Time-of-flight camera system and method for operating such a system

Abstract

A method is provided for operating a time-of-flight camera system (20) with a photodetector (21) and an illumination device (12), the illumination device (12) comprising a modulator (30) and at least one laser diode. The method provides for determining a distance by means of a time-of-flight measurement, wherein the time-of-flight measurement comprises at least one distance measurement cycle (EMZ) with at least two phase measurements, in each of which a phase shift between a transmitted light (S p1 ) and a received light (S p2 ), whereby the received light for phase measurement is measured over an integration time t int is integrated, and wherein the at least one laser diode is supplied with a transmission current I S to emit the transmitted light (S p1 ), whereby the transmit current I Swith a modulation frequency f mod for measuring the time of flight, as well as with a pulse width modulation with a PWM frequency f PWM and a PWM period T PWM modulated. The PWM frequency f PWM , the modulation frequency f mod and the integration time t int are chosen in such a way that - there is a phase difference (σ, σ1, σ2, σ3) between the PWM frequency f PWM and the modulation frequency f mod during the integration time t int changed; - the phase difference (σ, σ1, σ2, σ3) over N PWM periods T PWM averaged is zero; and - the integration time t int a number N ON on-times T ON of pulse width modulation, where N ON is an integer multiple of the N PWM periods, so that N ON = k • N holds, with k ∈ ℕ, k > 0, and N = 2, 3, 4, ...,.

Description

[0001] The invention relates to a time-of-flight camera system and a method for operating a time-of-flight camera system, as used, for example, in process automation or in autonomous driving applications.

[0002] The light-time-of-flight camera system relates in particular to light-time-of-flight camera systems or 3D-TOF camera systems that obtain time-of-flight information from the phase shift of emitted and received radiation. PMD cameras with photonic mixer detectors (PMD) are particularly suitable as time-of-flight or 3D-TOF cameras, as described, among others, in applications EP 1 777 747 A1, US 6 587 186 B2 and also DE 197 04 496 A1 and are available, for example, from the company 'ifm electronic GmbH' or 'PMD-Technologies GmbH' as the O3D frame grabber or CamCube. The PMD camera allows in particular a flexible arrangement of the light source and the detector, which can be arranged both in one housing or separately. Of course, the terms camera or camera system are also intended to include cameras or devices with at least one receiving pixel, such as the applicant's O1D distance measuring device.

[0003] DE 197 04 496 A1 also discloses the determination of a distance or a corresponding phase shift of the light reflected from an object. In particular, it discloses shifting the phase position of the transmitter modulation by 90°, 180°, or 270° in order to determine a phase shift and thus a distance from these four phase measurements using an arctan2 function.

[0004] From DE 10 2010 053 993 A1 a device and a method for distance measurement according to the principle of phase measurement with additional carrier modulation to increase the distance range are known.

[0005] DE 10 2014 207 163 A1 discloses a method for operating a time-of-flight camera system in which a time-of-flight sensor is operated with a basic modulation and a light source is operated with an illumination modulation dependent on the basic modulation.

[0006] DE 10 2018 131 182 A1 shows a time-of-flight camera with an illumination for emitting a modulated light, a time-of-flight sensor, an illumination circuit for operating the illumination, and a clock generator for generating a modulation signal.

[0007] DE 10 2021 102 870 A1 discloses a distance measuring system with an illumination system comprising a VCSEL for emitting intensity-modulated light and a time-of-flight sensor for receiving and demodulating the emitted light reflected from a scene. Furthermore, a modulator is provided for generating a high-frequency modulation signal and a low-frequency modulation signal, wherein the time-of-flight sensor is operated with the high-frequency modulation signal and the VCSEL is operated with a multiplication signal composed of the high-frequency modulation signal and the low-frequency modulation signal.

[0008] US 2017 / 0 307 736 A1 discloses a multi-wavelength LIDAR system comprising a first laser generating a laser beam at a first frequency and a second laser generating a laser beam at a second frequency. The laser beams reflected from a scene are received by an optical receiver, and the angular resolution of the LIDAR system is improved by evaluating wavelength-specific detector signals.

[0009] When controlling a time-of-flight camera with an illumination device comprising one or more laser diodes, it is necessary to ensure that the laser power meets eye safety requirements. To limit the light power, the illumination device is often operated with a pulse-width modulated control signal. This can be particularly necessary when using surface emitters that emit, for example, in the red or infrared spectral range. Pulse-width modulation (PWM) can also be used in some applications to improve the signal-to-noise ratio or to reduce the power consumption of the time-of-flight camera system.

[0010] Furthermore, high-frequency modulation of the transmitted light of the lighting device is necessary for measuring the light delay time using the phase mixing principle.

