LiDAR device with a dynamic filter and method

Abstract

LIDAR device (1) for scanning a scanning angle (H, V) with at least one radiation source (2) for generating at least one electromagnetic beam (4), with a rotatable mirror (6) for deflecting the at least one electromagnetic beam (4) along the scanning angle (H, V), with a receiving unit (12) for receiving at least one incoming electromagnetic beam (10) and for deflecting the at least one incoming electromagnetic beam (10) onto at least one detector (18) and with at least one filter (14) characterized in that the at least one filter (14) is adaptable to the at least one incoming electromagnetic beam (10), wherein the generated beam has a wavelength in a non-visible infrared range, wherein the rotatable mirror (6) is configured such thatthat it deflects the generated beam along a defined horizontal scanning angle (H) and is pivotable along a rotational axis orthogonal to the horizontal scanning angle (H) to cover a vertical scanning angle (V), and wherein the at least one filter (14) is rotatably mounted and pivoted synchronously with the rotatable mirror (6) such that the angle of incidence of the incoming beam (10) with respect to the adjusted filter (14) is substantially 0°, so that the incoming beam can pass through the filter and enter the receiving unit (12).

Description

[0001] The invention relates to a LIDAR device for scanning a scanning angle and a method for scanning a scanning angle with a LIDAR device. State of the art

[0002] Typical LiDAR (Light Detection and Ranging) devices consist of a transmitter and a receiver. The transmitter generates and emits electromagnetic radiation continuously or in pulses. When this electromagnetic radiation encounters a moving or stationary object, it is reflected back towards the receiver. The receiver can detect the reflected electromagnetic radiation and assign a reception time to it. This can be used in a time-of-flight analysis to determine the distance between the object and the LiDAR device. Depending on the application, high demands are placed on signal quality. This is particularly true when the LiDAR device needs to have the longest possible range combined with a wide scanning angle.Signal quality determines, among other things, the distance, angle, and accuracy (or probability) at which objects can be detected. This signal quality largely depends on the quality of the optical filtering of the received reflected radiation. Crucially, this depends on the width of the spectral bandpass filter. The narrower the filter's spectral bandwidth, the less interference or ambient light reaches the detector, and the better the signal quality. Since this transmission range shifts to shorter wavelengths with increasing angle of incidence of the received radiation, the filter must have a certain width to transmit the received radiation even at large angles. Currently, the angle-dependent shift of the filter's transmission window represents a physical limitation of LiDAR devices.

[0003] Document DE 10 2009 023 066 A1 discloses a self-propelled device, in particular a floor dust collection device, with obstacle detection based on optical measurement methods such as phase correlation, time-of-flight measurement or heterodyne methods.

[0004] Document DE 20 2012 010 014 U1 discloses a laser scanner that emits polarized light and evaluates the orthogonal and parallel polarization components of the reflected light for reliable object recognition even under adverse outdoor conditions.

[0005] Document US 2008 / 0285010 A1 discloses an object detector system in which the vertical extent of a horizontally scanning detection beam is changed depending on the horizontal scanning direction.

[0006] Document US 2014 / 0240721 A1 discloses an object detector system that uses a controllable mask in an image plane to selectively direct light from an illuminated area of ​​a scene to a detector.

[0007] Document US 2016 / 0084945 A1 discloses a remote sensing system that determines the frequency of reflected light by systematically varying either the frequency of the emitted light beam or the center frequency of a narrowband filter.

[0008] Document JP 2016-217971 A discloses a laser measuring device with a rotatable bandpass filter whose transmission wavelength depends on its rotation angle, and with a counter-rotating correction plate that compensates for a beam displacement caused by the filter.

[0009] Document US 2014 / 0125990 A1 discloses an optical measuring system with a tunable filter unit designed as a Fabry-Perot interferometer with at least two mirror elements, wherein the optical thickness of the cavity between the mirror elements is variable to adjust the transmitted wavelength range.

