Method and device for taking distance images

Abstract

Method for recording distance images having several distance pixels, wherein a transmitter-receiver arrangement (11, 17) emits electromagnetic radiation in the form of transmission pulses (102) for each distance image to be recorded and detects reflected echo pulses (120), wherein pulse transit times between the transmitted pulses (102) and the reflected echo pulses (120) are measured with a time measuring device (45, 63) in order to determine a plurality of distance values, each representing a distance pixel, wherein the transmission pulses (102) for determining the distance values ​​are transmitted in a transmission pulse sequence (107) comprising a plurality of transmission pulse groups (100, 108, 110), and wherein a transmission pulse spacing (104) between successive transmission pulses (102) within at least one of the transmission pulse groups (100) is smaller than a predetermined maximum pulse transit time (106); wherein an echo pulse sequence (118) is determined on the basis of the echo pulses (120) detected for the transmission pulse sequence (107), wherein at least one of the measured pulse transit times represents an average pulse transit time for a first subset of transmission pulses (102) of the transmission pulse sequence (107) and a second subset of echo pulses (120) of the echo pulse sequence (118), and wherein at least one of the distance values ​​is determined on the basis of the average pulse transit time, wherein the first subset is formed by one of the transmission pulse groups (100); wherein echo pulse spacings (121) between echo pulses (120) of the second subset are adjusted by the respective transmission pulse spacing (104) between successive transmission pulses (102) of the first subset.

Description

[0001] The present invention relates to a method and a device for recording distance images having a plurality of distance pixels.

[0002] A common approach to distance measurement consists in measuring the time between the emission of pulsed electromagnetic radiation and the reception of the radiation reflected from objects. Due to the constant propagation speed of electromagnetic radiation, this time can be converted into distances to the objects. For example, a transmitter-receiver arrangement can emit electromagnetic radiation in the form of one or more transmitted pulses for a distance image to be recorded, and the subsequently reflected echo pulses can be detected. A time measuring device can then measure the pulse transit times between the transmitted pulses and the reflected echo pulses in order to determine several distance values, each representing a distance pixel.

[0003] There are various designs of devices that fulfill this task, which are also collectively referred to as "pulse-TOF sensors." Examples of devices of this type are often referred to as LIDAR (light detection and ranging) devices. TOF stands for "time of flight" and refers to the time it takes for a transmitted or signal pulse to travel to the target and back.

[0004] Range images include not only two-dimensional arrays of distance pixels, but also one-dimensional arrays, also called profiles, i.e., range images that each comprise a single row of adjacent distance pixels. Due to the distance and depth information, a two-dimensional array of distance pixels can also be referred to as a 3D point cloud, because each distance pixel represents spatial information in three different spatial dimensions. Accordingly, a one-dimensional array of distance pixels represents a 2D point cloud.

[0005] Currently, the acquisition of range images is subject to several limitations with regard to acquisition speed and distance measurement accuracy. The acquisition speed is limited, on the one hand, by the fact that after transmitting a transmission pulse, the maximum pulse propagation time must first be elapsed before another transmission pulse is transmitted. Otherwise, the echo pulses for a given transmitter-receiver pair can no longer be reliably assigned to the respective underlying transmission pulse. To increase the acquisition speed, it is possible to transmit transmission pulses in parallel with several transmitter-receiver pairs and to detect the corresponding echo pulses. However, this approach requires additional hardware.

[0006] The fundamental problem of limited acquisition speed increases with increasing distance values, meaning that as the range of the acquisition device increases, the acquisition speed decreases. The same applies to the achievable accuracy. Thus, increased accuracy requires either multiple measurements and / or increased transmission power. Both approaches are suboptimal for fast, efficient, and reliable distance measurement.

[0007] Against this background, the following requirements are placed on distance measurement. On the one hand, the distance pixels should be captured as quickly as possible in order to capture the distance images at a high scan or frame rate. On the other hand, the distance values ​​of the individual distance pixels should be as accurate as possible, i.e., in particular, they should exhibit the smallest possible deviation from the actual distance values. Furthermore, the distance pixels should be valid even at high distance values, i.e., the distance measurement should be able to be performed reliably over the greatest possible range.

[0008] Furthermore, there is a desire for the highest possible resolution of the range pixels, meaning that the target area should be scanned with the highest possible density of range pixels. Another requirement lies in so-called multiple target analysis, meaning that target objects located one behind the other at different distances should be able to be detected or resolved separately using the range images. For example, atmospheric disturbances such as rain or snow should be distinguishable from a fixed background.

[0009] Document EP 2 626 722 A1 discloses an optoelectronic sensor with a correlation unit for generating a correlation signal by correlating the sampled received signal with a transmitted pseudorandom code sequence, and with an evaluation unit configured to determine a reception time from a correlation maximum in the correlation signal and, based on the transmission time, to determine a measured value for the object distance via the light propagation time from the sensor to an object. Further and related prior art is described in documents DE 10 2018 203 584 A1, US 2018 / 0 259 645 A1, US 10 527 727 B2, and US 7 791 713 B2.

[0010] It is an object of the invention to provide a method for taking distance images which enables fast, reliable and efficient distance measurement for different ranges and accuracy requirements.

[0011] The problem is solved by a method according to claim 1.

[0012] The transmission pulses for determining the distance values ​​are transmitted in a transmission pulse sequence comprising several transmission pulse groups, wherein a transmission pulse spacing between successive transmission pulses within at least one of the transmission pulse groups is smaller than a predetermined maximum pulse transit time.

[0013] Multiple transmission of transmitted pulses, especially in transmitted pulse groups, is useful for distance measurement because the echo pulses received for each transmitted pulse group can be evaluated using statistical methods to obtain a more accurate measurement result than a single measurement with just one transmitted pulse. The accuracy and / or range of the distance measurement can be increased in this way. For example, the echo pulses received for a transmitted pulse group can be evaluated together using an averaging method to determine a particularly precise distance value for a wide distance range, taking the respective transmitted pulse group into account.

[0014] In addition, the distance values ​​can also be determined at high speed by selecting the transmission pulse spacing, i.e. the time duration between two consecutive transmission pulses, to be shorter than the predetermined maximum pulse transit time. After transmitting a transmission pulse, the system does not first wait for all possible corresponding echo pulses to be received. Instead, after transmitting a first transmission pulse, a second transmission pulse is transmitted, even though any echo pulses attributable to the first transmission pulse have not yet been received. The transmission pulse frequency is thus increased so that a large number of echo pulses can be detected within a very short time and used for reliably accurate and rapid determination of a distance value.

[0015] The maximum pulse transit time can be determined, in particular, depending on the maximum range of the transmitter-receiver arrangement and / or the maximum distances to objects within the transmission range. For example, the maximum pulse transit time can be the maximum time elapsed between the transmission of a transmitted pulse and the reception of the last corresponding reflected echo pulse. The "last" echo pulse represents the maximum expected pulse transit time, or TOF, that should be used to determine a distance value.

[0016] A respective transmission pulse group can be transmitted, in particular, by a single transmitter of the transmitter-receiver arrangement, i.e., the speed increase achievable by the short transmission pulse spacing can relate to a transmitter-receiver pair. A further increase in speed and / or accuracy can be achieved by parallel operation of several transmitter-receiver pairs. In this case, at least some or even all of the transmitter-receiver pairs can be operated according to the method according to the invention.

[0017] The method can be advantageously used to capture and process a large number of distance pixels in parallel. For this purpose, several transmitters and receivers arranged in arrays can be operated simultaneously. This allows for high-speed distance measurement and enables multiple target evaluation.

