DE102016002441B3 - Mechanically robust optical measuring system by means of light transit time and / or reflectivity measurement - Google Patents

Abstract

The invention relates to an optical measuring system and a measuring method for measuring measuring objects and / or arrangements of measuring objects, in particular of three-dimensional measuring objects and / or arrangements of measuring objects. The optical measuring system comprises at least one light source for emitting a transmission beam (50), a scanning or angled deflection device for scanning a measurement object (3) with the transmission beam (50) along two scan axes, a receiving device for detecting at least a part of the object reflected from the measurement object and / or scattered transmitted-beam light (51) and for generating at least one received signal, the receiving device having at least one detector or a detector array (9); a tracking device for tracking the transmitted beam light (51) reflected and / or scattered by the measurement object (3) in the direction of the receiving device, wherein the first of the two scan axes is tracked electronically and the second scan axis is electronically tracked; and a signal processing device for processing the at least one received signal in order to obtain information about the distance to individual measuring points of the measuring object (3) and / or about the reflectivity in individual measuring points of the measuring object (3). The tracking device comprises a transmissive optical beam deflection arrangement (4, 5) which is designed to deflect the transmitted beam light (51) reflected and / or scattered by the measurement object (3) in the direction of the receiving device in synchronism with the scanning process in the first scan axis; and an imaging optical element (7) configured to image the light propagated by the beam deflection assembly (4, 5) onto the detector or detector array (9). The tracking device is further designed to switch different subregions of the detector or of the detector array (9) synchronously to the scanning process in the second scanning axis, wherein the respectively connected subarea is in signal connection with the signal processing device.

Description

The invention relates to an optical measuring system and a measuring method for measuring measuring objects and / or arrangements of measuring objects, in particular of three-dimensional measuring objects and / or arrangements of measuring objects, eg. B. by means of light transit time measurement (time-of-flight measurement), Intentsitätsmessung, Reflektivitätsmessung, etc .. In particular, the invention relates to a laser scanner or a laser ranger scanner.

Laser scanners are known in the field of optical metrology. They are often used to perform a fast, two-axis beam deflection of a transmitting unit and a receiving unit. Typically, the beam is diverted in a line or spiral to simplify signal processing. Therefore, a fast and a slow scan axis are needed, which are usually perpendicular to each other. On the transmitter side, for example, mechanical scanning mirrors can be used, which have a fast and a slow scan axis due to their small mass and a gimbal suspension. However, since the receiving unit needs the largest possible area to receive enough stray light from a measuring object, a fast biaxial beam deflection for inertial reasons is problematic.

A laser scanner, which uses an array of synchronously oscillating micromechanical oscillating mirrors to achieve an increased receiving aperture, is e.g. In DE 10 2007 045 334 A1 described. However, the approach of such a micromechanical oscillating mirror array solves the problem of an enlarged receiving aperture only imperfectly, since the filling factor of the array is very low due to the control and the suspension of the oscillating mirror. This causes a relatively large space with the same receiving aperture. Another problem is the beam folding, which requires an additional increase in the installation space. In particular, at scan angle ranges below about 60 × 60 degrees, apart from the installation space, another disadvantage is essential: the wafer area required for micromechanical oscillating mirror arrays is relatively large. However, wafer area is an important measure of system cost.

The publication EP 2 708 914 A1 1 describes an opto-electronic sensor for recording a depth map of a three-dimensional scene. The sensor comprises a light source which generates a pulsed transmitted light beam, a deflection unit having a MEMS mirror for deflecting the transmitted light beam in two orthogonal directions, a light receiver formed as a photodiode array, and receiving optics for imaging the scene onto the receiver array. Furthermore, the sensor comprises a control and evaluation unit for controlling the light source, the deflection unit and the receiver, and for evaluating the received signals and for determining positions of reflections in the scene by determining distance values from the pulse transit time and by detecting the deflection angle of the deflection unit. When emitting the pulsed transmitted light beam only the photodiodes are activated, on which the illuminated with the transmitted light beam area of the scene is displayed.

The publication US 2008/0285010 A1 describes a tracking device for tracking the reflected from a measurement object in the direction of the receiving device and / or scattered transmission beam light, wherein both axes are tracked optically. The tracking device comprises a transmissive optical beam deflection arrangement for deflecting the transmitted beam light reflected and / or scattered by the measurement object in two orthogonal scan axes synchronously with the scanning process. Furthermore, the tracking device comprises a lens with which the light propagated by the beam deflection arrangement is imaged onto a detector.

An object of the present invention is to provide an improved optical system and a method for measuring measuring objects, in particular three-dimensional measuring objects, by means of time-of-flight measurement and / or reflectivity measurement.

According to a first aspect, an optical system for measuring measurement objects and / or arrangements of measurement objects, in particular of three-dimensional measurement objects, arrangements of measurement objects, 3D scenarios, etc. is provided. The optical system can be used in particular for determining the distance to one or more objects. The measurement can z. B. by means of light transit time measurement ("time-of-flight" measurement) and / or reflectivity measurement. In a reflectivity measurement, the measurement objects preferably have the same reflectivity.

The optical measuring system comprises:
at least one light source for emitting a transmission beam;
a (transmitter-side) scanning or angled deflection device for scanning or for optically scanning a measurement object with the transmission beam along two scan axes,
a receiving device for detecting or detecting at least part of the transmitted beam reflected and / or scattered by the measured object and for generating at least one received signal, wherein the receiving device has at least one detector or a detector array;
a (receiving side) tracking device (tracking device) for tracking the reflected and / or scattered by the measurement object in the direction of the receiving device and transmit beam beam light; and
a signal processing device for processing the at least one received signal to obtain information about the distance to individual measuring points of the measuring object.

The light source can be z. As a laser, an LED or other suitable light sources include. The scan or Winkelablenkvorrichtung can z. B. mirror, prisms or other suitable means to deflect the emitted from the light source and transmit the measurement object along two scanning axes optically. The optical system may also include a plurality of light sources (eg, multiple lasers, LEDs, etc.) each emitting a transmit beam. The individual light sources could scan the measurement object at different angles of incidence or in different directions. So z. For example, the optical system may include two or more light sources that scan the measurement object at different angles of incidence along one scan axis (eg, along the horizontal direction). The same applies to the second scan axis (eg for the vertical direction). The multiple light sources can be operated in parallel or in a time-shifted or multiplexed manner.

