Patent Publication Number: US-2021173051-A1

Title: Optoelectronic sensor and method for detecting an object

Description:
The invention relates to an optoelectronic sensor and to a method for detecting an object in a monitored zone. 
     Many optoelectronic sensors work in accordance with the scanning principle in which a light beam is transmitted into the monitored zone and the light beam reflected by an object is received again in order then to electronically evaluate the received signal. The time of flight is here often measured using a known phase method or pulse method to determine the distance of a scanned object. This method is also called LIDAR (light detection and ranging). 
     To expand the measured zone, the scanning beam can be moved, on the one hand, as is the case in a laser scanner. A light beam generated by a laser there periodically sweeps over the monitored zone with the help of a deflection unit. In addition to the measured distance information, a conclusion is drawn on the angular location of the object from the angular position of the deflection unit and the site of an object in the monitored zone is thus detected in two-dimensional polar coordinates. 
     Another possibility for extending the measured zone comprises simultaneously detecting measured points using a plurality of scanning beams. This can also be combined with a laser scanner that then does not only detect a monitored plane, but also a three-dimensional spatial zone via a plurality of monitored planes. 
     An optoelectronic multiplane sensor for monitoring a three-dimensional spatial zone is known from EP 1 927 867 B1. The sensor has a plurality of image sensors spaced apart from one another. A light source can be associated with each image sensor and a distance can be determined via a pulse time of flight or by a phase process with its aid. The multiplane sensor is basically set up as a multiplication of individual plane sensors and is thereby relatively bulky. 
     The scanning movement is achieved by a rotating mirror in most laser scanners. Particularly on the use of a plurality of scanning beams, however, it is also known in the prior art to instead have the total measurement head with the light sources and light receivers rotate, as is described, for example, in DE 197 57 849 B4. 
     In all the described cases of multiple scanning, a light source is required that generates a plurality of light beams having sufficient power and beam quality. The known solutions in which only single-beam transmission modules are replicated are too complex, however. 
     A distance image sensor is known from DE 101 56 282 A1 by which a matrix of, for example, vertical semiconductor lasers is imaged on the scene to be presented via a transmission optics. 
     DE 10 2004 014 041 A1 discloses a sensor system for obstacle recognition in which a sensor head rotated about its axis. A laser array having a plurality of single lasers that are imaged on the environment via an optics is located in the sensor head. 
     A device for detecting objects is described in U.S. Pat. No. 4,656,462 that has an LED array as the light source in whose beam path a collimator lens is arranged, whereby a light fan is produced. 
     The devices indicated use either a plurality of light sources or one light source whose light beam scans the monitored zone by movement of the light source itself or with the aid of movable optics such as rotating mirrors to scan an expanded monitored zone. The use of a plurality of light sources as a rule means increased space requirements for the sensor. Movable optical arrangements can, for example, be susceptible toward vibrations, which can be disadvantageous on a use in a mobile deployment in vehicles. 
     A LIDAR scanner is disclosed in U.S. Pat. No. 10,338,321 that scans a monitored zone by a transmitted beam without any movable optical arrangements while using optical switches. The monitored zone is here, however, only scanned by one single beam; a multiple scanning is not described. 
     It is therefore the object of the invention to simplify the setup of a multi-beam optoelectronic sensor. 
     This object is satisfied by an optoelectronic sensor and by a method for detecting an object in a monitored zone in accordance with the respective independent claim. The sensor in accordance with the invention is a multiple scanner that can transmit a plurality of transmitted light beams spaced apart from one another while using a switchable beam splitter arrangement having at least one light source. The transmitted light beams are not to be understood as beams in the sense of geometrical optics within a larger bundle of rays, but rather as mutually separate light beams and thus isolated scanning beams that generate correspondingly isolated, mutually spaced apart light spots in the monitored zone on impinging onto an object. An associated light receiver is able to generate a plurality of received signals from a plurality of light beams remitted from different directions, that is, for example, in that one respective photodiode is present per transmitted light beam or individual reception elements or also pixels of an image sensor are correspondingly grouped on a spatially resolved light receiver. These received signals are evaluated to acquire information on the object. 
