Abstract:
A measuring apparatus for optically measuring a distance to a target object is described. The measuring apparatus has a transmitting device for emitting periodically modulated optical measuring radiation towards the target object, a receiving device for detecting optical measuring radiation which returns from the target object, and an evaluation device for receiving and evaluating detection signals from the receiving device. The measuring apparatus also has a calibration device for calibrating the measuring apparatus, wherein the calibration device is designed to calibrate the measuring apparatus on the basis of detection of uncorrelated radiation which does not correlate with the modulated measuring radiation emitted by the transmitting device. In this case, the uncorrelated radiation may be in the form of background radiation. Alternatively, uncorrelated measuring radiation can be emitted by the transmitting device and can be detected by the receiving device.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a measuring device for measuring a distance between the measuring device and a target object with the aid of optical radiation. Such a measuring device is also designated as an optical distance measuring device and can be provided as a separate, for example handheld, device or in another device, for example a tool. In particular, the invention relates to an optical distance measuring device equipped with a calibration unit in order to be able to increase a measurement accuracy of the distance measuring device by calibrating an actual distance measurement. 
       BACKGROUND OF THE INVENTION 
       [0002]    Optical distance measuring devices are known which align a temporally modulated light beam in the direction toward a target object whose distance from the measuring device is intended to be determined. Returning light reflected or scattered back from the target object aimed at is at least partly detected by the measuring device and used for determining the distance to be measured. In this case, a typical measurement range is for distances from a few centimeters up to several hundred meters. 
         [0003]    US 2007/0182949 A1 discloses a distance measuring device comprising a light source for illuminating the target object using continuously modulated light, a solid-state image sensor comprising an array of avalanche photodiodes, and a plurality of circuits for processing signals that have been output by the avalanche photodiodes, in order to provide data which are dependent on the light reflected from the target object onto the photodiodes. The circuits have a multiplexer designed to accumulate signals output by the avalanche photodiodes during different subperiods in different storage devices. 
         [0004]    The avalanche photodiodes serving as photon counters in this case receive the light reflected back from the target object and also background radiation additionally present and generate at their output electrical pulses in each case, wherein the temporal pulse density correlates with the impinging light power. The read-out of the pulses from the avalanche photodiodes is effected with the aid of a multiplexer arrangement. The latter can be operated synchronously with a modulation of a laser used as light source in such a way that the pulses of the avalanche photodiodes increment different digital counters in a manner dependent on the point in time of the respective detection events, that is to say for example of a photon absorbed in the avalanche photodiode. A temporal period with which the light source illuminates the target object in a modulated fashion is in this case subdivided into a plurality of subperiods. A number of digital counters corresponding to the number of subperiods are provided, wherein, during each subperiod, a correspondingly assigned digital counter is in each case incremented in accordance with the detection pulses received during the subperiod. In this way, detection events can be accumulated over a measurement time. While an individual period can have, for example, time durations in the range of nanoseconds, the total measurement time can comprise many such periods and last, for example, several milliseconds or several seconds. By accumulating the measurement results in the digital counters, it is possible to record a type of histogram of the detection events relative to the temporal occurrence of detection events within subperiods. As soon as a modulation impressed on the modulated light emitted by the light source is present in the counter readings of the digital counters with sufficient statistical accuracy, it is possible, by means of a phase evaluation, to deduce a propagation time of the light between emission and detection and hence a distance between the distance measuring device and the target object. Such a principle of laser distance measurement is generally known by the designation “time of flight ranging” for example with continuous or pulsed modulation of the intensity of the laser beam. 
         [0005]    An evaluation unit which operates in this way and which, within a distance measuring device, receives detection signals from a light-sensitive detector and evaluates them by registering the detection signals in a manner synchronized with a reference, that is to say accumulating them in accordance with their temporal occurrence relative to the periodicity of the modulated measurement light used, is also designated as so-called “binning architecture”. Such a binning architecture can be realized for example with a delay locked delay line (DLL). 
         [0006]    It has been observed that distance measuring devices which operate for example in the manner described above on the basis of photon counters, multiplexer arrangements and binning architectures cannot always yield satisfactory measurement accuracies. 
