Fill-level meter

Abstract

The invention relates to an FMCW-based fill-level measuring device (1) comprising the following components: a signal generating unit (11) for generating the ramp-shaped signal (SHF); an antenna (12) for transmitting the signals towards the contents (2) and for receiving the corresponding received signal (rHF); a signal-processing unit (13) which generates a frequency spectrum (sm) of the analysis signal (sZF) per measurement cycle according to the FMCW principle; and a control / analysis unit (15) in order to determine the distance (d) using the frequency spectrum (sm). The fill-level meter (1) is characterized in that a frequency band (Δf) and a measurement range can be specified, and on the basis thereon the fill-level meter (1) can autonomously set or change the threshold frequencies (fstart, fstop) and / or the ramp gradient (f'). This is advantageous in that the use location of the fill-level meter (1) is not limited to specific regions, such as China, the USE, or Europe, after being manufactured. Additionally, the logistics for manufacturing the fill-level meter (1) are simplified.

Description

[0001] Level measuring device

[0002] The invention relates to a radar-based level measuring device that can be individually adapted with regard to its frequency band.

[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 devices to record process parameters such as level, flow, pressure, temperature, pH value, redox potential, or conductivity. The Endress + Hauser Group manufactures and distributes a wide variety of field device types.

[0004] Non-contact measuring methods have become established for level measurement of filling materials in containers because they are robust and low-maintenance. A further advantage of non-contact measuring methods is their ability to measure the fill level virtually continuously. In the field of continuous level measurement, radar-based measuring methods are therefore predominantly used (in the context of this patent application, the term "radar" refers to signals or electromagnetic waves with frequencies between 0.03 GHz and 300 GHz). In principle, the higher the frequency, the higher the measurement resolution that can be achieved. The pulse transit time method and FMCW ("Frequency Modulated Continuous Wave") have become established measurement methods. Radar-based level measurement is described in more detail, for example, in "Radar Level Detection, Peter Devine, 2000f".

[0005] Typical frequency bands approved for radar-based level measurement are 26 GHz, 60 GHz, 80 GHz, and 120 GHz, and increasingly also 180 GHz and 240 GHz. The exact location and range of the frequency band depend on the respective country or the precise measuring location where the level measuring device is used: For example, in the case of 80 GHz, the corresponding frequency band in Europe is 76-84 GHz, while in Canada, for example, it is 77-81 GHz. The measuring range that the level measuring device must cover depends on the container in which the level is to be determined.

[0006] Accordingly, the required measuring range that the level measuring device must cover is limited to 15 m for smaller containers, while it can be up to 100 m for larger applications.

[0007] The operating principle of FMCW is based on transmitting a radar signal with a ramp-like frequency change in the direction of the product. After reflection from the product surface, the corresponding received signal is mixed with the generated radar signal to obtain a low-frequency intermediate frequency signal. The frequency of the intermediate frequency signal represents the distance to the product or the fill level. However, presetting the cutoff frequencies makes it impossible to adapt the level gauge to new measuring conditions. Within the desired frequency band, the frequency of the transmitted radar signal has a defined lower and upper cutoff frequency and a defined ramp steepness, which are pre-adjusted during production to suit the later application location and the later measuring range.However, due to the presetting of these parameters, it is no longer possible to subsequently adapt the level gauge to different measuring environments or conditions. The invention therefore aims to provide a radar-based level gauge that overcomes these disadvantages.

[0008] The invention solves this problem by means of an FMCW radar-based distance measuring device for measuring a distance to an object, which comprises the following components:

[0009] A signal generation unit which is designed to cyclically generate an electrical high-frequency signal whose frequency has a temporal frequency ramp with a defined ramp steepness and a defined ramp duration between a lower limit frequency and an upper limit frequency, an antenna by means of which the high-frequency signal can be transmitted as a radar signal in the direction of the object and, after reflection from the object, can be received as a corresponding received signal, and a signal processing unit, with o a mixer stage which is designed to generate a low-frequency intermediate frequency signal per measuring cycle based on the electrical high-frequency signal and the received signal, in accordance with the FMCW principle, o a transformer stage which is designed to create a frequency spectrum of the evaluation signal per measuring cycle, and a control-evaluation unit which is designed to determine the distance based on the frequency spectrum.

