Aerial scanning instrument with attitude adjustment

Abstract

The invention discloses an aerial scanning instrument with attitude adjustment. A mobile scanning instrument is configured to be mounted on a carrier and to acquire point cloud data representative of a target area at an object distance as the carrier vehicle travels at a carrier speed relative to the target area. The mobile scanning instrument comprises: (i) a pulse source unit configured to generate a scan pulse having a pulse rate; and (ii) a beam deflection element configured to define an actual emission direction of the generated scan pulse, where the actual emission direction varies along the scan pattern at a scan rate. The scanning instrument is configured to shift the scan pattern by a twist angle representative of a rotation about the main axis, and to optimize the twist angle based on an optimization objective representative of a point density and / or uniformity of the point cloud data.

Description

Technical Field

[0001] The present invention relates to the structure and operation of a mobile scanning instrument configured to acquire point cloud data representing a target area. The mobile scanning instrument is configured to be mounted on a carrier vehicle (in particular an aircraft) to acquire a point cloud while the carrier vehicle is traveling at a carrier speed. Background Art

[0002] In order to capture two-dimensional or three-dimensional information about a target area, in particular an urban or agricultural area, scanning methods are typically utilized. Ideally, a uniform, dense point cloud is provided. Conventional scanning instruments utilize a scanning beam (in particular a laser beam) scanning arrangement and determine the distance of an object point based on, for example, time-of-flight measurements. The measured distance information is combined with the emission angle of the scanning beam to generate a point cloud representing the arrangement. The capture of the point cloud can also be performed during the spatial movement of the measuring device. The self-movement of the measuring device (correspondingly the movement of the mobile carrier) must be acquired and merged with the scanning data. In particular for aerial scanning, the fast movement of the carrier and the relatively long flight time of the scanning pulses may limit the achievable quality of the point cloud. Although the applicability of the present invention is not limited to the field of aerial scanning, many aspects of the present invention will be illustrated by examples from aerial scanning. Specific features of other mobile scanning methods, such as automotive or railway profiling, can be applied accordingly.

[0003] Aerial scanning is typically performed using an instrument whose beam deflection element generates a scanning pattern by rotation about the main axis of the scanning instrument (e.g., a rotating prism) or an oscillatory beam deflection mode, the main axis defining a reference system. Although for some embodiments the main axis corresponds to the physical rotation of the surveying instrument and / or a specific optical axis, a virtual axis can also act as the main axis. The main axis is typically aligned with the nadir ("straight down") direction, while the rotational or oscillatory movement provides a scanning field of view in the range of 20° - 90°, more specifically 40° - 60°. The scanning is performed by mechanical elements, i.e., a scanning rate higher than 150,000 revolutions per minute or 250 Hz is currently not feasible.

[0004] The scanning instrument generates scanning pulses in a periodic manner and emits them at a pulse rate. For aerial scanning, the periodic manner is understood in a broad sense, i.e., the generation of the pulses may involve periodic or pseudo-random modulation, or even a combination of both, as disclosed, for example, in EP4148458 A1. Since the pulse generation is provided by pure electronic components, the practical limit of the pulse rate is set by the detection and allocation of the returned pulses.

[0005] In the following, the relevant resolution in a general high-resolution survey mission will be illustrated by a simplified example. For example, a scanning instrument provides a line pattern along a circular one-way scan, with a scanning rate of 200 Hz and a pulse rate of 2 MHz. As the carrier vehicle, an aircraft travels at a flight speed of 100 m / s (i.e., the magnitude of the carrier speed), and the flight altitude near the lowest point (which corresponds to the object distance) is 500 m. -1 The line-to-line distance (d

[0006] ) or the distance between two scan lines depends only on the carrier speed (v) and the scanning rate (f L2L ) and the scanning rate (f s ):

[0007]

[0008] The point-to-point distance (d P2P ) depends on the pulse rate (f p ) and the scanning rate (f s ) and the object distance (h). Under the above conditions, it can be estimated as

[0009]

[0010] The achievable point-to-point distance (d P2P ) is inversely proportional to the object distance (h). However, the line-to-line distance (d L2L ) does not depend on the object distance. This means that the maximum resolution (especially the maximum uniform resolution) is limited by the achievable line-to-line distance. However, the achievable line-to-line distance depends on the carrier speed and the scanning rate (e.g., limited by the stall speed of a fixed-wing aircraft). In addition, without improving the line-to-line distance (d L2L ), the surveyor cannot utilize the capabilities of improved electronics or higher-performance aircraft.

[0011] Systems that allow greater flexibility in providing effective line-to-line density are desirable. Among them, such systems would benefit from the development of faster laser sources / detectors as they would enable higher pulse rates. Summary of the Invention

[0012] Object of the Invention

[0013] In view of the above, the object of the present invention is to provide greater flexibility in designing survey operations by moving a scanning instrument.

[0014] A specific object is to optimize the uniformity and / or point density and / or scanning cost of point cloud data.