The two scan axes may be orthogonal to each other. However, other arrangements are also possible. Further, one of the two axes may be a fast axis and the other a slow axis.

The tracking device is designed to track the first of the two scan axes optically and the second scan axis electronically or virtually.

The tracking device comprises a transmissive optical beam deflecting arrangement which is designed to deflect the transmitted beam light reflected and / or scattered by the measurement object in the direction of the receiving device (hereinafter also referred to as receive light or receive beam) synchronously with the (transmitter side) scan process in the one (first) scan axis (eg, to deflect along the one (first) scan axis in synchronism with the deflection of the transmit beam).

Furthermore, the tracking device comprises at least one imaging optical element which is designed to image or focus the light propagated by the beam deflection arrangement onto the at least one detector or the at least one detector array. As a result, an optical tracking for the one (first) scan axis can be realized.

The tracking device is further designed to switch or to activate different subregions of the detector or of the detector array synchronously to the (transmitter-side) scanning process in the second scanning axis, wherein the respectively switched or activated subarea is in signal connection with the signal processing device. The different subregions of the detector or the detector array can, for. B. serially in succession and synchronously switched to the deflection of a light emitted by a light source and deflected by the scanning or Winkelablenkvorrichtung transmission beam or activated. As a result, an electronic tracking for the other (second) scan axis can be realized.

The detector may be a linear detector, i. H. a detector that is (substantially) longer than it is wide. Also, the detector array may be a linear detector array, i. H. a row-shaped arrangement of individual detectors or detector elements. The individual detectors or detector elements of the array can each be (substantially) longer than they are wide. The linear detector or the linear detector array may be arranged substantially perpendicular to the optical axis of the tracking device or perpendicular to the optical axis of the receiving branch of the optical measuring system and / or substantially parallel to one of the scanning axes.

If there are a plurality of light sources which scan the measurement object at different angles, a plurality of detectors or detector arrays may be provided, each detector or each detector array being associated with a light source. It is also possible, different portions of a detector or detector array associated with different light sources, in parallel, z. B. by means of an N × X multiplexer, or offset in time to bring in signal communication with the signal processing means or read.

So z. For example, the optical system may include two (or more) light sources that scan or optically scan the measurement object along one of the scan axes (eg, along the horizontal scan axis) at different angles of incidence. In this case, the receiving device may comprise two detectors or two detector arrays, which are designed to detect the light reflected and / or scattered by the measurement object of the respective light source. The two (eg vertically arranged) detectors or detector arrays can be read out in parallel or in time, or read out in multipliexed form.

However, it is not necessary to increase the number of detectors with multiple light sources. Thus, the optical system may include two (or more) light sources that scan the measurement object along one of the scan axes (eg, the vertical scan axis) at different angles of incidence. The receiving device can, for. B. comprise a linear detector or a linear detector array which is arranged vertically. The detector or the detector array can be designed and arranged so that the spots of the respective light source on the detector or detector array are spatially offset from each other, for. B. by several detector elements. The corresponding portions of the detector or the detector array can, for. B. be read in parallel by means of an N × X multiplexer. It is likewise possible to read out the sections of the detector or of the detector array which correspond to the different light sources with a time offset or multiplexed.

The proposed optical system for measuring objects (optical measuring system) is thus based on a new system concept in which two different tracking techniques: a virtual tracking and an optical tracking are advantageously combined with each other, wherein the optical tracking is effected by means of transmissive optical elements. Preferably, the electronic tracking is used for the fast scan axis and the optical tracking for the slow scan axis.

The term "virtual" or "electronic" tracking is understood to mean the change of the reception angular range (viewing direction) of the detector-receiving optical system in synchronism with the scanning process in at least one scan axis by switching or activating partial regions of the detector or of the detector array. As a result, the change in the reception spot position is accommodated depending on the reception angle range. In other words, the term "virtual" or "electronic" tracking means the change in the directional dependence of the reception characteristic of the detector-receiving optical combination by means of electronic circuit or activation of partial areas of the detector synchronously to the scanning process in at least one scan axis (eg. synchronous to the beam deflection of the transmission beam in at least one scan axis). This is done z. B. in that at any time a portion of a detector or a detector array is connected to the signal processing of the optical measuring device. The subregion of the detector or of the detector array can consist of one or more detector elements. The portion of the detector or the detector array connected to the signal processing is switched synchronously to the scanning process. If z. For example, if a detector array is illuminated with a scan pattern, the respective currently illuminated detector element is connected to the subsequent signal processing at any time. The principle of virtual tracking is z. B. in the publications "laser rangefinder based on MEMS levels for adaptive robotics", S. Bogatscher et al., Microsystems Technology Congress October 14, 2013, pages 211-214 described. Further, reference is made to the publication "Large Aperture at Low Cost Three-Dimensional Time-of-Flight Sensor Using Scanning MEMS Micromirrors and Synchronous Detector Switching," Applied Optics, Vol. 53, No. 8, pp. 1570-1582.

The term "optical" or "angle-enhanced" tracking is understood to mean the change in the reception angle range or the viewing direction of the detector-receiving optical system in synchronism with the scanning process in at least one scan axis by optical means. This can be achieved by deflecting or changing the propagation direction of the part of the transmission beam light reflected and / or scattered by the measurement object in the direction of the receiving device about at least one axis, the deflection or the change of the propagation direction being synchronous with the scanning process along at least one scan axis takes place (eg synchronously with the deflection of a transmission beam along at least one scan axis). According to the invention, the change in the reception angular range is effected by means of a receiving-side optical beam deflection arrangement based on the principle of beam transmission. The transmitted beam light reflected and / or scattered by a measurement object in the direction of the reception device or the receiver passes through the transmissive beam deflection arrangement, is deflected by the latter and then focused onto the linear detector or the linear detector array. The angular range in which the receive beam is deflected, z. B. ± 80 °, ± 70 °, ± 60 °, ± 45 ° or another angle range.