       DISCLOSURE AND POSSIBLE EMBODIMENTS OF THE INVENTION 
       [0007]    There may therefore be a need for a distance measuring device in which a measurement accuracy, and in particular a reliability of a measurement accuracy, is improved. 
         [0008]    Such a need can be fulfilled with a measuring device according to claim  1 . Further configurations of the measuring device are specified in the dependent claims. 
         [0009]    Aspects of the measuring device proposed can be regarded as being based on the following insights and concepts: 
         [0010]    As a possible source of measurement errors or measurement inaccuracies in the above-described distance measuring device, for example, it has been recognized that a measurement result can be influenced greatly by temporal lengths of the subperiods into which the total period of the modulated measurement light is subdivided and during which detection signals are accumulated by incrementing an assigned counter. Different lengths of the subperiods can, particularly if the differences in length are randomly established and are not known, act as system-dictated error sources and bring about systematic errors when determining a distance to be measured. Such systematical errors should be differentiated, in principle, from noise-dictated errors, since they cannot be reduced by longer measurement times, but rather regularly only with the aid of a more accurate calibration of the distance measuring device. 
         [0011]    Therefore, a distance measuring device is proposed which additionally comprises a calibration unit, with the aid of which the measuring device can be calibrated in an advantageous manner. 
         [0012]    In this case, the proposed measuring device for optical distance measurement comprises a transmitting unit for emitting periodically modulated optical measurement radiation toward a target object, a receiving unit for detecting optical measurement radiation returning from the target object, and an evaluation unit for receiving and evaluating detection signals of the receiving unit. 
         [0013]    Moreover, the measuring device comprises a calibration unit for calibrating the measuring device, which is designed to calibrate the measuring device, and in particular the evaluation unit of the measuring device, on the basis of a detection of uncorrelated radiation which does not correlate with the modulated measurement radiation emitted by the transmitting unit. 
         [0014]    A fundamental concept in this case is to be able to carry out a calibration of the measuring device, in particular the evaluation unit of the measuring device, before, in the course of or after an actual distance measurement. In contrast to the actual distance measurement, in which a distance can be determined with the aid of periodically modulated measurement radiation on the basis of a phase shift between emitted and detected measurement radiation and a time of flight of the measurement radiation that can be calculated therefrom, the calibration is intended to be effected with uncorrelated, preferably unmodulated radiation. In this case, the term “uncorrelated” is intended to express the fact that the radiation used for the calibration is preferably temporally unmodulated with regard to a property detected by the receiving unit, such as, for example, an intensity of the radiation, or is at least not modulated with a modulation which correlates with the periodically operating evaluation unit. In other words, by way of example, the uncorrelated radiation is intended to be able to be regarded as substantially temporally constant within a period duration with which the transmitting unit periodically modulates the measurement radiation during a normal distance measurement. Alternatively, the uncorrelated radiation can be generated as radiation modulated at high frequency, under the condition that this does not run synchronously with the “binning” of the evaluation unit, and, if appropriate, is integrated over many periods. 
         [0015]    The unmodulated radiation used can be for example background radiation, for example in the form of normal ambient light. Such background radiation is by definition uncorrelated and in distance measurements usually always impinges on the receiving unit anyway and generates there a substantially constant background signal. While this background signal is regularly ignored during the distance measurement and can even make a distance measurement more difficult, it is now possible to use the background signal in the context of a calibration process on account of its temporally constant properties. 
         [0016]    Alternatively, the transmitting unit of the measuring device itself can be designed for emitting uncorrelated, preferably unmodulated measurement radiation. In other words, the transmitting unit can be designed, on the one hand, to emit measurement radiation in a periodically modulated manner during an actual distance measurement and, on the other hand, to turn off the modulation in a targeted manner during a calibration process, in order to emit measurement radiation in an unmodulated manner. By detecting such unmodulated measurement radiation, it is then possible to effect a desired calibration. By virtue of the fact that, in addition to background radiation possibly present, unmodulated radiation from the transmitting unit is also detected by the receiving unit, a measurement signal detected by the receiving unit can be increased and a calibration process can thus be accelerated. 
