MEASURING UNIT AND METHOD FOR OPTICAL MEASUREMENT OF OBJECTS

Abstract

The invention relates to a measuring apparatus (1) for optically measuring objects (2), having a camera (3) and a laser projection unit (4) which has a laser light source (5), wherein the laser projection unit (4) is designed to project laser light (L) onto an object (2) to be measured and the camera (3) is designed to record an image of the object (2) with the projected laser light (L), wherein the measuring apparatus (1) is designed to supply the at least one laser light source (5) with a driving power which varies during each exposure time of the camera (3), in particular a varying injection current and / or driving voltage, in order to increase the bandwidth of the projected laser wavelengths.

Description

[0001] Measuring unit and method for optical measurement of objects

[0002] The invention relates to a measuring device for the optical measurement of objects with an image recording unit and a laser projection unit having a laser light source, wherein the laser projection unit is designed to project laser light onto an object to be measured and the image recording unit is designed to record an image of the object with the projected laser light.

[0003] The invention further relates to a method for optically measuring objects by projecting laser light onto the object to be measured using a laser projection unit with a laser light source, and by capturing images of the object using the projected laser light with 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 of the object, for example, with computer support using suitable evaluation software.

[0004] Triangulating optical measurement with an image acquisition unit and a laser-based pattern generator is well known for measuring objects.

[0005] DE 10 2010 018 979 A1 describes a method and a device for determining the 3D coordinates of an object by scanning the 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. DE 198 55 478 B4 also describes a method and a device for optically detecting a contrast line with a laser scanner. The contrast line is detected by controlling the intensity of the beam emitted by the laser scanner such that the intensity of the remitted beam 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 capture processes, wherein the frequency of repeated irradiation of the scene with laser radiation and capture of the radiation reflected from the scene by the laser scanner in the successive capture processes is different from each other.

[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 that is spread 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 the working distance between the scanning module and the workpiece surface and the aperture angle of the spread-out laser beam.

[0008] DE 10 2018 211 913 A1 describes a device for detecting an object surface using a beam-generating device. Electromagnetic radiation with at least two different wavelengths is projected onto an object surface to generate separate measured values ​​based on the different reflected radiation components at 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 occur in the irradiated area of ​​the object surface, with spots of varying radiation intensity. They can be caused by interference in emitted radiation due to interaction with the optics of the radiation-generating device.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 corrupted by speckle or other wavelength-dependent errors to a lesser extent than the other measured value.

[0009] Based on this, it is the object of the present invention to provide an improved measuring device for the optical measurement of objects, which in particular reduces disturbances caused by interference.

[0010] The object is achieved 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 subclaims.

[0011] It is proposed that the measuring device is configured to supply the laser light source with a driver power that varies during each exposure time of the image recording unit, in particular a varying injection current and / or a varying driver voltage, in order to increase the bandwidth of the projected laser wavelengths.

[0012] For the purposes of the present invention, the indefinite term "a" is not to be understood as a number, but as an indefinite term in the sense of "at least one." Thus, the features designated by "an element" also encompass several such elements, such as multiple image recording units or an image recording unit with one or more cameras, laser projection units, and / or laser light sources. Furthermore, other elements are not excluded.

[0013] By varying the driver power, the effective spectral bandwidth of the light source is increased during the period in which the image of the object is captured by an image acquisition unit, which leads to a reduction in interference caused by microinterference. While the interference still occurs during image acquisition, it is recorded only for a very short time over a wavelength range and changes due to the wavelength change caused by the varying injection current. As a result, the attenuated interference smeares the image and is only visible in a significantly weaker form. By varying the injection current, it is possible to modulate the laser light source during the relatively short exposure times of image acquisition units 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.

[0014] Varying the drive voltage changes the band pattern of the laser diode, resulting in a change in the gain profile and thus in a change in the wavelength. This enables fast modulation.

[0015] Depending on the requirements, the driver power can be varied either by varying the injection current alone or by varying the driver voltage alone, 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 different timing behaviors.

[0016] Instead of operating the laser light source, in particular a laser diode, with a constant injection current as is usual, the current intensity is modulated during the exposure time of the image recording unit. At least two effects ensure the variation in the emitted wavelength: a) Thermal modulation of the laser junction temperature through changing heat input due to the losses caused by the current. This changes the length of the laser resonator constructed by the semiconductor laser, and thus the wavelength. The wavelength becomes longer with increasing temperature. b) Shift in 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 thus the wavelength of the induced photons 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 significant effect occurs in combination with a shortened wavelength caused by a short pulse, followed by heating and thus an increased wavelength.

[0017] By varying the injection current, it is possible to increase the bandwidth of the projected laser wavelengths by changing the wavelength of the laser light source very quickly by varying the injection current.

