Sensors are devices or instruments that can sense specified measured quantities and convert them into usable output signals according to certain rules. In modern industrial production, especially in automation processes, various sensors are used to monitor and control various parameters in the production process, ensuring that equipment operates in a normal or optimal state and that products achieve the best quality. Therefore, it can be said that without numerous excellent sensors, modern production would lose its foundation. There are many types of sensors, and this article summarizes some characteristics and types of sensors for future reference in applications.
1. Characteristics of Sensors
1. Dynamic characteristics of sensors. The dynamic characteristics refer to the response characteristics of the sensor to input quantities that change over time. When the input signal changes, the output signal changes accordingly over time, a process known as response. The dynamic characteristics of a good sensor allow it to accurately track the input signal in real-time when the input signal is a dynamic signal that changes over time. When the input signal changes slowly, tracking is easier, but as the input signal changes rapidly, the sensor’s ability to track in real-time gradually decreases. Typically, sensors are required not only to accurately display the magnitude of the measured quantity but also to reproduce the pattern of change over time, which is also one of the important characteristics of sensors.
2. Linearity of sensors. Generally, the actual static characteristic output of a sensor is a curve rather than a straight line. In practical work, to ensure that the instrument has uniform scale readings, a fitted straight line is often used to approximate the actual characteristic curve. Linearity (non-linear error) is a performance indicator of this approximation degree. There are various methods for selecting the fitted straight line, such as connecting the theoretical straight line between the zero input and full-scale output points or selecting the theoretical straight line that minimizes the sum of the squared deviations from each point on the characteristic curve; this fitted straight line is known as the least squares fitted line.
3. Sensitivity of sensors. Sensitivity refers to the ratio of the change in output quantity Δy to the change in input quantity Δx under steady-state working conditions. It is the slope of the output-input characteristic curve. If there is a linear relationship between the output and input of the sensor, then sensitivity S is a constant. Otherwise, it will change with the variation of the input quantity. The dimension of sensitivity is the ratio of the dimensions of output and input quantities. For example, in a displacement sensor, when the displacement changes by 1mm, the output voltage changes by 200mV, then its sensitivity should be expressed as 200mV/mm. When the dimensions of the output and input quantities are the same, sensitivity can be understood as a magnification factor.
4. Stability of sensors. Stability indicates the ability of a sensor to maintain its performance parameters over a long period. Ideally, the characteristic parameters of the sensor should not change over time. However, in reality, most sensors’ characteristics change over time. This is because the sensitive components or parts that constitute the sensor change over time, affecting the stability of the sensor.
5. Resolution of sensors. Resolution refers to the ability of a sensor to detect the smallest change in the measured quantity. In other words, if the input quantity changes slowly from a non-zero value, the sensor’s output will not change until the input variation exceeds a certain value, meaning the sensor cannot distinguish this change. Only when the change in input quantity exceeds the resolution will its output change. Typically, the resolution at various points within the full-scale range of the sensor is not the same, so the maximum change in input quantity that causes a step change in output is often used as a measure of resolution. If the above indicators are expressed as a percentage of the full scale, it is referred to as resolution.
6. Hysteresis of sensors. Hysteresis characterizes the degree of inconsistency between the output-input characteristic curves during forward (increasing input) and reverse (decreasing input) travel, usually expressed as the maximum difference ΔMAX between these two curves as a percentage of the full-scale output F·S. Hysteresis can be caused by energy absorption from internal components of the sensor.
7. Repeatability of sensors. Repeatability refers to the degree of inconsistency in the characteristic curves obtained when the input quantity varies continuously in the same direction over the full range. The closer the characteristic curves are to each other, the better the repeatability, and the smaller the random error.
2. Common Types of Sensors
The above summarizes some main characteristics of sensors; below are the common types of sensors.
1. Resistance-type sensors
Resistance-type sensors convert measured quantities, such as displacement, deformation, force, acceleration, humidity, temperature, etc., into resistance values. The main types include resistive strain gauges, piezoresistive sensors, thermistors, thermocouples, gas-sensitive sensors, and humidity sensors.
2. Frequency power sensors
Frequency power sensors sample AC voltage and current signals and transmit the sampled values through cables, optical fibers, and other transmission systems to digital input secondary instruments. The digital input secondary instruments perform calculations on the sampled voltage and current values to obtain parameters such as effective voltage, effective current, fundamental voltage, fundamental current, harmonic voltage, harmonic current, active power, fundamental power, and harmonic power.
3. Load cells
Load cells are force-to-electric conversion devices that convert weight into electrical signals and are a key component of electronic scales.
