DE102020120398B4 - Device and method for performing measurements on rotating objects - Google Patents

Abstract

Device (1) for carrying out measurements on rotating objects (2), having an object holding element (4) for receiving and rotating an object (2) to be measured, a derotator (5) designed as a Dove prism and an image acquisition unit (3), which are arranged along a beam path, wherein a correction optics unit (7) for refraction of the light passing through it is arranged on its surfaces with an object-side focal plane in the direction of the object mounting element (4) in the beam path between the derotator (5) and the object mounting element (4). , wherein the object-side focal plane of the corrective optics unit (7) is designed to be adjustable in such a way that it coincides with the surface of the object (2) to be examined and wherein the corrective optics unit (7) has a variable focal length, with changing the focal length changing the position of the focal plane is shifted to the correction optics unit (7).

Description

The invention relates to a device for carrying out measurements on rotating objects, which has an object holder element for holding and rotating an object to be measured, a derotator designed as a Dove prism and an image acquisition unit, which are arranged along a beam path, and a method for carrying out Measurements on rotating objects and the use of the device as well as the use of a correction optics unit.

Measurements on rotating objects are important tools for examining and analyzing components under load. In particular for special applications in highly developed technologies such as aviation, new materials are used for components whose properties have not yet been sufficiently researched. It is crucial to test the deformation and vibration behavior of the components under load in order to be able to assess the extent to which changes in the component and thus possible errors can occur. Both the vibrations parallel to the axis of rotation and changes perpendicular to it caused by the centrifugal force are examined.

In order to be able to carry out the non-destructive analysis of the components, optical methods such as laser vibrometry are particularly suitable. These methods also enable measurements to be taken on rapidly rotating objects. However, the fast movement of the objects leads to a blur in the measurement, so that only very fast measurement methods can be used. In order to be able to use a larger number of measuring methods, derotators are used [1], which influence the optical path of the light used for the measurement in such a way that the rotation of the object to be examined is compensated and the object appears to be stationary. Examination of objects using a derotator is discussed in the U.S. 3,625,612 A described. As derotators can continue mirror combinations, as in the U.S. 4,929,040 A are described are used.

Derotators consisting of an array of mirrors are used for this purpose in astronomy, for example. To compensate for the rotation of the earth, telescopes have to track the apparent movement of the stars. While in smaller telescopes the movement of the entire optical unit takes place around an axis parallel to the earth's axis, in larger telescopes the movement of the stars is followed by rotating the primary mirror in the horizontal and vertical directions. However, this leads to a rotation of the image, so that a longer exposure time would result in a blurred image. To avoid this, a derotator consisting of at least 3 mirrors is used to compensate for this movement. Such derotators are comparatively large and heavy, but this is not a problem in this application as the movement is extremely slow. However, the use of complicated derotators, which contain several mirrors, is not suitable for measurements at necessarily high rotation speeds. Although such complicated derotators have already been used in the past for measuring vibrations, they have a very high mass and have not become established because of the expensive and complicated production.

For this reason, the use of Dove prisms has proven particularly useful at high rotational speeds [1], whose moment of inertia is advantageous due to the low expansion. A design of a Dove prism for use as a derotator is also in DE 20 2006 017 412 U1 explained. Due to its cylindrical design, the derotator described is suitable for being fixed within a cylindrical holder and set in rotation by it.

The use of derotators is known from the prior art U.S. 5,981,937A describes an optical system with an object table, which carries a material to be observed and is rotatably mounted on a predetermined axis. The optical system also has a Dove prism rotatably mounted on an optical axis.

From the U.S. 2007/0 222 991 A1 an imaging method for generating signals for monitoring wafers is known. In this case, an imaging system using an optical unit and a Dove prism is used.

In the CN 1 07 314 742 A describes a rotating optical tomographic imaging system and method. A line scan camera is used to generate a two-dimensional image. Imaging errors are not relevant for such an application.

In the U.S. 4,637,718 A discloses an optical scanner for reading indicia printed on a circular planar surface. This disclosure relates to a hand held optical identification system developed for use in the manufacture of nuclear fuel bundles.

JP H04-278555 A proposes an optical microscope which includes an illumination optical system and an observation optical system. An image rotation device is provided, which is provided on a common light path of the illumination optical system and the observation optical system.

Due to the derotators used, which are mostly made of glass, there are also additional imaging errors [2]. This is how chromatic errors occur, which are relevant for measurement methods using broadband light. In addition, there are further errors for points that are not on the optical axis. The installation of a derotator in the optical path leads to a periodic change in the optical path of the light [1], resulting in signals with the period of the rotational speed of the derotator. With a glass prism with a typical size of 100 to 150 mm in length and an angle of view of 3°, there are already periodic changes in the optical length of 40 µm. This periodic change in length leads to two additional components in the signal, one having the frequency of the derotator and a larger one having twice the frequency of the derotator, ie the rotation frequency of the object. When analyzing the vibrations of the component, these errors must be taken into account accordingly. This causes problems in particular in material testing, in which the deformations due to very high centrifugal forces are also to be examined.

