Laser measuring device for measuring a distance to an object and method for operating the same

Abstract

Laser measuring device for measuring a distance (DIS) to an object (OBJ) comprising: a pulse laser (2) for emitting a laser pulse (LAP) at the beginning of a measuring cycle (MZ); an optical sensor (3) with at least one detection unit (4) for generating detection signals (DES), wherein the detection unit (4) has at least one detector (11) for detecting individual photons (PHO), wherein the detection unit (4) generates one of the detection signals (DES) each time during the measuring cycle (MZ) when one of the photons (PHO) is detected by the detector (11); a coincidence detection stage (5) for generating coincidence signals (KOS), wherein each time during the measuring cycle (MZ) the detection signals (DES) generated by the detection unit (4) reach at least a predetermined coincidence depth (KOT) within a coincidence time (KOZ), one of the coincidence signals (KOS) is generated; a coincidence time specification stage (6) for specifying the coincidence time (KOZ) to the coincidence detection stage (5), wherein the coincidence time specification stage (6) is designed such that the coincidence time (KOZ) increases monotonically during the measuring cycle (MZ); and a travel time measuring device (7) for determining the distance (DIS) on the basis of a travel time measurement of the coincidence signals (KOS).

Description

[0001] The invention relates to a laser measuring device for measuring the distance to an object, which operates according to a pulse time-of-flight method. Furthermore, the invention relates to a method for operating such a laser measuring device. In this context, this is referred to as Light Detection and Ranging (LIDAR).

[0002] From the source [4] a receiver arrangement for receiving light pulses and a LIDAR module with such a receiver arrangement are known, in which a reduction of the resolution of the distance determination is carried out from at least a predetermined distance.

[0003] In known laser measuring devices operating according to a pulse transit time method, the transit time of a laser pulse emitted by an active radiation source and reflected by the object to be measured is measured by detecting the reflected laser pulse.

[0004] Known laser measuring devices that operate according to a pulse transit time method have the disadvantage that with increasing distance and increasing background light, the reflected laser pulse can be distinguished less and less from the background light, so that the quality of the distance measurements decreases.

[0005] The object of the present invention is to provide an improved laser measuring device for measuring a distance to an object.

[0006] In a first aspect, the problem is solved by a laser measuring device for measuring a distance to an object, which has the following features: a pulse laser for emitting a laser pulse at the beginning of a measuring cycle; an optical sensor having at least one detection unit for generating detection signals, wherein the detection unit has at least one detector for detecting individual photons, wherein the detection unit generates one of the detection signals each time one of the photons is detected by the detector during the measuring cycle; a coincidence detection stage for generating coincidence signals, wherein each time during the measuring cycle when the detection signals generated by the detection unit reach at least a predetermined coincidence depth within a coincidence time, one of the coincidence signals is generated; a coincidence time specification stage for specifying the coincidence time to the coincidence detection stage, wherein the coincidence time specification stage is designed such that the coincidence time increases monotonically during the measurement cycle; and a time-of-flight measuring device for determining the distance based on a time-of-flight measurement of the coincidence signals.

[0007] The laser measuring device is used to measure the distance to an object without contact.

[0008] The pulsed laser can, in particular, be an infrared pulsed laser. The measurement cycle begins with the emission of a laser pulse and ends at the latest with the start of a subsequent measurement cycle, i.e., when a new laser pulse is emitted. The duration of the measurement cycle corresponds to the maximum duration of the laser pulse during which photons of the laser pulse can still be detected.

[0009] The optical sensor can be an integrated CMOS sensor, with the detection unit arranged entirely on a single semiconductor chip. However, the detection unit can also be distributed across different semiconductor chips. Furthermore, the detection unit can also comprise discrete components. The optical sensor can be an integrated 3D hybrid sensor manufactured using wafer-to-wafer, chip-to-wafer, or chip-to-chip bonding.

[0010] Any detector capable of detecting a single photon in the wavelength range of the pulsed laser can be used as the detector. The detector can be an avalanche diode, in particular a single-photon avalanche diode (SPAD). Single-photon avalanche diodes (SPADs) are avalanche diodes operated above their breakdown voltage. In this so-called Geiger region, a single photon is sufficient to be absorbed in the active region of the diode and generate a free charge carrier, leading to breakdown of the diode and thus to a macroscopic current flow through the diode. SPADs thus enable the detection of single photons. The detector can also be a silicon photomultiplier (SiPM).

[0011] The coincidence detection stage processes the detection signals from exactly one detection unit. This means that if multiple detection units are provided, multiple coincidence detection stages are also provided. The coincidence detection stage outputs a coincidence signal if and only if at least a predetermined number of detection signals are received by the associated detection unit within a coincidence time. The predetermined number is called the coincidence depth. This ensures that, at least when the coincidence depth is >1, a single detection signal generated by a photon originating from the background light does not lead to a (false) time-of-flight measurement. Background light is any light that does not originate from the pulsed laser of the laser measuring device.

[0012] The distance can then be determined based on a time-of-flight measurement of the coincidence signals, which are more reliable than the detection signals.

[0013] The coincidence time setting stage is designed to variably specify the coincidence time so that the coincidence time increases monotonically during the measurement cycle. Monotonically increasing means that the coincidence time increases during the measurement cycle and is at least as long at any point in time during the measurement cycle as it was at the previous points in the measurement cycle. The monotonous increase in the coincidence time results in a variable sensitivity of the receiving side of the laser measuring device within the measurement cycle, with the sensitivity increasing over time. This is achieved by variable attenuation, which is higher at the beginning of the measurement cycle than at the end.

[0014] At short distances or short travel times, the increased attenuation reduces the likelihood of false measurements due to background light. Since the reflected laser pulse has a high intensity at short distances due to its inverse square dependence, the increased attenuation has only a minor impact on the precision of the measurement at close range. At greater distances or longer travel times, the attenuation is reduced to enable improved detection of the reflected laser pulse. In this way, the method enables improved measurement at longer distances or travel times without significant deterioration at close range.

[0015] The result is an improvement in measurement accuracy at long distances and / or in bright background light through distance-dependent adjustment of the signal evaluation.

[0016] The invention can be used in particular when precise and reliable determination of larger distances is essential. For example, the laser measuring device according to the invention is suitable for emergency braking systems in land vehicles, where distances to foreign objects must be reliably detected in real time in order to predict possible movements of foreign objects in a timely manner, so that collisions can be prevented by appropriate interventions in the control system of the respective land vehicle. However, the invention is also useful in other driver assistance systems. The invention is also suitable for autonomous vehicles, i.e., driverless vehicles. Further areas of application can be found in aviation and medical technology.

[0017] According to an advantageous development of the invention, the laser measuring device has a background event rate determination stage for determining a background event rate of the detection signals, wherein the coincidence time specification stage is designed to specify the coincidence time taking into account the background event rate.

[0018] The background event rate indicates the number of detection signals per unit of time that are generated exclusively by detecting photons originating from the background light. The background event rate can be determined by temporarily turning off the pulsed laser and counting the detection signals received during a specific period of time. The coincidence time can be shortened as the background event rate increases.

[0019] By specifying the coincidence time taking into account the background event rate, the quality of the distance measurements can be further improved.

[0020] According to a preferred embodiment of the invention, the coincidence time specification stage is designed to specify the coincidence time taking into account the predetermined coincidence depth. It can be provided that the coincidence time is extended if the coincidence depth is increased.

[0021] By specifying the coincidence time taking into account the given coincidence depth, the quality of the distance measurements can be further improved.

