Fill-level meter

Abstract

The invention relates to a pulse propagation time-based fill-level meter (1), the functionality of which can be traceably checked. For this purpose, according to the invention, the fill-level meter (1) also comprises, in addition to the regular internal pulse generation unit (11), a reference signal unit (13) which is designed to generate a reference signal (rref) for predeterminable target values ​​at target distances (dsoll) or target signal propagation times (tsoll), which simulates the intermediate frequency signal (rZF) used for evaluation in the pulse propagation time method. Thus, an evaluation unit (16) of the fill-level meter (1) can not only determine the fill-level (L) based on the intermediate frequency signal (rZF), but also a reference value corresponding to the target value (tsoll, dsoll) can be determined based on the reference signal (rref). This makes it possible to compare the reference value with the target value (tsoll, dsoll) and to classify the fill-level meter (1) as functional, provided the comparison shows a match. The comparison can also be used to determine the measuring accuracy of the fill-level meter (1).

Description

[0001] Level measuring device

[0002] The invention relates to a radar-based level measuring device.

[0003] In process automation technology, appropriate field devices are used to record relevant process parameters. To record the respective process parameters, suitable measurement principles are implemented in the corresponding field device types to record process parameters such as level, flow, pressure, temperature, pH value, redox potential, or conductivity. A wide variety of field device types are manufactured and distributed by the Endress + Hauser Group.

[0004] Non-contact measurement methods have become established for level measurement of filling materials in containers because they are robust and low-maintenance. Another advantage of non-contact measurement methods is their ability to measure the level virtually continuously. Therefore, high-frequency-based measurement methods are predominantly used in continuous level measurement. In principle, the higher the frequency, the higher the measurement resolution. The pulse transit time method and FMCW (Frequency Modulated Continuous Wave) have proven to be the most popular measurement methods. ,r ). Radar-based level measurement is described in more detail in "Radar Level Detection, Peter Devine, 2000."

[0005] In addition to free-radiating radar measurement, in which the high-frequency signals are transmitted and received via an antenna, there is also the variant of guided radar. Here, instead of an antenna, an electrically conductive measuring probe (e.g., a coaxial cable or a metal rod) serves as the transmit / receive arrangement. This probe is lowered into the container to guide the high-frequency signals. Similar to free-radiating radar, the high-frequency signal is reflected in the measuring probe at the level of the product surface and guided back along the measuring probe to the level measuring device, so that the level can be determined based on the signal propagation time of the corresponding received signal. This variant of radar-based level measurement is also known under the term "TDR" ("Time Domain Reflectometry"). The advantage of this variant is that the received signal is significantly less affected by interference.

[0006] Depending on the application, the level measuring device must meet various requirements: In crude oil processing plants, the "Manual of Petroleum Measurement Standards" of the American Petroleum Institute (API), Chapter 18.2, must be observed, which requires a cyclical verification of the measurement accuracy of the level measuring device in the process plant. In relation to level measurement, for example, level measurement in crude oil tanks, this means that the level value determined by the level measuring device must be periodically traceable, i.e., verifiable with respect to an initial reference.

[0007] Various solutions exist to meet this requirement: In the simplest case, manual checking (also known as "wet dipping" in technical jargon) can be performed by periodically measuring the fill level with a tape measure and comparing it with the level value measured by the level measuring device. However, this has several disadvantages: Firstly, the ongoing process must be interrupted, which is financially disadvantageous. Secondly, manual intervention is associated with a corresponding risk. Furthermore, manual calibration is limited to an accuracy in the millimeter range.

[0008] Another established testing method involves decoupling the level gauge from the antenna or measuring probe in specified cycles and periodically feeding one or more reference signals using an appropriate external device. The reference signal is synthetically generated and represents the actual measurement signal for a specified target distance or the corresponding level value. Based on this, the accuracy of the level gauge is evaluated by comparing the level value obtained from the fed-in reference signal with the known target distance. Although this method is less hazardous, it still requires interrupting process operation and manually decoupling the level gauge.

