Method and sensor arrangement for monitoring protective field

Abstract

To show and describe a method and a sensor arrangement for monitoring a protective field on a system.SOLUTION: The protective field is formed by a light grid 11. The light grid 11 is created by a plurality of light beams 17. When an object 13 enters the protective field, the protective field is divided into a first range 18, a second range 19 and a tolerance range 21, where the second range 19 is defined by all light beams except the outermost light beams 17 interrupted by the object 13, and the first range 18 is always present. After release of an interrupted light beam in the second range 19, a shutdown signal is generated if not all light beams 17 in the protective field are free light beams after a predefined first time period.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The invention relates to a method and a sensor arrangement for monitoring a protective field according to the general provisions of claims 1 and 16. [Background technology]

[0002] One of the roles of a light curtain is to provide a protective field to protect hazardous areas in a system. If the protective field provided by the light grid is interrupted, the light grid generates a stop signal to stop the system as quickly as possible. However, the protective field can be designed to allow certain objects to pass through, so that, for example, a material processing machine can continue to supply the required material to the hazardous zone through the protective field or material can be removed through the protective field without the light curtain generating a stop signal.

[0003] Japanese Patent No. 3603084B2 proposes a light grid configuration in which the light grid and the protective field are arranged perpendicular to the transport direction. The first light grid is arranged to the left and right of the transport path, the second one above and below. This means that both the width and the height of the object can be recorded. The distinction between people and materials, or between permitted and unpermitted objects, is achieved by means of pre-indicated reference objects in the protective field. During operation, the light grid compares the object with the stored reference and decides whether the object is permitted or not.

[0004] In EP 2933662A1, the light curtain requires an additional external sensor to determine whether to admit an incoming object.

[0005] EP 2017524 B1 describes a light grid that is inclined vertically in the transport direction and does not require additional muting sensors. When rectangular objects pass through the protective field, they form a pattern that is divided into three stages. Each stage has specific characteristics: in the first stage, the number of blocked rays increases steadily; in the second stage, the number of covered rays remains constant; and in the third stage, the number of blocked rays decreases steadily. The distinction between permitted and non-permitted objects is achieved by storing a sequence of the number of blocked rays of permitted objects. No teaching with a reference object is necessary, which is generally considered to be an advantage.

[0006] European Patent Application 3 447 540 discloses a light curtain for detecting objects and / or persons in a protective field with the aid of a light curtain, in which at least one monitoring zone forms a first partial area of ​​the monitoring range of the light curtain, and a safety stop signal is generated by the control and evaluation unit if the light beam is interrupted in the monitoring zone. A warning zone forms a second partial area of ​​the monitoring range of the light curtain, and a warning signal is generated by the control and evaluation unit if the light beam is interrupted in the warning zone. A safety stop signal is generated by the control and evaluation unit if the light beam is interrupted in the warning zone for more than a predefined period of time. Furthermore, a blank zone forms a sub-range of the monitoring range, and a blank zone does not generate a safety stop signal by the control and evaluation unit if the light beam is interrupted in the blank zone. Blank zones are used to exclude from monitoring areas where the light curtain is permanently interrupted, for example because a necessary tool table or tool is temporarily in the monitoring range.

[0007] EP 3133423 describes a light curtain control system including a light curtain status component configured to generate status information for the light curtain based on determining whether one or more light rays of a series of light rays are blocked, and a muting control component configured to control a partial muting mode of the light curtain. A muting height setting component is used to set a muting height for the partial muting mode based on identifying the highest light ray of the series of light rays detected by the light curtain during the partial muting mode that is blocked by an object.

[0008] The muting control component is further configured to mute a subset of the light rays ranging from the lowest light ray of the light curtain to a light ray corresponding to the muting height when in the partial muting mode.