[0015] Another object of the present invention is to extend the operating range of a survey system, in particular for a survey aircraft, by achieving a higher flight speed while maintaining the point spacing.

[0016] Another object of the present invention is to reduce the number of survey runs required over the same target area, thereby reducing the environmental burden.

[0017] Overview of the present invention

[0018] The present invention relates to a mobile scanning instrument configured to be mounted on a carrier vehicle and configured to acquire point cloud data representing a target area. When the carrier vehicle travels relative to the target area at a carrier speed, the target area is at an object distance from the mobile scanning instrument.

[0019] Unless otherwise stated, from here on, the mobile scanning instrument is understood to include elements configured to provide said mounting to the carrier vehicle. The mounting elements may provide an adjustable (in particular maneuverably adjustable within a scanning mission or between two scanning missions) orientation of the mobile scanning instrument with respect to the carrier vehicle and / or the carrier speed. The reference system of the mobile scanning instrument includes the main axis of the scanning instrument. The main axis represents the viewing direction.

[0020] The mobile scanning instrument includes a pulse source unit configured to generate scanning pulses in a periodic manner with a pulse rate. In the sense of the present invention, periodicity also includes modulation sequences. The mobile scanning instrument further includes a beam deflection element configured to define the actual emission direction of the generated scanning pulses with respect to the main axis, wherein the actual emission direction varies along a scanning pattern in a periodic manner with a scanning rate. In the sense of the present invention, the scanning pattern corresponds to the above-mentioned variation of the emission direction. The actual point pattern generated on the target is the result of the interaction of the scanning pattern, the actual generation of pulses according to a specific pulse sequence, and the target topography. As an example, for many of the described embodiments, it is assumed that the pulse generation and beam deflection are strictly periodic. It will be clear to those skilled in the art that this is merely for the sake of clarity, and that the present invention and the disclosed embodiments are not limited to this case unless explicitly stated. Specific features such as modulated pulses should be applied accordingly.

[0021] For example, the combination of the pulse source and the beam deflection element will also be referred to as the emission unit. For many of the disclosed embodiments, the beam deflection element also interacts with the reflected scanning beam. The claimed invention applies in principle to any detection device, including embodiments in which the beam deflection element interacts with the reflected beam and embodiments including partially or fully separated detector beam paths.

[0022] The mobile scanning instrument presents a main axis. In a general sense, the main axis defines the reference system among the instruments. In addition, the carrier speed and the main axis define the mobile external reference system. The main axis may correspond to a part of the beam path, particularly a part of the beam entering the beam deflection element, and / or correspond to the imaginary beam path of the non-deflected outgoing beam, and / or the time average of the actual emission direction, and / or the average of the emission directions of the emitted pulses. Those skilled in the art understand that for general scanning instruments, many elements in the above list refer to some practically relevant choices of the main axis.

[0023] The scanning instrument is configured to offset the scanning pattern by a twist angle representing the rotation about the main axis. Alternatively, if the carrier speed is known, the orientation of the scanning pattern can be set with respect to the carrier speed. The offset can be provided by a mechanical assembly that rotates the beam deflection element / emission element about the main axis. Alternatively, the offset is provided by changing the operating mode / operating parameters of the beam deflection element.

[0024] The scanning instrument is configured to execute a twist angle optimization algorithm. The twist angle optimization algorithm includes (i) accessing input parameters including pulse rate, scan rate, carrier speed, and object distance, (ii) providing an optimization objective representing the point density and / or uniformity of the point cloud data, and (iii) providing an output twist angle based on the optimization objective and the input parameters. It is obvious to those skilled in the art that the use of numbers and letters does not represent, for example, the order of performing steps. These and all other numbers do not represent temporal and spatial connections, even in the form of a preferred sequence, and are only for the purpose of readability. The actions can be performed in any reasonable manner, especially continuous adjustment.

[0025] Due to the twist, at least in the middle part of the swath, the effective line-to-line distance (d L2L ,eff) (i.e., the distance in the direction perpendicular to the scanning direction) will be d L2L ,eff = d L2L ·cos(α), where α is the angle between the scanning direction and the carrier speed, and d L2L is the line-to-line distance measured in the direction of the carrier speed. In other words, the twist can overcome the limitations of the speed of mechanical elements.

[0026] The optimization objective may include: preferred intervals (such as "field of view 25° - 40°" and / or "swath width 150 - 250 meters"), maximization objectives, minimization objectives, or threshold levels (such as "maximize point density" and / or "per m 2"at least 5 points"), constraints on the scan rate or pulse rate, or forced relationships between different parameters and / or navigation objectives (e.g., "uniform point density" and / or "scan direction perpendicular to the carrier velocity"). Those skilled in the art can provide many reasonable combinations and alternatives based on the above non-exhaustive list.

