Optoelectronic sensor and method for detecting a monitored area

Abstract

Optoelectronic sensor (10) for detecting a monitoring area (20), comprising at least one light transmitter (22) for emitting several separately emitted light beams (26), a common light receiver (34) for generating a received signal from light beams (28) reflected from objects in the monitoring area (20), and an evaluation unit (42) configured to obtain information about the objects at the respective point of impact of an emitted light beam (26) from the received signal, characterized in that an optical concentrator element (32) is arranged upstream of the light receiver (34), which has several separately separated light entry surfaces (54) for each reflected light beam (28) as well as a common light exit surface (56, 56a), and that a common receiving optic (30) is arranged upstream of the concentrator element (32).

Description

[0001] The invention relates to an optoelectronic sensor and a method for detecting a monitoring area according to the preamble of claim 1 and 15, respectively.

[0002] Many optoelectronic sensors operate on the touch principle, in which a light beam is emitted into the monitoring area and the light beam reflected by objects is received and then electronically evaluated. One possible result of this evaluation is a binary object detection. Often, the light travel time is measured using a known phase or pulse method to determine the distance to a touched object.

[0003] To extend the measuring range, the scanning beam can be moved, as is done in a laser scanner. There, a light beam generated by a laser, deflected by a deflection unit, periodically sweeps across the monitoring area. In addition to the measured distance information, the angular position of the object is deduced from the angle of the deflection unit, thus capturing the object's location within the monitoring area in two-dimensional polar coordinates. In most laser scanners, the scanning movement is achieved by a rotating mirror. However, especially when using multiple scanning beams, it is also known in the prior art to rotate the entire measuring head, including the light emitters and receivers, as described, for example, in DE 197 57 849 B4.

[0004] Another way to extend the measurement range is to simultaneously capture multiple measurement points with multiple scanning beams. This can also be combined with a laser scanner, which then captures not just one monitoring plane, but a three-dimensional spatial area across a multitude of monitoring planes.

[0005] Typically, in such a multi-beam system, a separate light transmitter and receiver are provided for each measuring beam. This means a multiplication of the components involved in the transmission and reception path, i.e., the optics, the transmitting and receiving elements themselves, and the associated electronics, and therefore high costs. For the transmission path, there are proposals, for example in DE 10 2015 121 839 A1, to guide several transmitted light beams through a common optic or even to generate them from a common transmitting element.

[0006] Attempts to construct a multi-beam receiver module with only a single receiver element encounter difficulties and entail significant disadvantages. Specifically, the question arises of suitable receiving optics. One possibility is to assign each light beam to be received its own receiving lens, which focuses its light onto the common light receiver. Aside from the obvious disadvantage of multi-part receiving optics, each individual receiving lens has a significantly reduced receiving aperture, and consequently, the received signal level and thus the sensor's range are reduced. Multiple receiving lenses with a receiving aperture comparable to a single-beam system are not feasible due to the required size.

[0007] An alternative, shared receiving lens does not exhibit these disadvantages. However, to actually focus all light rays onto the common light receiver, a very short focal length must be chosen, resulting in a very small image size. In a typical optical design for a sensor of this type, the focal length is already chosen to be as short as the desired aperture allows. An even shorter focal length would no longer permit processing all the light from the necessary large receiving aperture at the steep convergence angles between the receiving lens and the light receiver. Therefore, the consequence here, too, would be a smaller receiving aperture and thus a reduced sensor range. In principle, the requirements for the receiving lens could be eased by choosing a very large light receiver, but the costs and the unfavorable high-frequency characteristics of such a receiver argue against this.

[0008] In principle, other optical elements besides lenses are known to combine the incoming light rays by deflecting them onto a common light receiver. Examples include optical funnels or compound parabolic concentrators (CPCs) with various round or rectangular cross-sectional shapes. A significant disadvantage of these devices is their low efficiency, as a large proportion of the light is reflected back out of the entrance aperture. Furthermore, the required entrance area is relatively large, since it covers not only the cross-sections of the light rays but also the spaces between them. Due to the conservation of light intensity, it is impossible to transfer this lossless efficiency to the smaller area of ​​the light receiver.

[0009] EP 2 910 969 B1 discloses an optoelectronic sensor which, in its receiving path, includes a receiving lens and an optical funnel element with an aperture at its entrance surface. This is intended to illuminate a relatively large-area SPAD detector, thereby mixing the received light and simultaneously suppressing ambient light. This is a single-beam system, and the aperture would suppress additional measuring beams from other angular directions, such as ambient light. Furthermore, the aperture ensures that the funnel element does not act as a concentrator, as the entrance area is effectively considerably smaller than the exit area. If the aperture were omitted, purely hypothetically, the essential purpose pursued by EP 2 910 969 B1 would be defeated. Moreover, the problem of the low efficiency of an optical funnel due to light reflected from the entrance aperture, as explained above, would remain completely unresolved.

