DE102021117139A1 - Time-of-flight camera system and method of operating same - Google Patents

Abstract

Method for operating a time-of-flight camera system having lighting and a time-of-flight sensor with time-of-flight pixels that have integration nodes (A, B, Ga, Gb) for the accumulation of photogenerated charges,
- A modulation signal being applied to the lighting and the time-of-flight sensor or the light-time-of-flight pixels at integration intervals to determine the distance
and a distance is determined based on the charge differences at the integration nodes (A, B, Ga, Gb),
- wherein a CM measurement is provided with at least two CM integration intervals and the time-of-flight camera system is operated with differently encoded modulation signals in these CM integration intervals,
- and a CW measurement is provided with at least two CW integration intervals and the time-of-flight camera system is operated in these CW integration intervals with different phase angles of a periodic CW modulation signal,
- whereby a distance value is determined from the charge differences of the CM and CW measurement.

Description

The invention relates to a time-of-flight camera system and a method for operating such a system according to the species of the independent claims.

Time-of-flight camera or time-of-flight camera system is intended to include systems that obtain distances from a phase shift of emitted and received radiation. PMD cameras with photomixing detectors (PMD) are particularly suitable as time-of-flight or 3D cameras, as they are used, for example, in DE 197 04 496 A1 are described.

A method for distance measurement by means of a light time-of-flight distance-measuring system having illumination and a time-of-flight detector is known, for example, from the patent specification DE 10 2014 210 750 B3 famous. A time-of-flight camera based on the photomixing element principle serves as the time-of-flight detector. In the method, starting from a basic PN sequence, a modulation signal for the illumination and for the time-of-flight detector is generated by means of a modulator. The basic PN sequence is output to the lighting and the time-of-flight camera in the form of sub-bit sequences that result according to a substitution rule.

The object of the invention is to improve the distance measurement of a time-of-flight camera system.

The object is advantageously achieved by the time-of-flight camera system according to the invention and the method according to the species of the independent claims.

• - wobei zur Entfernungsbestimmung die Beleuchtung und der Lichtlaufzeitsensor bzw. die Lichtlaufzeitpixel in Integrationsintervallen mit einem Modulationssignal beaufschlagt werden und eine Entfernung anhand der Ladungsdifferenzen an den Integrationsknoten ermittelt wird,
• - wobei eine CM-Messung mit wenigstens zwei CM-Integrationsintervallen vorgesehen ist und in diesen CM-Integrationsintervallen das Lichtlaufzeitkamerasystem mit jeweils unterschiedlich kodierten Modulationssignalen betrieben wird,
• - und eine CW-Messung mit wenigstens zwei CW-Integrationsintervallen vorgesehen ist und in diesen CW-Integrationsintervallen das Lichtlaufzeitkamerasystem mit jeweils unterschiedlichen Phasenlagen eines unkodierten, periodischen CW-Modulationssignals betrieben wird,
• - wobei aus den Ladungsdifferenzen der CM- und CW-Messung ein Entfernungswert bestimmt wird.

A method for operating a time-of-flight camera system is advantageously provided, having an illumination and a time-of-flight sensor with light-time-of-flight pixels, which have integration nodes for the accumulation of photogenerated charges,

• - Wherein to determine the distance, the lighting and the time-of-flight sensor or the light-time-of-flight pixels are acted upon by a modulation signal at integration intervals and a distance is determined on the basis of the charge differences at the integration nodes,
• - wherein a CM measurement is provided with at least two CM integration intervals and the time-of-flight camera system is operated with differently encoded modulation signals in these CM integration intervals,
• - and a CW measurement with at least two CW integration intervals is provided and in these CW integration intervals the time-of-flight camera system is operated with different phase angles of an uncoded, periodic CW modulation signal,
• - whereby a distance value is determined from the charge differences of the CM and CW measurement.

This procedure has the advantage that, for example, a distance range can be roughly determined in the CM measurement and the distance resolution can be increased in the CW measurement. Compared to a typical rough/fine measurement, the procedure according to the invention has the advantage that the CM measurement can be carried out significantly more quickly than usual rough distance measurements.

