[b]1.1 Sensors[/b] Advances in information processing technology and the rapid development of microprocessors and computer technology necessitate corresponding progress in sensor development. Microprocessors are now widely used in measurement and control systems. As the capabilities of these systems increase, the role of sensors, as front-end units of information acquisition systems, becomes increasingly important. Sensors have become key components in automation systems and robotics, and their importance as a structural component of the system is becoming increasingly apparent. In the broadest sense, a sensor is a device that converts physical or chemical quantities into easily usable electrical signals. The International Electrotechnical Commission (IEC) defines it as: "A sensor is a front-end component in a measurement system that converts input variables into measurable signals." According to Gopel et al., "A sensor is a sensitive element that includes a carrier and circuit connections," while "a sensor system is a sensor system that combines some information processing (analog or digital) capabilities." The sensor is a component of a sensor system and is the first point of entry for the measured signal. The block diagram of a sensor system is shown in Figure 1-1. The signal amplitude entering the sensor is very small and is mixed with interference signals and noise. To facilitate subsequent processing, the signal must first be shaped into a waveform with optimal characteristics. Sometimes, signal linearization is also required. This is accomplished by amplifiers, filters, and other analog circuits. In some cases, a portion of these circuits is directly adjacent to the sensor component. The shaped signal is then converted into a digital signal and input to a microprocessor. German and Russian scholars believe that a sensor should consist of two parts: a sensitive element that directly senses the measured signal and a circuit that initially processes the signal. According to this understanding, the sensor also includes the circuitry of a signal shaper. The performance of a sensor system depends primarily on the sensor itself, which 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 requiring an external energy source or excitation source (see Figure 1-2(a)). Passive sensors cannot directly convert energy forms, but they can control the energy or excitation energy input from another input terminal (see Figure 1-2(b)). Sensors are responsible for converting specific characteristics of an object or process into quantities. The "object" can be a solid, liquid, or gas, and its state can be static or dynamic (i.e., a process). Once the object's characteristics are converted and quantified, they can be detected in various ways. The object's characteristics can be physical or chemical. According to its working principle, the sensor converts the object's characteristics or state parameters into measurable electrical quantities, 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 create sensors with different functions. Sensors can directly contact the measured object or not. The working mechanisms and effect types used in sensors are constantly increasing, and the processing procedures they incorporate are becoming increasingly sophisticated. Sensors are often compared to the five human senses: Photosensitive sensors – vision; Acoustic sensors – hearing; Gas sensors – smell; Chemical sensors – taste; Pressure-sensitive, temperature-sensitive, and fluid sensors – touch. While human senses are far superior to modern sensors, some sensors are more advanced. For example, humans cannot perceive ultraviolet or infrared radiation, electromagnetic fields, or colorless and odorless gases. Many technical requirements are set for sensors; some are applicable to all types of sensors, while others are specific requirements applicable only to certain types. The basic requirements for sensor operation and structure in different applications are: high sensitivity; stable anti-interference (insensitive to noise); linearity; easy adjustment (simple calibration); high accuracy; high reliability; no hysteresis; long service life (durability); repeatability; anti-aging; high response rate; resistance to environmental influences (heat, vibration, acid, alkali, air, water, dust); selectivity; safety (the sensor should be pollution-free); interchangeability; low cost; wide measurement range; small size, light weight, and high strength; wide operating temperature range. 1.2 Sensor Classification Sensors can be classified from different perspectives: their conversion principles (the basic physical or chemical effects of sensor operation); their applications; their output signal types; and the materials and processes used to manufacture them. Based on their working principles, sensors can be broadly classified into two categories: physical sensors and chemical sensors, as shown in Figure 1-3. Physical sensors utilize physical effects such as piezoelectricity, magnetostriction, ionization, polarization, thermoelectricity, photoelectricity, and magnetoelectricity. Even minute changes in the measured signal are converted into electrical signals. Chemical sensors include those based on phenomena such as chemical adsorption and electrochemical reactions; minute changes in the measured signal are also converted into electrical signals. Some sensors cannot be classified into either physical or chemical categories. Most sensors operate based on physical principles. Chemical sensors face more technical challenges, such as reliability, the feasibility of mass production, and price. Solving these challenges will lead to significant growth in the application of chemical sensors. The application areas and working principles of common sensors are listed in Table 1.1. Table 1.1 Sensors and Their Possibilities Sensor Types Working Principles Non-electrical quantities that can be measured: Force-sensitive sensors (resistive resistors, thermistors (NTC), PTC, semiconductor sensors) Resistance changes Force, weight, pressure, acceleration, temperature, humidity, gas Capacitive sensors