         [0017]    In this case, the measurement radiation used for the calibration process need not necessarily leave the measuring device toward the outside, as would be the case during the actual distance measurement. Instead, the measurement radiation can be guided within the measuring device directly onto the detector. A measurement signal independent of background radiation can thereby be generated, for example, at the detector. 
         [0018]    The measuring device can be designed to the effect that unmodulated radiation impinges on the receiving unit with an intensity adapted to a detection sensitivity of the receiving unit. By way of example, the receiving unit can have a paralyzable radiation detector such as, for example, an SPAD (single photon avalanche diode) which, given a specific impinging radiation intensity, has a maximum of a detection event rate as detector signal. Advantageously, in this case the unmodulated radiation is directed onto the receiving unit in such a way that its intensity is adapted to the maximum of the detector signal. If measurement radiation from the transmitting unit is used as uncorrelated radiation, its intensity can be set in a targeted manner by the corresponding driving of the transmitting unit. Alternatively, corresponding optical elements such as, for example, absorption elements in the form of diaphragms or filters can also be integrated into a beam path of the radiation impinging on the receiving unit, in order to be able to adapt the intensity of the radiation to be detected in a targeted manner. 
         [0019]    The measuring device can furthermore be designed to determine a duration of a calibration process to be performed by the calibration unit on the basis of a predefined calibration accuracy and an intensity of the unmodulated radiation detected by the receiving unit. In this case, the calibration accuracy can, for example, be preset in a device-specific manner or be predefined by a user via an associated input device before the calibration is carried out. The higher the desired calibration accuracy and the lower the intensity of the detected unmodulated radiation, the longer the duration of the calibration process should be chosen. 
         [0020]    Possible aspects, advantages and configurations of the invention have been described above with reference to individual embodiments of the invention. The description, the associated figures and the claims contain numerous features in combination. A person skilled in the art will also consider these features, in particular also the features of different exemplary embodiments, individually and combine them to form expedient further combinations. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0021]    Embodiments of the invention and partial aspects contained therein are described below with reference to the accompanying figures. The figures are merely schematical and not true to scale. 
           [0022]      FIG. 1  shows a measuring device for optical distance measurement in accordance with one embodiment of the present invention. 
           [0023]      FIG. 2  shows a schematic illustration of digital gate signals of a binning architecture such as can be used in a measuring device according to the invention. 
           [0024]      FIG. 3  shows by way of example a temporal dependence of a counting rate of a photon counter used as a receiving unit in the case of illumination with modulated measurement radiation. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0025]      FIG. 1  schematically illustrates a measuring device for optical distance measurement according to the invention with the most important components for describing its function. 
         [0026]    The measuring device  10  comprises a housing  11 , in which a transmitting unit  12  for emitting optical measurement radiation  13  and a receiving unit  14  for detecting measurement radiation  16  returning from a target object  15  are arranged. 
         [0027]    The transmitting unit  12  comprises a light source, which is realized by a semiconductor laser diode  18  in the exemplary embodiment illustrated. The laser diode  18  emits a laser beam  20  in the form of a light bundle visible to the human eye. For this purpose, the laser diode  18  is operated by means of a control unit  24 , which, by means of corresponding electronics, generates a temporal modulation of an electrical input signal  19  of the laser diode  18 . What can be achieved by such modulation of the diode current is that the optical measurement radiation  13  utilized for distance measurement is likewise modulated temporally in terms of its intensity in a desired manner. 
         [0028]    The control unit  24  and the transmitting unit  12  can be designed to emit unmodulated measurement radiation  13 , for example during a calibration process. For this purpose, the control unit  24  can operate the laser diode  18  with a constant diode current. Alternatively, the control unit  24  can turn off the transmitting unit  12  in a targeted manner during the calibration process, such that only substantially unmodulated background radiation impinges on the receiving unit  14 . 
         [0029]    In particular during a distance measuring process, the laser beam bundle  20  subsequently passes through a collimation optical unit  26  in the form of an objective  28 , which is illustrated in the form of an individual lens in a simplified manner in  FIG. 1 . In this exemplary embodiment, the objective  28  is optionally situated on an adjusting assembly  32 , which, in principle, makes it possible to change the position of the objective in all three spatial directions, for example for alignment purposes. Alternatively, however, the collimation optical unit  26  can also already be part of the laser diode  18  or fixedly connected thereto. 
