Light time-of-flight camera system

Abstract

Method for operating a time-of-flight camera system, in which a time-of-flight sensor (22) is operated with a basic modulation or modulation signal (M0) and a light source (12) is operated with an illumination modulation (Sp1) dependent on the basic modulation (M0), wherein the time-of-flight sensor (22) has a first and second accumulation gate (Ga, Gb), characterized in that a duty cycle (DC) of the illumination modulation (Sp1) is smaller than the duty cycle (DC) of the basic modulation or modulation signal (M0) with which the time-of-flight sensor (22) is operated and the accumulation gates (Ga, Gb) are controlled.

Description

The invention relates to a time-of-flight camera system and a method for operating such a system according to the preamble of the independent claims.The aim with the time-of-flight camera system is to include not only systems which determine distances directly from the time-of-flight, but in particular also all the time-of-flight or 3D TOF camera systems which acquire time-of-flight information from the phase shift of emitted and received radiation. Suitable as time of flight or 3D TOF cameras are, in particular, PMD cameras with photomix detectors (PMD), as described, inter alia, in the applications EP 1 777 747 B1, U.S. Pat. No. 6,587,186 B2 and also DE 197 04 496 A1 and available, for example, from the company 'ifm electronic GmbH' or 'PMD Technologies GmbH' as frame Digber O3D or as CamCube. The PMD camera allows in particular a flexible arrangement of the light source and the detector, which can be arranged both in a housing and separately. Of course, the term camera or camera system is also intended to include cameras or devices having at least one reception pixel, such as, for example, the distance measuring device O 1D of the applicant.Furthermore, DE 197 04 496 A1 discloses the determination of a distance or a corresponding phase shift of the light reflected by an object. In particular, it is disclosed to shift the transmitter modulation specifically by 90°, 180° or 270° in order to determine a phase shift and thus a distance from these four phase measurements via an arctan2function.US 2012 / 0205723 A1 discloses a distance sensor in which charges generated in a photosensitive region are conducted via switches to integration nodes synchronously with the illumination modulation. In the readout phases, the photosensitive region is set to a positive potential in order to dissipate the charges generated in this phase. Furthermore, in one exemplary embodiment, it is proposed to change the duty cycle of the modulation in order to be able to preferably increase the power of the illumination.Furthermore, US 2013 / 0116977 A1 discloses a distance measuring device in which a PMD sensor is used for determining a distance and an intensity of an object. By using at least two modulation frequencies, the effects such as multipath propagation and / or crosstalk can be reduced, among other things. In order to generate preferred correlation functions, it is proposed, among other things, to reduce the duty cycle to 25%.It is an object of the invention to improve the energy efficiency of a time-of-flight camera system.The object is advantageously achieved by the method and time-of-flight camera system according to the invention as generically defined by the independent claim.Advantageously, a method for operating a time-of-flight camera system is provided, 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, wherein a duty cycle of the illumination modulation is smaller than a duty cycle of the basic modulation.This procedure has the advantage that the duty cycle and thus also the required illumination energy are reduced.In particular, it is advantageous to choose the duty cycle of the illumination modulation to be less than 50%, in particular 35% and particularly preferably equal to 25%. With such a modulation of the illumination, it is possible to save illumination energy with the same measurement accuracy or to increase the measurement accuracy with the same energy input.Likewise advantageously, a time-of-flight camera system is provided which is designed to carry out one of the aforementioned methods, having an illumination for emission and a time-of-flight camera for receiving a modulated light, having a modulator for generating a first modulation signal for operating the time-of-flight camera, wherein a control device is designed such that, starting from the first modulation signal, a second modulation signal for operating the illumination is generated which has a lower duty cycle than the first modulation signal. The invention is explained in more detail below on the basis of exemplary embodiments with reference to the drawings.The following are shown: FIG. 1 schematically shows a time-of-flight camera system, FIG. 2 shows a modulated integration of the generated charge carriers, FIG. 3 shows two time curves of charge integration with different phase positions, FIG. 4 shows the relationship of the integration in an IQ diagram, FIG. 5 shows an intensity profile of an illumination modulation according to the invention, FIG. 6 shows an intensity profile of a delay-time-dependent phase shift, FIG. 7 shows a correlation of the adapted modulation with the basic modulation, FIG. 8 shows a correlation shifted by 90° FIG. 9 shows intensity profiles for different duty cycles FIG. 10 shows correlation functions for different duty cycles, FIG. 11 shows a relative distance error as a function of the phase angle.In the following description of the preferred embodiments, like reference numerals designate like or comparable components.FIG. 1 shows a measurement situation for an optical distance measurement using a time-of-flight camera, as is known, for example, from DE 197 04 496.The time-of-flight camera system 1 comprises a transmitting unit or an illumination module 10 with an illumination 12 and an associated beam shaping optical unit 15 and a receiving unit or time-of-flight camera 20 with a receiving optical unit 25 and a time-of-flight sensor 22. The reception optics 25 typically consist of a plurality of optical elements to improve the imaging properties. The beam shaping optics 15 of the transmission unit 10 can be designed, for example, as a reflector or lens optics. In a very simple embodiment, optical elements can optionally also be dispensed with both on the receiving side and on the transmitting side.The measuring principle of this arrangement is essentially based on the fact that the propagation time and thus the