Method and sensor arrangement for monitoring protection field

Abstract

A method and a sensor arrangement for monitoring a protection field at a facility are shown and described. The protection field is formed by a grating 11. The grating (11) is configured from a plurality of light beams (17). When the object (13) enters the protection field, the protection field is subdivided into a first region (18), a second region (19) and a tolerance region (21), the second region (19) is defined by all the light beams (17) interrupted by the object (13) except the outermost light beams interrupted by the object (13), and the first region (18) always exists. After the release of the interrupted light beam in the second region 19, a turn-off signal is generated if not all of the light beams 17 are free in the protection field after a predefined first duration.

Description

Technical Field

[0001] The invention relates to a method and a sensor device for monitoring a protective field according to the preambles of claims 1 and 16 . Background Art

[0002] The task of the light grid is to guard hazardous areas in a facility by generating a protective field. When the protective field generated by the light grid is interrupted, the light grid generates a switch-off signal and causes the facility to stop as quickly as possible. However, the protective field can be designed to allow specific objects to pass through, so that, for example, in a material handling machine, material that is still needed can be conveyed into the hazardous area through the protective field or removed through the hazardous area without the light grid generating a switch-off signal.

[0003] JP 3 603 084 B2 describes a light barrier and a light barrier arrangement, the protection field of which is arranged perpendicular to the transport direction. The first light barrier is arranged to the left and right of the transport path, while the second light barrier is arranged above and below. In addition to the height, the width of the object can thus also be advantageously detected. The distinction between people and materials and / or between permitted and impermissible objects is achieved with the aid of pre-learned reference objects in the protection field. During operation, the light barrier compares the object with the stored reference and decides whether the object is permitted.

[0004] In patent EP 2 933 662 A1, the light barrier requires an additional external sensor to determine whether an incoming object is permitted.

[0005] Patent EP 2 017 524 B1 describes a light barrier which is tilted relative to the vertical in the transport direction and which also works without an additional muting sensor. As a result, a rectangular object generates a pattern which is divided into three phases when it moves through the protective field. Each phase has specific characteristics. In the first phase, the number of interrupted light beams increases continuously. In the second phase, the number of covered light beams remains constant. In the third phase, the number of interrupted light beams decreases continuously. The distinction between permitted and impermissible objects is achieved by storing the course of the number of interrupted light beams in the case of permitted objects. This is generally considered to be advantageous because no learning with the aid of reference objects is required.

[0006] European patent application EP 3 447 540 discloses a light curtain for detecting objects and / or persons in a protection field by means of a light curtain, wherein at least one monitoring zone forms a first sub-area of ​​the monitoring area of ​​the light curtain and a safety switch-off signal is generated by a control and evaluation unit in the event of a light beam interruption in the monitoring zone. Furthermore, a warning zone forms a second sub-area of ​​the monitoring area of ​​the light curtain, wherein a warning signal is generated by the control and evaluation unit in the event of a light beam interruption in the warning zone. If the light beam interruption in the warning zone exceeds a predetermined duration, a safety switch-off signal is generated by the control and evaluation unit. Furthermore, a muting zone forms a further sub-area of ​​the monitoring area, wherein no safety switch-off signal is generated by the control and evaluation unit in the event of a light beam interruption in the muting zone. The muting zone serves to exclude permanently interrupted sub-areas of the light curtain from monitoring, for example due to workbenches or tools that are necessary to temporarily enter the monitoring area.

[0007] Patent EP 3 133 423 describes a light curtain control system having a light curtain state component configured to generate state information of the light curtain based on determining whether one or more light beams in a set of light beams are interrupted, and having a mute control component configured to control a partial mute mode for the light curtain. A mute level configuration component is used to set a mute level for the partial mute mode based on an identification of the highest light beam in the set of light beams that is interrupted by an object identified by the light curtain during the partial mute mode.

[0008] The muting control assembly is further configured to mute a subset of the beams from a lowest beam of the light curtain to a beam corresponding to the muting level for the partial muting mode.

[0009] In patent EP 3133423 A1, only objects arranged facing the initial beam of the grating are allowed. Objects facing the final beam of the grating and objects that cover neither the initial beam nor the final beam are not considered for detection. Summary of the invention

[0010] The object of the present invention is therefore to avoid the disadvantages of the cited prior art and to provide a method for monitoring a protective field which can be operated with a light barrier and is independent of reference objects for defining the permitted area. In addition, the object of the present invention is to provide a sensor device which can be operated with this method.