Such vibrations or deformations within the plane of rotation are usually analyzed using camera systems with very short exposure times without the use of derotators. With a rotation speed of 6000 revolutions per minute and a still tolerable movement of the test body of 1°, an exposure time of less than 25 µs is necessary. The motion blur is then only 2 mm for an object with a radius of 100 mm. Even then, minor damage can no longer be assessed, also because the image resolution of such fast systems is not high enough. Here the use of a derotator brings a significant advantage.

The use of glass Dove prisms is therefore necessary when it comes to examining rotating components, in particular using camera systems. However, when used with camera systems, the periodic errors in the evaluated signal shown above lead to a periodic deformation of the image, which is caused by the derotator and the optical path of the light that is changed by it. This deformation is mainly a consequence of the astigmatic image elevation. This special case of image location shift is the apparent heave of an object when viewed through a refracting medium. As a result of image elevation, a body of water appears shallower and a mirror thinner than they actually are. The reason for this is the changed beam path and thus the difference between the angle of the beam within the medium to the perpendicular of the interface compared to the angle of the beam outside of the medium to the perpendicular of the interface. This causes the object to appear closer in the plane of the prism than perpendicular to it. For a right-angle glass Dove prism with an opening of 30 mm, this results in a different image elevation for the two planes of approx. 2 cm.

Another, albeit smaller, error results from an asymmetry in the 180° rotation of the derotator. These two effects mean that in a camera system, all points outside the axis cover an approximately circular path. This effect is intensified with increasing distance of the image points from the axis of rotation and the radius of the path of the image points increases. At a distance of 2 m from the derotator to the object to be examined, each point already performs a movement which corresponds to about 1% of the distance from the center of rotation. However, this means that this effect is greater for rigid objects than the actual deformation caused by the rotational forces that occur. This makes it impossible to measure and determine the deformation effects in such objects.

Even when using a derotator for vibration measurements at one point, the measuring point periodically moves over the object to be examined and thus leads to further errors, especially if the surface is not perfectly flat. It is possible to reduce these effects by increasing the distance between the derotator and the object. However, at a distance of 10m, the effect is still 0.2% of the distance from the rotation point. For an object to be examined with a radius of 100 mm, i.e. already up to 0.2 mm. In addition, telephoto lenses with a large focal length must be used for such a large distance. Due to the small diameter of the derotator, only a small aperture of the lens can then be used, which further complicates the recordings.

Due to the astigmatism, the image is never really sharp, which leads to problems, especially with a shallow depth of field, i.e. if the distance to the test object is small and the aperture of the lens is large.

In order to be able to determine, especially when examining the internal structure of rotators made of glass fiber reinforced plastic (GRP), whether fibers break and the connection between fibers and matrix breaks during fast rotation, the use of optical coherence tomography (OCT ) required. This requires a resolution in the range of less than 50 µm. While only very fast and expensive OCT systems are suitable for the high rotation speed of the test object, comparatively slow OCT systems can also be used thanks to the derotator.

With the use of a derotator and the resulting limited diameter and the limited beam angle in the derotator, the high resolution can only be achieved for a comparatively small area around the axis. With a width of the derotator of approximately 30 mm and radiation in the near infrared range, an area of approximately 100 mm around the center can be examined. Also, due to the astigmatism caused by the derotator, the beam is not focused on the sample, but has two foci in and perpendicular to the plane of the derotator that are about 2 cm apart. There is also significant (almost circular) wandering of the beam on the specimen.

It is therefore the object of the invention to carry out measurements on a rapidly rotating object. It is essential that the deformation and vibration behavior of metallic objects (e.g. blisks) or fiber composite rotors, such as a disc made of glass fiber reinforced plastic, can be examined. The superficial structural changes are to be displayed and analyzed in particular using a video imaging method. It is crucial for sufficient sensitivity of the measurements that the movement artifacts described above, i.e. the image errors caused by the use of the derotator, are avoided.

According to the invention, the object is achieved by a device, a method and uses according to the independent claims. Advantageous developments of the invention are specified in the dependent claims.

The object is achieved in particular by a device for carrying out measurements on rotating objects, which has an object holder element for receiving and rotating an object to be measured, a derotator designed as a Dove prism and an image acquisition unit, the individual components being arranged along a beam path. Furthermore, a corrective optics unit for refraction of the light passing through it is arranged on its surfaces with an object-side focal plane in the direction of the object holder element in the beam path between the derotator and the object holder element and the object-side focal plane of the corrective optics unit is designed to be adjustable, i.e. can be set in such a way that it coincides with the surface of the object to be examined. The object-side focal plane of the correction optics unit then intersects the object, ie consequently coincides with the object. The object holding element is thus also arranged close to the focal plane on the object side. In one possible embodiment, the object-side focal plane also intersects and consequently coincides with the object-mounting element.

When used in a vibrometer or by means of OCT, the emitted light beam first traverses the derotator in a first direction and then the correction optics unit, after which it is reflected by the rotating object and, after traversing the correction optics unit and the derotator again, is detected by the vibrometer or the OCT unit .