[0011] This presents the problem that synchronizing the PWM frequency of the pulse width modulation and the modulation frequency of the transmitted light is complex. Various specifications and limitations of the components and control circuits used must be taken into account. For example, possible PWM frequencies may be predetermined by the laser diode. Likewise, a base frequency of a PWM controller may be predetermined, or the selection of possible PWM frequencies may be restricted. Likewise, the selection of possible modulation frequencies may be restricted by a modulator of the lighting device. This can lead to variable deviations and beats between the modulation frequency and the PWM frequency of the pulse width modulation, resulting in inaccuracies in the distance measurement of a time-of-flight camera.

[0012] The object of the invention is to improve the time-of-flight camera system so that distance measurements can be improved even without synchronization of the PWM frequency and the modulation frequency.

[0013] The object is achieved by a method according to claim 1 and a time-of-flight camera system according to claim 6. Advantageous embodiments of the invention are specified in the subclaims.

[0014] Advantageously, a method is provided for operating a time-of-flight camera system with a photodetector and an illumination device, the illumination device comprising a modulator and at least one laser diode, wherein the method provides for determining a distance by means of a time-of-flight measurement, wherein the time-of-flight measurement comprises at least one distance measurement cycle with at least two phase measurements, in each of which a phase shift between a transmitted light transmitted by the illumination device and a received light received by the photodetector is measured, wherein the received light for the phase measurement is in each case integrated over an integration time t int is integrated, and wherein the at least one laser diode is supplied with a transmission current I S to emit the transmitted light, whereby the transmitted current is modulated with a frequency f mod, as well as with a pulse width modulation with a PWM frequency f PWM and a PWM period T PWM modulated, where the PWM frequency f PWM , the modulation frequency f mod and the integration time t int are chosen such that - there is a phase difference between the PWM frequency f PWM and the modulation frequency f mod during the integration time t int changed; - the phase difference over N PWM periods T PWM averaged is zero; and - the integration time t int a number N ON of PWM turn-on times of the pulse width modulation, where N ON is an integer multiple of the N PWM periods, so that N ON = k • N holds, with k ∈ ℕ, k > 0, and N = 2, 3, 4, ...,.

[0015] The method enables accurate distance measurement without the need for complex synchronization of the PWM frequency fPWM and the modulation frequency f mod This can, for example, keep the costs of a control circuit low. Furthermore, in some embodiments, a superposition of the modulation frequency f mod with the PWM frequency f PWM and a signal-to-noise ratio can be reduced. Advantageously, the at least one laser diode is designed as a surface emitter (VCSEL). This method makes it possible to meet eye safety requirements, particularly for surface emitters that emit light in the red or infrared spectral range.

[0016] The integration time t int can, for example, have a duration of a few µs. The modulation frequency f mod can be used, for example, in the range 3 MHz ≤ f mod ≤ 250 MHz, especially in the range 5 MHz ≤ f mod ≤ 125 MHz. The PWM frequency f PWM is smaller than the modulation frequency fmod and for example in the range 10 kHz ≤ f PWM ≤ 500 kHz, especially in the range 40 kHz ≤ f PWM ≤ 100 kHz. In some versions of the method, k can be selected in the range 500 ≤ k ≤ 5000, in particular in the range 1200 ≤ k ≤ 2000. For example, one or more possible PWM frequencies f PWM be specified by a PWM circuit. Restrictions regarding the modulation frequencies f mod by the modulator and / or a desired distance measuring range. To carry out the procedure, the PWM frequency f PWM and the modulation frequency f mod be adjusted so that the switch-on time of the laser diode(s) per integration time is constant during the distance measurement. Since a synchronization of the modulation frequency f mod and the PWM frequency f PWMis not provided, a phase difference that varies over time results. By choosing k appropriately, the PWM frequency f PWM and the modulation frequency f mod However, it must be set in such a way that the occurring phase differences are distributed over N PWM switch-on times of the N PWM period durations and thus also over the k · N PWM switch-on times of the integration time t int cancel each other out. In other words, a signal between the modulation frequency f mod and the PWM frequency f PWM developed beat frequency f S be chosen so that per integration time t int all occurring phase positions between the PWM frequency f PWM and the modulation frequency f mod summed up again to zero. The beat frequency f S results in f S = |f Mod - f Pwm |. The number N ONof PWM switching times, or the PWM periods, should be divisible by N, so that for the beat frequency f S the following relation must apply: ƒs=!(k±1N)⋅fPWM, k∈ℤ0+

[0017] Finally, a relation between the PWM frequency f PWM and the modulation frequency f mod , with f mod := f mod,k be specified as follows: ƒmod,k=(k+1N)ƒPWM

[0018] The choice of k can be made independently of the PWM duty cycle η. Thus, the PWM turn-on time can be chosen arbitrarily.

