Vibration measurement for object in the liquid with Laser Doppler Vibrometer


Laser Doppler vibrometer can measure vibrating objects in the liquid by adjusting the conditions. There are a variety of devices used in liquid. Typical devices are ultrasonic cleaning tanks that clean semiconductors and machine parts and fish detectors. They are products that radiate and vibrate ultrasonic waves in liquids and usage their characteristics in various ways. Since these devices use ultrasonic vibrators that vibrate at a few tens of kHz to a few MHz, they cannot be measured with contact-type vibration detectors. Also when they are used in the air, it is difficult to operate correctly and demonstrate the characteristics as these devices are premised on usage in the liquid and the great load differences between liquid and air cause impedance mismatch. Therefore, in order to design and verify these devices, it is necessary to use them in actual operation in the liquid. By using laser Doppler vibrometer under appropriate conditions, it enables to obtain highly accurate vibration measurement to solve these problems.

System configuration

  Type Product name Remarks


LV-1800 Laser Doppler vibrometer  


LV-0160 20 MHz Wideband unit Measurement speed range: min. 2 mm/s, frequency range: up to 20 MHz


LV-0030 Large size Magnet stand Attach LV-0015 fine positioning XY stage if necessary
4 DS-3000 Data station Up to 40kHz or 100kHz
5   Digital Oscilloscope Select if necessary

Example of analysis data

Display screen of High speed Digital Oscilloscope


Vibration waveform of ultrasonic vibrator in water (upper ch1), drive signal (lower), each after averaging.


  • Enter laser beam into liquid effectively. → Use glass plate for liquid surface to enter/ receive light.
    The condition of measurement availability is that the laser beam can be transmitted through the liquid and it can receive sufficiently the light radiated and reflected against target. That is the minimum condition. Light is most reflected, scattered at the entrance of target and attenuated. Thus, it is most lost at the liquid surface. To avoid this, it is very efficient to make the liquid surface flat with a glass plate.

  • Place the target at shallow as much as possible.

    Even in the case of normal water (underwater), water itself has a characteristic that it is difficult to transmit light having a wavelength longer than about 450 nm. Blue light is easily transmitted, but red and infrared light are attenuated. The He-Ne (helium neon) laser used in the laser Doppler vibrometer with its wavelength of about 633 nm and red light has the characteristic of being greatly attenuated in water. Although the spectral transmission characteristics differs depending on the nature of the liquid, entering the light to excess into the liquid for a long time will make, S / N ratio worse.

    The laser Doppler vibrometer enables to measure the vibration frequency of the liquid surface of ultrasonic cleaning tank, etc. The following two points are important when measuring vibration of liquid surface.


  • Liquid surface with low frequency
    It is important to treat the liquid surface as a mirror surface and receive the laser light radiated from the sensor with regular reflection. When the laser beam is reflected obliquely by low-frequency vibrations (waves) other than ultrasonic vibration given for cleaning, it is difficult to be detected.

  • Shorten distance from liquid surface, radiate vertically against liquid surface.
    The laser beam should be entered into vertically against liquid surface and the regular reflection should be returned to the sensor. The fine wave on the liquid surface makes laser beam scattered In order to make the sensor receive these scattered light, the distance between the liquid surface and the sensor is shortened.




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