[0009] The invention is therefore based on the object of improving pulse transit time-based level measuring devices in such a way that the measuring accuracy can be checked with little effort, traceability and reliability.

[0010] The invention solves this problem by a pulse transit time-based level measuring device comprising the following components:

[0011] A pulse generation unit which is designed to generate pulse-shaped high-frequency signals according to the pulse transit time method, a transmitting / receiving arrangement by means of which the pulse-based high-frequency signals can be transmitted towards the filling material and received as received signals after reflection on the filling material surface, a reference signal unit which is designed to generate a corresponding reference signal for at least one setpoint value which represents a setpoint distance or a corresponding setpoint signal transit time, analogous to the received signal or to an intermediate frequency signal, a first clock generator which at least the pulse generation unit orthe reference signal unit clocks, a database unit in which the target value is stored, and an evaluation unit which is designed to o determine the fill level based on the received signal in accordance with the pulse transit time method, o determine a reference value corresponding to the target value (i.e. a reference distance or a corresponding signal transit time) based on the reference signal in accordance with the pulse transit time method, o compare the reference value with the target value, and o classify the fill level measuring device as functional if the comparison between the target value and the reference value shows a match, and / or to determine a measurement accuracy based on this comparison.

[0012] Within the scope of the invention, the term "unit" essentially refers to those electronic circuit components that are intended for a specific application, e.g., for high-frequency signal processing or as an interface. Depending on the application, the respective unit can therefore comprise corresponding analog circuits for generating or processing corresponding analog signals. However, the unit can also comprise digital circuits, such as FPGAs, microcontrollers, or storage media, in conjunction with corresponding programs. The program is designed to carry out the required method steps or apply the necessary computing operations. In this context, various electronic circuits of the unit within the meaning of the invention can potentially also access a common physical memory or be operated using the same physical digital circuit.It is not relevant whether different electronic circuits within the unit are arranged on a common circuit board or on several connected circuit boards.

[0013] By providing a separate reference signal unit in the level measuring device according to the invention and comparing it with predefined target values, the level measurement can be traceably tested for functionality and measurement accuracy, provided the target value was determined or specified during production via calibration. This means that the target value is determined by the evaluation unit based on a received or intermediate frequency signal recorded during a calibration measurement at known calibration distances. Within the scope of the invention, it is not strictly prescribed whether the functionality or measurement accuracy is tested cyclically.Accordingly, the reference signal unit can either be designed to automatically generate the first reference signal repeatedly in a defined cycle, with the evaluation unit comparing the correspondingly determined reference value with the first setpoint according to this cycle. Alternatively, the reference signal unit can be designed to generate the reference signal upon command or manual input. In this case, the evaluation unit must be designed to compare the correspondingly determined reference value with the setpoint upon this command.

[0014] Since pulse transit time-based level measuring devices usually include a sampling unit which converts the received signal into a time-stretched and possibly additionally digitized intermediate frequency signal, and the evaluation unit determines the level value based on the intermediate frequency signal, the reference signal unit according to the invention must in these cases be designed for the purpose of comparability so that the first reference signal is generated in a correspondingly time-stretched manner.

[0015] The method for evaluating the functionality and measurement accuracy of the level measuring device according to the invention provides the following process steps:

[0016] - Specification of a target value for a target distance or a corresponding target signal propagation time,

[0017] - Entering the target value into the database unit,

[0018] - Generating the reference signal using the reference signal unit,

[0019] - Determining the reference value based on the reference signal, and

[0020] - Comparison of the reference value with the target value, and

[0021] Classification of the level measuring device as functional if the comparison of the target value and the reference value shows a match, and / or determination of a measurement accuracy based on the comparison.

[0022] In order for this classification to be considered traceable, the target value must be determined in advance by the evaluation unit using such a calibration reception signal, which was received during calibration at a known calibration distance.

