EV3 Robotics›Level 2 · Intermediate›Lesson 17
Level 2 · Lesson 17 · EV3-L02-1760 minutes · Ages 9–16 · Model: Lawnmower
The Lawnmower: a two-motor driving base with a Medium Motor spinning a cutter underneath. It drives, it cuts, and — for the first time in this level — it has to get round a corner.
A mower cannot just drive in straight lines. A lawn is an area, and covering an area means turning at the end of every run.
And not every turn is the same turn. A mower that swings round in a wide arc leaves an uncut crescent at the end of each strip. One that spins on the spot with the blade running scrubs a bare patch into the grass. Today you learn the difference, and how to pick.
Robot mowers work football pitches and gardens now, quietly, for hours. Watch one long enough and the pattern gives it away: long straight runs, and a deliberate manoeuvre at each end.
A mower is judged on coverage — whether every blade of grass got cut — and coverage is decided almost entirely at the ends of the runs. The straight bit is easy. The turn is where you either overlap neatly with the last strip or leave a stripe of long grass behind.
Which is why a groundsman mowing a pitch by hand does a three-point turn at the end of each strip rather than a sweeping arc. It takes longer and it puts the machine exactly where the next strip starts.
Turn the wrong way for the job and you get one of two failures, both visible from across the garden: uncut crescents at every end, or a worn circle of mud where the machine pivots with its blade down.
The straight lines are not the hard part. Everything that goes wrong happens at the corners.
The block is the one you met on the Tricycle: two numbers, a left speed and a right speed. What is new is knowing where the robot turns about for each pair, because that is what decides whether the manoeuvre is any use.
move for (1) [rotations v] at (40) (40) % speed :: movement
| Manoeuvre | Left / right | It turns about… | Use it when |
|---|---|---|---|
| Curved turn | 40 / 20 | A point outside the robot, off to the slow side. Both wheels move forward; the robot travels while it turns. | You want to change direction without stopping — smooth, and it covers ground. |
| Point turn | 40 / 0 | The stopped wheel. That wheel stays exactly where it is and the robot swings round it. | You need one particular corner of the machine to stay put. |
| Spin | 40 / −40 | The middle of the axle, between the two wheels. The robot rotates and goes nowhere. | You want a new heading from the same spot. The mower’s end-of-strip turn. |
Read the third column rather than the second. The numbers are just how you ask; the pivot point is what you actually get, and it is the thing you should be choosing between.
Drag the robot, or use the arrow keys — it moves in half rotations. Rotations above the line, centimetres below.
1.5 — a half rotation past 1. Half a turn of the wheels is 8.75 cm, so halves matter.
35 ÷ 17.5 = 2. That is a number you can type straight into the block.
Set the two numbers in the lab and watch the trail. A curve, a swing about one wheel, and a rotation that leaves the robot where it started — three different shapes out of one block.
The block still ends on rotations, and those rotations count the faster wheel. So a spin of half a rotation is not half a circle — it is however much of a circle your robot happens to sweep in half a wheel-turn, which depends on how far apart its wheels are.
Nobody can tell you the number for a 90° turn. It depends on your wheel size and your track width, and it is different for every build in the room. You are going to find yours by testing, and then it will be right for every program you write on this model.
Choose the pivot first, the numbers second.
A driving base has two Large Motors, one per wheel. You could drive them with two separate motor blocks — but they would never start at quite the same instant, and the robot would curve away. EV3 has a separate family of Movement blocks that treat the pair as a single driving base.
port Bport C| Block | What it does |
|---|---|
set movement motors to [B v] and [C v] :: movement | Tells the Movement blocks which two ports are the driving wheels. Put it at the top of the program, before any movement. |
move [forward v] for (2) [rotations v] :: movement | Drives both wheels together, so the robot travels in a straight line. |
move for (1) [rotations v] at (75) (25) % speed :: movement | Drives the two wheels at different speeds, which makes the robot curve. |
set movement speed to (50) % :: movement | Sets how fast the driving base travels from now on. |
set movement motors to [B] and [C] is the block that decides which two motors every Movement block after it will drive. It does not look at the robot. It drives the two ports you name, whether or not there is a motor in them — so those two letters have to be the two sockets the cables are actually in.
