Method for diagnosing erroneous distance measurement of laser range finder unit, laser range finder unit and computer program

Abstract

The invention relates to a method for diagnosing erroneous distance measurements of a laser range finder unit (1, 105-109) due to contamination, in particular for use in the field of product dimensioning. The method comprises the following steps: emitting a continuous laser beam (7) onto the detection surface (20) by operation of a laser emitting unit (4) of the laser rangefinder unit; receiving, by a radiation sensor unit (5) of the laser rangefinder unit, a superposition of the laser light reflected from the detection surface (20) and the laser light reflected from the contaminated area (30) on the path of the laser beam (7); determining a phase difference between the reference phase and the phase of the received laser light, thereby deriving a signal indicative of the effect of the laser light reflected from the contaminated region (30) on the phase difference; and diagnosing an erroneous distance measurement due to contamination when the signal meets a fault criterion. The invention also relates to a laser range finder unit and a computer program for carrying out the method.

Description

Technical Field

[0001] The present invention relates to a method for diagnosing incorrect distance measurements caused by contamination in a laser rangefinder unit, and is particularly applicable to the field of product dimension measurement. In addition, the present invention relates to a laser rangefinder unit and a computer program for performing the method. Background Art

[0002] Laser rangefinder units are used for distance measurement and have various applications, including distance measurement, speed measurement, and dimension measurement. The technique of using a laser rangefinder unit for distance measurement is also known as LIDAR (Light Detection and Ranging).

[0003] Product dimension measurement involves estimating the amount of space occupied by a transported object in a trailer or a warehouse. For this purpose, a laser rangefinder unit can be used to create a 3D point cloud image of the object based on the measured distance to the object, and the dimensions of the object can be calculated using the 3D point cloud.

[0004] A laser rangefinder unit typically includes a laser emission unit and a radiation sensor unit. Possible implementations of the laser rangefinder unit can be based on, for example, time-of-flight or phase-shift measurements. In an implementation of a phase-shift based laser rangefinder unit, which is typically used for product dimension measurement, the laser emission unit can be operated to emit a continuous laser beam onto a detection surface, and the radiation sensor unit can be configured to receive the reflected portion of the laser beam, where, under ideal conditions, the reflected portion is or at least is a very large component of the laser reflected by the detection surface. Then, the laser rangefinder unit can determine the phase difference between the phase of the emitted laser (reference phase) and the phase of the received laser, and derive a distance value of the detection surface based on the phase difference. For product dimension measurement, a phase-shift based laser rangefinder unit generally has higher resolution, lower noise, and higher accuracy than a time-of-flight based laser rangefinder unit.

[0005] For example, when used for product dimension measurement, a laser rangefinder unit may operate in a dirty environment, such as an environment contaminated with dust, exhaust gas, etc. Then, a contamination area may be formed in the path of the emitted laser beam, for example, due to exhaust particles of a vehicle or dust particles accumulated on the window of the rangefinder unit housing, or due to a cloud of contamination particles floating in the air. It has been observed that using a laser rangefinder for distance measurement in a contaminated environment may result in incorrect distance values compared to the correct distance values measured by the laser rangefinder in a non-contaminated environment.

[0006] The above problems are known in the art, and various possible solutions have been proposed in the past, especially for laser rangefinders operating with pulsed laser beams. WO2021 / 065998A1 discloses a method for detecting dirt on the window of a laser rangefinder operating with a pulsed laser beam. The intensity histogram of the pulsed laser beam received from the window and / or the intensity histogram of the laser beam received from the target object are monitored. This histogram is compared with a reference histogram created using a rangefinder with a clean and non-degraded window.

[0007] US9772399B2 discloses a method in which the intensity of the laser pulse reflected from the window of the laser rangefinder unit is measured independently of the intensity of the laser pulse reflected from the object to be measured. If the intensity of the window-reflected pulse exceeds a threshold, it indicates that the window needs to be cleaned.

[0008] US2021 / 0223374A1 discloses a method in which the time of flight of the reflected laser beam is measured and compared with a threshold time to determine whether the detected laser is the laser reflected from the window. Since the window may always reflect some light even in a clean state, the amplitude of the received backscattered laser is also measured and compared with a minimum threshold amplitude. If the amplitude is greater than the minimum threshold amplitude, the window is diagnosed as being contaminated.

[0009] In the method disclosed in US11061121B2, a 3D imaging device measures the time of flight of the light beam reflected from the dirt on the window of the imaging device and determines the presence of dirt based on the magnitude of the reflected light.

[0010] EP3914932A1 discloses a method in which, based on a predetermined feature of an object scanned by a specific part of the housing of a rangefinder unit operating with a pulsed laser beam, it is determined whether the specific part is occluded.

[0011] The above methods are all applicable to laser rangefinder units operating with pulsed laser beams and require distinguishing between the laser reflected from the window and the laser reflected from the object to be measured.

[0012] In CN111429400B, the distances of multiple objects in the scene from the origin of the rangefinder coordinate system are determined based on the point cloud data created by the rangefinder system. When the measured distance of a certain object is approximately equal to the distance to the window, it can be diagnosed that there is dirt / obstacle on the window. This method requires analyzing the complete point cloud data, so the computational cost is high. Summary of the Invention

[0013] In view of these problems existing in the prior art, the object of the present invention is to provide a reliable and easily implementable method for diagnosing incorrect distance measurements caused by contamination in a phase-shift based laser rangefinder unit, and further to provide a laser rangefinder unit and a computer program for it to perform the said method.

