The invention relates to a distance measuring device and a method for determining a distance with the distance measuring device.

Distances can be measured between a measuring device and an obj ect without a physical contact between the device and the obj ect by optical methods . In these methods , the obj ect is illuminated by a light source of the device and the light back reflected from the obj ect is then captured by a light detector of the device .

Distances can for example be determined by periodically

modulating the light intensity which is emitted from the light source and by measuring the phase difference between the emitted light and the back reflected light arriving on the detector . However, due to the periodicity of the light

intensity this method results in an ambiguous distance

measurement . Distances can be unambiguously determined by measuring the time of flight between the emission of a light pulse and the arrival of a back reflected light pulse on the detector . Ambient light , for example sun-light , can interfere with the distance measurement and therefore result in a reduction of the precision for the distance measurement . Conventionally, a background measurement is carried out without illuminating the obj ect with the light pulse . The background measurement leads to an irregular operation of the light source . The irregular operation is disadvantageous because it results in a reduction of the life time of the light source and to fluctuations of the parameters of the light pulses , in particular intensity, pulse width, rise times and/or fall times. These fluctuations cause a reduction of the precision for the distance measurement .

It is an obj ect of the invention to provide a distance

measuring device and a method for measuring a distance with the distance measuring device, wherein the distance can be measured with a high precision.

The distance measuring device according to the invention comprises a light source adapted to illuminate an object with light pulses having a duration T _p , at least one photo element adapted to capture the light pulses after being back reflected from the object, a trigger generator for controlling the emission of the light pulses and for activating the photo element during temporal integration gates, wherein the photo element is adapted to output a signal value U at the end of each integration gate with the signal value U being

proportional to the energy of the light arriving on the photo element during its activation, and wherein the trigger

generator stores a trigger scheme to control the emission of the light pulses such that a sequence of the light pulses comprising four consecutive light pulses consisting of two light pulses having an intensity I _e ,i and two light pulses having an intensity I _e , _h being higher than I _e ,i is emitted and that the repetition rate 1/A _rep of the light pulses is constant, and to activate the photo element such that the delays between the integration gates and the emission start points in time of the four light pulses are such that the light pulses arriving on the photo element are captured such that one arriving light pulse corresponding to the intensity I _e ,i and one arriving light pulse corresponding to the intensity I _e , _h are captured by the photo element within first integration gates with an

integration start point in time ΤΊ, _s and an integration end point in time Ti, _e as well as the other arriving light pulse corresponding to the intensity I _e ,i and the other arriving light pulse corresponding to the intensity I _e , _h are captured by the photo element within second integration gates with an

integration start point in time T ₂ , _s and an integration end point in time T ₂ , _e? wherein the delay for the first integration gates is chosen such that either Ti _)S or χ _|6 is between A _t0f and t _of +T _p and the delay for the second integration gates is chosen such the respective light pulses are at least partially within the second integration gates (12), wherein Ti, _s , ΊΊ, _Θ , T _2fS , T ₂ , _e are the delays from the emission start point in time and A _t0f is the first point in t ime the arriving light pulses arrive on the photo element , and a processing unit adapted to calculate the distance between the distance measuring device and the obj ect by using the difference of the signal values U being output at the end of the first integration gates and the difference of the signal values U being output at the end of the second integration gates . The duration Ti, _e -Ti _(S of the first

integration gates can be equal to or can be different from the duration ₂ , _e - 2, _s of the second integration gates .

The method according to the invention for determining a

distance by means of the distance measuring device comprises the steps : a) illuminating the obj ect with the sequence of the light pulses ; b) captur ing one arriving light pulse

corresponding to the intensity I _e ,i within one of the first integration gates , and outputting a signal value Ui at the end of the first integration gate ; c) capturing the other arriving light pulse corresponding to the intensity I _e ,i within one of the second integration gates , and outputting a signal value U2 at the end of the second integration gate ; d) capturing one arriving light pulse corresponding to the intensity I _e , _h within the other first integration gate and outputting a signal value U3 at the end of the first integration gate ; e ) capturing the other arriving light pulse corresponding to the intensity I _e , _h within the other second integration gate and outputting a signal value U ₄ at the end of the second integration gate f ) calculating the distance between the distance measuring device and the obj ect by using the difference of the signal values U ₂ and Ui and the difference of the signal values U4 and U3.

