Which One, Not How Much: The Sorter

The Colour Sensor

The Colour Sensor looks down at a surface and can answer three quite different questions: what colour is this?, how bright is this? and how light is the room? Choosing the wrong one is the usual reason a line-following robot refuses to work.

front

side

Blocks reference

([3 v] color :: sensors)

([3 v] reflected light intensity :: sensors)

<[3 v] is color [red v]? :: sensors>

([3 v] ambient light intensity :: sensors)

Which colour, exactly?

The sensor does not describe a colour — it picks one from a list of eight, and that list is the whole of what it can ever say:

Anything you put under it is forced into one of those eight. There is no orange and no purple: an orange brick comes back as red or as yellow, and often as red one moment and yellow the next as the robot creeps along. Light blue and grey are the other classic pair to avoid — grey is neither black nor white, so it flips between them.

This is why colour mode is a good fit for a task you control and a bad fit for one you do not. Sorting the LEGO bricks that come in the set works, because they are made in exactly these colours. Reading a printed sheet, a coloured tile from another set, or anything pastel is asking the sensor to answer a question it does not have a word for.

Two practical points follow from how it decides. It shines its own lamp and looks at how much red, green and blue comes back, so it must be close — about half a centimetre, and no more than a centimetre. Lift it and the answer decays to 0. And because it takes those three readings before it can answer, colour mode is the slowest thing this sensor does; a robot driving quickly can pass right over a small patch without ever reporting it.

When a colour must be recognised reliably, test it. Drive the robot slowly over the real surface with color shown on the screen and watch what it actually says — including what it says at the edges between two colours, which is where the wrong answers live.

Colour, or brightness?

Both questions are asked of the same surface at the same moment. Watch the two answers travel across a strip of colours and then over the edge of a black line.

Over the patches, both read-outs are useful. Over the edge of the line they part company: the colour name has only two answers to give and jumps between them, while the number slides smoothly from 88 down to 8. Every value in that slide tells you how far onto the line the sensor is — which is information the name simply does not carry.

• Use colour when the answer really is a name — sorting red bricks from blue ones, stopping on a green square.
• Use reflected light when the answer is a matter of degree — following the edge of a black line, where the useful readings are all the greys between black and white.

A line follower built on colour names only knows “black” or “not black”, so it can only lurch. Built on reflected light it can tell how far onto the line it has drifted, which is what makes smooth following possible.

The third mode: ambient light

The first two modes both switch the sensor’s own lamp on and measure what bounces back off the surface. Ambient light intensity does the opposite: the lamp goes off, and the sensor simply reports how much light is arriving from wherever — 0 in the dark, up to 100 in bright light.

That makes it the only one of the three that is not really about the floor. A number between 0 and 100 means very little on its own, so watch the same sensor sit through five different rooms — nothing underneath it changes at any point.

Those are the shape of the scale rather than exact figures, but the shape is the useful part: a lit room is nowhere near 100, and the top of the range is reserved for a light pointed straight at the sensor. Cover it with your hand and the number drops to near zero — which is the easiest way to check the sensor is doing what you think.

Point it at the ceiling and it tells you whether the room lights are on; point it forwards and a torch will spike the reading, which is a way of signalling to a robot without touching it.

Do not reach for it as a substitute for reflected light. Room light falling on a black line and on white paper is almost the same, so ambient mode can barely tell them apart — the reason reflected light works is precisely that the sensor brings its own light and measures how much of it survives.

It is also the mode most at the mercy of the room. A reading taken by a window in the morning will not match the same spot in the afternoon, so anything built on ambient light needs measuring on the day, in the place, with the lights as they will be.

Light and height matter

Even the two lamp-on modes are affected by room lighting — a reading taken by a sunny window differs from one taken in a corner. The sensor must also sit close to the surface and at a constant height, because lifting it changes the reading even though the surface has not changed.

Making a decision

Up to now a robot has been able to wait for a sensor. Deciding is different: the robot checks the sensor and does one thing or another depending on the answer — and then carries on either way.

Blocks reference

if <> then
end

if <> then
else
end

Deciding again and again

A decision made once, at the start, is almost never what you want. Here are two robots with the identical if-else, testing the identical sensor against the identical number — one inside a loop and one not.

Nothing is wrong with the right-hand program’s decision. It asked the question, got a truthful answer, and acted on it correctly. It simply never asked again, and the world moved on. Decisions belong inside a loop, so the robot keeps re-deciding as things change.

when program starts :: events hat
forever
  if <([4 v] distance in [cm v] :: sensors) < (15)> then
    stop moving :: movement
  else
    start moving [straight: 0] :: movement
  end
end

Four shapes, and how to choose

Once there is more than one question, decisions can be arranged in four ways. They look nearly identical stacked up in the editor, which is exactly why they get muddled — the thing that differs is not what the blocks say, it is which routes through them exist.

One question sorts them almost completely:

Separate ifs — independent questions

Each if is asked no matter what the others answered, so any number of them can fire on the same pass. That is the right shape when the conditions genuinely have nothing to do with each other.

forever
  if <[3 v] is ambient light intensity [< v] (20) %? :: sensors> then
    [A v] start motor [clockwise v] :: motors
  end
  if <[4 v] is distance [< v] (15) [cm v]? :: sensors> then
    [D v] start motor [clockwise v] :: motors
  end
end

if / else — one question, two answers

Exactly one branch runs, every time. Reach for this whenever the robot must do something either way — and in preference to two ifs testing opposite conditions, which is the same idea written twice and can drift apart.

