WO2007065643A1

WO2007065643A1 - Vibrometer

Info

Publication number: WO2007065643A1
Application number: PCT/EP2006/011685
Authority: WO
Inventors: Joachim Barenz; Rainer Baumann; Hans Dieter Tholl
Original assignee: Diehl Bgt Defence Gmbh & Co. Kg
Priority date: 2005-12-09
Filing date: 2006-12-06
Publication date: 2007-06-14
Also published as: DE102006003877A1; DE102006003877B4

Abstract

A vibrometer (1, 23) is provided, which comprises a radiation source (e.g. 2) for coherent radiation, a beam splitter (4) for decomposing the output beam (13) of the radiation source (e.g. 2) into a measurement beam (14) for observing a vibrating object (17) and into a reference beam (15), a beam guiding device for superposing the measurement beam (18) returning from the vibrating object (17) with the reference beam (15), and a detector (10) for time-resolved and/or position-resolved recording of the superposition patterns. In this case, a micromirror grid (8) arranged in the beam path of the returning measurement beam (18), a receiving optical means (7) for imaging the vibrating object (17) onto the micromirror grid (8) and an imaging optical means (9) for imaging the micromirror grid (8) onto the detector (10) are provided. The imaging vibrometer (1, 23) is distinguished by a short measurement time. Furthermore, a method for producing a vibration image of the vibrating object (17) and also the use of the vibrometer (1, 23) are stated.

Description

Diehl BGT Defense GmbH & Co.KG, 88662 Überlingen

Vibrometer

The invention relates to a vibrometer according to the preamble of claim 1. The invention further relates to the use of such a vibrometer and a method for generating a vibration image of a vibrating object. A vibrometer of the type mentioned at the beginning can be found in the prospectus

"Possibilities and fundamentals of vibrometry" from Polytec GmbH, available in the download area at www.polytec.com. Coherent light is generated in such a vibrometer, for example by means of a laser, and by means of a beam splitter into a measuring and a reference beam The measuring beam strikes a vibrating object, runs back from it and is caused to overlap with the reference beam based on the principle of an interferometer. Due to the vibrations of the object, a characteristic overlay or interference pattern results, which indicates the oscillation behavior of the vibrating object If, for example, the intensity variation of the overlay pattern is observed over time by means of a detector, this variation can be used to draw direct conclusions about the oscillation frequency of the object or its illuminated surface tion frequency of the interference pattern is directly proportional to the speed of the vibrating object.

To detect the three-dimensional vibration behavior of the vibrating object, it is known to direct the measuring beams from orthogonal directions onto the object, in order to be able to observe path differences in three spatial directions. To generate a spatially resolved vibration image of the vibrating object, it is also known to scan the vibrating object or its surface with the measuring beam and thus to determine the specific vibration behavior for each measuring point. Unfortunately, both require 3D and spatially resolved vibrometry take up a lot of time to determine the vibration behavior.

It is an object of the invention to provide a vibrometer which is improved in terms of the necessary measuring time compared to a vibrometer of the conventional type. A use for such a vibrometer is also to be specified. It is also an object of the invention to provide a method for generating a vibration image of a vibrating object which is improved with regard to the necessary measurement time.

The object directed to a device for a vibrometer according to the preamble of claim 1 is achieved according to the invention in that a micromirror raster is arranged in the beam path of the returning measuring beam, with receiving optics for imaging the vibrating object onto the micromirror raster and imaging optics Image of the micromirror grid on the detector are provided.

The invention is based on the consideration that, in order to analyze the vibration behavior of the object to be examined, it should not be scanned by means of the measuring beam, but replaced by imaging the interference pattern on the detector. This eliminates the time-consuming scanning process, which not only reduces the measuring time, but also results in a simpler structure of the vibrometer. In a further step, the invention is based on the consideration that a simple and fast detector can be used to evaluate the interference pattern if a micromirror grid is used in the imaging beam path. Such a micromirror raster, which comprises micromirrors arranged in columns and rows, allows a selective selection of individual image sections, in that only the desired micromirrors are directed onto the detector.

In other words, the invention allows a vibration image of the object to be examined to be recorded, in which the measurement beam is not guided over the object in a time-consuming manner, but instead the object is selectively imaged on the detector. The necessary measuring time is shortened by at least the part time that would be required to guide the measuring beam over the object.