[0023] Overall, the invention is not limited to classifying the level measuring device or determining its measurement accuracy based on a single target value, i.e., based on a single calibration distance. If appropriate calibration measurements have been performed at two or more calibration distances and corresponding calibration reception signals have been recorded, a corresponding reference signal can be generated for each of the corresponding target values, so that the corresponding reference values ​​for runtime or distance can be determined from this. All reference values ​​are compared with the associated target values: If all target values ​​match the corresponding reference values, the level measuring device can be classified as functional. Similarly, the measurement accuracy can be determined separately for each target value.

[0024] The invention is explained in more detail with reference to the following figures. They show:

[0025] Fig. 1 : A TDR-based level gauge on a container,

[0026] Fig. 2: a block diagram of the level measuring device according to the invention, and

[0027] Fig. 3: a time graph with relevant signals within the level gauge.

[0028] For a basic understanding of the invention, Fig. 1 shows a container 3 with a filling material 2, the filling level L of which is to be determined. Depending on the type of filling material 2 and the area of ​​application, the container 3 can be up to more than 100 m high. The type of filling material 2 and the area of ​​application also determine the type of level measuring device 1 used. As a rule, the level measuring device 1 is connected to the measuring device via a separate interface unit, such as "4-20 mA", "PROFIBUS", "HART" 1, or "Ethernet" to a higher-level unit 4, such as a local process control system or a decentralized server system. The measured fill level value L can be transmitted via this, for example, to control inflows or outflows of the container 3. However, other information about the general operating status of the level measuring device 1 can also be communicated, such as parameterization data.

[0029] In the present illustration, the level measuring device 1 is based on the TDR principle and is accordingly mounted above the filling material 2 at a known installation height h above the brine of the container 3. The level measuring device 1 is attached to a corresponding opening of the container 3 in such a way that a metallically conductive measuring probe 12 of the level measuring device 1 is directed vertically downwards into the container 3 towards the filling material 2 as a transmitting / receiving arrangement, while the other components of the level measuring device 1 are arranged outside the container 3. High-frequency signals SHF are guided towards the surface of the filling material 2 via the measuring probe 12. After reflection at the level of the filling material surface, the level measuring device 1 receives the reflected received signals THF from the measuring probe 12. The signal propagation time t between transmission and reception of the respective high-frequency signal s, THF according to proportional to the distance d between the level gauge 1 and the filling material 2, where c is the situation dependence and usually at least roughly known

[0030] The signal propagation time t is determined by the level gauge 1 using the pulse transit time method:

[0031] This means that the high-frequency signals SHF are pulsed with a pulse repetition frequency fpRF in the MHz range and converted into a time-stretched intermediate frequency signal TZF by subsampling compared to the received signal THF. The time-stretch factor k is

[0032] In this context, fsample is the sampling frequency at which the received signal THF is sampled. The reason for this time expansion is that the signal propagation time t can be determined with significantly less technical effort. Thus, using the intermediate frequency signal TZF, the distance d can be determined from the intermediate frequency signal rzF d according to be determined.

[0033] In practice, this relationship between the distance d and the signal propagation time t must be learned in the level measuring device 1 through a series of calibration measurements: For this purpose, the measuring probe 12 is replaced by at least one calibration probe whose length represents a defined calibration distance. For more precise calibration, several calibration probes can be used, thereby correspondingly representing several calibration distances. This allows the level measuring device 1 to receive corresponding calibration reception signals THF during the calibration process after transmitting the high-frequency signal SHF and store them in a database unit 15 designed for this purpose. An evaluation unit 16 of the level measuring device 1 can complete the calibration by determining the corresponding signal propagation times t from the calibration reception signals THF. S0 n are determined. In this process, these calibration signal propagation times t S0n the underlying calibration distance as target distance d S0 n assigned.

[0034] Based on such a calibration, the level measuring device 1 can assign the determined signal propagation time t to the corresponding distance d during regular measuring operation. This allows the level measuring device 1 to

[0035] L = h — d in turn determines the fill level L, provided that the installation height h is stored as a parameter in the level measuring device 1.