[0011] This object is achieved by a method for monitoring a protective field having the features of claim 1 .

[0012] The invention relates to a method for monitoring a protective field at a facility. The protective field is formed by a light barrier. The light barrier is implemented by a plurality of light beams, which are located in a plane and are sent from a transmitting device to an oppositely arranged receiving device. The light barrier has a control unit for receiving and processing signals of the transmitting and / or receiving device. When an object enters the protective field, the protective field is divided into a first area, a second area and a tolerance area, wherein the second area is defined by all light beams interrupted by the object except the outermost light beam interrupted by the object. The first area always exists. If there are free adjacent light beams, the outermost light beam interrupted by the object forms a tolerance area together with the adjacent free light beams. The remaining free light beams form the first area. When a light beam is interrupted in the first area, a shutdown signal is generated by the light barrier. Free and interrupted light beams in the tolerance area are ignored, and no shutdown signal is generated for these light beams. As long as the object is in the protective field, the light barrier checks the number of interrupted light beams in the second area. After the interrupted light beams in the second area are released, if not all light beams are free in the protective field after a predefined first duration, a shutdown signal is generated.

[0013] The protection field is designed to generate a shutdown signal for stopping the facility when an object passes through the protection field or enters the protection field in an unauthorized manner. Objects entering the protection field are divided into permitted objects and unauthorized objects. The requirement for the light barrier is that permitted objects can pass through the protection field smoothly, while unauthorized objects cause the generation of a shutdown signal, thereby causing the facility to stop.

[0014] In the method for monitoring a protective field according to the invention, the light grid does not need to have information about the dimensions of the allowed objects. In the case of the light grid of the prior art, this information is provided to the light grid by providing the dimensions of the allowed objects or by scanning the reference object in advance. The method according to the invention makes it possible to determine whether the object is allowed or not allowed when the object passes through the protective field. For this purpose, the time point at which the object enters the protective field and interrupts at least one light beam is determined. After a predefined duration, the light grid checks how many light beams are interrupted by the object and subdivides the protective field into three areas according to this. The light beams interrupted by the object after the predefined time form the so-called second area of ​​the protective field, except for the two outermost light beams. The outermost interrupted light beams form the tolerance area together with the adjacent free light beams. The remaining light beams of the protective field form the first area. If one of the light beams in the first area is interrupted, the light grid generates a shutdown signal, which immediately stops the facility. The interruption or release of the light beam in the tolerance area has no effect on the generation of the shutdown signal. The condition resulting from the definition of the subdivision of the protective field is that the object guided through the protective field passes through the protective field as vertically as possible. The object should have edges that can be oriented perpendicular to the protective field as much as possible. This ensures that the same light beam remains interrupted over a longer period of time, while the object is in the protective field.

[0015] When the object leaves the protective field, the interrupted light beams in the second area are released. In order not to generate a switch-off signal when releasing the interrupted light beams in the second area, the light barrier waits for a predefined time interval after releasing the first light beam in the second area. At the end of this time interval, no light beams may be interrupted, otherwise a switch-off signal will be generated. Therefore, after releasing the first light beam, all interrupted light beams must be released within a predefined time interval.

[0016] The method according to the invention enables a local and temporary bypassing of the protective field, wherein the bypassed area is limited to the dimensions of the object passing through the protective field. The bypassing of the protective field can take place at any point in time. At the same time, the use of a predefined duration until the final subdivision of the protective field provides a reliable mode of operation and helps to reduce the complexity of the sensor device and its programming. The protective field plane expanded by the light barrier can be arranged perpendicular to the conveying movement of the object, which reduces the expenditure on the installation of the light barrier with the method according to the invention.

[0017] When an object enters the protective field, it is possible that at the time of the interruption of the first light beam, the complete widening of the object in the lateral direction has not yet been detected. Preferably, when the object enters the protective field, after the light beam interruption and the end of a predefined second time period, the protective field is subdivided into a first area, a second area and a tolerance area. The time period predefined for this depends in particular on the speed of the object. The slower the object moves through the protective field, the longer the time period must be selected in order to subdivide the protective field. The use of a time period from the interruption of the first light beam to the subdivision of the protective field provides uniform conditions for all objects, thereby reducing the complexity of identifying permitted objects. The advantage of the present invention is that, in contrast to the teaching of EP 2 933 662 A1, the method and sensor device according to the present invention do not require any additional external sensors to determine whether an entering object is permitted or not.

