DE102018126522A1 - Runtime-based distance measurement using modulated pulse trains of laser pulses - Google Patents

Abstract

A method for time-based distance measurement comprises two correlations, one with a coarse time resolution and one with a fine time resolution. As a result, the computing resources required to determine the distance to an object can be reduced. LIDAR beam steering can be used.

Description

TECHNICAL AREA

Various examples generally relate to a distance measurement based on transit time (TOF) by transmitting a pulse train of laser pulses. Light detection and ranging (LIDAR) methods are described. Various examples relate specifically to the processing of measured photon events.

BACKGROUND

Light detection and ranging (LIDAR; sometimes also called laser ranging or LADAR) makes it possible to create a 3D point cloud of a scene. Objects can be recognized precisely. A distance measurement is possible. Pulsed or continuous laser light is transmitted along a transmission beam and, after reflection from an object, is detected along a reception beam. This makes it possible to determine the distance to the object (z position). Determining the z position is also referred to as a TOF distance measurement.

Methods for modulating pulses are known for expanding the measuring range of the LIDAR. See for example: US2018238998A ; US2017269209A ; WO2017132703 ; WO2018050906 .

It was observed that the amount of data to be processed is very high when using modulated pulse sequences.

SUMMARY

There is therefore a need for advanced methods of TOF distance measurement using modulated pulse trains of laser pulses. In particular, there is a need for advanced methods that simplify the efficient and rapid processing of measured photon events.

This need is met by the features of the independent claims. The features of the dependent claims define embodiments.

According to one example, a method for TOF distance measurement includes sending a pulse train of laser pulses. The pulse train is modulated in accordance with a modulation reference code. The method also includes measuring a sequence of photon events over a measurement period using a detector. The method further comprises binning the measured photon events into a sequence of bins. Each bin in the sequence of bins has a binning period. The method also includes performing a first correlation between a number of photons in each bin of the sequence of bins and a correlation code. The correlation code is assigned to the modulation reference code. The method further comprises performing one or more second correlations between one or more partial sequences of photon events and the correlation code. The one or more partial sequences of photon events are cut out of the sequence of photon events based on one or more correlation maxima of the first correlation. The method further includes determining at least one distance based on the one or second correlations.

For example, an integrated gain detector can be used, e.g. an avalanche detector. For example, an array detector can be used.

For example, the method can be implemented by a computing unit, e.g. through a Field Programmable Gated Array (FPGA).

A computer program, a computer program product and a computer readable medium, which comprise program code, are provided. The program code can be executed by at least one computing unit. Execution of the program code causes the at least one computing unit to carry out a method for TOF distance measurement, which comprises sending a pulse sequence of laser pulses. The pulse train is modulated in accordance with a modulation reference code. The method also includes measuring a sequence of photon events over a measurement period using a detector. The method further comprises binning the measured photon events into a sequence of bins. Each bin in the sequence of bins has a binning period. The method also includes performing a first correlation between a number of photons in each bin of the sequence of bins and a correlation code associated with the modulation reference code. The method further comprises performing one or more second correlations between one or more partial sequences of photon events and the correlation code. The one or more partial sequences of photon events are cut out of the sequence of photon events based on one or more correlation maxima of the first correlation. The method further includes determining at least one distance based on the one or second correlations.

A device comprises a storage medium and at least one computing unit. The at least one computing unit can access the storage medium in order to load program code. Execution of the program code causes the at least one computing unit to carry out a method for TOF distance measurement, which comprises sending a pulse train of laser pulses. The pulse train is modulated in accordance with a modulation reference code. The method also includes measuring a sequence of photon events over a measurement period using an array detector. The method further comprises binning the measured photon events into a sequence of bins. Each bin in the sequence of bins has a binning period. The method also includes performing a first correlation between a number of photons in each bin of the sequence of bins and a correlation code associated with the modulation reference code. The method further comprises performing one or more second correlations between one or more partial sequences of photon events and the correlation code. The one or more partial sequences of photon events are cut out of the sequence of photon events based on one or more correlation maxima of the first correlation. The method further includes determining at least one distance based on the one or second correlations.

It is to be understood that the above-mentioned features and those which are still to be explained below can be used not only in the respectively specified combinations but also in other combinations or in isolation without departing from the scope of the invention.

Figure list

• 1 illustrates schematically a scanning system with a coaxial optical structure and a friction-free steering mirror according to various examples.
• 2nd is a flow diagram of a method according to various examples.
• 3rd illustrates schematically a transmission and a reception phase according to different examples.
• 4th schematically illustrates a laser pulse that is transmitted during the transmission phase, according to various examples.
• 5 schematically illustrates a modulation reference code for a pulse train with several laser pulses according to various examples.
• 6 schematically illustrates a sequence of photon events according to various examples.
• 7 is a flow diagram of a method according to various examples.
• 8th schematically illustrates a sequence of bins into which the sequence of photon events is binned, according to various examples.
• 9 schematically illustrates correlation maxima obtained from a first correlation between the modulation reference code and the sequence of bins, according to various examples.
• 10th is a flow diagram of a method according to various examples.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The embodiments of the invention are described in detail below with reference to the accompanying drawings. It is to be understood that the following description of the embodiments is not to be taken in the narrow sense. The scope of the invention is not intended to be limited by the embodiments described below or by the drawings, which are used only for the purpose of illustration.