The light passing through the correction optics unit is refracted on the surfaces of the correction optics unit. The correction optics unit is preferably arranged adjacent to the derotator in the beam path. In particular when used in a vibrometer, the correction optics unit is arranged adjacent to the derotator in an advantageous embodiment, as a result of which the diameter hardly has to be larger than the diameter of the derotator. When used in OCT, the distance between the derotator and the correction optics unit is preferably chosen such that a telecentric image is produced, i.e. the light bundles always hit the examination object parallel to the axis of rotation. Although this requires a larger diameter for the correction optics unit, it creates the same optical paths for all points in the focal plane. With this arrangement, the three-dimensional image of a plane remains a plane and does not have to be corrected afterwards.

When using an image capture unit, light from the rotating object is guided through the correction optics unit and the rotating Dove prism to the image capture unit. The rotating object is preferably illuminated by a stationary lighting unit which is arranged largely rotationally symmetrically around the object.

Within the meaning of the invention, the focal plane is understood to mean a plane which is arranged at the focal point perpendicularly to the optical axis. This is also referred to as the Fourier plane, focal surface or focal plane. The optical axis is the axis of symmetry of a rotationally symmetrical optical system. The important thing here is the symmetry of the surfaces, not their boundaries. In a device according to the invention, the optical axis runs from the image acquisition unit through the derotator and the correction optics unit to the object.

The correction optics unit can have one or more lenses and/or lens elements. A focal plane of the corrective optics unit is thus preferably understood to mean a plane that runs through the focal point of the corrective optics unit and that runs parallel to the main planes of the corrective optics unit.

An object-side focal plane is arranged in the beam path from the correction optics unit in the direction of the object.

According to the invention, the focal plane coincides with the surface of the object to be examined even if there is a deviation between the focal plane and a point on the surface of the object. Such deviations typically occur either due to an inexact setting of the focal plane or due to a non-flat or flat surface of the object. Furthermore, various imaging methods such as coherence tomography can ensure that the light is not only reflected directly on the surface of the object, but that the light penetrates the object to a certain extent. This can lead to a deviation in the position of the focal plane from the reflection plane of the light.

In the device according to the invention, what is particularly important is that the plane of reflection of the electromagnetic radiation on the object coincides with the focal plane of the correction optics unit. The plane of reflection of the electromagnetic radiation is understood to be a plane which is arranged perpendicularly to the beam path and which coincides with the point or area of the object at which the light is reflected. An object can thus have several, possibly wavelength-dependent, reflection planes. The exact position of the focal planes can also depend on the wavelength of the electromagnetic radiation or light used.

However, an arrangement according to the invention still reduces the image errors even if the examined point of the object deviates from the focal plane. The coincidence of the object-side focal plane of the correction optics unit with the surface of the object to be examined also includes a certain deviation within the meaning of the invention. The deviation is preferably less than 10% or particularly preferably less than 1% of the focal length of the correction optics unit.

For a derotator with an astigmatism of 2 cm, the distance between the derotator and the object under examination appears to vary by 2 cm. For example, if the object is at a distance of 2 m, the error is just 2 cm/2 m=1% without the use of the correction optics unit according to the invention. This error relates to the distance to the axis of rotation. For an object with a radius of 10 cm, the error is 1 mm.

If you then use a corrective optics unit that is set to 2 m, the error is 0 for objects in this plane. However, if you have not set the distance exactly or the object is not flat or flat, there will also be a certain deviation here. This can be calculated by considering how far away the image is from the object. For example, if the object (or parts of it) is at a distance of 1 cm from the focal plane, the error has been reduced to 5 × 10 ^-5 . With the specified size of the object with a radius of 10 cm, it is only 5 µm, which is smaller than the achievable resolution.

According to the invention, the measurements on the rotating objects are carried out using optical methods. By evaluating the light reflected from the objects to be measured, conclusions can be drawn about the properties of the object.

For example, laser vibrometry can be used to determine point-by-point vibrations of the object or test specimen in the direction of the axis of rotation, i.e. parallel to the axis of rotation. Deformations in the plane of rotation, on the other hand, can be determined with camera systems. A conventional video camera can be used for image acquisition, which generates a 2D representation of the test specimen, or a system can be used that enables a 3D representation of the inner structure of the test specimen, such as optical coherence tomography. However, coherence tomography, which has the advantage that all components can be recorded with one method due to the three-dimensional image acquisition, has the disadvantage of the very high information density. The invention is expressly not limited to this type of optical measurement method, but alternative measurement methods can also be used be used with the derotator and corrective optics unit.

Within the meaning of the invention, an object-holder element refers to one or more object-holder elements.

The device according to the invention has a derotator with a Dove prism. The Dove prism is preferably designed in such a way that a base reflection for each light beam occurs within the Dove prism when used as a derotator.

According to the invention, a Dove prism is a prism with the base of a rectangle or a trapezium with side surfaces inclined at 45°. In one possible embodiment, it is designed as an isosceles, right-angled prism in which the optically non-effective area is cut off. Other forms of such a prism can also be used, such as a form in which certain areas are rounded. In one possible embodiment, the prism is made of glass. In a Dove prism with a base reflection, when a ray of light traveling along the long axis of the prism strikes one of the inclined incident surfaces, it is first refracted into the prism and directed onto the longest side of the prism. There, the light beam undergoes total reflection and is refracted again in the inclined exit surface. The incoming and outgoing beams have the same angle to the base, whereby an image experiences a reflection on a surface parallel to the base due to the one-time reflection in the prism.

