Fill-level measuring device

Abstract

The invention relates to an FMCW-based fill-level measuring device (1) which can operate in two frequency bands which are clearly delimited from one another. For this purpose, the fill-level measuring device (1) comprises two high-frequency units (11, 12), each having a frequency-controllable oscillator (111, 112), in order, in accordance with the FMCW principle, to generate radar signals (SHF1, 2) in the frequency bands or to receive same after reflection at the filling material surface. An antenna arrangement (13) is used to transmit and receive the radar signals (SHF1, 2) towards the filling material (L) and to receive same after reflection at the filling material surface, wherein a control / evaluation unit (14) can determine the fill level (L) on the basis of the received signal (RHF1, 2) of the first frequency band or of the second frequency band. According to the invention, the fill-level measuring device (1) is characterised by a common clock unit (15). This is used to form a phase-locked loop with both the first oscillator (111) and the second oscillator (121) in order to generate, in accordance with the FMCW principle, the corresponding radar signals (SHF1, 2, RHF1, 2) of the two frequency bands. As a result, it is unnecessary to provide a complete phase-locked loop for each of the two frequency bands. The number of required components is thus reduced.

Description

[0001] Level gauge

[0002] The invention relates to a radar-based level measuring device that can be adapted to a wide variety of applications.

[0003] In process automation technology, field devices are used to acquire relevant process parameters. Suitable measurement principles are implemented in these field devices to acquire process parameters such as fill level, flow rate, 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 measuring methods have become established for measuring the fill level of contents in containers due to their robustness and low maintenance requirements. A further advantage of non-contact measuring methods is their ability to measure the fill level almost continuously. Therefore, radar-based measuring methods are predominantly used in the field of continuous level measurement. In the context of this invention, the term "radar" refers to signals or electromagnetic waves with frequencies between 0.03 GHz and 300 GHz. By design, the higher the frequency, the higher the measurement resolution achievable. Pulse time-of-flight measurement and FMCW (Frequency Modulated Continuous Wave) have become established as measurement methods. Radar-based level measurement is described in more detail, for example, in "Radar Level Detection," Peter Devine, 2000.

[0005] Typical frequency bands approved for radar-based level measurement are 26 GHz, 60 GHz, 80 GHz, and 120 GHz, as well as increasingly 180 GHz and 240 GHz. Higher frequency bands are advantageous for many applications because, for a given antenna size, a more focused beam is achieved, and generally more bandwidth is available, which can be used for greater distance resolution. One such application is, for example, high-precision level measurement in refinery tanks.

[0006] However, several disadvantages of radar signals with higher frequencies or in higher frequency bands are also known, which can lead to impairments or even the complete loss of the level measurement in certain applications. These disadvantages are largely due to interactions between the radar measurement and the contents being measured, the atmosphere above the contents, and partly also to container shapes, environmental and installation conditions, as well as regulatory requirements. Level measurement in grain silos, for example, is an application where a wide beam cone or a low frequency band is advantageous: Due to the granular nature of the contents, this can lead to diffuse reflection of the radar signal, which can result in poor accuracy with a narrow beam cone or a low frequency band.In a high-frequency bath, the reflected received signal can be deflected so strongly from the vertical that it is not received by the antenna arrangement of the level gauge.

[0007] To take advantage of the benefits of different frequencies or frequency bands, a level measuring device is described in publication WO 2023099269 A1 that can determine the level in several clearly distinct frequency bands, depending on the situation or application. For the purposes of the invention, frequency bands are considered clearly distinct if their center frequencies differ by at least a factor of two and their bandwidth is less than one-fifth of their center frequency. A clear distinction is also considered to exist if the center frequencies of the frequency bands differ by at least a factor of four and their bandwidth is less than half of their center frequency. The center frequency of a frequency band is defined as the frequency that lies exactly in the middle of the frequency band.According to this definition, for example, a frequency band with a center frequency of 26 GHz and a bandwidth of 2 GHz extends from 25 GHz to 27 GHz.

