Radar level gauge system and method for product level estimation under high and / or low level measurement conditions

Abstract

A method of estimating a level of a product in a tank using a radar level gauge system and a radar level gauge system for estimating a level of a product in a tank are disclosed. Estimating the level of the product in the tank by transmitting an electromagnetic transmission signal through the feedthrough to the probe; directing the transmission signal towards and into the product in the tank; receiving an electromagnetic reflection signal generated by reflection of the transmission signal; determining an echo representation comprising a reference echo local extremum; and estimating the level of the surface based on a propagation parameter for a local extremum within a first distance from the reference echo local extremum in a case where the echo represents that a local extremum of the same type as the reference echo local extremum within a predetermined first distance from the position of the reference echo local extremum toward the probe tip.

Description

Technical Field

[0001] The invention relates to a radar level gauge system and a method of estimating the level of a product in a tank. Background Art

[0002] Radar level gauge systems are widely used to measure the fill level in tanks. Radar level measurement is usually performed by non-contact measurement, in which an electromagnetic signal is radiated towards the product contained in the tank, or by contact measurement, which is usually called guided wave radar (GWR), in which the electromagnetic signal is guided towards the product and into the product by a probe. The probe is usually arranged at least approximately vertically in the tank. The electromagnetic signal is reflected at the surface of the product, and the reflected signal is received by a receiver or transceiver included in the radar level gauge system. Based on the transmitted signal and the reflected signal, the distance to the surface of the product can be determined.

[0003] More specifically, the distance to the surface of the product is usually determined based on the time between the emission of an electromagnetic signal and the reception of a reflection of the electromagnetic signal in the interface between the atmosphere in the tank and the product contained in the tank. In order to determine the actual filling level of the product, the distance from a reference position to the surface can be determined based on the above-mentioned time (the so-called time of flight) and the propagation speed of the electromagnetic signal along the probe.

[0004] Most radar level gauge systems on the market today are either so-called pulse radar level gauge systems that determine the distance to the surface of the product contained in the tank based on the time difference between the emission of a pulse and the reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the phase difference between the emitted frequency modulated signal and its reflection at the surface. The latter type of system is generally referred to as the FMCW (Frequency Modulated Continuous Wave) type.

[0005] In any case, a propagating electromagnetic signal is typically reflected not only at the impedance transition constituted by the interface between the atmosphere and the surface, but also at several other impedance transitions encountered by the signal. In the case of a GWR system, one such impedance transition is typically caused by the connection between the transceiver and the probe. Typically, the transceiver is located outside the tank and is connected to the probe via a feed-through through the wall (usually the roof) of the tank.

[0006] Such a feedthrough is usually formed of a coaxial line having the probe as its inner conductor, a tank wall or a connection attached to the tank as its outer conductor, and a dielectric member disposed between the inner and outer conductors.

[0007] Feedthrough impedances much higher than about 50 Ω are not feasible due to the need for a sufficiently mechanically strong inner conductor and a practical outer conductor diameter. Therefore, due to its construction, the impedance of a feedthrough is usually similar to that of a typical coaxial cable, i.e., about 50 Ω.

[0008] Since the impedance of the probe is typically quite high (about 200Ω to 300Ω for a two-wire probe and about 300Ω to 400Ω for a single-wire probe), there will be a relatively large impedance step where the feedthrough ends and the probe enters the tank.

[0009] As described above, this impedance transition partially reflects the transmitted electromagnetic signal, generating an echo signal that may be significantly stronger than the surface echo signal generated by the reflection of the transmitted signal at the surface of the product in the tank, especially when the product to be metered is a material that generates a relatively weak echo signal. Examples of products that generate relatively weak echo signals are liquefied natural gas (LNG), liquefied petroleum gas (LPG), oil-based products, and solids such as plastic pellets or grains.

[0010] This in turn can make it difficult to determine the fill level when the surface of the product is close to the feedthrough. A similar situation can exist near the end of the probe, at the bottom of the tank or measuring chamber.

[0011] It is desirable to provide improved radar level gauge systems and methods that allow estimation of the level of product in a tank near a feedthrough and / or near a probe tail end. Summary of the invention

[0012] In view of the above, it would be desirable to provide improved radar level gauge systems and methods that allow for estimating the level of product in a tank near a feedthrough and / or near a probe tail end.

