DE102013207652B4 - Time of flight camera system - Google Patents

Abstract

A method of operating a time of flight camera system, the system being operable with at least three modulation frequencies, comprising the steps of: a) determining a phase shift (φ) of an emitted and received signal (Sp1, Sp2) for a modulation frequency (f, f, f) in one Phase measurement cycle (PM, PM, ...), b) performing several phase measurement cycles (PM, PM, ...), wherein in at least three consecutive phase measurement cycles (PM, PM, ...) different modulation frequencies (f, f, f c) determination of two distance values (EP, EP, d) in a distance measurement cycle (M, M,...) on the basis of the phase shifts (φ, φ) determined in two successive phase measurement cycles (PM, PM), d) assignment of a probability value (W (EP), W (EP)) to each of the two distance values (EP, EP), e) determination of an object distance from a group of distance values (EP, EP) after a fixed number of distance measurement cycles the highest wah f) performing several distance measuring cycles (M, M, ...).

Description

The invention relates to a light transit time camera system and a method for operating such according to the preamble of the independent claims.

With the time of flight camera system, not only systems should be included which determine distances directly from the light transit time, but in particular also all the time of flight or 3D TOF camera systems which acquire transit time information from the phase shift of an emitted and received radiation. In particular, PMD cameras with photonic mixer detectors (PMD) are suitable as the light transit time or 3D TOF cameras, as described, inter alia, in the applications EP 1 777 747 A1 . US Pat. No. 6,587,186 B2 and also DE 197 04 496 A1 described and, for example, by the company, ifm electronic GmbH 'or' PMD Technologies GmbH 'as a frame grabber O3D or as CamCube to obtain. In particular, the PMD camera allows 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 should also encompass cameras or devices with at least one receiving pixel, such as, for example, the distance measuring device O1D of the Applicant.

For determining a distance or a corresponding phase shift of the reflected light is, as in the DE 197 04 496 A1 described in detail, mixed in the PMD sensor, the reflected light with the modulating signal. This mixture provides an in-phase signal (0 °) and a signal offset by 180 °, from which a distance can be determined in a known manner. To improve the quality of the distance measurement, it may be provided to shift the transmission modulation selectively by 90 °, 180 ° or 270 °, for example, and preferably to determine a phase angle of the reflected signal in relation to the transmitted signal by means of IQ (Inphase, Quadrature) demodulation. This approach is particularly useful for obtaining redundant information, for example, to compensate for various parasitic effects such as fixed pattern noise (FPN), backlight, or sensor asymmetries.

Furthermore, from the DE 10 2010 003 409 A1 a time-of-flight camera and a method of operating such, in which phase shifts are determined for at least three different modulation frequencies. To detect interferers, the differences of phase shifts of adjacent modulation frequencies are examined.

The object of the invention is to improve the phase measurement and thus the distance measurement of a light transit time camera system.

The object is achieved in an advantageous manner by the method according to the invention and the time of flight camera system according to the preamble of the independent claims.

1. a) Ermittlung einer Phasenverschiebung eines emittierten und empfangenen Signals für eine Modulationsfrequenz in einem Phasenmesszyklus,
2. b) Durchführung mehrerer Phasenmesszyklen, wobei in mindestens drei aufeinander folgenden Phasenmesszyklen unterschiedliche Modulationsfrequenzen verwendet werden,
3. c) Ermittlung von zwei Entfernungswerten ausgehend von zwei in aufeinander folgenden Phasenmesszyklen ermittelten Phasenverschiebungen,
4. d) Zuordnung eines Wahrscheinlichkeitswerts zu jedem der beiden Entfernungswerte
5. e) Bestimmung einer Objektentfernung aus einer Gruppe von Entfernungswerten (EP_A, EP_B), die nach einer festgelegten Anzahl von Distanzmesszyklen die höchsten Wahrscheinlichkeitswerte aufweist,
6. f) Durchführung mehrerer Distanzmesszyklen.

