Sensor arrangement

Abstract

Sensor arrangement (15) with a position measurement system (17) formed by an arrangement of markers, wherein the position of each marker within the position measurement system (17) is encoded in the marker, and with an optical sensor (1) movably arranged relative to the position measurement system (17), which has a light beam emitting transmitter (4), a light beam receiving receiver (10), and an evaluation unit (11) for evaluating received signals from the receiver (10), wherein the optical sensor (1) detects markers within a reading field, wherein a position measurement value is generated in the evaluation unit (11) by detecting a position encoded in a marker and detecting the position of the marker within the reading field, characterized in that the evaluation unit (11) has two computer units (12a, 12b) connected via a data connection (14), and that the received signal of each computer unit (12a, 12b) is processed via an input circuit (13a,13b) is supplied so that in each computing unit (12a, 12b) the received signal is evaluated for a detected mark and a position measurement value (p, A , p B ) is determined that a computing unit (12a, 12b) whose position measurements (p A or p B ) to the other computing unit (12b, 12a), in which the two position measurements (p A , p B ) are compared with each other, so that if the position measurements match (p A , p B ) within a specified tolerance limit the optical sensor (1) outputs a valid output position measurement value and otherwise the optical sensor (1) enters a safe state.

Description

[0001] The invention relates to a sensor arrangement with a position measurement system formed by markings and an optical sensor arranged to be movable relative to this.

[0002] The markers encode their positions, especially their absolute positions. By reading the markers and determining the location of each marker within a reading field, the optical sensor determines its position relative to the positional measurement system.

[0003] Such a sensor arrangement is known from DE 102 11 779 B1. The sensor arrangement described therein comprises an optical sensor with a transmitter emitting light beams, a receiver receiving light beams, means for guiding the transmitted light beams within a scanning range, and an evaluation unit for evaluating the received signals at the receiver's output. The optical sensor detects markers that form a position measurement system. By detecting markers of the position measurement system at two different times, the speed of the optical sensor relative to the position measurement system is also determined.

[0004] The optical sensor is particularly advantageously mounted on a vehicle, with the position measurement system being stationary on the roadway along which the vehicle travels. In principle, a position measurement system can also be moved relative to a stationary optical sensor. Through the continuous detection of the position measurement system's markers, the optical sensor continuously determines the current speed and position values ​​and transmits them to the vehicle's control system. In the control system, these measured values ​​can be used directly for vehicle control without further evaluation and, in particular, without any synchronization effort. Specifically, the control system can continuously monitor compliance with predefined speed profiles for the vehicle.Together with the current information about the vehicle's position, it can be driven reproducibly and precisely to predetermined target positions.

[0005] DE 10 2005 047 658 A1 relates to a sensor arrangement comprising a position measurement system formed by an arrangement of markers and an optical sensor for detecting the markers of the position measurement system. The optical sensor and the position measurement system are arranged to be movable relative to each other. A sensor element is permanently assigned to the optical sensor, by means of which the markers of the position measurement system or markings of a measurement system permanently assigned to the position measurement system can be detected. The signals generated by the optical sensor and the sensor element are evaluated in an evaluation system.

[0006] The optical sensor and the sensor element form a diverse, dual-channel sensor structure that enables the use of the sensor arrangement in safety-related applications.

[0007] Although this sensor arrangement can detect errors in position determination, it requires complex sensor technology.

[0008] The invention is based on the objective of providing a sensor arrangement that has high fault tolerance and high availability at the same time.

[0009] The features of claim 1 are provided to solve this problem. Advantageous embodiments and expedient further developments of the invention are described in the dependent claims.

[0010] The invention relates to a sensor arrangement with a position measurement system formed by an arrangement of markers. Each marker's position within the position measurement system is encoded in its respective marker. An optical sensor, movably arranged relative to the position measurement system, comprises a light-emitting transmitter, a light-receiving receiver, and an evaluation unit for evaluating the receiver's received signals. The optical sensor detects markers within a reading field. In the evaluation unit, a position measurement value is generated by detecting the position encoded in a marker and the marker's position within the reading field. The evaluation unit includes two computer units connected via a data link. The received signal is fed to each computer unit via an input circuit.In each processing unit, the received signal for a detected marker is evaluated, and a position measurement is determined from it. One processing unit sends its position measurements to the other, where the two position measurements are compared. If the position measurements match within a predefined tolerance limit, the optical sensor outputs a valid position measurement. Otherwise, the optical sensor enters a safe state.

