Sensor arrangement, gas analyser, method for calibration, method for condition monitoring, computer program product and computer-readable medium

Abstract

The invention relates to a sensor arrangement (1) having at last two preferably thermal flow sensors (S1, S2), which are arranged in a bridge circuit with two branches (2, 3), wherein at least one flow sensor (S1) is located in each of the two branches (2, 3), and wherein the flow sensors (S1, S2) each have a basic resistance value, and the basic resistance values of the flow sensors (S1, S2) can deviate from one another, in particular due to the manufacturing process, wherein a compensation device (10) with at least one preferably digitally adjustable resistance element (11) is provided, by means of which the deviation of the basic resistance values of the at least two flow sensors (S1, S2) can be compensated for during measurement operation. The invention also relates to a gas analyser having at least one such sensor arrangement (1), a method for calibration and a method for condition monitoring of at least one such sensor arrangement (1), a computer program product and a computer-readable medium.

Description

[0001] Description

[0002] Sensor arrangement, gas analyzer, calibration method, condition monitoring method, computer program product and computer-readable medium

[0003] The invention relates to a sensor arrangement with at least two preferably thermal flow sensors which are arranged in a bridge circuit with two branches, wherein at least one flow sensor is located in each of the two branches, and wherein the flow sensors each have a basic resistance value, and the basic resistance values ​​of the flow sensors can or do differ from one another, in particular due to manufacturing reasons. Furthermore, the invention relates to a gas analyzer with at least one such sensor arrangement, a method for calibration and a method for condition monitoring of at least one such sensor arrangement, a computer program product and a computer-readable medium.

[0004] Sensor arrangements are known that utilize multiple flow sensors, particularly thermal ones, operated in a bridge circuit. For example, two or four flow sensors are arranged together in a Wheatstone bridge. Such sensor arrangements are used, among other things, in gas analyzers. Examples of commercially available products include the ULTRAMAT 23 and ULTRAMAT 6 from Siemens Aktiengesellschaft.

[0005] Thermal flow sensors utilize the fact that their resistance is highly temperature-dependent. When a fluid flow to be measured is passed over or through them, this creates a cooling effect, resulting in a change in the resistance of the sensors.

[0006] The production of flow sensors comprises various manufacturing steps, including manual handling and thermal cycling, which can lead to certain deviations in the basic resistances, even of identical flow sensors, and thus in the measuring bridge. The applicant is aware that the electrical conductivity between two wafers can be up to 20%. In other words, the relative deviation of the basic resistances of two flow sensors can be, for example, 20%. Since full-wafer bonding is often used to assemble hundreds of sensors at once, manual selection of individual sensors, especially with matching basic resistance values, is not possible.

[0007] Large static deviations in the sensor's base resistance lead to an unbalanced measuring bridge circuit, which is associated with various problems. The rejection of the bridge supply voltage is reduced, which leads to electrical noise and complicates electrical signal processing, especially of the measurement signal(s).

[0008] Particularly in the case of gas analyzers, alternating AG signals are used to detect gas concentrations. For example, a fluid flow to be measured or a reference gas is alternately passed from opposite directions over a pair of two flow sensors arranged one behind the other or one above the other, which are located in different bridge branches, so that the temperature-dependent resistances of the two flow sensors change alternately, resulting in an AG component of the bridge voltage.

[0009] In theory, an unbalanced bridge configuration should not have a strong influence on a low-frequency AG signal, for example one with a frequency in the range of less than 20 Hz. From an electronics point of view, however, the practical implementation requirements for a low-noise, low-drift implementation are not so simple. To counteract the disadvantage of a DC offset, which is associated with the static deviations in the sensor base resistances of a measuring bridge, particularly those caused by manufacturing, and their resulting unbalance, the signal could in principle be corrected using a coupling capacitor in order to be amplified in an amplifier stage. The coupling amplifier, in particular an AC-coupled amplifier, could then amplify only or mainly the AC component of the signal.Such an amplifier would be connected to the bridge via two capacitors, one for each of the two bridge arms. However, for low-frequency signals, this requires very large, highly stable capacitors. Physical limitations of real capacitors (particularly capacitance drift, thermal stability, dielectric absorption, leakage currents, and microphonic effects) limit the usability of AC coupling concepts in a bridge preamplifier configuration.

[0010] The applicant is aware of a concept which has been used to address this problem. This involves using a DC-coupled preamplifier with a low gain factor to amplify the bridge voltage, including the DC offset generated by the sensor resistance deviations. This first, DC-coupled preamplifier is followed by a second amplifier stage which uses an AC-coupled, second preamplifier with a higher gain factor. The first and second amplifiers can be connected to one another via just one high-quality coupling capacitance, in particular a film capacitor. Suitable capacitances, however, are characterized by a comparatively large spatial extent.

