Level measuring device

Abstract

Level measuring device (12) for determining the level of a medium (14) in a container (16), comprising: a signal generating device configured to generate an electrical signal comprising a step function or an impulse function; a measuring line (20) which is connected to the signal generating device, in particular electrically, and extends from a process connection (22) into the medium (14), wherein the measuring line (20) serves to conduct the electrical signal into the container (16) and towards the medium (14), wherein the electrical signal is at least partially reflected at the surface of the medium (14) and the reflected signal influences the electrical signal; and an evaluation unit (24) which is designed to determine the fill level of the medium (14) based on the influenced electrical signal.

Description

[0001] The present invention relates to a level measuring device for determining the level of a medium in a container.

[0002] Precisely determining the fill level in tanks is a common requirement in various industrial applications. Several technologies are available to address this challenge. For example, optical, radar-based, capacitive, or conductive sensors, tuning forks, floats, or wired time-of-flight (TOF) methods can be used to measure the fill level.

[0003] In wired TOF methods, an electrical pulse is sent through a cable that extends into the liquid in the tank. Since the cable has different effective impedances for the section that does not extend into the liquid and the section that does, the electrical pulse sent through the cable is reflected at the liquid surface, i.e., at the boundary between the two different impedances. Based on the travel time of the electrical pulse from a process port to the liquid surface and the travel time of the reflected signal from the liquid surface back to the process port, the tank level can be determined.

[0004] However, evaluating the pulse in wired TOF (Time-of-Flight) methods places high demands on the transmitting and receiving equipment, as it must provide high resolutions, often down to the picosecond range, which increases complexity and cost. Another disadvantage is that many excitation signals are often required sequentially to measure the time of flight, negatively impacting energy consumption. Furthermore, existing methods result in high emissions of high-frequency interference, as portions of the high-frequency pulse signal can be radiated by the measuring probe, which acts as an antenna. Therefore, such applications are often limited to use in metallic containers.

[0005] It is an object of the present invention to remedy the aforementioned disadvantages and to provide an improved level measuring device for determining the level of a medium in a container and a corresponding method.

[0006] This task is solved by the subject matter of the independent claims.

[0007] A first aspect of the invention relates to a level measuring device for determining the level of a medium in a container, comprising: a signal generating device configured to generate an electrical signal comprising a step function or an impulse function; a measuring line which is connected to the signal generating device, in particular electrically, and extends or can extend from a process connection into the medium, wherein the measuring line serves to guide the electrical signal into the container and in the direction of the medium, wherein the electrical signal is preferably reflected at least partially at the surface of the medium and the reflected signal influences the electrical signal; and an evaluation unit which is configured to determine the fill level of the medium based on the influenced electrical signal.

[0008] The invention is based on the understanding that the reflection of the electrical signal at the surface of the medium influences or alters the original electrical signal, and that this alteration is reflected in the form of the influenced electrical signal. This allows the transit time of the electrical signal, and thus the fill level of the medium, to be determined, particularly indirectly, based on the influenced electrical signal. Specifically, the fill level of the medium is thus determined without directly measuring or determining the transit time of the electrical signal to the medium's surface and back (Time of Flight). The influenced electrical signal is, in particular, the step response or impulse response of the system to the applied input signal.

[0009] When the transit time of the electrical signal is mentioned herein, unless otherwise stated, this refers to the transit time of the electrical signal from the process connection (or from the signal generation device) to the surface of the medium and the transit time of the reflected signal from the surface of the medium back to the process connection (and thus to the evaluation unit).

[0010] The signal generation device is, for example, a voltage source that generates a voltage having at least a partial step or pulse function and feeds this voltage into the level measuring device. The signal generation device is particularly capable of rapidly switching on or feeding the electrical signal into the system, i.e., with a rise time shorter than 10 ns, 1 ns, or 100 ps. This ensures, for example, that the level measuring device is capable of performing measurements with a corresponding level of accuracy and resolution. A rapid rise of the electrical signal can also be generated, for example, by abruptly feeding a constant electrical signal into the system by switching a switch. The switch can, for instance, be integrated into the signal generation device and switch the electrical signal on and off.In other words, the switch connects the signal generated by the signal generation device to a remaining part of the level measuring device and then disconnects it again, so that a corresponding electrical signal is generated.

[0011] The electrical signal or voltage is applied to or transmitted via the measuring lead. The measuring lead can be any suitable conductor, particularly a cable, capable of transmitting electrical signals, and must have a corresponding measuring lead impedance, which depends in particular on the permittivity of the surrounding medium. For example, the measuring lead can also comprise several interconnected metal parts that are electrically connected to a metal probe extending into the medium. The metal parts can, for example, be integrated into a wall of the container or arranged at a distance from the wall.

