Impedance level sensor

Abstract

Impedance limit level sensor (100) with - a measuring probe (102) which can be influenced by a medium surrounding the measuring probe (102) in a measuring capacitance (110), wherein the measuring probe (102) has a measuring electrode (106) and a reference electrode (108) insulated from the measuring electrode (106), between which the measuring capacitance (110) is formed, - a measuring resonant circuit in which the measuring probe (102) is arranged as a capacitance-determining element, - an electronic unit (101) with a signal generator (103) for exciting the measuring resonant circuit, wherein the signal generator (103) is designed to excite the measuring resonant circuit with a frequency sweep, - an evaluation and control unit (105) for generating a measurement signal, which is connected to the electronic unit (101), characterized in that the electronic unit (101) has a current measuring device (402) for measuring a current at an input of the signal generator (103).

Description

[0001] The present invention relates to an impedance limit level sensor according to the preamble of claim 1.

[0002] Impedance level sensors are a well-established technology, used, for example, to measure limit or fill levels. Typical applications for detecting a predefined fill level include process containers such as process tanks, storage tanks, silos, or pipelines in the process industry. Impedance level sensors are frequently used as so-called limit switches, i.e., to determine whether a fill medium exceeds or falls below a specific fill level, the so-called limit level, in various liquids, as well as granulated and powdered bulk materials.

[0003] Other types of level switches or level sensors are also known, selected according to the application, process conditions, and properties of the filling medium. Besides impedance level sensors, sensors operating on the TDR (Time Domain Reflectometry) principle, vibration level sensors, or capacitive sensors are used. A switching command from the level switch can, for example, start or stop filling or emptying devices to prevent the respective process container from overflowing or emptying.

[0004] For the sake of simplicity, the terms impedance sensor, limit switch and limit level indicator are used interchangeably in the present application instead of the term impedance limit level sensor.

[0005] The prior art is known from US 2,573,172 A, US 6,361,396 B1 and US 4,589,281 A. The present invention is based on the prior art described below, which is considered to be generic.

[0006] A well-known impedance sensor 100 is in Fig. 1 shown. Fig. Figure 1 shows a simplified sectional view of the circuit blocks of an impedance sensor 100 according to the prior art. The impedance sensor 100 essentially consists of an electronic unit 101 and a measuring probe 102. In the present embodiment, the measuring probe 102 is designed as a series resonant circuit. A measuring capacitance 110 is formed between a measuring electrode 106 and a reference electrode 108, which is connected to the series resonant circuit via a discrete inductance 109.

[0007] The measuring electrode 106 is rotationally symmetrical about a longitudinal axis L of the impedance sensor 100 and is separated from a process chamber 90 by an insulation 107. The reference electrode 108 of the present impedance sensor 100 is also rotationally symmetrical about the longitudinal axis L. In this embodiment, the reference electrode 108 is designed as a tube that simultaneously forms part of a sensor housing. Viewed along the longitudinal axis L, the measuring electrode 106 is positioned in front of the tube and surrounded by the insulation 107 attached to the tube. The insulation 107 also forms a front closure of the housing.

[0008] Advantageously, the discrete inductance 109 is chosen such that a resonant frequency fres of the resonant circuit is established for a wide variety of media or coverage conditions (empty, full and dirty) between 100 MHz and 200 MHz.

[0009] Advantageously, the magnitude of a complex-valued impedance |Z| of this measuring resonant circuit, which varies with frequency, is analyzed between 100 MHz and 200 MHz. This means that the measuring resonant circuit is excited by a frequency generator 103 with a frequency sweep at frequencies between 100 MHz and 200 MHz, and a response signal (frequency response) of the measuring resonant circuit is detected with a frequency detector 104. If a medium is located in the vicinity of the measuring probe 102, the impedance behavior of the measuring resonant circuit changes; in particular, its resonant frequency fres, at which a minimum of the impedance is formed, shifts.

[0010] A frequency sweep is understood to be the sequential excitation with a plurality of successive frequencies within a frequency range, whereby the frequency range should ideally contain all possible resonant frequencies of the measuring resonant circuit.

[0011] The change in the impedance of the measuring resonant circuit is used for evaluation in an evaluation and control unit 105. Specifically, the frequency response is evaluated with respect to a frequency change Δf and a change in the amplitude of a minimum of the impedance Z, also referred to as amplitude change, and a switching command is generated from this. Alternatively, the evaluation could also be performed at a maximum of the impedance Z.

[0012] The impedance sensors according to the state of the art are used at process temperatures between -40°C and +115°C. These temperature differences have a strong influence on the behavior of the frequency generator 103 and the frequency detector 104, which can lead to measurement errors and thus to incorrect switching commands. This is considered a disadvantage of the state of the art.