[0018] An object passing through the protective field must preferably have a minimum size in order to identify the object as permitted. This prevents the passage of small, impermissible objects. This limitation helps to increase operational safety. The minimum size of an object is defined by the distance of at least five adjacent light beams. If the number of interrupted light beams after the interruption of the first light beam and the expiration of a predefined duration is not at least five, a switch-off signal is generated. Three of the five interrupted light beams form the second area here, while the two outermost light beams are located within the tolerance area. Preferably, the minimum size of the second area is defined by at least three interrupted light beams.

[0019] After the object enters the protective field, the protective field is subdivided into three sub-areas. Here, the outermost interrupted light beam together with the adjacent free light beams define a tolerance area within the protective field. For this purpose, the number of adjacent free light beams can exceed 1. It is conceivable that up to three adjacent free light beams each form a tolerance area together with the outermost interrupted light beam. Advantageously, the tolerance area consists of the same number of free light beams and interrupted light beams.

[0020] Preferably, the first duration has a length of at most 4 seconds, in particular at most 2 seconds. This defines the following duration, after which the object must have completed its exit from the protective field. By determining a maximum value for this parameter, impermissible objects can be detected better and faster. Releasing the first light beam in the second area means that the object has begun to leave the protective field. This process should be completed within a predefined duration of at most 4 seconds. This allows better identification of permitted objects. At the same time, if the permitted object has a rear edge that is non-straight and arranged perpendicular to the conveying direction, the permitted object also forms a tolerance area.

[0021] Advantageously, the length of the first duration is at least 5 ms, in particular at least 25 ms. A minimum duration of the first duration provides greater freedom in designing the geometry of the object. The smaller the first duration is defined, the less time the object has to leave the protective field.

[0022] In a preferred embodiment, the length of the second duration is at most 4 seconds, in particular at most 2 seconds. Thus, a maximum duration is given, until which the object must penetrate deep into the protective field so that its extension perpendicular to the conveying direction is completely covered. The introduction of the maximum duration makes it possible to detect impermissible objects more quickly, since detection cannot be performed until the duration has expired.

[0023] In another preferred embodiment, the length of the second time period is at least 5 ms, in particular at least 25 ms. The minimum length of the time period for subdividing the protective field increases the reliable operation of the method according to the invention, because it takes into account the time for a permitted object to penetrate deeply into the protective field until the complete extension of the object in the lateral direction can be detected.

[0024] Preferably, the light barrier checks the number of interrupted light beams at regular time intervals, the time interval being 0.5 ms to 30 ms, as long as the object is within the protection field. Checking at a certain sampling rate enables a new interruption or a new release of the light beam to be detected. The shorter the time is selected, the faster the entry of an inadmissible object can be detected. The time interval is therefore selected to be as short as possible, wherein a time interval of at most 30 ms enables a rapid detection of inadmissible objects.

[0025] Preferably, the tolerance region includes two sub-regions. These sub-regions are arranged on opposite sides of the second region. The second region extends from one edge of the grating to the opposite edge. The second region always has a rectangular shape. In addition to the two sides of the second region formed by the edges of the grating, the other two sides form the edges of the second region, thereby forming a boundary with the tolerance region. Unlike the sides formed by the edges of the grating, the positions of the other two sides are not predetermined by the size of the device, but are determined by the object passing through the protection field. For this reason, these two sides are called free sides of the second region. These free sides are respectively adjacent to the tolerance region. Since the second region can have two free sides, the tolerance region can be divided into two sub-regions. It is usually designed that each free side of the second region is adjacent to a sub-region of the tolerance region.

[0026] Preferably, the tolerance region is formed by up to twelve beams in total. Seven beams can also be distributed over two sub-regions. The tolerance region should be kept as small as possible, thereby also ensuring the reliability of the function of the grating. Therefore, the maximum number of beams forming the tolerance region can be limited.

[0027] Preferably, two gratings located in the same plane each form a protection field. Using two gratings enables the extension of the object in two different directions to be checked. When the gratings each form a protection field located in the same plane, the space required by the gratings for checking two dimensions is minimal.

