Level gauge

Abstract

The invention relates to an FMCW-based level measuring device (1) that can operate in two clearly distinct frequency bands: For this purpose, the level measuring device (1) comprises two high-frequency units (11, 12) each with a frequency-controllable oscillator (111, 112) in order to transmit radar signals (S) in the frequency bands according to the FMCW principle. HF1,2 ) to generate or, after reflection from the surface of the contents, to receive. An antenna arrangement (13) serves to transmit and receive the radar signals (S HF1, 2) towards the filling material (L) and for reception after reflection at the filling material surface, wherein a control / evaluation unit (14) is used based on the received signal (R) HF1,2The level measuring device (1) can determine the fill level (L) using the first or second frequency band. According to the invention, the level measuring device (1) is characterized by a common clock unit (15). This unit serves to form a phase-locked loop with both the first oscillator (111) and the second oscillator (121) in order to generate the corresponding radar signals (S) according to the FMCW principle. HF1,2 , R HF1,2 ) of both frequency bands. This eliminates the need for a complete phase-locked loop for each frequency band. As a result, the number of required components is reduced.

Description

[0001] The invention relates to a radar-based level measuring device that can be adapted to a wide variety of applications.

[0002] 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.

[0003] 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.

[0004] 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.

[0005] 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 failure 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.

[0006] 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.

[0007] 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.

[0008] 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 aspects.

[0009] The invention solves this problem by means of a radar-based level measuring device for determining the fill level of a product, which comprises 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 towards the contents and received after reflection at the surface of the contents, - a control / evaluation unit designed to determine the fill level based on the first received signal and / or the second received signal.

[0010] 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.

[0011] The clock unit can be designed in such a way that it at least - a frequency divider for the one of the two oscillators that is 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 on the output side, and - a charge pump which is 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 components and space requirements. It goes without saying that the idea according to the invention is not limited to just two frequency bands or two high-frequency units, but in principle to any number, for example, three frequency bands.

[0012] 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 / evaluation unit sets the first division factor of the frequency divider depending on which of the oscillators is currently forming the phase-locked loop.

[0013] 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 to have a filter unit downstream of the level gauge'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 gauge can be additionally or alternatively equipped with a filter unit. - one of the first high-frequency units, the first filter stage, and / or - include a second filter stage upstream of the second high-frequency unit.

[0014] 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.

[0015] 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.

[0016] The invention is explained in more detail using the following figures. It shows: Fig. 1: A radar-based level gauge on a container, and Fig. 2: a level measuring device according to the invention, Fig. 3: a high-frequency unit of the level gauge, Fig. 4: a clock unit of the level gauge, Fig. 5: a first implementation variant of a filter unit after the clock unit or a filter stage before the respective high-frequency unit, and Fig. 6: a second variant of a filter unit after the clock unit or a filter stage before the respective high-frequency unit.

[0017] For a basic understanding of the invention, in Fig. Figure 1 shows a container 3 with a substance 2, the fill 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 fill 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 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.

[0018] The level gauge 1 is typically connected to a higher-level unit 4, such as a local process control system or a decentralized server system, via a separate interface unit implementing a transmission protocol such as "4-20 mA", "PROFIBUS", "HART", or "Ethernet". The measured level value L can be transmitted via this interface, 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 measuring device 1 is attached and aligned to a corresponding opening of the container 3 in such a pressure- and media-tight manner that only an antenna arrangement 13 of the level measuring device 1 is directed vertically downwards into the container 3 in the direction of the filling material 2, while the other components of the level measuring device 1 remain outside the container 3.

[0019] Radar signals are transmitted via the antenna arrangement 13 within predefined frequency bands. HF1,2 emitted in the direction of the surface of the fill material 2. After reflection of the radar signals S HF1,2 The level measuring device 1 receives the reflected reception signals R at the surface of the contents. HF1,2 again via the antenna arrangement 13. Here, the signal propagation time t between transmission and reception of the respective radar signal S,R is HF1,2 according to t=2∗dc proportional to the distance d between the level measuring device 1 and the material 2, where c is the medium-dependent and usually at least roughly known propagation speed of the respective radar signal S, R HF1,2 represents.