Within the meaning of the invention, an image capturing unit can mean a single image capturing unit or also a plurality of image capturing units. The image acquisition unit is preferably designed as a video camera for the two-dimensional representation of the surface of the object or as a system for optical coherence tomography (OCT) for the three-dimensional representation of the internal structure of the object. The image capturing unit particularly preferably has a lens.

In the context of the invention, light is understood to mean electromagnetic radiation. In this case, the electromagnetic radiation can be monochromatic, that is to say essentially have radiation of one wavelength, or also be broadband, that is to say have radiation of a multiplicity of wavelengths. In the context of the invention, light is in particular not limited to visible radiation, but the term also includes infrared and ultraviolet radiation. In a preferred variant, visible and/or ultraviolet and/or infrared light is used.

The device according to the invention has the advantage that measurements can be carried out on rotating objects even at high rotational speeds, periodic distortions of the object during imaging, which are caused by a derotator containing a Dove prism, being avoided.

The evaluation of deformations and vibrations of the rotating test body thus advantageously requires less image processing and vibrations at the rotational frequency of the object can also be examined more easily.

According to the invention, the correction optics unit of the device has a variable focal length. The position of the focal plane is shifted relative to the correction optics unit by changing the focal length.

By changing the focal length, the focal plane can be brought into line with the object to be measured. This makes it possible to change the distance between the correction optics unit and the object and to adjust the focal length to the changed distance, so that the focal plane running through the focal point and perpendicular to the optical axis corresponds to the surface of the object to be measured.

In particular, it is possible to react to a variable distance between the object and the image acquisition unit and, depending on the application, objects that are both close and further away can be measured.

A possible configuration of the device provides that the correction optics unit comprises a lens system made up of several lens elements. The variation in the focal length of the correction optics unit results from a change in at least one distance between at least two lens elements. In a lens system made up of a plurality of lenses and/or lens elements, a small variation in the distance between the lenses and/or lens elements can result in large changes in the focal length of the lens system. At least individual lenses and/or lens elements or all lenses and/or lens elements are preferably arranged at a distance from one another.

In another preferred embodiment, the correction optics unit has a liquid lens. With such a liquid lens, the focal plane is adjusted without shifting the correction optics unit, but by adjusting the curvature of at least one light-refracting surface and thus the focal length.

In a preferred embodiment, the liquid lens has a first and/or a second flexible plate, ie one or two flexible plates, with a liquid delimited by this plate or plates. The focal length of the correction optics unit results from the amount of liquid within the liquid lens and the resulting change in the curvature of the flexible plates. The principle of a liquid lens is based on the fact that a liquid is delimited on at least one side by a flexible plate, preferably on two sides by two flexible plates. Changing the amount of liquid changes the pressure acting on the flex plate or both flex plates, changing their curvature, which directly affects the focal length of the liquid lens.

The liquid and the plates are preferably formed from materials with an identical or similar refractive index, so that there is little or no reflection at the interfaces within the liquid lens.

In one possible embodiment, the bendable plates are made of a transparent, thermoplastic material. They are preferably formed from polymethyl methacrylate. A convenient way is to use glycerin as the liquid inside the liquid lens.

In a suitable variant, the two plates have a diameter of 5 cm and a thickness of 2 mm, in which case a pressure of 1 bar is already sufficient for a suitable refractive power. The setting of the refractive power of the liquid lens can be controlled very well by means of the amount of liquid injected. The liquid can be applied, for example, by means of a syringe, which makes it possible to set the desired focal length very precisely.

A preferred variant envisages using plates with a slightly different thickness, so to reduce spherical aberration, a curvature in the ratio of approx. 1 to 6 can also be set, with the thicker plate, which is less curved, on the object side, i.e. facing the object, should lie.

In a particularly preferred option, the corrective optics unit of the device is designed to be exchangeable in a modular fashion. In this case, the correction optics unit can be arranged in a modular manner between the derotator and the object, ie consequently in the beam path also between the derotator and the object holding element. It is thus also possible to additionally install the corrective optics unit in an already existing measuring arrangement, with a device according to the invention only being realized by the corrective optics unit.

In an advantageous embodiment, the correction optics unit is designed in a modular manner as an add-on optics unit. The correction optics unit can be retrofitted to an existing system for image acquisition using a derotator. The correction optics unit is arranged at a distance from the derotator and the object to be examined. The beam path of the system or the device, ie the path of the light bundles through the individual components of the system or the device, is decisive for the exact position of the correction optics unit. The position of the correction optics unit does not therefore have to be arranged spatially in every variant between the object and the derotator; the beam path of the light to be evaluated is relevant, which can be changed in particular by reflective components.

Individual modules or components are preferably connected to one another in a detachable manner. In particular, the correction optics unit can be detachably connected to one or more of the other components.

In order to avoid or reduce other image errors, such as different brightness for different pixels due to aperture effects in the beam path (vignetting), the use of other optical components such as lenses and/or lens systems and/or apertures can be useful. These additional optical components are preferably arranged between the camera and the derotator. The use of other optical components can also offer advantages for the image quality when using OCT or when using a scanning vibrometer.

