Answers

3.2 Proportional control

Control · Robot club · about 15 min

What this lesson is about

Correct in proportion to the error. Gain, sign, feedback.

• The robot that curves
• Steer by the error
• The sign
• Why it is called feedback

Questions 6 marks in all

1. [1 mark]What does a proportional controller do?

   1. APushes in proportion to the error: hard when far off, gently when close
   2. BGoes at one fixed speed until the error is zero
   3. CTurns a set amount every loop
   4. DWaits until the error is big before doing anything
   Answer: A. The correction is the error times a gain, so it fades away as the error does.

2. [1 mark]The gain is 4 and the heading error is 7.5 degrees. What rotation does drive(70, 0, error * 4) ask for?

   Answer: 30. 7.5 times 4 is 30. Half the error would give half the rotation.

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

   def wrapped(h):
       return (h + 180) % 360 - 180
   
   for heading in [10, 350]:
       error = wrapped(0 - heading)
       print(heading, error * 4)

   Answer:

   10 -40
   350 40

   Facing 10, the robot is right of the target, so the rotation is negative and turns it back left. Facing 350, it is left of the target, so it turns right.

4. [1 mark]You write drive(70, 0, -error * 4) by mistake. What happens?

   1. AThe controller runs away and the error grows
   2. BThe robot holds the heading more gently
   3. CNothing, the sign does not matter
   4. DThe robot drives backwards
   Answer: A. With the wrong sign every correction pushes further the wrong way. When a controller does something wild, check the sign first.

5. [1 mark]Why is it called a feedback loop?

   1. AThe heading comes out of the robot and goes back to the motors as a correction
   2. BThe robot prints its heading every loop
   3. CIt uses a for loop
   4. DThe robot drives in a circle
   Answer: A. Output is fed back to input. Open loop hopes; closed loop checks.

6. [1 mark]Why does the P controller not need bursts and settling like the crude fix?

   1. ASmall errors give small rotations, so the correction fades away with no overshoot
   2. BIt always drives at a low speed
   3. CIt stops after every loop
   4. DIt only runs once
   Answer: A. Four degrees off gives rotation 16, half a degree gives 2. The robot eases in rather than slamming on.

The task: hold the heading

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

# drive forward at 70 for 70 cm, then stop
forward(70, distance=70)

from bugbot import *
connect()
def wrapped(h):
    return (h + 180) % 360 - 180
while position()[1] < 65:
    error = wrapped(heading())
    drive(70, 0, -error * 4)
    wait(0.1)
stop()