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Module 5: Experimental Modal Analysis for SHM Lecture 36: Laser doppler vibrometry

The Lecture Contains:

Laser Doppler Vibrometry

Basics of Laser Doppler Vibrometry

Components of the LDV system

Working with the LDV system

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Laser Doppler Vibrometry

Laser Doppler Vibrometer (LDV) is a Laser based non-contact vibration measurement system. It consists of three measuring scan heads which are capable of measuring the movements in all the three orthogonal directions yielding full information of the three dimensional movements [Figure 36.1]. The system works on the principle of Doppler Effect and interferometry for vibration measurement.

Figure 36.1 Laser Doppler Vibrometer with composite test plate

The minimum detectable vibration speed using this system is 5 µm/s at1Hz resolution while the maximum speed is of 10 m/s. The LDV system software controls the entire measurement process with graphical user interface. The PSV system also has the provision for input channels which can be used for simultaneous acquisition of data from accelerometers, load cells etc. Transfer function between any of the input channels connected to the system can be obtained. Signal generator card (NI-671x) contained in the system is used for generating excitation signals in the frequency range of 0-80 kHz. LDVs can measure vibrations up to 30 MHz range with very linear phase response and high accuracy. Applications of LDV include modal analysis of automotive parts, car bodies and aircraft panels etc.


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Basics of Laser Doppler Vibrometry

● Doppler Effect ● Heterodyne Interferometry

Doppler Effect

Doppler Effect is the change in the frequency (or wavelength) of emitted waves as the source of the wave approaches or moves away from an observer. This effect was named after the Austrian physicist Christian Johann Doppler who first stated this physical principle in 1842. The change or shift in frequency observed depends on the speed and direction of travel of both source and observer. Helium-Neon (He-Ne) Laser beam is made to incident on the vibrating surface and the reflected Laser light from the surface is detected by the vibrometer scanning unit. Incident and reflected beams are made to interfere on the detector by suitable arrangement. A moving surface induces a frequency shift on the light received by Vibrometer optics.

(36.1)

where fD is the frequency shift in the reflected beam, V is the velocity of the surface and λ is the wavelength of the He-Ne Laser.


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Heterodyne Interferometry

The Laser Doppler Vibrometer works on the basis of optical interference requiring two coherent light beams. The interference term relates to the path difference between both the beams. If the path difference between the interfering beams is integral multiplier of Laser wavelength, constructive interference occurs.

(36.2)

Itotal is the resultant intensity, I1 and I2 are the intensities of two interfering Laser beams and ( r1 - r2 ) is the path difference. In

this case, overall intensity becomes four times the single intensity. If the path difference is odd multiplier of half the wave length, destructive interference occurs where the overall intensity becomes zero. The interference phenomenon is exploited technically in Laser Doppler Vibrometer as shown in the Figure 36.2.

Figure 36.2 Schematic system setup for measuring vibration using LDV

A He-Ne Laser beam is split by a beam splitter BS1 into a reference beam and a measurement beam. After passing the beam splitter BS2, the measurement beam is focused onto the object to be measured. The object to be investigated must be reflective. Surface of the object may be made reflective by applying Ardox spray coating or retro reflective tape. The reflected beam is deflected by BS2 and is merged with the reference beam by the third beam splitter BS3 and is then directed on to the detector. As the path length of the reference beam is constant over time, a movement of object under consideration generates a dark and

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bright fringe pattern on the detector. One complete dark–bright cycle corresponds to an object displacement of exactly half the wavelength of the light used. For a He-Ne Laser, this displacement is 316 nanometers. Change in the optical path length per unit time causes the Doppler frequency shift of the measured beam. The modulation frequency of the interferometer pattern is exactly proportional to the velocity of the object.

Same interference patterns (and frequency shifts) are generated as the object moves towards or moves away from the interferometer. A Bragg cell is placed in the reference beam to distinguish the direction of movement as it shifts the Laser frequency by 40 MHz. A modulation frequency of the fringe pattern of 40 MHz is generated when the object is at rest. Movement of the object towards the interferometer reduces the modulation frequency while it increases when the object moves away from the Vibrometer. The detector receives a frequency lower or higher than 40 MHz indicating the direction and amplitude of movement of the object.


