U.S. patent application number 09/803224 was filed with the patent office on 2001-07-26 for method and apparatus for two-dimensional vibration analysis.
This patent application is currently assigned to Polytec GmbH. Invention is credited to Schussler, Matthias, Wortge, Michael.
Application Number | 20010009111 09/803224 |
Document ID | / |
Family ID | 7857829 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009111 |
Kind Code |
A1 |
Wortge, Michael ; et
al. |
July 26, 2001 |
Method and apparatus for two-dimensional vibration analysis
Abstract
A method and an apparatus are provided for contact-free optical
displacement and/or vibration measurement of an object, in which
the object to be measured is scanned in the form of a grid. The
position of the measurement points and the contour of the grid are
freely selectable. Individual measurement point subquantities, to
be analyzed respectively in correlation with one another, can be
classified in different categories and analyzed as a function of
the category to which they are assigned.
Inventors: |
Wortge, Michael; (Karlsruhe,
DE) ; Schussler, Matthias; (Karlsbad-Spielberg,
DE) |
Correspondence
Address: |
AKIN, GUMP, STRAUSS, HAUER & FELD, L.L.P.
ONE COMMERCE SQUARE
2005 MARKET STREET, SUITE 2200
PHILADELPHIA
PA
19103
US
|
Assignee: |
Polytec GmbH
|
Family ID: |
7857829 |
Appl. No.: |
09/803224 |
Filed: |
March 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09803224 |
Mar 9, 2001 |
|
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09251191 |
Feb 16, 1999 |
|
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6209396 |
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Current U.S.
Class: |
73/657 ;
73/655 |
Current CPC
Class: |
G01H 9/002 20130101 |
Class at
Publication: |
73/657 ;
73/655 |
International
Class: |
G01H 001/00; G01B
009/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 16, 1998 |
DE |
198 06 240.0 |
Claims
We claim:
1. A method for contact-free, optical displacement and/or vibration
measurement of an object (3) by means of at least one
interferometer (1) with at least one laser, at least one control
unit which guides the laser beam to a plurality of points of the
object (3) to be measured so that said object is scanned by the
laser beam, and at least one output unit (5) for the
high-resolution display of the measurement results, wherein, for a
scanning process, the measurement points are selectively positioned
on the object (3) to be measured, individually and/or in at least
one grid adaptable in its contour, the measuring points are
displayed in the output unit (5) for their positioning on the
object (3) to be measured, and the position of the measurement
points on the object (3) to be measured in the output unit (5) is
calculated via a correction process.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of co-pending
U.S. patent application Ser. No. 09/251,191, filed Feb. 16,
1999.
BACKGROUND OF THE INVENTION
[0002] The invention relates to a method for contact-free, optical
displacement--and/or vibration measurement of an object by means of
at least one interferometer with at least one laser, at least one
control unit which guides the laser beam to a plurality of points
of the object to be measured, so that the object is scanned by the
laser beam, and at least one output unit for the high-resolution
display of the measurement results. The invention further relates
to an apparatus for the implementation of the method.
[0003] Such a method and apparatus are known from DE 31 13 090 A1
in which the following procedure is performed: a video image of the
structure to be investigated is recorded, and a rectangular grid of
measurement points is inserted into this video image using a
computer. In this way, a rectangular grid of measurement points is
superimposed upon the object to be measured. The object to be
measured is set in vibration, and the laser beam of an
interferometer is directed toward the pre-defined grid points under
the control of the computer. At each of these points the vibration
spectrum is recorded in a contact-free manner by this
interferometer. Following the measurement, the individual vibration
spectra are analyzed in the computer, and the vibration image of
the object is reconstructed for individual frequencies selected
from the vibration spectrum. These vibration images are output by
an output unit (image screen).
[0004] Optionally, individual measurement points can also be erased
from the measurement grid.
