Ultrasonic transducers are the core components that enable ultrasonic measurement and detection. Through electroacoustic conversion, they establish the link between electrical signals and ultrasonic waves in a medium. According to their structure and operating principle, they can be divided into several types, such as piezoelectric, electromagnetic, and electrostatic. Among them, piezoelectric transducers dominate industrial and consumer applications due to their mature manufacturing processes, controllable cost, and wide applicability.
In measurement and control systems, an ultrasonic transducer probe is typically used as the front-end sensing unit to acquire physical information related to distance, liquid level, flow rate, wind speed, or presence/absence states. Combined with a driving circuit, signal processing, and upper-level control, it provides detection and feedback control functions. The transducer’s center frequency, bandwidth, beam angle, sensitivity, matching impedance, and mechanical design directly affect the system’s usable range, resolution, anti-interference capability, and long-term stability.
Compared with other detection technologies such as photoelectric, radar, inductive, and capacitive methods, ultrasonic measurement is insensitive to the surface color and transparency of the target, and it adapts well to powders, liquids, and some porous media. It has distinct advantages in short- and medium-range measurements and applications with general accuracy requirements. At the same time, ultrasonic wave propagation depends strongly on the properties of the medium and environmental conditions, so its application boundaries must be carefully evaluated under high temperature, strong turbulence, highly absorptive media, or high-noise conditions.
Engineering practice shows that selecting a transducer solely based on the nominal range, frequency, or protection rating in a datasheet often fails to ensure system reliability and consistency under real operating conditions. Understanding the basic concepts, working mechanisms, typical application modes, and how key parameters affect performance is a prerequisite for solution design, device selection, and on-site system integration.
1.Overview and Basic Concepts
1.1 Definition and Classification of Ultrasonic Transducers
Ultrasonic transducers are used to convert energy between electrical signals and ultrasonic waves, serving as the front-end actuation and sensing units in ultrasonic measurement and detection systems. According to their energy conversion mechanisms and structural forms, they can be roughly divided into the following categories:
- Piezoelectric transducers
Relying on the piezoelectric effect to achieve electrical–mechanical–acoustic conversion, commonly used materials include PZT-based ceramics, piezoelectric single crystals, and polymer piezoelectric materials. Through resonant structure design, different types can be developed for air media, liquid media, and solid-coupled applications. This is currently the most widely used type. - Electromagnetic Acoustic Transducer (EMAT)
Generates surface or near-surface elastic waves through electromagnetic induction or Lorentz force. It is mostly used for ultrasonic testing of metallic materials and is suitable for high-temperature conditions, coated surfaces, or rough surfaces. However, its efficiency is relatively low and it places higher requirements on the driving and receiving circuits. - Electrostatic/Capacitive Transducers (such as CMUT)
Achieve electroacoustic conversion through changes in capacitance between a movable diaphragm and an electrode. They are suitable for higher frequency bands and array applications, and their fabrication mostly relies on microfabrication processes. They are commonly used in medical and high-resolution imaging fields. - Other Special Structures
Including photoacoustic types, magnetostrictive types, etc., mainly used in specific industrial or research scenarios, and are relatively few in number.
In general industrial measurement and process control, standard probes are mostly of piezoelectric construction. According to the working medium, installation method, and packaging form, they can be further subdivided into several subtypes, such as air probes, liquid probes, immersion/insertion probes, flange-mounted probes, and integrated distance-measuring probes.
1.2 Role in Measurement and Control Systems
In a typical measurement and control system, ultrasonic transducers mainly undertake the following functions:
- Transmitting end: Under the excitation of the driving circuit, it generates an ultrasonic beam with a certain frequency, sound pressure, and directivity, radiating it into the target area or space to be measured.
- Receiving end: Converts ultrasonic signals reflected, scattered, or transmitted back from the target or medium interfaces into electrical signals for subsequent amplification, detection, and digital processing.
- Duplex/integrated probes: In some structures, the same transducer element alternately performs transmission and reception to simplify mechanical installation and acoustic calibration.
