Method for operating an inductive conductivity measuring device and respective inductive conductivity measuring device

Abstract

A method for operating an inductive conductivity measuring device that has a transmitting coil with an input and a receiving coil, the transmitting coil and the receiving coil being inductively coupled to one another by an electrically conductive medium. An electrical preset alternating signal is generated and fed to the input of the transmitting coil. The method for operating an inductive conductivity measuring device is improved in that a frequency of a preset alternating signal is varied in a frequency interval, in the frequency interval, a frequency-dependent minimum input impedance at the input of the transmitting coil is determined using a response alternating signal, a minimum frequency of the response alternating signal is determined at the minimum input impedance at the input of the transmitting coil, and a conductivity of the medium is determined using the minimum frequency of the response alternating signal.

Claims

1. A method for operating an inductive conductivity measuring device that has a transmitting coil with an input and a receiving coil inductively coupled to the transmitting coil by an electrically conductive medium, comprising the steps of: generating an electrical preset alternating signal and feeding the signal to the input of the transmitting coil, varying a frequency of the preset alternating signal in a frequency interval, in the frequency interval, determining a frequency-dependent minimum input impedance at the input of the transmitting coil using a response alternating signal, determining a minimum frequency of the response alternating signal at the minimum input impedance of the input of the transmitting coil, and determining a conductivity of the medium using the minimum frequency of the response alternating signal.

2. The method according to claim 1, wherein the transmitting coil, the receiving coil and the medium form an electrical resonant circuit having a resonant frequency, wherein, in determining the minimum input impedance and the minimum frequency, the resonant frequency of the resonant circuit is determined.

3. The method according to claim 1, wherein the conductivity measuring device has a measuring resistor at the input of the transmitting coil, wherein a measuring voltage is measured across the measuring resistor as the response alternating signal.

4. An inductive conductivity measuring device, comprising: a transmitting coil having an input, a receiving coil and a control unit, wherein the control unit is adapted to generate an electrical preset alternating signal and to supply the preset alternating signal to the input of the transmitting coil, and wherein, during operation, the transmitting coil and the receiving coil are inductively coupled to one another by an electrically conductive medium, wherein the control unit, during operation, is adapted to vary a frequency of the preset alternating signal in a frequency interval, to determine, in the frequency interval, a frequency-dependent minimum input impedance at the input of the transmitting coil using a response alternating signal (U.sub.M), to determine a minimum frequency of the response alternating signal at the minimum input impedance at the input of the transmitting coil, and to determine a conductivity of the medium using the minimum frequency of the response alternating signal.

5. The inductive measuring device according to claim 4, wherein the receiving coil has an input and the input is terminated with a terminating resistor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically depicts an embodiment of an inductive conductivity measuring device,

(2) FIG. 2 is a representation of a part of the electrical circuit of the inductive conductivity measuring device of FIG. 1 and

(3) FIG. 3 is a flow chart of an embodiment of a method for operating the inductive conductivity measuring device of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

(4) FIG. 1 shows the inductive conductivity measuring device 1. The inductive conductivity measuring device 1 has the hollow-cylindrical carrier 2, on which the transmitting coil 3 and the receiving coil 4 are arranged. The transmitting coil 3 and the receiving coil 4 are arranged on the hollow cylindrical carrier 2 by being wound around the hollow cylindrical carrier 2, wherein the transmitting coil 3 has the number N.sub.S of turns and the receiving coil 4 has the number N.sub.E of turns. Furthermore, the transmitting coil 3 has the input 5 and the receiving coil 4 has the input 6. The inductive conductivity measuring device 1 also has the control unit 7. The control unit 7 is designed to control the transmitting coil 3 and the receiving coil 4, which is why the control unit 7 is also electrically connected to the input 5 of the transmitting coil 3, on the one hand, and to the input 6 of the receiving coil 4, on the other hand.

(5) The hollow cylindrical carrier 2 having the transmitting coil 3 and the receiving coil 4 is immersed in the medium 8 and the inductive conductivity measuring device 1 is in operation. The medium 8 surrounds the hollow cylindrical carrier 2 and is also present in the inner space of the hollow cylindrical carrier 2. The medium 8 is electrically conductive and thereby couples the transmitting coil 3 and the receiving coil 4 inductively with one another. Since this is an abstracted schematic representation of the inductive conductivity measuring device 1, a housing that is usually present, which prevents direct contact of the transmitting coil 3 and the receiving coil 4 with the medium 8, is not shown here. By avoiding contact of the transmitting coil 3 and the receiving coil 4 with the medium 8, it is possible to use the inductive conductivity measuring device 1, in contrast to conductive conductivity measuring devices, in aggressive and corrosive media such as industrial waste water, seawater and acidic solutions without the functionality of the inductive conductivity measuring device 1 being limited. The housing also makes it suitable for hygienic applications in processes in the branches of food, beverage and pharmaceuticals.

