Laser Doppler vibrometry is a recognized experimental method for non-contact vibration measurements. This technique measures frequency shifts of laser light reflected off moving objects to obtain velocity data.
Castellini, Paone and Tomasini have successfully utilized scanning laser Doppler vibrometry to detect nonintrusive damage in frescoes paintings and icons using mechanical mobility measurements and modal analysis as their methodology.
Castellini, Paone and Tomasini have successfully utilized scanning laser Doppler vibrometry to detect nonintrusive damage in frescoes paintings and icons using mechanical mobility measurements and modal analysis as their methodology.
Basics
Laser Doppler vibrometry (LDV) is an experimental technique developed over many years that measures vibrations by employing the Doppler effect. This occurs as a change in frequency when laser light reflected off moving objects changes; its frequency shift corresponds directly with their velocity; by measuring this shift we can ascertain surface amplitude by way of Doppler frequency shifting measurements.
Laser vibrometers provide precise surface vibration measurement at close proximity and high spatial resolution, and can capture large amounts of data (both temporal and spectral), which can then be processed to gain insight into the structure under investigation. With their broad frequency response capabilities, SLDVs make great non-destructive characterisation tools for structures as large as towers and churches.
SLDV systems consists of an interferometer equipped with a laser source emitting a coherent beam of light at a fixed frequency, and a laser receiver which detects any vibration signals reflected back from objects being measured. The interferometer signal is then converted to digital output which provides velocity measurements. These can be used to assess displacement or velocity on objects at specific points on their surfaces; or scanning LDVs can scan entire surfaces.
SLDV can also be used to detect modal analysis results. More specifically, it can help identify natural frequencies of objects by comparing measurements with those from reference objects.
SLDV can also be used to detect vibrations caused by phonation. When someone speaks aloud, their neck and head vibrate due to aerodynamic energy being converted to acoustic energy at their glottis; by analyzing vibration patterns on this area it’s possible to evaluate both vocal quality and performance of singer.
SLDV can also be an effective tool for detecting damage to artworks and Cultural Heritage artefacts. For instance, when fresco begins detaching from its heavy substrate wall, loose areas vibrate at higher velocities than those still securely connected; by targeting these areas with the SLDV device, a 3D map of surface velocity can be obtained which clearly highlights these defects.
Laser vibrometers provide precise surface vibration measurement at close proximity and high spatial resolution, and can capture large amounts of data (both temporal and spectral), which can then be processed to gain insight into the structure under investigation. With their broad frequency response capabilities, SLDVs make great non-destructive characterisation tools for structures as large as towers and churches.
SLDV systems consists of an interferometer equipped with a laser source emitting a coherent beam of light at a fixed frequency, and a laser receiver which detects any vibration signals reflected back from objects being measured. The interferometer signal is then converted to digital output which provides velocity measurements. These can be used to assess displacement or velocity on objects at specific points on their surfaces; or scanning LDVs can scan entire surfaces.
SLDV can also be used to detect modal analysis results. More specifically, it can help identify natural frequencies of objects by comparing measurements with those from reference objects.
SLDV can also be used to detect vibrations caused by phonation. When someone speaks aloud, their neck and head vibrate due to aerodynamic energy being converted to acoustic energy at their glottis; by analyzing vibration patterns on this area it’s possible to evaluate both vocal quality and performance of singer.
SLDV can also be an effective tool for detecting damage to artworks and Cultural Heritage artefacts. For instance, when fresco begins detaching from its heavy substrate wall, loose areas vibrate at higher velocities than those still securely connected; by targeting these areas with the SLDV device, a 3D map of surface velocity can be obtained which clearly highlights these defects.
Applications
Laser Doppler vibrometry (LDV) is a non-contact sensing method that measures surface displacement and velocity through Doppler effect on scattered laser beams from surfaces. As it requires no direct contact between sensor and surface being measured, LDV makes itself suitable for applications across a range of industries where contact measurements would otherwise not be possible or desirable.
LDV technology excels in vibration characterization for structural health monitoring. LDV gives maintenance engineers unparalleled insights into a component’s dynamic behavior, helping them predict impending failure and take corrective actions early.
