DE102018103877B3 - Inductive conductivity sensor - Google Patents

Abstract

The invention relates to an inductive sensor (1) for determining the conductivity of a medium, comprising a first coil (2), a second coil (3) and a third coil (4), at least one of the three coils (2, 3 , 4) serves as a transmitting coil (6) and another of the three coils (2, 3, 4) serves as a receiving coil (7), with a sensor electronics (8), wherein the sensor electronics (8) a driver stage (9) for applying the as a transmitting coil (6) serving coil (2, 3, 4) with an electrical drive signal and a first receiving stage (10) for amplifying an in the receiving coil (7) serving as a coil (2, 3, 4) induced electrical measurement signal and with a control and evaluation unit (11) for controlling the sensor electronics (8) and for evaluating the measurement signals.
The object to provide an inductive sensor (1), which has an increased reliability against the known from the prior art inductive sensors, is achieved in that the sensor electronics (8) has a second driver stage (12) and a second receiving stage (13) wherein the first driver stage (9) and the second driver stage (12) and the first receiving stage (10) and the second receiving stage (13) via such at least one with the control and evaluation unit (11) controllable switching device (14) three coils (2, 3, 4) are connected, that at least one of the coils (2, 3, 4) both as a transmitting coil (6) and as a receiving coil (7) is operated and that the control and evaluation unit (11) in such a way is configured such that it controls the switching device (14), the driver stages (9, 12) and the receiving stages (10, 13) such that at least a first, a second and a third measuring signal are recorded, wherein a measuring circuit of a Driver stage (9, 12), a transmitting coil (6), a receiving coil (7) and a receiving stage (10, 13) is formed, and wherein the three measuring signals of different measuring circuits result, and that the control and evaluation unit (11) three measurement signals or derived from the three measurement signals measured values of a comparative evaluation and generates an output depending on the comparative evaluation.

Description

The invention relates to an inductive sensor for determining the conductivity of a medium, with a first coil, with a second coil and with a third coil, wherein at least one of the three coils serves as a transmitting coil and another of the three coils serves as a receiving coil, with a sensor electronics in which the sensor electronics has a driver stage for acting on the coil serving as a transmitting coil with an electrical drive signal and a first receiving stage for amplifying an electrical measuring signal induced in the coil serving as a receiving coil and with a control and evaluation unit for controlling the sensor electronics and for evaluating the measuring signals ,

The measurement of the conductivity of a medium is often necessary in the process analysis. A conductivity measurement can be carried out with the help of inductive sensors. An inductive sensor regularly has two coils, wherein a first coil serves as a transmitting coil and a second coil as a receiving coil. The two coils are arranged relative to each other, that when immersing the sensor in the medium whose conductivity is to be determined, both coils are flowed through by the medium. The transmitter coil is acted upon via a driver stage with a driver signal. The drive signal is usually an alternating current through which an alternating magnetic field is generated. The alternating magnetic field in turn induces a current in the medium surrounding the coil. The current flow in the medium in turn generates a magnetic field which induces a voltage and thus a current flow in the receiving coil. A receiving stage amplifies the electrical signal of the receiver coil. From the ratio of injected primary voltage to received secondary voltage is concluded that the conductivity of the medium. The two coils are magnetically isolated from each other, so that the information between the transmitting coil and the receiving coil are transmitted only through transmitted through the medium eddy currents.

From the prior art is for example from the DE 690 22 397 T2 an inductive sensor is known which has not two, but three coils. A measurement with the sensor can be performed by first using one coil as a transmit coil and another coil as a receive coil to measure a first range of values. The third coil is then connected in parallel with the transmitting coil or in parallel with the receiving coil, thereby increasing the sensitivity of the inductive sensor.

From the DE 10 2009 026 998 A1 An inductive sensor is also known. The sensor has two coils and a switching means, wherein in a first switching state, the first coil acts as a transmitting coil and the second coil as a receiving coil and wherein in a second switching state, the second coil acts as a transmitting coil and the first coil as a receiving coil.

The DE 10 2011 002 766 A1 discloses an inductive sensor for determining the electrical conductivity of a medium having a transmitting coil and two receiving coils, wherein the two receiving coils are arranged symmetrically to the transmitting coil. With a measuring circuit, the signals of the receiver coils are detected and determines the electrical conductivity of the medium.

Another conductivity sensor is out of the JP H09-329663 A known.

A disadvantage of the sensors known from the prior art is that the functional safety of the sensors is not adequately ensured. If it comes to a failure of a sensor, it is questionable whether this is detected directly, or whether with a defective sensor continues to be measured. In addition, it is not possible with the known from the prior art sensors, in the case of failure of a component of the sensor, such as the transmitting coil or the receiving coil or the driver stage or receiving stage, nevertheless perform a reliable measurement of the conductivity of a medium.

Accordingly, the invention has for its object to provide an inductive sensor having an increased reliability against the known from the prior art inductive sensors.

