Patent Application
20220034701
The invention relates to a method for the fill-level measurement of a filling material located in a container, and to a fill-level measuring device for carrying out said method.
In process automation technology, field devices for capturing or modifying process variables are generally used. For this purpose, the functioning of the field devices is in each case based on suitable measuring principles in order to capture the corresponding process variables, such as fill level, flow rate, pressure, temperature, pH value, redox potential or conductivity. A wide variety of such field devices is manufactured and distributed by the Endress+Hauser company.
For measuring the fill level of filling materials in containers, radar-based measuring methods have become established since they are robust and are low maintenance. Within the scope of the invention, the term “container” also refers to containers that are not closed, such as basins, lakes, or flowing bodies of water. A central advantage of radar-based measuring methods is the ability to measure the fill level more-or-less continuously (in the context of this patent application, the term “radar” refers to signals or electromagnetic waves having frequencies between 0.03 GHz and 300 GHz).
In the case of radar-based fill-level measurement, the pulse transit time method is an established measuring principle. Microwave pulses are transmitted cyclically at a defined clock rate in the direction of the filling material, and the transit time until reception of the correspondingly reflected microwave pulse is measured.
In order to determine the transit time or the fill level, a corresponding evaluation signal is recorded based on the received, reflected microwave pulses. Due to the high pulse frequency, the evaluation signal is generated by undersampling the reflected microwave pulses. Undersampling is carried out by mixing the reflected microwave pulses with corresponding generated sampling pulses, wherein their sampling rate deviates by a defined relation from the clock rate of the emitted microwave pulses. To maintain this defined target phase change between the sampling rate and the clock rate, the sampling rate is readjusted by a control loop depending on the measured relation.
Overall, the evaluation signal thus represents the signal amplitude of the reflected microwave pulses in a time-expanded manner. The evaluation signal reflects the signal amplitude of the reflected microwave pulses depending on the measuring distance.
On the basis of the pulse transit time method, fill-level measuring devices with comparatively low circuitry complexity and a high fill-level resolution in the sub-millimeter range can be realized. A radar-based fill-level measuring device which operates according to the pulse transit time method is described, for example, in the unexamined patent application DE 10 2012 104 858 A1.
In addition to freely radiating radar measurement in which the microwave pulses are emitted or received via an antenna, the variant of guided radar also exists. In this case, the microwave pulses are guided via an electrically conductive probe (for example, a coaxial cable or a metal rod) which is lowered into the container. At the location of the filling material surface, the microwave pulses are reflected in the probe and guided back along the probe toward the fill-level measuring device. This variant of radar-based fill-level measurement is also known by the term TDR (time-domain reflectometry). This variant is advantageous in that due to the guided signal radiation, less power is required for operating the fill-level measuring device. Analogous to freely radiating radar devices according to the pulse transit time method, an evaluation signal for determining the fill level is also generated when implementing the TDR principle.
The fill level is determined from the evaluation signal by detection and local assignment of the corresponding local signal maximum. In cases in which the sampling rate for sampling the received microwave pulses does however not correspond to the target sampling rate, the measuring distance corresponding to the signal maximum and thus the fill-level measurement is distorted since the evaluation signal is temporally expanded or compressed depending on the deviation. One reason for a deviation from the target sampling rate can be an inaccurate or slow control of the sampling rate. As a result, the potentially attainable accuracy of the fill-level measuring device is reduced.
The object of the invention is therefore to provide a more accurate fill-level measuring device.
The invention achieves this object by a method for radar-based measurement of the fill level of a filling material located in a container, comprising the following method steps:
According to the invention, the method is characterized in that:
an evaluation curve is generated on the basis of the evaluation signal, the relation between the clock rate and the sampling rate, and a predefined target relation. The fill level is thus determined on the basis of the evaluation curve. In this case, the evaluation curve is generated in particular by means of temporal expansion or compression of the evaluation signal, the compression or the expansion being carried out as a function of a ratio between the measured relation and the target relation.
The advantage of the method according to the invention is that any inaccurate control of the sampling rate is compensated so that the attainable accuracy of the fill-level measurement is potentially increased. This simultaneously reduces the technical requirements for controlling the sampling rate. It goes without saying that the method according to the invention can be used not only for measuring a distance to the filling material surface but also for radar-based distance measurement in general.