Advantageously, a method for operating a light transit time camera system is provided, wherein the system is operable with at least three modulation frequencies, comprising the steps:

1. a) determining a phase shift of an emitted and received signal for a modulation frequency in a phase measurement cycle,
2. b) performing a plurality of phase measurement cycles, wherein different modulation frequencies are used in at least three consecutive phase measurement cycles,
3. c) determination of two distance values on the basis of two phase shifts determined in successive phase measurement cycles,
4. d) assignment of a probability value to each of the two distance values
5. e) determination of an object distance from a group of distance values (EP _A , EP _B ) which has the highest probability values after a defined number of distance measurement cycles,
6. f) carrying out several distance measuring cycles.

This approach has the advantage that distance measurements with different modulation frequency pairings can reliably detect and correct possible misalignments of distances.

In particular, it is advantageous if distance values (EP _A , EP _B , d _{n, n + 1} ) are determined from at least three consecutive distance measuring cycles (M ₁ , M ₂ ,...) And divided into distance value groups and one for each distance value group Overall probability is determined.

To avoid accidental mismatches, it is convenient to perform a minimum number of distance measurements

Preferably, the determined distance values can be divided into distance value groups with the aid of tolerance limits.

Advantageously, a light transit time camera system is provided with a modulator which is connected to a lighting and a receiver of the time of flight camera system, wherein a modulation control device is connected to the modulator and is configured such that the modulator is operable with at least three modulation frequencies, and the one evaluation unit associated with the receiver and configured such that in each distance measuring cycle two distance values and a probability associated with these distance values are determined.

The invention will be explained in more detail by means of embodiments with reference to the drawings.

• 1 schematisch das grundlegende Prinzip der Photomischdetektion,
• 2 eine modulierte Integration der erzeugten Ladungsträger,
• 3 zwei zeitliche Verläufe der Ladungsintegration mit unterschiedlichen Phasenlagen,
• 4 Relation der Integration in einem IQ-Diagramm,
• 5 eine Distanzmessung mit einer Wellenlänge,
• 6 eine Distanzmessung mit zwei unterschiedlichen Wellenlängen,
• 7 einen Verlauf der Phasenverschiebungen mit dem Abstand,
• 8 einen Verlauf der Distanzwerte für unterschiedliche Wellenlängen,
• 9 bis 11 schematisch eine Entfernungsbestimmung für unterschiedliche Wellenlängen,
• 12 einen zeitlichen Ablauf der Distanzmessungen

Show it:

• 1 schematically the basic principle of photomix detection,
• 2 a modulated integration of the generated charge carriers,
• 3 two temporal courses of the charge integration with different phase positions,
• 4 Relation of integration in an IQ-diagram,
• 5 a distance measurement with one wavelength,
• 6 a distance measurement with two different wavelengths,
• 7 a course of the phase shifts with the distance,
• 8th a progression of the distance values for different wavelengths,
• 9 to 11 schematically a distance determination for different wavelengths,
• 12 a time sequence of the distance measurements

In the following description of the preferred embodiments, like reference characters designate like or similar components.

1 shows a measurement situation for an optical distance measurement with a light time camera, as for example from the DE 197 04 496 is known.

The light transit time camera system 1 comprises a transmitting unit or a lighting module 10 with a lighting 12 and associated beam shaping optics 15 as well as a receiving unit or light runtime camera 20 with a receiving optics 25 and a light transit time sensor 22 , The light transit time sensor 22 has at least one light-propagation time pixel, preferably a pixel array, and is designed in particular as a PMD sensor. The receiving optics 25 typically consists of improving the imaging characteristics of multiple optical elements. The beam shaping optics 15 the transmitting unit 10 may be formed for example as a reflector or lens optics. In a very simple embodiment, if necessary, optical elements can also be dispensed with both on the receiving side and on the transmitting side.

The measurement principle of this arrangement is essentially based on the fact that, based on the phase shift of the emitted and received light, the transit time and thus the distance covered by the received light can be determined. For this purpose, the light source 12 and the light transit time sensor 22 via a modulator 30 together with a certain modulation signal M _o with a base phase position φ ₀ applied. In the example shown is also between the modulator 30 and the light source 12 a phase shifter 35 provided with which the base phase φ ₀ the modulation signal M ₀ the light source 12 around defined phase positions φ _var can be moved. For typical phase measurements, preferably phase angles of φ _var = 0 °, 90 °, 180 °, 270 ° used.

