Photoelectric sensor for detecting an object within a detection area and method for enhancing the location of the detection area - Patents.com

Abstract

To provide a photoelectric sensor for detecting an object in a detection area.SOLUTION: A sensor includes a light receiving element (12) for generating a received signal from light incident from a detection area (14), marking devices (22a-b) for highlighting the location of the detection area (14) for a human eye by means of a light pattern (26) projected in the detection area (14), and a control and evaluation unit (30) for evaluating the received signal. The light pattern (26) includes at least two partial patterns (26a-b) projected from different sources (22a-b). The two partial patterns jointly mark a center point (28) of the detection area (14), independent of the distance at which the light pattern (26) is projected in the detection area (14).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a photoelectric sensor for detecting an object in a detection area and a method for highlighting the position of the detection area according to the preambles of claims 1 and 15, respectively. [Background technology]

[0002] Making the detection area of ​​a photoelectric sensor visually perceptible is useful in many applications. One of many examples is the reading field of a code reader. In manual reading, this allows the code to be correctly positioned. In reading tunnels, where objects are transported through the reading field, it is useful during the adjustment phase. The marking device for visualizing the detection area is also called aiming device, since it allows the orientation of the sensor to be known. A single laser dot indicating its center may be sufficient to highlight the detection area, but more complex patterns are also used. During actual operation, e.g. when a camera is taking pictures, the aiming device is usually switched off to avoid disturbances.

[0003] The surrounding area of ​​the optical axis of the sensor is required for light reception, e.g. for the objective lens and the camera chip. The light source of the aiming device must therefore be mounted off-axis. However, this has the consequence that the visualization of the detection area can only be set for a certain distance, e.g. for the focal length. At distances outside this range, a lateral shift of the light spot or light pattern occurs.

[0004] Patent document 1 discloses an image capture device with adaptive illumination, i.e. objective illumination and / or positioning illumination. When changing the position of the focal point of the receiving optics, the objective and / or positioning illumination are automatically adapted to the changed focal point. This is therefore an example of an aiming device that is adjusted to the focal position, because the superposition of the projected light patterns occurs exactly at that position.

[0005] US 2007 / 0133933 A1 relates to a camera system which projects a glowing reference pattern onto a detection plane, where the focus of the camera and the reference pattern correspond to each other, so that here too a focal length is selected, and at other distances the reference pattern will not in any case provide an optimal visualization.

[0006] Unpublished European Patent Application No. 22185480.5 describes a code reading device that repositions the focus and reports on the goodness of the last adjusted focus position. This can be done through a light signal or light pattern that the marking device projects onto the optical code. This is another example of coupling with focal length. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] EP 2 136 248 A1 [Patent Document 2] WO 2009 / 007772 A2 Summary of the Invention [Problem to be solved by the invention]

[0008] SUMMARY OF THE DISCLOSURE It is therefore an object of the present invention to further develop an indication device of a photoelectric sensor for visualizing the detection area. [Means for solving the problem]

[0009] This problem is solved by a photoelectric sensor for detecting an object in a detection area and a method for highlighting the position of the detection area according to claims 1 and 15, respectively. Detection of an object here can mean, for example, determining the properties of the object (such as distance), taking an image of the object or a scene containing the object, reading an optical code, etc. Light from the detection area is incident on a light receiving element and is converted into a received signal, in particular for generating image data. An indication device makes the detection area visible to the human eye by means of a projected light pattern. The indication device is preferably configured as an aiming device for adjusting the photoelectric sensor. The orientation can be checked and readjusted via visual feedback. A control and evaluation unit evaluates the received signal or image data. This can be done internally in the photoelectric sensor or externally in a computing unit connected thereto, such as a control device, computer, network or cloud, or partly internally and partly externally. The specific evaluation and its purpose can be very diverse, but this is not important for the moment, since the invention relates to an optical highlighting of the position of the detection area.

[0010] The basic idea from which the invention starts is to synthesize a light pattern from at least two projected partial patterns. Each partial pattern originates from its own source, and is not meant to refer to a partial area of ​​one common projected light pattern. The partial patterns can be generated directly from the source, for example a line generator, or a pattern generating element is associated with each source or each partial pattern. This can be as simple as a cylindrical lens or a slit diaphragm, or more complex, such as a DOE (diffractive optical element). The two partial patterns jointly mark the center point of the detection area. Here, "marking" means visual enhancement or visualization for the human eye. The two partial patterns mark the center point independently of the distance at which the projected light pattern strikes the scene, object or background in the detection area.

