Avoiding obstacles autonomously

So far, you have created programs for moving the robot buggy and detecting objects at different distances. Now you are going to create a program for detecting and navigating around objects autonomously.

Programming the motors and the UDS

For this program, you will need to bring together some of the code you have written during the course to solve a new problem.

To help you begin this process, consider the different parts of the problem:

What components does your program need to interact with?
What should the robot do if an object is not too close to it?
How should the robot behave when an object is too close?

Setting up the program

1. Copy the code from the last step into a new Python file.

2. Change the first line of code so that Robot is also imported from the gpiozero library to control the motors.

from gpiozero import Robot, InputDevice, OutputDevice

3. After the trig and echo variables have been initialised, define the Robot using the GPIO pins for your left and right motors.

robin = Robot(left=(8,7), right=(9,10))

You can use one of your working programs from week one to check that the GPIO pins of your motors are set correctly, as they may be different to the ones I've used above.

Setting up a timer

Next, you are going to create a timer so that your robot doesn't run forever, which is especially useful during the testing phases.

4. Add in these three variables just after you have initialised Robot:

duration = 10
end_time = time() + duration
running = True

The value of duration is the number of seconds that the timer will run for. Calculate end_time by adding duration to the current time, which is obtained using the time() function.

You will use the running variable later to specify when the program (and the robot) should stop running.

Check your program so far against my version of the program after step 4.

The last changes you are going to make are going to all be within the while loop.

How should the robot behave?

The robot should, by default, move forwards, unless it detects an object that is too close. If an object is detected within a certain distance of the UDS, the robot should turn left or right to avoid the obstacle in front of it.

How close is too close?

The program currently calculates the distance of an object from the UDS in metres. You need to specify the threshold, in metres, at which an object is too close to the robot.

For now, the threshold value I'm going to choose is 20 cm (0.2 metres); you can experiment with this value later.

5. Inside the while loop and before the sleep(0.06) command, use a selection statement to check if an object is less than 0.2 metres away. Instruct the robot to turn left for half a second if the distance is below the threshold value, and to move forwards otherwise.

if distance < 0.2:
    robin.left()
    sleep(0.5)
else:
    robin.forward()

Using a sleep command here inside the if statement means that the robot will continue to turn left for a certain amount of time before checking for more obstructions. This should allow your robot enough time to turn clear of the object that was detected.

6. To stop the program from running forever, add the following code inside the while loop before the sleep(0.06) command.

    if time() >= end_time:
        running = False
        robin.stop()

This checks if the current time is more than or equal to the end_time value specified at the start of the program. If so, it will change the value of running to False (which will stop the while loop from repeating) and then stop the motors.

7. Modify the while loop condition so that it stops repeating once running is set to False.

Change the condition from:

while True:

to:

while running:

Your final code for the while loop should be:

while running:
    duration = get_pulse_time()
    distance = calculate_distance(duration)

    if distance < 0.2:
        robin.left()
        sleep(0.5)
    else:
        robin.forward()

    if time() >= end_time:
        running = False
        robin.stop()

    sleep(0.06)

Testing the program

Try running the program, with the robot on a suitable surface.

If the buggy is not behaving as expected, check that all the connections to the GPIO pins are correct before trying the code again.

It may also be beneficial to run an earlier program that you know was working, to test whether the motors and UDS are still working as expected.

Discussion

How effective is your robot at avoiding objects?

Can you think of any modifications that might help?

Share your answers in the comments below, or let us know if you are having any issues.