[0022] According to an advantageous development of the invention, the laser measuring device has a maximum value determination stage for determining a maximum value of a constant probability density function for the occurrence of a first coincidence signal among the coincidence signals when considering exclusively the background event rate of the detection signals at the predetermined coincidence depth, wherein the coincidence time specification stage is designed to specify the coincidence time taking into account the maximum value. The probability density function for the occurrence of a first coincidence signal among the coincidence signals when considering exclusively the background event rate of the detection signals is constant if it has a constant value for the entire measuring cycle. A constant probability density function leads to a runtime-independent quality of the distance measurements.The higher the constant value of the probability density function, the higher the quality. The maximum value is the maximum constant value that the constant probability density function can assume. The maximum value can be calculated as a function of the coincidence depth and the background event rate.

[0023] By specifying the coincidence time taking the maximum value into account, the quality of the distance measurements can be further improved.

[0024] According to an advantageous development of the invention, the laser measuring device has a background event rate determination stage for determining a background event rate of the detection signals. The laser measuring device has a coincidence depth specification stage for specifying the coincidence depth to the coincidence detection stage. The coincidence depth specification stage is designed to specify the coincidence depth taking the background event rate into account. It can be provided that the coincidence depth is increased when the background event rate increases.

[0025] By specifying the coincidence depth taking into account the background event rate, the quality of the distance measurements can be further improved.

[0026] In simpler versions, however, the coincidence depth can also be fixed, for example based on empirical values.

[0027] According to an advantageous development of the invention, the coincidence depth specification stage is designed to determine signal-to-noise ratios of probability density functions for the occurrence of a first coincidence signal of the coincidence signals at different values ​​for the coincidence depth. One of the signal-to-noise ratios is determined for each of the different values. The one of the different values ​​is specified as the coincidence depth that corresponds to a maximum signal-to-noise ratio of the signal-to-noise ratios. The quality of the distance measurement generally increases with the signal-to-noise ratio of the probability density function for the occurrence of a first coincidence signal of the coincidence signals. The signal-to-noise ratio depends on the coincidence depth.

[0028] By calculating the respective signal-to-noise ratio for a large number of possible coincidence depths before the actual measuring cycle, it is possible to specify the coincidence depth for the measuring cycle which leads to the best signal-to-noise ratio, so that the quality of the distance measurements can be further improved.

[0029] According to an advantageous development of the invention, the coincidence depth setting stage is designed such that the coincidence depth remains constant throughout the measurement cycle. This allows a high signal-to-noise ratio to be maintained throughout the entire measurement cycle, thus further improving the quality of the distance measurements.

[0030] According to an advantageous development of the invention, the coincidence time specification stage is designed such that the coincidence time increases strictly monotonically during the measurement cycle. Strictly monotonically increasing means that the coincidence time increases during the measurement cycle and is greater at each point in time than at the previous points in time. This can further improve the quality of the distance measurements.

[0031] According to an expedient development of the invention, the coincidence time specification stage is designed such that the coincidence time during the measuring cycle is specified such that a probability density function for the occurrence of a first coincidence signal of the coincidence signals deviates from a constant value by a maximum of 10% when considering only the background event rate of the detection signals during the measuring cycle. Ideally, the probability density function for the occurrence of a first coincidence signal of the coincidence signals has a constant value when considering only the background event rate of the detection signals during the measuring cycle. However, this is not always possible in practice. However, it has been shown that the quality of the distance measurement can be significantly improved compared to devices with a constant coincidence time, even if deviations of 10% upwards or downwards are maintained.

[0032] According to a preferred development of the invention, the coincidence time specification stage is designed such that the measurement cycle is divided into several partial measurement cycles, with the coincidence time being determined at the beginning of each of the partial measurement cycles and kept constant for the respective partial measurement cycle. In this way, the computational effort can be significantly reduced, while the quality of the distance measurement can still be significantly improved compared to devices with a constant coincidence time.

[0033] According to an advantageous development of the invention, the detection unit comprises a plurality of detectors for detecting individual photons, wherein the detectors have corresponding detection ranges, and wherein the coincidence depth is not greater than a predetermined number of detectors. This ensures that, provided a number of photons corresponding to the coincidence depth occurs in the detection range of the detection unit, the coincidence signal is generated even if the detectors have a dead time after detecting one of the photons, during which the detection of another photon is not possible.

[0034] According to an advantageous development of the invention, the optical sensor has a plurality of detection units with different detection ranges. This allows high-quality three-dimensional distance images to be captured.

[0035] According to an expedient development of the invention, the time-of-flight measuring device is designed such that the distance is determined based on a time-of-flight measurement of the detection signals of the detection unit when the predetermined coincidence depth is one, and such that the distance is determined based on the time-of-flight measurement of the detection signals of the coincidence signals when the predetermined coincidence depth is greater than one. This eliminates the need to determine the coincidence time in cases where it is not required. This can occur particularly when the background event rate is very low. In this way, the computational effort can be significantly reduced, while the quality of the distance measurement can still be significantly improved compared to devices with a constant coincidence time.

[0036] In a further aspect, the invention relates to a method for operating a laser measuring device for measuring a distance to an object, the method comprising the following steps: Emitting a laser pulse at the beginning of a measuring cycle using a pulsed laser; Generating detection signals by means of at least one detection unit of an optical sensor, wherein the detection unit has at least one detector for detecting individual photons, wherein one of the detection signals is generated by the detection unit each time one of the photons is detected by the detector during the measuring cycle; Generation of coincidence signals by means of a coincidence detection stage, wherein each time during the measuring cycle when the detection signals generated by the detection unit reach at least a predetermined coincidence depth within a coincidence time, one of the coincidence signals is generated; Specification of the coincidence time to the coincidence detection stage by a coincidence time specification stage, wherein the coincidence time is specified such that the coincidence time increases monotonically during the measurement cycle; and Determination of the distance based on a time-of-flight measurement of the coincidence signals using a time-of-flight measuring device.

[0037] In a further aspect, the invention relates to a computer program for carrying out a method according to the invention.

[0038] The present invention and its advantages are described in more detail below with reference to the figures. They show: Fig. 1 shows a first embodiment of a laser measuring device according to the invention in a schematic representation; Fig. 2 exemplary probability density functions for the occurrence of a first detection signal for different pulse transit times as a function of the transit time for a measuring cycle; Fig. 3 an exemplary histogram for the occurrence of a first detection signal after the accumulation of several measurement cycles; Fig. 4 an exemplary representation of a signal-to-noise ratio of a runtime measurement based on the detection signals as a function of the runtime for a measurement cycle; Fig. 5 an exemplary representation of a target value of a background event rate of the coincidence signals for a constant probability density function for the occurrence of a first coincidence signal when considering exclusively the background event rate of the detection signals as a function of the running time for a measurement cycle; Fig. 6 an exemplary representation of a coincidence time as a function of the running time for a measuring cycle, which results when using the setpoint of the Fig. 5 results; Fig. 7 exemplary probability density functions for the occurrence of a first coincidence signal for different pulse transit times as a function of the transit time for a measuring cycle, which can be calculated using the coincidence time of the Fig. 6 results; Fig. 8 an exemplary representation of signal-to-noise ratios of travel time measurements based on the coincidence signals for different coincidence depths as a function of the background event rate of the detection signals; Fig. 9 shows a second embodiment of a laser measuring device according to the invention in a schematic partial representation; Fig. 10 exemplary mean values ​​and error probabilities for distance measurements with constant coincidence time for different background illuminances for different reflectances of the respective object; Fig. 11 exemplary mean values ​​and error probabilities for distance measurements with monotonically increasing coincidence time for different background illuminances for different reflectances of the respective object; Fig. 12 exemplary mean values ​​and error probabilities for distance measurements with constant coincidence time and with monotonically increasing coincidence time for different background illuminance levels; Fig. 13 shows an exemplary optical sensor with a plurality of detection units in a schematic view; Fig. 14 shows a schematic representation of an exemplary detection unit with a plurality of detectors, the associated coincidence detection stage and the associated time-of-flight measuring device; Fig. 15 a third embodiment of a laser measuring device according to the invention in a schematic partial representation; and Fig. 16 exemplary probability density functions for the occurrence of a first coincidence signal for different pulse transit times as a function of the transit time for a measuring cycle, whereby the coincidence time is determined at the beginning of each of the partial measuring cycles and is kept constant for the respective partial measuring cycle.