In the system-level functional chain, the ultrasonic transducer is usually located at:
- a. The front-end acoustic interface, directly interacting with the measured medium and environment;
- b. A closed loop formed together with the power driving stage, low-noise receiving front-end, and signal processing unit;
- c. A decisive position where its acoustic performance and electrical characteristics determine the measurement range, blind zone, resolution, stability, and anti-interference capability.
The design and selection of the transducer itself must be considered simultaneously with the system’s operating frequency band, transmit power, echo path, signal processing algorithms, and overall mechanical layout. Optimizing parameters solely from a “sensor” perspective often fails to achieve system-level optimal results.
In applications such as distance, liquid level, material level, and presence detection, ultrasonic transducers often serve as alternatives or complements to the following types of technologies:
- Comparison with photoelectric/laser distance measurement
- Optical methods offer high resolution and fast response, and are suitable for targets with good surface reflectivity.
- Ultrasonic measurement is insensitive to the target’s surface color and transparency, and adapts better to powders, liquids, foam, and some porous media.
- In environments with dust, smoke, steam, or oil mist, ultrasound is usually less affected than optical methods; however, in conditions with strong turbulence or steep temperature gradients, variations in sound speed and refraction may significantly affect measurement accuracy.
- Comparison with radar/millimeter-wave sensing
- Radar is suitable for longer distances and more complex environments, but the system cost and implementation complexity are higher. In some applications, due to the high sensitivity of microwave radar, slight shaking, vibration, or the entry of irrelevant objects may trigger false alarms.
- Ultrasonic sensing is more suitable for short- to medium-range applications with engineering-grade accuracy requirements, and has clear advantages in cost, structural complexity, and commissioning/maintenance difficulty.
- For external level measurement of metal tanks, environments with strong electromagnetic interference, or extreme working conditions, radar is usually the more appropriate choice; for conventional distance measurement and level/bulk level detection in open spaces, ultrasound offers better cost performance and easier deployment.
- Comparison with contact sensors (float, pressure, weighing, etc.)
- Contact sensors have a short measurement chain and low dependence on the acoustic properties of the medium, but they must be in direct contact with the measured medium and are easily affected by corrosion, contamination, and mechanical wear.
- Ultrasonic sensing is non-contact, and thus has advantages in handling corrosive or contaminating media and in applications with hygiene requirements (such as food and pharmaceuticals).
Combining the above characteristics, the typical application boundaries for ultrasonic transducers include:
- The distance/range is usually from several centimeters to a few tens of meters;
- For ultra-high accuracy, ultra-long distance, or extreme conditions such as strong convection and high temperature/high pressure, ultrasonic solutions should be evaluated together with other technologies;
- For scenarios with complex surface conditions and variable media, but with sensitivity to cost and maintenance conditions, ultrasonic technology is often one of the preferred options.
2.Operating Mechanism and Key Characteristics
2.1 Electroacoustic Conversion Principle and Main Physical Mechanisms
The core function of an ultrasonic transducer is to complete energy conversion between electrical signals and ultrasonic waves. Focusing on piezoelectric transducers, their working mechanism can be summarized as follows:
- Transmission process (direct piezoelectric effect)
When a voltage with a certain amplitude and frequency is applied across the electrodes, mechanical strain is generated inside the piezoelectric material, driving the transducer to vibrate and radiate sound waves outward through the front vibrating surface. - Reception process (inverse piezoelectric effect)
When external ultrasonic waves act on the vibrating surface of the transducer, they induce mechanical strain in the piezoelectric material, which in turn generates a time-varying electrical signal across the electrodes, achieving acoustic-to-electric conversion.
Through structural design and material selection, transducers are usually designed to form a distinct mechanical resonance near the target operating frequency. The electromechanical conversion efficiency is highest near the resonance point, but the bandwidth is relatively limited, requiring a trade-off among efficiency, bandwidth, and signal processing strategies.
In air-medium applications, the acoustic impedance is very different from that of the piezoelectric material, so matching layers, front-cover structures, and the geometry of the vibrating surface are used to improve the efficiency of sound radiation from the piezoelectric body into the air, while also taking into account protection and mechanical robustness for installation.