[0011] Furthermore, it should be noted that with ever-increasing permissible deviations in the resistance of the flow sensors, an ever-decreasing gain factor of the first preamplification stage would be required to remain within the permissible amplifier voltage limits, since the DC imbalance must not saturate the amplifier. However, a larger gain stage would be preferable here, as this stage defines the noise performance of the system. Small signals lost due to noise in the first amplifier stage cannot be recovered later.

[0012] All of this means that only certain deviations in the static basic resistance are permissible. If the permissible limit is exceeded by a single flow sensor or a fully assembled sensor arrangement with two or more such sensors, the sensor or even the entire sensor arrangement is considered scrap and cannot be used.

[0013] It is therefore an object of the present invention to provide a sensor arrangement of the type mentioned at the outset which is characterized by the highest possible suppression of the bridge supply voltage and the lowest possible signal noise, and which is simultaneously accompanied by reduced rejects.

[0014] This object is achieved in a sensor arrangement of the type mentioned at the outset in that a compensation device is provided with at least one preferably digitally adjustable resistance element, by means of which the deviation of the basic resistance values ​​of the at least two flow sensors can be compensated during measuring operation.

[0015] The compensation device is expediently integrated into the bridge circuit, in particular connected to at least one of the branches, preferably both branches.

[0016] The compensation device is preferably designed and in particular arranged in the bridge circuit in such a way that it can be used to counteract a balanced power supply to the two branches in order to compensate for the deviation in the basic resistance values ​​of the flow resistances. In other words, the basic idea of ​​the present invention is to specifically counteract basic resistance deviations of flow sensors operated in a bridge configuration, or their negative effect during measuring operation, by providing a suitable compensation device. It has proven particularly expedient to use such a device to deliberately slightly imbalance the supply power balance on or across the two branches of the bridge circuit in order to counteract the imbalance resulting from the different basic resistances. The compensation device is expediently designed accordingly.

[0017] In this case, compensation means that undesirable effects resulting from existing deviations in the static basic resistances, such as a DC offset or DC signal component in the bridge voltage, are completely eliminated or at least reduced during measurement operation.

[0018] The compensation device comprises at least one adjustable resistance element. This can also be referred to as an adjustable or controllable resistive element. In particular, it is at least one element whose resistance can be varied by adjustment.

[0019] An adjustable resistance element can also have an arrangement of several individually controllable partial resistors, by switching them on or off the resulting (total) resistance value can be adjusted.

[0020] The compensation device can comprise only one or several adjustable resistance elements, for example for each of the two branches or for each of the at least two flow sensors of a .

[0021] The inventive use of a compensation device makes it possible, on the one hand, to tolerate flow sensors even with larger deviations in their static base resistance values. This can significantly reduce rejects. At the same time, the inventive sensor arrangement is characterized by a high suppression of the DC signal component. The bridge supply noise is balanced in a balanced bridge. This results in lower signal noise.

[0022] The basic resistance is to be understood in particular as a static resistance or the static resistance of the respective flow sensor. Two or more flow sensors are characterized by the same basic resistance if they have the same resistance value at the same temperature, for example 20 ° Celsius. There is a deviation in the basic resistances of two or more flow sensors if their resistance values ​​differ when viewed at the same temperature, for example 20 ° Celsius. For flow sensors of the same construction, such deviations can be due to manufacturing reasons, as explained above. The basic resistance can also be understood or described as static resistance and existing deviations in the basic resistances of two or more flow sensors as static deviations in their resistances.Deviations in the basic resistances of the flow sensors of a sensor arrangement also occur outside of the measuring operation, when no temperature-dependent, dynamic resistance changes of the flow sensors are used to measure other physical quantities.

[0023] The at least one adjustable resistance element can, for example, comprise at least one digital-to-analog converter (DAC for short) or be provided by such a converter. The DAC or, in the case of several DACs, for example two, the respective DAC can have a unipolar or bipolar voltage output. The respective DAC is expediently connected to a bridge arm. In particular, if the compensation device has only one DAC which is connected to only one of the two bridge arms, this DAC has a bipolar voltage output connected to the corresponding arm. If a DAC is provided for each of the two arms, the voltage output of which is connected to the respective arm, the voltage outputs can also be unipolar.

[0024] In a particularly advantageous embodiment of the invention, the at least one adjustable resistance element is designed as a preferably digitally adjustable potentiometer or comprises at least one such potentiometer.

[0025] With at least one potentiometer, in which the resistance value can be changed mechanically or electronically in a manner known per se, a supply power balance can in particular be easily shifted in a particularly suitable manner in order to compensate for the imbalance resulting from the deviating basic resistances.