[0013] According to a first aspect of the present invention, there is thus provided a method for estimating the level of a product in a tank using a radar level gauge system, the radar level gauge system comprising a transceiver arranged outside the tank; a probe arranged inside the tank to extend substantially vertically toward the surface of the product and into the product to the end of the probe, the probe having a probe impedance; a feedthrough connecting the transceiver and the probe, the feedthrough having a feedthrough impedance; and a processing circuit system arranged outside the tank and coupled to the transceiver, the method comprising: transmitting an electromagnetic transmit signal by the transceiver through the feedthrough to the probe; directing the transmit signal by the probe toward and into the product in the tank; receiving by the transceiver an electromagnetic reflection signal generated by reflection of the transmit signal at an impedance transition encountered; determining by the processing circuit system an echo representation of the echo signal strength as a function of a propagation parameter based on a timing relationship between the reflection signal and the transmit signal , the propagation parameter indicates the distance from the transceiver, the echo representation includes a reference echo local extremum indicating the reflection of the transmitted signal at the impedance transition from the feed-through impedance to the probe impedance; and when the echo representation has a local extremum of the same type as the reference echo local extremum within a predetermined first distance from the position of the reference echo local extremum toward the probe tail end, the processing circuit system estimates the surface material level based on the propagation parameters for the local extremum within the first distance from the reference echo local extremum; and / or when the echo representation includes a probe tail end echo local extremum indicating the reflection of the transmitted signal at the probe tail end, and the echo representation has a local extremum of the same type as the probe tail end echo local extremum within a predetermined second distance from the position of the probe tail end echo local extremum toward the transceiver, the processing circuit system estimates the surface material level based on the propagation parameters for the local extremum within the second distance from the probe tail end echo local extremum.

[0014] A local extrema can be a local minimum or a local maximum. If a local extrema is a local minimum, then a "local extrema of the same type" is also a local minimum. Similarly, if a local extrema is a local maximum, then a "local extrema of the same type" is also a local maximum.

[0015] The first distance from the position of the local extreme value of the reference echo may be a distance substantially corresponding to the upper boundary of the normal operating range of the radar level gauge system. Similarly, the second distance from the position of the local extreme value of the probe tail echo may be a distance substantially corresponding to the lower boundary of the normal operating range of the radar level gauge system. The first distance and the second distance may be different depending on the specific radar level gauge system configuration and may be 500 mm or less, for example 300 mm or less.

[0016] When the surface of the product approaches the feedthrough, the surface echo seems to disappear due to the superposition with the usually quite strong reference echo. The same situation may also occur when the surface of the product approaches the tail end of the probe. The present invention is based on the recognition that when the surface approaches the feedthrough, the above-mentioned superposition of the reference echo and the surface echo also leads to the formation of the same type of local extrema as the local extrema of the reference echo in the echo representation, and when the surface approaches the tail end of the probe, the superposition of the probe tail end echo and the surface echo can also lead to the formation of the same type of local extrema as the local extrema of the probe echo in the echo representation. Therefore, even when the surface echo seems to disappear due to the above-mentioned superposition with the reference echo, the surface level of the product can be estimated. This can provide an output of more reliable information from the radar level gauge system, and can also provide improved processing control and / or safety.

[0017] According to various examples, the method may include, when the surface level has been estimated based on propagation parameters for a local extrema within a first distance from a local extrema of a reference echo, providing, by the processing circuit system, a first signal indicating the surface level within a first distance; and / or when the surface level has been estimated based on propagation parameters for a local extrema within a second distance from a local extrema of a probe tail end echo, providing, by the processing circuit system, a second signal indicating the surface level within a second distance.

[0018] The first signal may encode an estimate of the level of the surface when the surface is within the first distance; and

[0019] The second signal may encode an estimate of the level of the surface when the surface is within the second distance. Although the level estimate encoded by the first and / or second signal may not be as accurate as the level estimate within the normal measurement range, even an approximate level indication may be very useful when the level is close to the top (or bottom) of the tank. Alternatively or in addition, the first and / or second signal may include an alarm or warning.