[0018] According to the invention, high accuracy of the distance values ​​can be achieved, on the one hand, by jointly evaluating the echo pulses per transmitted pulse group. On the other hand, the method can be implemented with a highly accurate pulse transit time measurement, in which the pulse transit time is determined as the sum of an integer multiple of a predetermined period length of a clock generator plus a fraction of the period length. This pulse transit time measurement can be performed both analogically and digitally.

[0019] As explained, the method according to the invention can be used to increase the range. The range can also be increased by optimizing the optical and optronic means of the transmitter-receiver arrangement. For example, powerful receiver diodes (e.g., so-called SPADs, i.e., single-photon avalanche diodes) can be used. Using such diodes, a sufficient range can be ensured even when the target object has comparatively unfavorable remission properties, such as coal.

[0020] The method according to the invention is also suitable for achieving high resolution. For example, for autonomous driving applications, a spatial resolution of 2.5 to 5 cm and an angular resolution of 3 to 6 mrad can be achieved. However, even higher resolutions can also be realized, e.g., a spatial resolution of 5 to 15 mm and an angular resolution of between 0.5 and 1.5 mrad.

[0021] The advantages of the invention also benefit the multiple target evaluation described above. For example, several targets, e.g. five targets staggered in the direction of range, can be detected one after the other using high-frequency range images in order to reliably, quickly and accurately detect the last target, which is usually the target of interest. In a current application, the method can be implemented in an aerial drone to determine ground profiles. This allows measurements to be taken through the vegetation above the ground (e.g. trees). A similar application is the recording of ground profiles underwater, where the echoes from the water surface and other disturbances and from the waterbed can be differentiated.

[0022] Further embodiments of the invention are disclosed in the description, the figures and the claims.

[0023] According to one embodiment, the predetermined maximum pulse transit time is greater than a multiple of the transmission pulse spacing. The advantages achievable with the invention can be increased accordingly in this way. For example, the multiple can be in a range between 2 and 20, preferably between 8 and 12.

[0024] Preferably, the plurality of transmission pulse groups comprise a first transmission pulse group and at least one second transmission pulse group, wherein the transmission pulse spacing between successive transmission pulses within the first transmission pulse group and the second transmission pulse group is in each case smaller than the predetermined maximum pulse transit time. The advantages of the invention can thus be achieved continuously with a plurality of successive transmission pulse groups. In order to limit the processing effort for the reflected echo pulses, it is also advantageous if the group spacing between the first transmission pulse group and the second transmission pulse group is adapted to the predetermined maximum pulse transit time. In particular, the group spacing, ie the time interval between two successive groups, can be greater than the predetermined maximum pulse transit time or substantially equal to the maximum pulse transit time.The echo pulses generated by a particular transmit pulse group can thus be completely received before a subsequent transmit pulse group is transmitted. This simplifies the evaluation of the echo pulses per transmit pulse group.

[0025] According to a further embodiment, at least one of the distance pixels is determined based on the transmission pulses from only one of the multiple transmission pulse groups. For example, the transmission pulses of a respective transmission pulse group and the echo pulses detected for this transmission pulse group can be processed together to determine precisely one distance value for a respective distance pixel. In other words, the transmission pulse groups and the associated echo pulse groups can each be processed group by group. In this case, a respective distance value can be determined for each group per distance pixel. Alternatively, the determination of a distance value can also be based on the transmission pulses and echo pulses from multiple groups, for example to maximize measurement accuracy.

[0026] It has been shown that the number of transmitted pulses per transmitted pulse group can be advantageously selected from a range between 5 and 25. The required processing resources in the form of hardware and the complexity of the evaluation are reasonable at this group size. At the same time, however, the performance of the distance measurement can be significantly increased. For many applications, it has proven effective to provide at least 10 transmitted pulses per transmitted pulse group.

[0027] According to a further embodiment, the transmission pulse spacing between a plurality of consecutive transmission pulses within the respective transmission pulse groups is smaller than the predetermined maximum pulse transit time. In particular, the transmission pulse spacing between all consecutive transmission pulses within the respective transmission pulse groups can be smaller than the predetermined maximum pulse transit time. The speed of the distance measurement can be increased even further in this way.

[0028] It is conceivable that the transmission pulse spacing varies within a transmission pulse group. However, it is preferred that the transmission pulse spacing between the transmission pulses within a respective transmission pulse group be constant. This allows the transmission pulse spacing between immediately consecutive transmission pulses to be the same. The associated echo pulses thus exhibit a repeating pattern with a constant time offset that corresponds to the constant transmission pulse spacing. This greatly simplifies the evaluation of the echo pulses with reference to the underlying transmission pulses. Furthermore, the distance can be determined with even greater accuracy.

[0029] According to a further embodiment, at least a part of the transmitter-receiver arrangement and / or a deflection device associated with the transmitter-receiver arrangement is moved between the transmission of the transmission pulses of at least two successive transmission pulse groups. This allows the distance values ​​for different distance pixels to be determined, i.e., a scan of the target area can be generated by, for example, successively moving the transmitters of the transmitter-receiver arrangement and / or a rotatable mirror (e.g., a polygon mirror) between the successive transmission pulse groups in order to direct the transmission pulses to a different point in the target area and to determine an associated distance pixel. Preferably, the transmitters or the rotatable mirror are at least substantially stationary during the transmission of a respective transmission pulse group, i.e.,The transmitters or the deflection device are not moved, or at most only moved very slightly, during the transmission of the transmission pulses. The transmission pulses of a respective transmission pulse group are thus directed to the same point in the target area, so that the reflected echo pulses are essentially identical, except for the time offset between the echo pulses and any noise deviations. The distance values ​​can thus be determined with high speed and accuracy even for a large number of different distance pixels. The transmission pulse spacing is preferably selected as a function of the movement speed of the deflection device.

[0030] Due to the short transmission pulse spacing, the transmitters or deflection device can be moved at a constant speed, provided the speed is sufficiently low to ensure that the movement path during transmission of the transmission pulses is negligibly small. The transmitter or deflection device can thus also be moved during the transmission of the transmission pulse group.

[0031] With regard to the receiver side, at least the following step is preferably also carried out within the framework of the method described here: An echo pulse sequence is determined on the basis of the echo pulses detected for the transmitted pulse sequence, such that at least one of the measured pulse transit times represents an average pulse transit time for a first subset of transmitted pulses in the transmitted pulse sequence and a second subset of echo pulses in the echo pulse sequence. Here, echo pulses are assigned to those transmitted pulses that caused these echo pulses. The assigned pulses are then evaluated together to determine the average pulse transit time. The average pulse transit time corresponds to an average of the individual pulse transit times resulting from the individual assignments between transmitted and echo pulses. Subsequently, at least one of the distance values ​​is determined on the basis of the average pulse transit time.The distance value determined in this way is based on several transmitted and received pulses, so that the distance value can be given greater accuracy.

[0032] The first subset of transmitted pulses can, in particular, be formed by one of the transmitted pulse groups. Accordingly, the second subset is preferably formed by the echo pulses belonging to the first subset. To expand the calculation basis, however, it is also possible for transmitted pulses from different transmitted pulse groups to belong to the first subset, and accordingly, for the second set to be formed by associated echo pulse groups. The accuracy of the distance measurement can be increased in this way, whereby only an insignificantly longer time period is required due to the short transmitted pulse spacing.

[0033] Preferably, the echo pulse spacings between echo pulses of the second subset are adjusted by the respective transmission pulse spacing between successive transmission pulses of the first subset. In this way, a time-correct echo pulse sequence can be formed in which the underlying transmission pulse spacings are compensated. The echo pulses can, for example, be combined in time as if they had been generated by just one transmission pulse. For this purpose, the echo pulses can be shifted and / or superimposed, with preferably those echo pulses that are reflected at the same point, i.e. that represent a common distance pixel, being fused to form one echo pulse, in particular by forming an average value of the relevant echo pulses. The calculation of the pulse transit times is considerably simplified in this way because only one pulse transit time needs to be calculated for the echo pulses reflected at one point.This pulse transit time then represents an average transit time for the corresponding transmission pulses.

