Ultrasonic waves can propagate in gases, liquids, and solids at different speeds. They also exhibit refraction and reflection, and attenuate during propagation. Attenuation is faster in air, while it is less attenuated and travels farther in liquids and solids. Utilizing these properties, various ultrasonic sensors can be made, and with different circuits, various ultrasonic measuring instruments and devices can be constructed.
There are three basic types of ultrasonic applications: transmissive type is used for remote controls, burglar alarms, automatic doors, proximity switches, etc.; split reflective type is used for distance measurement, liquid level or material level measurement; and reflective type is used for material flaw detection, thickness measurement, etc.
Applications of ultrasound
Ultrasonic thickness measurement
Ultrasonic measurement of the thickness of metal parts offers advantages such as high accuracy, simple operation, and continuous automatic detection. The pulse-echo method is commonly used in ultrasonic thickness measurement. The working principle of this method is shown in the figure. The ultrasonic probe contacts the surface of the object being measured. The main controller excites the piezoelectric probe with a pulse signal of a certain frequency, causing it to generate repetitive ultrasonic pulses. When the pulses are transmitted to the other side of the workpiece, they are reflected back and received by the same probe.
Ultrasonic ranging
An air ultrasonic probe emits an ultrasonic pulse, which is reflected back when it reaches the object being measured and received by another air ultrasonic probe. The time t required from the emission of the ultrasonic pulse to its reception is measured, and then multiplied by the speed of sound in air (340 m/s) to obtain the distance the ultrasonic pulse travels at the measured distance. Dividing this distance by 2 gives the distance.
Ultrasonic level measurement
The height and position of the liquid surface in various containers is called the liquid level; the height or position of the surface of solid particles, powders, or lumps is called the material level. Both are collectively referred to as the material level.
Ultrasonic level measurement works based on the reflection characteristics of ultrasonic waves at the interface between two media.
Based on the functions of the transmitting and receiving transducers, ultrasonic level sensors can be divided into two types: single transducer and dual transducer. A single transducer uses one transducer for both transmitting and receiving ultrasonic waves, while a dual transducer uses one transducer for transmitting and one transducer for receiving ultrasonic waves.
Ultrasonic sensors can be placed in water, allowing ultrasonic waves to propagate through the liquid. Because ultrasonic waves attenuate less in liquids, even small ultrasonic pulses can propagate. Ultrasonic sensors can also be mounted above the liquid surface, allowing the ultrasonic waves to propagate through the air. This method is easier to install and maintain, but ultrasonic waves attenuate significantly in air. If the time interval from the emission of the ultrasonic pulse to the reception of the reflected wave by the transducer is known, the location of the interface can be determined. This method can be used to measure the level of a material.
Ultrasonic level sensors offer advantages such as high accuracy, long service life, easy installation, insensitivity to the measured medium, and the ability to perform non-contact continuous measurement in hazardous locations. Their disadvantage is that the presence of air bubbles or fluctuations in the liquid level can introduce significant errors. Under normal operating conditions, their measurement error is ± 0.1 %, and their measurement range is 10⁻²–10⁴ m.
Ultrasonic flow measurement
Ultrasonic flow measurement utilizes the characteristic that ultrasonic waves travel at different speeds in stationary and flowing fluids to determine the fluid velocity and flow rate.
In the diagram, v represents the average velocity of the fluid being measured, c represents the speed of ultrasonic wave propagation in a stationary fluid, q represents the angle between the direction of ultrasonic wave propagation and the direction of fluid flow (must be less than 90 degrees) , A and B represent two ultrasonic transducers, and L represents the distance between them.
There are generally three methods for ultrasonic flow measurement: time difference method, phase difference method, and frequency difference method.
Ultrasonic flow sensors are characterized by their non-obstruction of fluid flow, enabling non-contact measurement. They can measure a wide variety of fluids, including non-conductive fluids, high-viscosity fluids, and slurry-like fluids—any fluid capable of transmitting ultrasonic waves can be measured. Ultrasonic flow meters can be used to measure the flow velocity of tap water, industrial water, agricultural water, and can also be used to measure the flow rate in sewers, agricultural irrigation systems, and rivers.
Ultrasonic flaw detection
Ultrasonic nondestructive testing is used in industrial manufacturing, including penetration testing and reflection testing.
Penetration testing: This method determines the internal quality of a workpiece based on the change in energy after ultrasonic waves penetrate it.
This method uses two ultrasonic transducers, placed on opposite surfaces of the workpiece being tested. One transducer emits ultrasonic waves, and the other receives them. The emitted ultrasonic waves can be continuous waves or pulse signals.
When the workpiece being tested is free of defects, the received ultrasonic energy is high, resulting in a high reading on the instrument. When the workpiece contains defects, some energy is reflected, thus the received ultrasonic energy is low, and the reading on the instrument is low. Based on this change, the presence or absence of defects inside the workpiece can be detected.