[0020] According to various examples, when the echo representation has no local extrema of the same type as the reference echo local extrema within the first distance, the reference echo local extrema may indicate a first maximum received energy from the reflection of the transmitted signal at the impedance transition from the feedthrough impedance to the probe impedance, and when the echo representation has no local extrema of the same type as the probe tail end echo local extrema within the second distance, the probe tail end local extrema indicates a second maximum received energy from the reflection of the transmitted signal at the probe tail end; and the method may include: selecting the reference echo local extrema only when the echo representation has the same type as the reference echo local extrema within the first distance. When the local extrema of the reference echo indicates that the received energy is lower than the first maximum received energy, the processing circuit system estimates the material level of the surface based on the propagation parameters for the local extrema within a first distance from the local extrema of the reference echo; and / or when the echo indicates that there is a local extrema of the same type as the local extrema of the probe tail echo within a second distance and the local extrema of the probe tail echo indicates that the received energy is lower than the second maximum received energy, the processing circuit system estimates the material level of the surface based on the propagation parameters for the local extrema within a second distance from the local extrema of the probe tail echo. In addition to the presence of the same type of local extrema near the reference echo, the finding that the absolute value of the amplitude of the reference echo has decreased increases the possibility that the product surface is indeed close to the feedthrough, for example in an area sometimes referred to as the "upper dead zone". Similarly, this also applies to the probe tail echo.

[0021] Furthermore, in various examples, the method may include estimating, by the processing circuit system, the level of the surface based on the propagation parameters of the echo local extrema when the echo local extrema in the echo representation meets a predetermined surface echo identification criterion including a minimum echo amplitude. This would correspond to normal operation when the surface of the product is within a normal operating range and is not in a lower dead zone or an upper dead zone or the like.

[0022] The transmitted signal may include a first pulse sequence having a first pulse repetition frequency; and the method may further include the following steps: generating, by the transceiver, an electromagnetic reference signal in the form of a second pulse sequence having a second pulse repetition frequency controlled to differ in frequency from the first pulse repetition frequency; and determining an echo signal based on the reflected signal, the reference signal and the frequency difference.

[0023] The pulses in the first pulse train may be so-called DC pulses.

[0024] It should be noted that the steps of the method according to the embodiments of the present invention do not necessarily need to be performed in any particular order, unless explicitly or implicitly required.

[0025] According to a second aspect of the present invention, there is provided a radar level gauge system for estimating the level of a product in a tank, the radar level gauge system comprising: a transceiver arranged outside the tank, the transceiver being configured to generate, transmit and receive electromagnetic signals; a probe having a probe impedance, the probe being arranged inside the tank to extend substantially vertically toward the surface of the product and into the product to a probe tail end, the probe being configured to guide an electromagnetic transmit signal from the transceiver toward and into the product, and to guide an electromagnetic reflection signal generated by reflection of the transmit signal at an impedance transition encountered toward the transceiver; a feedthrough connecting the transceiver and the probe, the feedthrough having a feedthrough impedance; and a processing circuit system, the processing circuit system being arranged outside the tank and coupled to the transceiver, the processing circuit system being configured to: determine an echo signal as a function of a propagation parameter based on a timing relationship between the reflection signal and the transmit signal The invention relates to an echo representation of signal intensity, wherein the propagation parameters indicate the distance from the transceiver, the echo representation including a reference echo local extremum indicating reflection of the transmitted signal at the impedance transition from the feed-through impedance to the probe impedance; and when the echo representation has a local extremum of the same type as the reference echo local extremum within a predetermined first distance from the position of the reference echo local extremum toward the probe tail end, estimating the surface material level based on the propagation parameters for the local extremum within the first distance from the reference echo local extremum; and / or when the echo representation includes a probe tail end echo local extremum indicating reflection of the transmitted signal at the probe tail end, and the echo representation has a local extremum of the same type as the probe tail end echo local extremum within a predetermined second distance from the position of the probe tail end echo local extremum toward the transceiver, estimating the surface material level based on the propagation parameters for the local extremum within the second distance from the probe tail end echo local extremum.

[0026] According to various embodiments, a transceiver may include: a transmit signal generating circuit for generating a transmit signal in the form of a first pulse train having a first pulse repetition frequency; and a reference signal generating circuit for generating an electromagnetic reference signal in the form of a second pulse train having a second pulse repetition frequency controlled to differ in frequency from the first pulse repetition frequency; and an echo signal forming circuit may be configured to form an echo signal based on a reflected signal, a reference signal and a frequency difference.

[0027] Further embodiments and effects obtained by the second aspect of the present invention are largely similar to those of the first aspect of the present invention described above.