[0011] A source can be a light source, such as an LED or laser, or can be derived from the light source, for example by a segmented mirror. The source cannot be arranged coaxially, since, as mentioned at the beginning, the optical axis is already occupied by the receiving path. The optical axis here is the optical axis of the photoelectric sensor or the optical axis of the receiving element or the receiving optics assigned to it. Despite this unavoidable deviation of the axis of the source relative to the optical axis of the photoelectric sensor, the invention succeeds in marking the center of the detection field, for example not only in the focal length but also independent of the distance.

[0012] The invention has the advantage that the center point of the projected light pattern coincides with the optical axis regardless of the distance, and therefore the position of the detection area can be estimated with the same accuracy and reliability regardless of the distance. Suitable sources for generating the partial patterns are small and inexpensively available, such as DOE pattern projectors or laser line projectors. Different colors or wavelength ranges can also be used for better visualization or to make the light pattern stand out so that it is clearly visible in different views. The construction of the sensor with the indicator device according to the invention is at most only slightly more expensive than the known solutions described at the beginning. A modular construction is also conceivable, in which the indicator device can simply be inserted into the basic construction of an existing sensor or its illumination.

[0013] Preferably, each partial pattern is a line, which lines are at an angle to each other, in particular at right angles to each other, and intersect at said center point. Two mutually perpendicular lines resemble a cross. However, unlike a single projected cross, the intersection point of the cross is not fixed but moves relative to the middle of the lines depending on the distance to which the partial pattern is projected. This ensures that the center point of the detection area remains marked regardless of the distance, which is not the case with a single projected cross, which is displaced from the center point depending on the distance. The principle that the intersection of the lines highlights the center point regardless of the distance is the same in the case of two non-perpendicular lines or in the case of more lines, for example three lines displaced at an angle of 120 degrees. In this embodiment, the line is the basic element of each partial pattern, which marks the center point. However, this does not exclude the inclusion of further elements (e.g. a point raster) in the partial or light pattern in order to better convey the impression of the surface, or even for reasons not necessary for marking the center point.

[0014] The sources are preferably arranged circumferentially around a central point at angles different from 0° and 180°. Thus, in this embodiment, the sources are not on a common radius, nor on a common diameter through the center. For example, when using lines as partial patterns, otherwise the two partial patterns would simply result in one common line as the overall light pattern, with no discernible intersection at the center. An example of an angular offset different from 0° and 180° would be sources arranged at 90° offset from each other. Note that the sources do not necessarily have to be on the same circle around the central point, but could be at different radial distances.

[0015] Preferably, each partial pattern marks its own partial pattern center point, whereby said center point is marked as the middle of said partial pattern center points. In this embodiment, an additional or only visual clue to the center point can be provided. The human eye can easily recognize each partial pattern center point and recognize their middle as the center point of the desired detection area. In this case, the center point in the light pattern can already be highlighted, and the partial pattern center point can provide an additional clue. Alternatively, the center point of the light pattern itself is not highlighted, but the observer is allowed to find said center point based on the middle of the partial pattern center points.

[0016] Each partial pattern is preferably a cross or an arrangement of concentric circles. These are examples of partial patterns that mark their own partial pattern center point. The partial pattern cross should not be confused with conventional aiming devices, where the intersection of the cross marks the center point, but in this case a deviation from the actual center point occurs if the source is placed off-axis. In contrast, in the embodiment described here the intersection of the cross marks the partial pattern center point, and the center point of the detection area is only determined in a second step as the middle of the partial pattern centers. The center point is therefore highlighted without deviation at any distance.

[0017] The partial patterns are preferably identical to one another. "Identical" here can be understood in particular as being congruent and of the same size. This simplifies the construction of the sensor and the recognition of the centre point. For example, if two crosses are projected, the centre point is in the middle of the intersection of the crosses, or if an arrangement of two concentric circles is projected, the centre point is in the middle of the line joining the centres of the circles.