Attaining this goal involves studying the vibration spectrum created by your system, identifying resonance frequencies and scanning at these frequencies to generate data that can then be used to locate defects such as cracks, erosion or corrosion.
Recently, this technique has been applied to measuring stress and strain distributions on components. This requires more sophisticated systems such as the Polytec PSV-400-3D Scanning Laser Doppler Vibrometer with StrainProcessor but has proven an invaluable way to obtain valuable data for updating and validating FEA models.
Continuous-scan LDV can be an impressively beneficial use for LDV, wherein its laser spot continuously scans over the surface of an oscillatory structure to speed up measurement time and enable more detailed mode shapes from laser signal extraction.
An excellent example is LDV’s ability to calculate natural frequencies and mode shapes of rotating shaft structures directly from their laser signal, an extremely challenging task for conventional experimental modal analysis methods which require physical testing in order to fully calculate vibration behaviour of systems. By contrast, LDV allows calculations without restriction from geometry restrictions of systems, providing another means for validating results obtained via FEA simulations.
LDV technology excels in vibration characterization for structural health monitoring. LDV gives maintenance engineers unparalleled insights into a component’s dynamic behavior, helping them predict impending failure and take corrective actions early.
Attaining this goal involves studying the vibration spectrum created by your system, identifying resonance frequencies and scanning at these frequencies to generate data that can then be used to locate defects such as cracks, erosion or corrosion.
Recently, this technique has been applied to measuring stress and strain distributions on components. This requires more sophisticated systems such as the Polytec PSV-400-3D Scanning Laser Doppler Vibrometer with StrainProcessor but has proven an invaluable way to obtain valuable data for updating and validating FEA models.
Continuous-scan LDV can be an impressively beneficial use for LDV, wherein its laser spot continuously scans over the surface of an oscillatory structure to speed up measurement time and enable more detailed mode shapes from laser signal extraction.
An excellent example is LDV’s ability to calculate natural frequencies and mode shapes of rotating shaft structures directly from their laser signal, an extremely challenging task for conventional experimental modal analysis methods which require physical testing in order to fully calculate vibration behaviour of systems. By contrast, LDV allows calculations without restriction from geometry restrictions of systems, providing another means for validating results obtained via FEA simulations.
Limitations
Laser Doppler vibrometers are powerful tools for measuring vibrations and performing modal analysis, but the instrument must only be used on structures that reflect laser beams; otherwise it won’t measure vibration velocities of objects wet with oil or water, dust or particles might interfere with measurements as well.
Laser Doppler vibrometers measure vibration by using two oscillators to send high-frequency signals into an object and detect any reflected laser light that comes back through to its optical sensor head of the instrument, while simultaneously tracking any frequency shifts caused by vibratory movement which are converted into voltage signals that correspond with vibration velocity.
Laser Doppler vibrometer sensitivity depends on the surface roughness and shape. If an object’s surface is particularly smooth and round, vibration sensors may fail to register any amplitude due to not enough light being reflected back from it to be coupled to its vibratory momentum.
On the other hand, when dealing with objects with rough and non-normal shapes, laser light reflected off them may become tightly coupled to their vibrational momentum causing high levels of vibration at the sensor head even in the absence of any amplitude signal. This results in extremely high rates of vibration at its sensor head.
However, continuous scanning laser Doppler vibrometers can overcome this limitation by moving their laser spot along an assigned scan path on a structure. When exposed to impact excitation, a spectrum produced from such measurements contains both natural frequencies of the structure as well as multiples of its scan frequency that behave like pseudo-natural frequencies – with these estimates used to produce estimates of mode shapes as functions of structural parameters and provide estimates of damage severity quickly and reliably. Chen et al. developed a baseline-free method for identifying damage with estimated mode shapes data using estimated mode shapes from these data collected data – combined with continuous monitoring systems providing quick and reliable damage identification methods of detection of any structural damages quickly and accurately.
Laser Doppler vibrometers measure vibration by using two oscillators to send high-frequency signals into an object and detect any reflected laser light that comes back through to its optical sensor head of the instrument, while simultaneously tracking any frequency shifts caused by vibratory movement which are converted into voltage signals that correspond with vibration velocity.