[0027] In some specific embodiments, the mobile scanning instrument is configured to be mounted on an aircraft. The carrier velocity is the flight velocity with respect to the ground. In some specific embodiments, the main axis corresponds to the lowest point direction. In some specific embodiments, the carrier is one of the following: (i) a helicopter; (ii) a light fixed-wing aircraft having a takeoff gross weight of less than 6000 kg; and (iii) a large UAV having a weight of more than 150 kg. Although not limited thereto, the claimed invention can be beneficially combined with new, higher-performance survey aircraft.

[0028] In some embodiments, the beam deflection element includes a first wedge and a second wedge mounted along the main axis. The first wedge is configured to rotate at a first wedge rate. The second wedge is configured to rotate at a second wedge rate such that the ratio of the first wedge rate to the second wedge rate is a proper fraction, particularly a unit root. Thus, the scan rate corresponds to the greatest common divisor of the first wedge rate and the second wedge rate.

[0029] In some specific embodiments, the wedges are designed to provide a generalized Risley prism pair, more specifically, a prism pair having the same wedges, which are configured to provide a linear pattern, a circular pattern, and a rose pattern according to the ratio of the wedge rates. Alternatively, the wedges are designed to provide a prism pair having different wedges, which are configured to provide an elliptical pattern, a circular pattern, and a rose pattern according to the ratio of the wedge rates.

[0030] In some specific embodiments, the mobile scanning instrument is configured to provide a twist angle based on a wedge phase shift representing the phase difference between the rotation of the first wedge and the rotation of the second wedge. One advantage of the above general Risley prism pair is the ability to provide a twist angle without rotating the scanning instrument. That is, the twist angle can be continuously monitored and adjusted during the scanning task. Alternatively or additionally, the twist angle can vary according to part of the scanning task, i.e., when a single scanning task involves urban and topographical scanning and / or an overview survey of high-altitude and high-resolution scans from the area of interest.

[0031] In some embodiments, the mobile scanning instrument is configured to provide a twist angle interval, wherein the scanning instrument is configured to offset the scanning pattern by any twist angle within the interval. In some specific embodiments, the mobile scanning instrument is configured to provide any possible twist angle within the 0-360° interval in a stepless manner. Alternatively, the mobile scanning instrument may provide a set of selectable twist angles (including a limited number of preset twist angles, particularly less than six preset twist angles) and means for selecting one twist angle from the set of selectable twist angles, particularly as a hardware switch or a software switch implemented as a drop-down menu.

[0032] In some embodiments, (i) the scanning pattern is a line pattern, (ii) the scanning rate and the carrier speed define the line-to-line distance, (iii) the pulse rate, the scanning rate, and the object distance define the point-to-point distance, and (iv) the optimization goal is the threshold uniform point density of the point cloud data. Obviously, the line pattern in the sense of the present invention covers one-way scanning, thereby generating a point pattern including parallel lines and an oscillating pattern generating a generalized zigzag point pattern. In an alternative embodiment, the scanning pattern is an elliptical pattern. Specific features corresponding to the line-to-line distance and the point-to-point distance can be defined accordingly. In particular, the linear displacement between points corresponding to the same scanning stage can define the equivalent of the line-to-point distance.

[0033] In some specific embodiments, the twist angle optimization algorithm includes (i) determining the effective line-to-line distance based on the line-to-line distance and the twist angle, and (ii) providing the optimal twist angle based on the effective line-to-line distance and the point-to-point distance, particularly by selecting the twist angle to set the effective line-to-line distance equal to the maximum point-to-point distance or the point-to-point distance corresponding to the target point density. In other words, the highest uniform point density is limited by the effective line-to-line distance, which may be the case for low-altitude scanning, or by the point-to-point distance, which may be the case for high-altitude scanning. The twist angle optimization algorithm provides the twist angle at which the two coincide, so that "empty optimization" does not occur. Considering the resolution loss caused by post-processing, the effective point-to-point distance can be equivalently used instead of the point-to-point distance. The resolution may be lost, for example, due to artifact removal or smoothing.

[0034] In some embodiments, the mobile scanning instrument includes a task recognition function. The task recognition function is configured to (i) determine the type of the scanning task, particularly by determining the representative object distance change; and (ii) provide an evaluation of the optimization goal based on the determined scanning task. The scanning instrument may be configured to provide an evaluation of the scanning task, whether a landscape or urban scan is performed, whether a high or low altitude scan is performed, whether the flight speed is high, etc. Such data can be provided by the operator; however, it can equally be identified from the point cloud data itself.

[0035] In some specific embodiments, the optimization objective includes maximizing the effective strip width of a scanning task with a low representative object distance change (e.g., less than 10 m / 100 m). The effective strip width is the extent of the scanning pattern perpendicular to the carrier velocity.

[0036] In some specific embodiments, the optimization objective includes maximizing the uniform point density of the point cloud data for a scanning task with a representative object distance change higher than 10 m / 100 m. The uniform point density of the point cloud data can be provided by setting the effective line-to-line distance equal to the maximum point-to-point distance or the point-to-point distance corresponding to the point density objective.

