Answers

6.5 Following a robot

Seeing more · Robot club · about 20 min

What this lesson is about

Two controllers that never finish: chase a moving tag at a set distance.

• Watch it go
• Two controllers, forever
• Corners
• Losing it

Questions 6 marks in all

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

   for dist in [50, 30, 20, 12]:
       speed = max(0, min(70, (dist - 20) * 4))
       print(dist, speed)

   Answer:

   50 70
   30 40
   20 0
   12 0

   50 gives 120, clamped to 70. 30 gives 40. At 20 the error is 0. At 12 it would be -32, and the clamp holds it at 0.

2. [1 mark]Why is the lowest speed in the follower 0, the max(0, ...)?

   1. ASo the robot stops when too close instead of reversing
   2. BSo it never goes faster than the leader
   3. CSo it keeps moving when the leader stops
   4. DBecause negative speeds are an error
   Answer: A. Closer than 20 cm, the controller would ask for a negative speed. The clamp holds it still instead.

3. [1 mark]With speed = max(0, min(70, (dist - 20) * 4)), what speed does the follower use when the leader is 26 cm away?

   Answer: 24. (26 - 20) * 4 is 24, which is inside the clamp.

4. [1 mark]How is following a robot different from docking at a marker?

   1. AThe target keeps moving, so the loop never finishes
   2. BIt uses a different detector
   3. CIt needs no rotation controller
   4. DThe distance is measured in pixels
   Answer: A. The same two controllers, but the distance and bearing are new every tick and there is no end point.

5. [1 mark]When the leader goes out of view, the lesson just turns right. What is a better recovery?

   1. ARemember the last bearing and turn that way
   2. BStop and wait for the leader to come back
   3. CDrive forward at full speed
   4. DSwitch to the line detector
   Answer: A. The leader is most likely where it was last seen. Better still, lesson 6.7 predicts where it is going.

6. [1 mark]The task starter always drives at 60 towards robot 101. What goes wrong?

   1. AIt rams the leader, because nothing slows it down as it gets close
   2. BIt never catches up
   3. CIt turns away from the leader
   4. DIt loses the leader immediately
   Answer: A. It only has the rotation controller. Add the distance controller so speed falls as the gap closes.

The task: follow the leader

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

# do this 390 times (tick counts from 0)
for tick in range(390):
    seen = [r for r in robots() if r[0] == "green"]
    if seen:
        # forward, sideways, rotation: -100 to 100 each, until the next command
        drive(60, 0, (seen[0][1] - 160) * 120 / 320 * 3)
    # pause 0.1 s (the robot keeps doing what it was told)
    wait(0.1)
# all motors off
stop()

from bugbot import *
connect()
for tick in range(390):
    seen = [r for r in robots() if r[0] == 'green']
    if not seen:
        turn_right(40)                     # lost it: turn until the green dome is back
    else:
        cx, dist = seen[0][1], seen[0][3]
        bearing = (cx - 160) * 120 / 320
        speed = max(0, min(70, (dist - 20) * 4))   # hold about 20 cm behind
        drive(speed, 0, bearing * 3)
    wait(0.1)
stop()