Definition
Infrared vibrometer is a non-contact vibration measurement instrument based on the principle of infrared radiation. It realizes the quantitative analysis of vibration parameters by detecting the changes in infrared radiation caused by vibration on the surface of the target object. The instrument has a wide range of applicability in industrial monitoring, mechanical fault diagnosis, structural health assessment, and other fields, especially for high-temperature, rotating, or difficult-to-contact measurement scenarios.
Principle
The working principle of infrared vibrometer is mainly based on the law of thermal radiation and optical detection technology. When the temperature of an object is higher than absolute zero, its surface will continuously emit infrared radiation, and the radiation intensity is related to the surface temperature and emissivity of the object. When an object vibrates, the relative distance between its surface and the instrument detector changes periodically, resulting in corresponding modulation of the infrared radiation flux received by the detector. The optical system focuses the infrared radiation, converts it into an electrical signal by the photoelectric sensor, and then demodulates it through the signal processing circuit, and the vibration frequency, amplitude and other parameters can be extracted. Its basic relationship can be expressed as:
V ∝ Φ(ε, T, d)
where V is the output signal, Φ is the radiation flux, ε is the emissivity of the object surface, T is the surface temperature of the object, and d is the vibration displacement.
Measurement method
Typical measurement methods of infrared vibrometers include direct detection and heterodyne interferometry. The direct detection method directly inverts the vibration parameters by measuring the radiation intensity changes caused by vibration, and is suitable for the measurement of medium and low frequency vibrations with large amplitudes. The heterodyne interferometry method uses the principle of optical path difference interference between the reference light and the measured light to achieve high-precision vibration measurement by detecting the phase change of the interference signal, and is often used for micro-vibration analysis. During measurement, the instrument should be aimed at the target area, the focus and field of view should be adjusted to ensure that the detection spot covers the vibration feature points, and the correspondence between the signal and the physical quantity should be established through the calibration procedure.
Influencing factors
Measurement accuracy is influenced by various factors. Changes in the emissivity of the target surface can alter the radiation characteristics, often requiring surface treatment or parameter compensation to reduce errors. Stray radiation in the environment, such as sunlight and heat source interference, may reduce the signal-to-noise ratio, which can be mitigated by optical filtering and environmental shielding. The measurement distance and angle affect the radiated flux reception efficiency and need to be operated within the operating range allowed by the instrument. The thermal conductivity characteristics of the target material may lead to the attenuation of the vibration temperature change signal, and frequency response correction needs to be considered in data analysis. In addition, the noise level, bandwidth limit, and calibration stability of the instrument itself are also technical parameters that need to be paid attention to.
Applications
In rotating machinery monitoring, the instrument can be used for non-contact vibration monitoring of steam turbines and generator rotors. In the aerospace field, it is suitable for dynamic characteristic testing of engine blades and wing structures. In civil engineering, it can be used for structural health monitoring of bridges and high-rise buildings. In the manufacturing industry, it is used in vibration analysis of precision machine tools and monitoring of the status of production line equipment. The power industry is often used for vibration evaluation of high-voltage electrical equipment and transmission lines. In addition, it is also used in the field of scientific research for the study of material dynamics and the testing of microelectromechanical systems.
Instrument selection
Measurement needs and environmental conditions should be considered in the selection process. The vibration frequency range needs to cover the fundamental frequency and harmonic components of the object to be measured, and the bandwidth of common instruments can range from a few hertz to megahertz. The amplitude measurement range should match the expected vibration magnitude, from sub-micron to millimeter. The working distance needs to meet the on-site installation conditions, the short throw type is suitable for close measurement, and the long throw type can be used for remote monitoring. In terms of temperature adaptability, attention should be paid to the operating temperature range of the instrument and the surface temperature range of the measured object. The signal output interface should be compatible with the data acquisition system, commonly in the form of analog voltage, digital bus, etc. Additionally, the instrument's protection level, long-term stability, and calibration maintenance requirements should be evaluated to ensure its suitability in specific application scenarios.