DE102013207648A1 - Method of operating light-time camera system such as three dimensional-time of flight camera system, involves determining phase shift for further modulation frequency, and determining distance value from last two measured phase shifts - Google Patents

Abstract

The method involves defining two modulation frequencies by which distance determination is carried out and determining a phase shift of an emitted or received signal for each of the two selected modulation frequencies, while determining a distance value (d) using the measured phase shifts. A further modulation frequency is determined based on predetermined selection rules, taking into account the last two modulation frequencies. A further phase shift is determined for further modulation frequency. The distance value is determined from last two measured phase shifts. An independent claim is included for a light-time camera system.

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 , US 6,587,186 and also DE 197 04 496 described and, for example, by the company 'ifm electronic GmbH' or 'PMD Technologies GmbH' as a frame grabber O3D or as CamCube relate. 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 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.

The object of the invention is to improve the phase measurement and thus the distance measurement.

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

• a) Festlegung von zwei Modulationsfrequenzen anhand derer eine Entfernungsbestimmung erfolgen soll,
• b) Ermittlung einer Phasenverschiebung eines emittierten und empfangenen Signals für jede der beiden ausgewählten Modulationsfrequenzen,
• c) Ermittlung eines Entfernungswerts anhand der beiden ermittelten Phasenverschiebungen,
• d) Bestimmung einer weiteren Modulationsfrequenz anhand vorgegebener Auswahlregeln unter Berücksichtigung der beiden letzten Modulationsfrequenzen
• e) Ermittlung einer weiteren Phasenverschiebung für die weitere Modulationsfrequenz
• f) Ermittlung eines Entfernungswerts anhand der letzten beiden ermittelten Phasenverschiebungen,
• g) Durchführung weiterer Entfernungsmessungen anhand der Schritte d) bis g)

Advantageously, a method for operating a light transit time camera system is provided, which is operable with a plurality of modulation frequencies, comprising the steps:

• a) Definition of two modulation frequencies on the basis of which a distance determination is to take place,
• b) determining a phase shift of an emitted and received signal for each of the two selected modulation frequencies,
• c) determination of a distance value on the basis of the two determined phase shifts,
• d) Determining a further modulation frequency based on predetermined selection rules taking into account the last two modulation frequencies
• e) determination of a further phase shift for the further modulation frequency
• f) determination of a distance value based on the last two determined phase shifts,
• g) Carrying out further distance measurements on the basis of steps d) to g)

This approach has the advantage that the distance measurements do not necessarily have to be made according to a fixed frequency pattern, but the selection of the modulation frequencies can essentially be done at random. To avoid unfavorable frequency pairings, it is particularly advantageous to make the choice of the modulation frequencies randomly but according to specific selection rules.

In a further refinement, it is provided to provide a predetermined number of modulation frequencies as a limited selection option, wherein the determination of a further modulation frequency according to step d) of the method according to the invention takes place within the provided selection of modulation frequencies.

Due to the limited choice, it is advantageously possible to shorten the time for selecting a suitable modulation frequency and thus allows faster measurement cycles. In addition, a limited choice also reduces the calibration effort.

It is particularly advantageous if the determination of the further modulation frequency takes place randomly taking into account the selection rules.

Preferably, it is useful to select a plurality of the modulation frequencies as being prime and alien due to the knowledge that divisive frequency ratios often form suitable frequency pairings.

In a further embodiment, it is advantageous to evaluate a distance value as valid only if two of the distance values determined immediately before lie within tolerated deviation limits. As a result of this procedure, jumps in the distance measurement due to distance mismatches can be detected and hidden in a simple manner.

The deviation limits can be determined, for example, as a function of the last determined distance value.

Particularly advantageously, a light transit time camera system is provided, with a modulator, which is connected to a lighting and a receiver of the system, wherein a modulation control device is connected to the modulator and configured such that the modulator is operable with at least three modulation frequencies, and that an evaluation unit such is configured that modulation frequencies are selected for a distance measurement using predetermined selection rules

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

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 cross-section of a PMD pixel with a spatial potential distribution,

6 a distance measurement with one wavelength,

7 a distance measurement with two different wavelengths,

8th a course of the phase shifts with the distance,

9 a distance determination according to the invention,

10 schematically a scheme of a selection structure,

11 an example of a possible flowchart according to the invention,

12 an example with unfavorable frequency pairings,

13 an example with a recursive step

14 an inventive flowchart,

15 a disturbance of the modulation with a signal of the same frequency,

16 a disturbance of the modulation with a signal of unequal frequency.