The drawings are to be regarded as schematic representations and the elements shown in the drawings are not necessarily to scale. Rather, the various elements are presented in such a way that their function and general purpose will be apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components or other physical or functional units, which are described in the drawings or herein, can also be realized by an indirect connection or coupling. A coupling between the components can also be established via a wireless connection. Function blocks can be implemented in hardware, firmware, software, or a combination thereof.

Methods of TOF distance measurement and LIDAR are described below. With these methods it is possible to determine the distance to an object in the environment with the help of the round-trip time (RTT) of photons of a laser pulse. Therefore, an active TOF measurement technique is used.

The methods described herein can also be used to determine an intensity of the reflected light and thus a reflectance to determine an object that is within range. It is also possible to determine multiple distances with multiple reflections.

According to various examples, a modulated pulse sequence of laser pulses is used. For example, code division multiple access (CDMA) modulation, e.g. with on-off keying (OOK). For example, pulse time modulation can be used in which the pulse interval between the pulses of the pulse train is varied. Different types of respective modulation reference codes can generally be used here. To give an example, a so-called gold sequence can be used. Further examples are Kasami sequences, Hadamard sequences or Zadoff-Chu sequences. Different modulation reference codes of the same type of code can be orthogonal to each other. This helps ensure interference resistance and facilitates CDMA.

Some of the methods described herein benefit from a combination of one or more beamline modulation schemes. Different pulse sequences are sent in different directions. A steering mirror can be used to direct the light. Alternatively or additionally, it would also be possible to rotate or otherwise move the light source or light sources. For example, a FLASH lidar system can benefit from the methods described here, in which multiple directions are simultaneously illuminated with primary laser light.

For example, the methods described herein can facilitate 1-D or 2-D steering of light. The scanning of light can correspond to the repeated deflection of light with different transmission angles. A scan rate or repetition rate of the scan can be defined by steering cycles. The steering mirror can be deflected accordingly.

It is possible to implement transmission angles by deflecting the steering mirror according to one or more degrees of freedom of a mass-spring system formed from an elastic mounting and the steering mirror. For example, the steering mirror can be rotated, tilted, moved, etc. Examples of the degrees of freedom with which light can be directed are the bending and torsion of the at least one spring of the mass-spring system.

According to some examples, a resonant movement of the mass-spring system is possible. In particular, the respective freedom of movement can have a corresponding resonance characteristic - sometimes also referred to as a frequency response, i.e. Distraction as a function of frequency. The resonance characteristic can have a peak of a certain width in the frequency range. It is possible to select a driving force that has a frequency within this resonance peak. For this purpose, an actuator can be controlled accordingly. Large changes in the transmission angle can be achieved through resonance movements. Large scan areas can be implemented.

Exemplary actuators that can be controlled to drive the elastic scanning unit resonantly are piezoelectric comb drives, magnetic drives, piezoelectric actuators, etc.

The steering mirror and the elastic holder can be part of a scanning unit. A scanning system (or just a scanner) may include the scanning unit, a light source configured to emit the light to be scanned, and / or a detector configured to receive secondary light. The scanning system may also include one or more actuators to actuate the elastic bracket to thereby deflect the steering mirror.

In order to increase the measuring range and / or the daylight suppression, spatial filtering can be used. For example, by using spatial filtering, daylight can be suppressed to a level that enables the individual return or daylight photon events to be counted and recorded individually without exceeding a limit on the bandwidth of the event recording of the detector or digital circuit. Spatial filtering refers to a scenario in which primary laser light - i.e. the transmitted laser pulses - is sent to the object along the transmission beam and via the steering mirror; and collecting secondary light reflected from the object along a receiving beam and over the same steering mirror. In other words, the transmission beam overlaps at least partially with the reception beam. In particular, the transmission beam and the reception beam can overlap in an overlap section that includes the reflecting surface of the steering mirror.

Such a technique of spatial filtering is sometimes referred to as a coaxial optical structure because the OA of the transmit beam and the OA of the receive beam are coaxially aligned. In general, with a coaxial optical structure, there can be an offset between the transmitted beam and the received beam as long as there is an overlap between the beam profiles of the transmitted beam and the received beam. In a coaxial optical arrangement, the transmit and receive beams can have at least one share common aperture, e.g. defined by the steering mirror.

Coaxial optical structures generally make it possible to limit the solid angle from which the light is collected and focused on the detector. This reduces background noise by reducing the number of background photons, e.g. by sun exposure.

Various methods described herein relate to the post-processing of the measurement data received from the detector. Various methods are based on the result that the number of calculations associated with making a comparison between the measured data and the expected data (based on the correlation) can be very high. According to various examples, this is mitigated by a two-stage approach: firstly by a rough search for candidate areas; and second, by one or more fine searches in the possible areas to resolve ambiguities and determine the area with greater accuracy.