2.2 Transmit and Receive Characteristics
Transmit characteristics mainly include:
- Center frequency and spectral characteristics
The transducer outputs the highest sound pressure near a certain frequency, which is called the center frequency. The actual transmitted signal has a finite-bandwidth spectral distribution: under pulse drive the spectrum is broadened, while with continuous wave or narrowband modulation the spectrum is relatively concentrated. - Sound pressure level and radiation efficiency
These are related to the drive voltage, transmit pulse width, electromechanical coupling coefficient of the transducer, and acoustic matching. The sound pressure level determines the usable range and SNR, but excessively high sound pressure may introduce nonlinear effects or impose stress on the structure. - Directivity and beam angle
The size of the vibrating surface, operating frequency, and front-end structure determine the beam angle and sidelobe characteristics. An overly large beam angle tends to cause multipath and stray reflections, while an overly narrow beam angle imposes higher requirements on installation orientation and alignment accuracy.
Receive characteristics mainly include:
- Receiving sensitivity
It characterizes the output voltage or current generated per unit incident sound pressure, and is related to the piezoelectric material properties, structural dimensions, resonance characteristics, and circuit matching. - Equivalent noise and minimum detectable signal
The noise floor of the system is jointly determined by the mechanical loss of the transducer itself, electrode and lead noise, and the noise of the front-end amplifier circuit. - Time and amplitude response
These are related to the mechanical quality factor Q: high Q helps increase peak sensitivity but results in long ringing; low Q gives faster response and better pulse characteristics, but with slightly reduced peak sensitivity.
In integrated transmit–receive structures, isolation circuits or switching devices are also required to switch effectively between strong transmit pulses and weak echo signals, to prevent the transmit side from saturating or damaging the receive front-end.
2.3 Relationship Between Frequency, Beam Angle, and Measurement Range
There is a clear coupling among the operating frequency, beam characteristics, and range performance of an ultrasonic transducer:
- Frequency vs. range
- Higher frequency: The wavelength becomes shorter, spatial resolution improves, and it is more favorable for detecting fine structures and small targets; however, absorption and scattering losses in the medium increase significantly, so the effective range decreases.
- Lower frequency: Propagation attenuation is reduced and the achievable distance increases, making it more suitable for medium- and long-range measurements; correspondingly, spatial resolution decreases, and the transducer volume and structural dimensions usually need to be larger.
In air:- General short- to medium-distance measurements mostly use transducers around 40 kHz.
- For longer range or applications requiring stronger penetration capability, lower-frequency ultrasound can be used.
In specific applications involving “measurement of energy” or high-frequency detection, common configurations include:- Wind speed and wind direction detection: Typically use around 200 kHz to obtain higher time resolution and measurement accuracy.
- Material/property inspection: To obtain higher resolution and more sensitive interface response, frequencies of about 300 kHz or higher are usually adopted.
- Double-sheet detection: Commonly use 200 kHz or 300 kHz to distinguish the ultrasonic transmission differences between single-sheet and double-sheet stacking.
- Edge detection and web guiding: Mostly use around 200 kHz, achieving stable detection while balancing response speed, resolution, and installation space.
- Frequency vs. dead zone
The ringing time of the transducer and structure is related to the frequency and quality factor (Q). The higher the frequency and the larger the (Q), the longer the possible ringing duration. Near-range echoes are easily buried by ringing, thereby enlarging the dead zone. - Beam angle vs. spatial coverage
- A larger beam angle helps cover a wider area and relaxes installation orientation requirements, but tends to generate more multipath echoes and background stray signals.
- A smaller beam angle is beneficial for long-distance directional measurement and interference suppression, but imposes stricter requirements on installation attitude and target position stability.
- Beam angle vs. effective range
For the same transmit power and receive sensitivity, the more concentrated the beam (smaller beam angle), the higher the acoustic energy per unit solid angle, and the better the signal-to-noise ratio of long-distance signals. However, in the presence of alignment errors or unstable target positions, an overly narrow beam may lead to larger fluctuations in echo amplitude.
In engineering design, the operating frequency and beam angle must be jointly selected and balanced according to the target range, acceptable dead-zone length, spatial environment, and target size.