Reflection testing: This method detects internal defects in a workpiece by observing how ultrasonic waves are reflected. It is further divided into single-pulse reflection and multi-pulse reflection methods.
During testing, an ultrasonic probe is placed on the workpiece and moved back and forth across it for inspection. A high-frequency pulse generator emits a pulse (transmit pulse T) which is applied to the ultrasonic probe, exciting it to generate ultrasonic waves. The ultrasonic waves emitted by the probe propagate into the workpiece at a certain speed. Part of the ultrasonic waves are reflected back when they encounter a defect, generating a defect pulse F, while the other part continues to the bottom surface of the workpiece and is also reflected back, generating a bottom pulse B. The defect pulse F and the bottom pulse B are received by the probe, converted into electrical pulses, and amplified together with the transmitted pulse T before being displayed on the monitor screen.
The multiple pulse reflection method is a flaw detection method based on multiple bottom echoes. When the ultrasonic waves emitted by the ultrasonic probe are reflected back to the ultrasonic probe from the bottom of the workpiece being tested, a portion of the ultrasonic waves are received by the probe, while the remaining portion is reflected back to the bottom of the workpiece. This process of reflection continues until all the sound energy is attenuated.
Ultrasonic testing is a widely used non-destructive testing method. It can detect defects on the surface of materials as well as internal defects several meters deep, a depth that X-ray testing cannot reach.
Ultrasonic cleaning
When a weak sound wave signal acts on a liquid, it creates a negative pressure, increasing the liquid volume and widening the gaps between molecules, forming numerous tiny bubbles. Conversely, when a strong sound wave signal acts on a liquid, it creates a positive pressure, compressing the liquid volume and crushing the tiny bubbles. Research has shown that when ultrasound acts on a liquid, the collapse of each bubble generates a highly energetic shock wave, equivalent to instantaneously generating temperatures of several hundred degrees Celsius and pressures of thousands of atmospheres. This phenomenon is called "cavitation," and ultrasonic cleaning utilizes the shock waves generated by the collapse of bubbles in the liquid to clean and scour the inner and outer surfaces of workpieces. Ultrasonic cleaning is widely used in industries such as semiconductors, machinery, glass, and medical instruments.
Having considered some of the basic functions of ultrasonic sensors, let's look at their application scenarios in various industries:
Application of ultrasonic sensors in autonomous obstacle avoidance of robots
With the development of computer technology, sensor technology, and artificial intelligence, significant research achievements have been made in obstacle avoidance and autonomous navigation technologies for mobile robots. The requirements for autonomous pathfinding of mobile robots have evolved from simple functional implementation to reliability, versatility, and high efficiency, thus placing higher demands on related technologies.
A fundamental requirement for achieving autonomous navigation in robots is obstacle avoidance. A necessary condition for obstacle avoidance and navigation is environmental perception, which requires sensors to acquire information about the surrounding environment, including the size, shape, and position of obstacles. Therefore, sensor technology plays a crucial role in obstacle avoidance for mobile robots.
The sensors used for obstacle avoidance mainly include ultrasonic sensors, visual sensors, infrared sensors, and laser sensors.
The principle of ultrasonic sensors for distance detection is as follows: It measures the time difference between the emission of an ultrasonic wave and the subsequent detection of the emitted wave, and then calculates the distance to the object based on the speed of sound. Since the speed of ultrasound in air is related to temperature and humidity, more accurate measurements require taking into account changes in temperature and humidity, as well as other factors.
Ultrasonic sensors typically have a short operating range, with an effective detection distance generally between 5 and 10 meters. However, they have a minimum detection blind zone, typically several tens of millimeters. Due to their low cost, simple implementation, and mature technology, ultrasonic sensors are commonly used in mobile robots.
Ultrasonic sensors solve the problem of high-altitude icing detection for aircraft.
Canada has developed two ice particle detection technologies for high-altitude aircraft icing, which have been approved by the National Research Council of Canada (NRC) for industrial application.
Aircraft icing caused by liquid water in clouds has plagued the aviation industry for decades. Recent research has focused on the dangers posed by ice crystals in clouds that are undetectable by aviation weather radar. Highly concentrated ice crystals can affect engine operation and clog pitot tubes. The NRC states that there are currently no reliable sensors that can provide early warnings of icing, and ultrasonic devices can fill this gap.
This ultrasonic sensor is mounted using a non-destructive patch method, resulting in a small, thin, and low-power design. Employing ultrasonic waves, it can function as both a speaker and a receiver. By sending sound waves onto a structural surface or wall, the waves can be reflected back by accumulated ice, thus revealing ice accumulation data on the other side.
The NRC states that this ultrasonic device does not need to be installed in the monitored environment. It can be used on any non-exposed surface of an engine or aircraft component, eliminating the risk of sensor damage due to ice, disassembly, or engine penetration. When de-icing personnel know which locations on the engine are prone to icing, they can narrow down the monitoring area, allowing them to detect icing by installing the sensor in a suitable location on the opposite side wall of the airflow channel.