[0010] German patent DE 10 2014 118 056 A1 describes an optoelectronic detection device for a motor vehicle. A bundle of optical fibers captures light from a forward-facing strip and directs it inwards to an associated detector of a line-type receiver for distance measurement. A receiving optic is provided at each entry point of the optical fibers. Thus, while it is a multi-beam system, it suffers from the previously explained disadvantageous multiplication of all components across the multiple receiving channels.

[0011] From DE 102 00 366 A1, a multi-channel receiver system for angle-resolved laser distance measurement is known. In this system, light is received by a bundle of closely spaced optical waveguides, each with an individual length and a preselected distribution ratio, which direct the received light to a common optical receiver. The individual optical waveguide properties result in a respective time delay and relative amplitude, making it possible to identify which optical waveguide contributed to a received signal and thus ultimately reconstruct the position information at the input of the optical waveguide bundle, even with only one receiving element. However, a dense bundle of optical waveguides would be completely unsuitable for receiving measurement beams separated by intermediate spacing, as in a multi-beam system of the type described above.Furthermore, a key aspect of DE 102 00 366 A1, namely the individual optical waveguide properties, would have different influences on the individual measuring beams and would therefore only cause interference.

[0012] From DE 10 2007 004 609 A1, a vehicle-based lidar system is known which has several individually activatable lasers for generating respective scan beams. On the receiving side, a photodetector with associated optics is used, which in most embodiments is a simple lens and in one embodiment a combination of a TIR lens with a secondary lens.

[0013] DE 101 46 808 A1 discloses an optical system for a motor vehicle. Sensor units with respective transmitting and receiving optics are provided at several positions, the optical signals of which are guided via fiber optic cables to a central measuring unit. In one embodiment, the fiber optic cables lead on the receiving side via an optical multiplexer to a common receiving element of the central measuring unit.

[0014] DE 10 2016 208 975 A1 discloses a detection device with a cap made of micro-optics in front of a pixel matrix, the pixels of which can have different distances between them.

[0015] US patent 2011 / 0085155 A1 introduces a LADAR system in which a tapered fiber bundle is provided in front of the detector to effectively increase the receiving area.

[0016] It is therefore the object of the invention to further improve a generic multi-jet system.

[0017] This problem is solved by an optoelectronic sensor and a method for detecting a monitoring area according to claim 1 and 15, respectively. The sensor according to the invention can be understood as a multi-sensor that emits several light beams from at least one light source. Thus, there are either several light sources, or the light from one light source is split into several light beams, or a combination of both is used. The light beams are not to be understood as rays in the sense of ray optics within a larger beam of light, but rather as separated beams of light and thus individual scanning beams that, upon striking an object in the monitoring area, generate correspondingly separated, spaced-apart spots of light. This creates a bundle of monitoring beams and, in the case of a scanning movement, a multitude of monitoring planes.

[0018] According to the invention, on the receiving side, only one common light receiver is provided for the multiple emitted light beams or measuring beams, in the sense that, unlike, for example, a pixel-resolved light receiver, only one common received signal is generated. However, this is preferably a light receiver with only a single pixel, such as a PIN diode or an APD. It remains possible to combine the signals from several light receiving elements. This is particularly advantageous in the case of SPADs (Single-Photon Avalanche Diodes), where a single light receiving element would be too sensitive to interference, and therefore, for more robust statistics, several SPADs contribute to the common received signal. It should be noted that the concentrator element and the light receiver are preferably, but not necessarily, responsible for all measuring beams of the sensor.Hybrid systems with additional transmitting and / or receiving modules for individual or groups of additional measuring beams remain conceivable.

[0019] The invention is based on the fundamental idea of ​​placing a non-imaging optical concentrator element upstream of the light receiver. On one side, facing the monitoring area, it has several separate light entry surfaces, each assigned to one of the reflected light rays to be received. "Separated" here, corresponding to the previously explained separate reflected light rays, means that there are free areas between the light entry surfaces. On the other side, facing the light receiver, the concentrator element has a common light exit surface. Thus, the concentrator element captures the reflected light rays at the light entry surfaces and directs them in a focused beam onto the light receiver.

[0020] The invention has the advantage that the sensor requires only one light receiver in its reception path and, in many embodiments, also only one receiving optic. This eliminates the need for separate receiving electronics. The sensor is thus significantly simplified and correspondingly less expensive. In contrast to rigid, conventional concepts with multiple measuring beams, the receiving circuit board does not need to be modified to create sensors with different arrangements of the measuring beams and the angles between them. It is sufficient to adapt the optical concentrator element. The disadvantages of conventional approaches for receiving multiple reflected light beams in a common light receiver, discussed at the outset, are not present in the concentrator element according to the invention, so there is no need to accept any compromises in sensitivity and range.