[0014] The present invention is based on the following understanding: A signal can be obtained that indicates the influence of the laser reflected from the contaminated area on the phase difference determined by the phase-shift based laser rangefinder unit, and thus on the distance, height, and dimensions derived from the phase difference, where no additional installation or modification of the laser rangefinder unit is required, nor an operating mode of the laser rangefinder device that is not default implemented on state-of-the-art phase-shift based laser rangefinder devices. In particular, only routine distance measurements are required when there is no product to be measured within the scanning range of the laser rangefinder unit.

[0015] According to a first aspect of the present invention, there is provided a method for diagnosing incorrect distance measurements in a laser rangefinder unit caused by contamination, the laser rangefinder unit comprising: a laser emitting unit operable to emit a continuous laser beam onto a detection surface; and a radiation sensor unit arranged to receive the superposition of the laser reflected from the detection surface and the laser reflected from a contaminated area in the path of the laser beam, the method comprising:

[0016] The step of emitting the laser beam onto the detection surface by operating the laser emitting unit.

[0017] The step of determining the phase difference between a reference phase and the phase of the laser received by the radiation sensor unit, thereby obtaining a signal indicating the influence of the laser reflected from the contaminated area on the phase difference.

[0018] The step of diagnosing incorrect distance measurements due to contamination when the signal indicating the influence of contamination meets a fault criterion.

[0019] The continuous laser beam emitted by the laser emitting unit can be a modulated laser beam. In one example, the modulated laser beam can comprise modes of multiple frequencies and amplitudes.

[0020] In one embodiment, the detection surface is a flat surface. In one example, the detection surface can be the ground, particularly a horizontal floor, such as the floor of a warehouse.

[0021] In one example, the detection surface can be understood as a reference surface for diagnosing incorrect distance measurements.

[0022] In one example, the laser rangefinder unit can be arranged at a fixed distance from the detection surface. For example, the laser rangefinder unit can be mounted on a wall or a fixed bracket above the floor of a warehouse.

[0023] The laser distance measuring unit is arranged such that the laser emitted by the laser emitting unit is incident on the detection surface. The laser is reflected on the detection surface, and a part of the reflected laser is received by the radiation sensor unit. For distance measurement, the phase difference between the reference phase and the phase of the laser received by the radiation sensor unit is determined. In one embodiment, the reference phase is the phase of the emitted laser. In one example, the reference phase is the phase of the emitted laser at the initial emission moment. The method for determining the phase difference is known in the art. Then, the distance value can be calculated based on the determined phase difference.

[0024] In one example, the contamination area may be located between the detection surface and the laser emitting unit. In other words, the distance between the contamination area and the laser distance measuring unit may be less than the distance between the detection surface and the laser distance measuring unit.

[0025] In one example, the contamination area may include a contaminated surface or contaminated air, but is not limited thereto. The contamination area reflects the laser incident thereon.

[0026] Compared with the case without a contamination area, the laser reflected from the contamination area may affect the determined phase difference, that is, the phase difference may change compared with the non-contaminated case. In one example, the reflection of the laser from the contamination area may be mainly attributed to diffuse scattering.

[0027] The influence of contamination on the phase difference can be understood through a simple example. Assume that the emitted laser contains a sine wave with a fixed frequency, such as E sin(2πft), where E is the amplitude, f is the frequency, and t is the time. The laser reflected from the detection surface and received by the radiation sensor unit can be described as A sin(2πft + θ1), where A is the amplitude and θ1 is the first phase shift. The laser reflected from the contamination area and received by the radiation sensor unit can be described as B sin(2πft + θ2), where B is the amplitude and θ2 is the second phase shift. Since the distances between the contamination area and the detection surface and the laser distance measuring unit are different, the first phase shift and the second phase shift are different from each other. Then, the superposition of the laser reflected from the detection surface and the laser reflected from the contamination area is a sine wave given by the following formula, with an amplitude of K and a phase shift of θ3

[0028] K sin(2πft + θ3) = A sin(2πft + θ1) + B sin(2πft + θ2)

[0029] where And θ3 = arccos((Acos(θ1)+Bcos(θ2)) / K). Thus, the greater the amplitude B of the laser reflected from the contaminated area and received by the radiation sensor unit, the greater the impact on the phase difference between the phase θ3 of the laser received by the radiation sensor unit and the reference phase. The present invention believes that the amplitude B depends not only on, for example, the reflectivity of the contaminated area (determined by, for example, the amount of contamination), but also on the relative position between the laser emitting unit and the radiation sensor unit and the angle at which the laser beam impinges on the contaminant. This dependency affects the phase difference measured by the rangefinder unit and, in turn, any value derived therefrom, such as a distance, dimension, or height value, which are characteristic of the contamination effect and can be derived from the measured phase difference or the value derived therefrom. In particular, the impact on the phase difference depends on the emission angle, i.e., the angle at which the laser emitting unit of the rangefinder unit emits the laser beam. In contrast, it has been observed that error situations different from the backscattering caused by contamination, such as misalignment of the laser rangefinder unit or the laser emitting unit, do not have the same dependency on the transmission angle as the error situation caused by contamination. It has been observed that these different error situations generally only result in an offset, which means it is angle-independent.

[0030] For example, if the height profile generated by the laser rangefinder unit is considered, the error caused by contamination peaks at the following emission angle of the laser beam: this angle causes the laser beam reflected by the contaminant to coincide with the detection axis of the radiation sensor unit.

[0031] Generally, the evaluation of the height profile depends on the spatial orientation (position), particularly on at least one axis of a (usually Cartesian) coordinate system (such as the rangefinder coordinate system), rather than on the emission angle. In this case, the error caused by contamination peaks at the orientation (position) corresponding to the emission angle that causes the laser beam reflected by the contaminant to coincide with the detection axis of the radiation sensor unit.