[0037] In some embodiments, the mobile scanning instrument is configured to acquire point cloud data at a scanning rate of at least 10 Hz, particularly 50 Hz. In some specific embodiments, the mobile scanning instrument is configured to acquire point cloud data at a scanning rate of at least 200 Hz.

[0038] The present invention also relates to a method for surveying a target area at an object distance from a mobile scanning instrument mounted on a carrier vehicle traveling at a carrier velocity. The method includes (i) generating scanning pulses in a periodic manner with a pulse rate, (ii) emitting the generated scanning pulses along an actual emission direction with respect to a main axis, wherein the actual emission direction changes along a scanning pattern in a periodic manner with a scanning rate, (iii) acquiring return pulses from the target area and providing return pulse data regarding the acquired return pulses, (iv) deriving point cloud data representing the target area based on the return pulse data, and (v) performing twist angle optimization. The twist angle optimization includes: (a) accessing input parameters including the pulse rate, the scanning rate, the carrier velocity, and the object distance, (b) providing an optimization objective representing the point density and / or uniformity of the point cloud data, (c) providing an output twist angle based on the optimization objective and the input parameters, wherein the twist angle represents the offset of the scanning pattern by rotation around the main axis, and (d) setting the twist angle to the output twist angle.

[0039] In some embodiments of the method, the optimization objective is the highest achievable uniform point density of the point cloud data. In other words, equal point spacing is required at the maximum collection speed to overcome the limitations of the scanning rate.

[0040] In some embodiments, the method further includes task identification. The task identification includes (i) determining the type of the scanning task, particularly by determining the representative object distance change, and (ii) providing an evaluation of the optimization objective based on the determined scanning task.

[0041] In some specific embodiments of the method, feedback data is provided to the training process. The training process is based on a machine learning algorithm and provides updated information for task recognition. The feedback data is provided by (i) explicit feedback by an operator of a mobile scanning instrument, in particular, by means of a feedback process, and / or (ii) implicit feedback. The implicit feedback can be based on the confirmation of the assignment and / or comparison of the lack of explicit feedback as the actual point cloud data (in particular, line-to-line distance, point-to-point distance, or strip width) with the evaluated optimization objective.

[0042] In some embodiments of the method, (i) the survey is an aerial scan, (ii) the vehicle speed is the flight speed, (iii) the main axis corresponds to the lowest point direction, and (iv) the method is performed with respect to a reference system defined by the flight speed and the lowest point direction. In particular, in the zero-twist state, the scan pattern has the maximum extension in a direction perpendicular to both the lowest point direction and the flight speed. In other words, the zero-twist state corresponds to an aerial scan with optimized strip width, in particular, a topographic scan.

[0043] In some embodiments, the method further includes (i) providing a pulse modulation arrangement, where the pulse modulation arrangement includes (a) a pseudo-random component and / or (b) a geometry-dependent component provided based on the actual emission direction, and (ii) modulating the generation of the scan pulses based on the pulse modulation arrangement. In other words, the method of the present invention can be performed with a modulated pulse train. The modulated pulse train can be configured to provide improved uniformity of the point cloud, in particular, by compensating for the flight time variation of the pulses according to the actual emission direction and / or by compensating for the reduced ground speed at the boundaries of the scan pattern in the strip. The modulated pulse train can also be configured to provide MPiA disambiguation. The modulation can also be configured to meet these and / or further requirements.

[0044] In some embodiments, the method further includes post-processing. The post-processing includes steps of discarding and / or smoothing most of the point cloud data to improve uniformity. The pulse rate is set to oversample the target area, thereby allowing post-processing to be performed. In other words, the point-to-point distance can be intentionally set to be lower than the line-to-line distance.

[0045] In some specific embodiments, the point-to-point distance can be set to a unit fraction of the line-to-line distance, i.e., 1:2, 1:3, and the corresponding number of points can be discarded / smoothed. Alternatively, the pulse rate can be set to the highest, and uniformity can be ensured by providing virtual object points as average points derived from a plurality of measured object points.

[0046] The present invention also relates to a computer program comprising program code stored on a machine-readable medium or embodied by an electromagnetic wave. The computer program includes program code segments and has computer-executable instructions for performing a twist angle optimization algorithm for a mobile scanning instrument mounted on a carrier vehicle. The mobile scanning instrument is configured to (i) generate scanning pulses in a periodic manner with a pulse rate, (ii) emit the generated scanning pulses along an actual emission direction with respect to a main axis, wherein the actual emission direction varies along a scanning pattern in a periodic manner with a scanning rate, (iii) acquire return pulses from a target area and provide return pulse data regarding the acquired return pulses, and (iv) derive point cloud data representing the target area based on the return pulse data. The mobile scanning instrument is in particular an embodiment of the mobile scanning instrument of the present invention.

[0047] The twist angle optimization algorithm includes: (i) accessing input parameters including the pulse rate, the scanning rate, the carrier speed, and the object distance, (ii) providing an optimization objective representing the point density and / or uniformity of the point cloud data, (iii) providing an output twist angle based on the optimization objective and the input parameters, wherein the twist angle represents the offset of the scanning pattern by rotation about the main axis.