2.4 Sensitivity, Bandwidth, SNR, and Other Core Parameters
The key parameters used to evaluate the performance of ultrasonic transducers mainly include sensitivity, bandwidth, quality factor, equivalent noise, and related signal-to-noise ratio (SNR) metrics. These parameters directly affect measurement accuracy and stability.
- Transmit sensitivity and receive sensitivity
- Transmit sensitivity reflects the sound pressure output capability under a given drive voltage or electrical power.
- Receive sensitivity reflects the amplitude of the output electrical signal under a given incident sound pressure.
High sensitivity helps increase range and anti-interference capability, but must be evaluated together with circuit voltage rating, mechanical strength, and the risk of nonlinearity.
- Bandwidth and quality factor (Q)
- Bandwidth is defined as the effective frequency range of the transducer within a given threshold (such as (-3\ \text{dB})), and determines its response capability to signals of different frequency components.
- The quality factor (Q) is related to the sharpness of the resonance peak and the ringing characteristics.
In engineering practice, most air-coupled ultrasonic transducers are narrowband structures to improve transmit and receive sensitivity at a specific frequency point; however, through transmit pulse design and signal processing, the limitations of narrow bandwidth can be partially compensated. - Signal-to-noise ratio (SNR)
The effective range and measurement accuracy largely depend on the SNR of the echo signal. The main factors affecting SNR include:
Transmit sound pressure level and target reflection characteristics;- Absorption, scattering, and multipath interference along the propagation path;
- Transducer receive sensitivity and its intrinsic noise;
- Noise performance and anti-interference capability of the receive amplifier and filtering circuits.
- Stability and repeatability
Long-term stability is closely related to temperature characteristics, material aging, sealing, and assembly processes. Temperature variations cause changes in sound speed, resonance frequency drift, and sensitivity variation, which must be controlled through structural design, compensation algorithms, or calibration mechanisms.
In engineering applications, transducer parameters should not be evaluated in isolation. Instead, they should be considered at the system level together with range requirements, resolution, environmental noise levels, and the supporting circuitry/PCB and signal processing methods, in order to achieve repeatable, maintainable, and economically reasonable overall performance under specific operating conditions.
3.Typical Application Scenarios and Functional Positioning
3.1 Distance and Proximity Detection
Ultrasonic transducers are primarily used in distance and proximity detection to measure the spacing between a target object and the
sensor, enabling presence/absence judgment and position monitoring.
Typical functional positioning:
- Fixed installation to realize absolute distance measurement to the target object;
- Detection of approach/departure of workpieces or machine components;
- Partial replacement of photoelectric switches within a certain range for presence detection and simple positioning.
Application examples:
- Production line workpiece position detection and stack height control;
- Distance detection and anti-collision control for pallets and boxes on logistics conveyor lines;
- Travel limit and proximity protection for moving machine components;
- Basic obstacle avoidance and safety distance monitoring for robots or AGVs.
In such applications, different operating frequencies of ultrasonic transducers can be selected according to the measurement distance:
- For short distances, higher frequencies (e.g., 200–300 kHz) are often used to obtain higher resolution and measurement accuracy;
- For general short- to medium-range distance measurement, frequencies around 40–65 kHz are commonly used to balance range and cost.
Overall, the shorter the distance, the higher the selectable frequency, and the higher the frequency, the higher the measurement accuracy. The system calculates the target distance via the echo time-of-flight (TOF), and the control system sets corresponding thresholds to realize approach/departure judgment or in-zone detection and control.
3.2 Liquid Level and Material Level Measurement
In liquid level and material level measurement, ultrasonic transducers emit sound waves toward the surface of the medium and monitor the echo time to calculate height or depth, thus realizing non-contact measurement.
Typical functional positioning:
- Continuous level measurement of various storage tanks and vessels;
- Level monitoring of bulk material silos, powder silos, and granular material silos;
- Level/water level control in wastewater treatment and water supply/drainage systems;
- High/low level alarm and process control in open or semi-open containers.
Advantages and features:
- Non-contact measurement, suitable for corrosive, contaminated, or hygienic-grade media;
- Mounted on the top or side of the container, easy to maintain, and relatively tolerant to changes in medium properties;
- Insensitive to color and transparency, suitable for measuring the surfaces of liquids, slurries, and some bulk materials.