1 schematically illustrates a scanning system 100 according to various examples. The scanning system 100 comprises a computing unit 90 , a laser diode 101 and a detector 102 . The scanning system 100 defines a coaxial optical structure.

The computing unit 90 can be implemented by an application-specific integrated circuit (ASIC) and / or a field-programmable array (FPGA) and / or a general-purpose processor. The computing unit 90 can include an analog-to-digital converter and / or a time-to-digital converter (Time to Digital: TDC).

The laser diode 101 is from the computing unit 90 controlled to primary laser light 111 along a broadcast beam 121 to send. The broadcast beam 121 passes a beam splitter 130 , then a steering mirror 150 , then a case window 151 the scanning unit and then reaches an environment 190 .

The primary laser light can strike an object (not in 1 shown) to be scattered to non-coherent secondary light 112 to create. The secondary light 112 - as well as background light and / or interfering light - travels along a receiving beam 122 through the window 151 , passes the mirror 150 and then reaches the beam splitter 130 . The beam splitter directs the secondary laser light 112 to a detector 102 from.

The OA of the broadcast beam 121 and the OA of the receive beam 122 are aligned; therefore, a coaxial optical structure is implemented. Both the transmitter aperture of the transmit beam 121 as well as the detector aperture of the receiving beam 122 are through the steering mirror 150 Are defined.

Generally the detector 102 an avalanche photodiode (APD) detector or a single photon APD detector (SPAD). Another example would be a silicon photomultiplier (SiPM). The detector 102 can have built-in physical reinforcement, for example it can be an avalanche detector. Another example would be a silicon photomultiplier (SiPM), a conventional photon multiplier tube, or any detector that could be invented in the future and capable of detecting single photons.

1 illustrates an overlap section 125 of the broadcast beam 121 and the receiving beam 122 . The overlap section 125 has an end to the beam splitter 130 .

The computing unit 90 is configured to the steering mirror 150 to operate to implement different scan angles. There is an actuator for this 901 with the computing unit 90 coupled. Example actuators 901 include electrostatic drives, magnetic drives or piezoelectric drives. The actuator 901 is configured to apply a force to a first end of an elastic bracket. This causes a reversible deformation of a spring of the elastic holder 902 that caused the steering mirror 150 deflects that with a second end of the elastic bracket 902 is coupled, which is opposite the first end. A spring element may extend between the first and second ends (not in 1 shown).

Exemplary implementations of the elastic bracket 902 are in US20180143322A1 : 62 and in DE 10 2016 014 001 A1 and in DE102009058762A1 and in US20100290142A1 described.

The computing unit 90 can the detector 102 read out.

In the example of 1 the optical structure is represented as a so-called optical pre-scanner structure. Because there are no lenses etc. that have a significant visual impact on the front of the steering mirror 150 along the transmit beam 121 arranged transmission beam 121 have (as would be the case with an optical post-scanner setup, not shown here). Thus the divergence corresponds to that on the steering mirror 150 existing broadcast beam 121 the divergence of the scanning system 100 leaving broadcasting beam 121 towards the surrounding area 190 . An additional beam shaping behind the steering mirror 150 is not provided in the optical pre-scanner setup.

2nd Figure 14 is a flow diagram of an exemplary method performed by the scanning system 100 following the example of 1 can be implemented. For example, the method of 2nd through the computing unit 90 be implemented.

First, at block 1001 a pulse train of laser pulses is sent. The pulse train can be modulated in accordance with one or more modulation schemes. For example, the pulse train can be modulated in accordance with a modulation reference code. For example, the computing unit 90 the laser diode 101 control accordingly to send the pulse train.

block 1001 can be generally referred to as the broadcast phase.

Then at block 1002 Detected photon events. For example, the computing unit 90 the array detector 102 control accordingly. Photon events relate to the detection of one or more photons per sampling time interval. For example, a typical sampling time interval can be in the range of 20 picoseconds, for example in the range of 10ps to 30ps.

The block 1002 can generally be called the measurement phase.

Generally the blocks 1001 and 1002 run at least partially in parallel. Therefore, the pulse train of laser pulses in the block 1001 be sent while photon events are already in the block 1002 can be detected.

Next up at Block 1003 processes the photon events in accordance with the modulation scheme. The block 1003 can be generally referred to as the post-processing phase.

For example, postprocessing by the computing unit 90 be implemented. In particular, one or more correlations between an expected signal pattern - according to the modulation scheme - and the measured signal pattern of the photon events can be carried out. Then one or more correlation maxima are obtained which indicate the RTT of the photons. The distance to the object can be determined based on the one or more correlation maxima.

Generally it would be possible for (i) the blocks 1001 and or 1002 and (ii) the block 1003 are at least partially implemented in parallel. It is therefore possible to initiate the postprocessing while still sending pulses and / or measuring for further photon events.

Further details on the transmission phase and the measurement phase are in connection with 3rd shown.

3rd illustrates aspects related to the timing of sending a pulse train of laser pulses and measuring a sequence of photon events. In particular, as in 3rd shown a broadcasting phase 201 (see. 2nd , Block 1001 ) during which a pulse train of primary light is sent (the pulse train is in 3rd not shown for the sake of simplicity). The pulse train has a specific pulse train duration, that of the transmission phase duration 202 corresponds.