[0010] Document US 2018 / 0081045 A1 shows a LADAR system with a dynamic receive filter whose transmitted central wavelength is adapted to a Doppler-shifted wavelength of a laser signal returning from a moving target. Disclosure of the invention

[0011] The object underlying the invention can be seen as creating a method and a LIDAR device which has at least one filter which always has an optimal transmission characteristic despite a varying angle of incidence of an incident beam.

[0012] This problem is solved by means of the respective subject matter of the independent claims. Advantageous embodiments of the invention are the subject matter of dependent claims.

[0013] According to one aspect of the invention, a LiDAR device is provided for scanning a scanning angle. The LiDAR device comprises at least one radiation source for generating at least one electromagnetic beam and a rotatable mirror for deflecting the at least one electromagnetic beam along the angle. Furthermore, the LiDAR device comprises a receiving unit for receiving at least one incoming electromagnetic beam and for deflecting the at least one incoming electromagnetic beam onto at least one detector and at least one filter, wherein the at least one filter is adaptable to the at least one incoming electromagnetic beam.

[0014] Such a lidar device incorporates a dynamic optical filter that can compensate for the angle-of-incidence wavelength shift of an incoming beam. Particularly at larger angles of incidence, the filter's transmission range for a specific wavelength can shift towards shorter wavelengths. To prevent an incoming electromagnetic beam from passing through the filter only partially or not at all at larger angles of incidence, the filter can be adjusted. This can be achieved, for example, by changing the filter's position or by modifying at least one of its material properties. This allows the filter's transmission range to be dynamically or statically adjusted or tracked. Alternatively or additionally, the entire receiver unit can be adjustable.The filter can be, for example, a dielectric filter with one or more defined transmission ranges. These transmission ranges refer to a wavelength or frequency of an electromagnetic beam. The electromagnetic beam can be, for example, a laser beam or a light beam in the visible or invisible wavelength range.

[0015] According to one embodiment of the LIDAR device, the at least one filter is rotatable along the scanning angle. This allows the filter to be rotated or rotatably mounted, enabling its orientation to be changed. In particular, this measure allows the angle of incidence of an incoming beam relative to the filter to be optimally adjusted. Thus, the wavelength of the incoming beam can always lie within at least one transmission range of the filter and pass through the filter with minimal loss. An optimal angle of incidence is ideally 0°. However, the angle of incidence can deviate from 0° depending on the transmission characteristics of the filter and the wavelength shift of the incoming beam. The change in the filter's orientation can be achieved, for example, by piezoelectric actuators, electrostatic motors, electromagnetic motors, or similar devices.

[0016] In another embodiment of the LiDAR device, the at least one filter can be rotated angularly relative to the rotatable mirror, either offset or angularly synchronized. The filter's orientation can be adjusted or adapted depending on the mirror. Depending on the requirements and design of the LiDAR device, the filter can also be rotated or pivoted independently of the mirror to deflect the generated electromagnetic beam. In this case, the filter can be adjusted, for example, over time, allowing for an angular offset between the filter and the mirror. Alternatively or additionally, the entire receiver or parts of the receiver can be rotatable or pivotable parallel to the filter.

[0017] According to another embodiment, the at least one filter is an adjustable Fabry-Perot cavity. The filter can be a conventional optical filter or an optical resonator. The Fabry-Perot cavity corresponds to an optical resonator consisting of at least two partially reflective mirrors. Depending on the distance or cavity length between the two partially reflective mirrors, only incoming electromagnetic radiation with a specific wavelength can pass through. To increase decoupling efficiency for larger angles of incidence, the partially reflective mirrors can have low reflectivity.

[0018] In another embodiment of the LIDAR device, the at least one filter has an adjustable cavity length. The distance between the two partially reflective mirrors can be changed so that the transmission wavelength of the Fabry-Perot cavity can be adjusted. For example, one or both partially reflective mirrors can be adjusted by piezoelectric actuators or electrostatic or electromagnetic linear actuators. By adjusting the partially reflective mirrors, the cavity length can be changed or adjusted, and thus also the transmission range.