         [0030]    After passing through the objective  28 , this results in a, for example amplitude-modulated, signal of the measurement radiation  13  in the form of an almost parallel light bundle  37 , which propagates along an optical axis  38  of the transmitting unit  12 . 
         [0031]    In addition, a preferably switchable beam deflector  40  can also be situated in the transmitting unit  12 , and allows the measurement radiation  13  to be deflected wholly or partly whilst bypassing the target object  15  directly, that is to say device-internally, onto the receiving unit  14 . In this way, it is possible to generate a device-internal reference path  42 , which allows calibration or adjustment of the measuring device. The possibility of device-internal light deflection can be used, in particular, during the calibration process with unmodulated measurement radiation. 
         [0032]    If a distance measurement is carried out by means of the measuring device  10 , the measurement radiation  13  leaves the housing  11  of the measuring device through an optical window  44  in the end wall  45  of the measuring device  10 . The opening of the optical window  44  can be protected for example by a shutter  46 . For the purpose of actual measurement, the measuring device  10  is then aligned toward a target object  15  whose distance  48  from the measuring device  10  is intended to be determined. The signal  16  reflected or scattered at the desired target object  15  forms returning optical measurement radiation  16  in the form of a returning beam bundle  49  or  50 , a certain portion of which passes back into the measuring device  10  again. 
         [0033]    Through an entrance window  47  at the end side  45  of the measuring device  10 , the returning measurement radiation  16  is coupled into the measuring device  10  and then impinges, as illustrated in  FIG. 1 , on a receiving optical unit  52 . 
         [0034]    Two returning measurement beam bundles  49  and  50  for two different target object distances  48  are depicted for illustration by way of example in  FIG. 1 . For large object distances, where large can be interpreted as large relative to the focal length of the receiving optical unit  52 , the optical measurement radiation  16  returning from the target object  15  is incident approximately parallel to the optical axis  51  of the receiving unit  14 . This case is represented by the measurement beam bundle  49  in the exemplary embodiment in  FIG. 1 . As the object distance becomes smaller, the returning measurement radiation  16  incident in the measuring device is inclined more and more relative to the optical axis  51  of the receiving unit  14  on account of a parallax. The beam bundle  50  is depicted in  FIG. 1  as an example of such a returning measurement beam bundle in the near range of the measuring device. 
         [0035]    The receiving optical unit  52 , which is likewise merely symbolized schematically by an individual lens in  FIG. 1 , focuses the beam bundle of the returning measurement radiation  16  onto the detection area  66  of a receiving detector  54  provided in the receiving unit  14 . The detector  54  has a multiplicity of pixels for detecting the optical measurement radiation. Each of the pixels has at least one light-sensitive SPAD. By means of the SPADs provided in the detection area  66 , which are arranged individually or in groups in combination in pixels in a matrix-like manner and are connected to an evaluation unit  36 , the incident returning measurement radiation  16  is converted into an electrical signal  55  and fed for further evaluation in the evaluation unit  36 . In this case, on account of inherent properties of the SPADs, the electrical signal  55  can be regarded as a digital signal that reproduces a counting rate of photons impinging on the respective pixels of the detection area  66 . 
         [0036]    The detection signals generated by an individual SPAD or a combination of SPADs can be fed to one or more distance determining unit(s) contained in an evaluation unit  36 . The distance determining unit can sum the detection signals and generate therefrom a signal corresponding to a time-dependent intensity of the light signal impinging on the respective SPADs or the light intensity. By relating this signal to an excitation signal indicating the temporal profile of the photon rate emitted by the transmitting unit  12 , it is possible to deduce a photon time of flight from the transmitting unit  12  toward the target object  15  and back again to the receiving unit  13 . If the transmitting unit  12  periodically modulates the emitted light sinusoidally, for example, it is possible to determine a time of flight from a phase difference between the emitted and detected measurement radiation. 
