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1. Definition of a Sensor According to the National Standard GB7665-87, a sensor is defined as "a device or apparatus that can sense a specified measured quantity and convert it into a usable signal according to a certain rule, typically consisting of a sensitive element and a conversion element." A sensor is a detection device that can sense the information of the measured object and convert it into an electrical signal or other required forms of information output, following certain rules, to meet the needs of information transmission, processing, storage, display, recording, and control. It is the primary component for achieving automatic detection and control.
In Webster's New World Dictionary, "sensor" is defined as: "A device that receives power from one system and usually transmits it in another form to a second system." Based on this definition, the function of a sensor is to convert one form of energy into another. For this reason, many scholars also refer to sensors as "transducers."
2. The Role of Sensors To acquire information from the external world, humans must rely on sensory organs. However, the capabilities of human sensory organs alone are far from sufficient in studying natural phenomena, laws, and production activities. To adapt to this, sensors are necessary. Therefore, sensors can be considered extensions of the human senses, often referred to as the "electronic senses."
With the advent of the new technological revolution, the world has entered the information age. In the process of utilizing information, the first problem to solve is obtaining accurate and reliable information, and sensors are the primary means of acquiring information in both natural and production fields.
In modern industrial production, especially automated production processes, various sensors are used to monitor and control various parameters during the production process, ensuring that equipment operates under normal or optimal conditions, and that products achieve the best quality. It can be said that without numerous excellent sensors, modern production would lack a foundation.
In fundamental scientific research, sensors play an even more prominent role. Modern science and technology have ventured into many new fields: for example, observing the vast universe thousands of light-years away on a macro scale, observing particles as small as nanometers on a micro scale, observing celestial evolution over hundreds of thousands of years longitudinally, and observing instantaneous reactions as short as a fraction of a second. Additionally, various extreme technology studies have emerged, such as ultra-high temperature, ultra-low temperature, ultra-high pressure, ultra-high vacuum, ultra-strong magnetic fields, ultra-weak magnetic fields, and so on, which are essential for deepening our understanding of matter, exploring new energy sources, new materials, etc. Clearly, acquiring a large amount of information that human senses cannot directly obtain is impossible without appropriate sensors. Many obstacles in basic scientific research stem from the difficulty of obtaining information from objects, and the emergence of new mechanisms and highly sensitive detection sensors often leads to breakthroughs in the field. The development of some sensors often serves as the precursor to the development of certain interdisciplinary fields.
Sensors have long permeated areas such as industrial production, space exploration, ocean exploration, environmental protection, resource surveys, medical diagnostics, bioengineering, and even cultural heritage preservation. It is no exaggeration to say that from the vast outer space to the boundless ocean, and various complex engineering systems, almost every modern project relies on various sensors.
Thus, it is evident that sensor technology plays a crucial role in developing the economy and advancing social progress. Countries worldwide attach great importance to the development of this field. It is believed that sensor technology will soon experience a leap forward, reaching a new level commensurate with its importance.
3. Principles of Sensors Classification of sensor working principles:
Physical sensors utilize physical effects, such as the piezoelectric effect, magnetostriction, ionization, polarization, thermoelectricity, photoelectricity, magnetoelectricity, and other effects. Small changes in the measured signal are converted into electrical signals.
Chemical sensors include those that operate based on phenomena like chemical adsorption and electrochemical reactions, converting small changes in the measured signal into electrical signals. When a ±15V power supply is provided to the sensor, the crystal oscillator in the excitation circuit generates a 400Hz square wave. After being amplified by the TDA2030 power amplifier, it produces an AC excitation power supply, which is transmitted from the stationary primary coil to the rotating secondary coil through the energy ring transformer T1. The AC power obtained is converted into a ±5V DC power supply through the rectification and filtering circuit on the shaft, which serves as the operating power supply for the operational amplifier AD822. The high-precision regulated power supply composed of the reference power supply AD589 and the dual operational amplifier AD822 generates a precise ±4.5V DC power supply, which serves as both the bridge power supply and the operating power supply for the amplifier and V/F converter. When the elastic shaft is subjected to torsion, the strain signal detected by the strain bridge is amplified by the instrumentation amplifier AD620 into a strong signal of 1.5v ±1v, which is then converted into a frequency signal by the V/F converter LM131. This frequency signal is transmitted from the rotating primary coil to the stationary secondary coil through the signal ring transformer T2, and after being filtered and shaped by the signal processing circuit on the housing, a frequency signal proportional to the torque on the elastic shaft is obtained. This signal is at TTL level and can be provided to a dedicated secondary instrument or frequency counter for display or sent directly to a computer for processing. Since the gap between the moving and stationary rings of the rotary transformer is only a few tenths of a millimeter, and the parts on the sensor shaft are sealed inside a metal housing, forming effective shielding, it has strong anti-interference capabilities.
