METHOD FOR OPERATING AN INDUCTIVE CONDUCTIVITY SENSOR AND RESPECTIVE INDUCTIVE CONDUCTIVITY SENSOR

Abstract

A method for operating an inductive conductivity sensor, wherein a first electric transmitter signal having a first signal frequency is generated and supplied to the transmitter coil, a first electric receiver signal is measured at the receiver coil and a first conductivity of the medium determined from the first electric receiver and first electric transmitter signals using a first calibration model. At least one further electric transmitter signal having a different signal frequency is generated and supplied to the transmitter coil, a further electric receiver signal is measured at the receiver coil and a further conductivity of the medium determined from the further electric receiver and electric transmitter signals using another calibration model, at least one conductivity difference is determined between each of the determined conductivities of the medium and when the at least one conductivity difference exceeds a threshold conductivity difference, the conductivity difference is signaled as an error.

Claims

1. A method for operating an inductive conductivity sensor, comprising: a transmitter coil and a receiver coil, the transmitter coil and the receiver coil being inductively coupled to one another by an electrically conductive medium, generating a first electric transmitter signal having a first signal frequency and supplying the first electric transmitter signal to the transmitter coil, measuring a first electric receiver signal at the receiver coil and determining a first conductivity of the medium determined using a first calibration model for the first signal frequency, generating at least one further electric transmitter signal having a signal frequency differing from the first signal frequency and supplying further electric transmitter signal to the transmitter coil, measuring a further electric receiver signal at the receiver coil and determining a further conductivity of the medium from the further electric receiver signal and the further electric transmitter signal using a further calibration model for the further signal frequency, determining at least one conductivity difference between the two determined conductivities of the medium, and when the at least one conductivity difference exceeds a threshold conductivity difference, a signal is issued indicating a sensor error has occurred.

2. The method according to claim 1, wherein that each of the calibration models determines a respective conductivity as a function of a quotient of the electric receiver signal and the electric transmitter signal, wherein the respective function has a linear interpolation or a polynomial.

3. The method according to claim 1, wherein the first electric transmitter signal and at least the at least one further electric transmitter signal are simultaneously generated.

4. The method according to claim 1, wherein the first electric transmitter signal and at least the at least one further electric transmitter signal are generated temporally directly consecutively.

5. The method according to claim 1, wherein at least two further electric transmitter signals are generated, wherein the signal frequencies of at least the first electric signal and the at least two further electric transmitter signals differ from one another, and wherein at least two conductivity differences are determined between each two of the determined conductivities of the medium.

6. The method according to claim 5, wherein an algebraic sign of the at least two conductivity differences is evaluated and wherein uniform algebraic signs indicate an error in the calibration models.

7. The method according to claim 6, wherein the algebraic signs of the at least two conductivity differences are evaluated and wherein non-uniform algebraic signs are signalized as an error in the mechanics of the conductivity sensor.

8. The method according to claim 1, wherein the method is carried out multiple times and the frequentness of the signalized error is indicated.

9. An inductive conductivity sensor, comprising: a transmitter coil, a receiver coil, a signal generator, a measuring unit, and a control unit, wherein the signal generator is constructed for generating electric transmitter signals and supplying the transmitter signals to the transmitter coil wherein the measuring unit is adapted for measuring electric receiver signals at the receiver coil, wherein, during operation of the conductivity sensor, the transmitter coil and the receiver coil are adapted to be inductively coupled to one another by an electrically conductive medium, wherein the control unit is adapted for controlling the signal generator and the measuring unit in such a manner that the signal generator generates a first electric transmitter signal having first signal frequency, the measuring unit measures a first electric receiver signal at the receiver coil and for determining a first conductivity of the medium from the first electric receiver signal and the first electric transmitter signal using a first calibration model for the first signal frequency stored in the control unit, wherein the control unit is additionally adapted for controlling the signal generator and the measuring unit in such a manner that the signal generator generates at least one further electric transmitter signal having a further signal frequency differing from the first signal frequency, for measuring a further electric receiver signal at the receiver coil and for determining a further conductivity of the medium from the further electric receiver signal and the further electric transmitter signal using a further calibration model for the further signal frequency stored in the control unit, wherein the control unit is adapted for determining at least one conductivity difference between each two of the determined conductivities of the medium, and wherein the control unit is adapted for signaling the occurrence of an error when the at least one conductivity difference exceeds a threshold conductivity difference stored in the control unit.

10. The inductive conductivity sensor according to claim 9, wherein the control unit is adapted for evaluating an algebraic sign of at least two conductivity differences and for indicating an error in the calibration models when the algebraic signs are uniform.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a schematic perspective view of an embodiment of an inductive conductivity sensor according to the invention and

[0023] FIG. 2 is a flowchart of an embodiment of the method in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] FIG. 1 shows an inductive conductivity sensor 1 that has a transmitter coil 2, a receiver coil 3, a signal generator 4, a measuring unit 5 and a control unit 6. The transmitter coil 2 and the receiver coil 3 are arranged on the hollow cylinder shaped carrier 7. In operation, the inductive conductivity sensor 1 and the carrier 7 with the transmitter coil 2 and the receiver coil 3 are immersed in the medium 8.