[0021] Preferably, a common receiving optic is arranged upstream of the concentrator element, with the light-entry surfaces being arranged particularly in its focal plane. The common receiving optic can be very simply designed as a single receiving lens. It ensures that the reflected light rays produce a focused light spot, so that, as far as possible, all the light from a reflected light ray is collected. An alternative embodiment with individual receiving optics in front of a light-entry surface or a group of light-entry surfaces is conceivable. This makes it possible, for example, to arrange the light-entry surfaces not in a plane, but on a curved line or surface, thus creating an additional degree of optical design freedom, which may compensate for the increased complexity and smaller receiving aperture of multiple receiving lenses.

[0022] The concentrator element preferably has several light guides, in particular one light guide for each reflected light beam, between the respective light entry surface and the light exit surface. The deflection by light guides overcomes the initial separation of the reflected light beams or the light entry surfaces at least partially, preferably completely. The reflected light beams are thus brought closer together.

[0023] At least one optical fiber preferably has a cross-section that tapers from the light-entry surface to the light-emission surface. Such tapered or conical optical fibers ensure that the cross-section of the respective reflected light beam is reduced. The receiving light spot on the output side is therefore smaller than the receiving light spot on the input side and thus more easily detected by the common light receiver. As with the optical fiber properties described below, this applies to at least one, preferably several or all, optical fibers, although in the latter case, individual exceptions are conceivable.

[0024] At least one light-entry surface is preferably tilted relative to the perpendicular to the incident reflected light beam. This can simplify the design of the concentrator element and, in particular, of light guides, since only the light-entry surface needs to be adapted. With a suitable angle of incidence of the reflected light beam, achieved by appropriately tilting the light-entry surface, the reflected light beam couples in as ideally as possible and is refracted in such a way that it subsequently follows a desired path through the concentrator element.

[0025] At least one optical fiber is preferably oriented in an entry region at the light entry surface according to the path of the reflected light beam incident there. This allows the reflected light beam to enter perpendicularly and continue its path directly in the entry region. Optimal coupling is also possible in this way.

[0026] At least some optical fibers are preferably arranged parallel to each other in an exit region on the light-emitting surface. This allows them to be packed particularly densely because no curvatures force mutual spacing. Furthermore, the parallel alignment typically corresponds to a common propagation direction perpendicular to the surface of the light receiver.

[0027] At least one optical fiber is preferably curved. This curvature ensures alignment between the respective light entry and exit surfaces, allowing for a suitable orientation at each, such as corresponding to the incident reflected light beam on the entry side and perpendicular to the light receiver on the exit side. A curved optical fiber exhibits, in particular, a constant curvature towards the center of the concentrator element in a first region beginning at the light entry surface and a constant curvature away from the center in a second region ending at the light exit surface. Preferably, this positive and negative curvature is distributed evenly along the optical fiber, i.e., with an inflection point in the center of the optical fiber, and each curvature has the same magnitude.This ensures optimal use of the available distance along the optical axis to overcome the lateral offset from the center with the smallest possible maximum curvature. Areas of high curvature would adversely affect the angular distribution within the reflected light beam.

[0028] At least one optical fiber has a higher refractive index in its core. An example of this is a cross-sectional structure with a core material of high refractive index and a cladding with a lower refractive index. More than these two refractive index levels, or a continuously increasing refractive index towards the center, are also conceivable. Such optical fibers are somewhat more complex to manufacture, but are more robust against damage to the outer sheath.

[0029] The light-entry surfaces preferably have a distance from one another corresponding to at least a quarter, half, or a whole diameter of a light-entry surface. The distances between the light-entry surfaces correspond to the separation of the light rays. With such pronounced separation, where in particular the light rays are further apart than their own diameter, it is especially difficult to manage with a single receiving element, and therefore the concentrator element according to the invention is particularly advantageous. The light-entry surfaces are preferably of the same size, which corresponds to equivalent light rays with at least approximately the same cross-section.

[0030] The concentrator element preferably includes a non-imaging second concentrator section comprising an inlet area adjoining the light guides and the light-emitting surface, tapering from the inlet area to the light-emitting surface. This second concentrator section complements the first concentrator section, namely the light guides. It captures the intermediate light spot at the end of the light guides and reduces its size due to its tapered geometry towards the exit surface. Simultaneously, the light rays can be mixed, for example, by multiple internal total internal reflections, so that, regardless of the position of the incident reflected light ray, the entire light receiver is illuminated as uniformly as possible. A funnel or a parabolic concentrator is suitable for this second concentrator section.The light losses explained in the introduction no longer occur, or at least remain very low, due to the controlled incidence of light from the light guides.

[0031] The concentrator element is preferably designed to arrange the receiving light spots of the reflected light rays on the light receiver one above the other. The initially separated reflected light rays should thus overlap at least partially or even completely on the common light receiver. For conventional processing of multiple receiving channels in separate light receivers or pixel areas, such an overlap would be unusable; according to the invention, a small area suffices for common reception. The mixing or superimposition of receiving light spots preferably takes place in the second concentrator section, after the optical fibers have overcome the separation of the light entry surfaces.