distance covered by the received light can be determined on the basis of the phase shift of the emitted and received light. For this purpose, the light source 12 and the time of flight sensor 22 are jointly supplied via a modulator 30 with a specific modulation signal M o having a basic phase position φ 0. In the example shown, a phase shifter 35 is furthermore provided between the modulator 30 and the light source 12, with which phase shifter the base phase φ 0 of the modulation signal M 0 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° are preferably used.In accordance with the set modulation signal, the light source 12 emits an intensity-modulated signal S p1 with the first phase position p1or p1=φ 0+ φ var respectively. This signal S p1 or the electromagnetic radiation is reflected by an object 40 in the case shown and, on account of the distance travelled, impinges on the time-of-flight sensor 22 in a phase-shifted manner Δφ(t L) with a second phase position p2=φ 0+ φ var+ Δφ(t L) as a received signal S p2 In the time-of-flight sensor 22, the modulation signal M 0 is mixed with the received signal S p2 wherein the phase shift or the object distance d is determined from the resulting signal.Furthermore, a modulation control device 38 is provided, with which the shape and in particular pulse and pause ratios of the modulation signal are predefined. The phase shifter 35 can also be controlled via the modulation control unit 38 as a function of the measurement task to be carried out.Suitable illumination source or light source 12 are preferably infrared light-emitting diodes. Of course, other radiation sources in other frequency ranges are also conceivable; light sources in the visible frequency range are also possible in particular.The basic principle of phase measurement is schematically illustrated in FIG. 2. The upper curve shows the time profile of the modulation signal M 0 with which the illumination 12 and the time of flight sensor 22 are controlled. The light reflected by the object 40 impinges as a received signal S p2 on the time of flight sensor 22 in a phase-shifted manner Δφ(t L) corresponding to its time of flight t L.The time of flight sensor 22 typically has a first and a second accumulation gate Ga, Gb in which the photonically generated charges q are alternately collected over a plurality of modulation periods as a function of the potential profile in the light-sensitive region. The charges q generated in the unshifted phase position are collected in the first accumulation gate Ga and the charges q generated in the second accumulation gate Gb in the phase position M 0+ 180 ° shifted by 180°. From the ratio of the charges qa, qbaccumulated in the first and second gates Ga, Gb, the phase shift Δφ(t L) and thus a distance d of the object can be determined.FIGS. 3 aand 3 b show curves of the charge difference Δq=q a- q b / ( q a+ q b) as a function of the phase shift Δφ(t L) of the received light signal S p2 with different phase positions. FIG. 3 ashows a curve for an unshifted modulation phase M 0 with a phase position φ var= 0 °.In the case of an impingement of the signal S p2 without a phase shift, i.e. Δφ(t L) = 0 °, for example when the transmission signal S p1 is directed directly onto the sensor, the phases of the modulation M 0 and of the received signal S p2 are identical, so that all generated charge carriers are detected in phase synchronous fashion at the first gate Ga and thus a maximum difference signal with Aq=1 is present.As the phase shift increases, the charge on the first accumulation gate Ga decreases and on the second accumulation gate Gb increases. With a phase shift of Δφ(t L) = 90 °, the charge carriers qa, qb are distributed equally at both gates Ga, Gb and the difference is thus zero and after 180° phase shift "-1". With a further increasing phase shift, the charge at the first gate Ga increases again, so that as a result the charge difference increases again, in order then to reach a maximum again at 360° or 0°.Mathematically, this is a correlation or autocorrelation of the received signal S p2 with the modulating signal M 0.In the case of modulation with a square-wave signal, a triangular function is obtained as the correlation function, as already shown. In a modulation with, for example, a sine signal, the result would be a cosine function.As FIG. 3 ashows, a measurement of the phase with a phase position is unambiguous only up to a phase shift Δφ(t L)≤180°.For maximum detection of the phase shift, for example, the IQ (in-phase-quadrature) method is known, in which two measurements are carried out with phase positions shifted by 90°, that is to say 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 shown in FIG. 3 b.The relationship of these two curves can be measured in a known manner, for example for sinusoidal curves in an IQ diagram. FIG. 4 illustrate. In a first approximation, this representation can also be readily applied for the illustrated triangular functions.The phase angle can then be determined in a known manner via an arctan or arctan2function:In order to compensate for asymmetry of the sensor, for example, additional phase measurements shifted by 180° can be carried out, so that the phase angle can be determined as follows as a result.By correlating the PMD modulation or basic modulation M 0 with the delay-dependent phase-shifted illumination modulation S p2 a phase shift and distance can thus be determined. The modulation frequency or the modulation wavelength defines a uniqueness range within which the phase shifts by 360°. Larger phases are folded back to this area. However, if only certain phase positions or distances which are smaller than the value range occur during the target application, this information can be used to generate a modulation signal which is customized to the working range.Usually, the time of flight sensor 22 and the lighting 10 are operated with a duty cycle of 50%. However, in potential applications that intrinsically exhaust only a fraction of this uniqueness range, this restricted range information is not utilized. As will be shown below, this costs energy and dynamics without improving the signal.The upper curve in FIG. 5 shows a modulation signal M 0 or a basic modulation, and the two curves following below show an illumination signal S p1(0 °) 50DC, S p1(90 °) 50DC. shifted by 0° and 90° in phase position.In the case where the complete value range of the delay-time-dependent phase shift Δφ(t L) lies between 0° and 90°, it is evident that the unlabeled range of the switch-on interval of the