Sensor circuit for a device performing a safety function, device and method for processing sensor measurements

Description

Technical field of the invention

[0001] The invention relates to a sensor circuit for a device performing a safety function, a corresponding device and a method for processing measured values ​​from sensors.

[0002] In the context of this application, safety functions are those functions that are designed to protect people from injuries or other impairments to health caused by various technical devices, or to prevent further damage to systems, buildings, etc.

[0003] Sensor systems that monitor the status of devices performing safety functions are known in the art. These systems can, for example, detect conditions in which the device is inadequately or no longer performing its safety function at all. By coupling the sensor with safety circuits, an emergency shutdown can then be initiated, for example, upon detection of an unsafe condition.

[0004] For example, machines that have electrically, pneumatically or hydraulically driven elements may have a cover or housing to protect the machine operator from injury.

[0005] Safety-relevant covers and enclosures are also known, for example, for optical devices containing high-power lasers. In this case, the cover ensures laser safety by protecting operators and other persons present from eye damage caused by the laser light.

[0006] However, in order to be able to carry out maintenance work, for example, such covers are often equipped with an opening mechanism.

[0007] To ensure that an unforeseen opening of the cover does not pose a danger, sensor systems (e.g. mechanically or magnetically triggered switches or Hall sensors triggered by magnets) can monitor the opening state of the cover and, when open, cause a transition to a safe state (e.g. an emergency shutdown or a transition to a standby state).

[0008] Furthermore, with optical devices such as light microscopes, which primarily use laser sources, there is the problem that, when optical elements such as mirrors or filter arrangements in the beam path need to be mechanically adjusted, reflections of the laser light can enter the eyepiece. This can cause eye damage to the user. For this reason, safety sensors are known that detect certain safety-critical arrangements of the optical elements in the beam path and subsequently prevent light reflections from occurring in the eyepiece, for example, by darkening the eyepiece or deliberately interrupting or redirecting the laser light at a suitable location.

[0009] To further increase safety even in the event of safety system malfunctions, the DIN EN 61508-2:2010 standard, for example, requires redundant systems that perform safety functions. This can be achieved by, for example, providing at least two sensors that monitor the same safety function. These at least two sensors can measure the same physical quantity or different physical quantities, which can be used to determine the status of the safety system.

[0010] Furthermore, it is known from the prior art to couple such redundant sensors antivalently (e.g., via antivalent data lines) to a monitoring circuit. "Antivalent" means that the signal levels of the sensors' output signals differ from one another when the system executing the safety functions is in the same state. For example, a first sensor can generate a low-voltage (LO) output signal when the cover is closed and a high-voltage (HI) output signal when the cover is open, while a second sensor can generate a high-voltage (HI) output signal when the cover is closed and a low-voltage (LO) output signal when the cover is open.

[0011] Such a monitoring system has the advantage of having defined permissible states, namely those in which the sensor output signals exhibit different signal levels (HI / LO or LO / HI), while states with two identical signal levels (LO / LO or HI / HI) are impermissible and are interpreted as errors. As a result of impermissible states, the monitoring circuit can then automatically initiate an emergency shutdown of the system or a transition to a safe state.

[0012] Safety-relevant systems with redundant sensors have the disadvantage that temporary impermissible states (states with the same signal level for both sensors) often arise because the sensors do not trigger simultaneously when their measured value changes. As a result, unnecessary emergency shutdowns or transitions to safe states are triggered, for example, even though no safety-relevant problem exists. State of the art

[0013] US Patent No. 4,088,900 discloses a circuit for evaluating safety-relevant signals in the control of an elevator. The circuit comprises a switching circuit, a monitoring circuit, and a test circuit, which are coupled to each other via diodes. The three subcircuits each have two different digital logic units. The two logic elements of the circuit receive antivalent signals from two sensors that monitor the status of the elevator door. The monitoring circuit and the test circuit can detect different error states and subsequently block the starting of the elevator motor. A delay circuit is provided to prevent the system from being shut down if the sensor signals are equivalent for only a short time.

[0014] US patent application 2005 / 0052083 A1 describes a method for detecting faulty antivalent key or switching signals and a corresponding circuit for implementing the method. According to the illustrated embodiment, the antivalent signal outputs are routed to logical AND gates in an evaluation unit in two parallel branches, with an inverter element arranged in one of the branches so that an error signal is generated in the event of faulty equivalent states of the two signal outputs. A delay circuit ensures that brief equivalent states do not result in an error signal. In a variant of the method, a further delay circuit can be used to detect whether the key or switching signal is present for too long, an error that can arise, for example, from an input key being blocked.Furthermore, the patent application discloses the use of the method in various production machines.

[0015] Patent DE 43 15 298 C1 discloses a circuit arrangement for generating two complementary signals. A signal and an inverted signal are each applied to a transfer transistor controlled by a clock. Each transfer transistor controls an inverter on its output side, and the inverters are cross-coupled. This improves the timing consistency of the complementary signals.

[0016] European patent EP 0 164 118 B1 describes a circuit for evaluating the signal of a Hall sensor with antivalent output signals, which monitors the state of a DC motor for a computer hard drive. The circuit comprises a commutation comparator and an index comparator. According to one embodiment, the output of the commutation comparator is dynamically coupled to an input of the index comparator, so that a positive edge of the output signal is coordinated with a zero crossing of the sensor signal with high accuracy.

[0017] The solutions described in the prior art are aimed at improving the temporal coordination of the antivalent signals themselves (e.g. by means of delay circuits) and thus have the disadvantage that, in the case of imperfect synchronization, short-term signal equivalence can still occur in certain situations and thus undesirable error conditions can occur.

[0018] DE 103 30 202 discloses a displacement sensor device with two magnetoresistive elements for scanning magnetic encoders, with measuring bridges having measurement signal amplifiers that amplify the bridge signals, and comparators or analog-to-digital converters for converting the analog alternating signals into digital signals. Furthermore, a switching device is provided that can change the current in a measuring bridge to increase accuracy and signal-to-noise ratio. The displacement sensor device also has voltage followers connected to a center potential of the respective measuring bridge and the reference potential of a comparator or analog-to-digital converter, which relate the reference potential to the respective measuring bridge.