In metal enclosed containers or under heavy dust and high-temperature conditions, it is necessary to evaluate, based on site conditions, whether to adopt radar or other technologies. For conventional tanks and general industrial sites,
ultrasonic level/bulk level solutions offer a high cost-performance ratio.
3.3 Flow Rate and Wind Speed Measurement
In flow rate and wind speed measurement scenarios, ultrasonic transducers are mainly used to measure the difference in sound wave propagation time in the fluid or the Doppler effect, thereby estimating flow velocity, volumetric flow rate, or wind speed and direction.
Typical functional positioning:
- Measurement of air velocity and air volume in gas pipelines or air ducts;
- Flow monitoring in open channels and partially enclosed pipelines (in combination with level/water level information);
- Air velocity/air volume control in air-conditioning and ventilation systems;
- Wind speed and direction measurement in environmental and meteorological applications (with multi-channel arrangements).
Frequency bands and configuration examples:
- Wind speed and direction detection commonly use a frequency band of around 200 kHz to obtain higher time resolution and measurement accuracy;
- In pipeline flow measurement, transducers are often arranged in a through-beam or inclined configuration to measure the time difference of sound paths in the downstream and upstream directions.
In such applications, the frequency stability and matching accuracy of the transducers, as well as the installation angle and temperature compensation, are key to ensuring calculation accuracy. The system needs to use algorithms to correct for changes in sound speed and the influence of turbulence.
3.4 Typical Energy-Measurement Applications: Double-Sheet Detection, Edge Position Control, and Material Identification
This type of application mainly utilizes the differences in ultrasonic transmission, reflection, and attenuation characteristics caused by different materials, different numbers of layers, or different positions to determine process states and perform quality monitoring. Typical cases include double-sheet detection, edge position control, and material identification.
3.4.1 Double-Sheet Detection
Double-sheet detection identifies whether double or multiple sheets have been fed by comparing the differences in ultrasonic signals between a single sheet and stacked sheets.
Function positioning:
- Detect whether double or multiple sheets of paper, film, metal sheet, etc. are being fed;
- Prevent issues such as jams, die damage, and register deviation in printing, stamping, cutting, packaging, and other processes;
- Improve production line stability and reduce material waste and downtime.
Technical features:
- Frequency selection:
- Ultrasonic transducers of around 200–300 kHz are commonly used to improve resolution for small thickness variations;
- Detection methods:
- Transmission type: the transmitter and receiver are placed on opposite sides of the material. Single and double sheets are distinguished by changes in transmitted signal amplitude and energy;
- Reflection type: the transmitter and receiver are arranged on the same side, and the difference in absorption and attenuation of the reflected signal by the material is used for determination.
- Determination principle:
- Use a single sheet as a reference by calibrating its characteristic signal;
- During online detection, compare the current signal amplitude, envelope, or other characteristic parameters with the set threshold to determine whether it is a double/multiple sheet.
3.4.2 Edge Detection and Correction
Edge detection and correction are mainly applied to various continuous strip materials, such as paper webs, films, fabrics, and metal strips, to perform real-time monitoring and automatic correction of their lateral position and edge trajectory.
Image source：Tougu
Function positioning:
- Realize online detection of strip edges and automatic edge guiding to ensure the material runs stably within the set trajectory;
- Applied in winding, coating, printing, slitting, laminating, and other processes to avoid deviation, wrinkling, and edge damage;
- Improve product appearance quality and dimensional consistency, and reduce scrap rate.
Technical features:
- Frequency selection:
- Ultrasonic transducers of around 200 kHz are typically used to achieve high spatial resolution and good stability;
- Detection method:
- A through-beam structure is mostly adopted. When the strip edge moves within the sound beam, it causes changes in the received signal energy and effective area;
- Signal and control:
- Output analog or digital position signals to the edge-guiding controller;
- The edge-guiding controller drives the actuator based on the deviation amount to realize closed-loop edge correction control.
3.4.3 Material Detection
Material detection uses differences in the reflection characteristics of different materials to identify and distinguish material types and states.
