A sensor is a common yet crucial device. It is a device or apparatus that senses various quantities to be measured and converts them into useful signals according to a certain rule. For sensors, inputs can be divided into static and dynamic quantities based on their state. We can obtain the static characteristics of a sensor by analyzing the relationship between the output and input quantities under stable conditions at various values. The main indicators of a sensor's static characteristics include linearity, hysteresis, repeatability, sensitivity, and accuracy. The dynamic characteristics of a sensor refer to its response to changes in the input quantity over time. Dynamic characteristics are typically described using automatic control models such as transfer functions. Often, the signals received by sensors contain weak, low-frequency signals, and external interference can sometimes exceed the amplitude of the measured signal. Therefore, eliminating crosstalk noise is a critical sensor technology.
physical sensors
A physical sensor is a sensor that detects physical quantities. It is a device that utilizes certain physical effects to convert the measured physical quantity into a signal in an easily processed energy form. Its output signal has a definite relationship with the input signal. Major physical sensors include photoelectric sensors, piezoelectric sensors, piezoresistive sensors, electromagnetic sensors, thermoelectric sensors, and fiber optic sensors. As an example, let's look at a commonly used photoelectric sensor. This type of sensor converts light signals into electrical signals. It directly detects radiation information from an object and can also convert other physical quantities into light signals. Its main principle is the photoelectric effect: when light shines on a material, the electrical effects on the material change. These electrical effects include electron emission, conductivity, and electrical potential/current. Obviously, devices that can easily produce such effects become the main components of a photoelectric sensor, such as a photoresistor. Thus, we know that the main working process of a photoelectric sensor is to receive the corresponding light, convert the light energy into electrical energy through a device like a photoresistor, and then obtain the desired output electrical signal through amplification and noise reduction. The output electrical signal here has a certain relationship with the original optical signal, usually a near-linear relationship, making the calculation of the original optical signal relatively simple. The principles of other physical sensors can be compared to those of photoelectric sensors.
The applications of physical sensors are very wide. If we only look at the applications of physical sensors from the perspective of biomedicine, it is not difficult to infer that physical sensors also have important applications in other fields.
For example, blood pressure measurement is one of the most common medical measurements. Our usual blood pressure measurements are indirect, measuring the blood pressure in the blood vessels by detecting the relationship between blood flow and pressure on the body surface. The sensors needed to measure blood pressure typically include an elastic diaphragm that converts the pressure signal into diaphragm deformation, and then converts the strain or displacement of the diaphragm into a corresponding electrical signal. At the peak of the electrical signal, we can detect the systolic pressure. After passing through an inverter and a peak detector, we can obtain the diastolic pressure from the sensor's shape, and then obtain the mean pressure through an integrator.
Let's take a look at respiratory measurement technology. Respiratory measurement is an important basis for clinical diagnosis of lung function and is essential in surgical procedures and patient monitoring. For example, when using a thermistor sensor to measure respiratory rate, the sensor's resistor is mounted on the outside of the front of a clip, which is then clipped to the nostril. When the respiratory airflow passes over the surface of the thermistor, the respiratory rate and the state of the air can be measured through the thermistor.
Take, for example, the most common process of measuring body surface temperature. Although it seems simple, it involves a complex measurement mechanism. Body surface temperature is determined by various factors, including local blood flow, the thermal conductivity of underlying tissues, and the heat dissipation of the epidermis. Therefore, measuring skin temperature requires considering multiple influences. Thermocouple sensors are widely used in temperature measurement, typically including rod-shaped thermocouple sensors and thin-film thermocouple sensors. Because thermocouples are extremely small, with high precision reaching the micrometer level, they can accurately measure the temperature at a specific point. Combined with subsequent analysis and statistics, this allows for comprehensive analytical results. This is unmatched by traditional mercury thermometers and demonstrates the broad prospects that new technologies bring to scientific development.
As can be seen from the above introduction, physical sensors have a wide variety of applications even in the biomedical field alone. The development trend of sensors is towards multifunctional, imaging, and intelligent sensors. Sensor measurement, as a crucial means of data acquisition, is an indispensable device in industrial production and even daily life. Physical sensors, being the most common family of sensors, can undoubtedly lead to the creation of more products and better benefits through their flexible application.