This is the single commonest reason a driving base does nothing, and it is invisible in the program: every block is spelled correctly and the robot still will not go.
the same program on both robots
The yellow outline marks the two ports the block is driving. A robot only goes straight when both of them have a motor in them.
Both robots run the identical program. The left one has its motors in B and C, so both named ports have a motor and the robot drives straight. The right one is plugged into A and B: the block drives B, finds nothing in C, and never mentions the motor in A at all — so one wheel turns, one wheel sits there, and the robot swings round the dead one instead of driving.
If neither named port has a motor in it — cables in A and D, say, with the block still set to B and C — the robot does not move at all. The program runs happily to the end and the Brick reports nothing wrong, because as far as it is concerned it did exactly what it was told.
So before you look for a bug in the program, look at the cables: read the port letters off the Brick, then make the block say those two. B and C are only the usual choice, not a rule — if your build has the driving motors in A and D, set the block to A and D and everything works.
Everything a driving base can do comes from those two speed numbers. Try each button below and watch the trail the robot leaves.
The path is worked out from the two wheel speeds in the block, not drawn by hand — so changing the preset changes the numbers and the shape together.
Notice there is no steering block anywhere in that program, and no steering part anywhere on the robot. The turn is made entirely by driving the two wheels different amounts.
when program starts :: events hat set movement motors to [B v] and [C v] :: movement move [forward v] for (2) [rotations v] :: movement move for (1) [rotations v] at (50) (-50) % speed :: movement
“Move forward 2 rotations” says nothing about the floor — it says how many times the wheels go round. How far the robot actually travels depends on how big the wheels are, and once round a standard EV3 driving wheel is about 17.5 cm.
Before doing any sums, get a feel for it. Drag the robot along and watch both scales at once — rotations on top, centimetres underneath, the same line. Land on the half marks: there is a number between 1 and 2, and it is 1.5.
Drag the robot, or use the arrow keys — it moves in half rotations. Rotations above the line, centimetres below.
1.5 — a half rotation past 1. Half a turn of the wheels is 8.75 cm, so halves matter.
35 ÷ 17.5 = 2. That is a number you can type straight into the block.
set movement motors to [B v] and [C v] :: movement set movement speed to (30) % :: movement move [forward v] for (2) [rotations v] :: movement
Steering at right 100 or left 100 drives the two wheels in opposite directions, so the base stops travelling and pivots where it stands. The angle it sweeps is a simple doubling of the rotations:
Half a rotation is the one worth remembering: a square corner, 90°.
| Rotations | The robot turns |
|---|---|
| 0.25 | 45° — half a corner |
| 0.5 | 90° — a square corner |
| 1 | 180° — turn round and face back |
| 2 | 360° — all the way round |
The one to memorise is 0.5 rotations = 90°. Everything else follows from doubling or halving it.
set movement speed to (20) % :: movement move [right: 100] for (0.5) [rotations v] :: movement
Turn slowly. The wheels on a driving base are big and the robot carries a lot of weight, so at full speed it keeps going after the motors stop and lands past the angle you asked for. Somewhere between 15% and 30% speed is where turns become repeatable. A turn that overshoots is almost always a turn taken too fast, not a wrong number.
These figures belong to your robot. The distance depends on the wheels and the angle depends on how far apart they are, so a wider base needs a different number for 90°. Test it, measure what actually happened, and adjust — the same discipline as the Gyro Sensor tolerance.
Every wheeled vehicle steers this way — a tank, a digger, an office chair with two driven castors. Cars use a steering rack instead, but a robot that turns by driving its wheels at different speeds needs no steering mechanism at all.