[0005] This method and the associated apparatus are disadvantageous
inasmuch as the arrangement of the measurement points in a
predetermined grid is unsatisfactory for the analysis of vibrations
with complicated geometric configurations. These can be recorded
only by means of an extremely large number of measurement points,
with the result that the measurement can no longer be performed in
a reasonable period of time.
[0006] Additionally, for the analysis of vibration modes, in
particular of car doors or the like, it is known to arrange
individual sensors at different points of the workpiece to be
measured. The sensors are accelerometers which, on the one hand,
falsify the measurement result by virtue of their own bulk and, on
the other hand, cannot be attached in the desired amount or at all
the desired locations. Particularly in the case of small
workpieces, the use of such bulky accelerometers rapidly becomes
subject to insuperable limitations.
BRIEF SUMMARY OF THE INVENTION
[0007] Proceeding from this, an object of the invention is to
provide a method and apparatus which enable vibration analysis to
be performed even in the case of workpieces which have a
complicated geometric configuration or are very small.
[0008] This object is achieved in accordance with the invention in
that, for a scanning process, the measurement points are freely
positioned on the object to be measured, individually and/or in at
least one grid adaptable in its contour, wherein different
measurement point sub-quantities to be analyzed in correlation with
one another are classified in different categories and are analyzed
as a function of the category to which they are assigned.
[0009] Whereas previously in scanning processes, a surface or an
object to be measured (also referred to herein as "measured
object") was covered with a rectangular and right-angled grid and
measured point by point, wherein it was possible to vary the size
of the grid, density of points and number of points at which
measurement was to take place, the method according to the
invention facilitates a substantially more adapted procedure.
[0010] In this procedure one generally simply defines the surface
on which scanning is to take place, for example by marking its
outline on the screen with the mouse, and then fills this freely
defined surface with measurement points using the computer. For
this purpose, for example, the number of measurement points or
their density and other parameters are input into the computer.
[0011] Thus, for example, in the case of a circular measured
object, a circular grid pattern is defined, the contour of which is
thus adapted to the measured object. On the other hand, in the case
of an obliquely extending, rod-shaped measured object, a grid of
the same configuration is created. In this way, the measurement
points can be positioned precisely on the measured object, whereas
in the conventional procedure surface-covering and precise
positioning of the measurement points on the measured object is not
possible in the case of complicated geometric configurations.
[0012] If the vibration characteristic of a measured object is to
be investigated, it is generally insufficient to define a number of
measurement points in an arbitrarily selected geometric shape on
the two-dimensional image of the measured object. Rather, an
interpolating correlation must be established between the
individual measurement points of a measured object or a part of the
measured object to permit analysis of the vibration characteristic.
Here, it is particularly advantageous to define different
categories in which different sub-quantities of the measurement
points to be analyzed in correlation with one another are
classified.
[0013] Thus, for example, it is possible for a three-dimensional
measured object to be covered with a two-dimensional measurement
point grid which also takes into consideration the third dimension
in the analysis: for example, a cable could extend at some distance
in front of a planar measured object in the beam path of the laser
beam. From the imaging on a two-dimensional grid, the analysis unit
does not initially recognize that the measurement points situated
on the cable have nothing to do with the planar measured object and
must not be correlated in the analysis, so as not to falsify the
measured vibration characteristic of the planar measured object.
However, it is necessary for the measurement points situated on the
planar measured object on both sides of the cable to be correlated
respectively with one another and to be interpolated in order to
permit a complete analysis of the vibration characteristic of the
planar measured object.
[0014] The method according to the invention now offers the
possibility, for example, of defining a rectangular grid on the
planar measured object considered in the present example, and at
the same time of placing a polygon of measurement points over the
cable considered in the present example, wherein the measurement
points situated on the cable are classified in a first category,
and the remaining measurement points of the grid are classified in
a second category and are respectively separately analyzed.
[0015] To simplify handling, category types assigned to different
geometric objects, such as circles, ellipses, lines, etc., can be
pre-defined. The individual category types can be provided with
grid types, such as rectangular, hexagonal, polar, etc., and with
standardized features for the data acquisition, such as the number
of averaging operations, the measurement duration, and the number
of measured values.