[0008] A disadvantage of implementing multiple frequency bands is that a separate high-frequency unit is required for each band. This increases the space required on the corresponding circuit board. Furthermore, it raises component costs compared to radar-based level gauges that operate in only a single frequency band.

[0009] The invention is therefore based on the objective of providing a radar-based level measuring device for several frequency bands, which is advantageously designed in these respects.

[0010] The invention solves this problem by means of a radar-based level measuring device for determining the level of a product, comprising at least the following components: a first high-frequency unit, with a first frequency-controlled oscillator to generate a first radar signal in a first frequency band according to the FMCW principle and to receive it after reflection; a second high-frequency unit, with a second frequency-controlled oscillator to generate a second radar signal in a second frequency band according to the FMCW principle and to receive it after reflection; an antenna arrangement by means of which the first radar signal and the second radar signal can be transmitted to the product and received after reflection at the surface of the product; and a control-evaluation unit designed to determine the level based on the first received signal and / or the second received signal.

[0011] According to the invention, the level measuring device is characterized by a clock unit which can form a first phase-locked loop and a second phase-locked loop, respectively, with the first and second oscillators in order to generate the corresponding radar signal or received signal according to the FMCW principle. Which of the oscillators is currently forming the phase-locked loop, or in which frequency band the level is currently to be determined, can, with appropriate design, be controlled by the control-evaluation unit of the level measuring device.

[0012] The clock unit can be configured to include at least one frequency divider for the oscillator currently forming the phase-locked loop, a phase detector with a first input for a reference oscillator and a second input to which the frequency divider is connected as an output, and a charge pump connected downstream of the phase detector. The invention is thus based on the idea that the individual phase-locked loops for their respective frequency bands can, in principle, share these components. This results in a corresponding reduction in the number of components and the space required. It goes without saying that the invention is not limited to just two frequency bands or two high-frequency units, but can, in principle, be used for any number of them, for example, three frequency bands.

[0013] In order for the clock unit to function for at least two phase-locked loops, it is advantageous if the frequency divider has an adjustable division factor, and the control-Z evaluation unit sets the first division factor of the frequency divider depending on which of the oscillators is currently forming the phase-locked loop.

[0014] It is also advantageous if the clock unit's charge pump is powered by the supply voltage of the oscillator currently forming the phase-locked loop. This primarily reduces phase noise. From this perspective, it is also essential that a filter unit is connected downstream of the level sensor's clock unit, as the filter unit converts the charge pump's current-based output into a corresponding voltage signal. If the filter unit cannot be adequately tuned to the first and second oscillators, the level sensor can additionally or alternatively include a first filter stage upstream of the first high-frequency unit and / or a second filter stage upstream of the second high-frequency unit.

[0015] A simple embodiment of the level measuring device according to the invention consists of controlling the high-frequency units by means of the control-evaluation unit in such a way that the radar signals of both frequency bands are transmitted simultaneously. At least in this case, the first oscillator and the second oscillator can be coupled at a common fundamental frequency, i.e., operated in so-called "injection-locking" mode. If simultaneous transmission in both frequency bands is not desired, the level measuring device must again include a switching unit to connect the clock unit to the first oscillator and / or the second oscillator, thus forming the corresponding phase-locked loop. The switching unit can also be controlled accordingly by the control-evaluation unit. The switching unit can be implemented, for example, based on planar RF power dividers or broadband RF switches.

[0016] With regard to the level measuring device according to the invention, the term "unit" is understood to mean, in principle, any circuit group intended for a specific purpose, e.g., as an interface or for high-frequency signal processing. Depending on the intended use, the respective unit may therefore comprise corresponding analog circuits for generating or processing corresponding analog signals. However, the respective unit may also comprise digital circuits, such as FPGAs, microcontrollers, or storage media, in conjunction with corresponding programs. The program is designed to perform the necessary process steps or to apply the required arithmetic operations. In this context, various electronic circuits of the unit according to the invention can potentially also access a common physical memory or be operated by means of the same physical digital circuit.It is irrelevant whether different electronic circuits within the unit are arranged on a common circuit board or on several interconnected circuit boards.

