Challenge 1
Run three pallet cycles and finish on your tape mark, with the fork's count back at zero.
What you are building 3 min
The Forklift with Pulley: a driving base with a mast, and a fork raised by a string wound onto a drum by a Medium Motor underneath the Brick.
Level 2’s Forklift back in Lesson 14 lifted with gears. This one lifts with rope — which is lighter, smoother, much better at height, and much worse at knowing where it is.
By the end of the lesson your forklift will run a complete pallet cycle three times over — and you will have measured how much it drifts, which is the question this whole level has been asking.
In the real world 5 min
Where you have seen it
A warehouse is built around repeatability. Pallets are one size. Racking beams sit at fixed heights. The gap a fork has to enter is a few centimetres, and a driver hits it hundreds of times a shift.
Why it is built that way
None of that standardisation is for the forklift’s benefit. It is so that the same movement works every time. A pallet is 1,200 mm across everywhere in the world, so the fork spacing never changes, so the approach never changes.
Automated warehouses take it further. A robot shuttle does not look at the racking; it goes to a height it was told, because the racking is guaranteed to be there. That only works if the machine is repeatable — the same command producing the same result on the thousandth run as on the first.
And they all home before a shift. Drive to a known stop, zero everything, and count from there — exactly what Lesson 32’s balancing act taught you.
What would go wrong without it
A machine that is accurate gets it right once. A machine that is repeatable gets it the same every time, and a repeatable machine that is slightly wrong can be corrected once and stays corrected. A machine that drifts cannot be corrected at all — by the time you have adjusted it, it has moved on.
Repeatable and wrong is a machine you can fix. Right on average is a machine you cannot.
The main concept — does it do the same thing twice? 6 min
Level 1 asked whether you could make it move. Level 2 has been asking something harder the whole way through: does it do the same thing every time? This is where you find out.
Everything you need is already in your hands
| To make it repeatable | You use |
|---|---|
| Start from the same place every run | homing — Lesson 32 |
| Stop where you meant to, not where you drifted to | hold position at stop — Lesson 2 |
| Travel a distance rather than run a motor | circumference maths — Lesson 42 |
| Approach gently instead of arriving fast | lower speeds — Lessons 37 and 43 |
| Know how bad it is | measure it, several times — every lesson since 30 |
The string is the new problem
A string winding onto a drum does not have a fixed ratio, and this is a real difference from Lesson 39. The first turn winds string onto the bare drum. The next winds it on top of the first — so the drum is now thicker, and the same rotation pulls in more string. The fork rises further per degree the higher it goes.
The scissor door had a ratio you could measure once. The wheel had a circumference you could calculate. The pulley has neither: it has a ratio that changes as it works.
What repeatability actually means
Run the same program five times from the same start and mark where it finishes each time. Two different things can go wrong:
- The five marks are spread out. The machine is not repeatable. No number you type will fix this — you need a sensor, or a better mechanism.
- The five marks are tight together, in the wrong place. The machine is repeatable and mis-set. This is good news — change one number and all five move together.
Measure the spread before you adjust anything. Adjusting a machine that is not repeatable just moves the middle of the mess.
Pulley systems
A pulley is a wheel with a rope over it. On its own that sounds like it could not possibly matter, and yet the right arrangement lets a small motor lift something it has no business lifting. The whole of it comes down to one question: how many pieces of rope are holding the load up?
Count the supporting ropes
Look at the load and count the rope segments running down to it. If the load hangs from two segments, each of them carries half the weight, so you only have to pull half as hard. Four segments, a quarter each.
effort = load ÷ number of supporting ropes
And the price, which is exactly as large as the saving:
rope you must pull = distance lifted × number of supporting ropes
2supporting segments1⁄2of the weight, on you×2rope pulled per cm lifted
pull half as hardpull 2× as far
A movable pulley rides on the rope and the load hangs from it. Now two segments are holding the weight, and they share it.
Count the blue rope segments running down to the load. That number divides the force and multiplies the rope you have to pull.
The three arrangements
| Type | Supporting ropes | What it gives you |
|---|---|---|
| Fixed — the wheel is bolted to something solid | 1 | No force saved at all. What it changes is the direction you pull in — down instead of up, or round a corner — which is very often the whole reason it is there. |
| Movable — the wheel travels with the load | 2 | Half the force, twice the rope. The pull direction is unchanged. |
| Combined — fixed and movable together (a block and tackle) | 2, 4, 6… | Both at once: the force divided by the count and a convenient direction to pull in. |
A fixed pulley being “no help” is worth dwelling on, because it is where the counting rule proves itself. One rope, one whole load — and it is still the pulley you reach for most often, because pulling downwards with your weight behind it beats pulling upwards every time.