[0016] The measurement points can also be classified in different
categories in accordance with the expected vibration
characteristic, or can also be classified subsequently in
accordance with the measured vibration characteristic, and the
measurement points can be individually measured, displayed and/or
analyzed in accordance with the category to which they are
assigned. It is thus possible to separately record and display
different regions of the measured object which vibrate with a
common phase in themselves, albeit not with one another. For
example, in the case of the measurement of a disc brake of a motor
vehicle, the brake disc can be covered with measurement points of a
circular category type filled with a polar grid. In the same
operating step, the calliper is then measured with a polygonal
category type filled with a dense, hexagonal grid and also having
additional measurement points at the edge.
[0017] As already shown by the described examples, within the scope
of the invention it is possible to define a plurality of grids for
a scanning process, for example if a circular and a rod-shaped
object are to be measured simultaneously.
[0018] Alternatively, it is also possible to select a plurality of
individual measurement points which then together form a grid.
[0019] It is equally possible to apply measurement points to freely
defined curves (polynomials), for example in order to measure
narrow objects or the edge of an object.
[0020] According to our experiments, discrepancies can occur
between the position of the measurement points in the output unit,
on the one hand, and the position of said measurement points on the
measured object, on the other hand. Such discrepancies can be
caused by distortions of lens systems used in the imaging of the
object or distortions in the projection of the laser beam onto the
object to be measured. The scanning of the object to be measured is
also affected by the motor-driven rotation of mirrors. Upon the
rotation of the mirrors, the laser beam moves over an approximately
cylindrical surface. As the measured object generally has a
different geometric configuration, further distortions arise. Such
distortions would manifest themselves in a disadvantageous manner,
in particular in association with the newly achieved free
positionability of measurement points and groups of measurement
points, for example if measurement points extend along the edge of
a measured object or for example along a welding seam or a row of
rivets; in such cases it is particularly important that the
measurement actually be performed in a geometrically correct manner
and not be falsified by optical distortions.
[0021] Therefore, it is proposed that the position of the
measurement points in the output unit should not be directly taken
from their position on the object to be measured, but be calculated
via a correction process, in particular via one or more coordinate
transformations. The coordinate transformations can be performed
using transformation matrices and, if necessary, are also
non-linear.
[0022] To create the correction process, a set of values is formed
containing positions of measurement points on the object and their
associated positions in the high-resolution display of the output
unit. Depending upon the type of the distortions, different
correction algorithms, such as for example multidimensional,
polynomial, compensating curves, can be used.
[0023] In the case of complicated geometric configurations, which
can be measured particularly well by means of the process according
to the invention, complicated vibration phenomena can occur. To
take this better into account, as already mentioned in the
foregoing, it is preferably provided that the measurement points be
classified in different categories in accordance with the expected
or the measured vibration characteristic, and that the measurement
points be individually measured, displayed and/or analyzed in
accordance with the category to which they are assigned.
[0024] If the classification is performed prior to the scanning
process, the measurement duration or other measurement parameters
can also be specifically preselected for individual categories. For
example, regions of the measured object which vibrate particularly
slowly can thus be measured for a longer period of time.
[0025] For the positioning of the measurement points, their high
resolution display in the output unit is advantageously based on an
image of the measured object. This can be a stored image from a CAD
system or a previously made digital or digitalized recording. It is
also possible to gate-in a real-time image from a synchronously
operating video camera.
[0026] If such an image is available, the classification can
naturally be performed on the basis of this image. Here, it is also
possible to use modern imaging processes which simplify the
definition of the grid contour in computer assisted manner.