In order to arrange the integration gates with respect to the emission start point in time a distance range in which the obj ect can be located is predetermined . From the distance range p, i _{f s} and Ti, _e can be chosen such that Τχ ₍₃ or T _{1( e} is between A _t0f and A _t0f +T _P for all possible distances of the distance range . 2, _s and T ₂ , _e can then be chosen such that the respective light pulses are at least partially within the second integration gates for all possible distances of the distance range .

It is preferred that A _t0 f and A _t0f +T _p are between T ₂ , _s and T ₂ , _e · For the case that Ti, _s is between A _t0f and A _t0 f+Tp, the time of flight Δ-to _f f om the emission of the light pulses to the arrival of the light pulses on the photo element is calculated by

For the case that _i/ is between A _t0f and A _t0f +T _f

calculated by

(eq. 2)

Also for this embodiment the durations of the first and second integration gates can equal or can be different . If the durations are different , the duration T ₂ , _e -T ₂ , _s can be longer than the duration Ti, _e -Ti, _s to ensure that the complete light pulses are within the second integration gate .

Alternatively, it is preferred that in case T _1/S is between A _t0 f and Ato _f + p, T _2;S is between A _t0f and A _t0f +T _p , T ₂ , _e is later than Ato _f +Tp and T ₂ , _s is different from Ti _(S , and in case ΊΊ _{; β} is between A _t0f and A _t0 f+T _p , T ₂ , _e is between A _t0f and A _t0 f+T _p , T ₂ , _s is earlier than A _t0f and T ₂ , _e is different from Τχ, _e .

For the case that T _{if e} is between A _t0f and A _t0f +T _p , A _t0 f is

calculated by

For the case that T ₂ , _e is between A _t0f and A _t0f +T _p , A _t0 f is

calculated by

^=T^-T _p -(T _2j -T^) ₇₇ - ₇ ^ _r i^— _rr (eq. 4) .

^' (£/«-£/,)-(£/,-*/,)

For all cases, the distance r between the distance measuring device and the object is then calculated by r - 0.5 * c * Atof (eq. 5) , wherein c is the speed of light in the medium in which the distance measurement is carried out.

With the distance measuring device and method according to the invention it is possible to eliminate the influence of

background light, for example sun light, without taking a background measurement. The background measurement would include outputting a signal value U at the end of an

integration gate without illuminating the object with a light pulse . Since it is not necessary to take the background

measurement it is advantageously possible to operate the light source with the constant repetition rate 1/A _repr which results in a long life time of the light source . Δ _Γερ denotes the duration between two consecutive emission start points in time. Another advantage of the constant repetition rate is that the intensity fluctuations of the light pulses are reduced . Both, the elimination of the influence of the background light and the reduction of the intensity fluctuations result in a high precision for the distance measurement . The trigger scheme preferably controls the emission of the sequence such that single light pulses having the intensity I _e ,i are emitted alternating with single light pulses having the intensity I _e , _h - This results in a particular regular operation of the light source which results in a particular long life time of the light source and in particular small intensity fluctuations of the light pulses . It is preferred that the trigger scheme controls the emission of the light pulses such that the sequence comprises a dummy light pulse preceding the four light pulses . Since the dummy light pulse is not used for the distance measurement it is advantageously achieved that only light pulses with small intensity fluctuations are used for the distance measurement . It is preferred that the trigger scheme controls the emission of the light pulses such that the sequence comprises a plurality of dummy light pulses, wherein at least one dummy light pulse precedes each of the four light pulses. By using the multitude of dummy light pulses it is advantageously possible to maintain a stable and constant repetition rate 1/A _xep and to carry out the measurements of the signal values U at a measurement frequency, even if the

measurement frequency of the photo element is lower than repetition rate 1/A _rep for a stable operation of the light source .

The light source preferably comprises light emitting diodes, VCSELs (vertical-cavity surface-emitting laser) and/or lasers that are in particular adapted to emit in the visible and/or infrared spectral region. It is preferred that the distance measuring device comprises a CCD chip with an image intensifier and/or a CMOS chip that comprise the at least one photo

element .

