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    • 专利摘要:
    Method for measuring the distance of a target (Ui) by means of laser pulses (Sm) emitted by a transmitter (2), reflected at the target (Ui) and received in a receiver (4), the transmitter (2) and receiver (4) a common support structure (7) at which spurious reflections (Xm) of the laser pulses (Sm) occur, comprising: emitting a laser pulse (Sm) from the transmitter (2) to a target (Ud at a transmission time (tsml on a time scale ( obtaining a received signal (e (t)) representing a target reflection (Ern) of the laser pulse (Sm) picked up by the receiver (4) as an amplitude variation with respect to the transmission time (tsml); subtracting a stored reference Disturbing signal (Xref (t)) from the received signal (e (t)) to obtain an adjusted received signal (f (t)); detecting a pulse (Ern) and its occurrence time point (tEml in the adjusted received signal (f (t )) or a signal derived therefrom, andM eat the distance (Dm) of the target (Ui) from the runtime between the time of sending and the time of occurrence
    公开号:AT513402A1
    申请号:T504302012
    申请日:2012-10-05
    公开日:2014-04-15
    发明作者:Peter Dipl Ing Rieger；Andreas Dr Ullrich
    申请人:Riegl Laser Measurement Sys；
    IPC主号:
    专利说明:

    PATENT OFFICER DIPL.-ING. Dr.techn. ANDREAS WEISER EUROPEAN PATENT AND TRADEMARK ATTORNEY A-l 130 VIENNA · KOPFGASSE 7 05199 RIEGL Laser Measurement Systems GmbH A-358 0 Horn (AT)
    The present invention relates to a method for measuring the distance of a target by means of transit time measurement on laser pulses which are emitted by a transmitter, reflected at the target and received in a receiver.
    Methods of this type are used both in laser rangefinders and laser scanners, which measure a plurality of adjacent targets in the environment by means of a pulsed, mostly fan-shaped scanning laser beam in order to create a 3D image ("point cloud") of the environment. This often results in the problem that laser transmitters and receivers are mounted on a common support structure, for example in a protective housing, on which spurious reflections of the laser beam occur that simulate a very close target. For example, housing internal surfaces, internal deflection mirrors or a glazed housing exit window may cause backscattering or "window reflections" of the laser pulses directly to the receiver. By constructive measures such as tilting the exit window relative to the laser beam, applying anti-reflective coatings, etc., the spurious reflections may be reduced, but not completely eliminated. Even small amounts of the emitted laser power, which are backscattered to such a short distance, but TEL .: (+43 1) 879 17 06 · FAX: (+43 1) 879 17 07 · E-MAIL: MAIL@PATENTE.NET -WEB: WWW.PATENTE.NET FIRST BANK: 038-56704 BLZ: 20111 TBAN: ATIO2 / 4y303856704 BIC: GIBAATWW · VAT: AT U 53832900 2 Significant glitches in the receiver trigger, which falsely specify short-term goals.
    A known measure for the treatment of this problem is the hiding such Störreflexionen by so-called. "Gating". Any pulses that arrive in the receiver within a short time window ("lock window") after the transmission of a laser pulse are ignored. However, this has the disadvantage that true short-term goals can not be measured. While the measurement of such short-range goals could still be dispensed with in practice, this problem also occurs at regular intervals by the transceiver, for example. each at so-called. MTA zone boundaries: Modern laser range finders or scanners operate with high pulse power over long distances and / or high pulse repetition rate for the rapid creation of a variety of distance measuring points. In this case, the next pulse can be sent even before the reflection of the last pulse was received, so that the incoming received pulses can not be clearly assigned to their respective transmit pulse. This is known as "multiple time around" (MTA) or "multiple pulses in the air" problem. The maximum size dmax of the clearly measurable distance range, the so-called MTA zone, results from the pulse repetition rate PRR and the speed of light c to dmax = c / (2-PRR).
    In order to overcome the MTA limit of the pulse repetition rate, it is known to vary the individual pulses by varying their 3/47 3
    Polarization, amplitude or frequency distinguishable from each other in order to assign the echo pulses accordingly. In the documents AT 510 296 B1 (EP 2 469 297 A1) and AT 511 310 A1 (not prepublished) by the same Applicant, further methods are presented which make possible an MTA-zone-correct allocation of the received pulses; With these methods, distances can be measured correctly over MTA zones.
    For MTA cross-zone measurements, the gating " however, particularly unpleasant effects: long-range targets that are near an MTA zone boundary result in receive pulses that fall near the time of the transmission of a subsequently transmitted laser pulse, and are therefore suppressed by its blocking window. Remote targets located near MTA zone boundaries can therefore not be measured using the known gating range finding methods, which have "blind ranges". For example, if a terrain is surveyed from an aircraft-based laser scanner and the altitude over ground is passing through an MTA zone boundary, e.g. because the terrain is increasing or decreasing or the aircraft is changing altitude, there will be measurement failures in the survey point cloud of the terrain.