[0019] In some developments of the method, the duty cycle η of the PWM can also be adjusted during operation of the time-of-flight camera for power control. The duty cycle η can, for example, be set or adjusted in the range of 0.02 ≤ η ≤ 0.6, in particular in the range 0.05 ≤ η ≤ 0.25. In some developments, the method can provide for the duty cycle η to be adjusted as a function of temperature, in particular as a function of a temperature drift of the laser diode(s). This can be particularly advantageous when the laser diode(s) are designed as surface emitters. In particular with surface emitters that emit light in the red or infrared spectral range, constant operation without PWM is often not possible or not possible over a necessary temperature range. Adjusting the duty cycle, for example between two phase measurement cycles, can therefore be particularly advantageous when controlling surface emitters.

[0020] Furthermore, the time-of-flight camera may include a monitor diode with which the emitted transmitted light can be monitored. In some embodiments of the method, the duty cycle η may therefore be regulated depending on the light detected by the monitor diode.

[0021] Advantageously, N can be chosen in the range 2 ≤ N ≤ 5. N = 2 is particularly advantageous, where the modulation frequency f mod then by f mod := f mod,k , with ƒmod,k=(k+1N)ƒPWM is given, with k ∈ ℕ.and k > 0. The modulation frequency f mod can therefore be adjusted so that phase differences between the PWM frequency f PWM and the modulation frequency f mod averaged over two PWM on-times or over two PWM periods. In particular, the phase differences then also cancel out over the integration time t int, or over all in the integration time t int The PWM turn-on times are averaged. By selecting k, the modulation frequency can be adjusted to suit the time-of-flight measurement.

[0022] Preferably, the method is designed such that a distance measurement cycle comprises at least two of the three phase measurement cycles, wherein the phase measurements of different phase measurement cycles are carried out with different modulation frequencies f mod.k be performed, with k = k1, k2, k3, ...,. For example, two or three phase measurement cycles can be provided. Each phase measurement cycle can comprise at least two, in particular four, phase measurements. For example, a first phase measurement cycle can be a coarse measurement with a first modulation frequency f mod.k1 , a second phase measurement cycle is an intermediate measurement with a second modulation frequency f mod.k2, and a third phase measurement cycle a fine measurement with a third modulation frequency f mod.k3 include, where f mod.k1 < f mod.k2 < f mod.k3 The rough measurement can, in particular, have a larger range of uniqueness than the subsequent measurements. Furthermore, the rough measurement can include a plausibility check of the determined distance. Likewise, to validate the rough measurement, a comparison with a predetermined criterion, such as a brightness threshold, can be performed. This allows an invalid or faulty measurement to be quickly identified in the process. If an implausible or invalid measurement is detected, the distance measurement can be aborted and an error message can be displayed or output, for example, stored in a log file.

[0023] Table 1 illustrates possible values ​​of k for the case N = 2, as well as corresponding modulation frequencies f mod. A fine measurement F (first phase measurement cycle), by means of which a precise distance measurement is possible, should be carried out in the example with a modulation frequency f mod of approximately 100 MHz. The PMW frequency f PWM can be derived, for example, from a PWM base frequency. The time-of-flight camera can comprise a corresponding frequency generator which generates the PWM frequency f PWM based on the PWM base frequency. The PWM base frequency can be in the order of 100 MHz. For example, this results in a PWM frequency f PWM from f PWM = 62.5 kHz.

[0024] The modulation frequencies f mod,k1 for k1 = 1599 and 1600 cannot be provided by the modulator 30 and are therefore crossed out. The modulation frequencies f mod,k1 = 100.09375 (k1=1601) and f mod,k1= 100.15625 MHz (k1 = 1602) are possible. An intermediate measurement Z (second phase measurement cycle) should be performed at a modulation frequency f mod,k2 of approximately 25 MHz. For example, k2 = 399, 400, 401 or 402 can be selected. A coarse measurement G (third phase measurement cycle) should be carried out with a modulation frequency f mod,k3 of approximately 6.3 MHz. Accordingly, k3 = 99, 100, 101 or 102 can be selected. A distance measurement cycle with a coarse measurement G, an intermediate measurement Z and a fine measurement F can, for example, have the modulation frequencies f mod,k1 = 100.09375 MHz (k = 1601), f mod,k2 = 25.09375 MHz (k = 401) and f mod,k2 = 6.34375 MHz (k3 = 101).