[0039] Identical or similar elements or elements with identical or equivalent functions are provided with identical or similar reference symbols below.

[0040] In the following description, exemplary embodiments with a variety of features of the present invention are described in more detail to provide a better understanding of the invention. However, it should be noted that the present invention can also be implemented without individual features of the described embodiments. It should also be noted that the features shown in various exemplary embodiments can also be combined in other ways, unless this is expressly excluded or would lead to contradictions.

[0041] Fig. 1 shows a first embodiment of a laser measuring device 1 according to the invention for measuring a distance DIS to an object OBJ in a schematic representation.

[0042] The laser measuring device 1 for measuring a distance DIS to an object OBJ has the following features: a pulse laser 2 for emitting a laser pulse LAP at the beginning of a measuring cycle MZ; an optical sensor 3 with at least one detection unit 4 for generating detection signals DES, wherein the detection unit 4 has at least one detector 11 for detecting individual photons PHO, wherein the detection unit 4 generates one of the detection signals DES each time during the measuring cycle MZ when one of the photons PHO is detected by the detector 11; a coincidence detection stage 5 for generating coincidence signals KOS, wherein each time during the measuring cycle MZ, when the detection signals DES generated by the detection unit 4 reach at least a predetermined coincidence depth KOT within a coincidence time KOZ, one of the coincidence signals KOS is generated; a coincidence time specification stage 6 for specifying the coincidence time KOZ to the coincidence detection stage 5, wherein the coincidence time specification stage 6 is designed such that the coincidence time KOZ increases monotonically during the measuring cycle MZ; and a travel time measuring device 7 for determining the distance DIS on the basis of a travel time measurement of the coincidence signals KOS.

[0043] In a further aspect, the invention relates to a method for operating a laser measuring device 1 for measuring a distance DIS to an object OBJ, the method comprising the following steps: Emitting a laser pulse LAP at the beginning of a measuring cycle MZ by means of a pulse laser 2; Generating detection signals DES by means of at least one detection unit 4 of an optical sensor 3, wherein the detection unit 4 has at least one detector 11 for detecting individual photons PHO, wherein one of the detection signals DES is generated by the detection unit 4 each time during the measuring cycle MZ when one of the photons PHO is detected by the detector 4; Generation of coincidence signals KOS by means of a coincidence detection stage 5, wherein each time during the measuring cycle MZ, when the detection signals DES generated by the detection unit 4 reach at least a predetermined coincidence depth KOT within a coincidence time KOZ, one of the coincidence signals KOS is generated; Specification of the coincidence time KOZ to the coincidence detection stage 5 by a coincidence time specification stage 6, whereby the coincidence time KOZ is specified in such a way that the coincidence time KOZ increases monotonically during the measuring cycle; and Determination of the distance DIS based on a travel time measurement of the coincidence signals KOS using a travel time measuring device 7.

[0044] In a further aspect, the invention relates to a computer program for carrying out a method according to the invention.

[0045] The invention is based on the further development of known time-of-flight-based distance measurement methods. While in known laser measuring devices 1, the time-of-flight measuring device 7 is designed to determine the distance DIS based on a time-of-flight measurement of the detection signals DES, in the laser measuring device 1 according to the invention, the time-of-flight measuring device 7 is designed to determine the distance DIS based on a time-of-flight measurement of the coincidence signals KOS. The coincidence signals KOS are generated using a coincidence time KOZ that is variable in the measuring cycle MZ, namely with a monotonically increasing coincidence time KOZ.

[0046] In order to better understand the invention, the determination of the distance DIS based on a runtime measurement of the detection signals DES will first be explained: Existing laser measurement devices 1 are based on various principles. In the direct method considered here, the propagation time of a laser pulse LAP from emission via reflection at the target object OBJ to detection in sensor 3 is recorded using an electronic timer (e.g., time-to-digital converter, TDC). The time measurement is started with the emission of a short laser pulse LAP and stopped with the reception of the reflected pulse LAP [1]. The time measurement is stopped using the first-photon method with the first event detected by sensor 3 after the start. Ideally, the measured time corresponds to the light propagation time and can be directly converted into the distance DIS between sensor 3 and target object OBJ using d = ct / 2.Due to this procedure, a high intensity of the background light leads to the fact that an event resulting from the background light can be detected before the arrival of the reflected laser pulse LAP at the sensor 3 and thus leads to an erroneous detection signal DES, so that an erroneous measurement occurs.

[0047] To tolerate such false measurements and statistical fluctuations, several of these time stamps are typically first collected in a histogram, from which the actual light propagation time is then determined using an algorithm. By appropriately reducing the sensitivity of sensor 3, the rate of events generated by background light and consequently the number of false measurements can be reduced to a tolerable level. However, a lower sensitivity of sensor 3 also reduces the reception rate of the events or detection signals DES generated by the detected laser pulse, which complicates reliable and precise measurement, especially at long distances.

[0048] In the direct runtime-based method for distance measurement based on the detection of the first event or detection signal DES in each measurement cycle, the probability density function (PDF) of the first event P(t) is given by P(t)=R(t)(1−∫0tP(t)dt) with the time-dependent event rate R(t). Assuming a time-invariant event rate, which is a good approximation when considering only the background light due to the short measurement duration, P(t) results according to an exponential distribution corresponding to P(t)=R exp(−Rt).

[0049] When the event rate R is also taken into account A of the reflected laser pulse, the PDF is P(t)={RB exp(−RBt)for 0≤t <tTOFRAB exp(−RAB(t−TTOF))exp(−RBTTOF)fu¨r TTOF≤t<TTOF+TPRB exp(−RBt)exp(−RATP)fu¨r TTOF+TP≤t with the background event rate R B, the running time T TOF , the pulse width T P and R AB = R A + R B This means that with increasing measurement distance and background intensity, the probability of detecting an event originating from the background increases. Accordingly, the probability of detecting an event from the reflected laser pulse decreases with increasing distance.

[0050] Fig. Figure 2 shows exemplary probability density functions WDS1 and WDS2 for the occurrence of a first detection signal DES for different pulse transit times as a function of the transit time for a measurement cycle MZ. The probability density functions WDS1 and WDS2 are calculated according to (3) for background event rates R B and for event rates R A of 10 MHz each and a pulse width T P of 16 ns. The propagation time T TOFfor the probability density function WDS1, 100 ns, and for the probability density function WDS2, 200 ns. This shows that the resulting pulse is lower at greater distances, which corresponds to a reduced probability of detection.