[0028] In another preferred embodiment, the beams of the two gratings are perpendicular to each other. Thus, for example, the height of an object can be checked with the first grating and the width of the same object can be checked with the second grating. The mutually perpendicular arrangement provides information about the width and height of the object, thereby checking the two most important dimensions of the object in addition to the length.

[0029] Two or more light barriers can be provided. The light barriers are preferably arranged one after the other in the direction of movement of the object, so that the object is guided through the second light barrier after passing through the first light barrier. The light beams can be arranged differently between the two light barriers. Ideally, the light barriers are arranged parallel to one another. The first light barrier enables the extension of the object in one transverse direction to be measured, while the extension of the object in another transverse direction can be measured with the other second light barrier. The arrangement of the second light barrier increases the reliability of the method for detecting impermissible objects.

[0030] Preferably, the installation has a conveyor belt and is arranged relative to the light barriers in such a way that the running direction of the conveyor belt is perpendicular to the plane in which the light barriers are extended. The perpendicular arrangement of the light barriers relative to the running direction of the conveyor belt forms the simplest structural form of the light barriers on the installation. As a result, the costs for manufacturing the light barriers and the costs for mounting the light barriers on the installation are kept low. The simple construction of the light barriers on the installation is an advantage resulting from the method according to the invention.

[0031] The subject of the invention is also a sensor device having at least one light barrier for monitoring a protective field, the light barrier having two first and second strips which can be arranged opposite one another and are equipped with a transmitter and a receiver. During operation, a monitoring area is defined between the transmitters and the receivers of the two strips. A control and monitoring unit, which is preferably arranged in the strips of the light barrier, has a memory, which is connected to the first and second strips and in which a program is stored which ensures that a switching signal is generated when an object intervenes in the protective field, by means of which a safety function can be triggered. The function allows the monitoring function to be at least partially bypassed, which is referred to as "muting" in the technical terminology.

[0032] The mentioned sensor device is further characterized in that, when an object enters the protective field, a program stored in the memory subdivides the protective field into a first area, a second area and a tolerance area. The second area is defined by all beams interrupted by the object except the outermost beam interrupted by the object. If there is a free adjacent beam, the outermost beam interrupted by the object forms a tolerance area together with the adjacent free beam. The remaining free beams form a first area, wherein a switch-off signal is generated by the light barrier when a beam is interrupted in the first area. Interrupted and free beams in the tolerance area are ignored and no switch-off signal is generated. As long as the object is located in the protective field, the control and monitoring unit checks the number of interrupted beams in the second area and generates a switch-off signal after the interrupted beams in the second area are released if not all beams are free in the protective field after a first predefined duration. The sensor device has the following advantages: the sensor device can reliably distinguish between permitted objects and impermissible objects. Permitted objects can pass through the protective field smoothly, while impermissible objects lead to the generation of a switch-off signal and thus cause the facility to be monitored to stop. An advantage of the sensor arrangement is that the light barrier does not need to have information about the dimensions of the admissible objects.

[0033] The described light grid preferably comprises an emitter strip with a plurality of emitters and a receiver strip with a plurality of receivers. In principle, the emitters of the emitter strip can emit light pulses independently of the receiver strip. This means that the emitter strip does not have to be electrically connected to the receiver strip, but is optically connected, i.e. the synchronization is achieved via optical signals.

[0034] However, it is also conceivable to provide a transmitting element and a receiving element in both the first strip and the second strip. In this case, these strips do not have to be electrically connected to each other, but must be optically connected in order to achieve synchronization. However, in the case of the same arrangement, i.e., in the case of a transmitting element and a receiving element arranged in both the first strip and the second strip, these strips can also be electrically connected to achieve synchronization via electrical signals. There is then also a variant in which the transmitter of the transmitter strip transmits independently of the receiver strip. In this case, the transmitter is electrically connected to the receiver (synchronized via electrical signals).

[0035] According to a preferred variant of the method, when an object enters the protective field, after the light beam is interrupted and a second predefined time period has expired, the protective field is subdivided into a first region, a second region and a tolerance region. The predefined time period for this purpose depends in particular on the speed of the object. This method variant has the advantage that uniform conditions are achieved for all objects, thereby reducing the complexity of detecting permitted objects.

[0036] The tolerance region is preferably formed by an equal number of free light beams and interrupted light beams.

[0037] Preferably, the light barrier checks the number of interrupted light beams at regular time intervals as long as the object is located within the protection field, wherein the time interval is preferably between 0.5 ms and 30 ms.