    The object of the invention is to overcome the described disadvantages of the prior art and to provide a distance measuring method which can also detect near targets and in particular does not cause measured value failures in the case of MTA zone crossing measurement.
    This object is achieved according to the invention by a method of measuring the distance of a target by means of laser pulses emitted by a transmitter, reflected at the target and received in a receiver, the transmitter and receiver having a common support structure at which spurious reflections of the laser pulses occur, comprising:
    Emitting a laser pulse from the transmitter to a target at a time of transmission on a time scale /
    Obtaining a received signal representing a target reflection of the laser pulse intercepted by the receiver as an amplitude versus time dependency related to the time of transmission;
    Subtracting a stored reference interfering signal representing a spurious reflection of the laser pulse that can be captured by the receiver as an amplitude versus time characteristic of the laser pulse from the received signal to obtain an adjusted received signal;
    Detecting a pulse and its occurrence time in the adjusted received signal or a signal derived therefrom; and
    Measuring the distance of the target from the transit time between the time of transmission and the time of occurrence.
    According to the invention, a pre-stored spurious reflection model "is used. Time-correctly deducted from the received signal and 5/47 5 compensated with the Störreflexion directly in the received signal. Thus, a complete suppression (lock window) of the received signal after each transmission time is no longer required, so that also Nahziele can be measured correctly. With repetitive laser pulse emission, which leads to the discussed MTA zones, it is thus possible for the first time to measure long-range targets also at the MTA zone transitions. This makes it possible for the first time to use MTA-zone-based distance measuring and scanning methods without any "blind ranges". be designed.
    The reference interference signal can be obtained and pre-stored in various ways. According to a first embodiment of the invention, the reference interference signal is approximately calculated and stored from design parameters of the support structure. For example, from the knowledge of the position and Streufeflektivität a glass exit window relative to the transmitter and receiver, the expected amplitude, pulse shape and timing of the interference pulse in the receiver calculated and stored as the reference to the transmission time reference interference signal in a memory.
    According to a second embodiment of the invention, a noise signal caused by spurious reflection of a previous laser pulse is recorded and stored as a reference spurious signal. The method of the invention then has a first calibration phase in which the reference interference signal is recorded and a second measurement phase in which the reference interference signal recorded is used to compensate for the current measurement.
    According to a third embodiment of the invention, the reference interference signal may also be "online". during a plurality of measuring operations, e.g. in distance measuring or laser scanners with repetitive laser pulses. For this purpose, a plurality of laser pulses are preferably emitted at successive transmission times, and a plurality of time segments of the received signal, each starting after a transmission time, are checked for pulses occurring therein with the same amplitude and / or time slot and, if such are present, a reference interference signal is formed and stored therefrom ,
    In this embodiment, the reference interference signal is generated from a statistical evaluation of the reception pulses from the near-target and MTA zone boundary regions. The statistical analysis is based on the knowledge that "real" Close-or. MTA zone boundary targets - compared to the permanent support structure - comparatively "rare" are from which the reference interference signal due to the support structure can be determined.
    The described online determination of the reference interference signal may be performed once in the course of a repetitive range finding or scanning operation or may be repeated, e.g. every other, every second, every third, etc. laser pulse. A preferred variant of the invention is accordingly characterized in that the formation and storage of the reference interference 7
    Signal is repeated after a predetermined first number of laser pulses, each of the successively formed reference noise is formed on the basis of a predetermined second number of each preceding laser pulses.
    In a further embodiment of the invention, impulses from further past periods of time can be weighted weaker in the said statistical examination to a majority in order to take greater account of the respective current reflection state of the support structure.
    If the reference noise signal is not pre-modeled by calculation but is determined from records of spurious reflections, be it in an upstream calibration phase or "online", an alarm message may be generated if it exceeds a predetermined threshold. As a result, for example, excessive contamination, damage o the aging of the exit window of a laser range finder or scanner, a mechanical defect in the housing, a visual obstruction, etc. detected and the user to the need for cleaning, maintenance or repair are pointed.
    If the reference noise signal is repeated "online" is determined, alternatively or additionally, an alarm message can be generated even if the difference between two successively formed reference noise exceeds a predetermined threshold. Thereby, for example, a sudden deterioration of the exit window transmittance of a laser rangefinder or scanner can be detected, e.g. because of dirt, fogging, damage etc.
    As discussed, the method of the invention is particularly suitable for repetitive laser pulse measurements in which a plurality of laser pulses are transmitted at successive transmission times and the received signal accordingly represents a plurality of successively captured target reflections, wherein for each laser pulse the reference interfering signal is time-referenced once at the time of transmission of that laser pulse from the received signal is subtracted.
