Transcript Application of Laser Vibrometer

The basic physical principle
of a laser (light amplification
by stimulated emission of
radiation) is the induced
emission of photons. With
induced emission the emitted
photons
have
identical
properties and thus produce
coherent light of the same
wavelength.
A laser consists of an optical cavity which contains the lasing material
with a mirror placed at each end. The light, which is repeatedly reflected
between the two mirrors, is amplified. As one of the mirrors is only
partially reflecting, a small laser beam emanates from the cavity. To
keep the process going, energy is supplied in order to excite the atoms
in the lasing material.
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coherence length
The wave along the laser cavity is a standing
wave and the cavity of length L only
resonates when there is an integral number
n of half wavelengths l between the mirrors:
n = 2L / l
The frequency W is given by W = nc/ 2L
where c is the velocity of the light. The
separation of the multiple wavelengths
(longitudinal modes) is DW = c / 2L.
However, not all possible modes in a laser
cavity will be excited. Only those that are
within the gain profile of the lasing medium
will generate an emission.
The maximum distance over which the phase relationship will exist is called coherence length.
The relation between the coherence length and the bandwidth is c/Dl
The laser used for laser Doppler vibrometers is a helium neon (He-Ne) laser. This laser produces
a visible red beam (l = 0.6328 µm). This gas laser is an extremely low-noise light source and
therefore ideally suited for this application.
Such a laser can be stabilized so that only a single mode is excited. The line width is then a
couple of MHz which yields coherence lengths of about 200 to 300m.
Laser vibrometers are usually operated with multimode lasers where the lasers oscillate at 2-3
modes at the same time. The coherence length is only 10-20 cm because of the interference of
the beat frequencies of these different modes. However, this beat frequency generates a cos2
dependence of the visibility as the distance to the object is varied. The signal has a maxima at
distances of 2mL and minima at (2m-1)L where m is an integer. It is therefore possible to make
measurements at very long distances with such a laser. The measuring distance should be
adjusted to a visibility peak for maximum signal strength.
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A laser Doppler vibrometer is based on the principle of the detection of the Doppler shift of coherent
laser light that is scattered from a small area of a test object. The object scatters or reflects light from
the laser beam, and the Doppler frequency shift is used to measure the component of velocity which
lies along the axis of the laser beam.
As the laser light has a very high frequency W (approx. 4.74 x1014 Hz), a direct demodulation of the
light is not possible. An optical interferometer is therefore used to mix the scattered light coherently
with a reference beam. The photo-detector measures the intensity of the mixed light whose beat
frequency is equal to the difference frequency between the reference and the measurement beam.
Heterodyne and Homodyne detection
In homodyne detection, for a given relative phase shift the output is a constant (DC) signal level. This
level is indirectly related to the phase shift.
In heterodyne detection one modulates, usually by a frequency shift, one of two beams prior to
detection. Optical heterodyne detection detects the interference at the beat frequency. The AC signal
now oscillates between the minimum and maximum levels every cycle of the beat frequency. Since the
modulation is known, the relative phase of the measured beat frequency can be measured very
precisely even if the intensity levels of the beams are (slowly) drifting. This phase is identical in value to
the phase one measures in the homodyne case. There are many additional benefits of Optical
heterodyne detection including improved signal to noise when one of the beams is weak.
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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.
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A photodiode is a type of photodetector
capable of converting light into either current
or voltage, depending upon the mode of
operation.
One difference from Bragg diffraction is that the
light is scattering from moving planes. A
consequence of this is the frequency of the
diffracted beam f in order m will be Dopplershifted by an amount equal to the frequency of
the sound wave F.
Photovoltaic mode
a solar cell is just an array of large area
photodiodes.
Photoconductive mode
In this mode the diode is often reverse
biased. This increases the width of the
depletion layer, which decreases the
junction's capacitance resulting in faster
response times.
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In radio and signal processing, heterodyning is the generation of new frequencies by mixing, or multiplying, two
oscillating waveforms. It is useful for modulation and demodulation of signals, or placing information of interest
into a useful frequency range. This operation may be accomplished by a vacuum tube, transistor, or other signal
processing device. Mixing two frequencies creates two new frequencies, according to the properties of the sine
function: one at the sum of the two frequencies mixed, and the other at their difference. Typically only one of
these frequencies is desired—the higher one after modulation and the lower one after demodulation. The other
signal is either not passed by the tuned circuitry that follows, or may be filtered out.