[0009] EP 3133423 A1 only allows objects that are positioned towards the first ray of the light grid to be detected, and does not detect objects that are facing the last ray of the light grid, or that do not cover either the first or last ray. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent No. 3603084B2 [Patent Document 2] European Patent No. 2933662A1 [Patent Document 3] European Patent No. 2017524B1 [Patent Document 4] European Patent Application No. 3447540 [Patent Document 5] European Patent No. 3133423 [Patent Document 6] European Patent No. 3133423A1 Summary of the Invention [Problem to be solved by the invention]

[0011] The object of the present invention is therefore to provide a method for monitoring a protective field which avoids the drawbacks of the cited prior art, which is operable together with a light grid and which does not rely on a reference object for defining the permitted area. Furthermore, it is an object of the present invention to propose a sensor arrangement which operates with this method. [Means for solving the problem]

[0012] This problem is solved by a method for monitoring a protective field having the features of claim 1.

[0013] The invention relates to a method for monitoring a protection field of a system. The protection field is formed by a light grid. The light grid is constructed of a number of light rays, which are in one plane and are sent from a light emitter to a light receiver arranged on the opposite side. The light grid has a control unit for recording and processing signals from the light emitter and / or the light receiver. When an object enters the protection field, the protection field is divided into a first zone, a second zone and a tolerance zone, the second zone being defined by all light rays except the outermost light rays blocked by the object. The first zone remains constant. The outermost light rays blocked by the object form the tolerance zone together with adjacent free rays, if any. The remaining free rays form the first zone. When a light ray is blocked in the first zone, a stop signal is generated by the light grid. The free rays and blocked rays in the tolerance zone are ignored and do not generate a stop signal. The light grid checks the number of blocked rays in the second area as long as the object is in the protection field, and if all of the rays in the protection field have not become free rays after a first predetermined time has elapsed after the blocked rays in the second area are released, a stop signal is generated.

[0014] The protective field is intended to generate a stop signal to shut down the system if an object passes through or enters the protective field in an illegal manner. Objects that enter the protective field are separated into allowed and unallowed objects. The requirement of the light grid is that allowed objects can pass through the protective field without any problems, while unallowed objects generate a stop signal to shut down the system.

[0015] In the method according to the invention for monitoring the protective field, the light grid does not need to have information about the dimensions of the permissible objects. In the light grids of the prior art, this information is provided to the light grid by providing the dimensions of the permissible objects or by previously scanning a reference object. The method according to the invention makes it possible to determine whether an object is permissible or not while it is passing through the protective field. For this purpose, the time at which the object enters the protective field and blocks at least one light beam is determined. After a certain period of time has elapsed, the light grid checks how many light beams are blocked by the object and divides the protective field accordingly into three zones. The light beams blocked by the object after the certain time form the so-called second zone of the protective field, to which the two outermost light beams are excluded. The blocked outer light beams together with the adjacent free light beams form the tolerance zone. The remaining light beams of the protective field form the first zone. If one of the beams is blocked in the first zone, the light grid generates a stop signal and initiates an immediate shutdown of the system. The blocking or release of the beams in the tolerance zone does not affect the generation of the stop signal. One condition that arises from the subdivision of the protective field is that the object passing through it must be guided as upright as possible. If possible, the edge of the object must be aligned perpendicular to the protective field, so that the same light rays remain blocked for a longer period while the object is in the protective field.

[0016] When the object leaves the protection field, the blocking rays in the second zone are released. To avoid generating a stop signal when a blocking ray is released in the second zone, the light grid waits a predefined time after releasing the first ray in the second zone. After this time, the ray may not be blocked, but if it is blocked, a stop signal is generated. This means that all blocking rays must be released within a predefined time after the first ray is released.

[0017] The method according to the invention allows a localized temporary application of a protective field, the application range of which is limited to the dimensions of the object passing through the protective field. The protective field can be applied at any time. At the same time, the use of a predefined time for a clearly subdivided protective field provides a reliable function and helps to reduce the complexity of the sensor technology and its programming. The protective field surface spread by the light grid can be arranged perpendicular to the object-carrying movement, thereby reducing the effort required to install the light grid using the method according to the invention.