An essential advantage of the proposed system concept is that the strengths of the two tracking techniques used are optimally utilized while at the same time causing the greatest principle Weaknesses can be minimized or avoided. The particular strengths lie in particular in the decoupling between receiving aperture and scanning frequency (virtual or electronic tracking) and between receiving aperture and field of view (optical or angle-enhanced tracking). Thus, the field of view of the electronically tracked scan axis can be made smaller than that of the optically or angularly enhanced tracking scan axis. On the other hand, the scanning frequency of the electronically tracked axis can be chosen to be significantly higher than that of the optical or angle-enhanced (eg mechanically-translationally) tracked axis. The advantageous utilization of these relationships thus makes it possible to realize a 3D scanner with a larger receiving aperture compared with known systems without having to reduce the frame rate or the field of view and / or to increase the costs significantly.

Due to the use of transmissive optical elements (transmission optics or transmission elements), in particular in combination with a single transmitter-side micromechanical element for deflecting the transmission beam, the installation space of the optical measuring device can be significantly reduced. A space saving is achieved in particular by the fact that a filling factor of the receiving optics can be achieved up to 100% and the receiving side only transmission elements are used, which can be arranged directly behind each other to save space.

Another advantage is the small wafer area required for a given receive aperture at certain scan angle ranges. The required wafer area is z. B. much smaller than in the DE 10 2007 045 334 A1 proposed approach with an array of synchronously vibrating micromechanical vibrating mirrors.

Another advantage is the long service life, since bearings or other components subject to wear can be largely avoided.

The deflection of the reflected from the measurement object in the direction of the receiving device or the receiver and / or scattered transmission beam light (receiving beam) can, for. Example, be achieved by a mechanical translational movement of at least two transmissive optical elements relative to each other. This type of tracking is also referred to below as mechanical-translational tracking.

The mechanical movement can z. B. be a translational movement of two microlens arrays or lenticular lens arrays relative to each other. The microlens or lenticular lens arrays can be arranged essentially parallel to one another and perpendicular to the optical axis of the receiving branch and in particular of the receiving device.

The size of the micro or lenticular lens array is preferably in the range of 5 × 5 mm ² to 50 × 50 mm ² . The individual lenses of the micro or lenticular lens array may be elongated lenses, such as. As cylindrical or other aspherical lenses. The longitudinal axis of the individual lenses of the micro- or lenticular lens array may be substantially parallel to the linear detector or detector array and substantially perpendicular to the translational motion of the micro- or lenticular lens array.

The period of the micro- or lenticular lens array may, for example, be from 50 μm to 500 μm, preferably approximately 200 μm. The maximum translational movement of the micro or lenticular lens array is approximately half the diameter of the individual lenses of the micro or lenticular lens array. The speed or frequency of the translatory movement can be in the range of sub- to kilohertz, z. In the range of 1 Hz to 50 Hz. The maximum deflection angle may be in the range of several tens of degrees. For example, the maximum deflection angle can be ± 25 °, preferably ± 26.5 °.

An advantage of the mechanical-translational tracking by means of microlenses or lenticular lenses is that with relatively small translational movements, a relatively large scan area can be covered. As a result, the optical tracking can be implemented very space-saving. Furthermore, it is possible to make the mechanical drive due to the relatively small movement necessary simple and inexpensive.

A deflection of the transmitted beam light (receiving beam) reflected and / or scattered by the measurement object in the direction of the receiving device or the receiver can likewise be achieved by an opposite rotation of two transmission gratings or by at least one temporally variable or programmable transmission grating. It is Z. B. possible, for the deflection controllable transmissive spatial light modulators such. As LCD modulators to use, in which grids are written with variable direction and period. An advantage of using electronically controlled transmission gratings or other electronically controllable light modulators is that the tracking device does not require mechanically moving components. Thereby, the scanning speed of the tracking device can be increased and its structure can be simplified. Furthermore, the construction volume of the optical measuring device can be reduced and its robustness can be increased.

The imaging optical element may be a converging lens, in particular a Fresnel lens. The advantages of a Fresnel lens are, in particular, its low weight, its simple and cost-effective production (even of complex aspherical lenses) and its very low installation space requirements. Furthermore, Fresnel lenses can be very well integrated into convenient brackets due to their planarity.

The transmitter-side scan or angle deflection device may comprise a vibrating and / or rotating scanning mirror. The Scanspielgel can z. As a micromirror or other micromechanical scanning element. The micromirror can z. B. have a size of 0.4 mm to 4 mm in diameter, preferably from 1 mm to 3 mm, more preferably 1.5 mm to 2 mm in diameter. The scanning mirror can z. B. electrostatically, electromagnetically or piezo-electrically driven. The fast scan axis can z. B. oscillate resonantly, the slow scan axis can z. B. be statically controlled. The scanning or Winkelablenkvorrichtung may also include a plurality of scanning mirror, z. B. a one- or two-dimensional mirror array. The scanning or angled deflection device may further comprise a position device for measuring the position of the scanning mirror, which may be in signal communication with the tracking device and / or the signal processing device. As a result, the synchronization between the scanning operation and the tracking can be improved, which leads to an increase in the accuracy of the measurement.

The transmission-side scan or angle deflection device and the reception-side tracking device can be components of a scan unit or of a scan module. In one example, a scan mirror of the scanning or angled deflection device may be disposed in a recess in at least one optical transmission element (such as in the double-sided and / or single-sided micro or lenticular lens array).

Thus, the construction volume of the optical measuring system can be reduced.

The transmitter-side scanning or angular deflection device and the receiving-side tracking device and / or receiving device can be arranged such that their optical axes are coaxial. In other words, the transmitter and the receiver can be arranged coaxially. Due to the coaxial arrangement of the transmitter-side scan or Winkelablenkvorrichtung and / or the receiving side tracking device and / or receiving device (ie the transmitter and the receiver) can occur due to the triangulation unwanted distance-dependent displacement of the receiving beam spot on the detector (or a parallax error) avoided or be minimized. This distance-dependent spot shift is accompanied by an unintentional distance-dependent shift of the optimal switchover times, which is particularly noticeable at small measurement distances. A coaxial arrangement of the scan or Winkelablenkvorrichtung and the tracking device can, for. B. be realized by the above-described arrangement of the scanning mirror in a recess in one or more transmissive optical elements of the transmissive beam deflection arrangement.