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 the base phase φ _{0 of} the modulation signal M _{0 of} the light source 12 can be moved by defined phase positions φ _var . For typical phase measurements, phase positions of φ _var = 0 °, 90 °, 180 °, 270 ° are preferably used.

The light source transmits according to the set modulation signal 12 an intensity-modulated signal S _p1 with the first phase position p1 or p1 = φ ₀ + φ _var . This signal S _p1 or the electromagnetic radiation is in the illustrated case of an object 40 reflects and hits due to the distance traveled corresponding 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 light 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. The evaluation unit 27 may 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 profile of the modulation signal M ₀ with the illumination 12 and the light transit time sensor 22 be controlled. The object 40 Reflected light impinges on the light transit time sensor as received signal S _{p2 in} accordance with its light transit time t _L phase-shifted Δφ (t _L ) 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 180 ° shifted phase position M ₀ + 180 ° in a second accumulation Gb. From the ratio of the charges qa, qb collected in the first and second gate Ga, Gb, the phase shift Δφ (t _L ) and thus a distance d of the object can be determined.

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 ) of the received light signal S _p2 with different phase angles. The 3a shows a curve for an unshifted modulation phase M ₀ with a phase angle φ _var = 0 °

When the signal S _{p2 strikes} without a phase shift, ie 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 of the received signal S _{p2 are} identical, so that all generated charge carriers are detected synchronously at the first gate Ga and thus a maximum difference signal with Δq = 1 is applied.

As the phase shift increases, the charge on the first accumulation gate Ga decreases and on the second accumulation gate Gb. With a phase shift of Δφ (t _L ) = 90 °, the charge carriers qa, qb are equally distributed at both gates Ga, Gb and the difference is thus zero and after 180 ° phase shift "-1". With 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 function of the received signal S _p2 with the modulating signal M ₀ .

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, a phase phase measurement is unique 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 performed with shifted by 90 ° phase angles, so for example with the phase 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 or arctan2 function: φ = arctan Δq (90 °) / Δq (0 °)

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. φ = arctan Δq (90 °) - Δq (270 °) / Δ (0 °) - Δq (180 °)

5 shows a cross section through a pixel of a photonic mixer as it is for example from the DE 197 04 496 C2 is known. The modulation photographs yarn, G0, Gbm are substantially transparent and arranged in a light-sensitive area of a light transit time or 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 the other accumulation gate Ga, Gb. Here, "accumulation gate" generally means a structure in which the charge accumulation takes place, not necessarily a gate of a MOS (Metal-Oxide-Semiconductor) structure. As a specific embodiment, a diode structure is preferably used.

5b shows a potential curve in which the charges q flow in the direction of the first accumulation gate Ga, and 5c a complementary potential profile, which allows the charges q to flow in the direction of the second accumulation gate Gb. The potentials can be specified, for example, according to the applied modulation signal. Depending on the application, the frequency of the modulation signal is preferably in a range of 1 to 1000 MHz. At a modulation frequency of For example, 1 MHz results in a period of one microsecond, so that the modulation potential changes accordingly every 500 nanoseconds.

In 5a is also a readout device 400 shown, which may already be part of a designed as a CMOS light transit time sensor 22 or PMD sensor can be. The accumulation gates Ga, Gb designed as capacitances integrate the photonically generated charges over a large number of modulation periods. In a known manner, the voltage Ua, Ub which is then applied to the accumulation gates Ga, Gb can be determined, for example, via the read-out device 400 be tapped high impedance. The integration times are preferably to be selected so that the light transit time sensor or the accumulation gates and / or the light-sensitive areas do not saturate for the expected amount of light.

From the in 2 shown runtime-dependent phase shift Δφ (t _L ) can be for object distances d ≤ λ / 2 in a known manner to determine a distance. d = Δφ (t _L ) λ / 2π × 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.