[0032] Therefore, the above fault criterion may depend on the measured variable, and the signal indicating the impact of the laser reflected from the contaminated area on the phase difference is derived from this measured variable, and it can be or include, for example:

[0033] · Observation of the angle dependency, where the angle is the emission angle, which refers to the angle at which the laser emitting unit emits the laser beam.

[0034] · Observation of the non-linear dependency of the signal on the spatial orientation (position), particularly the orientation along the scan line or on the emission angle.

[0035] · Observation of signal peaks. The peak can be a bulge or a depression. For example, if the signal contains height information, a bulge can be observed. For example, if the signal contains distance information, especially the distance information about the distance between the detection surface and the laser rangefinder unit, a depression can be observed.

[0036] As described above, the amplitude B of the laser reflected from the contaminated area and received by the radiation sensor unit also depends on the relative arrangement between the laser emitting unit and the radiation sensor unit. Therefore, by knowing the above relative arrangement, the specific angular dependence, non-linear characteristics or peak orientation can be understood. Therefore, the signal is not necessarily or does not originate from a series of determined phase differences. Instead, an incorrect distance measurement can be diagnosed by determining a single phase difference. However, in many embodiments, the signal is or originates from a series of determined phase differences. This is because a series of phase differences can be determined by the normal operation of the laser rangefinder unit, and because the diagnosis becomes more reliable.

[0037] In one embodiment, the laser emitting unit and the radiation sensor unit are arranged coaxially with each other, thus sharing a common optical axis.

[0038] Furthermore, if the signal is or originates from a fan-shaped scanning manner of emitting a laser beam along a scanning line extending on the detection surface, the peak in the signal indicating the influence of the laser reflected from the contaminated area on the phase difference will appear at the following emission angle or azimuth: the laser beam impinges on the contaminated area at a 0° incident angle, which is defined with respect to the surface normal of the real surface or the imaginary surface at the laser beam incident position of the contaminated area. If the contaminated area is in a fixed position relative to the laser rangefinder unit, such as the window of the laser rangefinder unit housing, this angle or azimuth is particularly known.

[0039] Examples of signals indicating the influence of the laser reflected from the contaminated area on the phase difference will be introduced in more detail below.

[0040] The method according to the first aspect of the present invention allows for reliable diagnosis of incorrect distance measurement based on a signal indicating the influence of the laser reflected from the contaminated area on the determined phase difference. The method can be easily implemented using an existing laser rangefinder unit. The sensitivity of the method can be controlled by selecting a fault criterion.

[0041] In one embodiment of the method according to the first aspect of the present invention, emitting the laser beam onto the detection surface may include: emitting the laser beam in a fan-shaped scanning manner along a scanning line extending on the detection surface. This laser emission method can be used for dimensional measurement applications.

[0042] In one example, the detection surface for diagnosing incorrect distance measurement can be the ground, especially a horizontal ground.

[0043] The laser distance measuring unit can be arranged above the ground, and the scan line can extend on the ground, where the ground can be the ground on which the object to be measured is transported in a direction transverse to the scan line (e.g., by a forklift).

[0044] In another example, the detection surface for diagnosing an incorrect distance measurement can be the belt surface of a belt conveyor, in particular the horizontal surface of the belt or the belt conveyor.

[0045] The scan line can extend in a direction transverse to the conveying direction of the belt of the belt conveyor. Then, when no object is transported through the scan line, the method according to the first aspect of the present invention can be implemented.

[0046] In one embodiment, the signal indicating the effect of contamination can be derived from a series of phase differences corresponding to a series of positions along the scan line.

[0047] When the laser beam is emitted in a fan-shaped scanning manner along the scan line, the radiation sensor unit can receive the laser reflected from a series of positions along the scan line. The positions in this series of positions can be spaced apart from each other by a certain distance. Then, for each position in the series of positions, the phase difference corresponding to the position can be determined, and the signal indicating the effect of contamination can be derived from the series of phase differences. In other words, in one embodiment, the method includes emitting the laser beam in a fan-shaped scanning manner, where emitting the laser beam in a fan-shaped scanning manner includes emitting the laser beam to a series of positions along the scan line, where the method includes generating a series of phase differences by determining the phase difference between the reference phase and the laser phase received by the radiation sensor unit for each position in the series of positions, and where the signal is derived from the series of phase differences.

[0048] In one embodiment of the method according to the first aspect of the present invention, the signal can be derived based on a first phase difference corresponding to a position within a local area of the scan line, where the first phase difference is more strongly affected by the contamination compared to other phase differences corresponding to areas of the scan line outside the local area.

[0049] As described above, when the laser beam is emitted in a fan-shaped scanning manner along the scanning line, the influence of contamination on the phase difference may depend on the emission angle of the laser beam throughout the scan. Therefore, the influence of contamination on the phase difference associated with a position on the scanning line may be position-dependent. In particular, there may be a local region of the scanning line such that the first phase difference value corresponding to a position within this local region is more affected by contamination than other phase difference values corresponding to other positions outside this local region. In one example, the other phase difference values may be only minimally affected by the contamination or not affected at all, and the contamination may only affect the first phase difference value. In one example, the local region may be the region between a first point and a second point on the scanning line, where the first point and the second point are separated by a certain finite distance from each other. In one example, the size of the local region (e.g., the finite distance) may be much smaller than the size of the scanning line (i.e., the distance between the first end point and the second end point of the scanning line).

[0050] A laser rangefinder can be used for distance measurement. Thus, in an embodiment of the method according to the first aspect of the present invention, the method may further include: calculating a distance value for each position in the series of positions based on the determined phase difference value, and wherein the signal may be derived based on the distance value. When the distance value output by the laser rangefinder unit is used as the measurement result, this output can also be used to diagnose an incorrect distance measurement. Additionally, a fault criterion can be selected according to the desired accuracy for distance measurement.