illumination modulation S p1(0 °) 50DC, S p1(90 °) 50DC lies completely within a clock phase or switch-on interval of the basic modulation M 0 and therefore supplies only a constant amount in the correlation, which as a result does not contain any distance information. The marked region, on the other hand, scans the falling edge of the basic modulation M 0 to a certain extent and contains the entire range information.If the unlabeled region is omitted during the modulation of the illumination, a pulse shape S p1(0 °) 25DC, S p1(90 °) 25DC with a duty cycle of 25% results, as depicted in the two lower curves of FIG. 5.With an identical exposure time or integration time, both methods have the same resolution, since the marked areas are, to a certain extent, the same. In the procedure according to the invention, however, the power consumption is significantly reduced on account of the lower duty cycle. The signal which is thus smaller, with the same information content, also permits a significantly longer integration time and thus an increase in dynamics.In the measurements at different phase positions required for distance measurement, the range of the signal which effectively carries the information naturally changes. However, this can be taken into account, so that only the region of the signal which carries no information is removed at all times.The modified form of the illumination modulation S p1 naturally also has an influence on the correlation or autocorrelation function ACF. This influence is illustrated by way of example in FIGS. 6 and 7.FIG. 6 shows pulse sequences of different delay-time-dependent phase shifts Δφ(t L). The first pulse sequence shows the unshifted transmission pulse sequence S p1(0 °) 25DC with a 0° phase position and 25% dutycycles. The subsequent pulse sequences S p2, which are then ultimately detected by the sensor 22, are each shown shifted by 90° due to the propagation time.The correlation of the received, phase-shifted illumination modulation S p2 with the basic modulation M 0 leads to a correlation function according to FIG. 7. As the phase shift Δφ(t L) increases, the correlation decreases in order to achieve a flattened minimum at 90°. From 180° on, the correlation increases again and reaches a flattened maximum at 270°, which remains at this level by up to 360° or 0°.FIG. 7 shows, by way of example, a correlation function for a phase position shifted by 90°.The correlation functions according to FIGS. 7 and 8 are unambiguous only in a smaller range than the autocorrelation functions according to FIG. 3. In a simple application, it can therefore be provided to provide the reduced illumination modulation only for delay-time-dependent phase shifts of 0° to 90°. If the wavelength of the basic modulation M 0 is, for example, 8 m, this results in a measurement interval of 0 to 1 m. This measurement interval naturally does not have to begin at an absolute distance of 0 m, but can be shifted within the possible measurement range of 0 to 4 m.Due to the limitation to a small range of the uniqueness range, the aforementioned "arctan method" can be omitted, if necessary, and direct "fit algorithms" can also be used. In a simple application, for example, the correlation function for the 0° to 90° range can be provided in a characteristic value table. It is likewise conceivable to store the correlation as a mathematical function. Of course, further evaluation methods are also conceivable.It should also be noted that the slope of the correlation function in the relevant measurement range is substantially identical to the case of the normal measurement, so that the error of both methods is basically the same in the relevant measurement range.FIG. 9 shows exemplary intensity profiles for different duty cycles. The solid corresponds to the usual operation with a duty cycle of 50%, i.e. the illumination is operated to a proportion of 50% of the period duration T. The dotted line corresponds to a duty cycle of 35% and the dotted line to 25%. If the energy with which the light source is operated is kept constant for the various duty cycles, the light intensity correspondingly increases with a constant amount of light.If no higher light intensity is required, this can remain at the level of normal operation, with energy then being saved on account of the shorter switch-on duration.FIG. 10 shows curves of the correlation function for different duty cycles. The duty cycle of 50% yields the triangular function already discussed above, which is shown here with a solid line. If the duty cycle is shortened, a plateau forms at the extreme points at 90° and 270°, which plateau increases as the duty cycle becomes smaller.FIG. 11 shows an estimate of a relative distance error for different duty cycles as a function of the phase shift to be measured. The relative distance error is normalized to the maximum distance error of the 50% duty cycle. The distance error of the phase measurement periodically fluctuates depending on the phase to be measured. The minima of the distance errors have a phase spacing of 90° for all the duty cycles shown. The distance error of the 50% dutycycle varies between 0.7 and 1, while the 25% and 35% dutycycles vary between 0.5 and 0.7 and are thus always more accurate than the 50% dutycycle.The distribution of the minima can also be used advantageously, in particular, for certain measurement tasks. Thus, for example, it can be provided to shift the phase positions of the illumination modulation relative to the phase positions of the basic modulation such that the minimum of the distance errors falls within a preferred measurement range.For example, the phase position could be shifted in such a way that the accuracy is the greatest at the end of the preferred operating range, since there typically one has the smallest amplitudes and the greatest noise. This procedure advantageously makes it possible to ensure high measurement accuracy over the entire working range or measurement range.Of course, other configurations are also conceivable in order to optimize the respective measurement task.List of reference characters10 Illumination module 12 Illumination 22 Time of flight sensor 27 Evaluation unit 30 Modulator 35 Phase shifter, illumination phase shifter 38 Modulation control unit Δφ(t L) Time-of-flight-related phase shift φ var Phase position φ 0 Base phase M 0 Modulation signal p 1 First phase p 2 Second phase Sp 1 Transmission signal having first phase Sp 2 Reception signal having second phase Ga, Gb Accumulation gates Ua, Ub Voltages at the modulation gate