[0051] Fig. Figure 3 shows an example histogram HIG for the occurrence of a first detection signal DES after the accumulation of several measurement cycles MZ. In the direct method for distance measurement, the arrival time of the first received event, measured from the time of pulse emission over several measurement cycles MZ, is stored in a histogram HIG, from which the actual propagation time is then determined using a suitable evaluation rule. One possible variant of the evaluation is a mean filtering of the histogram HIG, which serves to reduce the variance of the individual bins, followed by a maximum value determination, whereby the temporal position of the maximum represents the measured propagation time. To determine the expected values ​​of all bins of the histogram HIG, the PDF P(t) is considered. An integration of this function over the time range of the bin multiplied by the number of summed time values ​​yields the respective expected value of the bin.Based on the histogram HIG and assuming Poisson-distributed counts in the bins, a measure of the quality of the measurement can be defined.

[0052] In Fig. 3, the bright areas of the bins represent events caused by background light, while events from the reflected and detected laser pulse are shown as dark areas. To reliably determine the position of the pulse LAP in the histogram HIG, the first bin after the arrival of the reflected pulse LAP must have a higher value than bins that only detected background. The quotient of the pulse-generated count value N puls and the standard deviation of the entire bin N Plus + N Hintergrund , which is given by the square root of the expected value according to the Poisson distribution, provides a measure for this. Accordingly, the SNR is defined as SNR=NPulsNPulse+Nbackground with the number of counted events due to the pulse N Puls and the background N Hintergrund . Since these count values ​​are a function of time and the aim of the data evaluation is to determine the pulse arrival time, the count values ​​at the time of pulse arrival, which corresponds to the pulse transit time T TOF If the exponential distribution according to (3) is used as a basis, it follows NBackground=NCycle∫TTOFTTOF+TBinP(t)|RA=0dt≈NCycleTBinP(TTOF)|RA=0=NCycleTBinRB exp(−RBTTOF) with the number of accumulated time values ​​N Zyklus and the width of the bins T Bin as well as NPuls=NCycle∫TTOFTTOF+TBinP(t)|RA≠0dt−NBackground≈NCycleTBinP(TTOF)|RA≠0−NBackground=NCycleTBinRA exp(−RBTTOF).

[0053] The approximations made in (5) and (6) assume a constant PDF over the considered bin and are thus valid for short bins or low rates. The SNR according to (4) is thus SNR=NCycleTBinP(TTOF)|RA≠0−P(TTOF)|RA=0P(TTOF)|RA≠0=NCycleTBinexp(−RBTTOF)RARA+RB with the event rate R A of the reflected laser pulse and the event rate R B the background light.

[0054] Fig. Figure 4 shows an example of a signal-to-noise ratio (SDS) of a runtime measurement based on the detection signals (DES) as a function of the runtime for one measurement cycle (MZ), where SNR indicates the value of the signal-to-noise ratio (SDS) over time. This shows a decrease in the signal-to-noise ratio (SDS), and thus in the quality of the measurement, with increasing runtime T. TOF . Fig. 4 shows the signal-to-noise ratio SDS calculated according to (7) for an event rate R B of 10 MHz for the background, for an event rate R A of the laser pulse of 10 MHz, at 400 cycles and a bin width of 312.5 ps.

[0055] The signal-to-noise ratio (SDS) will also serve as a comparison with the method according to the invention. In real-world applications, the intensity of the reflected laser pulse (LAP) scales with the inverse square of the distance or propagation time. This further exacerbates the problem of the decreasing signal-to-noise ratio (SDS), which further reduces the measurement quality for longer ranges, resulting in correspondingly shorter ranges.

[0056] Fig. 5 shows an example of a curve SW of a setpoint R Soll (t) of a background event rate of the coincidence signals KOS for a constant probability density function WKS (shown in Fig. 7) for the occurrence of a first coincidence signal KOS when considering only the background event rate of the detection signals DES as a function of the running time for one measurement cycle MZ.

[0057] The method according to the invention now reduces the distance dependence of the measurement quality. For the basic description of the method, the dependence of the pulse intensity on the distance DIS is initially neglected. To obtain a consistent measurement quality over the entire measurement distance, according to (3), the probability density function WKS of the first event leading to the generation of a coincidence signal KOS must be independent of the propagation time, neglecting the reflected laser pulse LAP. Considering (1), we get P(t) = P const immediately that this can only be achieved by varying the event rate according to RSoll(t)=Pconst1−Pconstt can be achieved.

[0058] Fig. 5 shows the event rate according to (8) for a PDF P const = 1.08 * 10 6 s -1 .

[0059] Fig. 6 shows an example of a coincidence time KOZ as a function of the running time for a measuring cycle MZ, which is determined when using the setpoint R Soll (t) the Fig. 5 results.

[0060] According to a preferred development of the invention, the coincidence time specification stage 6 is designed such that the coincidence time KOZ increases strictly monotonically during the measuring cycle MZ.

[0061] In order to be able to vary the event rate over time, not individually detected photons PHO or detection signals DES are considered as events, but photon coincidences or correlation signals KOS are considered [2]. An event occurs precisely when at least a defined number of individual photons, the so-called coincidence depth KOT, is received within a defined time, the so-called coincidence time KOZ. The event rate resulting from this method is a function of both the coincidence depth KOT and the coincidence time KOZ. However, since only the latter allows for a sufficiently finely resolved, continuous variation, which is necessary according to (8) to achieve the goal of a constant probability density function WKS, the event rate R(t) is adjusted by varying the coincidence time KOZ.To determine the required variation of the coincidence time KOZ, a model can be used that calculates the event rate according to the coincidence method as a function of the parameter n, which specifies the coincidence depth KOT, the parameter ϑ, which specifies the coincidence time, and the parameter R, which describes the single-photon detection rate. For this purpose, a simple model is first considered. RC(t)=Rnϑ(t)n−1(n−1)! with the resulting event rate R C The model is based on exponentially distributed interarrival time of the individual photons according to (2) and can be derived from the model in [3] by further approximation according to RC=Rnϑn−1n(1+Rϑ)(n−1)!n−(Rϑ)n≈Rnϑn−1(n−1)! for Rϑ << 1. The procedure shown here is generally valid and can also be transferred to more complex models of photon coincidence, thus exploiting the full potential of the method. Since the event rate according to the correlation method R C the target value of the event rate according to (8) R Soll (t) accordingly, in order to obtain a constant PDF, (8) and (9) are equated below. This provides the rule for varying the coincidence time KOZ according to ϑ(t)=(n−1)!Pconst(1−Pconstt)Rnn−1

[0062] Fig. 6 shows the coincidence time according to (11) for P const = 1.08 * 10 6 s -1 , n = 2 and R = 10 MHz. The shape of the curve corresponds to Fig. 5, since for the chosen coincidence depth KOT of n = 2 the event rate R C (t) after the photon correlation according to (9) by R C (t)| n=2 = R 2ϑ(t) is given and is thus directly proportional to the coincidence time KOZ. For higher coincidence depths KOT, the proportionality exhibits a higher order, which results in qualitatively different curves.

[0063] Fig. 7 shows an example of a probability density function WKS1 and WKS2 for the occurrence of a first coincidence signal KOS for different pulse transit times as a function of the transit time for a measuring cycle MZ, which results when using the coincidence time KOZ of the Fig. 6 results.

[0064] According to a preferred development of the invention, the coincidence time specification stage 6 is designed such that the coincidence time KOZ is specified during the measuring cycle MZ such that a probability density function WKS for the occurrence of a first coincidence signal KOS of the coincidence signals KOS deviates from a constant value by at most 10% when considering only a background event rate HGE of the detection signals DES during the measuring cycle MZ. The probability density functions WKS1 and WKS2 have values ​​that deviate from a constant value by at most 10%, at least outside of the pulse reception times from 100 ns or from 200 ns. The quality of the transit time measurement is thus almost independent of the transit time, which is also noticeable in the approximately equal height of the pulses of the probability density functions WKS1 and WKS2.

