Commonly used ultrasonic sensors consist of piezoelectric crystals and can both emit and receive ultrasonic waves. Low-power ultrasonic probes are mostly used for detection. They come in many different structures, including straight probes (longitudinal waves), angle probes (transverse waves), surface wave probes (surface waves), Lamb wave probes (Lamb waves), and dual probes (one probe emits and the other receives), etc.
An ultrasonic sensor is a sensor that converts ultrasonic signals into other energy signals (usually electrical signals). Ultrasonic waves are mechanical waves with vibration frequencies higher than 20 kHz. They are characterized by high frequency, short wavelength, minimal diffraction, and, most importantly, good directionality, enabling them to propagate directionally as rays. Ultrasonic waves have a strong penetrating ability in liquids and solids, especially in solids that are opaque to sunlight. When ultrasonic waves encounter impurities or interfaces, they are significantly reflected, forming echoes. When they encounter moving objects, they produce the Doppler effect. Ultrasonic sensors are widely used in industry, defense, and biomedicine.
Applications of ultrasonic sensors
Sensor: "Can sense a specified measurand and convert it into usable data according to a certain rule."
A signal device or apparatus typically consists of a sensing element and a conversion element. A sensor is a detection device that can sense the information being measured and transform that information into an electrical signal or other required form of information output according to a certain rule, in order to meet the requirements of information transmission, processing, storage, display, recording, and control. It is the primary link in realizing automatic detection and automatic control.
There is currently no unified classification method for sensors, but the following three are commonly used:
1. Based on the physical quantities measured by the sensor, sensors can be classified into displacement, force, velocity, temperature, flow rate, and gas composition sensors.
2. According to the working principle of the sensor, it can be classified into resistance, capacitance, inductance, voltage, Hall effect, photoelectric, grating, thermocouple and other sensors.
3. According to the nature of the sensor output signal, it can be classified into: switch type sensors with output as switching quantity ("1" and "0" or "on" and "off"); analog type sensors; and digital type sensors with output as pulse or code.
Here, we will mainly introduce a type of sensor that is widely used in daily life and brings great convenience to human society—the ultrasonic sensor and its application in reversing radar.
Basic Introduction to Ultrasonic Sensors
An ultrasonic sensor is a sensor developed using the properties of ultrasound. To use ultrasound as a detection method, it is necessary to generate and receive ultrasound waves. The device that performs this function is an ultrasonic sensor, commonly referred to as an ultrasonic transducer or ultrasonic probe.
An ultrasonic probe mainly consists of a piezoelectric crystal, which can both emit and receive ultrasonic waves. The core of the ultrasonic probe is a piezoelectric crystal within its plastic or metal casing.
A piezoelectric crystal is a component of an ultrasonic sensor. Many materials can be used to make up the crystal. The main materials for ultrasonic sensors are piezoelectric crystals (electrostrictive) and nickel-iron-aluminum alloys (magnetostrictive). Electrostrictive materials include lead zirconate titanate (PZT). An ultrasonic sensor composed of piezoelectric crystals is a reversible sensor; it can convert electrical energy into mechanical oscillations to generate ultrasonic waves, and when it receives ultrasonic waves, it can also convert them back into electrical energy. Therefore, it can be divided into a transmitter or a receiver. Some ultrasonic sensors function as both transmitters and receivers. An ultrasonic sensor consists of a transmitting sensor (or wave transmitter), a receiving sensor (or wave receiver), a control section, and a power supply section. The transmitter sensor consists of a transmitter and a ceramic transducer with a diameter of approximately 15mm. The transducer converts the electrical vibration energy of the ceramic transducer into ultrasonic energy and radiates it into the air. The receiving sensor consists of a ceramic transducer and an amplifier circuit. The transducer receives the wave, generates mechanical vibrations, converts them into electrical energy, and uses this as the output of the sensor receiver to detect the transmitted ultrasonic waves. The control section mainly controls the pulse train frequency, duty cycle, sparse modulation, counting, and detection distance emitted by the transmitter.
Working principle of ultrasonic sensors
Ultrasonic sensors are sensors developed using the properties of ultrasound. Sound waves are the propagation form of the mechanical vibration of an object. Ultrasound refers to sound waves with a vibration frequency greater than 20,000 Hz. Its vibration frequency per second is very high, exceeding the upper limit of human hearing; these inaudible sound waves are called ultrasound.