[0032] The area of ​​the light-sensitive surface of the light receiver is preferably smaller than the sum of the areas of the light-entry surfaces. This means that, in addition to the convergence of the light rays or the elimination of intermediate regions of light-ray separation, an additional concentration effect is achieved, which also reduces the area through which the reflected light rays pass towards the light receiver. Instead of the light-sensitive surface of the light receiver, the exit surface could also be used as a reference point, but at a finite distance between the optical concentrator element and the light receiver, a difference can arise depending on the divergence or convergence of the light exiting the concentrator element.

[0033] The area of ​​a light-entry surface is preferably smaller than that of the light-sensitive area of ​​the light receiver. This no longer refers to the total area of ​​all light-entry surfaces, but rather to a single light-entry surface. The result is a diaphragm effect. This is particularly helpful with large-area light receivers, such as a SPAD detector consisting of several SPAD receiving elements that provide a common receiving signal. The diaphragm effect limits the respective reception angle for a reflected light beam to the necessary minimum, thus reducing susceptibility to ambient light.

[0034] The evaluation unit is preferably designed to activate multiple light emitters sequentially in a multiplexing process to emit several separately emitted light beams. Time-division multiplexing enables the individual emitted light beams to be distinguished within the evaluation unit. The conventional approach of using a separate receiver element or at least a separate pixel area for each beam is not possible due to the shared light receiver and is thus replaced. Alternatively, it is conceivable to encode the emitted light beams, for example with a pulse sequence, and to reconstruct the identity of the emitted light beams generating the respective received signal within the evaluation unit based on this encoding.

[0035] The evaluation unit is preferably designed to determine the distance to an object from the time of flight of light between the emission of the emitted light beams and the reception of the reflected light beams. This makes the sensor capable of measuring distances. The distance is determined at multiple sampling points using the multiple light beams, with individual evaluation at each sampling point, as well as averaging or other combined evaluations being conceivable. Alternatively, instead of measuring distance, the presence of an object is simply detected and output, for example, as a switching signal.

[0036] The sensor is preferably designed as a laser scanner and features a movable deflection unit that periodically guides the emitted light beams through the monitored area, thus forming multiple scanning planes. In particular, the deflection unit is designed as a rotatable scanning unit in which the light transmitter and / or the light receiver are housed. The emitted light beams preferably do not run parallel to each other, but rather form a beam with an angular separation. Strictly speaking, this does not create scanning planes, but rather the lateral surfaces of a cone or sections thereof—visually speaking, a nesting of hourglasses. For the sake of simplicity, this is nevertheless referred to here as a scanning plane.The deflection unit is preferably designed in the form of a rotatable scanning unit, which practically forms a movable measuring head in which at least the light transmitter and possibly also the light receiver and the evaluation unit or parts thereof are housed.

[0037] The method according to the invention can be further developed in a similar manner and exhibits similar advantages. Such advantageous features are described by way of example, but not exhaustively, in the dependent claims following the independent claims.

[0038] The invention is further explained below with regard to additional features and advantages by way of example embodiments and with reference to the accompanying drawing. The illustrations in the drawing show: Fig. 1 a schematic cross-sectional view of a laser scanner with an optical concentrator element; Fig. 2. An enlarged view of the optical concentrator element; Fig. 3 a view of another embodiment of an optical concentrator element with eight instead of four light guides; Fig. 4 a view of another embodiment of an optical concentrator element with tapered light guides; Fig. 5 a view of another embodiment of an optical concentrator element with beveled light entry surfaces; Fig. 6 a view of another embodiment of an optical concentrator element with light guides which are oriented in their entry area in extension of the incident light beam; Fig. 7 a view of another embodiment of an optical concentrator element with a stabilizing connecting bridge at the ends of the light guides; Fig. 8 a view of another embodiment of an optical concentrator element with light guide ends of different lengths; Fig. 9 a view of another embodiment of an optical concentrator element with relatively closely spaced light entry surfaces and tapered light guides; Fig. 10 a representation of an optimal curvature over the length of an optical fiber; and Fig. 11 a view of another embodiment of an optical concentrator element with individual receiving optics per light guide.

[0039] Fig. Figure 1 shows a schematic cross-sectional view through an optoelectronic sensor 10 in an embodiment as a laser scanner. The sensor 10 comprises, in broad terms, a movable scanning unit 12 and a base unit 14. The scanning unit 12 is the optical measuring head, while the base unit 14 houses other components such as a power supply, evaluation electronics, connections, and the like. During operation, a drive 16 of the base unit 14 sets the scanning unit 12 into a rotary motion about a rotary axis 18 in order to periodically scan a monitoring area 20.

[0040] In the scanning unit 12, a light transmission module 22, which in this example has four individual light sources 24, generates several separated transmitted light beams 26 with a mutual angular offset, which are emitted into the monitoring area 20. The light sources 24 are, for example, LEDs or lasers in the form of edge emitters or VCSELs. The number of light sources 24 and transmitted light beams 26 need not match the figure shown, as a transmitting optic can combine or split transmitted light beams 26. Some non-exhaustive examples of a possible embodiment of the light transmission module 22 are explained in the introductory reference to DE 10 2015 121 839 A1.

