MEASURING UNIT AND METHOD FOR OPTICAL MEASUREMENT OF OBJECTS

Description

[0001] The invention relates to a measuring device for the optical measurement of objects with an image acquisition unit and a laser projection unit, which has a laser light source, wherein the laser projection unit is designed to project laser light onto an object to be measured and the image acquisition unit is designed to capture an image of the object with the projected laser light.

[0002] The invention further relates to a method for the optical measurement of objects by projecting laser light onto the object to be measured using a laser projection unit comprising a laser light source, and by capturing an image of the object with the projected laser light using an image capture unit. In particular, the invention relates to a triangulating 3D measuring unit which then determines the 3D coordinates of the object from the captured images, for example, using suitable evaluation software.

[0003] For measuring objects, triangulating optical measurement with an image acquisition unit and a laser-based pattern generator is well known.

[0004] DE 10 2010 018 979 A1 describes a method and a device for determining the 3D coordinates of an object by scanning its surface with a line scanner. The line scanner comprises a projector with a laser light source and a camera for recording a projection line generated by the projector on the surface of the object.

[0005] DE 198 55 478 B4 also discloses a method and a device for optically detecting a contrast line with a laser scanner. The detection of the contrast line is achieved by controlling the intensity of the beam emitted by the laser scanner such that the intensity of the beam reflected from the detector of the laser scanner always assumes a constant value.

[0006] DE 10 2017 212 371 B4 proposes, for scanning a scene by a laser scanner, to capture the surface coordinates successively in several acquisition processes, wherein the frequency of repeated irradiation of the scene with laser radiation and acquisition of the radiation reflected from the scene by the laser scanner is different from each other in the successive acquisition processes.

[0007] DE 10 2018 127 221 B4 describes a coordinate measuring system with a laser line scanner and a projection unit. The laser line scanner generates a laser beam which is fanned out by suitable optics and projected onto the surface of the workpiece as a laser line. The length of the laser line is defined by a working distance between the scan module and the workpiece surface as well as an opening angle of the fanned-out laser beam.

[0008] DE 10 2018 211 913 A1 describes a device for detecting an object surface using a beam generating unit. Electromagnetic radiation with at least two different wavelengths is projected onto the object surface to generate separate measurements based on the different reflected radiation components of each of these wavelengths. The occurrence of radiation interference, e.g., in the form of speckle phenomena, depends on the selected wavelength of the emitted radiation. Speckle phenomena appear as spots with varying radiation intensity in the irradiated area of ​​the object surface. They can be attributed to interference caused by the emitted radiation interacting with the optics of the beam generating unit.By using radiation with at least two wavelengths, the probability is increased that the reflected radiation of at least one of the wavelengths and at least one of the measured values ​​generated from it is distorted to a lesser extent by speckle or other wavelength-dependent errors compared to the other measured value.

[0009] US 2011 / 0292406 A1 discloses a measuring device for the optical measurement of objects with a camera and a laser projection unit, wherein the power of the laser light source is changed depending on the detected intensity of the back-reflected light in order to keep the intensity of the back-reflected light constant regardless of the reflectance of the object surface under investigation.

[0010] DE 10 2012 113 021 A1 discloses a measuring device for the optical measurement of objects with a camera and a laser projection unit for emitting broadband laser light radiation.

[0011] US 2019 / 0049237 A1 discloses a measuring device in which the injection current and the temperature of the laser light source are controlled based on a detected light signal from the light source in order to keep the wavelength of the laser light source constant.

[0012] Based on this, the object of the present invention is to create an improved measuring device for the optical measurement of objects, which in particular reduces disturbances caused by interference.

[0013] The problem is solved by the measuring device having the features of claim 1 and by the method having the features of claim 11. Advantageous embodiments are described in the dependent claims.

[0014] It is proposed that the measuring device be set up to supply the laser light source with varying driver power, in particular varying injection current and / or varying driver voltage, during each exposure time of the image acquisition unit, in order to increase the bandwidth of the projected laser wavelengths.