[0065] In (11) the values ​​P const and n. To determine P constthe signal-to-noise ratio SNR (shown in Fig. 8) a time-of-flight measurement based on the coincidence signals according to (4). The count value of the background N Hintergrund can be calculated under the premise of a constant probability density function P const by simple multiplication with the width of the bin T Bin and the number of measuring cycles N Zyklus determine. It is NBackground=NCycleTBinPconst.

[0066] For the number of events due to the laser pulse, (1) is used. Up to time T TOF P(t) is a constant function and given to P const , which gives the integral to P const T TOF The rate R(t) at time T TOF results from the model of photon correlation according to (9) with R = R AB and the coincidence time according to (11) for t = T TOF and R = R B This results in NPuls=NCycleTBinPconst(RARB+1)n−NBackground

[0067] With (12) and (13) it follows from (4) SNR=NCycleTBinPconst(RA+RBRB)n(1−(RBRA+RB)n).

[0068] It turns out that the signal-to-noise ratio SRV is now below the achieved constant probability density function P const independent of the running time T TOF and only proportional to the square root of P const - a constant - is. Fig. Figure 7 shows the probability density functions WKS1 and WKS2 of the first event when varying the rate according to (8) by adjusting the event rate by varying the coincidence time KOZ according to (11) for a coincidence depth KOT of n = 2 at a constant value of the probability density functions WKS1 and WKS2 of P const = 1.08 * 10 6 s -1 . The single photon rates R B and R Aare 10 MHz each for the background and the laser pulse. The height of the resulting pulse and the background are independent of the propagation time, which corresponds to a constant signal-to-noise ratio (SNR) and, as a result, the desired consistent quality of the distance measurement.

[0069] In general, even taking into account the distance dependence of the laser pulse event rate (LAP), a constant signal-to-noise ratio (SNR) can be achieved according to the definition in (4). However, this requires a significantly higher dynamic range of the event rate after coincidence detection. This results in a high dynamic range of the coincidence time (KOZ), necessitating a more complex implementation of the method.

[0070] Fig. 8 shows an exemplary representation of signal-to-noise ratios (SRV) of time-of-flight measurements based on the coincidence signals KOS for different coincidence depths KOT as a function of the background event rate of the detection signals DES, where SNR indicates the value of the signal-to-noise ratio (SRV) over time.

[0071] Since this method aims at a maximum signal-to-noise ratio (SRV), it follows from (14) that P const should be as high as possible. The product P const T, where T is the measurement time, which is determined by the range of the system over T = 2d max / c is defined, provides the number of detected events per measurement cycle MZ. Since in the case considered only the first and thus a maximum of one event of each cycle is recorded, the product can assume a maximum value of 1. If this condition is violated, negative values ​​of the target event rate would result for longer times, which would not be feasible. The influence of the choice of P const The following calculation shows the effect of the target event rate R on the dynamic range of the event rate or the coincidence time KOZ. The quotient of the target event rate R Soll (t) according to (8) at time t = 0 as well as at time t = T provides the desired dynamic range, since this function is a monotonically increasing function of t for 0 ≤ t ≤ T under the condition P const T ≥ 1. The dynamics of the rate is therefore DRR=RSoll(T)RSoll(0)=11−PconstT

[0072] It turns out that P constT = 1 would result in an infinite dynamic range. Analogously, the dynamic range of the coincidence time φ(t) can be determined from the quotient of the coincidence time according to (11) for t = 0 and t = T. The following applies to the dynamics of the coincidence time DRϑ=ϑ(T)ϑ(0)=1(1−PconstT)n−1 which allows the same conclusion. In real applications, the range of possible coincidence time KOZ is corresponding to ϑ min ≤ ϑ(t) ≤ ϑ max limited by technical or physical parameters. From the two limits ϑ min and ϑ max Using (11) the limits for the parameter P const by appropriate transformation. The assumption is that the minimum value ϑ min the coincidence time KOZ at time t = 0 and the maximum value ϑ max the coincidence time must be present at time t = T. This means that from (11) Pmin=ϑminn−1Rn(n−1)! as well as Pmax=ϑmaxn−1Rn(n−1)!+Tϑmaxn−1Rn.

[0073] Since the coincidence time KOZ is a monotonically increasing function of time according to (11), a constant probability density function WKS can only be achieved if P min ≤ P max This ensures that the event rate according to (9) can be set small or large enough to achieve a constant probability density function WKS over the time range 0 ≤ t ≤ T. This means that from (17) and (18) Pmin≤Pmax→1≤(ϑmaxn−1−ϑminn−1)(n−1)!ϑminn−1ϑmaxn−1TRn∼R−n.

[0074] This condition can also be derived from ϑ(0, P const = P max ) ≥ ϑ min or ϑ(T, P const = P min ) ≤ ϑ max derive, ie at maximum value P max of the probability density function WKS, the minimum necessary coincidence time KOZ must not be less than ϑ min or at the minimum value P minof the probability density function WKS, the maximum coincidence time KOZ must not exceed ϑ max The inequality shows that there is only a limit in the direction of increasing single photon rate R. For decreasing rates, a constant P const reach, but at the same time P const itself, whereby the signal-to-noise ratio SRV according to (14) and thus the quality of the measurement decreases.

[0075] To determine up to which event rate the variation of the coincidence time KOZ or the application of photon correlation is actually meaningful, the signal-to-noise ratios SDS and SRV are compared according to equations (7) and (14). Since the true travel time is unknown, T is used in (7). TOF = T and in (14) n = 2 and P const = P max. The analogous procedure is used to determine the optimal value n for the coincidence depth KOT. Here, the signal-to-noise ratio (SNR) is determined according to (14) for all available n, and the depth KOT with the highest signal-to-noise ratio (SNR) is selected. Fig. Figure 8 shows the signal-to-noise ratios SRV1-4 for the coincidence depths KOT from n = 1 (no coincidence) to n = 4 for N Zyklus = 400, T Bin = 312.5 hp, P const = P max , T TOF = T = 660 ns and R A = R B It is shown that, when using the presented method, the signal-to-noise ratio (SNR) remains constant at a high level, but, in contrast to measurements without coincidence, does not decrease again for higher event rates. It should be noted that the signal-to-noise ratio (SNR) calculated according to (14) is only valid as long as (19) is fulfilled.

[0076] Fig. Figure 9 shows a second embodiment of a laser measuring device 1 according to the invention in a schematic partial view. The second embodiment is based on the first embodiment, so only the modifications and additions to the second embodiment are explained below.

[0077] According to an expedient development of the invention, the laser measuring device 1 has a background event rate determination stage 8 for determining a background event rate HGE of the detection signals DES, wherein the coincidence time specification stage 6 is designed to specify the coincidence time KOZ taking into account the background event rate HGE. The value R used above corresponds to B the background event rate HGE.

[0078] According to an expedient development of the invention, the coincidence time specification stage 6 is designed to specify the coincidence time KOZ taking into account the predetermined coincidence depth KOT.

[0079] According to an expedient development of the invention, the laser measuring device 1 has a maximum value determination stage 9 for determining a maximum value MAW of a constant probability density function WKS for the occurrence of a first coincidence signal KOS of the coincidence signals KOS when exclusively considering a background event rate HGE of the detection signals DES at the predetermined coincidence depth KOT, wherein the coincidence time specification stage 6 is designed to specify the coincidence time KOZ taking into account the maximum value MAW. The above-calculated value P max can correspond to the maximum value MAW.