[0010] The distance measuring device according to the invention is characterized in that the desired frequency band and the desired measuring range can be specified for the signal generation unit. This allows the control / evaluation unit to control the signal generation unit in such a way that the cutoff frequencies and / or the ramp steepness are adjusted depending on the specified frequency band and the measuring range.

[0011] The advantage of this is that the location of use of the distance measuring device is not limited to a specific region. Furthermore, this simplifies the logistics during the production of the distance measuring device. Therefore, it is particularly suitable to use the distance measuring device according to the invention for measuring the fill level of a product in a container. If at least the ramp duration of the frequency ramp is preset in the signal generation unit, the signal generation unit can adjust the cutoff frequencies to set, provided that the ramp steepness f is according to below a maximum ramp steepness (f' ma x). In the other case, the cut-off frequencies are determined according to the formula where the difference between the cut-off frequencies (f stop - f start is thus set smaller than the specified frequency band Af. Both cutoff frequencies f stop and f start must be within frequency band A.

[0012] If the distance measuring device according to the invention comprises an analog / digital converter stage which digitizes the intermediate frequency signal in the signal direction upstream of the transformer stage, and if the distance measuring device comprises an 11 R or FIR filter with integrated decimator arranged downstream of the analog / digital converter stage in the signal direction, which subjects the digitized intermediate frequency signal to low-pass filtering, the distance measuring device according to the invention can be advantageously further developed: In this case, the control / evaluation unit can be designed such that the decimation factor of the decimator is set, in particular antiproportionally, depending on the ramp steepness or the measuring range.

[0013] Furthermore, the distance measuring device can be further developed so that the control / evaluation unit includes a position sensor for determining the current location. This allows the signal generation unit to independently adjust or change the cutoff frequencies and ramp duration depending on the determined location. The position sensor can be, for example, a GPS module, a WLAN module, and / or a GSM module.

[0014] Distance measuring device Alternatively or additionally, the signal generation unit can be assigned an input unit by means of which the cut-off frequencies can be manually adjusted or changed at the respective location. The term "unit 1In the context of the invention, a "unit" is understood to mean, in principle, a separate arrangement or encapsulation of those electronic circuits 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, different 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.

[0015] The invention is explained in more detail with reference to the following figures. They show:

[0016] Fig. 1 : A radar-based level gauge on a container,

[0017] Fig. 2: a possible design variant of the antenna arrangement, and

[0018] Fig. 3: a schematic representation of a typical FMCW frequency ramp.

[0019] For a basic understanding, Fig. 1 shows a container 3 containing a medium 2, whose fill level L is to be determined. Depending on the type of medium 2 and the area of ​​application, the container 3 can be up to more than 100 m high. The conditions within the container s also depend on the type of medium 2 and the area of ​​application. For example, exothermic reactions can lead to high temperature and pressure loads. For dusty or flammable substances, appropriate explosion protection conditions must be observed inside the container.

[0020] In order to be able to determine the fill level L independently of the prevailing conditions, a level gauge 1 operating according to the FMCW principle is mounted above the filling material 2 at a known installation height h above the brine of the container s. The level gauge 1 is attached and aligned in a pressure- and media-tight manner to a corresponding opening of the container 3 such that only the antenna 12 of the level gauge 1 is directed vertically downwards into the container 3 toward the filling material 2, while the other components of the level gauge 1 are arranged outside the container 3.

[0021] Radar signals SHF are transmitted via antenna 12 toward the surface of the medium 2. According to the FMCW principle, the frequency of the radar signal SHF is ramped between a lower limit frequency fstart and an upper limit frequency fstop per measurement cycle. The limit frequencies fstart and fstop are based on a frequency band Af specified during production.

[0022] After reflection of the radar signal SHF at the surface of the filling material, the level measuring device 1 receives the reflected reception signals RHF again via the antenna 12. The resulting signal propagation time t between transmission and reception of the respective radar signal SHF, RHF is according to accordingly proportional to the distance d between the level measuring device 1 and the filling material 2, where c corresponds to the radar propagation speed of the respective speed of light. The signal propagation time t can be determined by the level measuring device 1 using the FMCW method indirectly from the received signal RHF, as explained with reference to Fig. 2. For example, based on an appropriate calibration, the level measuring device 1 can assign the measured propagation time t to the respective distance d. Using this, the level measuring device 1 can in turn determine the filling level L according to d = h - L, provided the installation height h is stored in the level measuring device 1.