The object is initially and essentially solved by the inductive sensor according to the invention in that the sensor electronics has a second driver stage and a second receiver stage. The first driver stage and the second driver stage, as well as the first receiving stage and the second receiving stage, are connected to the three coils via a switching device which can be controlled by the control and evaluation unit such that at least one of the coils is operated both as a transmitting coil and as a receiving coil.

With the three coils, the two driver stages and the two receiving stages a total of three measuring circuits are realized, wherein a measuring circuit is formed in each case of a driver stage, a transmitting coil, a receiving coil and a receiving stage. The coil, which is operated both as a transmitting coil and as a receiving coil, is then a transmitting coil of a first measuring circuit and a receiving coil of a second measuring circuit.

According to the invention, the control and evaluation unit is designed such that at least one first measuring signal, one second measuring signal and one third measuring signal are recorded, the measuring signals resulting from different measuring circuits. In particular, the measurement signals are recorded successively in time and the switching device is brought from a first switching state to a second switching state at least between the recording of two successive measurement signals, so that one of the coils initially serves as a transmitting coil and then as a receiving coil. The control and evaluation unit is further configured such that it subjects the three measurement signals or measured values derived from the three measurement signals to a comparison evaluation and generates an output as a function of the comparison evaluation.

If this refers to measurement signals or measured values derived from the measurement signals, then this is based on the understanding of a customary measurement path in which a raw measurement value is first detected, for example in the form of an induced voltage at a receiver coil, whereby the raw measurement value may then be different Measures of signal processing is converted (high-impedance tapping of the induced voltage, amplify the voltage, analog-to-digital conversion of the amplified voltage) until finally - from the measurement signal - the actual measured value is determined.

The erfmdungsgemäße realization of three measuring circuits, a redundant conductivity sensor is realized, which has an overall increased reliability against the known conductivity sensors and thus increased reliability.

In the context of the comparative evaluation, the three measuring signals or the measured values derived from the three measuring signals can be compared and possibly rejected.

In an embodiment, it is provided that the control and evaluation unit carries out a comparative evaluation of the measurement signals or the measured values derived from the measurement signals and signals a fault condition in the case of discarding a measurement signal or a derived measurement value.

A particularly preferred embodiment of the inductive sensor according to the invention is distinguished in that the control and evaluation unit outputs a message if one of the measurement signals or one of the measured values derived from the measurement signals is rejected as being defective. Due to the development of the inductive sensor according to the invention the user is informed by the message directly to the presence of a fault condition.

A message in a preferred embodiment is a recalibration request. With the sensor according to the invention in particular wear phenomena on the coils or the other components of the sensor can be detected early. An exchange of worn components is thus possible before a total failure of the components, whereby the downtime of the sensor can be minimized as part of the so-called "predictive maintenance" (English: "predictive maintenance").

A message can be issued in different ways. An embodiment provides that the message is output by an acoustic or an optical signal.

In a further embodiment, it is provided that the control and evaluation unit carries out a comparative evaluation of the measurement signals and determines the conductivity measurement value of the medium on the basis of a measurement signal found to be free of errors.

In a preferred embodiment, it is provided that the control and evaluation unit derives measured values for the conductivity from the measuring signals and subjects the derived measured values to a comparative evaluation. The measured value found to be faultless is output as the conductivity measured value of the medium.

The inductive sensor according to the invention has the advantage that errors in individual measuring circuits, in particular in individual components of the measuring circuits, can be detected by the realization of three measuring circuits and the comparative evaluation. In addition, the inventive inductive sensor has the advantage that in the event of an error, ie in the case of detecting an error in one of the measuring circuits - or in two of the measuring circuits - nevertheless the reliability of the conductivity measurement is ensured because at least by a faultless measuring circuit, the functionality of the Sensor is maintained.

There are various ways in which the comparative evaluation is performed. In a first embodiment, the control and evaluation unit subjects the three measurement signals or the measured values derived from the three measurement signals to a relative comparison evaluation. Here, the basic assumption is made that the three measured signals obtained in a measurement situation or the derived measured values are the same or at least very similar. If it is said that the three measurement signals or the derived measurements are the same, then that does not necessarily mean that identity must exist. With regard to the measurement signals obtained in a specific measurement, it is rather meant that the ratio of the measurement signals to one another is assumed to be plausible. In particular, the ratio of the measurement signals to one another can be assumed to be constant. The ratio of the measurement signals to one another is dependent, in particular, on the geometric arrangement of the components relative to one another, in particular on the distance of the coils from one another. In the case of the measured values derived from the measuring signals, it is meant that the measured values can deviate from one another within the scope of the measuring accuracy. In the relative comparative evaluation, the three measurement signals or the three measurement values derived from the measurement signals are compared with one another. A measurement signal or a derived measurement value is rejected as faulty if it deviates from the other measurement signals / measured values beyond a predetermined deviation tolerance range.