3rd also illustrates aspects related to measuring photon events caused by secondary light from laser pulses. The photon events are triggered by secondary light and ambient or interfering light, ie background photons, for example from the sun or other light sources. 3rd schematically illustrates a measurement phase 205 that have a measurement phase duration 206 having. During the measurement phase 205 the detector detects 102 Photon events and outputs corresponding measurement signals, which are later analyzed.

In the example of 3rd the measurement phase overlap 205 and the broadcast phase 201 partly in the period. For example, the transmission phase duration 202 are in the range from 200 ns to 600ns. This corresponds to distances from 30 meters to 90 meters. On the other hand, it is desirable to recognize objects that are up to 10 m to 30 m close. The detector 102 is activated accordingly early. It is therefore possible that the first reflected secondary light is detected as a photon event before the last pulse of the primary light has been fired.

It cannot be said a priori at which RTT and area the object is located. Thus, the measurement phase has 206 a significant length. Therefore, it is necessary for photon events over the extended measurement phase 206 to measure - which corresponds to the ability to detect objects in a significant range of, for example, up to 200 meters or 300 meters or even up to 600 meters. In the case of closer objects, for example, the respective photon events occur earlier during the measurement phase 205 on; while with distant objects the respective photon events later during the measurement phase 205 occur.

An optional post-processing step will be explained next. As in 3rd shown, can sometimes a photon event 516 comparatively early in the measurement phase 205 (early photon event) can be detected. In particular, as in 3rd shown, the early photon event recorded at a time when the transmission phase 201 still persists. The early photon event 516 has a comparatively large signal amplitude, for example greater than a predefined threshold value. For example, the predefined threshold may correspond to a number of no less than 100 or 1,000 photons, which is the early photon event 516 assigned. The large signal amplitude of the early photon event 516 can be easily detected since it is significantly above the noise level given by interference and / or background photons. Secondary light that is reflected on a nearby object - for example at distances of up to 30 meters or up to 60 meters - is expected to have such a comparatively large signal amplitude. Therefore, from the presence of the early photon event 516 can be concluded that the object has already been detected and an accurate distance measurement based solely on the early photon event 516 can be carried out - without a correlation or the like having to be carried out. This means that it is not necessary to collect the coherence of several time-separate photon events, which are induced by the modulation of the pulse sequence, since the object is comparatively close and the signal is strong. In other words, the distance to the object can be determined without taking into account the pulse time that is assigned to the respective laser pulse of the primary light within the pulse sequence. Therefore, the modulation scheme that varies the interpulse distance can be used in the detection of such an early photon event 516 be ignored.

In some examples, it is in response to detecting the early photon event 516 possible the broadcast phase 201 for a period of time 202A cancel prematurely. Then the laser pulse (s) of the pulse train will become after the time period 202A not sent. This is advantageous because the cooling of a corresponding laser light source is made easier. Safety precautions for the eyes can also be better met.

Such an approach can be particularly helpful when using beam guidance (cf. 1 ). In particular when using beam steering, the period of time in which the beam steering mechanism is static is comparatively limited (for example, the steering mirror moves with a certain fast scan frequency when resonating, for example at 100 Hz to 500 Hz). It is therefore not easy to first send an isolated test pulse to find a nearby object, and then secondly to conditionally send the modulated pulse train. An integrated approach is therefore helpful, in which the early photon event is recognized from the primary light of a laser pulse of the pulse train.

4th schematically illustrates aspects related to a laser pulse 400 . The laser pulse 400 may be part of the broadcast phase 201 be. The laser pulse 400 can be sent as the primary light.

The laser pulse 400 has a certain duration 401 and a certain amplitude 402 . The time position 403 of the laser pulse 400 is also displayed, eg defined arbitrarily in relation to the maximum position.

The time position 403 several laser pulses 400 a pulse train can be set in accordance with a modulation reference code. Details of the modulation reference code are in connection with 5 shown.

5 schematically illustrates aspects related to a modulation reference code 300 . As in 5 shown, the modulation reference code 300 a certain length according to the measurement phase duration 202 (see. 3rd ) and includes several pulse times 301-305 . At the respective pulse times 301-305 becomes a laser pulse 400 (see. 4th ) is sent, ie primary light is sent. Also in 5 an exemplary pulse time gap is shown 310 . In the 5 Interpulse time gap shown 310 is the minimum interpulse time gap of the modulation reference code 300 ; other interpulse time gaps have longer durations, eg between the time of the pulse 304 and pulse time 305 .

The function of the modulation will be explained next. The photon events, which correspond to the secondary light reflected on the object, have the same or at least assigned pulse instants as the pulse instants 301-305 the modulation reference code 300 . This provides a priori knowledge of which signal pattern can be expected in the period. On the other hand, background or interference photon events are unlikely to have the same pulse instants. Therefore, a filter can be used that is specifically sensitive to the pulse times 301-305 the modulation reference code 300 is to filter out background or interference photon events. An exemplary implementation of such a filter corresponds to the implementation of a time period correlation between the measured photon events and a correlation code that corresponds to the modulation reference code 300 is assigned: This correlation detects suitable patterns in the measured photon events and the modulation reference code 300 . Such Adjustment patterns (identified by a maximum in the correlation) occur at certain times during the measurement phase 205 on. The best fit pattern - with the highest level of correlation, ie providing a maximum of correlation - is expected to correspond to the RTT of the light to and from the object. It is therefore possible to determine the distance of the object based on the maximum of the correlation.