[0034] The adjustment of the echo pulse spacing by the transmission pulse spacing can be carried out in particular on the basis of an autocorrelation analysis of the echo pulses. Alternatively or additionally, the echo pulse spacing can be adjusted by the transmission pulse spacing by first iteratively assigning the echo pulses of the second subset to the underlying transmission pulses of the first subset and then shifting the echo pulses belonging to a respective transmission pulse by the number of transmission pulse spacings that form the total distance of the respective transmission pulse to a reference, e.g. the first transmission pulse of the first subset. For example, after the first transmission pulse has been transmitted, the first received echo pulse can be reliably assigned to the first transmission pulse. In other words, the first echo pulse of the second subset is assigned to the first transmission pulse of the first subset. Then, the echo pulse of the second subset which is approximatelyone transmit pulse interval after the first echo pulse is assigned to the second transmit pulse of the first subset. In this way, the corresponding "first" echo pulses can first be identified for each transmit pulse. Then, of any remaining echo pulses, the first echo pulse in time can be assigned to the first transmit pulse of the first subset. The echo pulse received one transmit pulse interval after the last assigned echo pulse is in turn assigned to the second transmit pulse of the first subset. This assignment process is continued for all "second" echo pulses of the second subset. Any further echo pulses are assigned to the corresponding transmit pulses accordingly until all echo pulses have been assigned. In general, the predetermined transmit pulse intervals are used to assign the echo pulses to the underlying transmit pulses.

[0035] After all echo pulses have been assigned to the corresponding transmit pulses, the echo pulses assigned to a respective transmit pulse are each shifted by the number of transmit pulse intervals that the respective transmit pulse is away from a reference, in particular the first transmit pulse of the first subset. The echo pulses of the second subset shifted in this way can then be combined into an echo pulse sequence, which is used to determine the pulse transit times. Preferably, the shifted echo pulses are added and weighted by the inverse number of transmit pulses of the first subset to determine the average pulse transit time.

[0036] The echo pulses can generally be contained in a received signal of the transmitter-receiver arrangement, so that the received signal can generally also be corrected for the transmitted pulse spacing. The offset between recurring patterns in the echo pulse sequence, which is due to the transmitted pulse spacing, can also be efficiently determined and compensated in this case. Compensation can be carried out particularly easily if the transmitted pulse spacing between the transmitted pulses of a transmitted pulse group is always the same, i.e. constant. The transmitted pulse spacing is thus reflected many times over in the echo pulse sequence, so that the time-correct shifting of the echo pulses to correct the transmitted pulse spacing is simplified. Variable transmitted pulse spacings are also conceivable, which can be used as prior knowledge to correct the respective transmitted pulse spacing.

[0037] The determination of the distance values ​​is preferably based on at least one pulse pair in each case, whereby a detected echo pulse is assigned to a transmission pulse of the transmission pulse sequence and the pulse transit time is measured based on the pulse pair. The pulse transit time can be determined as the difference between the reception time of the echo pulse and the transmission time of the corresponding transmission pulse. The pulse transit time can then be converted into a distance value depending on the speed of light.

[0038] To further increase the accuracy of distance measurement, it is possible to determine a pulse width for each detected echo pulse and to measure the pulse transit times, at least partially depending on the pulse width. For example, the actual reception time of a wide echo pulse can be determined with greater accuracy by taking the pulse shape into account. The actual reception time can deviate significantly from the detection time for relatively large pulse widths, particularly when evaluating the received signal using a threshold comparison, as explained in more detail below.

[0039] The pulse shape can be determined by varying the threshold value for detecting the echo pulses. For example, the echo pulses for each transmit pulse in a transmit pulse group can be detected by applying a different threshold value. The essentially identical echo pulses are thus detected at different times in time, depending on their amplitude. Based on the different times, the pulse shape can then be reconstructed, taking the threshold values ​​into account.

[0040] A respective threshold value can, in particular, be formed by a reference of the transmitter-receiver arrangement. For example, eight different threshold values ​​or receiver references can be used for a pulse shape analysis.

[0041] Taking the pulse shape into account also allows for the detection and separation of double pulses. Double pulses are formed by two echo pulses received in close succession. These pulses typically overlap significantly and cannot be reliably detected with simple signal detection using only a single threshold. However, pulse shape analysis can also reliably resolve double pulses, allowing two detection times to be determined and evaluated for precise distance measurement.

[0042] According to a further embodiment, the electromagnetic radiation of the transmitted pulses each has a wavelength that is varied within the transmitted pulse sequence. For example, individual transmitted pulses can each be emitted at different wavelengths to ensure that at least some reflected echo pulses have the highest possible intensity. In this way, the pulse transit times can be measured with greater accuracy and reliability.

[0043] Varying the wavelength is based on the realization that a transmission pulse with a fixed wavelength is reflected with varying degrees of efficiency by different surfaces. In other words, variable remission properties in the target area can result in only weak or even no measurable echo pulses being received for certain wavelengths. By varying the wavelength, e.g., using a configurable NIR (near-infrared) laser, this effect can be reduced, meaning that sufficiently strong echo pulses can always be received to ensure reliable distance measurement, even with varying remission properties.

[0044] Preferably, the transmitter-receiver arrangement comprises a plurality of transmitters, each configured to emit pulses in the form of NIR laser beams. In a special design, the laser beams can be formed by high-power fiber laser beams. This allows even long distances to be reliably measured.

[0045] With regard to determining the distance values, according to a further embodiment, it is preferred that the transceiver arrangement comprises a plurality of transmitters arranged in a transmitter array and a plurality of receivers arranged in a receiver array. Compared to just one transmitter-receiver pair, the distance values ​​for a plurality of distance pixels can be determined significantly faster, in particular by parallel transmission of transmission pulses and reception of the associated echo pulses.

[0046] According to a further embodiment, a group measurement is performed for at least one of the transmission pulse groups, in which at least one logical start pulse derived from the transmission pulse group and several logical receiver pulses are generated. The receiver pulses are each generated using a reference of the transmitter-receiver arrangement, which is interrupted by a received signal from the transmitter-receiver arrangement. Exceeding the reference forms a positive edge of the respective receiver pulse, defining an up event. Conversely, falling below the reference forms a negative edge of the respective receiver pulse, defining a down event. The start pulse and the associated receiver pulses are then combined at the correct time to determine at least one of the distance values.

[0047] On the basis of the generated start and receiver pulses, preferably also a plurality of time periods are determined, each elapsed from a time before the start pulse to the respective receiver pulses, by counting at least the clock pulses provided by a central clock generator with a known frequency for each up event and / or each down event, wherein the distance value is determined on the basis of the counted clock pulses.

[0048] To further accelerate the distance value determination, the start and receiver pulses generated for different transmitter-receiver pairs can be distributed according to a predeterminable measuring sequence to an array of timing channels formed by the timing device, whereby the time periods between the start and receiver pulses are determined for each timing channel.

[0049] According to a further embodiment, when determining the time spans, both the elapsed entire periods of the clock pulses are counted, as well as the partial period that was overcounted at the time of the up event and / or overcounted at the time of the down event, so that two partial results are obtained for each up event and / or each down event. This enables a particularly precise determination of the time spans and, accordingly, a particularly precise determination of the distance values.

[0050] To obtain the first partial result, the number of positive or negative edges of the measurement clock must be counted from the start time set shortly before the start pulse until the first positive edge of the measurement clock after all events of all pulses in the pulse chain. The width of the counters for the measurement clock edges is selected so that the pulse transit time (TOF) can be clearly counted.