[0012] The impedance of the measuring line can also depend on its geometry, which is preferably assumed to be constant here. The impedance of the measuring line can be described as the characteristic impedance of the transmission path, e.g., a cable, as a function of the surrounding material properties along this path. It can change if the medium—and in particular its permittivity—in which the signal propagates changes, for example, when transitioning from air to a liquid medium. The electrical signal is carried via the measuring line from the process connection towards the medium. The measuring line is preferably electrically insulated from both the process connection and the medium.The process connection is, for example, a mechanical interface through which the level measuring device is securely and tightly mounted to the container and through which the measuring cable is connected to the rest of the level measuring device. The electrical signal thus travels along the measuring cable towards the medium. Since the measuring cable impedance inside the medium typically differs from the measuring cable impedance outside the medium, an impedance jump occurs during the transmission of the electrical signal at the medium's surface, i.e., the interface between the medium and air, or at the point where the measuring cable penetrates the medium's surface. This impedance change causes a reflection of the electrical signal at the medium's surface, with the reflected signal influencing the original electrical signal.In the following, the impedance of the measuring lead for the section extending into the medium is referred to as the medium impedance, while the impedance of the measuring lead for the section not extending into the medium is referred to as the reference impedance. The strength of the reflection depends in particular on the impedance difference between the reference impedance and the medium impedance. A greater impedance difference leads to a correspondingly stronger reflection. In particular, the reflected signal is superimposed on the originally generated electrical signal, which can attenuate or amplify the original electrical signal. Amplification is expressed, for example, by an increase in the amplitude of the electrical signal, while attenuation is expressed by a decrease in the amplitude of the electrical signal.An attenuation of the electrical signal occurs, particularly when the reference impedance is greater than the medium impedance. Conversely, an amplification of the electrical signal occurs, particularly when the reference impedance is less than the medium impedance. Specifically, reflection results in a superposition of the reflection with the excitation signal, i.e., the electrical signal, whereby the reflection exhibits the same or opposite polarity to the excitation signal depending on the magnitude of the impedance change and is superimposed accordingly.

[0013] The reflected signal thus modifies the original electrical signal, particularly its amplitude. The time of this change in the altered or influenced electrical signal can therefore be used to determine, at least indirectly, the propagation time of the electrical signal and thus the fill level of the medium. Since the strength of the reflection, and therefore the effect of the reflected signal on the electrical signal, depends on the impedance difference between the reference impedance and the medium's impedance, the reference impedance can be determined based on the medium's impedance, which depends on the known medium. In particular, the reference impedance can be set such that the strongest possible reflection of the electrical signal at the medium's surface is achieved, in order to facilitate the detection of the resulting change in the electrical signal.

[0014] The evaluation unit is thus able to determine the fill level of the medium based on the affected electrical signal. The evaluation unit is primarily used for processing and analyzing signals such as the affected voltage signal, particularly in real time. It can process both analog and digital signals. Additionally, various calculations or comparisons can be performed using predefined algorithms or stored rule sets to detect deviations, make decisions, or send further control commands to other system components. The evaluation unit can therefore also function as a control and evaluation unit. For example, the evaluation unit can comprise a microcontroller, a digital signal processor (DSP), a programmable logic controller (PLC), or a computer.By integrating storage and processing functions, the evaluation unit can also store data and make it available for analysis.

[0015] Advantageously, the variable transit times in the electronics of the level measuring device do not influence the determination of the level or the transit time, since, according to the invention, the level is determined based on the influenced electrical signal, wherein the first edge of the electrical signal represents the start time and the reflected signal influences the electrical signal in such a way that the time of change can be evaluated as the end of the transit time measurement. A further advantage of the invention is that a level determination can be carried out with significantly fewer transmission pulses than according to the prior art. In the simplest case, the level can be determined according to the invention using a single transmission pulse.This leads, on the one hand, to an improvement in the energy consumption of the level measuring device and, on the other hand, to a reduction in sensor-induced emissions, thereby enabling compliance with regulatory requirements regarding EMC compatibility. This allows the level measuring device to be used even without a metallic container. Furthermore, the level measuring device according to the invention achieves fast response times. In contrast to conventional wired time-of-flight (TOF) methods, the level measuring device according to the invention is simpler and more cost-effective to implement. The level measuring device according to the invention is particularly suitable for measuring greater depths with an accuracy of 1 to 10 cm. For example, the water level at shipping locks or reservoirs can be measured.But more compact applications are also possible in a rainwater storage system or for measuring the cooling water level in an engine compartment.

[0016] Further embodiments of the invention can be found in the description, the dependent claims and the drawings.

[0017] According to one embodiment, the evaluation unit is configured to determine a substitution quantity based on the influenced electrical signal. This substitution quantity is proportional to the transit time of the electrical signal from the process connection to the medium surface and back, and / or to the distance between the medium surface and the process connection. Based on this substitution quantity, the unit determines the fill level of the medium, preferably not corresponding to the transit time of the electrical signal. The substitution quantity can be different from the transit time of the electrical signal. Thus, the substitution quantity can be used to determine the fill level of the medium instead of the transit time of the electrical signal. However, the transit time of the electrical signal can also be determined, particularly indirectly, based on the substitution quantity, thereby allowing conclusions to be drawn about the fill level of the medium.The substitute variable can, for example, be a voltage value derived from or determined based on the influenced electrical signal. The determined value or magnitude of the substitute variable, such as a measured voltage value, can then be assigned to a fill level value using a mapping table or other suitable methods, which can then be defined as the determined fill level of the medium. The mapping table can, in particular, be empirically determined beforehand or calculated based on the known system properties.

[0018] According to a first embodiment, the influenced electrical signal has a pulse shape, wherein the evaluation unit is configured to generate a rectangular signal based on the pulse width of one, in particular a first, pulse of the influenced electrical signal, the width of which essentially corresponds to the pulse width of the influenced electrical signal, and to determine the substitution quantity based on the rectangular signal. As already described above, the reflected signal affects the electrical signal and, in particular, causes a decrease in the amplitude of the electrical signal. The influenced electrical signal thus has a pulse shape which is characterized at least by a rising edge when the electrical signal is generated and by a falling edge when the reflected signal is received.In particular, the pulse width corresponds to the transit time of the electrical signal from the process connection to the medium surface and back.