[0019] DE 10 2013 108 532 A1 describes a field device for indicating the level of liquids. A connector of the field device contains indicator elements, two of which signal complementary switching outputs of the field device. Another indicator element indicates a fault and / or malfunction of the field device. The indicator element for indicating the fault and / or malfunction is controlled by a NOR or XNOR circuit, which processes the complementary switching outputs.

[0020] US 2020 / 0195186 A1 discloses a control circuit for an inverter device that drives a motor by performing AC-DC conversion through a bridge circuit. The control circuit has a safety circuit with two comparators and a logic unit. The comparators each compare a DC voltage measurement and an output voltage of an isolated voltage source with respective reference values. The logic unit combines the comparator outputs using an AND operation.

[0021] DE 15 37 379 A1 describes a safety circuit for performing logical operations, particularly for railway safety systems. The safety circuit contains a NAND gate and a NOR gate, each with an output for binary switching variables and their complementary switching variables in the form of rectangular digital signals with a specified repetition frequency. A rectifier circuit is connected to the outputs of the two logic elements as a monitoring element. The rectifier circuit's output voltage is used as the supply voltage for the switching path of an electronic switch. For the control path, rectangular digital test signals with at least twice the specified repetition frequency are provided, which lie outside the edge range of the digital signals. Object of the invention

[0022] The object of the present invention is to provide a sensor circuit for a device performing a safety function with redundant sensors, in which principle-related, temporary error states occur less frequently, which improves the user-friendliness and reliability of the sensor circuit or the device. Solution

[0023] This object is achieved by the sensor circuit according to claim 1, the devices according to claims 12 and 13, the light microscope according to claim 14 and the method according to claim 15. Advantageous developments of the sensor circuit are the subject of subclaims 2 to 11. Further advantageous embodiments of the invention are described below. Description of the invention

[0024] A first aspect of the invention relates to a sensor circuit for a device performing a safety function.

[0025] The sensor circuit comprises at least two sensors, wherein each of the sensors is designed to detect a respective measured variable and to generate a measuring signal with a measured value that is monotonically dependent on the respective measured variable.

[0026] "Monotonically dependent" means that the measurement signal remains constant or increases as the measured variable increases (monotonically increasing), or that the measurement signal remains constant or decreases as the measured variable decreases (monotonically decreasing). In particular, the measured value is strictly monotonically dependent on the respective measured variable. This means that the measurement signal always increases as the measured variable increases and always decreases as the measured variable decreases. The sensors can measure the same measured variable or different measured variables, for example, to increase measurement redundancy. In particular, the sensors output an analog measurement signal.

[0027] Furthermore, the sensor circuit has at least two comparison circuits, wherein each of the comparison circuits is assigned to one of the sensors, and wherein each of the comparison circuits has a measurement input for receiving the measurement signal of the respective assigned sensor, a reference input for receiving a reference signal for setting a trigger threshold value of the comparison circuit, and a comparison circuit output.

[0028] The comparison circuit output can assume a first logical state and a second logical state, wherein the states can be represented, for example, by the parameters L=0 and L=1. The comparison circuit output is designed to generate an analog or digital comparison circuit output signal with a comparison circuit output value representing one of the logical states. The trigger threshold can be predefined or predefinable, i.e., variable, and influences the measured value received at the measurement input, at which the respective comparison circuit switches to a different logical state.

[0029] The comparison circuits are particularly designed to compare a signal level of the respective received measurement signal with a signal level of the respective received reference signal and, based on the comparison, to determine the logical state of the comparison circuit output and thus the generated comparison circuit output signal. For example, a comparison circuit according to the invention could switch from the state L=0 to the state L=1 if the measurement signal has a higher signal level than the reference signal.

[0030] The comparison circuits can be designed as comparators, particularly in an analog implementation of the safety circuit. Such comparators can be implemented, for example, as measuring amplifiers with two inputs and one comparator output, with the comparator output value outputted at the comparator output representing a comparison of the input signal levels present at the two inputs. Typically, one of the inputs is inverting and the other input is non-inverting.

[0031] Alternatively, the comparison circuit can also be formed, for example, by a subtractor, particularly in a digital implementation of the sensor circuit according to the invention. Such subtractors also have two inputs and one output and calculate the difference between the two input signals. The output level or output value of the subtractor is determined depending on whether the calculated difference is greater than or less than zero. For example, a digital subtractor can output an output value of 0 if the difference between the input values ​​is less than zero and an output value of 1 if the difference between the input values ​​is greater than zero.

[0032] The sensor circuit further comprises a logic unit configured to combine the logical states of the comparison circuit outputs of the at least two comparison circuits using a logical AND operation to form a circuit output signal of the sensor circuit. The circuit output signal, in particular, represents two alternative logical states that represent an AND operation of the logical states of the comparison circuit outputs.

[0033] The logic unit thus compares the comparison circuit output signals of two comparison circuits and outputs a circuit output signal that represents the logic state L=1 if the comparison circuit output signals of both comparison circuits represent the logic state L=1.

[0034] According to the invention, the sensor circuit is designed to scale the comparison circuit output value of at least a first of the comparison circuits and to feed it back to the measurement input or the reference input of at least a second of the comparison circuits, so that when the comparison circuit output of the first comparison circuit transitions between the logical states (ie in particular when transitioning from the state L=0 to the state L=1 and / or when transitioning from the state L=1 to the state L=0), 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.

[0035] In this way, the switching behavior of at least two sensors is temporally coordinated, thus avoiding short-term system fault conditions. This can often prevent unnecessary emergency shutdowns or transitions to a safe state, improving the reliability of the safety device.

[0036] Temporal coordination is achieved by deliberately increasing the sensitivity of the other sensor subsystem (second sensor and comparison circuit) when a change in the measured variable causes one sensor subsystem (first sensor and comparison circuit) to switch to a different logical state. Scaling the feedback signal ensures that the second sensor subsystem is not simply automatically activated, but remains largely dependent on the measured variable. This maintains the independence of the sensor channels despite the temporal coordination, and actual error states are still detected as such.

[0037] If the measured value acquired by the second sensor is already just below the trigger threshold when the first comparison circuit is triggered, the feedback of the scaled output signal from the first comparison circuit may reverse the sign of the difference between the measured value and the reference value of the second comparison circuit. In this case, the second comparison circuit also immediately switches to the other logical state. Otherwise, the difference between the measured value and the reference value is simply reduced, so that the second comparison circuit triggers more quickly than without the feedback when the measured value changes further.