[0026] In a further development, the at least one potentiometer can be connected as a voltage divider, expediently as a voltage divider for the two branches of the bridge circuit. The at least one potentiometer can have three terminals in a manner known per se. In this case, it is preferred that one of the three terminals is connected to one of the bridge branches, optionally via at least one fixed resistor. Such a resistor can serve as a limiting resistor.

[0027] If at least one potentiometer is digitally adjustable, in other words if its resistance value can be changed by digital control, a particularly convenient and reliable compensation, for example also software-supported, is possible.

[0028] Digital potentiometers, also referred to as digipots or digipotis for short, are digitally controllable elements or components that "imitate" the analog function of a classic, mechanically adjustable potentiometer. Digital potentiometers can, for example, comprise at least one resistive digital-to-analog converter, in particular with several resistors connected in series, or can be designed as such a converter. Digital potentiometers can also comprise at least one resistance ladder, in particular at least one R-2R ladder, or can be provided thereby.

[0029] It is possible for the compensation device to comprise or be provided by only one digital or mechanical potentiometer, or for it to comprise several digital and / or mechanical potentiometers, e.g. one for each of the two branches of the bridge circuit or one for each of the at least two flow sensors.

[0030] The two or more flow sensors of the sensor arrangement according to the invention are expediently designed to detect the flow velocity of a fluid and / or the concentration of such a fluid.

[0031] As a rule, the flow sensors of the sensor arrangement according to the invention are constructed identically. This allows the sensor arrangement to be manufactured particularly easily and cost-effectively, and a symmetrical arrangement can be achieved.

[0032] The invention has proven to be particularly suitable for thermal flow sensors. Such sensors usually comprise at least one line through which a direct current flows during measuring operation and which heats up. The at least one line can in particular be metallic or comprise or consist of doped silicon. If a fluid flows around or over the at least one line, it experiences a cooling effect, which leads to a change in resistance which can be measured very precisely in the bridge configuration, in particular as a bridge detuning dependent on the fluid concentration, for example. The at least one line is sometimes also referred to as a filament. A thermal flow sensor can comprise only one such line, in particular only one such filament, or several lines or filaments connected in parallel. The (respective) line or the (respective) filament can form a meander.

[0033] If the sensor arrangement according to the invention comprises two or more thermal flow sensors, it is further preferred that these are designed as microflow sensors or include at least one such sensor. Microflow sensors can also be used, in particular, to detect particularly small flows. Microflow sensors such as those described in WO 2009 / 153099 A1 can be used, among others, although this is to be understood purely as an example, and microflow sensors of other designs can also be used.

[0034] In an advantageous development, the sensor arrangement according to the invention further comprises a preamplifier, for example, precisely one, with which the voltage difference between the two branches, in other words, the bridge voltage, can be amplified. The preamplifier is preferably provided by a DC-coupled preamplifier or is connected accordingly. If (exactly) one preamplifier is present, it can be characterized, for example, by an amplification factor of 1000 or more, preferably by an amplification factor in the range from 1000 to 5000.

[0035] In other words, the compensation device provided according to the invention also makes it possible to use just one DC-coupled preamplifier with a high gain factor. This is because the risk of saturation of the preamplifier as a result of the imbalance can be eliminated or at least greatly reduced. The sensor arrangement according to the invention can therefore manage with fewer components than the conventional solution with two amplifier stages, which can reduce its cost and size and simplify production. Furthermore, a DC-coupled preamplifier can be connected to the branches without large, highly stable capacitors. It should be noted that, even if one preamplifier can be sufficient in principle, it is by no means impossible for more than one preamplifier to be used.

[0036] The sensor arrangement according to the invention can further comprise exactly two or more flow sensors that are part of the bridge circuit. If two flow sensors are present, one is expediently arranged in each branch. One of the two flow sensors can then, for example, be connected in series with at least one fixed resistor in one of the branches.

[0037] Particularly in the case where the compensation device comprises or is provided by at least one potentiometer, it is then further preferred that the potentiometer be connected in parallel with the two fixed resistors. One terminal of the potentiometer can, for example, be connected—together with the two branches of the bridge circuit—to a direct current source, and two further terminals of the potentiometer can each be connected to a branch, in particular at a connection point located between the fixed resistor and the flow sensor of the respective branch.

[0038] It is also possible, for example, for the sensor arrangement according to the invention to have four flow sensors, two of which are then expediently connected in series in each of the branches.

[0039] In particular, if the compensation device comprises or is provided by at least one potentiometer, it is then further preferred that the potentiometer is connected in parallel to two of the four flow sensors. One connection of the potentiometer can, for example, be connected to a direct current source - together with the two branches of the bridge circuit - and two further connections of the potentiometer can each be connected to a branch, in particular at a connection point which lies between the two flow sensors of the respective branch. The invention also relates to a gas analyzer comprising at least one sensor arrangement according to the invention. The sensor arrangement can then be used, for example, to determine a flow value or a concentration for at least one measuring gas, for example oxygen.