[0015] The indefinite term "ein" is not to be understood as a numeral in the context of the present invention, but rather as an indefinite term meaning "at least one".

[0016] This means that the features designated as "one element" also include multiple such elements, such as multiple image acquisition units or one image acquisition unit with one or more cameras, laser projection units, and / or laser light sources. Furthermore, other elements are not excluded.

[0017] Varying the driver power increases the effective spectral bandwidth of the light source during the period when the object is being imaged by an image acquisition unit, thus reducing interference from micro-interference. While interference still occurs during image acquisition, it is captured for only a very short time within a specific wavelength range and changes due to the wavelength shift caused by the varying injection current. This attenuated interference is then smeared across the image and is only visible in a significantly weakened form.

[0018] By varying the injection current, it is possible to modulate the laser light source during the relatively short exposure times of image acquisition units, which are on the order of milliseconds. The varying current intensity changes the temperature, which leads to a change in the resonator length and thus to a change in the wavelength. This enables slow modulation.

[0019] Varying the driver voltage changes the band model of the laser diode, leading to a change in the gain profile and thus a change in the wavelength. This enables fast modulation.

[0020] Depending on the requirements, the driver power can be varied either solely by varying the injection current or solely by varying the driver voltage, in order to achieve either slow or fast modulation. However, a combination of varying the injection current and the driver voltage to vary the driver power is also conceivable, in order to combine the different effects with their different time responses.

[0021] Instead of operating the laser light source, especially a laser diode, with a constant injection current as usual, the current intensity is modulated during the exposure time of the image acquisition unit. At least two effects ensure the variation of the emitted wavelength: a) Thermal modulation of the laser junction temperature due to changes in heat input caused by current losses. This alters the length of the laser resonator formed by the semiconductor laser, and thus the wavelength. The wavelength decreases with increasing temperature. b) Shift of the energy level of the semiconductor junction of the laser diode depending on the modulated current density. This is known as the quantum-confined Stark effect. This changes the energy and therefore the wavelength of the photons induced at the semiconductor. This effect depends on the semiconductor material. In blue laser diodes, the wavelength is shortened with higher current density. In red laser diodes, the effect is inverse: higher currents produce longer wavelengths.c) A particularly large effect occurs in combination with a shortened wavelength (due to a short pulse) followed by heating and thus an increased wavelength.

[0022] By varying the injection current, it is possible to increase the bandwidth of the projected laser wavelengths by rapidly changing the wavelength of the laser light source through the varying injection current.

[0023] The laser light source can preferably be a semiconductor laser. This allows the aforementioned effects of thermal modulation and the quantum-confined Stark effect to be utilized.

[0024] A particularly simple way to modulate the laser light source with a varying injection current is through a pulse-modulated injection current. The injection current can even be significantly increased and exceed the permissible current for the laser light source, as long as the energy remains below the damage level of the laser light source (e.g., laser diode).

[0025] Pulse modulation can be achieved, for example, through pulse width modulation, pulse density modulation, and similar methods of manipulating the injection current and / or the driver voltage. Other pulse shapes are also conceivable, such as a triangular, sawtooth, or sinusoidal injection current or driver voltage waveform.

[0026] The injection current can also be varied using time-shaped injection current pulses, each with a changing current intensity. Such current-pulse shaping can further improve efficiency and reduce the current load on the laser light source. The same applies to varying the driver voltage using time-shaped driver voltage pulses.

[0027] The laser projection unit can be configured to supply power to the laser light source with modulated current pulses with pulse trains ranging from 10 nanoseconds to 10 microseconds and a duty cycle ranging from 5 to 500. This makes it possible to achieve a sufficiently wide bandwidth of laser wavelengths projected onto the object within the short exposure times typical for optical surveying, thus avoiding effects caused by micro-interference, in a particularly efficient and reliable manner.

[0028] The laser projection unit can be configured to project blue laser light in the wavelength range of 440 to 470 nanometers. With blue light in this wavelength range, the effect of changing the wavelength due to variations in the injection current is very pronounced.

