LiDAR DEVICE AND CONTROL METHOD FOR LiDAR DEVICE

A LiDAR device according to an embodiment includes a rotating mirror having reflective surfaces, first/second light emitters each emitting light toward the rotating mirror, and first/second light receivers each receiving light reflected by the rotating mirror and converting the received light into an electrical signal. The first light emitter emits light in an orientation where an upper section of a distance measurement range is scanned. The second light emitter emits light in an orientation where a lower section of the distance measurement range is scanned. The first light receiver is provided at a position where light emitted by the first light emitter and reflected at the distance measurement range is received via the rotating mirror. The second light receiver is provided at a position where light emitted by the second light emitter and reflected at the distance measurement range is received via the rotating mirror.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2022-044475, filed on Mar. 18, 2022, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a LiDAR device and a control method for a LiDAR device.

BACKGROUND

Devices called LiDAR (light detection and ranging or laser imaging detection and ranging) devices are known. The LiDAR device is a device that calculates the distance to a distance measurement target by irradiating a distance measurement range with pulsed light (for example, laser light) emitted by a light source, capturing light (reflected light or scattered light) reflected (or scattered) by an object (as the distance measurement target) present in the distance measurement range, and measuring the time (round trip time of light) from emission to reception of light.

LiDAR devices are used in various technologies such as automated driving. The LiDAR device determines the position or even the shape of a distance measurement target by, for example, scanning the surface of the target with light.

The LiDAR device scans a distance measurement target with light by reflecting light (for example, laser light) with a mirror and changing the orientation of the mirror. When the scanning direction is a lateral direction (substantially horizontal direction), the resolution in the longitudinal direction (substantially vertical direction) depends on the number of pixels arranged in the longitudinal direction of a light-receiving element.

Possible methods to increase the resolution in the longitudinal direction (substantially vertical direction), that is, the vertical resolution in the LiDAR device are, for example, increasing the number of pixels of the light-receiving element, reducing the pixel size, or using, as the mirror, a polygon mirror having a plurality of mirror surfaces with different vertical tilt angles.

However, with such methods, it is difficult to avoid increase in costs for manufacturing new light-receiving elements or mirrors. Using a polygon mirror as the mirror incurs size increase and thereby reduces vibration resistance performance and rigidity.

Moreover, increasing the number of pixels not only incurs size increase of the light-receiving element but also causes the need for increasing the focal length of a lens for forming an image on the light-receiving element and inevitably increases the size of the device. Reducing the pixel size reduces the amount of light received by each pixel and therefore requires an imaging optical system (including lenses, etc.) that does not deteriorate the performance, thereby making the device expensive.

DETAILED DESCRIPTION

A LiDAR device according to an embodiment includes a rotating mirror, light emitters, and light receivers. The rotating mirror is driven to rotate around an axis of rotation oriented longitudinally. The light emitters are each configured to emit light toward the rotating mirror. The light receivers are each configured to receive light reflected by the rotating mirror and convert the received light into an electrical signal. The rotating mirror includes reflective surfaces reflecting light. Each of the reflective surfaces is configured to, with rotation of the rotating mirror, cause light emitted by a corresponding one of the light emitters to scan a distance measurement range in a lateral direction, and cause light reflected from the distance measurement range to travel toward a corresponding one of the light receivers. The light emitters include a first light emitter and a second light emitter. The first light emitter is configured to emit light in an orientation where an upper section of the distance measurement range is scanned. The distance measurement range is divided into the upper section and a lower section. The second light emitter is configured to emit light in an orientation where the lower section of the distance measurement range is scanned. The first light emitter is provided at a position facing one of the reflective surfaces. The second light emitter is provided at a position facing another one of the reflective surfaces whose orientation is different from the one of the reflective surfaces to which the first light emitter faces. The light receivers include a first light receiver and a second light receiver. The first light receiver is provided at a position where light emitted by the first light emitter and reflected at the distance measurement range is received via the rotating mirror. The second light receiver is provided at a position where light emitted by the second light emitter and reflected at the distance measurement range is received via the rotating mirror.