[0028] In summary, the present invention therefore relates to a method for estimating the level of a product in a tank by transmitting an electromagnetic transmit signal through a feedthrough to a probe; directing the transmit signal toward and into the product in the tank; receiving an electromagnetic reflection signal generated by reflection of the transmit signal; determining an echo representation including a reference echo local extrema; and in the case of a local extrema of the same type as the reference echo local extrema within a predetermined first distance from the position of the reference echo local extrema toward the tail end of the probe, estimating the level of the surface based on propagation parameters for the local extrema within the first distance from the reference echo local extrema. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] These and other aspects of the present invention will now be described in more detail with reference to the accompanying drawings showing example embodiments of the invention, in which:

[0030] Figure 1 schematically illustrates an exemplary tank arrangement including a radar level gauge system according to an embodiment of the present invention;

[0031] Figure 2 yes Figure 1 A schematic illustration of a measuring unit included in a radar level gauge system;

[0032] Figure 3 is a schematic block diagram of a transceiver included in a radar level gauge system according to an embodiment of the present invention;

[0033] Figure 4 is a flow chart schematically illustrating an example of a method according to the present invention;

[0034] Figure 5 is a schematic illustration of an example of an echo representation when the level of the product is within the normal measurement range;

[0035] Fig. 6A is a schematic illustration of an example of an echo representation when the level of the product is in a high level measurement range; and

[0036] Figure 6B yes Fig. 6A Schematic illustration of an enlarged portion of the echo representation in FIG. DETAILED DESCRIPTION

[0037] Figure 1 Schematically shown is a radar level gauge system 1 according to an example of the invention, which is mounted on a measuring chamber 3 for metering the level of a product 5 in a tank 7. The radar level gauge system 1 of the GWR (Guided Wave Radar) type comprises a measuring electronics unit 9 arranged outside the tank 7 and a transmission line probe 11 arranged inside the tank 7 (in this particular example, the probe 11 is arranged inside the chamber 3 considered to be part of the tank 7). Figure 1As shown schematically, the probe 11 extends toward the surface 13 of the product 5 and extends into the product 5 to the probe tail end 15. In addition, the radar level gauge system 1 includes a feedthrough 17, where the probe 11 passes from the outside of the tank 7 to the inside of the tank 7. When the radar level gauge system 1 is in operation, the probe emits an electromagnetic signal S T is directed toward and into the product 5, and is directed in the emission signal S T The impedance transition / discontinuity encountered is caused by the transmitted signal S T The electromagnetic reflection signal S generated by the reflection R Now we will refer to Figure 2 The schematic block diagram in the Figure 1 An exemplary radar level gauge system in.

[0038] Reference Figure 2 The schematic block diagram in Figure 1 The measuring unit 9 of the radar level gauge system 1 comprises a transceiver 19 and a processing circuit system 21. In addition, Figure 2 The block diagram in shows the following optional functional parts: wireless communication control unit (WCU) 23, communication antenna 25 and energy storage, such as battery 27. As explained in some detail in the background section, probe 11 has a probe impedance, and feedthrough 17 has a feedthrough impedance, which may be significantly different from the probe impedance.

[0039] like Figure 2 As schematically shown, processing circuitry 21 may control transceiver 19 to generate, transmit and receive electromagnetic signals. Transmit signals pass through feedthrough 17 to probe 11 , and receive signals pass from probe 11 through feedthrough 17 to transceiver 19 .

[0040] The processing circuitry 21 may be configured to determine the level of the product 5 and provide a value indicative of the level to an external device, such as a processing unit of a control center, via the WCU 23 through the communication antenna 25. The radar level gauge system 1 may advantageously be configured according to the so-called WirelessHART communication protocol (IEC 62591).

[0041] Although the measuring unit 9 is shown comprising an energy storage 27 and comprising devices for allowing wireless communication (such as WCU 23 and communication antenna 25), it should be understood that power supply and communication can be provided in a different way, such as by a communication line (such as a 4-20mA current line).

[0042] The local energy storage does not have to comprise (only) batteries but may alternatively or in combination comprise capacitors or supercapacitors.

[0043] Now refer to Figure 3The schematic block diagram in the Figure 1 and Figure 2 Radar level gauge system 1. Now refer to Figure 3 , showing that it can be included in Figure 2 A more detailed block diagram of an example transceiver 19 in a radar level gauge system is shown in FIG.

[0044] like Figure 3 As schematically shown in FIG. 1 , the example transceiver 19 may include a device for generating and transmitting a transmission signal S T The transmitter branch, and the one for receiving and operating the reflected signal S R The receiver branch. Figure 3 As shown, both the transmitter branch and the receiver branch are connected to a directional coupler 29 to direct the signal from the transmitter branch to the probe 11 and to direct the reflected signal returned by the probe 11 to the receiver branch.