[0029] The laser projection unit can have a diffractive optical element (DOE), a power lens, and / or a wavelength-dependent grating through which the laser light generated by the laser light source is passed. This allows the smearing effect of the microinterference points, caused by the wavelength change during image acquisition, to be further enhanced by interference at the DOE or grating even before the light exits the projector. For a diffractive optical element (DOE), lower orders are advantageous to avoid excessive smearing of the laser lines.

[0030] Suitable laser projection units include, for example, a laser line generator, a multi-line generator, or a random dot matrix generator. Other types of laser projection units are also conceivable.

[0031] The effects caused by microinterference can be further reduced by connecting the laser light source to an optical fiber. The path length difference then leads to a significant change in spatial and temporal coherence at different wavelengths through the optical fiber. This depends on the length of the optical fiber, which should be chosen to be sufficiently long to achieve a perceptible effect. The light coupled into the optical fiber is mixed by multiple reflections within the fiber. Each partial wavelength has its own reflection angle and thus path lengths as it travels through the fiber material. As the fiber length increases, the path length difference becomes greater, resulting in the light exiting the optical fiber having a spatially and temporally reduced degree of coherence compared to the coupled-in light.This process must be considered over a defined period of time during which the wavelength changes occur.

[0032] This effect can be further enhanced by laying the optical fibers in the form of loops.

[0033] The laser light emerging from the optical fiber can be collimated by an optical lens. The collimated, point-like laser light can then be guided through a power lens to create a laser line.

[0034] The invention is explained in more detail below with reference to the accompanying drawings and an exemplary embodiment. The drawings show: Figure 1 - Block diagram of a measuring device for the optical measurement of objects with a camera and a laser projection unit; Figure 2 - Exemplary diagram of the spectral bandwidth of a laser diode as a function of the injection current; Figure 3 - Diagram of an exemplary phase contrast as a function of the normalized beam diameter for determining spatial coherence; Figure 4 - Sketch of a laser beam with two different wavelengths directed at an object and guided through a diffractive optical element; Figure 5 - Sketch of a laser light source with an attached optical fiber to reduce the degree of coherence; Figure 6 - Sketch of an arrangement consisting of a laser light source, optical fiber, collimating lens, and Powell lens for generating a laser line.

[0035] Figure 1A block diagram of a measuring device 1 for the optical measurement of objects 2 with an image acquisition unit 3 and a laser projection unit 4 can be identified. It is conceivable that the image acquisition unit uses a single camera or several cameras, such as a stereo camera, to capture the object 2 or a part of the object 2.

[0036] The laser projection unit 4 has a laser light source 5, which is designed to emit a laser light beam. The laser light source 5 can, for example, be a semiconductor laser.

[0037] The laser projection unit 4, like the image acquisition unit 3, is aligned with the object 2 such that the laser light beam L strikes the surface of the object 2 and illuminates it. The image acquisition unit 3 is aligned with the surface of the object 2 such that it captures the projected laser light structures, such as lines 6, so that the images thus captured can be used by triangulation methods to determine properties of the object's surface, such as object coordinates.

[0038] Due to the coherent properties of laser light, interference patterns appear on the illuminated surface of the object. These are also known as microinterference points. They arise from coherent laser light beams reflected by non-reflective surfaces. The size of these microinterference points correlates with the wavelength, the temporal and spatial coherence length, and the roughness of the reflecting surface. Microinterference points distort the measurement result through constructive or destructive interference. This results in a large contrast difference for the human eye or an image acquisition unit. The position of a microinterference point also depends on the angle and distance at which it is viewed.When viewing one and the same microinterference point with an image acquisition unit 3, which has two cameras, each from a different direction, the position of the microinterference point is in a different position for each of these cameras.

[0039] The image acquisition unit 3 and the laser projection unit 4 are connected to a control unit 7, which is designed to control the laser projection unit 5 and the image acquisition unit 3. The images acquired by the image acquisition unit 3 can be received by the control unit 7 and at least temporarily stored. The control unit 7 can also have an evaluation unit and, for example, be configured by a suitable computer program to perform triangulation-based evaluation of the images acquired by the image acquisition unit 3.