    Particularly suitable is the method for use in laser scanning, in which a plurality of laser pulses are emitted in different directions to different transmission destinations and the received signal correspondingly represents a plurality of successively captured target reflections from different targets, with a separate reference interference signal is stored for each transmission direction, and wherein each laser pulse, the reference interference signal associated with its transmission direction is time-referenced to the transmission time of this laser pulse is subtracted from the received signal. As a result, scanning angle-dependent interfering reflections can be taken into account, as occur, for example, at a glass exit window when it is irradiated at different angles of incidence. 9/47 9
    The method of the invention is particularly advantageous in connection with MTA zone-selective or MTA zone-exceeding measurement methods, as described in the specifications of Applicants AT 510 296 Bl and AT 511 310 A1 (not prepublished). In the MTA zone selective measurement, for a selected MTA zone, the sensitivity for the receive pulse detection can be increased by emitting the laser pulses with varying pulse intervals and a plurality of time segments of the adjusted receive signal, each starting at a predetermined distance from the transmission time of a laser pulse to a sum signal are superimposed, in which subsequently a pulse is detected.
    The emission of laser pulses with varying pulse intervals ("phase jitter") is known per se as PRR modulation and is used in this embodiment, from "false". Receive pulses originating from MTA zones "jitter", while receive pulses from "right". MTA zones remain un-jittered and thereby additively overlap to a larger, more easily detectable receive pulse.
    Alternatively, laser pulses with varying pulse intervals are emitted for MTA-zone-exceeding measurement and transit times are respectively measured between a laser pulse and that detected in the adjusted received signal pulse, which lies in a time window which starts at a predetermined distance from the transmission time of this laser pulse. In this case, a first sequence of transit times is preferably measured on the basis of a first predetermined distance and a second sequence of transit times on the basis of a second predetermined distance, and that sequence of transit times which is least affected by the variation of the pulse intervals for the Distance measurement used. This allows receive pulses to be automatically assigned to the correct MTA zone, as only the "correct" Receive pulses in each "correct" MTA zone resulted in unjointed runtimes.
    The aforementioned MTA zone-selective and MTA zonenüberschrei-border measuring methods build on the invention according to the invention noise-corrected received signal and thereby draw great advantage of the invention Störreflexionskompensation: For the first time, even marginal areas of the MTA zones can be measured correctly.
    The method of the invention is suitable for both real-time and batch processing. In the case of real-time processing, the reference interference signal is preferably progressively retrieved from its memory and, with reference to the time of transmission of the laser pulse, subtracted in real time from the incoming reception signal. In batch processing, the entire received signal is first recorded and then the reference interfering signal is time-referenced subtracted to the transmission time of the laser pulse thereof.
    The invention will be explained in more detail with reference to embodiments illustrated in the accompanying drawings. In the drawings show: 11/47 11
    1 shows a laser rangefinder for carrying out the method of the invention in block diagram form with schematically drawn beam ratios when surveying an environmental target.
    FIG. 2 shows timing diagrams of exemplary waveforms in the process of the invention using the arrangement of FIG. 1; FIG.
    FIG. 3 shows the measurement of environmental targets in various MTA zones from an aircraft-based laser scanner; FIG.
    FIG. 4 shows timing diagrams of exemplary waveforms for the surveying situations of FIG. 3; FIG. Figures 5 and 6 are scan images of the laser scanner of Figure 3 in the region of an MTA zone junction without (Figure 5) and with (Figure 6) application of the method of the invention;
    FIG. 7 shows the online determination of reference interference signals in a repetitive embodiment of the method of the invention; FIG.
    FIG. 8 shows a signal histogram for forming the reference interference signals in FIG. 7 by means of statistical evaluation; FIG. Figures 9 and 10 show a laser scanner for carrying out the method of the invention in block diagram form and schematic side view (Figure 9) and in a fragmentary plan view (Figure 10).
    Fig. 11 illustrates the application of the method of the invention to an MTA zone selective ranging method; and 12/47 12
    Figure 12 illustrates the application of the method of the invention to an MTA cross-zone range finding method.
    1 shows a laser range finder 1 for measuring the distance Di to a target Ui of an environment U. The laser range finder 1 includes a laser transmitter 2 having one or more laser pulses Si, S2, S3 /..
    Target Ui sends out. The laser pulses Sm are respectively emitted at a transmission time tSi, ts2, ..., generally tSm / in a system-wide time scale t, e.g. The world time or a system clock of the laser rangefinder 1. The emitted laser pulses Sm can also be understood as a directed to the target Ui pulsed laser measuring beam 3. The laser pulses Sm reflected by the target Ui are received in a receiver 4 of the laser rangefinder 1 as receive pulses Ei, E2,..., Generally Em.
    From the transit time ATm of each target-reflected laser pulse Sm, i. The difference between the time of occurrence tEm of the receiving pulse Em and the sending time tSm of the transmitting pulse Sm, can be calculated in a known manner the distance Dm = c -ATm / 2 (c = speed of light) from the laser rangefinder 1 to the measured ambient target Um. Running times ATm and distance measuring values Dm are directly proportional to each other and are therefore used synonymously and interchangeably below.
    The transmission signal s (t) of the laser transmitter 2 with the transmission pulses Sm is carried as a waveform in the time scale in Fig. 2a to 13/47 13, for example, as the amplitude A over time t. The term " amplitude " or "amplitude curve " In this case, the received signal e (t), that at the light-sensitive input of the laser receiver 4 as an optical signal and at the electrical output of the laser receiver 4 is obtained as an electrical signal, are plotted as Ampiitudenverlauf A over time t, see Fig. 2b.
