Worksheet

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

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

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

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.

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)

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

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

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

The task: gentle stop

Stop with the robot's centre within 2 cm of the small square 50 cm ahead, having driven at least 40 cm. The starter times the drive; timing is never that accurate.

# 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()

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

Do it on the robot
www.bugbotlab.com/learn/1-5-speed-and-precision/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

1. Get the gentle stop within 1 cm.
2. Write precise_forward(cm) that does the fast-then-slow trick for any distance.
3. Do the same for turns: precise_turn(deg).