[0040] The measuring device 1 is configured to supply the laser light source 5 of the laser projection unit 4 with a varying injection current during each exposure time of the image acquisition unit 3, in order to increase the bandwidth of the projected laser wavelengths of the laser beam L. The variation of the injection current can be carried out by electronics of the laser projection unit 4. It can also be carried out by the control unit 7 or by a modulation specified by the control unit 7 in combination with electronics of the laser projection unit 4.

[0041] By supplying the laser light source with a varying injection current, the current intensity is modulated during the exposure time of the image acquisition unit 3, thus varying the emitted wavelength of the laser light beam L.

[0042] The relationship used here between the thermal modulation of the laser junction temperature due to a change in heat input caused by current losses and the shift in the energy level of the semiconductor junction of the laser diode, depending on the modulated current density, is demonstrated using the Figures 2 and 3 explained.

[0043] Figure 2 shows an exemplary diagram of the spectral bandwidth of a laser diode, which depends on the injection current.

[0044] Every laser has a defined spectral width, which depends, among other things, on the injection current and the cooling of the laser diode. From the example diagram of the Figure 2Different spectra of a laser diode can be observed at varying currents. A distinct peak wavelength becomes visible with varying injection current. The peak wavelength increases with increasing injection current. Simultaneously, the intensity of the laser light emitted by the laser also increases with increasing injection current.

[0045] Additionally, the wavelength also increases over the short period of laser heating, even if the current is constant. This dynamic effect further contributes to increasing the bandwidth. However, this effect is insufficient for the relatively short exposure times, so this inherent property is enhanced by varying the driver power.

[0046] Figure 3 shows a diagram of an exemplary phase contrast as a function of the normalized beam diameter to determine spatial coherence.

[0047] By increasing the temperature due to a higher injection current, the laser resonator of the semiconductor laser, constructed from a semiconductor element, lengthens, and the band gap of the pn junction decreases, leading to a shift of the peak wavelengths into longer wavelength ranges. By appropriately modulating the amplitude of the injection current, different wavelengths are emitted by the laser diode. A time-averaged result is a combined spectrum.

[0048] If coherence is determined via the sum spectra, modulated by changes in current intensity, a reduction in coherence can be observed.

[0049] This is in Figure 3 to recognize. There, the spatial (lateral) coherence is shown, which is represented on the one hand as modulated (ARB) or on the other hand as normalized intensity measured in continuous operation (CW).

[0050] It is clearly evident that there is a difference in the degree of coherence. The achieved reduction in coherence, achieved through a variation of the injection flow, leads to a reduction in microinterference.

[0051] The influence of changes in wavelength on the contrast caused by microinterference can be enhanced by using a so-called diffractive optical element DOE and / or a wavelength-dependent grating.

[0052] Such optical elements, especially wavelength-dependent optical gratings, deflect the laser light into different directions of motion. The angle of deflection is defined by both the grating constant and the wavelength. With a constant grating constant but a changing partial wavelength, the laser beams are diffracted or deflected differently.

[0053] At a wavelength λ1, the laser beams are deflected onto an impact point on a surface. The reflected coherent laser light creates several microinterference points in front of the surface, which are detected by an image acquisition unit 3.

[0054] Changing the injection current when supplying the laser light source, and thus the wavelength of the laser light source, deflects the same beam of light, which was previously deflected at the point of impact, to a new, shifted point of impact on the surface. This results in the points caused by microinterference shifting and generating new points, which are detected by the image acquisition unit 3.

[0055] Due to the variations in the injection current during the illumination time of the image acquisition unit 3, the wavelength change in the camera image during the exposure time leads to a smearing of the microinterference points that shift due to the variations in the injection current.

[0056] The more wavelength changes are performed within an exposure time of the image acquisition unit 3, the better the interfering microinterference points are smeared out and thus averaged out over time. This can also be described as angular diversification within a defined time window, i.e., the exposure time of the image acquisition unit 3.

