First, the definition of sensor
The progress of information processing technology and the rapid development of microprocessor and computer technology require the development of sensors to make corresponding progress. Microprocessors have been widely used in measurement and control systems. With the enhancement of the performance of these systems, sensors, as the front-end units of information acquisition systems, play an increasingly important role. Sensor has become a key component in automation system and robot technology. As a structural component in the system, its importance is more and more obvious.
In the broadest sense, a sensor is a device that can convert physical or chemical quantities into easy-to-use electrical signals. International Electrotechnical Commission (IEC: International
electrotechnical
Committee) is defined as: "the sensor is the pre-element in the measurement system, which converts the input variables into measurable signals." According to Gopel et al, "sensor is a sensitive redundant component, including carrier and circuit connection", and "sensor system is a sensor with some information processing (analog or digital) ability". The sensor is an integral part of the sensor system, and it is the first channel to measure the signal input of collapse.
The principle block diagram of the sensor system is shown in figure 1- 1. The amplitude of the signal entering the sensor is very small, mixed with interference signals and noise. In order to facilitate the subsequent processing, it is necessary to shape the signal into a waveform with the best characteristics, and sometimes it is necessary to linearize the signal. This work is done by amplifiers, filters and other analog circuits. In some cases, a portion of these circuits is directly adjacent to the sensor assembly. Then, the shaped signal is converted into a digital signal and transmitted to the microprocessor.
German and Russian scholars believe that the sensor should be composed of two parts, namely, the sensitive component part that directly senses the measured signal and the circuit part that preliminarily processes the signal. According to this understanding, the sensor also includes the circuit part of the signal shaper.
The performance of sensor system mainly depends on the sensor that converts one form of energy into another. There are two types of sensors: active and passive. Active sensors can directly convert one form of energy into another without external energy or excitation source (see figure 1-2(a)).
Signal flow of active (a) and passive (b) sensors
Passive sensors can't directly convert energy forms, but can control energy input from another input or excitation energy.
Sensors undertake the task of converting specific characteristics of an object or process into quantities. Its "object" can be solid, liquid or gas, and their state can be static or dynamic (that is, process). After transformation and quantization, object features can be detected in many ways. The characteristics of an object can be physical or chemical. According to its working principle, the sensor converts the characteristics or state parameters of the object into measurable electric quantity, and then separates the electric signal and sends it to the sensor system for evaluation or marking.
Various physical effects and working mechanisms are used to manufacture sensors with different functions. The sensor may or may not be in direct contact with the measured object. The working mechanism and effect types of sensors are more and more, and the processing technology involved is more and more perfect.
The functions of sensors are often compared with the five sensory organs of human beings:
Photosensitive sensor-vision? Auditory sensor
Gas sensor-smell? Chemical sensor-taste
Pressure-sensitive, temperature-sensitive and fluid sensors-touch.
Compared with contemporary sensors, human beings' perception ability is much better, but some sensors are superior to human beings' perception function, such as human beings can't perceive ultraviolet or infrared radiation, electromagnetic fields, colorless and odorless gases and so on.
There are many technical requirements for sensors, some of which are applicable to all types of sensors, while others are only applicable to specific types of sensors. According to the working principle and structure of the sensor, the basic requirements for different occasions are:
High sensitivity, anti-interference stability (insensitive to noise) and easy linear adjustment (simple calibration)
High precision, high reliability and long service life (durability) without hysteresis.
Repeatability, anti-aging, high response rate and resistance to environmental influences (heat, vibration, acid, alkali, air, water and dust).
Selective safety (sensors should be pollution-free), interchangeability and low cost.
Wide measuring range, small volume, light weight, high strength and wide working temperature range.
Second, the classification of sensors
Sensors can be classified from different angles: their conversion principle (the basic physical or chemical effect of sensor work); Their uses; Their output signal types, materials and processes for manufacturing them, etc.