It is preferred that the light source comprises a first group with at least one light emitting diode, VCSEL and/or laser and a second group with at least one light emitting diode, VCSEL and/or laser, wherein the trigger scheme controls the emission of the light pulses such that the first group emits light pulses with the repetition rate 1/Δ _Γθρ and with the intensity I _e ,i and such that the second group emits light pulses with the repetition rate 0.5/A _rep and with the intensity I _e ,h ^_ Ie,i so that the overlap of the emission of the first group and second group results in the light pulses with the intensity I _e ,h. Here, it is advantageously achieved that the first group and the second group are operated perfectly regularly so that the life times of the first group and the second group are particularly increased .

It is preferred that A _t0 f and A _t0f +T _p are between T ₂ , _s and T ₂ , _e · Alternatively, it is preferred that in case ΤΊ, _s is between A _t0 f and Atof+Tp, T ₂ , _s is between A _t0 f and A _t0f +T _p , T ₂ , _e is later than Δ-tof+Tp and T _2)S is different from T _(S , and in case T _{if e} is between A _t0f and A _t0f +T _pf T ₂ , _e s between A _t0f and A _t0f +T _p , T ₂ , _s is earlier than A _t0 f and T ₂ , _e is different from T _{1; e} .

It is preferred that in step a) the sequence is such that single light pulses having the intensity I _e ,i are emitted alternating with single light pulses having the intensity I _e , _h · The sequence preferably comprises a dummy light pulse preceding the four light pulses, wherein the dummy light pulse is not used for the determination of the distance. It is preferred that the sequence comprises a plurality of dummy light pulses , wherein at least one dummy light pulse precedes each of the four light pulses, wherein the dummy light pulses are not used for the determination of the distance. It is preferred that in step a) the sequence comprises a multitude of sets of the four light pulses and a respective distance is determined for each set by repeating steps b) to f ) .

In the following the invention is explained on the basis of schematic drawings.

Figure 1 shows temporal profile diagrams with integration gates and intensities of light pulses, and

Figure 2 shows a schematic cross section through a distance measuring device.

As it can be seen in Figure 2 a distance measuring device 14 comprises a light source 15, a photo element 16, a trigger generator 17, a memory unit 18 and a processing unit 19. The light source 15 comprises light emitting diodes, VCSEL

(vertical-cavity surface-emitting laser) and/or lasers, wherein the light emitting diodes, the VCSELs and/or the lasers are adapted to emit in the visible and/or infrared spectral region. The distance measuring device 14 comprises a CCD chip with an image intensifier and/or a CMOS chip that comprise the at least one photo element 16. The CMOS chip comprises at least one condenser that can be discharged via a photodiode. The trigger generator 17 provides an activation signal 25 for controlling the emission of the light source 15 and an activation signal 26 for activating the photo element 16 during a temporal

integration gate 6. The CCD chip is activated by switching on the image intensifier and the CMOS chip is activated by closing a switch in the circuit of the condenser and the photodiode, which allows that the condenser is discharged via the

photodiode. The photo element 16 is adapted to output a signal value U at the end of the integration gate 6, wherein the signal value U is proportional to the energy of the light arriving on the photo element during its activation . The signal value U is readout in a readout operation 27 and stored in the memory unit 18. The memory unit 18 is adapted to store a multitude of signal values U . The multitude of the signal values U can then be processed by the processing unit 19 in a processing operation 28 in order to determine a distance between the distance measuring device 14 and an obj ect 22.

The signal value U can be measured directly, for example if a CCD chip or CMOS image sensor is used . The charge measured at the end of the integration gate is proportional to the energy of the light arriving on the photo element during its

activation and therefore the signal value U, which is

proportional to the charge, is proportional to the energy of the light . On the other hand, the signal value U can be determined indirectly if the relation between a measured value and the energy of the light arriving on the photo element during its activation is known . For example , if the photo element comprises a condenser that is discharged via a

photodiode during the activation of the photo element the measured value is a voltage that is approximately inversely proportional to the energy of the light arriving on the photo element during its activation .