[0018] When an object enters the protection field, the full lateral extent of the object may not yet be detected when the first light beam is interrupted. Advantageously, when the object enters the protection field after the light beam is interrupted and a second predetermined time has elapsed, the protection field is divided into a first zone, a second zone and a tolerance zone. The length of the predetermined time for this depends, among other things, on the speed of the object. The slower the object passes through the protection field, the longer this time that must be set to subdivide the protection field. Using the time from the first light beam interruption to the subdivision of the protection field provides uniform conditions for all objects and reduces the complexity of detecting permitted objects. An advantage of the invention is that the method and sensor arrangement according to the invention do not require additional external sensors to determine whether an incoming object is permitted or not, which is in contrast to the teaching of EP 2 933 662 A1.

[0019] Objects passing through the protective field must preferably be of a minimum size to be recognized as acceptable objects, thereby preventing unacceptably small objects from passing through. This restriction helps to increase operational safety. The minimum size of an object is defined by a distance of at least five adjacent beams. If the number of blocked beams is less than five after a predefined time has elapsed since the first beam was blocked, a stop signal is generated. Three of the five blocked beams form a second zone, with the outermost two beams being within the tolerance zone. The minimum size of the second zone is preferably defined by at least three blocked beams.

[0020] The protection field is divided into three sections after the object enters the protection field. The outermost blocking ray together with the adjacent free ray defines a tolerance zone in the protection field. The number of adjacent free ray may be more than one. It is conceivable that up to three adjacent free ray forms the tolerance zone together with the outermost blocking ray. Advantageously, the tolerance zone is formed by an equal number of free ray and blocking ray.

[0021] The first time period is preferably a maximum of 4 seconds long, in particular a maximum of 2 seconds long. This determines the time until the object completely leaves the protective field. Calculating the maximum value of this parameter allows a better and faster detection of unauthorized objects. The release of the first light beam in the second zone means that the object has started to leave the protective field. This process must be completed within a predefined maximum time of 4 seconds. This ensures good detection of authorized objects. At the same time, a tolerance zone is formed if the non-straight rear edge of the authorized object is arranged perpendicular to the transport direction.

[0022] Advantageously, the first time period is at least 5 ms long, in particular at least 25 ms long. A minimum first time period allows greater freedom in designing the shape of the object. The shorter the first time period, the shorter the time given for the object to leave the protective field.

[0023] In a preferred embodiment, the second time period is at most 4 seconds long, in particular at most 2 seconds long. This means that there is a maximum time for an object to enter the protective field and for the extension of the object perpendicular to the conveying direction to be completely covered. Since the detection time has to be waited, the introduction of a maximum time allows unauthorized objects to be detected more quickly.

[0024] In a further preferred embodiment, the second period of time is at least 5 ms, in particular at least 25 ms. A minimum period of time for subdividing the protective field increases the reliability of the operation of the method according to the invention, since it takes into account the time required for an admissible object to enter the protective field far enough to allow detection of its entire lateral extent.

[0025] The light grid preferably checks the number of blocked rays at a certain time interval as long as the object is within the protection field, the time interval being 0.5-30 ms. Testing at a certain sampling rate allows detection of new blocking or new release of rays. The shorter this time is set, the faster the unauthorized object can be detected entering. Therefore, this time interval is set to be as short as possible, allowing for a rapid detection of unauthorized objects with a time interval of up to 30 ms.

[0026] The tolerance zone preferably includes two subzones, which are arranged on opposite sides of the second zone. The second zone extends from one edge of the light grid to the opposite edge. The second zone is always rectangular in shape. In addition to the two sides of the second zone being formed by both ends of the light grid, the remaining two sides form the edges of the second zone, thus forming the boundary of the tolerance zone. In contrast to the sides formed by both ends of the light grid, the position of the remaining two sides is not predetermined by the dimensions of the device, but by the object passing through the protective field. For this reason, the two sides are called free sides of the second zone. The tolerance zone borders on each of these free sides. Since the second zone can have two free sides, the tolerance zone can be divided into two subzones. Usually, the partial zone of the tolerance zone borders on the free sides of the second zone.

[0027] The tolerance zone preferably consists of a maximum of 12 rays in total. The 6 rays can also be divided into two sections. The tolerance zone should be kept as small as possible to ensure that the function of the light grid remains reliable. Therefore, the maximum number of rays forming the tolerance zone may be limited.

