SENSOR CIRCUIT FOR A DEVICE PERFORMING A SAFETY FUNCTION, DEVICE AND METHOD FOR PROCESSING MEASUREMENT VALUES OF SENSORS

Abstract

The invention relates to a sensor circuit for a device performing a safety function, comprising at least two sensors, at least two comparison circuits, each of the comparison circuits being assigned to one of the sensors, and a linking unit for combining logic states (L=0, L=1) of comparison circuit outputs of the comparison circuits to form a circuit output signal, the sensor circuit being configured to scale the comparison circuit output value of at least a first one of the comparison circuits and to feed it back to a measurement input or a reference input of at least a second one of the comparison circuits so that, when the comparison circuit output of the first comparison circuit transitions between the logic states (L=0, L=1), the difference between the measurement signal and the reference signal of the second comparison circuit is reduced or the sign of the difference between the measurement signal and the reference signal of the second comparison circuit is reversed.

The invention further relates to a device comprising the sensor circuit and a method for processing measurement values from sensors.

Claims

1. A sensor circuit for a device performing a safety function comprising at least two sensors, wherein each of the sensors is configured to detect a respective measured variable and to generate a measurement signal comprising a measurement value which is monotonically dependent on the respective measured variable, at least two comparison circuits, wherein each of the comparison circuits is assigned to one of the sensors and comprises a measurement input for receiving the measurement signal of the respective assigned sensor, a reference input for receiving a reference signal to define a trigger threshold value of the comparison circuit, and a comparison circuit output, wherein the comparison circuit output can assume a first logic state and a second logic state, and wherein the comparison circuit output is configured to generate a comparison circuit output signal comprising a comparison circuit output value representing one of the logic states, a linking unit which is configured to combine the logic states of the comparison circuit outputs by means of a logic AND operation to form a circuit output signal of the sensor circuit, wherein the sensor circuit is configured to scale the comparison circuit output value of at least a first one of the comparison circuits and to feed back the comparison circuit output value to the measurement input or the reference input of at least a second one of the comparison circuits, so that, when the comparison circuit output of the first comparison circuit transitions between the logic states, the difference between the measurement signal and the reference signal of the second comparison circuit is reduced or the sign of the difference between the measurement signal and the reference signal of the second comparison circuit is reversed.

2. The sensor circuit according to claim 1, wherein the sensor circuit is configured to scale the comparison circuit output value of the first comparison circuit with a factor of <0.5.

3. The sensor circuit according to claim 1, wherein the sensor circuit is configured to generate the reference signal of the second comparison circuit by a weighted sum of the trigger threshold value of the second comparison circuit and the comparison circuit output value of the first comparison circuit weighted by a weighting factor and optionally the comparison circuit output value of a further one of the comparison circuits weighted by a weighting factor, and/or in that the sensor circuit is configured to generate the measurement signal of the second comparison circuit by a weighted sum of the measurement signal of the sensor assigned to the second comparison circuit and the comparison circuit output value of the first comparison circuit weighted by a weighting factor and optionally the comparison circuit output value of at least a further one of the comparison circuits weighted by a weighting factor.

4. The sensor circuit according to claim 3, wherein the weighting factor used in generating the reference signal from the weighted sum is less than or equal to zero or the weighting factor used in generating the measurement signal from the weighted sum is larger than or equal to zero if the comparison circuit output value of the first comparison circuit is smaller in the first logic state than in the second logic state and the measurement value of the sensor assigned to the second comparison circuit increases monotonically as a function of the measured variable or if the comparison circuit output value of the first comparison circuit is greater in the first logic state than in the second logic state and the measurement value of the sensor assigned to the second comparison circuit falls monotonically as a function of the measured variable.

