DISTANCE MEASUREMENT OF AN OBJECT USING A LIGHT TIME-STAGE METHOD

Description

[0001] The invention relates to an optoelectronic sensor and a method for measuring the distance of an object in a detection area using a time-of-flight method according to the preamble of claim 1 and 9, respectively.

[0002] Distance measurement can be used in a wide variety of fields, such as factory automation, logistics automation, and safety technology. Based on a variation of the direct time-of-flight (dToF) principle, a short light pulse is emitted, and the time until the remitted or reflected light pulse is detected is measured. Possible applications of distance measurement include modified light barriers that monitor the distance between their transmitter and receiver or reflector, or switching systems with binary object presence detection, where the switching state depends on whether an object is within a specific distance range. These latter sensors are also known as background-suppressing light sensors. Single-beam or one-dimensional distance measurement can be extended to line-based or area-based distance measurement by using appropriately spatially resolved receivers.Laser scanners also rely on measuring the time of flight of light to determine distances at corresponding angular positions.

[0003] The detection sensitivity of simple photodiodes as light receivers is insufficient in many applications. In an avalanche photodiode (APD), the incident light triggers a controlled avalanche breakdown (avalanche effect). This multiplies the charge carriers generated by the incident photons, creating a photocurrent that is proportional to the light reception intensity but significantly larger than that of a simple PIN diode. In so-called Geiger mode, the avalanche photodiode is biased above the breakdown voltage, so that even a single charge carrier released by a single photon can trigger an avalanche, which then recruits all available charge carriers due to the high field strength. The avalanche photodiode thus counts individual events, just like the Geiger counter from which it takes its name. Avalanche photodiodes in Geiger mode are also known as SPADs (single-photon avalanche diodes).

[0004] Geiger-APDS or SPADs are therefore very fast, highly sensitive photodiodes based on semiconductors. A disadvantage of this high sensitivity is that not only a useful light photon, but also a weak interference event from ambient light, optical crosstalk, or dark noise can trigger an avalanche breakthrough. Each individual SPAD therefore measures not only the desired light transit times but also interference events as apparent transit times. An interference event then contributes to the measurement result with the same relatively strong signal as the received useful light and is indistinguishable from it within the signal itself. After an avalanche breakthrough, regardless of its cause, the sensitivity of the avalanche photodiode is drastically reduced for a dead or recovery time of approximately 5 to 100 ns, rendering it practically unusable for further measurements during this period. The pool of SPADs available for measurement is thus reduced by ambient light (pile-up effect).

[0005] Therefore, for a measurement to be effective, the desired events must be separated from the unwanted events. Traditionally, numerous individual measurements are performed using multiple SPADs and / or repeated measurements to enable statistical analysis, particularly by compiling the individual measurements in a histogram. A peak emerges from the desired events in this histogram, which, in principle, can be detected using a threshold. However, both the desired light and unwanted events are highly dependent on the specific measurement situation. Slower drifts, such as degradation of the light source, as well as variations in manufacturing and component tolerances, also play a role. Defining the threshold is therefore extremely challenging, and even an optimally adjusted static threshold is only effective under tightly controlled measurement conditions.

[0006] DE 10 2021 118 660 A1 describes a laser scanner with single-photon avalanche diodes and histogram evaluation using an optimal filter.

[0007] EP 3 428 683 A1 discloses an optoelectronic sensor with a light receiver comprising a plurality of SPADs. A selection of these SPADs is connected in a 1:1 connection to a time-of-flight measurement unit. This selects an area of ​​interest within the light receiver, upon which the time-of-flight measurement is based. The document does not offer a solution to the described threshold problem.

[0008] In EP 3 418 767 B1, parameters for describing the exponentially decreasing frequency of background events are estimated in another optoelectronic sensor of the same type, based on a binomial model, and further selected using a median filter. The estimation is based on a histogram in which measured and disturbance events are mixed; the median filter is intended to separate them. Firstly, the statistical basis is very limited with only one histogram. In certain measurement situations, such as an object moving laterally into the detection area with edge hits, there are also quite arbitrary and multiple peaks, which the median filter does not handle as desired. Furthermore, a median filter is comparatively computationally intensive because sorting is required, which reduces the response time of a sensor in limited hardware, especially embedded hardware.

