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Operational Principle and Technology of an Ultrasonic Sensor

Ultrasonic sensors measure distances in a contactless manner by using ultrasonic wave reflection. They detect transparent, dark, glossy or complex objects as well as liquids. They can detect, position, test for presence and perform distance measurements regardless of contamination such as dust, mist, vapor or ambient light.

What Are the Operational Principles of Ultrasonic Sensors?

Reflex Principle with an Ultrasonic Sensor

Distance sensors with the reflex principle are suitable for distance measurement, which includes detecting, distinguishing and measuring objects by means of ultrasound. In this case, the emitter and receiver are combined in the same housing.

Through-Beam Principle with Two Ultrasonic Through-Beam Sensors

With two ultrasonic sensors

In through-beam mode, two ultrasonic sensors are mounted facing each other. As a result, the emitter and receiver are directly across from each other and the signal sent by the emitter is tested to determine whether it has been detected by the receiver. With wenglor ultrasonic sensors, it is possible to set by means of parametrization whether this acts as an emitter or a receiver. In principle, it cannot measure distances, but rather detect or distinguish between objects.


Fork Sensors for Label Detection

Ultrasonic fork sensors are special sensors that work according to the through-beam principle. They detect labels on any label material regardless of color, transparency or surface finish. The emitter and receiver are mounted directly across from each other, but are located in the same housing.

How Does an Ultrasonic Sensor Work?

Functionality and Structure of an Ultrasonic Distance Sensor

Detect and Measure with One Sensor

An ultrasonic distance sensor detects objects in a contactless manner and measures the distance between the sensor and the measured object. To do this, it cyclically emits a short, high-frequency sound wave at the sensor head. This travels through the air at the speed of sound. If the sonic pulse encounters an object, this object reflects it back to the ultrasonic sensor. The ultrasonic probe then internally calculates the distance to the target object by measuring the time between the emission and reception of the sonic pulse.

Various Switch Outputs

Two independent digital switching outputs can be used to detect two positions (positioning sensor) or filling levels (fill level sensor). The actual distance/measured value can be output via an analog output  – either as current (4…20 mA) or voltage (0…10 V). This same value can also be output via the IO-Link. The switching outputs can be configured as NPN (low side), PNP (high side) or push-pull.

How Does an Ultrasonic Sensor Measure the Distance to the Object?

Time is used to determine the distance between the sensor and the object. The distance is calculated with the following equation:

Distance L = ½ × T × C 

In this case, the distance L, the time it takes for the ultrasonic wave T to be emitted and received and the speed of sound C are used to form the equation.

What Operating Modes Does an Ultrasonic Sensor Have?

What Is a Through-Beam Mode?

In through-beam mode (also common mode or opposing installation), two ultrasonic sensors acting as emitter and receiver are either mounted directly opposite each other or are arranged at an angle. In this mode of operation, the sensors achieve a larger working range and a higher switching frequency.

Example from the field: Detecting a slide

What Is Synchronous Operation?

Ultrasonic sensors in synchronous operation emit their ultrasonic pulses simultaneously (synchronously). This makes it possible to detect one or more objects over a larger area. In synchronous operation, up to 40 sensors can run at the same time in one application.

Example from the field: Detecting a long wooden board with no temporal offset (synchronous)

What Is Multiplex Mode?

In the multiplex mode, ultrasonic sensors emit their pulses alternately. This mode of operation prevents sensors located in direct proximity to each other from interfering with one another. In multiplex mode, up to 16 sensors can be used in one application.

Practical example: a large area of a liquid in a container is being monitored with regard to its level.

What Is the Difference Between Ultrasonic Sensors, Distance Sensors, Reflex Sensors and Fork Sensors?

wenglor ultrasonic expert Dominik Jeßberger knows the difference.

“Reflex distance sensors are also called ultrasonic reflex sensors, ultrasonic proximity switches or ultrasonic distance sensors. Different terms are used depending on the industry. Ultimately, these products are suitable for measuring or controlling distance, checking levels, as well as counting or detecting objects. 

However, only ultrasonic fork sensors are suitable for detecting labels. This is due to their structural design: they have an extremely small fork width, a transmitter and receiver, and an elevated sound frequency.”


What Is a Sonic Cone?

The sonic cone determines the area within which objects can be reliably detected by ultrasonic sensors. The sonic cones on wenglor sensors can be adjusted to suit the application. The picture shows the adjustable sonic cone on the UMS123U035 distance sensor.

What Is an Aperture Angle?

Parameter α defines the aperture angle of the cone-shaped sonic cone emitted by the ultrasonic sensor.