It is also provided for the CM measurement to divide a specified distance range into distance sectors, with a distance sector being able to be determined on the basis of the charge differences detected in the CM measurement, with the distance sector also being able to be determined on the basis of the signs of the detected charge differences or their binary representation.

This procedure has the advantage that for a rough determination of the distance, the usual time-consuming measurements with several phase angles of the modulation signal and a typical arctan calculation do not have to be carried out, but rather an at least rough distance can be determined from a sequence of the signs of the charge differences of the CM measurement is.

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

• 1 ein Lichtlaufzeitkamerasystem,
• 2 eine modulierte Integration erzeugter Ladungsträger,
• 3 einen Querschnitt durch einen PMD-Lichtlaufzeitsensor mit Potentialverteilung,
• 4 einen zeitlichen Verlauf der Integrationsspannungen an einem Lichtlaufzeitpixel,
• 5 Verläufe der Ladungsintegration abhängig von der Phasenverschiebung und -lage,
• 6 eine Relation der Phasenverschiebung in einem IQ-Diagramm,
• 7 einen Modulationsverlauf über vier Phasenlagen,
• 8 einen Verlauf von erfindungsgemäßen kodierten Modulationen (CM),
• 9 ein Modulationsschema und eine Einteilung eines Eindeutigkeitsbereiches in Sektoren A-P, die sich eindeutig über die CM Messung zuordnen lassen,
• 10 eine Modulation eines Pixel (links) sowie einer Beleuchtung (rechts) um das Modulationsschema aus 9 zu erhalten.
• 11 ein alternatives Modulationsschema mit erhöhtem Signalpegel und eine Einteilung eines Eindeutigkeitsbereiches in Sektoren A-P, die sich eindeutig über die CM Messung zuordnen lassen,
• 12 eine Modulation eines Pixel (links) sowie einer Beleuchtung (rechts) mit mehreren Pulsen um das Modulationsschema aus 11 zu erhalten.

They show schematically:

• 1 a time-of-flight camera system,
• 2 a modulated integration of generated charge carriers,
• 3 a cross-section through a PMD time-of-flight sensor with potential distribution,
• 4 a time course of the integration voltages at a light transit time pixel,
• 5 Courses of the charge integration depending on the phase shift and position,
• 6 a relation of the phase shift in an IQ diagram,
• 7 a modulation course over four phase angles,
• 8th a course of coded modulations (CM) according to the invention,
• 9 a modulation scheme and a division of a unique area into sectors AP, which can be clearly assigned via the CM measurement,
• 10 a modulation of a pixel (left) and a lighting (right) around the modulation scheme 9 to obtain.
• 11 an alternative modulation scheme with an increased signal level and a division of a unique area into sectors AP, which can be clearly assigned via the CM measurement,
• 12 a modulation of a pixel (left) and an illumination (right) with several pulses around the modulation scheme 11 to obtain.

In the following description of the preferred embodiments, the same reference symbols designate the same or comparable components.

1 shows a measurement situation for an optical distance measurement with a time-of-flight camera, such as that from DE 197 04 496 A1 is known.

The time-of-flight camera system 1 comprises a transmission unit or an illumination module 10 with an illumination 12 and associated beam-shaping optics 15 and a receiving unit or time-of-flight camera 20 with a receiving optics 25 and a time-of-flight sensor 22.

The time-of-flight sensor 22 has at least one transit-time pixel, preferably also 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 transmission unit 10 can be designed, for example, as a reflector or lens optics. In a very simple embodiment, it is also possible to dispense with optical elements both on the receiving 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 based on the phase shift of the emitted and received light. For this purpose, the light source 12 and the time-of-flight sensor 22 are supplied via a modulator 30 together with a specific modulation signal M _o with a basic phase position φ ₀ . In the example shown, a phase shifter 35 is also provided between the modulator 30 and the light source 12, with which 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 angles of φ _var =0°, 90°, 180°, 270° are preferably used.