[0013] In Fig. 2. The frequency responses of the impedance sensor 100 are listed as examples for the medium ketchup, according to the state of the art.

[0014] A first curve 200 shows the resonance behavior of a clean measuring probe 102. The magnitude of the impedance Z is shown as a function of the frequency f.

[0015] The behavior of a measuring probe 102 contaminated with ketchup residue is shown in a second curve 201 and that of a measuring probe 102 completely covered with ketchup is shown in a curve 202.

[0016] Switching commands (empty, full) are implemented by the evaluation and control unit 105, whereby, according to the state of the art, only the minima of the resonance curves are used for evaluation. These are evaluated with respect to a frequency change Δf and an amplitude change ΔZ. If the minimum of the resonance curve is located in a first range I, the evaluation and control unit 105 outputs the switching command "empty". However, if the minimum is located in a second range II, the switching command "full" is output. The two defined switching ranges I and II can be permanently programmed into the impedance sensor 100 at the factory or adjusted and modified by the customer. Ideally, the ranges should be defined in such a way that the standard settings are sufficient for as many different media as possible, since customer adjustments are time-consuming and therefore undesirable.

[0017] The level sensors 100 are used at process temperatures between -40°C and +115°C. As a result, the electronic unit 101 of the impedance sensor 100 is exposed to relatively high temperature fluctuations.

[0018] In Fig. Figure 3 shows two resonance curves of the impedance sensor 100 in the uncovered state of the measuring probe 102, where curve 300 describes the behavior at +25°C and curve 300' the behavior at +115°C. As can be seen from Fig. As can be seen in Figure 3, the minimum of curve 300 is at +25°C in region I and the minimum of curve 300' is at +115°C in region II, which corresponds to different switching states of the impedance sensor 100. Due to the temperature dependence of the electronic unit 101, temperature influences can therefore lead to incorrect switching decisions of the level sensor 100.

[0019] This is where the present invention comes in.

[0020] An impedance limit level sensor according to the invention, comprising a measuring probe which can be influenced by a medium surrounding the measuring probe in a measuring capacitance, wherein the measuring probe has a measuring electrode and a reference electrode insulated from the measuring electrode, between which the measuring capacitance is formed, a measuring resonant circuit in which the measuring probe is arranged as a capacitance-determining element, an electronic unit with a signal generator for exciting the measuring resonant circuit, wherein the signal generator is configured to excite the measuring resonant circuit with a frequency sweep, and an evaluation and control unit for generating a measuring signal, which is connected to the electronic unit, is characterized in that the electronic unit has a current measuring device for measuring a current at an input of the signal generator.

[0021] The current at the input of the signal generator, hereinafter also referred to as input current or a signal corresponding to the input current, is supplied to the evaluation and control unit as an input signal, which determines the resonant frequency from the input signal.

[0022] The present invention is based on the finding that the current draw of the signal generator increases significantly when the measuring resonant circuit is excited at its resonant frequency. This effect is used to determine the resonant frequency by measuring the current draw. For this purpose, it is assumed that the excitation frequency generated by the signal generator at the time of the increased current draw corresponds to the resonant frequency.

[0023] The evaluation and control unit can, for example, include an A / D converter that digitizes the measured current waveform and, together with the information about the control of the signal generator, which can be designed, for example, as a voltage-controlled oscillator (VCO), determines the resonant frequency and outputs a switching signal based on this.

[0024] By determining the resonant frequency based on a current measurement at the signal generator, it is no longer necessary to install a separate signal detector. Since, in the prior art, the signal detector is the component with the strongest temperature dependence in the measurement setup used there, omitting the signal detector and determining the resonant frequency based on the current draw of the signal generator can reduce the temperature dependence of the measurement setup many times over. Furthermore, current measurement can be implemented with few and very inexpensive components, thus significantly reducing the cost of the impedance limit sensor and the complexity of the measurement setup.

[0025] The current measuring device can be designed, for example, as a current sensor, particularly as a current-sensing resistor, or as a magnetic field sensor. A current sensor allows for simple current measurement. Such current measurement is particularly easy and cost-effective to implement using a current-sensing resistor. By using a resistor, for example, in the supply line of the signal generator, the current flowing through the resistor can be determined by measuring the voltage across the resistor.

[0026] If it is not possible to insert a resistor into the supply line of the signal generator, a magnetic field sensor can be used to determine the magnetic field generated by the current flowing in the supply line and to deduce the flowing current from this.