In a preferred embodiment, in a relative comparative evaluation, in addition to the basic assumption that the three measurement signals obtained or the derived measurement values are the same or at least very similar, it is also assumed that an error in one of the components of a measurement circuit is present in a reduced measurement signal or a reduced reading. When it comes to the components of a measuring circuit, among the components, the individual components, namely the coils and / or the driver stages and receiving stages are meant. A measurement signal or a measurement value derived from a measurement signal is rejected as being erroneous if it deviates downward from another or the other measurement signals or measured values beyond a predetermined deviation tolerance range. By means of this preferred embodiment, in particular the defective components can be at least partially identified.

In a particularly preferred embodiment, the control and evaluation unit subjects the three measurement signals to an absolute comparison evaluation. In the context of the absolute comparative evaluation, the measurement signals are examined for characteristic features. A measurement signal is rejected as erroneous if a detected characteristic feature of the measurement signal deviates from an expected characteristic feature of the measurement signal. In contrast to the relative comparative evaluation, the measurement signals in the absolute comparison evaluation are not compared with each other, but evaluated on the basis of objective criteria, namely the presence of certain features characteristic of the measurement signals. By such a configuration, in particular the case can be recognized, in which two of the three measurement signals are faulty, but in particular have the same error. In the relative comparison evaluation, the individual deviating measurement signal could be erroneously recognized as faulty. If the comparative evaluation takes place on the basis of objective criteria, a wrong prognosis can be avoided.

In one embodiment, the presence of a measurement signal as such is checked as a characteristic feature of a measurement signal. Therefore, if no measurement signal is output at one of the measurement paths, or a measurement signal equal to zero is output, then the control and evaluation unit rejects this measurement signal as defective and uses the non-zero measurement signal to determine a conductivity measurement value.

In a particularly preferred embodiment, the driver stage acts on the transmitter coil with a driver signal whose fundamental wave has a sinusoidal profile. In this embodiment of the inductive sensor is checked in the context of the absolute comparison evaluation, whether the measurement signal has a sinusoidal waveform analogous to a sinusoidal waveform of the fundamental wave of the driver signal. If the course of a measurement signal deviates from the expected sinusoidal profile, then this measurement signal is rejected as faulty.

The invention is not limited to sinusoidal driver signals. Rather, all driver signal waveforms are included. The comparison evaluation is then based on the test of the expected due to the driver waveform measurement waveform.

A further preferred embodiment of the inductive sensor is characterized in that the limitless freedom of the measurement signal is checked as a characteristic feature of the measurement signal. Limiting freedom means that the measuring signal is not limited due to metrological reasons. This includes, in particular, that it is checked whether the measuring signal processed in the measuring chain has clipping. Clipping refers to the cutting off of signals at the output of a transmission element, when the transmission element is controlled outside of its permissible input range, ie is overdriven. The transmission element can be, for example, an amplifier or an analog-to-digital converter, the overdrive driving the amplifier into saturation and operating the analog-to-digital converter outside its conversion range.

If it is detected that one of the measurement signals is not limitless, in particular has clipping, then this measurement signal is rejected as being faulty. The freedom of limitation of a measuring signal can be tested in various ways.

In one embodiment, the freedom of limitation of a measurement signal, which, in particular, is expected to have a sinusoidal profile, is checked in the time domain of the signal by checking whether the measurement signal has successively measured almost identical measured values. If this is the case, the measuring signal has clipping.

In the frequency range of the measuring signal, the freedom of limitation of the measuring signal is checked by checking whether a rise of the harmonics is detected. An unexpected increase in harmonics of a signal is also due to a clipping.

In an embodiment, it is provided that the user is shown all measurement signals or all measured values derived from the measurement signals. In particular, the user is shown which measurement signals or measured values are evaluated as defective and discarded.

A particularly preferred embodiment of the inductive sensor is characterized in that in the case of discarding at least one measurement signal or at least one derived measurement value, a user is prompted to select a measurement signal for determining a conductivity measurement value or to select a measurement value as the conductivity measurement value. In this embodiment of the inductive sensor, the user therefore has the option of defining a measured value as error-free if it appears realistic to him on the basis of his expectations of the measured value. A user is particularly given the opportunity to qualify a measuring circuit as faulty or error-free, for example via a control panel input or via a connected control unit or via a connected control room. Due to the qualification, the inductive sensor then only outputs measured values which have been obtained via the measuring circuit qualified as error-free, up to a fault correction.

In one embodiment of the inductive sensor, it is provided that a temperature compensation is carried out in the determination of the conductivity measured value of the medium. It is further provided that the control and evaluation unit then classifies a measured value as defective if it lies outside of a value range of the conductivity of a compensation function for the temperature compensation stored in the control and evaluation unit.