Another aspect of the invention relates to a method for measuring measurement objects and / or arrangements of measurement objects, in particular of three-dimensional measurement objects and / or arrangements of measurement objects, eg. Example by means of light transit time measurement ("time-of-flight" measurement) and / or by reflectivity measurement. The method can be carried out, for example, with the optical measuring system described above. The method comprises:
Transmitting at least one transmission beam;
Scanning a measurement object with the at least one transmission beam along two scan axes,
Tracking the transmitted object reflected and / or scattered by the measurement object in the direction of a receiving device, wherein the first of the two scan axes is optically and the second of the two scan axes electronically or virtually tracked,
Detecting or detecting at least part of the transmitted beam light (or at least part of the receiving beam) reflected and / or scattered by the measured object by at least one (eg linear) detector or at least one (eg linear) detector array of the receiving device and Generating at least one received signal; and
Processing the at least one received signal by means of a signal processing device.

The optical tracking of the first scan axis comprises:
Deflecting the transmission beam light (or the reception beam) reflected and / or scattered by the measurement object in the direction of the reception device synchronously to the scanning process in the first scan axis (eg synchronously with the beam deflection of the transmission beam along the first scan axis) by means of a transmissive optical deflection arrangement; and
Imaging the propagated by the beam deflection assembly light or receiving beam to the detector or the detector array.

The electronic tracking of the second scan axis comprises switching or activating different subareas of the detector or of the detector array (for example serially one after the other) synchronously with the scanning process in the second scan axis, wherein the respective switched or activated subarea is in signal connection with the signal processing device. The different subregions of the detector or detector array can be successively serially, z. B. synchronously switched to the beam deflection of the transmission beam along the second scan axis or activated.

As described above, the deflection of the transmitted beam light or the receiving beam reflected and / or scattered by the measurement object in the direction of the receiving device can take place, for example, by means of a translatory movement of a double-sided micro- or lenticular lens array and a single-sided micro- or lenticular lens array relative to each other. It is also possible to realize the deflection by means of an opposite rotation of two transmission gratings or by means of an electronically switchable (electronic) transmission grating.

Furthermore, as described above, the imaging of the light or the reception beam propagated by the beam deflection arrangement onto the detector or the detector array can take place by means of a Fresnel lens. The scanning of the measurement object with the transmission beam can be performed by means of an oscillating and / or rotating scanning mirror, for. B. an electrostatically or electromagnetically operated micromirror.

The measuring system according to the invention and the measuring method according to the invention can be used in automation technology, robotics, driverless transport systems, automobile production, logistics, etc. Uniaxial-scanning laser scanners are currently used in most of these areas because the problem of providing efficient, robust and fast two-axis laser scanners has not been satisfactorily solved. Another application of the optical measuring system is autonomous driving, z. As for the localization of vehicles side by side and at different distances and / or to distinguish road users and their distance.

Thus, the measuring system according to the invention and the measuring method according to the invention for determining a 3D point cloud of an object can be used, for. B. an object to be transported in driverless transport systems. A metrologically determined 3D point cloud of the object to be transported helps to implement the charge carrier recording more reliably and faster, which saves costs and time.

The invention will be explained in more detail below by way of example with reference to exemplary embodiments and the figures. Show it:

1 an exemplary scan pattern;

2A the principle of virtual tracking;

2 B the principle of optical or angle-enhanced tracking;

2C a portion of the beam path of the receiving beam through two microlens arrays at maximum deflection angle of the slow axis;

3 and 4 1 shows the schematic structure of an exemplary optical measuring system, wherein the receiving beam for a substantially perpendicular angle of incidence (FIG. 3 ) and for an oblique angle of incidence ( 4 ) of the slow scan axis is shown;

5 a perspective view of the in 3 and 4 shown measuring system;

6 an exemplary microlens array disposed in front of a detector array;

7 an exemplary embodiment of a microlens array with a recess for the transmitter-side scanning mirror; and

8th the schematic structure of another exemplary optical measuring system;

9 the light diffraction on a transmission grating;

10 an exemplary beam deflection assembly comprising two counter-rotating transmission grating.

In the figures, corresponding or functionally similar components are provided with the same reference numerals.

The optical measuring system and the optical measuring method in the following examples are based on the time of flight measurement. However, the measuring system and measuring methods can be based on the reflectivity measurement.

Using the transit time measurement of an optical signal, the distance covered can be determined at a known propagation speed. In the case of air as a propagation medium, the speed of light is c ≈ 3 · 10 ⁸ m / s. If the orbital period τ is measured, then the measuring distance d can be calculated from the following relationship: d = c · τ / 2τ (1)

To measure the transit time, both pulse transit time measurement method and phase transit time measurement method can be used. Alternatively, methods based on frequency modulation, such as. B. FMCW Radar (English: Frequency Modulated Continuous Wave) are used.

The principle of the pulse transit time measurement is based on the direct time measurement. Therefore, this method is often referred to as direct time-of-flight measurement (English: "direct time-of-flight" or "ToF"). For a single-point measurement, in the simplest case, only a single pulse is transmitted by the transmitter. This pulse is scattered back and / or reflected by a measured object at a distance d and registered by the receiver in a weakened form. The measured time difference between the transmitted and received pulses corresponds to the round trip time r. The sought measuring distance can finally be calculated according to equation (1).

In contrast to the pulse transit time measurement, an indirect measurement of the light transit time is carried out during the phase delay measurement via a phase measurement. Therefore, this method is often referred to as an indirect "time-of-flight" method. In this case, one uses, usually sinusoidal, amplitude modulation of the emitted optical power with the modulation frequency f _m . The phase φ between transmitted and received signal corresponds to the round trip time τ via the following relationship

The measuring devices and measuring methods in the following examples are scanning methods and are based on a beam deflection of the transmitting and receiving beam and thus on a sequential measurement of the field of view. Thus, the pixels or the measuring points of the measuring object are measured serially one after the other.

An exemplary line scan pattern is shown in FIG 1 shown. In this example, the measurement object is scanned or optically scanned along a fast axis y and a slow axis x. The horizontal scan motion may be a sinusoidal motion, while the vertical scan motion may be a linear motion. The scan pattern is processed once per frame. At the end of the frame, a return to the starting position or the frame runs counter to the pattern "backwards".

The reception concept of the optical measuring system and the measuring method is based on a combination of two different tracking techniques: a virtual or electronic tracking and an optical or angle-enhanced tracking.