[0038] The scaling of the comparison circuit output value is performed, in particular, by a driver of the sensor circuit with a gain factor other than one, which acts as a scaler. The driver is connected to the measurement input or the reference input of the second comparison circuit in order to feed the scaled comparison circuit output value back to it.

[0039] According to one embodiment, the sensor circuit is configured to scale the comparison circuit output value of the second comparison circuit and to feed it back to the measurement input or the reference input of the first comparison circuit, so that when the comparison circuit output of the second comparison circuit transitions between the logical states, the difference between the measurement signal and the reference signal of the first comparison circuit is reduced or the sign of the difference between the measured value signal and the reference signal of the first comparison circuit is reversed. In other words, according to this embodiment, cross-feedback is implemented as positive feedback between the comparison circuits.

[0040] According to one embodiment, the sensor circuit, in particular the driver or scaler, is configured to scale the comparison circuit output value of the first comparison circuit (or the second comparison circuit) by a factor of <0.5, in particular <0.2, and more particularly <0.1. The smaller the scaling factor, the more the other comparison circuit remains dependent on the measured variable of the sensor assigned to it, thus further improving the independence of the sensor channels. In a specific case, the value of the scaling factor could be adjusted to achieve an optimum balance between temporal coordination and independence of the sensors.

[0041] According to a further embodiment, the sensor circuit is designed to form the reference signal of one of the comparison circuits, in particular the second comparison circuit, by a weighted sum of the trigger threshold value of this comparison circuit, in particular the second comparison circuit, and the comparison circuit output value, weighted with a weighting factor, of another of the comparison circuits, in particular the first comparison circuit, and optionally the comparison circuit output value, weighted with a weighting factor, of at least one further comparison circuit. This means, in particular, that the reference signal r j the comparison circuit with the index j is determined by the sum rj = aj + g ij Y i determined, whereby aj the trigger threshold of the comparison circuit j, g ij the weighting factor and Y ithe comparison circuit output signal of the comparison circuit is denoted by the index i.

[0042] Alternatively or additionally, the sensor circuit can be designed to form the measurement signal of one of the comparison circuits, in particular the second comparison circuit, by a weighted sum of the measurement signal of the sensor assigned to this comparison circuit, in particular the second comparison circuit, and the comparison circuit output value, weighted with a weighting factor, of another of the comparison circuits, in particular the first comparison circuit, and optionally the comparison circuit output value, weighted with a weighting factor, of at least one further comparison circuit. This means, in particular, that the measurement signal mj the comparison circuit with the index j is determined by the sum m j = a j + g ij ′ Y i determined, whereby aj the measured value of the comparison circuit jassigned sensor, g ij ′ the weighting factor and Y i the comparison circuit output signal of the comparison circuit is denoted by the index i.

[0043] The weighting factors can be defined by a matrix g ij (in the case of feedback to the reference input of the respective comparison circuit) or g ij ′ (in the case of feedback to the measurement input of the respective comparison circuit). The index i denotes the comparison circuit whose comparison circuit output value is scaled and fed back and the index j refers to the comparison circuit whose reference signal or measurement signal is formed by means of the weighted sum. The formation of the weighted sums can be carried out in particular by summers of the sensor circuit. In particular, the main diagonal of the matrix g ijonly values ​​of zero, ie the comparison circuit outputs are not fed back to themselves.

[0044] According to a further embodiment, the weighting factor used in forming the reference signal from the weighted sum is less than or equal to zero or the weighting factor used in forming the measurement signal by the weighted sum is greater than or equal to zero if the comparison circuit output value of the first comparison circuit in the first logical state (L=0) is less than in the second logical state (L=1) and the measured 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 in the first logical state (L=0) is greater than in the second logical state (L=1) and the measured value of the sensor assigned to the second comparison circuit decreases monotonically as a function of the measured variable.

[0045] According to a further embodiment, the weighting factor used in forming the reference signal from the weighted sum is greater than or equal to zero or the weighting factor used in forming the measurement signal by the weighted sum is less than or equal to zero if the comparison circuit output value of the first comparison circuit is greater in the first logical state (L=0) than in the second logical state (L=1) and the measured 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 logical state (L=0) than in the second logical state (L=1) and the measured value of the sensor assigned to the second comparison circuit decreases monotonically as a function of the measured variable.

[0046] These relationships can also be summarized in the following way, where g ijthe matrix of weighting factors when feeding back the comparison circuit output signal of the i -th comparison circuit to the reference input of the j -th comparison circuit, g ij ′ the matrix of weighting factors in the feedback of the comparison circuit output signal of the i -th comparison circuit to the measuring input of the j -th comparison circuit, Y ( L = 0) the comparison circuit output value in the first logical state, Y ( L = 1) the comparison circuit output value in the second logic state and y ( x ) the function of the measured value of the respective sensor from the measured quantity x designated: g ij ≤ 0 , g ij ′ ≥ 0 wenn Y L = 0 < Y L = 1 und y x monoton steigend g ij ≤ 0 , g ij ′ ≥ 0 wenn Y L = 0 > Y L = 1 und y x monoton fallend g ij ≥ 0 , g ij ′ ≤ 0 wenn Y L = 0 > Y L = 1 und y x monoton steigend g ij ≥ 0 , g ij ′ ≤ 0 wenn Y L = 0 < Y L = 1 und y x monoton fallend

[0047] According to a further embodiment, the first logical state and the second logical state are represented by electrical signal levels, in particular by TTL, LVTTL, CMOS, ECL, PECL, LVECL, LVPECL or LVDS signal levels.

[0048] According to a further embodiment, logically identical logical states of the comparison circuit outputs of different comparison circuits are represented by different, in particular complementary, electrical signal levels.

[0049] According to a further embodiment, the first logical state and the second logical state are represented by optical signal states, in particular by intensity, wavelength or polarization states.

[0050] According to a further embodiment, logically identical logical states of the comparison circuit outputs of different comparison circuits are represented by different, in particular complementary, optical signal states.

[0051] According to a further embodiment, the measured values ​​are represented by analog voltage or current values.

[0052] According to a further embodiment, the measured values ​​are represented by numbers in a binary representation.