[0028] Preferably, two light grids in the same plane each create a protective field. Using two light grids makes it possible to see the extent of an object in two different directions. If each light grid creates a protective field in the same plane, the space required by the light grids to see the two dimensions is minimized.

[0029] In a further preferred embodiment, the light rays of the two light grids are perpendicular to each other, which makes it possible, for example, to ascertain the height of an object with a first light grid and the width of the same object with a second light grid. The perpendicular arrangement provides information about the width and height of the object, which means that the two most important dimensions of the object are ascertained in addition to the length.

[0030] There may be more than one light grid. The light grids are preferably arranged side by side in the direction of object movement, so that the object passes through the first light grid and then the second light grid. The light rays may be arranged differently between the two light grids. Ideally, the light grids are arranged parallel to each other. The first light grid can measure one lateral extent of the object, and the second light grid can be used to measure another lateral extent of the object. The installation of a second light grid increases the reliability of the process of detecting unauthorized objects.

[0031] Preferably, the system comprises a conveyor belt, which is arranged relative to the light grid such that the direction of movement of the conveyor belt is perpendicular to the plane spanned by the light grid. Arranging the light grid perpendicular to the running direction of the conveyor belt is the simplest design of the light grid on the system. This minimizes the effort required for both manufacturing the light grid and installing the light grid on the system. The simple structure of the light grid on the system is an advantage obtained from the process of the present invention.

[0032] Another object of the invention is a sensor arrangement comprising at least one light grid for monitoring a protective field, where a first strip and a second strip can be arranged opposite each other, both strips being equipped with a light emitter and a light receiver. During operation, a monitoring field is defined between the emitters and receivers of the two strips. A control and monitoring unit is preferably arranged on a strip of the light grid and connected to the first and second strips and has a memory, in which a program is stored that ensures the generation of a switching signal when an object enters the protective field, with which a safety function can be activated. A function allows the monitoring function to be at least partially avoided, which is technically called "muting".

[0033] The aforementioned sensor arrangement is further characterized in that, when an object enters the protective field, the program stored in the memory divides the protective field into a first zone, a second zone and a tolerance zone. The second zone is defined by all light rays except the outermost light rays blocked by the object. The outermost light rays blocked by the object form a tolerance zone together with adjacent free light rays, if there are adjacent free light rays. The remaining free light rays form the first zone, and a stop signal is generated by the light grid if a light ray is blocked in the first zone. The blocked and free light rays in the tolerance zone are ignored and no stop signal is generated. The control and monitoring unit checks the number of blocked light rays in the second zone as long as the object is in the protective field and generates a stop signal if not all of the light rays in the protective field have become free light rays after a first predetermined time after the blocked light rays in the second zone are released. This sensor arrangement has the advantage that it can reliably distinguish between permitted and non-permitted objects. Permitted objects are allowed to pass through the protective field without issue, while non-permitted objects generate a stop signal to shut down the monitored system.One advantage of the sensor arrangement is that the light grid does not require information about the dimensions of the permitted objects.

[0034] The described light grid preferably comprises a projector strip, which comprises a number of projectors, and a receiver strip, which comprises a number of receivers. In principle, the projectors of the projector strip can emit light pulses independently of the receiver strip. This means that the projector strip does not have to be electrically connected to the receiver strip, but must be optically connected, i.e. the synchronization takes place via an optical signal.

[0035] However, it is also conceivable that the light-emitter and light-receiving elements are provided on both the first and the second strip. In this case too, the strips do not have to be electrically connected to each other, but must be optically connected to enable synchronization. Even in the same arrangement, i.e. the light-emitter and light-receiving elements are present on both the first and the second strip, the strips can also be electrically connected to enable synchronization via electrical signals. There is also a variant in which the emitters of the emitter strip emit light independently of the receiver strip. In this case the emitters are electrically connected to the receivers (synchronization by electrical signals).