5. The sensor circuit according to claim 3, wherein the weighting factor used in generating the reference signal from the weighted sum is greater than or equal to zero or the weighting factor used in generating the measurement signal by the weighted sum is smaller than or equal to zero, if the comparison circuit output value of the first comparison circuit is larger in the first logic state than in the second logic state and the measurement value of the sensor assigned to the second comparison circuit increases monotonically as a function of the measured variable or if the comparison circuit output value of the first comparison circuit is smaller in the first logic state than in the second logic state and the measurement value of the sensor assigned to the second comparison circuit falls monotonically as a function of the measured variable.

6. The sensor circuit according to claim 1, wherein the first logic state and the second logic state are represented by electrical signal levels.

7. The sensor circuit according to claim 1, wherein the first logic state and the second logic state are represented by optical signal states.

8. The sensor circuit according to claim 6, wherein logically identical logic states of the comparison circuit outputs of different ones of the comparison circuits are represented by different signal levels or optical signal states.

9. The sensor circuit according to claim 1, wherein the measurement values are represented by analog voltage or current values.

10. The sensor circuit according to claim 1, wherein the measurement values are represented by numbers in a binary representation.

11. The sensor circuit according to claim 1, wherein the measured variables are independently selected from the group: mechanical displacement, mechanical force, pressure, light intensity, polarization, temperature, loudness, capacitance, inductance, magnetic flux, electric voltage, electric current or electric field strength.

12. The sensor circuit according to claim 1, wherein the sensors are independently selected from the group: hall sensors, photodiodes, phototransistors, photoresistors, thermocouples, capacitive or inductive distance sensors, strain gauges, microphones, adjustable resistors, adjustable capacitors, adjustable inductors.

13. The sensor circuit according to claim 1, wherein the sensors are configured to detect different measurement variables.

14. The sensor circuit according to claim 1, wherein the sensors are configured to detect the same measurement variable, but are based on different measuring principles or are different sensor types.

15. The sensor circuit according to claim 1, wherein the sensor circuit comprises a fault detection unit, wherein the fault detection unit is configured to link the logic states of the comparison circuit outputs by means of a logic exclusive-OR operation to form a fault signal of the sensor circuit.

16. A device for performing a safety function comprising a sensor circuit according to claim 1.

17. The device according to claim 16, wherein the device is a machine comprising a shielding element for protecting against contact of electrically, pneumatically or hydraulically driven elements, and wherein the sensor circuit is configured to monitor a state of the shielding element.

18. The device according to claim 16, wherein the device is a laser device, and wherein the laser device comprises a laser of laser class 3, 3B, 3R or 4 and a shielding element for protection against laser radiation escaping from the laser device and/or an optical element that can be moved into various adjustment positions, and wherein the sensor circuit is configured to monitor a state of the shielding element or the movable optical element.

19. A light microscope comprising a sensor circuit according to claim 1.

20. A method for processing measurement values from sensors for a device performing a safety function, comprising the steps of detecting a respective measured variable and generating a respective measurement signal comprising a measurement value which is monotonically dependent on the respective measured variable by means of at least two sensors, receiving the measurement signals by respective measurement inputs of at least two comparison circuits, each of the comparison circuits being associated with one of the sensors, receiving reference signals for setting a trigger threshold value by respective reference inputs of the comparison circuits, outputting comparison circuit output signals by respective comparison circuit outputs of said comparison circuits, said comparison circuit outputs each being capable of assuming a first logic state and a second logic state, said comparison circuit output signals each comprising a comparison circuit output value representing one of said logic states, combining the logic states of the comparison circuit outputs to a circuit output signal by means of a logical AND operation, scaling the comparison circuit output value of at least a first one of the comparison circuits and feeding it back to the measurement input or the reference input of at least a second one of the comparison circuits, so that during the transition of the comparison circuit output of the first comparison circuit between the logic states, the difference between the measurement signal and the reference signal of the second comparison circuit is reduced or the sign of the difference between the measurement signal and the reference signal of the second comparison circuit is reversed.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0077] FIG. 1 shows a block diagram of a sensor circuit according to the invention.

[0078] FIG. 2 shows a sensor unit with an alternative wiring of the comparison circuits.