[0051] To obtain the second partial result, the pulse width of a partial period pulse that was counted too much at the beginning of each pulse and the pulse width that was counted too much at the end are measured. The pulse width of the partial period of the up event as well as the pulse width of the partial period of the down event must therefore always be subtracted. Because the pulses are asynchronous to the measurement clock signal, the partial periods have a random pulse width, with the period of the measurement clock being the upper limit. The partial periods are converted, for example, using TDCs (time-to-digital converters) into digital values ​​as multiples of delay times of delay elements of the TDC. In this way, the resolution of the time measurement for the first partial result is increased by adding the second partial result to the delay time, e.g., 10 ps, ​​corresponding to a fictitious measurement clock of 100 GHz.The period of the measurement clock divided by the number of delay times results in the delay time, which is preferably continuously recalibrated in the processor system between measurements due to slight temperature dependence.

[0052] According to a further embodiment, the reference of the transmitter-receiver arrangement is smaller than a noise threshold, so that the logical receiver pulse is due to an echo pulse or a noise pulse. The noise threshold is preferably set to 4.5 NEP (noise equivalent power). The further the reference lies below this threshold, the higher the possible range of the distance measurement. This is because with increasing range, the relative intensity of the echo pulses decreases, so that the echo pulses are increasingly masked by noise and can therefore only be detected with a correspondingly lower reference. Negative impairments of the distance measurement due to the higher noise component can, however, be avoided by basing the distance measurement on several individual measurements, with a transmission pulse being emitted for each individual measurement.

[0053] For example, with a transmission pulse group comprising n transmission pulses, the associated echo pulses can be evaluated together to increase the range by a factor that can be specified as the inverse of the square root of n. In the case of n=10 transmission pulses, the factor is approximately 1.7. If the maximum range of the distance measurement based on just one transmission pulse is, for example, 1500 m, this range can be increased by a factor of 1.7 with a joint evaluation of the echo pulses generated by 10 transmission pulses, which corresponds to a range of 2500 m. The joint evaluation can, in particular, include averaging in order to average out the noise components.

[0054] According to an alternative embodiment, the reference of the transmitter-receiver arrangement is greater than or equal to the noise threshold, so that the logical receiver pulses are each traced back to an echo pulse. The receiver pulses are thus not subject to noise-related uncertainties and are therefore, in principle, particularly well suited for precise distance measurement. For example, the accuracy of the measurement can be maximized by jointly evaluating the noise-free receiver pulses generated by a transmission pulse group. If the transmission pulse group comprises n transmission pulses, the statistical error of the distance measurement can be approximately reduced by a factor that can be approximately specified as the square root of 1 / n. Due to the short transmission pulse spacing between the transmission pulses, this improvement requires no or only an insignificantly longer measurement time compared to the case with only one transmission pulse.

[0055] According to a further aspect, the invention relates to a device for recording distance images having a plurality of distance pixels, wherein the device has a transmitter-receiver arrangement for emitting electromagnetic radiation in the form of transmission pulses and for detecting reflected echo pulses, and wherein the device has a time measuring device which is configured to carry out the method according to one of the described embodiments.

[0056] The invention is explained below merely by way of example with reference to the drawings, in which the drawings show in detail the following: Fig. 1 schematically shows the structure of a distance measuring device according to an embodiment of the invention; Fig. 2a-2c using the example of a measurement without a signal the analog measurement itself ( Fig. 2a), the logical measurement derived from it ( Fig. 2b) and the individual measurement derived from it ( Fig. 2c); Fig. 3a-3c using the example of ten individual measurements without signal, the ten individual measurements themselves ( Fig. 3a), the averaging of these ten individual measurements ( Fig. 3b) and the software amplitude derived from the averaging by integration ( Fig. 3c); Fig. 4a-4b show, for example, the integration of an average of several individual measurements ( Fig. 4a) to the software amplitude ( Fig. 4b) in a opposite Fig. 3 enlarged scale; Fig. 5a-5f using the example of a measurement with signal the signal itself without noise ( Fig. 5a), an analog measurement that includes noise and the signal ( Fig. 5b), the software amplitude for a single measurement ( Fig. 5c), the software amplitude determined by averaging and integrating 10 individual measurements ( Fig. 5d), the software amplitude determined by averaging and integrating 100 individual measurements ( Fig. 5e) and the software amplitude determined by averaging and integrating 1000 individual measurements ( Fig. 5f); Fig. 6 shows a schematically illustrated transmission pulse group according to an embodiment of the invention; Fig. 7 shows a plurality of schematically illustrated transmission pulse groups according to a further embodiment of the invention; Fig. 8a-8b a received signal without noise ( Fig. 8a) and an echo pulse sequence; and Fig. 9a-9b the echo pulse sequence of Fig. 8b in a time-correct overlay without noise ( Fig. 9a) and in an averaged superposition with the received signal of Fig. 8a.

[0057] Functionally identical parts are provided with the same reference symbols.

[0058] Based on the Fig. 1-5, a method for measuring distance according to an embodiment is first explained. The representations of the Fig. 2-5 were obtained by simulation calculations.

[0059] Fig. 1 schematically shows a device for distance measuring, ie a sensor which is designed to carry out the distance measuring method according to the invention.

[0060] The sensor comprises a transmitter 11 with a laser diode (not shown) for emitting pulsed electromagnetic radiation 13.

[0061] Signal pulses 15 reflected from objects 19 located within the range of the sensor reach a receiving diode 61 of a receiver 17 of the sensor, ie the transmitted beam covers the field of view determined by the receiving diode 61.

[0062] A transmitting optics 53 assigned to the transmitter 11 and a receiving optics 55 assigned to the receiver 17 are arranged behind a sensor cover 51 that is permeable to the radiation used.

[0063] An amplifier 35 and a comparator 39 are arranged downstream of the receiving diode 61. The amplifier 35 provides an analog received signal, also referred to below as an analog measurement, which contains noise in addition to the received signals or signal pulses, which will be discussed in more detail below. The signal pulses can, in particular, be formed by echo pulses.

[0064] The reference of the comparator 39, also referred to below as the hardware or HW threshold, which forms a reference of the receiver, is set depending on the respective application, in particular according to the desired sensitivity.

[0065] Using the hardware threshold, a sequence of logic pulses is generated from the analog reception signal of amplifier 35. These pulses are fed to a downstream IC component 45 of the sensor, which is part of a control and evaluation device 41 of the sensor. The IC component 45 includes a memory 25, which will be discussed in more detail below. The memory 25 serves to store so-called individual measurements, which consist of characteristic times of the analog measurement, which are derived from the analog measurement via the logic pulses by means of the comparator 39.

[0066] The object distance to be determined by the sensor is obtained by averaging a plurality of such individual measurements. For this purpose, a processor system 63 is connected downstream of the IC module 45 via a parallel interface 49, to which the individual measurements contained in the memory 25 of the IC module 45 are transferred. A shift register multiplexer is also provided between the IC module 45 and the processor system 63.

[0067] The processor system 63 comprises a microprocessor 47 for controlling all relevant operations of the sensor, a central measuring clock 43 serving as a time clock generator for establishing a time base for the runtime measurement, a counter 67 for counting the time cycles of the measuring clock 43 and a memory system 27 comprising several time grid memories, which will also be discussed in more detail below, in which the averaging of the individual measurements transmitted by the IC module 45 takes place.

[0068] The object distances determined by the processor system 63 can be output via an interface 59.

