Overview of Vibration Measurement and Related Equipment

Ono Sokki offers a wide range of measuring instruments, from acceleration detectors that serve as the entry point for capturing vibrations, to impulse hammers used in natural frequency measurement and modal analysis, and Laser Doppler Vibrometer used for high-sensitivity vibration measurement, as well as equipment for analyzing acquired vibration data. We provide total vibration measurement and analysis systems that combine all of these instruments for various applications. We also offer consulting services from vibration data acquisition to analysis.

When sound waves, radio waves, or light waves with a certain frequency component are directed at an object moving at a certain speed, the frequency changes in proportion to the velocity component of the moving object. This is called the Doppler shift or Doppler effect.

At this time, the following relationship holds between the radiated frequency and the reflected frequency:

When an object is approaching, the radiation frequency < the reflected frequency.
When an object moves away, the radiation frequency > the reflected frequency.

The difference between the radiated frequency and the reflected frequency is related to the speed of the object's movement, and generally, the frequency difference increases with increasing speed. Laser Doppler Vibrometer utilize this principle. When a laser beam is shone on a moving target, the frequency of the reflected light from the target is Doppler-shifted from the original frequency of the irradiated light due to the Doppler effect.
Regarding the amount of Doppler shift at this time, if we let fD be the shifted frequency, V be the velocity of the target, λ be the wavelength of the irradiated light, and θ be the angle between the direction of the irradiated light and the direction of movement of the target, then the following equation holds:

Here, if we let the frequency of the laser beam be f0, then the frequency of the reflected light will be f0 + fD.
Because the wavelength λ of the laser light used in Laser Doppler Vibrometer is extremely stable, the Doppler frequency fD and the target's moving velocity V are directly proportional.
Furthermore, in Laser Doppler Vibrometer, the angle θ between the direction of laser irradiation and the direction of target movement is usually set to 0 degrees (detecting only the parallel component of reflected light relative to the incident light: out-of-plane vibration*). Therefore, by measuring the Doppler frequency fD, the target's movement velocity in the direction of irradiation can be determined.
However, the frequency of the laser light itself is extremely high and difficult to measure directly, so the detection of the Doppler frequency fD is usually done as follows:
Detection is performed by interfering the emitted light (f0) and the reflected light (f0+fD).

For more details, please read our technical report, "Vibration and its Sensors."

If you are considering purchasing this product, please contact your nearest sales office or agent.