• 1 eine schematische Darstellung eines induktiven Sensors,
• 2 eine schematische Darstellung einer ersten Verschaltung einzelner Komponenten des induktiven Sensors,
• 3a eine schematische Darstellung eines sinusförmigen Treibersignals,
• 3b eine schematische Darstellung eines sinusförmigen Messsignals,
• 4a eine schematische Darstellung eines nicht begrenzungsfreien Messsignals im Zeitbereich,
• 4b eine schematische Darstellung eines nicht begrenzungsfreien Messsignals im Frequenzbereich
• 5 eine schematische Darstellung einer zweiten Verschaltung einzelner Komponenten des induktiven Sensors,
• 6 eine schematische Darstellung einer dritten Verschaltung einzelner Komponenten des induktiven Sensors und
• 7 eine schematische Darstellung einer vierten Verschaltung einzelner Komponenten des induktiven Sensors.

In particular, there are a variety of ways to design and further develop the inductive sensor according to the invention. Reference is made to the claims subordinate to claim 1 and to the following description of preferred embodiments in conjunction with the drawings. In the drawing show

• 1 a schematic representation of an inductive sensor,
• 2 a schematic representation of a first interconnection of individual components of the inductive sensor,
• 3a a schematic representation of a sinusoidal driver signal,
• 3b a schematic representation of a sinusoidal measuring signal,
• 4a a schematic representation of a non-limit-free measurement signal in the time domain,
• 4b a schematic representation of a non-limit-free measurement signal in the frequency domain
• 5 a schematic representation of a second interconnection of individual components of the inductive sensor,
• 6 a schematic representation of a third interconnection of individual components of the inductive sensor and
• 7 a schematic representation of a fourth interconnection of individual components of the inductive sensor.

1 shows a schematic representation of an inductive sensor 1 , The sensor 1 has three coils 2 . 3 . 4 on, around a measuring tube 5 are arranged. The medium to be measured flows through the measuring tube 5 , The flow direction is shown schematically by arrows. The first coil 2 serves as a transmitter coil 6 , the third coil 4 serves as a receiver coil 7 , The second coil 3 can both as a transmitting coil 6 as well as a receiving coil 7 operate.

A sensor electronics 8th has a first driver stage 9 for applying the transmitting coil 6 with an electrical driver signal and a first receiving stage 10 for amplifying one in the receiving coil 7 induced electrical measurement signal. A control and evaluation unit 11 serves to control the sensor electronics 8th and for evaluating the measurement signals. To a redundant operation of the inductive sensor 1 to enable, the sensor electronics 8th also a second driver stage 12 and a second receiving stage 13 on. The two driver stages 9 . 12 and the two receiving stages 10 . 13 are about one in 1 not shown switching device with the three coils 2 . 3 . 4 connected to that at least one of the three coils 2 . 3 . 4 - In the illustration the coil 3 - both as a transmitting coil 6 as well as a receiving coil 7 can be operated. The switching device is with a control and evaluation unit 11 driven. Due to the configuration, at least three measuring circuits can be realized, wherein a measuring circuit is formed in each case from a driver stage of a transmitting coil, a receiving coil and a receiving stage.

The control and evaluation unit 11 is designed so that it the switching device, the driver stages 9 . 12 and the reception levels 10 . 13 such that at least three measurement signals are recorded, the measurement signals resulting from different measurement circuits. Furthermore, the control and evaluation unit 11 is configured such that it subjects the measurement signals or the measured values derived from the measurement signals to a comparison evaluation and outputs a conductivity measurement value for the medium as a function of the comparison evaluation. The comparative evaluation may be a relative comparison evaluation in which the measurement signals or the measured values are compared with one another, or the comparative evaluation may be an absolute comparative evaluation in which the individual measurement signals or measured values are checked on the basis of objective criteria, namely on the basis of characteristic features. In particular, both a relative comparison evaluation and an absolute comparison evaluation can be performed.

2 shows a schematic representation of a first interconnection of individual components of the inductive sensor 1 , The first coil 2 is with the first driver stage 9 connected. The second coil 3 is about the switching device 14 with the first reception stage 10 and the second driver stage 12 connected. In a first switching state of the switching device 14 is the second coil 3 with the first reception stage 10 connected and in a second switching state of the switching device 14 is the second coil 3 with the second driver stage 12 connected. The third coil 4 is with the second receiving stage 13 connected. In total, three measuring circuits are realized, with the first measuring circuit from the driver stage 9 , the coil 2 as a transmitting coil 6 , the coil 3 as a receiving coil 7 and the receiving level 10 is formed, the second measuring circuit from the driver stage 12 , the coil 3 as a transmitting coil 6 , the coil 4 as a receiving coil 7 and the receiving level 13 is formed and the third measuring circuit from the driver stage 9 , the coil 2 as a transmitting coil 6 , the coil 4 as a receiving coil 7 and the receiving level 13 is formed.