Fiber optic sensor
In recent years, sensors have been developing towards greater sensitivity, accuracy, adaptability, compactness, and intelligence. In this process, fiber optic sensors, a new member of the sensor family, have gained considerable popularity. Fiber optics possess many superior properties, such as resistance to electromagnetic interference and atomic radiation, thin diameter, softness, and light weight, insulating and non-inductive electrical properties, and water resistance, high-temperature resistance, and corrosion resistance. They can act as the eyes and ears of humans in places inaccessible to humans (such as high-temperature areas) or harmful areas (such as nuclear radiation zones), and can even transcend human physiological limits to receive external information imperceptible to human senses.
Fiber optic sensors are a relatively new technology that has emerged in recent years. They can be used to measure a variety of physical quantities, such as sound fields, electric fields, pressure, temperature, angular velocity, and acceleration, and can also perform measurement tasks that are difficult to accomplish with existing technologies. Fiber optic sensors have demonstrated unique capabilities in confined spaces, environments with strong electromagnetic interference, and high voltage. Currently, there are more than 70 types of fiber optic sensors, broadly categorized into sensors that utilize the fiber itself and sensors that utilize optical fibers.
A sensor inherent to optical fiber is one in which the fiber itself directly receives the measured quantity from the external environment. The external measured physical quantity can cause changes in the length, refractive index, and diameter of the measuring arm, thereby altering the amplitude, phase, frequency, and polarization of the light transmitted within the fiber. The light transmitted through the measuring arm interferes (compares) with the reference light from the reference arm, causing a change in the phase (or amplitude) of the output light. This change can be used to detect the change in the measured quantity. Optical fibers are highly sensitive to external phase influences; interferometry can detect physical quantities corresponding to minute phase changes of up to 10⁻⁴ radians. Utilizing the flexibility and low loss of optical fibers, very long fibers can be coiled into small-diameter loops to increase the usable length and achieve even higher sensitivity.
Fiber optic acoustic sensors are sensors that utilize the inherent properties of optical fibers. When an optical fiber is subjected to a very small external force, it will bend slightly, significantly altering its light transmission capacity. Sound is a mechanical wave; its effect on the optical fiber is to cause it to bend, and the intensity of the sound can be determined by this bending. Fiber optic gyroscopes are also a type of fiber optic sensor. Compared to laser gyroscopes, fiber optic gyroscopes offer higher sensitivity, smaller size, and lower cost, making them suitable for high-performance inertial navigation systems in aircraft, ships, and missiles. The diagram illustrates the principle of a fiber optic sensor turbine flow meter.
Another major category of fiber optic sensors utilizes optical fibers. Their structure is roughly as follows: the sensor is located at the end of the optical fiber, which acts merely as a transmission line for the light, transforming the measured physical quantity into changes in the amplitude, phase, or amplitude of the light. In this type of sensor system, traditional sensors are combined with optical fibers. The introduction of optical fibers makes probe-based telemetry possible. These fiber optic transmission sensors have a wide range of applications and are easy to use, but their accuracy is slightly lower than that of the first type of sensor.
Optical fiber is a relatively new addition to the sensor family. Thanks to its superior performance, it has been widely used and is a sensor that deserves attention in production practice.
Bionic sensors
Bionic sensors are a new type of sensor that employs novel detection principles. They combine immobilized cells, enzymes, or other bioactive substances with a transducer to form the sensor. This type of sensor represents a new information technology that has developed in recent years through the interdisciplinary integration of biomedicine, electronics, and engineering. These sensors are characterized by high performance and long lifespan. Among bionic sensors, those that mimic biological organisms are commonly used.
Bionic sensors can be categorized according to the medium used, including enzyme sensors, microbial sensors, organelle sensors, and tissue sensors. As shown in the diagram, bionic sensors are closely related to all aspects of biological theory and are a direct result of its development. Among biomimetic sensors, the urea sensor is a recently developed type. The following section will use the urea sensor as an example to illustrate the application of bionic sensors.