    The laser rangefinder 1 further includes a control and evaluation 5 with a data memory 6 for controlling the laser transmitter 2 for the delivery of the transmission signal s (t) and for the evaluation of the received signal e (t) of the receiver. 4
    Laser transmitter 2 and receiver 4 are added to protect against environmental influences in a common housing 7 of the laser rangefinder 1, which has a glazed passage window 8 for the exit and entry of the laser measuring beam 3. At the passage window 8 parasitic interference reflection of the laser measuring beam 3 occurs, i. the laser pulses Sm are backscattered directly into the receiver 4 (arrow 9), even before they leave the housing 7.
    The spurious reflections of the transmission pulses Sm lead to interference signals ("window echoes") Xi, X2,..., Generally Xm, in the received signal course e (t), which occur shortly after the transmission times tsm at disturbance times tXm (FIG. 2b). However, interfering reflections Xm or interference pulses Xm can not only be caused by the passage window 8 but also by any other parts of a supporting structure common to the transmitter 2 and receiver 4, for example from inside surfaces of the housing 7 of the parts projecting into the laser measuring beam 3.
    The glitches Xm in the received signal e (t) simulate alleged ("wrong") short-range targets. A known measure therefore consists in evaluating the received signal e (t) only within time windows Wi, W2,..., Generally Wm, which each begin at a predetermined interval (blocking time) TG from the respective transmission time tSm. As a result, the spurious reflections Xm are indeed hidden, but so can no real targets in the near range measured more ("blind ranks"). Furthermore, in repetitive distance measuring methods, as used in particular for laser scanning, further "blind ranges" result. in the area of MTA zone boundaries. This problem will now be explained in detail with reference to FIGS. 3 and 4.
    According to FIGS. 3 and 4, by way of example, an aircraft-supported lase range finder or scanner 1 continuously transmits transmit pulses Sm having mutual pulse intervals T = 1 / PRR to environmental targets Ui, U2, Ui ', U2' in different MTA zones Z, Z '. , Z " sent. FIGS. 3a and 4a show the situation when environmental targets Ui, U2 are located in a first MTA zone Z closest to the laser scanner 1. Here, the received pulses Em are always received in time before the next transmit pulse Sm is emitted, so that each receive pulse Em is assigned to the immediately preceding transmit pulse Sm and the distance Dm is determined directly from the signal delay between transmit pulse Sm and subsequently received receive pulse Em can be.
    Figures 3b and 4b show the situation when environmental targets Ui ', U2' are in a second more distant MTA zone Z '. Here, the receive pulse Ei of the first transmit pulse Si is received only after the second transmit pulse S2 has already been transmitted, so that the assignment of the receive pulse Ei to the "correct". Transmission pulse Si is not so easy anymore; if it were erroneously associated with the immediately preceding transmit pulse S2, then instead of the correct distance Di ', the wrong distance Di would be measured. To measure environmental targets in different MTA zones Z, Z ', Z " Therefore, either a prior knowledge of the MTA zone is required, in which the objectives can be expected, or additional measures are taken to correctly assign the transmit and receive pulses to each other, for example by the individual transmit pulses Sm by varying their polarization, Amplitude, wavelength, etc. are made distinguishable from each other in order to assign the received pulses Em accordingly. Further, particularly advantageous possibilities are the methods explained later with reference to FIGS. 11 and 12 for MTA-zone-selective or MTA-zone-exceeding distance measurement, which can be combined with the method provided here for disturbing reflection compensation.
    Referring to Figs. 1 and 2, the MTA zone problem of Figs. 3 and 4 shows that environmental targets Ui, Ui 'lying on or near an MTA zone boundary 10 may result in receive pulses Em on or very near a next, after next, etc. transmit pulse Sm and thus fall into the blocking time TG of one of the gating time windows Wm. In other words, the known gating methods have not only periodically repeated around the MTA zoom limits 10 "blind ranges" in which no environmental targets can be measured, i. E. Measured value failures occur. The method described here overcomes this problem.
    Returning to FIG. 2, instead of using inhibit times TG and gating windows Wm, a model of the spurious reflections Xm in the form of a reference spurious signal xref (t) is used, which is a spurious reflection Xm that can be picked up by the receiver 4 as being at the time of transmission tsm of the originating transmit pulse Sm represents related amplitude variation over time t (Figure 2c).
    The reference interference signal xref (t) is subtracted from the received signal e (t) for each transmitted pulse Sm, each time-referenced to its transmission time tSm. Fig. 2d shows the " cleaned " Receive signal f (t). As can be seen, the spurious reflections Xm are eliminated and only more " pay " reception pulses Em of target reflections occur in the adjusted 17/47 17
    Receive signal f (t) on. The adjusted received signal f (t) may then be evaluated in any manner, both conventional and described below, to determine target distances Dm to both one and many different environmental targets.
