response?

   1. AIt is probably a robot, so wait rather than shove
   2. BIt is a wall, so back off and go round
   3. CDrive harder to push through
   4. DTurn the camera off
   Answer: A. A wall stays put, a robot moves. Give it a second and the way may clear.

The task: bump and back

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

while position()[1] < 80:
    # drive forward at 60 (keeps going until the next command)
    forward(60)
    # pause 0.1 s (the robot keeps doing what it was told)
    wait(0.1)
# all motors off
stop()

from bugbot import *
connect()
state = 'go'
ticks = 0
while position()[1] < 80:
    if state == 'go':
        forward(60)
        if bumped():
            print('bumped')
            state = 'back'
            ticks = 0
    elif state == 'back':
        backward(50)
        ticks += 1
        if ticks > 8:
            state = 'side'
            ticks = 0
    elif state == 'side':
        right(60)
        ticks += 1
        if ticks > 18:
            state = 'go'
    wait(0.1)
stop()