Say this back before moving on: “Opposite speeds spin about the middle.”
| Part | What it is doing here |
|---|---|
| EV3 Intelligent Brick | Keeps the drive pair matched. Where its weight sits decides how cleanly the mower spins — weight over the axle spins tidily, weight out at one end drags. |
| Large Motor ×2 — the drive | Ports B and C. Today they are deliberately allowed to disagree, and the size of the disagreement is the manoeuvre. |
| Medium Motor — the cutter | Spins the blade. Runs the whole time, at its own speed, using the non-waiting start from Lesson 7. |
| The track width (not electronic) | The distance between the two driving wheels. It is not a part you fitted, but it decides every turn this robot will ever make. |
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 (Large) | B | B is left, and B is the first of the two numbers in the block. Get this wrong and every turn mirrors. |
| Right drive (Large) | C | C is right, and the second number. |
| Cutter (Medium) | D | Off the movement pair entirely, so a Movement block can never drive the blade by accident. |
Check your own build now:
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.
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.
EV3 until somebody changes it.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. A tethered mower drags its cable through its own turns, and a cable pulling on one side is indistinguishable from a steering error you did not write.
One square lap of the lawn: four straight runs with a spin at each corner, cutter running throughout.
when program starts :: events hat set movement motors to [B v] and [C v] :: movement [D v] start motor at (85) % speed :: motors repeat (4) move for (4) [rotations v] at (40) (40) % speed :: movement move for (0.5) [rotations v] at (35) (-35) % speed :: movement end [D v] stop motor :: motors
What success looks like: the mower finishes back on its starting tape, facing the way it started. That is the test, and it is a strict one — four corners means any error in your turn gets multiplied by four, which is exactly what makes it easy to see.
Tune the corner, not the straight. If it finishes turned too far clockwise, your spin is too big — try 0.45. Too far the other way, try 0.55. Change one thing, run the whole square, look at where it ends. Three or four goes will get you within a few degrees.
Same starting tape every run, and watch where the machine ends up rather than watching it go round.
Step 5 is the honest one. A turn that is one degree out is invisible on one corner, obvious after eight, and the reason competition robots eventually stop counting rotations and start using a gyro — which is waiting for you later in this level.
A small error you cannot see is still there. Run it four more times and it will introduce itself.
Build the model before you read any further. Everything after this is about making it do something, and none of it will make much sense with nothing on the table in front of you.
Use the viewer's own controls to zoom and turn pages. Fullscreen makes it big enough to build from.
Check the finished build against the picture before you switch anything on. A motor mounted the wrong way round is far easier to spot now than it is to debug later, when it looks like a program fault.
Work through the challenges in order — each is harder than the last. The mission comes after all three, and it is meant to make you plan before you build.
Tune your 90 degree turn. Run the square lap and adjust only the corner number until the mower finishes on its starting tape, facing the way it began. Write the number on the board — it belongs to your machine and you will use it all level.
Mow a rectangle instead of a square. Two long sides and two short ones, corners still square. The turn number does not change; work out what does.
Cover an area, not a line. Mow three parallel strips that sit next to each other with no uncut grass between them and no strip mown twice. You will need a turn, a short shift sideways, and another turn at each end — draw the route before you build it.
Mow a lawn with something in the middle of it. Mark out an area about a metre square and put an obstacle somewhere inside it — a box, a book, a shoe. Your mower has to cut the whole lawn and never touch the obstacle. Plan the route on paper first, as a drawing with arrows and the obstacle marked. Decide at every corner which of the three manoeuvres you are using and why. A curved turn covers ground and needs room; a spin needs none but leaves the machine standing still. There is no single right answer, which is why you have to justify yours. Then run it, and mark on your drawing anywhere it missed or anywhere it hit. Two questions when you present it. Which corner gave you the most trouble, and did you fix it by changing the turn or by changing the route? And your mower does not know the obstacle is there — it only avoids it because you counted rotations correctly. Describe what you would add so it could find one it had not been told about.