[0025] Additionally, the method can provide for a detection phase between two phase measurement cycles of a distance measurement cycle, during which no modulated transmitted light is transmitted. The detection phase can, for example, be implemented between the coarse measurement and the intermediate measurement. If modulated light is detected during the detection phase, an external source of interference, in particular another time-of-flight camera, can be inferred. The method can then be continued with slightly modified modulation frequencies f mod,k , for example with the modulation frequencies f mod,k corresponding to k1 = 1602, k2 = 402 and k3 = 102. This prevents future interference from the detected additional time-of-flight camera. Table 1 F (MHz) Z (MHz) G (MHz) k2=399 24,96875 k3=99 6,21875 k2=400 25,03125 k3=100 6,28125 k1=1601 100,09375 k2=401 25,09375 k3=101 6,34375 k1=1602 100,15625 k2=402 25,15625 k3=102 6,40625

[0026] According to a further aspect of the invention, a time-of-flight camera system is further specified with a photodetector and an illumination device, the illumination device comprising at least one laser diode, and a control circuit with a modulator, wherein the control circuit is designed and configured to control the time-of-flight camera system for measuring a distance by means of a time-of-flight measurement, wherein the time-of-flight measurement comprises at least one distance measurement cycle with at least two phase measurements, in each of which a phase shift between a transmission light transmitted by the illumination device and a reception light received by the photodetector is measured, wherein the reception light for the phase measurement is in each case integrated over an integration time t int is integrated, and wherein the laser diode is driven with a transmission current for emitting the transmitted light, wherein the transmission current has a modulation frequency fmod for measuring the time of flight, as well as with a pulse width modulation with a PWM frequency f PWM and a PWM period T PWM modulated, where the PWM frequency f PWM , the modulation frequency f mod and the integration time t int are chosen such that - there is a phase difference between the PWM frequency f PWM and the modulation frequency f mod during the integration time t int changed; - the phase difference over N PWM periods T PWM averaged is zero; and - the integration time t int a number N ON on-times T ON of pulse width modulation, where N ON is an integer multiple of the N PWM periods, so that N ON = k • N holds, with k ∈ ℕ, k > 0, and N = 2, 3, 4, ...,.

[0027] The time-of-flight camera system can comprise an illumination device and a photodetector, wherein both units are arranged in the same housing. Alternatively, separate units can also be provided, wherein the illumination device is arranged in a first housing and the photodetector in a second housing. The time-of-flight camera system can further comprise one or more control circuits, for example with a microcontroller or an ASIC, which are designed and configured to carry out an above-mentioned method. In particular, the control circuits can comprise a data memory on which an executable computer file is stored, which is designed to carry out the method. In the memory, furthermore, predetermined modulation frequencies f modor corresponding values ​​for k can be stored that are suitable for performing the process at a predetermined PWM frequency. Alternatively, the frequencies or values ​​for k can also be hard-coded into the executable computer file.

[0028] Preferably, the laser diode(s) are designed as surface emitters. Particularly advantageously, the illumination device comprises at least one surface emitter which transmits light S p1 emitted in the red or infrared spectral range. In such configurations of the time-of-flight camera system, the above-mentioned method can be used particularly advantageously.

[0029] In some advantageous embodiments, the time-of-flight camera system is designed and configured to vary the period of the PWM in steps of less than 40 ns, in particular in steps of less than 20 ns. It is also advantageous if the modulation frequency can be adjusted as precisely as possible. For example, the period of the modulation signal can be adjusted in steps of less than 40 ns, in particular in steps of less than 20 ns.

[0030] The invention is explained in more detail below using exemplary embodiments with reference to the drawings.

[0031] They show schematically: Fig. 1 schematically shows the basic principle of photonic detection, Fig. 2 a cross-section of a time-of-flight pixel; Fig. 3 a modulated integration of the generated charge carriers; Fig. 4 two time courses of the charge integration with different phase positions; Fig. 5 a relation of integration in an IQ diagram; Fig. 6 a) a phasor diagram relating to the phase differences occurring between a PWM frequency and a modulation frequency during an integration time, the phase difference vanishing when averaged over two PWM periods; and b) a phasor diagram relating to the phase differences between a PWM frequency and a modulation frequency occurring during an integration time, wherein the phase difference disappears when averaged over three PWM periods; Fig. 7 a PWM control signal and a modulation signal for controlling a lighting device, plotted over 2 PWM periods; and Fig. 8 an exemplary sequence of a distance measuring cycle with three phase measuring cycles and one detection phase.

[0032] In the following description of the preferred embodiments, like reference numerals designate like or comparable components.

[0033] Fig. 1 shows a measuring situation for an optical distance measurement with a time-of-flight camera system, as known, for example, from DE 197 04 496 A1.

[0034] The time-of-flight camera system 20 comprises a transmitting unit or an illumination module 10 with an illumination 12 and associated beam-shaping optics 15, as well as a receiving unit or a photodetector 21 with receiving optics 25 and a time-of-flight sensor 22. The time-of-flight sensor 22 has at least one time-of-flight pixel, preferably a pixel array, and is designed in particular as a PMD sensor. The receiving optics 25 typically consist of several optical elements to improve the imaging properties. The beam-shaping optics 15 of the transmitting unit 10 can, for example, be designed as a reflector or lens optics. In a very simple embodiment, optical elements on both the receiving and transmitting sides can be omitted if necessary.