[0041] If the transmitted light beams 26 encounter an object in the monitoring area 20, corresponding reflected light beams 28 return to the sensor 10. The reflected light beams 28 are guided by a common receiving optic 30 to an optical concentrator element 32 and then to a light receiver 34. The optical concentrator element 32 will be described later with reference to the Fig. Sections 2 to 10 are explained in more detail. In one embodiment according to Fig. 11 Furthermore, instead of a common receiving optic 30, several receiving optics are used for each remitted light beam 28.

[0042] The light receiver 34 is a single, unified light receiver. For example, it could be a photodiode or an avalanche photodiode (APD). It is also possible that the light receiver 34 has multiple light receiver elements but generates a single, summed signal. Unlike a conventional pixel-resolved light receiver, there is no spatial resolution. Multiple light receiver elements that together form a single light receiver are particularly useful for increasing the event statistics in a SPAD receiver (single-photon avalanche diode).

[0043] The light transmitter module 22 and the light receiver 34 are arranged on a circuit board 36, which lies on the rotary axis 18 and rotates with the shaft 38 of the drive 16. A contactless power supply and data interface 40 connects the movable scanning unit 12 to the stationary base unit 14. A control and evaluation unit 42 is located there, which controls the light transmitter module 22 and receives the signal from the light receiver 34 for further evaluation. It also controls the drive 16 and receives the signal from an angle measuring unit (not shown), commonly used in laser scanners, which determines the respective angular position of the scanning unit 12.

[0044] For evaluation, the distance to a scanned object is preferably measured using a known time-of-flight method. Together with the information about the angular position from the angle measuring unit, two-dimensional polar coordinates of all object points in a scanning plane are available after each scan period, including angle and distance. The respective scanning plane is also known via the identity of the respective transmitted light beam 26 or reflected light beam 28 on the light receiver 34, so that a three-dimensional spatial region is scanned overall.

[0045] Since only one common light receiver 34 is used, the identity of the reflected light beams 28, and thus the scanning plane or angular direction in elevation, cannot be determined conventionally by the location of the light incidence or the identity of the respective light receiver. Therefore, in an advantageous embodiment, the individual light sources 24 are operated using a time-division multiplexing method. In this case, there is always only one transmitted light beam 26 and consequently one reflected light beam 28, whose identity is therefore known at the light receiver 34 or in the downstream evaluation unit. Alternatively, different codes, such as pulse sequences, can be imprinted on the transmitted light beams 26, which are then used by the control and evaluation unit 42 to identify the reflected light beam.

[0046] Thus, the object positions or object contours are known and can be output via a sensor interface 44. Conversely, the sensor interface 44, or another connection not shown, serves as a parameterization interface. The sensor 10 can also be configured as a safety sensor for use in safety engineering to monitor a hazard source, such as a dangerous machine. In this case, a protective field is monitored that must not be entered by the operator during machine operation. If the sensor 10 detects an unauthorized intrusion into the protective field, such as an operator's leg, it triggers an emergency stop of the machine. Sensors 10 used in safety engineering must operate with exceptional reliability and therefore meet high safety requirements, such as the EN13849 standard for machine safety and the EN61496 standard for non-contact protective devices (NCPDs).In particular, the sensor interface 44 can then be configured as a safe output interface (OSSD, Output Signal Switching Device) in order to output a safety-oriented shutdown signal in the event of a protective field intrusion by an object.

[0047] The sensor 10 shown is a laser scanner with a rotating measuring head, namely the scanning unit 12. Alternatively, periodic deflection by means of a rotating mirror is also conceivable. However, with multiple transmitted light beams 26, this has the disadvantage that the way in which the multiple transmitted light beams 26 fall into the monitoring area 20 depends on the respective rotational position, since their arrangement rotates due to the rotating mirror, as known geometric considerations show. Another alternative embodiment pivots the scanning unit 12 back and forth, either instead of the rotational movement or additionally about a second axis perpendicular to the rotational movement, in order to generate a scanning movement even in elevation.

[0048] The Fig. The laser scanner setup shown in Figure 1 is purely illustrative. Alternatively, any other optical arrangement known from one-dimensional optoelectronic sensors or laser scanners, such as the use of a deflecting or beam-splitter mirror, would be possible. Instead of the circuit board 36 located on the axis of rotation, a horizontal circuit board and, in principle, any single or multiple circuit board arrangement are also conceivable. Furthermore, the control and evaluation functionality can be distributed virtually arbitrarily across the scanning unit 12, the socket unit 14, and, if necessary, external components.

[0049] The implementation as a laser scanner is also exemplary. A one-dimensional probe without periodic movement is also possible, which then consists practically only of the stationary scanning unit 12 with corresponding electronics, but without the socket unit 14.