[0045] like Figure 3 As schematically indicated in FIG. 1 , the exemplary transceiver 19 comprises a pulse generating circuit, here in the form of a first pulse forming circuit 31, a second pulse forming circuit 33 and a timing control unit 35 for controlling the transmission signal output by the first pulse forming circuit 31 and the frequency offset reference signal S output by the second pulse forming circuit 33. REF The timing relationship between them.

[0046] The transmitter branch comprises a first pulse forming circuit 31 , and the receiver branch comprises a second pulse forming circuit 33 and measurement circuitry 37 .

[0047] like Figure 3 As schematically indicated in FIG. 3 , the measurement circuitry 37 comprises a time correlator, here in the form of a mixer 39 , a sample-and-hold circuit 41 , and an amplifier circuitry 43 . In an example, the measurement circuitry 37 may also comprise an integrator 45 .

[0048] In addition, as mentioned above Figure 2 Briefly described, the radar level gauge system 1 comprises processing circuitry 21 connected to measurement circuitry 37 for determining the level of the surface 13 of the product 5 .

[0049] when Figure 3 When the radar level gauge system 1 in FIG. 1 is operated to perform filling level determination, the reflected signal S is processed in the mixer 39. R The reference signal S output by the second pulse forming circuit 33 REF The time correlation between the reference signal S REF is a pulse train with a pulse repetition frequency that is controlled to be consistent with the transmit signal S TThe pulse repetition frequencies of the reference signal S REF and the transmitted signal S T In phase, then determine the reference signal and the reflected signal S R The time-dependent parameter value between Δf and Δf is used to form an echo representation together with the frequency difference Δf. As will be described further below, based on the analysis of the echo representation, the fill level of the surface 13 of the product 5 can be determined.

[0050] The time expansion technique briefly described in the previous paragraph is well known to those skilled in the art and is widely used in pulse radar level gauge systems.

[0051] As is clear from the above discussion, the output of mixer 39 will be a sequence of values, each of which represents a reference signal S REF The pulse and reflected signal S R The values ​​in the sequence of values ​​may be bound together using a sample and hold circuit 41 to form a continuous signal.

[0052] In this context, it should be noted that the sample and hold circuit 41 is merely an illustrative example of a device capable of holding a voltage level for a given time, and that a variety of other devices may provide the desired functionality, as will be appreciated by those skilled in the art.

[0053] exist Figure 3 In the example embodiment of FIG. 4 , the time-dependent signal output from the sample-and-hold circuit 41, the correlation signal S C - is supplied to the integrator 45 to form the measurement signal S M , the measurement signal S M The signal is amplified by the low noise amplifier LNA43. The echo signal forming circuit 47 can be based on the measurement signal S M The above-mentioned echo signal is formed by the frequency difference Δf. According to an example of the present invention, the filling level can be determined by the level determination circuit system 49.

[0054] Now refer to Figure 4 Schematic flow chart in Figure 2 The method includes transmitting 100 an electromagnetic transmission signal S from the transceiver 19 to the probe 11 through the feedthrough 17. T . Transmit signal S T It can be a pulse sequence of transmitting pulses. The probe 11 transmits a signal S T The signal S is directed 101 towards the product 5 in the tank 7 and into the product 5 in the tank 7. The transceiver 19 receives 102 the transmitted signal S T The reflection at the impedance transition thus encountered generates an electromagnetic reflection signal SR .

[0055] The processing circuit system 21 is based on the reflection signal S R With the transmission signal S T The echo representation is a representation of the echo signal strength as a function of a propagation parameter indicative of the electrical distance from the transceiver 19 through the feedthrough 17 and along the probe 11 .

[0056] To illustrate normal measurement conditions, when the level of the product 5 is within the normal measurement range, Figure 5 is a schematic illustration of an example of an echo representation 51 of echo amplitude as a function of distance from the transceiver 19 . Figure 5 The echo representation in includes a reference echo local extremum 53 (local minimum), which indicates that the transmit signal S T The reflection at the impedance transition from the feedthrough impedance of the feedthrough 17 to the probe impedance of the probe 11. Under this normal measurement condition, the reference pulse 53 has a reference amplitude A f,ref . Figure 5 The example echo representation 51 in also includes an indication of the transmitted signal S T The probe end echo local extrema 55 of the reflection at the probe end 15 .

[0057] As stated, Figure 5 is a schematic illustration of an example of an echo representation 51 when the level of the product is within the normal measurement range 57. Thus, the processing circuit system 21 can estimate the level of the surface 13 using a technique known per se to search for an echo local extremum 59 that satisfies predetermined surface echo identification criteria, including a minimum echo amplitude A th The distance of the product 5 from the surface 13 can then be estimated based on the position 61 of the local extrema 59 of the echo.