[0035] The measuring principle of this arrangement is essentially based on the fact that, based on the phase shift of the emitted and received light, the propagation time and thus the distance traveled by the received light can be determined. For this purpose, the illumination device 12 and the light propagation time sensor 22 are jointly subjected to a specific modulation signal M0 with a base phase position φ0 via a modulator 30. In the example shown, a phase shifter 35 is also provided between the modulator 30 and the illumination device 12, with which the base phase φ0 of the modulation signal M0 of the light source 12 can be shifted by defined phase positions φ var For typical phase measurements, phase positions of φ var = 0°, 90°, 180°, 270°.

[0036] According to the set modulation signal, the lighting device 12 sends an intensity-modulated signal S p1(transmitted light) with the first phase position p1 or p1 = φ0 + φ var This signal S p1 or the electromagnetic radiation is reflected by an object 40 in the case shown and, due to the distance travelled, arrives with a phase shift Δφ(t L ) with a second phase position p2 = φ0 + φ var + Δφ(t L ) as received signal S p2 (received light) to the time-of-flight sensor 22. In the time-of-flight sensor 22, the modulation signal M0 is combined with the received signal S p2 mixed, whereby the phase shift or the object distance d is determined from the resulting signal.

[0037] Furthermore, a modulation control unit 38 is provided, which specifies the shape and, in particular, the pulse and pause ratios of the modulation signal M0. The phase shifter 35 can also be controlled via the modulation control unit 38 depending on the measurement task to be performed.

[0038] Laser diodes are particularly suitable as the light source of the illumination device 12. In some embodiments, the illumination device 12 can comprise one or more surface emitters.

[0039] Fig. Figure 2 shows a cross section through a time-of-flight pixel of a photodetector 21 designed as a photonic mixer, as known, for example, from DE 197 04 496 A1. The modulation photogates G am , G0, G bm form the light-sensitive area of ​​a PMD pixel. According to the modulation gates G am , G0, G bm applied voltage, the photonically generated charges q are transferred either to one or the other accumulation gate or integration node G a , G b directed.

[0040] Alternatively, such a time-of-flight pixel can also be designed without modulation gates, as shown and described, for example, in EP 1 332 594 B1.

[0041] Fig. Figure 2b shows a potential curve where the charges q are directed towards the first integration node G a while the potential according to Fig. 2c the charge q towards the second integration node G b The potentials are specified according to the applied modulation signals. Depending on the application, the modulation frequencies are preferably in the range of 1 to 100 MHz. For example, a modulation frequency of 1 MHz results in a period of one microsecond, so the modulation potential changes every 500 nanoseconds.

[0042] In Fig. 2a also shows a readout unit 400, which may already be part of a PMD time-of-flight sensor designed as a CMOS device. The integration nodes G, designed as capacitors or diodes, a , G bintegrate the photonically generated charges over a large number of modulation periods. In a known manner, the charge then applied to the gates G a , G b The applied voltage can be tapped with high impedance, for example, via the readout unit 400. The integration times should preferably be selected so that the light transit time sensor or the integration nodes and / or the light-sensitive areas do not reach saturation for the expected amount of light.

[0043] The basic principle of phase measurement is shown schematically in Fig. 3. The upper curve shows the time course of the modulation signal M0 with which the lighting device 12 and the time-of-flight sensor 22 are controlled. The light reflected from the object 40 arrives as the received signal S p2 according to its light travel time t L phase shifted Δφ(t L ) to the time-of-flight sensor 22.

[0044] The time-of-flight sensor 22 typically has a first and second accumulation gate G a , G b in which, depending on the potential profile in the light-sensitive region, the photonically generated charges q are collected alternately over several modulation periods. The charges q generated in the unshifted phase position are collected in the first accumulation gate Ga, and those in the 180° shifted phase position M0 + 180° are collected in the second accumulation gate G b The ratio of the first and second gate G a , G b collected charges q a , q b the phase shift Δφ(t L ) and thus determine a distance d of the object.

[0045] Fig. 4a and Fig. 4b shows the charge difference Δq = (q a - q b ) / (q a + q b ) depending on the phase shift Δφ(t L) of the received light signal S p2 with different phase positions. The Fig. 4a shows a curve for an unshifted modulation phase M0 with a phase position φ var = 0°.

[0046] When the signal S p2 without phase shift i.e. Δφ(t L ) = 0°, for example, if the transmitted signal S p1 is directed directly to the sensor, the phases of the modulation M0 and of the received signal S p2 identical, so that all generated charge carriers are detected in phase synchronism at the first gate Ga and thus a maximum difference signal with Δq = 1 is present.

[0047] With increasing phase shift, the charge at the first accumulation gate G a and at the second accumulation gate G b At a phase shift of Δφ(t L ) = 90° the charge carriers q a , q b at both Gates G a , G bequally distributed and the difference is thus zero and after 180° phase shift "-1". With further increasing phase shift, the charge at the first gate G a increases again, so that the charge difference increases again and then reaches a maximum again at 360° or 0°.