According to the set modulation signal, the light source 12 emits an intensity-modulated signal S _p1 with the first phase position p1 or p1=φ ₀ +φ _var . In the case shown, this signal S _p1 or the electromagnetic radiation is reflected by an object 40 and, due to the distance covered, arrives as a received signal with a phase shift Δφ(t _L ) with a second phase position p2=φ ₀ +φ _var +Δφ(t _L ). Sp ₂ to the time-of-flight sensor 22. In the time-of-flight sensor 22, the modulation signal M _o is mixed with the received signal Sp ₂ , the phase shift or the object distance d being determined from the resulting signal.

The system also has a modulation control device 27 which changes the phase position φ _var of the modulation signal M ₀ depending on the measurement task at hand and/or sets the modulation frequency via a frequency oscillator 38 .

Infrared light-emitting diodes are preferably suitable as the illumination source or light source 12 . Other radiation sources in other frequency ranges are of course also conceivable, in particular light sources in the visible frequency range can also be considered.

The basic principle of phase measurement is shown schematically in 2 shown. The upper curve shows the time profile of the modulation signal M ₀ with which the lighting 12 and the time-of-flight sensor 22 are controlled. The light reflected from the object 40 hits the time-of-flight sensor 22 as a received signal S _p2 with a phase shift Δφ(t _L ) according to its time-of-flight t _L . The time-of-flight sensor 22 collects the photonically generated charges q over several modulation periods in the phase position of the modulation signal M ₀ in a first accumulation gate Ga and in a phase shifted by 180° M ₀ +180° in a second accumulation gate Gb. The phase shift Δφ(t _L ) and thus a distance d of the object can be determined from the ratio of the charges qa, qb collected in the first and second gate A or Ga and B or Gb.

3 shows a cross section through a pixel of a photomixing detector such as that shown in FIG DE 197 04 496 C2 is known. The modulation photogates Gam, G0, Gbm form the light-sensitive area of a PMD pixel. According to the voltage applied to the modulation gates Gam, G0, Gbm, the photonically generated charges q are directed either to one or to the other accumulation gate or integration node Ga, Gb. The integration nodes can be formed as a gate or as a diode.

3b shows a potential curve in which the charges q flow in the direction of the first integration account Ga, while the potential according to 3c allows the charge q to flow towards the second integration node Gb. The potentials are specified according to the modulation signals present. Depending on the application, the modulation frequencies are preferably in a range from 1 to 100 MHz. A modulation frequency of 1 MHz, for example, results in a period of one microsecond, so that the modulation potential changes accordingly every 500 nanoseconds.

In 3a A readout unit 400 is also shown, which may already be part of a PMD time-of-flight sensor designed as CMOS. The integration nodes Ga, Gb designed as capacitors or diodes integrate the photonically generated charges over a large number of modulation periods. In a known manner, the voltage then present at the gates Ga, Gb can be tapped at high resistance, for example via the readout unit 400 . The integration times should preferably be selected in such a way that the time-of-flight sensor or the integration nodes and/or the light-sensitive areas do not become saturated for the amount of light to be expected.

In 4 is a typical time profile of the voltage U _a , U _b present at the integration nodes Ga, Gb during a phase measurement. Starting from a positive reset voltage U _DRS present at the integration node after a reset, the voltage drops at both integration nodes Ga, Gb due to the accumulated photoelectrons. Depending on the phase shift Δφ(t _L ) of the received signal, the voltages at the integration nodes Ga, Gb drop to different degrees. At the end of the integration time t _int , the voltage U _a , U _b present at the integration nodes Ga, Gb is read out. The voltage difference ΔU of the two voltages U _a , U _b corresponds in a known manner to the difference Δq of the charge q accumulated at the integration nodes Ga, Gb. The integration time t _int is preferably dimensioned in such a way that no integration node Ga, Gb reaches its saturation potential Us during normal exposure. A so-called SBI circuit for signal compensation can also be provided for larger signal strengths. Such circuits are, for example, from DE 10 2004 016 626 A1 or DE 10 2005 056 774 A1 famous.