[0053] According to a further embodiment, the measured variables are selected independently of one another from the group: mechanical displacement, mechanical force, pressure, light intensity, polarization, temperature, volume, capacitance, inductance, magnetic flux, electrical voltage, electrical current, or electrical field strength. In other words, according to this embodiment, each of the at least two sensors can detect one of the stated measured variables independently of the other sensor or sensors. Therefore, according to this embodiment, any combination of the stated measured variables is possible for the at least two sensors. In this embodiment, the avoidance of systematic failures is particularly taken into account by using diverse components and measuring principles.

[0054] According to a further embodiment, the sensors are independently selected from the group consisting of Hall sensors, photodiodes, phototransistors, photoresistors, thermocouples, capacitive or inductive distance sensors, strain gauges, microphones, adjustable resistors, adjustable capacitors, and adjustable inductors. This means that each of the at least two sensors can correspond to one of the aforementioned types, independently of all other sensors.

[0055] According to a further embodiment, the sensors are designed to detect different measured variables.

[0056] According to a further embodiment, the sensors are designed to detect the same measured variable, but are based on different measuring principles or belong to different sensor types.

[0057] Both latter embodiments improve the redundancy of the sensor circuit.

[0058] According to a further embodiment, the sensor circuit has an error detection unit, wherein the error detection unit is configured to combine the logical states of the comparison circuit outputs using a logical exclusive-OR operation to form an error signal of the sensor circuit. The error detection unit can advantageously detect error states in which the comparison circuit output signals represent different logical states.

[0059] A second aspect of the invention relates to a device for performing a safety function comprising a sensor circuit according to the first aspect of the invention.

[0060] According to one embodiment, the device is a machine with a shielding element for protection against contact with electrically, pneumatically, or hydraulically driven elements, wherein the sensor circuit is designed to monitor a state of the shielding element. In particular, the shielding element is designed to be movable, wherein the shielding element has at least a first state in which the driven elements are accessible from the outside and a second state in which the driven elements are covered by the shielding element to protect against contact. The shielding element can be, for example, a removable, pivotable, or movable cover, a lockable door, flap, or hatch in a housing of the machine, or the like. Suitable sensors for monitoring the state include, for example, magnetic sensors combined with permanent magnets or electromagnets, e.g., Hall sensors. Alternatively, for example,Optical sensors such as photodiodes, phototransistors, or photoresistors are used in combination with a light source (e.g., an LED). The light source can be arranged relative to the optical sensor in such a way that, in a first state of the shielding element, light from the light source falls onto the sensor, while in a second state, no or less light reaches the sensor. Alternatively, mechanical displacement sensors such as linear potentiometers can also be used.

[0061] Another possible sensor principle is the change in capacitance of conductive surfaces that are displaced relative to one another. In such capacitive sensors, a capacitor can be formed, for example, from a tubular electrode and a rod-shaped electrode. One of the electrodes can be connected to the shielding element, and the other electrode, for example, to the device's housing. If the rod-shaped electrode dips into the tubular electrode when the shielding element is displaced relative to the housing, the state of the shielding element can be detected by a sensor.

[0062] According to a further embodiment, the device is a laser device, wherein the laser device comprises a laser of laser class 3, 3B, 3R or 4.

[0063] According to a further embodiment, the device further comprises a shielding element for protecting against laser radiation escaping from the laser device, wherein the sensor circuit is designed to monitor a state of the shielding element. In particular, the shielding element is designed to be movable, wherein the shielding element has at least a first state in which laser radiation can escape and a second state in which the shielding element prevents the laser radiation from escaping. Here, too, the shielding element can be, for example, a removable, pivotable, or movable cover, a lockable door, flap, or hatch in a housing of the laser device, or the like. Sensors for monitoring the state can also be, for example, magnetic sensors combined with permanent or electromagnets, e.g., Hall sensors, optical sensors, mechanical displacement sensors, and capacitive sensors (see above).

[0064] According to a further embodiment, the device comprises an optical element that can be moved into various adjustment positions, wherein the sensor circuit is configured to monitor a state of the optical element. The state of the optical element is, in particular, an arrangement of the optical element in a beam path of the laser device.

[0065] According to one embodiment, the laser device further comprises an eyepiece for viewing objects illuminated by the laser radiation, wherein the sensor circuit is designed to detect states of the movable optical element in which reflections of the laser radiation can reach the eyepiece. The movable optical element can be, for example, a displaceable or rotatable mirror or a movable filter arrangement such as a filter wheel for selectively arranging various optical filters in a beam path. Possible sensors here include, for example, light sensors such as photodiodes or magnetic sensors such as Hall sensors, which can be arranged, for example, at a suitable position on the movable optical element.

[0066] A third aspect relates to a light microscope comprising a sensor circuit according to the first aspect of the invention and / or a device according to the second aspect of the invention, in particular comprising the laser device described above.

[0067] The light microscope, in particular, comprises at least one pulsed or continuous laser for illuminating a sample with laser light. The laser light can be used, for example, to excite emitters, such as fluorophores, in the sample to emit light (especially fluorescent light). Furthermore, the sample can be illuminated with additional laser light to deactivate or deplete the emission of the emitters in specific sample areas, as is known, for example, from STED and RESOLFT microscopy.

[0068] The light microscope further comprises an objective lens for concentrating the laser light. In particular, a focus of the laser light can be created in the sample. This focus can be moved over or through the sample using a scanner. The light emitted by the emitters can reach a detector (e.g., through the same objective lens), in particular via a confocal pinhole, as is common in confocal laser scanning microscopy. As an alternative to scanning the laser light over the sample, wide-field illumination of the sample with the laser light is also possible. In this case, the emitted light is often detected by a camera. The light microscope may further comprise optical filters, beam-shaping devices such as phase modulators, and the like.