[0057] Changing the laser diode wavelength, both thermally and through the quantum-confined Stark effect, can reduce coherence and thus the number of microinterference points. This effect can be further enhanced over time by wavelength-dependent angular diversification at a grating.

[0058] Figure 4 Figure 1 shows a sketch of a laser beam with two different wavelengths λ1 and λ2, directed at an object 2 and guided through a diffractive optical element DOE. The transmitted laser beam of diffraction order 0 (not shown) strikes the (uneven) surface at a first point of impact. Due to the differences in the laser beam's travel time at the point of impact caused by the surface roughness, interference with microinterference points occurs.

[0059] Due to diffraction at the diffractive optical element DOE, additional points of impact for the laser beams of higher diffraction orders are created, shifted relative to the main point of impact. The first-order diffraction laser beams are shown, exiting the diffractive optical element DOE at an acute angle to the transmitted first-order laser beam. The exit angle of the higher-order laser beams, and thus the points of impact on the object, depend on the wavelengths λ1 and λ2.

[0060] Microinterference points S1 λ1 , S2 λ1 with the first wavelength λ1 and, shifted in this respect, microinterference points S1 λ2 , S2 λ2 with the second wavelength λ2 are created in front of object 2.

[0061] Due to the differences in surface elevation, the wavelength-dependent shifts of the micro-interference points mentioned above also occur around the main point of impact in a corresponding manner. This is not shown in the diagram for the sake of clarity.

[0062] Therefore, a variation in wavelength during the recording of an image has an effect on the through-beam main laser beam of the 0th diffraction order, even without the diffractive optical element DOE diffractive optical element.

[0063] Figure 5The diagram shows a laser light source 5 with an attached optical fiber 8, through which the coupled laser light L from the laser light source 5 is mixed by multireflections, and the laser light emerging from the optical fiber 8 has a significantly reduced degree of coherence in space and time compared to the coupled laser light. It becomes clear that the path length difference of the laser light L1, L2, L3 at different wavelengths λ1, λ2, λ3 through the optical fiber 8 leads to a significant change in spatial and temporal coherence. The light L1, L2, L3 in the optical fiber 8 is mixed by multireflections. Each partial wavelength λ1, λ2, λ3 has its own reflection angles and thus path lengths on its path through the fiber material.Depending on the fiber length, the path length difference increases, resulting in a significantly reduced degree of coherence in the laser light L exiting the optical fiber 8 (light guide). This can be further intensified by laying the optical fiber 8 in loops, i.e., along a winding circular or meandering path. This effect must be considered over the defined period during which the wavelength changes occur. Over an exposure period in which the driver power is changed, for example, by varying the injection current and / or the driver voltage, a sum spectrum of all partial wavelengths is generated. Averaged over time, this leads to a broadening of the overall spectrum and thus to a shortening of the coherence length, resulting in a reduction of microinterference and its associated effects.

[0064] Figure 6Figure 1 shows a sketch of an arrangement consisting of a laser light source 5, an optical fiber 8, a collimating lens 9, and a Powell lens 10 for generating a laser line. The laser light L emitted from the laser light source 5 is coupled directly into an optical fiber 8 to reduce the degree of coherence. The laser light L emitted at the end of the optical fiber 8 is collimated by an optical lens 9 and directed onto a Powell lens 10. This Powell lens 10, in turn, generates a laser line 6 from the point light source, which is projected onto an object 2 to be measured and used to generate surface data in the measurement system. For the camera to clearly and accurately detect the laser line 6 and determine its center point, the laser line 6 should be as noise-free as possible. Points caused by microinterference, which behave differently for different materials, significantly impair the detection result.A reduction in microinterferences produced by the described laser modulation therefore has a positive effect on the measurement result.

Claims

1. Measuring device (1) for optically measuring objects (2), comprising a camera (3) and a laser projection unit (4) having a laser light source (5), wherein the laser projection unit (4) is configured for projecting laser light (L) onto an object (2) to be measured and the camera (3) is configured to record images of the object (2) with the projected laser light (L), characterized in that the measuring device (1) is designed to supply the laser light source (5) with a driver power varying during an exposure time of the camera (3) in each case, in particular varying injection current and / or driver voltage, to increase the bandwidth of the projected laser wavelengths.