Claims

Method for operating a time-of-flight camera system, in which a time-of-flight sensor (22) is operated with a basic modulation or a modulation signal (M 0) and a light source (12) is operated with an illumination modulation (Sp1) dependent on the basic modulation (M 0) wherein the time-of-flight sensor (22) has a first and second accumulation gate (Ga, Gb), characterized in that a duty cycle (DC) of the illumination modulation (Sp1) is smaller than the duty cycle (DC) of the basic modulation or of the modulation signal (M 0) with which / the time-of-flight sensor (22) is operated and the accumulation gates (Ga, Gb) are actuated.Method according to Claim 1, in which the duty cycle (DC) of the illumination modulation (Sp1) is less than 50%.Method according to claim 1, wherein the duty cycle (DC) of the illumination modulation (Sp1) is 25%.Time-of-flight camera system (1) which is designed to carry out one of the aforementioned methods, having an illumination (10) for transmission and a time-of-flight camera (20) for receiving a modulated light, having a modulator (30) for generating a basic modulation or a first modulation signal (M 0) for operating the time-of-flight camera (20), characterized in that a control unit (38) is designed in such a way that, starting from the basic modulation or from the first modulation signal (M 0) an illumination modulation or a second modulation signal (Sp1) for operating the illumination (10) is generated, wherein the illumination modulation has a lower duty cycle (DC) than the first modulation signal (M 0) with which the time of flight sensor (22) is operated and the accumulation gates (Ga, Gb) are driven.

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