[0079] FIG. 3 shows a sensor unit with a further alternative wiring of the comparison circuits.

[0080] FIG. 4 shows a sensor unit with a further alternative wiring of the comparison circuits.

[0081] FIG. 5 shows a sensor unit in digital design.

[0082] FIG. 6 shows the circuit diagram of an analog sensor circuit according to the invention.

[0083] FIG. 7 shows the simulation of a sensor circuit according to the invention.

[0084] FIG. 8A-D shows a device with a sensor circuit according to the invention.

DESCRIPTION OF THE FIGURES

[0085] FIG. 1 shows a sensor circuit 1 according to the invention as a block diagram. The embodiment shown comprises two sensors 2, each with an associated comparison circuit 3. According to this example, the comparison circuits 3 are designed as comparators. The measurement values 4 of the sensors 2 are fed to the measurement value inputs 8 of the comparison circuits 3 via drivers 5, which also assume the function of a scaler 7 when the amplification factor 6 is not equal to one.

[0086] In the embodiment shown, the measurement value 4 of the sensors 2 is proportional to the value of the respective measured variable, i.e., the measurement value y(x) 4 increases monotonically as a function of the measured variable x. For each comparison circuit 3, the switching points of the comparison circuits 3 are specified by individually adjustable or preset trigger threshold values 9, which are fed to the reference value inputs 11 of the comparison circuits 3 via a respective summator 10.

[0087] The measurement inputs 8 of the comparison circuits 3 are non-inverting inputs 12, whereas the reference inputs 11 are inverting inputs 13, so that the comparison circuit output values 14 of the comparison circuits 3 assume logical “1” values, when the level present at the measurement input 8 of the respective comparison circuit 3 is greater than the level present at the reference input 11 of the respective comparison circuit 3, and a logical “0” value when the level present at the measurement input 8 is less than the level present at the reference input 11.

[0088] The comparison circuit output value 14 of each comparison circuit 3 is individually scaled by a scaler 7 and fed back to the reference value inputs 11 of the respective other comparison circuit 3 via the summers 10. In this way, the switching of a comparison circuit 3 between the logic states leads to a change in the sensitivity of the other comparison circuit 3. The switching processes of the two redundant sensor channels are thus synchronized in time and transient error states, which are based on a delay in the switching behavior of one sensor channel relative to the other sensor Zo channel, occur with a lower frequency or for a shorter time.

[0089] The logic states of the comparison circuit outputs 14 are combined in the linking unit 15 with a logic AND operation, and the linking unit 15 outputs a sensor output value 16 representing the logic state obtained by the AND operation.

[0090] Optionally, the sensor circuit may further comprise an error detection unit 50 which links the comparison circuit output signals of the comparison circuit outputs 14 by an exclusive OR operation and outputs a corresponding error signal 51 representing the logic state L=1 when the two comparison circuit output signals represent different logic states. In this way, error states can be detected.

[0091] FIGS. 2 to 4 each show part of a sensor circuit 1 according to further embodiments of the present invention, comprising a sensor 2, a comparison circuit 3 (which is also designed here as a comparator), a summer 10 and several drivers 5 which perform the function of a scaler 7.

[0092] The embodiment shown in FIG. 2 differs from the embodiment shown in FIG. 1 and described above in that two comparison circuit outputs 14a, 14b of comparison circuits 3 not shown in FIG. 2 are scaled and fed back to the inverting reference input 11, 13 via the summer 10.

[0093] Here, one of the comparison circuit outputs 14a is connected to an adding input (+), the other comparison circuit output 14b is connected to a subtracting input (−) of the summer 10. A further adding input (+) of the summer 10 receives the trigger threshold value 9, which may have been scaled via a further driver/scaler 5,7. Thus, the summing circuit 10 calculates the sum of the scaled trigger threshold value and the scaled first comparison circuit output value minus the scaled second comparison circuit output value.