[0069] A fourth aspect of the invention relates to a method for processing measured values ​​from sensors for a device performing a safety function, in particular using the sensor circuit according to the first aspect of the invention. The method comprises the following steps, which may not necessarily be performed consecutively, but may also be performed in a different order, simultaneously, or with temporal overlap: Detecting a respective measured variable and generating a respective measurement signal with a measured value monotonically dependent on the respective measured variable by means of at least two sensors, receiving the measurement signals through respective measured value inputs of at least two comparison circuits, wherein each of the comparison circuits is assigned to one of the sensors, receiving reference signals for setting a trigger threshold through respective reference value inputs of the comparison circuits, outputting comparison circuit output signals through respective comparison circuit outputs of the comparison circuits, wherein the respective comparison circuit outputs can assume a first logical state and a second logical state, and wherein the comparison circuit output signals each have a comparison circuit output value that represents one of the logical states,Linking the logical states to form a circuit output signal by means of a logical AND operation and scaling the comparison circuit output value of at least a first of the comparison circuits and feeding back the scaled comparison circuit output value to the measurement input or the reference input of at least a second of the comparison circuits, so that when the comparison circuit output of the first comparison circuit transitions between the first logical state and the second logical state, 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 measured value signal and the reference signal of the second comparison circuit is reversed.

[0070] Advantageous developments of the invention emerge from the patent claims, the description, the drawings, and the associated explanations of the drawings. The described advantages of features and / or combinations of features of the invention are merely exemplary and may be effective alternatively or cumulatively. Regarding the disclosure content (but not the scope of protection) of the original application documents and the patent, the following applies: Further features can be found in the drawings—in particular, the relative arrangements and operative connections shown.

[0071] The combination of features of different embodiments of the invention or of features of different patent claims is also possible, deviating from the chosen references of the patent claims, and is hereby encouraged. This also applies to features shown in separate drawings or mentioned in their description. These features can also be combined with features of different patent claims. Likewise, features listed in the patent claims can be omitted for further embodiments of the invention; however, this does not apply to the independent claims of the granted patent.

[0072] The reference signs contained in the patent claims do not represent a limitation of the scope of the subject-matter protected by the patent claims. They serve solely to make the patent claims easier to understand. Short description of the characters

[0073] Fig. 1shows a block diagram of a sensor circuit according to the invention. Fig. 2 shows a sensor unit with an alternative wiring of the comparison circuits. Fig. 3 shows a sensor unit with another alternative wiring of the comparison circuits. Fig. 4 shows a sensor unit with another alternative wiring of the comparison circuits. Fig. 5 shows a sensor unit in digital design. Fig. 6 shows the circuit diagram of an analog sensor circuit according to the invention. Fig. 7 shows the simulation of a sensor circuit according to the invention. Fig. 8 shows a device with a sensor circuit according to the invention. Description of the characters

[0074] In Fig. 1A sensor circuit 1 according to the invention is shown as a block diagram. The embodiment shown comprises two sensors 2, each with an associated comparison circuit 3. According to this exemplary embodiment, the comparison circuits 3 are designed as comparators. The measured values ​​4 of the sensors 2 are fed to the measured value inputs 8 of the comparison circuits 3 via drivers 5, which, with an amplification factor 6 not equal to one, also assume the function of a scaler 7.

[0075] In the embodiment shown, the measured value 4 of the sensors 2 is proportional to the value of the respective measured variable, i.e., the measured value y(x) 4 increases monotonically with the measured variable x. The switching points of the comparison circuits 3 are specified by trigger thresholds 9 that are individually adjustable or preset for each comparison circuit 3 and are fed to the reference value inputs 11 of the comparison circuits 3 via a summer 10.

[0076] The measuring 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 applied to the measuring input 8 of the respective comparison circuit 3 is greater than the level applied to the reference input 11 of the respective comparison circuit 3, and a logical "0" value when the level applied to the measuring input 8 is lower than the level applied to the reference input 11.

[0077] The comparison circuit output value 14 of each comparison circuit 3 is individually scaled using a scaler 7 and fed back to the reference value inputs 11 of the other comparison circuit 3 via the summers 10. In this way, switching one comparison circuit 3 between the logical states results in 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 based on a delay in the switching behavior of one sensor channel compared to the other sensor channel occur with less frequency or for a shorter time.

[0078] The logical states of the comparison circuit outputs 14 are combined in the combination unit 15 with a logical AND operation, and the combination unit 15 outputs a sensor output value 16 that represents the logical state obtained by the AND operation.

[0079] Optionally, the sensor circuit may further comprise an error detection unit 50, which combines the comparison circuit output signals of the comparison circuit outputs 14 using an exclusive-OR operation and outputs a corresponding error signal 51 representing the logic state L=1 if the two comparison circuit output signals represent different logic states. In this way, error states can be detected.

[0080] The Figures 2 to 4each show a 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 as a comparator here), a summer 10 and several drivers 5 which perform the function of a scaler 7.

[0081] The Fig. 2 The embodiment shown differs from that shown in Fig. 1 illustrated and described above embodiment that two comparison circuit outputs 14a, 14b of in Fig. 2 not shown comparison circuits 3 and fed back to the inverting reference input 11, 13 via the summer 10.

[0082] One of the comparison circuit outputs 14a is connected to an adding input (+), and the other comparison circuit output 14b is connected to a subtracting input (-) of the summer 10. Another adding input (+) of the summer 10 receives the trigger threshold 9, which may be scaled via another driver / scaler 5, 7. Thus, the summer 10 calculates the sum of the scaled trigger threshold and the scaled first comparison circuit output value minus the scaled second comparison circuit output value.

[0083] Fig. 3shows an embodiment in which the signal of the comparison circuit output 14a of one comparison circuit 3 (not shown) is fed back to the measured value input 8 of the other comparison circuit 3. The comparison circuit output value 17a and the measured value 4 of the sensor 2 are scaled with respective weighting factors by means of respective drivers 5, which are designed as scalers 7, and added by means of the summer 10.

[0084] The added signal is then fed to the measured value input 8 of the comparison circuit 3, which is configured as a non-inverting input 12. The trigger threshold 9, scaled by another driver 5 / scaler 7, is fed to the reference input 11.

[0085] The comparison circuit output 14 thus changes to the logic state L=1 when the sum between the measured value 4 and the scaled feedback comparison circuit output value 17a is greater than the trigger threshold 9.

[0086] In the Fig. 4 an embodiment is shown 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 fed to the reference input 11 of the comparison circuit 3, which is designed as a non-inverting input 12.

[0087] The measured value 4 measured by sensor 2 is scaled via another driver 5 / scaler 7 and is applied to the measured value input 8 of the comparison circuit 3, which is implemented 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 9 and the scaled comparison circuit output value 17a exceeds the scaled measured value 4.

[0088] Fig. 5 shows a digital embodiment 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.