Various sensors can achieve force-to-electric conversion, commonly including resistive strain gauges, electromagnetic force sensors, and capacitive sensors. Electromagnetic force sensors are mainly used in electronic balances, while capacitive sensors are used in some electronic hanging scales. However, the vast majority of weighing products still use resistive strain gauge load cells. Resistive strain gauge load cells have a simple structure, high accuracy, wide applicability, and can be used in relatively poor environmental conditions.
4. Resistive strain sensors
Resistive strain gauges in sensors exhibit the strain effect of metals, meaning they produce mechanical deformation under external forces, causing the resistance value to change accordingly. Resistive strain gauges mainly consist of metal and semiconductor types, with metal strain gauges categorized into wire, foil, and thin-film types. Semiconductor strain gauges have the advantages of high sensitivity (usually tens of times higher than wire and foil types) and low transverse effect.
5. Piezoresistive sensors
Piezoresistive sensors are devices made based on the piezoresistive effect of semiconductor materials, created by diffusing resistors on the semiconductor substrate. The substrate can serve directly as the sensing element, and the diffused resistors are connected in a bridge configuration within the substrate. When the substrate is subjected to external forces and deforms, the resistance values change, resulting in a corresponding unbalanced output from the bridge. The materials used for the substrate (or diaphragm) in piezoresistive sensors are mainly silicon and germanium, with silicon piezoresistive sensors, made from silicon as the sensitive material, gaining increasing attention, especially for measuring pressure and speed.
6. Thermoresistive sensors
Thermoresistive temperature measurement is based on the property that the resistance value of metal conductors increases with temperature. Most thermoresistive sensors are made from pure metal materials, with platinum and copper being the most commonly used. Additionally, materials such as nickel, manganese, and rhodium have begun to be used to manufacture thermoresistive sensors. They mainly utilize the property of resistance changing with temperature to measure temperature and related parameters. These sensors are particularly suitable for applications requiring high temperature detection accuracy.
7. Laser sensors
Laser sensors use laser technology for measurement. They consist of a laser, a laser detector, and measurement circuitry. Laser sensors are a new type of measuring instrument with advantages such as non-contact long-distance measurement, fast speed, high precision, large measurement range, and strong resistance to light and electrical interference. During operation, the laser diode emits laser pulses aimed at the target, which are reflected by the target and scatter in various directions, with some scattered light returning to the sensor receiver, where it is captured by the optical system and imaged onto an avalanche photodiode.
8. Hall sensors
Hall sensors are magnetic field sensors made based on the Hall effect, widely used in industrial automation technology, detection technology, and information processing. The Hall effect is a fundamental method for studying the properties of semiconductor materials. The Hall coefficient determined through Hall effect experiments can be used to assess the conductivity type, carrier concentration, and carrier mobility of semiconductor materials.
9. Temperature sensors
Temperature sensors are primarily based on the principle that the resistance value and the potential of thermocouples change regularly with temperature, allowing us to obtain the desired temperature value. Temperature sensors come in various types and combinations, and suitable products should be selected based on different environments.
10. Wireless temperature sensors
Wireless temperature sensors convert the temperature parameters of the controlled object into electrical signals and send wireless signals to receiving terminals, performing detection, adjustment, and control of the system. They can be directly installed in the junction box of general industrial thermoresistive sensors or thermocouples, forming an integrated structure with the field sensing elements. Typically, they are used in conjunction with wireless repeaters, receiving terminals, communication serial ports, and electronic computers, which not only saves compensation wires and cables but also reduces signal transmission distortion and interference, resulting in high-precision measurement results.
11. Intelligent sensors
The function of intelligent sensors is proposed by simulating the coordinated actions of human senses and brains, combined with long-term research and practical experience in testing technology. They are relatively independent intelligent units, and their emergence alleviates the stringent requirements for hardware performance, while software assistance can significantly enhance the sensor’s performance.
12. Photoelectric sensors
Photoelectric sensors are among the most common sensors, with various types including phototubes, photomultiplier tubes, photoresistors, phototransistors, solar cells, infrared sensors, ultraviolet sensors, fiber optic photoelectric sensors, color sensors, CCD, and CMOS image sensors. Their sensitive wavelengths are around the visible light wavelength, including infrared and ultraviolet wavelengths. Photo sensors are not limited to light detection; they can also serve as detection elements in other sensors, detecting many non-electric quantities by converting these non-electric quantities into changes in light signals. Photo sensors are currently one of the most produced and widely used sensors, playing a very important role in automatic control and non-electric measurement technologies.
13. Visual sensors
Visual sensors refer to those capable of capturing light from a complete image with thousands of pixels. The clarity and detail of the image are often measured by resolution, represented by the number of pixels. Visual sensors have the ability to capture light from a complete image with thousands of pixels, and the clarity and detail of the image are usually measured by resolution, represented by the number of pixels.