[0023] Typically, a control / evaluation unit of the level measuring device 1 is connected to a higher-level unit 4, such as a local process control system or a decentralized server system, via a separate interface unit, 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 if necessary. However, other information about the general operating status of the level measuring device 1 can also be communicated.

[0024] The circuit design of the level measuring device 1 is shown in Fig. 1: Accordingly, the radar signal SHF to be transmitted is generated as an electrical high-frequency signal SHF in a signal generation unit 11, which in the embodiment shown is based on a phase-locked loop (known primarily in English as a “Phase Locked Loop, PLL”).

[0025] The high-frequency signal SHF is generated according to the FMCW principle with a corresponding ramp-shaped frequency change. The high-frequency signal SHF is then fed to the antenna 12 via a transmit / receive combiner 14, from where the high-frequency signal SHF is transmitted as a radar signal SHF. The transmit / receive combiner 14 can be designed, for example, as a diplexer or a duplexer.

[0026] The corresponding received signal r received by the antenna 12 is first fed to a mixer stage 131 via the signal splitter 14 in a signal processing unit 13 of the level measuring device. There, the received signal THF is mixed with the high-frequency signal SHF generated by the signal generation unit 11, thereby generating a low-frequency intermediate frequency signal SZF according to the FMCW principle.

[0027] The intermediate frequency signal SZF is time- or value-discretized in the signal direction behind the mixer stage 131 by an analog-to-digital converter stage 132, so that a transformer stage 134 can generate a frequency spectrum s m In the signal direction, a digital low-pass filter stage 133 can optionally be arranged behind the analog-to-digital converter stage 132 in order to filter out any noise and other interference. From the frequency spectrum s m the control-evaluation unit 15 can determine the maximum which can be assigned to the reflection on the filling material 2 in order to determine the distance d or the filling level L on the basis of this.

[0028] Fig. 3 shows the time-linear frequency change of the high-frequency or radar signal SHF, SHF, typical for FMCW, in the form of a ramp, as generated by the signal generation unit 11 per measurement cycle: The frequency ramp is characterized by a ramp duration T, a ramp steepness f and a lower limit frequency fstart as well as an upper limit frequency fstop, whereby the limit frequencies fstart, fstop, depending on the selection of the respective frequency band Af, do not have to coincide with its limits, but may be located within the frequency band Af. These parameters depend according to together.

[0029] According to the invention, the desired frequency band Af can be specified for the level measuring device 1 even after its completion, for example via an input mask of the control / evaluation unit 15, or indirectly via the higher-level unit 4. As shown in Fig. 2, the control / evaluation unit 15 controls the signal generation unit 11 accordingly. Furthermore, the desired measuring range of the level measuring device 1 can be adjusted manually or automatically. This can also be done, for example, via the operating and input mask, or indirectly via the higher-level unit 4. The measuring range must be set so that at least the installation height h is detected. In contrast to the measuring range and the frequency band Af, it is advantageous to preset the ramp duration T of the frequency ramp in the signal generation unit 11 to a defined value due to the upper limit of the sampling frequency of the analog / digital converter 132.The value of the ramp duration T is selected taking into account the required computing power, the necessary storage capacity, and the available energy. Under these boundary conditions, the level gauge 1 sets the ramp slope f according to the input of the measuring range h and the frequency band Af. The ramp steepness f is determined according to limited by the previously selected measuring range. Accordingly, the level measuring device 1 sets the limit frequencies fstart, fstop according to fstop ~ fstart &f, provided that the selected frequency band Af is not so wide that the maximum ramp steepness f'max is exceeded. Otherwise, the limit frequencies fstart, fstop are set within the frequency band Af according to limited, as shown in Fig. 3.

[0030] An automatic setting or changing of the frequency band Af and the measuring range h, if required, can be implemented by storing an assignment table in the control / evaluation unit 15. The table assigns different regions as potential locations to the frequency band Af of the respective region. In this case, a position sensor implemented in the control / evaluation unit 15 can be used to determine the region in which the level measuring device is currently located. Based on the current region, the level measuring device 1 can thus assign the associated frequency band Af using the table. As an alternative to a position sensor, it is also conceivable for the level measuring device 1 according to the invention to determine its current position and thus the respective region through contact with a possible mobile radio or WLAN network. For this purpose, the level measuring device 1 must comprise a corresponding GSM or WLAN module.List of reference symbols.