[0018] The sources are preferably positioned equidistant from the center point on either side of the center point. In this embodiment, the sources are positioned approximately to the left and right of the optical axis or receiving path on a common diameter, rather than at angular offsets different from 0 and 180 degrees as described above. This is particularly suitable in connection with a partial pattern that marks its own partial pattern center point.

[0019] The sources preferably each comprise a single sign light source, whereby each partial pattern originates from its own light source (LED, laser diode, etc.), but the light source may also comprise several individual light sources, e.g. to increase the light intensity.

[0020] Preferably, at least one source separates the light of another source with a beam splitter. In this embodiment, each source no longer has its own light source. At least two sources share one common light source. In addition to the beam splitter, there may be other optical elements, for example other mirrors, to bring the partial patterns to the desired position and / or orientation.

[0021] It is preferred that the sensor comprises an illumination device for illuminating the detection area, the indication device being part of the illumination device. The illumination device assists the actual detection of the sensor. Many illumination devices operate in the invisible wavelength range (especially in the infrared range) or are always only active for a short time. They are therefore not suitable for visually highlighting the detection area, especially since they do not provide a pattern that makes it easier for the human eye to locate the detection area. However, the indication device, in particular its indication light source, can be integrated into the illumination device. In particular a common illumination / indication module can be constructed.

[0022] It is preferred that the lighting device comprises a number of light sources distributed around the periphery, in particular distributed in a circular manner, and that the sources of the indicator device are arranged between the light sources distributed around the periphery. The light sources distributed around the periphery, preferably around the optical axis of the sensor, illuminate the detection area in accordance with the optical axis. The sources of the indicator device can be arranged in a distributed manner around the periphery. This is on the one hand a suitable location for the projection of the partial patterns, and on the other hand simplifies the overall construction. Other arrangements of the light sources of the lighting device are also possible, for example in a matrix with sources in rows and columns or between them.

[0023] The sensor is preferably configured as a camera and the light receiving element is preferably configured as an image sensor having a number of pixels for generating image data, the received signal then being the respectively captured image or a part thereof, the pixels being preferably arranged in a matrix.

[0024] Preferably, the sensor is configured as a code reading device for reading the optical code, and the control and evaluation unit is configured to read the optical code by evaluation of the received signal by means of a decoding method, which code reading device can be a barcode scanner or a camera-based code reader.

[0025] The sensor is preferably fixedly mounted near a conveying device which guides the object to be detected in the conveying direction through the detection area. This is a very common use situation, in particular with a sensor configured as a code reader in a reading tunnel. By marking the center point of the detection area according to the invention, the adjustment of the sensor in a fixed working position is facilitated and improved.

[0026] The method according to the invention can be implemented in a similar manner and thus exhibits similar advantages, such advantageous features being exemplarily recited, for example but not exclusively, in the dependent claims following the independent claims of this application.

[0027] The invention, also taking into account further characteristics and advantages, will now be described in greater detail, exemplarily on the basis of embodiments, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0028] [Figure 1] FIG. [Diagram 2] A photoelectric sensor configured as a camera is typically mounted near the conveying device. [Diagram 3] A plan view of an illumination device with a circular arrangement of light sources and two line projectors embedded within them. [Figure 4] Similar to Fig. 3, but now with three line projectors. [Diagram 5] Figure 3 with dot patterns added. [Figure 6] A plan view of an illumination device having light sources arranged in a matrix and two line projectors embedded within them. [Figure 7] Schematic diagram of a photoelectric sensor configured as a camera with two lateral projectors parallel to its axis. [Figure 8] Similar to FIG. 7, but with only one projector and additional splitting by a split mirror. [Figure 9] Two crosses as exemplary partial patterns. [Figure 10] An arrangement of two sets of concentric circles as an exemplary partial pattern. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] 1 shows a schematic cross-sectional view of a photoelectric sensor 10, here exemplarily configured as a camera, in particular as a camera-based code reader. A light receiving element 12, in particular an image sensor with pixels arranged in a matrix, generates a receiving signal or image data from the incident light from a detection area. A receiving optics 16 is arranged in front of the light receiving element 12. The optics is preferably configured as an objective lens consisting of several lenses and other optical elements such as apertures, prisms, etc., but is represented here for simplicity by just one lens.