With regard to the structure of the beam guiding device, which is necessary for superimposing the measuring and reference beams, the invention is not limits. For example, the structure can be the same as that of a known Michelson interferometer, with the measuring beam being reflected by the vibrating object in one branch and the reference beam being reflected by a mirror in the other branch. The beam splitting can be carried out with conventional beam splitters, but also with non-linear optical elements. Mirrors, prisms and in particular also optical fibers can be used for beam guidance.

The necessary measuring time of the vibrometer depends on the desired resolution of the oscillation frequency and the number of evaluated measuring points. The frequency resolution defines the measuring time per measuring point; the whole

Measuring time per area depends linearly on the number of measuring points. The measurement time for a desired resolution of the oscillation frequency of 1 Hz is, for example, 1 second per measurement point. It takes N seconds to evaluate N measuring points. If the N measuring points could be observed in parallel with a mosaic of detector elements, the required one would be

Reduce measurement time to 1 second. For each raster element, a converter for optical to electrical energy and electronics would be required to process the recorded information, such as intensity, amplitude or frequency. Such an elaborate element is not suitable for commercial implementation.

However, the use of a micromirror grid advantageously allows the use of a simple detector having a few grid elements, and in particular a single detector. For this purpose, only a selective mapping of the interference image onto the detector by aligning individual micromirrors needs to take place with the advantages described at the beginning with regard to the measuring time. With a micromirror grid, imaging of the vibrometer with an inexpensive detector is therefore possible with a reasonable measuring time. In a further advantageous embodiment of the invention, a transmitting optic is provided for aligning the measuring beam running away with the oscillating object. This makes it possible, for example, to widen or focus the measuring beam. However, it is equally conceivable to design the radiation source in such a way that the measuring beam striking the object already has a desired cross-sectional area.

A frequency shifter, in particular a Bragg cell, is expediently arranged in the reference beam path. By means of such a frequency shift In addition, which can in particular also be designed as a non-linear optical element, the frequency of the reference beam is shifted relative to the frequency of the measuring beam. In the case of the Bragg cell, this is done by a passing acoustic wave on which the reference beam is diffracted. By shifting the frequency of the reference beam, it is possible not only that

Analyze the speed or frequency of the vibrating object, but also its direction of movement. The modulation frequency of the interference pattern is dependent on the direction of movement of the object. To detect the direction of movement of the vibrating object, the polarization direction of the radiation can also be advantageously used. For this purpose, the beam splitter can be designed as a polarization-sensitive element. For beam splitting, the beam splitter can generally be designed as a partially reflective or diffractive element.

The measurement and reference beam can be superimposed on the detector itself. However, it is equally possible to design the beam guidance device in such a way that the measurement and reference beam are superimposed on the micromirror grid. In the former case, a pixel of the vibrating object imaged by the micromirror raster in the detector interferes with the reference beam only in the detector. In the second case, the interference pattern already arises on the micromirror grid before it is imaged, in particular, selectively on the detector. The second variant results in a simplified beam guidance, since the measuring beam and the reference beam are guided together in the optical system. However, image information can be deleted by interference.

A control unit for controlling the micromirror grid and a signal processing unit for evaluating the detector signals are expediently integrated in the vibrometer. In a preferred variant, the control unit is set up to align the micromirrors individually one after the other in time at the detector, and the signal processing unit is set up to generate a spatially resolved vibration image of the vibrating object from the detector signals obtained in this way. In this way, pixels of the vibrating object with the possible switching frequency of the

Micromirror grids are imaged one after the other on the detector, so that their interference pattern can be observed with the reference beam. Already Compared to a scanning vibrometer, the measurement time is shortened because there is no need to shift the measurement beam.