[0058] When the surface 13 approaches the feedthrough 17, the reflection from the surface 13 is superimposed with the reflection from the impedance transition from the feedthrough impedance of the feedthrough 17 to the probe impedance of the probe 11. Since the amplitude of the reference echo 53 is generally substantially higher than the amplitude of the surface echo 59, this superposition will result in the amplitude of the surface echo 59 first failing to meet the surface echo identification criterion (not reaching the minimum echo amplitude A). th ), and ultimately cannot be identified as a "positive" pulse at all.

[0059] exist Figures 6A to 6B An example of such a situation is schematically shown in FIG. 6A , where 6A is a schematic illustration of an example of an echo representation 63 when the level of the product 5 is above the normal measurement range 57 , and Figure 6B yes Fig. 6AThe echo in FIG. 6 represents an enlarged portion 65 of FIG. 63 .

[0060] From the corresponding high level measurement conditions Fig. 6A The echo in the figure 63 corresponds to the normal measurement condition. Figure 5 It can be clearly seen from the comparison between the echo representation 51 in FIG. 6 and FIG. 7 that in the echo representation 63 corresponding to the high level measurement condition, the amplitude A of the reference pulse 53 is f has decreased somewhat and the surface echo 59 appears to have disappeared.

[0061] It is desirable to be able to determine with relatively high confidence whether the surface 13 of the product 5 is near the feedthrough 17, and therefore possibly near the top of the tank 7. It is also desirable to be able to determine with relatively high confidence whether the surface 13 of the product 5 is near the probe tail 15, and therefore possibly near the bottom of the tank 7.

[0062] Return to Figure 4 , and additionally refer to Figure 6B The method includes: when the echo representation 63 has a local extrema 67 of the same type as the reference echo local extrema 53 (in this case, a local minimum) within a predetermined first distance D from the position 69 of the reference echo local extrema 53 toward the probe tail end 15, the processing circuit system 21 estimates 104 the material level of the surface 13 of the product 5 based on the propagation parameters 71 for the local extrema 67 within the first distance D from the reference echo local extrema 53.

[0063] After the level of the surface 13 has been estimated, the method may optionally include the step of providing 105 by the processing circuitry 21 a first signal indicating that the level of the surface 13 is within the first distance D from the reference echo local extrema 53, when the level of the surface 13 has been estimated based on the propagation parameters 71 for the local extrema 67 within the first distance D from the reference echo local extrema 53. The first signal may encode an estimate of the level of the surface 13 when the surface 13 is within the first distance D.

[0064] In order to improve the reliability of the filling level estimation, the reference echo is estimated only when the local extreme value 53 of the reference echo indicates, for example, the amplitude A of the reference pulse 53. f The received energy indicated is lower than the reference amplitude A under normal measurement conditions, for example f,ref An estimation based on the propagation parameters 71 for the local extrema 67 within the first distance D from the reference echo local extrema 53 can only be performed when the maximum received energy indicated is reached.

[0065] Optionally, when the echo representation includes an indication transmission signal S TWhen the reflected probe tail echo local extremum 55 at the probe tail end 15 and the echo representation has a local extremum of the same type as the probe tail echo local extremum 55 (in this case, a local minimum) within a predetermined second distance from the location of the probe tail echo local extremum 55 toward the transceiver 19, the level of the surface 13 of the product 5 can be estimated based on the propagation parameters for the local extremum within the second distance from the probe tail echo local extremum 55. This low level measurement condition is not explicitly shown in the drawings because it is similar to the reference Figures 6A to 6B The high level measurement conditions are described and are therefore easy to understand for a skilled person based on this description.

[0066] Also in this case, after the level of the surface 13 has been estimated, the method may optionally include the following step: when the level of the surface 13 has been estimated based on the propagation parameter 71 for the local extrema within a second distance from the local extrema of the probe tail echo, a second signal indicating that the level of the surface 13 is within the second distance is provided by the processing circuit system 21. The second signal may encode an estimate of the level of the surface 13 when the surface 13 is within the second distance.

[0067] In order to improve the reliability of the level estimation in other optional situations of low level measurement conditions, the estimation based on the propagation parameters of the local extrema within the second distance from the local extrema 55 at the probe tail end can be performed only when the local extrema of the probe tail end echo indicates that the received energy is lower than the maximum received energy from the reflection at the probe tail end 55 under normal measurement conditions.

