Distance Sensor

An ultrasonic sensor that looks like a pair of eyes and measures the distance to objects ahead.

How it works, and range

It emits inaudible ultrasound and times the echo:

• Range 5-200 cm (official spec: 50-2000 mm, accuracy ±2 cm).
• Each "eye" has 4 LED segments you can light up from code (decoration / status display).
• When there's no target (too far, too close, sound-absorbing surface) it returns an invalid reading.

What it detects well

Target	Result
Flat hard surfaces (walls, model sides)	✅ accurate
Surfaces facing the sensor	✅ accurate
Angled surfaces (> ~30°)	⚠️ echo bounces away, may miss
Thin poles, grids, soft fabric	❌ unreliable

Word Blocks

(distance sensor [D] distance in cm)              ← round reporter
<distance sensor [D] distance < [10] cm>           ← condition
wait until <distance sensor [D] distance < [10] cm>

Example 1: approach and slow down

Full speed toward the model, slow near it, avoid knocking it over:

when program starts
set movement motors [A+E]
set movement speed [80] %
start moving [forward]
wait until <distance sensor [D] distance < [20] cm>
set movement speed [25] %
start moving [forward]
wait until <distance sensor [D] distance < [6] cm>
stop moving

Example 2: keep distance (follow)

forever
  if <distance sensor [D] distance < [10] cm> then
    start moving [backward]
  else
    if <distance sensor [D] distance > [15] cm> then
      start moving [forward]
    else
      stop moving

Python

import runloop, distance_sensor
from hub import port

async def main():
    d = distance_sensor.distance(port.D)   # millimeters; -1 when no target
    if d != -1 and d < 100:                # under 10 cm
        pass

runloop.run(main())

Python returns millimeters

Word Blocks show cm; Python's distance() returns mm and -1 when nothing is detected. Check for -1 first, otherwise "-1 < 100" is always true and the robot thinks it has arrived.

FLL uses

• Precise stops: more slip-proof than counting rotations, ideal for "stop 5 cm before the model".
• Crash guard: a parallel stack doing "distance < 5 cm → emergency stop" during long sprints.
• Localization: measure to a wall whose position you know, and you know where the robot is.

Common pitfalls

• The sound cone spreads: the floor up close or a neighboring model can be "seen". Mount the sensor a bit higher, not at mat level.
• Two robots' distance sensors can interfere (rare but real on back-to-back tables).
• Sampling has latency - leave braking distance at high speed.

Next sensor: Force Sensor.