[0036] Fig. 2 shows a block diagram of the level measuring device 1 typical of the pulse transit time method, from which its individual units 11, 12, 13, 14, 15, 16, 17, 17, 18, 19 can be seen in more detail: Accordingly, the level measuring device 1 comprises a pulse generation unit 11 for generating the pulsed high-frequency signal SHF. The pulse generation unit 11 is clocked by means of a first clock signal clki of a first clock generator 14, which has the pulse repetition frequency fpRF. The first clock generator 14 can, for example, be based on a corresponding quartz oscillator or an electrical resonant circuit. The pulse generation unit 11 itself is designed, for example, as a high-frequency switching transistor, by means of which correspondingly short pulses are generated.The high-frequency signal SHF thus generated is coupled into the measuring probe 12 via a transmit / receive switch 18, whereby the receive signal THF is also branched off via this after reflection at the filling material 2 and fed to a scanning unit 17.

[0037] Analogous to the signal generation path 14, 11, 18, the sampling unit 17 is based on generating a sampling signal s S on P ie on a sampling pulse generation unit 172, which in turn is clocked by a clock signal clk2 of a sampling clock generator 171. For the previously described subsampling, this clock signal clk2 has the sampling frequency fsampie. A sample pulse generated by the sampling signal s saThe sampler 173 of the sampling unit 17, controlled by a piezoelectric system, samples the received signal r^ accordingly, generating the time-extended intermediate frequency signal TZF. As shown in Fig. 2, the generated intermediate frequency signal TZF is in turn fed to the evaluation unit 16 to determine the signal propagation time t or to determine the fill level L.

[0038] In order for the fill level L to be determined with the required accuracy using the intermediate frequency signal TZF, it is necessary for the time expansion factor k to correspond to the specified value. As can be seen from the above formula, the frequency difference between the pulse repetition frequency fpRF and the sampling frequency fsampie must be exactly correct. To ensure this, the evaluation unit 16 adjusts the sampling clock generator 171 accordingly, with the direct frequency difference between the pulse repetition frequency fpRF and the sampling frequency fpRF serving as the controlled variable. As can be seen from Fig. 2, the evaluation unit 16 receives this frequency difference from a mixer 19, which accordingly mixes the clock signals clk-1, 2 of the clock generator 14 and the sampling clock generator 171 and outputs them as a synchronized clock signal sync. However, the evaluation unit 16 itself is clocked by an independent, third clock generator 161 using a corresponding third clock signal clka.

[0039] Despite this regulation, any error that the first clock generator 14 does not clock correctly will result in an inaccurate level measurement. In order to be able to detect such a case in a traceable manner, the level measuring device 1 according to the invention comprises a reference signal unit 13, which, when triggered accordingly, generates a defined reference signal r re f is generated. The reference signal r simulates re f the intermediate frequency signal rzp for the case that the underlying high frequency signal SHF was emitted at a level value L which corresponds to the target distance d S0 n corresponds to the calibration measurement.

[0040] For this purpose, the reference signal unit 13 can be based, for example, on a high-frequency switching transistor, analogous to the pulse generation unit 11, which is triggered accordingly. In the embodiment of the level measuring device 1 according to the invention shown in Fig. 2, the reference signal r re f or the intermediate frequency signal rzp is fed to the evaluation unit 16 via a multiplexer 162. In order to detect any incorrect timing of the first clock generator 14 by means of the reference signal unit 13, the latter also clocks the reference signal unit 13, as shown in Fig. 2.