[0048] Mathematically, this is a correlation function of the received signal S p2 with the modulating signal M0. q(τ)=∫0tSp2(t−τ)M0(t)dt

[0049] When modulating with a square wave, the correlation function is a triangular function, as already shown. When modulating with a sine wave, for example, the result would be a cosine function.

[0050] Fig. Figure 4 shows two temporal profiles of the charge integration with different phase positions. Fig. As shown in Figure 4a, a measurement of the phase with a phase position is only possible up to a phase shift Δφ(t L ) ≤ 180° clearly.

[0051] For maximum detection of the phase shift, the IQ (in-phase quadrature) method is known, in which two measurements are carried out with phase positions shifted by 90°, for example with the phase position φ var = 0° and φ var = 90°. The result of a measurement with the phase position φ var = 90° is in Fig. 4b.

[0052] The relationship between these two curves can be shown in a known manner, for example for sinusoidal curves in an IQ polar diagram according to. Fig. 5. As a first approximation, this representation can also be easily applied to the triangular functions shown.

[0053] The phase angle can then be determined in a known manner using an arctan or arctan2 function: φ=arctanΔq(90°)Δq(0°)

[0054] For example, to compensate for asymmetry of the sensor, additional phase measurements shifted by 180° can be carried out, so that the phase angle can be determined as follows. φ=arctanΔq(90°)−Δq(270°)Δ(0°)−Δq(180°)

[0055] Or generally represented in an arctan2 function: φ=arctan 2(X,Y) with X=Δq(90°)−Δq(270°) and X=Δq(0°)−Δq(180°) or more generally: X=X1−X2=(A1−B1)−(A3−B3) and Y=Y1−Y2=(A2−B2)−(A4−B4)

[0056] The amplitude or length of the phase vector r is then determined in a known manner from: A=X2+Y2

[0057] When correlating a sinusoidal modulation, this amplitude is ideally constant. If the time-of-flight camera is additionally modulated with a coded binary sequence or pseudorandom noise, the asymmetry is compensated using phase measurements performed with an inverted modulation or demodulation signal. The phase measurements with an inverted demodulation or modulation signal then correspond to a phase measurement shifted by 180° relative to the phase measurements at 0° or 90°.

[0058] The Fig. 6a and Fig. 6b illustrates different phase differences σ between the PWM frequency f PWM and the modulation frequency f mod , which occurs during an integration time t int The phase factors e iσof the sum oscillations in a vector diagram. The phase differences σ are each determined by the imaginary part Im of the displayed phase factors e iσ given.

[0059] Fig. Figure 6a illustrates the case where the phase differences σ of N = 2 consecutive PWM periods T PWM cancel each other out. At a first PWM switch-on time T ON,1 (cf. Fig. 7) of the pulse width modulation PWM, a sum oscillation results with a first phase difference σ = σ1, which is chosen as σ1 = π / 2. During a subsequent second PWM switch-on time T ON,2 This results in a second phase difference σ = σ2 = - π12. Over both PWM periods T PWM averaged, the phase differences σ1 and σ2 cancel each other out. The integration time t int a phase measurement is now chosen such that it has a number N ON on PWM switch-on times T ONcomprises, where N ON is an integer multiple of N = 2 PWM periods (N ON = 2 · k, with k = 1, 2, 3, ...). The integration time t int can therefore be in the range 2 · (k-1) · T PWM + T ON ≤ tint ≤ 2 · k · T PWM (generally in the range N · (k-1) · T PWM + T ON ≤ t int ≤ N · k · T PWM ) lying horizontally. Alternatively, it can also be provided that the integration, or rather the integration time t int after every Nth PWM switch-on time T ON In this case, the integration time t int by an effective integration time t int,e which is determined by the sum of the time periods t int,e can be in the above-mentioned case with N = 2 then in the corresponding range k · (T PWM + T ON ) ≤ t int,e < 2 · k · T PWM (generally in the range k · ((N - 1) · T PWM + T ON) ≤ t int,e < N · k · T PWM ) lying. In a further alternative embodiment, it can be provided that the integration time t int after each PWM switch-on time T ON is interrupted. In case N = 2, the integration time t int , or the effective integration time t int,e then in the range 2 · k · T ON ≤ t int,e < 2 · k · T PWM (generally in the range N · k · T ON ≤ t int,e < N · k · T PWM ) lying down.

[0060] In particular, the integration time t int , or t int,e in the range N · k · T ON ≤ t int,e < k · ((N - 1) · T PWM + T ON ) or in the range k · ((N - 1) · T PWM + T ON ) ≤ t int,e < N · (k-1) · T PWM + T ON be chosen lying down.

[0061] For example, the signal-to-noise ratio can be improved by interrupting the integration after each or some PWM turn-on times.