The light source transmits according to the set modulation signal 12 an intensity modulated signal S _p1 with the first phase position p1 respectively. p1 = φ ₀ + φ _var out. In the case shown, this signal S _p1 or the electromagnetic radiation is reflected by an object 40 and hits due to the distance traveled Distance corresponding to phase-shifted Δφ (t _L ) with a second phase position p2 = φ ₀ + φ _var + Δφ (t _L ) as a received signal S _p2 to the Transit Time Sensor 22 , In the time of flight sensor 22 the modulation signal M _{o is} mixed with the received signal S _p2 , wherein the phase shift or the object distance d is determined from the resulting signal.

To improve the measurement accuracy and / or to expand the uniqueness range, it may also be provided to carry out light transit time measurements with different modulation frequencies. For this purpose, the modulator 30 with a modulation control unit 38 connected, which can preferably specify within a predetermined frequency spectrum modulation frequencies.

The modulator 30 could for example be designed as a frequency synthesizer, via the modulation control unit 38 is controlled for the respective measuring task.

Furthermore, the receiving unit 20 with an evaluation unit 27 connected. If necessary, the evaluation unit 27 can also be part of the receiving unit 20 and in particular also part of the light transit time sensor 22 be. Task of the evaluation unit 27 It is to determine based on the received signals in relation to the modulation frequency phase shifts and / or evaluate. The mixture of the received light beams with the modulation frequency is preferably carried out in the light transit time sensor 22 or PMD sensor. Furthermore, the modulation control unit 38 also part of the evaluation unit 27 be. In particular, it may also be provided that the evaluation unit 27 the function of the modulation control unit 38 completely or partially takes over.

As illumination source or light source 12 are preferably infrared light emitting diodes. Of course, other sources of radiation in other frequency ranges are also conceivable; in particular, light sources in the visible frequency range are also possible.

The basic principle of phase measurement is schematically in 2 shown. The upper curve shows the time course of the modulation signal M ₀ with the lighting 12 and the light transit time sensor 22 be controlled. The reflected light from the object 40 hits as a received signal S _p2 according to its light transit time t _L phase-shifted Δφ (t _L ) on the light transit time sensor 22 , The light transit time 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 angle shifted by 180 ° M ₀ + 180 ° in a second accumulation gate gb , From the ratio of the first and second gate ga . gb collected charges qa, qb can be the phase shift Δφ (t _L ) and thus determine a distance d of the object.

3a and 3b 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 ) the received light signal S _p2 with different phase positions. The 3a shows a course for a non-shifted modulation phase M ₀ with a phase angle φ _var = 0 °.

At a hitting the signal S _p2 without phase shift so Δφ (t _L ) = 0 °, for example, when the transmission signal S _{p1 is directed} directly to the sensor, the phases of the modulation M ₀ and the received signal S _{p2 are} identical, so that all the charge carriers generated phase-synchronously at the first gate ga are detected and thus a maximum difference signal with Δq = 1 is applied.

As the phase shift increases, the charge on the first accumulation gate decreases ga from and to the second accumulation gate gb to. At a phase shift of Δφ (t _L ) = 90 ° are the charge carriers qa, qb at both gates ga . gb equally distributed and the difference thus zero and after 180 ° phase shift "-1". As the phase shift increases, the charge on the first gate decreases ga again, so that as a result, the charge difference increases again, then at 360 ° or 0 ° again to reach a maximum.

Mathematically, this is a correlation function of the received signal S _p2 with the modulating signal M ₀ , $$q\left(\tau \right)={\displaystyle \underset{0}{\overset{\tau }{\int }}{S}_{p2}\left(t-\tau \right)M_0\left(t\right)dt}$$

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

As 3a shows, is a phase phase measurement only up to a phase shift Δφ (t _L ) ≤ 180 ° clearly.

For maximum detection of the phase shift, for example, the IQ (in-phase quadrature) method is known in which two measurements are performed with shifted by 90 ° phase angles, so for example with the phase angle φ _var = 0 ° and φ _var = 90 °. The result of a measurement with the phase angle φ _var = 90 ° is in 3b shown.

