Rotations and degrees
There are two ways to tell an EV3 motor how much to move: give it a length of time, or give it an amount of turn. Only the second one is repeatable, and that difference decides whether a machine works reliably or only sometimes.
Blocks reference
| Block | What it does |
|---|---|
[A v] run [clockwise v] for (1) [rotations v] :: motors | One full turn of the motor shaft, then stop. The program waits for it. |
[A v] run [clockwise v] for (180) [degrees v] :: motors | Half a turn. 360 degrees is one rotation. |
Count it as it turns
The counters below tick up in step with the shaft, so you can watch a rotation being counted rather than take it on trust.
the same movement, written in degrees
The two motor read-outs are the same measurement — 1 rotation is 360°. Use whichever makes the number easier to read.
Rotations or degrees?
Notice that the two motor counters finish together: 2.00 rotations and 720° are the same instant, because they are the same measurement in different units. So pick whichever makes the number easier to read. A winch that needs eight full turns is clearest in rotations. A gate that lifts a quarter turn is clearest in degrees — 90 rather than 0.25.
Gearing changes the answer
The third counter is the one that catches people out. The block counts turns of the motor, not of the thing it drives — and the gear in the demo is three times the size, so two motor rotations move it only 0.67 of a turn. Ask for two rotations and the mechanism does not move two rotations’ worth unless the gearing is one to one.
That is why a number that works on one build has to be retuned when the gearing changes — the program is right, the mechanism is different.
Why it matters
A printer feeds paper an exact distance; a lift stops level with the floor; a robot arm returns to the same place a thousand times. None of that is possible by timing a motor — they all count turns.
More motion tutorials
- One motor, on and off — Start one motor, let it run, and stop it again.
- The Medium Motor — The small, fast motor that drives arms, gates, jaws and winches.
- The Large Motor — The big, strong motor used for wheels and heavy lifting.
- Driving with two motors — Drive a robot in a straight line, and turn it, using two Large Motors.
- Speed and power — Make a motor go faster or slower.
- Brake or coast — Stop a motor dead, or let it roll to a halt.
- Moving to an exact angle — Send a motor to an exact position instead of a distance from here.