     The invention starts from the basic idea of using a beam splitter arrangement for the splitting of the light transmitted by the light source into a plurality of transmitted light beams, with the beam splitter arrangement having at least one input for receiving transmitted light and a plurality of outputs for emitting transmitted light. The number of outputs is here larger than the number of inputs. The beam splitter arrangement has at least one switchable beam splitter to distribute the transmitted light over the outputs. 
     Switchable beam splitters are here to be understood as elements that can have one or more beam inputs and a plurality of beam outputs, with one or more of the beam outputs being able to be activated so that light entering into the beam input only exits the activated beam outputs. 
     Switchable waveguide couplers such as are known from the field of telecommunications engineering can preferably be used as switchable beam splitters. They are characterized by robustness and compact construction shapes. A switchable waveguide coupler substantially has two adjacent light guide fibers between which an electric field can be built up by applying a voltage. If light is coupled at the input of the first fiber and if a voltage is applied, the light is transmitted via an evanescent field toward the second fiber and can be decoupled at its end. If no voltage is applied, the light remains in the first fiber and can again be decoupled at its end. Switchable waveguide couplers are typically used as optical switches, with light coupled in being either switched to the one first output or to a second output. Switchable waveguide couplers can be designed as integrated optics. This has the advantage that moving optical components such as rotating mirrors can be dispensed with and a robust system can thus be implemented. 
     The light of a light source can be split into almost any desired number of transmitted light beams by cascading a plurality of switchable beam splitters in a beam splitter arrangement so that one or more outputs of the beam splitter arrangement transmit transmitted light beams depending on the control. The number of activated outputs simultaneously transmitting transmitted light beams is here substantially given by the required transmitted light power in the individual transmitted light beams, by the available light power of the light source, and by the coupling losses between the beam splitters. 
     The degree of coupling in the waveguide coupler can depend on the following parameters: distance of the fibers from one another; length of the coupling regions disposed in parallel with one another; wavelength of the transmitted light; material between the fibers; material of the fibers; applied voltage. On a use of switchable waveguide couplers, there is therefore the possibility of simultaneously activating both outputs on application of a suitable voltage to simultaneously illuminate the monitored zone with a plurality of transmitted light beams. This has the advantage that a sequential scanning of a monitored zone with a plurality of transmitted light beams can take place and thus an increased scanning rate can be achieved. 
     Typical switching times of switchable waveguide couplers are in the range from 0.5 to 5 Mhz. A fast scanning of the monitored zone is thus possible. 
     In an embodiment of the invention, the optoelectronic sensor has a light source for transmitting transmitted light. The transmitted light is coupled into a beam splitter arrangement arranged downstream of the light source. The beam splitter arrangement has at least one input for coupling transmitted light and a plurality of outputs for decoupling transmitted light, with the number of outputs being greater than the number of inputs. The beam splitter arrangement furthermore has a plurality of switchable beam splitters for splitting the transmitted light over the outputs. The switchable beam splitters can be controlled such that the transmitted light can be simultaneously distributed over at least two outputs of the beam splitter arrangement. Outputs from which transmitted light exits are also called activated outputs. The optoelectronic sensor can furthermore have a transmission optics for projecting the transmitted light beams into a monitored zone and a light receiver having a reception optics arranged upstream for receiving light beams remitted from the monitored zone. Received signals generated by the light receiver can be communicated to a control and evaluation unit. To acquire information on an object in the monitored zone, the control and evaluation unit can calculate a time of flight between transmission of the transmitted light beams and reception of the light beams remitted by the object from the received signals and can thereby determine a distance of the object from the sensor. 
     A plurality of switchable beam splitters can be arranged cascading in a plurality of planes to generate a plurality of outputs of the beam splitter arrangement, with the respective two outputs of a switchable beam splitter being able to be connected to the inputs of following switchable beam splitters. On the use of beam splitters that split an input beam over two output beams, 2 n  outputs are thus obtained with n planes. The connection of the switchable beam splitters can take place via optical fibers, waveguides, or in a free beam. 