[0069] The sensor can be designed as a multi-channel variant, allowing distance measurement simultaneously in several parallel measuring channels. In this case, the receiver 17 comprises an array of receiving diodes 61 and amplifiers 35 assigned to them, as well as comparators 39, while the IC module 45 contains a memory 25 for each measuring channel, and the memory system 27 of the processor system 63 also has a number of identical, parallel-operating memory arrays corresponding to the number of measuring channels. In the case of such a multi-channel sensor, the transmission beam 13 emitted by the transmitter 11 is preferably shaped such that it covers the fields of view of all receiving channels, i.e., all receiving diodes 61. Alternatively, the transmitter 11 can be designed as a laser array with several laser transmitters, so that each laser transmitter is assigned a receiving diode.

[0070] Since distance measurement is preferably based on an average of individual measurements, constant conditions should prevail during the measurement, i.e., the object should neither change nor move relative to the sensor during the measurement, at least within a period sufficient to emit as many radiation pulses toward the object as are required for the averaging depth, which depends on the respective conditions. However, after determining a distance value based on several individual measurements, the sensor or an associated deflection device can be moved to determine additional distance values ​​for additional distance pixels, as explained in more detail below.

[0071] The following explanation of the measurement method that can be performed using the sensor according to the invention described above refers to processes in a single measurement channel. In a multi-channel sensor, these processes occur simultaneously in the parallel measurement channels.

[0072] Fig. Figure 2a shows the temporal behavior of the output signal 37 of an amplifier connected to a receiving diode when the receiving diode does not receive a signal, ie is not exposed to electromagnetic radiation of a signal of interest. Fig. Figure 2a thus shows the ever-present noise, which includes statistically distributed noise pulses located above and below the zero line. The zero line corresponds to 0 NEP.

[0073] The output signal 37 of the amplifier is fed to a comparator, whose reference 21, hereinafter referred to as hardware threshold or HW threshold, is set to a value which lies within the noise, as Fig. 2a shows. "Within noise" specifically means that the HW threshold 21 lies between -4.5 NEP and +4.5 NEP. The HW threshold 21 can, for example, be 0 NEP. With this setting, the detection sensitivity is comparatively high, allowing even relatively large distance values ​​to be measured.

[0074] Setting the HW threshold 21 to a value below 0 NEP is also conceivable. Furthermore, it is possible to set the HW threshold 21 to a value above +4.5 NEP, i.e., well above the noise level. This allows a particularly accurate measurement result to be achieved with a comparatively small number of individual measurements.

[0075] That the HW threshold 21 according to Fig. 2a is in the noise, means that the HW threshold 21 is repeatedly breached by the noise pulses. Due to the finite slope of the edges of the noise pulses and the different amplitudes of the noise pulses, the times 33 ( Fig. 2c), at which the HW threshold 21 is breached, depends on the position of the HW threshold 21, more precisely on the relative position between the HW threshold 21 and the analog measurement 37.

[0076] From the analog measurement 37 according to Fig. 2a, a logical measurement is generated using the HW threshold 21 of the comparator, which consists of a sequence of logical pulses 23 of the same height, as shown in Fig. 2b. The edges of the logic pulses 23 are at the times 33 ( Fig. 2c), at which the edges of the noise pulses of the analog measurement 37 have passed through the HW threshold 21, whereby a rising edge of a noise pulse corresponds to a rising edge of a logical pulse 23 and a falling edge of a noise pulse corresponds to a falling edge of a logical pulse 23.

[0077] The fineness or temporal resolution of the method is thus so high that not only is the exceeding of the hardware threshold 21 itself detected and fed into further analysis by generating a logical pulse 23, but much more information contained in the analog measurement 37 is utilized by distinguishing between exceeding and falling below the hardware threshold 21 by the analog measurement 37, or by detecting when the hardware threshold 21 is exceeded and then again fallen below it, or vice versa. Consequently, logical pulses 23 are obtained that have a different width, such as Fig. 2b shows. The width of the logical pulses 23 or the "pauses" between the logical pulses 23 indicate how long the analog received signal 37 is above or below the hardware threshold 21.

[0078] Then the logic pulses 23 of the logic measurement are calculated according to Fig. 2b, a so-called single measurement is generated by digitizing the times 33 of the rising and falling edges of the logical pulses 23 and storing them in a so-called event list. This event list represents the information contained in a single measurement. In the following, the rising or positive edges are also referred to as up events, and the falling or negative edges are also referred to as down events.

[0079] Fig. Figure 2c is a graphical representation of the stored event list forming the individual measurement, where the upward-pointing lines, hereinafter referred to as “needles” 33 for illustrative purposes only, represent the up-events and the downward-pointing needles 33 represent the down-events. In this respect, the individual measurement can be Fig. 2c can be considered as the time derivative of the logical measurement formed by the logical pulses 23, ie as obtained by differentiation of the logical measurement.

[0080] The generation of the individual measurement, i.e. the event list, according to Fig. 2c is carried out in an IC module, which has a comparatively small memory for storing the event list, hereinafter also referred to simply as IC memory, whose number of memory locations is only in the order of magnitude of the noise pulses expected per individual measurement, i.e., in terms of magnitude, corresponds to the expected sum of up-events and down-events. For each event, a memory location is assigned a number that corresponds to the counter reading of the clock pulses of the central measurement clock ( Fig. 1) counter. Thus, the numbers stored in the IC module's memory contain time information, namely information about the time of occurrence of the respective up or down event with respect to the emission of the radiation pulse that starts the individual measurement. The entire event list according to Fig. 2c is therefore stored in the IC's memory as a set of counter values. To distinguish between up events and down events, the memory is divided into two memory areas.

[0081] The content of the IC module's memory representing the individual measurement therefore only contains the information about when and in which direction the HW threshold 21, which is located in the noise, was penetrated by the noise pulses of the analog measurement 37. The averaging of individual measurements and a subsequent integration of the result of the averaging to a software amplitude are Fig. 3 illustrates.

[0082] The object distances are determined by averaging a plurality of consecutive individual measurements, with each individual measurement representing characteristic times 33 of what was received by the receiving diode after the transmission of one or more transmission pulses. It is preferred, but not mandatory, that the individual measurements to be averaged be generated from the analog measurements of immediately consecutive transmitted radiation pulses. The individual measurements can also be performed in a time-compressed manner by evaluating a received signal after the transmission of a transmission pulse group, as explained in more detail below.

[0083] The averaging of the individual measurements is no longer carried out in the IC component, but in a processor system connected to the IC component via a parallel interface, to which a time-grid memory is assigned.

[0084] An event list generated in the IC component is then immediately transferred via the parallel interface into a time-frame memory of the processor system, so that the IC component is ready to generate the next event list, in particular the one belonging to the radiation pulse emitted immediately afterwards.

[0085] In the processor system's time-grid memories provided for averaging the individual measurements, each memory cell corresponds to a time window of finite length. The measurement time during which the analog measurement 37 is performed is thus divided into a plurality of successive time windows, i.e., according to the invention, a time grid is used whose raster corresponds to the division of the time-grid memories. The number of memory cells in the time-grid memory is significantly larger than the number of memory locations in the IC memory, since the dimensioning of the IC memory is determined solely by the relatively small number of expected events, whereas the comparatively large number of time windows determines the dimensioning of the time-grid memory.

[0086] The time-frame memories can also be referred to as delay lines. The memory cells of the time-frame memory can accordingly be referred to as delay elements.

[0087] Fig. Figure 3a shows ten consecutively generated individual measurements, each individual measurement being generated via a sequence of logical pulses corresponding to Fig. 2b from an analog measurement 37 according to Fig. 2a, which in this example contains only noise, ie no signal. Due to the statistical distribution of the noise pulses, the values ​​derived from the noise pulses, Fig. 3a again shows the up-events and down-events of the individual measurements, represented by the needles pointing upwards and downwards, randomly distributed in time.

