Ultrasonic Sensors

Overview

Ultrasound is sound waves that are at a frequency above the audible range. This is usually frequencies above 18kHz. Also note that infrasound are sound waves with a frequency of 16Hz or below, which is below the audible range.

Terminology

Term Description
Acoustic Impedance The acoustic impedance of a material is a measure of it’s opposition to displacement by ultrasonic waves. The symbol for acoustic impedance is \(Z\).
Apodization When referring to ultrasound, apodization is usually referring to the technique of shaping the wave so that the intensity of the side lobes is reduced. Used in medical ultrasound machines.
Bandwidth The bandwidth of a ultrasonic transducer is the frequency range at which the transducer can operate within.
Bulk Waves Bulk waves are ultrasonic waves that do not depend on the surface or boundary of the medium it is travelling through. Two types of bulks waves can exist in an isotropic medium, longitudinal and shear waves.
Capacitance When talking about piezo-electric transducers, this refers to the capacitance of the transducer, which is typically around 180pF-2.2nF.
CMUT CMUT (or cMUT) is an initialism for “Capacitive Micromachined Ultrasonic Transducers“. They emit ultrasonic waves from a cavity in silicon as their capacitance changes. They work easily at high frequencies due to their small size.
Collapse Voltage Applies to CMUT transducers only. Also see Snapback Voltage.
Couplant A medium which is used to transfer ultrasonic energy from the transducer to the medium under investigation. Common couplants are water, oil or glycerine.
Directivity A measure of the interaction angle of a ultrasonic sensor with the medium under investigation. Normally defined as the angle range at which the signal is no less the -3dB than the peak. Normally 5-15°.
Fixed Capacitance Applies to CMUT transducers only.
Free Capacitance Applies to CMUT transducers only.
Heterodyne Laser Interferometer A laser-based tool that can be used to detect ultrasonic waves.
Input Voltage When talking about piezo-electric ultrasonic sensors, this is normally the maximum peak-to-peak voltage that can be applied across it’s two pins. Normally between 30-200Vp2p.
Kundt’s Tube Kundt’s tube is a long glass tube held in the horizontal direction with lycopodium powder sprinkled inside it. When ultrasonic waves pass through the tube the power forms heaps at the nodal points and is blown off the antinodal points. This works as long as the wavelength of the ultrasound is not too small.
LNA LNA is an initialism for “low noise amplifier“. LNAs are used on the front-end of ultra-sonic transducer circuits.
Longitudinal Waves Longitudinal waves (also called distortional, compression or primary waves) are waves whose particles motion is parallel to the direction of the wave propagation. This is opposed to shear waves.
Phased Array An array of ultrasonic transducers that is designed to produce a linear wavefront, as opposed to a singular transducer which produces a circular wavefront.
PRF PRF is an initialism for “pulse repetition frequency“. It is the number of ultrasound pulses per second.
Ringing The is the self-oscillation of a piezo-electric sensor after it has just been driven. The ringing limits the minimum distance that can be measured.
Sensitivity Used to describe an ultrasonic transducers electrical response to received ultrasonic energy, usually measure in dB. Common values are -75dB.
Shear Waves Shear waves (also called distortional, transverse or secondary waves) are waves whose particle motion is perpendicular to the direction of the wave propagation. This is opposed to longitudinal waves.
Snapback Voltage Applies to CMUT transducers only. Also see Collapse Voltage.
Sonotrode A sonotrode is a tool used in ultrasonic welding to apply the vibration energy to the material to be welded.
SPL SPL is an initialism for “Sound Pressure Level“. A common value for an ultrasonic transducer is 110dB.
Temporal Resolution A ability to accurately locate events at a particular instant in time. Related to Spatial Resolution.
TGA TGA is an initialism for “time gain amplifier“. They are used on the front-end of ultrasonic transducer circuits to compensate for the fact that returning ultrasonic signals are attenuated the further they travel.
TOF TOF is an initialism for “Time Of Flight“. It is when the time is measured between emitted and received waveforms, and distance information is determined from this, when knowing the speed and which the wave travels through the medium.
TOFD TOFD is an initialism for “Time Of Flight Diffraction“. It is used for the non-destructive testing of weld defects.