[0094] FIG. 3 shows an embodiment in which the signal of the comparison circuit output 14a of one comparison circuit 3 (not shown) is fed back to the measurement input 8 of the other comparison circuit 3. In this case, the comparison circuit output value 17a and the measurement value 4 of the sensor 2 are scaled by respective weighting factors by means of respective drivers 5, which are configured as scalers 7, and added by means of the summer 10.

[0095] The added signal is then fed to the measurement value input 8 of the comparison circuit 3, which is configured as a non-inverting input 12. The trigger threshold value 9 scaled by means of a further driver 5/scaler 7 is fed to the reference input 11.

[0096] Thus, the comparison circuit output 14 changes to the logic state L=1 when the sum between the measurement value 4 and the scaled fed-back comparison circuit output value 17a is greater than the trigger threshold value 9.

[0097] FIG. 4 shows an embodiment in which the trigger threshold value 9 and the comparison circuit output value 17a of the comparison circuit output 14a (from the comparison circuit 3 not shown here) are scaled via respective drivers 5/scalers 7, summed by means of the summer 10, and supplied to the reference input 11 of the comparison circuit 3, which is designed as a non-inverting input 12.

[0098] The measurement value 4 measured by means of the sensor 2 is scaled via a further driver 5/scaler 7 and reaches the measurement input 8 of the comparison circuit 3, which is designed here as an inverting input 13. Therefore, the comparison circuit output 14 changes to the logic state L=1 when the sum of the scaled trigger threshold value 9 and the scaled comparison circuit output value 17a exceeds the scaled measurement value 4.

[0099] FIG. 5 shows a digital example of the sensor circuit 1 according to the invention. The analog measurement signals of the two redundant sensors 2 of the sensor circuit are scaled by respective scalers 7 and then digitized by a respective analog-to-digital converter 18.

[0100] The analog-to-digital converters 18 may also be integrated in the respective sensors 2, so that the sensors 2 already provide a digital output signal. Furthermore, the scaling may also be implemented internally in the sensor, so that the scalers 7 may be dispensed with if necessary. In particular, sensors 2 with certain measuring ranges may also simply be selected to define the scaling factors without providing scalers 7.

[0101] The digitized measurement signals are fed by means of data lines 19 via a respective summer or adder 10 for the addition of a bias value (see below) to a measuring input 8 of a comparison circuit 3, which is designed as a subtractor.

[0102] Reference inputs 11 of the comparison circuits 3 receive digitally coded reference signals (by the binary numbers D1 to Dn) from data outputs 34 of a respective reference value register 30, which define the respective trigger threshold value 9 of the comparison circuits.

[0103] The comparison circuits 3 calculate the difference between the respective measurement signal and the respective reference signal and output a binary signal at the comparison circuit output 14, which in particular has the value 0 if the difference between the measurement signal and the reference signal is less than zero and has the value 1 if the difference is greater than zero.

[0104] A linking unit 15, which is formed as a digital AND gate, links the comparison circuit output signals of the comparison circuits 3 by means of a logical AND operation and outputs a corresponding binary circuit output signal 16 having a value of 1 when both comparison circuits 3 output the comparison circuit output value representing a difference greater than zero.

[0105] The trigger threshold value stored in the reference value register 20 may be changed by applying a data signal to the data input 21 and a clock signal to the clock input 22 of the reference value register 20. In particular, the individual bits D1 to Dn of the reference value register 20 receive data signals in known manner one after the other in a sequence predetermined by the clock signal, the level of which is assigned the value 0 or the value 1. In particular, this step is already carried out when the sensor circuit 1 is put into operation, which facilitates data synchronization during subsequent operation.

[0106] In order to implement the feedback according to the invention, the comparison circuit outputs 14 of the comparison circuits 3 are additionally connected to an enable input 33 of the bias register 30, which is assigned to the respective other sensor 2.

[0107] The bias register 30 stores a bias value in digitally encoded form. In the same way as for the reference value register 20, this bias value may be changed by applying a data signal to a data input 31 and applying a clock signal to the clock input 32 of the bias register 30.

