Definition
Single-point laser vibrometer is a non-contact vibration measurement instrument based on the principle of laser interference. It emits a laser beam to the surface of the object to detect the Doppler shift of reflected or scattered light, so as to accurately calculate the vibration velocity or displacement of the irradiation point along the direction of the laser beam. This technology plays an important role in vibration analysis in many industrial and scientific fields due to its high precision, high spatial resolution, and non-contact characteristics.
Measurement principle
The core physics of a single-point laser vibrometer is the optical Doppler effect. When the laser beam hits the surface of a vibrating object, the frequency of the reflected light will have a small offset due to the speed of the object's motion, that is, the Doppler frequency shift. Inside the instrument, a Michelson interferometer or similar optical path structure is usually used to interfere with the reflected light (signal light) that carries the vibration information of the object with the reference light. The interferometric light intensity signal is converted into an electrical signal by a photodetector, and the voltage signal directly proportional to the vibration velocity is extracted through the demodulation circuit. For displacement measurements, they are usually obtained by integrating the velocity signal.
Its basic relationship can be expressed as:
Δf = (2v/λ) * cosθ
Δf is the Doppler frequency shift, v is the vibration velocity of the object surface, λ is the laser wavelength, and θ is the angle between the laser beam and the normal direction of the object surface. When the laser is perpendicular incidence, cosθ=1.
Measurement method
When measuring with a single-point laser vibrometer, a standardized operating procedure is usually followed. First, it is necessary to select the appropriate measurement mode (e.g., using reflective lenses or direct measurement) according to the optical characteristics of the surface of the object to be measured (e.g., reflectivity, roughness), and if necessary, a special reflective foil can be applied at the measurement point to enhance the signal. Secondly, the focus position of the laser beam needs to be accurately adjusted to ensure that the spot falls on the point to be tested and the signal intensity is optimized. Instrument calibration is a critical step before measurement, and the sensitivity of the system is calibrated using a standard vibration source. During the data acquisition process, the measurement bandwidth, filtering parameters and sampling frequency need to be set reasonably to match the frequency range of the target vibration signal. Measurement results are usually presented and analyzed in the form of time-domain waveforms, spectrograms, or vibration magnitudes.
Influencing factors
The accuracy and reliability of measurement results are constrained by a variety of factors. In terms of environmental factors, air turbulence and temperature gradients can lead to beam drift or wavefront distortion. Strong ambient light or electromagnetic interference can affect signal quality. The characteristics of the measured object are the key factors, and the surface roughness, inclination angle, material light transmittance and reflectance will directly affect the return intensity and signal-to-noise ratio. For large amplitude or high-frequency vibrations, ensure that the vibration velocity does not exceed the maximum measurable speed limit of the instrument. In addition, the angular deviation of the laser beam from the direction of vibration introduces cosine errors. Operators need to identify and control these factors in their measurements in accordance with the requirements of relevant test standards, such as the ISO 5347 series of reference methods for vibration sensor calibration.
Applications
Single-point laser vibrometers have a wide range of applications. In precision manufacturing and mechanical engineering, it is used for vibration mode analysis, bearing condition monitoring and fault diagnosis of rotating machinery (such as turbines and motors). In the field of acoustics and transducers, it is used to evaluate the vibration characteristics of speaker diaphragms and ultrasonic transducers. In the field of microelectronics and microelectromechanical systems, it is used for dynamic performance testing of microstructures. In civil engineering, it can be used to measure micro-vibrations of large structures (e.g. bridges, buildings) under environmental excitation or load. In materials science research, it can be used to characterize the mechanical properties of composites and thin films. Its non-contact measurement feature makes it particularly suitable for vibration testing of high-temperature, light, soft or non-loadable objects.
Instrument selection considerations
When choosing a suitable single-point laser vibrometer, a comprehensive evaluation of various aspects is required. Technical parameters are the foundation, and attention should be paid to the measurement range (including maximum speed, displacement, and acceleration), frequency bandwidth, linearity, resolution, and working distance. Laser wavelength is an important consideration, with visible lasers facilitating light path alignment and infrared lasers having better adaptability to certain material surfaces. The instrument's ability to measure different surfaces (e.g., highlights, darkness, roughness) needs to be evaluated. The ease of use of the system, including the simplicity of the calibration process, the completeness of the software functions, and the openness of the data interface, should also be taken into account. In addition, it is important to confirm that the instrument meets the requirements of the standard method referenced in the specific industry or test project. Finally, a comprehensive comparison and selection should be made on the premise of clarifying one's own measurement requirements (such as target amplitude, frequency, accuracy, and the conditions of the measured object).