[0050] Fig. Figure 2 shows the concentrator element 32 again in an enlarged view. The common receiving optics 30 focuses the reflected light rays 28 onto spatially separated receiving light spots 46. The concentrator element 32 is therefore preferably located with its input region in the focal plane of the common receiving optics 30. The receiving light spots 46 are separated from each other according to the separated transmitted light rays 26; thus, there is a distance between each spot that corresponds to a fraction, for example, at least a quarter or half, of the diameter of a receiving light spot 46. Preferably, the receiving light spots 46 are even further apart than a diameter, as shown. Furthermore, the receiving light spots 46 preferably form a row, as shown. A two-dimensional arrangement with mutual offset perpendicular to the plane of the paper is also possible. Fig. However, option 2 remains conceivable.

[0051] The concentrator element 32 has a first concentrator section 48, which collects the receiving light spots 46 and focuses them into an intermediate light spot 50. For this purpose, a light guide 52 is provided for each receiving light spot 46, each light guide having a light-entry surface 54 at the receiving light spot 46. The light-entry surface 54 is large enough for the receiving light spot 46. It can be intentionally chosen to be smaller than the light-sensitive area of ​​the light receiver 34, which then leads to a diaphragm effect for each reflected light ray 28 and thus improved resistance to ambient light. The light guides 52 are curved towards the center, so together they fill a smaller cross-section in the direction of the light receiver 34. Therefore, the distances between the receiving light spots 46 are partially or even almost completely eliminated in the intermediate light spot 50. Advantageous curvature properties of the light guides 52 will be discussed later, particularly with reference to the Fig. 10, still under discussion.

[0052] The exit areas of the light guides 52 at the intermediate light spot 50 can together already form a light exit surface 56a of the concentrator element 32, provided that the total area fits sufficiently onto the light receiver 34.

[0053] Otherwise, an optional second concentrator section 58 preferably follows without gaps, and only its exit area forms the light exit surface 56. The second concentrator section 58 is designed to taper, so that the light exit surface 56 is smaller than the entrance area at the intermediate light spot 50. This reduces the cross-sectional area of ​​the intermediate light spot 50, so that as much light as possible reaches the light receiver 34.

[0054] The funnels or parabolic concentrators mentioned in the introduction are also suitable for the geometry of the second concentrator section 58. Within the concentrator element 32, a lower luminous efficiency due to light reflected from the second concentrator section 32 is practically irrelevant, since the beam path is predetermined and the transition is seamless. The tapering can additionally or alternatively occur in the transverse dimension (not shown) perpendicular to the plane of the paper. Fig. 2. The second concentrator section 58, in addition to its effect of reducing the area, can also mix the remitted light rays 28 and thus distribute the received light largely independently of the specific remitted light ray 28 over the entire light receiver 34 at least somewhat uniformly.

[0055] Fig. Figure 3 shows another embodiment of the optical concentrator element 32. In this case, eight instead of four light guides 52 are provided for a corresponding number of reflected light beams 28. In addition, simulated beam paths in the concentrator element 32 are shown, from which, among other things, the mixing in the second concentrator section 58 can be seen.

[0056] As a numerical example, the simulation assumes light entry surfaces 54, each with an area of ​​0.5 x 0.5 mm. The eight light guides 52, each 30 mm long, terminate at an intermediate light spot with an area of ​​1.0 x 2.0 mm; they are arranged in two slightly offset layers, each with four light guides. The second concentrator section 58, with a length of 10 mm, further reduces the cross-sectional area to 1.0 x 1.0 mm, which is a realistic area size for a light receiver 34 designed as a SPAD detector. Smaller areas, for example for a photodiode or APD, can also be achieved with different numerical values.

[0057] Even with a full volume of PMMA plastic, an overall efficiency of 70% can be achieved. By supplementing this with measures such as avoiding intersections of the light guides 52 and applying anti-reflective coatings to the entry and exit facets of the light guides 52, an increase to 85% is easily possible. For less demanding applications, for example, with only three arms with light entry areas of 0.5 x 0.5 mm and a light receiver area of ​​0.8 x 0.8 mm, an efficiency of 88% can be achieved over a length of 10 mm + 5 mm. Since a Fresnel reflection loss of 2 x 4% is unavoidable, this value is already very close to the optimal efficiency of 92%.

[0058] Fig. Figure 4 shows an embodiment of the optical concentrator element 32 in which the light guides 52 already taper in the first concentrator section 48. For the sake of simplicity, only three light guides 52 are shown in this and the following examples. This number is still exemplary; the sensor 10 can have any number of light beams 26, 28.

[0059] The tapered light guides 52 in the first concentrator section 48 ensure that the receiving light spots 46 not only move closer together but also become smaller individually. The second concentrator section 58 complements this effect, or the second concentrator section 58 can be omitted.

[0060] Fig. Figure 5 shows an embodiment of the optical concentrator element 32 with light entry surfaces 54 that are angled differently depending on the position of the light guide 52. This achieves optimal coupling of reflected light rays 28 arriving from different angles. The angled light entry surfaces 54 refract the main ray of the respective reflected light ray 28, so that it then runs parallel to the entry area of ​​the light guide 52.

