Technical Report: Vibration and its Sensors 3

Laser Doppler Vibrometer consists of an optical head that emits laser light and a conversion unit that processes the Doppler frequency from the reflected light.

The signal from the conversion unit is a voltage signal proportional to the movement speed of the target object. By inputting this signal into an FFT analyzer or similar device, it can be converted into acceleration and displacement. Furthermore, by performing frequency analysis on this signal, it is possible to analyze the behavior of the target object.

When sound waves, radio waves, or light waves with certain frequency components strike 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 emitted frequency and the frequency of the reflected wave.

(1)	When an object is approaching →	Radiation frequency < Reflection frequency
(1)	When an object moves away →	Radiation frequency > Reflection frequency
(2)	The difference between the radiated frequency and the reflected frequency is related to the speed of the object's movement; generally, the frequency difference increases with increasing speed.

Laser Doppler Vibrometer uses this principle. When a laser beam is shone onto a moving target, the frequency of the reflected light from the target is Doppler-shifted from the original frequency of the shone light due to the Doppler effect. Regarding the amount of this Doppler shift, if we let f _D be the shifted frequency, V be the velocity of the target, λ be the wavelength of the shone light, and θ be the angle between the direction of the shone light and the direction of the target's movement, then the following equation holds:

Here, if the frequency of the laser beam is _f0, then the frequency of the reflected light is _f0 + _fD. Since the wavelength λ of the laser beam used in Laser Doppler Vibrometer is extremely stable, the Doppler frequency _fD and the target's moving velocity V are proportional. Also, 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 the reflected light relative to the incident light: out-of-plane vibration*), so by measuring the Doppler frequency _fD, the target's moving velocity in the direction of irradiation can be determined. However, the frequency of the laser beam itself is extremely high and difficult to measure directly, so the Doppler frequency _fD is usually detected by interfering the irradiation light (_f0) and the reflected light (_f0 + _fD).

<Type>

Laser Doppler Vibrometer can be classified as follows based on their structure. Our corresponding models are listed on the right.

kinds	Compatible models from our company
Out-of-plane vibration, reference optical type	LV-1710, LV-1720A
3D vibration	LV-3300

The laser beam emitted from the light source is split into two channels via a beam splitter: one becomes the incident beam directed at the object not being measured, and the other becomes the reference beam returned within the instrument. The reflected beam returning from the target undergoes a Doppler shift according to the vibration velocity of the object not being measured. When this is interfered with the reference beam, which has been pre-frequency shifted by an AOM (Acousto-Optical Modulator), a beat frequency is obtained. Only the Doppler-shifted frequency component of this beat signal is extracted by a detection circuit and output as a voltage signal corresponding to the vibration velocity by an FM (Fiber-to-Motion) demodulation circuit.

The vibration of an object is not always unidirectional, but rather involves complex three-dimensional movements. In recent years, as products have become increasingly miniaturized and precision-driven, the need to measure and analyze such complex three-dimensional movements has been growing. The LV-3300 uses three reference-type Laser Doppler Vibrometer and performs vector calculations on the signals obtained from each to simultaneously measure the vibration velocity and direction of the object in the X, Y, and Z directions. The configuration of the optical head of the LV-3300 and the vector calculation formula are shown below.

LV-1710/1720 optical head ×3

One of the three optical heads is positioned so that the angle between the incident beam and the target's movement direction (Z direction) matches, while the other two are positioned at specific angles (ZX and ZY). The signals obtained from these two angled heads represent the vibration velocity and direction in the ZX and ZY directions, respectively. Therefore, the reflected beams from these three optical heads will have vibration components in the Z, ZX, and ZY directions of the target. By inputting these three signals into a vector processor, vibrations in the X, Y, and Z directions can be measured simultaneously.

Advantages	Non-contact measurement Wide measurement dynamic range High spatial resolution The sensor head is small.
Points to note	The sensor head needs to be directly facing the target. Ensuring sufficient reflected light from the laser beam. Affected by oil and water on the target Caution is required when measuring rotating bodies (due to the influence of noise caused by surface roughness).

Because it is a non-contact measurement method, it does not affect the object being measured. Furthermore, since the distance between the sensor head and the target can be appropriately maintained (standard distance of 100 mm to 5 m for the LV-1710/1720), it is possible to measure even high-temperature targets without affecting the sensor head.
The LV-1710 has a dynamic range of 0.3 μm/s to 10 m/s for velocity measurement and 1 Hz to 3 MHz (extendable up to 20 MHz) for frequency measurement. This means that in the high-frequency range, it can measure displacement down to 0.01 nm (0.00001 μm).
The laser spot size on the target is extremely small, ranging from tens of micrometers to hundreds of micrometers (20 micrometers to 400 micrometers for the LV-1710/1720), making it possible to measure vibrations of even minute targets. Furthermore, by integrating it with a dedicated microscope, it is possible to measure the spot vibrations of even smaller targets.
Measurements cannot be taken if oil, water, or other substances are flowing on the surface of the object being measured. Furthermore, if oil or water is present on the object being measured, it can cause errors, so it is necessary to remove them as much as possible. This is because unnecessary laser reflection occurs on the oil or water surface, which not only hinders the accurate capture of the vibration of the object itself, but may also make measurement impossible due to diffuse reflection.
For objects that have a significant component of movement or vibration laterally relative to the laser beam, such as rotating bodies, the reflected light may momentarily drop out depending on the surface roughness and shape of the object, causing whisker-like noise in the output signal, which may appear as a measurement error.