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I. Description of the characteristic parameters of the fault 1. Output parameters of the device or component: The relationship between the output and input of a device, as well as among the output variables, can all reflect the operating condition of the device. 2. Degree of damage to equipment components: Deformation, wear, cracks, and corrosion are all characteristic parameters used to assess the technical condition of equipment. 3. Secondary effect parameters during equipment operation mainly include vibration, noise, temperature, power consumption, etc., generated by the equipment while it is in use. The output parameters of equipment or components, as well as the degree of damage to those components, are direct characteristic parameters of failures, whereas secondary effect parameters are indirect characteristic parameters. The advantage of using indirect characteristic parameters for fault diagnosis is that it can be carried out while the equipment is in operation, without the need to disassemble it. The drawback is that the relationship between the indirect characteristic parameters and the fault is not entirely deterministic. II. Process of fault diagnosis 1. Condition monitoring: Sensors are used to collect various types of information regarding the equipment’s operation, converting it into electrical signals or other physical quantities; these signals are then fed into a signal processing system for further analysis. 2. Analytical diagnosis: By examining the changes in the signs or characteristic parameters that reflect the operating condition of the equipment, or by comparing these signs with various patterns, it is possible to determine the presence of faults, their nature, causes, severity, as well as their development trend. 3. Governance prevention: Determine the methods for governance corrections and prevention based on the conclusions drawn from the analytical diagnosis. Condition monitoring is the foundation and prerequisite for fault diagnosis; fault diagnosis involves further analysis and processing of the monitoring results, with diagnosis being the goal. III. Vibration measurement: Based on whether it can be described by a function of definite temporal relationships, vibrations are divided into deterministic vibrations and random vibrations. 1. Basic parameters of vibration (1) Amplitude: The distance of the vibrating body or particle from its equilibrium position. (2) Frequency: the number of vibrations per second, expressed in HZ. (3) Period: The time required for one vibration; frequency and period are reciprocals of each other. (4) Phase: Indicates the position of the vibrating part relative to other vibrating parts or fixed parts. 2. The first derivative of vibration displacement with respect to time is velocity; the first derivative of velocity with respect to time is acceleration. Velocity is obtained by integrating acceleration with respect to time, and displacement is obtained by integrating velocity with respect to time. Therefore, for displacement, velocity, and acceleration, as long as one of them is measured, the other two physical quantities can be determined using the relationships of differentiation and integration. 3. Common vibration sensing sensors (structure and applications): Piezoelectric acceleration sensors operate on the piezoelectric effect of piezoelectric crystals. Piezoelectric accelerometers do not require an external power source and are considered energy-converting sensors. It consists of a compression spring, a mass block, a piezoelectric crystal, and a base, among other components. The piezoelectric crystal is the core of the accelerometer; the charge generated by the piezoelectric crystal is proportional to the acceleration of vibration, offering high sensitivity and stability. Magnetoelectric velocity sensors operate on the principle of magnetoelectric induction; they do not require an external power source and are also considered energy-converting sensors. It consists of magnets, coils, damping rings, springs, mandrels, a housing, and output wires. When the sensor vibrates along with the system being measured, relative motion occurs between the sensor coil and the magnetic field, which cuts through the magnetic flux lines and generates an induced electromotive force, thereby producing a voltage that is proportional to the vibration velocity. Vibration displacement signals are typically acquired using eddy current displacement sensors. It consists of a coil, a housing, and leads. It operates on the basis of the eddy current effect of a metal body in an alternating magnetic field. During operation, it converts the changes in the distance between the tip of the sensor and the surface of the object being measured into an electrical signal that is proportional to those changes. This type of sensor can not only measure the vibration and axial displacement of certain rotating shaft systems, but also the rotational speed. Eddy current displacement sensors are a type of non-contact measurement device, but they require an external power source and belong to the category of energy-controlled sensors. 