Milestone — Collect and Deliver

The mission 4 min

Drive from HOME to the solar panel at F, collect the energy module, bring it back, and release it in the delivery yard — three times in a row without touching the robot.

Nothing new is taught today. That is what makes it hard. Calibration, line following, squaring and distance all have to hold together for a whole round trip, and the failure you meet will almost certainly be in the joins rather than in any one piece.

The route 5 min

HOME → F → HOME. Out along the line, collect, come back. Write it as legs before you program it:

Test each leg on its own before joining any of them. A team that writes all six and then presses play has six suspects when it fails.

The technique — joining legs together 10 min

Individually your legs work. Joined up they often do not, and there are three usual reasons.

1 · The handover position

Leg 2 ends wherever the follower happened to stop. Leg 3 assumes the robot is somewhere specific. Every join needs an agreed handover state: where the robot is, which way it faces, and what the sensor should be reading. Write it between each pair of legs.

2 · Momentum

A robot at 40% speed does not stop where stop moving is called — it slides a little further. Add a short wait (0.2) seconds after a stop and before a measurement, so the robot has actually finished moving before the next leg assumes it has.

3 · The return trip inherits everything

All the error from legs 1 to 4 is still there when leg 5 starts. Square on a line at the start of the return and the robot comes home from a known position instead of a hopeful one.

Build it — the collector 12 min

The attachment mounting points from lesson 2 are what today is for. Build the simplest thing that traps a module — a passive fork, no motor.

1. Two beams forming a fork, wider than the module, narrowing so it cannot roll back out.
2. Low enough to slide under or around the module without riding over it. Check the height against the real module, not by eye.
3. It must not block the Colour Sensor. Put the robot on the line and confirm the live readout still sweeps 0 to 100.
4. It must still fit the starting area. Check with the paper rectangle from lesson 2.
5. Release is by reversing — the fork slides off the module and leaves it behind. No motor needed.

The program 12 min

when program starts :: events hat
set movement motors to [B v] and [C v] :: movement
clear display :: display
write [LEG 1 - LEAVE HOME] at line (1) :: display
move [forward v] for (1.5) [rotations v] at (30) % speed :: movement

write [LEG 2 - FOLLOW] at line (1) :: display
reset timer :: sensors
repeat until <(timer :: sensors) > (8)>
  if <([3 v] reflected light intensity :: sensors) < (50)> then
    start moving [right: 50] at (30) % speed :: movement
  else
    start moving [left: 50] at (30) % speed :: movement
  end
end
stop moving :: movement
wait (0.2) seconds

write [LEG 4 - COLLECT] at line (1) :: display
move [forward v] for (0.5) [rotations v] at (20) % speed :: movement
wait (0.5) seconds

write [LEG 5 - RETURN] at line (1) :: display
move [backward v] for (2) [rotations v] at (30) % speed :: movement
turn [right v] for (1) [rotations v] at (25) % speed :: movement
stop moving :: movement
play sound [Communication / Goodbye v] until done :: sound

The turn and reverse numbers above are a starting point, not an answer. Yours come from your own distance-per-rotation measurement in lesson 3.

Tune it — one failure at a time 9 min

Run it. It will fail. Work the failure properly instead of changing numbers hopefully:

1. Read the screen to see which leg was running.
2. Write down what you saw, physically — “it stopped 4 cm short”, not “it did not work”.
3. Change one thing, and predict out loud what should change.
4. Run again and check the prediction, not just the outcome.

A prediction that comes true means you understand the robot. A fix that works for a reason you cannot state will come back in Block 4.

Run it — three in a row 8 min

Place the robot at HOME, press play, and do not touch it. Reset the module by hand between runs; the robot goes back to the start but is not straightened, adjusted or nudged.

• Run 1 — time, and did the module end fully inside the yard?
• Run 2 — same.
• Run 3 — same.

If run 2 or 3 fails where run 1 succeeded, something is not repeatable — usually the starting position, or a battery that has dropped since you tuned. Both are real competition problems and both belong in the journal.

The journal 2 min

• A photograph of the collector attachment.
• The six legs, with the handover state written between each pair.
• Your failure log: what failed, what you changed, whether your prediction held.
• Three run times, and whether the module scored each time.