Some sensors cannot be classified as either physical or chemical. Most sensors operate based on physical principles. Chemical sensors face challenges such as reliability issues, the possibility of mass production, and cost issues. Once these challenges are addressed, the application of chemical sensors will see significant growth.
4. Applications of Sensors Common examples include:
Intelligent sensors are now widely used in aerospace, aviation, defense, science and technology, and various industrial and agricultural production fields. For example, they have broad application prospects in the field of robotics, where intelligent sensors endow robots with sensory capabilities akin to human senses and brain functions, enabling them to perceive various phenomena and perform various actions. In industrial production, traditional sensors cannot quickly and directly measure and control certain product quality indicators (e.g., viscosity, hardness, surface smoothness, composition, color, and taste). However, intelligent sensors can directly measure certain quantities (e.g., temperature, pressure, flow rate) during the production process, which have a functional relationship with product quality indicators.
Cygnus has produced a "glucose watch" that looks like a regular wristwatch and can perform painless, blood-free, continuous blood glucose testing when worn. The "glucose watch" has a pad coated with a reagent, which, when in contact with the skin, absorbs glucose molecules from the skin and undergoes an electrochemical reaction, generating an electric current. The sensor measures this current, and the processor calculates the corresponding blood glucose concentration from it, displaying the result as a digital reading.
5. Sensor Classification Sensors can be classified from different perspectives: their conversion principles (the basic physical or chemical effects sensors operate on); their purposes; the type of output signal; and the materials and processes used to create them.
Classification based on operating principles:
Some sensors cannot be classified as either physical or chemical. Most sensors operate based on physical principles. Chemical sensors face challenges such as reliability issues, mass production potential, and cost considerations. Once these challenges are addressed, the application of chemical sensors will grow significantly.
Classification by purpose:
Classification based on output signal type:
Classification by material: All materials exhibit characteristic responses to external factors, and those most sensitive to external influences, i.e., functional materials, are used to create the sensitive elements of sensors. Based on the materials used, sensors can be classified as follows:
Classification by process:
"Random" refers to something arbitrary and without a fixed pattern. Random vibration is, therefore, an irregular and chaotic type of vibration.
Describing a random vibration is more complex than describing a sinusoidal vibration. A sinusoidal vibration can be described using just its frequency and amplitude or acceleration.
Before discussing random vibration, let's first talk about non-sinusoidal periodic vibration. Periodic vibration includes a fundamental frequency corresponding to its period and several frequencies that are integer multiples of the fundamental frequency. Each frequency has its own amplitude. The intensity of periodic vibration can be represented by the root mean square (RMS) amplitude or RMS acceleration. However, random vibration does not have a fixed period, and its frequency components are continuous rather than discrete like in periodic vibration. Therefore, amplitude or acceleration must be represented by a curve that varies with frequency, known as the frequency spectrum curve.
We often use RMS acceleration to represent the intensity of random vibration and use what is called the "acceleration power spectral density" (PSD) curve to express its frequency characteristics instead of the frequency spectrum curve.
Additionally, when the RMS acceleration of random vibration is expressed in units of (m/s²), the acceleration power spectral density is expressed in units of (m²/s³). However, it is also common to use gravitational acceleration (G) as the unit for RMS acceleration, with the corresponding unit for acceleration power spectral density being (G²/Hz).
Sweep Mode: Linear Sweep (Hz/min or Min/Sweep), Logarithmic Sweep (Oct/min or Min/Sweep)
Octave (Oct): When A is the lower frequency limit and B is the upper frequency limit, log2 is approximately 0.3.
When B is twice A, it is equivalent to 1 octave (oct). For example, 10 to 20 Hz is 1 oct, 20 to 40 Hz is another 1 oct, and so on.
A cycle is equivalent to sweeping from A to B and back from B to A twice. If the number of cycles is 10, it implies the sweep will occur 20 times.
For example, for a sweep range of 10-150 Hz at 1 oct/min, with 10 cycles per axis and 3 axes:
For a single sweep from 20-2000 Hz, the time is 6 min 39 sec.
A rough estimation method: Use one finger to represent 1 oct, which corresponds to 1 minute.
Number of cycles: The number of sine wave cycles within the sweep time; commonly used for durability evaluation (e.g., 10 Hz fixed frequency, 10^6 cycles).
1. Question: What is vibration?
Answer: Vibration is the repetitive motion of an object or particle relative to its equilibrium position.
2. Question: How many parameters are there to describe vibration?
Answer: The main parameters to describe vibration are amplitude, velocity, and acceleration.