Generally it would be possible that the correlation code with the modulation reference code 300 matches. In other examples, it would be possible to base the correlation code on the modulation reference code 300 to determine. For example, it would be possible to consider a physical model of the distortion, for example the operating properties of the laser and / or detector. For example, a time characteristic of the laser and / or detector can be taken into account, which leads to delay or trembling, etc. This would have the advantage of an even more precise measurement.

6 illustrates schematically aspects related to the measured photon events 511 - 515 . Illustrated in particular 6 the time sequence of the measurement signals from the array detector 102 be issued. The detector 102 is characterized by a sampling time interval 520 .

Generally the detector 102 be designed so that it has a sufficient dynamic range to detect photons at any time. Due to the shape of the enhanced detector response, which is between 100ps and a few nanoseconds in length, it is sometimes not possible to distinguish two photons with a distance of less than the response length. Therefore, the length of the amplified response limits the sampling time interval time 520 .

The number of photons per sampling time interval 520 is in 6 depicted over time. As illustrated, the number of photons is mostly zero. As in 6 shown, there are a total of five photon events 511 - 515 with a non-zero number of photons.

In general it is possible to change the signal time of the photon events 511 - 515 to determine, for example a time stamp for each photon event 511 - 515 to provide; and / or the signal amplitude of the photon events 511 - 515 .

According to various examples, it is possible for at least some of the photon events 511 - 515 to determine a respective signal amplitude - ie the number of photons - by looking at the respective one from the array detector 102 measurement signal obtained with at least three mutually offset baselines 501-503 (dashed lines in 6 ) compares. For example, the baselines 501-503 be generated with a digital signal generator in combination with a digital-to-analog converter. The amplitude of the baselines 501-503 can be set so that it corresponds to different signal amplitudes of the measurement signal. In particular, the baseline corresponds 501 , as in 6 shown, a one-photon event: For example, the photon events exceed 512 - 514 the baseline 501 , but do not cross the baseline 502 ; they are therefore called one-photon events 512 - 514 identified. Then it is possible to do a threshold comparison between each of the baselines 501-503 and that from the array detector 102 perform received measurement signal. Every time one of the baselines 501-503 is crossed, it is possible to draw conclusions about the signal amplitude and the corresponding signal can be obtained from the computing unit 90 implemented TDC. Using one or more TDCs, it is possible to set the signal time for each of the photon events 511-515 to determine.

Generally it would be possible for there to be one TDC per baseline 501-503 there to facilitate parallel processing. Thus, a corresponding signal time can be set for at least some of the photon events and for each baseline 531 (for the sake of simplicity only in 6 for the photon event 511 shown) can be determined by an intersection of the measurement signal with the respective baseline 501-503 is fed to the respective TDC.

Typically, the TDCs are dead time 521 connected. The dead time 521 is larger than the sampling time interval 520 . For the sake of dead time 521 can the photon event 513 not be reworked because of the baseline 501 assigned TDC by processing the photon event 512 is still locked.

As will be noted, it is therefore necessary that the dead time 521 the TDCs under normal conditions, such as the light intensity of the surrounding daylight, at an average background photon rate at the array detector 102 (otherwise the system would practically always be "blinded"). For example, the light intensity of the surrounding daylight can be defined as follows: a light intensity of 1 kW / m ² . Using a coaxial optical design with spatial filtering and a narrow bandpass filter centered on the wavelength of the primary light, the light intensity at the array detector can 102 - with a surrounding light intensity of 1 kW / m ² - down to 15 W / m ² .

Various methods described herein are based on the finding that such a low background photon rate at the array detector 102 can normally only be achieved using a colinear or coaxial optical design, such as in connection with 1 shown. Especially in other scenarios where the reception path is not via the steering mirror 150 , it is typically necessary to record light from a comparatively large solid angle in order to cover the entire scan area. Then the average background photon rate at the array detector 102 extraordinarily large. This can be an implementation of a baseline 501 prevent that corresponds to a single photon event. And that, in turn, can severely limit the achievable measuring range.

In general, the number of sampling time intervals 520 and photon events 511 - 515 over the entire measurement phase 206 be very big. If, for example, the measurement phase duration 206 20th Is microseconds long and the sampling time interval 520 Is 20ps long, there would be 100,000 sampling time intervals. Then performing a correlation between the correlation code and the sequence of photon 511 - 515 with a time resolution equal to the sampling time interval 520 or dead time 521 corresponds to require considerable computing resources. Sampling time interval. In the following, methods are described which make it possible to determine the distance in a computationally less complex manner and therefore more quickly. This enables real-time processing of the array detector 102 received measurement signals. A similar approach is related to 7 explained.

7 10 is a flowchart of a method according to various examples. Optional blocks are identified by dashed lines.

