Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
  • G01H17/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves, not provided for in the preceding groups
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
  • G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
  • G01S17/50—Systems of measurement based on relative movement of target
  • G01S17/58—Velocity or trajectory determination systems; Sense-of-movement determination systems
• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
  • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
  • G03H1/0005—Adaptation of holography to specific applications
  • G03H2001/0033—Adaptation of holography to specific applications in hologrammetry for measuring or analysing

Definitions

• the present invention relates to the field of metrology, and more precisely to the measurement of the speed of movement of objects animated by a vibratory movement.
• US Pat. No. 3,899,921 describes an assembly using a stationary hologram and implementing the principle of holography in medium time.
• the invention described in this patent of the prior art relates to equipment using an annexed arrangement which makes it possible to compensate for a more or less significant part of the moving the target.
• the assembly of the reference makes it possible to have a cartography of the displacement of the object but does not make it possible to follow the temporal variation of this vibration.
• the mounting of the reference does not work, for example, on objects moving transiently.
• the assembly described in this document of the prior art operates in displacement measurement while the ours measures the instantaneous speed of the vibrating target.
• the system presented in the article cannot measure displacements greater than the wavelength (approximately 0.5 ⁇ m) while the invention is designed for the analysis of large displacements (greater than several ⁇ m).
• Pieces of equipment according to the state of the art are suitable for vibratory displacements of low amplitude and of high frequency.
• the equipment according to the state of the art is however not suitable for speed measurement for targets vibrating at low frequency, with movements of possibly large amplitude.
• the object of the invention is to measure the speed of objects animated by a vibratory movement around a mean position, without contact and without prior preparation, and thus make it possible to establish, among other things, a spectrum of the speeds of vibration of this object.
• the device will make it possible to measure vibrations of amplitude of the order and greater than the wavelength of the light used, the study frequencies will belong to the fields of low frequencies (typically 0 to 10 kHz).
• Another object of the invention is to allow a measurement which is not sensitive to the surface state of the object and can therefore work without prior treatment of the surface of the object (polishing).
• the invention relates in a first sense to a method consisting in mixing two waves in a dynamic holographic material already used to develop an ultrasonic vibration sensor.
• the measurement point on the object is illuminated by a beam from a laser source.
• the reflected wave, diffused by the measurement point is sent into the dynamic holographic material at the same time as a pump wave from the same laser source.
• These waves register in the material dynamic holographic a hologram of the wavefront structure of the signal wave coming from the object.
• This hologram is read by the pump beam to give a local oscillator beam which is sent to a detector at the same time as the signal beam transmitted by the dynamic holographic material. Due to the properties of holography, the two waves (local oscillator and transmitted signal wave) are identical and phase shifted by a value which depends on the assembly and the material used. This phase shift will be chosen so as to make the operation of the invention optimal.
• the vibration of the measuring point modulates in phase 1 the signal wave returned by the object.
• the aim of the system will be to transform this phase modulation into intensity modulation which will be transformed into an electrical signal characteristic of the vibration. This transformation will depend on both the frequency of the vibration and its amplitude.
• vibrations of very low amplitude compared to the wavelength of the light used
• very high frequency compared to the inverse of the response time of the dynamic holographic material
• This one is therefore stationary from the point of view of vibration and the local oscillator resulting from the diffraction of the stationary pump beam on the stationary hologram, is itself stationary. It is sent to the detector at the same time as the transmitted signal beam which is modulated in phase by the ultrasonic vibration. If the phase shift ⁇ ⁇ between the two beams sent to the detector is taken equal to ⁇ / 2 (quadrature waves), the interference between the two signal and local oscillator beams transforms the phase modulation into intensity modulation, then into an electrical signal directly proportional to the amplitude of the ultrasonic vibration.
• the two-wave mixing arrangement is therefore very well suited for measuring ultrasonic vibrations.
• the assembly according to the state of the art will no longer work. Indeed when the vibration has a large amplitude (compared to the wavelength of light), first of all the response of the device is no longer linear, and then the hologram is erased by the phase modulation, which removes the local oscillator and suppresses the electrical measurement signal. It is therefore not possible to measure vibrations of large amplitude and high frequency with this device.