Nothing is gained
Multiply the two formulas together and the gain cancels the cost exactly: half the force over twice the distance is the same amount of work. A pulley system does not create effort out of nothing — it lets you spend the same effort in a form you can actually manage, a little at a time instead of all at once.
That is the same bargain gears and levers make, in three different shapes. Once you have seen it in one of them, the other two stop being new.
Building them out of LEGO
- Use a proper pulley wheel or a smooth free-spinning wheel on an axle. A rope dragged over a fixed beam is a friction brake with extra steps.
- String stretches, and stretch eats your lift. Braided cord is far better than the thin white string for anything carrying weight.
- Every extra wheel adds friction. A four-rope block and tackle does not quite give you the full quarter — measure what you actually get rather than trusting the arithmetic.
- Wind the rope onto a spool neatly. A rope that piles up on itself changes the effective spool radius as it winds, so the lift speed drifts through the run.
Why it matters
Cranes, lifts, sailing boats, gym machines and window blinds all count ropes for a living. The counterweight on a lift is a pulley system; so is the tackle a single person uses to raise an engine out of a car.
Brake or coast
Telling a motor to stop does not say how it should stop. There are two very different behaviours, and which one you want depends entirely on what the mechanism is holding up.
- Brake — the motor actively holds its position. A lifted arm stays lifted; a robot stops dead where it is.
- Coast — the motor lets go. A lifted arm sags under its own weight; a moving robot rolls on a little before stopping.
Watch what happens after the stop
Both arms below are lifted by the same program and stopped at the top. The only difference is one word in the stop block. Everything worth seeing here happens after the movement has finished.
when program starts
A run clockwise for 1 rotations
A stop motor brake
the only change on the other arm
A stop motor coast
0%braked, held0%coasting
Brake: still where it was putCoast: back on the floor
Two identical arms, each with a weight on the end.
If a mechanism you just lifted sags the moment it stops, the stop action is the first thing to check — not the lifting movement.
The coasting arm does not slide down steadily — it starts slowly and picks up speed, because it is gravity pulling it and not a motor lowering it. That is also why a coasting robot rolls a little past where its measured movement ended.
Which to choose
If gravity is pulling on the mechanism, you almost always want brake — otherwise the thing you just lifted comes back down on its own. If you want a smooth, natural halt, or you want to be able to push the robot by hand afterwards, coast is friendlier.
Coasting also makes a robot travel slightly further than its measured movement, because it keeps rolling after the motor has finished. On a precise task that error matters.
Why it matters
A crane must brake — a coasting crane drops its load. A train coasts into a station rather than stopping dead, because passengers would be thrown forward. The same choice, made differently, for good reasons.
First find out whether it does the same thing twice. Only then find out whether that thing is right.
Brake or coastFrom Lesson 2 — hold position at stop, and why a coasting motor never lands twice in the same place.Show meHide
Brake or coast
Telling a motor to stop does not say how it should stop. There are two very different behaviours, and which one you want depends entirely on what the mechanism is holding up.
- Brake — the motor actively holds its position. A lifted arm stays lifted; a robot stops dead where it is.
- Coast — the motor lets go. A lifted arm sags under its own weight; a moving robot rolls on a little before stopping.
Watch what happens after the stop
Both arms below are lifted by the same program and stopped at the top. The only difference is one word in the stop block. Everything worth seeing here happens after the movement has finished.
when program starts
A run clockwise for 1 rotations
A stop motor brake
the only change on the other arm
A stop motor coast
0%braked, held0%coasting
Brake: still where it was putCoast: back on the floor
Two identical arms, each with a weight on the end.
If a mechanism you just lifted sags the moment it stops, the stop action is the first thing to check — not the lifting movement.
The coasting arm does not slide down steadily — it starts slowly and picks up speed, because it is gravity pulling it and not a motor lowering it. That is also why a coasting robot rolls a little past where its measured movement ended.
Which to choose
If gravity is pulling on the mechanism, you almost always want brake — otherwise the thing you just lifted comes back down on its own. If you want a smooth, natural halt, or you want to be able to push the robot by hand afterwards, coast is friendlier.