2A schematically illustrates the principle of the virtual or electronic tracking, which takes place without a beam deflection of the receiving beam. The term "receiving beam" is understood to mean the part of the light (receiving light) which is reflected and / or scattered by the measuring object in the direction of the receiving device or of the receiver and detected by the receiving device or the receiver. In other words, the receiving device or the receiver "looks" in the direction from which the receiving beam comes. The term "receive beam" can thus be equated in some cases with the viewing direction of the receiving device or the receiver.

The entire scan angle range of a scan axis (eg the fast axis y) is converted to an elongated or linear detector or to a linear or one-dimensional detector array by an imaging optical element (eg a converging lens) 9 displayed. While on the detector or on the detector array 9 focused reception beam 51 due to the scan across the detector or detector array 9 moves, each is the most illuminated detector element by a multiplexer 10 with the subsequent signal processing 11 connected. By switching between the individual detectors of the detector array 9 can the total amount of light of the receiving beam 51 pointing to the condenser lens 7 meets, be received. The two largest receiving angles of the receiving beam 51 that can be received are shown in dashed and solid lines, respectively.

In particular, due to the resonant oscillation of the scanning mirror, the focus of the receiving beam moves with a z. B. sinusoidal velocity profile over the detector array 9 time. In order to enable a continuous distance measurement, the maximum illuminated detector is connected to the signal processing at any time. This can be done through a multiplexer 10 respectively. If the detector does not consist of a detector array, the multiplexer is eliminated 10 and it does not have to be switched. In this case, however, the stray light of the entire angular range falls on the detector and increases the noise.

Due to the avoidance of movable tracking mechanisms, a decoupling between scanning frequency and receiving aperture is achieved in the electronic or virtual tracking. The achievable receive aperture is determined only by the size of the detector array and the field of view. Due to the achievable fast switching times of the multiplexer, the scanning frequency is thus limited only by the characteristics of the scanning mirror. High scanning frequencies allow z. B. the registration of fast movements in the far distance. For example, it is possible to measure an image field of 100 × 100 pixels at a frame rate of f _FR = 100 Hz at a scan frequency of the fast axis of f _fa = 5 kHz and a pixel rate of f _px = 1 MHz.

Furthermore, it is possible to use only small wafer areas with expensive MEMS structures and large wafer areas with favorable detector structures, which reduces the costs for the entire optical measuring system. Furthermore, the synchronization effort can be shifted from the mechanical to the electrical domain. Thus, it is possible to synchronize even with only one trigger pulse per oscillation period of the micromirror, the micro-mirror oscillation and the switching times. The synchronization accuracy can z. B. be further improved by a sensory detected position signal of the micromirror. As a result of the shifting of the synchronization effort into the electrical domain, it is also possible to track any scan pattern, which considerably increases the flexibility.

As described above, the achievable receive aperture is determined by the size of the detector array. However, increasing the number of detector elements can lead to complication of circuit design and cost increase.

A disadvantage of the electronic tracking can be the reduced measurement accuracy in the switching ranges. One reason for this reduction of the measurement accuracy may be a falsification of the phase measurement during switching between two detectors or detector elements. A possible solution to this problem is to replace the measured values obtained during the switching process with an in-phase delayed copy of the last measured values before a quadrature demodulation. Another solution is to adapt the circuit design.

Further, part of the light in the switching area may be lost by distributing the spot illuminated by the receiving beam to two or more detector elements, which may result in a reduction of the signal-to-noise ratio. By evaluating the measurement result in the illuminated detector elements and means of the detected values, the reduction of the signal-to-noise ratio in the switching region can be reduced. In a distribution of the illuminated spot on two detector elements z. B. the reduction of the signal-to-noise ratio of 2 can be reduced.

Furthermore, a desynchronization between the oscillation of the scanning mirror and the detector switching can be avoided by tracking the switching times with corresponding position sensors of the scanning mirror.

2 B schematically shows the principle of optical or angle-enhanced tracking and in particular the mechanical-translational tracking. At the in 2 B In the example shown, the mechanical-translational beam tracking is realized by means of a transmissive beam deflection arrangement comprising three microlens arrays. 2 B shows the case of the maximum deflection angle β _max . The first lens array LA1 focuses the incident beam (receive beam 51 ) in the field plane at a distance f _ml , in which the field lens array LA2 is the same focal length f _ml . This forms the associated microlens of the first microlens array LA1 on the associated microlens of the third microlens array LA3, so that a "microlensenkanal" is formed, which can not leave the incident beam in the ideal case. The lens of the third microlens array LA3 collimates the deflected divergent beam. A lateral displacement s of the third microlens array LA3 relative to the first two arrays LA1 and LA2 changes the deflection angle.

The relationship between the lateral displacement s and the deflection angle β of the incident beam is given by the following equation: s = f _ml · tan (β) (3)

Wird die laterale Verschiebung s größer als der halbe Mikrolinsendurchmesser D_ml, so überschreitet der Zwischenfokus den Rand der Feldlinse und kann somit nicht mehr im Mikrolinsenkanal verbleiben. Daher berechnet sich der maximale Ablenkwinkel β_max dieses Strahlablenkungsprinzips mit

The maximum displacement s _{max of} the microlens array LA3 is given by

If the lateral displacement s is greater than half the microlens diameter D _ml , then the intermediate focus exceeds the edge of the field lens and thus can no longer remain in the microlens channel. Therefore, the maximum deflection angle β _{max of} this beam deflection principle is also calculated

2C shows a possible implementation of the in 2 B shown principle of mechanical-translational tracking with a double-sided structured microlens array 4 and a one-sided structured microlens array 5 , In particular shows 2C a portion of the beam path of the receiving beam 51 through the two microlens arrays 4 and 5 at maximum deflection angle of the slow scan axis. The one of a measuring object 3 reflected and / or scattered receive beam 51 is due to the double-sided structured microlens array 4 divided into individual partial beams, which are focused in an intermediate plane and then on a second, one-sided structured microlens array 5 again collimated and then focused on a detector or on a detector array (not shown). By a lateral translational displacement of the two microlens arrays relative to each other, a lateral beam deflection can be achieved, ie a beam deflection in a plane perpendicular to the optical axis z of the receiving device. The shift takes place synchronously with the beam deflection along one of the scan axes (eg the slow scan axis).