[0018] 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.

[0019] A particularly simple method for modulating the laser light source with a varying injection current is a pulse-modulated injection current. The injection current may even be significantly increased and exceed the permissible current for the laser light source, as long as the energy is below the damage level of the laser light source (e.g., laser diode).

[0020] Pulse modulation can be achieved, for example, by pulse-width modulation, pulse-density modulation, and the like of the injection current and / or the drive voltage. However, other pulse shapes are also conceivable, such as a triangular, sawtooth, or sinusoidal injection current waveform or drive voltage waveform.

[0021] The injection current can also be varied using temporally shaped injection current pulses, each with a varying current intensity. This type of current-pulse shaping can further improve efficiency and reduce the current load on the laser light source. The same applies to varying the drive voltage using temporally shaped drive voltage pulses.

[0022] The laser projection unit can be configured to power the laser light source with modulated current pulses with pulse sequences in the range of 10 nanoseconds to 10 microseconds and a duty cycle in the range of 5 to 500. This makes it possible to achieve a sufficiently wide bandwidth of laser wavelengths projected by the laser light onto the object in a particularly efficient and reliable manner within the short exposure times typically used for optical measurement, thus avoiding effects caused by microinterference.

[0023] The laser projection unit can be configured to project blue laser light in the wavelength range of 440 to 470 nanometers. For blue light in this wavelength range, the wavelength change effect resulting from varying the injection current is very pronounced.

[0024] 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 of the micro-interference points caused by the wavelength change during image acquisition to be further amplified by interference at the DOE or grating before the light exits. With a diffractive optical element (DOE), low-order resolutions are recommended to avoid excessive smearing of the laser lines.

[0025] A laser line generator, a multi-line generator, or a random dot matrix generator are suitable laser projection units. Other types of laser projection units are also conceivable.

[0026] The effects caused by microinterferences can be further reduced by connecting the laser light source to an optical fiber. The difference in path length then leads to a significant change in the 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 multi-reflections within the optical fiber. Each partial wavelength has its own reflection angle and thus path length on its path through the fiber material. Depending on the fiber length, the difference in path length increases, which means that the light leaving the optical fiber has a spatially and temporally reduced degree of coherence at the light exit compared to the coupled light.This process must be observed over a defined period of time, during which the wavelength changes occur. This effect can be further enhanced by laying the optical fibers in loops.

[0027] The laser light emerging from the optical fiber can be collimated by an optical lens. The collimated point-shaped laser light can be directed through a Powell lens to create a laser line.

[0028] The invention is explained in more detail below with reference to an exemplary embodiment of the invention, which is shown in the accompanying drawings.

[0029] Figure 1 - Block diagram of a measuring device for optical measurement of objects with a camera and a laser projection unit;

[0030] Figure 2 - exemplary diagram of a spectral bandwidth of a laser diode dependent on the injection current;

[0031] Figure 3 - Diagram of an exemplary phase contrast as a function of the normalized beam diameter for determining spatial coherence;

[0032] Figure 4 - Sketch of a laser beam with two different wavelengths directed at an object and guided through a diffractive optical element;

[0033] Figure 5 - Sketch of a laser light source with an optical fiber connected to it to reduce the degree of coherence;

[0034] Figure 6 - Sketch of an arrangement of laser light source, optical fiber, collimation lens and Powell lens for generating a laser line.

[0035] Figure 1 shows a block diagram of a measuring device 1 for optically measuring objects 2, comprising an image acquisition unit 3 and a laser projection unit 4. It is conceivable that the image acquisition unit uses a single camera or multiple cameras, such as a stereo camera, to capture the object 2 or a portion of the object 2.

[0036] The laser projection unit 4 has a laser light source 5 configured to emit a laser light beam. The laser light source 5 can comprise, for example, a semiconductor laser. The laser projection unit 4, like the image recording unit 3, is aligned with the object 2 such that the laser light beam L impinges on the surface of the object 2 and illuminates the surface. The image recording unit 3 is aligned with the surface of the object 2 such that it records the projected laser light structures, such as lines 6, so that the images thus recorded can be used by triangulation methods to determine properties of the object's surface, such as object coordinates.

[0037] Due to the coherent properties of the laser light, interference phenomena arise on the illuminated surface of the object 2. These are also referred to as micro-interference points. They are created by coherent laser light beams reflected from non-reflective surfaces. The size of these micro-interference points correlates with the wavelength, the temporal and spatial coherence length, and the roughness of the reflecting surface. Micro-interference points distort the measurement result through constructive or destructive interference. This creates a significant contrast difference for the human eye or an image acquisition unit 3. The position of a micro-interference point also depends on the angle and distance at which it is viewed.When viewing one and the same micro-interference point with an image recording unit 3 having two cameras, each from a different direction, the position of the micro-interference point is in a different position for each of these cameras.