4. Methods for analyzing abnormal vibrations (1) Total vibration value method: Direct measurement is carried out using sensors, and the trends are presented in tables or graphs; then, by referring to the \"criteria for determining abnormal vibrations\", it is determined whether the equipment is operating normally. (2) Frequency analysis method: The measured vibration signal is taken for frequency analysis, and by comparing the spectrum with the normal spectrum, it is possible to identify the source of vibration, the affected area, and the severity of the vibration. The purpose of the Fourier transform is to convert a time-domain signal into a frequency-domain signal. In a time-domain signal, the horizontal axis represents time; in a frequency-domain signal, the horizontal axis represents frequency or angular frequency. A frequency analyzer is an instrument that converts time-domain signals into frequency-domain signals. A frequency analyzer can decompose the waveform of a vibration signal into its various frequency components, thereby determining the frequency structure of the signal as well as the amplitude and phase of each harmonic that makes up the signal, and thus identifying the characteristics of the signal. Vibration pulse measurement method: It is mainly used for the measurement of rolling bearings, with the vibration peak value serving as the basis for evaluation. IV. Noise Measurement Noise: Vibration waves generated in the air by irregular mechanical vibrations. Sound pressure level, sound intensity level, and sound power level are objective measures of the intensity of noise; frequency or spectrum indicates the components of noise, and subjective perceptions such as loudness can also be used for measurement. 1. Physical measures of noise (1) Sound pressure: The increase in pressure (in Pa) resulting from the pressure fluctuations generated by the vibration of air particles as sound waves propagate. (2) Sound pressure level (dB): 20 times the logarithm to the base 10 of the ratio of sound pressure to a reference sound pressure. (3) Sound intensity: The energy of sound waves per unit area per unit time – sound intensity (W/㎡). (4) Sound intensity level: 10 times the logarithm to the base 10 of the ratio of sound intensity to a reference sound intensity – sound intensity level (dB). (5) Sound power: The total sound energy radiated by a sound source per unit of time – sound power (W). (6) Sound power level: 10 times the logarithm to the base 10 of the ratio of sound power to reference sound power – sound power level (dB). 2. Subjective measures of noise (1) Isochronic curves: The human ear’s perception of noise is influenced not only by the sound pressure level but also by the frequency of the noise. Loudness curve: The curve showing the relationship between sound pressure level and frequency for pure tones that are perceived as having the same loudness by a typical listener. The curve obtained by connecting the sound pressure levels at the hearing threshold at various frequencies, all with the same loudness, is known as the hearing threshold curve. Since the loudness is set to 0, the hearing threshold curve is also referred to as the zero-loudness line. Similarly, the curve that shows the relationship between the sound pressure level at which pain is perceived and frequency at different frequencies is called the pain threshold curve, or also known as the 120-dB loudness line. Between the hearing threshold and the pain threshold, there are 13 loudness levels, with loudness values of 0, 10, 20, 30 ┉┉ 110, 120. Points on the same curve have different frequencies and sound pressure levels, but the loudness is the same. (2) Weighted sound level: A sound level meter uses different circuits to apply varying degrees of attenuation to sounds of different frequencies, thereby enabling an approximate representation of people’s perception and response to sound. Three weighting networks, A, B, and C, are commonly used in sound level meters. Among them, the C-weighted network allows audible sounds at all frequencies to pass through to the same extent, so it represents the total sound level ； The B-weighted network causes sounds in the low-frequency range to experience a certain degree of attenuation as they pass through ； A weighted network causes greater attenuation in the low-frequency range of sound. In noise measurement, if: ① LC = LB = LA, it indicates that the acoustic energy of the noise is primarily concentrated in the high-frequency range ； ② When LC = LB > LA: it indicates that the acoustic energy of the noise is primarily concentrated in the medium frequency range ； ③ When LC > LB > LA: it indicates that the acoustic energy of the noise is primarily concentrated in the low-frequency range. 3. Noise measurement instruments (1) Microphone: Its function is to convert acoustic energy into electrical energy. A diaphragm is typically used to sense sound pressure, converting changes in sound pressure into vibrations of the diaphragm. Microphones are divided into three categories: ① Pressure type, where the diaphragm senses sound pressure ； ②Differential pressure type: the vibration of the diaphragm is determined by the pressure difference across the diaphragm ； ③Pressure and pressure difference combination type. Capacitive microphone: high sensitivity, wide dynamic range, stable output characteristics, strong adaptability to the surrounding environment, and small size. Piezoelectric microphone: It has a simple structure and low cost, low output impedance, high capacitance, and relatively high sensitivity; its performance is greatly affected by temperature and humidity. (2) Sound level meter: A sound level meter can be used to measure sound levels, perform spectral analysis, record the temporal characteristics of noise, and measure vibrations. The measured sound pressure signal is converted into a voltage signal by a microphone, and after passing through attenuators, amplifiers, and weighting networks, it is finally displayed on a decibel meter. 