Embodiments of a LiDAR device will be described with reference to the drawings. In this specification, components according to embodiments and explanation of the components may be described in multiple expressions. The components and the description thereof are by way of example and are not intended to be limited by the expressions in the specification. The components may also be identified by names different from those in the specification. The components may also be explained by expressions different from the expressions in the specification.

First Embodiment

FIG.1Ais a plan view illustrating scanning of a distance measurement target by a LiDAR device100.FIG.1Bis a side view illustrating scanning of a distance measurement target by the LiDAR device100. The LiDAR device100is installed in, for example, a self-driving car to measure various targets, such as roads, buildings, pedestrians, other vehicles, and obstacles. Note that “LiDAR” is an acronym for light detection and ranging or laser imaging detection and ranging.

The LiDAR device100calculates the distance to the distance measurement target by irradiating a distance measurement range20with light30emitted by a light source, detecting light (reflected or scattered light) reflected (or scattered) by an object (distance measurement target)21present in the distance measurement range20, and measuring the time (round trip time of light) from emission to reception of light. The distance measurement range20is the “field of view” of the LiDAR device100.

The light30emitted by the LiDAR device100is linear in a planar view (seeFIG.1A). Additionally, the light30diverges in the longitudinal direction (substantially vertical direction) in a side view (seeFIG.1B). The LiDAR device100scans the rectangular distance measurement range20in the lateral direction (substantially horizontal direction) while illuminating it in the longitudinal direction by moving radiation light as depicted by arrow A inFIG.1Ato determine the position and the shape of the distance measurement target21.

FIG.2is a side view illustrating scanning by a LiDAR device101according to a first embodiment. The LiDAR device101in the present embodiment scans two divided sections of the distance measurement range20: an upper section and a lower section, individually, rather than simultaneously. With this configuration, the LiDAR device101substantially doubles the vertical resolution compared to the conventional one while using a light-receiving element with the number of pixels and pixel size equivalent to the conventional ones.

A configuration of the LiDAR device101will now be described.FIG.3Ais a plan view illustrating a structure of the LiDAR device101in the first embodiment.FIG.3Bis a front view illustrating a structure of the LiDAR device101in the first embodiment. The structure of the LiDAR device101is not limited to the example illustrated inFIG.3AandFIG.3B.

The LiDAR device101includes a light source1, a collimator lens2, a rotating stand3, a double-sided mirror4, an imaging lens5, and a line sensor6, and further includes a light source11, a collimator lens12, an imaging lens13, and a line sensor14. The LiDAR device101may further include other parts and devices.

The light sources1and11and the collimator lenses2and12are examples of light emitters each emitting light toward the double-sided mirror4. The light sources1and11are, for example, laser diodes (LD) capable of pulsed emission. The light source1operates under control of a control unit110(described below) of the LiDAR device101to emit light (for example, laser light)31. Similarly, the light source11operates under control of the control unit110(described below) to emit light (for example, laser light)32. The light31and32is, for example, visible light. The light31and32may be infrared rays, ultraviolet rays, or X-rays.

The collimator lens2collimates (converts light into parallel light) the light31emitted from the light source1and incident on the collimator lens2, and emits the collimated light toward the double-sided mirror4. In other words, the collimator lens2converts light passing through the collimator lens2into light focused at infinity.

Similarly, the collimator lens12collimates the light32emitted from the light source11and incident on the collimator lens12and emits the collimated light toward the double-sided mirror4. In other words, the collimator lens12converts light passing through the collimator lens12into light focused at infinity.

The double-sided mirror4and the rotating stand3constitute an example of a rotating mirror that is driven to rotate around an axis of rotation oriented longitudinally (for example, substantially vertical direction). The rotating stand3rotates the double-sided mirror4around the axis of rotation along the substantially vertical direction. The axis of rotation is the virtual central axis of rotation of a reflective surface. The double-sided mirror4has, for example, a rectangular shape and is provided upright on the rotating stand3with its lengthwise direction along the substantially vertical direction. The double-sided mirror4is a mirror having substantially planar reflective surfaces on both sides to reflect laser light. With rotation of the double-sided mirror4, each of the reflective surfaces causes light emitted by the light sources1and11to scan the distance measurement range20in the lateral direction (for example, in the substantially horizontal direction) and causes light reflected from the distance measurement range20to travel toward the line sensors6and14.