[0017] The invention is explained in more detail with reference to the following figures. Figure 1 shows a radar-based level gauge on a container, and

[0018] Fig. 2: a structure of the level measuring device according to the invention,

[0019] Fig. 3: a high-frequency unit of the level gauge,

[0020] Fig. 4: a clock unit of the level gauge,

[0021] Fig. 5: a first embodiment of a filter unit after the clock unit or a filter stage before the respective high-frequency unit, and

[0022] Fig. 6: a second embodiment of a filter unit after the clock unit or a filter stage before the respective high-frequency unit.

[0023] For a basic understanding of the invention, Fig. 1 shows a container 3 with a substance 2, the level L of which is to be determined by a radar-based level gauge 1. Depending on the type of substance 2 and the application, the container 3 can be up to 100 m high. The optimal frequency band in which the level gauge 1 determines the level L also depends on the type of substance 2 and the application: In the case of a coarse-grained substance 2 and correspondingly diffuse reflection, a relatively low frequency band, for example 6 GHz, is generally suitable. Lower frequency bands are also more appropriate for foaming substances 2, since the foam does not reflect the signal. In the case of a refinery tank as the container 3, a higher frequency band is advantageous due to the flat surface of the substance, as this inherently allows for a potentially higher distance resolution.

[0024] The level sensor 1 is usually connected via a separate interface unit, using a transmission protocol such as "4-20 mA", "PROFIBUS", or "HART". 1 , or “Ethernet 1The level gauge 1 is implemented and connected to a higher-level unit 4, such as a local process control system or a decentralized server system. The measured level value L can be transmitted via this connection, for example, to control the inflow or outflow of the container 3. Other information about the general operating status of the level gauge 1 can also be communicated. To determine the level L, the level gauge 1 is mounted above the contents 2 at a known installation height h above the bottom of the container 3. The level gauge 1 is attached and oriented to a corresponding opening in the container 3 in such a pressure- and media-tight manner that only an antenna assembly 13 of the level gauge 1 is directed vertically downwards into the container s towards the contents 2, while the other components of the level gauge 1 remain outside the container 3.Radar signals SHFI,2 are transmitted via the antenna arrangement 13 within predefined frequency bands towards the surface of the material 2. After reflection of the radar signals SHFI,2 from the surface of the material, the level measuring device 1 receives the reflected received signals RH I ,2 again via the antenna arrangement 13. The signal propagation time t between transmission and reception of the respective radar signal S,RHFI,2 is given by... proportional to the distance d between the level measuring device 1 and the contents 2, where c represents the medium-dependent and usually at least roughly known propagation speed of the respective radar signal S, RH I ,2.

[0025] The signal propagation time t is determined by the level gauge 1 using the FMCW method. Accordingly, the frequency fzpi,2 represents the intermediate frequency signal ZFI ,2 that is obtained after receiving and mixing with the outgoing radar signal S, RHFI,2, according to

[0026] _ fzFl,2

[0027] f'1,2 is the signal propagation time t between transmission and reception. f'1,2 is the preset and therefore known frequency scaling rate of the transmitted radar signal SHFI,2. The frequency fzpv of the intermediate frequency signal ZFI,2 can be determined, for example, by its Fourier transform. This allows the level gauge 1 to assign the measured propagation time t to the respective distance d, based on appropriate calibration. Using this, the level gauge 1 can then determine the fill level L according to d = h - L, provided the installation height h is stored in the level gauge 1. To determine the signal propagation time t or the corresponding fill level value L based on the low-frequency intermediate frequency signal ZFI,2, the level gauge 1 includes a suitably designed control / evaluation unit 14.

[0028] In the level gauge 1, a high-frequency unit 11, 12 is used to generate the radar signal SHFI,2 to be transmitted and to create the corresponding intermediate frequency signal ZFI,2. According to the prior art, each high-frequency unit 11, 12 comprises a correspondingly designed phase-locked loop (PLL) for implementing the FMCW method for the desired frequency band. On the receiving side, a Fourier transform logic is used in the control / evaluation unit 14 to detect the frequency fzpi,2 of the intermediate frequency signal ZFI,2 corresponding to the distance d.

