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DISTANCE MEASURING DEVICE AND METHOD FOR DETERMINING A DISTANCE

20170307359 · 2017-10-26

    Inventors

    Cpc classification

    International classification

    Abstract

    A distance measuring device and a method for determining a distance are provided. The method includes: illuminating an object with a sequence of the light pulses, capturing one arriving light pulse corresponding to an intensity T.sub.e,l within a first integration gate, and outputting a signal value U.sub.1, capturing another arriving light pulse corresponding to the intensity T.sub.e,l within a second integration gate, and outputting a signal value U.sub.2, capturing one arriving light pulse corresponding to an intensity I.sub.e,h within the first integration gate and outputting a signal value U.sub.3, capturing the other arriving light pulse corresponding to the intensity I.sub.e,h within the second integration gate and outputting a signal value U.sub.4, and calculating the distance between the distance measuring device and the object based on U.sub.1, U.sub.2, U.sub.3, and U.sub.4.

    Claims

    1. A distance measuring device comprising: a light source configured to illuminate an object with emitted light pulses, the emitted light pulses having a pulse duration T.sub.p; at least one photo element configured to capture arriving light pulses after being back reflected from the object; the arriving light pulses including first arriving light pulses and second arriving light pulses; a trigger generator configured for controlling an emission of the emitted light pulses and for activating the at least one photo element during temporal integration gates; the temporal integration gates including first and second temporal integration gates; the at least one photo element being configured to output a signal value U at an end of each of the temporal integration gates, the signal value U being proportional to an energy of light arriving on the at least one photo element when the at least one photo element is activated; the trigger generator being configured to store a trigger scheme to control the emission of the emitted light pulses such that a sequence of the emitted light pulses is emitted and a repetition rate 1/Δ.sub.rep of the emitted light pulses is constant; the sequence of the emitted light pulses including four consecutively emitted light pulses; the four consecutively emitted light pulses including two first emitted light pulses having a first intensity I.sub.e,l and two second emitted light pulses having a second intensity I.sub.e,h; the second intensity I.sub.e,h being higher than the first intensity I.sub.e,l; the trigger generator being configured to activate the at least one photo element such that delays between the temporal integration gates and emission start points in time of the four consecutively emitted light pulses are such that the arriving light pulses arriving on the photo element are captured such that one first arriving light pulse corresponding to the first intensity I.sub.e,l and one second arriving light pulse corresponding to the second intensity I.sub.e,h are captured by the at least one photo element within the first integration gates with a first integration start point in time T.sub.1,s and a first integration end point in time T.sub.1,e and another first arriving light pulse corresponding to the first intensity I.sub.e,l and another second arriving light pulse corresponding to the second intensity I.sub.e,h are captured by the at least one photo element within the second integration gates with a second integration start point in time T.sub.2,s and a second integration end point in time T.sub.2,e; a delay for the first integration gates being selected such that either the first integration start point in time T.sub.1,s or the first integration end point in time T.sub.1,e is between a time of flight Δ.sub.tof and a sum of the time of flight and the pulse duration Δ.sub.tof+T.sub.p; a delay for the second integration gates being selected such that respective arriving light pulses are at least partially within the second integrations gates; the first integration start point in time T.sub.1,s, the first integration end point in time T.sub.1,e, the second integration start point in time T.sub.2,s, and the second integration end point in time T.sub.2,e being the delays from the emission start points in time and Δ.sub.tof being a first point in time the arriving light pulses arrive on the at least one photo element, and a processing unit configured to calculate a distance between the distance measuring device and the object based on a difference of signal values U being outputted at the end of the first integration gates and the difference of the signal values U being outputted at the end of the second integration gates.

    2. The distance measuring device of claim 1, wherein the trigger scheme controls the emission of the sequence of the emitted light pulses such that the time of flight Δ.sub.tof and the sum of the time of flight and the pulse duration Δ.sub.tof+T.sub.p are between the second integration start point in time T.sub.2,s and the second integration end point in time T.sub.2,e.

    3. The distance measuring device of claim 1, wherein the trigger scheme controls the emission of the sequence of the emitted light pulses such that: when the first integration start point in time T.sub.1,s is between the time of flight Δ.sub.tof and the sum of the time of flight and the pulse duration Δ.sub.tof+T.sub.p, the second integration start point in time T.sub.2,s is between the time of flight Δ.sub.tof and the sum of the time of flight and the pulse duration Δ.sub.tof+T.sub.p, the second integration end point in time T.sub.2,e is later than the sum of the time of flight and the pulse duration Δ.sub.tof+T.sub.p and the second integration start point in time T.sub.2,s is different from the first integration start point in time T.sub.1,s, and when the first integration end point in time T.sub.1,e is between the time of flight Δ.sub.tof and the sum of the time of flight and the pulse duration Δ.sub.tof+T.sub.p, the second integration end point in time T.sub.2,e is between the time of flight Δ.sub.tof and the sum of the time of flight and the pulse duration Δ.sub.tof+T.sub.p, the second integration start point in time T.sub.2,s is earlier than the pulse duration Δ.sub.tof and the second integration end point in time T.sub.2,e is different from the first integration end point in time T.sub.1,e.