[0019] Based on the influenced electrical signal, a square wave signal can be generated, specifically an electrical one. Its rising edge indicates the application of the electrical signal, and its falling edge indicates the detection of the reflected signal. In other words, the influenced electrical signal is converted into a square wave signal that represents only the width of the first pulse. The width of the square wave signal thus corresponds precisely to the propagation time of the electrical signal from the process connection to the medium surface and back. The square wave signal contains, in particular, the propagation time information necessary for determining the fill level of the medium. Instead of extracting this propagation time information in a complex and costly manner using a dedicated ADC, the square wave signal can be used to determine the substitution parameter, which allows for a simpler determination of the fill level.

[0020] According to one embodiment, the evaluation unit is configured to generate the square wave signal based on the influenced electrical signal using a threshold filter, wherein the square wave signal essentially assumes a value of zero when the influenced electrical signal is less than a predetermined threshold, and assumes a predefined non-zero value when the influenced electrical signal is greater than the predetermined threshold, wherein the evaluation unit is configured to perform an integration of the square wave signal using an integrator component in order to determine an integration value, and to determine the fill level of the medium based on the integration value.The square wave signal exhibits the predefined value only if the influenced electrical signal is greater than the specified threshold, thus ensuring that only the time interval before a decay caused by the reflected signal and after an initial rise of the electrical signal is captured. In particular, the specified threshold is chosen such that it is greater than the signal amplitude after a decay of the influenced electrical signal caused by the reflected signal.

[0021] To determine the fill level of the medium, a substitution variable can be calculated instead of the transit time of the electrical signal. This substitution variable is proportional to the transit time of the electrical signal and thus to the distance to be determined. For this purpose, the evaluation unit is designed to perform an integration of the square wave signal using the integrator component to determine an integration value that is proportional to the transit time of the electrical signal. Based on this integration value, the fill level of the medium can then be determined. In this case, the integration value is used as the substitution variable.

[0022] The threshold filter can, for example, comprise a comparator, a diode, or a digital gate. The integrator component can further comprise a capacitor and / or an operational amplifier. The following explanation of the level measuring device's operation is purely exemplary, using a comparator as the threshold filter and a capacitor as the integrator component; however, the explanations also apply accordingly to a diode or operational amplifier.

[0023] As soon as the comparator receives the influenced electrical signal as a voltage signal, it compares this signal, for example, with a predefined reference voltage corresponding to a predefined threshold, and outputs the square wave signal described above, which is also a voltage signal. Based on this square wave signal, a current source can then be driven to charge the capacitor. The capacitor is thus charged only for the period during which the square wave signal has a non-zero value. In other words, the capacitor is charged for a duration corresponding to the propagation time. The voltage drop across the capacitor is therefore proportional to the propagation time of the electrical signal. This voltage drop across the capacitor can then be sampled, for example, using an ADC.The voltage of the capacitor can be measured, and the fill level of the medium can be determined based on the measured voltage value. Specifically, a corresponding fill level value can be assigned to the measured voltage value using a mapping table, which can then be defined as the determined fill level. The determination of the medium's fill level is thus based primarily on a capacitor voltage measurement. Based on the voltage measurement, the pulse width and therefore the propagation time of the electrical signal can be determined indirectly or directly. Furthermore, the evaluation unit can be configured to discharge the capacitor, particularly completely, until the next measurement, using a discharge circuit or a resistor connected in parallel with the capacitor.

[0024] According to one embodiment, the evaluation unit is configured to determine the predetermined threshold value as a function of the impedance difference between a reference impedance, which represents the impedance of the measuring line for the section that does not extend into the medium, and a medium impedance, which represents the impedance of the measuring line for the section that does extend into the medium. This follows in particular from the fact that the strength of the reflection of the electrical signal, and thus the influence of the reflected signal on the electrical signal, depends on the impedance difference between the medium impedance and the reference impedance. If the effect of the reflected signal on the signal amplitude of the electrical signal, i.e.,If the magnitude of the electrical signal decay due to the reflected signal, which is defined by the impedance difference, is known, the threshold can be adjusted so that the square wave signal captures or replicates the corresponding rising and falling edges of the affected electrical signal as precisely as possible. The specified threshold must be set such that it is greater than the signal amplitude of the affected electrical signal after a decay caused by the reflected signal. This ensures, in particular, that signal values ​​after the decay are disregarded when determining the width of the square wave signal.

[0025] According to one embodiment, the evaluation unit is configured to continuously adjust, or at predetermined time intervals reduce, the predefined threshold of the threshold filter. The predefined threshold can therefore be a variable threshold. By changing the threshold, the shape of the affected electrical signal can be determined, and multiple reflection points can be identified based on this shape. In such a case, for example, several electrical signals could be generated sequentially by the signal generation device, with a different predefined threshold being set for each generated electrical signal. In this case, a deliberate deterioration of the response time of the level measuring device could be accepted in order to detect multiple reflection points.