[0088] The probability that an event occurs in a time window of the measurement time beginning with the emission of a radiation pulse is therefore the same for all time windows, i.e., across all individual measurements, each time window contains a certain number of events that is proportional to the width of the time window.

[0089] The example of Fig. 3 has only an averaging depth of ten, so that the needle density is not exactly constant over time, as Fig. 3b shows.

[0090] Depending on the fineness of the time grid, ie depending on the size of the time windows, it is more or less likely that more than one event falls into a time window in the case of a finite averaging depth. Where this is the case in the example of the Fig. 3 is the case, this was done in Fig. 3b illustrated by needles of twice the normal length.

[0091] Technically, the averaging of the individual measurements is achieved by sequentially entering the individual measurements into the same time-frame memory of the processor system. For each event falling within the i-th time window, the i-th memory cell of the time-frame memory is modified by increasing the value by 1 in the case of an up event (i.e., a rising edge) and decreasing it by 1 in the case of a down event (i.e., a falling edge).

[0092] For each time window represented by a memory cell of the time-grid memory, the number of times an event occurs within this time window is counted during averaging, with up events being counted positively and down events being counted negatively. If only noise is measured, i.e., if no signal is present, then in the case of an infinite averaging depth, the occurrence of up events and down events is equally likely for each time window, so that for this theoretical limiting case, the initial value of each memory cell of the time-grid memory would again be present at the end of averaging.

[0093] Since each individual measurement is Fig. 2c the time derivative of the underlying logical measurement according to Fig. 2b and only individual measurements are averaged, the averaging explained above takes place in a sense in the “differentiated world”.

[0094] The return to the “real world” takes place after the averaging by integrating the averaged individual measurements ( Fig. 3b) to a so-called amplitude function 29, which in Fig. 3c and is also referred to as software amplitude or SW amplitude.

[0095] During integration, for each time window, i.e. for each memory cell of the time grid memory, the sum of the contents of all memory cells corresponding to the previous time windows and the contents of the respective memory cell itself is calculated, i.e. the sum of memory cells 1 to i is written into the i-th memory cell. Each summand is only different from zero if, within the respective time window, the number of up events that occurred is different from the number of down events that occurred, and the value of a summand that is not zero corresponds to the average number of excess up events or down events in the respective time window, namely the average number because not every individual measurement is integrated, but rather the averaged individual measurements, i.e. the integration takes place after averaging.

[0096] A reversal of this order, ie an averaging of previously integrated individual measurements, would lead to the same result, since averaging and integration are linear arithmetic operations, but would involve a considerably greater computational effort.

[0097] The SW amplitude 29 according to Fig. 3c represents the noise of the analog measurement 37 according to Fig. 2a, where, however, the noise was reduced by a factor dependent on the averaging depth, i.e., the number of individual measurements used for averaging, due to the averaging performed on the individual measurements. With an infinitely large averaging depth, the SW amplitude 29 would be Fig. 3c a smooth horizontal line. The noise in the SW amplitude 29 can therefore be referred to as SW noise.

[0098] For this reduction of the noise, only a comparatively small amount of computation is required, since only the edges of the logical pulses 23 ( Fig. 2b) corresponding characteristic times 33, which form the individual measurements, are used as the basis for the averaging, wherein, moreover, the averaging only requires operations in the time grid memory comprising adding and subtracting the value 1.

[0099] The Fig. The result of the averaging of the plurality of individual measurements graphically represented in Figure 3b can be clearly described as a packet average if the majority of individual measurements are considered as packets of individual measurements, ie the inventive averaging of the individual measurements is thus carried out packet by packet. The transition to the amplitude function or SW amplitude 29 according to Fig. 3c is then performed by integrating the averaged packet of individual measurements, or—in other words—by integrating the packet mean. Overall, this reduces the clock rate for further processing of the time-frame memory by the averaging depth.

[0100] From the example of Fig. 4 the principle of integration of averaged individual measurements ( Fig. 4a) to a SW amplitude of 29 ( Fig. 4b) becomes clearer.

[0101] With the exception of a single time window, each memory cell of the has the averaging according to Fig. 4a contains the value 1. The exception is illustrated by a downward-pointing needle with twice the normal length, ie exactly two down events fell into this time window, which consequently occurred at the same time within the accuracy of the time grid, each calculated from the emission of the corresponding radiation pulse.

[0102] For the SW amplitude 29 this means that the values ​​obtained in the above-mentioned connection with Fig. 3b, the sums formed do not change by the value 1 between two consecutive time windows only once. This exception is a change by the value 2, which results from the time window mentioned above containing two down events.

[0103] Fig. Figure 5 shows that the method described here can detect echo or signal pulses that are in the noise. In particular, signal pulses with a maximum amplitude significantly smaller than 4.5 NEP can be detected.

[0104] Such a signal 15 is in Fig. 5a in the form of a signal pulse 15 with a short duration compared to the measurement time without noise.

[0105] Fig. 5b shows an analog measurement 37 corresponding to Fig. 2a, but the noise and the signal 15 of Fig. 5a are superimposed on each other. This superposition means that the noise is altered by signal 15, or—in other words—signal 15 is contained in the noise.

[0106] The averaging depth required to detect a signal 15 depends, among other things, on the height of the signal 15. This is determined by the Fig. 5c-5f illustrates the SW amplitudes 29 for different averaging depths.

[0107] Fig. 5c corresponds to an averaging depth of 1, ie the SW amplitude 29 shown was obtained by integrating a single measurement. For this special case, the SW amplitude 29 is compared with the logical measurement ( Fig. 2b) is identical, where it would be impossible to detect a signal.

[0108] The Fig. 5d, Fig. 5e and Fig. 5f show SW amplitudes 29 for averaging depths of 10, 100, and 1,000, respectively. With increasing averaging depth, the signal 15 grows further out of the noise. While at an averaging depth of 10 ( Fig. 5d) the signal 15 is not yet recognizable, is at an averaging depth of 100 ( Fig. 5e) the signal 15 is already well above the noise level. At an averaging depth of 1,000 ( Fig. 5f), the noise is already reduced to such an extent that signal 15 can be clearly identified. The averaging depths mentioned are merely examples and can be selected differently depending on the transmission power.

[0109] To determine the distance at which the object 19 is located, from which the emitted radiation pulse 13 is reflected and transmitted as a signal pulse 15 ( Fig. 5a and Fig. 5f) was received ( Fig. 1), the time of the beginning of the leading edge of the signal pulse 15 is required when the distance is calculated, taking into account the speed of light, from the time period that elapses between the leading edge of the emitted radiation pulse 13 and the leading edge of the signal pulse 15.

[0110] In order to determine the point in time constituting the end of this period, a software threshold or SW threshold, also referred to below as detection threshold 31, is applied to the SW amplitude 29, which is Fig. 5f is shown as an example. This detection threshold 31 is a SW threshold 31 insofar as its application to the SW amplitude 29 occurs within the framework of the software-supported evaluation process running in the microprocessor.

[0111] From the time 65 at which the leading edge of signal pulse 15 crosses detection threshold 31, the desired object distance can be calculated using the speed of light. Since this time 65 is determined from the intersection of the leading edge of signal pulse 15 and SW threshold 31 using software, time 65 is also referred to as a software or SW event.

[0112] The distance calculation is carried out by measuring the time period from the emission of the respective radiation pulse 13 ( Fig. 1), in particular from its leading edge, to a significant base point of a SW event, which can be formed in particular by the time 65. For this purpose, a counter is used to count the number of the central measuring clock 43 ( Fig. 1) are counted. From the known width and frequency of these clock pulses, the elapsed time between the emitted radiation pulse 13 and the received signal pulse 15 can then be determined, and the corresponding object distance can be calculated using the speed of light.