The relationship of these two curves can be in a known manner, for example, for sinusoidal waveforms in an IQ diagram gem. 4 represent. In a first approximation, this representation is readily applicable to the triangular functions shown.

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

In order to compensate, for example, for asymmetry of the sensor, additional phase measurements shifted by 180 ° can be performed so that, as a result, the phase angle can be determined as follows. $$\phi =\text{arctan}\frac{\Delta q\left(90\text{°}\right)-\Delta q\left(270\text{°}\right)}{\Delta q\left(0\text{°}\right)-\Delta q\left(180\text{°}\right)}$$

From the in 2 shown runtime-related phase shift Δφ (t _L ) can be determined for object distances d, which are smaller than half the wavelength λ of the modulation frequency d ≦ λ / 2 in a known manner a distance. $$d=\Delta \phi \left(t_L\right)\frac{\mathrm{<figure-callout\; id="2"\; label="\lambda "\; filenames="DE102013207652B4\_0019.png,DE102013207652B4\_0020.png"\; state="\{\{state\}\}">\lambda </figure-callout>}}{2\pi }\cdot \frac{1}{2}$$

For distances d> λ / 2, there is generally no possibility of absolutely measuring the phase shift, so that the determined phase shift can no longer be unambiguously assigned to a distance value.

aufweist, wobei selbstverständlich die bis zum Empfänger 20 zurückgelegt Wegstrecke doppelt so groß ist, nämlich D = 2d = 4λ + R Zur Erhöhung des Eindeutigkeitsbereichs ist es, wie in 6 schematisch dargestellt, vorgesehen, mit mindestens zwei Modulationsfrequenzen respektive Modulationswellenlängen eine Objektabstand d zu bestimmen. Der Einfachheit halber ist in 6 die Gesamtstrecke D zwischen Sender 10 und Empfänger 20 dargestellt. Innerhalb des Eindeutigkeitsbereichs der beiden Wellenlängen λ₁, λ₂, der typischerweise durch das kleinste gemeinsame Vielfache der Wellenlängen λ₁, λ₂ aufgespannt wird, gilt folgende Distanzgleichung: $$D=2d=n_1{\lambda }_1+R_1=n_2{\lambda }_2+R_2$$

mit $$R_i=D\cdot \left(\text{mod}{\lambda }_i\right)={\phi }_i\left(f_i\text{,}D\right)\frac{{\lambda }_i}{2\pi }$$

wobei für die von der Modulationsfrequenz und dem Objektabstand abhängige relative Phasenverschiebung φ_i(f_i,D) gilt: $${\phi }_i=\frac{D\cdot f_i}{c}\cdot 2\pi \left(\text{mod 2}\pi \right)=\frac{D}{{\lambda }_i}\cdot 2\pi \left(\text{mod }2\pi \right)$$

5 shows an example in which the object 40 is a distance d from the transmitter 10 from $d=2\lambda +\frac{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="DE102013207652B4\_0019.png,DE102013207652B4\_0020.png"\; state="\{\{state\}\}">R</figure-callout>}}{2}$

which, of course, to the receiver 20 distance traveled is twice as large, namely D = 2d = 4λ + R To increase the uniqueness range, it is as in 6 shown schematically, provided to determine an object distance d with at least two modulation frequencies respectively modulation wavelengths. For the sake of simplicity, is in 6 the total distance D between transmitters 10 and receiver 20 shown. Within the uniqueness range of the two wavelengths λ ₁ , λ ₂ , which is typically spanned by the smallest common multiple of the wavelengths λ ₁ , λ ₂ , the following equation applies: $$D=2d={\mathrm{<figure-callout\; id="1"\; label="n"\; filenames="DE102013207652B4\_0016.png,DE102013207652B4\_0019.png"\; state="\{\{state\}\}">n</figure-callout>}}_1{\mathrm{<figure-callout\; id="1"\; label="\lambda "\; filenames="DE102013207652B4\_0016.png,DE102013207652B4\_0019.png"\; state="\{\{state\}\}">\lambda </figure-callout>}}_1+{\mathrm{<figure-callout\; id="1"\; label="R"\; filenames="DE102013207652B4\_0016.png,DE102013207652B4\_0019.png"\; state="\{\{state\}\}">R</figure-callout>}}_1={\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="DE102013207652B4\_0019.png,DE102013207652B4\_0020.png"\; state="\{\{state\}\}">n</figure-callout>}}_2{\mathrm{<figure-callout\; id="2"\; label="\lambda "\; filenames="DE102013207652B4\_0019.png,DE102013207652B4\_0020.png"\; state="\{\{state\}\}">\lambda </figure-callout>}}_2+{\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="DE102013207652B4\_0019.png,DE102013207652B4\_0020.png"\; state="\{\{state\}\}">R</figure-callout>}}_2$$