[0026] According to one embodiment, the evaluation unit comprises several threshold filters with different threshold values, particularly for detecting a sequence of different reflected signals. For example, the electrical signal can be reflected at different locations on the surface of the medium, generating several different reflected signals. Alternatively, multiple media can be present within the container, so that the electrical signal is reflected at the surface of each medium, resulting in a distinct reflected signal. For example, a first medium and a second medium can be present in the container, the first medium having a lower density than the second medium, so that the first medium "floats" on top of the second medium.The first medium and the second medium can have different impedances, so that the strength of the reflection of the electrical signal is different for the two media, which in turn can be detected in the affected electrical signal.

[0027] The level measuring device is therefore particularly multi-echo capable, meaning it can detect and process multiple reflections or echoes from a single electrical signal or pulse. As already described, one advantage of a multi-echo capable system is that multiple reflections from different surfaces or materials within a container can be detected. This allows for the acquisition of information about different fill levels or layer boundaries within a single measurement or within several consecutive measurement processes. Furthermore, the reliability of the level measurement can be improved by detecting multiple reflected signals, as the additional information can be used to obtain a more precise measurement.Additionally or alternatively, in a first step a number of media can be determined and then in a second step the comparator can be set so that only the distance to a selected medium is measured.

[0028] According to one embodiment, the evaluation unit is configured to sample an integration signal generated by the integration of the square wave signal using an analog-to-digital converter (ADC) with a sampling frequency of less than 10 MHz, less than 1 MHz, less than 100 kHz, less than 10 kHz, or less than 1 kHz, in order to determine the integration value. For example, if the integration of the square wave signal is performed using a capacitor by charging the capacitor for a predetermined time period corresponding to the width of the square wave signal, the voltage drop across the capacitor can be sampled using an ADC to determine the corresponding voltage value, which is proportional to the propagation time of the electrical signal. The ADC can, in particular, be integrated into a microcontroller or a system-on-a-chip (SoC).Advantageously, the fill level is thus determined not by using expensive and energy-intensive evaluation components, but can be carried out using simple ADCs, which, due to the simplicity of the signal, have reduced requirements. This is primarily because no directly time-resolved methods are used to determine the fill level.

[0029] According to one embodiment, the measuring line comprises a coaxial cable. Coaxial cables have the advantage of possessing a defined characteristic fundamental impedance, typically between 50 and 75 ohms, which is particularly constant along the entire cable length. This is important for determining the fill level because the reflected signal is not influenced by the characteristic properties of the measuring line and thus depends primarily on the impedance difference between the reference impedance and the medium impedance. In other words, the constant fundamental impedance of the coaxial cable ensures that no additional reflections occur due to fluctuations in the fundamental impedance. The fundamental impedance is, for example, the impedance of an electrical component without the influence of the surrounding medium, e.g., in an ideal vacuum or an empty space.Another advantage of coaxial cables is that the signal is protected from electromagnetic interference (EMI). The consistent properties of the coaxial cable ensure precise and reliable transmission of both the electrical and reflected signals.

[0030] According to one embodiment, the measuring line does not include a coaxial cable. In this case, the level measuring device can, for example, be tolerant of tolerance jumps in the process connection, i.e., signal deviations that can be caused, for example, by inconsistent transmission of the electrical signal or the reflected signal. Advantageously, using a measuring line that does not include a coaxial cable reduces costs. However, it should be noted that the affected electrical signal, and thus its processing, becomes more complex. After the electrical signal is applied, i.e., after an initial rising edge of the electrical signal, a reflection occurs from the measuring line with the same polarity as the electrical signal, resulting in an increase in the amplitude of the affected electrical signal.After receiving the signal reflected from the medium's surface, a drop in the affected electrical signal occurs, as described above. To better evaluate the resulting affected electrical signal using a comparator circuit, it can be differentiated beforehand. If the system is not capable of multiple echoes, the system's impedance must be designed to prevent reflections of the reflected signal back towards the process connection, as these multiple reflections would impair the simple evaluation of the affected electrical signal using the comparator circuit.

[0031] According to one embodiment, the electrical signal comprises a plurality of rectangular pulses, and the influenced electrical signal comprises a plurality of pulses, particularly of the same type. The evaluation unit is configured to determine a DC component of the influenced electrical signal and, based on this DC component, to determine the fill level of the medium. The individual rectangular pulses can, in particular, be assigned to different measurements. The width of a rectangular pulse can, for example, be designed such that the pulse width of the corresponding pulse of the influenced electrical signal can be uniquely determined. In other words, the individual rectangular pulses are used as separate step functions with which the system is subjected for a predetermined time period, the predetermined time being defined by the width of the respective rectangular pulse.The affected electrical signal can be further processed using a comparator circuit, as described above. In this case, the output signal of the comparator circuit would comprise a series of square wave pulses, with each square wave corresponding to a specific pulse of the affected electrical signal and representing a specific measurement. To determine an average of the measurements, the DC component of the comparator's output signal can be measured, for example, using a low-pass filter. A particularly advantageous implementation involves using a simple low-pass filter whose output signal is digitized using an ADC, allowing the DC component to be determined digitally.For example, the DC component can be determined by performing a Fourier transform of the digitized signal at 0 Hz, which essentially corresponds to a moving average of the time signal. The average can then be calculated using simple, cost-effective hardware.

[0032] Alternatively, several individual level measurements can be taken according to one of the above embodiments and an average of the individual levels determined can be calculated to determine a final level value.