[0051] In an embodiment of the method according to the first aspect of the present invention, particularly in an embodiment including calculating a distance value for each position in the series of positions, the method may further include calibrating the laser rangefinder unit such that a series of distance values calculated based on the phase difference values obtained from the operation of the laser rangefinder unit without contamination are consistent with a series of specified reference values. In the absence of contamination, the contaminated region does not exist.

[0052] In one example, a series of specified reference values may define a baseline curve in the rangefinder coordinate system, such as a straight line with a slope of zero. Then, a series of distance values calculated based on the phase difference obtained from the operation of the laser rangefinder unit in the presence of contamination may define a contamination curve (also referred to as an error curve) different from the baseline curve in the rangefinder coordinate system. Then, a signal indicating the influence of contamination can be derived by comparing the contamination curve with the baseline curve. The fault criterion may include a threshold for the curve comparison according to a certain deviation metric.

[0053] In one embodiment, compared with other phase differences corresponding to scan line regions outside the local region, the first phase difference in the local region is more strongly affected by the contamination, and thus the influence of the contamination can be reflected in the local peak of the distance value corresponding to the position within the local region. For example, a contamination curve defined by a series of distance values calculated from the phase differences determined by the operation of the laser rangefinder unit in a contaminated environment may exhibit a local peak, which does not exist in the curve defined by a series of distance values calculated from the phase differences obtained in the absence of contamination.

[0054] Therefore, according to another embodiment of the first aspect of the present invention, the signal indicating the influence of the contamination can be obtained as the height value of the peak in the distance values corresponding to the positions in the local region, and when the height value exceeds a threshold, the fault criterion can be satisfied.

[0055] As described above, the peak can be the peak on the curve defined by the series of distance values. In one embodiment, the height value can be determined as the difference between the value at the peak and a specified reference value at the corresponding position on the scan line. Preferably, the value at the peak is the maximum value. In another embodiment, the height value can be determined as the difference between the value at the peak and the distance value corresponding to the position outside the local region.

[0056] In one embodiment of the method, the laser beam is emitted in a fan-shaped scanning manner to a series of positions along the scan line, and the method includes: for each position in the series of positions, generating a series of phase differences by determining the phase difference between a reference phase and the phase of the laser received by the radiation sensor unit, and obtaining a signal from the series of phase differences. Obtaining the signal includes: determining the height curve of the scan line using the series of phase differences, and obtaining an error curve from the height curve.

[0057] The height curve of the scan line can be determined in the same manner as the scan line of the object to be measured. In particular, the height curve can be given with respect to the coordinate system of the rangefinder unit, especially a three-dimensional coordinate system, such as a Cartesian coordinate system, where one axis is the height axis, which means that the values assigned to this axis represent the height of the object.

[0058] In one embodiment, the height curve is given by a two-dimensional point cloud.

[0059] The error curve represents the influence of the error situation existing when generating a series of phase differences. In other words, the error curve includes factors that may cause incorrect distance measurements, such as contamination, misalignment of the laser rangefinder unit, and misalignment of the laser emission unit. As described above, due to the characteristic dependence of the error curve, the incorrect distance measurement caused by contamination can be diagnosed through this error curve, which contains the significant contribution of the incorrect distance measurement caused by contamination to a series of positions (or a series of emission angles, as the case may be).

[0060] In one embodiment, the error curve is a signal, and the method includes determining whether the error curve exhibits linear characteristics, where the failure criterion is non-linear characteristics.

[0061] In particular, when determining whether the error curve exhibits linear characteristics, the entire range covered by a series of positions can be considered.

[0062] Linear characteristics mean that the error curve can be approximated by a function of the type h(x) = ax + b, where a and b are constants.

[0063] In particular, determining whether the error curve exhibits linear characteristics can include determining whether the error curve contains peaks.

[0064] In one embodiment, deriving the error curve from the height curve includes determining the difference between the height curve and the target height curve.

[0065] The target height curve can be the height curve of a scan line, which is generated without any potential sources that may cause incorrect distance measurements.

[0066] For example, the target height curve can be determined immediately after setting (especially calibrating) the laser rangefinder unit.

[0067] The target height curve can include or be a series of specified reference values.

[0068] In one embodiment where the error curve is a signal, the method includes determining whether the error curve exhibits linear characteristics, where the failure criterion is non-linear characteristics, and the scan line extends on the above-mentioned horizontal ground or the horizontal surface of a belt (conveyor). In this case, the target height curve is a height curve with a constant height, which means h(x) = b, where b can be zero (b = 0).

[0069] In particular, in this case, the error curve corresponds to the height curve, and the incorrect distance measurement caused by contamination can be diagnosed by determining whether the height curve has a constant height.

[0070] Optionally, the method may include determining whether misalignment will result in an incorrect distance measurement by determining whether there is a constant offset of the height curve relative to a target height curve having a constant height.

[0071] In an embodiment of the first aspect of the present invention, the laser rangefinder unit may include a housing having a window that is at least partially transparent to the laser emitted by the laser emitting unit. The laser emitting unit and the radiation sensor unit may be disposed inside the housing, and the contaminated area may include a soiled area of the window of the housing. The window is located on the path of the laser from the laser emitting unit to the detection surface and from the detection surface to the radiation sensor unit. In this embodiment, when an incorrect distance measurement is diagnosed, the window may be cleaned to remove the contaminated area.

[0072] In an embodiment where the contaminated area includes a soiled area of the window of the housing, the signal indicating the influence of the laser reflected from the contaminated area on the phase difference and the fault criterion may be carried out according to any disclosed embodiment.