[0011] The operating principle of the sensor arrangement according to the invention is such that marks within a reading field can be detected with the optical sensor.

[0012] Advantageously, the optical sensor has a deflection unit by which the light rays are periodically guided within the reading field.

[0013] The reading field is limited by two reading field boundaries Δl located on either side of a reference point.

[0014] The light beams can be guided periodically within a scanning area using the deflection unit, with the reading field being a section of the scanning area.

[0015] The position measurement system consists of a regular, in particular linear, arrangement of markers, in particular barcodes, in which the positions, in particular absolute positions of the markers within the position measurement system are encoded.

[0016] The optical sensor and the position measurement system are arranged to be movable relative to each other. In particular, the optical sensor can be installed on a vehicle that moves past the position measurement system.

[0017] The optical sensor determines the position relative to the position measurement system by detecting (i.e., decoding) a marker and determining its location within the reading field. The optical sensor outputs this position as a measured position value, particularly to a vehicle control system, enabling the vehicle to be positioned relative to the position measurement system.

[0018] According to the invention, the optical sensor for evaluating received signals from the receiver has a redundant two-channel evaluation unit, consisting of two computer units connected via a data connection, which are preferably identical in design.

[0019] The data connection, preferably wired, allows for bidirectional data exchange, enabling the computer units to monitor each other.

[0020] Advantageously, each computer unit is formed by a DSP (Digital Signal Processor) and / or an ARM (Advanced RISC Machine) processor, where RISC = reduced instruction set computer.

[0021] Due to the redundant design in the form of several mutually monitoring computer units of the evaluation unit, it has a fail-safe design, so that the optical sensor can be used in safety-related applications.

[0022] A key aspect of the invention is a reliable position determination based on the received signals of the optical sensor receiver, by deriving a position measurement value p from the received signal in each computing unit. A , P B is determined, whereby these position determinations are carried out in the same way in both computing units.

[0023] The received signal generated in the receiver during the detection of a tag is simultaneously fed to both processing units of the evaluation unit. Each processing unit has an input circuit for this purpose, which includes, in particular, an analog-to-digital converter. The input circuits are identical to the processing units.

[0024] Despite the identity of the computing units and the input circuits, and despite the identical evaluation of the received signal in the computing units, different position measurements p can be obtained in the computing units. A ,p B be determined.

[0025] This is because the position of the mark in the reading field is evaluated differently by the processing units. This can be caused by slightly different clock rates or fluctuations in the clock rates of the processing units and component tolerances in electronic components such as the analog-to-digital converter. These differences in clock rates have a particularly strong impact on the evaluation in the processing units when the detected mark is not in the center of the reading field, but at its edge. In such cases, imaging errors in the receiving optics located upstream of the receiver can lead to signal distortion.

[0026] According to the invention, such errors are detected or corrected by having a computer unit measure the position value p when a mark is detected. A or p Bsends to the other computer unit, whereby the sending and receiving computer unit can be chosen arbitrarily and this can also change.

[0027] In which the position measurements p A or p B The receiving computer unit compares the two position measurements p. A , p B .

[0028] Do the position measurements p match? A , p B If the values ​​match within a specified tolerance limit, an output position measurement value is calculated from these and output by the optical sensor.

[0029] The output position measurement value is advantageously the arithmetic mean of the position measurements p. A , p B Also, only one of the position values ​​p can be used. A , p B be selected as the position value.

[0030] The output position measurement is sent to an external unit, in particular to a control unit of a vehicle on which the optical sensor is installed, where the output position measurements can be used for positioning tasks.

[0031] Do the position measurements p match? A , p B If the readings do not match within the specified tolerance limit, the optical sensor enters a safe state.

[0032] Advantageously, the optical sensor outputs a message in a safe state indicating an invalid position measurement p. A , p B signaled.

[0033] The interference signal is read into the external unit, in particular the vehicle's control unit, so that the external unit can trigger a safety function. Specifically, the safety function consists of stopping the vehicle.

[0034] In principle, the optical sensor itself could enter a safe state by being switched off.

[0035] By specifying the tolerance limit, the availability of the sensor arrangement, especially the optical sensor, can be specifically determined and, in particular, increased, since it is not required that the two position measurements must match exactly.

[0036] Advantageously, the tolerance limit is determined by a threshold value. The threshold value defines the difference in magnitude between the two position measurements p. A , p B rated.

[0037] According to an advantageous embodiment of the invention, the distance Δd of the detected mark is determined in each computing unit. A , Δd B determined as the reference point and used as a criterion for a valid output position measurement value.