[0033] According to one embodiment, the signal generation device is configured to generate the rectangular pulses of the electrical signal at variable time intervals. This is intended to ensure that coherent EMC interference, which cannot be suppressed by multiple measurements and subsequent averaging, is compensated. In other words, the generation of the rectangular pulses does not occur according to a fixed transmission period, but can be arbitrary or according to another non-periodic pattern. For example, the time interval between two successive rectangular pulses can be randomly selected from a time interval that includes interval values ​​between a minimum and a maximum time interval. In the proposed evaluation, or rather...Processing an aperiodic signal using a comparator is particularly easy, as the associated evaluations can be implemented asynchronously and do not have a fixed clock reference to the electrical signal. Consequently, the system's emissions can be further reduced.

[0034] According to another embodiment, the reference impedance is larger, in particular 5, 10, 12, or 15 times larger, than the medium impedance. In particular, the reference impedance can be matched to the medium impedance in such a way that the electrical signal is reflected as strongly as possible at the medium's surface, so that the corresponding reflection can be easily detected from the affected electrical signal. If the medium is water, the reference impedance can be, for example, 50 ohms, 60 ohms, or 75 ohms.

[0035] In one embodiment, the measuring lead is short-circuited at the end facing the medium. The short circuit is located within the medium itself, such that in the event of incomplete reflection at the medium boundary, a residual portion or wave of the electrical signal is reflected by the short circuit. If the affected electrical signal shows an influence solely due to the reflection caused by the short circuit, and if the influence of this reflection on the electrical signal is known, it can be determined that the fill level is below the detection range. Furthermore, based on the affected electrical signal, a sensor defect can be ruled out.

[0036] According to one embodiment, the measuring line, particularly as a coplanar line or microstrip line, is arranged on the same circuit board as the evaluation unit and the signal generation device. The level measuring device can therefore be very compact and manufactured cost-effectively. This is particularly advantageous when used in smaller tanks.

[0037] The level measuring device can be designed in such a way that the electronic components, with the exception of the measuring line, are protected from water, for example by means of potting, and housed in a suitable enclosure, so that the level measuring device can be housed directly in the container.

[0038] According to one embodiment, the electrical signal is carried with a substantially constant fundamental impedance. The level measuring device can, for example, be designed such that the fundamental impedances of the electrical components are matched. In particular, the electrical signal or the reflected signal is carried along the measuring line with a substantially constant fundamental impedance. As already described, this can be achieved using a coaxial cable.

[0039] It should be noted that any combination of the above embodiments is possible, unless explicitly excluded.

[0040] The invention is described below by way of example only, with reference to the drawings. The drawings show: Fig. 1 a schematic representation of a level measuring device, Fig. 2 a detailed schematic representation of a level measuring device, Fig. 3 a circuit diagram of a level measuring device (a), an influenced electrical signal for different levels (b) and the associated output voltages (c), Fig. 4 a circuit diagram of a level measuring device (a), an influenced electrical signal for different levels (b), an output signal of a first comparator for the different levels (c), an output signal of a second comparator for the different levels (d) and the associated output voltages (e), Fig. 5 a representation of the step response for a level measuring device with a coaxial cable (a) and for a level measuring device without a coaxial cable (b) as well as a representation of an impulse response (c), Fig. 6 a schematic representation of an evaluation unit for processing multiple level measurements, and Fig. 7 an embodiment of the level measuring device.

[0041] Fig. Figure 1 shows a schematic representation of a level measuring device 12 for determining the level of a medium 14 in a container 16. The level measuring device 12 comprises a signal generation device in the form of a voltage source 18, which is configured to generate an electrical signal, i.e., a voltage signal, comprising a step function or a pulse function. The level measuring device 12 further comprises a measuring line 20, which is electrically connected to the voltage source 18 and extends from a process connection 22 into the medium 14, wherein the measuring line 20 serves to conduct the voltage signal into the container 16 and towards the medium 14, the voltage signal is reflected at least partially at the surface of the medium 14, and the reflected signal influences the voltage signal.The level measuring device 12 also includes an evaluation unit 24, which is designed to determine the level of the medium 14 based on the influenced voltage signal.

[0042] Fig. Figure 2 shows a detailed schematic representation of the individual components of the level measuring device 12. The voltage source 18 of the level measuring device 12 generates a voltage signal in the form of a step function. The voltage signal is applied to the measuring line 20 via a first resistor 26 and the process connection 22. In particular, the voltage wave of the voltage signal generated by the voltage source 18 travels via the first resistor 26 to the process connection and via a second resistor 28 to a comparator 30. A discontinuity in the impedance profile can occur at the process connection 22 if the impedance of the circuit at the interface to the measuring line differs from the impedance of the measuring line 20, especially the reference impedance 46. Preferably, the impedance of the circuit and the reference impedance 46 are matched so that there is no discontinuity or impedance step at the process connection 22.The voltage wave thus travels from the process connection 22 via the measuring line 20 to the medium 14 and is reflected by the medium surface upon reaching the medium surface, so that a reflected signal is created which travels from the medium surface towards the process connection 22.