With $$R_i=D\cdot \left(\text{mod}{\lambda }_i\right)={\phi }_i\left(f_i\text{.}D\right)\frac{{\mathrm{<figure-callout\; id="2"\; label="\lambda \; i"\; filenames="DE102013207652B4\_0019.png,DE102013207652B4\_0020.png"\; state="\{\{state\}\}">\lambda \; </figure-callout>}}_i}{2\pi }$$

wherein for the relative to the modulation frequency and the object distance dependent relative phase shift φ _i (f _i , D) applies: $${\phi }_i=\frac{D\cdot f_i}{c}\cdot 2\pi \left(\text{<figure-callout id="2" label="mod" filenames="DE102013207652B4\_0019.png,DE102013207652B4\_0020.png" state="{{state}}">mod</figure-callout> 2}\pi \right)=\frac{D}{{\lambda }_i}\cdot 2\pi \left(\text{<figure-callout id="2" label="mod" filenames="DE102013207652B4\_0019.png,DE102013207652B4\_0020.png" state="{{state}}">mod</figure-callout>}2\pi \right)$$

The relative phase shift φ _i (f _i , D) is thus a measure of the remainder piece R _i remaining in the distance measurement. For the distance determination, a solution for the above-described distance equation can now be found with two phase shifts φ _1/2 (f _1/2 , D) detected for different modulation frequencies f ₁ , f ₂ .

One possible solution is in 7 shown schematically. The 7 shows two relative phase shift φ _1/2 (f _1/2 , D) as a function of the double object distance 2d = total path length D for two different frequencies f ₁ , f ₂ . With a solid line is the phase shift φ ₁ for f ₁ = 7.5 MHz corresponding to a wavelength λ ₁ = 40 m and shown with a dashed line for f ₂ = 5 MHz corresponding to a wavelength λ ₂ = 60 m. The unambiguity range EB for the two frequencies results in a known manner from the smallest common multiple of the two wavelengths A ₁ , λ ₂ , ie here 120 m.

For each distance value or total path length D within the common unambiguous region EB, there is exactly one phase difference pair (φ ₁ , (φ ₂ ).) For the exemplary distance value of 23 m, a phase difference pair of approx. 1 2 | 0.8).

In one possible application example, it could be provided, for example, to store a suitable number of phase difference pairs with their associated distance value d in a value table. In the case of a distance measurement, it can then be determined, for example, which tabulated phase difference pair comes closest to the determined phase difference pair with a correspondingly assigned distance value.

8th corresponds to the illustration according to 7 with the difference that the length of the respective remaining piece is plotted on the y-axis with: $$R_i=D\cdot \left(\text{mod}{\lambda }_i\right)={\phi }_i\left(f_i\text{.}D\right)\frac{{\mathrm{<figure-callout\; id="2"\; label="\lambda \; i"\; filenames="DE102013207652B4\_0019.png,DE102013207652B4\_0020.png"\; state="\{\{state\}\}">\lambda \; </figure-callout>}}_i}{2\pi }$$

In the example shown, up to a total path length D, which corresponds to the smallest wavelength, ie here 40 m, both residual pieces are the same length. For a total distance D of, for example, 70 m, however, the remaining pieces are of different sizes.