[0027] A method according to the invention for operating an impedance limit level sensor with a measuring probe which can be influenced by a medium surrounding the measuring probe in a measuring capacitance, wherein the measuring probe has a measuring electrode and a reference electrode insulated from the measuring electrode, between which the measuring capacitance is formed, with a measuring resonant circuit in which the measuring probe is arranged as a capacitance-determining element, an electronic unit with a signal generator for exciting the measuring resonant circuit, and an evaluation and control unit for generating a measuring signal which is connected to the electronic unit, wherein the measuring resonant circuit is successively excited with a plurality of different frequencies via the signal generator, is characterized in that a resonance point, in particular a resonance frequency of the measuring resonant circuit, is determined by determining a current at an input of the signal generator.

[0028] The present method is based on the finding that the current draw of the signal generator increases significantly when the measuring resonant circuit is excited at its resonant frequency. This effect is used to determine the resonant frequency or resonant point by measuring the current draw. It is assumed that the excitation frequency generated by the signal generator at the time of the increased current draw corresponds to the resonant frequency. The evaluation and control unit can, for example, include an analog-to-digital converter (ADC) that digitizes the measured current waveform and, together with information about the control of the signal generator (which can be, for example, a voltage-controlled oscillator, VCO), determines the resonant frequency and outputs a switching signal based on this.

[0029] The input current can be determined, for example, by measuring the voltage across a current-sensing resistor.

[0030] The present invention is explained in detail below with reference to exemplary embodiments and the accompanying figures. These show: Fig. 1. A block diagram of an impedance sensor according to the state of the art (already covered), Fig. 2 Measurement curves of the frequency response for impedance sensors uncovered with medium, contaminated and covered with medium (already treated), Fig. 3 Measurement curves of an uncovered impedance sensor according to the state of the art at different temperatures (already discussed), Fig. 4. A block diagram of an impedance sensor according to the state of the art. Fig. 5 a block diagram of an impedance sensor according to the present application and Fig. 6. the current waveform at the input of the signal generator for an impedance sensor according to Fig. 5.

[0031] In the figures, unless otherwise stated, the same reference symbols denote the same components with the same function.

[0032] The division into individual functional blocks is used below only for better clarity and illustration and may differ from the division used in individual cases.

[0033] To illustrate, in Fig. Figure 4 shows a block diagram of an impedance limit level sensor 100, hereinafter also referred to as impedance sensor 100 for simplicity. The impedance sensor 100 has a signal generator 103 and a signal detector 104, which are connected via a sensor block 303, representing the measuring probe 102, which in this case is designed as a series resonant circuit consisting of a measuring capacitor 110, which forms between a measuring electrode 106 and a reference electrode 108, and an inductor 109.

[0034] The signal generator 103 shows in the block diagram of the Fig. Figure 4 includes a digital-to-analog converter (DAC) 301 and a voltage-controlled oscillator 302. The voltage-controlled oscillator 302 is powered by a power supply 307 and receives a voltage ramp input from the DAC 301. The output of the voltage-controlled oscillator 302 provides a frequency ramp, a so-called frequency sweep. This frequency sweep is fed to the sensor block 303.

[0035] The sensor block 303 essentially consists of a series resonant circuit comprising a discrete inductor 109 and the measuring capacitance 110. The measuring capacitance 110 is formed from the measuring electrode 106, an insulator 107, and the reference electrode 108 (see figure). Fig. 1) wherein the measuring capacitance 110 is formed between the measuring electrode 106 and the reference electrode 108. The value of the measuring capacitance 110 is variable due to the filling medium to be monitored, which acts as a dielectric, so that a resonance frequency fres of the series resonant circuit changes depending on the coverage state of the sensor.

[0036] The resonance behavior of sensor block 303 is processed and analyzed by signal detector 104. Signal detector 104 is included in the block diagram of the... Fig. 4 according to the state of the art, an AM demodulator 304 and a downstream amplifier circuit 305. The demodulated resonance curve is finally digitized with an A / D converter 306 and the evaluation and control unit 105 (see Fig. 1) supplied.

[0037] The signal detector 104, and in particular the AM demodulator 304, exhibits a strong temperature dependence, which leads to a significant temperature shift in the measurement results and thus to the in Fig. This can lead to the 3 false detections shown.

[0038] In Fig. Figure 5 shows a block diagram of an impedance sensor 100 according to the present application.

[0039] In contrast to the block diagram of the Fig. 4 is according to Fig. 5 Only the signal generator 103 is connected to the sensor block 303. A signal detector 104, as is common in the prior art, does not exist.

[0040] To determine the resonant frequency fres or resonance point of the sensor block 303, a current sensor 402, configured here as a current-sensing resistor, is arranged between the power supply 307 and the voltage-controlled oscillator 302. Depending on the resonance behavior of the sensor block 303, the voltage-controlled oscillator 302 draws a different current I. A voltage U corresponding to the current I is dropped across the current-sensing resistor 402, which is measured by a measuring circuit 403. The measuring circuit 403 is connected to the input and output sides of the current-sensing resistor 402.