     In an embodiment, the outputs of the beam splitter arrangement can be arranged one-dimensionally in a line. High resolution line scanners typically have a field of view (FOV) of 60 degrees at an angular resolution of 0.25 degrees, which can be advantageously implemented by a beam splitter arrangement with 240 linearly arranged outputs, that is four outputs per degree of field of view. In simpler arrangements, angular resolutions of one to two degrees are frequently sufficient so that the number of outputs of the beam splitter arrangement can be correspondingly reduced. An output of the beam splitter arrangement can therefore scan a typical field of view of at most two degrees, preferably of at most one degree, particularly preferably of at most 0.25 degrees. 
     In a further embodiment, the outputs of the beam splitter arrangement can be arranged two-dimensionally in a matrix. A two-dimensional monitored zone can thus be scanned. The considerations on the advantageous number of outputs of the beam splitter arrangement apply analogously to the above-described one-dimensional case. 
     The control and evaluation unit can be adapted to control each switchable beam splitter individually, in particular to set an individual splitting ratio for each switchable beam splitter, that is how many percent of the light power coupled into the switchable beam splitter exits the respective output of the switchable beam splitter. The transmitted light power of the transmitted light beams can thus be varied for every output of the beam splitter arrangement. The marginal regions of a monitored zone can thus be scanned, for example, at a higher transmitted light power than the middle zones. 
     The control of the switchable beam splitters in the beam splitter arrangement can also take place in dependence on evaluation results of the control and evaluation unit. If the control and evaluation unit, for example, detects an object in the monitored zone, the splitting ratios of the switchable beam splitters can be set such that the transmitted light power in the object zone is increased to enable a more exact position determination. 
     The outputs of the beam splitter arrangement can be sequentially activated and deactivated. The monitored zone can then be scanned in a linear or grid manner as with an optical scanner. 
     The switchable beam splitters can also be controlled such that the monitored zone is first scanned with a low resolution, for example in that only every second output of the beam splitter arrangement is activated and transmits transmitted light beams. On a detection of an object by the control and evaluation unit, the resolution in the object zone can then be increased as required. 
     A transmission optics can be arranged downstream of the beam splitter arrangement, with a separate transmission optics being used for each output or a common transmission optics for groups of or for all of the outputs of the beam splitter arrangement. 
     With a one-dimensional linear arrangement of the outputs of the beam splitter arrangement, a diffractive optical element can additionally be arranged downstream of the outputs that again splits a respective one output beam into a defined number of transmitted light beams. A two-dimensional scan field can be scanned with such an arrangement using a one-dimensional arrangement of beam splitter outputs, with a respective line of transmitted light beams being able to be simultaneously activated. 
     A spatially resolving area detector, preferably a matrix of photodiodes or APDs (avalanche photodiodes) or also an image sensor having correspondingly associated individual pixels or pixel groups, can be provided for the detection of the light beams remitted by objects from the monitored zone. A further conceivable embodiment provides a SPAD (single-photon avalanche diode) receiver having a plurality of SPADs. 
     The control and evaluation unit can be connected to the light source, to the beam splitter arrangement, and to the detector and is preferably configured to measure the time of flight between the transmission of the light beams and the reception of the remitted light beams and thus to determine a distance of an object in the monitored zone using a known phase or pulse method. The sensor thereby becomes distance measuring. Alternatively, only the presence of an object can be determined and output as a switching signal, for example. 
     The detector can be synchronized with the scanning of the monitored zone to suppress interfering or extraneous light. For example, only the receiver groups or pixel groups of the detector can thus be activated that receive remitted light from the zones of the monitored zone scanned by the activated beam splitter outputs. 
     The method in accordance with the invention can be further developed in a similar manner and shows similar advantages in so doing. Such advantageous features are described in an exemplary, but not exclusive manner in the subordinate claims dependent on the independent claims. 