[0089] The analog-to-digital converters 18 can also be integrated into the respective sensors 2, so that the sensors 2 already provide a digital output signal. Furthermore, the scaling can also be implemented internally within the sensors, so that the scalers 7 can be omitted if necessary. In particular, sensors 2 with specific measuring ranges can simply be selected to determine the scaling factors without the need for scalers 7.

[0090] The digitized measurement signals are fed by means of data lines 19 via a respective summer or adder 10 for adding a bias value (see below) to a measurement input 8 of a comparison circuit 3, which is designed as a subtractor.

[0091] Reference inputs 11 of the comparison circuits 3 receive digitally coded (with the binary numbers D1 to Dn) reference signals from data outputs 34 of a respective reference value register 30, which determine the respective trigger threshold value 9 of the comparison circuits.

[0092] 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 has the value 0 in particular when the difference between the measurement signal and the reference signal is less than zero and has the value 1 when the difference is greater than zero.

[0093] A logic unit 15, which is designed as a digital AND gate, combines 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, which has the value 1 if both comparison circuits 3 output the comparison circuit output value that represents a difference greater than zero.

[0094] The trigger threshold stored in the reference value register 20 can 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 a known manner, one after the other, at a rate specified by the clock signal, whose level is assigned the value 0 or the value 1. This step is performed, in particular, during commissioning of the sensor circuit 1, which facilitates data synchronization during subsequent operation.

[0095] 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.

[0096] A bias value is stored in digitally encoded form in the bias register 30. In the same way as with the reference value register 20, this bias value can be changed by applying a data signal to a data input 31 and a clock signal to the clock input 32 of the bias register 30.

[0097] If a signal representing the value or logical state 1 is present at the enable input 33 of the respective bias register 30, the bias value is passed via the data outputs 34 to an input 10a of the respective adder 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.

[0098] By activating the enable input due to the inventive feedback, 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.

[0099] Of course, the bias value can also be a negative number, which then leads to a lower output value of the summer 10 compared to the value of the measurement signal of the sensor 2.

[0100] In addition, as an alternative to the Fig. 5 In the embodiment shown, 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 by feedback.

[0101] For the data lines 19 of the Fig. 5The digital sensor circuit 1 shown may in particular be a data bus. Alternatively, the entire sensor circuit 1 or parts thereof may also be integrated into a circuit, e.g., a microcontroller, a so-called field-programmable gate array (FPGA) or a so-called application specific integrated circuit (ASIC).

[0102] Fig. 6 shows an analog embodiment of the sensor circuit 1 according to the invention, in which the trigger threshold values ​​are preset or adjustable by electrical resistors.

[0103] The redundant sensors 2a, 2b are connected via resistors R4 and R9 respectively to the measured value inputs 8 of the comparison circuits 3a, 3b, which are designed as comparators.

[0104] Resistors R2 and R3, as well as R7 and R8, each form a voltage divider. The trigger threshold for the respective comparison circuit 3a can be set using the reference voltages U1 and U2.

[0105] The comparison circuit outputs 14a, 14b of the comparison circuits 3a, 3b are connected via resistors R1 and R6, respectively, to the reference input 11 (here, an inverting input 13) of the other comparison circuit 3a, 3b, 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 adjusting the resistors R1 and R6.

[0106] The logical states of the comparison circuit outputs 14a, 14b are combined in the combination unit 15 with a logical AND operation, and the combination unit 15 outputs a sensor output value 16 that represents the logical state obtained by the AND operation.

[0107] Finally, sensors 2a, 2b are each directly connected to logic unit 15 via additional resistors R5, R10, resulting in a hysteresis of the switching process. In other words, resistors R5, R10 cause the trigger threshold of each sensor to differ at least slightly between a switch-on process (switching from the logic state L=0 to the logic state L=1) and a switch-off process (switching from the logic state L=1 to the logic state L=0).

[0108] For example, a slightly lower trigger threshold may be provided for a switch-on process than for a switch-off process.

[0109] Fig. 7 shows a simulation result of a sensor circuit 1 according to the invention, which can be used, for example, to monitor the opening state of a shielding element 40, such as a housing cover (see Fig. 8). For this purpose, a permanent magnet is inserted or pivoted into a gap between two oppositely arranged Hall sensors 2a, 2b when the cover is closed; when the device cover is opened, the magnet is pulled out of the gap or pivoted out.

[0110] When the cover is open (i.e., when the magnet is swung out), the Hall sensors 2a, 2b output a rest voltage of 2.5 V as a measured value. Depending on the magnet pole facing the sensor 2a, 2b, this value 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 magnets are swung in). Sensors 2a, 2b thus have antivalent signal levels.

[0111] For the simulation of the sensor circuit, the Fig. 6 The circuit diagram shown was used as a basis and simulated using the circuit simulation software Ngspice version 33.

[0112] The trigger threshold value for the comparison circuit 3a was set to approximately 1.3 V and for the comparison circuit 3b to approximately 3.8 V via the resistors R2 and R3, each relative to the idle state.

[0113] In Fig. 7 Time-voltage curves are shown for the first sensor 2a and the first comparison circuit 3a (lower diagram), as well as for the second sensor and the second comparison circuit 3b (upper diagram). The solid curves represent the temporal progression of the measured values ​​4a, 4b of the sensors 2a, 2b. The dashed lines represent the temporal progression of the trigger thresholds 9a, 9b of the comparison circuits 3a, 3b. Finally, the temporal progression of the comparison circuit output values ​​17a, 17b is shown.

[0114] 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.

[0115] The measured value 4a of the first sensor drops continuously from a starting value of 2.5 V to 0 V within 250 ms and then continuously rises back to 2.5 V within the same time. Such behavior could occur, for example, due to the rapid closing and subsequent reopening of a housing cover, the opening state of which is monitored by a Hall sensor. Accordingly, the measured value 4b of the complementary sensor 2b (upper diagram) rises from 2.5 V to 5 V within 250 ms and then falls back to 2.5 V within the same time.

[0116] At the first triggering time t 1 , the measured value 4b of the second sensor 2b exceeds the specified triggering threshold 9b. This causes the second comparison circuit 3b to switch to the logic state L=1, which is represented by the comparison circuit output value 17b of 5 V. As a result, the comparison circuit output value 17b rises abruptly from 0 V to 5 V at time t 1 (or correspondingly later depending on the delay of the comparison circuit).