In the present embodiment, as illustrated inFIG.3B, the optical axis of the light source1and the axis of rotational symmetry of the collimator lens2are tilted upward with respect to a virtual plane orthogonal to the axis of rotation of the double-sided mirror4and the rotating stand3. With this configuration, the light source1and the collimator lens2constitute an example of a first light emitter to emit the light31in an orientation where the upper section of the distance measurement range20, which is divided into upper and lower sections, is scanned.

Similarly, the optical axis of the light source11and the axis of rotational symmetry of the collimator lens12are tilted downward with respect to a virtual plane orthogonal to the axis of rotation of the double-sided mirror4and the rotating stand3, as illustrated inFIG.3B. With this configuration, the light source11and the collimator lens12constitute an example of a second light emitter to emit the light32in an orientation where the lower section of the distance measurement range20is scanned.

The first light emitter including the light source1and the second light emitter including the light source11are provided at positions where they do not simultaneously irradiate the same reflective surface of the double-sided mirror4. In other words, the first light emitter is provided at a position facing one of the reflective surfaces of the double-sided mirror4, while the second light emitter is provided at a position facing another one of the reflective surfaces whose orientation is different from the one of the reflective surfaces to which the first light emitter faces. The first light emitter and the second light emitter are a pair of light emitters that emit light in two different directions and irradiate the double-sided mirror4in two different directions. The double-sided mirror4is disposed at a position where light emitted in two different directions by the pair of light emitters can be received.

The imaging lens5and the line sensor6are an example of a light receiver and also constitute a first light receiver that receives light reflected by the double-sided mirror4and converts the received light into an electrical signal. The imaging lens5and the line sensor6receive the light31, which is emitted by the first light emitter and reflected at the distance measurement range20, via the double-sided mirror4. Similarly, the imaging lens13and the line sensor14are an example of a light receiver and also constitute a second light receiver that receives light reflected by the double-sided mirror4and converts the received light into an electrical signal. The imaging lens13and the line sensor14receive the light32, which is emitted by the second light emitter and reflected at the distance measurement range20, via the double-sided mirror4.

The imaging lens5collects the light31, which is reflected by the distance measurement target21and incident via the double-sided mirror4, and forms an image on the line sensor6. Similarly, the imaging lens13collects the light32, which is reflected by the distance measurement target21and incident via the double-sided mirror4, and forms an image on the line sensor14(an image22formed on the line sensor14is schematically illustrated inFIG.2).

The line sensors6and14are each examples of a light-receiving element having a plurality of pixels aligned in the longitudinal direction. The line sensors6and14detect the received light for each pixel and generate and output an electrical signal corresponding to the received light. The arrangement of the light-receiving element is not limited to this example.

In the LiDAR device101with such a configuration described above, light emitted from the light source1has a light distribution shaped by the collimator lens2and is reflected by the upper portion of the rotating double-sided mirror4to scan the distance measurement range20in the substantially horizontal direction. The reflected light from the distance measurement target21is reflected by the lower portion of the double-sided mirror4and forms an image on the line sensor6by the imaging lens5.

Similarly, light emitted from the light source11has a light distribution shaped by the collimator lens12and is reflected by the upper portion of the double-sided mirror4to scan the distance measurement range20in the substantially horizontal direction. The reflected light from the distance measurement target21is reflected by the lower portion of the double-sided mirror4and forms an image on the line sensor14by the imaging lens13.

FIG.4is a block diagram illustrating an electrical configuration example of the LiDAR device101in the first embodiment. The LiDAR device101further includes a control unit110and a motor18.

The motor18is driven to rotate the rotating stand3, and thereby the double-sided mirror4rotates around the axis of rotation in the longitudinal direction (substantially vertical direction). With this rotation, the light31and32emitted by the light sources1and11scans the distance measurement range20in the lateral direction (substantially horizontal direction).