3. Question: How is the unit of acceleration expressed?
Answer: In the field of vibration engineering, acceleration is commonly represented by 'g'. In the International System of Units (SI), the unit is expressed in meters per second squared (m/s²). In China, 1g is typically equal to 9.80665.
4. Question: What is the working principle of a piezoelectric accelerometer?
Answer: A piezoelectric accelerometer operates based on the piezoelectric effect of crystals. When a piezoelectric crystal is deformed by an external force, it becomes polarized internally, generating an electric charge on its surface. When the force is removed, the crystal returns to its original state. The crystal and a mass are tightly pressed together and housed in a metal casing, forming the piezoelectric accelerometer. When subjected to vibration or shock, inertia causes the mass to exert a force on the piezoelectric crystal proportional to the vibration or shock. By measuring the change in charge on the crystal, the force can be determined, allowing the acceleration of the vibration or shock to be calculated. This is the basic working principle of the piezoelectric accelerometer.
5. Question: In terms of natural frequency and damping coefficient, where do displacement, velocity, and acceleration sensors operate relative to their natural frequency? What are their damping coefficients?
Answer: For general inertial sensors, accelerometers operate from extremely low frequencies to below their natural frequency. Displacement sensors operate above their natural frequency to very high frequencies, while velocity sensors work near their natural frequency. The damping coefficients of accelerometers and displacement sensors are generally less than 1, whereas the damping coefficient of velocity sensors is greater than 1. This setup ensures flat amplitude-frequency and phase-frequency characteristics within the working frequency range, allowing accurate measurement of the corresponding vibration quantities without distortion.
6. Question: How should the measured acceleration be considered when the mass of the accelerometer is non-negligible?
Answer: Generally, when an accelerometer is attached to the object being measured, the measured acceleration value is lower than it would be without the accelerometer attached. In special cases, the installation of the accelerometer may cause resonance in the component, significantly exceeding the expected measurement. In such cases, a different accelerometer model should be considered, or alternative methods should be explored.
7. Question: What is the amplitude linearity of a piezoelectric accelerometer? What is the purpose of verifying it?
Answer: Within the working range of a piezoelectric accelerometer (such as within the limit acceleration range), amplitude linearity is defined as the ratio of the deviation in sensor sensitivity at different accelerations relative to the reference sensitivity. The purpose of verifying amplitude linearity is to determine the dynamic range of the accelerometer. For example, standards specify that for vibration and shock measurements, amplitude linearity should be within ±5% and ±10%, respectively, while for standard piezoelectric accelerometers, it should be within ±3%.
8. Question: What is the definition of reference sensitivity for a working piezoelectric accelerometer?
Answer: The reference sensitivity of a working accelerometer is defined as the ratio of the electrical output to the acceleration applied to its mounting surface under specified conditions (amplitude, frequency, temperature, total capacitance, amplifier input resistance, mounting torque, etc.).
9. Question: What is mounting stiffness? What is its impact on vibration and shock measurement?
Answer: Mounting stiffness refers to the rigidity of the connection between a contact-type vibration sensor and the object being measured. If the sensor and the object are perfectly joined together as one, it forms a rigid body, meaning the mounting stiffness is very high and the elasticity is very low. However, in practical installations, mounting stiffness cannot be infinitely high, forming a spring-mass system with the sensor’s mass. If this system’s resonant frequency is very low, the measured vibration includes not only the object’s vibration but also the sensor-mounting spring system’s vibration, leading to significant distortion if resonance occurs. Different mounting methods, such as hand-held contact, clay mounting, wax bonding, magnetic mounting, 502 glue bonding, insulated screw mounting, and steel screw mounting, provide increasing mounting stiffness and resonant frequency, in that order.
10. Question: A factory-produced piezoelectric accelerometer has a frequency response curve from 200Hz to 35kHz. Can this accelerometer be used below 200Hz? Answer: Yes, it can be used below 200Hz because the frequency response of the accelerometer is flat down to below several hundred Hertz, generally extending to around ten Hertz. Further down, the amplifier characteristics should be checked for suitability. If the amplifier is good, the lower frequency limit can be below 1Hz. The factory provides the frequency response curve starting from 200Hz due to the limitations of the testing device, not because of the sensor.
11. Question: List sensors with output proportional to acceleration, velocity, and displacement.
Answer: Sensors with output proportional to acceleration include piezoelectric accelerometers, servo accelerometers, and strain-gauge accelerometers. Sensors with output proportional to velocity include electromagnetic sensors, velocity sensors, and seismometers. Sensors with output proportional to displacement include eddy current sensors, capacitive sensors, differential transformers, optical displacement sensors, and laser interferometers.