6 shows an example where the object 40 a distance d from the transmitter 10 from d = 2λ + R / 2 which, of course, to the receiver 20 traveled distance is twice as large, namely D = 2d = 4λ + R

wobei für die von der Modulationsfrequenz und dem Objektabstand abhängige relative Phasenverschiebung φ_i(f_i, D) gilt:

To increase the uniqueness range, it is as in 7 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 7 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 = n ₁ λ ₁ + R ₁ = n ₂ λ ₂ + R ₂ With

wherein for the relative to the modulation frequency and the object distance dependent relative phase shift φ _i (f _i , D) applies:

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 8th shown schematically. The 8th shows two relative phase shift φ _1/2 (f _1/2 , D) as a function of the double object distance 2d = 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 uniqueness range EB for the two frequencies results in a known manner from the smallest common multiple of the two wavelength λ ₁ , λ ₂ , so here 120 m.

For each distance value D within the common unambiguous range 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 approximately (1.2 / 0.8) results.

In one possible application, 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 is then possible to determine, for example, which tabulated phase difference pair comes closest to the determined phase difference pair. The associated distance value can then simply be taken from the value table.

9 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 positions 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, it is provided here that a distance value d is only considered valid if essentially the same distance value is determined within three consecutive distance measuring cycles within tolerated limits.

A possible procedure according to the invention is illustrated by way of example in the following table: Distance measurement cycle 1 2 3 4 5 6 7 8th 9 EB 120 200 300 120 200 300 120 200 300 distance d ₁₂ d ₂₃ d ₃₁ d ₁₂ d ₂₃ d ₃₁ d ₁₂ d ₂₃ d ₃₁ D = 23 m 23.4 22.5 23.0 23.7 178 23.1 23.2 24 267 difference 23.4 0.9 0.5 0.7 155.3 145.9 0.1 0.8 243 Tolerance 20 m < < < > > < < > Valid value (-) (-) (+) (+) (-) (-) (-) (+) (-) output 23.0 23.7 24

To detect misalignments, the tolerance limit can be set clearly above a normal signal noise and, of course, below a minimum possible distance jump due to misallocation. In the illustrated example, for example, a tolerance limit Δd _{tol could be set} to ± 20 m. The starting point for the application of the tolerance limit in the example shown are the differences of the last three measurements.

At the beginning of the measurement, there are still no three distance measurements in the first and second distance measuring cycle, so that the actual distance values are not output. From the third measuring cycle there is then a sufficient number of distance values.

The distance values determined in the third and fourth measuring cycle lie within the tolerance limit, are thus valid and are output. It may also be provided for certain evaluation methods to recursively evaluate and output the two preceding distance values if they lie within the tolerance, as is the case in the illustrated first and second measurement cycle.

In the fifth measurement cycle, a mismatch occurred with a distance jump to 178 m, which exceeds the tolerance limit. This distance value is invalid and will not be output. A valid distance value is only present again in the eighth distance measuring cycle after the distance values in a row in the eighth measurement were within the tolerance limit. Possibly. Here, too, the leading distance values lying within the tolerance could be recursively output as valid. It is according to the invention intended to set the tolerance limit primarily so that misalignments are reliably detected. Of course, the tolerance limit can be narrowed depending on the accuracy requirement of the range measurements.

A further embodiment relates to a light transit time camera system that can be operated with a multiplicity of modulation frequencies. According to the invention, it is provided that successive modulation frequencies f _j , f _k are combined with one another on the basis of predetermined selection rules.

As a possible selection rule, for example, the size of the spanned uniqueness range can be used. If, for example, it is of interest to detect an area of up to 20 m, the wavelengths 3 m and 7 m, which span a range of 21 m or, if necessary, also 5 m and 7 m with a uniqueness range of 35 m, are particularly suitable a combination of 3 m and 4 m spans too small a uniqueness range. The following table shows some examples: Frequency / MHz 100, 42.9 60, 42,9 100, 75 60, 23 75, 68 75, 71 Wavelength / m 3; 7 5; 7 3; 4 5; 13 4; 4.4 4; 4.2 unambiguous range 21 35 12 65 44 84 suitable + + - - + -

In principle, it can be seen that modulation frequencies or wavelengths lying close to one another span an increasingly larger uniqueness range, so that the determination of the distance in the preferred range of distances becomes increasingly inaccurate. Likewise, combinations are unfavorable which span a large or too small uniqueness range despite the large frequency spacing. In the case shown frequency combinations were considered favorable, the uniqueness range is between 20 and 50 m.

It is also conceivable to allow only frequency combinations that are particularly robust in terms of distance determination with regard to noisy or disturbed signal.