[0117] The switching of the second comparison circuit 3b leads to the feedback of the scaled comparison circuit output value 17b to the measuring input 8 or reference input 11 of the first comparison circuit 3a and thus to the raising of the first trigger threshold value 9a (see lower diagram).

[0118] At the first triggering time t1, the measured value 4a of the first sensor 2a was still just above the initially set triggering threshold 9a. However, due to the feedback of the other switching signal, the triggering threshold 9a exceeds the measured value 4a shortly after the first triggering time t1. This causes a change in the sign of the difference between the measured value 4a and the triggering threshold 9a, which leads to the switching of the first comparison circuit 3a.

[0119] The switching of the first comparison circuit 3a also leads to a reduction of the trigger threshold 9b of the second comparison circuit 3b through feedback. However, since this has already switched to the state L=1, this does not cause any further change at the first triggering time t1.

[0120] At the second triggering time t 2 , the measured value 4b of the second sensor 2b again reaches the triggering threshold 9b, while the measured value 4a of the first sensor 2a is still below its triggering threshold 9a. Thus, initially only the second comparison circuit 3b switches to the logical state L=0. However, the feedback reduces the triggering threshold 9a of the first comparison circuit 3a, which again results in a change in the sign of the difference between the measured value 4a and the triggering threshold 9a. As a result, the first comparison circuit 3a also switches to the logical state L=1.

[0121] The simulation therefore shows how the triggering time of the redundant sensors 2a, 2b can be synchronized with the sensor circuit 1 according to the invention while maintaining the independence of the sensors 2a, 2b.

[0122] Fig. 8shows various views of an exemplary device 100 with a sensor circuit 1 according to the present invention. Fig. 8A and Fig. 8B Side views of the device 100 in different states. Fig. 8C and Fig. 8D show a detail of the device 1 in different states.

[0123] The device 100 has a housing 41, which at least partially encloses internal components (not shown) of the device 100. A shielding element 40, e.g. a cover, is pivotally connected to the housing 41 via a pivot connection 42. In the Fig. 8B In the state shown, the shielding element 40 covers the internal components.

[0124] The internal components may be, for example, moving mechanical parts such as shafts, gears, etc. In this case, the shielding element 40 functions in the Fig. 8Bshown state as protection against injury to a user by the mechanical parts during operation of the device 100.

[0125] Alternatively, the device 100 may also be a laser device that contains a laser or a beam path as an internal component into which laser light is coupled from an external source. In this case, the shielding element 40 prevents Fig. 8B shown state the escape of laser light from the device 100 and thus possible eye damage to a user.

[0126] The device 100 further comprises a first sensor 2a and a second sensor 2b (see Fig. 8C and Fig. 8D ), which redundantly monitor the state of the shielding element 40. According to the Fig. 8In the example shown, 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 shaft 44, measurement signals can be transmitted to the previously described components of the sensor circuit 1.

[0127] A magnet 43 is connected to the shielding element 40, which is arranged in the Fig. 8B and Fig. 8D shown state of the shielding element 40 is arranged in the shaft 44, but in the Fig. 8A and Fig. 8C shown state is positioned outside the shaft 44. Thus, depending on the state of the shielding element 40, a magnetic field of varying strength acts in the shaft 44, which can be measured by the sensors 2a, 2b.

[0128] If the sensor circuit 1 detects through the evaluation of the measurement signals of the sensors 2a, 2b that the shielding element 40 is in a state in which no sufficient protection is offered for the user ( Fig. 8A, Fig. 8C ), for example, a warning message can be issued and / or an emergency shutdown of the device 100 or a transition to a safe state can be automatically initiated. This means, in particular, 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 symbols

[0129] 1Sensor circuit 2Sensor 2aFirst sensor 2bSecond sensor 3Comparison circuit 3aFirst comparison circuit 3bSecond comparison circuit 4Measured value 4aFirst measured value 4bSecond measured value 5Driver 6Gain factor 7Scaler 8Measurement input 9Trigger threshold 9aFirst trigger threshold 9bSecond trigger threshold 10Summator 10aInput 10bOutput 11Reference input 12Non-inverting input 13Inverting input 14Comparison circuit output 14aFirst comparison circuit output 14bSecond comparison circuit output 15Logic unit 16Circuit output signal 17Comparison circuit output value 17aFirst comparison circuit output value 17bSecond comparison circuit output value 18Analog-to-digital converter 19Data line 20Reference value register 21Data input 22Clock input 23Data output 30Bias register 31Data input 32Clock input 33 Enable-Input 34Data output 40Shielding element 41Housing 42Pivoting connection 43Magnet 44Shaft 50Fault detection unit 51Fault signal 100Device t 1 First tripping time t 2 Second tripping time U 1 First reference voltage U 2 Second reference voltage

Claims

1. A sensor circuit (1) for a device (100) performing a safety function comprising - at least two sensors (2), wherein each of the sensors (2) is configured to detect a respective measured variable (x) and to generate a measurement signal comprising a measurement value (4) which is monotonically dependent on the respective measured variable (x), - at least two comparison circuits (3), wherein each of the comparison circuits (3) is assigned to one of the sensors (2) and comprises a measurement input (8) for receiving the measurement signal of the respective assigned sensor (2), a reference input (11) for receiving a reference signal to define a trigger threshold value (9) of the comparison circuit (3), and a comparison circuit output (14), wherein the comparison circuit output (14) can assume a first logic state (L=0) and a second logic state (L=1), and wherein the comparison circuit output (14) is configured to generate a comparison circuit output signal comprising a comparison circuit output value (17) representing one of the logic states (L=0, L=1), - a linking unit (15) which is configured to combine the logic states (L=0, L=1) of the comparison circuit outputs (14) by means of a logic AND operation to form a circuit output signal (16) of the sensor circuit (1), characterized in that the sensor circuit (1) is configured to scale the comparison circuit output value (17) of at least a first one of the comparison circuits (3) and to feed back the comparison circuit output value (17) to the measurement input (8) or the reference input (11) of at least a second one of the comparison circuits (3), so that, when the comparison circuit output (14) of the first comparison circuit (3) transitions between the logic states (L=0, L=1), the difference between the measurement signal and the reference signal of the second comparison circuit (3) is reduced or the sign of the difference between the measurement signal and the reference signal of the second comparison circuit (3) is reversed.