The urea sensor mainly consists of two parts: a biomembrane and its ion channels. The biomembrane can sense external stimuli, and the ion channels can receive, amplify, and transmit information from the biomembrane. When the sensing sites within the membrane are affected by external stimuli, the membrane's permeability changes, allowing a large influx of ions into the cell, thus transmitting information. Membrane proteins, a component of the biomembrane, play a crucial role in this process, causing conformal network changes that alter membrane permeability and facilitate information transmission and amplification. The ion channels of the biomembrane are composed of amino acid polymers. Poly(L-glutamic acid) polymers (PLG), readily synthesized in organic chemistry, can be used as alternatives due to their better chemical stability than enzymes. While PLG is water-soluble and unsuitable for motor modification, PLG and polymers can be synthesized into block copolymers to form the sensing membrane used in the sensor.
The principle of ion channels in biofilms is basically the same as that of biofilms. After the block copolymer membrane is fixed on the electrode, if a substance that senses changes in the PLG protective network is added, the permeability of the membrane will change, thereby generating a change in current. The change in current can be used to detect irritating substances.
The urea sensor has been proven to be a stable biomimetic sensor with a detection limit on the order of 10 to the power of -3. It can also detect irritants, but it is not yet suitable for biomimetic measurements.
Currently, although many biomimetic sensors have been successfully developed, their stability, reproducibility, and mass production capabilities are still insufficient. Therefore, biomimetic sensing technology is still in its infancy. Thus, in addition to continuing to develop new series of biomimetic sensors and improving existing series, the immobilization technology of bioactive membranes and the solidification of biomimetic sensors deserve further research.
In the near future, biomimetic sensors mimicking biological functions such as smell, taste, hearing, and touch will emerge, potentially surpassing the sensitivity of the human five senses and enhancing current robotic capabilities in vision, taste, touch, and object manipulation. We can see the broad prospects for biomimetic sensor applications, but these all require further development in biotechnology. We eagerly await that day.
Infrared sensor
Infrared technology has become widely known and has been extensively applied in modern science and technology, national defense, and industry and agriculture. Infrared sensing systems are measurement systems that use infrared light as a medium. They can be divided into five categories according to their functions: (1) radiometers, used for radiation and spectral measurements; (2) search and tracking systems, used for searching and tracking infrared targets, determining their spatial location, and tracking their movement; (3) thermal imaging systems, which can generate images of the distribution of infrared radiation of the entire target; (4) infrared ranging and communication systems; and (5) hybrid systems, which refer to a combination of two or more of the above systems.
The core of an infrared system is the infrared detector, which can be divided into two main categories based on its detection mechanism: thermal detectors and photon detectors. The following analysis uses a thermal detector as an example to explain its working principle.
Thermal detectors utilize the radiative thermal effect. When a detection element receives radiant energy, its temperature rises, causing changes in the detector's temperature-dependent properties. By detecting these changes in a particular property, radiation can be detected. In most cases, radiation is detected through thermoelectric changes. When the element receives radiation, causing a non-electrical physical change, the corresponding electrical change can be measured after appropriate transformation.
Electromagnetic sensor
Magnetic sensors are among the oldest types of sensors, with the compass being one of their earliest applications. However, modern sensors, for ease of signal processing, need to convert magnetic signals into electrical signals. The earliest applications were magnetoelectric sensors based on the principle of electromagnetic induction. These magnetoelectric sensors made outstanding contributions to industrial control, but today they have been largely replaced by newer magnetic sensors made primarily of high-performance magnetically sensitive materials.