14. Displacement sensors
Displacement sensors, also known as linear sensors, convert displacement into electrical quantities. Displacement sensors are linear devices that belong to metal induction, and their role is to convert various measured physical quantities into electrical quantities. They are divided into inductive displacement sensors, capacitive displacement sensors, photoelectric displacement sensors, ultrasonic displacement sensors, and Hall displacement sensors.
15. Grating sensors
Grating sensors are usually used in digital detection systems to detect high-precision linear displacement and angular displacement, and are one of the commonly used detection devices in CNC machine tools. The spatial resolution of grating sensors can generally reach around 1μm, and the length of a single grating can exceed 600mm, with the main grating capable of being spliced to measure ranges exceeding several meters.
16. Infrared sensors
Infrared sensors detect infrared radiation through the physical effects presented by the interaction between infrared radiation and materials, usually utilizing the electrical effects arising from this interaction. They measure the temperature difference between the target object and the sensor or between the object and the environment. The principle of thermocouples involves two different metals A and B forming a closed loop; when the temperatures of the two contact ends are different (T>To), a thermoelectric potential Eab is generated in the loop, where T is the hot end, working end, or measuring end, and To is the cold end, free end, or reference end. A and B are called thermoelectrodes. The size of the thermoelectric potential is determined by the contact potential (also known as Peltier potential) and the temperature difference potential (also known as Thomson potential).
17. Vacuum sensors
Vacuum sensors are produced using advanced silicon micromechanical processing technology, with integrated silicon piezoresistive sensitive elements as the core components of absolute pressure transmitters. Due to the use of silicon-silicon direct bonding or silicon-Pyrex glass electrostatic bonding to form vacuum reference pressure chambers, along with a series of stress-free packaging technologies and precise temperature compensation techniques, they exhibit excellent stability and high precision, making them suitable for measuring and controlling absolute pressure under various conditions.
18. Pressure sensors
Pressure sensors are the most commonly used sensors in industrial practice, widely applied in various industrial automation environments, involving water conservancy and hydropower, railway transportation, smart buildings, production automation, aerospace, military, petrochemical, oil wells, electric power, ships, machine tools, pipelines, and many other industries.
19. Ultrasonic distance measuring sensors
Ultrasonic distance measuring sensors use the principle of ultrasonic echo ranging, employing precise time difference measurement technology to detect the distance between the sensor and the target object. They use small angle, small blind zone ultrasonic sensors, featuring accurate measurement, non-contact, waterproof, corrosion-resistant, and low-cost advantages, making them suitable for liquid level and material level detection. Their unique liquid level and material level detection methods ensure stable output even in conditions where foams or significant fluctuations on the liquid surface make echo detection difficult.
20. Capacitive level sensors
Capacitive level sensors consist of capacitive sensors and electronic module circuits, with a basic two-wire 4~20mA constant current output. After conversion, they can output in three-wire or four-wire configurations, forming standard signals such as 1~5V, 0~5V, and 0~10mA. Capacitive sensors are composed of insulated electrodes and cylindrical metal containers containing the measured medium. As the material level rises, the dielectric constant of non-conductive materials is significantly smaller than that of air, causing the capacitance to change with the height of the material.
21. Antimony electrode acidity sensors
Antimony electrode acidity sensors are industrial online analyzers that integrate pH detection, automatic cleaning, and electrical signal conversion. They consist of an antimony electrode and a reference electrode as a pH measurement system. In the measured acidic solution, an oxide layer of antimony trioxide forms on the surface of the antimony electrode, creating a potential difference between the metallic antimony surface and the antimony trioxide. The magnitude of this potential difference depends on the concentration of antimony trioxide, which corresponds to the concentration of hydrogen ions in the measured acidic solution.
22. Conductivity sensors
Conductivity sensors are process instruments (integrated sensors) that indirectly measure ion concentration by measuring the conductivity of solutions, capable of continuously detecting the conductivity of aqueous solutions in industrial processes.
Since electrolyte solutions are good conductors of electricity like metallic conductors, there is always resistance when current flows through the electrolyte solution, conforming to Ohm’s law. However, the temperature characteristics of the resistance of liquids are opposite to those of metallic conductors, exhibiting negative temperature characteristics. To differentiate from metallic conductors, the conductivity of electrolyte solutions is represented by conductivity (the inverse of resistance) or specific conductivity (the inverse of resistivity). When two insulated electrodes form a conductivity cell and a test solution is placed in between, applying a constant pressure alternating current forms a current loop. If the voltage and electrode sizes are fixed, the loop current has a certain functional relationship with the conductivity.
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