[0048] The computer program can be implemented as part of a general driver of the mobile scanning instrument configured to control other aspects of the mobile scanning instrument. Alternatively, the computer program can also be implemented as an independent product configured substantially for performing the twist angle optimization algorithm. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] By way of example only, specific embodiments of the present invention will be described more fully hereinafter with reference to the drawings, in which:

[0050] Figure 1 A schematic view of an aerial scan of a target area is depicted.

[0051] Figures 2a to 2j Several point patterns with respect to the main axis ( Figures 2a to 2e ) and several point patterns with respect to the target area ( Figures 2f to 2j ) are depicted.

[0052] Figure 3 A schematic view of a possible implementation of the scanning instrument and the beam deflection element is depicted.

[0053] Figure 4 The offset of an elliptical scanning pattern is schematically depicted.

[0054] Figure 5 The influence of offsetting a linear scanning pattern by a twist angle on the uniform point density and the strip width is schematically depicted.

[0055] Figure 6aSome key aspects of the twist angle optimization algorithm are schematically depicted by a flow chart.

[0056] Figure 6b An algorithm for providing a uniform point density by adjusting the twist angle and the pulse rate is schematically depicted. Detailed Description

[0057] Figure 1 A mobile scanning instrument 1 carried by an aircraft 4 is shown to acquire point cloud data from a target area 2 as the aircraft 4 travels relative to the target area 2 at a vehicle speed 40. The mobile scanning instrument 1 is typically mounted together with additional survey elements and cameras facing downward or in an inclined direction. The main axis 10 of the mobile scanning instrument 1 is aligned in a known manner relative to the vehicle speed 40. In the illustrated embodiment, the main axis is aligned with the lowest point. The object distance 21 between the target area 2 and the scanning instrument 1 can be defined with respect to the main axis 10, as shown. Alternative mountings (e.g., the main axis 10 can be pointed obliquely forward or backward with respect to the vehicle speed 40) are also within the scope of the present invention.

[0058] The scanning instrument 1 is configured to provide a scanning pattern 3, which is depicted as a line pattern and represented by a resulting point pattern 399. Although the present invention is not limited to such a line pattern, many features of the present invention will be shown using a line pattern for clarity. Many aspects of alternative scanning patterns, particularly elliptical patterns, can be applied accordingly. Key parameters of such scanning patterns are the field of view 30 defining the strip width, the line-to-line distance 31 of the resulting point pattern 399, and the point-to-point distance 32 of the resulting point pattern 399. Throughout the specification, the field of view 30 is defined by an angular dimension, while the strip width, the line-to-line distance 31, and the point-to-point distance 32 are defined by linear dimensions. This is only for clarity reasons, and suitable alternative conventions are also applicable. Although Figure 1 is schematically represented and not drawn to scale, the reasonably achievable point-to-point distance 32 is typically less than the line-to-line distance 31.

[0059] Figures 2a to 2j A non-exclusive example of a point pattern is depicted. The top row ( Figures 2a to 2e ) depicts the point pattern with respect to the main axis, i.e., as a fixed pattern. The bottom row ( Figures 2f to 2j ) depicts the corresponding pattern with respect to the target area, i.e., considering the vehicle speed 40. Solid circles represent the last scan cycle, while previous scan cycles are marked with empty circles.

[0060] The first pair (a - f) is a line pattern scanned in a single scan direction 11. The second pair (b - g) corresponds to a zigzag pattern, i.e., the scan direction 11 is reversed during scanning. The third pair (c - h) is a circular pattern with a strictly periodic pulse rate, i.e., the time interval between two pulse emissions is always the same. The fourth pair (d - i) is a circular scan / dot pattern; however, the pulse rate is modulated to provide a uniform distance between pulse rows. In the sense of the present invention, the pattern according to the fourth pair also has pulses generated in a periodic manner. The fifth pair (e - j) represents an elliptical pattern.

[0061] Figure 3 Schematically shown is a scanning instrument 1 with an emission unit 5, and the emission unit 5 has a beam deflection element 50 including a first wedge 51 and a second wedge 52. By way of example, the first wedge 51 and the second wedge 52 with different geometries are shown, and such a beam deflection element 50 is configured to provide an elliptical scan pattern. Using two optically identical wedges (i.e., a Risley prism pair), a linear scan pattern can be provided. For obvious reasons, the previous embodiment will be discussed in more detail. However, this should not be construed in a limiting manner. Specific aspects of the present invention apply to embodiments with identical wedges. In addition, generally speaking, the present invention also applies to scanning instruments 1 based on other beam deflection elements 50 (such as galvanometric mirrors or polygon mirrors).

