Ultrasonic Linear Position Sensors and Switches Information

Ultrasonic Poition SensorUltrasonic linear position sensors and switches use ultrasonic waves in combination with light and frequency readings. Specifically, ultrasonic linear position sensors and switches use surface acoustic waves (SAW) and lamb waves that reflect off the target. Ultrasonic sound is a vibration at a frequency above the range of human hearing, usually greater than 20 kHz.

Unlike other types of position sensors, ultrasonic linear position sensors and switches are not affected by color or material type. Instead, ultrasonic linear position sensors and switches only need an object to pass within their operating zone to bounce a signal.

Ultrasonic linear position sensors transmit a short burst of ultrasonic sound toward a target. When the sound is reflected, it returns to the sensor as an echo. The distance between the ultrasonic linear position sensor and the target is calculated from the signal's return time and the propagation velocity of the measurement medium. These sensors require a minimum distance to provide a time delay so that the "echoes" can be interpreted.

An oscillating cycle may be formed, incorporating a feedback loop which includes an acoustical oath, electrical elements, and optical elements. The feedback loop of ultrasonic linear position sensors and switches produces an output frequency that is determined by a frequency measurement of the desired distances or angles.

When used for sensing functions, the ultrasonic method has advantages over conventional sensors:

  • Discrete distances to moving objects can be detected and measured.

Less affected by target materials and surfaces, and not affected by color. Solid-state units have virtually unlimited, maintenance free life. Can detect small objects over long operating distances.

  • Resistance to external disturbances such as vibration, infrared radiation, ambient noise, and EMI radiation.


Ceramic and electrostatic sensing elements are common technologies for ultrasonic linear position sensors and switches.

  • Ceramic sensors are bonded to a metal case or cone and use the Piezo effect to create and measure ultrasonic pulses. Also called piezoelectric sensors.

  • Electrostatic sensors consist of a fixed aluminum plate and a moveable plate coated with a thin gold layer. When a signal is applied to the two plates, the gold foil is attracted to the back plate which displaces air and creates an ultrasonic burst.


The sensors operate at different frequencies and have different radiation patterns. Important specifications for ultrasonic linear position sensors and switches are the distance to be measured, the output signal desired, range resolution, and the sample rate as well as the mounting configuration, environmental sealing, and electronic features. 

Measurement Specifications

  • Measurement range - the range over which the linear output is needed
  • Sample rate - how frequently the device senses. It is measured in hertz (Hz).
  • Range resolution - the smallest unit of distance measurement.
  • Accuracy - how accurate the measurement of the device is. Accuracy of ultrasonic linear position sensors and switches is measured as a percentage of the full scale.
  • Temperature range and compensation - the range of which the device can function properly. Some sensors include a temperature compensation sensor which means the temperature sensor inside the sensor head sense the temperature and compensates for the change of sound velocity.

Electrical output

  • Analog current or frequency - outputs for ultrasonic linear position sensors and switches are useful when transmitting a signal long distances. Feedback from an analog current is used to provide the appropriate current regardless of noise, impedance, etc. Analog frequency or modulated frequency outputs are a variation of the analog current output. In analog frequency outputs the signal is encoded via an amplitude modulation (AM), frequency modulation (FM), or additional modulations.
  • Serial or Parallel digital output.
  • Switch or alarm output - when a change of state is detected.


Ultrasonic linear position sensors and switches are useful in environments that cause other sensing technologies to fail, such as dust, particle-laden air, moisture or wetness, or particularly arid conditions. The medium of sound travel should be considered when selecting an ultrasonic sensor, the following table shows the speed of sound though various gases.

Speed of Sound for Various Gases


Speed, in./s at 10°C


Speed, in./s at 10°C











Illuminating Gas


Carbon Dioxide

10,152(low frequency)10,572(high frequency)



Carbon Disulfide




Carbon Monoxide


Nitric Oxide








Nitrous Oxide






Steam (100°C)


Chart credit: Sensorsmag.

When separated from their targets by a substantial distance, ultrasonic linear position sensors and switches are more accurate than other types of sensors.


Migatron Corp. - About Ultrasonics

Image credit:

Baumer Ltd.