B - Broken Track

We only need the line follower module for this lesson - assemble your robot like the picture!

Dealing with breaks in the track is difficult. We can't just drive forward for 1 second using a wait block, as we won't be able to sense the line at the same time.

Build the test program in the picture.

It would be great if this program drove forwards and then stopped on the line - try it, see what happens.

It only works if one of the sensors is exactly on the line after 1 second - this is not very likely!

This is because the wait block stops anything else from happening whilst it is waiting for 1 second - so all the time we are driving forwards, we can't check the sensors - that's no good!

Things that stop other things from happening are called 'blocking' - they block everything else until they are finished.

We need to come up with a way to wait whilst still being able to do other things.

Replace the wait 1 second block with a do 10 times loop, and a 100 millisecond wait block inside the loop.

This code will wait for 1 second, just like before.

We now need to add a variable that acts as a counter - it will count how many milliseconds of delay have happened.

Above the do 10 times loop, add a block to let t = 0.

Inside the loop, increase t by 100.

Run the program and watch the value of t in the variables menu - it will now count the number of milliseconds of delay!

Now let's use the counter to control the number of times the loop runs.

Instead of using a do 10 times loop, replace it with a do until loop.

For the condition, use an = block from the operators menu so the loop runs until t = 1000.

We can now change this number to decide how long the wait is! For example, changing to t=2000 would run the loop for a total of 2000 milliseconds (2 seconds).

What's the point of making a really complicated wait block?

Anything we put in the loop will be run as the wait is happening - so we can check the sensors whilst we are driving forwards!

Move the IF block checking the sensors inside the loop.

Try it out - the robot should now drive forward and stop exactly on the line, every time!

Experiment with changing the length of the wait loop, so the robot can start further away from the line and still reach it.

We actually don't need the IF block in the loop - we can merge the conditions of the loop and the IF block together!

Let's think about this - we want to stop the loop (and then stop the motors) if:

t=2000, OR

Sensor A is LO, OR

Sensor B is LO

Luckily, we can use OR blocks inside each other to do this! Change your code to look like the picture, and test it out.

Let's merge our code with the 2 sensor line follower program to deal with simple breaks in the track.

Load up your code and add the line finder code you just wrote to the IF block where both sensors are off the track (HI).

It should look like the picture!

We need to make a few changes to make our code work with the line follower:

Add a wait block of 200 milliseconds before the wait loop (this makes sure both sensors are not on the line)

We only want to stop the motors if both sensors are still off the track after the wait loop.

Put the motor stop blocks in an IF block, that checks if both sensors are HI

After we have stopped the motors, we then want to wait until 1 of the sensors is LO before we continue

Add a wait until block to do this!

Cover a small section of straight track (about 5cm) with a piece of paper and tape it down to test the program.

You will probably need to make adjustments to speeds and timings to make it work reliably!

Keep experimenting until it works well.

Make sure the gap is on a straight section of track - this code won't work on gaps in curves! Can you work out why?

Once you can cross a gap in straight track, try a gap in curved track!

To do this, you will need to make the robot move side to side in the wait loop, instead of just moving forwards.

This can be done by making the robot turn to start with instead of going forwards, and then changing the direction of turn inside the loop every so often.

It works best if the robot goes left and right several times , in a kind of sweeping motion.

You can also experiment with other types of break like in the picture - offset lines and breaks that point the robot in the wrong direction like the middle example are particularly difficult to get right!