The control unit110is, for example, a computer including a processor such as a central processing unit (CPU), a storage device such as a read only memory (ROM), a random access memory (RAM), and a flash memory, and a bus connecting them. The control unit110is electrically connected to the light sources1and11, the line sensors6and14, and the motor18.

The processor in the control unit110executes a computer program read from the ROM or the flash memory to function as a drive control unit111and a post-processing unit112.

The post-processing unit112performs various processing on the basis of output of the line sensors6and14. For example, the post-processing unit112reconstructs an image that reflects a front view of the distance measurement target21. The post-processing unit112measures the distance to each portion of the distance measurement target21. Additionally, the post-processing unit112determines the shape of the distance measurement target21.

More specifically, the post-processing unit112calculates the shape of the target and the distance to the target on the basis of, for example, the difference between the time when the light sources1and11emit light31and32and the time when the line sensors6and14receive light31and32reflected from the target. The functional configuration of the control unit110is not limited to this example.

The drive control unit111is a functional unit that controls the light sources1and11and the motor18. More specifically, the drive control unit111appropriately synchronizes the orientation of the reflective surfaces of the rotating double-sided mirror4and the light emission timing of the light sources1and11.

In order to ensure eye safety, the light sources1and11are designed to emit light only when the light is incident on any of the reflective surfaces of the double-sided mirror4. The distance measurement range20is set on the front side of the LiDAR device101, whereas the distance on the rear side of the LiDAR device101is not measured.

The drive control unit111therefore alternately drives either the light source1or the light source11.

Supposing that the time difference set to prevent interference between the light31emitted by the light source1and the light32emitted by the light source11is A, the processing may generally be performed as described below.

FIG.5is a flowchart illustrating exemplary processing performed by the control unit110of the LiDAR device101in the first embodiment.

The control unit110functions as the drive control unit111that drives the motor18to rotate the double-sided mirror4(step S1). The control unit110then functions as the drive control unit111that causes the light source1to emit pulses at a predetermined timing (step S2), and functions as the post-processing unit112that obtains output of the line sensor6(step S3).

The control unit110then continues the above processing until the elapsed time from step S2becomes Δ (No at step S4). When the elapsed time from step S2becomes Δ (Yes at step S4), the control unit110switches the optical system to be driven. In other words, the control unit110functions as the drive control unit111that stops driving the light source1and starts driving the light source11(step S5). At step S5, the light source11emits pulses at a predetermined timing in the same manner as the light source1. Subsequently, the control unit110functions as the post-processing unit112that obtains output of the line sensor14(step S6).

The control unit110functioning as the post-processing unit112generates images, performs distance measurement, and performs AI analysis, by using the output (electrical signals) obtained from the line sensor6and the line sensor14.

The control unit110continues the above processing until the elapsed time from step S5becomes Δ (No at step S7). When the elapsed time from step S5becomes Δ (Yes at step S7), the control unit110determines whether the distance measurement is finished (step S8). In response to determining that the timing to finish the distance measurement has not been reached (No at step S8), the control unit110switches the optical system to be driven. In other words, the control unit110returns the processing to step S2.

In response to determining that the distance measurement is finished (Yes at step S8), the control unit110functions as the drive control unit111to stop the emission of the light source1or the light source11in operation (step S9), and then stop the motor18(step S10).

The LiDAR device101of the present embodiment operating described above is capable of doubling the vertical resolution, as illustrated inFIG.2, while using the light-receiving elements (line sensors6and14) equivalent to the conventional ones.

In addition, according to the present embodiment, the light sources1and11and the collimator lenses2and12are installed symmetrically on both sides across the double-sided mirror4in a tilted manner, and those on each side measures distance of the corresponding one of upper and lower sections of the field of view (distance measurement range20) of the LiDAR device101. This configuration slightly increases the size of the LiDAR device101compared with conventional devices, but does not require higher performance of the imaging lenses5and13and can measure distance of the vertical field of view as large as the conventional one at a double resolution at lower costs than other means since the configuration uses the same sensor as that of conventional devices.