[0073] In an embodiment where the laser emitting unit and the radiation sensor unit of the method are disposed inside a housing having a window that may be contaminated, the laser emitting unit and the radiation sensor unit may be coaxially arranged with each other so as to share a common optical axis. When the arrangement is used in the following structure of the laser rangefinder unit, in which the laser beam is deflected in a fan-shaped scanning manner along the detection surface and thus deflected through the window in a scanning manner, the local area where the influence of the contamination on the phase difference is strong may include the first position of the scanning line, which is the incident position of the laser on the detection surface, and the laser is incident on the window at an incident angle of approximately zero degrees when emitted through the laser emitting unit, and this incident angle is defined with respect to the surface normal of the window surface.

[0074] Irrespective of the specific implementation of the method according to the first aspect of the present invention, the laser rangefinder unit may further include a controller, and determining the phase difference according to any disclosed embodiment and / or obtaining the signal according to any disclosed embodiment and / or calculating the distance value according to any disclosed embodiment and / or diagnosing an incorrect distance measurement according to any disclosed embodiment may be implemented by the controller, preferably by electronic data processing.

[0075] According to a second aspect of the present invention, there is provided a laser rangefinder unit including a laser emitting unit, a radiation sensor unit, and a controller.

[0076] The laser emitting unit is operative to emit a continuous laser beam onto a detection surface under the control of the controller. In particular, according to any embodiment disclosed in the first aspect of the present invention, the laser emitting unit is operative to emit a continuous laser beam onto a detection surface under the control of the controller.

[0077] The radiation sensor unit is arranged to receive the superposition of the laser reflected from the detection surface and the laser reflected from the contamination area on the path of the laser beam. In particular, the radiation sensor unit may be arranged according to any of the embodiments disclosed in the first aspect of the present invention.

[0078] The controller is operative to determine (preferably by electronic data processing) the phase difference between a reference phase and the phase of the laser received by the radiation sensor unit, thereby deriving a signal indicative of the influence of the laser reflected from the contamination area on the phase difference, and preferably by electronic data processing, when the signal indicative of the influence of the contamination satisfies a fault criterion, diagnose an incorrect distance measurement due to contamination. In particular, the phase difference, the signal may be obtained and / or determined, and the diagnosis may be performed according to any of the embodiments disclosed with respect to the first aspect, and the controller may be configured accordingly.

[0079] In particular, the laser rangefinder unit according to the second aspect of the present invention is configured to perform the method according to the first aspect of the present invention according to any of the disclosed embodiments. All of the above described with respect to the method according to the first aspect of the present invention also applies to the laser rangefinder unit according to the second aspect of the present invention.

[0080] According to a third aspect of the present invention, there is provided a computer program for diagnosing an incorrect distance measurement of a laser rangefinder unit due to contamination. The laser rangefinder unit includes: a laser emitting unit operative to emit a continuous laser beam onto a detection surface; and a radiation sensor unit arranged to receive the superposition of the laser reflected from the detection surface and the laser reflected from a contamination area in the path of the laser beam. The computer program contains instructions which, when executed by a computer, cause the computer to perform the following steps:

[0081] Deriving a signal indicative of the influence of the laser reflected from the contamination area on the phase difference, wherein the signal is derived from a determined phase difference between a reference phase and the phase of the laser received by the radiation sensor unit, a height value derived from the phase difference, or a distance value derived from the phase difference. In particular, the signal may be derived according to any of the embodiments disclosed with respect to the first and second aspects.

[0082] Diagnosing an incorrect distance measurement due to contamination when the signal indicative of the influence of the contamination satisfies a fault criterion. In particular, the diagnosis may be performed according to any of the embodiments disclosed with respect to the first and second aspects.

[0083] In particular, a computer program according to a third aspect of the present invention is configured to be used with a laser rangefinder unit according to a second aspect of the present invention and a method according to a first aspect of the present invention, in particular for performing the method on the laser rangefinder unit. The above-described content regarding the method according to the first aspect of the present invention also applies to the computer program according to the third aspect of the present invention.

[0084] In one embodiment, the computer program includes further instructions that, when executed by a computer, cause the computer to perform the following steps: determining a phase difference between a reference phase and the phase of the laser received by the radiation sensor unit, thereby obtaining a signal indicating the influence of the laser reflected from the contaminated area on the phase difference. The phase difference can be obtained and / or determined according to any of the embodiments disclosed with respect to the first and second aspects.

[0085] In other words, the computer program includes instructions that, when executed by a computer, cause the computer to perform the following steps: obtaining, according to any of the embodiments disclosed with respect to the first and second aspects of the present invention, a signal representing the influence of the laser reflected from the contaminated area on the phase difference; diagnosing, according to any of the embodiments disclosed with respect to the first and second aspects of the present invention, an incorrect distance measurement due to contamination; and optionally, determining, according to any of the disclosed embodiments, a phase difference between a reference phase and the phase of the laser received by the radiation sensor unit.

[0086] The present invention also relates to a reproducible computer-readable signal encoding a computer program that, when loaded onto and executed by a computer, causes the computer to perform the following steps: obtaining, according to any of the embodiments disclosed with respect to the first and second aspects of the present invention, a signal indicating the influence of the laser reflected from the contaminated area on the phase difference; diagnosing, according to any of the embodiments disclosed with respect to the first and second aspects of the present invention, an incorrect distance measurement due to contamination; and optionally, determining, according to any of the disclosed embodiments, a phase difference between a reference phase and the phase of the laser received by the radiation sensor unit.