A mechanical-translational tracking can also be realized with other transmissive optical elements. Thus, the deflection angle β can be changed by an opposite rotational movement of two transmission grating relative to each other. It is also possible to change the deflection angle β by two electronically controllable or variable transmission gratings or other spatial light modulators. Since no mechanically moving parts are necessary in this case, the robustness and / or accuracy of the system can be increased.

Due to the necessary changes in direction of the movement, not so high scanning frequencies are generally possible with an optical and in particular a mechanical-translational tracking, but this beam deflection principle can be integrated, for example, in the slow scan axis of the measuring system. As explained above, the principle of virtual tracking can be used for the fast scan axis.

While the achievable receive aperture in the electronic tracking is limited by the size of the detector array and the field of view, the limit in the case of mechanical-translational deflection lies in the detector size and the mechanical properties of the translation. An important advantage of the mechanical-translatory beam deflection is the decoupling between field of view and achievable reception aperture. Furthermore, the method is characterized by low space requirements. A combination of the electronic tracking in the fast and the mechanical-translational tracking in the slow scan axis, as shown in the following figures, allows both the exploitation of the respective strengths of both tracking principles as well as avoiding the principle of conditional weaknesses.

3 shows a top view of an exemplary measurement system by means of light transit time measurement, wherein the receive beam 51 is shown for an angle of incidence of the slow scan axis y, which is substantially zero (vertical incidence). 4 shows the same structure as 3 , wherein the receiving beam 51 is shown for an oblique angle of incidence of the slow scan axis y. 5 shows a perspective view of the in 3 and 4 shown measuring system.

The measuring system comprises a transmitting unit with a laser diode 1 or another suitable light source. The transmitting unit may be in signal communication with a signal processing device and may be configured to convert an electrical signal generated by the signal processing device into a modulated optical signal (transmit beam). The optical signal can, for. B. be amplitude modulated.

A collimated transmission beam 50 is from the laser diode 1 transmitted and via a transmitter-side scanning mirror 2 deflected within the field of view in two axes (scan axes). The transmitter-side scanning mirror 2 can z. B. be a micromechanical, electrostatic or electromagnetically driven oscillating mirror. The fast scan axis (y) is a vibration about the x-axis, the slow scan axis (x) a vibration about the y-axis. Is there a scattering and / or reflective measurement object 3 in the field of view of the optical measuring system, the result is a received signal 51 which hits the tracking device. The tracking device comprises the transmissive optical elements successively positioned in the z-direction (ie in the direction perpendicular to the x-axis and y-axis, the z-direction preferably coinciding with the optical axis of the receiving device) 4 . 5 . 6 and 7 wherein the arrangement of the optical elements 4 . 5 . 6 and 7 except for one condition may be arbitrary. The condition that must be met is that the receive beam 51 first on the two microlens arrays 4 . 5 meets and passes through these before going to the condenser lens 7 meets. At the in 3 to 5 shown example, one of the two microlens arrays is double-sided structured (double-sided microlens array 4 ), the other one-sided (single-sided microlens array 5 ).

The two microlens arrays 4 and 5 comprise several individual microlenses, e.g. B. several cylindrical, lenticular, etc. microlenses. The longitudinal axis of each individual microlens is substantially perpendicular to the x-direction and parallel to the y-direction. The two microlens arrays 4 and 5 are arranged substantially parallel to each other and perpendicular to the optical axis z.

The two microlens arrays 4 and 5 become synchronous with the movement of the slow scan axis of the transmit-side scan mirror 2 moved relative to each other in the x direction (see 4 ). The relative movement can, as in 4 shown by a movement of the double-sided microlens array 4 , about a movement of the single-sided microlens array 5 , or via a movement of both microlens array 4 and 5 will be realized. To reduce the mass of the moving microlens array, the unilaterally structured microlens array 5 be moved, the thickness of which can be reduced to the minimum necessary value, which ensures the desired stability. For a given actuator drive force, the limiting mechanical factor for the receive aperture is the scan frequency.

After the receiving beam 51 the optical filter 6 has happened, he is facing the condenser lens 7 (eg a Fresnel lens) on the detector array 9 focused. The linear detector array 9 with the detector elements extends in the y-direction (see eg. 5 ). In front of the detector array 9 can be an additional micro or lenticular lens array 8th to increase the fill factor of the detector array 9 be positioned (such as in 6 shown). The additional microlens array 8th can z. B. be a one-sided micro- the lenticular lens array.

The transmissive optical filter 6 is designed to suppress unwanted wavelength ranges (such as extraneous light sources, solar radiation, etc.) of the electromagnetic spectrum and to allow the desired ranges to pass through. The optical filter 6 may be a narrowband bandpass filter whose transmission maximum at the wavelength of the laser diode 1 emitted light is. It can too Interference and absorption filters are combined to realize narrow band filters with high optical density in the stopband areas.

The focus of the receive beam 51 moves on the detector array 9 due to the vibration of the fast scan axis in the y-direction. At any point in time, the detector element or elements on which the center of gravity of the focus is located are via a multiplexer 10 with the subsequent signal processing 11 connected. In this way, virtual / electronic tracking along the fast axis x can be realized. If the detector is designed as an elongated detector extending along the y-axis, a multiplexer and a switching of the individual detector elements are not necessary.

The multiplexer 10 may be a semiconductor-based analog multiplexer, e.g. As a CMOS multiplexer, a buffered analog multiplexer ("buffered analog multiplexer" or "video multiplexer-amplifier"). Preferably, the number of multiplexer stages will be kept to a minimum to reduce noise. This can be achieved by minimizing the number of detector elements, resulting in an increase in the active area of each individual detector for a given detector length and thus a reduction of the bandwidth. However, with a linear (one-dimensional) detector array, an optimal trade-off between noise reduction and bandwidth enhancement can be found more easily than with a 2D detector array.

6 shows an exemplary single-sided microlens array 8th which optionally in front of the detector array 9 can be arranged. The microlens array 8th can increase the fill factor of the detector array 9 be used.

7 shows an exemplary embodiment of the microlens array 4 which has a recess. This recess can serve to the transmitting side scanning mirror 2 in the plane of the microlens array 4 to position. Thus, a particularly compact measuring system can be realized. Moreover, in this embodiment, the optical axes of the transmitter and the receiver and the transmitter-side scanning or Winkelablenkvorrichtung and the receiving side tracking device and / or receiving device are collinear.

