Gyro Move Straight

The mission 4 min

Drive a long open crossing and arrive facing the way you set off — even if somebody knocks the robot on the way.

“Drive straight” has always been a lie. Both motors get the same power, one wheel grips slightly better, and the robot curves. Over a short leg you never notice. Over a crossing from the reefs to the harbour dock, you miss the dock.

The crossing 5 min

Use the run from the reef square back to HOME at the harbour dock — the longest open stretch on the mat, and the one your route has to make every single round.

There is nothing to follow out there. No line, no wall to square against, nothing for the Colour Sensor to lock onto. Everything you built in Block 2 assumed a line existed; this leg is where that assumption runs out.

The lighthouse island at E sits between the reefs and the dock. If your route from lesson 17 includes it, this is the leg it interrupts — which makes holding a heading across it even more valuable, because the robot resumes on the same bearing after the stop.

The technique — a heading is just another error 12 min

Set the target heading to zero at the start of the leg. Then, over and over, all the way down the crossing:

error = current heading − target heading

Straight ahead, the error is zero. Drifted left, the error has one sign; drifted right, the other. Multiply by a gain and use the result as steering, and the robot corrects itself continuously — hard when it is badly off, gently when it is nearly right.

Compare that with lesson 12, where the error was the light reading minus 50. The shape is identical. Only the measurement changed. That is worth dwelling on, because the same three lines of thinking now solve three completely different-looking problems.

One consequence worth naming. Because the correction is against a heading and not a position, the robot will end up facing the right way but possibly a little to one side of where you wanted. Heading hold fixes rotation, not sideways drift. Squaring on a line, from lesson 6, is still the tool for fixing position — and that is exactly the trade-off lesson 22 makes you choose between.

Build it — nothing new, two checks 10 min

No building. Two checks, both of which invalidate the lesson if they fail.

The gyro is still rigid. You mounted it in lesson 19 and have driven with it since. Push it gently: if it moves relative to the frame at all, re-brace it now. Heading hold amplifies a loose gyro into a robot that weaves.

The attachment is loaded. Run today’s tests with the sorter carrying items, because a loaded robot pulls differently from an empty one. A gain tuned empty will be wrong on the leg that matters.

Program it — Move Straight 12 min

Build a new My Block called Move Straight, taking a distance the same way Move For Distance did in lesson 14 — and in the same units, which you settled then.

Inside, a loop that runs until the distance is covered, and each time round:

• read the gyro angle
• subtract the target heading to get the error
• multiply by the gain to get the steering
• drive with that steering at the leg speed

Zero the gyro at the start of the block, not at the start of the program — every leg gets its own fresh heading, so the errors of the last leg are not inherited by the next one.

Leave Turn By Degrees alone. It was rebuilt last lesson and works; changing two blocks in one hour means you will not know which one caused a problem.

Tune it — find the gain, again 9 min

Same procedure as lesson 12, on a different measurement. Run the full crossing and watch the shape of the path.

• Too small — the robot corrects lazily and arrives off to one side, having drifted most of the way.
• Too big — it weaves down the crossing, which should look familiar. It is the same over-correction that made bang-bang wobble in Block 1.
• About right — the robot tracks a visibly straight line and any disturbance fades out rather than growing.

Record the gain you keep. Note whether it differs from your line follower’s — it usually does, because the two errors are measured in different units, and expecting them to match is a common source of confusion.

Run it — the knock test 6 min

Run the crossing from the reefs to HOME. Then run it again, and nudge the robot sideways mid-leg with a finger.

Watch what happens. A blind robot keeps the new wrong heading and arrives somewhere else entirely. A heading-holding robot swings back and carries on to the dock. That difference is the whole lesson, and it is much more convincing seen than described.

Do it once more with the attachment loaded, because that is the condition the round will actually run in.

The journal 4 min

Today’s entry:

• the gains you tried on the crossing, and what each did to the path
• the gain you kept, and how it compares with your line follower’s gain from lesson 12
• what happened in the knock test, with and without heading hold
• one sentence on the difference between fixing your heading and fixing your position — lesson 22 turns that into a decision, leg by leg