Image Credits: Banner Engineering Corp., Baumer Ltd., Carlton-Bates Company

 

Ultrasonic 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.

Ultrasonic Switch. Image Credit: Omron 

 

 

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.

 

How Ultrasonic Linear Position Sensors and Switches Work

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.

 

Video Credit: SparkFun

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.

Types

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

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

  • Electrostatic sensorsconsist 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.

Specifications

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.

Application

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

Gas

Speed, in./s at 10°C

Gas

Speed, in./s at 10°C

Air

13,044

Helium

38,184

Ammonia

16,332

Hydrogen

49,980

Argon

11,886

Illuminating Gas

19,308

Carbon Dioxide

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

Methane

17,004

Carbon Disulfide

7,272

Neon

17,124

Carbon Monoxide

13,272

Nitric Oxide

12,792

Chlorine

8,088

Nitrogen

13,152

Ethylene

12,360

Nitrous Oxide

10,308

 

 

Oxygen

12,492

 

 

Steam (100°C)

15,876

Chart Credit: Sensormag.

 

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

References


Choosing an Ultrasonic Sensor for Proximity or Distance Measurement Part 1: Acoustic Considerations
Measurement Principle / Effective Use of Ultrasonic Sensor
Ultrasonic Sensing/Control Basics

 


Related Products & Services

  • Capacitive Linear Position Sensors

    Capacitive linear position sensors are devices that sense position / displacement using capacitance technology.

  • Eddy Current Linear Encoders

    Eddy current linear encoders detect the distance from a target by using magnetic fields generated by a reference and sensing coils.

  • Linear Variable Differential Transformers (LVDT)

    Linear variable differential transformer (LVDT) linear position sensors have a series of inductors in a hollow cylindrical shaft and a solid cylindrical core. These devices produce an electrical output proportional to the position of the core

  • Optical Linear Encoders

    Optical linear encoders use fiber optic technology to sense position and displacement.

  • Optical Triangulation Position Sensors

    Optical triangulation position sensors use reflected waves to pinpoint position and displacement. The source of these waves may be a light emitting diode (LED), infrared (IR) light, or laser.

  • Photoelectric Sensors

    Photoelectric sensors use emitters and receivers to detect the presence, absence, or distance of target objects.

  • String Potentiometers

    String Potentiometers are used to measure the movement and displacement of objects. A cable or wire is attached to an object, and as the object moves, the transducer produces an electrical signal proportional to the wire's linear extension.