Initially, the optional block 1010A early photon event detection performed. Here the signal output by the detector can be compared directly with a predefined threshold value, ie each measured photon event 511-515 isolated. If one of these photon events is greater than the predefined threshold value, this can correspond to an early photon event - in particular if the photon event occurs early, ie while the transmission phase is still active and before the transmission of the pulse sequence has been completed. Then this early photon event can be used to determine the distance to the object. Therefore, in block 1010B checked whether an early photon event was detected. The blocks 1011 and 1012 are only carried out if an early photon event was not detected (cf. 3rd , early photon event 516 ).

The blocks 1011 and 1012 correspond to a two-step approach to measuring an object, ie determining its distance. The blocks 1011 and 1012 are based on a modulation scheme, for example using a CDMA-modulated pulse sequence. The two-stage approach uses a first rough search that is performed with low time resolution to identify one or more possible distances, see block 1011 . The two-step approach also uses a second fine search, which is performed with the higher resolution at and around the one or more possible distances, see block 1012 .

Such a two-stage approach makes it possible to limit the number of calculation processes, since the high temporal resolution associated with the fine search at the higher resolution can be restricted to the one or more possible distances.

An exemplary implementation of the two-step approach is related to 8th and 9 shown.

8th illustrates schematically aspects related to binning. PICTURE 8th illustrates a sequence 570 the bins 571-582 . Each bin has a binning period 590 . The binning period 590 is much larger than the sampling time interval 520 and dead time 521 , for example by at least the factor 10th or at least the factor 100 .

Exemplary binning times 590 are in the range of 1ns to 5ns, for example around 3ns.

In practical implementation, it would be possible that the binning period 590 a clock frequency of the computing unit 90 corresponds. Then the different bins 571-582 be filled in real time, from cycle to cycle of the computing unit 90 . This enables computationally cost-effective binning.

As in 8th each photon event is shown 511 - 517 one of the bins 571-582 assigned (except those photon events 511 - 517 , due to the dead time 521 the TDCs cannot determine an appropriate signal time; however, they are in 8th not shown). With binning, it would be possible to obtain information about the signal amplitude of the respective photon events 511 - 517 to discard. In addition, the binning process provides information about the respective fine signal time of each photon event 511 - 517 within the respective bin 571-582 discarded - ie the time position of each photon event 511-517 binning can be neglected within a bin.

It is optional, as in 8th shown, also possible, information about the number of photon events per bin 571-582 to discard beyond a binary distinction. In particular, every bin gives 571-582 at how in 8th shown whether there are no photon events within the respective bin or whether they have one or more photon events 511 - 517 within the respective bin 571-582 are. In other words, every bin 571-582 can be in two states, populated or unpopulated (binary bins). For example, while there are actually two photon events 511 , 512 within the bin 571 gives, the bin shows 571 simply that there are one or more photon events.

Such a binary design of the bins 571-582 can be particularly applicable in a scenario where the minimum interpulse distance 310 (see. 5 ) greater than or equal to the binning period 590 is. Namely, it is only possible in this scenario that a photon event 511-517 per bin 571-582 comes from secondary light; excess photon events 511-517 must necessarily come from background photons.

In other scenarios, a multi-level fill level of each bin 571-582 be taken into account. For example, the signal amplitude of each photon event can be within a given bin 571-582 be summed up and every bin 581 - 582 can then have a corresponding fill state, which is the sum of the signal amplitudes of all photon events within this bin 571-582 corresponds. This prevents false alarms.

In any case, it is possible to carry out a first correlation between the sequence of bins and the expected signal pattern, ie between the photon number of each bin 571-582 and the correlation code (in 7 becomes the associated modulation reference code 300 reproduced as a guide for the eye). Because the bins 571-582 are binary, it is sufficient to make a one-step comparison between the correlation code and the binary fill state of the bins 571-573 perform. The one-step comparison cannot differentiate between different amplitudes of the transmitted pulses 400 or hit the detected photon events.

The first correlation can be described intuitively as shifting the correlation code against the binning sequence 510 in the period and, for each time step size, checking for a match between the binary fill state of the bins 571-582 and the pulse time of the correlation code. One or more rough maxima result from this comparison 591 - 593 (see. 9 ) within the measurement phase 206 are at precisely defined time positions. The rough maximum values 591 - 593 match the best matches. The rough maximum values 591 - 593 in other words, provide the possible RTTs. Using a one-step comparison can lead to ambiguities - it is not known with high certainty which of the possible RTTs corresponds to the distance of the object. The possible RTTs can also only be known at the rough point in time, that of the binning period 590 corresponds, ie the error limit of the rough maxima 591 - 593 is not less than the binning period.

Next, it is possible for the coarse maxima for each temporal position 591 - 593 a second correlation between the respective partial sequences of the photon events 511 - 515 perform that are cut out of the overall sequence of photon events. In other words, it is possible to limit the high-resolution correlations to the relevant time periods around the time position of the correlation maxima 591 - 593 on the sampling time interval 520 dissolve. The number of calculation steps is thus greatly reduced since only a small fraction of all photon events have to be taken into account.

