Ultrasonic sensor
An ultrasonic sensor measures distance by sending out a pulse of sound and timing the echo. It sees transparent, shiny, dark and liquid targets that trouble light-based sensors, so it is popular for tank levels and for detecting clear bottles.
How it works
A transducer emits a short burst of ultrasound (typically 40–400 kHz) and then listens. The time until the echo returns is proportional to the distance:
distance = (speed of sound × time) ÷ 2
Speed of sound in air is about 343 m/s at 20 °C, and changes with temperature, so good sensors have built-in temperature compensation.
Blind zone and beam angle
- Blind zone: right in front of the sensor it cannot measure, because the transducer is still ringing from its own pulse. Targets must stay further away than this minimum distance.
- Beam angle: the sound spreads in a cone. Keep tank walls, ladders and fittings out of the cone or they will echo.
Outputs
| Output | Use |
|---|---|
| Switching (PNP/NPN) | "object closer than X", high/low level alarms |
| Analog 4–20 mA | continuous level or distance into a PLC analog input; 4 mA = near, 20 mA = far (or reversed). Below about 3.6 mA indicates a fault. |
| Analog 0–10 V | same, for short cable runs |
| IO-Link | distance value plus diagnostics over a digital link |
Good and bad targets
Good: liquids, glass, clear plastic, metal, dark objects. Difficult: foam, soft fabric and cotton (they absorb sound), steep angled surfaces (the echo misses the sensor), and heavy dust or steam.
Scaling an analog level in the PLC
If a 4–20 mA sensor is set to 0–2 m and the analog input reads 0–27648 (Siemens) for 4–20 mA, then level = raw ÷ 27648 × 2 m. Use the brand's scaling instruction (SCALE / NORM_X + SCALE_X on Siemens, SCP on Allen-Bradley).
See also level sensors and the capacitive sensor for point level.
Try it yourself. Wire sensors, contactors and a PLC, write ladder logic and press RUN in the free PLC Sim Lab Playground. Nothing to install.
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