[0020] The signal propagation time t is determined by the level sensor 1 using the FMCW method. Accordingly, the frequency f represents ZF1,2 of that intermediate frequency signal IF 1,2 the S, R signal, each after receiving and mixing with the outgoing radar signal HF1,2 is obtained according to t=fZF1,2f'1,2 The signal propagation time t between transmission and reception. At f' 1,2 This refers to the preset and therefore known frequency change rate of the emitted radar signal S. HF1,2 The frequency f can be ZF1,2 of the intermediate frequency signal IF 1,2for example, by its Fourier transform. This allows the level gauge 1, for instance, to assign the measured transit time t to the respective distance d based on a corresponding calibration. Using this, the level gauge 1 can, according to d=h−L The fill level L is then determined, provided the installation height h is stored in the level gauge 1. The signal propagation time t and the corresponding fill level value L are determined using the low-frequency intermediate frequency signal ZF. 1,2 The level measuring device 1 includes a correspondingly designed control / evaluation unit 14.

[0021] To generate the radar signal S to be transmitted in each case HF1,2 and to generate the corresponding intermediate frequency (IF) signal 1,2In the level gauge 1, a high-frequency unit 11, 12 serves this purpose. 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 receive side, a Fourier transform logic is used in the control / evaluation unit 14 to determine the frequency f corresponding to the distance d. ZF1,2 of the intermediate frequency signal IF 1,2 to record.

[0022] The center frequency or frequency band of the radar signal S HF1,2The choice of antenna frequency depends primarily on the application area and, in particular, on the type of material being conveyed: For highly accurate level measurement, such as in oil storage tanks, a high frequency band is inherently advantageous, whereas for uneven or wavy material surfaces, a wide radiation angle of the antenna arrangement 13, and thus a comparatively low frequency band, is advantageous. In the context of this invention, the term "radiation 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.

[0023] In order to be used under these application conditions, the in Fig. 1 level gauge shown 1 capable of receiving radar signals S HF1 , S HF2to transmit in two different frequency bands, whereby the frequency bands do not overlap but are clearly distinct from one another. The choice 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.

[0024] As in Fig. As shown in Figure 2, the level measuring device 1 comprises two separate high-frequency units 11, 12, each designed to receive radar signals S within the corresponding frequency band. HF1,2to generate and, after reflection, the corresponding received signals R HF1,2 to receive or process. 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 S HF,2 The frequency generated by the second high-frequency unit 12, for example, is at a center frequency of 26 GHz. In this process, the following occur: Fig. 1 or Fig. In the second variant shown, all frequency bands or the underlying radar signals S, R are used. HF1,2 The same antenna arrangement 13 transmits and receives the signal. In contrast, it is also conceivable to use separate antennas for transmitting and receiving, thus eliminating the need for the transmit / receive switch 18.

[0025] 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-controllable oscillator 111, 121, the two high-frequency units 11, 12 share the other components of the phase-locked loop in a common clock unit 15, as shown in the figure. Fig. 2 can be seen. Fig. Figure 3 clarifies 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 advantageous for the charge pump 154 ​​to be operated with the same supply voltage as the currently active oscillator 111, 121.

[0026] 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 divides the output signal S HF1,2 of the one of the two frequency-controlled oscillators 111, 121 which currently forms the phase-locked loop, frequency-wise with an adjustable division factor N.

[0027] This setup allows the clock unit 15 to generate a first phase-locked loop for the first radar signal S with both the first oscillator 111 and the second oscillator 121. HF1 in the first frequency band or a second phase-locked loop for the second radar signal S HF2 in the second frequency band. The frequencies of the output signals S HF1,2Oscillators 111 and 121 are significantly spaced apart due to the position of their frequency bands. Therefore, the division factor N of the frequency divider 153 must be adjusted, depending on which oscillator 111 or 121 is currently active, 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.

[0028] Which of the two phase-locked wipers is currently configured, or which of the two high-frequency units 11, 12 is currently active, is determined by the Fig. In the embodiment shown in Figure 2, the circuit is controlled by a switching unit 16. For this purpose, the switching unit 16 is arranged behind the charge pump 154, between the clock unit 15 and the respective frequency-controllable oscillators 111, 121 of the high-frequency units 11, 12. To control the circuit accordingly, the switching unit 16 comprises... Fig. 2 Switching unit 16 shown 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.

[0029] The first switch 161 controls which of the two frequency-controlled oscillators 111, 121 the charge pump 154 ​​is driven by. The second switch 162 controls which of the two frequency-controlled oscillators 111, 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. The second switch must be switched so that the output signal S HF1The signal 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 in Fig. As shown in Figure 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 to adjust the incoming frequency of the output signal S HF1,2 to reduce it in advance by a defined factor x.