Although distance measurement results similar to those of the LiDAR device101can be achieved using two conventional LiDAR devices, the LiDAR device101is preferable in that image processing is relatively easy because the two optical systems share a rotating mirror whereby the frame rates are automatically synchronized.

Other embodiments will now be described. The following embodiments are modifications to the first embodiment, so that the parts already described in the first embodiment are denoted by the same reference signs and will not be further elaborated. The parts different from those in the first embodiment will be described in detail.

Second Embodiment

FIG.6is a diagram illustrating a double-sided mirror401and the rotating stand3of a LiDAR device according to a second embodiment.FIG.7is a side view illustrating scanning by a LiDAR device102in the second embodiment.

In the LiDAR device102, one reflective surface41of the double-sided mirror401is inclined in a tilted direction with respect to the axis of rotation Ax, and the other reflective surface42is inclined in the opposite direction. The reflective surfaces41and42and the direction of incidence of light on the reflective surfaces41and42are not orthogonal but are tilted obliquely.

In the LiDAR device102with such a configuration, light emitted by the light source1and incident obliquely upward on the double-sided mirror4is further directed upward when reflected by the reflective surface41. The upward orientation is slightly canceled out when the light is reflected by the reflective surface42. Light emitted by the light source11and incident obliquely downward on the double-sided mirror4is further directed downward when reflected by the reflective surface42. The downward orientation is slightly canceled out when the light is reflected by the reflective surface41.

The angles of the reflective surfaces41and42are set on the basis of the mechanism described above, whereby the LiDAR device102is able to perform scanning with light311,312,321, and322in four directions illustrated inFIG.7. The situation illustrated inFIG.7represents that the scanning with light311emitted by the light source1and reflected by the reflective surface41and the scanning with light312emitted by the light source1and reflected by the reflective surface42are finished, the scanning with light321emitted by the light source11and reflected by the reflective surface41is in progress, and, subsequently, the scanning with light322emitted by the light source11and reflected by the reflective surface42will be performed.

The LiDAR device102in this manner can increase the vertical resolution twice higher than using the double-sided mirror4and therefore can provide a vertical resolution four times higher than conventional devices.

Modification

FIGS.8A to8Care each a diagram illustrating a modification of a rotating mirror (constituted with the double-sided mirror4and the rotating stand3) of the LiDAR device102in the second embodiment.FIG.8Ais a plan view,FIG.8Bis a front view, andFIG.8Cis a side view. The rotating mirror may be a polygon mirror. A polygon mirror402illustrated in each ofFIGS.8A to8Chas a substantially quadrangular prism shape and has four reflective surfaces43,44,45, and46. The reflective surfaces43and46are inclined obliquely upward with respect to the axis of rotation Ax, and the reflective surfaces44and45are inclined obliquely downward with respect to the axis of rotation Ax. The degree of inclination differs between the reflective surface43and the reflective surface46facing upward, and the degree of inclination also differs between the reflective surface44and the reflective surface45facing downward. In other words, the inclination states of the reflective surfaces43to46are different from each other.

The polygon mirror402in this manner can further increase the vertical resolution twice higher than using the double-sided mirror401having two reflective surfaces41and42and therefore can provide a vertical resolution eight times higher than conventional devices.

A polygon mirror having more surfaces may be used, or a polygon mirror having three surfaces may be used. However, a polygon mirror tends to become larger as the number of surfaces increases, and may involve a tradeoff between vertical resolution and size.

Third Embodiment

FIG.9is a front view illustrating a structure of a LiDAR device103according to a third embodiment. The optical axes of the light sources1and11and the axis of rotational symmetry of the collimator lenses2and12in the present embodiment are not tilted but are substantially parallel to the virtual plane orthogonal to the axis of rotation of the double-sided mirror4and the rotating stand3. The term “substantially parallel” is not limited to perfectly parallel but is intended to permit slight deviations from being perfectly parallel. In addition, the axis of rotational symmetry of the collimator lenses2and12is shifted from the optical axes of the light sources1and11by a predetermined distance in a direction along the axis of rotation of the double-sided mirror4.