5a and 5b show curves of the normalized 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 Sp ₂ with different phase positions. the 5a shows a curve for an unshifted modulation phase M ₀ with a phase angle φ _var =0°.

If the signal Sp ₂ arrives without a phase shift, i.e. Δφ(t _L ) = 0°, for example if the transmission signal S _p1 is sent directly to the sensor, the phases of the modulation M _o and of the received signal Sp ₂ are identical, so that all generated charge carriers are detected phase-synchronously at the first integration node Ga and thus a maximum difference signal with Δq=1 is present.

As the phase shift increases, the charge accumulated at the first integration node Ga decreases and increases at the second integration node Gb. With a phase shift of Δφ(t _L ) = 90°, the charge carriers qa, qb are equally distributed at both integration nodes Ga, Gb and the charge difference is therefore zero and “-1” after a 180° phase shift. As the phase shift continues to increase, the charge at the first gate Ga increases again, so that the result is that the charge difference increases again, in order then to reach a maximum again at 360° or 0°.

Mathematically, this is a correlation function of the received signal Sp ₂ with the modulating signal M ₀ . $$q\left(\tau \right)={\int }_0{S}_{p2}\left(t-\tau \right)M_0\left(t\right)\mathrm{i.e}t$$

In the case of modulation with a square-wave signal, a triangular function results as a correlation function, as already shown. If modulated with a sine signal, for example, the result would be a cosine function.

how 5a shows, a measurement of the phase with a phase position is only clear up to a phase shift Δφ(t _L )≦180°.

The IQ (in-phase quadrature) method, for example, is known for maximum detection of the phase shift, in which two measurements are carried out with phase positions shifted by 90°, ie 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 5b shown.

The relationship between these two curves can be calculated in a known manner, for example for sinusoidal moderate curves in an IQ diagram acc. 6 represent. In a first approximation, this representation can also be used without further ado for the triangular functions shown.

The phase angle can then be determined in a known manner using an arctan function or arctan2 function: $$\phi =a\mathrm{right}ctan\frac{\Delta q\left(90°\right)}{\Delta q\left(0°\right)}$$

Due to the linear relationship between charge and voltage, the phase angle can also be determined via the voltage differences: $$\phi =a\mathrm{right}ctan\frac{\Delta u\left(90°\right)}{\Delta u\left(0°\right)}$$

In order to compensate, for example, for sensor asymmetries, additional phase measurements shifted by 180° can be carried out, so that the phase angle can be determined as follows. $$\phi =\Delta \phi \left(t_L\right)=a\mathrm{right}ctan\frac{\Delta q\left(90°\right)-\Delta q\left(270°\right)}{\Delta q\left(0°\right)-\Delta q\left(180°\right)}$$

Or in short: $$\phi =\Delta \phi \left(t_L\right)=a\mathrm{right}ctan\frac{a_2-a_4}{a_1-a_3}$$

Where the indices indicate the respective phase position of the differences a _i , with $$a_1=\Delta q\left(0°\right)\text{etc}.$$

From the phase shift φ or Δφ(t _L ), a distance can be determined in a known manner for object distances d that are less than half the wavelength λ of the modulation frequency d≦λ/2. $$\mathrm{i.e}=\Delta \phi \left(t_L\right)\frac{\mathrm{<figure-callout\; id="2"\; label="\lambda "\; filenames="DE102021117139A1\_0012.png"\; state="\{\{state\}\}">\lambda </figure-callout>}}{2\pi }\cdot \frac{1}{2}$$

In 7 a complete set of a distance measurement with four phase positions of 0°, 90°, 180° and 270° is shown. In the case shown, charge carriers are integrated over modulation periods and a value a ₁ , a ₂ , a ₃ , a ₄ corresponding to the charge difference is read out in each phase position, from which a phase shift and a corresponding distance value can be determined, as already shown.

In previous ToF systems, the 4-phase algorithm is used to determine the phase position within an unambiguous range. With this algorithm, four interpolation points are determined equidistantly in the unambiguity range from 0 to 2π with the same modulation frequency and then the phase position of the fundamental wave that runs through these interpolation points is calculated. The uniqueness range is converted from radians to a distance by a transformation factor.