[0062] In particular, the phase difference σ is also constant over the integration time t int averaged equal to zero. This ensures that the switch-on time of the laser diodes over the integration time t int averaged for all phase measurements of a phase measurement cycle, the same. Using this method, the time-of-flight measurement can be improved and a precise distance measurement can be performed.

[0063] In the pointer representation of the Fig. 6b shows an example of the case in which the phase differences σ of N = 3 consecutive PWM periods T PWM cancel each other out. At the first switch-on time T ON,1 For example, the phase difference σ1 = 0 results here, while at the second switch-on time T ON,2and to a third switch-on time T ON,3 The phase differences σ2 = π / 3 and σ3 = 2 / 3 π / 2 are now calculated. Over the three PWM periods T PWM When averaged, the phase differences σ1 to σ3 cancel each other out.

[0064] Fig. 7 shows schematically a possible course of a PWM control signal S PWM and the modulation signal M0 to irradiate one or more laser diodes of a lighting device 12 with a transmission current I S for emitting modulated transmitted light S p1 The laser diodes are advantageously designed as surface emitters. The modulation occurs at the modulation frequency f mod , corresponding to the modulation period T mod . The modulation frequency f mod A pulse width modulation (PWM) is superimposed to control the power of the laser diode(s). The PWM operates at a PWM frequency f PWM , corresponding to the period T PWM. For the time of flight measurement, the received light S hitting the photodetector 21 is p2 over an integration time t int integrated. The integration time t int is again chosen so that this N ON PWM switch-on times T ON comprises, where N ON is an integer multiple of the N PWM periods, or the N PMW turn-on times. The end of the integration time t int can therefore be chosen in a range that extends from the end of the N ON -th PWM switch-on time until the end of the N ON -th complete PW period. The last (N ON -th) PWM period of a distance measuring cycle does not have to be completed, as shown in the example, but can be started after its PWM switch-on time T ON This allows the distance measurement to be carried out particularly quickly. In the example, N = 2, so t intcan comprise a total of 2 · k PWM switching times, where k = 1, 2, 3, ...,. For the sake of clarity, only two of the 2 · k PWM periods, or PWM switching times T ON shown.

[0065] The procedure described corresponds to that described in Fig. 6a. For example, k = 1600 is selected. The PWM frequency f PWM and the modulation frequency f mod are chosen in such a way that a phase difference σ occurring between the two frequencies over two consecutive PWM periods T PWM averaged. In a first on-time T ON,1 In the example shown, the PWM has four off-times and 3.5 on-times of the modulation signal M0. In the second on-time T ON,2The PWM, on the other hand, has four on-times and 3.5 off-times of the modulation signal M0. During the first PWM period, a rising edge of the PWM signal is followed by a rising edge of the modulation signal M0. In the second PWM periods T PWM follows the rising edge of the PWM signal S PWM a falling edge of the modulation signal M0. A signal between the PWM frequency f PWM and the modulation frequency f mod occurring phase σ thus changes between successive PWM periods T PWM . At the first PWM switch-on time T ON,1 the PWM, the phase difference σ takes a value σ = σ1 and in a second PWM switch-on time T ON,2 the subsequent second PWM period T PWM a value σ = σ2 = - σ1. For consecutive PWM periods T PWM this corresponds to a change in the phase difference Δσ of the modulation frequency f mod relative to the PWM frequency f PWMof Δσ = 180°. The PWM turn-on times T ON In pairs, they each comprise the same number of on-times and off-times of the modulation signal M0. This means that even without complex synchronization of the PWM signal S PWM and the modulation signal M0, an accurate time-of-flight measurement can be performed. The PWM turn-on time T ON can be chosen arbitrarily.

[0066] In the Fig. Figure 8 shows an example of a distance measurement cycle EMZ comprising three phase measurement cycles and a detection phase D. During a phase measurement cycle designed as a coarse measurement G, the modulation frequency f mod to f mod = f mod,k1 The modulation frequency f mod during a second phase measurement cycle designed as an intermediate measurement Z to f mod = f mod,k2 , and during a third phase measurement cycle designed as a fine measurement F to f mod = fmod,k3 elected, with f mod,k1 < f mod,k2 < f mod,k3 . Between the coarse measurement G and the intermediate measurement Z, the detection phase D is provided, during which an influence by a modulated light, for example transmitted light emitted by a second light delay camera, can be detected. List of reference symbols 10 lighting module 12 Lighting device 20 Time-of-flight camera system 21 Photodetector 22 Light transit time sensor 27 Evaluation unit 30 Modulator 35 phase shifters, lighting phase shifters 38 Modulation control unit 40 objects 50 differential circuit 60 inverters Δφ(t L ) runtime-related phase shift φ var Phase position φ0 base phase M0 modulation signal p1 first phase p2 second phase Sp1 Transmission signal with first phase (transmission light) S p2 Received signal with second phase (received light) S p2ab Scattered light G a , G b Accumulation gates' G am , G0, G bm Modulation gates t int Integration time T PWM PWM period T ON , T ON,1 , T ON,2 PWM turn-on time T mod Modulation period σ, σ1, σ2, σ3 phase difference I S Transmission current f PWM PWM frequency f mod Modulation frequency G Phase measurement cycle / coarse measurement Z Phase measurement cycle / intermediate measurement F Phase measurement cycle / fine measurement D Detection phase S PWM PWM signal