    
    
     
       The invention will be explained in more detail in the following also with respect to further features and advantages by way of example with reference to embodiments and to the enclosed drawing. The Figures of the drawing show in: 
         FIG. 1  a schematic representation of an optoelectronic sensor; 
         FIG. 2  a schematic representation of a beam splitter arrangement; 
         FIG. 3 a    a control example for a beam splitter arrangement with a uniform distribution of the transmitted light power over all outputs; 
         FIG. 3 b    a control example for a beam splitter arrangement with a variable distribution of the transmitted light power; 
         FIGS. 4 a -4 c    a control example for a beam splitter arrangement for scanning a monitored zone; 
         FIG. 5 a    an example for a beam splitter arrangement with linearly arranged outputs; 
         FIG. 5 b    an example for a beam splitter arrangement with outputs arranged in a two-dimensional matrix; and 
         FIG. 6  a beam splitter arrangement with a diffractive optical element for further beam splitting arranged downstream. 
     
    
    
       FIG. 1  shows a schematic representation of an optoelectronic sensor  10  in an embodiment as a multi-beam distance scanner. The sensor  10  has a light source  12 , for example a laser diode, whose transmitted light is coupled into an input  13  of a beam splitter arrangement  14  having a plurality of outputs  16 . 1 ,  16 . 2 ,  16 . 3 ,  16 . 4 . The beam splitter arrangement  14  can have a plurality of switchable beam splitters (not shown) that can split the transmitted light over one or more of the outputs  16 . 1 ,  16 . 2 ,  16 . 3 ,  16 . 4 . The transmitted light exiting the outputs  16 . 1 ,  16 . 2 ,  16 . 3 ,  16 . 4  can be collimated with a transmission optics  18  and can be projected as a transmitted light beam  20  into a monitored zone  22 . The light remitted at an object  24  in the monitored zone  22  is conducted as received light  26  to a light receiver  30  via a reception optics  28 . 
     The light receiver  30  is configured as a matrix of a plurality of light reception elements  32 , preferably as a matrix of photodiodes APDs (avalanche photodiodes) or SPAD (single photon avalanche diode) receivers or also as an image sensor having correspondingly associated single pixels or pixel groups. 
     A control and evaluation unit  40  that is connected to the light source  12  and to the light receiver  30  is furthermore provided in the sensor  10 . The control and evaluation unit  40  comprises a light source control  42 , a beam splitter control  44 , a time of flight measuring unit  46 , and an object distance estimation unit  48 , with them initially only being functional blocks that can also be implemented in the same hardware or in other functional units as in the light source  12 , in the beam splitter arrangement  14 , or in the light receiver  30 . The control and evaluation unit  40  can output measured data via an interface  50  or can conversely accept control and parameterization instructions. The control and evaluation unit  40  can also be arranged in the form of local evaluation structures on a chip of the light receiver  30  or can interact as a partial implementation with the functions of a central evaluation unit (not shown). 
     The function of the beam splitter arrangement  14  of  FIG. 1  should now be explained with reference to an exemplary beam splitter arrangement having one input and eight outputs. 
       FIG. 2  shows a beam splitter arrangement  14  having seven switchable beam splitters  64   a ,  64   b . 1 ,  64   b . 2 ,  64   c . 1 ,  64   c . 2 ,  64   c . 3 ,  64   c . 4  that are arranged cascaded in three planes. 
     The first plane has a switchable beam splitter  64   a  having one input  62   a  and two outputs  66   a . 1 ,  66   a . 2 . The second plane has two switchable beam splitters  64   b . 1 ,  64   b . 2 , each having one input  62   b . 1 ,  62   b . 2  and two outputs  66   b . 1 ,  66   b . 2 ,  66   b . 3 ,  66   b . 4 . The third plane has four switchable beam splitters  64   c . 1 ,  64   c . 2 ,  64   c . 3 ,  64   c . 4 , each having one input  62   c . 1 ,  62   c . 2 ,  62   c . 3 ,  62   c . 4  and two outputs  66   c . 1 ,  66   c . 2 ,  66   c . 3 ,  66   c . 4 ,  66   c . 5 ,  66   c . 6 ,  66   c . 7 ,  66   c . 8 . 