In order to compensate for inaccurate control, it is advantageous to carry out the expansion or compression of the evaluation signal to form the evaluation curve proportionally as a function of the ratio between the measured relation and the target relation. That is to say, the greater the ratio of the measured relation to the target relation, the more expansion is necessary. If the ratio is greater than 1, the evaluation signal must be expanded; if the ratio is less than 1, the evaluation signal must be compressed.
On the basis of the evaluation signal, the measured relation and the target relation, the evaluation curve can be generated, for example, by:
The type of approximation is not prescribed in a fixed manner within the scope of the invention. The approximation can be implemented, for example, as an optionally partially polynomial approximation, such as a spline. Preferably, a polynomial approximation of the 3rd order or higher is to be implemented. In case of limited computing power, however, an incremental linear approximation of the evaluation signal can also be used, for example.
The fill level can be determined on the basis of the evaluation curve by ascertaining the evaluation curve maximum that is caused by the filling material surface. The corresponding signal transit time is then assigned to this maximum, the fill level being determined on the basis of the measuring distance that corresponds to the signal transit time.
Analogous to the method according to the invention, the object on which the invention is based is achieved by a radar-based fill-level measuring device designed to carry out the method according to at least one of the preceding embodiments. Accordingly, the fill-level measuring device according to the invention comprises the following components:
In this case, the invention does not rule out that the fill-level measuring device additionally controls the sampling rate as a function of the measured relation such that the relation corresponds to the target relation. This compensates for any temperature dependencies or aging effects of the sampling unit and thus likewise reduces the accuracy losses of the fill-level measurement.
Within the context of the invention, the term “unit” is understood to mean in principle an electronic circuit that is designed to be suitable for its intended purpose. Depending on the requirement, it can therefore be an analog circuit for generating or processing corresponding analog signals. However, it can also be a (semiconductor-based) digital circuit, such as an FPGA or a storage medium in interaction with a program. In this case, the program is designed to carry out the corresponding method steps or to apply the necessary calculation operations of the unit in question. In this context, various electronic units of the fill-level measuring device in the sense of the invention can potentially also access a common physical memory or be operated by means of the same physical digital circuit.
The invention is explained in more detail with reference to the following figures. The following is shown:
To fundamentally understand the invention,
As a rule, the fill-level measuring device 1 is connected via a bus system, such as “Ethernet,” “PROFIBUS,” “HART” or “Wireless HART,” to a higher-level unit 4, such as a process control system or a decentralized database. On the one hand, information about the operating status of the fill-level measuring device 1 can thus be communicated. On the other hand, information about the fill level L can also be transmitted in order to control any inflows and/or outflows that may be present at the container 2.
Since the fill-level measuring device 1 shown in
The microwave pulses EHF are reflected at the surface of the filling material 3 and, after a corresponding signal transit time, are received as corresponding electrical receive signals eHF at the antenna 121. In this case, the signal transit time of the microwave pulses SHF, EHF is a function of the distance d=h−L of the fill-level measuring device 1 from the filling material surface.
A circuitry design of the fill-level measuring device 1 with which the microwave pulses SHF can be generated and with which the fill level L can be determined on the basis of the electrical receive signals eHF is shown in
In order to generate the microwave pulses SHF, the circuit of the fill-level measuring device 1 shown in
The high-frequency pulses sHF thereby generated by the high-frequency oscillator 111 are supplied to the antenna 121 via a transmitting/receiving switch 120 so that they are correspondingly emitted as microwave pulses SHF. Since the reflected microwave pulses EHF are also received via the antenna 121, the transmitting/receiving switch 120 supplies the corresponding electrical receive signal eHF to a mixer 14.
