Guided wave radar level gauge and method for controlling a guided wave radar level gauge

Abstract

The invention relates to a guided wave radar level gauge and a method for controlling a guided wave radar level gauge. A guided wave radar level gauge (1) for determining a fill level of an article contained in a storage tank comprises a transceiver (17) configured to provide a transmission signal Tx signal in the form of a sequence of pulses with a controllable pulse repetition frequency f Tx and to receive a reflection signal resulting from a reflection of the transmission signal at a surface of the article, a probe (11) connected to the transceiver and configured to propagate the Tx signal to the surface and to return the reflection signal to the transceiver, the probe having a known length, and a control circuit (19) configured to determine the fill level based on the received reflection signal, wherein the control circuit is further configured to provide the pulse repetition frequency to the transceiver based on the length of the probe.

Description

TECHNICAL FIELD

[0001] The present invention relates to guided wave radar level gauges and to a method for controlling a guided wave radar level gauge. In particular, the present invention aims at reducing the influence of a probe end reflection in a guided wave radar level gauge. BACKGROUND

[0002] Radar level gauge (RLG) systems are widely used to determine the fill level of an article contained in a tank. Radar level gauging is typically performed by a non-contact measurement whereby electromagnetic signals are radiated towards the article contained in the tank, or by a contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards the article and into the article by a probe acting as a waveguide. The probe is typically arranged to extend vertically from the top towards the bottom of the tank.

[0003] The transmitted electromagnetic signals are reflected at the surface of the article and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge. Based on the transmitted and reflected signals, the distance from the surface of the article can be determined. More specifically, the distance from the surface of the article is typically determined based on the time between the transmission of the electromagnetic signal and the reception of the reflection of the electromagnetic signal at the interface between the atmosphere in the tank and the article contained therein. In order to determine the actual fill level of the article, the distance from a reference position to the surface is determined based on the above-mentioned time (so-called time of flight) and the propagation speed of the electromagnetic signal.

[0004] One class of RLG systems involves so-called pulsed RLG systems which determine the distance to the surface of the article contained in the tank based on the time difference (time of flight) between the transmission of a pulse and the reflection of the pulse at the surface of the article.

[0005] Most pulsed radar level gauge systems employ a time domain reflectometer (TDR) which provides a time expansion of the (very short) time of flight. Such TDR radar level gauge systems generate a sequence of transmitted pulses with a first pulse repetition frequency Tx and a sequence of reference pulses with a second pulse repetition frequency Rx which differs from the repetition frequency of the transmitted pulses by a known frequency difference Af. This frequency difference Af is typically in the range of a few Hz or tens of Hz.

[0006] The sequence of transmitted pulses is emitted towards the surface of the article contained in the tank (non-contact or probe) and the reflected signals are received and sampled using the sequence of reference pulses. At the start of the measurement scan, the transmitted and reference signals are synchronized to have the same phase. Due to the frequency difference, the phase difference between the transmitted and reference signals will gradually increase during the measurement scan. The time sampling of this gradual shift of the reflected signals will provide a time expanded version of the time of flight of the reflected pulses from which the distance to the surface of the article contained in the tank can be determined.

[0007] The transmitted signal will typically not only be reflected at the impedance transition constituted by the interface between the tank atmosphere and the surface of the product, but also at several other impedance transitions encountered by the transmitted signal. In particular for guided wave radar systems, the signal is reflected at the end of the probe acting as a waveguide. Thus, the probe end reflection can interfere with the measurement since the probe end echo can be mistaken for a surface echo.

[0008] US 2002 / 0026828 addresses the above problem by arranging an absorbing means at the end of the probe such that the electromagnetic waves reaching the end of the probe can only be reflected to a limited extent, thereby effectively reducing the false echoes.

[0009] However, since the reflected signal is only partially absorbed by the described means, there is still a residual reflection which in some applications and under certain circumstances can affect the product level measurement.

[0010] Therefore, for some applications it is desirable to further reduce the influence of the probe end reflection in a guided wave radar level gauge. SUMMARY

[0011] In view of the above-mentioned and other drawbacks of the prior art, it is an object of the present application to provide a solution to the influence of the probe end reflection in a guided wave radar level gauge.

