Laser scanner

The disclosed subject matter relates to a laser scanner for scanning a ground from a seaborne or airborne vehicle, comprising a scanning unit for emitting a fan-shaped scan pattern made of laser beams fanned out about a scan axis and for receiving the laser beams reflected off the ground and an evaluation unit connected to the scanning unit for evaluating the laser beams that are received. The laser scanner is characterized by a measuring unit that is designed to measure the height of the vehicle above ground, and an actuation device that can be anchored to the vehicle and that is connected to the measuring unit. The actuation device is designed to rotate the fan-shaped scan pattern of the scanning unit with respect to the vehicle about a first actuation axis that is different from the scan axis, depending on the measured height above the ground.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase application of International Application No. PCT/AT2017/060015 filed Feb. 3, 2017 which claims priority to Austrian Patent Application No. A 50334/2016 filed Apr. 15, 2016, the disclosures of which are incorporated herein by reference.

TECHNICAL FIELD

This disclosed subject matter relates to a laser scanner for scanning a ground from a seaborne or airborne vehicle, comprising a scanning unit for emitting a fan-shaped scan pattern made of laser beams fanned out about a scan axis and for receiving the laser beams reflected off the ground and an evaluation unit connected to the scanning unit for evaluating the laser beams that are received.

BACKGROUND

The prior art discloses putting such a laser scanner on an airplane or a ship to scan the underlying ground as the airplane flies over it or the ship it passes over it, this scanning being done in scanning strips running along the flight path or the navigation path. The strip width of the scanning strip depends on the aperture angle of the fan-shaped scan pattern, i.e., the fan angle, which is usually determined by an optical system, e.g., an oscillating mirror or a continuously rotating polygon mirror wheel, which fans out the laser beams of a single laser source. Alternatively, it is also possible for multiple laser sources to be aligned as a fan-shaped scan pattern. In this way, the overflown or passed-over ground is scanned line by line within each scanning strip.

The laser beams used are, for example, modulated or pulsed laser beams. The time-of-flight of every laser beam from when it is emitted until when its reflection off the ground is received and the emission direction are used to calculate the distance of the ground from the laser scanner in the emission direction, and the many laser beam measurements are used to determine a three-dimensional terrain model of the ground. In order to calculate a terrain model of a larger area, the ground is overflown or passed over multiple times in adjacent paths, and the adjacent scanning strips that are scanned in this way are later assembled by computer.

As the scanning is being performed, if the height of the vehicle above the ground should vary, e.g., as a consequence of the terrain course of the ground, then the width of the scanning strip and simultaneously the scanning resolution correspondingly change for a given fan angle. Since it is difficult or even impossible, depending on the laser scanner used, to change the scanning fan angle, the prior art requires a complicated flight path or route of the scanning vehicle and/or may even require scanning multiple times to achieve the desired scanning resolution and scanning coverage; this often results in substantial areas of overlap of adjacent or crossing scanning strips, which has the consequence of an abrupt change in the scanning resolution at the borders of every overlap area, an overall non-uniform scanning resolution, and high scanning expense.

BRIEF SUMMARY

The disclosed subject matter has the goal of creating a laser scanner that overcomes these disadvantages, in particular one that allows laser scanning from a seaborne or airborne vehicle with uniform, good scanning resolution and coverage, and that allows the selection of simple routes for the scanning vehicle.

This goal is achieved with an inventive laser scanner of the type mentioned at the beginning comprising:

a measuring unit that is designed to measure the height of the vehicle above the ground; and

an actuation device configured to be anchored to the vehicle and that is connected to the measuring unit;

wherein the actuation device is designed to rotate the fan-shaped scan pattern of the scanning unit with respect to the vehicle about a first actuation axis that is different from the scan axis, depending on the measured height above the ground.

In this way, given a constant fan angle of the fan-shaped scan pattern, suitable rotation of the fan-shaped scan pattern with respect to the vehicle about the first actuation axis keeps the strip width of the scanning strip constant and the scanning resolution at least approximately unchanged, even if there is a change in the height of the (airborne) vehicle above the ground or, conversely, the depth of the ground beneath the (water-based) vehicle during the laser scanning. For a predefined scanning resolution, this yields a substantially higher surface area during scanning, i.e., a larger scanned surface per unit time. This makes it possible to do without multiple scans and/or overlaps of adjacent scanning strips, and allows selection of very simple, rectilinear flight or navigation routes which nevertheless cover the entire ground to be scanned with largely uniform scanning resolution. For example, the rotation of the fan-shaped scan pattern is selected in such a way that during laser scanning at the lowest height above the ground the fan-shaped scan pattern is approximately transverse to the direction of motion of the vehicle.

