Sensors are an important component of industrial automation and control, as well as a key part of industrial robots, serving as the robot's execution unit.
Sensors are essential products of the information age, almost ubiquitous. They are key to humans acquiring information from the outside world. Currently, relying solely on our own sensory organs is far from sufficient for studying natural phenomena and laws, as well as for production activities. To adapt to this situation, sensors are needed. Therefore, it can be said that sensors are an extension of human senses, also known as electrical senses.
Miniaturization, digitization, intelligence, multifunctionality, systematization, and networking. Sensors are the primary link in achieving automatic detection and control. The existence and development of sensors have given objects senses such as touch, taste, and smell, gradually bringing them to life. They are typically classified into ten major categories based on their basic sensing functions: thermal sensors, photosensors, gas sensors, force sensors, magnetic sensors, humidity sensors, acoustic sensors, radiation sensors, color sensors, and taste sensors.
The advent of the new technological revolution has ushered in the information age. In utilizing information, the first challenge is obtaining accurate and reliable data, and sensors are the primary means of acquiring information in both natural and industrial fields. In modern industrial production, especially automated production, various sensors are used to monitor and control parameters throughout the process, ensuring equipment operates at its normal or optimal state and that products achieve the highest quality. Therefore, it can be said that without numerous high-quality sensors, modern production would lack its foundation.
Sensors are characterized by miniaturization, digitization, intelligence, multifunctionality, systematization, and networking. They not only promote the transformation and upgrading of traditional industries but also have the potential to establish new industries, thus becoming a new economic growth point in the 21st century. Miniaturization is based on microelectromechanical systems (MEMS) technology and has been successfully applied to silicon devices to create silicon pressure sensors. Several sensor classifications are introduced below.
1. Resistive type
A resistive sensor is a device that converts physical quantities such as displacement, deformation, force, acceleration, humidity, and temperature into resistance values. The main types of resistive sensors include strain gauge, piezoresistive, resistance temperature detectors (RTDs), thermistors, gas sensors, and humidity sensors.
2. Weighing
A load cell is a force-to-electrical conversion device that converts gravity into an electrical signal, and it is a key component of electronic weighing instruments. There are various types of load cells capable of force-to-electrical conversion, the most common being resistance strain gauge load cells, electromagnetic load cells, and capacitive load cells. Electromagnetic load cells are mainly used in electronic balances, capacitive load cells are used in some electronic crane scales, while the vast majority of weighing instruments use resistance strain gauge load cells. Resistance strain gauge load cells have a relatively simple structure, high accuracy, wide applicability, and can be used in relatively harsh environments. Therefore, resistance strain gauge load cells are widely used in weighing instruments.
3. Piezoresistive type
A piezoresistive sensor is a device made by spreading resistors on a semiconductor substrate based on the piezoresistive effect of semiconductor materials. The substrate can be directly used as the sensing element, and the spreading resistors are connected in a Wheatstone bridge configuration within the substrate. When the substrate is subjected to external force and deforms, the resistance values will change, and the Wheatstone bridge will produce a corresponding unbalanced output.
The substrate (or diaphragm) materials used in piezoresistive sensors are mainly silicon and germanium wafers. Silicon piezoresistive sensors, made with silicon wafers as the sensing material, are receiving increasing attention, especially solid-state piezoresistive sensors for measuring pressure and speed.
4. Laser
A laser sensor is a sensor that uses laser technology for measurement. It consists of a laser, a laser detector, and a measurement circuit. Laser sensors are a new type of measuring instrument with advantages such as enabling non-contact, long-distance measurement, high speed, high accuracy, large measuring range, and strong resistance to light and electrical interference.
When a laser sensor operates, a laser emitting diode first emits a laser pulse at the target. After reflection from the target, the laser light scatters in all directions. Some of the scattered light returns to the sensor receiver, where it is received by the optical system and imaged onto an avalanche photodiode. An avalanche photodiode is an optical sensor with internal amplification capabilities, enabling it to detect extremely weak light signals and convert them into corresponding electrical signals. Utilizing the high directionality, high monochromaticity, and high brightness of laser light, non-contact, long-distance measurements can be achieved.
5. Hall
Hall sensors are magnetic field sensors based on the Hall effect, widely used in industrial automation, detection technology, and information processing. The Hall effect is a fundamental method for studying the properties of semiconductor materials. The Hall coefficient, measured through Hall effect experiments, can determine important parameters of semiconductor materials, such as conductivity, carrier concentration, and carrier mobility.
6. Biology
Biosensors are an interdisciplinary field that organically combines bioactive materials (enzymes, proteins, DNA, antibodies, antigens, biomembranes, etc.) with physicochemical transducers. They are an essential advanced detection and monitoring method for the development of biotechnology, as well as a rapid and trace analysis method at the molecular level.
Various biosensors share the following common structure: they consist of one or more related bioactive materials (biomembranes) and a physical or chemical transducer (sensor) that can convert the signals expressed by bioactivity into electrical signals. These two components are combined and the biological signals are further processed using modern microelectronics and automated instrumentation technology to form various usable biosensor analysis devices, instruments, and systems.
7. Temperature
A temperature sensor is a sensor that can sense temperature and convert it into a usable output signal. Temperature sensors are the core component of temperature measuring instruments and come in a wide variety. They can be broadly classified into two categories based on the measurement method: contact and non-contact. Based on the sensor materials and electronic component characteristics, they are further divided into resistance temperature detectors (RTDs) and thermocouples.