[0009] From EP 4 249 950 A1, a distance-measuring optoelectronic sensor is known which evaluates a reception time with a threshold that is dynamically adjusted by means of level information.

[0010] The dissertation by Maik Beer, "SPAD-based sensors for time-of-flight-based distance measurement at high background light intensity", Duisburg, Essen, University of Duisburg-Essen, 2018 deals with the influence of background light on the distance measurement of a sensor of this type.

[0011] It is therefore the object of the invention to further improve the light time-of-flight measurement of a SPAD-based distance-measuring sensor.

[0012] This problem is solved by an optoelectronic sensor, in particular one of the sensor types mentioned in the introduction, and a method for measuring the distance of an object within a detection range using a time-of-flight method according to claim 1 and 9, respectively. As is typical for time-of-flight measurement, a light transmitter emits a light signal which is received by a light receiver after diffuse remission or direct reflection from an object whose distance is to be measured. Preferably, the light signal has a short pulse, so that a pulse-based method (Direct Time of Flight, dToF) is used. The light receiver comprises a first plurality of avalanche photodiodes or pixels which can be operated in a Geiger mode in which they are biased with a voltage greater than the breakdown voltage in order to trigger an avalanche event upon light reception.The term SPAD is frequently used below to refer to avalanche photodiodes in Geiger mode. A second set of time-of-flight measuring units determines the individual transit times between the emission and reception of a light signal. These time-of-flight measuring units are assigned to specific avalanche photodiodes, depending on the embodiment, in a 1:n, 1:1, or n:1 ratio, in varying or uniform group sizes. It is possible that only a selection of avalanche photodiodes are evaluated by a time-of-flight measuring unit in order to define a relevant area of ​​the light receiver, particularly as described in EP 3 428 683 A1. Individual transit times are initially uninterpreted measurement results that can correspond to both useful events of the light signal returning from an object and to interference events.

[0013] A control and evaluation unit analyzes the individual light transit times, initially collecting them in a histogram via numerous time-of-flight measurement units and / or measurement repetitions, in which a light signal is emitted and a measurement period is observed. The histogram sorts the individual light transit times into a discretization of the measurement period (temporal bins) and counts the frequency of each individual light transit time (count per bin). A useful light signal or peak is then located in the histogram based on a threshold. The distance value is determined from the temporal position of the useful light signal.

[0014] The invention is based on the fundamental idea of ​​first estimating the ambient light level as information about the current measurement situation in order to dynamically adjust the threshold. The defined threshold takes into account the model assumptions regarding the pile-up effect with an exponential decay of background events over time and maintains a buffer distance to the interfering events, i.e., noise and ambient light events, defined by a safety margin. The threshold's behavior is parameterized based on the currently measured ambient light level. In the concrete implementation, it can be advantageous to use a more easily computable function, such as a polynomial, as an approximation of the exponential function.

[0015] The invention offers the advantage of enabling reliable measurement even under the influence of ambient light. Thanks to dynamic threshold adjustment, low-reflecting objects at great distances can be detected in low ambient light conditions, while in high ambient light conditions, the threshold is set to prevent false detections. The threshold used, which depends on the current ambient light level, can be carefully prepared and defined during the development phase, if necessary requiring considerable time and a broad database. This allows for the consideration of drift, degradation processes, and variations between individual sensors within a series. At runtime, only minimal computational effort is required to select the appropriate threshold for the current ambient light level.

[0016] The time-of-flight measurement units preferably incorporate a TDC (Time-to-Digital Converter). This is a well-known and relatively simple component capable of determining individual light transit times with high temporal resolution. TDCs can be directly integrated monolithically into a crystal of the light receiver. The respective TDC is preferably started at the transmission time and stopped at the reception time by the received single light pulse, or in the case of interference from ambient light or dark noise. Other operating modes are conceivable, such as starting the TDCs by triggering an avalanche and then stopping them at a known time, like the end of the measurement period.

