Worksheet

5.2 States

Behaviours · Robot club · about 20 min

What this lesson is about

A plan as a state variable, transitions, drawing the machine.

• A state is a variable
• Sliding to a point
• The machine
• Drawing it

Questions 7 marks in all

1. [1 mark]In a state machine, what is the state?

   1. AA variable holding a word that says which part of the plan the robot is in
   2. BThe robot's position on the mat
   3. CThe last sensor reading
   4. DA function that drives the robot

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

   import math
   
   route = {"up": (0, 60), "right": (60, 60)}
   state = "up"
   for p in [(0, 20), (0, 57), (30, 60), (57, 61)]:
       tx, ty = route[state]
       if math.hypot(tx - p[0], ty - p[1]) < 6:
           if state == "right":
               print("done")
               break
           state = "right"
       print(p, state)

3. [1 mark]In the "up" branch of a patrol, the transition is if near(0, 60): state = "up". The robot reaches (0, 60) and just sits there. Why?

   1. AThe transition sets the state back to "up", so it never leaves that state
   2. Bnear needs a cm argument
   3. Cgo_to cannot drive to (0, 60)
   4. DThe loop is missing wait(0.1)

4. [1 mark]target = route[state] gives a pair like (60, 60). What does near(*target) do?

   1. AUnpacks the pair into two arguments, the same as near(60, 60)
   2. BMultiplies the target by something
   3. CPasses the pair as one argument
   4. DGives the distance to the target

5. [1 mark]The task's patrol goes round the box and back. Put its states in order.

   Number the lines 1 to 4 to put them in the right order.

   1. up
   2. right
   3. down
   4. left

6. [1 mark]You draw a state machine before writing it. Which belong in the drawing?

   Tick every answer that is true.

   1. AA circle for each state
   2. BAn arrow for each transition
   3. CThe condition written on each arrow
   4. DA box for every tick of the loop

7. [1 mark]Why does the patrol keep the corners in a dictionary, route, instead of writing a separate driving block for each state?

   1. AThe loop body is the same for every state, and only the transitions differ
   2. BDictionaries make the robot faster
   3. Cgo_to only accepts a dictionary
   4. DA state machine is not allowed to use if

The task: patrol in states

From A to B and back, round the box: up, right, then left, down. Without touching the box.

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

# maths: atan2, hypot, sin, cos, radians
import math

def wrapped(h):
    return (h + 180) % 360 - 180

def go_to(x, y, speed=70):
    # where am I?
    px, py = position()
    a = math.radians(wrapped(math.degrees(math.atan2(x - px, y - py)) - heading()))
    # forward, sideways, rotation: -100 to 100 each, until the next command
    drive(speed * math.cos(a), speed * math.sin(a), wrapped(0 - heading()) * 3)

def near(x, y, cm=6):
    # where am I?
    px, py = position()
    return math.hypot(x - px, y - py) < cm

# do this 100 times (tick counts from 0)
for tick in range(100):
    if near(0, 60):
        # leave the loop
        break
    go_to(0, 60)
    # pause 0.1 s (the robot keeps doing what it was told)
    wait(0.1)
# all motors off
stop()

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

Do it on the robot
www.bugbotlab.com/learn/5-2-states/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

1. Add a "pause" state: stop for one second at B before coming back.
2. Patrol twice, with a lap counter.
3. Make the LED a different colour in each state.