[0038] 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 triangulate the images acquired with the image acquisition unit 3.

[0039] 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, thereby increasing the bandwidth of the projected laser wavelengths of the laser beam L. The variation of the injection current can be achieved by electronics of the laser projection unit 4. It can also be achieved 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.

[0040] By supplying the laser light source with a varying injection current, the current intensity is modulated during the exposure time of the image recording unit 3 and thus the emitted wavelength of the laser light beam L is varied.

[0041] The relationship used for this purpose between the thermal modulation of the laser junction temperature by a changed heat input due to the losses caused by the current as well as the shift of the energy level of the semiconductor junction of the laser diode, depending on the modulated current density, are explained using Figures 2 and 3.

[0042] Figure 2 shows an exemplary diagram of spectral bandwidth of a laser diode, which depends on the injection current.

[0043] Every laser has a defined spectral width, which depends, among other things, on the injection current and the cooling of the laser diode. The example diagram in Figure 2 shows various spectra of a laser diode at different currents. A distinct peak wavelength becomes visible with varying injection current. Thus, the peak wavelength increases with increasing injection current. At the same time, the intensity of the laser light emitted by the laser also increases with increasing injection current.

[0044] In addition, the wavelength also increases over the short period of laser heating, even when the current is constant. This dynamic effect further helps increase the bandwidth. However, with relatively short exposure times, this effect is not sufficient, so this inherent property is enhanced by varying the driver power. Figure 3 shows a diagram of an example phase contrast as a function of the normalized beam diameter for determining spatial coherence.

[0045] The increase in temperature due to a higher injection current lengthens the laser resonator of the semiconductor laser, which is constructed from a semiconductor element, and narrows the band gap of the pn junction, which leads to a shift in the peak wavelengths into longer wavelength ranges. By appropriately modulating the amplitude of the injection current, different wavelengths are emitted by the laser diode. Averaged over time, this results in a cumulative spectrum.

[0046] If the coherence is now determined via the sum spectra, modulated by current changes, a reduction in coherence can be detected.

[0047] This can be seen in Figure 3. 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 mode (CW).

[0048] It is clearly visible that there is a difference in the degree of coherence. The shortening of the coherence achieved by varying the injection current leads to a reduction in microinterference.

[0049] The influence of changes in wavelength on the contrast caused by microinterferences can be enhanced by using a so-called diffractive optical element DOE and / or by a wavelength-dependent grating.

[0050] Such optical elements, particularly wavelength-dependent gratings, direct the laser light into different motion orders. The angle of deflection is defined by both the grating constant and the wavelength. With a constant grating constant but changing partial wavelength, the laser beams are diffracted or deflected differently. At a wavelength of 1, the laser beams are deflected to an impact point on a surface. The reflected coherent laser light creates several micro-interference points in front of the surface, which are detected by an image acquisition unit 3.

[0051] By changing the injection current supplying the laser light source, and thus the wavelength of the laser light source, the same beam of light that was previously deflected at the point of impact is deflected to a new, shifted point of impact on the surface. This results in the points caused by microinterference shifting and creating new points that are detected by the image acquisition unit 3.

[0052] Due to the variations in the injection current during the illumination time of the image recording unit 3, the wavelength change in the camera image carried out during the exposure time leads to a smearing of the micro-interference points that shift due to the variations in the injection current.

[0053] The more wavelength changes are performed within an exposure time of the image acquisition unit 3, the better the interfering micro-interference points are smeared and thus averaged out over time. This can also be referred to as angular diversification within a defined time window, i.e., the exposure time of the image acquisition unit 3.

[0054] By changing the laser diode wavelength, both thermally and through the quantum-confined Stark effect, a reduction in coherence and thus in the number of microinterference points can be achieved. This effect can be further amplified by wavelength-dependent angular diversification on a grating, averaged over time.

[0055] Figure 4 shows a sketch of a laser beam with two different wavelengths A1 and A2, which is directed at an object 2 and guided through a diffractive optical element DOE. The transmitted laser beam of the third diffraction order (not shown) strikes the (uneven) surface at a first impact point. The time-of-flight differences of the laser beam at the impact point caused by the surface roughness create interference with micro-interference points. Diffraction at the diffractive optical element DOE creates additional impact points for the laser beams of a higher diffraction order, shifted from the main impact point. The first-order diffraction laser beams are shown emerging obliquely from 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 impact points on the object depend on the wavelength 1 and A2.

[0056] Microinterference points S1 are created in front of object 2 Ä i, S2 Ä1 with the first wavelength A1 and shifted microinterference points S1 Ä 2, S2 Ä 2 with the second wavelength A2.

[0057] At the main point of impact, the aforementioned wavelength-dependent shifts of the microinterference points also occur around the main point of impact due to the height differences on the surface. This is not shown for clarity.

[0058] Therefore, a variation in wavelength when recording an image has an effect on the transmitted main laser beam of the diffraction order, even without the diffraction effects caused by the diffractive optical element DOE.

[0059] Figure 5 shows a sketch of a laser light source 5 with an optical fiber 8 connected to it, through which the coupled laser light L of the laser light source 5 is mixed by multi-reflections and the laser light L emerging from the optical fiber 8 has a spatially and temporally significantly reduced degree of coherence compared to the coupled laser light. It is clear that the path length difference of the laser light L1, L2, L3 at different wavelengths A1, A2, A3 through the optical fiber 8 leads to a significant change in the spatial and temporal coherence. The light L1, L2, L3 in the optical fiber 8 is mixed by multi-reflections. Each partial wavelength A1, A2, A3 has its own reflection angle and thus path length on its path through the fiber material.Depending on the fiber length, the path length difference increases, which means that the laser light L leaving the optical fiber 8 (light guide) has a significantly reduced degree of coherence in space and time. This can be further enhanced by laying the optical fiber 8 in the form of loops, i.e., on a winding, circular or meandering path. This effect must be observed over the defined period of time in 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 is generated from all partial wavelengths. Averaged over time, this leads to a broadening of the overall spectrum and thus to a shortening of the coherence length, which reduces microinterference and the associated effects.

[0060] Figure 6 shows a sketch of an arrangement comprising a laser light source 5, an optical fiber 8, a collimation 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 via 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 measuring system. For the camera to clearly and accurately detect the laser line 6 and determine the line center, 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 caused by the described laser modulation has a correspondingly positive effect on the measurement result.

Claims

Patent claims 1. Measuring device (1) for optical measurement of objects (2) with a camera (3) and a laser projection unit (4) having a laser light source (5), wherein the laser projection unit (4) is designed to project laser light (L) onto an object (2) to be measured and the camera (3) is designed to capture an image of the object (2) with the projected laser light (L), characterized in that the measuring device (1) is configured to supply the laser light source (5) with a driver power, in particular a varying injection current and / or driver voltage, during each exposure time of the camera (3), in order 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 configured to supply the laser light source (5) with a pulse-modulated injection current, a triangular, sawtooth or sinusoidal injection current waveform or with time-shaped injection current pulses and / or voltage pulses, which have a changing current intensity in each current pulse or a changing voltage in each voltage pulse.

4. Measuring device (1) according to one of claims 1 to 3, characterized in that the laser projection unit (4) is used to supply the laser light source (5) is set up with modulated current and / or voltage pulses with pulse trains in the range of 10 nanoseconds to 10 microseconds and a duty cycle in the range of 5 to 500.

5. Measuring device (1) according to one of claims 1 to 4, characterized in that the laser projection unit (4) is configured for the projection of blue laser light in the wavelength range of 440 to 470 nanometers.

6. Measuring device (1) according to one 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 passed.

7. Measuring device (1) according to one 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 one of the preceding claims, characterized in that the laser light source (5) is connected to an optical fiber (8).

9. Measuring device according to claim 8, characterized in that the optical fiber (8) is wound in a loop shape.

10. Measuring device according to claim 8 or 9, characterized in that the output of the optical fiber (8) is directed to an optical lens (9) for collimating the laser light (L) and the collimated laser light (L) exiting the optical lens (9) is directed to a Powell lens (10) for generating a laser line (6).

11. Method for optically measuring objects (2) by projecting laser light onto the object (2) to be measured using a laser projection unit (4) having a laser light source (5) and by capturing an image of the object (2) with the projected laser light (L) using a camera, characterized in that the laser light source (5) is illuminated during each exposure time of the camera (3) is operated 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 or sinusoidal injection current waveform or with time-shaped injection current pulses and / or voltage pulses, which have a changing current intensity in each current pulse or a changing voltage in each voltage pulse.

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

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

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

16. Method according to one 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 of 440 to 470 nanometers.

17. Method according to one of claims 11 to 16, characterized by coupling the laser light (L) of the laser light source (5) into an optical fiber (8) for mixing the coupled laser light (L) by multireflections in the optical fiber (8) such that the light leaving the optical fiber (8) has a spatially and temporally reduced degree of coherence at the light exit of the optical fiber (8) compared to the coupled laser light (L).

18. A method according to claim 17, characterized by guiding the laser light (L) in loops in the optical fiber (8) along a path that is curved at least in partial sections.

19. A method according to claim 17 or 18, characterized by collimating the laser light (L) emerging from the optical fiber (8) and generating a laser line (6) from the collimated point-shaped laser light (L) by means of a Powell lens (10).