[0043] The reflection of the voltage wave is caused by the impedance difference between the impedance of the measuring line outside the medium 14, i.e., the reference impedance 46, and the impedance of the measuring line 14 inside the medium, i.e., the medium impedance 48. The stronger, or larger, the impedance difference, the stronger the reflection of the voltage wave. The reflection of the voltage wave, i.e., the reflected signal, affects the voltage signal by causing a drop in the voltage signal. The affected voltage signal, which is present particularly at the input of the comparator 30, is further processed by the comparator 30 to extract information from it that can be used to determine the fill level of the medium 14 in the container 16.For this purpose, the comparator 30 compares the influenced voltage signal with a predefined reference voltage, which in this case is generated by a digital-to-analog converter (DAC) 32, which can be implemented as part of a microcontroller 34. The reference voltage of the comparator 30 can, in particular, be variable and can be continuously reduced or increased by means of the DAC 32 or at predefined time intervals.

[0044] As previously described, the reflected signal affects the voltage signal by causing a voltage drop. The magnitude of this voltage drop depends on the impedance difference between the reference impedance 46 and the medium impedance 48. For example, if the reference impedance 46 is 10 times greater than the medium impedance 48, this results in a drop of up to 90% in the original voltage. Based on this knowledge, the reference voltage of the comparator 30 can be set such that the output signal of the comparator 30, which is a square wave, reflects both the rising edge of the affected voltage signal and the falling edge of the affected voltage signal caused by the reflected signal. For example, if the reflected signal causes a voltage drop of 90%, the reference voltage can be set to a value of 20% of the initial voltage signal.The width of the square wave signal output by comparator 30 corresponds to the propagation time of the voltage wave from the process connection to the medium surface and back. The output signal of comparator 30 displays a voltage value V for all values ​​of the influenced voltage signal that are greater than the specified reference voltage. high and for all values ​​of the influenced voltage signal that are smaller than the specified reference voltage, a voltage value V is assigned. lowor a voltage value of 0 V. Based on the output signal of the comparator 30, a current source 38 is then controlled via a switch 36, which charges an output capacitor 40 for the duration defined by the square wave signal. The voltage drop across the output capacitor 40 is proportional to the transit time of the voltage wave from the process connection 22 to the medium surface and back, or inversely proportional to the fill level of the medium 14 in the container 16. Accordingly, the fill level of the medium 14 in the container 16 can be determined based on a measurement of the voltage of the output capacitor 40. The time measurement, i.e., the measurement of the transit time of the voltage wave, has thus been converted into a voltage measurement. The measurement or detection of the voltage of the output capacitor 40 can be carried out, for example, by a corresponding ADC 62 (not shown) of the microcontroller 34.

[0045] Fig. Figure 3(a) shows a circuit diagram of a level measuring device 12. The voltage signal from the voltage source 18 is abruptly applied to the measuring line 20 via the first resistor 26 by means of a fast switch 42. The switch 42 is moved to the closed and open positions by a pulse generator 44. By alternately switching the switch 42 on and off, a sequence of square wave pulses can be generated. As described above, a voltage wave of the voltage signal can be guided via the measuring line 20 to the medium 14, whereby the measuring line 20 has a corresponding reference impedance 46 outside the medium 14, whereas the measuring line 20 has a corresponding medium impedance 48 inside the medium 14. Due to the impedance difference between the reference impedance 46 and the medium impedance 48, the voltage wave is reflected back to the process connection, which causes a voltage drop in the voltage signal.The influenced voltage signal can be used to charge an output capacitor 40 and thereby determine the fill level of the medium 14 in the container 16. In the present embodiment, a diode 50 is used instead of the previously described comparator 30 to define a voltage threshold and to generate the square wave signal, which has a width corresponding to the propagation time of the voltage wave.

[0046] Fig. Figure 3(b) shows the influenced voltage signal tapped at the input of diode 18. As can be seen from the graph, after an initial steep rise, the voltage signal drops back down to a value approximately 20% of the initial voltage value, with the voltage drop being caused by the reflected signal. Further drops and rises of the influenced voltage signal, which are shown in the graph in Fig. The effects described in section 3(b) are due to further reflections of the voltage wave. However, to determine the propagation time of the voltage wave and thus the fill level of the medium, the time of the first falling edge is particularly relevant. Accordingly, diode 50 is adjusted such that the reference voltage is greater than 20% of the output voltage value of voltage source 18, so that the resulting square wave signal has a width equal to the width of the influenced voltage signal from a first rising edge to a first falling edge. In this case, the reference voltage of the diode is 30% of the switch-on voltage of voltage source 18. The influenced voltage signal is, according to Fig. 3(b) is shown for different fill levels. The fill levels comprise a first, a second, and a third fill level, where the first fill level is higher than the second fill level and the second fill level is higher than the third fill level. As shown in Fig. As can be seen in section 3(b), the voltage signal for the first level drops earliest due to the shorter transit time of the voltage wave. Accordingly, the voltage signal for the second level drops earlier than for the third level.

[0047] Fig. Figure 3(c) shows the output capacitor voltage curve for different fill levels. As can be seen, the voltage of the output capacitor 40 increases linearly for the duration during which the square wave signal maintains a voltage value V. highexhibits this behavior. As soon as the square wave signal drops back to 0V, the output capacitor 40 is no longer charged and remains at the achieved voltage level. As can be seen from... Fig. As can be seen from 3(c), the voltage drop across the output capacitor 40 is higher for lower fill levels than for higher fill levels. This is because the charging time of the output capacitor ideally corresponds to the transit time of the voltage wave and is therefore longer for lower fill levels than for higher fill levels.