[0025] The medium 8 surrounds and is within the carrier 7. The medium 8 is electrically conductive, and thus, inductively couples the transmitter coil 2 and the receiver coil 3 to one another. What is not shown in the figure is the housing, which prevents direct contact between the conductivity sensor 1 and the medium 8.

[0026] The signal generator 4 is designed for generating electric transmitter signals u.sub.S and supplying the transmitter signals u.sub.S to the transmitter coil 2 and the measuring unit 5 is designed for measuring electric receiver signals u.sub.M at the receiver coil 3. Both the electric transmitter signals u.sub.S and the electric receiver signals u.sub.M are electric alternating voltages. The conductivity of the electrically conductive medium 8 is determined by the inductive conductivity sensor 1, in that first an electric transmitter signal u.sub.S having a signal frequency is generated and supplied to the transmitter coil. The electric transmitter signal is transmitted to the medium 8 by the transmitter coil 2 and the medium 8 conveys the transmitter signal to the receiver coil 3.

[0027] In the receiver coil 3, the transmitter signal conveyed by the medium 8 induces a receiver signal u.sub.S, which is measured by the measuring unit 5. The magnitude of the receiver signal u.sub.M in respect to the magnitude of the transmitter signal u.sub.S is a measure for the electric conductivity of the medium 8. A calibration model stored in the control unit 6 accordingly determines the conductivity of the medium 8 from the electric receiver signal u.sub.M and the electric transmitter signal u.sub.S. The calibration model describes the relation between, on the one hand, the electric conductivity of the medium 8, and on the other hand, the electric receiver signal u.sub.M and the electric transmitter signal u.sub.S for the frequency of the transmitter signal u.sub.S. A calibration model valid for a signal frequency other than the signal frequency used for determining the conductivity of the medium 8 provides conductivities that differ from the actual conductivity of the medium 8.

[0028] When the inductive conductivity sensor 1 is in operation, the conductivity sensor 1 carries out the method shown in the flowchart in FIG. 2 with the following method steps.

[0029] In the first method step 9, the control unit 6 controls the signal generator 4 and the measuring unit 5 in such a manner that the signal generator 4 generates a first electric transmitter signal u.sub.S,1 having a first signal frequency, the measuring unit 5 measures a first electric receiver signal U.sub.M,1 at the receiver coil 3 and the control unit 6 determines a first conductivity of the medium from the first electric receiver signal u.sub.M,1 and the first electric transmitter signal u.sub.S,1 using a first calibration model stored in the control unit 6 for the first signal frequency.

[0030] In the second method step 10, the control unit 6 controls the signal generator 4 and the measuring unit 5 in such a manner that the signal generator 4 generates a second electric transmitter signal u.sub.S,2 having a second signal frequency that is different from the first signal frequency, temporally directly following the first electric transmitter signal u.sub.S,1, and generates a third electric transmitter signal u.sub.S,3 having a third signal frequency differing from the first signal frequency and the second signal frequency temporally directly following the second electric transmitter signal u.sub.S,2. The measuring unit 5 measures a second electric receiver signal u.sub.M,2 caused by the second electric transmitter signal u.sub.S,2 and a third electric receiver signal u.sub.M,3 caused by the third electric transmitter signal u.sub.S,3. The control unit 6 determines a second conductivity of the medium from the second electric receiver signal u.sub.M,2 and the second electric transmitter signal u.sub.S,2 by using a second calibration model for the second signal frequency stored in the control unit 6 and determines a third conductivity of the medium from the third electric receiver signal u.sub.M,3 and the third electric transmitter signal u.sub.S,3 by using a third calibration model for the second signal frequency stored in the control unit 6.

[0031] Each of the three calibration models describes the correlation between, on the one hand, the electric conductivity of the medium 8, and on the other hand, the respective electric receiver signal u.sub.M and the respective electric transmitter signal u.sub.S for each one of the three frequencies. A calibration model that has been determined for a signal frequency other than signal frequency used for determining the conductivity of the medium provides conductivities deviating from the actual conductivity of the medium 8.

[0032] It holds true for each of the three calibration models that each conductivity is determined as a function of a quotient of the electric receiver signal u.sub.M and the electric transmitter signal u.sub.S. Thereby, the respective function has a polynomial. Thus, the polynomial is a function of the quotient.

[0033] In the third method step 11, the control unit 6 determines a first conductivity difference between the third conductivity and the second conductivity and a second conductivity difference between the second conductivity and the first conductivity of the medium 8. The conductivity differences are thus each determined between two temporally consecutively determined conductivities of the medium 8.

[0034] In the fourth method step 12, the control unit 6 signalizes, when at least one of the two conductivity differences exceeds a conductivity difference threshold stored in the control unit 6, this conductivity difference indicates the existence of an error. The conductivity difference threshold is a predeterminable, still tolerable conductivity difference between each two of the determined conductivities of the medium 8, the conductivity difference being dependent on the respective application of an inductive conductivity sensor 1.