In the relative comparison evaluation, a measurement signal or a measurement value is rejected as being erroneous if it deviates from another or the other measurement signals / measured values beyond a predetermined deviation tolerance range. With the help of some basic assumptions, it is also possible to use the relative comparison evaluation to identify the measuring circuit, in particular some of the faulty components in a faulty measuring circuit, which is the cause of the fault. This is illustrated by an example of the in 2 shown interconnection in which a comparison of the three measured values derived from the measurement signals is performed:

measuring circuit 1 is through the driver stage 9 , the sink 2 , the sink 3 and the receiving level 10 educated. measuring circuit 2 is through the driver stage 12 , the sink 3 , the sink 4 and the receiving level 13 educated. measuring circuit 3 is through the driver stage 9 , the sink 2 , the sink 4 and the receiving level 13 educated. As a basic assumption, it is assumed that an error in one of the components affects a reduced measured value. If there is an error in one of the coils 2 . 3 . 4 before, namely in particular a short circuit, makes itself felt in a reduced reading. In addition, as a simplified basic assumption, it is further assumed that the three measured values of the three measuring circuits are the same. An error in the first driver stage 9 , the second receiving stage 13 or in one of the three coils 2 . 3 . 4 always affects two measuring circuits, as these elements are all part of two different measuring circuits. An error in the first reception stage 10 or the second driver stage 12 On the other hand, it only affects one measuring circuit at a time, since these two elements are only part of one measuring circuit. If, therefore, the comparison evaluation shows that one or two of the measuring circuits give a smaller measured value than the other measuring circuits, then it can be ruled out that the error lies in the measuring circuit with the larger measured value. Overall, the following results: (measured value 1 results from measuring circuit 1 , Measured value 2 results from measuring circuit 2 , Measured value 3 results from measuring circuit 3 ):

reading 1 and reading 3 are less than reading 2 : Measuring circuit 2 is error-free, the error is either in the first driver stage 9 or the first coil 2 , both part of the measuring circuit 1 and measuring circuit 3 are.

reading 1 is less than reading 2 and reading 3 : Measuring circuit 2 and measuring circuit 3 are error-free, the error is in the receiving stage 10 , as these are the only component only in measuring circuit 2 is included.

reading 2 is less than reading 1 and reading 3 : Measuring circuit 1 and measuring circuit 3 are error-free, the error lies in the second driver stage 12 , as these are the only component only in measuring circuit 2 is included.

reading 2 and reading 3 are less than reading 1 : Measuring circuit 1 is error-free, the error is either in the first reception stage 10 or the third coil 4 , both part of the measuring circuit 2 and measuring circuit 3 are.

reading 1 and reading 2 are none as a reading 3 : Measuring circuit 3 is error-free, the error lies in the second coil 3 because only these are components of the measuring circuit 1 and measuring circuit 2 is.

By means of such a comparison evaluation, the faulty components can be unambiguously identified or at least partially uniquely identified. In all cases shown, however, a high probability of correct measurement result can be determined.

In the absolute comparison evaluation, a measurement signal or a measurement value is rejected as erroneous if a detected characteristic feature of a measurement signal deviates from an expected characteristic feature of the measurement signal. It is therefore checked whether the measurement signal has a specific characteristic feature. Some examples of characteristic features or the error detection on the basis of characteristic features are described in US Pat 3 and 4 shown. Exemplary is in this 3a a sinusoidal drive signal is shown. If the transmitting coil is subjected to such a sinusoidal driver signal, the measuring signal must, as expected, have an analogue sinusoidal profile. As a characteristic feature, for example, the shape of the measurement signal is checked. 3b shows a measuring signal. Here, the measurement signal has the expected sinusoidal profile, which is analogous to the sinusoidal profile of the driver signal 3a is. Since the measurement signal has the characteristic feature, it is not rejected as faulty.

Another characteristic feature may be the freedom of restriction of the measurement signal. 4 shows a measurement signal that is not restriction-free. In the time domain - shown in 4a it is possible to detect such an error on the basis of values which are constant over time, namely in the region in which the signal is overdriven. In the frequency domain - shown in 4b - A faulty signal is identified by the occurrence of unexpected harmonics.

In the 5 to 7 are other possible interconnections of the coils 2 . 3 . 4 , the driver stages 9 . 12 and the receiving stages 10 . 13 shown. The additional interconnection options enable further measurement circuits to be implemented, with each of the measurement circuits supplying a measurement signal, so that further measurement signals can be generated which are subjected to the comparative evaluation. The additional measuring paths ensures that the reliability of the inductive sensor 1 is further increased.