The rotation of the fan-shaped scan pattern is especially effective if the first actuation axis is essentially vertical. It is also favorable if the first actuation axis intersects the scan axis at the origin of the fan-shaped scan pattern. In this case, even when the fan-shaped scan pattern is rotated about the first actuation axis, the origin of the fan-shaped scan pattern does not undergo any displacement, which would otherwise additionally have to be taken into consideration when calculating the terrain model.

The measuring unit can be constructed according to different embodiments and variants.

According to a first, especially simple embodiment, the measuring unit is designed to measure the height of the vehicle above ground by measuring the time-of-flight of an emitted measurement beam that has been reflected off the ground and received. This requires no prior knowledge of the ground or its course.

To accomplish this, in one variant a separate measuring unit can be provided, the measurement beam being a radar, laser, or sonar measurement beam. Radar, laser, or sonar distance measuring devices are well-known and proven for many applications, so that a corresponding device that meets the specific requirements can be effectively used in the laser scanner.

In another variant, the measurement beam is one of the laser beams that is emitted by the scanning unit and reflected off the ground and received, e.g., a vertically emitted laser beam. Thus, the scanning unit itself is used as part of the measuring unit to measure the height of the vehicle above the ground; there is no additional, separate device.

According to a second embodiment of the disclosed subject matter, the measuring unit comprises a satellite navigation receiver to measure the three-dimensional position, and the measuring unit is designed to use the position measured by this satellite navigation receiver and a stored terrain model of the ground beneath the vehicle to measure the height of the vehicle above the ground. Seaborne or airborne vehicles scanning the ground usually have such satellite navigation receivers to create points of reference for the scanning. Therefore, this embodiment of the measuring unit can also be realized without special accessories, and can—if a correspondingly precise satellite navigation receiver and a terrain model that is already sufficiently detailed are used—be used even if high requirements are placed on the accuracy of the measured height.

In one variant of this embodiment that uses especially little computing power, said terrain model of the ground is predefined and is stored in a memory of the laser scanner. In most cases at least the rough course of the ground, i.e., at least a rough terrain model, is available anyway, e.g., since such a terrain model was used to plan the scanning. Such a rough terrain model, even merely in tabular or vectorized form, can already be sufficient for the measurement of the height of the vehicle above the ground that is required here. The stored terrain model can then be used in the laser scanner without high computational effort.

Another especially advantageous variant of this embodiment of the measuring unit results if the terrain model created by the evaluation unit of the laser scanner itself is used to determine the height above ground. That is, in this case the evaluation unit is designed to use the emission direction and time-of-flight of the laser beams to calculate said terrain model of the ground and to store it in a memory; the measuring unit has access to this memory. On the one hand, this approach measures the height of the vehicle above the ground very precisely, which can even be done in real time and, if it is desired to increase accuracy, with additional interpolation or extrapolation of the measurements, and on the other hand it does not require any separate prestored terrain model.

In every one of the mentioned embodiments of the disclosed subject matter, the actuation device can further be designed to use the terrain model also to determine a slope of the ground beneath the measured position and transverse to a direction of motion of the vehicle and to rotate the fan-shaped scan pattern of the scanning unit with respect to the vehicle about a second actuation axis that is different from the first one and that lies essentially in the direction of motion, depending on the slope that is determined. This also allows a slope of the ground transverse to the direction of flight or travel to be taken into consideration, to achieve, by suitable rotation about the second actuation axis, a straight course of the scanning strip with constant strip width over the entire flight or navigation path.

According to another advantageous embodiment, the laser scanner has an inertial measurement unit for determining at least one of the values pitch angle, roll angle, and yaw angle, the actuation device being connected to the inertial measurement unit and designed to rotate the fan-shaped scan pattern also to compensate for the determined pitch, roll, and/or yaw angle/s. This makes it possible to compensate for deviations of the vehicle from the horizontal position and direction of motion, e.g., as a consequence of turbulence or crosswind or waves or currents, in one, two, or all three spatial directions.