Nested if — a follow-up question

Putting one if inside another means the inner question is only ever asked when the outer one is true. Use it when the second question is meaningless otherwise: there is no point asking which side an obstacle is on when there is no obstacle.

if <[4 v] is distance [< v] (15) [cm v]? :: sensors> then
  if <([2 v] angle :: sensors) < (0)> then
    start moving [right: 50] :: movement
  end
end

When not to nest. If you only want “both true” and nothing happens at the outer level, an and says it in one block and reads better:

if <<[4 v] is distance [< v] (15) [cm v]? :: sensors> and <([2 v] angle :: sensors) < (0)>> then
  start moving [right: 50] :: movement
end

Nesting earns its place when something happens at the outer level too, or when there is an else at each level and the two mean different things.

Chained if / else — one winner out of several

This is the shape for a list of cases where exactly one should win: colour bands, distance bands, speed ranges. EV3 Classroom has no else-if block, so you build a chain by putting the next if inside the else of the last one.

And here is why it matters, because this is the single commonest bug in this whole module. Three bands written as three separate ifs are each perfectly correct, and together they are wrong: a reading of 20 is under 30 and under 60 and under 90, so all three run and the last one to run is the one that sticks.

The rule to carry away: if the cases are meant to be exclusive, they must be made exclusive. Chaining does it by construction. Separate ifs only work if you are careful to write non-overlapping bands yourself — light < 30, 30 to 60, 60 and over — which is more to get right and easy to break later.

Why it matters

This is the point at which a machine stops following a script and starts responding. A thermostat, an automatic door, a robot vacuum — all of them are a decision inside a loop.

Variables

Why anybody needs one

Long before there were computers, people had exactly this problem. A shepherd counting sheep through a gate, a trader counting sacks of grain, a builder counting days — none of them can hold the number in their head while they get on with the work. So they scratched a mark on a wall, cut a notch in a stick, or wrote a number on a piece of paper. The number lived outside the person, in a place they had agreed on, and they could go back to it, read it, and change it.

Better still, once the number is written down somebody else can use it. Watch these two: one of them counts and writes, the other never sees a single animal and simply reads the wall.

Neither Abby nor Ben is holding the number — the wall is. And notice what never happens: Ben does not ask Abby. He does not need to, because the count is not in her head. It is in a place they both agreed on, which is what makes it useful to more than one of them.

That is all a variable is. The robot cannot hold a number in its head either, so you give it a wall of its own, write a name at the top so everyone knows which wall is which — score, count, degree_turn — and the program can read what is on it and write something new. One part of the program writes; another part reads. Exactly Abby and Ben.

The paper, and the two things you can do to it

Say we are counting rotations of a motor. Before we start we write 0 on the paper. Every time the motor completes a turn we cross out what is there and write one more: 0 becomes 1, then 2, then 3. That is change — it has to read the old number to work out the new one.

set is the other thing you can do, and it is completely different: rub the whole paper out and write the number you want. It does not care what was there. Press the buttons and watch what happens to the crossings-out.

• change leaves a trail — every value follows from the one before it. This is what counting is.
• set wipes the sheet. Use it to start a count, never to continue one.
• Press set score to 0 after counting up a few times and watch the whole history vanish. That is what happens to a count when a set block ends up in the wrong place — and it is the commonest variable bug there is.

Blocks reference

set [count v] to (0)

change [count v] by (1)

(count)

Set, or change?

set replaces; change adds. Counting things needs change. Starting a count needs set. Both programs below have both blocks — the only difference is whether the set block is inside the loop or above it.

The count on the right is not broken; it is being told to start again on every pass. Each time round the loop it is wiped back to zero and then changed by one, so the honest answer is always 1 — while the motor cheerfully turns four times. A counter stuck at 1 almost always means a set block that has slipped inside the loop.

Anything oval is a number you can pick up

EV3 Classroom tells you what a block does by its shape, and once you have noticed that, a whole set of questions answers itself:

• Oval — reports a number. The blue degrees counted, the timer, a distance, your own variable.
• Pointed — reports true or false. These go in an if or a wait until, not in a variable.
• Block-shaped — does something. These stack up; they do not fit inside anything.

So when a slot is oval, any oval fits it — and it does not matter in the least where that number came from. You can take the motor’s own A degrees counted and keep it in a variable you named degree_turn, then compare that with a number later. Pick an oval below and watch the same one drop into all three kinds of slot.

This is what makes a variable more than a counter. A sensor reading is true only at the instant you read it; copying it into a variable freezes it, so the robot can compare where it is now against where it was when something happened:

when program starts :: events hat
[A v] reset degrees counted :: motors
set [degree_turn v] to ([A v] degrees counted :: sensors)
start moving [right: 30] :: movement
wait until <(([A v] degrees counted :: sensors) - (degree_turn)) > (400)>
stop moving :: movement

Read the condition aloud: how far the motor has gone now, minus where it was when we started, is more than 400. Both are ovals, so both can go into a subtraction, and the subtraction is an oval too — which is why it can go into a comparison. Ovals nest inside ovals as deep as you need.

Reset at the start, every time

A variable keeps its value after the program ends. Run the program again without setting it back and the second run begins where the first left off — the count starts at 14, the robot thinks it has already done the job. Every variable a program changes must be set to its starting value at the top.

Why it matters

A variable is the difference between a machine that repeats a fixed routine and one that responds to how things have gone — counting parts, tracking a score, remembering where it started.