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Components of the LDV system

The main components of the Polytec Scanning Laser Doppler Vibrometer are the scan heads (top, left and right), Vibrometer controllers (OFV-5000) for each scan head, junction box connecting all these controllers and a personal computer (PC) containing the PSV software.

PSV-I-400 Scan head

Each scanning head has four major inbuilt units. They are video, scan electronics, scanning mirrors and photo sensor (OFV-505) units as shown in the Figure 36.3.

Figure 36.3 Scan head and its inbuilt units.

Salient features of the scanning head are

● Scan angle ± 20o with 0.02o resolution ● 72 x Zoom video camera● Large working distances 0.5 m to 50 m ● Close up unit for scanning small objects in millimeter range ● Laser focus, alignment procedures can be remote controlled using PDA which connects directly to the PSV software

Vibrometer Controller (OFV-5000)

The Vibrometer controller provides signals and power for the sensor head which is present in the scan head and processes the vibration signals. They are electronically converted by specially developed analog and digital decoders within the controller to obtain velocity and displacement information about the test structure. Information provided by controller is available in analog or digital form for further evaluation of data. The analog output is provided at standard BNC connectors.


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Junction Box

The junction box acts as an interface between the Vibrometer controller and the PSV software. Vibrometer controller of each scan head is connected to the junction box. Input for up to 8 analog signals and trigger are available on BNC connectors.


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Working with the LDV system

The sequence of steps followed while workings with the PSV system are shown in Figure 36.4.

Figure 36.4 Workflow during the experiment on LDV

Details of the test setup

Composite plates are coated with white spray for the purpose of better reflection. Plates are experimented in cantilever position as shown in the Figure 36.5 (a).


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Figure 36.5 (a) Composite test plate with dynamic shaker

For the dynamic excitation of the composite plate an electro-dynamic shaker is used. A power amplifier (make: LDS, PA500L series) is connected to the shaker for the purpose of amplifying the excitation signal generated by the LDV system. During the experiment, a pseudo random signal in the frequency range of 0-2000 Hz is used for the excitation of the composite plate. Experiment is carried out in two steps. In the first step the plate is excited from 0-800 Hz with a signal voltage of 0.2 V. In the second step the plate is excited from 800-2000 Hz with increased amplifier gain in order to excite the high frequency modes.


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Alignment procedure

The 2-D alignment step is carried out to relate the video and scanner coordinate system. This is done by selecting 10-15 points for each Laser head on the scan area. PSV software stores the video coordinates and the scan angles of these points on the scan area and calculates the polynomial interpolation. 2-D alignment step performed for one of the scan heads is shown in Figure 36.5(b).

Figure 36.5 (b) 2-D alignment step performed for the composite plate

Alignment procedure for 3D

In 3D alignment, coordinate system for the scan area is defined. This is done by selecting origin, point on X-axis, and point on X-Y plane on the scan area. After this step, all the three Lasers are made to coincide at other four to five points. The purpose of using these points is to back calculate the position and angle between the Laser heads by PSV software. After 3-D alignment procedure, the software is able to provide the coordinates (X, Y, and Z) of any point on the scan area with respect to the chosen coordinate system.

Geometry scan

All the scan points in the mesh grid are accessed by the three Lasers in order to estimate the surface of the test object. The three Lasers coincide at each and every scan point on the mesh grid [Figure 36.5(c)]. The accuracy of the surface estimated depends on the previous steps. This step is crucial to obtain the exact simulations of the test surface during the test.


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Figure 36.5 (c) Status points on the scanned composite plate

Rectangular mesh grid of size 250 mm x 200 mm was done in ABAQUS. This grid was imported into the PSV software to define the scan area as shown in Figure 6.5(d). The grid has 356 scan points in the present case.

Figure 36.5 (d) Mesh grid with the scan points on the rectangular composite plate.

Data acquisition parameters

Data acquisition properties such as the excitation signal to be used, frequency range, parameters for the FFT analysis (number of FFT lines, bandwidth etc.), averages, windows, velocity decoder are given in this step. Time required for the complete scan depends on the number of scan points defined and FFT parameters. In the present analysis, 356 scan points were defined on the composite plate and it took about one hour to complete the scan. Response plots, mode shapes animations are visualized after the scan.


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