[0027] The apparatus according to the invention for the
implementation of the previously described method is characterized
in that for a scanning process, measurement points can be
positioned individually and/or in at least one grid adaptable in
its contour, that the measurement points for this positioning can
be displayed in the output unit so that they can be placed at the
desired location, and that different measurement point
sub-quantities to be analyzed in correlation with one another can
he classified in different categories by means of a classifying
device and can be analyzed as a function of the category to which
they are assigned.
[0028] Further advantageous embodiments of the apparatus are
disclosed below in the detailed description and in the sub-claims
relating to the apparatus, corresponding to the detailed
description and sub-claims of the process.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0029] The foregoing summary, as well as the following detailed
description of the invention, will be better understood when read
in conjunction with the appended drawings. For the purpose of
illustrating the invention, there are shown in the drawings
embodiment(s) which are presently preferred. It should be
understood, however, that the invention is not limited to the
precise arrangements and instrumentalities shown.
[0030] In the drawings:
[0031] FIG. 1 is a block diagram of the apparatus according to the
invention; and
[0032] FIG. 2 shows an arrangement of grids, which are adapted in
their contour, on an object to be measured.
DETAILED DESCRIPTION OF THE INVENTION
[0033] FIG. 1 illustrates a laser interferometer 1 which preferably
has the form of a heterodyne vibrometer and from which a laser beam
2 is transmitted, for interferometric measurement, to an object 3
to be measured. For the scanning of the measured object 3 with the
laser beam 2, the interferometer 1 comprises a lens and mirror
arrangement (not shown). This is set such that the laser beam 2
strikes the object 3 to be measured at a desired measurement point.
It remains here for a few vibration cycles in order to perform an
interferometric measurement of vibration frequency, amplitude and
phase.
[0034] Then the lens and mirror arrangement of the interferometer 1
is adjusted by motor control and/or via piezo elements, such that
the laser beam is guided towards the next measurement point on the
object 3 to be measured, where it scans the aforementioned
variables in the new position 2'. This process is repeated until
all the measurement points have been recorded.
[0035] The measured values are forwarded from the interferometer 1
to an analysis unit 4 in the form of a computer which conditions
the data for the high-resolution display in an output unit 5
(screen).
[0036] Here, it is of essential significance that the lens and
mirror arrangement does not control the laser beam in such manner
that the beam scans the object 3 to be measured in a fixed,
rectangular grid pattern. Rather, the laser beam is deflected such
that for a scanning process, measurement points are positioned
individually and/or in at least one quasi-variable grid whose
contour is adapted to the region to be measured.
[0037] To define this contour, it is also essential that the
measurement points for this positioning can be displayed in the
output unit 5.
[0038] This high-resolution display of the measurement points is
based on an image of the measured object, so that the desired
position of each individual measurement point can easily be found.
This image can be produced particularly well by means of a video
camera connected to the output unit 5.
[0039] In general, the optical system of the video camera and the
lens and mirror arrangement of the interferometer 1 are subject to
faults. In order nevertheless to produce congruency between the
actual position of the measurement points on the measured object 3,
their setting in the lens and mirror arrangement of the
interferometer 1, and their display in the output unit 5, the
device comprises a calibrator 6. Via transformation matrices this
calibrator respectively performs a coordinate transformation
between two of the three aforementioned systems. The position of a
measurement point on the measured object 3 is thereby assigned the
correct position in the high-resolution display of the output unit
5.
[0040] The setting of the calibrator is effected by recording a set
of values of positions of a few measurement points on the object
and the positions of said measurement points in the output unit.
The transformation matrices are calculated therefrom. In addition,
here it is possible to include the particular setting of the lens
and mirror arrangement of the interferometer 1 for each measurement
point, so that calibration also takes place for this purpose.