[0030] The sensor 10 includes an illumination unit 18 with a number of light sources 20 for illuminating the detection area 14. In addition, the illumination unit 18 is provided with two indicator light sources 22a-b, shown in dashed lines. Alternatively, the indicator light sources 22a-b can be separate from the illumination unit 18. The light sources 20 and the indicator light sources 22a-b can be, for example, LEDs or laser diodes. The geometrical arrangement of the light sources 20 and the indicator light sources 22a-b is insufficient from the merely schematic drawing of FIG. 1. They are, for example, arranged in a circle around the optical axis 24 of the sensor 10. This and other arrangements will be illustrated and discussed later.

[0031] The marking light sources 22a-b, possibly with the aid of their respective pattern generating elements (not shown), jointly generate a light pattern 26 in the detection area 14. The light pattern 26 consists of at least two partial patterns 26a-b, each of which is projected by one of the marking light sources 22a-b. The two partial patterns 26a-b mark a center point 28 of the detection area 14, which is a substantially two-dimensional observation of the detection area 14 at a given distance in a plane 29. The light pattern 26 is formed in this plane 29 because there is an object or a background (e.g. a wall, a conveyor belt, etc.) therein.

[0032] The marking light sources 26a-b cannot be arranged coaxially on the optical axis 24, because this would cause the light receiving element 12 or the receiving path of the receiving optics 16 to be shaded. Thus, even though the marking light sources 26a-b are arranged off-axis, i.e. offset from the optical axis 24, the center point 28 is marked regardless at the distance of the plane 29. In the example shown in FIG. 1, the two partial patterns 26a-b are lines with different orientations, in particular perpendicular to each other, so that they cross each other and mark the center point 28 at the intersection. The cross of the two lines is distorted asymmetrically depending on the distance of the plane 29, but the intersection remains at the center point 28. The observer can therefore determine the position of the detection area 14 at any distance based on the center point 28 and can, for example, center an object in the detection area 14 or control or adjust the mounting position and orientation of the sensor 10.

[0033] A control and evaluation unit 30 with at least one digital computing component is connected to the light receiving element 12 and the illumination unit 18 and is responsible for the tasks of control, evaluation and further regulation within the sensor 10. It generates the light pattern 26 as required, illuminates the detection area 14, records the received signals or images and evaluates the received signals or images. In the embodiment as a code reader, an optical code captured in the received signals or image data is read. For this purpose, a segmentation is preferably performed in a manner known per se to find the code area, which is then passed to at least one decoder. The read code and optionally also the raw data or preprocessing results can be stored or output to the interface 32. Several components can be provided for different control and evaluation tasks, for example the preprocessing of the image data can be performed on a separate FPGA and the actual evaluation can be performed by a microprocessor. At least part of the control and evaluation unit 30 can also be provided externally.

[0034] Said interface 32 or another interface can be used to activate and deactivate the sign light sources 22a-b from the outside. Departing from Fig. 1, the lighting unit 18 and / or the unit with the sign light sources 22a-b can be an external part with appropriate connections for activating the lights or the light pattern 26. However, it is also possible to waive individual control of the sign light sources 22a-b, by automatically deactivating the sign light sources 22a-b when the lights are activated, for example in order not to interfere with the actual detection during operation.

[0035] The sensor 10 is protected by a casing 34 which is closed in the front area where the light enters and exits by a front panel 36. The illustrated constructional arrangement is to be understood as purely exemplary.

[0036] FIG. 2 shows an exemplary mounting of a photoelectric sensor 10 configured as a camera near a transport device or belt conveyor 38. The sensor 10 is shown here only symbolically, its structure as explained on the basis of FIG. 1 is no longer shown. The belt conveyor 38 transports an object 40 through the detection area 14 as indicated by the arrow 42. In one application, the object 40 has a code field 44 on its outer surface. The task of the sensor 10 in this application is to recognize the code field 44, read and decode the code applied therein and assign it respectively to the relevant object 40. In order to also capture the code field 46 applied on the side, preferably additional sensors 10 (not shown) are used from different directions. Furthermore, several sensors 10 can be arranged side by side to cover a wider detection area 14 overall. FIG. 2 shows an application example, the invention is not limited to the mounting position near the belt conveyor 38 and to the function of the sensor 10 being code reading. For example, other object characteristics such as the object's shape and size or specific qualitative features may be captured, even in use situations where the objects reach the detection area 14 in a manner other than by conveyor belt 38, particularly by manual presentation.