Capacitive sensors Capacitive changes Force, weight, pressure, acceleration, liquid level, humidity Sensing sensors Inductance changes Force, weight, pressure, acceleration, rotational speed, torque, magnetic field Hall sensors Hall effect Angle, rotational speed, force, magnetic field Piezoelectric sensors, ultrasonic sensors Piezoelectric effect Pressure, acceleration, distance Thermoelectric sensors Thermoelectric effect Smoke, open flame, heat distribution Photoelectric sensors Photoelectric effect Radiation, angle, rotational speed, displacement, torque According to their applications, sensors can be classified as: Pressure-sensitive and force-sensitive sensors Position sensors Liquid level sensors Energy sensors Speed ​​sensors Thermistors Acceleration sensors Radiation sensors Vibration sensors Humidity sensors Magnetic sensors Gas sensors Vacuum sensors Biosensors, etc. Based on their output signal, sensors can be classified as follows: Analog sensors—convert the measured non-electrical quantity into an analog electrical signal. Digital sensors—convert the measured non-electrical quantity into a digital output signal (including direct and indirect conversion). Semi-digital sensors—convert the measured signal quantity into a frequency signal or short-period signal output (including direct and indirect conversion). Switch sensors—when a measured signal reaches a specific threshold, the sensor outputs a set low-level or high-level signal accordingly. Under the influence of external factors, all materials will exhibit corresponding and characteristic responses. Those materials most sensitive to external influences, that is, those with functional properties, are used to make the sensing elements of sensors. From the perspective of the materials used, sensors can be classified into the following categories: (1) According to the type of materials used: Metals, Polymers, Ceramics, Mixtures (2) According to the physical properties of the materials: Conductors, Insulators, Semiconductors, Magnetic Materials (3) According to the crystal structure of the materials: Single Crystals, Polycrystalline, Amorphous Materials Sensor development work closely related to the use of new materials can be summarized into the following three directions: (1) Exploring new phenomena, effects, and reactions in known materials and then making them practically applicable in sensor technology. (2) Exploring new materials and applying those known phenomena, effects, and reactions to improve sensor technology. (3) Exploring new phenomena, effects, and reactions based on the research of new materials and implementing them in sensor technology. The progress of modern sensor manufacturing depends on the intensity of development of new materials and sensitive elements for sensor technology. The basic trend of sensor development is closely related to the application of semiconductors and dielectric materials. Table 1.2 lists some materials that can be used in sensor technology and are capable of converting energy forms. According to their manufacturing process, sensors can be classified as: Integrated sensors, Thin-film sensors, Thick-film sensors, and Ceramic sensors. Table 1.2 Energy Conversion (Modulation) of Semiconductor and Dielectric Materials Energy Conversion (Modulation) Conversion Element Material Mechanical → Electrical (Voltage) Mechanical → Electrical (Impedance) Piezoelectric Element Mechanism Resistor PbTiO3, PbZrO3, PZT (PbZr1-xTixO3) Si, Ge, InSb Thermal → Electrical (Voltage) Thermal → Electrical (Impedance) Thermal → Electrical (Capacitive Reactance) Thermal → Electrical (Voltage) Thermistor Capacitor Thermoelectric Effect Element Bi2Te3, Sb2Te3 NiO, CoO, MnO BaSrTiO3 LiTaO3, PbTiO3 Light → Electrical (Voltage) Light → Electrical (Current) Photovoltaic Cell Photovoltaic Converter CbS Si, GaAa Magnetism → Electrical (Voltage) Magnetism → Electrical (Impedance) Hall Element Magnetoresistive Element InSb, InAs Ge, Si Gas → Electricity (Impedance) Humidity → Electricity (Impedance) Humidity → Electricity (Capacitive Reactance) Gas Sensing Element Humidity Sensing Resistor Capacitor SnO2, ZnO MgCr2O4-TiO2 Al2O3 Integrated sensors are manufactured using standard processes for producing silicon-based semiconductor integrated circuits. Typically, some circuitry for preliminary signal processing is also integrated onto the same chip. Thin-film sensors are formed by depositing a thin film of the corresponding sensitive material onto a dielectric substrate. When using hybrid processes, some circuitry can also be fabricated on this substrate. Thick-film sensors are made by coating a paste of the corresponding material onto a ceramic substrate, typically made of Al2O3, followed by heat treatment to form the thick film. Ceramic sensors are produced using standard ceramic processes or some variants thereof (sol-gel, etc.). After appropriate preparatory operations, the formed element is sintered at high temperature. Thick-film and ceramic sensor processes share many characteristics; in some respects, thick-film processes can be considered a variation of ceramic processes. As can be seen from the comparison in Table 1.3, each process technology has its own advantages and disadvantages. Due to the lower capital investment required for research, development, and production, and the high stability of sensor parameters, ceramic and thick-film sensors are more reasonable. Table 1.3 Comparison of Sensor Manufacturing Processes Integrated Sensor Thin-Film Sensor Thick-Film Sensor Ceramic Sensor Parameter Repeatability High High Medium Parameter Stability High Very High High Operating Temperature Range Below 150°C Below 600°C Below 600°C Below 700～800°C Capital Investment Required for Development and Production High High Medium Low Production Investment per Sensor in Small-Batch Production Low Low Low Low Investment per Sensor in Small-Batch Production High High Low Low Research and Development Investment High Very High Medium Medium Flexibility of Process Technology (Possibility of Adaptation) Low High Medium Medium High Production Scale (Annual Output) 10⁵～10⁶ 10⁴～10⁶ 10⁵～10⁷ 10⁵～10⁷