The in the 7 and 8th shown relationship of phase and distance can be advantageous in a so-called modulo diagram according to the 9 to 11 represent. The phase values φ ₁ and φ ₂ for a first and second modulation frequency are shown on the x and y axes, and the distance values or remaining value d ₁ , d ₂ corresponding to the phase values are shown on the secondary x and y axes. As already mentioned, only a single pair of phase values exists for a removal value within the uniqueness range.

9 shows a modulo diagram for the frequencies f ₁ = 7.5 MHz corresponding to a wavelength λ ₁ = 40 m and f ₂ = 5 MHz corresponding to a wavelength λ ₂ = 60 m as in 7 and 8th shown. The curve starts with the phase difference pair (0 | 0) for D = 0. If the total path length of the light reflected by the object reaches the wavelength λ ₁ = 40 m, the phase value of the first modulation frequency also reaches its maximum value, namely 2π with the phase value pair (2 | 1,35). As the distance increases, the curve always jumps at the points or total path lengths at which one of the two phase values passes through a 2π value.

A distance can be determined, for example, by assigning a determined phase value pair to a distance point of the distance curve. Im in 9 is shown case an example of a measured phase value pair with (1.05 0.85) drawn. This phase value pair is not on the distance curve and is now assigned to a distance point on one of the two curve sections. The distance d _AB between the curve sections is known. For the assignment, it is therefore sufficient to determine the distance d _A , d _B to one of the two curve sections. The object distance is then determined from the closest distance point. In the case shown, the phase value pair can be assigned a distance of 23 m.

Furthermore, a probability can be determined from the distance of the measured phase value from the curve section. For example, according to the following relation: $$W\left(e{P}_{A\text{/}B}\right)=1-\frac{d_{A\text{/}B}}{d_{AB}}$$

The probability W (EP _A ) for the determined distance point is 1 if the measured phase value pair is on the curve section A and corresponding to zero if the phase value pair lies on the opposite section B. The consideration for the opposite distance point EP _{B is} calculated accordingly.

In the present example the probability for the distance point EP _A = 23 is calculated as 77% and for EP _B = 93 accordingly as 23%.

To increase the measurement accuracy and reliability, this distance assignment is repeated with further different modulation frequency pairings.

In 10 a modulo diagram for the frequency pairing f ₁ = 7.5 MHz corresponding to a wavelength λ ₁ = 40 m and f ₂ = 6 MHz corresponding to a wavelength λ ₂ = 50 m is shown. The phase value pair determined with this frequency pairing can be assigned to a distance point EP _A of 23.4 m with a probability of 80% and EP _B = 178.3 with W (EP _B ) = 20%.

In 11 is a modulo diagram for the frequency pairing f ₁ = 6 MHz corresponding to a wavelength λ ₁ = 50 m and f ₂ = 5 MHz corresponding to a wavelength λ ₂ = 60 m. The phase value pair determined with this frequency pairing can be assigned to a distance point EP _A of 22.9 m with a probability of 75% and EP _B = 267.9 with W (EP _B ) = 25%.

The results of all three measurements can be displayed in a table as follows: EP _A W (EP _A ) EP _B W (EP _B ) 23 m 77% 93 m 23% 23.4 m 80% 178.3 m 20% 22.9 m 75% 267.9 m 25%

gem. Beispiel $d_{mean}=\frac{23+\mathrm{23,4}+\mathrm{22,}\text{9}}{3}=\mathrm{23,1}$

In the present case, the selection of a plausible distance value is easy to make, for example by averaging the last three distance values with the highest probability: $d_{mean}=\frac{{\mathrm{<figure-callout\; id="1"\; label="d"\; filenames="DE102013207652B4\_0016.png,DE102013207652B4\_0019.png"\; state="\{\{state\}\}">d</figure-callout>}}_1+{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="DE102013207652B4\_0019.png,DE102013207652B4\_0020.png"\; state="\{\{state\}\}">d</figure-callout>}}_2+d_3}{3}$

gem. example $d_{mean}=\frac{23+23.4+22\text{9}}{3}=23.1$

gem. Beispiel $d_{mean}=\frac{23\cdot \mathrm{0,77}+\mathrm{23,4}\cdot \mathrm{0,8}+\mathrm{22,9}\cdot \mathrm{0,75}}{\mathrm{0,77}+\mathrm{0,8}+\mathrm{0,75}}=\mathrm{23,11}$