Objects at distances that are multiples of this unambiguous range (0 to 2π) lead to ambiguities in the phase position. These ambiguities can be converted into an unambiguous phase position by a further phase measurement or by a renewed application of the 4-phase algorithm with a different, typically lower, modulation frequency. This procedure therefore implies at least 8 measurements before a clear distance value can be calculated.

According to the invention, it is now provided to combine the continuous wave CW measurements described above with a special coded modulation CM measurement.

The use of such pseudo-random binary sequences (PN sequences) in time-of-flight measurement systems is quite widespread, and the substitution of an underlying basic PN sequence by means of sub-bit sequences is also well known in connection with such measurement systems. The use of such pseudo-random binary sequences, which are also known as "pseudo-noise sequences", such as "Maximum Length Sequences" (MLS) so-called maximum sequences and "Barker codes" for the modulation of a time-of-flight sensor (depth image sensor) offer due to their advantageous auto-correlation properties decisive advantages over conventional modulation sequences in the form of simply periodic square-wave or sinusoidal signal sequences.

In indirect ToF, the distance noise and unambiguity range are inherently linked. Low noise always requires the use of high frequencies and thus always reduces the uniqueness range.

• - Je höher der gewünschte Eindeutigkeitsbereich-Gewinn ist, umso höher sind auch die Anforderungen an den benötigten Signalpegel, um Falschzuordnungen zu vermeiden.
• - Bei der Verwendung von zwei hohen Frequenzen ist der Grenz-Rauschpegel besonders ungünstig klein.
• - Bei der Verwendung von großen und kleinen Frequenzen ist das Rauschen wiederum nicht optimal.
• - Systematische Distanzfehler Fehler wie Streulicht oder Mehrwege-Reflexionen können ebenfalls Falschzuordnungen verursachen.
• - Der erreichbare Eindeutigkeitsbereich-Gewinn ist limitiert und skaliert schlecht!

Multi-frequency methods try to solve the problem, but they also have disadvantages:

• - The higher the desired unambiguous range gain, the higher the requirements for the required signal level in order to avoid incorrect assignments.
• - When using two high frequencies, the limit noise level is particularly unfavorably small.
• - When using large and small frequencies, the noise is again not optimal.
• - Systematic distance errors Errors such as scattered light or multipath reflections can also cause incorrect assignments.
• - The achievable uniqueness range gain is limited and scales poorly!

• - Die Pulsform / Flankensteilheit muss gut angepasst werden, damit ein einigermaßen homogenes Rauschverhalten über den Messbereich sichergestellt wird. Wenn zum Beispiel die Flanke zu steil sind, dann wird das Korrelationssignal in bestimmten Phasenlagen sich nur wenig (bzw. gar nicht ändern) was das Rauschen hochtreibt.
• - Insbesondere treten periodische Distanz/Phasen abhängige Fehler verstärkt auf und bedürfen besonderer Aufmerksamkeit
• - Die Pixel- / Beleuchtungs- Modulations-Schema sind je nach Messung unterschiedlich (unterschiedliche Frequenzen, unterschiedliche Puls-Muster). Das bedingt weitere Asymmetrien, die sich nur schwer korrigieren lassen und bei klassischen CW Messungen nicht auftauchen.

In the past, the use of coded modulation for distance determination has had an impact on possible masking properties or has also attempted to determine the exact distance directly. However, all CM approaches have so far suffered from the following problems:

• - The pulse shape / edge steepness must be well adjusted to ensure a reasonably homogeneous noise behavior over the measuring range. For example, if the flanks are too steep, the correlation signal will change only slightly (or not at all) in certain phase positions, which increases the noise.
• - In particular, periodic distance/phase-dependent errors occur more frequently and require special attention
• - The pixel / illumination modulation schemes are different depending on the measurement (different frequencies, different pulse patterns). This causes further asymmetries, which are difficult to correct and do not appear in classic CW measurements.

The combination of CM and CW outlined here combines advantages from both worlds.