• the problem is solved by the invention in the case of low frequency vibrations (compared to the inverse of the response time of the dynamic holographic material) and one can then measure vibrations of large amplitude.
• the registered hologram At low frequency, the registered hologram is no longer stationary, it moves according to a law related to phase modulation. However, it follows this modulation with a certain delay which depends on the frequency of variation of this phase modulation (it follows it all the better as it is slow).
• the delayed phase modulation due to the movement of the hologram is then transmitted to the local oscillator which will again interfere in quadrature with the transmitted signal wave.
• the electrical signal is then proportional to the derivative of the phase modulation and therefore proportional to the instantaneous speed of the vibrating object. Saturation at large displacement amplitude still exists but is considerably relaxed and can be controlled by the response time of the dynamic holographic material. At a given frequency, the value corresponding to saturation will be greater the shorter the response time of the holographic material.
• the invention is an original adaptation of the two-wave mixing arrangement in a dynamic holographic material allowing the extension of the operating regime to the measurement of low frequency vibrations (compared to the response time of the dynamic holographic material) and of high amplitude. (compared to the wavelength of the light used). All the montages actually used have always considered the hologram as fixed to the time scale of the vibration. No assembly used a mobile hologram for the detection of vibration (except for the very different assembly using the transient photoelectromotive force for the measurement of Doppler shift).
• the invention also has the advantage of allowing the monitoring of transient signals throughout their time development.
• vibration sensors There are currently commercially available vibration sensors. They are based on coherent detection devices (homodyne or heterodyne). Although very effective, these sensors see their effectiveness drop sharply as soon as they are used with diffusing surfaces, which often prevents measurement unless the surface of the object is polished. However, there are a number of areas where it is not possible to polish the object under study.
• FIG. 3 Response signal to a high amplitude sinusoidal phase modulation, for a high illumination value (dotted curve) and low illumination (solid lines curves). The curve in bold line represents a measurement with low illumination and with a phase shift divided by 10.
• - Figure 4 Response to a low sinusoidal phase shift with low illumination (bold line) and high illumination (grayed line). The thin line indicates the signal sent to the piezoelectric block.
• FIG. 1 represents the diagram of an exemplary embodiment of a two-wave velocimeter according to the invention.
• the light from a laser (1) is split in two by a beam splitter (2).
• the reference beam (3) is sent into the dynamic holographic material (4) to form the pump beam.
• the other beam, the measurement beam (5) is sent to the target (6) whose vibration is to be measured.
• the light returned by the target (6) is collected and sent onto the dynamic holographic material (4) to form the signal beam (14).
• This beam transport (both for the signal and for the pump) can be done with multi-mode fibers of large core diameter, to have a remote measurement.
• the light of the signal beam (7) after passing through the holographic material (4), is sent to a detector (8) to give an electrical signal which will then be processed by an electronic system.
• This basic assembly is completed by optical means to ensure that there is a diffracted beam (7) in quadrature with the transmitted signal (7).
• ⁇ ⁇ ⁇ / 2. Their locations will mainly depend on the nature of the dynamic holographic material (4).
• the first possibility is the use of a dynamic holographic material (4) in which the registered hologram is local, that is to say in phase with the illumination figure, such as for example a photorefractive material with a voltage electrical applied.
• a dynamic holographic material (4) in which the registered hologram is local, that is to say in phase with the illumination figure, such as for example a photorefractive material with a voltage electrical applied.
• a second possibility is the use of a material in which the hologram is phase shifted by ⁇ / 2 relative to the illumination network, as in the case of a photorefractive material without an applied electric field.
• ⁇ ⁇ 0. This means zero installation efficiency. It will therefore be necessary to add an additional phase shift so external to obtain ⁇ ⁇ ⁇ ⁇ / 2.
• a first solution consists in polarizing the incident beams (3, 5 or 14), the pump beam in a direction X and the signal beam (14) at 45 ° from this direction.