[0012] According to a first aspect of the present application, there is provided a guided wave radar level gauge for determining a fill level of a product contained in a tank, comprising a transceiver configured to provide a transmitted signal Tx signal in the form of a sequence of pulses having a pulse repetition frequency f Tx and to receive a reflected signal resulting from a reflection of the transmitted signal at a surface of the product; a probe connected to the transceiver and configured to propagate the Tx signal towards said surface and to return the reflected signal to the transceiver, the probe having a known length; and a control circuit configured to determine the fill level based on the received reflected signal, wherein the control circuit is configured to provide the pulse repetition frequency to the transceiver based on the length of the probe.

[0013] The present invention is based on the insight that depending on the probe length and the pulse repetition frequency, the two and three bounces from the probe tip reflection can not be suppressed before the next pulse is transmitted. The effect of this is that the two and three bounces from the old pulse are measured in the next tank signal. The radar level gauge system of the described type can be used in very large tanks, for example tanks for containing liquid natural gas (LNG), where the probe can be up to 50 m or longer. This in turn has the effect that the pulse repetition frequency of the TDR that can be suitable for other applications will be disturbed by the two and three probe tip reflection forms when using longer probes. In particular, longer probes require a lower pulse repetition frequency to avoid undesired interactions.

[0014] In the present context, the control circuit is considered to control the transceiver such that the pulse repetition frequency is determined by, stored in or received by the control circuit and subsequently provided to the transceiver forming the transmit signal. However, the same functionality can equally be provided in a system having a different architecture, for example in an integrated module comprising both the functionality of the transceiver and the associated control circuit.

[0015] Thus, by controlling the pulse repetition frequency based on the length of the probe, the frequency can be controlled such that the probe tip reflections do not disturb the level measurement. Thereby, in addition to improving the measurement quality, the present invention is advantageous in that because the pulse repetition frequency can be set based on the size of the tank, the described radar level gauge can be used in tanks of different sizes, which is not possible in a pulsed radar level gauge system having a fixed pulse repetition frequency. In short, the described system and method can reduce the effect of disturbing bounce echoes in long probes by having a controllable pulse repetition frequency.

[0016] For example, the transceiver having a controllable pulse repetition frequency can be implemented using a PLL (Phase-Locked Loop) circuit configured to receive an oscillator frequency as an input frequency and provide an adjusted output frequency.

[0017] According to one embodiment of the application, the control circuit is configured to set the pulse repetition frequency such that the travel time of two probe tip reflections is less than the period of the Tx signal. The travel time of two probe tip reflections is the time it takes for a pulse to travel from the transceiver to the probe tip, back to the transceiver, again to the probe tip, and back to the transceiver. It is thus two reflections at the probe tip. The period of the Tx signal is simply the time between two successive pulses of the pulse sequence, i.e. the inverse of the pulse repetition frequency. The travel time also depends on the dielectric properties of the material surrounding the probe. However, since, as a first approximation, the dielectric constant can be assumed to be known, and with sufficient accuracy to accurately determine the travel time of the probe tip reflections and to set the pulse repetition frequency thereafter.

[0018] According to one embodiment of the application, the control circuit is configured to set the pulse repetition frequency such that the travel time of three probe tip reflections is less than the period of the Tx signal. The travel time of three probe tip reflections is thus the time it takes for a signal to travel the length of the probe six times. In certain applications, it can not be sufficient to eliminate the effect of only two probe tip reflections, but it is necessary to take into account three probe tip reflections as well. The effect of three probe tip reflections can thus be eliminated by further reducing the pulse repetition frequency.

[0019] According to one embodiment of the application, the control circuit is further configured to determine the length of the probe and to set the pulse repetition frequency based on the determined length of the probe. The length of the probe can be determined, for example, if it is known that the tank is empty, or if both the fill level and the dielectric properties of the medium in the tank are known. However, the length of the probe can also be set manually by an operator and / or the control circuit uses a stored value for the length of the probe when setting the pulse repetition frequency.

[0020] According to one embodiment of the application, the control circuit is further configured to determine the dielectric constant of the medium surrounding the probe and to set the pulse repetition frequency based on the determined dielectric constant. Since the propagation speed depends on the dielectric properties of the medium surrounding the probe, the pulse repetition frequency can be set with higher accuracy if the propagation speed is known. If both the fill level and the length of the probe are known, the dielectric constant can be determined, for example. There can also be several different media surrounding the probe, for example the contents of the tank and the tank atmosphere, in which case the effective propagation speed can be estimated using the known or estimated fill level of the tank.