[0029] The center frequency or frequency band of the radar signal SHFI,2 must be selected primarily depending on the application area and, in particular, on the type of material 2 being measured: For highly accurate level measurement, such as in oil storage tanks, a frequency band that is as high as possible is inherently advantageous, whereas for an uneven or wavy surface of the material being measured, a wide beam angle of the antenna arrangement 13, and thus a comparatively low frequency band, is advantageous. In the context of the present invention, the term "beam angle" refers to the solid angle within which the antenna arrangement 13 exhibits a defined, uniform transmit intensity or receive sensitivity of, for example, -3 dB.

[0030] To be used under these application conditions, the level gauge 1 shown in Fig. 1 is capable of transmitting radar signals SHFI and SHF2 in two different frequency bands, whereby the frequency bands do not overlap but are clearly distinct from one another. The selection of the frequency band on which the level gauge 1 bases its determination of the level value L can either be manually specified, or the level gauge 1 can select the most suitable frequency band itself. In the latter case, the level gauge 1 can be designed to automatically select the underlying frequency band depending on certain parameters, such as a possible rate of change of the level value L. This is also described in publication DE 10 2021 131 690 A1.

[0031] As shown in Fig. 2, the level gauge 1 comprises two separate high-frequency units 11 and 12, each designed to generate radar signals SHFI,2 ZU within the corresponding frequency band and, after reflection, to receive and process the corresponding received signals RHFI,2 ZU. In the illustrated embodiment, the first high-frequency unit 11 operates, for example, at a center frequency of 180 GHz or in a corresponding first frequency band. The second frequency band, in which the second radar signal SHF,2 is generated by the second high-frequency unit 12, has, for example, a center frequency of 26 GHz. In the embodiment shown in Fig. 1 and Fig. 2, all frequency bands and the underlying radar signals S and RHFI,2 are transmitted and received via the same antenna arrangement 13.In contrast, it is also conceivable to use separate antennas for sending and receiving, so that the transmit / receive switch 18 can be dispensed with.

[0032] According to the invention, the two high-frequency units 11, 12 do not each comprise a complete phase-locked loop. Rather, with the exception of a first and second frequency-controlled oscillator 111, 121, respectively, the two high-frequency units 11, 12 share the other components of the phase-locked loop in a common clock unit 15, as can be seen in Fig. 2. Fig. 3 illustrates in this context that the clock unit 15 comprises a phase detector 152 and a charge pump 154 ​​connected downstream of it in the signal direction. The output of the charge pump 154 ​​can be switched to the frequency-controlled oscillator 111, 121 of both high-frequency units 11, 12. To minimize phase noise, it is advisable to operate the charge pump 154 ​​with the same supply voltage that supplies the currently active oscillator 111 , 121.

[0033] The phase detector 152 has two inputs. A reference oscillator 151 is connected to the first input of the phase detector 152 to provide a reference frequency. The output of a frequency divider 153 is connected to the second input of the phase detector 152. This divider divides the output signal SHFI,2 of the oscillator 111 or 121, which is currently forming the phase-locked loop, by an adjustable division factor N.

[0034] This configuration allows the clock unit 15 to generate a first phase-locked loop for the first radar signal SHFI in the first frequency band and a second phase-locked loop for the second radar signal SH2 in the second frequency band, respectively, using both the first oscillator 111 and the second oscillator 121. Due to the position of the frequency bands, the frequencies of the output signals SHFI,2 of oscillators 111 and 121 are significantly separated. Therefore, the division factor N of the frequency divider 153 must be adjusted, depending on the currently active oscillator 111 or 121, such that the frequency applied to the second input of the phase detector 152 lies within the same frequency range as that of the reference oscillator 151. This is controlled by the control-evaluation unit 14.