[0017] The control and evaluation unit is preferably designed to estimate the ambient light level by summing the first bins of the histogram. The first bins are used because no useful light has yet returned at this stage, and the pile-up effect has only sent a small number of avalanche photodiodes into their dead time. It should be noted that the avalanche photodiodes are preferably inactive until a measurement starts, in order to initially provide the full pool of recruitable avalanche photodiodes. An ideally, a falling exponential function could be reconstructed from just two bins. In reality, the measurement is a random experiment, so a larger number of bins should be used. Nevertheless, summing the first bins should also include a partial selection, for example, of the second, fifth, and eighth bins, and only preferably refer to the inclusion of all 1 to N first bins.In any case, the counts in the first bins are highly correlated with the ambient light level, and this relationship is used to determine it. Alternatively, the ambient light level can be determined by an additional dedicated ambient light receiver or at least by a dedicated avalanche photodiode of the light receiver.

[0018] The control and evaluation unit preferably maintains a first lookup table that assigns an ambient light level to a summed number of detection events. "Maintaining" means that the first lookup table (LUT) is stored in the control and evaluation unit or in memory that it can access. Determining the ambient light level at runtime is then limited to summing the first bins and performing a simple table access to obtain the appropriate ambient light level value from the sum of the first bins. This is a very fast and easy-to-use implementation that requires minimal hardware resources. Intermediate values ​​missing from the first lookup table can be interpolated. For this purpose, a predefined function, in particular a polynomial, is preferably used; even more preferably, the same function that was already used to train the first lookup table, as explained below.However, a simple linear interpolation will often suffice.

[0019] The first reference table is preferably trained prior to distance measurement by repeatedly exposing the light receiver to a defined ambient light level with the light source inactive and determining the sum of the first bins of the resulting histogram. The ambient light is generated, in particular, by an additional light source. This training takes place before the actual measurement operation, for example, during development or factory production. The light receiver is systematically exposed to varying ambient light of known intensity, without superimposition of the useful light from the inactive light source. A histogram is recorded each time, and its first bins are summed. This creates data pairs that assign a sum of the first bins to a given ambient light level.From this, the relationship between the ambient light level and the sum of the first bins at any desired grain size from the first reference table can be derived by averaging or, preferably, by using a function or polynomial fit. The fitted function can be stored in the sensor for later use in interpolation.

[0020] The control and evaluation unit is preferably designed to delay the emission of a light signal relative to a start signal, ensuring that no useful light signal is registered in the first bins of the histogram. This allows the distance measurement range to begin immediately in front of the sensor, while still guaranteeing that the first bins are free of useful light, enabling the estimation of the ambient light level. The additional delay can be easily calculated from the measured light transit times. Appropriate calibration is generally required anyway for unavoidable internal signal delays, which can, incidentally, replace or supplement the artificial delay.

[0021] The control and evaluation unit preferably maintains a second lookup table that assigns at least one suitable parameter of a calculation formula for the threshold to each ambient light level. The threshold is thus defined as a calculation formula with at least one parameter that is not predetermined, in particular with a component of a decreasing exponential function and a safety margin component, which is preferably also based on an exponential function. The at least one parameter is then read from the second lookup table based on the ambient light level, and the threshold is thus dynamically adjusted to the currently prevailing ambient light level. Regarding the storage location, the type of use, the possible interpolation, and the advantages, the same applies to the second lookup table as to the first.

[0022] The second reference table is preferably learned in advance of the distance measurement by repeatedly exposing the light receiver to a defined level of ambient light while the light source is inactive and calculating the respective sum. HS = ∑ i = 1 M Histogram i as well as a focus λ = ∑ i = 1 M Histogramm i ∗ i HS of the histogram generated in the process is determined, whereby Histogram ( i ) the i = 1 ... M Bins of the histogram are designated. The sum HS and the focus λ They are then used as parameters or within the threshold parameters. The learning scenario is comparable to that for the first lookup table; in particular, both lookup tables can be learned simultaneously.