Detection optics 21 are arranged in front of the photo element 16 in order to image a field of view 24 onto the photo element 16. Illumination optics 20 are arranged in front of the light source 15 in order to shape the light emitted by the light source 15 such that an illumination area 23 can be illuminated by the light source 15. The illumination area 23 and the field of view 24 are shaped such that the field of view 24 is

substantially completely covered by the illumination area 23. The distance measuring device 14 is adapted such that the light emitted by the light source 15 impinges onto the object 22 located within the field of view 24 , and arrives on the photo element 16 after being back reflected from the object 22. The illumination optics 20 and the detection optics 21 are

preferably a respective lens. It is also possible to use a single lens for both the illumination optics 20 and the

detection optics 21.

In Figure 1 three temporal profile diagrams are shown, wherein an intensity 1 and an integration gate 2 are plotted versus time 3. The first temporal profile diagram is a plot of the intensity 4 of the emitted light pulses 7, 8 versus the time 3, the second temporal profile diagram is a plot of the intensity 5 of the light pulses 9, 10 arriving on the photo element 16 after being back reflected from the object 22 versus the time 3, and the third temporal profile diagram is a plot of the integration gates 6 versus the time 3.

The first temporal profile diagram shows that the light source 15 emits a sequence of consecutive light pulses 7, 8. The light pulses 7, 8 have a preferably rectangular temporal profile so that the light source 15 switches the intensity of the light pulses 7, 8 at an emission start point in time 13 from a lower intensity to a higher intensity and after a pulse duration T _p from the emission start point in time 13 back to the lower intensity. The pulse duration T _p is preferably the same for all the light pulses 7, 8 and is in the order of picoseconds or nanoseconds. The repetition rate 1/Δ _Γβρ for all the light pulses in the sequence is constant, wherein Δ _Γβρ is the duration between two consecutive emission start points in time 13. The repetition rate 1/A _rep for the light pulses 7, 8 is from 1 Hz to 20 kHz.

In the following it is assumed that the lower intensity is zero. The sequence comprises a set of four consecutive light pulses 7, 8, wherein two 7 of the four light pulses 7, 8 have the intensity I _e ,i and the other two 8 of the four light pulses 7, 8 have the intensity I _e , _h/ wherein I _e , _h > I _e , ι · In the sequence a single light pulse 7 with the intensity I _e ,i and a single light pulse 8 with the intensity I _e , _h are always emitted

alternating. After the emission the light pulses 7, 8 impinge on the object 22 located within the field of view 24 and are back reflected from the object 22. Afterwards the light pulses 9, 10 arrive on the photo element 16, wherein A _t0f is the first point in time from the emission start point in time 13, when the light pulses 9, 10 arrive on the photo element 16. The two light pulses 9 arriving on the photo element 16 and

corresponding to the light pulses 7 with the intensity I _e ,i have the intensity I _a ,i, wherein I _a ,i < I _e ,i- The two light pulses 10 arriving on the photo element 16 and corresponding to the light pulses 8 with the intensity I _e ,h have the intensity I _a , _h , wherein

Ia,h < Ie,h · The third temporal profile diagram shows that the set of the four light pulses 9, 10 arriving on the photo element 16 are captured within two first integration gates 11 and two second integration gates 12. The first integration gates 11 have an integration start point in time Τχ, _s and an end integration start point in time Ί _ίιβ , wherein Ti _{( s} and Ti, _e are the delays from the emission start point in time 13. The second

integration gates 12 have an integration start point in time T ₂ ,s and an integration end point in time T ₂ ,e _? wherein T ₂ , _s and T ₂ , _e are the delays from the emission start point in time 13. One of the four light pulses 9 having the intensity Ι ₃₍₁ and one of the four light pulses 10 having the intensity I _a , _h are captured by the photo element 16 within a respective first integration gate 11 such that Ti, _e is between A _t0f and A _t0f +T _p and that i _fS is earlier than A _t0 f · Alternatively, it is possible that the respective first integration gate 11 is such that Ti _/S is between A _t0f and A _t0f +T _p and that Τχ, _e is later than A _t0f +T _p . The other of the four light pulses 9 having the intensity I _a ,i and the other of the four light pulses 10 having the intensity I _a ,h are captured by the photo element 16 within a respective second integration gate 12 such that A _t0f and A _t0f +T _P are between T _2(S and T ₂ , _e .