    4. The distance measuring device of claim 1, wherein the trigger scheme controls the emission of the sequence of the emitted light pulses such that single first light pulses having the first intensity I.sub.e,l are emitted alternating with single second light pulses having the second intensity I.sub.e,h.

    5. The distance measuring device of claim 1, wherein: the trigger scheme controls the emission of the light pulses such that the sequence includes a dummy light pulse preceding the four consecutively emitted light pulses or such that the sequence includes a plurality of dummy light pulses, and at least one dummy light pulse precedes each of the four consecutively emitted light pulses.

    6. The distance measuring device of claim 1, wherein the light source includes at least one of light emitting diodes, VCSELs and lasers that are configured to emit light in at least one of a visible spectral region and an infrared spectral region.

    7. The distance measuring device of claim 1, wherein: the light source includes a first group with at least one of a light emitting diode, a VCSEL and a laser, and a second group with at least one of the light emitting diode, the VCSEL, and the laser, the trigger scheme controls the emission of the light pulses such that: the first group emits the light pulses with the repetition rate 1/Δ.sub.rep and with the first intensity I.sub.e,l, and the second group emits the light pulses with a repetition rate 0.5/Δ.sub.rep and with an intensity I.sub.e,h-I.sub.e,l such that an overlap of the emission of the light pulses of the first group and second group results in the light pulses with the second intensity I.sub.e,h.

    8. The distance measuring device of claim 1, further comprising at least one of a CCD chip with an image intensifier, and a CMOS chip that includes the at least one photo element.

    9. A method for determining a distance by the distance measuring device of claim 1, the method comprising the steps of: a) illuminating the object with the sequence of the emitted light pulses; b) capturing one arriving light pulse corresponding to the first intensity T.sub.e,l within one of the first integration gates, and outputting a first signal value U.sub.1 at an end of the one of the first integration gates; c) capturing another arriving light pulse corresponding to the first intensity T.sub.e,l within one of the second integration gates, and outputting a second signal value U.sub.2 at an end of the one of the second integration gates; d) capturing one arriving light pulse corresponding to the second intensity I.sub.e,l within another one of the first integration gates and outputting a third signal value U.sub.3 at an end of the other one of first integration gates; e) capturing another arriving light pulse corresponding to the second intensity I.sub.e,h within the another one of the second integration gates and outputting a fourth signal value U.sub.4 at an end of the other one of the second integration gates; f) calculating the distance between the distance measuring device and the object based on a difference between the second signal value U.sub.2 and the first signal value U.sub.1 and a difference between the fourth signal value U.sub.4 and the third signal value U.sub.3.

    10. The method of claim 9, wherein the time of flight Δ.sub.tof and the sum of the time of flight and the pulse duration Δ.sub.tof+T.sub.p are between the second integration start point in time T.sub.2,s and the second integration end point in time T.sub.2,e.

    11. The method of claim 9, wherein: when the first integration start point in time T.sub.1,s is between the time of flight Δ.sub.tof and the sum of the time of flight and the pulse duration Δ.sub.tof+T.sub.p, the second integration start point in time T.sub.2,s is between the time of flight Δ.sub.tof and the sum of the time of flight and the pulse duration Δ.sub.tof+T.sub.p, the second integration end point in time T.sub.2,e is later than the sum of the time of flight and the pulse duration Δ.sub.tof+T.sub.p and the second integration start point in time T.sub.2,s is different from the first integration start point in time T.sub.1,s, and when the first integration end point in time T.sub.1,e is between the time of flight Δ.sub.tof and the sum of the time of flight and the pulse duration Δ.sub.tof+T.sub.p, the second integration end point in time T.sub.2,e is between the time of flight Δ.sub.tof and the sum of the time of flight and the pulse duration Δ.sub.tof+T.sub.p, the second integration start point in time T.sub.2,s is earlier than the time of flight Δ.sub.tof and the second integration end point in time T.sub.2,e is different from the first integration end point in time T.sub.1,e.

    12. The method of claim 9, wherein in step a) the sequence of the emitted light pulses is such that single light pulses having the first intensity T.sub.e,l are emitted alternating with single light pulses having the second intensity I.sub.e,h.