    The described method of Störreflexionskompensation is of great benefit in conjunction with repetitive, in particular MTA zone-selective or -überschreitenden measurement method: Here also environmental targets around around MTA zone boundaries 10 can be measured. FIGS. 5 and 6 show exemplary laser scans (3D point clouds) of the terrain U, which was height-measured by the airborne laser scanner 1 of FIGS. 3 and 4. Figs. 5 and 6 are plan views of a portion of the terrain U and the blackening represents the height. Here, the terrain U rises from a region B 'in the second MTA zone Z' over a zone boundary 10 to a region B in the first MTA zone Z. 5 shows the result of a conventional gating process in which 10 measured value failures (shown in white) occur around the MTA zone boundary. FIG. 6 shows the result of the interference reflection compensation method presented here, in which no measurement value failures occur around the MTA zone boundary 10, but a continuous measurement of the terrain U is possible.
    The reference interference signal xref (t) can be stored directly in the memory 6 of the laser rangefinder or scanner 1 and, as it were "in real time", during the reception or recording of the received signal e (t), progressing therefrom deducted for each transmit pulse Sm each new time-referenced to the transmission time tSm. Alternatively, first of all the entire received signal e (t) can be recorded in the memory 6 of the laser rangefinder or scanner 1 (or a separate memory connected to it) and then the reference interference signal xref (t) - as it were in "batch processing". in an "offline" evaluation phase - are subtracted from the recorded received signal e (t) as often as mentioned in the manner mentioned in order to produce the adjusted received signal f (t). The offline evaluation may also be performed at a location remote from the laser rangefinder or scanner 1, e.g. in an evaluation data center.
    The reference interference signal xref (t) can be obtained in a variety of ways and pre-stored for the purposes mentioned. A first possibility is to calculate (at least approximately) the reference interference signal Xref (t) from design parameters of the laser rangefinder or scanner 1, for example based on the known relative positions of transmitter and receiver 2, 4 with respect to those parts of the support structure 7, which Cause interference reflection, eg the exit window 8, and their reflectivity. Thus, from the reflectivity of the exit window 8 under a certain transmission angle of the laser measuring beam 3, the knowledge of the transmission amplitude, the geometric dimensions of the housing 7 and an assumed pulse shape, the expected amplitude 19/47 19th
    Axref and time tx of the Störreflexionsimpulses Xref calculated in the reference noise signal Xref (t) and stored in their calculated course over time as a reference interference signal xref (t).
    A second option is to record the reference noise Xref (t) using calibration measurements in an upstream calibration phase of the process. For this purpose, the laser rangefinder or scanner 1 is directed to one (or more) target (s) Um, which are not known to be in the vicinity of or in the area of an MTA zone boundary 10, and any within a short time after a transmission time tsmr. within the time TG after a transmission time tSm / einlangende pulse is considered as interference reflection or interference pulse Xm and recorded in its amplitude variation over time as a reference interference signal xref (t) and stored. Several such reference interfering signals xref (t) may be combined (statically "consolidated") for equalization purposes to exclude outlier measurements and to obtain the most representative reference interference signal xref (t).
    A third possibility is, in repetitive range finding or scanning methods in which a plurality of transmit pulses Sm are emitted as the pulsed measurement beam 3 ("pulse train"), the reference noise signal xref (t) during the measurement ("online") with the pulse train mitzuvermessen and save. This will now be explained in more detail with reference to FIGS. 7 and 8. 20/47 20
    FIG. 7 schematically shows a sequence of transmission pulses S 1, S 2,..., S m,..., To which interference pulses Χχ, X 2,..., X m, - and (for the sake of simplicity, only in the first MTA zone Z lying) receive pulses Ei, E2, ..., Em, ... follow. After every a number a of transmission pulses Sm (here: a = 2, i.e. every third transmission pulse Sm), the reference interference signal Xref (t) is newly determined, i.a. for use with the respectively following transmission pulses, see arrows 11 for the pulse group Sm + i, Sm + 2, Sm + 3 following the transmission pulse Sm. This results in a sequence of successively determined reference interference signals xref, 1 (t), xref , 2 (t), etc., etc. The number a can be arbitrary, for example also a = 0, ie Even after each transmit pulse Sm, a new reference interference signal xref, m (t) for the next transmit pulse Sm + i can be determined immediately, if desired.
    The respective reference interference signal xref, m (t) is in turn determined on the basis of a number b of preceding transmission pulses Sm, b = 4 in the example shown. Preferably, b > 1, more preferably b »1. In this case, by means of a statistical evaluation of b last time intervals 12 of the received signal e (t), which each start after a transmission time tsm and preferably only a fraction of the transmission pulse distance T, e.g. TG, spurious reflections Xm are distinguished from true target reflections Em. The statistical evaluation is based on the realization that targets in the vicinity and around the MTA zone boundaries 10 - in comparison 21/47 21 to targets of the support structure 7 - comparatively "rare". and can therefore be identified by means of a frequency estimate. Fig. 8 shows this in detail.