[0080] According to an advantageous development of the invention, the laser measuring device 1 has a background event rate determination stage 8 for determining a background event rate HGE of the detection signals DES, wherein the laser measuring device 1 has a coincidence depth specification stage 10 for specifying the coincidence depth KOT to the coincidence detection stage 5, and wherein the coincidence depth specification stage 10 is designed to specify the coincidence depth KOT taking into account the background event rate HGE.

[0081] According to an advantageous development of the invention, the coincidence depth specification stage 10 is designed to determine signal-to-noise ratios SRV of probability density functions WKS for the occurrence of a first coincidence signal KOS of the coincidence signals KOS at different values ​​for the coincidence depth KOT, wherein for each of the different values ​​one of the signal-to-noise ratios SRV is determined, wherein the one of the different values ​​is specified as the coincidence depth KOT which is associated with a maximum signal-to-noise ratio SRV of the signal-to-noise ratios SRV.

[0082] According to an expedient development of the invention, the coincidence depth specification stage 10 is designed such that the coincidence depth KOT is constant during the measuring cycle MZ.

[0083] An example of a distance measurement procedure using the direct runtime method using the coincidence time KOZ adjustment is shown in Fig. 9. The background light and the laser source used result in the values ​​R A and R B of the event rates. Since the value R B the background event rate HGE is required, this is first determined. From the value R B the background event rate HGE of the background, the optimal value n of the coincidence depth KOT is determined based on equations (7) and (14) as well as inequality (19). This and the maximum value ϑ max the coincidence time KOZ and the measurement duration T, the value P const the constant probability density function WKS. From the value P const , the value R Bthe background event rate HGE and the previously determined value n of the coincidence depth KOT, the value ϑ(t) of the coincidence time KOZ is determined as a function of the transit time, which is fed to the coincidence detection stage 5 for the detection of events. There, the incoming detection signals DES with the event rates R A and R B the coincidence signals KOS are formed, whereby the coincidence signals KOS are correlated with the event rate with the value R C a probability density function WKS with the constant value P const for the background light component, so that after summing several runtime measurements a histogram HIG with a constant background count value is obtained.

[0084] The Fig. 10, Fig. 11 and Fig. 12 show a comparison of the method according to the invention with the prior art using a simulation.

[0085] To compare the presented method with the state of the art, a simulation of the direct measurement method is carried out. For this purpose, an optical sensor 3 with a detection unit 4 (sensor pixel 4), which comprises four detectors 11, is simulated, with each detector 11 generating events at a rate of R / 4. The simulation of the events is carried out using exponentially distributed random variables according to (2). From the events of the individual detectors 11, the coincidence events for a given coincidence depth KOT and coincidence time KOZ are then determined. The simulation is carried out with a constant coincidence time KOZ and with a variable coincidence time KOZ according to (11). To assess the quality, the mean value according to d¯=1N∑i=1Ndi with the measured distance DIS of the i-th measurement d i and the total number of measurements N as well as the error probability from N = 200 individual measurements with N each zyklus= 400 individual time measurements for each distance point. The error probability is defined as the probability that the measured distance deviates from the actual distance by more than 3%. This means PErr=1N∑i=1Nxi with xi={0 for |di−dtrue| / dtrue≤0.031 otherwise with the actual distance d true .

[0086] Fig. Figure 10 shows exemplary mean values ​​and error probabilities for distance measurements with constant coincidence time KOZ for different background illuminances in kilolux (klx) for different reflectances of the respective object OBJ.

[0087] Fig. Figure 11 shows exemplary mean values ​​and error probabilities for distance measurements with monotonically increasing coincidence time KOZ for different background illuminances in kilolux (klx) for different reflectances of the respective object OBJ.

[0088] The Fig. 10 and Fig. Figure 11 shows the mean value according to (20) and the error probability according to (21) for background intensities of 30 klx, 50 klx, and 80 klx, as well as for reflection coefficients from 5% to 95% for a constant and variable coincidence time KOZ at a constant coincidence depth KOT of n = 2. The background event rate can be determined from the intensity using the selected system parameters. For an intensity of 100 klx and a reflectance of 100%, the rate is R B = 108.96 MHz. The event rate of the laser pulse is R A = 1863 MHz at a distance of d = 10 m for a reflectance of 100 %. For the simulations, a pulse duration of 16 ns was assumed, which corresponds to the constant coincidence time KOZ in the case of the simulation without variable coincidence time KOZ as well as the maximum value ϑ maxcorresponds to the coincidence time KOZ when varied according to (11). Depending on the background intensity, a different minimum value ϑ min The coincidence time KOZ is required to ensure a constant probability density function for the background component. From the condition in (19), these can be determined to be 1324 ps, 504 ps, and 200 ps for 30 klx, 50 klx, and 80 klx, respectively.

[0089] Fig. 12 shows exemplary mean values ​​and error probabilities for distance measurements with constant coincidence time KOZ and with monotonically increasing coincidence time KOZ for different background illumination intensities.

[0090] Comparing the curves for constant and variable coincidence time (KOZ), an increase in range is evident for medium and high reflection coefficients or background event rates when applying the presented method. For a constant coincidence time, high rates ensure that, due to the shape of the probability density function (WDS), Fig. 2 the maximum of the histogram is found at distances close to zero. Consequently, the mean curves bend downwards and tend towards zero as soon as the reflected signal pulse can no longer be reliably detected. In the case of a variable coincidence time KOZ, the maxima are due to the probability density function WKS according to Fig. 7 approximately uniformly distributed (ie P(t) = 1 / T for 0 ≤ t ≤ T) in case of non-detection of the signal pulse, whereby the mean value tends towards the mean system range (∫0TP(t)t dt=∫0Tt / T dt=T / 2). In the simulations shown, this is 50 m. In order to make a quantitative statement about the range gain, a criterion for the maximum range must be defined. For example, a maximum error probability of 10% should define the maximum range. With a constant coincidence time, this results in a maximum range at a reflectance of 50% of 50 m, 27 m, and < 10 m for background intensities of 30 klx, 50 klx, and 80 klx. Using a variable coincidence, however, ranges of 75 m, 65 m, and 54 m can be achieved. For the sake of clarity, the curves of the above values ​​are shown in Fig. 12 shown again separately.

[0091] As discussed at the beginning, the simulation of the increasing coincidence time (KOZ) method clearly demonstrates the robustness gain of the maximum range for high background light intensities. For a constant coincidence time (KOZ), the range drops to below 10 m at 80 klx, whereas it only drops to 54 m with increasing coincidence time (KOZ). Overall, the range drops from 50 m to below 10 m—i.e., by more than 80%—with a constant coincidence time (KOZ) as the background light intensity increases from 30 klx to 80 klx. However, with increasing coincidence time (KOZ), the range drops from 75 m to 54 m, i.e., by less than 30%.

[0092] Fig. 13 shows an exemplary optical sensor 3 with a plurality of detection units 4 in a schematic view.

[0093] According to an advantageous development of the invention, the optical sensor 3 has a plurality of detection units 4 which have different detection ranges.

[0094] The optical sensor 3 consists of several independent detection units 4 (pixels), allowing a three-dimensional distance image to be captured. Each detection unit 4 can have several detectors 11.

[0095] Fig. 14 shows a schematic representation of an exemplary detection unit 4 with a plurality of detectors 11, the associated coincidence detection stage 5 and the associated time-of-flight measuring device 7.