[0113] The procedure is described below with reference to Fig. 6 to 9 are further described with regard to a group-wise transmission of several transmission pulses, wherein a transmission pulse spacing is less than a maximum pulse transit time.

[0114] Fig. 6 shows purely schematically several transmission pulses 102, which are transmitted one after the other and together form a transmission pulse group 100. By way of example, the transmission pulse group 100 in Fig. 6 four transmission pulses 102. The transmission pulse group 100 can generally also have more or fewer transmission pulses 102. The transmission pulses 102 are each spaced apart from one another by a transmission pulse spacing 104, ie the immediately consecutive or adjacent transmission pulses each have a time interval equal to the transmission pulse spacing 104. This is, as in Fig. 6 qualitatively indicated, significantly smaller than a maximum pulse transit time 106. The transmission pulse spacing 104 is also constant for the transmission pulse group 100.

[0115] The maximum pulse transit time 106 represents the time from the transmission of the first transmission pulse 102 of the transmission pulse group 100 to the reception of the latest echo pulse that could be generated by the transmission pulse group 100 and is used for distance measurement. The maximum pulse transit time 106 thus corresponds to the maximum time required for a transmission pulse 102, starting from the transmitter-receiver arrangement, to the furthest point in the target area and back to the transmitter-receiver arrangement. Due to the significantly shorter transmission pulse interval 104, after the transmission of a respective transmission pulse 102, the last possible echo pulse is not initially waited for, but rather a new transmission pulse 102 is transmitted after the transmission pulse interval 104 has elapsed. In this way, the distances to the target area can be measured quickly and with high accuracy using a plurality of transmission pulses 102, as can be seen using an exemplary echo pulse sequence with reference to Fig. 8 and Fig. 9 is explained in more detail.

[0116] Deviating from the provisions in Fig. 6, the sum of all transmission pulse intervals 104 of the transmission pulse group 100 can be equal to or at least substantially equal to the maximum pulse transit time 106.

[0117] In Fig. Figure 7 schematically illustrates the transmission of two temporally successive transmission pulse groups 108 and 110. The transmission pulse groups 108 and 110 are each structurally configured corresponding to the transmission pulse group 100 and have a group spacing 112.

[0118] The transmission pulses 102 of the transmission pulse group 108 are transmitted over a group duration 114, which results from the sum of the transmission pulse intervals 104 within the transmission pulse group 108. The group duration 114 is adapted to the maximum pulse transit time. In particular, the group duration 114 is substantially equal to or less than the maximum pulse transit time.

[0119] The transmission pulse group 110 is designed correspondingly to the transmission pulse group 108. Preferably, each transmission pulse group 108, 110 comprises at least ten transmission pulses 102, wherein the number of transmission pulses between the transmission pulse groups 108, 110 is preferably the same. In addition to the transmission pulse groups 108, 110, further transmission pulse groups will subsequently be transmitted in a corresponding manner (not shown), wherein these are spaced apart in time by the group spacing 112 and together with the transmission pulse groups 108, 110 form a transmission pulse sequence for distance measurement. The transmission pulse sequence can be used in particular within the framework of the Fig. 1 to 5 explained procedure.

[0120] Each of the transmitted pulse groups 108, 110 serves to determine a distance value for a respective distance image point. For this purpose, the transmitted pulses 102 are directed to different points in the target area using a deflection device (not shown), e.g., a movable mirror. For this purpose, the deflection device can be moved at a constant speed, which is selected depending on the group duration 114. In particular, the speed of movement is adjusted so that the transmitted pulses 102 of a respective transmitted pulse group 108, 110 are directed essentially to the same point in the target area. The deflection device is therefore considered stationary during the group duration 114.

[0121] Accordingly, the group spacing 112 is many times greater than the group duration 112, so that the deflection device can cover the distance required to record a new distance pixel.

[0122] For example, a value of 100 µs can be provided for the group spacing 112 and a value of 10 µs for the group duration 114. Within the group duration 114, ten transmission pulses 102 are emitted at a frequency of 1 MHz, i.e., the transmission pulse spacing 104 is 1 µs. The deflection device is rotated at a speed of 1.5 mrad per second, so that a total of 1000 transmission pulses 102 are emitted in a 90-degree scan. This corresponds to an average of one transmission pulse 102 per 1.5 mrad. However, due to the compressed transmission of the transmission pulses 102 in groups, it should be noted that 10 transmission pulses 102 are already transmitted in the first 10 µs and no transmission pulses 102 are transmitted in the remaining 90 µs until the next transmission pulse group 110 is transmitted.

[0123] Using the numerical example explained, the distance measurement can advantageously be performed with a range of 700 m. Due to the round trip that a transmission pulse 102 must travel due to its reflection, the total distance is 1400 m. Within 1 µs, a transmission pulse 102 covers a distance of approximately 150 m, so that the transmission pulse 102 requires approximately 9.2 µs to cover the distance of 1400 m. The last possible echo pulse of the transmission pulse group 108 is thus received approximately 10 µs after the last transmission pulse 102 of the transmission pulse group 108 and thus reliably before the group spacing 112 expires.

[0124] With a conventional distance measurement without group measurement, the same laser technology would only be able to measure distances of up to 400 m within 10 µs. The group-based transmission of pulses thus enables a range increase by a factor of 1.7.

[0125] The following are based on Fig. 8-9 aspects of the receiver side are highlighted.

[0126] At the transmitter-receiver arrangement, after and / or during the transmission of a respective transmission pulse group 108, 110, a reception signal is received which is composed of a noise signal component and an echo signal component. Fig. Figure 8a shows an example of a simulated noise signal 116 received in response to a transmit pulse group comprising eight transmit pulses 102, each spaced apart by a constant transmit pulse spacing. Fig. Figure 8b shows an echo pulse sequence 118, which was also received in response to the transmit pulse group. As can be seen from Fig. As can be seen in Figure 8b, the echo pulse sequence 118 comprises a plurality of echo pulses 120 that have been received successively at variable intervals. However, due to the constant transmission pulse spacing of the transmission pulse sequence, groups of echo pulses 120 can be identified in the echo pulse sequence 118, each of which is spaced from one another by a constant echo pulse spacing 121. The echo pulse sequence 118 therefore has a repeating pattern of echo pulses 120, which is attributable to the repeated transmission of transmission pulses with a constant transmission pulse spacing. It should be noted that the transmission pulse spacing does not necessarily have to be constant. However, the identification of the echo pulse groups belonging to the individual transmission pulses is simplified and can be carried out, for example, by means of correlation analysis or an iterative assignment method, which has already been described above.

[0127] The echo pulse sequence 118 is adjusted for the transmission pulse spacing before evaluation. For this purpose, the echo pulse sequence 118 is processed such that the echo pulse spacing 121 caused by the transmission pulse spacing between the groups of echo pulses belonging to a respective transmission pulse is compensated. This is done in particular by shifting the groups of echo pulses by an integer multiple of the transmission pulse spacing in order to standardize the groups of echo pulses to a common reference time, in particular the time of the first transmission pulse, as described in detail above. To average the echo pulses, they are preferably divided by the number of transmission pulses of the associated transmission pulse group.

[0128] The result is a cleaned echo pulse sequence 122 (in Fig. 9a), which, as explained, is based on the echo pulse sequence 118 of Fig. 8b. The cleaned echo pulses 124 of the echo pulse sequence 122 are each shifted relative to the echo pulses 120 as if they were caused by only one transmit pulse. For this purpose, the groups of echo pulses, each assigned to one of the transmit pulses, are shifted by a multiple of the echo pulse spacing 121. With a constant transmit pulse spacing, the echo pulse spacing 121 corresponds to the transmit pulse spacing and is also constant. As can be seen from Fig. As can be clearly seen in Figure 9a, each group of echo pulses comprises six echo pulses 124, each of which is spaced differently and represents a distance pixel.

