Definition
A vibration displacement meter is an instrument used to measure the amount of position change in an object during vibration relative to a reference point. It usually takes displacement peak-to-peak or RMS as the main output parameter, and belongs to one of the basic equipment in the field of vibration measurement. The instrument is widely used in industrial equipment condition monitoring, structural dynamics analysis, and precision manufacturing process control, providing key data support for evaluating the stability and structural safety of machinery.
Measurement principle
Vibration displacement measurement is mainly based on non-contact sensing technology, among which the eddy current sensor principle is widely used. When the sensor coil with a high-frequency current is close to the surface of the conductor, an eddy current will be induced in the conductor, and the magnetic field generated by the eddy current will act on the original coil and change its equivalent impedance. There is a functional relationship between the impedance change and the distance from the coil to the conductor surface, and the displacement change can be reflected by converting the measurement circuit into a voltage signal. Its basic relationship can be expressed as:
ΔV = k · Δd
ΔV is the output voltage change, k is the system sensitivity coefficient, and Δd is the displacement change. Another common principle is laser interferometry, which calculates the displacement value by measuring the phase difference between the reference light and the measured light, which is suitable for higher precision measurement scenarios.
Measurement method
In actual measurement, the relative measurement method is usually used, where the sensor is fixed on a stationary reference frame and the probe is aligned with the surface of the object to be measured for continuous monitoring. For the axial displacement measurement of rotating machinery, sensors are often installed on the bearing housing or casing to measure the axial movement of the shaft relative to the chassis. System calibration is required prior to measurement, including sensitivity calibration and linearity verification. When measuring dynamically, it should be noted that the sampling frequency should meet the Nyquist sampling theorem, that is, the sampling frequency should be at least twice the highest frequency component of the measured vibration. For complex vibration waveforms, frequency domain analysis can be performed by Fourier transform to separate the frequency components.
Influencing factors
Measurement accuracy is influenced by various factors. In terms of sensor characteristics, the linear range, frequency response characteristics, and temperature stability of the probe will directly affect the measurement results. In mounting conditions, deviations in the verticality of the probe can lead to cosine errors, and the resonance of the mounting bracket may introduce additional vibrations. In terms of the measured object, the conductivity, surface roughness, and residual magnetic effect of the material will change the response characteristics of the eddy current sensor. Among environmental factors, strong electromagnetic interference may drown out the ineffective signal, and thermal expansion caused by temperature changes will introduce measurement errors. During signal transmission, cable impedance matching issues can lead to signal attenuation.
Applications
In the power industry, this instrument is used to monitor the axial displacement of the rotor of a steam turbine generator set to prevent friction between moving and static components. In the field of rail transit, it is used in the vibration characteristics test of bogies and the evaluation of track flatness. In aerospace, it is used for engine blade vibration monitoring and fuselage structure modal analysis. In precision manufacturing, assist in evaluating the accuracy of CNC machine tool spinal runout and guide rail movement. In the field of civil engineering, it is used for dynamic deflection measurement of bridges and health monitoring of building structures. In addition, it provides data support for material fatigue testing and shaker performance verification in the field of scientific research.
Selection considerations
When selecting a model, it is necessary to comprehensively consider the measurement requirements and technical parameters. The range should cover the maximum possible displacement of the object under test with an appropriate margin. The frequency response needs to meet the frequency characteristics of the measured vibration signal, and the natural frequency of the sensor is usually higher than the highest frequency to be measured. The resolution selection should take into account the actual measurement accuracy required, as too high a resolution may increase unnecessary costs. The output interface type needs to be compatible with the data acquisition system, and the common ones are analog voltage output and digital bus interface. In terms of environmental adaptability, it is necessary to select the corresponding products according to the requirements of on-site temperature, humidity and protection level. For special application scenarios, such as high-temperature environments or small displacement measurements, it is necessary to choose a special model of sensor.