[0038] The tolerance region expediently comprises two sub-regions which are arranged on opposite sides of the second region.

[0039] Further preferred embodiments of the sensor device have already been discussed above in the description of the method. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The invention will be described in more detail below with reference to the accompanying drawings in the form of schematic diagrams. The preferred features described can be implemented in any combination - as long as they are not mutually exclusive. In the form of schematic diagrams, which are not true to scale:

[0041] Figure 1 A cross-sectional view showing a grating with an object in front of it and a subdivision of the grating;

[0042] Figure 2 A cross-sectional view showing a grating with an object at the edge of the grating and a corresponding subdivision of the grating;

[0043] Figure 3 A front view of the grating with subdivided protection fields is shown;

[0044] Figure 4 shows a front view of the protection field formed by two gratings;

[0045] Figure 5 A diagram showing the time profile of the beam state when an object enters the grating is shown. DETAILED DESCRIPTION

[0046] In the following, the same reference numerals denote identical or functionally identical elements (in different figures). Additional primes denote multiple occurrences of the same element in one figure and are used to distinguish identical elements where necessary.

[0047] exist Figure 1 1 shows a cross-section of a grating 11 with an object 13 moving on the grating 11. The grating comprises a transmitter and a receiver 15 for light beams 17, which are arranged perpendicularly with uniform spacing from one another. The light beams 17 form a protective field. The object 13 moves perpendicularly to the plane of the protective field. Figure 1 In the embodiment of the present invention, the light beam 17 is subdivided into different regions. The division of the light beam 17, as shown in Figure 1 , are only used to illustrate different areas. Because the subdivision of the light beam 17 only occurs when the object 13 enters the protection field. The size of these areas is determined by the geometry of the object 13. The first area 18 is usually formed by free light beams. The second area 19 includes those light beams that are interrupted by the object 13 when the object 13 is located in the protection field. The outermost light beams of the second area and their adjacent light beams that are not located in the first area form a tolerance area 21. In the example shown here, the tolerance area includes two light beams twice. Therefore, the first area 18 is formed by all free light beams that do not belong to the tolerance area 21. The interruption of the light beam in the first area 18 leads to the generation of a shutdown signal, which causes the monitored facility to stop as soon as possible. The interruption and release of the light beam in the tolerance area 21 are ignored by the grating 11, so that the light beam in the tolerance area 21 does not affect the generation of the shutdown signal.

[0048] exist Figure 2 An example is shown in FIG. 1 , where the shadow of an object 13 falls on the edge of the grating 11. The grating 11 is subdivided into three regions as already described. Figure 1 As shown in . Figure 1 Unlike the example in Figure 2 The tolerance regions 21 on both sides of the second region are different in size. The tolerance region 21′ at the edge of the grating 11 includes only one beam in the embodiment shown. This beam is the outermost beam that is covered by the shadow of the object 13. The tolerance region 21″ on the other side is as shown in FIG. Figure 1 As in the embodiment in which two light beams are included.

[0049] exist Figure 3, a front view of a light barrier 11 is shown which is temporarily subdivided according to the invention into different zones. To simplify the illustration, objects in the protection field are not shown. The second zone 19 has a tolerance zone 21 which is generated in the form of two strips at the upper and lower edges of the second zone 19. Since the second zone 19 is formed by beam interruption, the second zone 19 is extended over the entire working distance of the light barrier 11 from the transmitting device 15 ′ to the receiving device 15 ″.

[0050] In order to limit the second region 19 in width, a second grating 11" may be arranged. Ideally, the second grating 11" is arranged so that the light beam 17' of the first grating 11' is perpendicular to the light beam of the second grating 11". Figure 4 1 shows an embodiment with two such light barriers 11. The second region 19 is defined by the light beams 17 of the two light barriers 11, 11' which are interrupted by the object. The second region 19 is therefore limited both in height and in width of the light barriers. In the case of using a tolerance region 21, this is formed as a frame around the second region in front view. The objects are usually designed to be carried on a conveyor belt. In the case of Figure 4 In the arrangement of the light barriers 11, 11" shown in FIG, it is possible that a conveyor belt arranged below the object interrupts a large number of light beams, thus not allowing the width of the object to be checked. This can be eliminated if the conveyor belt is subdivided into two conveyor belts arranged one behind the other. Here, the light barriers form the transition from one conveyor belt to the next, so that no conveyor belt is present at the light barriers and the light beams cannot be interrupted by the conveyor belt.