[0041] The calibration of the laser positioning is performed most
easily using an image of the measured object continuously produced
by a video camera. The laser is directed toward a selected
calibration point marked in the video image. Here, the absolute
position of the lens and mirror arrangement is recorded. This
process is repeated for all the desired calibration points. Then
the calibration parameters are calculated. The occurring
distortions are interpolated and corrected by a polynomial
compensating curve. An example of a correcting algorithm will be
described in detail in the following for the frequently occurring
pin-cushion distortion of a scanner:
[0042] The user moves the laser beam to an arbitrary number of
calibration points and marks these in the video image. A polynomial
actuation system is set up in order to perform a mathematical
regression by means of which the pin-cushion structure, which the
laser beam would approach instead of the desired rectangle in the
absence of a correcting algorithm, is restored to a rectangle.
Taking only one axis into consideration, this yields for
example:
U.sub.x=arc tan
(.alpha..sub.o.multidot.X+.alpha..sub.2.multidot.X.sup.2+.-
alpha..sub.3X.sup.3+ . . . )
[0043] wherein U.sub.x is the mirror voltage, .alpha..sub.o,
.alpha..sub.1, etc are the parameters defined in the correcting
algorithm and x is the coordinate of the laser on the video image.
With such a correcting algorithm, in particular disturbances such
as distortion of the -video image, pin-cushion distortion of the
scanners, non-linear response of the scanner and inaccuracies in
the user input, can be corrected simultaneously. Here, the accuracy
of the correcting algorithm is greater, the larger the number of
selected reference points.
[0044] Reference will be made to FIG. 2 in explanation of the
classification of measurement points. A portion of the left-hand
side of a vehicle can be seen here. This portion comprises a part
of a car door 8, in the lower region, and the left-hand A-column 9
of the vehicle, in the upper region. In accordance with the
invention, the scanning grid is adapted to the measured object,
here comprising the car door 8 and the A-column 9. Correspondingly,
the grid comprises two sub-grids. Of these, the first uniformly
covers a rectangular surface of the car door 8. The second is a
polynomial freely defined on the A-column 9.
[0045] In the vibration analysis, the amplitude, frequency and
phase of the vibration are of interest. For an effective analysis,
points assigned to the same vehicle part or points with a fixed
phase relationship are to be combined with one another and
classified in the same category. In the subsequent optical display
(animation) this category can then be respectively displayed and
analyzed independently, or optionally also in combination.
[0046] As it is to be expected that the vehicle door 8 and the
A-column 9 will possess a different vibration characteristic, the
measurement points located thereon are assigned to different
categories via a classifying device 7 (FIG. 1). The categorization
via the classifying device 7 is normally carried out by an
operator. However, it can also take place automatically using
modern imaging methods, or the operator can be assisted by these
imaging methods in the classification process.
[0047] In addition to the illustrated example, it is also possible
to form categories, for example of measurement points, respectively
located on different read heads of a hard disc or on different
cooling ribs of a cylinder.
[0048] Within the scope of the invention, the analysis unit 4, the
calibrator 6 and the classifying device 7 can also be combined to
form a central unit, for example in a computer.
[0049] An advantageous embodiment of the method comprises the
following steps:
[0050] The measurement points are defined in the video image of the
output device 5 via a routine. They are either selected manually or
are arranged on the video image by an imaging program. The video
image can consist of a stored or reconstructed real image. For
real-time processing, simultaneous recording of the video image is
advisable.
[0051] The points thus defined are assigned to their respective
category (optionally with graphic support). This can preferably be
effected by marking on the screen, and specifically before or after
the measurement, as the classification is of primary importance for
the analysis. If different categories are to be measured in
relation to one another, or different measurement parameters are to
be set for different categories, it is advisable for the
classification to take place prior to the measurement.
[0052] Then, the measurement is carried out in a manner known per
se, possibly taking into account the assigned categories.
[0053] Finally, the vibration modes are graphically displayed,
separately according to categories.
[0054] It will be appreciated by those skilled in the art that
changes could be made to the embodiment(s) described above without
departing from the broad inventive concept thereof. It is
understood, therefore, that this invention is not limited to the
particular embodiment(s) disclosed, but it is intended to cover
modifications within the spirit and scope of the present invention
as defined by the appended claims.
* * * * *