DE102014207163A1 - Time of flight camera system - Google Patents

Abstract

Method for operating a light transit time camera system, in which a light transit time sensor (22) with a basic modulation (M0) and a light source (12) with a basic modulation (M0) dependent illumination modulation (Sp1) is operated, wherein a duty cycle (DC) of the illumination modulation ( Sp1) is smaller than the duty cycle (DC) of the basic modulation (M0).

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 Pat. No. 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.

From the DE 197 04 496 Furthermore, the determination of a distance or a corresponding phase shift of the reflected light from an object is known. In particular, it is disclosed to selectively shift the transmitter modulation by 90 °, 180 ° or 270 ° in order to determine a phase shift and thus a distance from these four phase measurements via an arctan2 function.

The object of the invention is to improve the energy efficiency 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 claim.

Advantageously, a method is provided for operating a light transit time camera system, in which a light transit time sensor with a basic modulation and a light source with a basic modulation dependent illumination modulation is operated, wherein a duty cycle of the illumination modulation is smaller than a duty cycle of the basic modulation.

This procedure has the advantage that the duty cycle and thus also the required illumination energy are reduced.

In particular, it is advantageous to choose the duty cycle of the illumination modulation of less than 50%, in particular 35% and particularly preferably equal to 25%. With such a modulation of the illumination, it is possible to save illumination energy with the same measurement accuracy or to increase the measurement accuracy with the same energy input.

Likewise advantageously a light transit time camera system is provided, which is designed to carry out one of the aforementioned methods, with a lighting for transmission and a light runtime camera for receiving a modulated light, with a modulator for generating a first modulation signal for operating the light transit time camera, wherein a control device is designed in that, starting from the first modulation signal, a second modulation signal for operating the illumination is generated, which has a lower duty cycle than the first modulation signal. The invention will be explained in more detail by means of embodiments with reference to the drawings.

Show it:

1 schematically a light transit time camera system,

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 an intensity profile of a lighting modulation according to the invention,

6 an intensity profile of a time-related phase shift,

7 a correlation of the matched modulation with the fundamental modulation,

8th a correlation shifted by 90 °

9 Intensity profiles for different duty cycle

10 Correlation functions for different duty cycle,

11 a relative distance error depending on the phase angle.

In the following description of the preferred embodiments, like Reference signs the same 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.

Further, a modulation control device 38 provided, with which the shape and in particular pulse and pause ratios of the modulation signal are specified. Also, via the modulation control unit 38 the phase shifter 35 be controlled depending on the measurement task to be performed.

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

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, with} phase shift Δφ (t _L ) 22 ,

The light transit time sensor 22 typically has a first and second accumulation gate Ga, Gb in which, depending on the potential curve in the photosensitive region, the photonically generated charges q are alternately collected over a plurality of modulation periods. The charges q generated in the unshifted phase position are accumulated in the first accumulation gate Ga and the phase position M ₀ + 180 ° shifted in the second accumulation gate Gb by 180 °. 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 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. At a phase shift of Δφ (t _L ) = 90 ° are the Charge carriers qa, qb equally distributed on both gates Ga, Gb and the difference 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 or autocorrelation 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 a 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 °)

By correlating the PMD modulation or basic modulation M ₀ with the time-dependent phase-shifted illumination modulation S _p2 , a phase shift and distance can thus be determined. The modulation frequency or the modulation wavelength defines a uniqueness range within which the phase shifts by 360 °. Larger phases are folded back to this area. However, if the target application only has certain phase positions or distances that are smaller than the value range, this information can be used to generate a modulation signal tailored to the work area.

Usually, the light transit time sensor 22 and the lighting 10 operated with a duty cycle of 50%. However, in potential applications that intrinsically exploit only a fraction of this uniqueness range, this limited range information is not used. As shown below, this costs energy and dynamics without improving the signal.

The upper curve in 5 shows a modulation signal M ₀ and a basic modulation and the two subsequent curves below a shifted by 0 ° and 90 ° in the phase position illumination _signal S _p1 (0 °) _50DC , S _p1 (90 °) _50DC .

In the event that the complete value range of the transit-time-dependent phase shift Δφ (t _L ) is between 0 ° and 90 °, it can be seen that the unmarked range of the switch-on _{interval of} the illumination _modulation S _p1 (0 °) _50DC , S _p1 (90 °) _{50DC is} completely within a clock phase or switch-on interval of the basic modulation M ₀ and thus provides only a constant amount in the correlation, which contains in the result no distance information. The marked area, on the other hand, effectively scans the falling edge of the basic modulation M ₀ and contains all the distance information. If the unmarked area is omitted during the modulation of the illumination, the result is a pulse shape S _p1 (0 °) _25DC , S _p1 (90 °) _25DC with a duty cycle of 25%, as in the two lower curves of FIG 5 is shown.

With identical exposure time or integration time, both methods have the same resolution, since, as it were, the marked areas are the same. In the procedure according to the invention, however, the power consumption is significantly reduced due to the lower duty cycle. The thus smaller signal, with the same information content, also allows a much longer integration time and thus an increase in dynamics.