[0036] According to a preferred variant of the method, the protection field is divided into a first zone, a second zone and a tolerance zone when an object enters the protection field after the light beam is interrupted and a second predetermined time has elapsed, the length of which depends, inter alia, on the speed of the object. The advantage of this variant of the process is that uniform conditions are applied to every object, reducing the complexity of recognizing permitted objects.

[0037] Advantageously, the tolerance zone is formed with an equal number of free rays and blocked rays.

[0038] The light grid preferably checks the number of blocked rays at regular intervals, preferably every 0.5 to 30 milliseconds, as long as the object is in the protection field.

[0039] Advantageously, the tolerance zone includes two sub-zones located on opposite sides of the second zone.

[0040] Further advantageous embodiments of the sensor arrangement have already been considered above in the description of the method.

[0041] The invention will now be explained in more detail with reference to the drawings in schematic form, in which the preferred features described above can be realised in combination, unless they are mutually exclusive. The drawings are schematic and not to scale. [Brief description of the drawings]

[0042] [Figure 1] FIG. 2 is a cross-section of a light grid with an object in front of it and the light grid subdivided. [Diagram 2] FIG. 2 is a cross-sectional view of a light grid with an object at the edge of the light grid, also illustrating the subdivision of the light grid. [Diagram 3] FIG. 2 shows a front view of a light grid into which protective fields are subdivided. [Figure 4] FIG. 2 shows a front view of a protective field formed by two light grids. [Diagram 5] FIG. 13 is a diagram showing changes over time in the state of light rays when an object enters a light grid. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] In the following, identical reference numbers represent identical or functionally identical elements (in different figures). An additional apostrophe refers to the presence of multiple identical elements in a single figure and serves to distinguish between the identical elements.

[0044] FIG. 1 shows a cross-section of a light grid 11, towards which an object 13 moves. The light grid includes emitters and receivers 15 of a light beam 17, which are arranged vertically at a uniform distance from each other. The light beam 17 forms a protection field. The object 13 moves perpendicular to the plane of the protection field. In FIG. 1, the light beam 17 is divided into various zones. The division of the light beam 17 as shown in FIG. 1 is merely to illustrate the various zones, since the division of the light beam 17 only occurs when the object 13 enters the protection field. The size of these zones is determined by the three-dimensional shape of the object 13. A first zone 18 is formed by mostly free rays. In contrast, a second zone 19 contains the rays that are blocked by the object 13 when it is in the protection field. The outermost rays of the second zone and the adjacent rays that are not in the first zone form a tolerance zone 21. In the illustrated example, the tolerance zone includes two rays. A first zone 18 is thus formed by all free rays that do not belong to the tolerance zone 21. Any interruption of a ray in the first zone 18 generates a stop signal, which shuts down the monitored system as soon as possible. The rays within the tolerance zone 21 do not influence the generation of the stop signal, since interruption or release of rays in the tolerance zone 21 are ignored by the light grid 11.

[0045] FIG. 2 shows an example where a shadow cast by an object 13 falls on the edge of the light grid 11. The light grid 11 is divided into three zones, as already shown in FIG. 1. In contrast to the example of FIG. 1, the tolerance zone 21 is not the same on both sides of the second zone in FIG. 2. The tolerance zone 21' at the edge of the light grid 11 contains only one light ray in the illustrated case. This light ray is the outermost ray that is covered by the shadow cast by the object 13. The tolerance zone 21'' on the other side of the tolerance zone contains two rays as in the example shown in FIG. 1.

[0046] 3 shows a front view of the light grid 11 which is temporarily divided into various zones according to the invention. To simplify the drawing, objects in the protective field are not shown. The second zone 19 has a tolerance zone 21, which is provided at the top and bottom in the form of two strips of the second zone 19. The second zone 19 is caused by the interruption of the light beam, so that the second zone 19 extends over the entire working distance of the light grid 11 from the emitter 15' to the receiver 15".