(6) FIG. 2 shows a representation of a part of the electrical circuit of the inductive conductivity measuring device 1. The control unit 7 is designed to generate the electrical preset alternating signal U.sub.V and to supply the preset alternating signal U.sub.V to the input 5 of the transmitting coil 3, wherein the transmit alternating signal U.sub.S is then applied at the input 5 of the transmitting coil 3. For this, the control unit 7 has the alternating signal source 9 and the first amplifier 10. The alternating signal source 9 generates a preset alternating signal U.sub.V=1.5V+1.4V sin(cot), which is comprised of the constant voltage of 1.5V and the sinusoidal voltage of 1.4V modulated with the angular frequency co. The first amplifier 10 has a gain of 1 and thus serves as a buffer amplifier. The measuring resistor 11 is arranged between the amplifier 10 and the input 5 of the transmitting coil 3. The capacitor 12 is electrically connected in parallel to the transmitting coil 3. To set an operating point, the control unit 7 also has the DC signal source 13 and the amplifier 14. The DC signal source 13 generates a constant DC voltage of 1.5V and the second amplifier 14 has a gain of 1 and thus also serves as a buffer amplifier. The first amplifier 10 and the second amplifier 14 are fed with a DC voltage of 3V by the supply source 15. The terminating resistor 16 with the resistance R.sub.A is electrically connected in parallel with the receiving coil 4. The electrical resistance of the medium 8 between the transmitting coil 3 and the receiving coil 4 is R.sub.W. The conductivity of the medium 8 is generally determined from the electrical resistance value R.sub.W of the medium 8 and the geometry of the medium 8 between the transmitting coil 3 and the receiving coil 4.

(7) The transmitting coil 3 and the receiving coil 4 are inductively coupled by the electrically conductive medium 8as already mentioned. The inductive coupling takes place in that the transmit alternating signal U.sub.S fed into the transmission coil 3 generates eddy currents in the medium 8, and the eddy currents in the receiving coil 4 induce the receive alternating signal U.sub.E. Thus, the preset alternating signal U.sub.V causes the receive alternating signal U.sub.E via the transmit alternating signal U.sub.S.

(8) The control unit 7 is designed to vary the angular frequency of the preset alternating signal U.sub.V in a frequency interval, to determine, in the frequency interval, a frequency-dependent minimum input impedance at the input 5 of the transmitting coil 3 using the response alternating signal U.sub.M across the measuring resistor 11, to determine a minimum frequency of the response alternating signal U.sub.M at the minimum input impedance at the input 5 of the transmitting coil 3 and to determine a conductivity of the medium 8 using the minimum frequency of the response alternating signal U.sub.M. In this case, the alternating signal source 9 generates the preset alternating signal U.sub.V, the first amplifier 10 amplifies the preset alternating signal U.sub.V by a factor of 1 and the preset alternating signal U.sub.V causes the response alternating signal U.sub.M across the measuring resistor 11 in the form of a voltage, on the one hand, and on the other hand, the transmit alternating signal U.sub.S in the form of a voltage.

(9) In this embodiment of the inductive conductivity measuring device 1, the transmitting coil 3, the receiving coil 4, the medium 8 and the capacitor 12 form a resonant circuit with a resonant frequency. The resonant circuit is a function which, taken in isolation, has no spatial configuration. In this case, both the transmitting coil 3 and the receiving coil 4 have, in addition to inductive components, parasitic capacitive components that affect the resonant circuit. Since the resonant frequency corresponds to the minimum frequency, the minimum input impedance at the input of the transmitting coil 3 is given at resonance of the resonant circuit, i.e., at resonant frequency, wherein the minimum input impedance is accompanied by a maximum amplitude of the response alternating signal U.sub.M. The amplitude of the response alternating signal U.sub.M over the frequency interval is determined, for example, with an analog lock-in amplifier and then the maximum amplitude in the course of the amplitude over the frequency interval.

(10) Since the inductive conductivity measuring device 1 is in operation, the control unit 7 carries out the method shown in the flow chart in FIG. 3 with the following process steps.

(11) In a first method step 17, the electrical preset alternating signal U.sub.V is generated by the alternating signal source 9 and fed to the input 5 of the transmitting coil 3 and the angular frequency of the preset alternating signal U.sub.V is varied in a frequency interval.

(12) In a second method step 18, a frequency-dependent minimum input impedance at the input 5 of the transmitting coil 3 is determined in the frequency interval using the response alternating signal U.sub.M.

(13) In a third method step 19, a minimum frequency of the response alternating signal U.sub.M is determined at the minimum input impedance at the input 5 of the transmitting coil 3.

(14) In a fourth method step 20, a conductivity of the medium 8 is determined using the minimum frequency of the response alternating signal U.sub.M.