[0023] The control and evaluation unit is designed to determine the threshold based on the calculation rule. a exp − b i + S ∗ a exp − b i to define, with the parameters a and band a scaling factor S for the safety margin. This is a concrete calculation rule that accurately models the behavior of the light receiver used. The parameters a and b are set according to the prevailing ambient light level, preferably simply read from the second reference table. The first term models the expected behavior of the noise threshold. To compensate for variations across individual measurements, the threshold is shifted upwards by the second term as a buffer. The safety margin S is in contrast to a and b No parameter is dependent on the ambient light level; this dependency is already accounted for in the factor under the square root. Rather, it is determined via the safety margin. SThe system weighs the relationship between Type I and Type II errors, i.e., whether it is more acceptable to miss an object as noise or, conversely, to falsely detect a rare, strong noise event as an object. The sensor is configured accordingly, either at the factory or on-site, by setting S.

[0024] For the parameters a and b applies a = H a λ and b = 1 λ . This is λ preferably derived from the center of gravity of at least one histogram determined at a defined ambient light level. These parameters result from a model of the light receiver's behavior that has proven very effective in practice.

[0025] The control and evaluation unit is preferably designed to compare the number of individual light transit times with the threshold for each bin in the histogram and to assign suprathreshold bins to the useful light signal. The threshold is thus applied bin by bin to locate those bins in which returning transmitted light has been detected. This allows the useful light signal to be temporally localized. In many measurement situations, there is only one such cluster of bins corresponding to a single useful light peak. The light transit time can then be determined as the maximum, center point, or similar measure of these bins, or by fitting a peak function to these bins. If there are several suprathreshold bins that are not adjacent, multiple distance values ​​can be output, or, for example, the first, last, or most pronounced peak can be used as the basis for the distance value.

[0026] The method according to the invention can be further developed in a similar manner and exhibits similar advantages. Such advantageous features are described by way of example, but not exhaustively, in the dependent claims following the independent claims. Fig. 1 a schematic representation of an optoelectronic sensor with time-of-flight measurement; Fig. 2 a schematic representation of a light receiver and downstream components for measurement evaluation; Fig. 3 an exemplary histogram of measured individual time-of-flights with only ambient light incident without useful light, illustrating the summation of the first bins; Fig. 4 a representation of six exemplary histograms similar to Figure 3for different ambient light levels; Fig. 5 shows the relationship between the summed first bins and the ambient light level; Fig. 6 illustrates a model of the ambient light component in a histogram and a threshold with a safety margin above it; Fig. 7 shows six exemplary thresholds similar to Figure 6 for different levels of ambient light; and Fig. 8 a representation similar to Figure 6 , but now with a useful light peak that is detected by the threshold.

[0027] Figure 1Figure 1 shows a schematic representation of an optoelectronic sensor 10 for distance measurement based on the time-of-flight principle in a one-dimensional embodiment. The sensor 10 is described as representative of other time-of-flight sensors, as mentioned in particular in the introduction. A light emitter 12, for example an LED or a laser light source, sends a light signal 14 into a monitoring area 16. If an object 18 is located there, part of the light is diffusely remitted or reflected and returns to the sensor 10 as a remitted light signal 20, where it is registered in a light receiver 22.

[0028] The light receiver 22 comprises a plurality of pixel elements 24, also called SPADs (Single-Photon Avalanche Diodes), which can be operated in a Geiger mode to trigger an avalanche event upon light reception by biasing them with a voltage greater than their breakdown voltage. Some basic SPAD properties have already been described in the introduction. The pixel elements 24 are preferably arranged in a matrix. The number of pixel elements 24 can vary; the matrix can, for example, be a square or rectangular arrangement with several dozen, hundreds, or even thousands of pixel elements 24 or more.