[0048] Fig. 4(a) illustrates a circuit diagram of a level measuring device 12. In contrast to the circuit diagram of the Fig. 3(a) The level measuring device 12 comprises a first comparator 52 and a second comparator 54. A further difference is that, instead of an output capacitor 40, an operational amplifier 56 is used to integrate the square wave signal generated by the first comparator 52. In this embodiment, however, the integrated value must be accessed directly, since the signal value is not stored as with the output capacitor 40. As previously explained, the first comparator 52 compares the influenced voltage signal, which is in Fig. As shown in Figure 4(b), a reference voltage is used that is greater than the voltage value of the influenced voltage signal after a first voltage drop induced by the reflected signal. The output signal of the comparator thus replicates the first rising edge and the first falling edge of the influenced voltage signal, as shown in Figure 4(b). Fig. 4(c) is evident. The second comparator 54, however, is set such that its comparison voltage is lower than the voltage level after a first drop in the influenced voltage signal, but higher than the voltage level of the influenced voltage signal after a second drop, which is caused, for example, by further reflections. The signal generated in this way, i.e., the output signal of the second comparator 54, is shown in Fig. Figure 4(d) shows that additional comparators can be used to capture multiple reflections and thus create a multi-echo capable system. Further processing of the output signal of the second comparator 54 is shown in Fig. 4(a) not provided for.

[0049] In principle, the output signal of the second comparator 54 can be processed in the same way as the output signal of the first comparator 52, for example, by means of a suitable operational amplifier. Fig. Figure 4(e) shows the output signal of the operational amplifier 56, where a voltage signal of higher magnitude, as already shown in Fig. 3(c), indicates a lower fill level and thus a longer runtime of the voltage wave.

[0050] In Fig. Figure 5(a) shows a sketch of the influenced voltage signal, i.e., the step response, for a step function as an input voltage signal when the level measuring device 12 is equipped with a coaxial cable as the measuring line 20. The influenced voltage signal has a pulse shape characterized by a rising edge when the electrical signal is generated and a falling edge when the reflected signal is received. The pulse width corresponds to the transit time of the voltage wave.

[0051] In Fig. Figure 5(b) shows a sketch of the influenced voltage signal, i.e., the step response, for a step function as an input voltage signal when the level measuring device 12 is not equipped with a coaxial cable as the measuring line 20. After the application of the voltage signal, i.e., the rising edge of the voltage signal, a reflection with the same polarity as the voltage signal occurs at the measuring line 20, causing the influenced voltage signal to exhibit an amplitude increase. After receiving the signal reflected from the medium surface, a subsequent decrease in the influenced voltage signal is observed.

[0052] In Fig. Figure 5(c) shows a sketch of the influenced voltage signal, i.e., the impulse response, for an impulse function as the input voltage signal. As can be seen, an input pulse is applied, which, after a corresponding propagation delay, can be detected as a reflected pulse in the influenced voltage signal. An advantage of using an impulse function as the input voltage signal is that, as expected, media with small dielectric constants can be detected, even if the amplitude of the reflected pulse decreases in these cases. Furthermore, small impedance differences between the medium impedance 48 and the reference impedance 46 can be detected.

[0053] Fig. Figure 6 shows a schematic representation of an evaluation unit 24 for processing multiple level measurements. The evaluation unit 24 comprises a comparator 30 and an averaging unit 58, which includes a low-pass filter 60 and an analog-to-digital converter (ADC) 62. It receives the influenced voltage signal via a preprocessing circuit 64, which includes, among other things, the voltage source 18 and the measuring line 20. The voltage signal generated by the voltage source 18 was periodically switched on and off, or periodically connected to and disconnected from the voltage source 18, via the fast switch 42, so that the output signal, i.e., the uninfluenced voltage signal, comprises several rectangular pulses. Each rectangular pulse can be assigned to a specific measurement. Accordingly, the voltage signal influenced by the reflected signal also has a pulse shape, which is shown schematically in Figure 6. Fig. Figure 6 illustrates this. Each pulse of the influenced voltage signal exhibits a first rising edge, which is due to the switching on of the voltage by the fast switch 42 and which leads to a rise in the voltage to a first voltage level; a first falling edge, which is caused by the reflected signal and which leads to a drop in the voltage to a second voltage level; and a second falling edge, which is caused by further reflections of the voltage signal or voltage wave and / or by the disconnection of the voltage by the fast switch 42 and which leads to a drop in the voltage to an output level or a zero level.

[0054] The influenced voltage signal is further processed by comparator 30, whereby the reference voltage of comparator 30 is set such that it is greater than the second voltage level. The signal components after the first drop of the influenced voltage signal are thus filtered out, so that the signal output by comparator 30 remains, which comprises a sequence of square wave pulses.

[0055] The individual pulses of the comparator 30's output signal can be assigned to a specific measurement. The individual pulses of the comparator 30's output signal can differ from one another, at least slightly, in their width and amplitude. To improve the accuracy of the overall measurement, it is therefore advantageous to calculate an average of the individual measurements. For this purpose, an average of the individual measurements can be determined using the averaging unit 58. This can be done particularly easily by determining the DC component G of the pulsed output signal of the comparator 30. The low-pass filter 60 removes the high-frequency signal components, so that the output of the low-pass filter 60 mainly comprises the DC component G of the input signal.Using the ADC 62, the signal can then be digitized and the DC component G digitally determined, for example by performing a Fourier transform of the digitized signal at 0 Hz, which corresponds to a moving average of the time signal. The average value can thus be determined using simple, cost-effective hardware.