[0047] To limit the width of the second area 19, a second light grid 11" can be arranged. Ideally, the second light grid 11" is arranged so that the light rays 17' of the first light grid 11' are perpendicular to the light rays of the second light grid 11". An example with two such light grids 11 is shown in Fig. 4. The second area 19 is defined by the light rays 17 of both light grids 11, 11' that are blocked by the object. The second area 19 is thereby limited both in height and in width of the light grid. If a tolerance area 21 is used, it is formed as a frame around the second area in the front view. The object is usually intended to be transported on a conveyor belt. If the light grids 11, 11" are arranged as in Fig. 4, it may be impossible to see the width of the object, since the conveyor belt arranged under the object blocks many light rays. This can be solved by splitting the conveyor belt into two conveyor belts and arranging them side by side. The light grid forms the transition from one conveyor belt to the next. This means that there are no conveyor belts in the light grid and the light rays cannot be blocked by conveyor belts.

[0048] FIG. 5 shows the state of the light beam 17 in time sequence while the object 13 enters the protection field. The front edge 25 of the object 13 is not parallel to the plane of the protection field. It is therefore necessary, among other things, to wait a certain predefined time until the beam is divided into zones. The vertical lines in the figure show the scans 27 of the light grid, which represent the examination of the state of the beam at regular time intervals. After the fifth scan 27 of the light grid, the object 13 has passed through the protection field to such an extent that both the number and the position of the blocked beams remain constant. The light grid divides the beam into the three previously mentioned zones after a predefined time. The blocked beams, except for the two outermost beams, form the second zone 19. The outermost blocked beams, including the adjacent free beams, form the tolerance zone 21, and the remaining beams form the first zone 18. The time until the complete scan 27 of the object by the light grid is completed must be equal to or shorter than the predefined time. Waiting a certain time after the first ray is blocked before the protective field splits allows for a tolerance on the shape of the front of the object. The front edge may be curved and does not necessarily have to be parallel to the light grid. From the time course of the blocked rays in Figure 5 it is clear that the object must be placed against the light grid so that the edges moving away from the front of the object are oriented perpendicular to the light grid. In the tolerance zone a certain curvature of these edges is allowed, but their extent lateral to the direction of object movement must not vary so much that rays outside the tolerance zone are affected.

[0049] The method according to the invention is therefore particularly suitable for objects with a constant cross section.

[0050] Although the present invention has been described above with reference to specific embodiments, it is apparent that changes, modifications, and combinations may be made without departing from the spirit and scope of the present invention. [Explanation of symbols]

[0051] 11. Light Grid 13 Object 15 Emitter / Receiver 17 rays of light 18 First Area 19 Second Area 21 Tolerance area 25 Front edge of object 27 Scan

Claims

1. 1. A method for monitoring a protective field of a facility, comprising: - the protective field is formed by a light grid (11) which generates a number of light beams (17) which lie in one plane and are projected from a light projecting device (15') to a light receiving device (15'') arranged on the opposite side, The method, wherein the light grid (11) comprises a control unit for receiving and processing signals from the light projecting and / or receiving devices, when an object (13) enters said protective field, said protective field is divided into a first zone (18), a second zone (19) and a tolerance zone (21); the second area (19) is defined by all rays other than the outermost rays (17) that are blocked by the object; - the outermost rays blocked by the object form a tolerance zone (21) with adjacent free rays, if any; the remaining free light rays form the first area (18), - a stop signal is generated by said light grid (11) when the light beam (17) is interrupted in said first area (18), - blocked and free rays within said tolerance zone (21) are ignored and no stop signal is generated; - said light grid (11) checks the number of blocked rays in said second area as long as said object (13) is in said protection field; If after the blocking light beams in the second area (19) are released and after a first predetermined time has elapsed, not all of the light beams in the protective field have become free light beams, a stop signal is generated. A method comprising:

2. 2. The method according to claim 1, characterized in that when an object (13) enters the protective field, after a second predetermined time has elapsed since the light beam (17) is interrupted, the protective field is divided into the first zone (18), the second zone (19) and the tolerance zone (21).

3. 3. The method according to claim 1 or 2, characterized in that the minimum size of the second area (19) is defined by at least three blocking rays (17).

4. Method according to any one of claims 1 to 3, characterized in that the tolerance zone (21) is formed with an equal number of free rays and blocked rays.