[0096] According to an expedient development of the invention, the detection unit 4 has a plurality of detectors 11 for detecting individual photons PHO, wherein the detectors 11 have detection ranges corresponding to one another, and wherein the coincidence depth KOT is not greater than a number of detectors 11.

[0097] The detection unit 4 consists of several individual detectors 11. The detectors 11 each have a single-photon avalanche diode 21, since thanks to their high sensitivity for detecting individual photons PHO and thus are suitable for the described method. Each detector 11 of a detection unit 4 delivers detection signals DES as soon as an incident photon PHO has been detected. After the detection of a photon PHO, the detector 11 is inactive for the duration of the dead time; only then can the next photon PHO be detected. In order to realize coincidence times below the dead time, the detection of photon correlation is therefore carried out by linking the detection signals DES of several detectors 11. In the present embodiment, four detectors 11 are used per detection unit 4.Each of these has a separate circuit 22 for quenching the current after the detection of a photon PHO and for resetting the detector 11 (Active Quenching and Reset, AQR). This circuit 22 provides detection signals DES of the duration of the dead time, which are fed to the coincidence detection stage 5 for detecting photon coincidence. This stage detects whether at least a number of detection signals DES corresponding to the predetermined coincidence depth KOT has been received within a defined coincidence time KOZ. If this condition is met, the coincidence detection stage 5 generates a coincidence signal KOS. This signal is fed to the time-of-flight measuring device 7, which has a time measuring unit 12, a memory device 13, and a control and processing unit 14. The time measuring unit 12 starts the time measurement upon receipt of the coincidence signal KOS.At the end of a defined measurement window, the time measurement of all detection units 4 is stopped synchronously, and the measured time value is stored in memory elements 13. The data is read from the memory elements 13 at the appropriate time and transferred to a control and processing unit 14. This calculates an individual time for each detection unit 4 from several time stamps, which is converted into the measured distance DIS.

[0098] In the exemplary embodiment, the coincidence detection unit 5 generates coincidence signals KOS from the detection signals DES of the detectors 11 of a detection unit 4. To do this, the duration of the detection signals DES is first adjusted to the desired coincidence time KOZ using a pulse shaper. The coincidence time KOZ depends on the time elapsed since the start of the measurement according to (11). The resulting detection signals DES with adjusted duration are then fed to a logical operation. This generates an output signal when at least a number of detection signals DES corresponding to the selected coincidence depth KOT is present. By using several logical operations for different coincidence depths KOT and selecting one of the output signals using a multiplexer, the coincidence depth KOT can be varied.

[0099] Fig. Figure 15 shows a third embodiment of a laser measuring device according to the invention in a schematic partial view. The third embodiment is based on the second embodiment, so only the modifications and additions to the third embodiment are explained below.

[0100] According to an advantageous development of the invention, the travel time measuring device 1 is designed such that the distance DIS is determined on the basis of a travel time measurement of the detection signals DES when the predetermined coincidence depth KOT is one, and that the distance is determined on the basis of the travel time measurement of the coincidence signals KOS when the predetermined coincidence depth KOT is greater than one.

[0101] Fig. Figure 15 shows a flowchart of a possible algorithm for the distance-dependent variation of the coincidence time of a laser measuring device 1 according to the invention. In the first step, the background event rate HGE is determined by means of the background event rate determination stage 8. This can be done by counting events within a defined time window without active coincidence or based on the histogram of a previous distance measurement. To select the coincidence depth KOT, the signal-to-noise ratio SDS without coincidence according to (7) and the signal-to-noise ratio SRV with coincidence according to (14) are calculated for all available values ​​of n using the signal-to-noise ratio determination stage 15 of the coincidence depth specification stage 10. The necessary variables "number of cycles N Zyklus “, “Width of bins T Bin ” and the “measurement duration T” are defined by the measuring system and therefore known. For P const becomes P maxaccording to (18), which is calculated from the known quantities. The value R A the event rate of the reflected laser pulse LAP. Since this cannot be measured directly or determined from the histogram HIG, the case R A = R Bbe assumed. This case is considered the worst case, up to which a measurement is possible. For higher intensities of the laser pulse LAP, the limit values ​​shift to lower rates, so that a coincidence depth KOT is set that is too low rather than too high. This ensures that a measurement is always possible, even if the optimal signal-to-noise ratio SNR is not always achieved. An alternative possibility is to estimate the event rate of the laser pulse LAP from the histogram HIG of a previous measurement, although this requires a certain correlation between the measurements. If the setting of the coincidence depth KOT shows that a measurement without coincidence promises the highest quality, the further steps using the first decision stage 16 are skipped and a time-of-flight measurement without coincidence can be carried out directly.

[0102] If coincidence is reasonable, the limit factor determination stage 17 is used to check whether a constant probability density can be achieved with the available dynamic range of the coincidence time KOZ. For this purpose, the criterion according to (19) is checked. If this criterion is not met, the coincidence depth KOT is increased using the incrementation stage 20 until the criterion is met or its maximum value n max is reached, whereby reaching the maximum value n max is checked by means of the third decision stage 19. Subsequently, the coincidence time KOZ is determined by means of the coincidence time specification stage 6 according to (11) under the assumption P const = P maxbefore the time-of-flight measurement is performed based on the direct method. Depending on the system requirements and the target application, this sequence can be performed before each MZ measurement cycle, i.e., before each shot of Laser 2, before a group with a defined number of MZ measurement cycles, or simply before a complete distance measurement consisting of a large number of MZ measurement cycles.

[0103] Fig. 16 shows exemplary probability density functions for the occurrence of a first coincidence signal for different pulse transit times as a function of the transit time for a measuring cycle, wherein the coincidence time KOZ is determined at the beginning of each of the partial measuring cycles TMZ and is kept constant for the respective partial measuring cycle TMZ.

[0104] According to an advantageous development of the invention, the coincidence time specification stage 6 is designed such that the measuring cycle MZ is divided into several partial measuring cycles TMZ, wherein the coincidence time KOZ is determined at the beginning of each of the partial measuring cycles TMZ and is kept constant for the respective partial measuring cycle TMZ.

[0105] The continuous variation of the coincidence time KOZ as a function of the runtime described above places high demands on the hardware. An alternative variant is the variation of the coincidence time KOZ in fixed steps. The more steps used, the greater the advantage of the presented method compared to a fixed coincidence time KOZ. Fig. Figure 16 shows the probability density functions WKS1 and WKS2 of the first event when the coincidence time KOZ is varied in four steps. The values ​​of the coincidence time KOZ are calculated according to (11) for the times 0, T / 4, T / 2 and T3 / 4. These values ​​also correspond to the times at which the coincidence time KOZ is changed. Compared to the probability density functions WKS1 and WKS2 of the Fig. 7, the probability density functions WKS1 and WKS2 of the Fig. 16 only has a minor change. However, the hardware effort is reduced considerably.