    13. The method of claim 9, wherein: the sequence of the emitted light pulses includes a dummy light pulse preceding the four consecutively emitted light pulses or a plurality of dummy light pulses, at least one dummy light pulse precedes each of the four consecutively emitted light pulses, and none of the dummy light pulses is used for the determination of the distance.

    14. The method of claim 9, wherein in step a) the sequence of the emitted light pulses includes a plurality of sets of the four consecutively emitted light pulses and a respective distance is determined for each set by repeating steps b) to f).

    15. The method of claim 9, wherein: the light source includes a first group with at least one of a light emitting diode, a VCSEL and a laser, and a second group with at least one of the light emitting diode, the VCSEL, and the laser, the emission of the light pulses is controlled such that: the first group emits the light pulses with a repetition rate 1/Δ.sub.rep and with the first intensity I.sub.e,l, and the second group emits the light pulses with a repetition rate 0.5/Δ.sub.rep and with an intensity I.sub.e,h-I.sub.e,l such that an overlap of the emission of the light pulses of the first group and second group results in the light pulses with the second intensity I.sub.e,h.

    Description

    BRIEF DESCRIPTION OF THE DRAWINGS

    [0023] The invention will now be described with reference to the drawings wherein:

    [0024] FIG. 1 shows temporal profile diagrams with integration gates and intensities of light pulses, and

    [0025] FIG. 2 shows a schematic cross section through a distance measuring device.

    DESCRIPTION OF EXEMPLARY EMBODIMENTS

    [0026] FIG. 2 shows a distance measuring device 14 including 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 includes light emitting diodes, VCSEL (vertical-cavity surface-emitting laser) and/or lasers, wherein the light emitting diodes, the VCSELs and/or the lasers are configured to emit in the visible and/or infrared spectral region. The distance measuring device 14 includes a CCD chip with an image intensifier and/or a CMOS chip that includes the at least one photo element 16. The CMOS chip includes 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 configured 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 read out in a readout operation 27 and stored in the memory unit 18. The memory unit 18 is configured to store a multitude of signal values U. The multitude of the signal values U is then 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 object 22.

    [0027] 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 includes 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.

    [0028] 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 respective lenses. It is also possible to use a single lens for both the illumination optics 20 and the detection optics 21.

    [0029] In FIG. 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.

    [0030] 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.sub.p from the emission start point in time 13 back to the lower intensity. The pulse duration T.sub.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.sub.rep for all the light pulses in the sequence is constant, wherein Δ.sub.rep is the duration between two consecutive emission start points in time 13. The repetition rate 1/Δ.sub.rep for the light pulses 7, 8 is from 1 Hz to 20 kHz.

    [0031] In the following it is assumed that the lower intensity is zero. The sequence includes a set of four consecutive light pulses 7, 8, wherein two light pulses 7 of the four light pulses 7, 8 have the intensity I.sub.e,l and the other two light pulses 8 the four light pulses 7, 8 have the intensity I.sub.e,h, wherein I.sub.e,h>I.sub.e,l. In the sequence, a single light pulse 7 with the intensity I.sub.e,l and a single light pulse 8 with the intensity I.sub.e,h are always emitted alternatingly. 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 Δ.sub.tof 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.sub.e,l have the intensity I.sub.a,l, wherein I.sub.a,l<I.sub.e,l. The two light pulses 10 arriving on the photo element 16 and corresponding to the light pulses 8 with the intensity I.sub.e,h have the intensity I.sub.a,h, wherein I.sub.a,h<I.sub.e,h.

    [0032] 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 T.sub.1,s and an end integration end point in time T.sub.1,e, wherein T.sub.1,s and T.sub.1,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.sub.2,s and an integration end point in time T.sub.2,e, wherein T.sub.2,s and T.sub.2,e are the delays from the emission start point in time 13. One of the four light pulses 9 having the intensity I.sub.a,l and one of the four light pulses 10 having the intensity I.sub.a,h are captured by the photo element 16 within a respective first integration gate 11 such that T.sub.1,e is between Δ.sub.tof and Δ.sub.tof+T.sub.p and that T.sub.1,s is earlier than Δ.sub.tof. Alternatively, it is possible that the respective first integration gate 11 is such that T.sub.1,s is between Δ.sub.tof and Δ.sub.tof+T.sub.p and that T.sub.1,e is later than Δ.sub.tof+T.sub.p. The other of the four light pulses 9 having the intensity I.sub.a,l and the other of the four light pulses 10 having the intensity I.sub.a,h are captured by the photo element 16 within a respective second integration gate 12 such that Δ.sub.tof and Δ.sub.tof+T.sub.p are between T.sub.2,s and T.sub.2,e.