A normal (e.g. 4-phase) CW measurement is carried out with a (possibly) high frequency. This results in a low and optimal distance noise. Using a four-phase measurement guarantees low asymmetries.

An additional CM measurement takes care of the problem of range mapping. This only addresses the allocation to different area sectors, but makes no further contribution to the measurement accuracy. In this respect, weaker claims apply here in terms of accuracy, stability, etc. This is a key point.

8th shows a possible approach according to the invention, in which different PN functions are used over the desired distance range. In contrast to CW modulation, in which a phase shift and from this a distance is typically determined by measuring the charge differences at different phase positions, the invention provides for the coded modulation (CM) only the charge differences at the integration nodes A, Ga, B, Gb to determine. In 8a and 8b two different coded modulations CM4, CM3 are shown as an example, which are repeated after a certain time. The period duration or the 'wavelengths' of the encoded modulations accordingly also represents a maximum resolvable distance range d _max .

For the evaluation according to the invention, it is sufficient to determine the sign sgn(x) of the charge difference Δq(CM) at the integration nodes A, B, Ga, Gb. Instead of the sign, a '+' can also be substituted with a '1' and a '-' with a '0' or, of course, vice versa. In the example shown, at a distance of d _max for CM4 there would be a '+' for CM3 a '-', so that this position or distance could also be marked with the binary number '10'.

According to the invention, it is provided as in 9 shown to divide the desired uniqueness range 0 to d _max into sectors. In the example shown, the maximum distance range d _max is divided into 16 distance sectors AP. With a suitable choice of the CM modulation pattern, the corresponding sector can be clearly assigned solely based on the signs of the CM measurements. This enables a purely binary evaluation of the CM results. For example, in 9 from the measurement (- + + -) or 0110 for the exemplary four CM measurements directly to sector J. The coded modulations and their number are selected in such a way that each distance sector AP has a clear binary assignment.

It is advantageous to use Gray codes because this ensures that only one measurement ever changes sign at the area boundaries and noise-induced sign changes in this area allow at most incorrect allocation to the neighboring area.

The determination of the sectors is now combined with a classic CW measurement (see 9 below), which determines the exact position of the object within the sector at a high frequency. This allows the low noise of a high-frequency CW measurement to be combined with the advantageous properties of the CM measurement in order to expand the unambiguity range.

The combination with the CW measurement then even makes it possible to recognize the wrong assignments at the sector boundaries and thus to always provide highly accurate stable distance data.

For example, it is advantageous to select the frequency of the CW measurement in such a way that the unambiguous range of the CW measurement is greater than one CM distance sector AP. The CW unambiguous range preferably covers at least two CM distance sectors. But it is sensibly smaller than the sum of all CM distance sectors. Through this choice of the CW unambiguous range, in particular, distances that lie on a border between two CM distance sectors AP can be unambiguously assigned to a distance sector.

So shows 9 a modulation scheme and a division of a unique area into sectors AP, which can be clearly assigned via the CM measurement. For example, in sector B, the expected phase of the CW measurement is in the range 0 - π. If the phase measurement is close to zero, you can conclude that you are on the border between sectors A and B.

In particular, it can be advantageous to use other error-correcting codes that further increase the stability of the measurement.

By increasing the number of CM measurements, the maximum measurement range/unambiguity range d _max can be increased as desired without affecting the accuracy of the CW measurement. With a suitable implementation, the uniqueness range can be doubled with each additional CM measurement without losing accuracy.

10 shows a modulation of a time-of-flight pixel in the left image and a corresponding modulation of the illumination in the right image, in accordance with the modulation scheme 9 to obtain. In the CW integration interval, the time-of-flight sensor and the lighting are operated with simple periodic signals.

With the coded CM modulation, on the other hand, it is provided that the modulation signals of the lighting deviate in duration and frequency from the modulation of the time-of-flight sensor in the CM integration intervals. In comparison to CW measurement, the lighting is typically operated with a lower number of pulses.

12 shows a modulation of a light transit time pixel in the left image and a corresponding modulation of the illumination with several pulses in the right image in order to comply with the modulation scheme 11 to obtain.