The measurement time can advantageously be reduced further if the micromirrors are aligned in succession with a frequency on the detector which is higher than a limit frequency required to resolve a maximum oscillation frequency. For example, if a maximum oscillation frequency of 10 Hz is to be detected at a resolution of 1 Hz, then the cut-off frequency required for the resolution is double the maximum oscillation frequency, namely 20 Hz, according to the Nyquist theorem.Therefore, each measurement point no longer has to be at a time interval are directed at the detector as 50 milliseconds over a period of 1 second in order to obtain the information necessary for the vibration pattern from the overlay pattern for each measurement point. If the micromirrors are successively aligned with the detector at a higher frequency than the required cutoff frequency, this enables a reduction in the measurement time. This succeeds in that in the time between two measuring cycles of a measuring point, with the micro mirror being aligned with the detector in each measuring cycle, further micro mirrors are aligned with the detector. In other words, each micromirror is aligned with the detector with a time offset, each micromirror, viewed separately, aligned with the clocking of the cutoff frequency and the time interval between the alignment of successive micromirrors corresponds to a higher frequency, the maximum value of which is the maximum possible control frequency of the micromirror grid given is. As a result, the measurement time can be significantly reduced.

In a further advantageous embodiment, the micromirror grid is controlled in such a way that the micromirrors are aligned with the detector in chronological succession in a respectively predetermined grouping, the signal processing unit using the mathematical transformation to generate a spatially resolved vibration image of the vibrating object from the detector signals obtained in this way. In this variant, groups of micromirrors are aligned together on the detector. In this way, several pixels of the overlay pattern are recorded together by the detector. If the micromirrors that are aligned jointly on the detector are specified accordingly, a mathematical transformation can be used to calculate back from the total of the detector signals recorded to the intensity curve in each individual pixel of the overlay pattern. Since the micro aligned in groups of mirrors on the detector, it is hereby possible to significantly reduce the measurement time required to achieve a desired resolution. A Hadamard transformation is particularly suitable as a mathematical transformation for backward calculation. This is described, for example, in M. Harwit, Hadamard Transform Optics, Academic Press,

1979.

To generate the spatially resolved vibration image of the object to be examined, e.g. the respective vibration spectrum can be inferred from the intensity course of each individual image point of the superimposition pattern by means of a frequency analysis, for example by means of a Fourier transformation. In the case of a linear mathematical transformation for back calculation from the group information, it is irrelevant whether the frequency analysis matches the received detector signals, i.e. the raw data, and then the transformation is applied, or whether the mathematical transformation is first applied to the raw data and the frequency analysis is carried out with the transformed raw data.

To reduce the signal-to-noise ratio, it is advisable to suppress stray light. For this purpose, the micromirrors of the micromirror grid are designed such that they can each assume two tilt positions, each micromirror directing incoming radiation in the first tilt position onto the detector and in the second tilt position into a light trap. This configuration ensures that only that radiation reaches the detector that originates from micromirrors aligned with the detector. Scattered radiation from micromirrors not aligned with the detector is reduced by the light trap.

With regard to the indication of use, the object is achieved according to the invention in that the vibrometer described is used to identify a vibrating object or a significant property of the vibrating object on the basis of its vibration characteristics.

For example, an active vehicle can be distinguished from a mere dummy by the vibration of a running engine. Particularly when the vibrometer is used in the military, it is thus possible to use a further property, namely its vibration, to identify real vehicles. In an expedient embodiment, the vibration characteristic of the object to be identified is specified in particular by a defined vibration device. For this purpose, a specific vibration characteristic can be impressed on the objects to be identified, such as in particular vehicles, aircraft or buildings, by means of which they can be identified on the basis of their own side. With such a use, a friend-foe distinction is possible. The task directed to a method is achieved by a method according to the

The preamble of claim 14 is achieved according to the invention in that the oscillating object is imaged on a micromirror grid by means of the measuring beam, and in that the micromirror grid is imaged on the detector. Further advantageous refinements can be found in the subclaims directed to a method.

The advantages described for the vibrometer are to be applied analogously to the procedural configurations.

Embodiments of the invention are explained in more detail with reference to a drawing. Show:

1 schematically shows a first variant of an imaging vibrometer with a micromirror grid,

2 schematically shows a second variant of an imaging vibrometer with a micromirror grid and

3 schematically shows the use of a vibrometer to identify a vibrating object.

1 schematically shows a vibrometer 1, which comprises a laser 2, a beam splitter 4, a transmitting optic 5, a receiving optic 7, a micromirror laser 8, an imaging optic 9 and a detector 10 as the radiation source.

The laser 2 emits a coherent output beam 13 which is split into a measuring beam 14 and a reference beam 15 by the beam splitter 4. The measuring beam 14 is widened by means of the transmitting optics 5 and onto the investigative vibrating object 17 directed. The oscillating object 17 carries out oscillations, as indicated by the arrows.