[0051] Figure 5 The time course of the state of the light beam 17 during the entry of the object 13 into the protection field is shown. The object 13 has a front edge 25, which has a parallelism error with the protection field plane. Therefore, in particular, a specific predefined time must be waited until the light beam is subdivided into regions. The vertical lines in the diagram show the sampling 27 of the grating, which shows that the state of the light beam is checked at regular time intervals. After the fifth sampling 27 of the grating, the object 13 has penetrated deep into the protection field so that both the number and the position of the interrupted light beams remain constant. The grating subdivides the light beam into the above-mentioned three regions after a predefined duration. Except for the two outermost light beams, the interrupted light beams form the second region 19. The outermost interrupted light beams form the tolerance region 21 together with the adjacent free light beams, while the remaining light beams form the first region 18. The time until the grating completes the sampling 27 of the object must be equal to or shorter than the predefined duration. By waiting for a predefined duration until the protection field is subdivided after the interruption of the first light beam, a tolerance is generated with respect to the shape of the front side of the object. The edges of the front side can have an arc and do not necessarily have to extend parallel to the grating. However, from Figure 5From the time course of the interrupted light beam in the grating, it is known that the object must be arranged relative to the grating in such a way that the edges leading away from the front side of the object must be oriented perpendicular to the grating. The tolerance range allows a certain curvature of these edges, but the extension transversely to the direction of movement of the object must not change so much that it affects the light beam outside the tolerance range.

[0052] The method according to the invention is therefore particularly suitable for objects with a constant cross-sectional profile.

[0053] Although the invention has been described in conjunction with specific embodiments, it is apparent that changes, modifications, variations and combinations may be made without departing from the spirit of the invention.

[0054] Reference numerals list

[0055] 11 Grating

[0056] 13 Objects

[0057] 15 Transmitter / Receiver

[0058] 17 Beam

[0059] 18 First Area

[0060] 19 Second Area

[0061] 21 Tolerance area

[0062] 25 The leading edge of the object

[0063] 27 Sampling

Claims

1. A method for monitoring a protection field at a facility, wherein the protective field is formed by a light barrier (11), and the light barrier (11) is formed by a plurality of light beams (17) which are located in one plane and are transmitted from a transmitting device (15') to an oppositely arranged receiving device (15"), the light barrier (11) has a control unit for receiving and processing the signals of the transmitting and / or receiving device, It is characterized in that - when an object (13) enters the protection field, subdividing the protection field into a first area (18), a second area (19) and a tolerance area (21), wherein the second region (19) is defined by all the light beams (17) interrupted by the object except the outermost light beam interrupted by the object, - if there are free adjacent light beams, the outermost light beam interrupted by the object together with the adjacent free light beams forms the tolerance region (21), - the remaining free beam forms the first region (18), wherein - when the light beam (17) is interrupted in the first region (18), a switch-off signal is generated by the light barrier (11), - interrupted and free light beams in the tolerance zone (21) are ignored and no switch-off signal is generated, - the light barrier (11) checks the number of interrupted light beams in the second area as long as the object (13) is within the protection field, and After the interrupted light beams in the second region (19) are released, a switch-off signal is generated if not all light beams are free within the protection field after a predefined first time duration.

2. The method according to claim 1, characterized in that: When an object (13) enters the protective field, after the light beam (17) is interrupted and a predefined second time period has elapsed, the protective field is subdivided into the first area (18), the second area (19) and the tolerance area (21).

3. The method according to claim 1 or 2, characterized in that: The minimum size of the second region (19) is defined by at least three interrupted light beams (17).

4. The method according to any one of claims 1 to 3, characterized in that The tolerance region (21) is formed by an equal number of free light beams and interrupted light beams.

5. The method according to any one of claims 1 to 4, characterized in that The length of the first duration is at most 4 seconds, in particular at most 2 seconds.

6. The method according to any one of claims 1 to 5, characterized in that The length of the first time duration is at least 5 ms, in particular at least 25 ms.

7. The method according to any one of claims 1 to 6, characterized in that The length of the second duration is at most 4 seconds, in particular at most 2 seconds.

8. The method according to any one of claims 1 to 7, characterized in that The length of the second time duration is at least 5 ms, in particular at least 25 ms.