Ultrasound is a mechanical oscillation in an elastic medium, and it exists in two forms: transverse oscillation (transverse waves) and longitudinal oscillation (longitudinal waves). In industrial applications, longitudinal oscillation is primarily used. Ultrasound can propagate in gases, liquids, and solids, with varying propagation speeds. Furthermore, it exhibits refraction and reflection, and its propagation is subject to attenuation.
The propagation laws of ultrasound in a medium, such as reflection, refraction, diffraction, and scattering, are not fundamentally different from those of audible sound waves. However, compared to audible sound waves, ultrasound possesses many unique characteristics: its propagation properties—the diffraction of ultrasound waves...
Ultrasonic waves have poor propagation ability; they can propagate in a straight line in a homogeneous medium, and this characteristic is more pronounced the shorter the wavelength of the ultrasound. Power characteristics—When sound propagates in air, it drives airborne particles to vibrate back and forth, doing work on the particles. At the same intensity, the higher the frequency of the sound wave, the greater its power. Because ultrasound has a very high frequency, its power is much greater than that of ordinary sound waves. Cavitation—When ultrasound propagates in a liquid, the intense vibration of liquid particles creates small cavities within the liquid. These cavities rapidly expand and close, causing violent collisions between the liquid particles, generating pressures of several thousand to tens of thousands of atmospheres. This intense interaction between particles causes a sudden increase in the liquid temperature, leading to emulsification of two immiscible liquids (such as water and oil), accelerating the dissolution of solutes, and accelerating chemical reactions. These various effects caused by ultrasound in liquids are called ultrasonic cavitation.
Characteristics of ultrasound:
(1) Ultrasonic waves have strong directionality and their energy is easily concentrated during propagation;
(2) Ultrasound can propagate in various media and can travel a sufficiently long distance; (3) Ultrasound interacts moderately with the transmission medium and is easy to carry information about the state of the transmission medium (diagnosis or effect on the transmission medium).
Applications of ultrasonic sensors
Ultrasonic sensing technology is applied in various aspects of production practice.
1. Application of ultrasonic distance sensor technology
An ultrasonic sensor consists of three parts: an ultrasonic transducer, a processing unit, and an output stage. First, the processing unit applies voltage excitation to the ultrasonic transducer, which then emits ultrasonic waves in pulse form. Next, the transducer switches to a receiving state, and the processing unit analyzes the received ultrasonic pulses to determine if the received signal is an echo of the emitted ultrasonic wave. If so, the travel time of the ultrasonic wave is measured, and the travel time is converted to the distance to the object reflecting the ultrasonic wave. By installing the ultrasonic sensor in a suitable location and emitting ultrasonic waves in the direction of change of the object being measured, the distance between the object's surface and the sensor can be measured. An ultrasonic sensor has a transmitter and a receiver, but a single ultrasonic sensor can also perform the dual function of transmitting and receiving sound waves. The ultrasonic sensor utilizes the piezoelectric effect to convert electrical energy and ultrasonic waves into each other; that is, when emitting ultrasonic waves, electrical energy is converted into ultrasonic waves; and when receiving the echo, the ultrasonic vibration is converted into an electrical signal.
2. 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 procedure, clear imaging, and high diagnostic accuracy.
3. Application of ultrasonic sensors in liquid level measurement
The basic principle of ultrasonic liquid level measurement is: the ultrasonic pulse signal emitted by the ultrasonic probe propagates in the gas, is reflected after encountering the air-liquid interface, and the echo signal is received. The round-trip propagation time of the ultrasonic wave is calculated, and the distance or liquid level height can be calculated. The ultrasonic measurement method has many advantages that other methods cannot match: (1) There are no mechanical transmission parts and no contact with the liquid being measured. It is a non-contact measurement method. It is not afraid of electromagnetic interference and strong corrosive liquids such as acids and alkalis. Therefore, it has stable performance, high reliability and long service life; (2) Its short response time can easily realize real-time measurement without lag.