According to the working principle of sensors, they can be divided into physical sensors and chemical sensors.
Classification of sensor working principle Physical sensors apply physical effects, such as piezoelectric effect, magnetostrictive phenomenon, ionization, polarization, thermoelectric, photoelectric, magnetoelectric and other effects. Small changes in the measured signal amount will be converted into electrical signals.
Chemical sensors include sensors with chemical adsorption, electrochemical reaction and other phenomena as causal relations, and small changes in the measured signal amount will also be converted into electrical signals.
Some sensors can neither be classified as physical sensors nor as chemical sensors. Most sensors work according to physical principles. Chemical sensors have many technical problems, such as reliability, possibility of mass production, price and so on. After solving these problems, the application of chemical sensors will greatly increase.
The application fields and working principles of common sensors are listed in table 1. 1.
According to the use, sensors can be divided into:
Pressure and force sensors? position detector
Liquid level sensor? Energy consumption sensor
Speed sensor? thermal element
Acceleration sensor? Ray radiation sensor
Vibration sensor? humidity sensor
Magnetic sensor? gas sensor
Vacuum sensor? Biosensors, etc. ?
According to its output signal, the sensor can be divided into:
Analog sensor-converts measured non-electric quantity into analog electric signal. ?
Digital sensor-converts the measured non-electric quantity into digital output signal (including direct and indirect conversion). ?
Pseudo-digital sensor-converts the measured signal into frequency signal or short-period signal for output (including direct or indirect conversion). ?
Switch sensor-When the measured signal reaches a certain threshold, the sensor outputs a set low-level or high-level signal accordingly.
Under the influence of external factors, all substances will make corresponding and characteristic reactions. Among them, those materials that are most sensitive to external effects, that is, materials with functional characteristics, are used to make sensitive elements of sensors. From the perspective of application materials, sensors can be divided into the following categories:
(1) According to the type of materials used?
Metal? Polymer? Ceramics? Mixture?
(2) Divide conductors according to the physical properties of materials? Insulator? Semiconductor? Magnetic material?
(3) According to the crystal structure of the material?
Single crystal? Polycrystalline amorphous material?
The development of sensors closely related to the adoption of new materials can be summarized into the following three directions:?
(1) Explore new phenomena, effects and reactions in known materials, and then apply them in sensor technology. ?
(2) Explore new materials and apply those known phenomena, effects and reactions to improve sensor technology. ?
(3) Explore new phenomena, new effects and new reactions on the basis of studying new materials, and realize them in sensor technology. ?
The progress of modern sensor manufacturing industry depends on the development intensity of new materials and sensitive components used in sensor technology. The basic trend of sensor development is closely related to the application of semiconductors and dielectric materials. Table 1.2 gives some materials that can be used in sensor technology and can convert energy forms. ?
According to its manufacturing process, sensors can be divided into:
Integrated sensor? Thin film sensor? Thick film sensor? Ceramic sensor
Integrated sensors are manufactured by standard technology for producing silicon-based semiconductor integrated circuits. Usually, some circuits used for preliminary processing of measurement signals are also integrated on the same chip. ?
The thin film sensor is formed by a thin film of a corresponding sensitive material deposited on a dielectric substrate (substrate). When a hybrid process is used, a part of the circuit can also be fabricated on the substrate. ?
Thick-film sensor is made by coating the slurry of the corresponding material on a ceramic substrate usually made of Al2O3, and then heat-treating it to form a thick film.
Ceramic sensors are produced by standard ceramic technology or some variant technologies (sol-gel, etc.). ).？
After the proper preparation operation is completed, the molded parts are sintered at high temperature. There are many similarities between thick film technology and ceramic sensor technology. In some aspects, thick film technology can be considered as a variant of ceramic technology. ?
Each technology has its own advantages and disadvantages. Due to the low investment in R&D and production and the high stability of sensor parameters, it is more reasonable to use ceramic and thick film sensors.