The measuring beam 14 is reflected on the vibrating object 17. The returning measuring beam 18 passes through the receiving optics 7, which images the illuminated surface of the vibrating object 17 onto the micromirror grid 8. The micromirror laser 8 is imaged on the detector 10 by means of the imaging optics 9. On the detector 10, the part of the object image which is oriented by the micromirror grid 8 onto the detector interferes with the reference beam 15, so that the detector 10 observes the interference or superimposition pattern which is formed.

In order to evaluate the detector signals contained, the detector 10 is connected to a signal processing unit 20. The signal processing unit 20 is combined with a control unit 21 which is connected to the micromirror grid 8 for control purposes. The control unit 21 controls the micromirror grid 8, for example, in such a way that individual micromirrors are aligned with the detector 10 one after the other in a respectively predetermined grouping. In a measurement cycle, a predetermined number of groups defined by the respective arrangement of the micromirrors in the micromirror grid 8 are aligned with the detector 10 at the actuation frequency possible by the micromirror laser 8. From the detector signals obtained in this way, a Hadamard transformation in the signal processing unit 20 is used to deduce the temporal intensity profile of the superimposition pattern of each imaging point defined by the size of the micromirror. A spatially resolved vibration image of the vibrating object 17 is generated by a frequency analysis.

An alternative vibrometer 23 is shown schematically in FIG. The vibrometer 23 differs from the vibrometer 1 shown in FIG. 1 in that the transmitting optics 5 are arranged in the output beam 13 and in that the beam splitter 4 directs the divided reference beam 15 onto the micromirror grid 8. A Bragg cell 22 is also inserted into the reference beam 15, as a result of which the reference beam 15 experiences a frequency shift. This enables the direction of vibration to be detected.

As a result of this configuration, the returning measuring beam 18 and the reference beam 15 are superimposed on the micromirror in the vibrometer 23. grid 8 instead. In this case, one or more pixels of the interference pattern are imaged into the detector 10 by selective activation of the individual micromirrors of the micromirror grid 8. The detectors 10 in the vibrometers 1 and 23 are each designed as individual detectors in the form of fast photomultipliers. The time course of the recorded intensity is recorded in each case.

FIG. 3 shows the use of the vibrometer 1 according to FIG. 1 for obtaining reconnaissance data. For this purpose, a drone 24 flies over a site 26 with open vegetation 27 and trees 28. Furthermore, a truck 29 is located in the site 26, from which it is unclear whether it is real or a dummy. The drone 24 flying over the terrain 26 is one of a number of others

Sensors, such as cameras and infrared detectors, are equipped with a vibrometer 1 according to FIG. 1. The vibrometer 1 measures vibrations of the targeted object via the drawn beam path 30. In the present case, the vibrometer 1 detects an engine vibration of the truck 29, so that it can now be clearly concluded that a real truck 29 is present. The

Use of the vibrometer 1 thus supplements the information received further from the truck 29 with the other sensors, so that the necessary reasons for the decision are available for corresponding measures to be initiated.

Reference list

1 vibrometer

2 lasers

4 beam splitters

5 transmission optics

7 receiving optics

8 micromirror grids

9 imaging optics

10 detector

13 output beam

14 measuring beam

15 reference beam

17 swinging object

18 returning measuring beam

20 signal processing unit

21 control unit

22 Bragg cell

23 vibrometer

24 drone

26 terrain

27 vegetation

28 tree

29 trucks

30 beam path

Claims

1. vibrometer (1, 23), comprising a radiation source (eg 2) for coherent radiation, a beam splitter (4) for splitting the output beam (13) of the radiation source (eg 2) into a measuring beam (14) for observing a vibrating object ( 17) and into a reference beam (15), a beam guiding device for superimposing the measuring beam (18) returning from the vibrating object (17) with the reference beam (15), and a detector (10) for the time and / or spatially resolved recording of the Overlay pattern, marked,

by means of a micromirror grid (8) arranged in the beam path of the returning measuring beam (18), by receiving optics (7) for imaging the vibrating object (17) onto the micromirror grid (8), and by imaging optics (9) to depict the micromirror grid (8) on the detector (10).