9. The method according to any one of claims 1 to 8, characterized in that As long as an object (13) is located within the protective field, the light barrier (11) checks the number of interrupted light beams (17) at regular time intervals, wherein the time intervals are between 0.5 ms and 30 ms.

10. The method according to any one of claims 1 to 9, characterized in that The tolerance region (21) comprises two sub-regions, and the two sub-regions are arranged on opposite sides of the second region (19).

11. The method according to any one of claims 1 to 10, characterized in that The tolerance region (21) is formed by a total of up to twelve light beams (17).

12. The method according to any one of claims 1 to 11, characterized in that Two gratings (11) are used, and the protection fields of the two gratings are located in one plane.

13. The method according to claim 12, characterized in that The light beams (17) of the two gratings (11) are perpendicular to each other.

14. The method according to any one of claims 1 to 13, characterized in that Two or more gratings (11) are provided which are arranged parallel to each other.

15. The method according to any one of claims 1 to 14, characterized in that The installation has a conveyor belt, and the conveyor belt is arranged relative to the light barrier (11) in such a way that the running direction of the conveyor belt is substantially perpendicular to a plane spanned by the light barrier (11).

16. A sensor device (1) having at least one light barrier (8) for monitoring a protective field, the sensor device comprising: - two first and second strips arranged opposite one another, which are equipped with transmitters and receivers, so that during operation a monitoring area is defined between the transmitters and receivers of the two strips, and a control and monitoring unit, which is connected to the first strip and the second strip and has a program stored in a memory, which ensures that a switching signal is generated when an object intervenes in the protective field, by means of which a safety function can be triggered, and in which a muting function is provided, by means of which the monitoring function can be at least partially bypassed, It is characterized in that - when an object (13) enters the protection field, the program subdivides the protection field into a first area (18), a second area (19) and a tolerance area (21), wherein the second region (19) is defined by all beams (17) interrupted by the object except the outermost beam interrupted by the object, - if there are free adjacent beams, the outermost beam interrupted by the object together with the adjacent free beams forms the tolerance region (21), and the remaining free light beam forms the first region (18), wherein when the light beam (17) is interrupted in the first region (18), a switch-off signal is generated by the grating (11), - interrupted and free light beams in the tolerance zone (21) are ignored and no switch-off signal is generated, - As long as the object (13) is within the protective field, the light barrier (11) checks the number of interrupted light beams in the second area and, after the interrupted light beams in the second area (19) are released, generates a switch-off signal if not all light beams are free in the protective field after a predefined first duration.

17. The sensor device (1) according to claim 16, characterized in that When an object (13) enters the protective field, after the light beam (17) is interrupted and a predefined second time period has expired, the protective field is subdivided into the first area (18), the second area (19) and the tolerance area (21).

18. The sensor device according to claim 16 or 17, characterized in that The length of the first duration is at least 5 ms, in particular at least 25 ms, and at most 4 seconds, in particular at most 2 seconds.

19. The sensor device according to claim 17 or 18, characterized in that The length of the second duration is at least 5 ms, in particular at least 25 ms, and at most 4 seconds, in particular at most 2 seconds.

20. The sensor device (1) according to any one of claims 17 to 19, characterized in that The minimum size of the second region (19) is defined by at least three interrupted light beams (17).

21. The sensor device (1) according to any one of claims 16 to 20, characterized in that The tolerance region (21) is formed by an equal number of free light beams and interrupted light beams.

22. The sensor device (1) according to any one of claims 16 to 21, characterized in that As long as the object (13) is in the protective field, the light barrier (11) checks the number of interrupted light beams (17) at regular time intervals, wherein the time intervals are between 0.5 ms and 30 ms.

23. The sensor device (1) according to any one of claims 16 to 22, characterized in that The tolerance region (21) comprises two sub-regions, and the two sub-regions are arranged on opposite sides of the second region (19).

24. The sensor device (1) according to any one of claims 16 to 23, characterized in that The tolerance region (21) is formed by a total of up to twelve light beams (17).

25. The sensor device (1) according to any one of claims 16 to 24, characterized in that Two gratings (11) are used, the protection fields of which lie in a common plane.

26. The sensor device (1) according to any one of claims 16 to 25, characterized in that The light beams (17) of the two gratings (11) are arranged substantially perpendicular to each other.

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