• the signal beam (14) comprises two components of equal amplitudes along two polarization directions X and Y, Y designating the direction orthogonal to X.
• the polarized component along X of the signal beam (14) writes the hologram with the pump beam ( 3), and thus creates a diffracted beam polarized along X.
• the beam (7) transmitted comprises it, the two components along X and Y.
• a phase plate (13) whose axes are in the directions X and Y and which phase ⁇ / 2 la component along Y of the signal beam (7) transmitted with respect to the diffracted beam (7) which is polarized along X.
• the two components are then made to interfere by placing a polarization splitter cube (18) oriented at 45 ° from the X and Y directions and the two beams from the cube are sent to two detectors (8).
• a polarization splitter cube 18
• the photorefractive crystal can also be used in a particular configuration called an anisotropic diffraction configuration in which the diffracted beam is polarized orthogonally to the writing beam.
• an anisotropic diffraction configuration in which the diffracted beam is polarized orthogonally to the writing beam.
• phase plate has a variable phase shift which is used simply to bring the total phase shift ⁇ ⁇ between the diffracted beam and the signal transmitted to its optimal value of ⁇ / 2.
• the material that will be used will generally be any dynamic holographic material (4), that is to say a material in which an illumination structure given by the interference of two beams will generate a hologram of a of these beams, this hologram being dynamic, that is to say that it will follow the changes in the illumination structure quickly enough so that we have a system that meets the specifications laid down.
• dynamic holographic materials it is advantageous to use photorefractive materials.
• the photorefractive crystals it is possible to use sillenites, as well as III-V semiconductor compounds such as GaAs, InP and II-VI such as CdTe, ZnTe or quantum well structures.
• This montage like any montage based on holographic techniques, allows measurements to be made in parallel on several points in the context of multichannel or imaging processing.
• the method according to the invention is based on the use of a dynamic holographic material (4) in which we write a hologram. It may be a phase or amplitude hologram. The rest of the explanation will be made by considering a phase hologram or index network. It can easily be adapted to consider the case of an amplitude hologram or absorption network.
• This hologram is obtained by interfering on the dynamic holographic material (4) the signal beam (14) (of amplitude E s and intensity I s ) coming from the target (6) with a pump beam (3) (d 'amplitude E p and intensity I p ) from the same laser.
• the hologram In stationary regime, the hologram is a variation of index whose amplitude ⁇ n is proportional to m, interference term between the two beams (given by the product 2E s E p * / (I s + I p )) .
• the hologram is dynamic, which means that it reaches steady state after a while. This dynamic will be given (without loss of generality) by a first order kinetics law with a real time constant.
• the hologram being characterized by the quantity ⁇ n, this quantity therefore follows the following law of variation (G is the proportionality constant giving the strength of the index network): d ⁇ n ⁇ n G m
• the signal (14) and pump (3) beams interfering in the dynamic holographic material (4) will write a dynamic hologram, which will therefore follow more or less easily the phase modulation generated by a vibration of the target (6). Furthermore, the two beams will diffract on the hologram that they write. In particular, the pump beam (3) will diffract on the hologram to give a beam propagating in the direction of the transmitted signal beam.
• the condition for using the velocimeter depends on the response time ⁇ of the dynamic holographic material. For example, for a sinusoidal vibration of amplitude ⁇ and frequency f, the relation between ⁇ and f ⁇ is fixed by the condition:
• the assembly (Fig.l) is a two-wave mixing assembly in a CdTe photorefractive crystal.
• Only one mounting channel is used with a single detector (8).
• a delay line is installed, it can be easily removed and replaced. When it is removed the pump harness (3) is sent to a mirror
• FIG. 2 represents the comparison of the response obtained with the two-wave mixture assembly (curve in bold) with that obtained with a Mach-Zehnder interferometer
• the measurement is made with the Mach Zehnder interferometer (adjusted roughly in quadrature) and it is compared with the response given by the two-wave mixing velocimeter.