In an advantageous embodiment, two lenses are arranged between the diaphragm and the derotator to reduce the vignetting, so that the image of a diaphragm arranged in the image acquisition unit is imaged in the derotator.

When using a derotator in front of a camera with a lens for imaging, both the field of view and the diameter of the light beam are limited by the size of the derotator. The light beam hits the camera lens from a point on the rotating object. There are several optical components that exert an aperture effect on the light beam, i.e. limit it in the outer area. The entry and exit surfaces of a derotator designed as a Dove prism, as well as the aperture of the camera, are particularly relevant. An axially parallel bundle of rays is limited only by the smallest of the apertures. However, obliquely incident light beams are also transmitted the other apertures limited. In order to obtain an image of the same brightness, this effect should be reduced. For this reason, one possible embodiment provides for a reduction in the image field with a simultaneous reduction in the size of the aperture of the camera. In an alternative embodiment, a screen is arranged between the derotator and the image acquisition unit, as close as possible to the derotator. The disadvantage of this is that the image field that can be achieved becomes smaller and the beam bundle has a smaller area, which results in a darker image and consequently in a longer necessary exposure time.

An alternative embodiment provides two lenses or two objectives between the derotator and the image acquisition unit, which preferably have an identical focal length and are particularly preferably arranged at a distance equal to the sum of their focal lengths. The beam is limited by the diaphragm arranged in the image capturing unit, which is usually close to the outer lens of a lens combination inside the image capturing unit, such as a camera lens, for example. The aperture of the image acquisition unit is imaged through the two lenses or through the two lenses into the center of the derotator. It is possible to increase the field of view by 25% and obtain more than four times the cross-sectional area of the light beam. Despite the loss of light in the two lenses or objectives, a quadrupled image brightness or a reduction of the exposure time by a factor of 4 can be achieved. It is relevant that the diameter of the lenses or objectives is sufficiently large. Adequate imaging quality of the lenses or objective lenses is also required.

Even when using a scanning vibrometer or OCT, in which the measuring beam is guided over the object or the object surface, additional imaging between the deflection system and the derotator can increase the beam cross-section and/or the area that can be measured without limiting the Lichtstreits can be measured, can be achieved. Due to the larger beam cross-section, a better resolution over the image field can be achieved, especially for OCT. Advantageously, the rotating mirror or mirrors used for deflection should be placed at or near the position of the stop 20.

1. a) Bereitstellen einer erfindungsgemäßen Vorrichtung zur Durchführung von Messungen an rotierenden Objekten,
2. b) Einbau des Objektes in das Objekthalterungselement,
3. c) Einstellen der Korrekturoptikeinheit, sodass die objektseitige Brennebene der Korrekturoptikeinheit mit der Oberfläche des zu untersuchenden Objektes zusammenfällt und
4. d) Messungen an dem rotierenden Objekt mittels der erfindungsgemäßen Vorrichtung.

The object is also achieved by a method for carrying out measurements on rotating objects using a device according to the invention, which comprises the following steps:

1. a) providing a device according to the invention for carrying out measurements on rotating objects,
2. b) installation of the object in the object holder element,
3. c) Adjusting the correction optics unit so that the object-side focal plane of the correction optics unit coincides with the surface of the object to be examined and
4. d) Measurements on the rotating object using the device according to the invention.

The method preferably serves to determine the deformation and vibration behavior of a rapidly rotating object.

The image capturing unit preferably also includes a lens. In a rather advantageous variant of the method, the lens can be set to infinity when measuring the rotating object.

According to the invention, method step c) comprises setting the focal length.

An advantageous variant of the method provides for the measurement at individual points of the rotating object to be focused on this point of the object. In a particularly preferred option of the method, the light beam is scanned over the surface of the object for the measurement on the rotating object.

Furthermore, the object is achieved by using a device according to the invention for laser Doppler vibrometry, in particular by means of a scanning vibrometer.

Alternatively, the object is also achieved by using a device according to the invention for two-dimensional display using video imaging. A further preferred variant is the use of a device according to the invention for the three-dimensional representation by means of optical coherence tomography.

The object is also achieved in particular through the use of a correction optics unit according to the invention, which has at least one lens and/or a lens element, the light passing through the correction optics unit being refracted at boundary surfaces of the correction optics unit, in order to eliminate periodic deformations caused by the derotator during measurements on rotating objects solved. The correction optics unit then corrects image errors that are caused by the use of a derotator in the form of a Dove prism.

The corrective optics unit, which has at least one lens and/or one lens element, is preferred for imaging methods or for used in scanning vibrometry to eliminate periodic deformations caused by the derotator.

The principle is that derotators made of prisms lead to a beam shift, but not to a change in the angle of the beams with respect to the axis of rotation, since a symmetrical prism with a reflection at the base does not change the angle of the beam with respect to this base. Therefore, the periodic deformation can be eliminated by a lens or a combination of lenses between the object and the derotator, which has a focal length corresponding to the lens-object distance. Effectively, the lens or lens combination makes the object behind the lens appear to be at infinity, so an astigmatic difference of, say, 2 cm does not change the size of the image.