[0062] The emission unit further includes a pulse source 340 and a focusing / collimating element 341 as well as a mirror 342 to direct the beam towards the beam deflection element 50. A part of the beam path 34 corresponds to the main axis 10 of the emission unit 5 or the scanning instrument 1. The beam deflection element 50 defines the actual emission direction 33 of the scanning pulse. The actual emission direction 33 is provided with a periodic change along the scan pattern by rotating 510 the first wedge 51 and the second wedge 52 with corresponding motors 61, 62.

[0063] The scanning instrument 1 further includes a detection unit 20, which has a sensing element 200 and appropriate focusing optics 201. The detection unit 20 is configured to acquire return pulses from a target area (not shown). The scanning instrument includes a calculation unit 6, and the calculation unit is configured to control the pulse source 340, the motors 61, 62 driving the wedges 51, 52 and access data from the detection unit (not shown).

[0064] Those skilled in the art understand that the above schematic representation may omit some basic features of the claimed invention and / or display said features in a simplified manner. In addition, some of the depicted features, although advantageous, may not be strictly necessary for implementing the present invention.

[0065] Figure 4illustrates the concept of offsetting the scan patterns 3, 300 by a twist angle 301 corresponding to a rotation about the main axis 10. For example, the main axis 10 points to the lowest point. As an example, Figure 4 is explained based on the principle of a scanning instrument similar to Figure 3 that shown. As will be shown, such a scanning instrument is well-suited for performing the present invention because it can provide the twist angle 301 without any additional mechanical components. However, the present invention can be performed by a scanning instrument based on other operating principles. The present invention can in particular be applied to an instrument in which the beam deflection element or the housing of the scanning instrument is mechanically rotated by an angle corresponding to the twist angle 301.

[0066] The scanning instrument is configured to provide a first beam deflection angle 515 and a second beam deflection angle 525. The net beam deflection angle 535 is the vector sum of the first beam deflection angle 515 and the second beam deflection angle 525.

[0067] The beam deflection angles 515, 525 rotate 510, 520 in opposite directions at respective first and second rates 519 and 529. The magnitudes of the first rate 519 and the second rate 529 are equal. This means that during one complete rotation, when the net beam deflection angle 535 scans the elliptical second scan pattern 3, 300, the first beam deflection angle 515 and the second beam deflection angle 525 experience all orientation differences. The major axis 326 of the ellipse corresponds to the sum of the beam deflection angles 515, 525, while the minor axis 327 corresponds to the difference between the beam deflection angles 515, 525.

[0068] The top row depicts the case where the beam deflection angles 515, 525 are in phase with each other in the first set (panel), while the bottom row depicts the case where a wedge phase shift 539 is introduced between the two beam deflection angles 515, 525. The shape of the scan patterns 3, 300 characterized by the major axis 326 and the minor axis 327 is the same. However, the offset scan pattern 300 is offset by rotating about the main axis 10 by the twist angle 301. The twist angle 301 in the depicted embodiment is half of the wedge phase shift 539. The offset of the linear scan pattern provided by the Risley prism pair can be performed in a similar manner.

[0069] Figure 5 (a) and (b) of illustrate the effect of offsetting the linear scan patterns 3, 302 by the twist angle 301 on the line-to-line distance 31, the effective line-to-line distance 312 (i.e., uniform point density), and the strip widths 39, 392, respectively. As an example, Figure 5 (a) of shows a case where the scan is performed by a fixed-wing aircraft traveling at a carrier speed 40, and the scan direction 11 is set perpendicular to the carrier speed 40.

[0070] The point-to-point distance 32 is limited by the achievable pulse rate and the flight altitude, the latter of which can vary according to the mission, while the former depends on the performance of the electronic components. The line-to-line distance 31 depends on the scan rate and the vehicle speed 40, the latter of which is constrained by the type of aircraft, while the former depends on the performance of the mechanical components.

[0071] This means that for high-resolution scanning, situations such as those shown in Figure 5 (a) of the zero-twist state are often encountered. In the zero-twist state, the scan pattern 3 has the maximum extension in the direction perpendicular to the lowest point direction and the direction of the vehicle speed 40. The object points 35 of the uniform point cloud are represented by solid circles. Their density is limited by the achievable line-to-line distance 31. The empty circles refer to additional target points 36, which are usually measured for improved filtering of the point cloud, however, they do not contribute to the resolution. In other words, the pulse rate is set to oversample the target area, which allows post-processing to be performed. The point-to-point distance 32 is usually set to be related to the line-to-line distance 31.

[0072] In Figure 5 (b), the scan direction 11 is offset by the twist angle 301. The effective line-to-line distance 312 in the twist pattern 302 is reduced by the cosine of the twist angle 301. This allows an increase in the density of the target points 352 of the uniform point cloud. The price to pay is to reduce the strip width 392 by the square of the cosine to the twist angle 301. However, this is usually an acceptable compromise, especially for urban surveys, because the advantage of a large strip width 392 cannot be utilized due to the shadow effect of buildings. The effective point-to-point distance 322 is set to be related to the effective line-to-line distance 312.