The hatched areas in the second temporal profile diagram are proportional to the energy of the light arriving on the photo element 16 during its activation. Since the signal value U is proportional to the energy of light arriving on the photo element during its activation, the signal value U is also proportional to the hatched areas. A signal value Ui is output at the end of the first integration gate 11 that captures one of the light pulses 9 with the intensity I _a ,i . A signal value U ₃ is output at the end of the first integration gate 11 that captures one of the light pulses 10 with the intensity I _a , _h - A signal value U ₂ is output at the end of the second integration gate 12 that captures the other of the light pulses 9 with the intensity I _a ,i. A signal value U ₄ is output at the end of the second integration gate 12 that captures the other of the light pulses 10 with the intensity I _a<h .

As it can be seen from Figure 1, it is A _t0f + Ui/I _a ,i = T _i<e and it is A _t0 f + U ₃ /Ia ₍ h = Ti _{( e} · These two equations are equivalent to

Furthermore, it can be seen from Figure 1 that

U ₄ = T _p *Ia _fh . These two equations are equivalent

By subtracting 4 from eq. 5 and eq. 6 from eq. 7 it follows :

1

/ (t/ ₃ -£/, ) (eq. 8) and

By equalizing the ride hand sides of eq. 8 and eq. 9 it is then possible to derive eq. 2. Eq. 3 can be derived in an analogous manner. By subtracting the equations 4 to 7 it is

advantageously achieved that the influence of background light is eliminated. The sequence can comprise a dummy light pulse preceding the set of the four light pulses 7, 8, wherein the dummy light pulse is not used for the determination of a distance. In this case, the dummy light pulse has an emission start point in time that is a duration A _rep earlier than the emission start point in time 13 of the earliest of the four light pulses 7, 8.

The sequence can also comprise a multitude of dummy pulses preceding each of the four light pulses 7, 8 wherein the dummy light pulses are not used for the determination of a distance. In these cases the dummy light pulses have emission start points in time that are earlier than the emission start point of the respective measurement light pulse 7, 8.

Furthermore, the sequence can comprise a multitude of the sets of the four light pulses 1 , 8 and a respective distance is calculated for each of the sets. If the sequence comprises the multitude of the sets, the repetition rate of all the light pulses is maintained constant 1/A _rep . The distance measuring device 14 can comprise a plurality of photo elements 16 and a respective distance is determined for each of the photo elements 16.

It is preferred that the light source comprises a first group with at least one light emitting diode, VCSEL and/or laser and a second group with at least one light emitting diode, VCSEL and/or laser, wherein the emission of the light pulses is controlled such that the first group emits light pulses with the repetition rate 1/Δ _Γ6ρ and with the intensity I _e , i and such that the second group emits light pulses with the repetition rate 0.5/Δ _Γβρ and with the intensity I _e , ^~ Ie,i so that the overlap of the emission of the first group and second group results in the light pulses with the intensity I _e , _h ·

List of reference signs

1 intensity

2 integration gate

3 time

4 intensity of emitted light pulses

5 intensity of arriving light pulses

6 integration gates

7 emitted light pulse with low intensity

8 emitted light pulse with high intensity

9 arriving light pulse with low intensity

10 arriving light pulse with high intensity

11 first integration gate

12 second integration gate

13 emission start point in time

14 distance measuring device

15 light source

16 photo element

17 trigger generator

18 memory unit

19 processing unit

20 illumination optics

21 detection optics

22 object

23 illumination area

24 field of view

25 activation signal for light source

26 activation signal for photo element

27 readout operation

28 processing operation

A _rep repetition duration

T _p pulse duration

t ₀ f time of flight

Ti,s integration start point in time of first integration gate Ti, _e integration end point in time of first integration gate

T ₂ ,s integration start point in time of second integration gate ₂ , _e integration end point in time of second integration gate I _e , 1 intensity of emitted light pulse

I _e , _h intensity of emitted light pulse

I _a ,i intensity of arriving light pulse

I _a , _h intensity of arriving light pulse