4. Fault noise identification methods can establish a limit based on the characteristic parameters of the noise signal to determine whether there is a fault or not. To identify the nature of the fault, its location, and severity, it is also necessary to extract the noise signal for spectral analysis. There are absolute, relative, and analog standards for noise judgment. The three methods correspond respectively to comparing the characteristic values of the noise signal obtained from measurements with standard characteristic values, characteristic values under normal operation, or characteristic values of similar devices under the same operating conditions. V. Temperature Measurement Methods 1. Temperature measuring instruments: Contact-type temperature measurement devices: The temperature sensing element is in direct contact with the object being measured, and temperature is determined through heat exchange. (1) Thermal expansion type (mercury, bimetal, liquid, gas, etc.). (2) Pressure type. (3) Thermal resistance type (platinum, nickel, copper, semiconductors, etc.): The resistance of the material changes with temperature; by utilizing this property, temperature can be converted into an electrical signal. (4) Thermocouple type (nickel-chromium-copper, nickel-chromium-nickel-silicon, platinum-rhodium-platinum, etc.): Measurement is based on the thermoelectric effect; that is, when two conductors made of different materials form a circuit and the temperatures at their ends differ, an electromotive force is generated, the magnitude of which depends on the materials and the temperature difference between the two ends. Once the material is determined, the thermoelectromotive force is only a function of the temperature being measured, and is independent of diameter and length. Non-contact temperature measurement devices: radiant pyrometers, optical pyrometers, colorimetric pyrometers, and infrared temperature measuring instruments. An infrared temperature measuring instrument consists of an infrared detector, an infrared optical system, a signal processing system, and a display system. Commonly used infrared temperature measurement instruments include: infrared thermometers and infrared thermal cameras (which can measure the distribution of temperature on an object’s surface or in a space). The core of an infrared temperature measuring instrument is the infrared detector, which converts the incident infrared radiation into electrical energy or other forms of energy. Based on their different radiation response mechanisms, they are divided into two categories: photodetectors and thermosensitive detectors. Infrared optical systems include reflective, refractive, and refract-reflective types. Commonly used infrared temperature measurement instruments include: infrared thermometers and infrared thermal cameras. The latter can measure the distribution of temperature on an object’s surface or in a space. The infrared radiation from the object under test is concentrated, filtered, and focused onto an infrared detector through an optical system. Then, an optical-mechanical scanning system arranges the infrared radiation fluxes from various points on the object’s surface in chronological order; these fluxes are converted into electrical pulses by the infrared detector, and the resulting signals are sent to a display via video signal processing to show a thermal image. 2. Common faults that can be detected through temperature measurement include bearing damage, fluid system failures, abnormal heating, accumulation of contaminants, damage to insulation materials, electrical component failures, failures in non-metallic parts, internal defects in machinery, and crack detection. VI. Non-destructive testing methods for cracks Cracks are the most serious defects in machine components. Cracks can occur at various stages, including raw material production, component processing, and equipment use. Possible methods include visual-optical inspection, penetrant testing, magnetic particle testing, radiographic testing, ultrasonic testing, eddy current testing, and acoustic emission testing. Among them, the acoustic emission detection method is a dynamic detection technique that is carried out under loading or operating conditions; the cracks play an active role by providing information on their movement; it features high sensitivity, a wide coverage area, and no missed detections ； However, it cannot reflect the situation of static defects. As for the eddy current detection method: it is suitable for flaw detection on the surface or near the surface of conductive materials ； High sensitivity, with automatic alarm display ； Contactless, suitable for fault diagnosis of typical components in high-temperature measurement applications ； It can be used for display, recording, and alarm generation, and it can estimate the location and size of defects. Shortcomings: Deep defects are difficult to detect, there are many influencing factors, and boundary effects exist.