[0040] The invention further relates to a method for calibrating at least one sensor arrangement according to the invention, comprising the steps: the bridge circuit comprising the flow sensors is fed with a direct current, in a calibration operation with known or without fluid flow through the flow sensors the voltage difference between the two branches of the bridge circuit, bridge voltage, is tapped and amplified, at least one resistor of the compensation device is changed if necessary, in particular by digital control, in such a way that the bridge voltage assumes a smaller value in the calibration operation, preferably drops to zero.

[0041] In other words, it can be provided that the sensor arrangement according to the invention is subjected to a calibration process, for example during initial commissioning immediately after production, in the course of which an optimal setting of the compensation device for the given sensor arrangement is determined.

[0042] If the compensation device includes at least one potentiometer or is provided thereby, it can be done, for example, that the potentiometer is first moved to a neutral middle position. In other words, no "disturbance" is then introduced into the bridge. The amplified DC signal is then measured and the potentiometer - particularly by digital control - is moved out of the middle position in order to reduce the DC offset. This can be repeated in several iterations until the bridge adjustment has been brought as close to an optimal adjustment as possible. In particular, if a digital potentiometer is used, it may not be possible to achieve a 10 ohm adjustment with the selectable digital resistance values. In other words, the setting that comes closest to this is then selected.

[0043] The setting value, in particular the resistance value, of the compensation device which provides an optimal or as optimal a calibration as possible can be saved as the setting or target value.

[0044] The calibration steps mentioned can also be carried out automatically, in particular completely software-based, without human intervention.

[0045] The calibration according to the invention can be performed only once, for example, after completion of production or upon commissioning. It is, of course, also possible for the calibration to be performed repeatedly. Thus, in a further development, it can be provided that the steps of the calibration method according to the invention are repeated at least once more, preferably cyclically, over the operating life of the sensor arrangement. Any deviations that may occur during the operating life can then be taken into account.

[0046] The invention further relates to a method for monitoring the condition of at least one sensor arrangement according to the invention, comprising the steps: the bridge circuit comprising the flow sensors is fed with a direct current, the voltage difference between the two branches, bridge voltage, is tapped and amplified, at least one resistor of the compensation device is changed by a predetermined value, in particular by digital control, the resulting change in the bridge voltage is recorded and compared with an expected value and the condition of the sensor arrangement is determined based on the result of the comparison.

[0047] In an advantageous development, it is provided that, if the resulting change in the bridge voltage does not correspond to the expected value, it is assumed that there is a defect in the sensor arrangement, in particular in at least one flow sensor thereof. Then, at least one resistance of the compensation device can be changed, in particular by a predetermined value, in order to compensate for the defect—at least partially, in other words to a certain extent.

[0048] Furthermore, it can be provided that a user is informed of the defect and / or a maintenance event is scheduled to correct it. The user can be informed, for example, by the sensor arrangement issuing at least one message for this purpose.

[0049] Condition monitoring has proven particularly advantageous for flow sensors which have a number of lines or filaments connected in parallel. If one of n filaments (n = 2, 3, 4, 5, ...) is defective, for example if it breaks, the effective resistance increases by 1 / n. The value of the change in resistance caused by the defect is known based on the number of filaments. The condition monitoring method according to the invention can be used both to detect such a defect with a known change in resistance and to compensate for it. By specifically controlling the compensation device, it can be determined whether such a change in the effective resistance by 1 / n exists and, if this is the case, it can be compensated for using the compensation device and the operation of the sensor arrangement can continue despite the defect.This can happen, for example, until an already scheduled maintenance check is carried out, during which the affected flow sensor can be replaced. The condition monitoring method according to the invention can also be used to check the integrity of the signal chain, including any preamplifier that may be present. By deliberately “adjusting” the compensation device, in particular the at least one potentiometer, by a predetermined value, which can also be done during regular operation, for example, the DC offset after the preamplifier shifts by a predetermined value. This depends on the bridge resistance itself and the DC gain of the amplifier. If a bridge is perfectly balanced, the DC offset is +- 0V. As soon as the bridge is deliberately detuned, the DC offset is not equal to 0V. Since all values ​​are known or...can be calculated, a known change in the setting of the compensation device, in particular of the at least one potentiometer, should also result in a known DC offset change. This can be used specifically to detect problems in the configuration, such as gain drift or ADC problems. An ADC problem can, for example, arise if an ADC outputs incorrect, albeit apparently plausible, values ​​due to an internal defect, which could be detected in this way.

[0050] The condition monitoring options described above can also be fully automated, particularly software-supported, without user intervention. If a defect is identified, a user can be informed, particularly automatically. Recalibration, maintenance, or repair can also be planned or performed.