Coasting also makes a robot travel slightly further than its measured movement, because it keeps rolling after the motor has finished. On a precise task that error matters.
Why it matters
A crane must brake — a coasting crane drops its load. A train coasts into a station rather than stopping dead, because passengers would be thrown forward. The same choice, made differently, for good reasons.
Say this back before moving on: “Spread first, then aim.”
What’s in this build 4 min
Look at your forklift. Compare it with the Forklift you built in Lesson 14. What is doing the lifting now, and what has that changed?
| Part | What it is doing here |
|---|---|
| EV3 Intelligent Brick | The body and the counterweight. A real forklift’s heaviest part is behind the rear axle for exactly this reason. |
| Large Motor ×2 — the drive | A matched pair. Their wheel diameter is the number your distances come from. |
| Medium Motor — the hoist | Winds the string. It must hold position at stop, or a loaded fork descends every time the program pauses. |
| The string and drum (not electronic) | Where the repeatability goes. It stretches, it slips, and it winds on top of itself. |
| The mast and fork (not electronic) | Must slide freely. A sticky mast turns a smooth lift into a series of jerks, and jerks are not repeatable. |
Wind it properly — this is not fussiness
- Lower the fork fully and wind the string on in a single neat layer, side by side, no crossing over.
- Raise and lower it three times by hand and watch the string. If it piles up in one place, guide it — a pile is a thicker drum and a changing ratio.
- Check the fork slides the whole mast without catching.
- Measure the wheel diameter and work out the circumference, as in Lesson 42. You will need it in a moment.
Ports — and the rule 4 min
Sensors go in ports 1, 2, 3, 4. Motors go in ports A, B, C, D. They are not interchangeable, and nothing will tell you politely if you swap them.
| Part | Port | Why this one |
|---|---|---|
| Left drive wheel (Large) | B | The pair. Two motors, one job. |
| Right drive wheel (Large) | C | The other half of the pair. |
| Hoist (Medium) | A | Its own job, and the only motor with a load hanging off it. |
| Sensors | none | Nothing on this machine can see the fork’s height. That is the honest limit of Level 2, and the reason Level 3 opens where it does. |
Check your own build now:
- Wheels in B and C, hoist in A.
- Route the hoist cable so it cannot be wound onto the drum. It is right beside the string, and it will go round with it.
- Lower the fork fully before every run. The program homes there and counts from there.
- Mark the start position on the floor with tape, and mark which way the robot faces. You will be measuring against both.
Connect the Brick 4 min
Two routes, and either is fine. USB is the reliable one and the one to fall back on when a room’s Bluetooth is busy; Bluetooth leaves the robot free to move, which some models need.
How to connect the BrickUSB and Bluetooth, step by step, with a photograph of every screen. Open it if you have not done this before — or if pairing is not working.Show meHide
USB — the reliable one
- Switch the Brick on with the dark grey centre button.
- Cable into the Brick’s PC port — the small square socket beside the numbered ports, not one of the numbered ones.
- Other end into the computer.
Bluetooth — name it first
Do these in order. Naming the Brick after you go looking for it in the list is how groups end up driving each other’s robots.
- Name your Brick. On the Brick: Settings (the spanner) → Brick Name. Type something nobody else will pick, then press the tick. Every Brick is called
EV3until somebody changes it. - Turn Bluetooth on. Settings → Bluetooth. Tick Bluetooth and Visibility. Leave iPhone/iPad/iPod unticked.
- Connect from EV3 Classroom. Click the Brick icon at the top of the programming area, find your Brick by name, and click Connect.
- Say yes on the Brick. It asks “Connect?” with the computer’s name — choose the tick, then accept the passkey, which is already 1234.
EV3.The two failures, every class, every time. The Brick has gone to sleep while you were building — press the centre button to wake it. Or you have paired with the group at the next table, which is why the name matters.
The long version, including Port View and how to read the port tiles, is in the Brick & Bluetooth guide.
Bluetooth, and this time it is not optional. Today’s whole measurement is how much the robot drifts between runs. A trailing cable adds a different pull on every run, which is a source of drift you invented yourself.
Confirm the connection 2 min
- Check the Brick icon: connected, or not.
- Three motor tiles — A, B and C.
- Turn each drive wheel by hand and confirm B is left and C is right. Then wind the hoist and confirm A.