The process of cutting out and performing the second correlations is related to 9 explained in more detail, especially for the rough correlation maximum 591 . This process enables the height and the time position of the rough correlation maximum with lower error limits 591 to be determined with limited computing resources.

The binning sequence includes in detail 570 in the example of 9 a number of four bins 571-574 which are expected to include the photon events corresponding to the respective laser pulses of the reference code 300 are assigned (also in 9 as a guide for the eye), which leads to the rough correlation maximum 591 leads. It should be noted that between the bins 571-574 Gaps 580 are present in which no photon events are to be expected (since no laser pulses were fired). Given this finding, it is possible to (i) all photon events outside of duration 202 of the reference code 300 (or more specifically, the correlation code) and (ii) cut out all photon events that are in bins within the gaps 580 are included. In other words, as in the inset of 9 (dashed line; enlarged view) illustrates the partial sequences 701-703 of photon events from each of the four bins 571-574 that are expected to include the photon events of the secondary light, the maximum of the coarse ones Correlation is assigned. Then the correlation can be carried out to find a fine correlation maximum 595 to search. The fine correlation maximum 595 lies within the period of time caused by the rough correlation maximum 591 the first correlation and the binning period 590 is defined: For example, in the example of 9 the fine correlation maximum 595 by the second correlation based on the rough correlation maximum 591 was identified for a period of time 590A compared to longer RTTs in terms of the rough correlation maximum 591 transferred. The error limit with which the time position of the fine correlation maximum 595 is known is in the order of the sampling time interval 520 . This is much smaller than the error limit with which the time position of the rough correlation maximum 591 is known to be of the order of magnitude of the binning period 590 lies.

An ambiguity between the several rough maxima 591 - 593 that are output from the first correlation - that is, serve as candidates - can be resolved by the one or more second correlations that take into account the signal time of each photon event that occurs in the sampling time interval 520 is dissolved. For example, the respective fine correlation maximum 595 with the greatest amplitude to determine the distance to the object. In some examples, in which there are several correlation maxima, they can originate from several reflections on, for example, different objects. Instead of discarding distance values that are associated with less pronounced coarse or fine correlation maxima, it would be possible to output corresponding distances for further processing by applications at a higher level.

Furthermore, it would be possible to determine the signal amplitudes of the photon events that result from the multiple baselines 501-503 be taken into account. For example, the signal amplitudes of the photon events corresponding to the secondary light can be compared to the expected signal amplitudes due to divergence and absorption along the line of motion of primary and secondary light. On the other hand, the signal amplitudes of the background light tend to deviate from these expected signal amplitudes, which can be taken into account in the second correlation. In general, the second correlations can implement a multi-level comparison.

Furthermore, in some examples it would be possible to determine the amplitude of the photon events that correspond to the fine correlation maximum (s) 595 and derive a reflectance of the object for this (taking into account the baseline of the reduced signal level for more distant objects). Alternatively or additionally, it would be possible to determine the amplitude of the fine correlation maximum (s) 595 and to derive the reflectance from this.

10th schematically illustrates a method according to various examples. For example, the method of 10th through the system 100 according to 1 be implemented. In particular, it would be possible for the method of 1 through the computing unit 90 is implemented.

At the beginning, block 1021 a pulse train of laser pulses is sent. The block thus corresponds 1021 the broadcast phase 201 . The pulse train is modulated according to a modulation reference code.

Then at block 1022 measured a sequence of photon events using an array detector for a specific measurement phase duration. The block thus corresponds 1022 the measurement phase 205 .

Then it would be possible to determine at least some of the photon events, signal times and signal amplitudes. This can be achieved by using one or more TDCs. For example, several baselines can be determined in analog space and compared against the measurement signal received from the array detector. An intersection between the measurement signal and the baselines can then be processed by a corresponding TDC, each baseline being assigned to a respective TDC. This enables the signal times to be determined and a time stamp to be assigned to each photon event. By differentiating between several staggered baselines, it is possible to determine the signal amplitudes. Appropriate methods are discussed above in connection with 5 described.

Then the photon events can block based on the respective signal times 1023 , be binned. Binning can correspond to the act of assigning a corresponding bin of a binning sequence to each photon event based on the respective signal time. The binning sequence can thus correspond to a downsampling of the time resolution. For example, where time stamps are used to keep track of signal times, a number of most significant bits of the time stamp can be used to decide on the bin.

Then at Block 1024 a first correlation can be performed at the bin level. In other words, it would be possible to make a first correlation between the number of photons (possibly defined in binary) of each bin of the sequence of bins and the modulation reference code used to modulate the pulse train at block 1021 is used.

Then in block 1024 one or more rough correlation maxima are identified. For one or more of these rough correlation maxima, Block 1025 a second, high-resolution correlation can be performed. Here, the one or more second correlations are carried out at the photon event level, ie based on the complete information about the signal times available from the TDC operation, as explained above. At the same time, only a reduced number of photon events per second correlation can be taken into account, based on the cut out around the respective correlation maximum of the first correlation. The second correlation yields one or more fine correlation maxima.