[0019] In another embodiment, the cavity length is adjustable depending on the orientation of the rotatable mirror. Here, the cavity length of the Fabry-Perot cavity can be reduced or increased according to the deflection of the mirror to deflect the generated beam. The reflected electromagnetic beam has a similar angle of incidence to the orientation of the mirror. This allows the transmission range to be adapted to the angle of incidence. In this way, it can be ensured that the incoming beam can always pass through the filter.

[0020] According to another embodiment, the at least one filter has an adjustable refractive index. A change in the refractive index(s) can be achieved, for example, by dynamically aligning liquid crystals using electric or magnetic fields. Since, in addition to the wavelength shift of the incoming beam, the transmission range of the filter can depend on its temperature, this effect can be used to adjust the transmission range or the refractive index. In particular, the refractive index depends on the density of the material and thus also on the temperature. Therefore, the refractive index can also be adjusted by the temperature of the filter or a part of the filter. For example, in a Fabry-Perot cavity, the partially transparent mirrors are vapor-deposited or applied to a glass or transparent substrate.This substrate material can undergo a change in its refractive index when exposed to temperature and / or electric or magnetic fields. In a standard filter, the refractive index can also influence the transmission range. Therefore, the filter, or at least a portion of it, can be cooled or heated to compensate for undesirable effects or to adjust the filter's performance. The temperature can be lowered, for example, by air or water cooling. Similarly, the filter can be heated by heated water or air. Alternatively, the filter can be heated by an electrically conductive layer or coating. In this way, one or more glass elements of the filter can be heated by Joule heating.

[0021] According to another embodiment of the LIDAR device, the refractive index is adjustable depending on the orientation of the rotatable mirror. Advantageously, the refractive index is adjusted such that the transmission range of the filter is adapted to the wavelength shift or the angle of incidence of the incoming beam. This allows the refractive index of the filter, or a part of a filter such as glass, to be adjusted to the transmission range so that the incoming beam can pass through the filter as completely and without loss as possible.

[0022] According to another embodiment, at least two filters for filtering the at least one incoming electromagnetic beam can be arranged at an angle to each other. Alternatively or additionally to the examples described, a plurality of filters arranged at an angle to each other can be used. This allows each filter to react less dynamically to an incoming beam, since the angle of incidence can no longer be as large as with a single static filter. Thus, the angle of incidence of the incoming beam relative to a filter is progressively reduced by an increasing number of static or dynamic filters, which are, for example, arranged in a semicircle. This allows each filter to be adjusted or changed depending on or independently of the angle of incidence or at least one neighboring filter.Alternatively or additionally, all or some of the filters can be regular optical filters that do not have dynamic adjustment.

[0023] According to a further embodiment, the at least one filter for filtering the at least one incoming electromagnetic beam has a curvature. Advantageously, a filter can be used that has a curvature which covers at least a portion of the scanning angle of the LIDAR device and thus covers an angular range within which an incoming reflected beam can strike the filter. The filter can be oriented such that, regardless of the angle of incidence of the incoming beam, the incoming beam always strikes the filter perpendicularly. The filter can additionally feature dynamic adjustment of the refractive index, for example, by changing the temperature. Alternatively, such a filter can have a curvature with a varying radius of curvature. In this case, it can be advantageous to move the filter along at least one length to adapt the filter to an incoming beam.