- Wind the fork from the bottom to the top by hand and read tile A. Do it three times, resetting at the bottom each time. Are the three numbers the same? If they are not, your string is slipping — and you have found today’s answer before writing a program.
Stuck? The long version, with a photograph of every screen, is in the Brick & Bluetooth guide.
Make it move 10 min
A pallet cycle, three times: home the fork, drive out, lift, drive back, lower.
when program starts :: events hat set movement motors to [B v] and [C v] :: movement [A v] set motor to [hold position v] at stop :: motors [A v] run [counterclockwise v] for (2) [rotations v] at (20) % speed :: motors [A v] reset degrees counted :: motors clear display :: display write [KITARAN] at line (1) :: display repeat (3) move [forward v] for (2.8) [rotations v] at (30) % speed :: movement [A v] run [clockwise v] for (400) [degrees v] at (30) % speed :: motors move [backward v] for (2.8) [rotations v] at (30) % speed :: movement [A v] run [counterclockwise v] for (400) [degrees v] at (30) % speed :: motors write ([A v] degrees counted) at line (3) :: display end play sound [Communication / Goodbye v] until done :: sound
- The homing run comes first, at 20% speed. It winds down past the bottom and lets the string go slack, so every run starts from the same place. Low speed, because it is deliberately running into a stop.
hold position at stopbefore anything moves. Lesson 2. Without it the fork sags a little at every pause, and three cycles of sag is a visible error.- Line 3 is the instrument. The lift is +400 and the lower is −400, so after each cycle the count should be back to 0. If it is not, the motor did not complete a movement — usually because it hit a stop.
- Everything is slow. 30% throughout. Speed is the enemy of repeatability, and today repeatability is the whole point.
What success looks like: the fork winds to the bottom, then the robot drives out, lifts, drives back and lowers — three times — and finishes on your tape mark facing the way it started, with line 3 reading 0.
If the fork ends up higher every cycle, the lower is hitting its stop and losing movement the encoder counted. Reduce the 400. If the robot walks forward across the three cycles, the forward and backward moves are not equal on the floor — which is exactly what you are about to measure.
Change it and test 8 min
This is the measurement Level 2 has been building towards. Rule up a table. Tape on the floor. Run, mark, measure, repeat.
- Measure the spread. Run the whole program five times from the same tape mark, and mark where the robot finishes each time. How far apart are your five marks? That number is your machine’s repeatability, and it is the most honest thing you will measure this level.
- Now decide what kind of problem you have. Five marks close together but off the tape? Adjust the 2.8 and all five will move. Five marks scattered? No number will help — that is slip, and it needs a sensor.
- Watch the string ratio change. Lift 400 degrees from the bottom and measure the fork height. Lift another 400 and measure again. The second 400 lifts further, because the drum is thicker. Lesson 39’s fixed ratio does not exist here.
- Take out
hold position at stopand run all five again. The spread gets worse. One block, and it is measurable. - Raise every speed to 80% and measure the spread again. Faster, and worse — and now you have the number to prove it rather than an opinion.
- Delete the homing run and start each of five runs with the fork at a slightly different height. The spread explodes. Homing is not tidiness; it is most of your repeatability.
A machine you can measure is a machine you can improve. Measure the spread before you touch a number.
Where Level 2 stops 3 min
Look at your program again — not at what it does, at what it looks like.
The four blocks inside repeat (3) are one idea: fetch a pallet. You wrote it once and asked for it three times, and that worked because all three cycles were identical.
Now try to make the second cycle go somewhere else. A different shelf, a different height. The repeat is useless immediately, and you have to write those four blocks out again, in full, with different numbers. Three shelves, twelve blocks. Six shelves, twenty-four — and if you ever change how a pallet is fetched, you change it in six places and miss one.
What you want is to give that sequence a name — fetch pallet — and then say it three times with different numbers. That is a My Block, and it does not exist in Level 2. It is the first thing Level 3 gives you, and this is the moment you can feel why you need it.
There is a second thing missing, and you met it in step 10. Nothing on this forklift can see the fork. The string’s ratio changes as it winds and the program cannot know, because it has no way to check the height against the world. The answer is calibration — measuring the machine against something real and correcting for it as you go — and that is Level 3 too.
Level 1 ended stuck for a missing Switch and a missing variable. Level 2 ends stuck for a missing name and a missing measurement. Both times, feeling the gap is the lesson.
You can now build a machine that does the same thing every time. Level 3 is about machines that do the right thing when every time is different.