Worksheet

U1.1 Sense, decide, act

The robot as a system · University · about 20 min

What this lesson is about

The control loop, its period, and the deadman that stops a robot whose program has gone quiet.

• A loop you can watch
• The period
• The deadman
• Blocking and non-blocking

Questions 7 marks in all

1. [1 mark]Put the body of the control loop in order, as it runs on every pass.

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

   1. act(command)
   2. wait(period)
   3. command = decide(measurement)
   4. measurement = sense()

2. [1 mark]A loop ends each pass with wait(0.1) and does nothing else that takes noticeable time. How often does it run, and roughly what is the fastest disturbance its measurements can represent properly?

   1. A10 Hz, and disturbances slower than about 5 Hz
   2. B10 Hz, and disturbances up to about 10 Hz
   3. C0.1 Hz, and disturbances slower than about 0.05 Hz
   4. D10 Hz, and disturbances up to about 20 Hz

3. [1 mark]A loop has a period of 0.04 s. What is its rate in Hz?

4. [1 mark]A loop calls wait(0.1), and the sensor reads and arithmetic in each pass take a further 0.03 s. This works out the rate it really runs at. What does it print?

   work = 0.03
   period = 0.1 + work
   print("rate:", round(1 / period, 1), "Hz")

5. [1 mark]A program calls forward(60) once and then wait(2), with no other commands. What does the robot do?

   1. ADrives for about half a second, then stops itself because no new command arrived
   2. BDrives at 60 for the full 2 seconds, then stops
   3. CDoes not move at all, because wait() blocks the command from being sent
   4. DDrives 60 cm and then stops

6. [1 mark]Why does every controller in the course use forward(60) followed by wait(0.1), rather than forward(60, distance=40)?

   1. AThe non-blocking form returns at once, so the loop can change the command on the next pass
   2. BThe blocking form is less accurate about the distance it drives
   3. CThe deadman does not apply to blocking calls, which makes them unsafe
   4. DThe non-blocking form drives the motors harder

7. [1 mark]Which of these are set directly by the loop's period?

   Tick every answer that is true.

   1. AHow often the controller can react to what the world does
   2. BThe fastest motion the measurements can represent properly
   3. CThe robot's top speed
   4. DThe size of the drive's dead band

The task: run a loop at 10 Hz

Without driving, read the depth sensor thirty times at ten times a second, print each reading, and print how long the loop actually took on average, as period: 0.1.

from bugbot import *
connect()

# clock() is the seconds since the program started
start = clock()

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

Do it on the robot
www.bugbotlab.com/learn/u1-1-the-loop/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

1. Print clock() inside the loop. Is the spacing exactly 0.1 s?
2. Change the wait to 0.02 s and run it. What is the period now, and what does that tell you about how much work the loop itself costs?
3. Put forward(60) in a loop of thirty with no wait at all in it. Why does the robot not move in the simulator, and what would a real BugBot do?