Basic Principle

Ultrasonic sensors work by emitting high-frequency sound waves (18-50MHz), and analysing the reflected waves  (including time of flight). Ultrasonic waves are longitudinal sound waves and require an elastic medium to travel through. They are normally made with a piezo-electric material that vibrates when a AC voltage is applied to it. Because piezo-electric materials convert energy in both ways (they also convert mechanical vibration into electricity), the same sensor can be used as the receiver, while some setups use separate transmitters and receivers. Smooth surfaces are hard to detect on inclined levels (e.g. only up to 10°). Rough/jagged surfaces are much easier to detect on inclined angles. The higher the ultrasound frequency, the better it will reflect of smaller objects (such as defects in materials).

Comparison Against Other Sensors

Ultrasound sensors are not effected by the translucence of the target object. For this reason they are better then photo-sensitive sensors are detecting transparent objects, or objects where there is high glare. They are also relatively cheap compared to other forms of distance sensors. Piezo-electric ultrasonic transducers are larger than most other photo-sensitive sensors.

Variations

Open and closed types. The following image shows a open type piezo-electric ultrasonic transducer commonly used by electronic hobbyists, robiticists, and the maker community, called the HC-SR04. These modules integrate easily with the Arduino microcontroller platform.

An Arduino compatible ultrasound module.
An Arduino compatible ultrasound module.

High-power types have a power rating between 40-150W. These types of ultrasonic transmitters are used for ultrasonic cleaning.

Uses

  • Ultrasound modules for tank level measurement. They normally can measure a range of liquids including water, black water, oil, petrol, diesel without any adjustment (liquid type agnostic). Range is typically from 100mm to 4m. Some have focusers made of materials such as glass-reinforced polypropylene to produce narrow beams.
  • Ultrasound cleaners are used to clean objects by vibrating the dirt/greese from them. They are commonly used to clean the resin of circuit boards after soldering.
  • High-end wind sensors (anemometers) use ultrasound to measure the wind speed and direction (or more concisely, it’s velocity).
    A 3D sonic anemometer. Image from http://www.thiesclima.com/ultrasonic_anemometer_3d_e.html.
    A 3D sonic anemometer. Image from http://www.thiesclima.com/ultrasonic_anemometer_3d_e.html.
  • Similar to anemometers, ultrasonic flowmeters can be used to determine the speed and direction of liquid within pipes.
  • Ultrasonic devices are used in the medical world for getting images of body parts (e.g. an unborn baby). Human tissue attenuates ultrasound by approximately 1dB/cm/MHz.
    The C60X medical ultrasound sensor by Sonosite. Image from http://www.sonosite.com/accessories/c60x-0.
    The C60X medical ultrasound sensor by Sonosite. Image from http://www.sonosite.com/accessories/c60x-0.
  • Parking sensors in vehicles commonly use ultrasound sensors for distance detection.
  • Manufacturing processes such as flaw detection, thickness gauging, weld inspection.
  • Ultrasonic welding is the process of using high-energy ultrasonic sound waves (20-40kHz) to soften or melt thermoplastics to join them together. Welds are very fast (typically less than a second to completion).
  • Gas leak detection (leaks usually produce a high-frequency sound).
  • Ultrasonic fuel vaporizers. They use ultrasound to vaporize fuel, usually before entering the combustion chamber on a vehicle.
  • Bat detectors. Ultrasonic bat detectors listen for the ultrasonic noises made by bats, and either graph them or convert them to the audible spectrum (heterodyning). The circuit below converts ultrasonic waves into the audible spectrum.

Common Outputs For Modules

  • 4-20mA current output
  • Voltage output (max voltage varies, usually 3.3-12V)
  • Relay outputs (both NC and NO)

Beam Pattern And Range

With a simple single transducer acting as both the receiver and transmitter, the further away the object you wish to detect, the slower your maximum sample frequency can be, as you have to wait longer for the ultrasonic wave to return.