     Transmitted light from the light source  12  can be coupled into the beam input  62   a  of the first switchable beam splitter  64   a  via the input  13  of the beam splitter arrangement  14 . The beam splitter  64   a  has two outputs  66   a . 1 ,  66   a . 2  from which the transmitted light can be coupled into the input  62   b . 1  of the beam splitter  64   b . 1  and/or into the input  62   b . 2  of the beam splitter  64   b . 2  of the second plane. The transmitted light from the beam splitter  64   b . 1  can be coupled via the output  66   b . 1  into the input  62   c . 1  of the beam splitter  64   c . 1  and/or via the output  66   b . 3  into the input  62   c . 2  of the beam splitter  64   c . 2  of the third plane. 
     The transmitted light from the beam splitter  64   b . 2  can be coupled via the output  66   b . 3  into the input  62   c . 3  of the beam splitter  64   c . 3  and/or via the output  66   b . 4  into the input  62   c . 4  of the beam splitter  64   c . 4  of the third plane. 
     The transmitted light from the outputs  66   c . 1 ,  66   c . 2 ,  66   c . 3 ,  66   c . 4 ,  66   c . 5 ,  66   c . 6 ,  66   c . 7 ,  66   c . 8  of the switchable beam splitters  64   c . 1 ,  64   c . 2 ,  64   c . 3 ,  64   c . 4  of the third plane can be decoupled from the beam splitter arrangement  14  via the outputs  16 . 1 ,  16 . 2 ,  16 . 3 ,  16 . 4 ,  16 . 5 ,  16 . 6 ,  16 . 7 ,  16 . 8 . 
     The outputs  16 . 1 ,  16 . 2 ,  16 . 3 ,  16 . 4 ,  16 . 5 ,  16 . 6 ,  16 . 7 ,  16 . 8  of the beam splitter arrangement  14  can be designed as grid couplers. The construction size of a grid coupler output amounts to between 10 μm and 25 μm depending on the type. The grid couplers can be positioned next to one another at very small intervals of typically 5 μm. A length of 3.6 mm (grid coupler output with a construction size of 10 μm and an interval of 5 μm to the next output) up to 7.2 mm (grid coupler output with a construction size of 25 μm and an interval of 10 μm to the next output) thus results for a beam splitter arrangement  14  having a one-dimensional linear arrangement of 240 outputs. 
     The switchable beam splitters  62   a ,  62   b . 1 ,  62   b . 2 ,  62   c . 1 ,  62   c . 2 ,  62   c . 3 ,  62   c . 4  are connected to the beam splitter control  44  via a wired or wireless (radio for example) connection  68 . The splitting ratio for individual ones or groups of switchable beam splitters can be set using the beam splitter control  44 . The splitting ratio can preferably be set individually for every switchable beam splitter. 
       FIG. 3 a    shows an exemplary setting of the switchable beam splitters  62   a ,  62   b . 1 ,  62   b . 2 ,  62   c . 1 ,  62   c . 2 ,  62   c . 3 ,  62   c . 4 , with a splitting ratio of the beam splitter outputs of 50/50 being set for all the beam splitters. At 100% input power of the transmitted light from the light source  12 , a power of 12.5% of the input power is thus applied at every output, with coupling losses being left out of consideration in this representation and in the following representations. A monitored zone could therefore be simultaneously scanned over a large area at this setting. 
       FIG. 3 b    shows a further exemplary setting, with a splitting ratio of the beam splitter outputs of 50/50 being set for the beam splitters of the first plane  64   a  and of the third plane  62   c . 1 ,  62   c . 2 ,  62   c . 3 ,  62   c . 4 , a splitting ratio of the beam splitter outputs of 80/20 being set for the first beam splitter of the second plane  62   b . 1 , and a splitting ratio of the beam splitter outputs of 20/80 being set for the second beam splitter of the second plane  62   b . 2 . The outer outputs  16 . 1 ,  16 . 2 ,  16 . 7 ,  16 . 8  of the beam splitter arrangement  14  are thereby acted on by a power of 20% of the input power than the inner outputs  16 . 3 ,  16 . 4 ,  16 . 5 ,  16 . 6  that are acted on by a power of 10% of the input power. The marginal zone of a monitored zone could thus be scanned at a higher power than the center. 