2. Vibrometer (1,23) according to claim 1,

characterized,

that the detector (10) is designed as a detector with a few raster elements, in particular as a single detector.

3. vibrometer (1,23) according to claim 1 or 2,

featured,

by means of a transmission optics (5) for aligning the measuring beam running away onto the vibrating object (17).

4. vibrometer (1, 23) according to any one of the preceding claims,

featured,

by means of a frequency shifter arranged in the reference beam path, in particular a Bragg cell (22).

5. vibrometer (1, 23) according to any one of the preceding claims,

characterized,

that the beam splitter (4) is designed as a partially reflective, diffractive or polarization-sensitive element.

6. vibrometer (1, 23) according to any one of the preceding claims,

characterized,

that the beam guiding device is designed for superimposing the measurement and reference beam (14, 15) on the micromirror grid (8).

7. vibrometer (1, 23) according to any one of the preceding claims,

characterized,

that a control unit (21) for controlling the micromirror grid (8) and a signal processing unit (20) for evaluating the detector signals are included.

8. vibrometer (1, 23) according to claim 7,

characterized,

that the control unit (21) is set up to align the micromirrors one after the other individually to the detector (10), and that the signal processing unit (20) is set up to use the detector signals obtained in this way to provide a spatially resolved vibration image of the vibrating object (17) produce.

9. vibrometer (1, 23) according to claim 8,

characterized,

that the control unit (21) is set up to align the micromirrors one after the other with a frequency on the detector (10) that is required to resolve a maximum oscillation frequency

Cutoff frequency is increased.

10. vibrometer (1,23) according to claim 7,

characterized,

that the control unit (21) is set up to align the micromirrors one after the other in a respectively predetermined grouping towards the detector (10), and that the signal processing unit (20) is set up to use the mathematical transformation, in particular by means of a mathematical transformation, of the detector signals obtained in this way a Hadamard transformation to generate a spatially resolved vibration image of the vibrating object (17).

11. Vibrometer (1, 23) according to one of the preceding claims,

characterized,

that the micromirrors of the micromirror grid (8) are designed such that they can each assume two tilt positions, each micromirror Radiation incident on the mirror directs the detector (10) in the first tilt position and into a light trap in the second tilt position.

12. Use of a vibrometer (1, 23) according to one of the preceding claims for identifying a vibrating object (17) or a significant property of the vibrating object (17) based on its vibration characteristics.

13. Use of a vibrometer (1,23) according to claim 12,

wherein the vibration characteristic of the object to be identified is specified in particular by a defined vibration device.

14. Method for generating a vibration image of a vibrating object (17),

an output beam of coherent radiation is generated and broken down into a measuring beam (14) and a reference beam (15),

- The measuring beam (14) is directed onto an oscillating object (17),

the measuring beam (18) returning from the object (17) being superimposed on the reference beam (15), and

- wherein the overlay pattern is observed in a temporally and / or spatially resolved manner by means of a detector (10) and the oscillation pattern is deduced therefrom,

characterized,

- That the vibrating object (17) by means of the measuring beam (14) on

Micromirror grid (8) is shown, and

that the micromirror grid (8) is imaged on the detector (10).

15. The method according to claim 14,

characterized,

that the frequency of the reference beam (15) is shifted.

16. The method according to claim 14 or 15,

characterized,

that the measuring (14) and the reference beam (15) overlap on the micromirror grid (8).

17. The method according to any one of claims 14 to 16,

characterized,

that the micromirrors of the micromirror grid (8) are aligned one after the other in each case individually to the detector (10), and that a spatially resolved vibration image of the vibrating object (17) is generated from the detector signals obtained in this way.

18. The method according to claim 17,

characterized,

that the micromirrors are aligned in succession with a frequency on the detector (10) which is higher than a limit frequency required to resolve a maximum oscillation frequency.

19. The method according to claim 14 to 16,

characterized,

that the micromirrors are aligned one after the other in a respectively predetermined grouping on the detector (10), and that a spatially resolved vibration image of the vibrating object (17) is generated from the detector signals obtained in this way by means of a mathematical transformation, in particular by means of a Hadamard transformation becomes.

20. The method according to any one of claims 14 to 19,

characterized,

that the micromirrors of the micromirror grid (8) direct radiation arriving in a first tilt position onto the detector (10) and in a second tilt position into a light trap.