[0041] The measuring circuit 403 can be implemented, for example, with a discrete differential amplifier or a commercially available current measuring module.

[0042] As an alternative to determining the current via the current sensing resistor 402 and measuring the voltage, any other method of current measurement can also be used.

[0043] The resonance behavior of the sensor block 303 is clearly visible in the measured values ​​of the measuring circuit 403 and is digitized with an A / D converter 306.

[0044] Fig. Figure 6 shows the current waveform as measured by the current sensor 402 at the input of the voltage-controlled oscillator 302 with an impedance sensor 100 with a setup according to Fig. 5. The diagram shows the course of the current I[mA] versus the frequency f[Hz] for three different coverage states of the measuring probe 102 with ketchup as the filling medium.

[0045] The first curve 500 describes the resonance behavior of a clean measuring probe 102. If the measuring probe 102 is completely covered with ketchup, the resonance behavior is shown in the second curve 501. The behavior of a measuring probe 102 contaminated with ketchup is characterized by the third curve 502. The switching commands (empty, full) are implemented by the evaluation and control unit 105, whereby only the maxima of the current waveform are relevant for the evaluation. During current measurement, the frequency change Δf and the change in the measured current ΔI, i.e., the amplitude change, are also evaluated.

[0046] Out of Fig.Figure 6 shows that the resonance behavior of the sensor block 303, and thus the coverage state of the impedance sensor 100, can be determined from the current waveform at the input of the signal generator or at the voltage-controlled oscillator 302 just as well as with the prior art measurement method. However, compared to the prior art, the present setup has the advantage of exhibiting a significantly lower temperature dependence and being considerably more cost-effective to manufacture. Reference symbol list 90 Process room 100 Impedance Limit Level Sensor, Impedance Sensor 101 Electronic unit 102 Measuring probe 103 Signal generator 104 Signal detector 105 Evaluation and control unit 106 Measuring electrode 107 Insulation 108 Reference electrode housings 109 Inductance 110 measuring capacity 200 first curve 201 second curve 202 third curve 300 curve 300' curve 301 D / A converter 302 voltage-controlled oscillator 303 Sensor block 304 Demodulator 305 Amplifier Circuit 306 A / D converter 307 Energy supply 402 Current measuring device, current sensor 403 Measuring circuit 500 first curve 501 second curve 502 third turn I first area II second area A f frequency Δf frequency change free resonant frequency I Current intensity U voltage Z impedance ΔZ Impedance change

Claims

[1] Impedance limit level sensor (100) with - a measuring probe (102) which can be influenced by a medium surrounding the measuring probe (102) in a measuring capacitance (110), wherein the measuring probe (102) has a measuring electrode (106) and a reference electrode (108) insulated from the measuring electrode (106), between which the measuring capacitance (110) is formed, - a measuring resonant circuit in which the measuring probe (102) is arranged as a capacitance-determining element, - an electronic unit (101) with a signal generator (103) for exciting the measuring resonant circuit, wherein the signal generator (103) is designed to excite the measuring resonant circuit with a frequency sweep, - an evaluation and control unit (105) for generating a measurement signal, which is connected to the electronic unit (101), characterized by, that the electronic unit (101) has a current measuring device (402) for measuring a current at an input of the signal generator (103). [2] Impedance limit level sensor (100) according to claim 1, characterized by , that the current measuring device (402) is designed as a current sensor (402). [3] Impedance limit level sensor (100) according to claim 2, characterized by , that the current sensor (402) is designed as a current sensing resistor. [4] Impedance limit level sensor (100) according to claim 1, characterized by , that the current measuring device (402) is designed as a magnetic field sensor. [5] Method for operating an impedance level sensor (100) with - a measuring probe (102) which can be influenced by a medium surrounding the measuring probe (102) in a measuring capacitance (110), wherein the measuring probe (102) has a measuring electrode (106) and a reference electrode (108) insulated from the measuring electrode (106), between which the measuring capacitance (110) is formed, - a measuring resonant circuit in which the measuring probe (102) is arranged as a capacitance-determining element, - an electronic unit (101) with a signal generator (103) for exciting the measuring resonant circuit, - an evaluation and control unit (105) for generating a measurement signal, which is connected to the electronic unit (101), in which the measuring resonant circuit is excited via the signal generator (103) with a plurality of different frequencies, characterized by , that a resonance point of the measuring resonant circuit is determined by determining a current (I) at an input of the signal generator (103). [6] Method for operating an impedance limit level sensor (100) according to claim 5, characterized by , that the current (I) is determined by a voltage measurement across a current-sensing resistor.

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