[0051] The invention also relates to a sensor arrangement according to the invention with means adapted to carry out the steps of the calibration method according to the invention and / or the steps of the condition monitoring method according to the invention. A further subject of the invention is a computer program product comprising instructions that cause the sensor arrangement to carry out the steps of the calibration method according to the invention and / or the steps of the condition monitoring method according to the invention.

[0052] The aforementioned, optionally present means of the sensor arrangement, which are adapted to carry out the steps of the method according to the invention for calibration and / or the steps of the method according to the invention for condition monitoring, can, for example, comprise or be provided by a computer program product according to the invention.

[0053] The sensor arrangement according to the invention can also have storage means on which the computer program according to the invention is stored. It can further comprise a processor and RAM for its execution.

[0054] Finally, the invention relates to a computer-readable medium comprising instructions which cause the sensor arrangement to carry out the steps of the method according to the invention for calibration and / or the steps of the method according to the invention for condition monitoring.

[0055] The computer-readable medium can be, for example, a CD-ROM or DVD, or a USB or flash memory. It should be noted that a computer-readable medium is not exclusively understood to mean a physical medium, but can also be in the form of a data stream and / or a signal representing a data stream.

[0056] Further features and advantages of the present invention will become apparent from the following description of embodiments of the invention with reference to the accompanying drawing. FIG. 1 shows, in a purely schematic representation, the circuit diagram of a sensor arrangement with two thermal flow sensors operated in a bridge circuit, whose static basic resistances differ from one another due to manufacturing reasons.

[0057] FIG 2 is a circuit diagram of the bridge circuit of FIG 1 with an AG-coupled preamplifier for the bridge voltage, which is connected via a capacitor to each of the two branches,

[0058] FIG 3 is a circuit diagram of the bridge circuit of FIG 1 with a two-stage amplifier configuration known to the applicant with a first DC-coupled and a second AG-coupled preamplifier connected via a capacitance,

[0059] FIG 4 shows the circuit diagram of a first embodiment of a sensor arrangement according to the invention with two thermal flow sensors operated in a bridge arrangement, whose static basic resistances differ from one another due to manufacturing reasons, a compensation device in the form of a digitally adjustable potentiometer and a DC-coupled preamplifier with an amplification factor of more than 2000,

[0060] FIG 5 shows the circuit diagram of a second embodiment of a sensor arrangement according to the invention with two thermal flow sensors operated in a bridge arrangement, whose static basic resistances differ from one another due to manufacturing reasons, and a compensation device with a digital-to-analog converter, and

[0061] FIG 6 shows the circuit diagram of a third embodiment of a sensor arrangement according to the invention with two thermal flow sensors operated in a bridge arrangement, whose static basic resistances differ from one another due to manufacturing reasons, and a compensation device with two digital-to-analog converters.

[0062] In the figures, identical or corresponding components are provided with the same reference numerals.

[0063] FIG. 1 shows a sensor arrangement 1 with two identical thermal flow sensors S1 and S2, designed as microflow sensors, which are arranged together with two fixed resistors RI and R2 in a bridge circuit, specifically a Wheatstone bridge. The fixed resistors RI and R2 are also identical and have a resistance of 100 ohms, although this is only an example. One fixed resistor RI, R2 and one flow sensor SI, S2 of the sensor arrangement 1 are each connected in series in one of the two branches 2, 3 of the bridge circuit: the fixed resistor RI is connected to the flow sensor S1 in branch 2, and the resistor R2 is connected to the flow sensor S2 in branch 3.

[0064] The sensor arrangement 1 is part of a gas analyzer not further shown in the figures.

[0065] The flow sensors SI, S2 each comprise a metal filament (not visible in the figures), through which a direct current flows and is heated for measurement purposes. A DC voltage source 4 is provided for power supply.

[0066] The flow sensors SI, S2, and their filaments, are characterized by a highly temperature-dependent resistance. Therefore, the flow sensors SI, S2 are represented in the figure with the circuit symbol of a variable resistor with an arrow.

[0067] If the flow sensors SI, S2 or filaments are brought into contact with a gas to be measured, which for example flows around or over them, a cooling effect occurs and thus a change in resistance which is easily measurable in the bridge configuration. An alternating AG signal is used here, for example to detect a gas concentration. Specifically, the gas to be measured or a reference gas is alternately passed in opposite directions over the two flow sensors SI, S2. For example, first in such a way that the gas flows from S1 to S2 (in FIG 1 from left to right), then in the inverted direction from S2 to SI (in FIG 1 from right to left), then vice versa and so on. The alternation takes place at a frequency. The sensor arrangement 1 or the gas analyzer comprising it are designed accordingly. The flow sensors SI, S2 can, for example, be arranged next to one another or one above the other.