    In FIG. 8, the maximum amplitudes Axref and timings tx of all pulses occurring in the time segments 12 - be they spurious reflections Xm or true target reflections Em - are plotted as points in an amplitude / time plane ("histogram"). As can be seen, spurious reflections Xm are minimally scattered with respect to their maximum amplitude Axref and time slot tx, i. fall together in a small area 13 (points PXm), while "real" Receive pulses Em from "real" Ambient targets, which happen to be in a near or MTA zone boundary area, can assume completely different, strongly scattered amplitudes and timings, see the points PEm of the received pulses Em.
    With the aid of a statistical evaluation, for example by means of the histogram of FIG. 8 or also by means of other statistical methods, the amplitude profiles of the spurious reflections Xm can thus be distinguished from the amplitude characteristics of the target reflections Em in the time segments 12 of the received signal e (t) and thus the glitches Xm and their amplitude curves are identified as interference signals x (t). For example, those pulses Xm are selected from all the pulses Xm, Em in the periods 12, which have mostly the same maximum amplitude Axref and / or the same time slot tx. 22/47 22
    In the example of FIG. 7, the pulse patterns in the first, second and fourth time sections 12 before the transmit pulse Sm and the pulse waveforms from all four time sections 12 before the transmit pulse Sm are identified from the thus identified correct interference signals x (t) in the time segments 12 +3, then the respective reference interference signals xref, i (t) and Xref, 2 (t) is formed. For example, a particularly representative interfering signal characteristic x (t) can be selected or all interfering signal profiles can be superimposed. Again, statistical methods can be used to eliminate outlier waveforms to form a particularly significant reference noise xref, m (t). It is also possible, Störsignalverläufe x (t) from further past periods 12 to assign less importance, i. weight them weaker than from more recent time periods 12, if desired.
    The described method for "online" determination of the reference interference signals in this way provides as "by-product". also a sequence of progressive reference interference signals xref, m (t), the tendency of which can also be evaluated on its own in order to measure the disturbing influence of the support structure of the laser rangefinder or scanner 1 over time. Thus, for example, a sudden or creeping progressive contamination of the exit window 8, damage to the device, which leads to partial blockages of the laser measuring beam 3, or an incorrect operation ("finger in the measurement beam") 23/47 23 and a corresponding alarm for the user be issued.
    For example, the respective maximum amplitude Axref of the successively determined reference interference signals xrefm (t) can be evaluated over the time t, as plotted in the lowest diagram of FIG. 7, and compared with a threshold value c; if the threshold c is exceeded, an alarm is triggered. Alternatively or additionally, the difference AAxref of two successive reference interference signal maximum amplitudes can also be monitored, and e.g. an alarm is triggered when the threshold exceeds a threshold.
    FIGS. 9 and 10 show an expansion of the previously described methods for taking into account a scanning angle dependence of spurious reflections in a laser scanner. In the case of the laser scanner 1 exemplified in FIG. 9, the pulsed laser measuring beam 3 is detected by means of a rotating deflection mirror 14, e.g. a mirror pyramid, periodically fan-shaped pivoted to scan a variety of juxtaposed environmental targets Ui and measure. When the laser measuring beam 3 passes through the exit window 8 of the housing 7 of the laser scanner 1, different interference reflections X (a) occur as a function of the scanning angle α and thus the passage angle through the window 8. The respective reference interference signals Xref (t, a) are thus also dependent on the transmission direction, here: scan angle α. It goes without saying that the laser scanner 1 can pivot the measuring beam 8 in more than one angular dimension α. The term "transmission direction " therefore includes any form of solid angle definition of the emission direction of the pulsed laser measurement beam 3 with respect to the support structure of the laser scanner 1.
    In the memory 6, a separate, assigned reference interference signal xref (t, a) is stored for each possible or occurring transmission direction (here: scan angle oc). This can in turn be obtained in any of the ways discussed above, whether by pre-calculation (modeling), measurement in a calibration phase or "online". As described with reference to FIGS. 7 and 8, only with additional consideration of the respective transmission direction α of the laser scanner. 1
    Starting from the memory 6 in the laser scanner 1 or another memory, e.g. an offline evaluation center, stored set of senderichtungsabhängigen reference interference signals xref (t, a) is determined for each laser pulse Sm of its respective transmission direction α associated reference noise xref (t, a) and as shown in FIGS. 2c and 2d described time-referenced to the respective transmission time tSm subtracted from the received signal e (t) to produce the adjusted received signal f (t).
    FIGS. 11 and 12 show two different evaluation methods that can be performed on the adjusted received signal f (t), u.zw. once for MTA zone selective ranging (Figure 11) and once for MTA cross-zone ranging (Figure 12). 25/47 25
    In both variants, FIGS. 11 and 12, the transmission pulses Sm no longer have a constant pulse spacing τ = 1 / PRR but a pulse interval τχ = 1 / PRRi, t2 = 1 / PRR2, etc. varying from transmission pulse to transmission pulse, generally in = l / PRRm sent out. In other words, the pulse repetition rate PRR or the pulse spacing τ is modulated with a signal ("pulse position modulated"), as it were, in order to achieve the mentioned variation of the pulse spacings Tm. The variation of the pulse interval in the pulse to pulse is preferably only slightly, for example, +/- 1%, +/- 5% or +/- 10% to the average (average) pulse spacing τ.