[0108] If a signal representing the value or logic state 1 is applied to the enable input 33 of the respective bias register 30, the bias value is routed via the data outputs 34 to an input 10a of the respective summer 10 and added to the digitized measurement signal of the respective sensor 2. The enable input 33 thus controls in particular only the output of the stored bias value, but not the overwriting of the stored bias value via the data input 31.

[0109] By activating the enable input due to the feedback according to the invention, the difference between the measurement input 8 and the reference input 11 is reduced or the sign of this difference is reversed. In this way, the temporal response of the sensors 2 can be synchronized while maintaining their independence.

[0110] Of course, the bias value may also be a negative number, which then results in a lower output value of the summer 10 compared to the value of the measurement signal of the sensor 2.

[0111] Furthermore, as an alternative to the embodiment shown in FIG. 5, the output signal of the reference value register 20 can also be linked to the output signal of the bias register 30 via a summer 10 or adder in order to achieve an adjustment of the reference signal via feedback.

[0112] The data lines 19 of the digital sensor circuit 1 shown in FIG. 5 may in particular be a data bus. Alternatively, the entire sensor circuit 1 or parts thereof may also be integrated on a circuit, e.g., on a microcontroller, a so-called field programmable gate array (FPGA) or a so-called application specific integrated circuit (ASIC).

[0113] FIG. 6 shows an analogous example of the sensor circuit 1 according to the invention, in which the trigger threshold values are preset or adjustable by electrical resistors.

[0114] The redundant sensors 2a, 2b are connected via resistors R4 and R9 respectively to the measurement value inputs 8 of the comparison circuits 3a, 3b, which are designed as comparators.

[0115] The resistors R2 and R3 as well as R7 and R8 each form a voltage divider. The reference voltages U.sub.1 and U.sub.2 can be used to set the trigger threshold value for the respective comparison circuit 3a.

[0116] The comparison circuit outputs 14a, 14b of the comparison circuits 3a, 3b are connected to the reference input 11 (here an inverting input 13) of the respective other comparison circuit 3a, 3b via resistors R1 and R6, respectively, so that the corresponding comparison circuit output values 17 are fed back to the respective reference inputs 11. The scaling factor of the feedback can be determined by selecting or setting the resistors R1 and R6.

[0117] The logic states of the comparison circuit outputs 14a, 14b are combined in the linking unit 15 with a logic AND operation, and the linking unit 15 outputs a sensor output value 16 representing the logic state obtained by the AND operation.

[0118] Finally, the sensors 2a, 2b are each directly connected to the linking unit 15 via the additional resistors R5, R10, which results in a hysteresis of the switching operation. In other words, the resistors R5, R10 cause the trigger threshold value of each sensor to differ at least slightly between a switch-on operation (switching from logic state L=0 to logic state L=1) and a switch-off operation (switching from logic state L=1 to logic state L=0). For example, a slightly lower trigger threshold value may be provided for a switch-on operation than for a switch-off operation.

[0119] FIG. 7 shows a simulation result of a sensor circuit 1 according to the invention, which can be used, example. g., to monitor the opening state of a shielding element 40, such as a housing cover (see FIG. 8). For this purpose, when the cover is closed, a permanent magnet is inserted or pivoted into a gap between two Hall sensors 2a, 2b arranged opposite each other; when the device cover is opened, the magnet is pulled out of the gap or pivoted out.

[0120] When the cover is open (i.e. when the magnet is pivoted out), the Hall sensors 2a, 2b output a quiescent voltage of 2.5 V as a measurement value which, depending on the pole of the magnet facing the sensor 2a, 2b, drops to 0 V (sensor 2a) or rises to the operating voltage VCC=5 V (sensor 2b) when the cover is closed (i.e. when the magnet is pivoted in). Sensors 2a, 2b thus have antivalent signal levels.