[0068] The skilled person realizes that the present invention is by no means limited to the above examples.For example, other probe configurations than a single-wire transmission line probe may be used, such as a two-wire transmission line probe or a coaxial probe.

[0069] A "transceiver" may be one functional unit capable of transmitting and receiving electromagnetic signals, or may be a system comprising separate transmitter and receiver units.

[0070] A can may be any container or tank capable of containing a product and may be metallic or partially or completely non-metallic, open, semi-open or closed.

[0071] The probe should be understood as a waveguide designed to guide electromagnetic signals.The probe may be rigid or flexible and may advantageously be made of metal, such as stainless steel.

[0072] The propagation parameter may be any parameter indicative of the position along the probe.

Claims

1. A method for estimating the level of a product in a tank using a radar level gauge system, the radar level gauge system comprising: a transceiver disposed on the exterior of the tank; a probe arranged inside the tank to extend substantially vertically toward the surface of the product and into the product to a probe tail end, the probe having a probe impedance; a feedthrough connecting the transceiver and the probe, the feedthrough having a feedthrough impedance; and processing circuitry disposed outside the tank and coupled to the transceiver, the method comprising: transmitting, by the transceiver, an electromagnetic transmission signal through the feedthrough to the probe; directing, by the probe, the transmit signal toward and into the product in the tank; receiving, by the transceiver, an electromagnetic reflection signal generated by reflection of the transmit signal at an encountered impedance transition; determining, by the processing circuitry, an echo representation of echo signal strength as a function of a propagation parameter based on a timing relationship between the reflected signal and the transmitted signal, the propagation parameter indicative of a distance from the transceiver, the echo representation comprising a reference echo local extrema indicative of reflection of the transmitted signal at an impedance transition from the feed-through impedance to the probe impedance; and When the echo representation has a local extrema of the same type as the reference echo local extrema within a predetermined first distance from the position of the reference echo local extrema toward the probe tail end, the processing circuit system estimates the surface level based on the propagation parameters for the local extrema within the first distance from the reference echo local extrema; and / or When the echo representation includes a probe tail end echo local extremum indicating reflection of the transmitted signal at the probe tail end, and the echo representation has a local extremum of the same type as the probe tail end echo local extremum within a predetermined second distance from the position of the probe tail end echo local extremum toward the transceiver, the processing circuit system estimates the surface material level based on propagation parameters for the local extremum within the second distance from the probe tail end echo local extremum.

2. The method according to claim 1, wherein: The method comprises: providing, by the processing circuitry, a first signal indicating that the level of the surface is within the first distance from the reference echo local extrema when the level of the surface has been estimated based on the propagation parameters for the local extrema within the first distance from the reference echo local extrema; and / or When the level of the surface has been estimated based on the propagation parameters for the local extrema within the second distance from the probe tail echo local extrema, a second signal is provided by the processing circuitry indicating that the level of the surface is within the second distance.

3. The method according to claim 2, wherein: the first signal encodes an estimate of the level of the surface when the surface is within the first distance; and The second signal encodes an estimate of the level of the surface when the surface is within the second distance.

4. The method according to claim 1, in, When the echo representation has no local extrema of the same type as the local extrema of the reference echo within the first distance, the local extrema of the reference echo indicates a first maximum received energy, and when the echo representation has no local extrema of the same type as the local extrema of the probe tail end echo within the second distance, the local extrema of the probe tail end indicates a second maximum received energy; as well as Wherein, the method comprises: estimating, by the processing circuit system, the level of the surface based on the propagation parameters for the local extrema within the first distance from the reference echo local extrema only when the echo representation has a local extrema of the same type as the reference echo local extrema within the first distance and the reference echo local extrema indicates a received energy lower than the first maximum received energy; and / or The processing circuit system estimates the material level of the surface based on the propagation parameters for the local extrema within the second distance from the local extrema of the probe tail end echo only when the echo representation has a local extrema of the same type as the local extrema of the probe tail end echo within the second distance and the local extrema of the probe tail end echo indicates that the received energy is lower than the second maximum received energy.

5. The method according to claim 1, wherein: The method comprises: When an echo local extrema in the echo representation meets predetermined surface echo identification criteria including a minimum echo amplitude, a level of the surface is estimated by the processing circuitry based on propagation parameters of the echo local extrema.