[0087] The present invention also relates to a computer-readable medium storing instructions that, when executed by a computer, cause the computer to perform the following steps: obtaining, according to any of the embodiments disclosed with respect to the first and second aspects of the present invention, a signal representing the influence of the laser reflected from the contaminated area on the phase difference; diagnosing, according to any of the embodiments disclosed with respect to the first and second aspects of the present invention, an incorrect distance measurement due to contamination; and optionally, determining, according to any of the disclosed embodiments, a phase difference between a reference phase and the phase of the laser received by the radiation sensor unit.

[0088] The invention also relates to a data carrier signal carrying instructions which, when executed by a computer, cause the computer to perform the following steps: obtaining a signal indicating the influence of the laser reflected from the contaminated area on the phase difference, according to any embodiment disclosed in the first and second aspects of the invention; diagnosing an incorrect distance measurement due to contamination, according to any embodiment disclosed in the first and second aspects of the invention; and optionally, determining the phase difference between a reference phase and the phase of the laser received by the radiation sensor unit, according to any embodiment disclosed.

[0089] The invention also relates to a method of manufacturing a non - transitory computer - readable medium, the method comprising storing computer - executable instructions on the computer - readable medium which, when executed by a processor of a computing system, cause the computing system to perform the following steps: obtaining a signal indicating the influence of the laser reflected from the contaminated area on the phase difference, according to any embodiment disclosed in the first and second aspects of the invention; diagnosing an incorrect distance measurement due to contamination, according to any embodiment disclosed in the first and second aspects of the invention; and optionally, determining the phase difference between a reference phase and the phase of the laser received by the radiation sensor unit, according to any embodiment disclosed. BRIEF DESCRIPTION OF THE DRAWINGS

[0090] In the following description, the invention will be described in more detail by way of example with reference to the accompanying drawings. In the drawings,

[0091] Figure 1 a simplified schematic view of a warehouse working environment in which the method, the laser rangefinder unit and the computer program according to the invention are applied is shown;

[0092] Figure 2 a schematic cross - sectional view of a laser rangefinder unit according to the second aspect of the invention operating in a clean, non - contaminated environment is shown;

[0093] Figure 3 schematically shows a laser rangefinder unit operating in a contaminated environment Figure 2 ; and

[0094] Figure 4 shows a height profile obtained by the rangefinder unit shown in Figure 3 operating in a contaminated environment. DETAILED DESCRIPTION

[0095] Figure 1 A simplified schematic view of the working environment of a warehouse 100 in which the method, the laser rangefinder unit and the computer program according to the invention can be applied is shown. In Figure 1In it, four laser rangefinder units 105, 106, 107, and 108 are installed on the wall 109 of the warehouse 100. The package 103, which is the measurement object of the laser rangefinder units 105, 106, 107, 108, is loaded on the pallet 102 by the forklift 101, and the forklift 101 moves on the ground 104 of the warehouse 100. In the illustrated embodiment, the ground 104 serves as the detection surface 20.

[0096] Each of the four laser rangefinder units 105, 106, 107, 108 is configured to emit a continuous laser beam 7 onto the detection surface 20, as Figure 2 shown. In one example, the four laser rangefinder units 105, 106, 107, 108 can be configured to emit a modulated laser beam 7 within the visible spectrum, and its frequency is preferably in the range of 10 to 100 MHz. The laser rangefinder units 105, 106, 107, 108 can be configured to emit a visible modulated beam 7, for example, with a wavelength of 660 nm.

[0097] The forklift 101 can transport the package 103 in a transport direction transverse to the scan lines 25 (see Figure 2 ) projected by the laser rangefinder units 105, 106, 107, 108 onto the detection surface 20. Thus, the laser rangefinder units 105, 106, 107, 108 can calculate distance values based on the laser reflected by the package during transportation, thereby creating a 3D point cloud image of the package 103. In this way, dimensional measurement can be performed.

[0098] Figure 2 Fig. shows a schematic cross-sectional view of the laser rangefinder unit 1 according to an embodiment of the second aspect of the present invention, and this laser rangefinder unit is suitable for implementing the method according to the first aspect of the present invention. The configurations of the laser rangefinder units 105, 106, 107, 108 can be the same as the configuration of the laser rangefinder unit 1.

[0099] As Figure 2 shown, the laser rangefinder unit 1 in this embodiment includes a housing 2 with a window 3. A laser emitting unit 4, a radiation sensor unit 5, and a controller 6 are arranged inside the housing 2.

[0100] The laser emitting unit 4 is operated under the control of the controller 6 to emit a continuous laser beam 7. In one example, the laser beam 7 can be a modulated laser beam. The window 3 is at least partially transparent to the emitted laser 7. Preferably, the window 3 is completely or almost completely transparent to the emitted laser.

[0101] The laser rangefinder unit 1 can also be operated under the control of the controller 6 to deflect the laser beam 7 emitted through the window 3 of the housing 2 so as to perform a fan-shaped scanning manner along the scanning line 25 extending on the detection surface 20 from the first end point 23 to the second end point 24. The detection surface 20 can be Figure 1 the ground 104 or the wall surface 109 in

[0102] In Figure 2 the illustrated embodiment, the laser emitting unit 4 and the radiation sensor unit 5 are coaxially arranged with each other, so as to share a common optical axis. However, the present invention is not limited to such an arrangement.

[0103] From Figure 2 it can be seen that emitting the laser beam 7 in a fan-shaped scanning manner can include emitting the laser beam 7 to a series of positions 26 along the scanning line 25.

[0104] The radiation sensor unit 5 is arranged to receive the superposition of the laser reflected from the detection surface 20 and from a contamination area ( Figure 2 not shown in Figure 3 ) in the path of the laser beam 7. The controller 6 is preferably operated by electronic data processing to determine the phase difference between the reference phase and the phase of the received laser. When the radiation sensor unit 5 receives the reflected laser from each position 26 in the series of positions, the controller 6 can be configured to generate a series of phase difference values by determining the phase difference between the reference phase and the phase of the laser received by the radiation sensor unit 5 for each position 26 in the series of positions. Then, in one example, the phase difference value associated with the position 26 can indicate the height value or the distance value associated with the position 26.