[0024] According to a further aspect of the invention, a method for scanning a scanning angle with a LIDAR device is provided. In one step, at least one electromagnetic beam is generated and deflected along the scanning angle. The at least one deflected electromagnetic beam can be reflected by an object positioned within the scanning angle. The at least one reflected electromagnetic beam becomes the at least one incoming beam and is received and filtered. Subsequently, the at least one incoming beam is detected, with at least one filter being adapted according to a wavelength and / or an angle of incidence of the at least one incoming beam. This allows the filter to be dynamically changed. In particular, the filter can be adapted to a wavelength of the incoming beam depending on an angle of incidence of the incoming beam.This allows the filter to be moved or rotated so that an incoming beam strikes it at the smallest possible angle of incidence relative to the filter. Alternatively or additionally, the filter's material properties can be adjusted. In the case of an optical resonator as a filter, the cavity length can be dynamically varied to provide a transmission range adapted to the wavelength of the incoming beam. This adaptation can be performed continuously according to a sampling rate of the process.

[0025] In the following, preferred embodiments of the invention are explained in more detail with reference to highly simplified schematic representations. These show Fig. 1 a schematic representation of a LIDAR device according to a first embodiment, Fig. 2 a schematic representation of a receiving unit of the LIDAR device according to the first embodiment, Fig. 3 a schematic representation of the receiving unit of the LIDAR device according to a second embodiment, Fig. 4 a schematic representation of the receiving unit of the LIDAR device according to a third embodiment, Fig. 5a,b a schematic representation of the receiving unit of the LIDAR device according to a fourth embodiment, Fig. 6 a schematic representation of the receiving unit of the LIDAR device according to a fifth embodiment and Fig. 7 a schematic representation of the receiving unit of the LIDAR device according to a sixth embodiment.

[0026] In the figures, the same constructive elements each have the same reference numerals.

[0027] Fig. Figure 1 shows a first embodiment of a LIDAR device 1. The LIDAR device 1 has a radiation source 2 for generating an electromagnetic beam 4. According to this embodiment, the radiation source 2 is a laser 2. The laser 2 is used to generate a beam 4 with a wavelength in the non-visible infrared range. The wavelength can, for example, be greater than 800 nm. The beam 4 generated by the laser 2 is deflected by a rotatable mirror 6. The mirror 6 is pivotable along a rotational axis R. Thus, the mirror 6 can deflect the generated beam 4 along a defined horizontal scanning angle H. Additionally, the mirror 6 is pivotable orthogonally to the horizontal scanning angle H and thus covers a vertical scanning angle V. This allows the LIDAR device 1 to scan a solid angle V×H and locate any objects 8 positioned within this solid angle V×H.The generated beam 4 is at least partially reflected by the object 8 and becomes the reflected or incoming beam 10. The incoming beam 10 is received by a receiving unit 12.

[0028] In the Fig. Figure 2 is a schematic representation of the receiver unit 12 of the LIDAR device 1 according to the first embodiment. For illustrative purposes, objects 8 are also shown. The receiver unit 12 is shown in an xy-plane. The axis of rotation R runs orthogonally through the xy-plane. The receiver unit 2 has a filter 14 that preferably allows the incoming beam 10 to pass through while blocking stray light or reflections. Since the transmission range of such filters 14 shifts towards shorter wavelengths with increasing angle of incidence, either the transmission range must be large or the possible angles of incidence β must be small. The angle of incidence β includes both a horizontal scanning angle component H and a vertical scanning angle component V. This component can also be 0°.According to the exemplary embodiment, the filter 14 is rotatably mounted and is rotated or periodically pivoted by a piezoelectric actuator (not shown) synchronously with the mirror 6 along an axis of rotation parallel to the axis of rotation R. This allows a large horizontal scanning angle H to be scanned. In particular, the filter 14 is tracked such that an incoming beam 10 strikes the filter 14 as perpendicularly as possible. This allows a narrow transmission range of the filter 14 to be selected, since there is no or only a negligible angle-dependent change in the wavelength of the incoming beam 10. The filter 14, shown with dashed lines, does not adjust its angle when an object 8 is positioned frontally or slightly offset from an optical axis A. For an object 8 located further from the optical axis A, an incoming beam 10 has a larger angle of incidence β.Since the filter 14 is pivoted synchronously with the mirror 6, the angle of incidence β relative to the optical axis A is large, for example greater than 20°, but relative to the tracking filter 14, the angle of incidence β is 0°. The incoming beam 10 can thus transmit through the filter 14 and reach a receiving optic 16. The receiving optic 16 directs the incoming beam 10 onto a detector 18. The detector 18 registers the incoming beam 10 and assigns it, for example, a reception time and a scanning angle H, V of the mirror 6.

