First, the definition of the sensor
The progress made in information processing technology and the rapid development of microprocessors and computer technologies all require corresponding advances in the development of sensors. Microprocessors are now widely used in measurement and control systems. With the enhancement of these system capabilities, the role of the sensor is becoming more and more important as the front end unit of the information acquisition system. Sensors have become a key component in automation systems and robotics. As a structural component of the system, its importance has become increasingly apparent. In its broadest sense, a sensor is a device that converts a physical or chemical quantity into an electrical signal that is easy to use. The International Electrotechnical Commission (IEC: International Electrotechnical Committee) is defined as: "A sensor is a front-end component of a measurement system that converts input variables into signals that can be measured." According to Gopel et al., "a sensor is a sensor that includes a carrier and a circuit," and "a sensor system is a sensor that combines some kind of information processing (analog or digital)." The sensor is an integral part of the sensor system and it is the first gate to which the measured signal is input. The block diagram of the sensor system is shown in Figure 1-1. The signal amplitude entering the sensor is very small and mixed with interfering signals and noise. In order to facilitate the subsequent processing, the signal must first be shaped into a waveform with the best characteristics, and sometimes the signal needs to be linearized. This work is done by amplifiers, filters, and other analog circuits. In some cases, some of these circuits are directly adjacent to the sensor components. The shaped signal is then converted into a digital signal and input to the microprocessor.
German and Russian scholars believe that the sensor should be composed of two parts, that is, the sensor part that directly senses the measured signal and the circuit part that initially processes the signal. In this understanding, the sensor also contains the circuit portion of the signal shaper.
The performance of the sensor system depends mainly on the sensor, which converts some form of energy into another form of energy. There are two types of sensors: active and passive. Active sensors can directly convert one form of energy into another without requiring an external source of energy or excitation (see Figure 1-2(a)).
Signal flow for active (a) and passive (b) sensors
Passive sensors cannot directly convert the energy form, but they can control the energy input from another input or the excitation energy sensor to take on the task of converting the specific characteristics of an object or process into quantities. Their "objects" can be solids, liquids, or gases, and their state can be static or dynamic (ie, process). Object characteristics can be detected in various ways after being converted and quantified. The characteristics of the object can be either physical or chemical. According to its working principle, the sensor converts object characteristics or state parameters into measurable electrical quantities, and then separates this electrical signal and sends it to the sensor system for evaluation or labeling.
Various physical effects and working mechanisms are used to make sensors with different functions. The sensor can be in direct contact with the object being measured or it may not be in contact. The type of working mechanisms and effects used for sensors are constantly increasing, and the processes involved are increasingly perfect. The function of the sensor is often compared with the human five major sensory organs: photosensitive sensor - vision, acoustic sensor - hearing, gas sensor - olfactory, chemical sensor - taste, pressure sensitive, temperature sensitive, fluid sensor - Touch, compared to contemporary sensors, humans have much better sensory abilities, but there are also sensors that are superior to human sensory functions. For example, humans do not have the ability to perceive ultraviolet or infrared radiation, do not feel electromagnetic fields, and have no color or odorless gas. Many technical requirements have been set for sensors, some of which apply to all types of sensors, and special requirements that apply only to specific types of sensors. The basic requirements for the working principle and structure of the sensor in different situations are: high sensitivity, anti-jamming stability (insensitive to noise), linear, easy to adjust (simple calibration), high precision and high reliability, no hysteresis , Long working life (durability), Repeatable anti-aging, High response rate, resistance to environmental influences (heat, vibration, acid, alkali, air, water, dust), selectivity, safety (sensors should be none Contaminated), interchangeable, low cost, wide measuring range, small size, light weight and high strength, wide operating temperature range
Second, the classification of sensors can be used to classify sensors from different perspectives: their conversion principles (basic physical or chemical effects of sensor work); their uses; their output signal types and the materials and processes used to make them. According to the working principle of the sensor, it can be divided into two major categories of physical sensors and chemical sensors. Classification of Sensor Operation Principles Physical sensors use physical effects such as piezoelectric effects, magnetostrictive phenomena, ionization, polarization, thermoelectricity, optoelectronics, and magnetoelectric effects. Small changes in the measured signal will be converted into electrical signals. Chemical sensors include those that are causally related to chemical adsorption, electrochemical reactions, etc. Small changes in the measured signal volume will also be converted into electrical signals. Some sensors can neither be classified into physical categories nor classified into chemical categories. Most sensors operate on the basis of physics. Chemical sensor technology problems, such as reliability issues, the possibility of scale production, price issues, etc., to solve such problems, the application of chemical sensors will have tremendous growth. The application fields and working principles of common sensors are listed in Table 1.1. According to their use, sensors can be categorized as: Pressure Sensitive and Force Sensors Position Sensors Level Sensors Energy Sensors Speed ​​Sensors Thermal Sensors Acceleration Sensors Radiation Sensors Vibration Sensors Humidity Sensors Magnetic Sensors Gas Sensors Vacuum Degree sensors, biosensors, etc.  Sensors can be categorized based on their output signals: Analog sensors—convert the measured non-electrical quantities into analog electrical signals.  Digital sensors - convert non-electrical quantities that are measured into digital output signals (including direct and indirect conversions).膺 膺 digital sensor - convert the measured signal into the output of the frequency signal or short-cycle signal (including direct or indirect conversion).开关 Switch Sensor - When a measured signal reaches a certain threshold, the sensor outputs a set low or high signal accordingly. Under the influence of external factors, all materials will make corresponding and characteristic responses. Among them, materials that are most sensitive to the outside world, that is, those that have functional properties, are used to make sensor sensitive components. From the point of view of the applied materials, the sensors can be divided into the following categories: (1) according to the category of the materials used: metals, polymers, ceramics, cerium mixtures; (2) physical properties of materials: conductors, insulators Semiconductors  Magnetic materials (3) According to the crystal structure of the material, single crystal  Polycrystalline  Amorphous materials  The development of sensors closely related to the use of new materials can be summarized in the following three directions:  (1) In already Known materials explore new phenomena, effects, and reactions, and then make them practical for use in sensor technology.  (2) Explore new materials and apply known phenomena, effects, and reactions to improve sensor technology.  (3) Explore new phenomena, new effects, and reactions based on new materials, and implement them in sensor technology.进展 The development of modern sensor manufacturing depends on the strength of the development of new materials and sensitive components for sensor technology. The basic trend of sensor development is closely related to the application of semiconductors and dielectric materials. Table 1.2 shows some of the materials that can be used in sensor technology and can convert energy forms.  According to its manufacturing process, sensors can be distinguished as: Integrated sensors, thin-film sensors, thick-film sensors, and ceramic sensors. Integrated sensors are manufactured using standard process technologies for producing silicon-based semiconductor integrated circuits. Parts of the circuits that are used to initially process the measured signal are also typically 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 using a hybrid process, some circuits can also be fabricated on this substrate. The thick film sensor is made by coating a ceramic substrate with a slurry of a corresponding material. The substrate is usually made of Al2O3 and then heat-treated to form a thick film. Ceramic sensors are produced using standard ceramic processes or some variant process (sol-gel, etc.). After an appropriate preliminary operation is completed, the formed element is sintered at a high temperature. There are many common characteristics between the two processes of thick film and ceramic sensor. In some aspects, it can be considered that the thick film process is a variation of the ceramic process. Each process technology has its own advantages and disadvantages. Due to the low capital investment required for research, development and production, as well as the high stability of sensor parameters, it is reasonable to use ceramic and thick-film sensors.