4. Application of ultrasonic sensors in ranging systems
Ultrasonic ranging generally includes the following methods:
① Take the average voltage of the output pulse. This voltage (whose amplitude is basically fixed) is proportional to the distance. The distance can be measured by measuring the voltage.
② Measure the width of the output pulse, i.e., the time interval t between transmitting and receiving the ultrasonic wave. Therefore, the measured distance is S = 1/2vt. If high ranging accuracy is required, it should be corrected using temperature compensation. Ultrasonic ranging is suitable for high-precision medium-to-long-distance measurements.
5. Industrial Applications of Ultrasonic Sensors
In industry, typical applications of ultrasound include non-destructive testing of metals and ultrasonic thickness measurement. Previously, many technologies were hampered by their inability to detect the internal structure of objects; the advent of ultrasonic sensing technology has changed this situation. Ultrasonic testing utilizes the characteristic that ultrasonic energy penetrates deep into metal materials and reflects off the interface when it travels from one section to another to inspect for defects in parts. When an ultrasonic beam travels from the surface of a part through a probe into the metal, it generates reflected waves when it encounters defects or the bottom surface of the part, forming pulse waveforms on a fluorescent screen. The location and size of the defects are determined based on these pulse waveforms.
6. Application of ultrasonic sensors in reversing radar
A reversing radar, also known as a "reversing collision avoidance radar" or "parking assistance device," is a safety aid for parking or reversing a car. It consists of ultrasonic sensors (commonly called probes), a controller, and a display (or buzzer). It informs the driver of surrounding obstacles through sound or a more visual display, eliminating the need for the driver to constantly look around when parking, reversing, or starting the vehicle. It also helps eliminate blind spots and blurred vision, improving driving safety.
The reversing radar is designed based on the principle that bats can fly at high speeds in the dark without colliding with any obstacles. The sensors are mounted on the rear bumper, and depending on the price and brand, there are two, three, four, six, or eight sensors, each covering the front, rear, left, and right sides. The sensors radiate at a 45-degree angle, searching for targets in all directions. Its biggest advantage is its ability to detect obstacles lower than the bumper and difficult for the driver to see from the rear window, and to issue an alarm, such as flower beds or children playing behind the car. The reversing radar display is mounted on the rearview mirror, constantly reminding the driver of the distance to objects behind the car. When the distance reaches a dangerous level, a buzzer sounds, prompting the driver to stop. The reversing radar automatically starts working when the gear lever is engaged in reverse, with a detection range of approximately 0.3 to 2.0 meters, making it very useful for drivers when parking.
A reversing radar is essentially an ultrasonic sensor. Generally speaking, ultrasonic sensors can be divided into two main categories: one uses electrical methods to generate ultrasonic waves, and the other uses mechanical methods to generate ultrasonic waves. Currently, the most commonly used type is the piezoelectric ultrasonic generator, which has two piezoelectric crystals and a resonant plate. When a pulse signal is applied to the two electrodes and its frequency is equal to the natural oscillation frequency of the piezoelectric crystals, the piezoelectric crystals will resonate and drive the resonant plate to vibrate. This process of converting mechanical energy into electrical signals is the working principle of an ultrasonic sensor.
To better study and utilize ultrasound, many ultrasonic transmitters and probes have been designed and manufactured, and these are used in automotive reversing radars. This principle, applied to a non-contact detection technology, is simple, convenient, and quick for distance measurement, allowing for real-time control and achieving distance accuracy that meets industrial application requirements. In reversing radar, an ultrasonic signal is emitted at a certain moment. The echo signal wave after encountering the object being measured is received by the reversing radar. The propagation speed of the ultrasonic signal in the medium is calculated using the time from emission to reception of the echo signal, thus determining the distance between the probe and the detected object.
Throughout the various industrial revolutions of human civilization, sensing technology has always played a pioneering role. It is a key technology that runs through all technological and application fields, and it is almost ubiquitous in all imaginable areas.
With advancements in sensor technology, sensors will evolve from simple judgment functions to learning capabilities, and ultimately, to creative abilities. Looking to the future in the new century, ultrasonic sensors, as a new, crucial, and useful tool, will have significant room for development in various aspects, moving towards higher positioning and accuracy. In meeting the ever-evolving needs of society, these revitalized sensors will play an even greater role.