Furthermore, the correction optics unit avoids chromatic errors caused by the prism contained in the derotator.

Advantageously, no fast and expensive video system is required for image acquisition when using the device according to the invention. Advantageously, it is even possible to carry out examinations using high-resolution 3D measurement methods such as optical coherence tomography.

Since the distance between the object to be examined and the derotator can vary greatly depending on the application, a variable distance lens combination or a focus variable liquid lens is preferably used so that the device or the correction optics unit can be adjusted accordingly for objects in a large range of distances can.

A combination of an achromatic converging lens with a focal length of 200 mm and an achromatic diffusing lens with a focal length of -150 mm with a distance between the adjacent main planes of between 5 and 7 cm allows focusing on objects between 1 m and infinity behind the correction optics. The orientation of the lenses to one another is particularly important for good image quality. In particular, the position of the main planes of the lens system must be taken into account when determining these numerical values.

$$-100{\text{ dpt}}^2<\mathrm{\phi }1+\mathrm{\phi }2<-20{\text{ dpt}}^2$$

In an advantageous embodiment, the following condition applies to the selection of the lenses: $$-5\text{dpt <}\mathrm{\phi }1+\mathrm{\phi }2<\mathrm{0,}$$

$$-100{\text{<figure-callout id="2" label="dpt" filenames="DE102020120398B4\_0003.png,DE102020120398B4\_0005.png" state="{{state}}">dpt</figure-callout>}}^2<\mathrm{\phi }1+\mathrm{<figure-callout\; id="2"\; label="\phi "\; filenames="DE102020120398B4\_0003.png,DE102020120398B4\_0005.png"\; state="\{\{state\}\}">\phi </figure-callout>}2<-20{\text{<figure-callout id="2" label="dpt" filenames="DE102020120398B4\_0003.png,DE102020120398B4\_0005.png" state="{{state}}">dpt</figure-callout>}}^2$$

Where φ1 = 1/f1 and φ2 = 1/f2 are the refractive powers of the lenses and f1 and f2 are the focal lengths of the lenses.

In the case of use for examination by means of optical coherence tomography, a focal length in the range of less than one meter is preferred. An achromat with a refractive power of approx. 2 dpt has proven to be a particularly advantageous variant.

For the realization of the invention, it is also expedient to combine the above-described configurations, embodiments and features of the claims with one another in an expedient arrangement.

exemplary embodiments

The invention will be explained in more detail below using exemplary embodiments. The exemplary embodiments are intended to describe the invention without restricting it.

• 1 eine Vorrichtung zur Durchführung von Messungen an rotierenden Objekten,
• 2 eine Korrekturoptikeinheit, welche als Linsensystem mit mehreren Linsenelementen ausgebildet ist,
• 3 eine alternative Ausgestaltung einer Korrekturoptikeinheit als Flüssiglinse,
• 4a eine Vorrichtung zur Durchführung von Messungen an rotierenden Objekten, bei der die Vignettierung durch eine Blende nahe am Derotator reduziert wird und
• 4b eine Vorrichtung zur Durchführung von Messungen an rotierenden Objekten, bei der die Vignettierung durch eine Blende und zwei weitere Linsen reduziert wird.

The invention is explained in more detail with reference to drawings. show it

• 1 a device for performing measurements on rotating objects,
• 2 a correction optics unit, which is designed as a lens system with several lens elements,
• 3 an alternative configuration of a correction optics unit as a liquid lens,
• 4a a device for performing measurements on rotating objects, in which the vignetting is reduced by an aperture close to the derotator and
• 4b a device for performing measurements on rotating objects, in which the vignetting is reduced by an aperture and two other lenses.

In 1 a device 1 for carrying out measurements on rotating objects 2 is shown. An image acquisition unit 3 is used to detect and evaluate the light beam 6 reflected by the object 2 . A derotator 5 is arranged between the rotating object 2 and the image acquisition unit 3 in order to enable or simplify a measurement despite the rapid rotation of the object 2 , which is mounted in or on an object holding element 4 . The derotator 5 is designed as a Dove prism. The derotator 5 designed as a Dove prism rotates at half the rotational speed of the object 2, as a result of which the beam path is changed in such a way that the object 2 appears to the image acquisition unit 3 to be stationary. The use of a larger number of variants for the image acquisition unit 3 is advantageously possible as a result. A measurement here is not limited to very fast systems, but slow-working image acquisition units 3 can also be used.

To reduce image errors caused by the derotator 5, a correction optics unit 7 is arranged between the derotator 5 and the object 2 to be measured. Image errors are avoided by this correction optics unit 7 and are therefore not included in the evaluation of the signal by the image acquisition unit 3 . For this purpose, optical systems are used, ie lens elements or combinations of lens elements with a specific or with a variable focal length. The object 2 is arranged in the focal plane of the correction optics unit 7 or the focal plane is set in such a way that the focal plane coincides with the object 2 to be examined. For the derotator 5, the object 2 appears to be at infinity, so that the reflected rays, starting from a point on the object, run parallel to one another through the derotator.

The course of the light beam 6 is shown schematically here, with a representation of the exact course including the refraction and the reflection within the derotator 5 and also within the correction optics unit 7 being dispensed with. Also, only the on-axis light beam is shown.