[0038] The distances Δd A , Δd BIn addition to the position measurements p, they therefore provide A , p B further validity criteria for position determination, which can also form separate output variables of the optical sensor.

[0039] The evaluation then takes place in such a way that the optical sensor outputs a valid position measurement value if the position measurements p A , p B within a specified tolerance limit and if the detected mark is recognized within the reading field boundaries in at least one computer unit.

[0040] The difference in the magnitude of the position measurements p is calculated. A , p B The value is evaluated using a threshold value that determines the tolerance limit. The magnitude of the threshold value depends on whether the distance Δd is measured in every computing unit, only in one computing unit, or in no computing unit. A and Δd B is detected within the reading field boundaries.

[0041] In particular, the difference in the magnitude of the position measurements p is A ,p B evaluated with a smallest threshold value when both distances Δd A and Δd B lie within the reading field boundaries. The difference in the magnitude of the position measurements p A , p B is evaluated with a mean threshold if one of the distances Δd A and Δd B lies within the reading field boundaries. The difference in the magnitude of the position measurements p A , p B is evaluated with a maximum threshold when both distances Δd A , Δd B lie outside the reading field boundaries.

[0042] This evaluation is adapted to the fact that the probability of an error in position determination is greater the further the detected mark is located at the edge of the reading field or even protrudes beyond the reading field.

[0043] This specific evaluation further increases the availability of the sensor arrangement and, in particular, the optical sensor.

[0044] According to a simplified version of the evaluation, the distances Δd A , Δd B divided into several classes. The optical sensor determines the classification based on the distances Δd. A , Δd B Different evaluations of the position measurements p for a class A , p B carried out.

[0045] In this process, the distances Δd are in an outer area extending beyond the reading field boundaries. A , Δd B The optical sensor switches to the safe state when the magnitude difference of the position measurements p A ,p B exceeds a threshold.

[0046] Furthermore, in a central area of ​​the distances Δd A and Δd B, within which the reference point lies, a position measurement value lying outside the tolerance limit is tolerated.

[0047] Finally, in a central area lying within the reading field boundaries, the distances Δd are generated. A , Δd B The optical sensor sends a warning signal when the magnitude difference of the position measurements p A ,p B exceeds a threshold.

[0048] These evaluations also take into account the fact that the probability of incorrect position determinations is minimal when the detected mark is in the center of the reading field and maximum when the mark is at the edge of the reading field.

[0049] The invention will be explained below with reference to the drawings. The drawings show: Fig. 1: Schematic representation of an optical sensor for detecting marks. Fig. 2: Exemplary embodiment of the sensor arrangement according to the invention with the optical sensor according to Fig. 1. Fig. 3: Partial representation of the sensor arrangement according to Fig. 2 when a brand is detected using the optical sensor within a reading field. Fig. 4: Order according to Fig. 3 with a division of the reading field into three areas: a) Detection of a brand in an internal area. b) Capturing a brand in a medium area. c) Detection of a brand in an outer area.

[0050] In Fig. Figure 1 shows the basic structure of an optical sensor 1 for detecting marks. In this example, the optical sensor 1 is designed as a barcode reader. Accordingly, the marks, which generally exhibit defined contrast patterns, are in this case designed as barcodes 2, consisting of a sequence of dark and light line elements of defined length and width.

[0051] The optical sensor 1 comprises a light beam emitter 4, preferably a laser diode, and a transmitting optic 5 arranged upstream of the emitter 4. The transmitting optic 5 serves to focus the light beams 3 emitted by the emitter 4. The focused light beams 3 are guided via a deflecting mirror 6 onto a deflection unit 7, which in this embodiment is formed by a rotating polygonal mirror wheel. The polygonal mirror wheel has a predetermined number of identically shaped mirror surfaces 8. The light beams 3 are deflected at the mirror surfaces 8 and periodically guided within a scanning range A.

[0052] The light rays 3 are reflected at the barcode 2 and from there guided via the deflection unit 7 and a receiving optic 9 to a receiver 10, which may be formed by a photodiode.

[0053] The received signals from receiver 10 are fed to an evaluation unit 11. The evaluation unit 11 is dual-channel and has two identical computer units 12a, 12b, each with an input circuit 13a, 13b. The input circuits 13a, 13b are also identical. The computer units 12a, 12b are connected via a data link 14, enabling bidirectional data exchange.

[0054] Advantageously, each computing unit 12a, 12b is formed by a DSP (Digital Signal Processor) and / or an ARM (Advanced RISC Machine) processor.