    The modulation signal for achieving said pulse spacing variation may be of any type, e.g. a sine signal, triangular signal, sawtooth signal, staircase signal, a data signal with its own information content, etc. Preferably, the modulation signal is a statistically random signal, such as white noise. With such a random signal, the pulse repetition rate PRRm or the pulse spacing in the manner of a random "phase jitter". the transmit pulses Sm statistically randomly varies. Within certain limits, such a random signal may also be merely pseudo-random.
    According to FIG. 11, regardless of the variation of the pulse interval Tm, time windows Wi, W2,... Of the adjusted received signal f (t) are considered, each starting at a predetermined distance AW from the transmission time tsm of a laser pulse Sm; the received signal f (t) is thus broken down into sections 15 26/47 26. The predetermined distance AW corresponds to the desired MTA zone to be measured: for the first MTA zone Z, AW = 0, for the second MTA zone Z ', AW = l-τ, for the third MTA zone Z " is AW = 2-T, etc., etc.
    If the sections 15 of the adjusted received signal f (t) are now temporally superimposed and summed up, a sum signal curve f2 (t) results, in which receive pulses Em from unwanted ("false") MTA zones due to the phase position jittering of the transmitted pulses Sm in their time also "jitter" and thus su-perponate only to a sum pulse 16 of relatively low maximum amplitude, whereas receive pulses Em from the desired "right". MTA zone remain unjitted and superposing to a significant pulse 17 which can be detected against threshold g (Figure 11b)
    Appearance time tE to determine (Fig. 11c).
    FIG. 12 shows an alternative use of the PRR modulation for automatic MTA zone assignment of received pulses Em detected in the adjusted receive signal f (t). From one and the same received receive signal f (t), a plurality of ( here: 4) different sequences F, F ', F ", F' " formed by runtime or distance measurement values Dm, once under assignment of the receive pulses Em of the time window Wm to the transmit pulse Sm of this time window (AW = 0) for the first sequence F; once with assignment of the received pulses Em of the time window Wm 'to the previous transmit pulse Sm-i (AW = l-τ) for the sequence F'; once by assigning the receive pulse Em in the time window Wm " to the previous transmit pulse Sm-2 (AW = 2 · τ) for the sequence F "; and once by assigning the reception pulse Em in the time window Wm '" to the pre-past transmission pulse Sm-3 (AW = = 3 · τ) for the sequence F '".
    That sequence F ', which is the "right one" Assignment of receive pulses to transmit pulses according to the "correct" MTA zone (here: Z ') is from the PRR modulation, i. the variation of the pulse intervals Tm, least affected (least "jittered"). By signal-analyzing comparison of the sequences F, F ', F ", F'", for example by recognizing the original PRR modulation signal by correlation, or by detecting a degree of "disorder". in the sequence, e.g. Measuring their noise or higher-frequency signal components, etc., the least affected by the PRR modulation sequence can be determined as the correct sequence with the correct distance measurements Dm. In the laser scanning example of FIGS. 3 and 4, it is thus possible for environmental targets Ui, U2, Ui ', U2' in any MTA zones Z, Z ', Z " etc., the correct distance Di, D2, Di ', D2', etc. can be determined without prior knowledge of the terrain. When flying over MTA zone boundaries 10, the then correct MTA zone is automatically selected according to FIG. 12; Due to the described interference reflection compensation measurement losses are avoided at the same time. 28/47 28
    The invention is not limited to the illustrated embodiment forms, but includes all variants and Modifi cations that falls within the scope of the attached claims len. 29/47
    权利要求:
    Claims (15)
    [1]
    Claims 1. A method for measuring the distance of a target (Ui) by means of laser pulses (Sm) emitted by a transmitter (2), reflected at the target (Ui) and received in a receiver (4), transmitter (2) and Receiver (4) have a common support structure (7) at which spurious reflections (Xm) of the laser pulses (Sm) occur, comprising: emitting a laser pulse (Sm) from the transmitter (2) to a target (Ui) at a transmission time (tSm) on a time scale (t); Obtaining a received signal (e (t)) representing a target reflection (Em) of the laser pulse (Sm) picked up by the receiver (4) as an amplitude variation with respect to the time of transmission (tSm); Subtracting a stored reference interfering signal (Xref (t)), which represents an interference reflection (Xm) of the laser pulse (Sm) which can be picked up by the receiver (4) as an amplitude progression with respect to the transmission time (tSm) over time, from the received signal (e (t)) to obtain an adjusted received signal (f (t)); Detecting a pulse (Em) and its occurrence time (tEm) in the adjusted received signal (f (t)) or a signal derived therefrom; and 30/47 30 measuring the distance (Dm) of the target (Ui) from the transit time (ATm) between the time of transmission (tSm) and the time of occurrence (tEm) ·
    [2]
    2. The method according to claim 1, characterized in that the reference interference signal (xref (t)) from design parameters of the support structure (7) is approximately calculated and stored.