[0068] Although the flow sensors SI and S2 are identical in design, their base resistance values ​​do not match. This is due to the manufacturing challenges explained above. In the arrangement shown in FIG. 1, the base resistance of flow sensor S1 is 62 ohms and the base resistance of flow sensor S2 is 58 ohms. It should be emphasized that these values ​​are only examples. Of course, other values ​​and deviations in the base resistance values ​​can also occur in a sensor arrangement 1.

[0069] Due to the deviation of the base resistances, the bridge is unbalanced and - in addition to the actual AG measurement signal - a DC offset of the bridge voltage is caused. Ideally, an oscillation around the zero point is present. With an offset, the center of the oscillation is shifted, e.g., into the positive range. FIG 1 also shows a DC voltmeter 5 and an AG voltmeter 6, by means of which the bridge voltage between the two branches 2, 3 can be measured. One can also say that the bridge voltage comprises an AC signal component corresponding to the measurement signal and a superimposed DC offset. For example, the AC measurement signal can be 1 millivolt and the DC offset due to the unbalance can be 100 millivolts.

[0070] It would be conceivable in principle to counter this problem by using an AC-coupled preamplifier 7 with a high amplification factor of, for example, 500 or 1000. A corresponding circuit diagram can be seen in Figure 2. By means of this, only the AC measurement signal component would be amplified, or more strongly. However, such an AC-coupled preamplifier would have to be connected to the two branches 2, 3 via two capacitors, in particular capacitors C1, C2, whereby the capacitors C1, C2 have to fulfill certain requirements. These would have to be very large and highly stable for the low-frequency AC signal. Due to physical limitations, real capacitors C1, C2 generally do not meet these requirements, which severely limits the usefulness of this approach.

[0071] An alternative concept is illustrated in FIG 3. This uses a DC-coupled preamplifier 8 with a low gain factor, followed by a second, AC-coupled preamplifier with a significantly higher gain factor. In the present case, the gain of the DC-coupled preamplifier 8 is 20 and that of the AC-coupled preamplifier 9 is 200, these being example values. Downstream of the first amplifier 8 - based on the aforementioned values ​​- the AC measurement signal would then be, for example, 2 volts and the DC component 20 millivolts. Downstream of the second amplifier 9, the DC component could be 0 volts and the AC measurement signal 4 volts.

[0072] In this scenario, as shown in the figure, only a high-quality coupling capacitance C between the two amplifiers 8, 9 is required. However, this is characterized by comparatively large dimensions.

[0073] To accommodate even large deviations in the base resistances of the flow sensors SI, S2, a particularly low amplification factor of the first stage 8 would be required. However, this is offset by the problem that this stage defines the noise performance. Small signals lost due to noise in the first amplifier stage 8 cannot be recovered later. As a result, in the configuration shown in FIG. 3, only flow sensors SI, S2 whose base resistances differ only comparatively little from one another can be used.

[0074] FIG 4 shows the circuit diagram of a first embodiment of a sensor arrangement 1 according to the invention.

[0075] This comprises - in complete analogy to the arrangements in Figures 1 to 3 - a bridge circuit with two identical flow sensors S 1 and S2 with different basic resistances, as well as two fixed resistors RI and R2, whereby the fixed resistor RI and flow sensor S 1 are connected in series in one branch 2, and the fixed resistor RI and flow sensor S2 are connected in the second branch 3. The fixed resistors RI, R2 and flow sensors S1, S2 can be designed in exactly the same way as explained above in connection with the previous figures, although this is only to be understood as an example. Here too, in analogy to the previous figures, a DC voltage source 4 is provided for feeding and thus heating the filaments during measuring operation.

[0076] It should be noted that, as an alternative to the two fixed resistors RI, R2, two further flow sensors could also be provided, in other words a bridge circuit with four flow sensors SI, S2, two of which are connected in series in each branch 2, 3.

[0077] In contrast to the arrangements 1 of Figures 1 to 3, the sensor arrangement 1 according to the invention has a compensation device 10 with at least one preferably digitally adjustable resistance element, by means of which the deviation in the basic resistance values ​​of the two flow sensors SI, S2 can be compensated for during measuring operation. The compensation device 10 is designed and arranged in the bridge circuit in such a way that it can be used to counteract a balanced power supply of the two branches 2, 3 in order to compensate for the deviation in the basic resistance values ​​of the flow sensors SI, S2.

[0078] In the embodiment shown in FIG. 4, the compensation device comprises a digitally adjustable resistance element 10 in the form of a digital potentiometer 1, whose resistance can be varied by digital control. In the embodiment shown here, the compensation device 10 further comprises two fixed resistors R3, R4, which can be, for example, 1 ohm and serve for limiting. These two fixed resistors R3, R4 are referred to below as further fixed resistors to distinguish them from the two fixed resistors R1 and R2.