Function positioning:
- Distinguish between different materials or specifications, such as different types of paper, plastic films, composite materials, metal/non-metal, etc.;
- Assist in determining whether a material meets the requirements of subsequent processes or products, supporting sorting and grading;
- In robotic vacuum cleaner applications, used to differentiate between carpets/rugs, wooden floors, marble, and other floor materials. This provides the basis for adaptive cleaning strategies such as suction adjustment, roller brush speed, and mopping water volume. The basic principle is to identify the material type by comparing the degree of attenuation of ultrasonic echo energy reflected from different floor surfaces.
Technical features:
- Frequency selection:
- Frequency bands of 300 kHz and above are usually adopted to enhance sensitivity to subtle structural and interface differences;
- Detection methods:
- Reflective type: Analyze the echo time, amplitude, and waveform characteristics from surfaces and internal interfaces. In robotic vacuum cleaners, the focus is on judging based on the magnitude and attenuation of the ground reflection echo: carpets/rugs absorb more ultrasonic energy and show larger echo attenuation; wooden floors have medium attenuation; hard and dense surfaces such as marble reflect more strongly with smaller attenuation;
- Transmission type: Compare differences in ultrasonic transmission attenuation, phase change, etc., across different materials;
- Determination methods:
- Extract characteristic parameters such as echo amplitude, arrival time, spectral distribution, and energy;
- Use calibration data to set thresholds or characteristic ranges to identify and distinguish different materials/states. In robotic vacuum cleaner scenarios, these are further mapped to floor types such as carpet/rug, wood floor, and marble, to drive the corresponding cleaning modes.
Ultrasonic transducers can realize online monitoring of process conditions, error-proof control, and material quality during production, providing important support for stable equipment operation and product consistency.
4.Application Prospects
With the continuous advancement of industrial automation, intelligent manufacturing, and logistics upgrading, ultrasonic ranging and energy-measurement sensing technologies will present the following development trends and application opportunities in the future:
4.1 Deep Integration with the Industrial Internet of Things
Ultrasonic sensors will be connected to upper-level systems via fieldbus and industrial Ethernet, enabling real-time status data acquisition, remote monitoring, and operation & maintenance, thus providing a more refined sensing foundation for production lines.
4.2 Integration of High-Precision and Multi-Dimensional Measurement
On the basis of single-distance detection, additional measurement functions such as speed, thickness, material type, and edge position will be further integrated to form “all-in-one” intelligent sensing units, reducing installation space and system complexity.
4.3 Enhanced Intelligent Algorithms and Adaptive Capabilities
With the help of signal processing and machine learning algorithms, ultrasonic systems will perform adaptive compensation and recognition for noise, temperature drift, material changes, and other factors under complex working conditions, thereby improving measurement stability and generalization capability.
4.4 Broader Extension into Various Industries
Beyond traditional manufacturing and logistics, ultrasonic sensing technology will gain more application opportunities in industries such as new energy battery manufacturing, semiconductor equipment, 3C electronics assembly, and medical and life science equipment.
4.5 Standardization and Modularization
Through the standardization of interfaces, protocols, and mechanical structures, quickly integrable modular products will be formed, shortening customers’ development cycles and reducing system integration costs.
Ultrasound-based ranging and energy-measurement sensing applications will gradually evolve from single-function, point-level products into comprehensive perception solutions oriented toward systems and scenarios, playing an increasingly important role in fields such as intelligent manufacturing and smart logistics.
5.Summary
In summary, the applications of ultrasonic transducers in ranging and energy measurement complement each other, each with its own strengths: ranging applications focus on non-contact detection of geometric quantities such as distance, position, and level. They are insensitive to the color, surface characteristics, and transparency of the measured target, providing a stable and reliable foundation of spatial and positional information for industrial environments. Energy-measurement applications, represented by double-sheet detection, edge correction, and material detection, analyze ultrasonic energy and its attenuation, transmission, and reflection characteristics to achieve online identification of material properties and process states, as well as error-proofing and quality monitoring.
Together, these two categories form a complete application system that spans from basic metrology to process assurance and quality control, demonstrating broad development prospects in intelligent manufacturing and smart logistics, and continuing to evolve toward higher integration, greater intelligence, and stronger scenario adaptability.
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