Of course, in the measurements at different phase angles required for a range finding, the area of the signal effectively carrying the information changes. However, this can be taken into account so that only the portion of the signal that carries no information is ever removed.

Of course, the changed form of the illumination _modulation S _p1 also has an influence on the correlation or autocorrelation function AKF. This influence is exemplary in 6 and 7 shown.

6 shows pulse sequences of different, time-related phase shifts Δφ (t _L ). The first pulse sequence shows the unshifted transmit pulse sequence S _p1 (0 °) _25DC with 0 ° phase position and 25% dutycycle. The following pulse sequences S _p2 , which then ultimately from the sensor 22 are detected, each shown shifted by 90 ° runtime.

The correlation of the received, phase-shifted illumination _modulation S _p2 with the basic modulation M ₀ leads to a correlation function according to FIG 7 , As the phase shift Δφ (t _L ) increases, the correlation decreases to reach a flattened minimum at 90 °. From 180 ° the correlation increases again and reaches at 270 ° a flattened maximum, which remains up to 360 ° or 0 ° at this level.

7 shows an example of a correlation function for a shifted by 90 ° phase angle.

The correlation functions according to 7 and 8th are contrary to the autocorrelation functions according to 3 unique only in a smaller area. In a simple application, it may therefore be provided to provide the reduced illumination modulation only for runtime-related phase shifts from 0 ° to 90 °. If the wavelength of the basic modulation M _{0 is,} for example, 8 m, this results in a measurement interval of 0 to 1 m. Of course, this measuring interval does not have to start at an absolute distance of 0 m, but can be shifted within the possible measuring range from 0 to 4 m.

Due to the limitation to a small area of the uniqueness range, it may be possible to dispense with the aforementioned "arctan method" and also use direct "fit algorithms". In a simple application, for example, the correlation function for the 0 ° to 90 ° range can be provided in a characteristic table. Likewise, it is conceivable to deposit the correlation as a mathematical function. Of course, further evaluation methods are conceivable.

It should also be noted that the slope of the correlation function in the relevant measurement range is substantially identical to the case of the normal measurement, so that the error of both methods is basically the same in the relevant measurement range.

9 shows examples of intensity profiles for different duty cycles. The solid corresponds to the usual operation with a duty cycle of 50%, ie the lighting is operated to a proportion of 50% of the period T. The dashed line corresponds to a duty cycle of 35% and the dotted line of 25%. If the energy with which the light source is operated is kept constant for the various duty cycle, the light intensity increases correspondingly with the same amount of light.

If no higher light intensity is required, this can remain at the height of normal operation, whereby energy is saved due to the shorter switch-on time.

10 shows curves of the correlation function for different duty cycle. The duty cycle of 50% results in the previously discussed triangular function, which is shown here by a solid line. If the duty cycle is shortened, a plateau forms at the extreme points at 90 ° and 270 °, which increases with the decreasing duty cycle.

11 shows an estimate of a relative distance error for different duty cycle as a function of the phase shift to be measured. The relative distance error is normalized to the maximum distance error of the 50% dutycycle. The distance error of the phase measurement varies periodically depending on the phase to be measured. The minima of the distance errors have a phase separation of 90 ° for all shown duty cycles. The distance error of the 50% Dutycycle varies between 0.7 and 1 while the 25% and 35% Dutycycle fluctuate between 0.5 and 0.7 and thus are always more accurate than the 50% Dutycycle.

The distribution of the minima can be advantageously exploited especially for certain measurement tasks. Thus, it may be provided, for example, to shift the phase positions of the illumination modulation with respect to the phase positions of the basic modulation such that the minium of the distance errors falls within a preferred measurement range.

For example, the phase angle could be shifted so that the accuracy is greatest at the end of the preferred operating range, because there one typically has the smallest amplitudes and the largest noise. As a result of this procedure, a high measuring accuracy over the entire working range or measuring range can be ensured in an advantageous manner.

Of course, other configurations are conceivable to optimize the respective measurement task.

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

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]
• US 6587186 [0002]
• DE 19704496 [0002, 0003, 0023]

Claims (4)

Method for operating a light transit time camera system, in which a light transit time sensor ( 22 ) with a basic modulation (M ₀ ) and a light source ( 12 ) is operated with one of the basic modulation (M ₀ ) dependent illumination modulation (Sp1), characterized in that a Dutycycle (DC) of the illumination modulation (Sp1) is smaller than the Dutycycle (DC) of the basic modulation (M ₀ ). Method according to Claim 1, in which the duty cycle (DC) of the illumination modulation (Sp1) is less than 50%.  The method of claim 1, wherein the duty cycle (DC) of the illumination modulation (Sp1) is equal to 25%. Time of Flight Camera System ( 1 ), which is designed to carry out one of the aforementioned methods with a lighting ( 10 ) and a photoelectric time camera ( 20 ) for receiving a modulated light, having a modulator ( 30 ) for generating a first modulation signal (M ₀ ) for operating the time of flight camera ( 20 ), characterized in that a control unit ( 38 ) is designed such that, starting from the first modulation signal (M ₀ ), a second modulation signal (Sp1) for operating the illumination ( 10 ) is generated, which has a lower duty cycle (DC) than the first modulation signal (M ₀ ).

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