[0055] Fig. Figure 2 shows an embodiment of the sensor arrangement 15 according to the invention with the optical sensor 1.

[0056] The optical sensor 1 is installed on a vehicle 16, wherein the vehicle 16 is a driverless transport vehicle controlled by a controller 16a.

[0057] In addition, the sensor arrangement 15 includes a stationary position measurement system 17 with a linear arrangement of markers, i.e. barcodes 2, in which the absolute positions of the individual markers are encoded.

[0058] Fig. Figure 3 shows the principle of detecting markers, which serve to determine the position of the optical sensor 1 and thus of the vehicle 16 relative to the position measurement system 17.

[0059] Fig. Figure 3 shows the scanning line A' along which the light rays 3 are guided periodically when scanning the scanning area A.

[0060] The optical sensor 1 detects marks within a reading field that is bounded by reading field boundaries Δl. The reading field boundaries Δl are symmetrical about a reference point 19 or a reference line.

[0061] Due to the dual-channel design of the evaluation unit 11, it has a fail-safe structure that enables the use of the optical sensor 1 in safety-related applications.

[0062] In particular, a two-channel position determination takes place in the evaluation unit 11. For this purpose, a received signal generated when a marker is detected in the receiver 10 is simultaneously fed to both computer units 12a and 12b of the evaluation unit 11.

[0063] To determine the position of a token, its absolute position in the position measurement system 17 is determined by decoding the code contained in the token. In addition, the token's position within the reading field is determined.

[0064] Despite the identity of the computing units 12a, 12b and the input circuits 13a, 13b, and despite the identical evaluation of the received signal in the computing units 12a, 12b, different position measurements p can be obtained in the computing units 12a, 12b.A , p B will be obtained.

[0065] This is because the position of the mark in the reading field is evaluated differently by the computer units 12a and 12b, which can be caused by slight variations in the clock rates of the computer units 12a and 12b and electronic components such as the analog-to-digital converter. These differences in clock rates have a particularly strong effect on the evaluation in the computer units 12a and 12b when the detected mark is not in the center of the reading field but at its edge. In this area, imaging errors of a receiving optic 9 located in front of the receiver 10 can lead to signal distortions in the received signal.

[0066] According to the invention, such errors are detected or corrected by having a computer unit 12a or 12b measure the positional measurements p when a mark is detected. A or p Bto the other computer unit 12b or 12a, whereby the sending and receiving computer unit 12a, 12b can be chosen arbitrarily and this can also change.

[0067] In the present case, the computer unit 12b transmits its position measurement value p. B to the computer unit 12a, which receives the position measurements p A , p B compares.

[0068] The following comparison function g(p) is used. A , p B ) determined. v=g(pA,pB)={1 for|pA−pB|≤tp0 otherwise

[0069] In equation (1) t p A threshold defining a tolerance limit for a permissible position difference between the two computer units 12a, 12b, i.e., the comparison function g(p). A , p B ) returns the value 1 if the two position values ​​p A and p Bare approximately equal. Otherwise, the value 0 is obtained. Then, the computer unit 12a determines the value using the function f(p). A , p B , v) the output position value p, which is output via an interface. The function f (p A , p B , v) could, for example, read as follows: p=f(pA,pB,v)={pA+pB2fu¨rv=1"safe state" otherwise

[0070] An output position value p = “safe state” means that the optical sensor 1 transitions into a safe state.

[0071] The output position measurement value obtained in the error-free case pA+pB2 is transmitted to the control unit 16a of the vehicle 16 and used there to position the vehicle 16.

[0072] Advantageously, the optical sensor 1 outputs a message in a safe state indicating an invalid position measurement p. A , p B signaled.

[0073] The interference signal is read into the external unit, in particular the control unit 16a of the vehicle 16, so that the external unit can trigger a safety function. In particular, the safety function consists of stopping the vehicle 16.

[0074] According to an advantageous further training, in addition to determining the position of a brand, its distance Δd to reference point 19 is evaluated ( Fig. 3).

[0075] This distance determination is performed in both computing units 12a and 12b. Due to the aforementioned sources of error, distances Δd are determined in computing units 12a and 12b. A , Δd B received, which can be different.

[0076] The values ​​p are then taken from computer unit 12b. B , Δd B determined and transmitted to computer unit 12a, where these values ​​are matched with the corresponding values ​​p A , Δd A can be compared.