[0061] Fig. Figure 6 shows an alternative embodiment. Here, the light guides 52 have perpendicular light entry surfaces 54, but are themselves bent differently so that the entry area corresponds to the respective direction of the incident reflected light rays 28. This also enables optimal coupling, now centrally located in the angular space.

[0062] Fig. Figure 7 shows an embodiment of the optical concentrator element 32, which now includes an additional bridge 60 extending over all light entry surfaces 54. Optically, this bridge 60 has practically no effect, but it provides improved mechanical stability and fixes the light guides 52 in their desired arrangement.

[0063] Fig. Figure 8 shows an embodiment of the optical concentrator element 32 in which the light entry surfaces 54 are not on a straight line. The light guides are therefore oriented in the X-direction of the Fig. 8 different lengths. A transverse curve of the light entry surfaces 54, for example a circular path, or a two-dimensional arrangement with a curved surface forms a further design degree of freedom, with which, for example, an image field curvature of the common receiving optics 30 can be compensated.

[0064] Fig. Figure 9 shows an embodiment of the optical concentrator element 32, which initially corresponds to that shown in Figure 9. Fig. 3 is similar because the optical fibers 52 already taper. Furthermore, in this case, the optical fibers 52 are straight, and their light entry surfaces 54 are only slightly separated from each other, meaning they can, in principle, be positioned almost without gaps next to each other. With such a concentrator element 32, a moving receiving light spot can be detected by a stationary light receiver 34, for example, in the case of a moving, scanning transmitting light beam 26 using an optical phased array or other methods. The second concentrator section 58 ensures that the optical fibers 52 do not need to be too tapered or too long, while still achieving a sufficient reduction in cross-section.

[0065] Fig. Figure 10 illustrates an advantageous curvature profile in the optical fibers 52. The upper part shows the slope, and the lower part shows the curvature as its derivative, each as a function of the lateral X-position on an optical fiber 52. In the entry region, at the light entry surface 54, the optical fiber 52 should be parallel to the main ray of the reflected light ray 28, which may be refracted upon entering the optical fiber 52. In the exit region, i.e., at the light exit surface 56a or at the transition to the second concentrator section 58, the optical fiber 52 should be parallel to the other optical fibers 52 and thus perpendicular to the light receiver 34.

[0066] If the optical fibers 52 do not run at least somewhat parallel to each other in the exit region, they will strike the light-emitting surface 56 at an angle that is no longer negligible from the vertical. At least a portion of the light beams will then be inclined more sharply than the critical angle for total internal reflection. These light components can then ultimately exit the concentrator element 32 at a different point, resulting in a reduced overall efficiency.

[0067] Therefore, in order to achieve the desired path of the light guides 52 at the entry and exit regions, a mutual lateral offset of the light guides 52 towards the center must be overcome by bending the light guides 52. However, excessive curvature would adversely alter the angular distribution of the guided light. Therefore, the maximum value of the curvature should be as small as possible. To achieve this, the necessary lateral offset of the light guides 52 must be distributed over a length of the light guides 52 limited by the available installation space. The optimal solution for this is shown in Fig. Figure 10 shows that the curvature remains constant, but changes sign at an inflection point in the middle. Any other distribution would have at least one point with greater curvature for the same integrated lateral offset over the length of the optical fiber 52.

[0068] Fig.Figure 11 shows another embodiment of the concentrator element 32. Here, individual receiving optics 30a are provided in front of each light-entry surface 54, which replace or supplement the common receiving optic 30. A hybrid form would also be conceivable, consisting of only one common receiving optic 30 and individual receiving optics 30a, for example, with several receiving lenses for each group of light-entry surfaces 54. Alternatively, any other embodiment presented could be used instead of the concentrator element 32 shown.

[0069] The optical concentrator element 32, described in various embodiments, can be made of glass or plastic and manufactured, for example, by injection molding, casting, or embossing. A solid volume, a solid volume with external mirroring, or a hollow body with internal mirroring are possible. The light guides 52 can be flexible, for example, in the form of thin glass fibers or elastic silicones. The now movable light entry surfaces 54 can then be used for adjustment.

[0070] Optical fibers 52 can also have a more elaborate cross-sectional structure, i.e., a core material with a high refractive index for total internal reflection and a cladding material with a lower refractive index, or even a multi-stage or continuous radial refractive index gradient. The more complex manufacturing process is then offset by the advantage that the outer surface is less susceptible to contamination and scratches, because the effective total internal reflection area is shifted inwards. The optical fibers 52 are preferably round, but other cross-sectional shapes or a change in the cross-sectional shape along the length of the optical fibers 52 are conceivable.