[0129] The noise signal 116 and the cleaned and averaged echo pulse sequence 122 are in Fig. 9b together as a superimposed signal 126. The superimposed signal 126 is compared with a detection or SW threshold to detect the echo pulses 130, as described in detail with reference to Fig.5f explains the principle. Based on the echo pulses 124 detected in this way, a pulse transit time is determined for each echo pulse 124 and converted into a distance value, as explained.

[0130] Because the transmission pulse spacing is significantly shorter than the maximum pulse propagation time, it is possible to perform a large number of individual measurements at a significantly higher speed, i.e., in a time-compressed manner. In addition to the possibility of particularly fast distance measurement, this also opens up a wide range of possibilities for improving distance measurement in terms of achievable range and / or accuracy, without having to accept compromises in acquisition speed. LIST OF REFERENCE SYMBOLS 11 channels 13 emitted radiation 15 signal pulse 17 recipients 19 objects 21 Receiver threshold, HW threshold 23 logical pulse of the logical measurement 25 storage 27 Storage system, time-frame storage 29 Amplitude function, SW amplitude 31 Detection threshold, SW threshold 33 Time of a single measurement, event 35 amplifiers 37 analog received signal, analog measurement 39 Device with threshold, comparator 41 Control and evaluation device 43 Clock generator, measuring clock 45 IC module 47 microprocessor 49 Interface 51 Cover 53 Transmitting optics 55 Receiving optics 57 shift register multiplexer 59 Interface 61 Receiving diode 63 processor system 65 SW Event 67 points 69 Pulse width 100 transmission pulse group 102 transmission pulse 104 transmit pulse spacing 106 Maximum pulse runtime 108 transmission pulse group 110 transmission pulse group 112 group distance 114 group duration 116 Noise signal 118 Echo pulse sequence 120 echo pulses 121 Echo pulse spacing 122 Cleaned echo pulse sequence 124 Adjusted echo pulse 126 Overlay signal 128 Detection threshold, SW threshold

Claims

[1] Method for recording distance images having several distance pixels, wherein a transmitter-receiver arrangement (11, 17) emits electromagnetic radiation in the form of transmission pulses (102) for each distance image to be recorded and detects reflected echo pulses (120), wherein pulse transit times between the transmitted pulses (102) and the reflected echo pulses (120) are measured with a time measuring device (45, 63) in order to determine a plurality of distance values, each representing a distance pixel, wherein the transmission pulses (102) for determining the distance values ​​are transmitted in a transmission pulse sequence (107) comprising a plurality of transmission pulse groups (100, 108, 110), and wherein a transmission pulse spacing (104) between successive transmission pulses (102) within at least one of the transmission pulse groups (100) is smaller than a predetermined maximum pulse transit time (106); wherein an echo pulse sequence (118) is determined on the basis of the echo pulses (120) detected for the transmission pulse sequence (107), wherein at least one of the measured pulse transit times represents an average pulse transit time for a first subset of transmission pulses (102) of the transmission pulse sequence (107) and a second subset of echo pulses (120) of the echo pulse sequence (118), and wherein at least one of the distance values ​​is determined on the basis of the average pulse transit time, wherein the first subset is formed by one of the transmission pulse groups (100); wherein echo pulse spacings (121) between echo pulses (120) of the second subset are adjusted by the respective transmission pulse spacing (104) between successive transmission pulses (102) of the first subset. [2] Method according to claim 1, wherein the predetermined maximum pulse transit time (106) is greater than a multiple of the transmission pulse spacing (104), in particular wherein the multiple is in a range between 2 and 20, preferably between 8 and 12. [3] Method according to claim 1 or 2, wherein the plurality of transmission pulse groups comprise a first transmission pulse group (108) and at least one second transmission pulse group (110), wherein the transmission pulse spacing (104) between successive transmission pulses (102) within the first transmission pulse group (108) and the second transmission pulse group (110) is in each case smaller than the predetermined maximum pulse transit time (106), and wherein a group spacing (112) between the first transmission pulse group (108) and the second transmission pulse group (110) is adapted to the predetermined maximum pulse transit time (106), in particular wherein the group spacing (112) is greater than the predetermined maximum pulse transit time (106). [4] Method according to one of the preceding claims, wherein at least one of the distance values ​​is determined on the basis of the transmission pulses (102) of only one of the plurality of transmission pulse groups (100) and / or wherein the number of transmission pulses (102) of the transmission pulse groups (100) is in the range between 5 and 25, in particular wherein the number of transmission pulses (102) of the transmission pulse groups (100) is preferably at least 10. [5] Method according to one of the preceding claims, wherein the transmission pulse spacing (104) between a plurality of consecutive transmission pulses (102) within the respective transmission pulse groups (100) is in each case smaller than the predetermined maximum pulse transit time (106), in particular wherein the transmission pulse spacing (104) between all consecutive transmission pulses (102) within the respective transmission pulse groups (100) is in each case smaller than the predetermined maximum pulse transit time (106). [6] Method according to one of the preceding claims, wherein the transmission pulse spacing (104) between the transmission pulses (102) within a respective transmission pulse group (100) is constant. [7] Method according to one of the preceding claims, wherein at least a part of the transmitter-receiver arrangement (11, 17) and / or a deflection device associated with the transmitter-receiver arrangement (11, 17) is moved between the transmission of the transmission pulses (102) of at least two successive transmission pulse groups (100), in particular wherein the part of the transmitter-receiver arrangement (11, 17) and / or the associated deflection device is at least substantially stationary during the transmission of the transmission pulses of a respective transmission pulse group (100). [8] Method according to at least one of the preceding claims, wherein pulse widths (69) for detected echo pulses (15) are determined and the pulse transit times are measured at least partially as a function of the pulse widths (69). [9] Method according to at least one of the preceding claims, wherein the electromagnetic radiation of the transmission pulses (102) each has a wavelength which is varied within the transmission pulse sequence (107). [10] Method according to at least one of the preceding claims, wherein a group measurement is carried out for at least one of the transmission pulse groups (100), in which at least one logical start pulse (73) derived from the relevant transmission pulse group and a plurality of logical receiver pulses (79) are generated, wherein the receiver pulses (79) are each generated by means of a reference (5) of the transmitter-receiver arrangement, which reference is interrupted by a reception signal of the transmitter-receiver arrangement, wherein exceeding the reference (5) forms a positive edge of the respective receiver pulse defining an up event (81) and falling below the reference forms a negative edge of the respective receiver pulse defining a down event (82), wherein the at least one start pulse () and the receiver pulses (79) are combined in a timely manner in order to determine at least one of the distance values. [11] Method according to claim 10, wherein a plurality of time periods are determined, each elapsed relative to a time before the start pulse () up to the respective receiver pulses (), by counting at least the clock pulses provided by a central clock generator (33) with a known frequency for each up event (81) and / or each down event (82), wherein the at least one distance value is determined on the basis of the counted clock pulses. [12] Method according to claim 10 or 11, wherein the reference (5) is smaller than a noise threshold value, so that the receiver pulses (79) each represent one of the echo pulses and / or a noise pulse, or wherein the reference (5) is greater than the noise threshold value or equal to the noise threshold value, so that the receiver pulses (79) each represent one of the echo pulses. [13] Device for recording distance images having several distance pixels, wherein the device comprises a transmitter-receiver arrangement for emitting electromagnetic radiation in the form of transmission pulses and for detecting reflected echo pulses, and wherein the device comprises a time measuring device which is adapted to carry out the method according to one of the preceding claims.

Images