[0079] The digital potentiometer 12 has three connections A1, A2, A3. Each of the connections A1, A2, A3 is connected to one of the branches 2, 3, namely connection A1 to the first branch 2, on the left in FIG. 4, and connection A3 to the second branch 3, on the right in FIG. 4, in each case via one of the two further fixed resistors R3, R4. The connection or connection point of the compensation device 10 in the respective branch 2, 3 is clearly located between the fixed resistor RI or R2 there and the flow sensor SI or S2. The remaining connection A2 of the digital potentiometer 11 is connected to the DC voltage source 4. The arrow at connection point A2 illustrates the selected position of the potentiometer 11 in a manner known per se. In the figure, the arrow is shown as an example in a central, neutral position.

[0080] By appropriately selecting the resistance value of the digital potentiometer 11, the deviation of the basic resistances of the two flow sensors S 1 and S2 can be compensated.

[0081] In order to optimally adjust the potentiometer 11, the exemplary embodiment of the calibration method according to the invention described below can be carried out. This can, for example, be carried out directly after the manufacture of the sensor arrangement 1. In this process, the bridge circuit is supplied with direct current, using the source 4. In a calibration operation with a known or no fluid flow through the flow sensors S1, S2, the voltage difference between the two branches 2, 3 of the bridge circuit, i.e. the bridge voltage, is tapped and amplified by the preamplifier 9.Starting from the neutral center position of potentiometer 11, in which it introduces no disturbance into the bridge circuit, the resistance value of potentiometer 11 is changed, in particular by digital control, such that the bridge voltage in calibration mode, i.e. the DC offset, assumes a smaller value, preferably dropping to zero. In other words, the DC offset is reduced by suitable resistance adjustment, preferably to the smallest possible value. These steps can also be repeated several times until the bridge adjustment is as close to an optimal value as possible.

[0082] The optimal resistance value determined during calibration can be determined and saved once. It can also be dynamically readjusted cyclically.

[0083] Calibration is preferably carried out entirely by software without any user intervention.

[0084] Due to the targeted compensation by means of the potentiometer 11, flow sensors S1, S2 can be installed in a sensor arrangement 1 according to the invention even with larger deviations. Furthermore, it is possible to use a DC-coupled preamplifier 12 with a high gain factor. In the embodiment shown in FIG. 4, the gain factor is 2000. At the same time, a high suppression of the bridge supply voltage is ensured. The bridge supply noise is balanced in a balanced bridge. This results in lower signal noise.

[0085] The compensation device 10 can also be used for the condition monitoring of the sensor arrangement 1.

[0086] For this purpose, the potentiometer 11 can be changed or adjusted, for example, by a predetermined value, and the effect on the amplified bridge voltage can be observed. In particular, the resulting change in the bridge voltage is recorded and compared with an expected value. The result of the comparison can then be used to determine the state of the sensor arrangement 1.

[0087] In other words, the bridge can be deliberately brought into an unbalanced state by adjusting the potentiometer 11. This is accompanied by a shift in the DC offset. This depends on the bridge resistance itself and the DC gain factor of the preamplifier 12. Since these variables are known or can be calculated, a known change in the resistance value of the potentiometer 11 should result in a known change in the DC offset. This can be used, for example, to detect gain drift or ADC problems, in particular with an ADC used to convert the output of the amplifier 12 into digital. This diagnostic can be used, in particular, in a secure environment to detect sensor and signal path problems.

[0088] If a problem or defect is detected, a user is conveniently informed. Alternatively, a maintenance event can be scheduled to correct the problem—in other words, predictive maintenance.

[0089] Figures 5 and 6 show two further exemplary embodiments of sensor arrangements 1 according to the invention in a schematic representation. These differ from the example in Figure 4 essentially only in a different design of the compensation device 10. This comprises at least one digital-to-analog converter 13 (DAC for short) with a voltage output. In the case of Figure 5, only one DAC 13 is provided, specifically - by way of example - for the left branch 2, and in the example in Figure 6, one DAC 13 is provided for each of the two branches 2, 3. In both cases, the DAC 13 is connected on the output side to the respective branch 2, 3, via a further fixed resistor R5, R6, which in turn serves to limit the maximum "compensation current". The one DAC 13 from FIG 5 has a bipolar voltage output connected to branch 2, in other words one whose polarity is adjustable (positive and negative).The two DACs 13 from FIG 6 each have a unipolar, in particular positive voltage output, which is connected to the respective branches 2, 3.

[0090] By means of the DAC(s) 13 shown in Figures 5 and 6, the deviation in the base resistances of the two flow sensors S1 and S2 can be compensated for in a similar manner, as described above in connection with the exemplary embodiment shown in Figure 4. These can also be used in a similar manner for condition monitoring.

[0091] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited to the disclosed examples and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

[0092] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identity are included.

