Answers

1.5 Speed and precision

Driving · Robot club · about 15 min

What this lesson is about

Coasting, overshoot, and why fast and accurate pull in different directions.

• Coasting
• Why: momentum
• The robot allows for it
• The trick: fast, then slow
• Turning has the same problem
• Measure it yourself

Questions 7 marks in all

1. [1 mark]The robot drives by time at speeds 30, 60 and 100, then stop(). Which slides furthest after the motors switch off?

   1. ASpeed 100
   2. BSpeed 30
   3. CSpeed 60
   4. DThey all slide the same
   Answer: A. The faster it is going, the more momentum it has, so the mat's friction takes longer to stop it.

2. [1 mark]How does distance= allow for coasting?

   1. AIt switches the motors off early, by the amount it expects the robot to slide
   2. BIt drives backwards a little after stopping
   3. CIt always drives at a slow speed
   4. DIt waits for the robot to hit the target and then brakes
   Answer: A. The robot knows how fast it is going and predicts the slide. The prediction is never perfect, so fast drives are still a little further off.

3. [1 mark]Why drive forward(100, distance=15) and then forward(25, distance=5), instead of 20 cm at 100?

   1. AMost of the way is at full speed, and the finish has the slow speed's accuracy
   2. BSpeed 100 cannot go further than 15 cm
   3. CIt makes the robot go perfectly straight
   4. DIt is the only way to drive 20 cm
   Answer: A. Most of the way is fast, and only the final part, where the stopping point is decided, is slow. Real robots do the same.

4. [1 mark]You want to stop 50 cm ahead, with the last 8 cm at a slow speed. What distance should the fast forward be? Give a whole number of centimetres.

   Answer: 42. The two legs add up to the whole journey, so the fast part is 50 minus 8, which is 42 cm.

5. [1 mark]What does this program print?

   errors = []
   for y in [20.1, 20.3, 19.6]:
       errors.append(round(y - 20, 1))
   print("errors:", errors)

   Answer:

   errors: [0.1, 0.3, -0.4]

   Each error is where the robot got to minus 20. A negative error, like -0.4, means it stopped short.

6. [1 mark]What does this program print?

   speed = 60
   y = 19.8
   print(f"speed {speed}: asked 20, got {y}")

   Answer:

   speed 60: asked 20, got 19.8

   In an f-string the names inside curly brackets are replaced by their values.

7. [1 mark]A fast turn overshoots by about a degree. Why does that matter?

   1. AOver a long route those degrees add up
   2. BIt does not matter, a degree is too small to notice
   3. CThe robot will refuse to turn again
   4. DIt makes the next forward go backwards
   Answer: A. One degree is tiny, but every turn adds its own. A slower turn overshoots by only a fraction of a degree.

The task: gentle stop

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

# drive forward at 100 (keeps going until the next command)
forward(100)
# pause 2.5 s (the robot keeps doing what it was told)
wait(2.5)
# all motors off
stop()

from bugbot import *
connect()
forward(100, distance=44)
forward(25, distance=6)