[0041] To what extent the reference signal r re f simulates the intermediate frequency signal rzp, is shown in more detail in the graph of Fig. 3: In principle, the graph represents the signal levels in the time domain, where the reference signal r re f at the target signal propagation time tsoii corresponding to the target distance d S0n, forms a rectangular pulse with a defined pulse length. The target signal propagation time t S0 n to the pulse of the synchronized clock signal sync, also shown in Fig. 2. This pulse defines the time at which the pulse-based high-frequency signal SHF is emitted. Overall, this triggers the start of the respective measurement, which is why the pulse generation unit 11 and the sampling pulse generation unit 172 are also triggered by the synchronized clock signal sync, as shown in Fig. 2. In the graph shown in Fig. 3, which reproduces the signal waveforms of the circuit shown in Fig. 2, the intermediate frequency signal TZF, in contrast to the reference signal r re f as an analog, non-discrete signal. The signal propagation time t of the intermediate frequency signal rzp in the situation shown in Fig. 2 corresponds to the desired signal propagation time t S0n or the corresponding target distance d S0 n, where calibration may also be performed. From the comparison of the intermediate frequency signal rzF and the reference signal r re f it follows that their signal characteristics do not have to be strictly identical in the sense of the invention, as long as the reference signal r re f triggers a signal-technically identical reaction in the evaluation unit 16.

[0042] The generation of the reference signal r re According to the invention, f is used to determine a reference value, which in turn is determined by the evaluation unit 16. The reference value represents, depending on how the setpoint t S0 n, d S0 n is defined, either a reference distance or a corresponding signal propagation time t. For this purpose, the evaluation unit 16 processes the reference signal r ref according to the pulse transit time method is technically identical to the determination of the fill level L using the intermediate frequency signal TZF:

[0043] Identification of the o signal maximum in the intermediate frequency signal TZF or the o rectangular pulse in the reference signal r re f,

[0044] Determination of the signal transit time t corresponding to the o signal maximum or o rectangular pulse, and determination of o the distance d / the fill level L or o the reference value based on the respective signal transit time t.

[0045] According to the invention, the evaluation unit 16 can compare the reference value thus determined with the previously entered target value t S0 n, d S0n in order to classify the level measuring device 1 as functional, provided the comparison results in sufficient agreement. Additionally, or alternatively, it is possible to determine the measuring accuracy of the level measuring device 1 based on this comparison. Both cases represent a traceable type of verification. Depending on the context in which this verification is required, the verification can be triggered either manually or automatically if the level measuring device 1 is designed accordingly. In the case of automatic verification, this can be carried out in fixed cycles, whereby the cycle length can in principle be freely selected depending on the area of ​​application.

[0046] As shown in Fig. 3 by the dashed rectangular pulses of the reference signal r re f is indicated, in the embodiment shown in Fig. 2 there are several setpoints t S0 n, d S0n, which are based on different calibration distances. This makes it possible to set the calibration values ​​within the measuring range for the different target values ​​t S0 n, d S0 n corresponding reference signals r re f or reference values. This allows an even more detailed check of the level measuring device 1 , since all reference values ​​are compared with the corresponding target values ​​t S0 n, d S0 n must match in order to be able to derive sufficient functionality or measurement accuracy.

[0047] List of reference symbols

[0048] 1 level gauge

[0049] 2 Filling material

[0050] 3 containers

[0051] 4 Superior unit

[0052] 11 Pulse generation unit

[0053] 12 measuring probe

[0054] 13 Reference signal unit

[0055] 14 First pacemaker

[0056] 15 Database unit

[0057] 16 Evaluation unit

[0058] 17 Sample Unit

[0059] 18 Transmit / receive switch

[0060] 19 mixers

[0061] 161 Third Clock

[0062] 162 multiplexers

[0063] 171 sampling clock

[0064] 172 Sampling pulse generation unit

[0065] 173 sampling samplers

[0066] Clkl ,2, 3 clock signals d distance fpRF pulse repetition frequency f sample sampling frequency h installation height k time expansion factor

[0067] L Fill level

[0068] PHF Receive signal ref Reference signal of the reference signal unit

[0069] PZF intermediate frequency signal

[0070] Ssample Sampling signal sync Synchronized clock signal

[0071] SHF High frequency signal t Signal propagation time tsoll Target signal propagation time