[0077] This results in, for example, the following comparison function. v=g(pA,pB,ΔdA,ΔdB)={1 for|pA−pB|≤tp,1,ΔdA≤Δl andΔdB≤l1 for|pA−pB|≤tp,2,ΔdA≤Δl andΔdB>l1 for|pA−pB|≤tp,2,ΔdA>Δl andΔdB≤l2 for|pA−pB|≤tp,3,ΔdA>Δl andΔdB>l0 otherwise

[0078] In equation (3) the following applies to the threshold values ​​t p = t p,1 ≤ t p,2 ≤ t p,3 By including the evaluation of the distances Δd A and Δd B The position of the mark in the reading field is checked. By using different thresholds depending on the position in the reading field, according to the exemplary implementation of the comparison function according to equation (3), the device availability is increased. The function f (p A , p B , v) can be implemented, for example, as follows: p=f(pA,pB,v)={pA+pB2 for¨rv=10 for¨rv=2"safe state" otherwise

[0079] An output position value p = 0 indicates an invalid position value. In this case, the device is not in a safe state.

[0080] An output position measurement value for v = 1 is used to position the vehicle 16.

[0081] The Fig. Figures 4a to 4c show another embodiment of determining the position of tokens within the reading field.

[0082] In this case, the reading field is divided into three areas I to III, namely an inner area I around the reference point 19, an outer area III at the reading field boundary Δl and an intermediate middle area II.

[0083] The evaluation of position measurements p A , p B This then depends on whether the determined distances Δd A , d A for a brand in the inner I, outer III, or middle area II. The actual distance Δd is in the Fig. 4a to 4c are marked.

[0084] Fig. Figure 4a shows the detection of a mark in the inner area I. In this case, if the difference in the amount of the position measurements determined in the computer units 12a and 12b is p A , p B the threshold t p,1 If the safe state is exceeded, the limit is reached. Otherwise, the mean of these position measurements p is used. A , p B Output as a measured value for the output position.

[0085] Fig. Figure 4b shows the detection of a mark in the middle area II. In this case, if the difference in the amount of the position measurements determined by the computer units 12a and 12b is p A , p B the threshold t p,1 If the safe state is exceeded, the limit is reached. Otherwise, the mean of these position measurements p is used. A , p B Output as a measured value for the output position.

[0086] Additionally, optical sensor 1 can generate a warning signal if one of the two computing units has detected area II. This signal is output to an external unit and indicates that the sensor is approaching its reading field limit Δl.

[0087] Fig. Figure 4c shows the detection of a mark in outer area III. In the case that the difference in the magnitude of the position measurements p A ,p B the threshold t p,3If the limit is exceeded, optical sensor 1 enters a safe state. Otherwise, the position value 0 is output. Additionally, optical sensor 1 can generate another warning signal if either of the two processing units has detected area III. This signal is output to an external unit and indicates that the sensor is outside its reading field limit Δl. If the two processing units detect different areas, the procedure described in equations 3 and 4 is followed. Reference symbol list 1 Optical Sensor 2 Barcode 3 light beam 4 transmitters 5 transmitting optics 6 deflecting mirrors 7 Deflection unit 8 Mirror surface 9 Reception optics 10 recipients 11 Evaluation unit 12a, b Computer unit 13a, b Input circuit 14 Data connection 15 Sensor arrangement 16 vehicles 16a Control 17 Position measurement system 19 Reference point A sampling range A' scanning line Δd, Δd A , Δd B Distance Δl Reading field boundary p A , p B Position measurements I inner area II middle range III outer area QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 102 11 779 B1

[0003] DE 10 2005 047 658 A1

[0005]