The surface structure of the measured object has no influence on the measurement result. This means uneven bulk materials, punched sheets with holes or moving and bouncing objects can be detected. Thus, for uneven surfaces, the aperture angle is set particularly wide, while for narrow and small surfaces, it is set particularly narrow.

What Happens If the Sonic Cone Is Larger than the Object?

To allow the ultrasonic sensor to measure the time between the transmitted and received signal, the object to be detected must reflect enough sound. The smaller the target’s surface area, the smaller the amount of sound reflected. If the object is too small, not enough sound can be reflected, which means the sensor can no longer detect the target. Therefore, sensors with a narrow sonic cone should be used for measuring small objects. Thanks to the focused sound beam, a large portion of the sonic energy then directly hits the object. As a result, almost the entire sonic energy can be reproduced by the target and perceived by the sensor. In general, it is not a problem if the target is smaller than the sonic cone. For the switching point, the sensor focuses on the earliest object it can detect. 

For detecting very small objects, photoelectronic sensors with laser light are the more suitable option.

How Is the Sonic Cone Affected by Accessories?

The ultrasonic sensor’s sonic cone can be affected if accessories are placed in front of the sensor’s active area. A soundpipe (or sonic tracker) is used to guide and narrow the sonic cone so that precise measurement is possible through small openings. The filling process in the food and pharmaceutical industries, in particular, requires exact fill level measurements in vessels with narrow openings such as bottles, cannulas or vials. The sonic tracker enables you to easily expand the ultrasonic sensor in the 1K miniature design without changing the installation size (32 × 16 × 12 mm).

What Is Sound?

Sound describes mechanical vibrations that spread in a medium such as gas or liquids in the form of acoustic waves, which are called sound waves.

What Is Ultrasound?

Ultrasound is sound with a frequency between 20 kHz and 1 GHz (ultrasonic frequency). Ultrasound is above the human hearing threshold and is therefore inaudible. 40 kHz to 400 kHz is typically used in sensor technology.

What Is an Ultrasonic Frequency?

The frequency indicates the number of oscillations per second, which is measured in Hertz. The higher the frequency, the higher the achievable measurement resolution. The lower the frequency, the closer the possible range.

Can the Ultrasonic Cone Be Redirected?

The ultrasonic waves can be deflected by another object. This object must have a hard flat surface that relays the signal well. Care should be taken to ensure that only one deflection is used, as multiple deflections result in a significant decrease in sound wave range. To ensure that no large debris falls onto the active surface, a baffle plate (e.g. Z0024) can be used.

What Is an Oscillator in an Ultrasonic Sensor?

In an ultrasonic sensor, the sensor surface on which the sound signal is generated is referred to as an oscillator, active surface, ceramic converter or transducer. Since this surface vibrates, the sensor is resistant to contamination to a certain degree: Dirt and grime do not stick to the oscillator, but instead are blown away by the fine movements.

Advantages of Ultrasonic Sensors

Excellent Background Suppression

Since the distance is determined by ultrasound waves, the background in front of which the object is located is almost irrelevant.

Detection of Almost All Materials

Any material that reflects sound is detected. However, hard material reflects the impulse energy particularly well. Colors, shapes or transparency do not play a role. In addition to wood, plastic and metal, thin films or glass can also be detected.

Wide Range of Distances

wenglor ultrasonic sensors detect objects in the immediate vicinity (3 cm) as well as those up to nearly six meters away.

Resistant to Interfering Influences

Dirt, mist and dust have almost no effect on the sensor’s function.

What Objects Do Ultrasonic Sensors Detect?

Ultrasonic distance sensors measure distances precisely, regardless of material, surface, color or transparency.

Detect almost any object

Ultrasonic waves are reflected by semi or fully transparent objects, such as glass or liquids. Even grainy, powdery and shiny objects can be reliably detected.

Dust, mist and dirt resistant

Dirt, dust, smoke or mist are irrelevant when detecting objects using ultrasound.

Detect complex forms

Ultrasound detection is a reliable presence check for complex object shapes such as grids or springs.

Which Objects Cannot Be Optimally Detected by Ultrasonic Sensors?

  • Soft material like cotton, fabrics, foam rubber or felt absorb the sound or diffusely reflect the sound. As a result, an ultrasonic sensor passes through the soft material and the hard surface (e.g. the table behind it) is detected instead.
  • Objects with an extremely high temperature result in the echo only reaching the sensor head diffusely or not at all.
  • Environmental factors such as air turbulence can have an impact on echo quality and therefore also the measurements. The ambient temperature influence is canceled by temperature compensation.