Claims

Patent claims 1. Pulse transit time-based level measuring device (1), comprising: A pulse generation unit (11) which is designed to generate pulse-shaped high-frequency signals (SHF) according to the pulse transit time method, a transmitting / receiving arrangement (12) by means of which the pulse-based high-frequency signals (SHF) can be transmitted towards the filling material (2) and, after reflection at the filling material surface, can be received as received signals (r), a reference signal unit (13) which is designed to be used for at least one setpoint value, which is a setpoint distance (d S0 n) or a corresponding target signal propagation time (tsoii), a reference signal (r re f) to generate, a first clock generator (14), which clocks at least the pulse generation unit (11) and / or the reference signal unit (13), a database unit (15), in which the setpoint value (t S0 n, d S0n), and an evaluation unit (16) which is designed to o determine the fill level (L) based on the received signal (r, TZF) or the intermediate frequency signal (rzp) according to the pulse transit time method, o based on the reference signal (r re f) according to the pulse transit time method, a value corresponding to the setpoint (t S0 n, d S0 n) to determine the corresponding reference value for a reference distance or a corresponding signal propagation time (t), o to compare the reference value with the target value (t S0 n, d S0 n) and o to classify the level measuring device (1) as functional if the comparison shows a match and / or to determine a measuring accuracy on the basis of the comparison.

2. Level measuring device according to claim 1, wherein the evaluation unit (16) is designed to determine the setpoint (t S0 n, d S0n) using a calibration reception signal or calibration intermediate frequency signal (F, TZF), which is recorded as part of a calibration measurement.

3. Level measuring device according to claim 1 or 2, wherein the transmitting / receiving arrangement comprises an electrically conductive measuring probe (12) or an antenna.

4. Level measuring device according to one of the preceding claims, comprising: - A sampling unit (17) which converts the received signal (r) into an intermediate frequency signal (rzp), wherein the reference signal unit (13) is designed to sample the reference signal (r re f) to generate analogue to the intermediate frequency signal (rzp), and wherein the evaluation unit (16) is designed to determine the fill level (L) based on the intermediate frequency signal (rzp).

5. Level measuring device according to one of the preceding claims, wherein the reference signal unit (13) is designed to generate the reference signal (r ref) to generate automatically and recurringly in a defined cycle, and wherein the evaluation unit (16) is designed to compare the correspondingly determined first reference value with the first target value in accordance with this cycle.

6. Level measuring device according to claim 1 to 5, wherein the reference signal unit (13) is designed to generate the reference signal (r re f) to generate on command or input, and wherein the evaluation unit (16) is designed to compare the correspondingly determined reference value with the setpoint value (t S0 n, d S0 n) to compare.

7. Method for functional testing of the level measuring device (1) according to at least one of the preceding claims, comprising the following method steps: - Input of the setpoint (t S0 n, d S0 n) into the database unit (15), - Generating the reference signal (r re f) by means of the reference signal unit (13), - Determine the reference value based on the reference signal (r re f), - Comparison of the reference value with the target value (t S0 n, d S0 n), and Classification of the level measuring device (1) as functional, provided that the comparison of the setpoint value (t S0 n, d S0 n) and the reference value, and / or determining a measurement accuracy based on the comparison.

8. The method according to claim 2 and 7, wherein the setpoint (t S0 n, d S0 n) is determined using a calibration reception or intermediate frequency signal (r, TZF), which is recorded as part of a calibration measurement at a known calibration distance.

9. The method according to claim 8, wherein a calibration measurement is carried out at at least two different calibration distances, Based on the corresponding calibration reception signals THF, TZF) two different target values ​​(t S0n, d S0 n) are determined for each of the target values ​​(t S0 n, d S0 n) each have a corresponding reference signal (r re f) is generated and from this the corresponding reference value is determined, all reference values ​​are compared with the corresponding target values ​​(t S0 n, d S0 n) are compared, and - Classification of the level measuring device (1) as functional, provided that all set values ​​(t S0 n, d S0 n) comply with the relevant reference values.

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