[0021] The control circuit can thus be configured to determine the effective length of the probe based on the properties of the medium surrounding the probe and to set the pulse repetition frequency based on the determined effective length, and / or to determine the effective length of the probe based on the fill level of the tank and to set the pulse repetition frequency according to the determined effective length.

[0022] According to an embodiment of the application, the control circuit is further configured to detect a change in the fill level and to set the pulse repetition frequency based on a current fill level of the tank. Since the dielectric surrounding of the probe changes as the fill level in the tank changes, the pulse repetition frequency can be set more accurately by determining the effective length of the probe from the effective propagation speed of the signal and taking into account the full length of the probe in the dielectric surrounding.

[0023] According to a second aspect of the application, there is provided a method of determining a fill level of an article contained in a tank using a guided wave radar level gauge. The method comprises forming, by a transceiver, a transmission signal Tx signal in the form of a sequence of pulses having a pulse repetition frequency f Tx ; receiving a reflected signal resulting from a reflection of the transmission signal at a surface of the article, wherein the transmission signal propagates along a probe connected to the transceiver and configured to propagate the transmission signal towards the surface and return the reflected signal to the transceiver; wherein forming the Tx signal comprises setting the pulse repetition frequency based on a length of the probe; and determining the fill level based on the received reflected signal.

[0024] The effects and features of the second aspect of the application are largely analogous to those described above in connection with the first aspect of the application.

[0025] Further features and advantages of the application will become apparent when the claims and following description are considered. It will be understood that different features of the application can be combined in different ways to create further embodiments of the application not expressly described in the following description. BRIEF DESCRIPTION OF DRAWINGS

[0026] These and other aspects of the application will now be described in more detail, with reference to the appended drawings showing example embodiments of the application, in which:

[0027] Figure 1 An exemplary tank arrangement comprising a guided wave radar level gauge system according to an embodiment of the application is schematically illustrated.

[0028] Figure 2 is a schematic illustration of a measurement unit comprised in a radar level gauge system according to an embodiment of the application. Figure 1

[0029] Figure 3 is a diagram schematically outlining a signal used in an embodiment of the application.

[0030] Figures 4A-4B Features of a radar level gauge according to an embodiment of the application are schematically illustrated; and

[0031] Figure 5 ​is a flow chart outlining the general steps of the method according to an embodiment of the application. DETAILED DESCRIPTION

[0032] In the present detailed description, various embodiments of the system and method according to the present application are mainly described with reference to a guided wave radar level gauge installed in a tank located on land. However, the described system and method are applicable in other fields, such as marine applications. Furthermore, the various embodiments of the present application are mainly discussed with reference to a pulsed radar level gauge system having a signal propagation device in the form of a probe and wireless communication capabilities.

[0033] It should be noted that this in no way limits the scope of the present application, which also encompasses pulsed radar level gauge systems having additional types of signal guiding devices and pulsed radar level gauge systems configured for wired communication, such as using a 4-20 mA current loop and / or other wired communication means.

[0034] Figure 1 An exemplary radar level gauge system 1 of the GWR (guided wave radar) type installed at a tank 3 having a tubular mounting structure 5 (commonly referred to as a "nozzle") extending substantially vertically from the top of the tank 3 is schematically shown.

[0035] The guided wave radar level gauge system 1 is installed to measure the fill level of an item 7 in the tank 3. The radar level gauge system 1 comprises a measuring unit 9 and a propagation device, here in the form of a single conductor probe 11, extending from the measuring unit 9 through the tubular mounting structure 5 towards and into the item 7 in the tank 3. In Figure 1 In the exemplary embodiment, the single conductor probe 11 is a wire probe with a weight 13 attached to the end of the wire to keep the wire straight and vertical. The probe 11 can also be attached to the bottom of the tank. However, the probe can equally be any other type of probe suitable for guided wave radar applications.

[0036] By analyzing a transmitted signal S T guided by the probe 11 towards the surface 15 of the item 7 and a reflected signal S R traveling back from the surface 15, the measuring unit 9 can determine the fill level L of the item 7 in the tank 3. It should be noted that although a tank 3 containing a single item 7 is discussed here, the distance to any material interface along the probe can be measured in a similar manner. The radar level gauge in Figure 2 will now be described in more detail with reference to the schematic block diagram in Figure 1

[0037] With reference to the schematic block diagram in Figure 2 , the radar level gauge in Figure 1 ​The measurement unit 9 of the exemplary radar level gauge system 1 in Fig. 1 comprises a transceiver 17, a measurement control circuit (here labelled measurement control unit (MCU) 19), a wireless communication control unit (WCU) 21, a communication antenna 23 and an energy store, e.g. a battery 25.