The image acquisition unit 3 used is embodied as a camera with a lens which is set to infinity for an inventive correction of the image errors, or it is embodied as a vibration measuring system which is used to measure vibrations of the object 2. The laser beam used for the vibration measurement is a parallel beam when exiting the image acquisition unit. Furthermore, the image acquisition unit 3 can be an OCT system. Even then, the light beam is a parallel beam when exiting the image acquisition unit. For a deflection of the light beam over the object 2, the pivot point of the light beam should be in the middle of the derotator 5 if possible.

In the preferred measurement with the device 1 according to the invention using a vibrometer or OCT, a light beam is emitted by the image acquisition unit 3, which passes through the derotator 5, is then influenced by the correction optics unit 7 in terms of radiation behavior, and is then reflected by the rotating object 2. The reflected light beam then traverses the correction optics unit 7 again, so that the radiation behavior of the light beam is again influenced, after which part of the light beam passes through the derotator 5 again and is then detected and processed by the image acquisition unit 3 . In the alternative variant of measurement by means of a camera, the object 2 is illuminated by a lighting unit (not shown) which illuminates the object 2 as rotationally symmetrically and uniformly as possible.

Possible embodiments of the correction optics units 7 are in the 2 and 3 shown.

2 shows a possible variant of the correction optics unit 7. This consists of two correction optics elements 8, 9, which each have two lens elements 10, 11 and 12, 13 in this exemplary embodiment. In this case, a correction optics element 8 is designed as an achromatic converging lens with a focal length of 200 mm and the other correction optics element 9 is designed as an achromatic scattering lens with a focal length of -150 mm. With a combination of these two correction optics elements 8, 9, which are arranged with a distance between the adjacent main planes of between 5 and 7 cm, depending on the distance between the correction optics elements 8, 9, objects 2 can be inspected at a distance of 1 m to infinity behind the rear ( object-side) lens 9 of the correction optics unit 7 are focused. In this context, particular attention must be paid to the orientation of the lenses, that is to say the corrective optical elements 8, 9, relative to one another. The refractive power of the diffusing lens 13 should be greater than that of the converging lens 10 in order to produce a sum of the refractive powers that is negative. The sum of the refractive power of the two correction optical elements 8, 9 is preferably in the range between -5 dpt and 0.

The product of the refractive powers of the two correction optical elements 8, 9 should preferably be between -100 dpt ² and -20 dpt ² . When using the correction optics unit 7 for an optical coherence tomography measurement, it is advantageous to use a correction optics element 8 with a refractive power of 1 dpt or higher in order to produce a focal length of less than 1 m.

The beam path 6a shows the course of an axis-parallel bundle of rays 6a, which leaves the derotator 5, which is not shown here and is arranged on the left, on the axis of rotation and is focused on the object 2, which is also not shown here and is arranged on the right. The two light bundles 6b show two bundles of rays which leave the derotator 5 at different angles and are each focused on different points of the object 2.

3 shows a further possible configuration of a corrective optics unit 7, which is known here as a liquid lens 7 known from the prior art is trained. A first and a second plate 14a, 14b delimit a liquid 15. The plates 14a, 14b are made of a deformable material, so that their shape changes as a function of the internal pressure. In the upper area, the liquid lens 7 has an air outlet 17 through which air located within the plates 14a, 14b can escape. The air outlet 17 is therefore necessary for venting and is closed after it has been completely filled with the liquid 15 . With further filling, the two plates 14a, 14b bulge more outwards due to the internal pressure and the curvature of the plates 14a, 14b increases. Sealing rings 18 are provided between the spacer 19, which contains the liquid inlet 16 and the liquid outlet 17, and the plates 14a and 14b, which prevent the liquid 15 from escaping undesirably. These sealing rings 18 are preferably designed as O-rings. Alternatively, these openings 16, 17, ie the liquid inlet 16 and the air outlet 17, can also be arranged on the edge of one or both plates 14a, 14b. An outer holder is not shown, which presses the two plates 14a, 14b, the spacer 19 and the sealing rings 18 together and thus seals the liquid lens 7. The curvature of the plates 14a, 14b can be adjusted very well via the volume of the injected liquid.

In 4a a device for carrying out measurements on rotating objects 2 , shown here only schematically, is shown, in which the vignetting is reduced by a diaphragm 20 . The device has a derotator 5 and a correction optics unit 7 . Electromagnetic radiation along the beam path 6 is used to carry out measurements on the object 2 . In this case, a diaphragm 20 is arranged adjacent to the derotator 5, as a result of which the vignetting that occurs for the image field shown can be avoided.

In 4b shows a possibility to reduce the vignetting by an alternative embodiment. In this embodiment, further lenses 21 or alternatively lenses 21 are arranged between the image acquisition unit 3 and the derotator 5 . This creates an additional image between the derotator 5 and the image capturing unit 3, such as a camera lens. Vignetting is avoided for a larger image field and the diameter of the light beam is increased. A possible embodiment has two objectives or lenses 21 of identical focal length, which are at a distance equal to the sum of their focal lengths. As a result, the diaphragm 20 arranged in the image acquisition unit 3 can be used to limit the beam. In combination with the additional lenses 21, the aperture 20 of the image acquisition unit 3 is imaged in the middle of the derotator 5. The delimitation of the light beams is thus advantageously effected only by the aperture effect of the aperture 20 and not additionally by the derotator 5, with the cross section of the derotator 5 being utilized in the best possible way for the edge points of the image field.