[0035] In the embodiment shown in Fig. 2, a switching unit 16 controls which of the two phase-locked wipers is currently configured, or which of the two high-frequency units 11, 12 is currently active. For this purpose, the switching unit 16 is arranged downstream of the charge pump 154, between the clock unit 15 and the respective frequency-controlled oscillator 111, 121 of the high-frequency units 11, 12. To control the units accordingly, the switching unit 16 shown in Fig. 2 comprises two changeover switches 161, 162. The individual changeover switches 161, 162 can be designed as high-frequency power dividers or as broadband high-frequency switches.

[0036] The first switch 161 controls which of the two frequency-controlled oscillators 111 and 121 the charge pump 154 ​​drives. The second switch 162 controls which of the two frequency-controlled oscillators 111 and 121 is connected to the frequency divider 153 of the clock unit 15 via feedback. It goes without saying that both switches 161 must be switched in the same direction to form or activate the respective phase-locked loop: To form the phase-locked loop belonging to the first high-frequency unit 11, the first switch 161 must be switched so that the charge pump 154 ​​is connected to the first frequency-controlled oscillator 111. For this purpose, the second switch must be configured so that the output signal SHFI of the first frequency-controlled oscillator 111 is fed back to the frequency divider 153.The same applies to the formation of the phase-locked loop belonging to the second high-frequency unit 12. As shown in Fig. 4, a prescaler 116, 126 can optionally be connected downstream of each of the frequency-controllable oscillators 111, 112 within the high-frequency unit 111, 112 in order to reduce the incoming frequency of the output signal SHFI, 2 by a defined factor x beforehand.

[0037] The frequency ramp characteristic of the FMCW method is set at the respective frequency-controlled oscillator 111, 121 via its supply voltage by the control-evaluation unit 14. Figure 4 illustrates that the high-frequency units 11, 12 each comprise more than just a frequency-controlled oscillator 111, 121. In the embodiment shown there, the output signal SHFI,2 of the frequency-controlled oscillator 111, 121 is fed to the antenna array 13 via an optional output signal amplifier 112, 122 and a transmit-receive switch 113, 123. After reception, the received signals RHFI,2 are fed via the respective signal switch 113, 123 and an optional received signal amplifier 114, 124 to a mixer 115, 125. As shown in Fig.As can be seen from 4, the signal SHFI,2 emanating from the frequency-controlled oscillator 111 , 121 is also supplied to the mixer 115, 125 in order to obtain the intermediate frequency signal ZFI ,2 characteristic of FMCW.

[0038] In the embodiment shown in Fig. 2, a filter unit 17 is connected downstream of the clock unit 15. If the different frequency bands of the two high-frequency units 11, 12 are far apart, it is possible that both frequency-controlled oscillators 111, 121 cannot be tuned equally to the filter unit 17, or vice versa. Therefore, each frequency-controlled oscillator 111, 121 in the high-frequency unit 11, 12 is preceded by an additional, individual filter stage 17', 17". In contrast to the embodiment shown in Fig. 2, depending on the design of the filter stages 17', 17" it is conceivable to omit the filter unit 17 altogether.

[0039] Possible embodiments of the filter unit 17, one of the filter stages 17', 17" or the filter unit 17 together with one of the filter stages 17', 17" are shown in Figs. 5 and 6: The embodiment shown in Fig. 5 is designed as a passive third-order filter. For this purpose, the filter shown in Fig. 5 is based on three capacitors C1, C2, C3 connected sequentially to ground in the signal direction. The embodiment shown in Fig. 6 is designed as an active filter and is based on a non-inverting operational amplifier p1. In the case of this embodiment, it is again advantageous to operate the operational amplifier p1 with the same supply voltage that also supplies the oscillator 111, 121 of the currently active phase-locked loop.

[0040] Regardless of the type of implementation, it is crucial that the filter unit 17, in conjunction with the respective filter stage 17', 17", is applied to the corresponding

[0041] The phase control loop is adapted.