If block 1025 provides several fine correlation maxima, it is possible to block 1026 filter the fine correlation maxima. This can be done to get a reliable result for the distance to the object. For example, a situation can sometimes occur in which there are several fine correlation maxima per measured distance, for example due to the non-orthogonality of the pulse sequence used, i.e. one fine main correlation maximum and one or more fine side lobe correlation maxima arranged in relation to the fine main correlation maximum. It would thus be possible, based on a type of non-orthogonality, for example derived from the modulation reference code, to filter the one or more fine side lobe correlation maxima.

At block 1027 the distance to the object is determined, for example based on one or more fine correlation maxima that are in the block 1026 have not been filtered. Optional, block 1027 , it would also be possible to determine a reflectance of the object.

Although the invention has been shown and described in relation to certain preferred embodiments, equivalents and changes will occur to other persons skilled in the art after reading and understanding the specification. The present invention encompasses all such equivalents and changes and is only limited by the scope of the appended claims.

For illustration purposes, various methods have been described which use an optical system with spatial filtering. However, such methods can easily be applied to other types and types of systems, e.g. FLASH systems in which several directions are simultaneously illuminated with primary light.

For further illustration, by using a CDMA modulation reference code to obtain a CDMA-modulated pulse sequence, it is also possible to transmit several pulse sequences of laser pulses simultaneously or during two overlapping transmission time phases in two or more different directions. Then the spatial resolution can be increased by using one and the same detector to resolve between the respective orthogonal codes.

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• US 2018238998 A [0003]
• US 2017269209 A [0003]
• WO 2017132703 [0003]
• WO 2018050906 [0003]
• US 20180143322 A1 [0036]
• DE 102016014001 A1 [0036]
• DE 102009058762 A1 [0036]
• US 20100290142 A1 [0036]

Claims (14)

1. Procedure for measuring the transit time, comprising: Sending a pulse train of laser pulses, the pulse train being modulated in accordance with a modulation reference code (300), Measuring a sequence of photon events (511-515) over a measurement period using a detector, Binning at least some of the photon events (511-515) of the sequence of photon events (511-515) into a sequence (570) of bins (571-582), Performing a first correlation between a photon number of each bin of the sequence (570) of bins (571-582) and a correlation code which is assigned to the modulation reference code (300), Performing one or more second correlations between one or more partial sequences of photon events (511-515), which are cut out from the sequence of photon events (511-515) based on one or more correlation maxima (591-593) of the first correlation, and the Correlation code, and Determining at least a distance of an object based on the one or more second correlations. Procedure according to Claim 1 , further comprising: - for at least some photon events (511-515) of the sequence of photon events (511-515): determining a respective signal amplitude by comparing a measurement signal obtained from the array detector with at least three mutually offset baselines (501-503). Procedure according to Claim 2 , wherein a first baseline (501) of the at least three baselines (501-503) corresponds to a single photon event (512-514). Procedure according to Claim 2 or 3rd , further comprising: - for the at least some photon events of the sequence of photon events and for each baseline (501-503): determining a respective signal time (531) by supplying an intersection of the measurement signal with the respective baseline (501-503) at a respective time -Digital converter. Procedure according to Claim 4 , wherein a dead time of the time-to-digital converter is less than an average background photon rate at the array detector at ambient light intensity. Method according to one of the preceding claims, wherein the first correlation works based on a one-step comparison between the correlation code and a binary fill state of the bins (571-582). Method according to one of the preceding claims, wherein a minimum interpulse interval (310) of the pulse train is equal to or greater than a binning time period (590) of each bin. Method according to one of the preceding claims, wherein a binning time period (590) of each bin (571-582) corresponds to a clock frequency of a computing unit (90) which carries out the first correlation and the second correlation. Method according to one of the preceding claims, the pulse train being emitted by laser pulses along a transmission beam, the array detector detecting light along a received beam, wherein the transmit beam and the receive beam are directed coaxially by a steering mirror. The method of any preceding claim, further comprising: - determining the correlation code based on the modulation reference code (300) based on a model of a pulse time distortion of at least one from a laser source used to transmit the pulse train of laser pulses and the detector. Method according to one of the preceding claims, wherein the cutting out for each of the one or more correlation maxima (591-593) of the first correlation comprises: selecting the one or more partial sequences (701-703) of the photon events (511-515) from bins of the sequence ( 570) of bins (571-582), which are expected to include photon events (511-515) from reflected laser pulses with a round trip time associated with the respective correlation maximum (591-593) of the first correlation. Method according to one of the preceding claims, each of the one or more second correlations yielding a respective further correlation maximum (595), wherein an error margin of the further correlation maxima (595) of the one or more second correlations is in the order of a sampling time interval (520) of the detector, wherein an error margin of the one or more correlation maxima (591-593) of the first correlation is in the order of a binning period (590) of each bin. Method according to one of the preceding claims, further comprising: during a transmission of the pulse sequence of laser pulses: detection of a photon event with an amplitude that exceeds a predetermined threshold, and - selectively aborting the transmission of the pulse train of laser pulses in response to the detection of the photon event with an amplitude that exceeds the predetermined threshold. Method according to one of the preceding claims: wherein a reflectance of the object is determined based on the second correlation.

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