It can be advantageous through a suitable combination of illumination and PMD modulation (e.g 12 ) to generate correlation functions ( 11 ), which no longer correspond directly to classical Gray codes but contain a higher number of optical pulses. By considering the sign, one again obtains binary codes, so that the assignment of the sectors in 11 analogous to 9 happens, just with a different mapping table. The higher number of optical pulses is advantageous for better signal-to-noise behavior. So the signal level for modulation is off 12 on average 3 times higher than 9 .

• - Unterteilung des gewünschten Eindeutigkeitsbereiches in verschiedenen disjunkte Sektoren (siehe 9). Verwendung von CM Messungen, um eindeutig den Sektor festzustellen in dem sich das Testobjekt befindet. Da die Auswertung der Messungen rein binär erfolgt (Messwert positiv oder negativ) ergibt sich eine hohe Robustheit.
  • ◯ Insbesondere die Verwendung von Gray-Codes, da es in diesem Fall an den Bereichswechseln nicht zu großen Distanzsprüngen kommen kann.
  • ◯ Es können dabei auch Korrelations-Signale verwendet werden, die keinen einfachen Binär-codes entsprechen, dafür aber einen vorteilhaft höheren Signalpegel besitzen.
  • ◯ Die Verwendung von weiteren fehlerkorrigierenden und/oder fehlererkennenden Codes, um die Robustheit bei Bedarf weiter zu erhöhen.
• - Kombination der groben CM-Sektoren-Messung mit einer hochgenauen klassischen CW Messung. Diese ist für die genaue Lokalisierung des Objektes innerhalb des vorher bestimmten Sektors zuständig. Die Messfrequenz ist an die Länge der Sektoren angepasst, so dass auch für Objekte nahe der Sektorengrenze keine Mehrdeutigkeiten entstehen können. Die Verwendung von möglichst hohen Frequenzen ist vorteilhaft, um das Rauschen zu minimieren. Die Kombination der bestimmten Phasenlage der CW-Messung mit dem CM Binärsignal ermöglicht es auch an den Sektorengrenzen stabil eine hochgenaue Distanz zu bestimmen.
• - Eine Pixelausleseschaltung, die so ausgelegt ist, dass sie wahlweise eine klassische hochaufgelöste Analog-Digital Umwandlung durchführen kann oder auch eine einfache, schnelle und stromsparende Vorzeichen Erkennung (z.B. mittels Komparator)
• - Verwendung von kurzen Pulsen für die Beleuchtung bei den CM-Messungen, trotz eventueller langsamerer Modulationsfrequenz für das Pixel. Insbesondere kann es sinnvoll sein, gleichkurze optische Pulse wie bei der CW Messung zu verwenden. Dadurch ist sichergestellt, dass die Übergangsbereiche zwischen den Sektoren möglichst kurz sind, schnell der maximale Signalpegel aufgebaut wird und sich insbesondere die einzelnen Übergangsbereiche maximalen Abstand zu einander haben.
• - Optimierung der Integrationszeit von CW und CM Messungen, um sicherzustellen, dass zu einem der CM-Signalpegel stets ausreicht, um eine eindeutige Sektorenzuordnung zu treffen, zum anderen aber nicht zu groß ist, um nicht unnötig Energie zu verschwenden.

In principle, the following procedure is advantageous:

• - Subdivision of the desired uniqueness area into different disjoint sectors (see 9 ). Use of CM measurements to unequivocally determine the sector in which the test object is located. Since the evaluation of the measurements is purely binary (measured value positive or negative), there is a high level of robustness.
  • ◯ In particular, the use of Gray codes, as in this case large jumps in distance cannot occur at the area changes.
  • ◯ Correlation signals can also be used that do not correspond to simple binary codes but have an advantageously higher signal level.
  • ◯ The use of additional error-correcting and/or error-detecting codes to further increase robustness if necessary.
• - Combination of the rough CM sector measurement with a high-precision classic CW measurement. This is responsible for the exact localization of the object within the previously determined sector. The measuring frequency is adapted to the length of the sectors so that no ambiguities can arise, even for objects close to the sector boundary. The use of frequencies that are as high as possible is advantageous in order to minimize noise. The combination of the determined phase position of the CW measurement with the CM binary signal makes it possible to determine a highly accurate distance even at the sector boundaries.
• - A pixel readout circuit that is designed in such a way that it can either perform a classic high-resolution analog-digital conversion or a simple, fast and power-saving sign recognition (e.g. using a comparator)
• - Use of short pulses for illumination in CM measurements, despite possible slower modulation frequency for the pixel. In particular, it can be useful to use optical pulses of the same short length as in the CW measurement. This ensures that the transition areas between the sectors are as short as possible, the maximum signal level is built up quickly and, in particular, the individual transition areas are at maximum distance from one another.
• - Optimization of the integration time of CW and CM measurements to ensure that one of the CM signal levels is always sufficient to make a clear sector assignment, but is not too high on the other hand to avoid wasting energy unnecessarily.

Reference List

1

time-of-flight camera system

10

lighting module

12

lighting

20

Receiver, time-of-flight camera

22

time-of-flight sensor

27

evaluation unit

30

modulator

35

Phase shifter, lighting phase shifter

38

modulation controller

40

object

400

evaluation unit

φ, Δφ(tL)

runtime-related phase shift

φvar

phasing

φ0

base phase

M0

modulation signal

p1

first phase

p2

second phase

Sp1

Transmission signal with first phase

Sp2

Receive signal with second phase

ga, gb

integration node

Ua, Ub

Stresses at the integration nodes

ΔU

voltage difference

Δq

charge difference

i.e

object distance

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely 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

• DE 19704496 A1 [0002, 0013]
• DE 102014210750 B3 [0003]
• DE 19704496 C2 [0021]
• DE 102004016626 A1 [0024]
• DE 102005056774 A1 [0024]

Claims (8)

Method for operating a time-of-flight camera system having lighting and a time-of-flight sensor with time-of-flight pixels that have integration nodes (A, B, Ga, Gb) for the accumulation of photogenerated charges, - Wherein to determine the distance, the lighting and the time-of-flight sensor or the light-time-of-flight pixels are supplied with a modulation signal at integration intervals and a distance is determined on the basis of the charge differences at the integration nodes (A, B, Ga, Gb), - wherein a CM measurement is provided with at least two CM integration intervals and the time-of-flight camera system is operated with differently encoded modulation signals in these CM integration intervals, - and a CW measurement is provided with at least two CW integration intervals and the time-of-flight camera system is operated in these CW integration intervals with different phase angles of a periodic CW modulation signal, - whereby a distance value is determined from the charge differences of the CM and CW measurement. procedure after claim 1 , in which a predetermined distance range (dmax) is divided into distance sectors (A - P) for the CM measurement and a distance sector (A - P) can be determined on the basis of the charge differences detected in the CM measurement. procedure after claim 2 , in which the distance sector (A - P) can be determined using the sign of the recorded charge differences or their binary representation. procedure after claim 3 , in which a number of distance sectors, a number of coded modulation signals and integration intervals and/or the coding of the CM modulation signals is selected in such a way that for each distance sector (AP) there is a unique sign sequence of the charge differences, with which a distance sector (A - P) is clearly identifiable. procedure after claim 4 , in which the modulation signals are selected in such a way that the sign sequence of the charge differences or their binary representation with which the distance sectors (AP) can be determined corresponds to a Gray code. Procedure according to one of claims 2 until 5 , in which the frequency of the uncoded CW modulation signal is selected for the CW measurement in such a way that an unambiguous distance range of the CW modulation signal is smaller than the specified distance range (dmax) and greater than or equal to a length of at least two distance sectors (A - P) is. procedure after claim 6 , in which the range sector (AP) is determined in the CM measurement, and a CW range value is determined in the CW measurement, a distance being determined from the combination of range sector and CW range value. Time-of-flight camera system designed to carry out one of the aforementioned methods.

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