[0073] Figure 6a The first exemplary embodiment of the twist angle optimization algorithm 7 is schematically depicted by a flow chart. The command / flow lines are shown in bold and data lines, such as dashed arrows. Those skilled in the art understand that the depicted flow chart focuses on the features of the claimed invention, and the actual implementation also includes elements not depicted, particularly command or data elements and / or data transmission lines. In addition, for reasons of clarity and conciseness, the command or data modules may be depicted in a simplified form.

[0074] In the depicted embodiment, accessing the input data is implemented in two steps. First, accessing 210 / 400 includes survey parameters such as object distance data 21 and carrier speed data 40. Further survey parameters can also be accessed, in particular the expected change in the object distance 21. This is followed by accessing 600 / 633 the instrument rate, i.e., the scan rate 60 and the actual pulse rate 634. Finally, accessing 303 the twist angle 301, in particular the orientation of the scan pattern with respect to the carrier speed 40. From the above parameters, the effective line-to-line 312 and effective point-to-point distances 322 are derived 313 / 320. Based on the effective line-to-line 312 and point-to-point distances 322, the achievable uniform point density data 361 is derived 362. Then, the uniform point density data 361 is compared with, for example, point density target data 70 provided by manual input 700, or as a scan task-related target. If the uniform point density 361 does not correspond to the point density target 70, output twist angle data 308 is provided 309, i.e., a command for adjusting the twist angle 301. It will be apparent to those skilled in the art that the depicted twist angle optimization algorithm 7 is effectively applicable if the uniform point density 361 is limited by the effective line-to-line distance 312.

[0075] Figure 6b A second exemplary embodiment of the twist angle optimization algorithm 7 is schematically depicted by a flow chart. In this embodiment, the goal is to provide a point cloud according to the point density target 70. First, the point density target 70 is accessed 700. Based on the point density target 70, the line-to-line distance target 71 is derived 710. The system accesses 400, 600 the carrier speed 40 and the scan rate 60. Then, the optimal twist angle 73 is derived 730, and the optimal twist angle provides the required line-to-line distance 71. Finally, based on the line-to-line density target 71, the object distance 21 accessed 210, and the scan rate 60, a pulse rate target 734 is provided 733. The pulse rate target 734 can be provided such that it oversamples the target, i.e., the pulse-to-pulse distance is a unit fraction of the line-to-line distance target 71.

[0076] Although the present invention has been described above in part with reference to some specific embodiments, it must be understood that many modifications and combinations of the different features of the embodiments can be made. All such modifications are within the scope of the appended claims.

Claims

1. A mobile scanning device (1) configured to be mounted on a carrier vehicle (4) and configured to acquire point cloud data representing a target area (2) when the carrier vehicle (4) travels at a carrier speed (40) relative to a target area (2) at an object distance (21) from the mobile scanning device (1), in, The mobile scanning device (1) presents a main axis (10) and comprises a pulse source unit (340) configured to generate scanning pulses in a periodic manner having a pulse rate (634), and a beam deflection element (50) configured to define an actual emission direction (33) of the generated scanning pulses about the main axis (10), wherein the actual emission direction (33) varies along the scanning pattern (3, 300, 302) in a periodic manner with a scanning rate (60), Features The scanning device (1) is configured as offsetting the scanning pattern (3, 300, 302) by a twist angle (301) representing a rotation about the main axis (10), and Execute the torsion angle optimization algorithm (7), including accessing (210, 400, 600, 633) input parameters including the pulse rate (634), the scan rate (60), the carrier speed (40) and the object distance (21), providing an optimization target representing point density and / or uniformity of the point cloud data, An output twist angle (308) is provided (309) based on the optimization objective and the input parameters.

2. The mobile scanning device (1) according to claim 1, wherein the mobile scanning device (1) is configured to be mounted on an aircraft, wherein: The carrier speed (40) is the flight speed above the ground, in particular wherein the main axis (10) corresponds to the nadir direction.

3. The mobile scanning device (1) according to claim 1 or 2, wherein: The scanning pattern (3, 300, 302) is a line pattern, The scanning rate (60) and the carrier speed (40) define the line-to-line distance (31), The pulse rate (634), the scan rate (60) and the object distance (21) define a point-to-point distance (32), and The optimization target is the threshold uniform point density of the point cloud data.

4. The mobile scanning device (1) according to claim 3, wherein: The torsion angle optimization algorithm (7) comprises: determining (313) an effective line-to-line distance (312) based on the line-to-line distance (31) and the torsion angle (301), An optimum twist angle (73) is provided (730) based on the effective line-to-line distance (312) and the point-to-point distance (32), in particular by selecting a twist angle that sets the effective line-to-line distance (312) equal to a maximum point-to-point distance (32) or a point-to-point distance (32) corresponding to a point density target (70).