In contrast to the shown embodiment variant, instead of the antenna 121, an electrically conductive probe such as a waveguide or a coaxial cable extending toward the container bottom can also be used. In the implementation of this embodiment variant known by the term TDR (time-domain reflectometry), the high-frequency oscillators 111, 131 are not required in contrast to the circuit shown in
The undersampling of the receive signal eHF characteristic for the pulse transit time method is performed by the mixer 14. For this purpose, the receive signal eHF is mixed with electrical sampling pulses s′HF by the mixer 14. In this case, the sampling rate f′c at which the sampling pulses s′HF are generated deviates by a defined, low relation of far less than 0.1 per thousand from the clock rate fc of the generated high-frequency pulses sHF.
The sampling pulses s′HF are generated by a sampling unit which, analogously to the pulse generating unit, comprises a second pulse generator 130 and a second high-frequency oscillator 131. Thus, correspondingly to the high-frequency pulses sHF, the frequency fHF of the sampling pulses s′HF is defined by the second high-frequency oscillator 131. The second pulse generator 130 controls the sampling rate f′c at which the sampling pulses s′HF are generated.
By mixing the receive signal eHF with the electrical sampling pulses s′HF by means of the mixer 14, an evaluation signal ZF is generated, which represents the receive signal eHF in a time-expanded manner. The time expansion factor is proportional to the relation φ between the clock rate fc and the sampling rate f′c.
The time expansion is advantageous in that the evaluation signal ZF can be evaluated considerably more easily technically due to the time expansion in comparison with the pure receive signal eHF: The reason for this is that the receive signal eHF has a correspondingly short time scale tin the nanosecond range due to the high speed of propagation of the microwave pulses SHF, EHF at the speed of light. The time expansion results in the evaluation signal ZF having a time scale in the millisecond range. An evaluation curve ZF is schematically illustrated in
To correctly determine the fill level L based on the evaluation signal ZF, it is essential that the sampling rate f′c of the sampling unit corresponds exactly to its target sampling rate: This means that the relation φ to the clock rate fc of the pulse generating unit corresponds to a required target relation φref. In order that the sampling rate f′c does not drift therefrom, it is correspondingly controlled. In the exemplary embodiment shown in
From the change in the time shift, the evaluation unit 16 calculates the relation φ between the clock rate fc and the sampling rate f′c. The evaluation unit 16 also compares the determined relation φ between the clock rate fc and the sampling rate f′c with a stored target relation φref. In this way, the evaluation unit 16 can control the sampling rate f′c at the second pulse generator 130 as a function of the measured relation φ such that the relation φ corresponds to the target relation φref. In the embodiment variant shown in
Depending on the technical implementation of the control, the control may have an imprecise, overcontrolling or delayed effect. These effects lead to a distorted measurement of the fill level L since the evaluation signal ZF is thereby inadvertently temporally expanded or compressed. The maximum MZF corresponding to the filling material surface is thereby shifted in the evaluation signal ZF.
According to the invention, this distortion is counteracted in that the evaluation unit 16 either temporally expands or temporally compresses the stored, digitized evaluation signal ZF as a function of the ratio φ/φref between the measured relation φ and the target relation φref. Since the measurement is a freely radiating radar measurement, the evaluation signal ZF is initially rectified before the A/D conversion, in contrast to guided radar.
The evaluation unit 16 can carry out the expansion or compression by the value Δt by generating a mathematical function using the rectified and digitized evaluation signal ZF. For this purpose, the evaluation signal ZF can be fitted by means of polynomial interpolation, for example. The evaluation curve ZF′ is subsequently generated by means of temporal expansion or temporal compression of the approximated mathematical function by the value Δt.
The temporal expansion or compression Δt is proportional to the ratio φ/φref between the measured relation φ and the target relation φref: The greater the ratio φ/φref is, the more the evaluation signal is expanded. Whether the evaluation signal ZF is expanded or compressed in the evaluation unit 16 depends on whether the resulting ratio φ/φref is greater or less than 1 or greater or less than 100%. In the exemplary illustration in
The evaluation unit 16 subsequently determines the fill level L by means of the evaluation curve ZF′ thus obtained. The fill level L is determined by the evaluation unit 16 ascertaining the maximum M−ZF′ of the evaluation curve ZF′ that is caused by the surface of the filling material 3 (in the schematic evaluation curve ZF′ illustrated in
As
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2018 123 429.5 | Sep 2018 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/072118 | 8/19/2019 | WO | 00 |