[0029] The Fig. Figure 3 shows a schematic representation of the receiver unit 12 of the LIDAR device 1 according to a second embodiment. In contrast to the first embodiment, the receiver unit 12 has an adjustable filter 14, which consists of a Fabry-Perot cavity 20. The Fabry-Perot cavity 20 has two semi-transparent mirrors 22, 24. Each of the semi-transparent mirrors 22, 24 consists of a glass substrate 26 and a semi-transparent coating 28. A first semi-transparent mirror 22 is stationary and cannot be repositioned. A second semi-transparent mirror 24 is displaceable by an actuator (not shown). Thus, by displacing the second semi-transparent mirror 24, the cavity length 30, or the distance between the two semi-transparent mirrors 22, 24, can be changed.Incoming rays 10 can pass through the Fabry-Perot cavity 20 if they have a specific wavelength relative to the cavity length 30. Consequently, by adjusting the cavity length 30, a transmission range for incoming rays 10 with a specific wavelength can be created. For example, to allow incoming rays 10 with a shorter wavelength to pass through the Fabry-Perot cavity 20, the cavity length 30 must be reduced. This is illustrated by the second partially reflective mirror 24, shown with a dashed line, in its original position.

[0030] In the Fig. Figure 4 shows a schematic representation of the receiver unit 12 of the LIDAR device 1 according to a third embodiment. In addition to the features of the second embodiment, the receiver unit 12 has a fan 32 that can cool the Fabry-Perot cavity 20. Furthermore, a heating element 34 is connected downstream of the fan 32 to heat the airflow generated by the fan 32. The arrows illustrate the airflow generated by the fan 32. In particular, the temperature of the semi-transparent mirrors 22, 24 is set by the heated airflow. Additionally, the air or a fluid between the semi-transparent mirrors 22, 24 can also be heated to the temperature of the airflow. This adjusts the density of the fluid or the semi-transparent mirrors 22, 24.Since the refractive index of the components of the Fabry-Perot cavity 20 depends on the density, the refractive index can be adjusted by changing the temperature. Thus, the transmission range of the Fabry-Perot cavity 20 can be adjusted by setting the temperature or adapted to an incoming beam 10.

[0031] The Fig. 5a and Fig. Figure 5b shows a schematic representation of the receiving unit 12 of the LIDAR device 1 according to a fourth embodiment. In contrast to the embodiments already mentioned, the receiving unit 12 is mounted to rotate as a whole along the axis of rotation R and can be tracked by an actuator (not shown) according to the deflection of the mirror 6 and thus according to an angle of incidence β of the incoming beam 10.

[0032] In the Fig. Figure 6 shows a schematic representation of the receiver unit 12 of the LIDAR device 1 according to a fifth embodiment. In contrast to the previous embodiments, the receiver unit 12 has three stationary filters 14. The filters 14 are arranged at a relative angle to each other. According to this embodiment, the filters 14 are arranged approximately in a semicircle with the axis of rotation R as one center point. As a result, incoming rays 10 have a small relative angle of incidence β to the respective filters 14. In this case, the incoming rays 10 strike a filter 14 angled relative to the optical axis A at a large angle of incidence β relative to the optical axis A.

[0033] In the Fig.Figure 7 shows a schematic representation of the receiving unit 12 of the LIDAR device 1 according to a sixth embodiment. In contrast to the sixth embodiment, the receiving unit 12 has a filter 14 which has a curvature. The filter 14 is a single piece and has such a curvature that incoming rays 10 have a relative angle of incidence β of 0° relative to the filter 14.

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