[0071] In the illustrated embodiments, the light guides 52 each lie in a single plane. However, deviations from this are possible. Firstly, it is possible that the light-entry surfaces 54 form a two-dimensional pattern and do not lie on a straight or curved line. Even if the light-entry surfaces 54 remain in a single plane, it can be advantageous to bend the light guides 52 in both transverse spatial directions, i.e., also perpendicular to the paper plane of the illustrations. Particularly in the case of a larger number of more than just three or four light guides 52, this creates space to guide the exit areas in a two-dimensional grid, rather than just a one-dimensional one, onto a more compact intermediate light spot 50, thus making better use of the two dimensions of the surface of the light receiver 34.

Claims

[1] Optoelectronic sensor (10) for detecting a surveillance area (20), which has at least one light transmitter (22) for emitting a plurality of emitted light beams (26) separated from one another, a common light receiver (34) for generating a received signal from light beams (28) remitted by objects in the surveillance area (20) and an evaluation unit (42) which is designed to obtain information about the objects at the respective point of incidence of an emitted light beam (26) from the received signal, characterized by that an optical concentrator element (32) is arranged upstream of the light receiver (34), which has a plurality of mutually separated light entry surfaces (54) for each remitted light beam (28) and a common light exit surface (56, 56a), and that a common receiving optics (30) is arranged upstream of the concentrator element (32). [2] Sensor (10) according to claim 1, wherein the light entry surfaces (54) are arranged in the focal plane of the common receiving optics (30). [3] Sensor (10) according to claim 1 or 2, wherein the concentrator element (32) has a plurality of optical fibers (52), in particular one optical fiber (52) for each remitted light beam (28), between a respective light entry surface (54) and the light exit surface (56). [4] Sensor (10) according to claim 3, wherein at least one light guide (52) has a cross-section tapering in the direction from the light entry surface (54) to the light exit surface (56a). [5] Sensor (10) according to one of the preceding claims, wherein at least one light entry surface (54) is tilted relative to the perpendicular to the remitted light beam (28) incident thereon. [6] Sensor (10) according to one of claims 3 to 5, wherein at least one optical fiber (52) is oriented in an entry region on the light entry surface (54) according to the light path of the remitted light beam (28) incident there and / or wherein at least some optical fibers (52) are arranged parallel to one another in an exit region on the light exit surface (56a). [7] Sensor (10) according to one of claims 3 to 6, wherein at least one optical fiber (52) is bent, in particular in a first region beginning with the light entry surface (54) with a constant curvature towards the center of the concentrator element (32, 48) and in a second region ending with the light exit surface (56a) with a constant curvature away from the center. [8] Sensor (10) according to one of claims 3 to 7, wherein at least one optical fiber (52) has a higher refractive index in the interior. [9] Sensor (10) according to one of the preceding claims, wherein the light entry surfaces (54) are spaced apart from one another by a distance corresponding to at least a quarter, a half or a whole diameter of a light entry surface (54). [10] Sensor (10) according to one of the preceding claims, wherein the concentrator element (32) has a non-imaging second concentrator section (58), in particular a funnel or a parabolic concentrator, which has an entrance region adjoining the light guides (52) and the light exit surface (56) and tapers from the entrance region to the light exit surface (56). [11] Sensor (10) according to one of the preceding claims, wherein the concentrator element (32) is designed to arrange the received light spots (46) of the remitted light beams (28) on the light receiver (34) one above the other. [12] Sensor (10) according to one of the preceding claims, wherein the surface area of ​​the light-sensitive surface of the light receiver (34) is smaller than the sum of the surface areas of the light entry surfaces (54) and / or wherein the surface area of ​​a light entry surface (54) is smaller than that of the light-sensitive surface of the light receiver (34). [13] Sensor (10) according to one of the preceding claims, wherein the evaluation unit (42) is designed to activate a plurality of light transmitters (24) for emitting the plurality of light beams (26) emitted separately from one another in a multiplexing process one after the other. [14] Sensor (10) according to one of the preceding claims, wherein the evaluation unit (42) is designed to determine a distance of the object from a light propagation time between emission of the emitted light beams (26) and reception of the remitted light beams (28) and / or which is designed as a laser scanner and has a movable deflection unit (12) with the aid of which the emitted light beams (26) are periodically guided through the monitoring area (20) and thus form a plurality of scanning planes, wherein in particular the deflection unit (12) is designed in the form of a rotatable scanning unit (12) in which the light transmitter (22) and / or the light receiver (34) is accommodated. [15] Method for detecting a surveillance area (20), in which a plurality of separate light beams (26) are emitted into the surveillance area (20), a reception signal is generated from the light beams (28) remitted by objects in the surveillance area (20) in a common light receiver (34), and the reception signal is evaluated in order to obtain information about the objects at the respective point of incidence of an emitted light beam (26), characterized by that the remitted light beams (28) are collected by an optical concentrator element (32) in front of the light receiver (34) at a plurality of mutually separated light entry surfaces (54) for one remitted light beam (28) in each case and are combined in the optical concentrator element (32) onto a common light exit surface (56a, 56), and that a common receiving optics (30) is arranged in front of the concentrator element (32).

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