Sectors and Industries Which Use Ultrasonic Sensors

In the beverage industry, objects such as bottles, cans and containers must be reliably detected. Ultrasonic sensors are ideal for detecting objects made of glass, aluminum or PET regardless of their shape, color, position, surface or size. A contamination-resistant ultrasonic distance sensor with a wide sonic cone is installed in deposit return machines, which also reliably detects moving and bouncing objects. The background is completely hidden from view. 
Ultrasonic sensors also function reliably when used for level detection and control in filling systems. 
The demands placed on today’s machinery manufacturing industry are manifold. For example, stand-alone machines or automated manufacturing cells must reliably and quickly meet the high production demands and different application requirements. Ultrasonic sensors ensure that any surfaces, materials, shapes and colors are detected based on the operational principle. Ultrasonic sensors are true all-rounders: They are ideally suited for detecting transparent, shiny and dark objects, reflective surfaces and materials of all kinds – whether solid or liquid, rough or smooth, porous or translucent. The surface structure has no influence on the measurement result. Ambient conditions such as dust, steam, contamination or the presence of ambient light do not interfere with ultrasonic sensors.
The challenges faced in the electronics industry are flexible production lines and little downtime. Therefore, modern ultrasonic sensors must work quickly and reliably.
An ultrasonic sensor with a large sound cone detects the laminated, perforated and punched bellows regardless of color, transparency and surface. 
In the automotive industry, ultrasonic sensors help reliably detect dark, transparent or reflective objects. Thanks to the operational principle and the sonic cone, they can detect objects with a wide variety of shapes and reflective properties. They work very efficiently and reliably due to their resistance to dust and contamination. This is how transparent glass sheets are reliably transported in automotive production. Similarly, object detection of irregular, small or black plastic parts is a challenge easily accomplished by the ultrasonic sensors. 
Ultrasonic sensors are very well suited for use in harsh industrial environments such as those found in the printing industry. They work very efficiently and reliably due to their resistance to dust and contamination. 
Ultrasonic fork sensors detect labels on any label material regardless of color, transparency or surface finish.
Thanks to their operational principle, they are ideal for reliable and contactless label detection. 
Glass bottles have different shapes, color and degree of transparency. Ultrasonic sensors evaluate the sound reflected off the object. As a result, they detect almost any object regardless of the material and composition. Ultrasonic sensors are therefore particularly suitable for flexible use in a wide variety of applications as well as with reflective, shiny, dark or transparent glasses and bottles. 

Possible Uses for Ultrasonic Sensors

Presence check

Film tear monitoring

Fill level monitoring

Sag detection

Robot positioning

Stacking height monitoring

Label detection

End position control

Through-beam sensor


What Should Be Taken into Account when Installing Ultrasonic Sensors?

General Use

  • When installing ultrasonic reflex sensors, avoid heavy dirt deposits on the active surface (transducer).
  • The active surface (oscillator) of the sensor must remain free.
  • The product must be protected against mechanical impact.
  • Make sure that the sensor is mounted in a mechanically secure fashion.

This image illustrates the optimal installation of an ultrasonic sensor. For very hard and smooth objects, the angle between the sound axis and the object surface should be within 90° ± 3°. The angle can be larger for most object surfaces.

Effects of External Influences

Air flows such as wind, drafts and compressed air can influence ultrasonic sensor measurements under certain conditions. However, for modern ultrasonic sensors, these special impairments are minor in conventional industrial environments. ​​​​​

Can You Hear Ultrasonic Sensors?

Although ultrasound is not audible to the human ear, ultrasonic sensors do generate low-frequency noise during operation by emitting sound packets. With modern ultrasonic sensors, the oscillator’s vibration is almost completely inaudible.

What Is the Difference Between Ultrasonic Sensors and Optical Sensors?

Object Detection

Ultrasonic sensors use sound waves for detection, while optical sensors typically use infrared light, red light, blue light or laser light. One key difference is the size of the query area. The specific application determines which sensor is ideal for the task.

Capture Speed

Since the speed of light is greater than the speed of sound, an optical sensor measures faster than an ultrasonic sensor.

Concrete Example: Perforated Plate Detection with Photoelectric and Ultrasonic Sensor

When it comes to detecting panels such as perforated plates, mesh boxes or printed circuit boards (PCB), photoelectronic sensors behave differently than ultrasonic sensors. Since a photoelectronic sensor measures with a precise point of light, it switches at each hole for this use. On the other hand, the ultrasonic sensor’s sonic cone covers a large area, which results in the continuous detection of the product rather than the holes for this use.
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