Technical Report on Vibration and Its Sensors 3

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

Principle

When a sound wave, radio wave, or light wave with a certain frequency component strikes an object moving at a certain speed, the frequency changes in proportion to the speed component of the moving object. This phenomenon is called the Doppler shift or Doppler effect. At this time, the following relationship holds between the emitted frequency and the frequency reflected back.

(1)	When an object is approaching →	Emission frequency < Reflection frequency
(1)	When an object moves away.	Radiation frequency > Reflection frequency
(2)	The difference between the radio frequency and the reflection frequency is related to the speed of the object, and generally, as the speed increases, the difference in frequency becomes larger.

The Laser Doppler Vibrometer uses this principle. When laser light is irradiated onto 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. Looking at the amount of Doppler shift at this time, let the shifted frequency be f _D, the speed of the target be V, the wavelength of the irradiated light be λ, and the angle between the direction of the irradiated light and the direction of the target's movement be θ, then the following equation holds.

Here, if the frequency of the laser irradiation light is f ₀, the frequency of the reflected light is f ₀ + f _D. Since the wavelength λ of the laser light used in the Laser Doppler Vibrometer is extremely stable, the Doppler frequency f _D and the target's velocity V are in a proportional relationship. Additionally, in the Laser Doppler Vibrometer, the angle θ between the direction of the laser irradiation and the direction of the target's movement is usually set to 0 degrees (detecting only the parallel component of the reflected light to the incident light: out-of-plane vibration ^*), allowing the target's velocity in the irradiation direction to be determined by measuring the Doppler frequency f _D. However, since the frequency of the laser light itself is extremely high and difficult to measure directly, the detection of the Doppler frequency f _D is usually done by interfering the irradiation light (f ₀) and the reflected light (f ₀ + f _D).

<type>

Laser Doppler Vibrometers can be classified as follows based on their structure. The corresponding models from our company are displayed on the right.

Type	Corresponding model of our company
Out-of-plane vibration reference light type	LV-1710, LV-1720A
three-dimensional vibration	LV-3300

The laser beam emitted from the light source is split into two paths via a beam splitter; one path is the incident beam directed at a non-measured object, while the other is the reference beam that is returned within the equipment. The reflected beam returning from the target undergoes a Doppler shift according to the vibration speed of the non-measured object, and when it interferes with the reference beam, which has been pre-shifted in frequency by an acousto-optic modulator (AOM), a beat frequency is obtained. This beat signal is processed in the detection circuit to extract only the frequency components that have undergone a Doppler shift, and is output as a voltage signal corresponding to the vibration speed through an FM demodulation circuit.

Object vibrations are not always limited to one direction; they exhibit complex movements in three dimensions. With the recent trend towards miniaturization and high precision of products, the necessity to measure and analyze such three-dimensional complex movements is increasing. The LV-3300 uses three reference light Laser Doppler Vibrometers, and by vector calculating the signals obtained from each, it simultaneously measures the vibration speed and direction of the target object in the X, Y, and Z axes. The configuration of the optical head of the LV-3300 and the vector calculation formula are shown below.

One of the three optical heads aligns the angle between the incident beam and the target's movement direction (Z direction), while the other two are installed at specific angles (ZX, ZY). The signals obtained from these two heads correspond to the vibration velocities and directions in the ZX and ZY directions, respectively. Therefore, the reflected beams from these three optical heads will contain vibration components in the Z, ZX, and ZY directions, enabling simultaneous measurement of vibrations in the X, Y, and Z directions by inputting these three signals into a vector processor.

advantage point	non-contact measurement The measurement dynamic range is wide. high spatial resolution The sensor head is compact.
Points to note	The sensor head needs to be aligned with the target. Ensure the amount of reflected laser beam light. affected by oil and water on the target Caution is necessary when measuring rotating bodies (influence of noise due to surface roughness).

Non-contact measurement does not affect the object being measured. Additionally, the distance between the sensor head and the target can be appropriately adjusted (for models LV-1710/1720, the standard range is from 100 mm to 5 m), allowing for measurements even if the target emits high temperatures without affecting the sensor head.
In the case of the LV-1710, it has a speed measurement range of 0.3 μm/s to 10 m/s and a measurement frequency range of 1 Hz to 3 MHz (extendable up to 20 MHz). This allows for displacement conversion measurements down to 0.01 nm (0.00001 μm) in the high-frequency range.
The diameter of the laser spot on the target is very small, ranging from several tens of micrometers to several hundred micrometers (20 μm to 400 μm for the LV-1710/1720), allowing for the measurement of fine vibrations of small targets. Furthermore, by incorporating it into a dedicated microscope, it is also possible to measure the vibrations of even smaller target spots.
Measurements cannot be taken when oil, water, or similar substances are flowing on the surface being measured. Additionally, if oil or water is adhered to the measurement target, it can also cause errors, so it is necessary to remove them as much as possible. This is because unnecessary laser reflections occur on the oil or water surface, which not only hinders the accurate capture of the vibration amount of the measurement target but can also render measurements impossible due to diffuse reflections.
In the case of measurement targets that have significant lateral movement or vibration components against a rotating laser beam, the roughness and shape of the target's surface can cause the reflected light to momentarily disappear, resulting in spike-like noise in the output signal, which may manifest as measurement errors.