         [0037]    In detail, the distance determining unit can forward the digital detection signals received by the receiving unit  14  to different digital counters during different subperiods. The distance determining unit is in some instances also designated herein as “binning architecture” and the subperiods are in some instances designated as “bin widths”. In this case, the sum of the subperiods should correspond to the period of the modulated measurement radiation. In other words, during a subperiod, a periodically repeating phase region of the periodically modulated detection signal is detected and the corresponding digital detection signals are accumulated in counters. For this purpose, the detection signals, temporally correlated with the periodicity of the driving signal to the laser diode  18 , can be conducted to corresponding digital counters via multiplexers during the different subperiods. From the counting results of the digital counters accumulated over many periods, it is then possible to deduce the phase difference between the emitted and detected measurement radiation and thus to determine the desired distance. 
         [0038]    The evaluation unit  36  furthermore has a calibration unit  80 . As described in detail hereinafter, the calibration unit is designed to calibrate the measuring device  10 , and in particular the evaluation unit  36  thereof, during a calibration process in which the detection unit is illuminated with unmodulated light. 
         [0039]      FIG. 2  shows a schematic illustration of digital gate signals of a binning architecture on the basis of the example of a four-fold multiplexer. The size and the stability of the bin widths actually realized can constitute a particularly large systematic error source. A deviation of the bin widths from their desired value can have a considerable effect on the measurement result, particularly in the case of strong background illumination. 
         [0040]    The bin widths can be calibrated by means of a measurement with non-modulated constant light. In this case, such a calibration measurement against the background of a high required accuracy with a low signal-to-noise ratio can take a very long time, for example longer than the actual distance measurement. 
         [0041]    Furthermore, unmodulated light generation can be effected for the calibration measurement by means of the non-modulated operation of the laser diode  18  of the measuring device  10 , said laser diode serving as a transmitting unit  12 . The intensity of the laser diode  18  can be chosen or even regulated in such a way that the paralyzable detector in the form of an SPAD serving as a receiving unit  14  is operated with a high counting rate. 
         [0042]    One advantage of this type of calibration by means of non-modulated laser radiation can reside in the fact that the measurement time required for the calibration can be shortened with the same accuracy. 
         [0043]    A further advantage can reside in the fact that the calibration can be carried out completely using system components already present. All that is required is to switch off the laser diode modulation, which can be realized very easily. Consequently, no additional components are required, which can mean a cost saving. 
         [0044]    One advantage of the invention in accordance with one embodiment is described below on the basis of an example with continuously modulated laser radiation, specifically with sinusoidal modulation. With the designations given in  FIG. 3 , the modulation M on the receiving side is defined by 
         [0000]    
       
         
           
             
               
                 
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         [0045]    In this case, m L1  is a factor describing the modulation depth of the laser radiation emitted on the device side, {dot over (N)} L  is the temporally averaged counting rate (in counts/s) with respect to the detected laser light, {dot over (N)} DL  is the temporally averaged counting rate with respect to background radiation, and DCR is a dark counting rate of the detector. 
         [0046]    Under typical measurement conditions, the modulation can assume values in the percent range, for example. 
         [0047]    Furthermore, an estimation of the error propagation of the bin width error δτ   w    to the phase error Δφ yields the following relationship: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    where T represents the period of the modulated measurement radiation. 
         [0048]    The phase error is antiproportional to the modulation of the signal received under measurement conditions and proportional to the relative accuracy of the bin width relative to the modulation period. The high sensitivity of the system to deviations of the bin widths from the desired value thus become clear: given strong background illumination, a uniform phase accuracy requires a higher accuracy during the bin width calibration. 
         [0049]    If the calibration measurement is effected with unmodulated constant light, as proposed, then counter readings which are directly proportional to the effective bin widths arise after a certain measurement time. Consequently, a calibration of the bin widths can be carried out. Assuming that the counter events exhibit Poisson distribution, the following arises for the measurement time required for calibration: 
         [0000]    
       
         
           
             
               
                 
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         [0050]    In this case, Δφ is the uncertainty of the phase which is still afforded tolerance with calibration, M is the modulation achievable under the measurement condition, and {dot over (N)} DC  is the counting rate used during the calibration measurement. 
         [0051]    If the non-modulated laser radiation is used for the calibration measurement, then by comparison with the measurement without a laser this results in a relative time saving of: 
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         [0052]    The time saving is all the greater, the smaller the background illumination proportion.