8. Contact type
A contact temperature sensor, also known as a thermometer, has good contact between its sensing element and the object being measured. The thermometer achieves thermal equilibrium through conduction or convection, allowing its reading to directly represent the temperature of the object being measured.
Generally, the measurement accuracy is relatively high. The carburized glass resistance thermometer, which is made by carburizing and sintering porous high silica glass, is a type of temperature sensing element in low-temperature thermometers and can be used to measure temperatures in the range of 1.6 to 300K.
9. Non-contact
Its sensing element does not contact the object being measured, hence it is also called a non-contact temperature measuring instrument. This type of instrument can be used to measure the surface temperature of moving objects, small targets, and objects with low heat capacity or rapidly changing (transient) temperatures. It can also be used to measure the temperature distribution of a temperature field. The most commonly used non-contact temperature measuring instruments are based on the fundamental law of blackbody radiation and are called radiation thermometers. Radiation thermometry includes the luminance method (see optical pyrometer), the radiation method (see radiation pyrometer), and the colorimetric method (see colorimetric thermometer).
Advantages of non-contact temperature measurement: The upper limit of measurement is not limited by the temperature resistance of the sensing element, so there is no limit to the highest measurable temperature in principle. For high temperatures above 1800℃, non-contact temperature measurement methods are mainly used. With the development of infrared technology, radiation thermometry has gradually expanded from visible light to infrared light, and it has been used for temperatures below 700℃ up to room temperature, with very high resolution.
10. Thermocouple
Thermocouples are the most commonly used temperature sensors in temperature measurement. Their main advantages are a wide temperature range and adaptability to various atmospheric environments; they are also robust, inexpensive, require no power supply, and are the cheapest option. A thermocouple consists of two different metal wires (metal A and metal B) connected at one end. When one end of the thermocouple is heated, a potential difference is created in the thermocouple circuit. The temperature can be calculated by measuring this potential difference.
11. Thermistor
Thermistors are made of semiconductor materials, most of which have a negative temperature coefficient, meaning their resistance decreases as temperature increases. Temperature changes cause significant resistance changes, making them the most sensitive temperature sensors. However, thermistors have extremely poor linearity and are highly dependent on the manufacturing process. Manufacturers do not provide standardized thermistor profiles.
12. Accelerometer
An accelerometer is a sensor that measures acceleration. It typically consists of a mass, a damper, an elastic element, a sensing element, and adaptive circuitry. During acceleration, the sensor measures the inertial force acting on the mass and uses Newton's second law to obtain the acceleration value. Depending on the sensing element, common accelerometers include capacitive, inductive, strain gauge, piezoresistive, and piezoelectric types.
13. Piezoelectric
A piezoelectric accelerometer, also known as a piezoelectric accelerometer, is a type of inertial sensor. The principle of a piezoelectric accelerometer is based on the piezoelectric effect of piezoelectric ceramics or quartz crystals. When the accelerometer is subjected to vibration, the force exerted by the mass on the piezoelectric element changes accordingly. When the frequency of the measured vibration is much lower than the natural frequency of the accelerometer, the change in force is directly proportional to the measured acceleration.
14. Piezoresistive type
Based on world-leading MEMS silicon micromachining technology, piezoresistive accelerometers are characterized by small size and low power consumption, and are easy to integrate into various analog and digital circuits. They are widely used in fields such as automotive crash tests, testing instruments, and equipment vibration monitoring.
15. Capacitor type
A capacitive accelerometer is a capacitance sensor based on the principle of capacitance and varying electrode spacing. It is a widely used accelerometer and is irreplaceable in certain fields, such as airbags and mobile devices. Capacitive accelerometers utilize microelectromechanical systems (MEMS) technology, making mass production economical and ensuring lower costs.
16. Servo
A servo accelerometer is a closed-loop testing system characterized by good dynamic performance, a large dynamic range, and good linearity. Its working principle involves a vibration system consisting of a "mk" system, similar to a general accelerometer, but with an electromagnetic coil connected to the mass m. When there is an acceleration input to the base, the mass deviates from its equilibrium position. This displacement is detected by a displacement sensor, amplified by a servo amplifier, and converted into a current output. This current flows through the electromagnetic coil, generating an electromagnetic restoring force in the magnetic field of a permanent magnet, attempting to keep the mass in its original equilibrium position within the instrument housing. Therefore, the servo accelerometer operates in a closed-loop state.
17. Fiber Optic Sensor
The basic working principle of an optical fiber sensor is to send an optical signal from a light source into a modulator through an optical fiber. After the parameter to be measured interacts with the light entering the modulation area, the optical properties of the light (such as light intensity, wavelength, frequency, phase, polarization state, etc.) change, becoming a modulated signal source. After passing through an optical fiber into a photodetector, the parameter to be measured is obtained after demodulation.
18. Thermocouple sensing
A thermocouple consists of two metal wires of different materials welded together at their ends. By measuring the ambient temperature of the unheated portion, the temperature of the heated point can be accurately determined. Because it requires two conductors of different materials, it is called a thermocouple. Thermocouples made of different materials are used in different temperature ranges, and their sensitivities also vary. The sensitivity of a thermocouple refers to the change in output potential difference when the temperature of the heated point changes by 1°C. For most thermocouples supported by metallic materials, this value is approximately between 5 and 40 microvolts per degree Celsius (µV/°C).
There are many types of sensors, and special sensors are required for specific application scenarios. With the development of third-generation sensors, these sensors are characterized by the combination of microcomputer technology and detection technology, giving them a certain degree of artificial intelligence. In the next few years, a large number of intelligent sensors will also emerge.
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