[0029] The light receiver 22 is connected to a sensor control block 26. In Figure 1 This is only a summary representation; a possible structure of the sensor control block 26 will be described later with reference to Figure 2The sensor control block 26 controls the light transmitter 12 such that the light signal 14 is emitted, preferably with a short pulse in the nanosecond or even picosecond range. The time at which a light signal 14 is triggered can be used as a reference for the time-of-flight measurement. In other embodiments, a portion of the light signal 14 can serve internally as an optical reference. The sensor control block 26 processes signals from the pixel elements 24, which are evaluated to determine the time of flight from a transmission time of the emitted light signal 14 to a reception time of the returned light signal 20. The time of flight can be converted into a distance using the speed of light. The determination of the reception time will be described later with reference to the Figures 2 to 8 explained.

[0030] In practice, the sensor comprises 10 additional elements, in particular transmitting and receiving optics as well as interfaces, which are known per se and have been omitted for the sake of simplicity. A division of the light receiver 22 and the sensor control block 26 as in Figure 1 This is possible in practical embodiments, but serves mainly for illustrative purposes. Preferably, these components are at least partially integrated on a common chip, the surface of which is used jointly by the pixel elements 24 and the circuits assigned or assignable to the pixel elements 24 or groups of pixel elements 24 for their control and evaluation.

[0031] In Figure 1A coaxial arrangement is shown in which the light source 12 is positioned in front of the light receiver 22. Other coaxial arrangements are possible, for example, using a beam splitter. A biaxial or triangulating arrangement is also conceivable, in which the light source 12 and light receiver 22 are positioned side by side with a mutual offset. The sensor 10 can be a one-dimensional sensor that is Figure 1The depicted type is used. Other embodiments, not exhaustively mentioned, include light barriers, light grids, and laser scanners. The sensor 10 can output or display a distance value or function as a switch by triggering a switching event when an object is detected within a specific distance range, including deviations from an expected distance range. Several sensors 10 can be combined, for example, to form a distance-measuring or distance-monitoring light grid. Mobile systems in which the sensor 10 is movably mounted, or scanning systems in which the emitted light signal 14 sweeps across the monitoring area 16 by means of a movable mirror or by moving the measuring system, in particular by a rotational movement, are also conceivable.

[0032] Figure 2Figure 1 shows a schematic representation of the light receiver 22 and downstream components of the sensor control block 26. The light receiver 22 is again represented as a SPAD matrix with a plurality of pixel elements 24. Some of the pixel elements 24 are connected to time-of-flight measurement units 28, which in this embodiment are time-to-digital converters (TDCs). A switching device 30 determines the connections, i.e., which pixel elements 24 are selected for evaluation and by which time-of-flight measurement unit 28 they are evaluated. The time-of-flight information generated by the time-of-flight measurement units 28 is accumulated and evaluated by a control and evaluation unit 32, preferably after storage of the accumulated time-of-flight information in a memory (not shown separately) and preferably in the form of a histogram.One result of the evaluation is a distance value, which can serve as a basis for further evaluations.

[0033] The switching device 30 can be configured as a programmable matrix or in some other way to connect selected pixel elements 24 to a selected time-of-flight measurement unit 28 according to a 1:1 or an n:1 scheme. It is not necessary to evaluate all pixel elements 24. One reason for forming only selected connections is that a large number of time-of-flight measurement units 28, corresponding to the number of pixel elements 24, is not feasible or at least too expensive and requires too much chip area. Therefore, the number of time-of-flight measurement units 28 is preferably only a fraction of the number of pixel elements 24. Furthermore, the signal-to-noise ratio can be improved by preferentially selecting pixel elements 24 in a region of interest (ROI) that actually receives the remitted light signal 20.

[0034] The time-of-flight measuring units 28 measure the individual light transit time between the transmission of the emitted light signal 14 and the reception of the returned light signal 20. In one embodiment, the time-of-flight measuring units 28 are started by the emission of the light signal 14 and stopped by an avalanche event in the connected pixel element(s) 24. In another embodiment, they are started by the avalanche event and stopped at a reference time, whereby the offset between the transmission time and the reference time is computationally compensated. Each individual light transit time is, in itself, very unreliable, since the measured avalanche breakthrough may be caused by ambient light or darkness noise instead of by the returned light signal 20, so that the corresponding individual light transit time may be completely uncorrelated with the distance to be measured.