• the phase shift greater than ⁇ causes a complex response from Mach Zehnder, with several folds of the response, corresponding to a phase shift due to the piezoelectric mirror estimated at approximately (3 ⁇ / 2) sin (2 ⁇ ft).
• the response is almost sinusoidal (a component at the second harmonic begins to be visible) and phase shifted by ⁇ / 2 compared to the initial phase shift.
• This phase shift of ⁇ / 2 corresponding to a derivation of the sine, there is indeed a response proportional to the instantaneous speed of the target (6) as predicted by theory and which moreover makes it possible to measure strong displacement.
• FIG. 3 represents the response signal to a high amplitude sinusoidal phase modulation, for a high value of the illumination
• the curve in bold line represents a measurement with low illumination and with a phase shift divided by 10.
• the assembly represented in FIG. 1 it is also possible to have an indication of the frequency response of the velocimeter by mixing with two waves.
• the illumination is lowered so that the frequency of the phase modulation becomes large in front of the inverse of the response time of the photorefractive crystal.
• the operation of the two-wave mixture no longer makes it possible to measure strong phase shifts.
• the vibration of the illumination figure on more than one fringe causes an erasure of the network which leads to a fall in the efficiency of the measurement of the phase at the same time as the appearance of higher harmonics in the response, it is what one observes on the curve in fine line of Figure 3.
• To find a linear behavior with the phase shift it is necessary under these conditions to lower the amplitude of the sinusoidal phase shift. This is what we observe on the curve in bold line in FIG. 3 for which the amplitude of the phase shift has been reduced by a factor of 10.
• FIG. 4 represents the response to a weak sinusoidal phase shift with low illumination (bold line) and with strong illumination (gray line).
• the thin line indicates the signal sent to the piezoelectric block.
• the signal is then in phase with the phase shift, unlike the dotted curve obtained with high phase shift and high illumination power (low frequency regime) (in the figure the two curves are in phase for purely coincidental reasons of change trigger level).
• a small phase shift ( ⁇ ) is measured and the incident illumination is varied, everything else remaining constant, in order to pass from the low frequency regime (high illumination) to the high frequency regime (low illumination).
• the two-wave mixture behaves as a diverter, which causes the phase shift of ⁇ / 2 between the response of the two-wave mixture (gray curve) compared to the initial phase shift (thin line curve). While at high frequency, it is directly proportional (bold curve) to the initial phase shift, which reflects the fact that the two curves are in phase.
• FIG. 5 represents the response to a low triangular phase shift with low illumination (bold line) and with high illumination (gray line).
• the signal sent to the piezoelectric setting In thin line is indicated the signal sent to the piezoelectric setting. This derivative behavior is even more evident when a triangular signal of low amplitude is sent to the piezoelectric mirror (Fig. 5).
• Fig. 5 At low frequency we measure the derivative of this signal (gray curve), which is a square signal, while at high frequency we are again proportional to the phase shift as shown by the triangular signal (bold curve) obtained on the detector (8).

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• General Physics & Mathematics (AREA)
• Computer Networks & Wireless Communication (AREA)
• Radar, Positioning & Navigation (AREA)
• Remote Sensing (AREA)
• Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
• Holo Graphy (AREA)

Applications Claiming Priority (3)

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• 1998
  • 1998-07-06 FR FR9808630A patent/FR2780794B1/fr not_active Expired - Fee Related
• 1999
  • 1999-07-06 CA CA002337267A patent/CA2337267C/fr not_active Expired - Fee Related
  • 1999-07-06 EP EP99929416A patent/EP1093569A1/fr not_active Withdrawn
  • 1999-07-06 JP JP2000558368A patent/JP4405673B2/ja not_active Expired - Fee Related
  • 1999-07-06 AU AU46241/99A patent/AU4624199A/en not_active Abandoned
  • 1999-07-06 WO PCT/FR1999/001631 patent/WO2000002023A1/fr active Application Filing
• 2001
  • 2001-01-04 US US09/754,615 patent/US6486959B2/en not_active Expired - Fee Related

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