The actuation device can be implemented designed in different ways. For instance, according to a first advantageous embodiment, the actuation device is designed to rotate the fan-shaped scan pattern of the scanning unit by adjusting a deflection mirror of the scanning unit with respect to the vehicle. This means that the actuation device moves only the small mass of the deflection mirror, and thus can be designed to be small and very dynamic. The deflection mirror can be, on the one hand, a separate mirror of the scanning unit, or, on the other hand, for example, a rotating polygon mirror wheel that is present anyway, whose position and/or rotational axis orientation is adjusted by the actuation device.

In a second embodiment that is an alternative to this, the actuation device is designed to rotate the entire scanning unit with respect to the vehicle. In this variant, the actuation device does not intrude into the laser beam course of the scanning unit, but rather holds the scanning unit, e.g., on a flange or in a holding frame. This simplifies the use of a selfcontained scanning unit, which can, if necessary, be quickly and simply replaced by another scanning unit that has, e.g. a different scanning fan angle.

It is especially favorable if the actuation device comprises a controller and an actuator controlled by the controller to rotate the fan-shaped scan pattern of the scanning unit with respect to the vehicle. In this case, it is more flexible if the controller and actuator are designed to be separate, so that, for example, the control unit can also be designed as a part of the evaluation unit. In the latter case, the required computing power is concentrated in a single unit, the evaluation unit; there is no duplication of computing power, so that the laser scanner can be more compact and economical overall.

DETAILED DESCRIPTION

According toFIG. 1, a seaborne or airborne vehicle1, here a manned airplane1, carries a downward directed laser scanner3to scan a ground2. To accomplish this, the laser scanner3produces, e.g., in a single laser source, pulsed or modulated laser beams4, which an oscillating or rotating optical system, e.g., a continuously rotating polygon mirror wheel, fans out about a scan axis5into a fan-shaped scan pattern6having a fan angle φ. Alternatively, the laser scanner3can have multiple laser sources, which in their totality form the fan-shaped scan pattern6by suitable alignment about the scan axis5. Thus, the fan angle φ is predefined by the structure of the laser scanner3, and has approximately the shape of a sector of a circle or of a sector of a lateral surface of a cone.

Scanning involves the laser scanner3emitting the laser beams4onto the overflown ground2and receiving the laser beams4reflected off the ground2. To accomplish this, the ground2beneath the vehicle1is sampled (“scanned”) line by line in a scanning strip7having the width w with the lines8being separated from one another by a distance d. Every line8represents the impingement of the laser beams4of a fan-shaped scan pattern6onto the ground2; the emission direction and time-of-flight of the laser beams4of the multiple lines8are used to calculate a three-dimensional terrain model of the ground2.

The distance d of the lines8results as a consequence of the travel of the airplane1and the scanning speed; the strip width w depends on the fan angle φ and on the height of the airplane1above the ground2(“above ground level”, AGL).

Thus, if the ground2comprises a mountain9, as in the example shown inFIG. 1, the change in strip width w as the mountain9is overflown—see, for example, the smaller strip width w1on the mountain9in comparison with the strip width w in the valley—must, according to the prior art, be compensated for, e.g., by suitable selection of the flight path or multiple overflights, to prevent gaps between adjacent scanning strips7during scanning. This results in overlaps of adjacent or crossing scanning strips7and, consequently, abrupt changes in the scanning resolution at the borders of the overlap areas and an uneven distribution of the scanning resolution over the ground2.

On the basis of the examples shown inFIG. 2 through 5, the discussion below describes various embodiments of an inventive laser scanner10that allows uniform scanning of the ground2. The same reference numbers are used to designate the same parts as inFIG. 1.

According toFIG. 2, the laser scanner10comprises a scanning unit11, which—comparable with the laser scanner3according toFIG. 1—emits the fan-shaped scan pattern6of laser beams4fanned-out about the scan axis5and receives the laser beams4reflected off the ground2. The received laser beams4are evaluated by an evaluation unit12that is connected to the scanning unit11. To accomplish this in the simplest case, the evaluation unit12takes the emission direction and time-of-flight of the laser beams4and also position values x/y/z, which are produced, for example, by a satellite navigation receiver13of the laser scanner10, and, if necessary, the pitch angle p, the roll angle r, and the yaw angle y of the vehicle1from an inertial measurement unit (IMU)14of the laser scanner10, and records all of these in a connected memory15. The memory15can be read out, and the recorded values can be used to calculate a three-dimensional terrain model 3D after the scanning, i.e., “offline”; optionally, the terrain model 3D can be calculated by the evaluation unit itself12immediately—that is, “online”- and the terrain model 3D can be recorded in memory15.