[0035] Therefore, the individual light transit times of the light transit time measurement units 28 are accumulated in a histogram, preferably also across measurement repetitions for improved statistics, and evaluated by the control and evaluation unit 32. The histogram divides a measurement period into temporal bins, in each of which the number of individual light transit times falling within the time interval of a bin is counted. Upon reception of the remitted light signal 20, a peak forms in this histogram, the temporal position of which determines the time of reception.

[0036] As mentioned previously, the light receiver 22 and the components of the sensor control block 26 can be integrated on the same chip. In a preferred embodiment, the light receiver 22, the time-of-flight measurement units 28, and the switching device 30 are part of an ASIC (Application-Specific Integrated Circuit), while the control and evaluation unit 32 is implemented on a microprocessor. In another embodiment, the control and evaluation unit 32 is also at least partially integrated into the ASIC. The memory for the histograms can be part of the ASIC, the microprocessor, or a separate component. This is only one preferred hardware implementation; the functionality of the sensor control block 26 can alternatively be implemented on one or more arbitrary hardware components, such as an ASIC, a CPU (Central Processing Unit), an FPGA (Field Programmable Gate Array), a DSP (Digital Signal Processor), or the like.

[0037] Figure 3Figure 1 shows an example histogram of measured individual light transit times with only ambient light incident without the useful light of the reflected light signal 20. The respective number of individual light transit times (count) is shown on the Y-axis as the height of the bars of the respective bin on the X-axis. Without useful light, the histogram contains only a noise component or background events (pile-up) in an exponential decay, resulting from the dead times of the pixel elements 24 after an avalanche event. There are a total of M Bins are provided according to a measurement period or maximum detectable distance. Due to unavoidable and / or intentional delays between the internal trigger for sending the light signal 14 and the actual transmission time, usable light reception is only possible from the bin onwards. N +1 possible. The first NBins can therefore be used at runtime for a pure ambient light estimation, whereby other noise events are subsequently added to the ambient light component for simplification. The summed first values ​​highlighted in Box 34 N Bins, i.e. FS = ∑ i = 1 N Histogramm i with Count histogram ( i ) in the i -ten bin, are strongly correlated with the ambient light level:

[0038] Figure 4 shows a representation of six exemplary histograms similar to Figure 3 for different ambient light levels. By exposing the sensor 10 or its light receiver 22 to such known and defined ambient light levels before measurement operation, for example during development, manufacturing or commissioning, data pairs can be generated ( FS n , FP n ) for different levels of ambient light such as FP 0 = 0 Klux, FP 1 = 1 KLux , ... win the desired number.

[0039] Figure 5shows a representation of the relationship between FP and FS, that is, the ambient light level and the accumulated first N Bins. The data pairs ( FS n , FP n ) can be used to fit a function, for example a straight line or a polynomial such as FS Fit ( x ) = p 0 + p 1 x + p 2 x 2< . For a particularly efficient implementation, a first lookup table LUT1 can be generated from this at any desired grain size. Alternatively, only the coefficients are used. p 0 , p 1 , p 2 stored. During measurement operation, the measured, summed first values ​​are stored. N Bins FS The first reference table LUT1 is translated into estimated ambient light levels FP. Intermediate values ​​can be rounded or interpolated.

[0040] As an alternative to a dynamic estimation of the ambient light level, a fixed configuration is conceivable, which sets a specific ambient light resistance at the factory or customer level and specifies a FP as a parameter. Furthermore, as an alternative to estimating the ambient light level via summed first values, N Bins FS The use of an additional receiving element or at least one pixel element 24 is conceivable, in which only extraneous light is measured by channel separation to the useful light.

[0041] Figure 6 Figure 1 shows an illustration of a model of the exponential curve 36 of the ambient light component in a histogram, as well as a threshold 38 above it with a safety margin. The determination of the threshold 38 is the actual goal; the estimation of the ambient light level FP is an intermediate step towards this.