5 shows an embodiment in which in addition to a first switching device 14 a second switching device 15 and a third switching device 16 are realized. The first coil 2 is about the second switching device 15 with the first reception stage 10 and the second receiving stage 13 connected. In a first switching state of the second switching device 15 is the first coil 2 with the first reception stage 10 connected, in a second switching state of the second switching device 15 is the first coil 2 with the second receiving stage 13 connected. The first coil 2 So so in both switching states as a receiving coil 7 used. The second coil 3 is about the first switching device 14 with the second receiving stage 13 and the second driver stage 12 connected. In a first switching state of the first switching device 14 is the second coil 3 with the second receiving stage 13 connected, in a second switching state of the first switching device 14 is the second coil 3 with the second driver stage 12 connected. Overall, the second coil 3 so as a transmission coil 6 and also as a receiver coil 7 be used. The third coil 4 is about the third switching device 16 with the second driver stage 12 and with the first driver stage 9 connected. In a first switching state of the third switching device 16 is the third coil 4 with the second driver stage 12 connected, in a second switching state of the third switching device 16 is the third coil 4 with the first driver stage 9 connected and so is exclusively as a transmission coil 6 used. By means of such an interconnection, six different measuring circuits can be realized, accordingly six measuring signals can also be recorded.

Also at the in 6 shown interconnection six different measuring circuits can be realized and therefore six different measurement signals are recorded. For this purpose, likewise a first switching device 14 , a second switching device 15 and a third switching device 16 intended. The first coil 2 is about the first switching device 14 with the first reception stage 10 and with the second driver stage 12 connected. In a first switching state of the first switching device 14 is the first coil 2 with the first reception stage 10 connected, in a second switching state of the first switching device 14 is the first coil 2 with the second driver stage 12 connected. The first coil 2 So it is both as a transmission coil 6 as well as a receiving coil 7 used. The second coil 3 is about the second switching device 15 with the second receiving stage 13 and with the first driver stage 9 connected. In a first switching state of the second switching device 15 is the second coil 3 with the second receiving stage 13 connected and in a second switching state of the second switching device 15 is the second coil 3 with the first driver stage 9 connected. Also the second coil 3 is both as a transmitting coil 6 as well as a receiving coil 7 used. The third coil 4 is about the third switching device 16 with the second receiving stage 13 and with the first driver stage 9 connected. In a first switching state of the third switching device 16 is the third coil 4 with the second receiving stage 13 connected and in a second switching state of the third switching device 16 is the third coil 4 with the first driver stage 9 connected. Also the third coil 4 is in the present embodiment both as a transmitting coil 6 as well as a receiving coil 7 used.

In the in 7 shown interconnection 12 measuring circuits are realized. Overall, so 12 recorded various measurement signals. The embodiment has a first switching device 14 , a second switching device 15 , a third switching device 16 and a fourth switching device 17 on. The first switching device 14 and the second switching device 15 are connected in series. Likewise, the third switching device 16 and the fourth switching device 17 connected in series. The first coil 2 is about the first switching device 14 and the second switching device 15 with the first reception stage 10 and with the second receiving stage 13 connected. The first coil 2 is in a first switching state of the first switching device 14 and in a first switching state of the second switching device 15 with the first reception stage 10 connected and in a first switching state of the first switching device 14 and in a second switching state of the second switching device 15 with the second receiving stage 13 connected. In a second switching state of the first switching device 14 is not a connection of the first coil 2 with the first reception stage 10 or the second receiving stage 13 realized. The first coil 2 becomes only as a receiving coil 7 used. The second coil 3 is about the first switching device 14 and the second switching device 15 such with the first receiving stage 10 and the second receiving stage 13 connected, that in a first switching state of the first switching device 14 no connection of the second coil 3 with the first reception stage 10 or the second receiving stage 13 realized is that the second coil 3 in a second switching state of the first switching device 14 and in a first switching state of the second switching device 15 with the first reception stage 10 is connected and in a second switching state of the first switching device 14 and in a second switching state of the second switching device 15 with the second receiving stage 13 connected is. The second coil 3 is also on the third switching device 16 and the fourth switching device 17 with the second driver stage 12 and with the first driver stage 9 connected. The connection is such that the second coil 3 in a first switching state of the third switching device 16 and in a first switching state of the fourth switching device 17 with the second driver stage 12 is connected and in a first switching state of the third switching device 16 and in a second switching state of the fourth switching device 17 with the first driver stage 9 is connected, and that in a second switching state of the third switching device 16 no connection between the second coil 3 and the first driver stage 9 and the second driver stage 12 is realized. The second coil 3 serves both as a transmitter coil 6 as well as a receiving coil 7 , The third coil 4 is about the third switching device 16 and the fourth switching device 17 with the second driver stage 12 and with the first driver stage 9 connected such that in a first switching state of the third switching device 16 no connection between the third coil 4 and the first driver stage 9 and the second driver stage 12 realized is that the third coil 4 in a second switching state of the third switching device 16 and in a first switching state of the fourth switching device 17 with the second driver stage 12 is connected and in a second switching state of the third switching device 16 and in a second switching state of the fourth switching device 17 with the first driver stage 9 connected is. The third coil 4 is used as a transmitting coil 6 used.