[0030] 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. This illustrates Fig. 4, that the high-frequency units 11, 12 each do not merely comprise a frequency-controllable oscillator 111, 121. In the embodiment shown there, the output signal S HF1,2The frequency-controlled oscillator 111, 121 is fed to the antenna array 13 via an optional output signal amplifier 112, 122 and via a transmit / receive switch 113, 123. After reception, the received signals R HF1,2 The signal is fed to a mixer 115, 125 via the respective signal switch 113, 123 and an optional receiving signal amplifier 114, 124. As shown in the diagram... Fig. As can be seen from section 4, the mixer 115, 125 is also supplied with the signal S emanating from the frequency-controlled oscillator 111, 121. HF1,2 supplied to generate the intermediate frequency signal (IF) characteristic of FMCW 1,2 to obtain.

[0031] At the in Fig. In the embodiment shown in Figure 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 one shown in Figure 2, this is... Fig. In the 2 shown design variant, depending on the implementation of the filter stages 17', 17'', it is conceivable to dispense with the filter unit 17.

[0032] Possible design variants 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 in Fig. 5 and Fig. 6 shown: The in Fig. The implementation variant shown in Figure 5 is designed as a passive third-order filter. This is based on the [missing information]. Fig. The 5 filters shown are connected to three capacitors C1, 2, 3, connected sequentially to ground in the direction of the signal. The in Fig. The embodiment shown in Figure 6 is designed as an active filter and is based on a non-inverting operational amplifier p1. In 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. Regardless of the implementation method, it is crucial that the filter unit 17, in conjunction with the respective filter stage 17', 17'', is adapted to the corresponding phase-locked loop. Reference symbol list 1 level gauge 2 Filling material 3 containers 4. Higher-level unit 11 First high-frequency unit 12 Second high-frequency unit 13 Antenna Arrangement 14 Control / Evaluation Unit 15-beat unit 16 switching unit 17 filter unit 17', 17'' filter stages 18 signal switches 111 First frequency-controlled oscillator 112 First output signal amplifier 113 First signal switch 114 First receiving signal amplifier 115 First mixer 116 First Prescaler 121 Second frequency-controlled oscillator 122 Second Output Signal Amplifier 123 Second signal switch 124 Second receive signal amplifier 125 Second mixer 126 Second Prescaler 151 Reference Oscillator 152 Phase detector 153 Frequency dividers 154 Charge pump Capacitors C1, C2, C3 c Propagation speed of radar signals d distance h Installation height L level p1 operational amplifier R HF1,2 Received signals S HF1,2 Radar signals QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] WO 2023099269 A1

[0006] DE 10 2021 131 690 A1

[0023]

Claims

[1] Radar-based level measuring device for determining the level (L) of a fill material (2), comprising: - a first high-frequency unit (11), with ◯ a first frequency-controlled oscillator (111) to generate a first radar signal (S) in a first frequency band according to the FMCW principle HF1 ) to generate and receive after reflection, - a second high-frequency unit (12), with ◯ a second, frequency-controlled oscillator (121) to generate a second radar signal (S) in a second frequency band according to the FMCW principle HF2 ) to generate and receive after reflection, - an antenna arrangement (13) by means of which the first radar signal (S HF1 ) and the second radar signal (S HF2 ) are capable of being emitted from the contents (L) and received after reflection at the surface of the contents, - a control / evaluation unit (14) designed to operate based on the first received signal (RHF1 ) and / or based on the second received signal (R HF2 ) to determine the fill level (L), characterized by - a clock unit (15) designed to form a first phase-locked loop and a second phase-locked loop respectively with the first oscillator (111) and with the second oscillator (121) in order to generate the corresponding radar signal (S) according to the FMCW principle HF1,2 ) or received signal (R HF1,2 ) to generate, 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) at least - a frequency divider (153) for the one of the two oscillators (111, 121) which is currently forming the phase-locked loop, - a phase detector (152) with, ◯ a first input for a reference oscillator (151) and ◯ a second input to which the frequency divider (153) is connected on the output side, and - includes 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 / 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 is currently forming the phase-controlled loop. [5] Level measuring device according to any 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-locked 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 (S HF1,2 ) both frequency bands are transmitted simultaneously. [8] Level measuring device according to any 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 any one of claims 1 to 8, comprising: - one of the first high-frequency unit (11) upstream, first filter stage (17'), and / or - one of the second high-frequency unit (12) upstream of the second filter stage (17'').

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