[0042] The exponential curve 36 can be estimated for each ambient light level FP using a few parameters. The density function of the exponential distribution has the general form: f i = 1 λ exp − 1 λ i .

[0043] The model parameter λ The maximum likelihood method can be used to estimate the parameters for each measured ambient light histogram, taking all measured values ​​into account. This method enables robust parameter estimation by utilizing all available measurement data within a histogram.

[0044] A reliable estimate for λ The exponential distribution is calculated as the mean or center point of all measured individual light travel times of a histogram. λ = ∑ i = 1 M Histogramm i ∗ i HS . HS = ∑ i = 1 M Histogram i is the sum of the counts of all bins in the histogram.

[0045] The exponential curve 36 of the extraneous light component is the expected value E ( i) for the number of background events in each histogram bin i and is calculated to E i = HS ∗ f i = HS λ exp − 1 λ i .

[0046] This is not yet the threshold of 38, as at least some bins are also 40, 42 with random results compared to the expected value. E ( i The increased number of counts is even higher. Therefore, a safety margin should be included.

[0047] Assuming a Poisson distribution of the extraneous light events across the bins, the standard deviation can be calculated directly, since the expected value and variance are identical here: Std i = E i = HS λ exp − 1 λ i .

[0048] By scaling the standard deviation by a desired factor S for example in the area S = 2 ... 6 for the safety margin, the corresponding threshold 38 can now be determined, which is referred to as the noise limit in the following formulas. RG ( i ) of the respective i-th bin is designated: RG i = E i + S ∗ Std i .

[0049] Will a certain level of ambient light now be used? FP n the parameter λ n and HS n If determined from an extraneous light histogram as described above via maximum likelihood, then the following applies to the threshold: 38 RG n i = HS n λ n exp − 1 λ n i + preFact ∗ HS n λ n exp − 1 λ n i .

[0050] If we now define parameters ( an , bn ) as b n = 1 λ n , a n = HS n λ n , This simplifies the threshold of 38 to RG n i = a n exp − b n i + S ∗ a n exp − b n i , and a second lookup table LUT2 can be created, which corresponds to a respective ambient light level FP n the parameters ( an , bn ) assigns. During measurement operation, only the parameters ( an , bn ) with the current ambient light level FP n to find the threshold 38 using the equation above for RG n ( i ) per bin i to be able to calculate the ambient light level FP nThis, in turn, is obtained from the first reference table LUT1 as described, or measured otherwise. To reduce the effort required to calculate an exponential function and a square root, an approximation, for example in the form of a polynomial, can be used.

[0051] Figure 7 shows a representation of six exemplary thresholds 38 similar Figure 6 for different levels of ambient light FP , which are determined according to the procedure just explained.

[0052] Figure 8 shows a representation similar Figure 6 To explain the further evaluation using the appropriate threshold 38. In contrast to the histograms shown so far, a useful light peak 44 was also registered here, which is reliably separated from the background by threshold 38. For example, the counts of each bin are i with the associated threshold RG n ( iThe bins with a suprathreshold number contain information about the temporal position of the desired useful light peak 44. If there is more than one suprathreshold bin, the maximum, a center point, or a comparable measure, especially weighted by the exponential decay, can be calculated from the suprathreshold bins to determine the reception time. Alternatively, a profile of the transmitted pulse can be fitted, even in a simplified form, such as a parabola. If there are several non-contiguous suprathreshold bins, the first, most pronounced, last, or otherwise predefined cluster is selected, or multiple reception times are calculated for several clusters. Such a multi-target measurement occurs, for example, with an object behind another (semi-)transparent object.

[0053] In a particularly preferred embodiment, the two lookup tables LUT1 and LUT2 are determined during the development phase as described, sometimes requiring considerable time and measurement effort, and stored in the respective sensor 10. During operation, it is then sufficient to sum the FS the first N to calculate the bins of a histogram and to determine an ambient light level from the first lookup table LUT1. FP n to read out. For this, the corresponding noise threshold is used. RG n read from the second lookup table LUT2, or from a few parameters of the second lookup table LUT2, such as ( an , bn ), a predefined calculation rule for threshold 38 was adapted. With this threshold 38, the useful light peak 44 is then reliably detected against the background in the current ambient light situation.