Claims

Patent claims 1. Sensor arrangement (1) with at least two preferably thermal flow sensors (S1, S2) which are arranged in a bridge circuit with two branches (2, 3), wherein at least one flow sensor (S1) is located in each of the two branches (2, 3), and wherein the flow sensors (S1, S2) each have a basic resistance value, and the basic resistance values ​​of the flow sensors (S1, S2) can differ from one another, in particular due to manufacturing reasons, wherein a compensation device (10) with at least one preferably digitally adjustable resistance element (11) is provided, by means of which the deviation of the basic resistance values ​​of the at least two flow sensors (S1, S2) can be compensated for during measuring operation, characterized in that the at least one adjustable resistance element is designed as an adjustable potentiometer (11) which is connected as a voltage divider, preferably,wherein the at least one potentiometer (11) has three terminations (A1, A2, A3), each of which is connected to one of the two branches (2, 3), in particular via an at least fixed resistor (R3, R4).

2. Sensor arrangement (1) according to claim 1, characterized in that the compensation device (10) is designed and arranged in particular in the bridge circuit in such a way that by means of this a balanced power supply of the two branches (2, 3) can be counteracted in order to compensate for the deviation of the basic resistance values ​​of the flow sensors (S1, S2).

3. Sensor arrangement (1) according to claim 1 or 2, characterized in that the adjustable potentiometer (11) is designed as a digitally adjustable potentiometer (11) and.

4. Sensor arrangement (1) according to claim 3, characterized in that the at least one potentiometer (11) is designed as a digital potentiometer, in particular as a resistive digital-to-analog converter.

5. Sensor arrangement (1) according to one of the preceding claims, characterized in that the sensor arrangement (1) comprises one, preferably exactly one preamplifier (12), with which the voltage difference between the two branches, bridge voltage, can be amplified, preferably, wherein the preamplifier (12) is DC-coupled and / or is characterized by an amplification factor of 1000 or more, preferably in the range from 1000 to 5000.

6. Sensor arrangement (1) according to one of the preceding claims, characterized in that two flow sensors (SI, S2) are provided, one of which is connected in series with at least one fixed resistor (RI, R2) in one of the branches (2, 3), or that four flow sensors (SI, S2) are provided, two of which are connected in series in one of the branches (2, 3).

7. Sensor arrangement (1) according to one of the preceding claims, characterized in that the flow sensors (SI, S2) are of identical construction and / or the flow sensors (SI, S2) each comprise a microflow sensor or are provided by such a sensor.

8. Sensor arrangement (1) according to one of the preceding claims, comprising means adapted to carry out the steps of the calibration method according to one of claims 12 or 13 and / or carry out the steps of the condition monitoring method according to one of claims 14 to 16.

9. Computer program product comprising instructions which cause the sensor arrangement (1) according to claim 8 to carry out the method steps according to one of claims 12 or 13 and / or the method steps according to one of claims 14 to 16.

10. Computer-readable medium comprising instructions which cause the sensor arrangement (1) according to claim 8 to carry out the method steps according to one of claims 12 or 13 and / or the method steps according to one of claims 14 to 16.

11. Gas analyzer comprising at least one sensor arrangement (1) according to one of claims 1 to 8.

12. Method for calibrating at least one sensor arrangement (1) according to one of claims 1 to 8, comprising the steps: the bridge circuit comprising the flow sensors (SI, S2) is supplied with a direct current, in a calibration operation with known or without fluid flow through the flow sensors (SI, S2), the voltage difference between the two branches (2, 3) of the bridge circuit, bridge voltage, is tapped and amplified, at least one resistor of the compensation device (10) is changed, if necessary, in particular by digital control, in such a way that the bridge voltage assumes a smaller value in calibration mode, preferably falling to zero.

13. Calibration method according to claim 12, characterized in that Steps are repeated at least once more, preferably cyclically, over the operating period of the sensor arrangement (1).

14. Method for monitoring the condition of at least one sensor arrangement (1) according to one of claims 1 to 8, comprising the steps: the bridge circuit comprising the flow sensors (S1, S2) is fed with a direct current, the voltage difference between the two branches of the bridge circuit (2, 3), bridge voltage, is tapped and amplified, at least one resistor (11) of the compensation device (10) is changed by a predetermined value, in particular by digital control, the resulting change in the bridge voltage is recorded and compared with an expected value and the condition of the sensor arrangement (1) is deduced from the result of the comparison.

15. Method for condition monitoring according to claim 14, characterized in that in the event that the resulting change in the bridge voltage does not correspond to the expected value, it is assumed that there is a defect in the sensor arrangement (1), in particular at least one flow sensor (SI, S2) thereof, preferably, wherein to compensate for the defect at least one resistance of the compensation device (10) is changed, in particular by a predetermined value.

16. A condition monitoring method according to claim 15, characterized in that a user is informed of the defect and / or a maintenance event is planned to rectify it.

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