5. The mobile scanning device (1) according to any one of the preceding claims, wherein: The beam deflecting element (50) comprises a first wedge (51) and a second wedge (52) mounted along the main axis (10), The first wedge (51) is configured to rotate at a first wedge speed (519), The second wedge (52) is configured to rotate at a second wedge rate (529) such that a ratio of the first wedge rate (519) and the second wedge rate (529) is a suitable fraction, in particular a unity root.

6. The mobile scanning device (1) according to claim 5, is configured to provide the twist angle (301) based on a wedge phase shift (539) representing a phase difference between a rotation of the first wedge (51) and a rotation of the second wedge (52).

7. The mobile scanning device (1) according to any one of the preceding claims, configured to provide a torsion angle interval, wherein: The scanning device (1) is configured to shift the scanning pattern (3, 300, 302) by any twist angle (301) within the twist angle interval.

8. The mobile scanning instrument (1) according to any one of the preceding claims, configured to acquire point cloud data at a scanning rate (60) of at least 10 Hz.

9. A method for surveying a target area (2) at an object distance (21) from the mobile scanning device (1) by means of a mobile scanning device (1) mounted on a carrier vehicle (4) travelling at a carrier speed (40), the method comprising: generating scanning pulses in a periodic manner having a pulse rate (634), The generated scanning pulses are emitted along an actual emission direction (33) about a main axis (10), wherein the actual emission direction (33) varies along a scanning pattern (3, 300, 302) in a periodic manner with a scanning rate (60), acquiring return pulses from the target area (2) and providing return pulse data regarding the acquired return pulses, deriving point cloud data representing the target area (2) based on the return pulse data, Features The method further comprises performing a torsion angle optimization (7), wherein the torsion angle optimization (7) comprises accessing (210, 400, 600, 633) input parameters including the pulse rate (634), the scan rate (60), the carrier speed (40), and the object distance (21); providing an optimization target representing point density and / or uniformity of the point cloud data, providing (309) an output twist angle (308) based on the optimization objective and the input parameters, wherein the twist angle represents a shift of the scan pattern (3, 300, 302) by a rotation about the main axis (10), The torsion angle (301) is set as the output torsion angle (308).

10. The method according to claim 9, wherein: The optimization goal is the highest achievable uniform point density (361) of the point cloud data.

11. The method according to claim 9, wherein: The method also includes task identification, wherein the task identification includes In particular, the type of scanning task is determined by determining the representative object distance variation. providing an evaluation of the optimization goal based on the determined scanning task, In particular, wherein feedback data is provided to a training process, said training process being based on a machine learning algorithm and providing updated information for said task identification, wherein said feedback data is provided by: By means of the feedback process, in particular by means of explicit feedback from an operator of the mobile scanning device (1), and / or In particular, implicit feedback based on at least one of the following: interpret the lack of explicit feedback as confirmation of the assignment, Comparison of actual point cloud data, in particular line-to-line distances (31), point-to-point distances (32) or strip widths (39, 392), with the evaluated optimization target.

12. The method according to any one of claims 9 to 11, wherein The survey is an aerial scan, The carrier speed (40) is the flight speed, The main axis (10) corresponds to the nadir direction, and The method is performed relative to a reference system defined by the flight speed and the nadir direction, in particular in a zero-twist state, the scanning pattern (3, 300, 302) having a maximum extension in a direction perpendicular to both the nadir direction and the flight speed.

13. The method according to any one of claims 9 to 12, wherein: The method further comprises: A pulse modulation arrangement is provided, wherein the pulse modulation arrangement comprises pseudo-random component, and / or Based on the geometry-dependent component provided by the actual emission direction (33), The generation of the scan pulses is modulated based on the pulse modulation arrangement.

14. The method according to any one of claims 9 to 13, further comprising post-processing, wherein The post-processing comprises the steps of discarding and / or smoothing a major portion of the point cloud data in order to improve homogeneity, The pulse rate (634) is set to oversample the target region (2) to allow the post-processing to be performed.

15. A computer program comprising a program code stored on a machine-readable medium or embodied by an electromagnetic wave comprising program code segments and having computer executable instructions for executing a torsion angle optimization algorithm (7) for a mobile scanning device (1) mounted on a carrier vehicle (4), The mobile scanning device (1) is configured as generating scanning pulses in a periodic manner having a pulse rate (634), The generated scanning pulses are emitted in an actual emission direction (33) about the main axis (10), wherein: The actual emission direction (33) varies along the scanning pattern (3, 300, 302) in a periodic manner with a scanning rate (60), acquiring a return pulse from the target area (2) and providing return pulse data regarding the acquired return pulse, deriving point cloud data representing the target area (2) based on the return pulse data; as well as Wherein, the torsion angle optimization algorithm (7) comprises: accessing (210, 400, 600, 633) input parameters including said pulse rate (634), said scan rate (60), carrier speed (40) and object distance (21); providing an optimization target representing point density and / or uniformity of the point cloud data, An output twist angle (308) is provided (309) based on the optimization objective and the input parameters, wherein the twist angle (301) represents a shift of the scan pattern (3, 300, 302) by a rotation about the main axis (10).

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