6. The method according to claim 1, wherein: The method comprises: When the echo representation has a local extrema of the same type as the reference echo local extrema within a predetermined first distance from the position of the reference echo local extrema toward the probe tail end, the processing circuit system estimates the material level of the surface based on the propagation parameters of the local extrema within the first distance from the reference echo local extrema.

7. The method according to claim 1, wherein: The method comprises: When the echo representation has a local extrema of the same type as the local extrema of the probe tail end echo within a predetermined second distance from the position of the local extrema of the probe tail end echo toward the transceiver, the processing circuit system estimates the material level of the surface based on the propagation parameters of the local extrema within the second distance from the local extrema of the probe tail end echo.

8. The method according to claim 1, wherein: The first distance is less than 500 mm; and The second distance is less than 500 mm.

9. A radar level gauge system for estimating the level of a product in a tank, the radar level gauge system comprising: a transceiver disposed on the exterior of the tank, the transceiver configured to generate, transmit and receive electromagnetic signals; a probe having a probe impedance, the probe being arranged inside the tank to extend toward the surface of the product and into the product to a probe tail end, the probe being configured to direct an electromagnetic transmission signal from the transceiver toward and into the product, and to direct an electromagnetic reflection signal generated by reflection of the transmission signal at an impedance transition encountered toward the transceiver; a feedthrough connecting the transceiver and the probe, the feedthrough having a feedthrough impedance; and a processing circuit system disposed outside the tank and coupled to the transceiver, the processing circuit system being configured to: determining an echo representation of echo signal strength as a function of a propagation parameter based on a timing relationship between the reflected signal and the transmitted signal, the propagation parameter being indicative of a distance from the transceiver, the echo representation comprising a reference echo local extrema indicative of reflection of the transmitted signal at an impedance transition from the feed-through impedance to the probe impedance; and When the echo representation has a local extrema of the same type as the reference echo local extrema within a predetermined first distance from the position of the reference echo local extrema toward the probe tail end, estimating the level of the surface based on the propagation parameters for the local extrema within the first distance from the reference echo local extrema; and / or When the echo representation includes a probe tail end echo local extremum indicating reflection of the transmitted signal at the probe tail end, and the echo representation has a local extremum of the same type as the probe tail end echo local extremum within a predetermined second distance from the position of the probe tail end echo local extremum toward the transceiver, the object level of the surface is estimated based on propagation parameters for the local extremum within the second distance from the probe tail end echo local extremum.

10. The radar level gauge system according to claim 9, wherein: The processing circuit system is configured to: providing a first signal indicating that the level of the surface is within the first distance from the reference echo local extrema when the level of the surface has been estimated based on the propagation parameters for the local extrema within the first distance from the reference echo local extrema; and / or When the level of the surface has been estimated based on the propagation parameters for the local extrema within the second distance from the probe tail echo local extrema, a second signal is provided indicating that the level of the surface is within the second distance.

11. The radar level gauge system according to claim 9, wherein: The processing circuit system is configured to: forming the first signal such that the first signal encodes a level of the surface when the surface is within the first distance; and The second signal is formed such that the second signal encodes a level of the surface when the surface is within the second distance.

12. The radar level gauge system according to claim 9, in, When the echo representation has no local extrema of the same type as the reference echo local extrema within the first distance, the reference echo local extrema indicates a first maximum received energy, and when the echo representation has no local extrema of the same type as the probe tail end echo local extrema within the second distance, the probe tail end local extrema indicates a second maximum received energy; and in, The processing circuit system is configured to: estimating the level of the surface based on the propagation parameters for the local extrema within the first distance from the reference echo local extrema only when the echo representation has a local extrema of the same type as the reference echo local extrema within the first distance and the reference echo local extrema indicates a received energy lower than the first maximum received energy; and / or The material level of the surface is estimated based on the propagation parameters for the local extrema within the second distance from the local extrema of the probe tail end echo only when the echo representation has a local extrema of the same type as the local extrema of the probe tail end echo within the second distance and the local extrema of the probe tail end echo indicates that the received energy is lower than the second maximum received energy.

13. The radar level gauge system according to claim 9, wherein: The processing circuit system is configured to: When an echo local extrema in the echo representation meets a predetermined surface echo identification criterion including a minimum echo amplitude, a material level of the surface is estimated based on a propagation parameter of the echo local extrema.

14. The radar level gauge system according to claim 9, wherein: The first distance is less than 500 mm; and The second distance is less than 500 mm.

15. The radar level gauge system according to claim 9, wherein: The probe is a single conductor probe.

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