12. Question: What are transverse sensitivity and transverse sensitivity ratio in a sensor?
Answer: For a uniaxial sensor, transverse sensitivity refers to the ratio of the sensor’s electrical output to the input vibration when subjected to lateral vibration, and it is a function of frequency and the sensor's lateral position. The transverse sensitivity ratio is expressed as the maximum transverse sensitivity divided by the axial sensitivity of the sensor, given as a percentage.
13. Question: What are the differences in excitation principles between mechanical and electromagnetic vibration tables? What are their main characteristics?
Answer: A mechanical vibration table generates excitation through the inertia force produced by a rotating eccentric mass or the eccentric distance created by an eccentric connecting rod in motion. Its main characteristics are low vibration frequency, narrow frequency band, large waveform distortion, no magnetic leakage, inconvenient amplitude adjustment, and difficulty in automatic frequency sweeping. An electromagnetic vibration table, on the other hand, generates excitation based on the principle of electromagnetic force produced by a current-carrying conductor in a magnetic field. Its main characteristics include a relatively high lower frequency limit, wide frequency band, small waveform distortion, convenient amplitude and frequency adjustments, and some models can perform automatic frequency sweeping, but the table surface may have magnetic leakage, and the cost is higher.
14. Question: What is the crossover frequency of a vibration table? What does a low crossover frequency imply for vibration tables with the same thrust?
Answer: In vibration environment testing, the crossover frequency refers to the frequency point where the vibration characteristic changes from one type of relationship to another, such as from constant displacement-frequency to constant velocity-frequency. For vibration tables with the same thrust, a low crossover frequency implies a higher load capacity (larger M) or larger displacement amplitude, or a wider frequency band. Therefore, a lower crossover frequency indicates better performance.
15. Question: What is the role of the air gap in an electromagnetic vibration table? Answer: The air gap in an electromagnetic vibration table couples the stationary components, such as the magnet, excitation coil, or permanent magnet cylinder, with the moving parts, such as the voice coil, suspension springs, and table surface. Though small, the air gap plays a crucial role by coupling the electrical system with the mechanical system. If the air gap is too small, friction damage and waveform distortion can occur, which is not permissible. If the air gap is too large, electrical coupling weakens, reducing the electrical-to-mechanical conversion efficiency. Therefore, the air gap must be kept clean, dry, and stable to avoid noise, drift, wandering, or waveform distortion.
16. Question: What is low-frequency crosstalk in a vibration table? What are its main causes?
Answer: Low-frequency crosstalk in a vibration table refers to the phenomenon where a low-frequency, large-amplitude vibration is superimposed on the specified vibration waveform. The causes of low-frequency crosstalk are many, including mechanical guiding, dry friction in the transmission parts, and resonance caused by the mechanical table’s transmission pulley. Sometimes, 50Hz interference can also cause crosstalk in an electromagnetic table. To eliminate low-frequency crosstalk, mechanical lubrication should be improved, the air gap in the electromagnetic table should be kept clean, and various mechanical transmission and 50Hz power supply interferences should be minimized.
17. Question: What are the main points to consider when using a cam-type shock table?
Answer: 1) Select the severity level and install the test piece according to the relevant standards based on the test requirements. 2) For shock tables with cushioning layers on both sides, the buffer layers must be arranged in the order required by the calibration severity level, with both sides corresponding one-to-one. 3) When changing the severity level, replacing the buffer layers, or stopping the use of the shock table, safety precautions must be taken. After the work is finished, the shock table surface must be separated from the buffer layers to avoid long-term compression of the layers, which could alter their characteristics.
18. Question: How should the mass of the moving system in an electromagnetic vibration table be determined?
Answer: The mass of the moving system can be determined by keeping the thrust of the vibration table constant and measuring the change in amplitude with and without the table surface, after measuring the air gap and ensuring it is within the normal range. To minimize the influence of electromagnetic damping, the measurement should be done at the crossover frequency. Additionally, the mass can be verified by measuring the low-frequency axial resonance of the electromagnetic moving coil and comparing it with the natural frequency of the moving system. The resonance can also be measured using a displacement sensor with constant acceleration excitation, such as by monitoring the output of a double-integrated accelerometer. The difference between the moving coil mass and the table surface mass should be small, to maintain accuracy.
19. Question: What is mechanical noise in a vibration table? How can it be reduced? Answer: Mechanical noise in a vibration table refers to unwanted vibrations or sounds that occur during operation. To reduce mechanical noise, the machine should be assembled correctly and tested. Components should be cleaned and checked for wear, with any worn parts replaced. The air gap should be kept within its normal range, and the moving coil should be centered. Additionally, the machine should be isolated from external vibrations and shock.
A complete vibration test system typically consists of the following key components:
A vibration test system can perform various types of vibration tests, including but not limited to:
When selecting the appropriate vibration test system, consider the following factors: