Device and method for scanning the distance to an object

Abstract

Device (14) for scanning the distance to an object (12), with a light source (16) which is configured to produce an optical signal with a varying frequency, a scanning device (28) which is configured to direct measuring light (29) in different directions, wherein the measuring light (29) is formed by a first part of the optical signal generated by the light source, a detector (32) configured to detect a superposition of reference light and reflected light, wherein the reference light is formed by a second part of the optical signal generated by the light source (16), which is not supplied to the scanning device (28), and wherein the reflected light is formed by the measuring light (29) after it has been at least partially reflected by the object (12), an evaluation device (34) which is configured to determine a distance to the object (12) from the superposition detected by the detector (32), a monitoring device with a light sensor (36a, 36b) which is arranged in the light path of the emitted measuring light (29) behind the scanning device (28) and is configured to detect a scanning movement of the emitted measuring light (29), wherein the light sensor (36a, 36b) is arranged such that it is exposed to the measuring light (29) only once per scan cycle, and with a shut-off device (38) connected to the light sensor (36a, 36b) and the light source (16) and configured to shut off the light source (16) or otherwise prevent the emission of measuring light (29) when the light sensor (36a, 36b) does not detect any scanning movement of the measuring light (29).

Description

BACKGROUND OF THE INVENTION 1. Field of the invention

[0001] The invention relates to a device and a method for scanning and measuring the distance to a moving or stationary object based on FMCW LiDAR technology. Devices and methods of this type can be used, for example, in autonomous vehicles. 2. Description of the state of the art

[0002] For optical distance measurement, a measurement principle known as FMCW LiDAR is known, in which optical signals with a time-varying frequency (FMCW stands for frequency-modulated continuous wave) are directed from a scanning device in different directions towards an object to be measured. After reflection from the object, these signals return to the scanning device at low intensity and are superimposed with a signal that was not emitted and is therefore called the local oscillator. The resulting beat frequency is detected by a detector and allows the distance between the scanner and the object to be calculated. If the Doppler shift is also taken into account, the radial relative velocity between the scanner and the object can also be calculated.

[0003] FMCW LiDAR devices typically contain a laser light source that generates measurement light with a wavelength of 1550 nm. Since this frequency lies in the infrared spectral range and thus outside the visible spectrum, the measurement light cannot be perceived by humans. Furthermore, infrared light only becomes harmful to the eyes at very high power levels. Therefore, FMCW LiDAR devices with high-intensity measurement light can measure distances of up to 300 m without posing a risk to eye safety.

[0004] Even greater ranges require correspondingly higher laser power. High intensity of the measuring light is also desirable with regard to a high signal-to-noise ratio and thus the reliability of the distance measurement.

[0005] Infrared measuring light with very high intensities is only harmless to the eyes if it briefly strikes the retina, as is normally the case during scanning. However, if the scanning device malfunctions, the measuring beam may become stationary. If such a stationary, high-intensity measuring beam enters a human eye, damage cannot be ruled out.

[0006] From DE 10 2019 210 999 A1 (corresponding to US 2021 / 0026014 A1) an FMCW LiDAR measuring device is known in which an optical position sensor is arranged in the signal path of a dispersive optical element. This allows the instantaneous beam direction of the measuring light to be continuously monitored, which can be used for improved image reconstruction.

[0007] German patent DE 10 2017 205 631 A1 discloses a LiDAR system based on the time-of-flight principle, in which a portion of the measurement light reflected by the scanning mirror is extracted using a semi-transparent element and directed onto a detector array comprising multiple sensors. By evaluating the signals provided by the sensors, the position of the measurement light beam and thus the instantaneous scan angle can be determined. The sensor signals can also be used to detect fault conditions, such as a broken mirror, a mechanical stop, and similar defects.

[0008] German patent DE 10 2017 218 587 A1 discloses a device for monitoring a scanning laser system, in which a holographic optical element directs part of the measuring light onto detector elements of a detector device. This makes it possible to detect when the scan mirror stops moving.

[0009] From DE 10 2020 110 142 A1, an FMCW LiDAR system is known in which the optical signals are distributed to several optical output waveguides via a distribution matrix. A deflection optic deflects the measurement light emerging from the optical output waveguides so that it is emitted in different directions.

[0010] German patent DE 10 2020 104 601 A1 discloses a LiDAR system with an actuated scanning mirror. Two optical markers are arranged at the outer edge of a window, the reflectivity of which differs from that of the surrounding window. When the measuring beam passes through an optical marker, this is detectable by a sudden change in intensity. This allows malfunctions of components to be detected. In particular, it can be determined whether the maximum deflection angle is still being achieved or not. It is also disclosed that when a fault is detected, measures such as switching off the light source are taken to prevent, for example, eye damage.

[0011] EP 2 983 004 A2 relates to a laser rangefinder not based on the FMCW principle. An oscillating scan mirror is used for scanning. A detector, which captures the scan beam, is located directly next to an exit lens. Since the detector is positioned at the edge of the scan area, but not at the reversal point, the oscillating scan beam sweeps over the detector twice in quick succession during each oscillation period. If the detector detects a disturbance because it no longer receives a signal, the laser can be switched off.

[0012] US Patent 2021 / 0316756 A1 discloses a device based on the FMCW principle for scanning and measuring the distance to an object. An optical distribution matrix distributes the optical signal generated by the light source to several optical output waveguides. Deflection optics deflect the optical signals emerging from the optical output waveguides so that they are emitted from the device in different directions. Detectors detect a superposition of the optical signal generated by the light source with optical signals reflected by the object and fed to the detectors, bypassing the optical distribution matrix. An evaluation unit determines the distance to the object from this superposition. SUMMARY OF THE INVENTION

[0013] The object of the invention is to provide a device and a method for scanning FMCW LiDAR measurement of the distance to an object, in which measuring light can be emitted with high intensity without damaging the eyes of people in the vicinity and without significantly disturbing the propagation of the measuring light.

[0014] With regard to the device, this problem is solved by a device for scanning the distance to an object, which includes a light source configured to generate an optical signal with a varying frequency. The device also includes a scanning unit configured to direct measuring light in different directions. The measuring light is formed by a first part of the optical signal generated by the light source. The device further includes a detector configured to detect a superposition of reference light and reflected light. The reference light is formed by a second part of the optical signal generated by the light source, which is not supplied to the scanning unit. The reflected light is formed by the measuring light after it has been at least partially reflected by the object.An evaluation unit of the device is configured to determine the distance to the object from the superposition detected by the detector. According to the invention, the device includes a monitoring unit with a light sensor, which can, for example, be a photodiode. The light sensor is arranged in the light path of the emitted measuring light downstream of the scanning unit and is configured to detect any scanning movement of the emitted measuring light. The light sensor is arranged such that it is exposed to the measuring light only once per scan cycle. The monitoring unit also includes a shutdown device, which is connected to the light sensor and the light source and is configured to switch off the light source or otherwise prevent the emission of measuring light if the light sensor does not detect any scanning movement of the measuring light.

[0015] The invention is based on the consideration that (especially infrared) measuring light may be emitted at a high intensity as long as the measuring beam(s) are moving, thus ensuring that the measuring light cannot enter the eyes of people for extended periods. Whether the scanning device, which generates the movements of the measuring light, functions flawlessly and whether the measuring light performs the desired scanning movements can only be reliably detected in the light path behind the scanning device by a light sensor. Detecting the scanning movements of the measuring light within the scanning device is particularly difficult when it contains no moving parts. Scanning devices without moving parts are preferably implemented as photonic integrated circuits, which are difficult to modify.

[0016] However, with a simple light sensor in the light path behind the scanning device, it is very easy to determine whether the measuring light is still performing the desired scanning movements or not.

[0017] According to the invention, the light sensor is arranged such that it is exposed to the measuring light only once per scan cycle. The underlying principle is that the measuring light does not need to be continuously monitored; it is sufficient to detect the measuring light with the light sensor only once per scan cycle. Since scan movements are generally periodic, a light sensor positioned at a reversal point of the measuring light beam makes it easy to determine whether the scanning process is still being carried out correctly. During a correct scan, the light sensor detects a signal at the reversal point at periodic intervals. If this signal is absent, it can be assumed that the scanning device is no longer functioning correctly and that the propagation of the measuring light beam must therefore be prevented. In this configuration, the light sensor is thus arranged in the region of a reversal point and therefore at the edge of the scan field.There, the light sensor does not significantly interfere with the propagation of the measuring light.

[0018] If scanning is to be performed in two orthogonal directions, at least one light sensor should be provided for each scan direction to monitor both scan movements independently. If the measuring light sweeps over a contoured area during the scanning process, two or more light sensors can be arranged around this area. For example, several light sensors could be positioned at the edge of a light-exit window of the device.

[0019] To prevent a faulty scanning device from emitting measuring light in a single direction, various measures can be taken. One option is to actively close an aperture that the measuring light must pass through during normal operation. Such a closable aperture could, for example, contain a shutter plate that is unlocked by an actuator when needed and then closes the aperture automatically under the influence of gravity.

[0020] However, it is simpler and safer to switch off the light source so that no measuring light is generated in the first place. For this purpose, the switching device can, for example, include a switching relay or a safety semiconductor switch, which is configured to interrupt the power supply to the light source depending on a control signal.

[0021] The invention can be used advantageously regardless of how the scanning device is constructed. A scanning device with an optical distribution matrix, comprising several optical switches and / or optical splitters, is particularly robust and can be manufactured cost-effectively in large quantities. This matrix is ​​configured to distribute the measuring light simultaneously or successively to several optical output waveguides. A deflection optic of the scanning device is configured to deflect the measuring light exiting the optical output waveguides so that it is emitted in different directions. Such a scanning device, which is known per se, can be used for one or both scanning directions. If scanning in two directions is required, the output waveguides must be arranged in two dimensions.

[0022] Alternatively or additionally, the scanning device can have a dispersive optical element which directs the measuring light in different directions depending on the wavelength.

[0023] Alternatively or additionally, the scanning device can include a rotatably mounted optical element with a reflective surface. Such a rotatably mounted optical element can be set into rotational oscillation by a galvanometer drive, be designed as a continuously rotating scanning prism, or be a micromirror of a micromirror array built using MEMS technology, as is known in the prior art.

[0024] Regarding the procedure, the aforementioned task is solved by a scanning method for measuring the distance to an object, which comprises the following steps: a) Generating an optical signal with a varying frequency; b) Directing measuring light in different directions, wherein the measuring light is formed by a first part of the optical signal; c) Detecting a superposition of reference light and reflected light, wherein the reference light is formed by a second part of the optical signal that is not directed in different directions, and wherein the reflected light is formed by the measuring light after it has been at least partially reflected by the object; d) Determining a distance to the object (12) from the superposition recorded in step c); e) Detecting a scan movement of the emitted measuring light with a light sensor arranged so that it is exposed to the measuring light only once per scan cycle; f) Prevent measuring light from being emitted as soon as the light sensor in step e) no longer detects any scanning movement of the measuring light.

[0025] The above remarks and advantageous embodiments mentioned for the device apply accordingly to the method. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Exemplary embodiments of the invention are explained in more detail below with reference to the drawings. These show: Fig. 1 a schematic side view of a vehicle approaching an object that is detected by a measuring device according to the invention; Fig. 2 a top view of the in the Fig. 1 measuring device shown; Fig. 3. The construction of the measuring device according to an exemplary embodiment in a schematic representation; Fig. 4 a graph showing the frequency of the emitted optical signals as a function of time; Fig. 5 parts of the in the Fig. 3 measuring device shown with additional details in a schematic representation; Fig. 6 a graph in which the photocurrent of two photodiodes used as light sensors is plotted as a function of time. DESCRIPTION OF PREFERRED EXECUTION EXAMPLES 1. Application Example

[0027] The Fig. Figure 1 shows a schematic side view of a vehicle 10 approaching an object 12, which is located in the Fig. 1. The vehicle 10 is equipped with at least one measuring device 14, which uses light beams L11, L21, L31, and L41 to scan the area in front of the vehicle 10, from which a three-dimensional image of the surroundings is calculated. The measuring device 14 also determines the relative velocity to the object 12. This information is particularly important if the object 12 is another vehicle or an animal that is also moving.

[0028] The information about the vehicle 10's surroundings, as determined by the measuring device 14, can be used, for example, to assist the driver of the vehicle 10 in controlling the vehicle by generating warning messages when a collision between the vehicle 10 and the object 12 is imminent. If the vehicle 10 is driving autonomously, the information about the surroundings is required by the control algorithms that control the vehicle 10.

[0029] As in the Fig. As can be seen, the scanning device 14 emits the light beams L11 to L41 in a vertical plane (in the Fig. 1 (this is the paper plane) in different directions, thus scanning the surroundings vertically. Simultaneously, scanning also takes place in a horizontal direction, as described by the Fig. Figure 2 shows a top view of the measuring device 14. Four light beams L11, L12, L13 and L14 are shown, emitted in different directions in a horizontal plane.

[0030] For the sake of clarity, the following is included in the Fig. 1 and Fig. 2 assumes that only four light beams Ln1 to Ln4 each in four different planes, i.e., a total of 16 light beams, are generated by the scanning device 14. Preferably, the measuring device 14 emits many more light beams. Preferably, for example, k·2 n Light rays, where n is a natural number between 7 and 13 and indicates how many rays are emitted in one of k planes, where k is a natural number between 1 and 16. Depending on the technology used, the different light rays Ln1 to Ln4 can be emitted successively or at least partially simultaneously. 2. Measuring device

[0031] Fig. Figure 3 schematically shows the setup of the measuring device 14 according to an embodiment of the invention. The measuring device 14 is designed as a LiDAR system and comprises an FMCW light source 16 which, during operation of the measuring device 14, emits measuring light with a varying frequency f. chirp generated. How the Fig. 4 illustrates how the frequency f varies (“chirps”) chirp periodically over time t between a lower frequency f l and a higher frequency f h The center frequency of the measuring light is 1550 nm, and therefore in the infrared spectral range.

[0032] Each measurement interval with a chirp duration T is divided into two halves of equal length T / 2. During the first interval, the frequency f increases. chirp linear with a constant and positive upchirp rate r chirp , dh df chirp / dt = r chirp During the second interval, the frequency f decreases chirplinear with a constant negative downchirp rate -r chirp , dh df chirp / dt = -r chirp The frequency of the measuring light can therefore be described by a periodic triangular function. However, other functional relationships are also possible, e.g., sawtooth functions.

[0033] How to in the Fig. 3 detects the light source 16, which is connected to a splitter 22 that divides the optical signals generated by the light source 16 into two parts. A smaller part of the optical signals is separated and is called the reference light or local oscillator. The remaining part of the optical signals, subsequently referred to as the measuring light, is first amplified in an optical amplifier 24 and then passes to an optical circulator 26, which directs the amplified measuring light to a scanning device 28. An optical circulator has at least three terminals and the property that light entering at one terminal exits at the next. Instead of the circulator, a 2x2 coupler, for example, can also be used, but this leads to higher light losses.

[0034] The scanning device 28 directs the measuring light 29 onto the object 12 - in Fig. 3 represented by a moving car - along different directions, as shown above with reference to the Fig. 1 and Fig. As explained in section 2, several measuring beams can be emitted simultaneously in different directions. Typically, the measuring light emitted by the scanning device 28 is at least partially diffusely reflected by the object 12. A small portion of the reflected light returns to the measuring device 14, where it can be coupled back into the scanning device 28.

[0035] The optical circulator 26 directs the coupled reflected light onto a combiner 30, which superimposes the reference light, previously separated from the optical signals by the splitter 22, with the coupled reflected light. Since the frequencies of the superimposed light components differ slightly from one another, a beat signal is generated, which is detected by a detector 32, preferably a symmetrical photodetector. The electrical signals generated by the detector 32 are fed to a processing unit 34, which calculates the distance R to the object and the relative velocity v between the scanning device 14 and the object 12 from the analysis of the beat frequencies.

[0036] Preferably, some or all of the components described above are implemented as photonic integrated circuits (PICs). This allows for a very compact design, high mechanical robustness, and low unit costs in mass production.

[0037] The measuring device 14 also includes a monitoring device, which comprises a light sensor 36 (schematically indicated by 36) and a shut-off device 38. The light sensor 36 is arranged in the light path of the measuring light behind the scanning device 28 and in front of an exit window 42 of the measuring device 14 and has the task of detecting the scanning movements of the emitted measuring light 29.

[0038] The light sensor 36 is connected via a data line 44 (indicated by dotted lines) to the shutdown device 38, which comprises an electronic control unit 46 and a switching relay 48. The switching relay 48 is connected between the light source 16 and a power source 50, which supplies power to the light source.

[0039] If the light sensor 36 no longer detects any scanning movements of the measuring light 29, the control unit 46 generates a control signal for the switching relay 48, which then immediately interrupts the power supply to the light source 16. This ensures that the measuring light 29 is not emitted in one direction (or, in the case of a multi-channel measuring device 14, in several fixed directions) for an extended period of time, which could lead to eye damage in persons in the vicinity of the measuring device 14.

[0040] The Fig. Figure 5 shows the scanning device 28 in a simplified schematic representation. In this embodiment, the scanning device 28 comprises a distribution matrix M in which several optical switches S11, S21, and S22 are arranged in a tree-like configuration. Using the optical distribution matrix M, measuring light from an input 56 of the distribution matrix M can be successively distributed to several output waveguides 58. For the sake of clarity, the optical distribution matrix M in the illustrated embodiment has only three optical switches S11, S21, and S22, so that a total of four output waveguides 58 can be controlled. In actual measuring devices 14, eight or more switching levels can be arranged in series, so that, for example, 256 output waveguides 58 can be selectively connected to the input 56.

[0041] In other embodiments, the distribution matrix M is located before the amplifier 24 or between the amplifier 24 and the circulator 26. This is particularly advantageous when several optical signals are to be emitted simultaneously by supplying optical signals in parallel to several distribution matrices. Alternative configurations for integrating distribution matrices into the measuring device 14 can be found in European patent application EP 20176355.4 and DE 10 2020 110 142 A1.

[0042] The output waveguides 58 terminate in free-space couplers 60, which couple the measurement light guided in the output waveguides 58 into free space. Such couplers are known in the prior art and can, for example, be designed as grid couplers, which have a widening waveguide section to which a grid structure is attached. Alternatively, the free-space couplers 60 can be edge couplers, which have a higher coupling efficiency than grid couplers.

[0043] In the Fig. Figure 5 shows that the measuring light beams emerging divergently from the free-space couplers 60 are collimated by a deflecting optic 64 and emitted in different directions. The further a free-space coupler 60 is from an optical axis 62 of the deflecting optic, the larger the angle at which the collimated measuring light is emitted by the deflecting optic 64.

[0044] In the illustrated embodiment, it is assumed that in a (at least approximately) horizontal plane, which is aligned with the plane of the paper, Fig. 5 coincides, is scanned. In order to also in the vertical plane perpendicular to it, i.e. perpendicular to the plane of the paper, the Fig. 5. To be able to scan, for example a rotating reflective optical element may be provided (not shown), as is known in the prior art.

[0045] In the illustrated embodiment, the scanning device 28 also serves to receive the optical signals reflected from the object 12 and to couple them back into the output waveguides 68 via the free-space couplers 60. In other embodiments, the reflected signals can be received by separate free-space couplers 60 and fed to the detector 32 via separate waveguides.

[0046] In this embodiment, the monitoring device has a total of four light sensors, two of which are located in the Fig. 5 are identifiable and designated 36a, 36b. The light sensors 36a, 36b can, for example, be designed as photodiodes sensitive to the infrared measuring light 29. The four light sensors 36a, 36b are arranged at the edges of a rectangular scan field, which is swept over by the measuring light 29 during a scan. For this purpose, the light sensors 36a, 36b can, for example, be attached to the edge of the rectangular exit window 42 of the measuring device 14.

[0047] The opposing light sensors 36a and 36b detect the proper scanning process in the horizontal plane. During the switching of the distribution matrix M, the measuring light beams pivot periodically back and forth, as described in the Fig. 5 is indicated. This fast scan movement is superimposed on a slower scan movement in the vertical direction.

[0048] The light sensors 36a, 36b are positioned such that at the reversal points of the horizontal scan movement they are each briefly exposed to a portion of the measuring light 29 and then each generate a short electrical measurement signal, which is monitored by the control unit 46. During a proper scan, the measurement signals generated by the light sensors 36a, 36b recur regularly with a period P, as shown by the graph of the Fig. Figure 6 illustrates the photocurrent I generated by light sensors 36a and 36b. phThe time t is plotted as a solid or dashed line. If the control unit 46 detects that after one period P one of the light sensors 36a, 36b no longer receives a measurement signal, the control unit 46 assumes that the scanning process is disrupted and the measuring light is only emitted in one direction.

[0049] The control unit 46 then generates the control signal for the switching relay 48 as described above, in order to immediately interrupt the power supply to the light source 16.

Claims

[1] Device (14) for scanning the distance to an object (12), with a light source (16) which is configured to produce an optical signal with a varying frequency, a scanning device (28) which is configured to direct measuring light (29) in different directions, wherein the measuring light (29) is formed by a first part of the optical signal generated by the light source, a detector (32) configured to detect a superposition of reference light and reflected light, wherein the reference light is formed by a second part of the optical signal generated by the light source (16), which is not supplied to the scanning device (28), and wherein the reflected light is formed by the measuring light (29) after it has been at least partially reflected by the object (12), an evaluation device (34) which is configured to determine a distance to the object (12) from the superposition detected by the detector (32), a monitoring device with a light sensor (36a, 36b) which is arranged in the light path of the emitted measuring light (29) behind the scanning device (28) and is configured to detect a scanning movement of the emitted measuring light (29), wherein the light sensor (36a, 36b) is arranged such that it is exposed to the measuring light (29) only once per scan cycle, and with a shut-off device (38) connected to the light sensor (36a, 36b) and the light source (16) and configured to shut off the light source (16) or otherwise prevent the emission of measuring light (29) when the light sensor (36a, 36b) does not detect any scanning movement of the measuring light (29). [2] Device according to claim 1, wherein several light sensors (36a, 36b) are arranged around a field which is swept over by the measuring light (29) during a scanning process. [3] Device according to one of the preceding claims, wherein the shutdown device (38) comprises a switching relay (48) or a safe semiconductor switch configured to interrupt the power supply to the light source (16) depending on a control signal. [4] Device according to one of the preceding claims, wherein the scanning device an optical distribution matrix (M) comprising several optical switches (S11, S21, S22) and / or optical splitters and configured to distribute the measuring light (29) simultaneously or successively to several optical output waveguides (58), and a deflection optic (64) which is configured to deflect the measuring light emerging from the optical output waveguides (58) so that it is emitted in different directions. [5] Device according to one of the preceding claims, wherein the scanning device (28) has a rotatably mounted optical element (72) which has a reflective surface. [6] Method (14) for scanning the distance to an object (12), comprising the following steps: a) Generating an optical signal with a varying frequency; b) Directing measuring light (29) in different directions, wherein the measuring light is formed by a first part of the optical signal; c) Detecting a superposition of reference light and reflected light, wherein the reference light is formed by a second part of the optical signal that is not directed in different directions, and wherein the reflected light is formed by the measuring light after it has been at least partially reflected by the object; d) Determining a distance to the object (12) from the superposition recorded in step c); e) Detecting a scan movement of the emitted measuring light (29) with a light sensor (36a, 36b) arranged so that it is exposed to the measuring light (29) only once per scan cycle; f) Prevent measuring light (29) from being emitted as soon as the light sensor (36; 36a, 36b; 36') in step e) no longer detects any scanning movement of the measuring light.

Images