2. The sensor circuit (1) according to claim 1, characterized in that the sensor circuit (1) is configured to scale the comparison circuit output value (17) of the first comparison circuit (3) with a factor of < 0.5, in particular of < 0.2, further in particular of < 0.1.

3. The sensor circuit (1) according to claim 1 or 2, characterized in that the sensor circuit (1) is configured to generate the reference signal of the second comparison circuit (3) by a weighted sum of the trigger threshold value (9) of the second comparison circuit (3) and the comparison circuit output value (17) of the first comparison circuit (3) weighted by a weighting factor (gij) and optionally the comparison circuit output value (17) of a further one of the comparison circuits (3) weighted by a weighting factor (gij), and / or in that the sensor circuit (1) is configured to generate the measurement signal of the second comparison circuit (3) by a weighted sum of the measurement signal of the sensor (2) assigned to the second comparison circuit (3) and the comparison circuit output value (17) of the first comparison circuit (3) weighted by a weighting factor ( g ij ′ ) and optionally the comparison circuit output value (17) of at least a further one of the comparison circuits (3) weighted by a weighting factor ( g ij ′ ).

4. The sensor circuit (1) according to claim 3, characterized in that - the weighting factor (gij) used in generating the reference signal from the weighted sum is less than or equal to zero or the weighting factor ( g ij ′ ) used in generating the measurement signal from the weighted sum is larger than or equal to zero if the comparison circuit output value (17) of the first comparison circuit (3) is smaller in the first logic state (L=0) than in the second logic state (L=1) and the measurement value (4) of the sensor (2) assigned to the second comparison circuit (3) increases monotonically as a function of the measured variable (x) or if the comparison circuit output value (17) of the first comparison circuit (3) is greater in the first logic state (L=0) than in the second logic state (L=1) and the measurement value (4) of the sensor (2) assigned to the second comparison circuit (3) falls monotonically as a function of the measured variable, and / or - the weighting factor (gij) used in generating the reference signal from the weighted sum is greater than or equal to zero or the weighting factor ( g ij ′ ) used in generating the measurement signal by the weighted sum is smaller than or equal to zero, if the comparison circuit output value (17) of the first comparison circuit (3) is larger in the first logic state (L=0) than in the second logic state (L=1) and the measurement value (4) of the sensor (2) assigned to the second comparison circuit (3) increases monotonically as a function of the measured variable (x) or if the comparison circuit output value (17) of the first comparison circuit (3) is smaller in the first logic state (L=0) than in the second logic state (L=1) and the measurement value (4) of the sensor (2) assigned to the second comparison circuit (3) falls monotonically as a function of the measured variable (x).

5. The sensor circuit (1) according to one of the preceding claims, characterized in that the first logic state (L=0) and the second logic state (L=1) are represented by electrical signal levels, in particular by TTL, LVTTL, CMOS, ECL, PECL, LVECL, LVPECL or LVDS signal levels, or by optical signal states, in particular by intensity, wavelength or polarization states.

6. The sensor circuit (1) according to claim 5, characterized in that logically identical logic states of the comparison circuit outputs (14) of different ones of the comparison circuits (3) are represented by different, in particular complementary, signal levels or optical signal states.

7. The sensor circuit (1) according to one of the preceding claims, characterized in that the measurement values (4) are represented by analog voltage or current values, or by numbers in a binary representation.

8. The sensor circuit (1) according to one of the preceding claims, characterized in that the measured variables (x) 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.

9. The sensor circuit (1) according to one of the preceding claims, characterized in that the sensors (2) 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.

10. The sensor circuit (1) according to one of the preceding claims, characterized in that the sensors (2) are configured to detect different measurement variables (x), or that the sensors (2) are configured to detect the same measurement variable (x), but are based on different measuring principles or are different sensor types.

11. The sensor circuit (1) according to one of the preceding claims, characterized in that the sensor circuit (1) comprises a fault detection unit (50), wherein the fault detection unit (50) is configured to link the logic states (L=0, L=1) of the comparison circuit outputs (14) by means of a logic exclusive-OR operation to form a fault signal (51) of the sensor circuit (1).

12. A device (100) for performing a safety function comprising a sensor circuit (1) according to one of claims 1 to 11, wherein the device (100) is a machine comprising a shielding element (40) for protecting against contact of electrically, pneumatically or hydraulically driven elements, and wherein the sensor circuit (1) is configured to monitor a state of the shielding element (40).

13. A device (100) for performing a safety function comprising a sensor circuit (1) according to one of claims 1 to 11, wherein the device (100) is a laser device, and wherein the laser device comprises - a laser of laser class 3, 3B, 3R or 4 and - a shielding element (40) 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 (1) is configured to monitor a state of the shielding element (40) or the movable optical element.

14. A light microscope comprising a sensor circuit (1) according to one of claims 1 to 11 and / or a device (100) according to claim 13.

15. A method for processing measurement values (4) from sensors (2) for a device (100) performing a safety function, comprising the steps of - detecting a respective measured variable (x) and generating a respective measurement signal comprising a measurement value (4) which is monotonically dependent on the respective measured variable (x) by means of at least two sensors (2), - receiving the measurement signals by respective measurement inputs (8) of at least two comparison circuits (3), wherein each of the comparison circuits (3) are associated with one of the sensors (2), - receiving reference signals for setting a trigger threshold value (9) by respective reference inputs (11) of the comparison circuits (3), - outputting comparison circuit output signals by respective comparison circuit outputs (14) of said comparison circuits (3), wherein the comparison circuit outputs (14) are each capable of assuming a first logic state (L=0) and a second logic state (L=1), wherein the comparison circuit output signals each comprise a comparison circuit output value (17) representing one of said logic states (L=0, L=1), - combining the logic states (L=0, L=1) of the comparison circuit outputs to a circuit output signal (16) by means of a logical AND operation, characterized in that - the comparison circuit output value (17) of at least a first one of the comparison circuits (3) is scaled and fed back to the measurement input (8) or the reference input (11) of at least a second one of the comparison circuits (3), so that during the transition of the comparison circuit output (14) of the first comparison circuit (3) between the logic states (L=0, L=1), the difference between the measurement signal and the reference signal of the second comparison circuit (3) is reduced or the sign of the difference between the measurement signal and the reference signal of the second comparison circuit (3) is reversed.