Likewise, it is conceivable to weight the determined distance values according to the probabilities: $d_{mean}=\frac{{\displaystyle \underset{i=1}{\overset{n}{\Sigma }}e{P}_{A\text{.}i}\cdot W_i}}{{\displaystyle \underset{i=1}{\overset{n}{\Sigma }}{W}_i}}$

gem. example $d_{mean}=\frac{23\cdot 0.77+23.4\cdot 0.8+22.9\cdot 0.75}{0.77+0.8+0.75}=\mathrm{23,11}$

12 shows by way of example a temporal sequence of a distance measurement according to the invention for different frequency pairings, in which the relative phase shift φ _i (f _i , D) for each Modulation frequency f _i with four phase angles φ _var = 0 °, 90 °, 180 °, 270 ° is performed. Of course, distance measurements with less and possibly more phase angles are conceivable.

In a first phase measuring cycle PM _1, a first phase shift for a first modulation frequency f ₁ φ ₁ and for the subsequent phase measuring cycles PM _2/3 for a second and third modulation frequency f _2, f ₃ a second and third phase shift φ _2, φ ₃ determined. After the third phase measurement cycle PM ₃ , the phase measurements start again at the first modulation frequency f ₁ and so forth. With more than three modulation frequencies, other frequency orders, in particular also random sequences, can be selected.

Two consecutive phase measurement cycles PM _n , _{n + 1} form a distance measurement cycle M _n from which a phase value pair (φ _n , φ _{n + 1} ) and a distance value d _n , _{n + 1} assigned to this pair is determined. According to the invention, not only a distance value is determined for each phase value pair but the two distance points EP _A , EP _B closest to the phase value pair on the next curve sections in the modulo diagram.

In a further embodiment, it is provided according to the invention that a distance value d is only considered valid if, in successive distance measuring cycles, at least three distance values lie with a high probability within an accepted tolerance limit Δd _tol .

The procedure according to the invention is illustrated in the following table: Distance measurement cycle 1 2 3 distance ₁₂ d d ₂₃ d ₃₁ Distance value EP _A 46 65 25 Prob. W 0.55 0.6 0.75 Distance value EP _A 24 23 66 Prob. W 0.45 0.4 0.25

With a tolerance limit Δd _tol of 5 m, the distance values can be grouped into three groups: Group 1 : "23, 24, 25 m" agree within the specified tolerance limit, with a cumulative probability of 1.6; Group 2: "65 and 66 m" with a cumulated probability of 0.85. Group 3 has only one distance value, namely 46 m with a probability of 0.55.

For the determination of the distance, the three distance values are now used which correspond within predefined tolerance limits and have the highest overall probability.

For the output of a valid distance value, as shown, the distance values can be combined into an average value. In a further embodiment, it is also conceivable to output the last distance value of the most probable distance group.

The averaging and outputting of the distance values preferably takes place in a sliding manner, so that only a specific number of distance values is always considered, for example the last three or four distance values. Distance measurement cycle 1 2 3 4 5 distance d ₁₂ d ₂₃ d ₃₁ d ₁₂ d ₂₃ Distance value EP _A 46 65 25 30 23 Prob. W 0.55 0.6 0.75 0.4 0.4 Distance value EP _A 24 23 66 60 65 Prob. W 0.45 0.4 0.25 0.6 0.6 ΣW 3-group 1 (about 23 m) 0.45 0.85 1.6 1.55 1.05 ΣW 3-group 2 (about 60 m) 0 0.6 0.85 1.45 1.45 Distance issue - - 25 30 65

In the table shown, the last distance value of a 3-distance group with the highest total probability is always output. In the first two measuring cycles, there is still no group of 3, so far no valid distance value is output. In the third measurement cycle, there are already 3 measurements in the 23 m range, which also have a high probability. The last distance value of 25 m is output as valid.