2. Measuring device (1) according to Claim 1, characterized in that the laser light source (5) is a semiconductor laser.

3. Measuring device (1) according to Claim 1 or 2, characterized in that the laser projection unit (4) is designed to supply the laser light source (5) with a pulse-modulated injection current, a triangular, sawtooth-shaped or sinusoidal injection current profile or with temporally shaped injection current pulses and / or voltage pulses, which have a varying current in in each case one current pulse or a varying voltage in one voltage pulse.

4. Measuring device (1) according to any of Claims 1 to 3, characterized in that the laser projection unit (4) is designed to supply the laser light source (5) with modulated current pulses and / or voltage pulses having pulse sequences ranging from 10 nanoseconds to 10 microseconds and a duty cycle ranging from 5 to 500.

5. Measuring device (1) according to any of Claims 1 to 4, characterized in that the laser projection unit (4) is designed to project blue laser light in the wavelength range from 440 to 470 nanometres.

6. Measuring device (1) according to any of Claims 1 to 5, characterized in that the laser projection unit (4) has a diffractive optical element (DOE), a Powell lens and / or a wavelength-dependent grating, through which laser light generated by the laser light source (5) is guided.

7. Measuring device (1) according to any of Claims 1 to 6, characterized in that the laser projection unit (4) is a laser line generator, a multi-line generator or a random dot matrix generator.

8. Measuring device according to any of the preceding claims, characterized in that the laser light source (5) is connected to an optical fibre (8).

9. Measuring device according to Claim 8, characterized in that the optical fibre (8) is coiled in a loop-shaped manner.

10. Measuring device according to Claim 8 or 9, characterized in that the exit of the optical fibre (8) is guided onto an optical lens (9) for collimating the laser light (L) and the collimated laser light (L) exiting the optical lens (9) is guided onto a Powell lens (10) for generating a laser line (6).

11. Method for optically measuring objects (2) by means of projecting laser light onto the object (2) to be measured using a laser projection unit (4) having a laser light source (5), and by means of recording images of the object (2) with the projected laser light (L) using a camera, characterized in that the laser light source (5) is operated during an exposure time of the camera (3) in each case with a varying driver power, in particular a varying injection current and / or a varying driver voltage, to increase the bandwidth of the projected laser wavelengths.

12. Method according to Claim 11, characterized by operating the laser light source (5) with a pulse-modulated injection current, a triangular, sawtooth-shaped or sinusoidal injection current profile or with temporally shaped injection current pulses and / or voltage pulses, which have a varying current in in each case one current pulse or a varying voltage in one voltage pulse.

13. Method according to Claim 12, characterized in that the injection current comprises during the exposure time at least one full wave of the injection current profile.

14. Method according to any of Claims 11 to 13, characterized in that the laser light source (5) is a semiconductor laser.

15. Method according to any of Claims 11 to 14, characterized by operating the laser light source (5) with modulated current pulses and / or voltage pulses having pulse lengths ranging from 10 nanoseconds to 10 microseconds and a duty cycle ranging from 5 to 500.

16. Method according to any of Claims 11 to 15, characterized by projecting laser light (L) onto the object (2) to be measured with the laser light source (5) in the wavelength range from 440 to 470 nanometres.

17. Method according to any of Claims 11 to 16, characterized by coupling the laser light (L) from the laser light source (5) into an optical fibre (8) to mix the input-coupled laser light (L) by multi-reflections in the optical fibre (8) in a manner such that the light leaving the optical fibre (8) has at the light exit of the optical fibre (8) a degree of coherence which is reduced spatially and temporally compared with the input-coupled laser light (L).

18. Method according to Claim 17, characterized by guiding the laser light (L) in the optical fibre (8) in loops along a path which is curved at least in sections.

19. Method according to Claim 17 or 18, characterized by collimating the laser light (L) exiting the optical fibre (8) and generating a laser line (6) from the collimated point-type laser light (L) by means of a Powell lens (10).