LIST OF REFERENCE NUMBERS

1

inductive sensor

2

first coil

3

second coil

4

third coil

5

measuring tube

6

transmitting coil

7

receiving coil

8th

sensor electronics

9

first driver level

10

first reception level

11

evaluation

12

second driver stage

13

second reception stage

14

first switching device

15

second switching device

16

third switching device

17

fourth switching device

Claims (13)

Inductive sensor (1) for determining the conductivity of a medium, comprising a first coil (2), a second coil (3) and a third coil (4), wherein at least one of the three coils (2, 3, 4) as Transmitter coil (6) is used and another of the three coils (2, 3, 4) as a receiving coil (7), with a sensor electronics (8), wherein the sensor electronics (8) a driver stage (9) for acting as a transmitting coil (6 ) serving coil (2, 3, 4) with an electrical drive signal and a first receiving stage (10) for amplifying an in the receiving coil (7) serving as a coil (2, 3, 4) induced electrical measurement signal and having a control and Evaluation unit (11) for controlling the sensor electronics (8) and for evaluating the measurement signals, characterized in that the sensor electronics (8) comprises a second driver stage (12) and a second receiving stage (13), wherein the first driver stage (9) and the second driver stage (12) and the first receiving stage (10) u the second receiving stage (13) are connected to the three coils (2, 3, 4) via at least one switching device (14) which can be controlled by the control and evaluation unit (11) such that at least one of the coils (2, 3, 4 ) is operated both as a transmitting coil (6) and as a receiving coil (7) and that the control and evaluation unit (11) is designed such that it the switching device (14), the driver stages (9, 12) and the receiving stages (10 13) in such a way that at least a first, a second and a third measuring signal are recorded, wherein a measuring circuit comprises a driver stage (9, 12), a transmitting coil (6), a receiving coil (7) and a receiving stage (10, 13 ), and wherein the three measurement signals result from different measurement circuits, and that the control and evaluation unit (11) subjects the three measurement signals or measurement values derived from the three measurement signals to a comparison evaluation and, depending on the comparison evaluation g produces an output. Inductive sensor after Claim 1 , characterized in that the output is realized by signaling an error condition and / or realized by an output of a conductivity measured value for the medium, in particular that the control and evaluation unit (11) outputs a message when one of the measurement signals or one of the measuring signals derived measured value is rejected as faulty. Inductive sensor (1) after Claim 1 or 2 , characterized in that the control and evaluation unit (11) subjects the three measurement signals or measured values derived from the three measurement signals to a relative comparison evaluation, for which purpose the three measurement signals or the three measurement values derived from the measurement signals are compared with one another and a measurement signal or off A measured value derived from a measuring signal is rejected as being erroneous if it deviates from the other measuring signals or measured values beyond a predetermined deviation tolerance range. Inductive sensor (1) after Claim 1 or 2 , characterized in that the control and evaluation unit (11) subjects the three measurement signals or measured values derived from the three measurement signals to a relative comparative evaluation, for which purpose the three measurement signals or the three measurement values derived from the measurement signals are compared with one another, it being assumed that an error in one of the components of the individual measurement circuits affects a reduced measurement signal or a reduced measurement value, and a measurement signal or a measurement signal derived from a measurement signal is rejected as being erroneous if it passes from one or the other beyond a predetermined deviation tolerance range Measurement signals or measured values deviates downward. Inductive sensor (1) according to one of Claims 1 to 4 , characterized in that the control and evaluation unit (11) subjects the three measurement signals to an absolute comparison evaluation, for which purpose the measurement signals are examined for characteristic features and a measurement signal is rejected as faulty if a detected characteristic feature of a measurement signal of an expected characteristic feature deviates from the measurement signal. Inductive sensor (1) after Claim 5 , characterized in that as a characteristic feature of the measuring signal, the presence of a measuring signal is checked as such. Inductive sensor (1) after Claim 5 or 6 , characterized in that as a characteristic feature of the measuring signal, a sinusoidal profile of the measuring signal is tested analogously to a sinusoidal course of the fundamental wave of the driver signal. Inductive sensor (1) according to one of Claims 5 to 7 , characterized in that as a characteristic feature of the measurement signal, the freedom of limitation of the measurement signal is checked, wherein the limitation freedom means that the measurement signal is not limited due to metrological reasons limited. Inductive sensor (1) according to one of Claims 2 to 8th , characterized in that the control and evaluation unit (11) in the event of a rejection of at least one measuring signal or at least one derived measured value a Prompting a user to select a measurement signal to determine a conductivity reading, or to select a reading as a conductivity reading. Inductive sensor (1) according to one of Claims 1 to 9 , characterized in that the first coil (2) is connected to the first driver stage (9), that the second coil (3) via the switching device (14) to the first receiving stage (10) and the second driver stage (12) is connected wherein the switching device (14) is formed by a switch, such that in a first switching state of the switch the second coil (3) is connected to the first receiving stage (10) and in a second switching state of the switch, the second coil (3) is connected to the second driver stage (12), and that the third coil (4) is connected to the second receiving stage (13). Inductive sensor (1) according to one of Claims 1 to 9 , characterized in that the sensor electronics (8) comprises a first switching device (14), a second switching