[0105] In one example, the controller 6 can be configured to preferably determine, by electronic data processing and using a series of phase difference values, a height curve that can indicate the height of the detection surface 20. Additionally or alternatively, the laser rangefinder unit 1 can be configured to, preferably through the controller 6, calculate a distance value for each position 26 in a series of positions based on a series of determined phase difference values. Each distance value can indicate the distance between the position 26 and the laser rangefinder unit 1.

[0106] Figure 2 The illustrated laser rangefinder unit 1 can be calibrated. For this purpose, the laser rangefinder unit 1 can be operated in a clean and pollution-free environment and can be calibrated so that the height curve or a series of distance values respectively coincide with a specified reference height curve or a series of specified reference values.

[0107] Figure 3 is schematically shown Figure 2 the situation when the laser rangefinder unit 1 shown operates in a contaminated environment. For example, a contamination area in the form of a dirt layer 30 may be formed on the window 3 of the housing 2 of the laser rangefinder unit 1. However, in other cases, the contamination area may be formed at different positions in the optical path of the laser rangefinder unit 1, for example, a dirt cloud is formed between the window 3 and the detection surface 20.

[0108] As Figure 3 shown, in the presence of contamination, the laser will not only be reflected from the detection surface 20 (for the sake of illustration, the reflections from three positions on the scan line 25, namely the first and second end points 23, 24 and the center point 22, are shown in the figure), but also the laser will be reflected from the dirt layer 30 on the window 3 (for the sake of illustration, the reflections from three positions 12, 13, 14 are shown in the figure). In Figure 3 the example shown, the reflections are shown as diffuse. However, the present invention is not limited thereto.

[0109] The radiation sensor unit 5 is arranged to receive the superposition of the laser reflected from the detection surface 20 and the laser reflected from the dirt layer (contamination area) 30. The laser reflected from the dirt layer 30 will affect the phase of the received laser, thereby affecting the phase difference between the reference phase and the phase of the received laser, as described in the overview section of the above specification. In Figure 3 the example shown, the laser reflected from the position 12 of the contamination area 30 (where the laser 7 is incident at an incident angle of 0° (the incident angle is defined with respect to the surface normal of the real or imaginary surface of the contamination area 30 at the laser incident position)) may have a greater impact on the phase difference than the laser reflected from other positions of the contamination area 30, especially the positions 13 and 14 (where the laser 7 is incident at an incident angle greater than 0°).

[0110] The controller 6 is also operable to preferably derive a signal indicating the influence of the laser reflected from the contamination area 30 on the phase difference through electronic data processing.

[0111] In one example, deriving the signal may include using a series of phase difference values to determine the height curve 200 of the scan line and deriving an error curve based on the height curve. Figure 4It is a diagram of such a height curve obtained in the presence of contamination. The height curve 200 is presented with respect to a coordinate system, where the X-axis represents the position 26 along the scan line 25. The minimum X value can be associated with the first end point 23 of the scan line 25, and the maximum X value can be associated with the second end point 24 of the scan line 25. Each position 26 along the scan line 25 can be associated with a height value Z(X). The laser rangefinder unit 1 can be calibrated such that in the absence of contamination, the height value Z(X) associated with each position 26 is zero. That is, in the absence of contamination, the height curve is a straight line passing through the origin Z(X)=0 and having a slope of zero. In some examples, the height curve in the absence of contamination can be defined as the target height curve.

[0112] Figure 4 The shown height curve 200 can be as Figure 3 shown obtained by the laser rangefinder unit 1 when there is a dirt layer 30 on the window 3. From Figure 4 it can be seen that the height curve 200 exhibits a non-linear characteristic, and the X value of the peak 201 corresponds to a local area of the scan line 25.

[0113] In one example, an error curve can be obtained by preferably determining the difference between the height curve 200 and the target height curve by the controller 6, and the signal can be the error curve. For Figure 4 the shown example, each height value of the target height curve may be zero, and the error curve may be the same as the height curve. The controller 6 can also be operable to preferably determine by electronic data processing whether the error curve exhibits a linear characteristic. When the signal meets the fault criterion, an incorrect distance measurement due to contamination can preferably be diagnosed by the controller 6, and the fault criterion is a non-linear characteristic. For example, the controller 6 can be configured to determine whether the error curve (in this case, the height curve) contains a peak 201 (in this case, a bulge) by looking for height values Z(X) above a threshold TV (especially in a local area near the X value corresponding to the local area of the scan line 25). As described above, when the laser is incident on the contaminated area at an incident angle of 0°, the influence of the laser reflected from the contaminated area on the phase difference may be the strongest. Since the position of the dirt layer 30 relative to the laser rangefinder unit 1 is fixed, this position is known. Therefore, in the presence of contamination, the local area of the expected peak of the scan line is also known.

[0114] In other embodiments, the signal can be obtained based on the distance values associated with the positions 26 along the scan line 25, the signal can be obtained as the height value of the peak among the distance values, and when the height value exceeds the threshold, the fault criterion can be satisfied.