As is shown inFIG. 2, the laser scanner10comprises a measuring unit16, which measures the height a of the vehicle1above the ground2. To accomplish this, the measuring unit16can use every measurement principle known in the art, e.g., a photogrammetric distance measurement method. In the example shown, the measuring unit16emits a measurement beam17, e.g., a radar, laser, or sonar measurement beam, e.g., vertically downward, and measures its height a (FIG. 4a)—and thus that of the laser scanner10or of the vehicle1—above the ground2by measuring the time-of-flight of the measurement beam17that has been reflected off the ground2and received. Through a wire18, the measuring unit16sends the value of the measured height a to a actuation device19.

The actuation device19comprises a controller20and an actuator21controlled by this controller20. The actuation device19or its actuator21is anchored to vehicle1so that it is rigid to movement with respect to the vehicle1. The controller20can optionally be a part of the evaluation unit12.

The actuation device19receives the height a above ground measured by the measuring unit16, and, depending on this height a, it now rotates the fan-shaped scan pattern6of the scanning unit11with respect to the vehicle1about a first actuation axis22, which is different from the scan axis5, by a first actuation angle α, i.e., α=f(a). In the example shown inFIG. 2, this first actuation axis22is essentially vertical.

FIG. 3illustrates the effect of this rotation: Suitable rotation of the fan-shaped scan pattern6about the first actuation axis22depending on the height a keeps the strip width w of the scanning strip7constant even when the mountain9is overflown. This makes it possible to scan the ground2with simple, adjacent flight paths or routes, and to do so without gaps and with uniformly good scanning resolution. The scanning strips7of constant width w that are produced in this way are substantially simpler to combine for effective calculation of the terrain model 3D than is possible if the same ground2is scanned with the laser scanner3inFIG. 1.

FIG. 4athrough 4cshow the example ofFIG. 3in detail. The ground2beneath the airplane1runs, e.g., in a first area A1approximately at sea level (0 m) and, in a following second area A2, up the mountain9to a—highest—third area A3at 1000 m. The airplane1flies in direction of motion23over all areas A1-A3at a constant absolute height of, e.g., 2000 m. Despite the fact that the fan angle φ remains the same and despite the change in the height a above the ground, the strip width w of the scanning strip7remains constant in all areas A1-A3(seeFIGS. 4band 4c), which is attributable to the height-dependent rotation of the fan-shaped scan pattern6about the first actuation axis22. In the rear view shown inFIG. 4b, the projection φ′ of the fan angle φ changes from a smaller value in the first area A1(α>>0, e.g., α=60°) to the full fan angle φ at the lowest height a above ground in the third area A3(α=0), without the real fan angle φ ever needing to be changed; the top view ofFIG. 4cillustrates this. In the third area A3(α=0) the scan axis5is aligned, e.g., directly in the direction of motion23of the airplane1.

Returning toFIG. 2, the measuring unit16can be designed not only as a separate, stand-alone unit, but rather also in one of the following alternative types; the laser scanner10could possibly even have more than one of these alternatives and select the one which is most suitable for measuring the height a or combine the measurement results of multiple alternatives.

According to one of these alternative variants, the measuring unit is formed by the scanning unit11itself, i.e., its measurement beam is one of the laser beams4emitted by the scanning unit11and reflected off the ground and received, e.g., a laser beam4emitted vertically downward. The controller20of the actuation device19can receive this information of the scanning unit11through a wire24. If necessary, evaluation of the information, e.g., by the evaluation unit12, can be interposed, so that in this variant the scanning unit11—optionally together with the evaluation unit12—forms the measuring unit.

According to another alternative variant, the measuring unit comprises the satellite navigation receiver13, which measures its three-dimensional position x/y/z, and thus the position of the laser scanner10or of the vehicle1. With the help of the position x/y/z measured by the satellite navigation receiver13and a stored terrain model 3D′ of the ground2beneath the vehicle1, the height a of the vehicle1above the ground is then determined.