Claims

[1] Sensor arrangement (15) with a position measurement system (17) formed by an arrangement of tokens, wherein the position of each token within the position measurement system (17) is encoded, and with an optical sensor (1) arranged to be movable relative to the position measurement system (17), which has a light beam emitting transmitter (4), a light beam receiving receiver (10) and an evaluation unit (11) for evaluating received signals from the receiver (10), wherein tokens within a reading field are detected with the optical sensor (1), wherein a position measurement value is generated in the evaluation unit (11) by detecting a position encoded in a token and detecting the position of the token within the reading field, characterized by, that the evaluation unit (11) has two computer units (12a, 12b) connected via a data connection (14), that the received signal is supplied to each computer unit (12a, 12b) via an input circuit (13a, 13b), that in each computer unit (12a, 12b) the received signal is evaluated for a detected mark and a position measurement value (p) is derived from it A , p B ) is determined that a computing unit (12a, 12b) whose position measurements (p A or p B ) to the other computing unit (12b, 12a), in which the two position measurements (p A , p B ) are compared with each other, so that if the position measurements match (p A , p B ) within a specified tolerance limit the optical sensor (1) outputs a valid output position measurement value and otherwise the optical sensor (1) enters a safe state. [2] Sensor arrangement (15) according to claim 1, characterized by, that the optical sensor (1) has a deflection unit (7) by means of which the light rays (3) are periodically guided within the reading field. [3] Sensor arrangement (15) according to one of claims 1 or 2, characterized by , that the optical sensor (1) outputs an error message in the safe state indicating an invalid position measurement value (p A , p B ) signals. [4] Sensor arrangement (15) according to one of claims 1 to 3, characterized by that the tolerance limit is determined by a threshold value with which the difference in magnitude between the two position measurements (p) A , p B ) is evaluated. [5] Sensor arrangement (15) according to any one of claims 1 to 4, characterized by , that the output position measurement is the arithmetic mean of the position measurements (p A , p B ) or one of the position measurements (p A , p B ) is. [6] Sensor arrangement (15) according to any one of claims 1 to 5, characterized by , that the reading field is bounded by two reading field boundaries (Δl) located on either side of a reference point (19). [7] Sensor arrangement (15) according to claim 6, characterized by , that for the detected mark in each computing unit (12a, 12b) its distance (Δd A , Δd B ) is determined to the reference point (19) and is used as a criterion for a valid output position measurement value. [8] Sensor arrangement (15) according to one of claims 6 and 7, characterized by , that the optical sensor (1) outputs a valid output position measurement when the position measurements (p A , p B ) within a specified tolerance limit and if at least in one computing unit (12a, 12b) the detected mark is recognized within the reading field limits Δl. [9] Sensor arrangement (15) according to claim 8, characterized by, that the difference in the amount of the position measurements (p A , p B ) is evaluated with a threshold value determining the tolerance limit, the size of which depends on whether the distance (Δd) is in every computing unit (12a, 12b), only in one computing unit (12a, 12b) or in no computing unit (12a, 12b). A and Δd B ) within the reading field boundaries Δl is detected. [10] Sensor arrangement (15) according to claim 9, characterized by , that the difference in the amount of the position measurements (p A , p B ) is evaluated with a smallest threshold value when both distances (Δd) A and Δd B ) within the reading field boundaries Δl, such that the magnitude difference of the position measurements (p A , p B ) is evaluated with a mean threshold if only one of the distances (Δd) A and Δd B ) lies within the reading field boundaries Δl, and that the magnitude difference of the position measurements (p A , pB ) is evaluated with a largest threshold when both distances (Δd A , Δd B ) lie outside the reading field boundaries Δl. [11] Sensor arrangement (15) according to claim 7, characterized by , that the distances (Δd A , Δd B ) are divided into several classes, and that the optical sensor (1) determines the membership of the distances (Δd) A , Δd B ) different evaluations of the position measurements for a class (p A , p B ) be performed. [12] Sensor arrangement (15) according to claim 11, characterized by , that in an outer area of ​​distances extending beyond the reading field boundaries Δl (Δd) A , Δd B ) the optical sensor (1) enters the safe state when the magnitude difference of the position measurements (p A , p B ) exceeds a threshold. [13] Sensor arrangement (15) according to one of claims 11 or 12, characterized by , that in a middle area of ​​the distances (Δd) lying within the reading field boundaries Δl A , Δd B ) the optical sensor (1) generates a warning signal when at least one of the two computing units detects the central area. [14] Sensor arrangement (15) according to one of claims 11 to 13, characterized by , that in an outer area of ​​the distances (Δd) lying outside the reading field boundaries A , Δd B ) the optical sensor (1) generates a further warning signal if at least one of the two computing units detects the outer area. [15] Sensor arrangement (15) according to any one of claims 1 to 14, characterized by , that the computing units (12a, 12b) forming the evaluation unit (11) are identical. [16] Sensor arrangement (15) according to any one of claims 1 to 15, characterized by, that each computing unit (12a, 12b) is formed by a DSP (Digital Signal Processor) and / or an ARM (Advanced RISC Machine) processor. [17] Sensor arrangement (15) according to any one of claims 1 to 16, characterized by , that bidirectional data transmission takes place between the computer units (12a, 12b) via the data connection (14). [18] Sensor arrangement (15) according to any one of claims 1 to 17, characterized by , that the absolute positions of the marks within the positional measurement system (17) are encoded.

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