Answers

3.4 Controlling speed

Control · Robot club · about 15 min

What this lesson is about

Speed from the remaining distance, the dead band, clamping.

• The bang-bang park
• Proportional speed
• The dead band
• The pattern

Questions 6 marks in all

1. [1 mark]The bang-bang park drives at 60 until distance() is 22 or less, then stops. Where does it end?

   1. ACloser than 22 cm, because it coasts
   2. BExactly 22 cm
   3. CFurther than 22 cm
   4. DTouching the wall every time
   Answer: A. Fast all the way and stop means a long slide. It is the overshoot from lesson 3.1, now driving forward.

2. [1 mark]Target 22, gain 4, and distance() reads 32. What speed does speed = (distance() - target) * 4 ask for?

   Answer: 40. The error is 32 minus 22, which is 10, and 10 times 4 is 40.

3. [1 mark]The plain proportional park stalls a little short of the target. Why?

   1. ANear the target it asks for speeds below about 15, which do not move the robot
   2. BThe gain is too high
   3. CThe sensor cannot see closer than 25 cm
   4. DThe loop runs out of ticks too early
   Answer: A. The vibration motors have a dead band. With 3 cm to go it asks for 12, which does nothing.

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

   for value in [4, 30, 250]:
       print(max(16, min(60, value)))

   Answer:

   16
   30
   60

   Clamping keeps a number between two limits. 4 is raised to 16, 30 is fine, and 250 is capped at 60.

5. [1 mark]In the clamped park, the robot overshoots and distance() reads 18. What does it do?

   error = distance() - target
   speed = max(16, min(60, abs(error) * 4))
   if error > 0:
       forward(speed)
   else:
       backward(speed)

   1. ABacks up slowly, at speed 16
   2. BStops, because it is past the target
   3. CDrives forward at speed 16
   4. DBacks up at speed 60
   Answer: A. The error is 18 minus 22, which is -4, so it goes backward. 4 times 4 is 16, which is exactly the lowest allowed speed.

6. [1 mark]Put the three lines of every P controller in order.

   Number the lines 1 to 3 to put them in the right order.

   1. output = clamp(error * gain)
   2. apply(output)
   3. error = target - measured
   Answer:

   error = target - measured
   output = clamp(error * gain)
   apply(output)

   Measure the error, work out how hard to push, then push. Steering and speed are the same loop with a different sensor and motor.

The task: park at twenty

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

# as long as the thing ahead is further than 22 cm
while distance() > 22:
    # 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()
target = 22
while True:
    error = distance() - target
    if abs(error) < 1:
        break
    speed = max(16, min(60, abs(error) * 4))
    if error > 0:
        forward(speed)
    else:
        backward(speed)
    wait(0.05)
stop()