Laser Doppler vibrometer sensitivity depends on the surface roughness and shape. If an object’s surface is particularly smooth and round, vibration sensors may fail to register any amplitude due to not enough light being reflected back from it to be coupled to its vibratory momentum.
On the other hand, when dealing with objects with rough and non-normal shapes, laser light reflected off them may become tightly coupled to their vibrational momentum causing high levels of vibration at the sensor head even in the absence of any amplitude signal. This results in extremely high rates of vibration at its sensor head.
However, continuous scanning laser Doppler vibrometers can overcome this limitation by moving their laser spot along an assigned scan path on a structure. When exposed to impact excitation, a spectrum produced from such measurements contains both natural frequencies of the structure as well as multiples of its scan frequency that behave like pseudo-natural frequencies – with these estimates used to produce estimates of mode shapes as functions of structural parameters and provide estimates of damage severity quickly and reliably. Chen et al. developed a baseline-free method for identifying damage with estimated mode shapes data using estimated mode shapes from these data collected data – combined with continuous monitoring systems providing quick and reliable damage identification methods of detection of any structural damages quickly and accurately.
Conclusions
Laser Doppler vibrometry is a non-contact method of measuring vibrations by emitting a laser beam at an object and monitoring changes in frequency due to movement on its surface. A vibrometer then uses this data to generate a voltage signal proportional to velocity – the resultant signal may then be used for vibration analysis or other purposes.
An LDV (laser Doppler vibrometer) can measure vibration velocities at multiple points simultaneously across a surface that has been sinusoidally excited, producing frequency domain data which are then used to produce vibration modes of that structure.
Vibrometer accuracy depends on a range of factors including its sensor geometry, surface material composition, size and shape of structure under test as well as underlying mechanical systems; however, Laser Doppler vibrometers boast relatively high overall accuracy compared to other measurement techniques.
Laser Doppler vibrometers can be an invaluable asset when used to validate natural frequencies, modes and mode shapes computed in finite element analysis environments, since their ability to simultaneously measure vibrations at multiple points allows it to produce spectral signals with amplitudes corresponding to acceleration/displacement can provide invaluable modal analysis data.
LDVs can also be beneficial in inspecting surfaces that are difficult to access, or cannot be reached using conventional contact and non-contact sensors like accelerometers and strain gages. Furthermore, Laser Doppler vibrometers offer accurate data regarding dynamic behavior of complex structures through measurements across a broad spectrum of frequencies.
Kitamura (2014) conducted an investigation of vocal dynamics using a scanning laser Doppler vibrometer. They measured vibration patterns on participants’ facial surfaces during phonation using this instrument; results demonstrated that its root-mean-square error for measuring vibration velocities at 60 points on participants’ faces was less than 4.0 dB.
An LDV (laser Doppler vibrometer) can measure vibration velocities at multiple points simultaneously across a surface that has been sinusoidally excited, producing frequency domain data which are then used to produce vibration modes of that structure.
Vibrometer accuracy depends on a range of factors including its sensor geometry, surface material composition, size and shape of structure under test as well as underlying mechanical systems; however, Laser Doppler vibrometers boast relatively high overall accuracy compared to other measurement techniques.
Laser Doppler vibrometers can be an invaluable asset when used to validate natural frequencies, modes and mode shapes computed in finite element analysis environments, since their ability to simultaneously measure vibrations at multiple points allows it to produce spectral signals with amplitudes corresponding to acceleration/displacement can provide invaluable modal analysis data.
LDVs can also be beneficial in inspecting surfaces that are difficult to access, or cannot be reached using conventional contact and non-contact sensors like accelerometers and strain gages. Furthermore, Laser Doppler vibrometers offer accurate data regarding dynamic behavior of complex structures through measurements across a broad spectrum of frequencies.
Kitamura (2014) conducted an investigation of vocal dynamics using a scanning laser Doppler vibrometer. They measured vibration patterns on participants’ facial surfaces during phonation using this instrument; results demonstrated that its root-mean-square error for measuring vibration velocities at 60 points on participants’ faces was less than 4.0 dB.