Claims

1. An optoelectronic sensor (10) for the distance measurement of an object (18) in a detection zone (16) using a time of flight method, wherein the sensor (10) has a light transmitter (12) for transmitting a light signal (14) into the detection zone (16), a light receiver (22) having a first plurality of Geiger-mode avalanche photodiodes (24) for detecting received light (20) from the detection zone (16), a second plurality of time of flight measurement units (28) for determining individual times of flight between a transmission of a light signal (14) and a triggering of a detection event in an avalanche photodiode (24), and a control and evaluation unit (32) which is configured to collect individual times of flight in a histogram, to localize a useful light signal (44) in the histogram with reference to a threshold (38), and to determine a distance value from the object (18) from the useful light signal (44), to first estimate an extraneous light level from the histogram for this purpose and then to fix the threshold (38) using the extraneous light level using the calculation rule a exp − b i + S ∗ a exp − b i such that it is above an expected exponentially decreasing number of noise and extraneous light events with a safety margin, using the parameters a = HS λ and b = 1 λ , a scaling factor S for the safety margin, HS = ∑ i = 1 M Histogram i and λ = ∑ i = 1 M Histogramm i ∗ i HS , where Histogram(i) designates the i = 1 ... M bins of the histogram.

2. A sensor (10) in accordance with claim 1, wherein the control and evaluation unit (32) is configured to estimate the extraneous light level by summing the first bins of the histogram.

3. A sensor (10) in accordance with claim 2, wherein the control and evaluation unit (32) stores a first lookup table which associates an extraneous light level with a summed number of detection events.

4. A sensor (10) in accordance with claim 3, wherein the first lookup table is taught in advance to the distance measurement in that the light receiver (22) is repeatedly exposed to a defined extraneous light level with an inactive light transmitter (12) and the respective sum of the first bins of a histogram generated in this process is determined.

5. A sensor (10) in accordance with any one of the preceding claims, wherein the control and evaluation unit (32) is configured to delay the transmission of a light signal (14) with respect to a start signal so that no useful light signal is registered in the first bins of the histogram.

6. A sensor (10) in accordance with any one of the preceding claims, wherein the control and evaluation unit (32) stores a second lookup table which associates at least one matching parameter of a calculation rule for the threshold (38) with an extraneous light level.

7. A sensor (10) in accordance with claim 6, wherein the second lookup table is taught in advance to the distance measurement in that the light receiver (22) is repeatedly exposed to a defined extraneous light level with an inactive light transmitter (12) and HS and λ of the histogram generated in this process are determined in each case.

8. A sensor (10) in accordance with any one of the preceding claims, wherein the control and evaluation unit (32) is configured to compare the number of individual times of flight per bin of the histogram with the threshold (38) and to associate bins above the threshold with the useful light signal (44).

9. A method for the distance measurement of an object (18) in a detection zone (16) using a time of flight method, wherein a light signal (14) is transmitted into the detection zone (16), a light receiver (22) having a first plurality of Geiger-mode avalanche photodiodes (24) detects received light (20) from the detection zone (16), a second plurality of time of flight measurement units (28) determines individual times of flight between a transmission of a light signal (14) and a triggering of a detection event in an avalanche photodiode (24), individual times of flight are collected in a histogram, a useful light signal (44) is localized in the histogram with reference to a threshold (38), and a distance value from the object is determined from the useful light signal (44), wherein an extraneous light level is first estimated from the histogram and then the threshold (38) is fixed using the extraneous light level using the calculation rule a exp − b i + S ∗ a exp − b i such that it is above an expected exponentially decreasing number of noise and extraneous light events with a safety margin, using the parameters a = HS λ and b = 1 λ , a scaling factor S for the safety margin, HS = ∑ i = 1 M Histogram i and λ = ∑ i = 1 M Histogramm i ∗ i HS , where Histogram(i) designates the i = 1 ... M bins of the histogram.