     The switchable beam splitters can also be controlled dynamically so that a scanning of the monitored zone by transmitted light beams of higher power is possible. Individual switchable beam splitters can be operated for this purpose at a splitting ratio of 100/0 as is described by way of example in  FIGS. 4 a    and  4   b.    
       FIG. 4 a    shows the setting of the switchable beam splitters at the start of a scan procedure in which the monitored zone is scanned with 2 transmitted light beams. For this purpose, the switchable beam splitter first splits the transmitted light at a splitting ratio of 50/50. The switchable beam splitters of the second plane  64   b . 1 ,  64   b . 2  and the switchable beam splitters  64   c . 1  and  64   c . 3  of the third plane are operated at a splitting ratio of 100/0 so that only the outputs  16 . 1  and  16 . 5  of the beam splitter arrangement are acted on by 50% of the input power in each case. A control of the switchable beam splitters  64   c . 2  and  64   c . 4  of the third plane is not necessary since they are not acted on by transmitted light. 
     In the next step shown in  FIG. 4 b   , the splitting ratio of the switchable beam splitters  64   c . 1  and  64   c . 3  of the third plane is switched to 0/100 so that only the outputs  16 . 2  and  16 . 6  of the beam splitter arrangement are acted on by 50% of the input power in each case. 
     The next step of the scan procedure is shown in  FIG. 4 c    in which the splitting ratio of the switchable beam splitters of the second plane  64   b . 1 ,  64   b . 2  is switched to 0/100 so that now the beam splitters  64   c . 2  and  64   c . 4  of the third plane are acted on by transmitted light, with a respective splitting ratio of 100/0 being set for the beam splitters  64   c . 2  and  64   c . 4  so that only the outputs  16 . 3  and  16 . 7  of the beam splitter arrangement are acted on by 50% of the input power in each case. 
     In the following step (not shown), the splitting ratio of the switchable beam splitters  64   c . 2  and  64   c . 4  of the third plane is then switched to 0/100 so that only the outputs  16 . 4  and  16 . 8  of the beam splitter arrangement are acted on by 50% of the input power in each case. The monitored zone was thus scanned by two transmitted light beams and the scan procedure can start again using the configuration shown in  FIG. 4   a.    
       FIGS. 5 a  and 5 b    show possible geometrical arrangements of the outputs  16  of a beam splitter arrangement  14 . The outputs can be arranged linearly (one-dimensional arrangement), for example, as shown in  FIG. 5 a    or in the form of a two-dimensional matrix, for example as a quadratic matrix in  FIG. 5 , with 64 times  64  outputs  16 . 1  to  16 . 64 . Other arrangement possibilities such as a circular arrangement of the outputs are likewise possible. 
       FIG. 6  shows a beam splitter arrangement  14  having a one-dimensional linear arrangement of outputs  16 . 1 ,  16 . 8 , with a static beam splitter  70 . 1 ,  70 . 8  being arranged downstream of each output that splits the transmitted light exiting the outputs  16 . 1 ,  16 . 8  of the beam splitter arrangement  14  into eight respective transmitted light beams  20 . The splitting into 8 transmitted light beams is also only to be understood as purely exemplary here. The static beam splitters  70 . 1 ,  70 . 8  can be designed as a diffractive optical element; they can have grid couplers (not shown) to decouple the transmitted light from the static beam splitter  70 . 1 ,  70 . 8 . The beam splitter arrangement  14 , the static beam splitters  70 . 1 ,  70 . 8 , and the grid couplers can be designed as an integrated optics. The construction size of a grid coupler output amounts to between 10 μm and 25 μm depending on the type. The grid coupler outputs can be positioned next to one another at very small intervals of typically 5 μm. A length of 3.6 mm (grid coupler output with a construction size of 10 μm and an interval of 5 μm to the next output) up to 7.2 mm (grid coupler output with a construction size of 25 μm and an interval of 10 μm to the next output) thus results for a beam splitter arrangement  14  having a one-dimensional linear arrangement of 240 outputs.