Among the electromagnetic sensors used today, the magnetic rotation sensor is an important type. A magnetic rotation sensor mainly consists of several parts: a semiconductor magnetoresistive element, a permanent magnet, a fixture, and a housing. A typical structure involves mounting a pair of magnetoresistive elements on the stimulus of a permanent magnet, connecting the input and output terminals of the elements to the fixture, then mounting them in a metal box, and finally sealing them with engineering plastic to form a hermetically sealed structure, which provides excellent reliability. Magnetic rotation sensors offer many advantages in terms of appearance that semiconductor magnetoresistive elements cannot match. Besides possessing high sensitivity and a large output signal, they also have a wide rotational speed detection range, a result of advancements in electronic technology. Furthermore, this type of sensor can be used over a wide temperature range, has a long operating life, and is highly resistant to dust, water, and oil, thus tolerating various environmental conditions and external noise. Therefore, this type of sensor is widely valued in industrial applications.
Magnetic rotary sensors have wide applications in factory automation systems because of their satisfactory characteristics and maintenance-free operation. Their main applications include detecting the rotation of machine tool servo motors, positioning of robotic arms in factory automation, detecting hydraulic strokes, position detection of related equipment in factory automation, detection units for rotary encoders, and various rotation detection units.
Modern magnetic rotary sensors mainly include four-phase sensors and single-phase sensors. During operation, a four-phase differential rotary sensor uses one pair of detection units for differential detection and another pair for inverse differential detection. Thus, the detection capability of a four-phase sensor is four times that of a single-element sensor. Two-element single-phase rotary sensors also have their advantages, namely compact size, reliability, large output signal, ability to detect low-speed motion, strong resistance to environmental influences and noise, and low cost. Therefore, single-phase sensors will also have a good market.
Magnetic rotation sensors also have great application potential in home appliances. In the commutation mechanism of cassette recorders, magnetoresistive elements can be used to detect the end of the magnetic tape. Most home video recorders have variable speed and high-speed playback functions, which can also be controlled by detecting and controlling the spindle speed using magnetic rotation sensors to obtain high-quality images. The forward and reverse rotation and high and low speed rotation functions of the motor in washing machines can be detected and controlled using servo rotation sensors.
This type of switch can detect metal objects entering its detection area and control the opening or closing of its internal circuitry. The switch itself generates a magnetic field, and when a metal object enters this field, it causes a change in the magnetic field. This change is converted into an electrical signal through the switch's internal circuitry.
It further highlights that electromagnetic sensors are a widely used high-tech field, with both domestic and international research efforts focused on their development. The application of these sensors is permeating all aspects of the national economy, national defense, and people's daily lives. With the advent of the information society, their status and role will inevitably increase.
Magneto-optical effect sensor
Modern electrical measurement technology has become increasingly mature, and due to its advantages such as high accuracy and ease of connection to microcomputers for automatic real-time processing, it has been widely used in the measurement of electrical and non-electrical quantities. However, electrical measurement methods are susceptible to interference, and in AC measurements, the frequency response is not wide enough, and there are certain requirements for withstand voltage and insulation. With the rapid development of laser technology, these problems can now be solved.
Magneto-optic sensors are high-performance sensors developed using laser technology. Lasers, a rapidly developing technology from the early 1960s, marked a new stage in humanity's mastery and utilization of light waves. Previously, the low monochromaticity of ordinary light sources limited many important applications, but the advent of lasers enabled rapid advancements in radio and optical technologies, fostering their mutual penetration and complementarity. Currently, lasers have been used to create numerous sensors, solving many previously insurmountable technical problems and making them suitable for use in hazardous and flammable environments such as coal mines, oil and natural gas storage facilities.
For example, fiber optic sensors made with lasers can measure parameters related to crude oil jets and cracks in large oil tanks. At the measurement site, no power supply is required, making it particularly suitable for petrochemical equipment clusters with stringent safety and explosion-proof requirements. It can also be used to implement remote chemical sensing technology using optical methods in certain stages of large steel plants.
The principle of magneto-optical sensors mainly utilizes the polarization state of light to achieve sensor functionality. When a beam of polarized light passes through a medium, if an external magnetic field exists in the direction of beam propagation, the light will rotate by an angle as it passes through the polarization plane; this is the magneto-optical effect. In other words, the applied magnetic field can be measured by the angle of rotation. Under specific experimental setups, the deflection angle is proportional to the output light intensity. By illuminating a laser diode (LD) with the output light, the digitized light intensity can be obtained and used to measure specific physical quantities.