    [0033] 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 U.sub.1 is put out at the end of the first integration gate 11 that captures one of the light pulses 9 with the intensity I.sub.a,l. A signal value U.sub.3 is output at the end of the first integration gate 11 that captures one of the light pulses 10 with the intensity I.sub.a,h. A signal value U.sub.2 is put out at the end of the second integration gate 12 that captures the other of the light pulses 9 with the intensity I.sub.a,l. A signal value U.sub.4 is put out at the end of the second integration gate 12 that captures the other of the light pulses 10 with the intensity I.sub.a,h.

    [0034] FIG. 1 shows that Δ.sub.tof+U.sub.1/I.sub.a,l=T.sub.1,e and Δ.sub.tof+U.sub.3/I.sub.a,h=T.sub.1,e. These two equations are equivalent to:

    [00005] I a , l = U 1 T 1 , e - Δ tof .Math. .Math. and ( equation .Math. .Math. 6 ) I a , h = U 3 T 1 , e - Δ tof . ( equation .Math. .Math. 7 )

    [0035] Furthermore, FIG. 1 shows that U.sub.2=T.sub.p*I.sub.a,l and U.sub.4=T.sub.p*I.sub.a,h. These two equations are equivalent to:

    [00006] I a , l = U 2 T p .Math. .Math. and ( equation .Math. .Math. 8 ) I a , h = U 4 T p . ( equation .Math. .Math. 9 )

    [0036] By subtracting equation 6 from equation 7 and equation 8 from equation 9 it follows:

    [00007] I a , h - I a , l = 1 T 1 , e - Δ tof .Math. ( U 3 - U 1 ) .Math. .Math. and ( equation .Math. .Math. 10 ) I a , h - I a , l = 1 T p .Math. ( U 4 - U 2 ) . ( equation .Math. .Math. 11 )

    [0037] By equalizing the right hand sides of equation 10 and equation 11, it is then possible to derive equation 2. Equation 3 can be derived in an analogous manner. By subtracting the equations 6 to 9, the influence of background light is eliminated.

    [0038] The sequence can include 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 Δ.sub.rep earlier than the emission start point in time 13 of the earliest of the four light pulses 7, 8.

    [0039] The sequence can also include 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.

    [0040] Furthermore, the sequence can include a multitude of the sets of the four light pulses 7, 8 and a respective distance is calculated for each of the sets. If the sequence includes the multitude of the sets, the repetition rate of all the light pulses is maintained constant with the repetition rate 1/Δ.sub.rep. The distance measuring device 14 can include a plurality of photo elements 16 and a respective distance is determined for each of the photo elements 16.

    [0041] In an exemplary embodiment, the light source includes 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.sub.rep and with the intensity I.sub.e,l and such that the second group emits light pulses with the repetition rate 0.5/Δ.sub.rep and with the intensity I.sub.e,h-I.sub.e,l so that the overlap of the emission of the first group and second group results in the light pulses with the intensity I.sub.e,h.

    [0042] It is understood that the foregoing description is that of the exemplary embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.

    LIST OF REFERENCE NUMERALS

    [0043] 1 intensity [0044] 2 integration gate [0045] 3 time [0046] 4 intensity of emitted light pulses [0047] 5 intensity of arriving light pulses [0048] 6 integration gates [0049] 7 emitted light pulse with low intensity [0050] 8 emitted light pulse with high intensity [0051] 9 arriving light pulse with low intensity [0052] 10 arriving light pulse with high intensity [0053] 11 first integration gate [0054] 12 second integration gate [0055] 13 emission start point in time [0056] 14 distance measuring device [0057] 15 light source [0058] 16 photo element [0059] 17 trigger generator [0060] 18 memory unit [0061] 19 processing unit [0062] 20 illumination optics [0063] 21 detection optics [0064] 22 object [0065] 23 illumination area [0066] 24 field of view [0067] 25 activation signal for light source [0068] 26 activation signal for photo element [0069] 27 readout operation [0070] 28 processing operation [0071] Δ.sub.rep repetition duration [0072] T.sub.p pulse duration [0073] Δ.sub.tof time of flight [0074] T.sub.1,s integration start point in time of first integration gate [0075] T.sub.1,e integration end point in time of first integration gate [0076] T.sub.2,s integration start point in time of second integration gate [0077] T.sub.2,e integration end point in time of second integration gate [0078] I.sub.e,l intensity of emitted light pulse [0079] I.sub.e,h intensity of emitted light pulse [0080] I.sub.a,l intensity of arriving light pulse [0081] I.sub.a,h intensity of arriving light pulse