[0121] The simulation of the sensor circuit was based on the circuit diagram shown in FIG. 6 and simulated with the circuit simulation software Ngspice version 33.

[0122] Resistors R2 and R3 were used to set the trigger threshold value to approximately 1.3 V for comparison circuit 3a and approximately 3.8 V for comparison circuit 3b, both relative to the quiescent state.

[0123] FIG. 7 shows time-voltage curves for the first sensor 2a and the first comparison circuit 3a (lower diagram) and for the second sensor and the second comparison circuit 3b (upper diagram). The solid curves represent the time course of the measurement values 4a, 4b of the sensors 2a, 2b. The dashed lines represent the time course of the trigger threshold values 9a, 9b of the comparison circuits 3a, 3b. Finally, the time course of the comparison circuit output values 17a, 17b is shown.

[0124] The comparison circuit output value 17a 5V of the first comparison circuit 3a represents the logic state L=0 and the comparison circuit output value 17a 0V represents the logic state L=1. Conversely, the comparison circuit output value 17b 0V of the second comparison circuit 3b represents the logic state L=0 and the comparison circuit output value 17b 5V represents the logic state L=1.

[0125] The measurement value 4a of the first sensor drops continuously from an initial value of 2.5 V to 0V within 250 ms and then rises continuously again to 2.5 V within the same time. Such a behavior could result, e.g., from rapid closing and subsequent reopening of a housing cover whose opening state is monitored by a Hall sensor. Accordingly, the measurement value 4b of the antivalent sensor 2b (upper diagram) rises from 2.5 V to 5 V within 250 ms and falls back to 2.5 V within the same time.

[0126] At the first triggering time t.sub.1, the measurement value 4b of the second sensor 2b exceeds the predetermined trigger threshold value 9b. This causes the second comparison circuit 3b to switch to the logic state L=1 represented by the comparison circuit output value 17b of 5V. As a result, the comparison circuit output value 17b increases abruptly from 0 V to 5 V at the time t.sub.1 (or correspondingly later depending on the delay of the comparison circuit).

[0127] The switching of the second comparison circuit 3b causes the scaled comparison circuit output value 17b to be fed back to the measurement input 8 or reference input 11 of the first comparison circuit 3a, thus raising the first trigger threshold value 9a (see lower diagram).

[0128] The measurement value 4a of the first sensor 2a was still just above the initially set trigger threshold 9a at the first triggering time t.sub.1. However, due to the feedback of the other switching signal, the trigger threshold value 9a exceeds the measurement value 4a shortly after the first triggering time t.sub.1. Thus, a sign change of the difference between the measurement value 4a and the trigger threshold value 9a is brought about, which leads to a switching of the first comparison circuit 3a.

[0129] The switching of the first comparison circuit 3a also leads to a reduction in the trigger threshold value 9b of the second comparison circuit 3b due to feedback. However, since it has already switched to the L=1 state, this does not cause any further change at the first triggering time t.sub.1.

[0130] At the second triggering time t.sub.2, the measurement value 4b of the second sensor 2b again reaches the trigger threshold value 9b, while the measurement value 4a of the first sensor 2a is still below its trigger threshold value 9a. Thus, initially only the second comparison circuit 3b switches back to the logic state L=0. However, the trigger threshold value 9a of the first comparison circuit 3a is reduced by the feedback, which again results in a sign change of the difference between the measurement value 4a and the trigger threshold value 9a. As a result, the first comparison circuit 3a also switches to the logic state L=1.

[0131] Thus, it can be seen from the simulation how the sensor circuit 1 according to the invention may be used to synchronize the triggering timing of the redundant sensors 2a, 2b while maintaining the independence of the sensors 2a, 2b.

[0132] FIG. 8 shows various views of an exemplary device 100 with a sensor circuit 1 according to the present invention. Here, FIG. 8A and FIG. 8B are side views of the device 100 in different states. FIG. 8C and FIG. 8D show a detail of the device 1 in different states.