In the fourth measurement below, the distance value measured at 60 m probably has a high probability of 0.6, but the 60 m group of the last 3 measurements has an overall probability of 1.45, while the 23 m group still has a total probability of 1.55. Thus, the last value of the group with a distance of 30 m is output here.

In the fifth measurement, the total probability of the 60 m group is highest, so that now the last value of this group is output with 65 m.

The tolerance limit Δd _tol is preferably determined as a function of the respective measurement situation. For example, in the case of a moving camera, the tolerance limit must be set such that the tolerance range is not already left by the camera movement or proper movement of the objects.

If, for example, the camera is moved at a maximum speed of 10 m / s and a detection rate of 1/50 s is assumed for each phase measurement, each distance point between two measurements shifts by 0.2 m. That for three consecutive measurements, a distance point shifts by 0.6 m due to camera movement alone. For the determination of the tolerance limit, it would thus be advantageous preferably to take account of an inherent camera movement, if necessary also an expected object movement and possible measurement errors. In the present example, for example, the tolerance limit could be set or extended to 1.2 m.

For example, the distance jump to the neighboring family of curves in the modulo diagram can be used as the uppermost limit for defining a tolerance limit. In the case under consideration, the distance values of adjacent curves jump by a minimum of 20 m, so that the tolerance limit should be less than 20 m for a clear distance selection.

LIST OF REFERENCE NUMBERS

10

lighting module

12

lighting

22

Transit Time Sensor

27

evaluation

30

modulator

35

Phase shifter, lighting phase shifter

38

Modulation controller

Δφ (t _L )

term-related phase shift

φ _var

phasing

φ ₀

base phase

M ₀

modulation signal

p1

first phase

p2

second phase

Sp1

Transmission signal with first phase

sp2

Received signal with second phase

Ga, Gb

accumulation gates

Ua, Ub

Voltages at the modulation gate

Claims (6)

A method of operating a time of flight camera system, the system being operable with at least three modulation frequencies, comprising the steps of: a) determining a phase shift (φ _i ) of an emitted and received signal (Sp1, Sp2) for a modulation frequency (f ₁ , f ₂ , f ₃ ) in a phase measurement cycle (PM ₁ , PM ₂ ,...), B) performing a plurality of phase measurement cycles (PM ₁ , PM ₂ ,...), Wherein in at least three consecutive phase measurement cycles (PM ₁ , PM ₂ , .. .) Different modulation frequencies (f ₁ , f ₂ , f ₃ ) are used, c) determination of two distance values (EP _A , EP _B , d _{n, n + 1} ) in a distance measuring cycle (M ₁ , M ₂ , ... ) based on the in two consecutive phase measurement cycles (PM _n , PM _{n + 1} ) determined phase shifts (φ _n , φ _{n + 1} ), d) assignment of a probability value (W (EP _A ), W (EP _B )) to each the two distance values (EP _A , EP _B ), e) determination of an object distance from a group of distances values (EP _A EP _B) having the highest probability values after a set number of distance measuring cycles, f) performing a plurality of distance measurement cycles (M _1, M _2, ...). Method according to Claim 1 in which the distance values (EP _A , EP _B , d _{n, n + 1} ) determined from at least three successive distance measuring cycles (M ₁ , M ₂ ,...) are divided into distance value groups and a total probability is determined for each distance value group. Method according to Claim 1 or 2 in which a valid distance value is determined on the basis of the distance value group with the highest overall probability. Method according to one of the preceding claims, in which the distance value groups are formed with the aid of tolerance limits (Δd _tol ). A light transit time camera system (1) having a modulator (30) connected to a lighting (10) and a receiver (20) of the time of flight camera system (1), characterized in that a modulation control device (38) is connected to the modulator (30) and is configured such that the modulator (30) with at least three modulation frequencies (f ₁ , f ₂ , f ₃ ) is operable, and the one evaluation unit (27) associated with the receiver (20) and configured such that in each distance measuring cycle two Distance values (EP _A , EP _B ) and a probability associated with these distance values (EP _A , EP _B ) (W (EP _A ), W (EP _B )) is determined. Light time camera system according to Claim 5 for carrying out a method according to any one of Claims 1 to 4 is trained.

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