Optical heterodyne detection
Since optical frequencies are far beyond any feasible electronic circuit bandwidth, all photon detectors are
inherently energy detectors not oscillating electric field detectors. However since energy detection is inherently
"square-law" detection, it intrinsically mixes any optical frequencies present on the detector. Thus sensitive
detection of specific optical frequencies is possible by Optical heterodyne detection when two different (closeby) wavelengths of light illuminate the detector so that the oscillating electrical output corresponds to their
difference frequency. This allows extremely narrow band detection (much narrower band than any possible color
filter can achieve) as well as precision measurements of phase and frequency of a signal light relative to a
reference light source.
The heterodyne detection of the vibrometer signal
The scattered or reflected 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 fo of the laser is very high (> 1014 Hz), which is
higher than the response of the detector. The resulted 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 by
demodulator in the instrument controller to derive the velocity vs. time of the vibrating target.
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•An excitation (shaker, loudspeaker,
hammer etc.) cause the object under
investigation to vibrate.
•The heterodyne detection signal is
demodulated by the decoder in the
controller. An output of which is
proportional to the velocity of the
vibration parallel to the measurement
beam is achieved at the vibrometer
channel.
The excitation type
•The voltage data on the vibrometer
channel is sampled by DAQ system in
the industrial PC. Final data is
presented by PSV software system.
•Periodic i.e. with a repeating signal (sinsusiodal, periodic chirp, periodic random, etc)
•Transient i.e. with a pulse (e.g. rectangular pulse, hammer blow, etc.)
•Stochastic (random) i.e. with noise (noise generator, self excited)
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Specular surfaces, i.e.
highly reflecting
surfaces, obey the
law: angle of
incidence = angle of
reflection. When
making
measurements from
such surfaces, the
optics of the LDV
need to be aligned so
that the reflected
light returns within
the aperture of the
collecting optics.
Diffuse surfaces scatter the
incident light over a large
angular area. The intensity
of the scattered light power
per unit solid angle follows
Lambert's cosine law. It can
vary greatly between shiny
surfaces and dull black
surfaces that absorb most
of the light.
Speckle patterns are
always produced when a
coherent light source is
focused onto a rough
surface. It is caused by
interference effects
between the beams
originating at the different
scattering centers on the
surface. If the focused
spot is very small, the
number of scattering
centers is small and the
angular dependence of the
path length differences in
a given direction is also
small. This leads to a large
angle over which the
interference condition is
reasonably constant and
thus a large solid angle
for the speckle.
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•Aerospace - LDVs are being used as tools in non-destructive
inspection of aircraft components.
•Acoustic - LDVs are standard tools for speaker design, and have
also been used to diagnose the performance of musical instruments.
•Automotive - LDVs have been used extensively in many automotive
applications, 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 and insect communication.
•Calibration - Since LDVs measure motion that can be calibrated
directed to the wavelength of the light, there are frequently used to
calibrate other types of transducers.
•Hard Disk Drive Diagnostics - LDVs have been used extensively in
the analysis of hard disk drives, specifically in the area of head
positioning.
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Polytec offers a comprehensive line of scanning laser Doppler vibrometers (SLDV).
Scanning offers all the advantages of a laser vibrometer together with speed, ease
of use, laser positioning accuracy and comprehensive data processing in a single
automated, turnkey package.
Users get a very quick, easily understood and accurate visualization of a
structure's vibrational characteristics without the inconvenience of attaching and
interpreting data from an array of transducers.
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Working distance
> 0.4 m
(shorter distances accessible by using close-up unit)
Laser wavelength
633 nm, visible beam
Laser protection class
Class II He-Ne laser, < 1 mW, eye-safe
Sample size
Several mm² up to m² range
Scan grid
Multiple grid densities and coordinate systems (polar, cartesian and
hexagonal) each with up to 512 x 512 points
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Position the PSV
The PSV system measures velocity component parallel to the laser beam. Position
scanning head and object such that:
The laser beam can cover the scanning area
The longitudinal axis of the scanning head is positioned perpendicular to the area to
be scanned
Optimize the signal level
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Observe the signal level indicator at the back side of the scanning head. The higher
the value, the better signal-to-noise ratio will be.
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The surface of the area can be treated. Highly reflective and transparent surfaced
usually do not scatter enought laser light back to the scanning head. Evenly matt
surfaces are ideal. Water-slouble white wall paint can be used for the surface
treatment.
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Enable laser and beam shutter
Adjust the camera view (Autofocus)
Laser autofocus (Signal Intensity)
Alignment (Scanning mirror-object surface)
Defined scanning area
Select the scanning parameter (excitation,
decoder, frequency, etc.)
Perform the scan
Post processing the data
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Alignment
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Define scanning patterns
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Scanning
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Post processing
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http://en.wikipedia.org/wiki/Vibrometer
Polytec Manual
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Thanks,
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