Cited non-patent literature:

• [1] Sebastian Boedecker, Alexander Dräbenstedt, Lars Heller, Arne Kraft, Andreas Leonhardt, Christian Pape, Sergej Ristau, Eduard Reithmeier, Christian Rembe, "Optical derotator for scanning vibrometer measurements on rotating objects," Proc. SPIE 6345, Seventh International Conference on Vibration Measurements by Laser Techniques: Advances and Applications, 63450M (21 June 2006)
• [2] Tino Wollmann, Edmund Koch, Julian Lich, Max Vater, Robert Ku Schmierz, Christian Schnabel, Jürgen Czarske, Angelos Filippatos, Maik Gude, "Motion blur suppression by using an optical derotator for deformation measurement of rotating components," Proc. SPIE 11380, Nondestructive Characterization and Monitoring of Advanced Materials, Aerospace, Civil Infrastructure, and Transportation IX, 1138016 (22 April 2020)

Reference List

1

contraption

2

object

3

image acquisition unit

4

object holding element

5

Derotator, Dove prism

6

path of rays, light

6a

paraxial light beam

6b

Rays in two different directions

7

Corrective optics unit, liquid lens

8th

corrective optics element

9

corrective optics element

10

lens element

11

lens element

12

lens element

13

lens element

14a

First plate, plate

14b

Second plate, plate

15

liquid

16

liquid inlet

17

air outlet

18

sealing ring

19

spacers

20

cover

21

lens, lens

Claims (13)

Device (1) for carrying out measurements on rotating objects (2), having an object holding element (4) for receiving and rotating an object (2) to be measured, a derotator (5) designed as a Dove prism and an image acquisition unit (3), which are arranged along a beam path, wherein a correction optics unit (7) for refraction of the light passing through it is arranged on its surfaces with an object-side focal plane in the direction of the object mounting element (4) in the beam path between the derotator (5) and the object mounting element (4). , wherein the object-side focal plane of the corrective optics unit (7) is designed to be adjustable in such a way that it coincides with the surface of the object (2) to be examined and wherein the corrective optics unit (7) has a variable focal length, with changing the focal length changing the position of the focal plane is shifted to the correction optics unit (7). Device (1) after claim 1 , characterized in that the correction optics unit (7) is designed to be exchangeable in modular form. Device (1) after claim 1 or 2 , characterized in that the correction optics unit (7) comprises a lens system made up of a plurality of lens elements (10, 11, 12, 13), the variation in the focal length of the correction optics unit (7) resulting from a change in at least one distance between at least two lens elements (10, 11 , 12, 13) results. Device (1) according to one of Claims 1 or 2 , characterized in that the correction optics unit (7) has a liquid lens (7). Device (1) after claim 4 , characterized in that the liquid lens (7) has a first and a second bendable plate (14a, 14b) with a liquid (15) delimited by these plates (14a, 14b), the focal length of the correction optics unit (7) being determined by the amount of liquid (15) within the liquid lens (7) and the resulting change in the curvature of the flexible plates (14a, 14b) is adjustable. Device (1) according to one of Claims 1 until 5 , characterized in that the image acquisition unit (3) is designed as a video camera for the two-dimensional display of the surface of the object (2) or as a system for coherence tomography for the three-dimensional display of the internal structure of the object (2). Method for carrying out measurements on rotating objects (2) by means of a device (1) according to one of Claims 1 until 6 , comprising the following steps: a) providing a device (1) for carrying out measurements on rotating objects (2) according to one of Claims 1 until 6 , b) installing the object (2) in the object mounting element (4), c) adjusting the corrective optics unit (7) so that the object-side focal plane of the corrective optics unit (7) coincides with the surface of the object (2) to be examined and d) measurements the rotating object (2) by means of the device (1), method step c) comprising the adjustment of the focal length procedure after claim 7 , characterized in that the image acquisition unit (3) comprises a lens (2) and that when measuring the rotating object (2), the lens is set to infinite distance. Use of a device (1) according to one of Claims 1 until 6 for laser Doppler vibrometry. Use of a device (1) according to one of Claims 1 until 6 for two-dimensional representation using video imaging. Use of a device (1) according to one of Claims 1 until 6 for three-dimensional representation using coherence tomography. Use of a correction optics unit (7) which has at least one lens element (10, 11, 12, 13) within a device (1) according to one of Claims 1 until 6 for eliminating periodic deformations caused by the derotator (5) during measurements on rotating objects (2), the light passing through the correction optics unit (7) being refracted at boundary surfaces of the correction optics unit (7). Use of a correction optics unit (7) which has at least one lens element (10, 11, 12, 13) within a device (1) according to one of Claims 1 until 6 to eliminate by the derotator (5) occurring, periodic deformations in imaging methods or scanning vibrometry, the light passing through the correction optics unit (7) being refracted at boundary surfaces of the correction optics unit (7).

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