Worksheet

7.3 Fetching a ball

Hands and feet · Robot club · about 20 min

What this lesson is about

See it, approach it, grip it, bring it home.

• Find it
• Approach and grip
• Bring it home

Questions 7 marks in all

1. [1 mark]Put the stages of a fetch in order.

   Number the lines 1 to 5 to put them in the right order.

   1. Stop and close the jaws once the ball is in reach
   2. Steer towards the blob, slowing as distance() falls
   3. Open the jaws
   4. Spin until a red blob is in view
   5. Drive home with go_to, holding the ball

2. [1 mark]What does this program print?

   def bearing_of(cx):
       return (cx - 160) * 120 / 320
   
   print(bearing_of(240))
   print(bearing_of(100))

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

   for d, error in [(30, 4), (30, 25), (8, 2)]:
       speed = 15 if d < 10 else (45 if abs(error) < 10 else 20)
       print(d, error, speed)

4. [1 mark]Why does fetch slow to 15 for the last few centimetres?

   1. AThe slow finish is what makes the grip reliable
   2. BThe distance sensor only works at low speed
   3. CThe jaws cannot close while the robot is moving
   4. DTo save the battery for the drive home

5. [1 mark]Inside fetch, the jaws close but holding() is None. What happens next?

   1. AIt creeps forward at 15 and tries again on a later tick
   2. BIt returns False straight away
   3. CIt opens the jaws and spins to search again
   4. DIt returns True anyway

6. [1 mark]fetch has used all 250 ticks without holding the ball. What does it do?

   1. AStops the motors and returns False
   2. BReturns True
   3. CStarts the 250 ticks again
   4. DKeeps driving forward at 15

7. [1 mark]fetch waits 0.1 s each tick for up to 250 ticks. Ignoring the time spent gripping, what is the longest it can take, in whole seconds?

The task: fetch the ball

Find the red ball, fetch it, and leave it in the home zone.

Plan your program here, then type it in and press Run.

Do it on the robot
www.bugbotlab.com/learn/7-3-fetching-a-ball/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

1. Print how long the fetch took.
2. Fetch a blue ball instead. (Add one in the free-play editor first.)
3. fetch gives up after 250 ticks, but find_ball spins forever if no ball is in view. Make both give up after 10 seconds, with fetch returning False.