In particular, it is favorable with regard to the proposed plausibility of the distance values to select a new modulation frequency taking into account the previously used frequency combination. Thus, it can advantageously be ensured that always a set of three modulation frequencies have similar properties within certain limits. For example, the combinations of three successive modulation frequencies with regard to a distance resolution of the defined unambiguous range could be similar within certain limits.

In principle, it is conceivable to randomly select the subsequent modulation frequency within a predetermined frequency range until the randomly selected frequency corresponds to the selection conditions.

However, it is preferably proposed to provide a selection table or characteristic diagram with suitable modulation frequencies or modulation frequency pairs in advance of a measurement, if necessary, from which it is possible to fall back in the course of a measurement.

Ideally, and in certain applications, the modulation frequencies may be selected such that all combinations of the modulation frequencies match each other. The following table shows a possible selection. f1 f2 f3 f4 Frequency / MHz 5.6 6.4 7.5 9.0 Wavelength / m 26.6 23.3 20.0 16.6

This results in the following six frequency combinations with the corresponding uniqueness ranges: Freq.paar f1 + f2 f1 + f3 f1 + f4 f2 + f3 f2 + f4 f3 + f4 EB / m 186 80 133 140 116 99

Preferably, the frequency pairs are chosen so that both frequencies are prime alien. This is the case in the example shown.

In 10 all branch paths with the possible frequency pairings are shown, whereby a direct reversal of the previous frequency pairing was allowed. The probability for each branch is 33%, already at the third branch the probability for the chosen path is only 4%.

For various reasons, it may be inconvenient to allow direct reversal of the previous frequency pairing. 11 shows a corresponding branch diagram. The probability of a selection is 50%. After five branches, a 3% probability is achieved for the chosen path.

According to the above frequency table, the combination of the two frequencies f1 and f3 has the smallest uniqueness range. Depending on the application, this combination may be unfavorable.

12 shows a branch diagram in which the selection f1 + f3 and the inversion f3 + f1 has been suppressed. As a result, the probability for certain frequency sequences is thus unevenly distributed. To mitigate this effect, for example, more than four modulation frequencies can be used.

In 13 schematically a possible selection structure is shown with more than four modulation frequencies. The measurement begins deliberately or randomly with a first and second modulation frequency f ₁ , f ₂ . The distance d will, as already in 9 represented by combination of the determined to the two modulation frequency pairs f ₁ , f ₂ phase shifts φ ₁ , φ ₂ determined. For the next distance determination, a third, fourth and seventh modulation frequency f ₃ , f ₄ , f _{7 are} proposed to be particularly favorable in the illustrated selection structure for a combination with the second modulation frequency f ₂ . Within these choices, you can now choose anything.

If, for example, a combination with the fourth modulation frequency f _{4 has been} selected, then only a single favorable combination results with the fifth modulation frequency f ₅ , with several combinations again being available thereafter.

If, according to the original combination of the first and second modulation frequencies f ₁ , f _2, the third modulation frequency f _{3 has been} selected, there are two possible combinations, namely with the first and fourth modulation frequencies. If, coincidentally, then the first and then the second modulation frequency has been selected f ₁ , f ₂ , an already used modulation frequency pair is again achieved. However, since each selection step has a random component, there is little chance that the same frequency sequence will be traversed several times in succession.

Of course, the selection structure is structured such that each modulation frequency pair is followed by at least one selection option, and thus all branching steps are self-contained.

14 schematically shows a possible flow chart of the procedure according to the invention. In a first step a), a specific number of modulation frequencies in a suitable selection structure is predetermined, for example, by an initialization program or possibly also at the factory. The further selection of the modulation frequencies then takes place on the basis of the predetermined selection structure.

In a second step b) two modulation frequencies f, f _k in the selection structure randomly or predetermined selected _i either with which in the subsequent step c) two phase shifts φ _i, φ _j and thereafter determines a distance in step d) from this combination becomes. The determined distance can be output, for example, to a subsequent evaluation unit and / or, for example, also temporarily stored.

For the subsequent distance determination, a subsequent modulation frequency f _{k is selected} in a fifth step e) as a function of the previously used modulation frequencies f _i , f _{j in} accordance with the selection structure, in particular randomly selected.

In a sixth step f), a further phase shift φ _{k is} determined with the new frequency f _k , from which a further distance value d _{j, k is obtained} in the seventh step g) in combination with the last determined phase shift φ _j . This distance can also be transmitted, for example, to a subsequent evaluation unit.