Ultrasonic sensors can detect ice buildup in real-world engine icing environments, and they are sensitive enough to distinguish the degree of buildup and effectively measure its intensity. While a single ultrasonic sensor is sufficient to detect icing, the NRC believes that an array of multiple sensors can be installed due to their small size, weight, and power consumption, thus providing detailed data on the extent of ice coverage.
Application of ultrasonic sensor ranging in intelligent parking
Car reversing radars utilize ultrasonic ranging systems, and there are currently two commonly used ultrasonic ranging solutions. One is an ultrasonic ranging system based on a microcontroller or embedded device, and the other is an ultrasonic ranging system based on a CPLD (Complex Programmable Logic Device).
The principle of ultrasonic ranging is based on the known speed of sound in air. It measures the time it takes for the sound wave to reflect back from an obstacle after being emitted, and calculates the actual distance from the emission point to the obstacle based on the time difference between emission and reception. First, an ultrasonic transmitter emits an ultrasonic wave in a certain direction, and timing begins simultaneously with the emission. The ultrasonic wave travels through the air, and upon encountering an obstacle, it immediately returns. The ultrasonic receiver stops timing as soon as it receives the reflected wave. The speed of sound in air is C = 340 m/s. Based on the time T seconds recorded by the timer, the distance L from the emission point to the obstacle can be calculated: L = C × T / 2. This is the so-called time-difference ranging method.
Since ultrasound is also a type of sound wave, its speed of sound, C, is related to temperature. Table 1 lists the speed of sound at several different temperatures. In practical applications, if the temperature change is small, the speed of sound can be considered essentially constant. If high ranging accuracy is required, it should be corrected using temperature compensation methods.
Because of its advantages of easy directional emission, good directionality, easy intensity control, and no need for direct contact with the object being measured, ultrasound is an ideal choice for measuring reversing distance.
Ultrasonic sensors are used for fingerprint authentication in smartphones.
The sensor, developed using micro-nano electromechanical systems (MEMS) technology, features a two-dimensional array of ultrasonic transmitters and receivers, using the reflection of ultrasonic waves incident on the fingertip to read fingerprints. It can accurately read fingerprints even when the finger is wet, and it can also detect attempts to cheat using paper or other materials bearing fingerprints. Furthermore, it can perform deep scans at a distance of approximately several hundred micrometers from the finger surface.
The ultrasonic transceiver array for the fingerprint sensor is formed on a 4.73mm × 3.25mm MEMS chip, superimposed on a CMOS IC for reading. A protective PDMS film is laid on the ultrasonic transceiver array. Ultrasonic waves are transmitted and received using 110 piezoelectric elements arranged vertically and 56 horizontally. The piezoelectric elements are configured with a vertical spacing of 43μm and a horizontal spacing of 58μm. To accurately read fingerprints with peak and trough spacing of several hundred μm, the ultrasonic frequency is set to 14MHz. This is the optimal frequency value to prevent ultrasonic beam propagation and to minimize attenuation on PDMS and skin.
Fingerprint reading utilizes the ultrasonic wave reflection from the boundary between PDMS and the fingertip, as well as the boundary between the fingertip epidermis and subcutaneous tissue. In a fingerprint, the wave crests in contact with PDMS and the wave troughs in contact with the air layer exhibit significantly different reflection characteristics. This allows for fingerprint identification using the reflected waves. Furthermore, the reflected waves generated at the boundary between the epidermis and subcutaneous tissue can be used to detect forged fingerprints on paper.
The piezoelectric elements forming the ultrasonic transceiver array are created by depositing piezoelectric material on a silicon thin film with a microcavity at the bottom, which can be vibrated by applying a voltage. During reading, the voltage changes caused by the vibration can be read. To reduce noise during reading, virtual elements without microcavities that cannot receive ultrasonic waves are placed next to each element. Signals other than ultrasonic waves received by the two types of elements are considered noise and are eliminated using a differential circuit.
Applications of ultrasonic sensors in medicine
The primary application of ultrasound in medicine is disease diagnosis, and it has become an indispensable diagnostic method in clinical medicine. The advantages of ultrasound diagnosis include: painless and non-invasive for the patient, simple method, clear imaging, and high diagnostic accuracy. Therefore, it is easily promoted and welcomed by medical professionals and patients. Ultrasound diagnosis can be based on different medical principles; let's look at a representative one called the so-called A-mode. This method utilizes the reflection of ultrasound waves. When ultrasound waves propagate through human tissue and encounter an interface between two media with different acoustic impedances, a reflected echo is generated at that interface. Each time an echo is encountered, it is displayed on the oscilloscope screen, and the impedance difference between the two interfaces determines the amplitude of the echo.
In real life, we can easily find applications of ultrasonic sensors. With the rapid development of technology, the application range of ultrasonic sensors is becoming increasingly wide. Throughout human history, the application of ultrasonic sensors has been ubiquitous; wherever humans can imagine, they have a place.