[0133] The device 100 comprises a housing 41 that at least partially encloses internal components (not shown) of the device 100. A shielding element 40, such as a cover, is pivotally connected to the housing 41 via a pivotal connection 42. In the state shown in FIG. 8B, the shielding element 40 covers the internal components.

[0134] The internal components may be, e.g., moving mechanical parts such as shafts, gears, and the like. In this case, the shielding element 40 in the state shown in FIG. 8B functions to protect a user from being injured by the mechanical parts during operation of the device 100.

[0135] Alternatively, the device 100 may be a laser device that includes as an internal component a laser or a beam path into which laser light is coupled from an external source. Then, in the state shown in FIG. 8B, the shielding element 40 prevents laser light from exiting the device 100, thereby preventing potential eye damage to a user.

[0136] The device 100 further comprises a first sensor 2a and a second sensor 2b (see FIG. 8C and FIG. 8D), which monitor the state of the shielding element 40 in a redundant manner. According to the example shown in FIG. 8, the sensors 2a, 2b are magnetic field sensors, e.g., Hall sensors. The sensors 2a, 2b are arranged on both sides of a shaft 44 in the housing 41 and are connected to a sensor circuit 1 according to the invention so that, depending on the magnetic field acting in the Zo shaft 44, measurement signals can be transmitted to the previously described components of the sensor circuit 1.

[0137] A magnet 43 is connected to the shielding element 40, which is arranged in the shaft 44 in the state of the shielding element 40 shown in FIG. 8B and FIG. 8D but is positioned outside the shaft 44 in the state shown in FIG. 8A and FIG. 8C. Thus, depending on the state of the shielding element 40, a magnetic field of different strength acts in the shaft 44, which can be measured by the sensors 2a, 2b.

[0138] If the sensor circuit 1 detects by evaluating the measurement signals from the sensors 2a, 2b that the shielding element 40 is in a state in which there is insufficient protection for the user (FIG. 8A, FIG. 8C), a warning message may be issued, for example, and/or an emergency shutdown of the device 100 or a transition to a safe state may be initiated automatically. In particular, this means that the moving mechanical components in the device 100 are brought to a standstill or that the laser light is switched off, shielded or redirected.

LIST OF REFERENCE SIGNS

[0139] 1 Sensor circuit [0140] 2 Sensor [0141] 2a First sensor [0142] 2b Second sensor [0143] 3 Comparison circuit [0144] 3a First comparison circuit [0145] 3b Second comparison circuit [0146] 4 Measurement value [0147] 4a First measurement value [0148] 4b Second measurement value [0149] 5 Driver [0150] 6 Gain factor [0151] 7 Scaler [0152] 8 Measurement input [0153] 9 Trigger threshold value [0154] 9a First trigger threshold value [0155] 9b Second trigger threshold value [0156] 10 Summer [0157] 10a Input [0158] 10b Output [0159] 11 Reference input [0160] 12 Non-inverting input [0161] 13 Inverting input [0162] 14 Comparison circuit output [0163] 14a First comparison circuit output [0164] 14b Second comparison circuit output [0165] 15 Linking unit [0166] 16 Circuit output signal [0167] 17 Comparison circuit output value [0168] 17a First comparison circuit output value [0169] 17b Second comparison circuit output value [0170] 18 Analog-to-digital converter [0171] 19 Data line [0172] 20 Reference value register [0173] 21 Data input [0174] 22 Clock input [0175] 23 Data output [0176] 30 Bias Register [0177] 31 Data input [0178] 32 Clock input [0179] 33 Enable input [0180] 34 Data output [0181] 40 Shielding element [0182] 41 Housing [0183] 42 Pivotal connection [0184] 43 Magnet [0185] 44 Shaft [0186] 50 Error detection unit [0187] 51 Error signal [0188] 100 Device [0189] t.sub.1 First triggering time [0190] t.sub.2 Second triggering time [0191] U.sub.1 First reference voltage [0192] U.sub.2 Second reference voltage