Laser Doppler Vibrometer

February 25, 2010

Laser Doppler Vibrometer

A Laser Doppler Vibrometer (LDV) is a scientific instrument that is used to make non-contact vibration measurements of a surface. The laser beam from the LDV is directed at the surface of interest, and the vibration amplitude and frequency are extracted from the Doppler shift of the laser beam frequency due to the motion of the surface. The output of an LDV is generally a continuous analog voltage that is directly proportional to the target velocity component along the direction of the laser beam.

Some advantages of an LDV over similar measurement devices such as an accelerometer are that the LDV can be directed at targets that are difficult to access, or that may be too small or too hot to attach a physical transducer. Also, the LDV makes the vibration measurement without mass-loading the target, which is especially important for MEMS devices.

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Principles of Operation

A vibrometer is generally a two beam laser interferometer that measures the frequency (or phase) difference between an internal reference beam and a test beam. The most common type of laser in an LDV is the Helium-neon laser[1], although laser diodes[2], fiber lasers, and Nd:YAG lasers are also used. The test beam is directed to the target, and scattered light from the target is collected and interfered with the reference beam on a photodetector, typically a photodiode. Most commercial vibrometers work in a heterodyne regime by adding a known frequency shift (typically 30-40 MHz) to one of the beams. This frequency shift is usually generated by a Bragg cell, or acousto-optic modulator.

A schematic of a typical laser vibrometer is shown below. The beam from the laser, which has a frequency fo, is divided into a reference beam and a test beam with a beamsplitter. The test beam then passes through the Bragg cell, which adds a frequency shift fb. This frequency shifted beam then is directed to the target. The motion of the target adds a Doppler shift to the beam given by fd = 2*v(t)*cos(α)/λ, where v(t) is the velocity of the target as a function of time, α is the angle between the laser beam and the velocity vector, and λ is the wavelength of the light.

Basic components of a laser Doppler vibrometer

Light scatters from the target in all directions, but some portion of the light is collected by the LDV and reflected by the beamsplitter to the photodetector. This light has a frequency equal to fo + fb + fd. This scattered light is combined with the reference beam at the photo-detector. The initial frequency of the laser is very high (> 1014 Hz), which is higher than the response of the detector. The detector does respond, however, to the beat frequency between the two beams, which is at fb + fd (typically in the tens of MHz range).

The output of the photodetector is a standard frequency modulated (FM) signal, with the Bragg cell frequency as the carrier frequency, and the Doppler shift as the modulation frequency. This signal can be demodulated to derive the velocity vs. time of the vibrating target.

Applications

LDVs are used in a wide variety of scientific, industrial, and medical applications. Some examples are provided below:

  • Aerospace - LDVs are being used as tools in non-destructive inspection of aircraft components[3].
  • Acoustic - LDVs are standard tools for speaker design, and have also been used to diagnose the performance of musical instruments[4].
  • Automotive - LDVs have been used extensively in many automotive applications[5], such as structural dynamics, brake diagnostics, and quantification of Noise, vibration, and harshness (NVH).
  • Biological - LDVs have been used for diverse applications such as eardrum diagnostics[6] and insect communication[7].
  • Calibration - Since LDVs measure motion that can be calibrated directly to the wavelength of light, they are frequently used to calibrate other types of transducers[8].
  • Hard Disk Drive Diagnostics - LDVs have been used extensively in the analysis of hard disk drives, specifically in the area of head positioning[9].
  • Landmine detection - LDVs have shown great promise in the detection of buried landmines. The technique uses an audio source such as a loudspeaker to excite the ground, causing the ground to vibrate a very small amount, and then the LDV to measure the ground vibration. Areas above a buried mine show an enhanced ground velocity at the resonance frequency of the mine-soil system. Mine detection with single-beam scanning LDVs[10], an array of LDVs[11], and multi-beam LDVs[12] has been demonstrated.

Types of Laser Doppler Vibrometers

  • Single Point Vibrometers - This is the most common type of LDV. Vendors include Polytec, MetroLaser, B&K, Brimrose, and Piezojena.
  • Scanning Vibrometers - A scanning LDV adds a set of X-Y scanning mirrors, allowing the single laser beam to be moved across the surface of interest.
  • 3-D Vibrometers - A standard LDV measures the velocity of the target along the direction of the laser beam. To measure all three components of the target's velocity, a 3-D Vibrometer measures a location with three independent beams, which strike the target from three different directions. This allows a determination of the complete in-plane and out-of-plane velocity of the target.
  • Rotational Vibrometers - A rotational LDV is used to measure rotational or angular velocity.
  • Differential Vibrometers - A differential LDV measures the out-of-plane velocity difference between two locations on the target.
  • Multi-Beam Vibrometers - A multi-beam LDV measures the target velocity at several locations simultaneously.[13]
  • Self-Mixing Vibrometers - Simple LDV configuration with ultra-compact optical head. These are generally based on a laser diode with a built-in photodetector, leading to a very rugged and compact optical system.[14][15]
 

Carleen Hutchins

February 25, 2010

Carleen Hutchins

Carleen Hutchins
Born May 5, 1911
Springfield, Massachusetts, United States
Died August 7, 2009 (aged 98)
Wolfeboro, New Hampshire, United States
Occupation acoustician, violinmaker and researcher

Carleen Maley Hutchins (May 24, 1911 – August 7, 2009) was an American former high school science teacher, violinmaker and researcher, best-known for her creation, in the 1950s/60s, of a family of eight proportionally-sized violins now known as the violin o...


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violin

February 25, 2010

Violin

Violin
Violin VL100.jpg
A standard modern violin shown from the front and the side
String instrument
Other names Fiddle, de: Violine or Geige, fr: Violon, it: Violino
Hornbostel-Sachs classification 321.322-71
(Composite chordophone sounded by a bow)
Developed Early 16th century
Playing range
Range violin.png
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Musicians

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Isaac Stern

February 25, 2010

Isaac Stern

 

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IsaacStern.jpg

Isaac Stern (Ukrainian: Стерн Ісаак; July 21, 1920 – September 22, 2001) was a Ukrainian-born violin virtuoso. He was renowned for his recordings and for discovering new musical talent.

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Posted by Marivic Morata.

 

Antonio Stradivari

February 25, 2010

Antonio Stradivari

Antonio Stradivari
Born 1644
Cremona, Italy
Died December 18, 1737 (aged 93)
Cremona, Italy
Occupation Luthier

Antonio Stradivari (1644 – December 18, 1737) was an Italian luthier, a crafter of stringed instruments such as violins, cellos, guitars and harps. Stradivari is generally considered the most significant artisan in this field. The Latinized form of his surname, Stradivarius, as well as the colloquial, "Strad", is often used to refer to...


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Posted by Marivic Morata.

 

Stradivarius

February 25, 2010

Stradivarius

Antonio Stradivari, by Edgar Bundy, 1893: a romanticized image of a craftsman-hero

A Stradivarius is a violin or other stringed instrument built by a member of the Stradivari family, particularly Antonio Stradivari. According to their reputation, the quality of their sound has defied attempts to explain or reproduce, though this belief is controversial. The name "Stradivarius" has also become a superlative applied to designate excellence. To be called "the Stradiva...


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Posted by Marivic Morata.

 

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