[0031] 1 level gauge

[0032] 2 Filling material

[0033] 3 containers

[0034] 4 Superior unit

[0035] 11 Signal generation unit

[0036] 12 Antenna

[0037] 13 Signal processing unit

[0038] 14 Transmit / Receive Switch

[0039] 15 Control / Evaluation Unit 131 Mixer Stage

[0040] 132 Analog / digital converter stage

[0041] 133 Low-pass filter stage

[0042] 134 T ransformator stage fstart, stop Cut-off frequencies C Radar propagation speed d Distance h Installation height L Level

[0043] PHF Received signal SHF Radar signal Sm Frequency spectrum SHF Electrical high-frequency signal SZF Intermediate frequency signal

[0044] Af Frequency band f , f max (Maximum) ramp steepness

[0045] T, Tmax (Maximum) ramp duration

Claims

Patent claims 1. FMCW radar-based distance measuring device for measuring a distance (d) to an object (2), comprising: A signal generation unit (11) designed to cyclically generate a high-frequency electrical signal (SHF) ZU, the frequency (fü ) of which is between a lower cutoff frequency (fstart) and an upper cutoff frequency (f s t op) a temporal frequency ramp with a defined ramp steepness (f') and a defined ramp duration (T), an antenna (12) by means of which the high-frequency signal (SHF) can be transmitted as a radar signal (SH) in the direction of the object (2) and, after reflection from the object (2), can be received as a corresponding received signal (THF), a signal processing unit (13) with: o a mixer stage (131) designed to generate a low-frequency intermediate frequency signal (SZ) per measurement cycle based on the electrical high-frequency signal (SHF) and the received signal (THF) according to the FMCW principle, o a transformer stage (134) designed to generate a frequency spectrum (s) per measurement cycle m ) of the evaluation signal (SZ ) to create, and a control / evaluation unit (15) is designed to use the frequency spectrum (s m) to determine the distance (d), characterized in that a frequency band (Af) and a measuring range can be specified to the signal generation unit (11), and that the control / evaluation unit (15) controls the signal generation unit (11) in such a way that the cutoff frequencies (fstart, fstop) and / or the ramp steepness (f) are set as a function of the specified frequency band (Af) and the measuring range.

2. FMCW radar-based distance measuring device according to claim 1, characterized in that a ramp duration (T) of the frequency ramp in the signal generation unit (11) is fixed preset.

3. FMCW radar-based distance measuring device according to claim 2, characterized in that the signal generation unit (11) sets the cutoff frequencies fstart, fstop according to fstop ~ fstart &f, provided that the ramp steepness (f') according to ff max = / l below a maximum ramp gradient (f' ma x) is located.

4. Distance measuring device according to claim 1, 2 or 3, comprising: An analog / digital converter stage (132) designed to digitize the intermediate frequency (IF) signal in the signal direction before the transformer stage (134).

5. Distance measuring device according to claim 4, comprising: A low-pass filter stage (133) arranged in the direction of the signal behind the analog-to-digital converter stage (132), which is designed to subject the digitized intermediate frequency (IF) signal to low-pass filtering.

6. Distance measuring device according to claim 5, wherein the low-pass filter stage (133) is designed as an HR or FIR filter with integrated decimator, and wherein the control / evaluation unit (15) is designed to set a decimation factor of the low-pass filter stage (133), in particular inversely proportionally, depending on the ramp steepness (f') or the measuring range.

7. Distance measuring device according to at least one of the preceding claims, wherein the control / evaluation unit (15) comprises a position sensor by means of which a momentary operating location of the distance measuring device (1) can be determined, and wherein the control / evaluation unit (15) is designed to set or change the cut-off frequencies (fstart, fstop) and the ramp duration (T) at the signal generation unit (11) depending on the determined operating location.

8. Distance measuring device according to claim 5, wherein the position sensor comprises a GPS module, a WLAN module and / or a GSM module, by means of which the place of use can be determined.

9. Distance measuring device according to one of the preceding claims, wherein the signal generation unit (11) is associated with an input unit by means of which the cut-off frequencies (fstart, fstop) can be manually set or changed.

10. Use of the distance measuring device (1 ) according to at least one of the preceding claims for measuring a fill level (L) of a fill material (2) in a container (3).

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