For this purpose it is possible to use, on the one hand, a fixed predefined terrain model 3D′ of the ground2, this terrain model 3D′ being stored in a memory25of the laser scanner10. It can be, e.g., a rough model of the ground2used for planning the scanning process, such as is commercially available in the form of a terrain model, e.g., from suppliers of navigation maps.

On the other hand, in the case described further above in which the evaluation unit12itself calculates the terrain model 3D as the received laser beams4are evaluated (“online”), this calculated terrain model 3D can be used as the terrain model 3D′ for determining the height a, see data line26.

Each of the calculation steps required for measuring the height a from the position data x/y/z of the satellite navigation receiver13and the terrain model 3D′ can be carried out in its own functional block27, which, however, can also be part of the controller20or even of the evaluation unit12. That is, in these cases the measuring unit is formed by the satellite navigation receiver13, the memory25or15with the terrain model 3D′ or 3D, and the functional block27.

As is shown inFIG. 2, the actuation device19can carry the entire scanning unit11on a movable arm28and rotate it with respect to the vehicle1. Alternatively, the scanning unit11is pivotably mounted on the vehicle1or on a housing part of the laser scanner10, and is merely rotated by the actuation device19. According to another alternative embodiment, the actuation device19rotates the fan-shaped scan pattern6of the scanning unit11merely by adjusting a deflection mirror of the scanning unit11with respect to the vehicle1. The deflection mirror can be inside or outside a housing of the scanning unit11.

FIGS. 5aand 5bshow another possible way of adjusting the laser scanner10or its fan-shaped scan pattern6depending on the terrain course of the ground2. The airplane1overflies a slope29in the ground2inclined transverse to the direction of motion23of the airplane1. As is shown inFIG. 5a, this produces an asymmetric position of the scanning strip7with respect to the vertical line30under the airplane1(see the sections w1and wrof the scanning strip7), which also displaces the scanning strip in the direction transverse to the direction of motion23.

To counteract this, the actuation device19according toFIG. 5bis designed to use the terrain model 3D′ to determine the slope29of the ground2beneath the measured position x/y/z and transverse to the direction of motion23of the airplane1. After that, the actuation device19rotates the fan-shaped scan pattern6of the scanning unit11with respect to the airplane1by a second actuation angle β, depending on the determined slope29, about a second actuation axis31(FIG. 2) lying essentially in the direction of motion23(normal to the plane of the drawing ofFIG. 5), to center the scanning strip7with respect to the vertical line30. As is shown inFIG. 2, the second actuation axis31can coincide with the scan axis5.

In another optional embodiment, the pitch, roll, and/or yaw angles p, r, and y of the airplane1measured by the inertial measurement unit14of the laser scanner10can also be used to rotate the fan-shaped scan pattern6to compensate for at least one of these angles. The rotation about the first actuation axis22or the angle α can be used to compensate for the yaw angle y, that about the second actuation axis31(angle β) can be used to compensate for the roll angle r, and that about a third actuation axis32(angle γ) can be used to compensate for the pitch angle p.

It goes without saying that in every embodiment the actuation device19sends the actuation angle/s α and, if present, β and γ through a corresponding wire33to the evaluation unit12, and the evaluation unit12takes these angles α, β, γ into consideration in the determination of the emission directions of the laser beams4, to create the terrain model 3D correctly.

To make it simpler for the evaluation unit12to take into consideration the rotation of the fan-shaped scan pattern6, the first actuation axis22and—if desired and present—also the second and/or the third actuation axes31,32can intersect the scan axis5at the origin34of the fan-shaped scan pattern6.

The laser scanner10can be used from an airborne vehicle1both to scan a terrain and also to scan the floor of a body of water. To scan the floor of a body of water, the laser scanner10can be used in the same way on a suitable seaborne vehicle, i.e., a ship or submarine. Optionally, the vehicle1is unmanned, i.e., an unmanned aerial vehicle (UAV), unmanned surface vehicle (USV), or unmanned underwater vehicle (UUV).

The disclosed subject matter is not limited to the presented embodiments, but rather comprises all variants, modifications, and combinations that fall within the scope of the associated claims.