8th shows an exemplary measuring system in which the transmitter-side scanning mirror 2 in addition to the receiving side tracking device (comprising the microlens arrays 4 and 5 , the filter 6 and the condenser lens 7 ) is arranged. In this case, to avoid shadow effects, an off-axis Fresnel lens as a condensing lens 7 be used.

In the above examples, microlens arrays are used for beam deflection and thus optical tracking. Instead of microlens arrays other transmissive optical elements such. As lenticular lens arrays or diffractive transmission grating can be used. Thus, it is possible to realize an optical, mechanical-translational tracking by a lateral displacement of two micro- or lenticular lens arrays relative to one another or by a rotation of two diffractive gratings in opposite directions. By using optical transmission elements and by the relatively small required range of motion of the optical transmission elements, the translational principle can be implemented in a very space-saving manner. Furthermore, it is possible to make the mechanical drive due to the relatively small movement necessary simple and inexpensive. So with dive coil drives favorable translational drive concepts for the micro or lenticular lens array (s) are available.

It is also possible to use electronically controllable spatial light modulators or transmission gratings. So z. B. by an electronically controlled change of the grating structures of two diffractive transmission grating relative to each other, also an optical tracking can be realized. An advantage of this approach is that the tracking device does not require any mechanical moving parts.

In the above examples, the switching of the detector elements takes place synchronously with the scanning process along one of the scanning axes. A time deviation between the optimal switching time at the transition of the focus between two detector elements and the actual switching time can lead to a deterioration of the received signal. This can be z. B. be minimized by considering the position of the scanning mirror in the processing of the received signal generated by the detector. The position of the scan mirror can, for. B. by means of a suitable position sensors (which can be integrated in the scanning mirror) are detected. This allows a temporal shift of the optimal Switching times, z. B. due to temperature or humidity fluctuations that affect, for example, the Oszillationsamplitude easily, are substantially compensated.

As described above, due to the parallax error, an unwanted shift of the receiving beam spot or focus on the detector may occur. This distance-dependent spot shift is also accompanied by a distance-dependent shift of the optimum switchover times, which is particularly noticeable at small measurement distances. There are several ways to solve this problem. On the one hand, the effect can be largely avoided or minimized by a coaxial arrangement of the optical axes of transmitter and receiver (cf. 7 ). It is also possible to shift the problem into the optically and in particular mechanically translationally guided axis. Thus, the beam deflection arrangement can be designed to substantially compensate for the triangulation-dependent distance-dependent displacement of the receiving beam spot or of the focus on the detector. Thanks to the flexibility of the switching approach, it is also possible to read two detectors or detector elements simultaneously and to use the better of both received signals for a quadrature demodulation. This approach increases the complexity of the switching as well as the signal processing, but offers the possibility of improving the SNR (signal-to-noise ratio) halved in the switching range by the factor by forming the average of both received signals (eg before quadrature demodulation). It is of course possible to read out more than two elements at the same time. It is also possible to calculate the switching times also from the distance information from the previous frame.

wobei k_min die minimal realisierbare Blendenzahl bezeichnet (z. B. k_min ≈ 1) und β_v der maximale Einfallswinkel (Halbwinkel) ist. Bei z. B. w_det = 4 mm, k_min ≈ 1 ist der maximale vertikale Durchmesser der Sammellinse ungefähr 11,3 mm.In the above examples, the maximum vertical diameter D _{v of} the condenser lens may be chosen as a function of the length of the detector array w _det , e.g. For example, according to the following formula:

where k _min denotes the minimum feasible f-number (eg k _min ≈ 1) and β _{v is} the maximum angle of incidence (half angle). At z. B. w _det = 4 mm, k _min ≈ 1, the maximum vertical diameter of the converging lens is about 11.3 mm.

Under certain circumstances, even smaller f-numbers can be achieved, but also increase the aberrations in such high-intensity optics drastically. This in turn reduces the optical efficiency as the spot size exceeds the detector size.

The condenser lens may be a Fresnel lens. One advantage of a Fresnel lens is its low weight, which is especially important for large lenses. In the mass production of plastic optics can be achieved with the injection molding technology with aspherical structures, a very low unit cost. In addition, Fresnel lenses also have very small installation space requirements and can be very well integrated into favorable support concepts due to their planarity. For the production of small quantities also cutting manufacturing processes can be used. However, a disadvantage of Fresnel lenses is the shadow areas that occur by total reflection on the sidewalls of the structures. The shadow effect can be z. As minimized by cleverly arranging the Fresnel structures, but in many cases can not be completely avoided.

In the above examples, that includes the receive beam 51 illuminated spot of the detector array, preferably a detector element. However, it is possible, in particular in certain areas of the detector array, that the illuminated spot comprises a plurality of detector elements. The reception power is distributed in this case to two or more detector elements and is reduced. This usually leads to a reduction of the signal-to-noise ratio. It is possible to add the signals of the illuminated detector elements. However, this does not lead to an improvement of the signal-to-noise ratio because the signal also doubles the noise. By means of an averaging of the signals, the reduction of the signal-to-noise ratio can be limited.

9 schematically shows the light diffraction on a transmission grating 12 , In the example shown, the grid is a blazed grid. Such gratings diffract the light substantially in a diffraction order and have a high diffraction efficiency. It is also possible to use other grids, but they have clutter and are less efficient.

The lattice diffraction can be described vectorially with the wave vector concept. The diffraction at the grating adds to the wave vector of the incident wavefront k → _in a grid vector g →, so that after the wavefront of the failing wavefront k → _out the following equation (6) satisfies: k → _out = k → _in + g → (6)

• 
  den Wellenvektor der einfallenden Wellenfront;
• g → = λ / p den Gittevektor;
• p die Periode des Gitters; und
• λ die Wellenlänge des einfallenden Lichts.

In equation (6) denote:

• 
  the wave vector of the incident wavefront;
• g → = λ / p the Gittevektor;
• p the period of the grid; and
• λ is the wavelength of the incident light.

10 shows an exemplary beam deflection assembly comprising two counter-rotating transmission gratings 13 and 14 , Preferably, the grids 13 and 14 Blazed grid.

The lattice vectors of the transmission gratings 12 and 14 are each g → ₁ and g → ₂ .