    [3]
    3. The method according to claim 1, characterized in that an interference signal (x (t)), which is caused by a spurious reflection (Xm) of a previous laser pulse (Sm), recorded and stored as a reference interference signal (xref (t)) ,
    [4]
    4. The method according to claim 1, characterized in that a plurality of laser pulses (Sm) at successive transmission times (t sm) emitted and a plurality of time sections (12) of the received signal (e (t)), each after a transmission time (tsm) begin on in the majority of pulses having the same amplitude (Axref) and / or timing (tx) are checked and, if any, from these a reference noise signal (Xref (t)) is formed and stored.
    [5]
    5. The method according to claim 4, characterized in that the forming and storing the reference interference signal (Xref (t)) after a predetermined first number (a) of laser pulses (Sm) is repeated, each of the successively formed reference interference signals ( xref, m (t)) is formed on the basis of a predetermined second number (b) of respective preceding laser pulses (Sm). 31/47 31
    [6]
    6. The method according to claim 4 or 5, characterized in that in the above-check on multi-identity pulses (Xm, Em) from further past periods (12) are weighted weaker.
    [7]
    7. The method according to any one of claims 1 to 6, characterized in that an alarm message is generated when the reference interference signal (xref (t)) exceeds a predetermined threshold value (c).
    [8]
    8. The method according to any one of claims 4 to 7, characterized in that an alarm message is generated when the difference between two successively formed reference interference signals (xref, i (t), (xref, 2 (t)) a predetermined threshold ü Bersch rides.
    [9]
    9. The method according to any one of claims 1 to 8, characterized in that a plurality of laser pulses (Sm) at successive transmission times (tSm) are emitted and the received signal (e (t)) accordingly a plurality of successively collected target reflections (Em) represents, and that for each laser pulse (Sm), the reference interference signal (xref (t)) is in each case once time-referenced to the transmission time (tSm) of this laser pulse (Sm) from the received signal (e (t)) is subtracted.
    [10]
    10. The method according to any one of claims 1 to 9, characterized in that a plurality of laser pulses (Sm) in different transmission directions (a) to different targets (Ui) are emitted and the received signal (e (t)) accordingly several successively collected target reflections (Em ) Of different targets (Ui) represents that for each transmit direction (a) a separate reference interference signal (xref (t, a)) is stored, and that for each laser pulse (Sm) that of its transmit direction (a) assigned reference interference signal (xref (t, a)) time-referenced to the transmission time (tSm) of this laser pulse (Sm) from the received signal (e (t)) is subtracted.
    [11]
    11. The method according to claim 9 or 10, characterized in that the laser pulses (Sm) with varying pulse intervals (im) are emitted, and that a plurality of time sections (15) of the adjusted received signal (f (t)), each in a predetermined Distance (AW) from the transmission time (tsm) of a laser pulse (Sm) begin to be superimposed to a sum signal (f2 (t)), in which subsequently a pulse (Em) is detected.
    [12]
    12. The method according to claim 9 or 10, characterized in that the laser pulses (Sm) with varying pulse intervals (im) are emitted, and that transit times (ATm) in each case between a laser pulse (Sm) and that in the adjusted received signal (f (t )) are detected, which is in a time window (W), which starts at a predetermined distance (AW) from the transmission time (tSm) of this laser pulse (Sm).
    [13]
    13. The method according to claim 12, characterized in that a first sequence (F) of terms (ATm) based on a first predetermined distance (AW) and a second sequence (F ') of maturities (ATm) based on a second predetermined Ab - 33/47 33 Stands (AW) is measured, and that sequence (F, F ') of terms (ATm), which is least affected by the variation of the pulse intervals (im), is used for the distance measurement.
    [14]
    14. The method according to any one of claims 1 to 13, characterized in that the reference interference signal (xref (t)) progressively retrieved from its memory (6) and time-referenced on the transmission time (tSm) of the laser pulse (Sm) in real time is subtracted from the incoming received signal (e (t)).
    [15]
    15. The method according to any one of claims 1 to 13, characterized in that the entire received signal (e (t)) is recorded and then the reference interference signal (xref (t)) time-referenced to the transmission time (tSm) of the laser pulse (Sm) thereof is subtracted. 34/47
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    引用文献:
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    DE20220710U1|2002-06-27|2004-03-11|Sick Ag|Laser distance measurement device couples light signal out for reference measurements before entry of light signal into measurement channel using controlled reflection at optical boundary surface|
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    法律状态:
    优先权:
    申请号 | 申请日 | 专利标题
    AT504302012A|AT513402B1|2012-10-05|2012-10-05|Method for distance measurement|AT504302012A| AT513402B1|2012-10-05|2012-10-05|Method for distance measurement|
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