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

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

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

8. The method according to any one of the preceding claims, characterized in that the second period of time is at least 5 ms long, in particular at least 25 ms long.

9. 9. The method according to claim 1, characterized in that as long as an object (13) is in the protection field, the light grid (11) checks the number of blocked rays (17) at regular time intervals, said time intervals being between 0.5 and 30 milliseconds.

10. The method according to any one of the preceding claims, characterized in that the tolerance area (21) comprises two partial areas, which are arranged on opposite sides of the second area (19).

11. Method according to any one of the preceding claims, characterized in that the tolerance zone (21) is formed by a maximum of 12 rays (17) in total.

12. Method according to any one of the preceding claims, characterized in that two light grids (11) are used, the protective fields of said light grids being in one plane.

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

14. Method according to any one of the preceding claims, characterized in that two or more light grids (11) are provided, said light grids being arranged parallel to one another.

15. The method according to any one of claims 1 to 14, characterized in that the installation comprises a conveyor belt, which is arranged relative to the light grid (11) such that the direction of travel of the conveyor belt is oriented essentially perpendicular to the plane spanned by the light grid (11).

16. A sensor arrangement (1) comprising at least one light grid (8) for monitoring a protective field, - two strips, a first and a second, which can be arranged opposite each other and which are provided with a light emitter and a light receiver, so that during operation a monitoring field is defined between the light emitters and the receivers of the two strips; a control and monitoring unit, connected to said first and second strips and having stored in its memory a program that ensures that a switching signal is generated when an object enters said protective field, whereby a safety function can be initiated, a muting function can be provided and whereby a monitoring function can be at least partially implemented; In a sensor arrangement comprising: The program divides the protection field into a first area (18), a second area (19) and a tolerance area (21) when an object (13) enters the protection field; the second area (19) is defined by all rays other than the outermost rays (17) that are blocked by the object; - the outermost rays blocked by the object form the tolerance zone (21) together with adjacent free rays, if any; the remaining free light beams form the first area (18), and if a light beam (17) is blocked in the first area (18), a stop signal is generated by the light grid (11); - blocked and free rays within said tolerance zone (21) are ignored and no stop signal is generated; - said light grid (11) checks the number of blocked rays in said second area as long as an object (13) is in said protection field; A stop signal is generated if, after a first predetermined time period, all of the rays in the protective field have not become free rays after the blocked rays in the second area (19) are released. A sensor arrangement (1).

17. 17. The sensor arrangement (1) according to claim 16, characterized in that when an object (13) enters the protective field, after a second predetermined time has elapsed since the light beam (17) is interrupted, the protective field is divided into the first area (18), the second area (19) and the tolerance area (21).

18. The sensor arrangement (1) according to claim 16 or 17, characterized in that the first period of time is at least 5 ms, in particular at least 25 ms long, and at most 4 seconds, in particular at most 2 seconds.

19. The sensor arrangement (1) according to claim 17 or 18, characterized in that the second period of time is at least 5 ms, in particular at least 25 ms long, and at most 4 s, in particular at most 2 s.

20. The sensor arrangement (1) according to any one of claims 17 to 19, characterized in that the minimum size of the second area (19) is defined by at least three blocking rays (17).

21. The sensor arrangement (1) according to any one of claims 16 to 20, characterized in that the tolerance zone (21) is formed by an equal number of free rays and blocked rays.

22. The sensor arrangement (1) according to any one of claims 16 to 21, characterized in that as long as an object (13) is in the protective field, the light grid (11) checks the number of blocked light rays (17) at regular time intervals, said time intervals being between 0.5 and 30 milliseconds.

23. The sensor arrangement (1) according to any one of claims 16 to 22, characterized in that the tolerance area (21) comprises two partial areas, the two partial areas being arranged on opposite sides of the second area (19).

24. The sensor arrangement (1) according to any one of claims 16 to 23, characterized in that the tolerance zone (21) is formed by a total of at most 12 light rays (17).

25. The sensor arrangement (1) according to any one of claims 16 to 24, characterized in that two light grids (11) are used, the protective fields of said light grids being in a common plane.

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

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