In an eighth step h), reference is again made to the fifth step e), in which case the last two modulation frequencies f _j , f _{k used} are to be taken into account for the selection of the next following modulation frequency.

The illustrated loop can now be run through several times for the duration of the measurement.

In a further possible embodiment, it can also be provided to store the last modulation frequency pair used and to begin a subsequent measurement with this frequency pair.

Advantageously, the described selection method with the 9 described distance value plausibility be combined.

The change in the modulation frequency has the particular advantage that the interference that can be reduced or even avoided by further light transit time camera systems that radiate a modulated light into their own detection range.

Such a disorder is exemplified in 15 shown. Parallel to a first time of flight camera system L1, a second time of flight camera system L2 with the same modulation frequency f ₁ radiates into the detection range of the first time of flight camera. Due to the same periodicity of the two same frequencies, the charge accumulation at both accumulation gates Ga, Gb is influenced in common mode, so that the charge ratio is shifted, and thus an incorrect phase shift and, as a result, a faulty distance value is determined.

In 16 is a coincidence of two modulation frequencies f ₁ , f _{2 shown} with different frequency and thus different periodicity. The disturbance thus does not occur in common mode, but is evenly distributed on average in both accumulation gates Ga, Gb, so that as a result only the total amount of light but not the charge distribution is influenced.

The randomly structured selection of the modulation frequencies according to the invention avoids with a high probability that, when a plurality of light transit time systems meet in a common detection area, modulated light having the same modulation frequency appears.

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

φ ₁ , φ ₂

phase shift

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

F1, f2

 modulation frequencies

PM

 Phase measurement cycle

d

 object distance

D

 Distance value = 2d

EB

 unambiguous range

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been 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.

Cited patent literature

• EP 1777747 [0002]
• DE 19704496 [0002, 0003, 0034]
• DE 19704496 C2 [0053]

Claims (9)

A method for operating a light transit time camera system which is operable with a plurality of modulation frequencies, a) fixing two modulation frequencies (f _n , f _{n + 1} ) on the basis of which a distance determination is to be carried out, b) determining a phase shift (φ _n , φ _{n + 1} ) of an emitted and received signal (Sp1, Sp2) for each of the two selected modulation frequencies (f _n , f _{n + 1} ) c) determination of a distance value (d _{n, n + 1} ) on the basis of the two determined phase shifts (φ _n , φ _{n +1} ), d) determination of a further modulation frequency (f _{n + 2} ) based on predetermined selection rules taking into account the two last modulation frequencies (f _n , f _{n + 1} ) e) determination of a further phase shift (φ _{n + 2} ) for the further modulation frequency (f _{n + 2} ) f) determination of a distance value (d _{n + 1, n + 2} ) on the basis of the last two determined phase shifts (φ _{n + 1} , φ _{n + 2} ), g) performing further distance measurements on the basis of S steps d) to g) Method according to claim 1, wherein a predetermined number of modulation frequencies (fm) are provided for selection and the determination of a further modulation frequency (f _{n + 2} ) according to step d) of claim 1 occurs within the provided selection of modulation frequencies (fm). Method according to Claim 1 or 2, in which the determination of the further modulation frequency (f _{n + 2} ) occurs at random taking into account the selection rules. Method according to one of Claims 2 to 3, in which the majority of the provided modulation frequencies are relatively prime. Method according to one of the preceding claims, wherein in determining the further modulation frequency (fn + 2) modulation frequencies are preferred, which are least common to the preceding modulation frequencies (f _n , f _{n + 1} ). Method according to one of the preceding claims, in which a distance value (d _{n, n + 1} ) is evaluated as valid only if two of the distance values (d _{n-2, n-1} , d _{n-1, n} ) determined immediately before within tolerated deviation limits. Method according to Claim 4, in which the tolerated deviation limits are determined on the basis of the last determined distance value (d _{n, n + 1} ). Time of Flight Camera System ( 1 ) with a modulator ( 30 ), which with a lighting ( 10 ) and a receiver ( 20 ) of the time of flight camera system ( 1 ), characterized in that a modulation control unit ( 38 ) with the modulator ( 30 ) and is configured such that the modulator ( 30 ) is operable with at least three modulation frequencies (f ₁ , f ₂ , f ₃ ), and that an evaluation unit is designed such that modulation frequencies for a distance measurement are selected based on predetermined selection rules. A light transit time camera system according to claim 8, adapted for carrying out a method according to one of claims 1 to 7.

Images