[0037] FIG. 3 is a plan view of the lighting device 18 and the two marking light sources 22a-b embedded therein, in an exemplary circular or annular arrangement of the light sources 20. The marking light sources are configured here as line projectors. The observation direction is here along the optical axis 24, so that some geometrical relationships can be better appreciated with the aid of FIG. 1. The two marking light sources 22a-b each generate lines extending in the radial direction relative to the optical axis 24 as partial patterns 26a-b of the light pattern 26, and the center point 28 of the detection area 14 is marked as the intersection of these lines. In a camera as the photoelectric sensor 10, the light receiving element 12 configured as an image sensor is usually arranged in the center, in which case the center point 28 is in the center of the image.

[0038] Also, the center point 28 is marked regardless of distance. When the light pattern 26 hits a closer or more distant object, the horizontal lines of the partial pattern 26a and the vertical lines of the partial pattern 26b shift or stretch in their horizontal and vertical directions, respectively, but the position of the intersection at the center point 28 remains unchanged.

[0039] Figure 4 is a view similar to figure 3, but now with three marking light sources 22a-c or line projectors and correspondingly three lines as partial light patterns 26a-c. It is thus illustrated firstly that the central point 28 can be marked with more than two partial patterns 26a-c and / or with lines that are not perpendicular to one another, regardless of the distance. On the other hand, this results in a light pattern 26 that is rotationally symmetrical overall with respect to the optical axis 24, which may be advantageous in interaction with the light source 20 of the lighting unit 18 as well as with regard to visibility.

[0040] Fig. 5 shows Fig. 3 with the addition of a dot pattern 48 to further illustrate a possible embodiment. The inventive principle of marking the central point 28 with the partial light patterns 26a-b regardless of the distance allows additional pattern elements like this dot pattern 48, which is drawn purely exemplarily. Such pattern elements can help in visualizing the position of the detection area 14, but can also be used for completely different purposes, for example to include sensor information.

[0041] 6 shows a plan view of a lighting device 18, this time with a rectangular or matrix arrangement of light sources 20, in which two indicator light sources 22a-b (again in the form of line projectors) are interleaved at or between matrix positions. Marking a central point 28 regardless of distance with partial patterns 26a-b is independent of the form of the lighting device 18 and can be combined with a wide variety of lighting devices 18.

[0042] FIG. 7 shows a schematic diagram of a photoelectric sensor 10 configured as a camera with two lateral marking light sources 22a-b. Each projection direction 50a-b is parallel to the optical axis 24, resulting in an axis-parallel projector. Alternatively, it is conceivable to tilt it with respect to the optical axis 24. In such an embodiment with symmetrical marking light sources 22a-b on both sides of the optical axis 24, it is preferable to use somewhat more complex partial patterns 26a-b, which will be considered shortly in connection with FIGS. 9 and 10. For this purpose, the marking light sources 22a-b are preferably each provided with a pattern generating element (not shown), for example a DOE (diffractive optical element), which can also be used in the previous embodiment, specifically for forming lines.

[0043] Before considering further preferred partial patterns 26a-b, FIG. 8 shows a similar view to FIG. 7, but with an alternatively only one indicator light source 22. Whereas up to now the source from which each partial pattern 26a-b is projected is each its own indicator light source 22a-b, this is not the case, but rather one source is generated by off-coupling using a split mirror or beam splitter 52. The indicator light source 22 is thereby duplicated, effectively resulting in two sources. When using a pattern generating element, it is preferable to perform off-coupling behind it, so that not just light but even light modulated or shaped in accordance with the partial patterns 26a-b is off-coupled. This results in two partial patterns 26a-b from the same physical projector. The light path after the off-coupling source can be adjusted once or more times by other optical elements, as exemplarily shown in FIG. 8 by a mirror 54. In principle, further off-coupling is also conceivable in order to create additional sources. Additionally, although the generation of additional sources has been described in relation to a symmetric, axis-parallel arrangement, this is also contemplated for each of the embodiments discussed above in connection with Figures 3 to 6. Wherever multiple marking light sources 22a-b are described herein, at least one of the marking light sources 22a-b can be replaced with a separate takeoff.