[0042] Reference symbol list

[0043] 1 level gauge

[0044] 2 Filling material

[0045] 3 containers

[0046] 4. Higher-level unit

[0047] 11 First high-frequency unit

[0048] 12 Second high-frequency unit

[0049] 13 Antenna Arrangement

[0050] 14 Tax Evaluation Unit

[0051] 15-beat unit

[0052] 16 switching unit

[0053] 17 filter unit

[0054] 17', 17" filter stages

[0055] 18 signal switches

[0056] 111 First frequency-controlled oscillator

[0057] 112 First output signal amplifier

[0058] 113 First signal switch

[0059] 114 First receiving signal amplifier

[0060] 115 First mixer

[0061] 116 First Prescaler

[0062] 121 Second frequency-controlled oscillator

[0063] 122 Second output signal amplifier

[0064] 123 Second signal switch

[0065] 124 Second receive signal amplifier

[0066] 125 Second mixer

[0067] 126 Second Prescaler

[0068] 151 Reference Oscillator

[0069] 152 Phase detector

[0070] 153 Frequency dividers

[0071] 154 Charge pump

[0072] C1, 2, 3 capacitors c propagation speed of radar signals d distance h installation height

[0073] L level p1 operational amplifier

[0074] RHFI,2 received signals

[0075] SHFI,2 radar signals

Claims

Patent claims 1. Radar-based level measuring device for determining the level (L) of a fill material (2), comprising: a first radio frequency unit (11), with a first frequency-controlled oscillator (111) to generate a first radar signal (SHFI) in a first frequency band according to the FMCW principle and to receive it after reflection; a second radio frequency unit (12), with a second frequency-controlled oscillator (121) to generate a second radar signal (SHF2) in a second frequency band according to the FMCW principle and to receive it after reflection; an antenna arrangement (13) by means of which the first radar signal (SHFI) and the second radar signal (SH2) can be transmitted to the fill material (L) and received after reflection at the surface of the fill material; a control-evaluation unit (14) designed to use the first received signal (RH) to I) and / or to determine the fill level (L) based on the second received signal (RH 2),characterized by a clock unit (15) designed to form a first phase-locked loop with the first oscillator (111) and a second phase-locked loop with the second oscillator (121) respectively, in order to generate the corresponding radar signal (SHFI,2) or receive signal (R HFI,2) according to the FMCW principle, wherein the control evaluation unit (14) is designed to control which of the oscillators (111 , 121) is currently forming the phase-locked loop.

2. Level measuring device according to claim 1, wherein the clock unit (15) comprises at least one frequency divider (153) for the one of the two oscillators (111, 121) which currently forms the phase-locked loop, a phase detector (152) with, o a first input for a reference oscillator (151) and o a second input to which the frequency divider (153) is connected on the output side, and a charge pump (154) which is connected downstream of the phase detector (152).

3. Level measuring device according to claim 2, wherein the frequency divider (155) has an adjustable division factor (N), and wherein the control-Z evaluation unit (14) is designed to adjust the first division factor (N) depending on which of the oscillators (111 , 121) is currently forming the phase-locked loop.

4. Level measuring device according to claim 2 or 3, wherein the charge pump (154) is operated with the supply voltage of the oscillator (111 , 121) which currently forms the phase control loop.

5. Level measuring device according to one of the preceding claims, comprising: a switching unit (16) designed to connect the clock unit (15) to the first oscillator (111) and / or the second oscillator (121) so that the corresponding phase control loop is formed, wherein the control evaluation unit (14) is designed to control the switching unit (16).

6. Level measuring device according to one of the preceding claims, wherein the first oscillator (111) and the second oscillator (121) are coupled at a common fundamental frequency.

7. Level measuring device according to claim 6, wherein the control-evaluation unit (14) is designed to control the high-frequency units (11, 12) such that the radar signals (SHFI,2) of both frequency bands are emitted simultaneously.

8. Level measuring device according to any one of the preceding claims, comprising: A filter unit (17) downstream of the clock unit (15).

9. Level measuring device according to claim 8, wherein the filter unit (17) is tuned to the first oscillator (111) and the second oscillator (111).

10. Level measuring device according to one of claims 1 to 8, comprising: a first filter stage upstream of the first high-frequency unit (11). (17'), and / or a second filter stage (17") upstream of the second high-frequency unit (12).

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