device (15) and a third switching device (16), wherein the three switching devices (14, 15, 16) of the control and evaluation unit ( 11) that the first coil (2) via the second switching device (15) with the first receiving stage (10) and the second receiving stage (13) is connected, such that in a first switching state of the second switching device (15) first coil (2) is connected to the first receiving stage (10) and in a second switching state of the second switching device (15), the first coil (2) is connected to the second receiving stage (13) that the second coil (3) via the first switching device (14) is connected to the second receiving stage (13) and the second driver stage (12) such that in a first switching state of the first switching device (14) the second coil (3) is connected to the second receiving stage (13) is connected and in a second switching state of the first switching device (14) the second coil (3) with the second driver stage (12) is connected, that the third coil (4) via the third switching device (16) with the second driver stage (12) and to the first driver stage (9), such that in a first switching state of the third switching device (16) the third coil (4) is connected to the second driver stage (12) and in a second switching state of the third switching device (16) the third coil (4) is connected to the first driver stage (9). Inductive sensor (1) according to one of Claims 1 to 9 , characterized in that the sensor electronics (8) comprises a first switching device (14), a second switching device (15) and a third switching device (16), wherein the three switching devices (14, 15, 16) of the control and evaluation unit ( 11) are controllable, that the first coil (2) via the first switching device (14) is connected to the first receiving stage (10) and to the second driver stage (12), such that in a first switching state of the first switching device (14) the first coil (2) is connected to the first receiving stage (10) and in a second Switching state of the first switching device (14) the first coil (2) to the second driver stage (12) is connected, that the second coil (3) via the second switching device (15) with the second receiving stage (13) and the first driver stage ( 9), such that in a first switching state of the second switching device (15) the second coil (3) is connected to the second receiving stage (13) and in a second switching state of the second switching device (15) the second coil (3) is connected to the first driver stage (9), that the third coil (4) via the third switching device (16) to the second receiving stage (13) and to the first driver stage (9) is connected such that in a first Schaltzusta and the third switching device (16), the third coil (4) is connected to the second receiving stage (13) and in a second switching state of the third switching device (16) the third coil (4) with the first driver stage (9) is connected. Inductive sensor (1) according to one of Claims 1 to 9 , characterized in that the sensor electronics (8) comprises a first switching device (14), a second switching device (15), a third switching device (16) and a fourth switching device (17), wherein the four switching devices (14, 15, 16, 17) of the control and evaluation unit (11) are controllable, wherein the first switching device (14) and the second switching device (15) are connected in series and the third switching device (16) and the fourth switching device (17) are connected in series in that the first coil (2) is connected via the first switching device (14) and the second switching device (15) to the first receiving stage (10) and to the second receiving stage (13), such that the first coil (2) in FIG a first switching state of the first switching device (14) and in a first switching state of the second switching device (15) is connected to the first receiving stage (10), in a first switching state of the first switching device (14) and in a second switching state of the second switching device (15) is connected to the second receiving stage (13) and in a second switching state of the first switching device (14) no connection of the first coil (2) with the first receiving stage (10) or the second receiving stage ( 13) is realized, that the second coil (3) via the first switching device (14) and the second switching device (15) to the first receiving stage (10) and the second receiving stage (13) is connected, such that in a first switching state the first switching device (14) no connection of the second coil (3) with the first receiving stage (10) or the second receiving stage (13) is realized that the second coil (3) in a second switching state of the first switching device (14) and in a first switching state of the second switching device (15) is connected to the first receiving stage (10) and in a second switching state of the first switching device (14) and in a second scarf tzustand of the second switching device (15) to the second receiving stage (13) is connected, that the second coil (3) via the third switching device (16) and the fourth switching device (17) with the second driver stage (12) and with the first driver stage (9) is connected such that the second coil (3) in a first switching state of the third switching device (16) and in a first switching state of the fourth switching device (17) is connected to the second driver stage (12) and in a first switching state the third switching device (16) and in a second switching state of the fourth switching device (17) to the first driver stage (9) is connected, and that in a second switching state of the third switching device (16) no connection between the second coil (3) and the the first driver stage (9) and the second driver stage (12) is realized that the third coil (4) via the third switching device (16) and the fourth switching device (17) with de r second driver stage (12) and with the first driver stage (9) is connected, such that in a first switching state of the third switching device (16) no connection between the third coil (4) and the first driver stage (9) and the second driver stage (12) is realized that the third coil (4) in a second switching state of the third switching device (16) and in a first switching state of the fourth switching device (17) with the second driver stage (12) is connected and in a second switching state of the third Switching device (16) and in a second switching state of the fourth switching device (17) with the first driver stage (9) is connected.

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