This configuration can achieve generally the same effects as the LiDAR device101in the first embodiment.

Fourth Embodiment

FIG.10is a front view illustrating a structure of a LiDAR device104according to a fourth embodiment. The optical axes of the light sources1and11and the axis of rotational symmetry of the collimator lenses2and12in the present embodiment are not tilted but are substantially parallel to the virtual plane orthogonal to the axis of rotation of the double-sided mirror4and the rotating stand3. In addition, in the LiDAR device104of the present embodiment, the optical axes of the light sources1and11and the axis of rotational symmetry of the collimator lenses2and12are not shifted but are substantially matched with each other, although they are shifted in the LiDAR device103according to the third embodiment described above.

The LiDAR device104also includes a prism7in the optical path between the collimator lens2and the double-sided mirror4to correct the orientation of the optical axis of the light source1upward. The LiDAR device104further includes a prism15in the optical path between the collimator lens12and the double-sided mirror4to correct the orientation of the optical axis of the light source11downward.

The LiDAR device104also includes a prism8in the optical path between the double-sided mirror4and the imaging lens5. The prism8corrects the orientation of light incident downward obliquely from above, slightly upward. The prism8thus substantially matches the orientation of the light incident obliquely from above with the axis of the imaging lens5. Here, the orientation of light corrected by the prism8in the present embodiment is substantially horizontal. The term “substantially horizontal” is not limited to perfectly horizontal but is intended to permit slight deviations from being perfectly horizontal.

The LiDAR device104includes a prism16in the optical path between the double-sided mirror4and the imaging lens13. The prism16corrects the orientation of light incident upward obliquely from below, slightly downward. The prism16thus substantially matches the orientation of the light incident obliquely from below with the axis of the imaging lens13. The orientation of light corrected by the prism16in the present embodiment is substantially horizontal.

This configuration can achieve the effects equivalent to the LiDAR device101in the first embodiment. In other words, in the LiDAR device104in the present embodiment, instead of the axis shift of the collimator lenses2and12in the foregoing third embodiment, the optical axis is corrected by the prism7or the prism15immediately before light is incident on the double-sided mirror4, and the optical axis is corrected by the prism8or the prism16immediately after light is incident on the double-sided mirror4. Accordingly, aberrations that may occur in the LiDAR device103in the third embodiment are unlikely to occur in the LiDAR device104in the present embodiment. With this configuration, the present embodiment enables the optical elements to be arranged upright while maintaining illuminance and resolution.

Fifth Embodiment

FIG.11Ais a plan view illustrating a structure of a LiDAR device105according to a fifth embodiment.FIG.11Bis a front view illustrating a structure of the LiDAR device105in the fifth embodiment. The LiDAR device105of the present embodiment includes cylindrical lenses9and17in the optical path between the collimator lenses2and12and the double-sided mirror4, instead of the prisms7and15in the LiDAR device104of the fourth embodiment.

The cylindrical lenses diverge the incident light into a sheet-like form. The direction of divergence of light by the cylindrical lenses9and17in the present embodiment is the up and down direction, and the cylindrical lenses9and17spread light longitudinally.

The cylindrical lenses9and17are arranged at positions where the main axis is shifted by a predetermined distance from the optical axis of the incident light (the optical axis of the light-emitting element) in a direction along the axis of rotation of the double-sided mirror4and the rotating stand3. With this configuration, the cylindrical lens9corrects the orientation of the optical axis of the incident light upward, and the cylindrical lens17corrects the orientation of the optical axis of the incident light downward.

The optical axes of the cylindrical lenses9and17are shifted in the longitudinal (substantially vertical) direction, whereby the cylindrical lenses9and17can serve to shape and tilt a beam. In a horizontal scanning LiDAR device, the horizontal divergence angle of illumination light needs to be kept small. Unlike the axis shift of the collimator lenses2and12in the third embodiment, the axis shift of the cylindrical lenses9and17hardly increases aberrations in the horizontal direction and therefore optically has less adverse effects.

Such a configuration therefore can achieve effects equivalent to or greater than the LiDAR device101in the first embodiment.