[0056] Fig.Figure 7 illustrates an embodiment of the level measuring device 12 in which the measuring line 20 is arranged together with the evaluation unit 24 and the voltage source 18 on the same circuit board 66. The level measuring device 12 can thus be designed to be very compact and manufactured cost-effectively. The electronic components, with the exception of the measuring line 20, are protected against moisture by a potting compound in the upper part 68 of the level measuring device 12, so that the level measuring device 12 can be installed directly in the container 16 of the medium 14. The measuring line 20 can, for example, be designed as a coplanar line or a microstrip line. Reference symbol list 12 Level measuring device 14 Medium 16 containers 18 Voltage source 20 measuring leads 22 Process connection 24 evaluation units 26 first resistance 28 second resistor 30 Comparator 32 Digital-to-Analog Converters 34 microcontrollers 36 switches 38 Power source 40 Output capacitor 42 quick switches 44 Pulse generator 46 Reference impedance 48 Medium impedance 50 diode 52 first comparator 54 second comparator 56 operational amplifiers 58 Mean calculation unit 60 low-pass filters 62 Analog-to-Digital Converters 64 Preprocessing circuit 66 circuit board 68 upper part of the level measuring device

Claims

[1] Level measuring device (12) for determining the level of a medium (14) in a container (16), comprising: a signal generating device configured to generate an electrical signal comprising a step function or an impulse function; a measuring line (20) which is connected to the signal generating device, in particular electrically, and extends from a process connection (22) into the medium (14), wherein the measuring line (20) serves to conduct the electrical signal into the container (16) and towards the medium (14), wherein the electrical signal is at least partially reflected at the surface of the medium (14) and the reflected signal influences the electrical signal; and an evaluation unit (24) which is designed to determine the fill level of the medium (14) based on the influenced electrical signal. [2] Level measuring device (12) according to claim 1, wherein evaluation unit (24) is designed to determine a substitution quantity based on the influenced electrical signal, which is proportional to a transit time of the electrical signal from the process connection (22) to the medium surface and back and / or to a distance between the medium surface and the process connection (22), and to determine the fill level of the medium (14) based on the substitution quantity, where the substitution quantity does not correspond to the propagation time of the electrical signal. [3] Level measuring device (12) according to claim 2, where the influenced electrical signal has a pulse shape, wherein the evaluation unit (24) is configured to generate a rectangular signal based on a pulse width of a, in particular first, pulse of the influenced electrical signal, the width of which essentially corresponds to the pulse width of the influenced electrical signal, and to determine the substitution quantity based on the rectangular signal. [4] Level measuring device (12) according to claim 3, wherein the evaluation unit (24) is configured to generate the square wave signal based on the influenced electrical signal using a threshold filter, wherein the square wave signal essentially assumes the value 0 when the influenced electrical signal is less than a predetermined threshold, and assumes a predefined non-zero value when the influenced electrical signal is greater than the predetermined threshold, wherein the evaluation unit (24) is configured to perform an integration of the square wave signal using an integrator component in order to determine an integration value, and to determine the fill level of the medium (14) based on the integration value. [5] Level measuring device (12) according to claim 4, wherein the evaluation unit (24) is configured to determine the predetermined threshold value as a function of the impedance difference between a reference impedance, which represents the impedance of the measuring line for the section which does not extend into the medium, and a medium impedance, which represents the impedance of the measuring line for the section which extends into the medium. [6] Level measuring device (12) according to claim 4 or 5, wherein the evaluation unit (24) is configured to continuously adjust the predetermined threshold value of the threshold filter or to reduce it at predetermined time intervals. [7] Level measuring device (12) according to one of claims 4 to 6, wherein the evaluation unit (24) comprises several threshold filters with different threshold values. [8] Level measuring device (12) according to one of claims 4 to 7, wherein the evaluation unit (24) is configured to sample an integration signal generated by the integration of the square wave signal by means of an ADC with a sampling frequency of less than 10 MHz, less than 1 MHz, less than 100 kHz, less than 10 kHz or less than 1 kHz in order to determine the integration value. [9] Level measuring device (12) according to one of the preceding claims, wherein the measuring line (20) does not comprise a coaxial cable. [10] Level measuring device (12) according to one of the preceding claims, wherein the electrical signal comprises a plurality of rectangular pulses and the influenced electrical signal comprises a plurality of, in particular similar, pulses, wherein the evaluation unit (24) is configured to determine a DC component of the influenced electrical signal and to determine the fill level of the medium (14) based on the DC component of the influenced electrical signal. [11] Level measuring device (12) according to claim 10, wherein the signal generation device is configured to generate the rectangular pulses of the electrical signal at variable time intervals. [12] Level measuring device (12) according to one of the preceding claims, wherein the reference impedance (20) is larger, in particular 5 times, 10 times, 12 times or 15 times larger, than the medium impedance (14). [13] Level measuring device (12) according to one of the preceding claims, wherein the measuring line (20), in particular as a coplanar line or micro-strip line, is arranged together with the evaluation unit (24) and the signal generation device on the same printed circuit board (66). [14] Level measuring device (12) according to claim 1, wherein the electrical signal is carried with a substantially constant basic impedance.

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