Claims

[1] Method for operating a time-of-flight camera system (20) with a photodetector (21) and an illumination device (12), the illumination device (12) comprising a modulator (30) and at least one laser diode, wherein the method provides for determining a distance by means of a time-of-flight measurement, wherein the time-of-flight measurement comprises at least one distance measurement cycle (EMZ) with at least two phase measurements, in each of which a phase shift between a transmitted light (S p1 ) and a received light (S p2 ), whereby the received light for phase measurement is measured over an integration time t int is integrated, and wherein the at least one laser diode is supplied with a transmission current I S to emit the transmitted light (S p1 ), whereby the transmit current I Swith a modulation frequency f mod for measuring the time of flight, as well as with a pulse width modulation with a PWM frequency f PWM and a PWM period T PWM modulated, where the PWM frequency f PWM , the modulation frequency f mod and the integration time t int are chosen such that - there is a phase difference (σ, σ1, σ2, σ3) between the PWM frequency f PWM and the modulation frequency f mod during the integration time t int changed; - the phase difference (σ, σ1, σ2, σ3) over N PWM periods T PWM averaged is zero; and - the integration time t int a number N ON on-times T ON of pulse width modulation, where N ON is an integer multiple of the N PWM periods, so that N ON = k ·N holds, with k ∈ ℕ, k > 0, and N = 2, 3, 4, ...,. [2] Method according to claim 1, wherein N = 2 and the modulation frequency f mod by f mod := f mod,k is given, where ƒmod,k=(k+12)ƒPWM with k ∈ ℕ and k > 0. [3] Method according to claim 1 or 2, wherein the distance measuring cycle (EMZ) comprises at least two of the three phase measuring cycles, wherein the phase measurements of different phase measuring cycles are carried out with different modulation frequencies f mod: = f mod.k be carried out, with k = k1, k2, k3, ...,. [4] Method according to one of claims 1 to 3, wherein a duty cycle η of the modulation signal (M0) is variable. [5] Method according to claim 4, wherein the duty cycle η for controlling a laser power of the laser diode is changed as a function of a temperature and / or as a function of a detector signal of a monitor diode of the illumination device (12). [6] Method according to one of claims 1 to 5, wherein the integration time t int in the range N · k · T ON ≤ t int < N · k · T PWM lying is selected. [7] Time-of-flight camera system (20) with a photodetector (21) and an illumination device (12), the illumination device (12) comprising at least one laser diode, and a control circuit with a modulator (30), wherein the control circuit is designed and configured to control the time-of-flight camera system (20) for measuring a distance by means of a time-of-flight measurement, wherein the time-of-flight measurement comprises at least one distance measurement cycle (EMZ) with at least two phase measurements, in each of which a phase shift between a transmission light (S p1 ) and a received light (S p2 ) is measured, whereby the received light (S p2) for phase measurement over an integration time t int is integrated, and wherein the laser diode is supplied with a transmitting current (I S ) to emit the transmitted light (S p1 ), whereby the transmit current I S with a modulation frequency f mod for measuring the time of flight, as well as with a pulse width modulation with a PWM frequency f PWM and a PWM period T PWM modulated, where the PWM frequency f PWM , the modulation frequency f mod and the integration time t int are chosen such that - there is a phase difference (σ, σ1, σ2, σ3) between the PWM frequency f PWM and the modulation frequency f mod during the integration time t int changed; - the phase difference (σ, σ1, σ2, σ3) over N PWM period T PWM averaged is zero; and - the integration time t int a number N ONon-times T ON of pulse width modulation, where N ON is an integer multiple of the N PWM periods, so that N ON = k · N holds, with k ∈ ℕ, k > 0, and N = 2, 3, 4, ...,. [8] Time-of-flight camera system (20) according to claim 7, wherein the at least one laser diode is designed as a surface emitter. [9] Time-of-flight camera system (20) according to claim 7 or 8, wherein the illumination device (12) comprises a monitor diode for controlling a laser power of the laser diode. [10] Time-of-flight camera system (20) according to one of claims 7 to 9, wherein the control circuit is designed and configured to determine the PWM period T PWM in time steps smaller than 20 ns, and / or one of the modulation frequencies f mod corresponding modulation period T mod in time steps smaller than 20 ns. [11] Time-of-flight camera system (20) according to one of claims 7 to 10, which is operated by means of a method according to one of claims 1 to 6.

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