[0115] List of reference numerals

[0116] 1 Laser rangefinder unit

[0117] 2 Housing

[0118] 3 Window

[0119] 4 Laser emitting unit

[0120] 5 Radiation sensor unit

[0121] 6 Controller

[0122] 7 Emitted laser beam

[0123] 8 Rotating polygon mirror

[0124] 12, 13, 14 Positions on the soiled window

[0125] 20 Detection surface

[0126] 22 Center point

[0127] 23 First endpoint

[0128] 24 Second endpoint

[0129] 25 Scanning line

[0130] 26 Position along the scanning line

[0131] 30 Soiled layer

[0132] 100 Warehouse

[0133] 101 Forklift

[0134] 102 Pallet

[0135] 103 Package (object to be measured)

[0136] 104 Ground

[0137] 105, 106, 107, 108 Laser rangefinder units

[0138] 109 Wall

[0139] 200 Height curve

[0140] 201 Peak

[0141] TV First threshold

Claims

1. A method for diagnosing an incorrect distance measurement caused by contamination in a laser rangefinder unit (1, 105 - 108), the laser rangefinder unit including a laser emission unit (4) and a radiation sensor unit (5), the laser emission unit being operative to emit a continuous laser beam (7) onto a detection surface (20), the radiation sensor unit being arranged to receive a superposition of the laser reflected from the detection surface and the laser reflected from a contamination area (30) in the path of the laser beam, the method comprising: emitting the laser beam onto the detection surface by operation of the laser emission unit; determining a phase difference between a reference phase and the phase of the laser received by the radiation sensor unit, thereby obtaining a signal indicating the influence of the laser reflected from the contamination area on the phase difference; when the signal meets a fault criterion, diagnosing an incorrect distance measurement due to contamination.

2. The method according to claim 1, wherein, The emission of the laser beam (7) onto the detection surface (20) includes emitting the laser beam in a fan-shaped scanning manner along a scan line (25) extending on the detection surface (20).

3. The method according to claim 2, wherein The detection surface (20) is the ground (104) or the surface of a belt or a belt conveyor.

4. The method according to claim 2 or 3, wherein, Emitting the laser beam (7) in a fan-shaped scanning manner includes emitting the laser beam to a series of positions (26) along the scan line (25), wherein the method includes generating a series of phase difference values by determining the phase difference between the reference phase and the phase of the laser received by the radiation sensor unit (5) for each position in the series of positions, and wherein the signal is obtained from the series of phase difference values.

5. The method according to claim 4, wherein, Obtaining the signal includes determining a height curve (200) of the scan line (25) using the series of phase difference values and obtaining an error curve from the height curve.

6. The method according to claim 5, wherein, The error curve is the signal, and the method includes determining whether the error curve exhibits a linear characteristic, wherein the fault criterion is a non-linear characteristic.

7. The method according to claim 5 or 6, wherein Obtaining the error curve from the height curve (200) includes determining the difference between the height curve and a target height curve.

8. The method according to claim 4, wherein The method further includes calculating a distance value for each position in the series of positions (26) based on the determined phase difference values, and obtaining the signal based on the distance values.

9. The method according to claim 8, wherein, The method further includes calibrating the laser rangefinder unit such that a series of distance values calculated based on phase difference values obtained from the operation of the laser rangefinder unit without contamination is consistent with a series of specified reference values.

10. The method according to any one of claims 1 to 9, wherein, The signal is obtained based on a first phase difference value corresponding to a position within a local area of the scan line (25), and within the local area, the first phase difference value is more strongly affected by the contamination compared to other phase difference values corresponding to areas of the scan line outside the local area.

11. The method according to claim 10, including calculating a distance value for each first phase difference value, wherein the signal is obtained as the height value of a peak (201) among the distance values corresponding to the positions in the local area, and when the height value exceeds a threshold (TV), the fault criterion is met.

12. The method according to claim 11, wherein, The method includes calibrating a laser rangefinder unit such that a series of distance values calculated based on the phase difference obtained from the operation of the laser rangefinder unit without contamination is consistent with a series of specified reference values, and wherein the height value is determined as the difference between the value of the peak and the specified reference value at the corresponding position on the scan line (25).

13. The method according to any one of claims 1-12, wherein, The laser rangefinder unit includes a housing (2) having a window (3), the window being at least partially transparent to the laser (7) emitted by the laser emitting unit (4), the laser emitting unit (4) and the radiation sensor unit (5) being disposed inside the housing, and the contamination area including a soiled area (30) of the window of the housing.

14. A laser rangefinder unit (1, 105 - 108), the laser rangefinder unit including a laser emitting unit (4), a radiation sensor unit (5), and a controller (6). The laser emitting unit is operative to emit a continuous laser beam (7) onto a detection surface (20) under the control of the controller. The radiation sensor unit can be arranged to receive the superposition of the laser reflected from the detection surface and the laser reflected from a contamination area (30) in the path of the laser beam. The controller is operative to, preferably by electronic data processing, determine the phase difference between a reference phase and the phase of the laser received by the radiation sensor unit, thereby obtaining a signal indicating the influence of the laser reflected from the contamination area on the phase difference, and when the signal meets a fault criterion, preferably by electronic data processing, diagnose an incorrect distance measurement due to contamination.

15. A computer program for diagnosing incorrect distance measurements due to contamination in a laser distance meter unit (1, 105 - 108), said laser distance meter unit comprising: A laser emitting unit (4) operative to emit a continuous laser beam (7) onto a detection surface (20). And a radiation sensor unit (5) arranged to receive the superposition of the laser reflected from the detection surface and the laser reflected from a contamination area (30) in the path of the laser beam, the computer program comprising instructions which, when executed by a computer, cause the computer to perform the following steps: Obtain a signal indicating the influence of the laser reflected from the contamination area on the phase difference, wherein the signal is obtained from the determined phase difference between the reference phase and the phase of the laser received by the radiation sensor unit, from the height value derived from the phase difference, or from the distance value derived from the phase difference. When the signal indicating the influence of contamination meets a fault criterion, diagnose an incorrect distance measurement due to contamination.

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