With two counter-rotating gratings of the same period, the Y components of the two grating vectors are lifted g → ₁ and g → ₂ each other up. The sum vector g → _s , in which g → _s = g → ₁ + g → ₂ , always points in the same direction but, depending on the angle of rotation, has a variable amount that changes periodically between 0 and 2π. This allows a linear scanner to be realized.

LIST OF REFERENCE NUMBERS

1

Laser diode with associated collimation optics

2

Transmission side scanning mirror

3

Measuring object with reflective and / or scattering properties

4

Receiving-side movable double-sided structured microlens array or lenticular lens array

5

Receiving-side unilaterally structured microlens array or lenticular lens array

6

Optical filter or filter arrangement

7

Condenser lens (eg Fresnel lens)

8th

Microlens array for increasing the fill factor of the detector array

9

Linear detector array

10

multiplexer

11

Signal processing and signal evaluation

12

transmission grid

13, 14

counter-rotating transmission gratings

50

transmission beam

51

receiving beam

LA1-LA3

Microlens arrays

Claims (14)

Optical system for measuring measuring objects and / or arrangements of measuring objects, in particular of three-dimensional measuring objects and / or arrangements of measuring objects, comprising: at least one light source for emitting a transmitting beam ( 50 ); a scanning or angled deflection device for scanning a measurement object ( 3 ) with the transmission beam ( 50 ) along two scan axes; a receiving device for detecting at least part of the transmitted object reflected and / or scattered by the measurement beam ( 51 ) and for generating at least one received signal, wherein the receiving device comprises at least one detector or a detector array ( 9 ) having; a tracking device for tracking the object to be measured ( 3 ) in the direction of the receiving device reflected and / or scattered transmission beam light ( 51 ), wherein the first of the two scan axes is tracked optically and the second scan axis electronically; and a signal processing device for processing the at least one received signal to obtain information about the distance to individual measuring points of the measuring object ( 3 ) and / or via the reflectivity in individual measuring points of the test object ( 3 ), the tracking device comprising: a transmissive optical beam deflection assembly ( 4 . 5 : 13 . 14 ), which is designed, that of the measuring object ( 3 ) in the direction of the receiving device reflected and / or scattered transmission beam light ( 51 ) to deflect in synchronism with the scan in the first scan axis; and an imaging optical element ( 7 ) designed by the beam deflecting arrangement ( 4 . 5 ; 13 . 14 ) propagated light onto the detector or detector array ( 9 ); and wherein the tracking device is further configured to detect different subregions of the detector or of the detector array ( 9 ) in synchronism with the scanning process in the second scan axis, wherein the respective switched subarea is in signal connection with the signal processing device. The optical system of claim 1, wherein the detector is a linear detector or the detector array is a linear detector array. An optical system according to claim 1 or 2, wherein the beam deflecting arrangement comprises: a double-sided micro- or lenticular lens array and a single-sided micro- or lenticular lens array, wherein the double-sided micro or lenticular lens array and the single-sided micro or lenticular lens array are arranged translationally relative to each other. An optical system according to claim 1 or 2, wherein the beam deflection assembly comprises: two counter-rotating transmission gratings ( 13 . 14 ); or an electronically controllable transmission grating. Optical system according to one of the preceding claims, wherein the imaging optical element ( 7 ) is a Fresnel lens. Optical system according to one of the preceding claims, wherein the scanning or angled deflection device comprises a vibrating and / or rotating scanning mirror ( 2 ). Optical system according to claim 3 and claim 6, wherein the double-sided and / or single-sided micro or lenticular lens array has a recess in which the scanning mirror ( 2 ) is arranged. Optical system according to one of the preceding claims, wherein the optical axes of the scan or Winkelablenkvorrichtung and the tracking device and / or receiving means are coaxial. Optical system according to one of the preceding claims wherein the measurement is carried out by means of light transit time measurement and / or reflectivity measurement. Arrangement of Measuring Objects, in Particular of Three-Dimensional Measuring Objects and / or. Arrangements of objects to be measured, comprising: transmitting at least one transmission beam ( 50 ); Scanning a DUT ( 3 ) with the at least one transmission beam ( 50 ) along two scan axes, tracking the object to be measured ( 3 ) in the direction of a receiving device reflected and / or scattered transmission beam light ( 51 ), wherein the first of the two scan axes is tracked electronically and the second scan axis is electronically tracked, detecting at least part of the object to be measured ( 3 ) reflected and / or scattered transmission beam light ( 51 ) by means of at least one detector or at least one detector array ( 9 ) the receiving device and generating at least one received signal; and processing the at least one received signal by means of a signal processing device, wherein the optical tracking of the first scan axis comprises: deflecting the object to be measured ( 3 ) in the direction of the receiving device reflected and / or scattered transmission beam light ( 51 ) in synchronism with the scanning process in the first scan axis by means of a transmissive optical beam deflection arrangement ( 4 . 5 ; 13 . 14 ); and imaging by the beam deflecting assembly ( 4 . 5 ; 13 . 14 ) propagated light ( 51 ) on the detector or detector array ( 9 ); and wherein the electronic tracking of the second scan axis comprises: Switching of different sections of the detector or the detector array ( 9 ) synchronously with the scanning process in the second scan axis, wherein the respective switched subarea is in signal connection with the signal processing device. Method according to claim 10, wherein the deflection of the object to be measured ( 3 ) in the direction of the receiving device reflected and / or scattered transmission beam light ( 51 ): takes place by means of a translational movement of a double-sided micro or lenticular lens array and a single-sided micro or lenticular lens array relative to each other; or by means of an opposite rotation of two transmission gratings ( 13 . 14 ) or by means of an electronically switchable transmission grating. A method according to claim 10 or 11, wherein imaging by the beam deflecting arrangement ( 4 . 5 ; 13 . 14 ) propagated light ( 51 ) on the detector or detector array ( 9 ) takes place by means of a Fresnel lens; and / or scanning the measurement object ( 3 ) with the transmission beam ( 50 ) by means of a vibrating and / or rotating scanning mirror ( 2 ) he follows. Method according to one of claims 10 to 12, wherein the detector is a linear detector or the Dektorarray is a linear detector array. Method according to one of claims 10 to 13, wherein the measurement is carried out by means of light transit time measurement and / or reflectivity measurement.

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