Since the late 1960s, RCLecraw's research report on the magneto-optical effect has attracted widespread attention. Japan, the Soviet Union, and other countries have conducted research, and domestic scholars have also explored its application. Magneto-optical sensors possess excellent electrical insulation properties, anti-interference capabilities, wide frequency response, fast response, and explosion-proof characteristics, making them uniquely effective for measuring electromagnetic parameters in special applications, particularly in high-voltage, high-current measurements in power systems, where they demonstrate significant potential advantages. Furthermore, by developing processing software and hardware, automatic real-time measurements in welding machine and robot control systems can also be achieved. In the use of magneto-optical sensors, the selection of the magneto-optical medium and laser is crucial, as different devices offer varying capabilities in terms of sensitivity and operating range. With the emergence of high-performance lasers and novel magneto-optical media in recent decades, the performance of magneto-optical sensors has become increasingly powerful, and their applications have become increasingly widespread.
As a specific type of sensor, magneto-optical sensors can function in specific environments and are also a very important type of industrial sensor.
pressure sensor
Pressure sensors are one of the most commonly used sensors in industrial practice. The pressure sensors we usually use are mainly made using the piezoelectric effect, and such sensors are also called piezoelectric sensors.
We know that crystals are anisotropic, while amorphous materials are isotropic. Some crystalline media, when deformed by mechanical force along a certain direction, exhibit a polarization effect; when the mechanical force is removed, they return to a neutral state. In other words, under pressure, some crystals may generate an electrical charge—this is the so-called polarization effect. Scientists have developed pressure sensors based on this effect.
The main piezoelectric materials used in piezoelectric sensors include quartz, sodium potassium tartrate, and ammonium dihydrogen phosphate. Quartz (silicon dioxide) is a natural crystal, and the piezoelectric effect was discovered in this crystal. Within a certain temperature range, the piezoelectric property persists, but it completely disappears above this range (this high temperature is known as the "Curie point"). Because the electric field changes only slightly with stress (meaning the piezoelectric coefficient is relatively low), quartz has gradually been replaced by other piezoelectric crystals. Sodium potassium tartrate has a high piezoelectric sensitivity and coefficient, but it can only be used in environments with low room temperature and low humidity. Ammonium dihydrogen phosphate is a synthetic crystal that can withstand high temperatures and relatively high humidity, and therefore has been widely used.
The piezoelectric effect is now also applied to polycrystalline materials, such as piezoelectric ceramics, including barium titanate piezoelectric ceramics, PZT, niobate-based piezoelectric ceramics, lead magnesium niobate piezoelectric ceramics, and so on.
The piezoelectric effect is the main working principle of piezoelectric sensors. Piezoelectric sensors cannot be used for static measurements because the charge generated after an external force is applied is only preserved when the circuit has infinite input impedance. This is not the case in reality, which means that piezoelectric sensors can only measure dynamic stress.
Piezoelectric sensors are primarily used in the measurement of acceleration, pressure, and force. A piezoelectric accelerometer is a commonly used type of accelerometer. It possesses excellent characteristics such as simple structure, small size, light weight, and long service life. Piezoelectric accelerometers have been widely used in vibration and shock measurements in aircraft, automobiles, ships, bridges, and buildings. The unique shape of piezoelectric sensors holds a special place in the aerospace field. (Piezoelectric sensor core...)
It can also be used to measure combustion pressure and vacuum levels inside engines. It can also be used in the military industry, for example, to measure the change in chamber pressure at the moment a bullet is fired and the shock wave pressure at the muzzle. It can be used to measure both large and small pressures.
Piezoelectric sensors are also widely used in biomedical measurements. For example, ventricular catheter microphones are made of piezoelectric sensors because measuring dynamic pressure is so common, so the application of piezoelectric sensors is very widespread.
Besides piezoelectric sensors, there are also piezoresistive sensors made using the piezoresistive effect, strain gauge sensors that utilize the strain effect, and other different pressure sensors that utilize different effects and materials to perform their unique applications in different situations.