[0106] Aspects of the invention described in connection with a device also relate to corresponding methods. Conversely, aspects of the invention described in connection with a method also relate to a corresponding device. Reference symbol: 1 laser measuring device 2 pulse lasers 3 optical sensor 4 Detection unit 5 Coincidence detection level 6 Coincidence time specification level 7 Runtime measuring device 8 Background event rate determination stage 9 Maximum value determination level 10 coincidence depth setting level 11 Detector 12 Time measurement unit 13 Storage device 14 Control and processing unit 15 Signal-to-noise ratio determination stage 16 first decision stage 17 Limit factor determination stage 18 second decision stage 19 third decision stage 20 increment level 21 single-photon avalanche diode 22 circuit DIS Distance OBJ object LAP laser pulse PHO photons DES detection signal KOS coincidence signal KOZ coincidence time KOT coincidence depth WDS probability density function for the occurrence of a first detection signal HIG histogram SDS signal-to-noise ratio of a time-of-flight measurement based on the detection signals SW Target value of a background event rate of the coincidence signals for a constant probability density function for the occurrence of a first coincidence signal when considering only the background event rate of the detection signals WKS probability density function for the occurrence of a first coincidence signal SRV Signal-to-noise ratio of a time-of-flight measurement based on the coincidence signals HGE background event rate of the detection signals MAW maximum value of a constant probability density function TMZ partial measurement cycle MZ measuring cycle Sources: [1] P. Seitz and A. J. P. Theuwissen, Eds., Single-photon imaging. Heidelberg ; New York: Springer, 2011. [2] M. M. Hayat, S. N. Torres, and L. M. Pedrotti, „Theory of photon coincidence statistics in photon-correlated beams,“ Opt. Commun., vol. 169, Nr. 1-6, S. 275-287, Oktober 1999. [3] M. Beer, O. M. Schrey, B. J. Hosticka, and R. Kokozinski, „Coincidence in SPAD-based time-of-flight sensors,“ in 2017 13th Conference on Ph.D. Research in Microelectronics and Electronics (PRIME), 2017, S. 381-384. [4] DE 10 2018 203 534 A1

Claims

[1] Laser measuring device for measuring a distance (DIS) to an object (OBJ) comprising: a pulse laser (2) for emitting a laser pulse (LAP) at the beginning of a measuring cycle (MZ); an optical sensor (3) with at least one detection unit (4) for generating detection signals (DES), wherein the detection unit (4) has at least one detector (11) for detecting individual photons (PHO), wherein the detection unit (4) generates one of the detection signals (DES) each time during the measuring cycle (MZ) when one of the photons (PHO) is detected by the detector (11); a coincidence detection stage (5) for generating coincidence signals (KOS), wherein each time during the measuring cycle (MZ) the detection signals (DES) generated by the detection unit (4) reach at least a predetermined coincidence depth (KOT) within a coincidence time (KOZ), one of the coincidence signals (KOS) is generated; a coincidence time specification stage (6) for specifying the coincidence time (KOZ) to the coincidence detection stage (5), wherein the coincidence time specification stage (6) is designed such that the coincidence time (KOZ) increases monotonically during the measuring cycle (MZ); and a travel time measuring device (7) for determining the distance (DIS) on the basis of a travel time measurement of the coincidence signals (KOS). [2] Laser measuring device according to the preceding claim, wherein the laser measuring device (1) has a background event rate determination stage (8) for determining a background event rate (HGE) of the detection signals (DES), wherein the coincidence time specification stage (6) is designed to specify the coincidence time (KOZ) taking into account the background event rate (HGE). [3] Laser measuring device according to one of the preceding claims, wherein the coincidence time specification stage (6) is designed to specify the coincidence time (KOZ) taking into account the predetermined coincidence depth (KOT). [4] Laser measuring device according to claim 1, wherein the laser measuring device (1) has a maximum value determination stage (9) for determining a maximum value (MAW) of a constant probability density function (WKS) for the occurrence of a first coincidence signal (KOS) of the coincidence signals (KOS) when exclusively considering a background event rate (HGE) of the detection signals (DES) at the predetermined coincidence depth (KOT), wherein the coincidence time specification stage (6) is designed to specify the coincidence time (KOZ) taking into account the maximum value (MAW). [5] Laser measuring device according to claim 1, wherein the laser measuring device (1) has a background event rate determination stage (8) for determining a background event rate (HGE) of the detection signals (DES), wherein the laser measuring device (1) has a coincidence depth specification stage (10) for specifying the coincidence depth (KOT) to the coincidence detection stage (5), and wherein the coincidence depth specification stage (10) is designed to specify the coincidence depth (KOT) taking into account the background event rate (HGE). [6] Laser measuring device according to the preceding claim, wherein the coincidence depth specification stage (10) is designed to determine signal-to-noise ratios (SRV) of probability density functions (WKS) for the occurrence of a first coincidence signal (KOS) of the coincidence signals (KOS) at different values ​​for the coincidence depth (KOT), wherein one of the signal-to-noise ratios (SRV) is determined for each of the different values, wherein that of the different values ​​is specified as the coincidence depth (KOT) which is associated with a maximum signal-to-noise ratio (SRV) of the signal-to-noise ratios (SRV). [7] Laser measuring device according to claim 5 or 6, wherein the coincidence depth setting stage (10) is designed such that the coincidence depth (KOT) is constant during the measuring cycle (MZ). [8] Laser measuring device according to one of the preceding claims, wherein the coincidence time setting stage (6) is designed such that the coincidence time (KOZ) increases strictly monotonically during the measuring cycle (MZ). [9] Laser measuring device according to claim 1, wherein the coincidence time specification stage (6) is designed such that the coincidence time (KOZ) during the measuring cycle (MZ) is specified such that a probability density function (WKS) for the occurrence of a first coincidence signal (KOS) of the coincidence signals (KOS) deviates from a constant value by at most 10% when considering only a background event rate (HGE) of the detection signals (DES) during the measuring cycle (MZ). [10] Laser measuring device according to one of the preceding claims, wherein the coincidence time specification stage (6) is designed such that the measuring cycle (MZ) is divided into several partial measuring cycles (TMZ), wherein the coincidence time (KOZ) is determined at the beginning of each of the partial measuring cycles (TMZ) and is kept constant for the respective partial measuring cycle (TMZ). [11] Laser measuring device according to one of the preceding claims, wherein the detection unit (4) has a plurality of detectors (11) for detecting individual photons (PHO), wherein the detectors (11) have mutually corresponding detection ranges, and wherein the coincidence depth (KOT) is predetermined to be no greater than a number of the detectors (11). [12] Laser measuring device according to one of the preceding claims, wherein the optical sensor (3) has a plurality of detection units (4) which have different detection ranges. [13] Laser measuring device according to one of the preceding claims, wherein the transit time measuring device (1) is designed such that the distance (DIS) is determined on the basis of a transit time measurement of the detection signals (DES) when the predetermined coincidence depth (KOT) is one, and that the distance is determined on the basis of the transit time measurement of the coincidence signals (KOS) when the predetermined coincidence depth (KOT) is greater than one. [14] Method for operating a laser measuring device (1) for measuring a distance (DIS) to an object (OBJ), the method comprising the following steps: Emitting a laser pulse (LAP) at the beginning of a measuring cycle (MZ) by means of a pulse laser (2); Generating detection signals (DES) by means of at least one detection unit (4) of an optical sensor (3), wherein the detection unit (4) has at least one detector (11) for detecting individual photons (PHO), wherein one of the detection signals (DES) is generated by the detection unit (4) each time during the measuring cycle (MZ) when one of the photons (PHO) is detected by the detector (4); Generation of coincidence signals (KOS) by means of a coincidence detection stage (5), wherein each time during the measuring cycle (MZ) when the detection signals (DES) generated by the detection unit (4) reach at least a predetermined coincidence depth (KOT) within a coincidence time (KOZ), one of the coincidence signals (KOS) is generated; Specification of the coincidence time (KOZ) to the coincidence detection stage (5) by a coincidence time specification stage (6), wherein the coincidence time (KOZ) is specified such that the coincidence time (KOZ) increases monotonically during the measuring cycle; and Determination of the distance (DIS) on the basis of a travel time measurement of the coincidence signals (KOS) by means of a travel time measuring device (7). [15] Computer program for carrying out a method according to the preceding claim.

Images