[0044] FIG. 9 shows an example of partial patterns 26a-b, which are particularly suitable for marking the center point 28 for the axis-parallel projectors of FIGS. 7 and 8. Here, each partial pattern 26a-b is already a cross, respectively, so that the respective partial pattern center point 56a-b is immediately visible. However, the partial pattern center point 56a-b still never coincides with the center point 28 regardless of distance, since the corresponding marking light source 22a-b is offset off-axis. Instead, the center point 28 is marked by being in the middle of the two partial pattern center points 56a-b. This middle, and thus the center point 28, is also immediately recognizable to the human eye, and the position of the middle does not depend on the distance.

[0045] 10 shows another example of sub-patterns 26a-b, this time using an arrangement of concentric circles rather than crosses. Sub-pattern center points 56a-b are readily recognizable as the centers of circles, and therefore the middles of sub-pattern centers 56a-b, and therefore center point 28.

Claims

1. A photoelectric sensor (10) for detecting an object in a detection area (14), comprising a light receiving element (12) for generating a received signal from light incident from said detection area (14), an indication device (22a-b) for highlighting for the human eye the position of said detection area (14) by means of a light pattern (26) projected in said detection area (14), and a control and evaluation unit (30) for evaluating said received signal, said light pattern (26) comprising at least two partial patterns (26a-b) projected from separate sources (22a-b, 52, 54), said two partial patterns jointly marking a center point (28) of said detection area (14) independent of the distance at which said light pattern (26) is projected in said detection area (14); and The center point of the projected light pattern (26) coincides with the optical axis of the photoelectric sensor (10). A photoelectric sensor (10) characterized by:

2. The sensor (10) according to claim 1, wherein each partial pattern (26a-b) is a line, which lines are at an angle to each other, in particular at a right angle to each other, and which intersect at said central point (28).

3. The sensor (10) of claim 1, wherein the sources (22a-b, 52, 54) are angularly offset from one another circumferentially about the central point (28) at angles different from 0 and 180 degrees.

4. The sensor (10) of claim 1, wherein each partial pattern (26a-b) marks a unique partial pattern center point (56a-b), whereby said center point (28) is marked as the middle of the partial pattern center points (56a-b).

5. The sensor (10) of claim 4, wherein each partial pattern (26a-b) is a cross or an arrangement of concentric circles.

6. The sensor (10) of claim 4, wherein the partial patterns (26a-b) are identical to one another.

7. The sensor (10) of claim 4, wherein the sources (22a-b, 52, 54) are positioned equidistant from the central point (24, 28) on either side of the central point (24, 28).

8. The sensor (10) of any one of claims 1 to 7, wherein the sources (22a-b, 52, 54) each comprise one indicator light source (22a-b).

9. The sensor (10) according to any one of the preceding claims, wherein at least one of the sources (22a-b, 52, 54) comprises a beam splitter (52), which separates out light of another of the sources (22a-b, 52, 54).

10. The sensor (10) of claim 1, further comprising an illumination device (18) for illuminating said detection area (14), said indicator devices (22a-b) being part of said illumination device (18).

11. The sensor (10) according to claim 10, wherein the lighting device (18) comprises a number of light sources (20) distributed around the periphery, in particular distributed in a circular manner, and the sources (22a-b) of the indication device (22a-b) are arranged between the light sources (22) distributed around the periphery.

12. The sensor (10) of claim 1 configured as a camera, the light receiving element (12) configured as an image sensor having a number of pixels for generating image data.

13. 2. The sensor (10) according to claim 1, configured as a code reading device for reading an optical code (44), the control and evaluation unit (30) being configured to read the optical code (44) by evaluation of the received signal using a decoding method.

14. 2. The sensor (10) of claim 1, wherein the sensor (10) is fixedly mounted near a conveying device (38) that guides an object (40) to be detected in a conveying direction (42) through the detection area (14).

15. A method for highlighting the position of a detection area (14) of a photoelectric sensor (10), in particular a sensor (10) according to any one of claims 1 to 7, in which a light pattern (26) visible to the human eye is projected into said detection area (14), at least two partial patterns (26a-b) of the light pattern (26) are projected from at least two separate sources (22a-b, 52, 54), the two partial patterns jointly marking a center point (28) of the detection area (14) independently of the distance at which the light pattern (26) is projected in the detection area (14); The center point of the projected light pattern (26) coincides with the optical axis of the photoelectric sensor (10). The method comprising:

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