[0038] As Figure 2 illustrated schematically in Fig. 1, the MCU 19 controls the transceiver 17 to generate, transmit and receive electromagnetic signals. The transmitted signals are passed to the probe 11 through a feedthrough, and the received signals are passed from the probe 11 to the transceiver 17 through the feedthrough.

[0039] The MCU 19 determines the fill level L of the goods 7 in the tank 3, and provides a value indicative of the fill level from the MCU 19 via the WCU 21 to an external device, such as a control centre, through the communication antenna 23. The radar level gauge system 1 can advantageously be configured in accordance with the so-called WirelessHART communication protocol (IEC 62591).

[0040] Although it is shown that the measurement unit 9 comprises an energy store (battery 25) and comprises means for allowing wireless communication, such as the WCU 21 and the communication antenna 23, it will be appreciated that the power supply and communication can be provided in different ways, e.g. through a communication line (e.g. a 4-20 mA line).

[0041] The local energy store 25 need not (only) comprise a battery, but can instead or in combination comprise a capacitor or a supercapacitor.

[0042] Furthermore, the measurement control unit (MCU) 19 can more generally be referred to as a control circuit 19, and the control circuit 19 can comprise a microprocessor, a microcontroller, a programmable digital signal processor or a further programmable device. The control circuit can also or instead comprise an application specific integrated circuit, a programmable gate array or programmable array logic, a programmable logic device, or a digital signal processor. Where the control circuit comprises a programmable device such as a microprocessor, microcontroller or programmable digital signal processor as described above, the processor can further comprise computer executable code that controls operation of the programmable device.

[0043] The control circuit 19 is further configured to set the pulse repetition frequency f TX based on the length of the probe 11. Tx The time T Tx between successive pulses in the sequence of transmitted pulses is the inverse of the pulse repetition frequency T Tx = 1 / f Tx , as Figure 3 illustrated.

[0044] As Figure 4AAs shown, the secondary probe tip reflection travels the probe length 4 times before reaching the transceiver. double Determined as:

[0045]

[0046] This means that the pulse repetition frequency is set to

[0047]

[0048] So that the travel time of the secondary probe tip reflection is less than the period of the Tx signal. Here, l probe is the length of the probe, c0 is the speed of light, ε r is the relative dielectric constant of the medium in which the probe is immersed. By setting the pulse repetition frequency as shown in the above relationship, it is ensured that the pulse from the secondary reflection reaches the transceiver before the next pulse in the transmitted pulse train. Therefore, the secondary probe tip reflection will not interfere with the measurement.

[0049] like Figure 4B As shown, the three probe tip reflections travel the probe length 6 times before reaching the transceiver. Therefore, the travel time t for the three probe tip reflections can be triple Determined as:

[0050]

[0051] This means that the pulse repetition frequency is set to:

[0052]

[0053] The travel time of three probe tip reflections is shorter than the period of the Tx signal.

[0054] By setting the pulse repetition frequency as shown in the above relationship, you can ensure that the pulse from the third probe tip reflection reaches the transceiver before the next pulse in the transmitted pulse sequence. Therefore, the third probe tip reflection will not interfere with the measurement. You can also set the pulse repetition frequency to avoid the influence of reflections with higher orders (for example, fourth, fifth, sixth, etc.).

[0055] The pulse repetition frequency is thus provided from the control circuit 19 to the transceiver, so that the transceiver 17 can form a signal with a specified pulse repetition frequency. The control circuit 19 can determine the pulse repetition frequency in many different ways. For known fixed applications with a probe length, the pulse repetition frequency can for example be stored in the control circuit during manufacturing or installation. The operator can also provide the pulse repetition frequency to the control circuit 19, either locally or remotely via the communication interface 21, 23 of the radar level gauge 1. The pulse repetition frequency can also be determined automatically by the control circuit 19, either based on a known probe length, or also based on a probe length that the radar level gauge 1 has determined in a measurement, for example in an empty tank measurement. Thereby, the transceiver 17 can form a transmit signal with a pulse repetition frequency that is based on the length of the probe.

[0056] Furthermore, the above calculations can be adapted to take into account the fill level of the tank by determining an effective final propagation speed based on the fill level and the dielectric properties of the material surrounding the probe above and below the fill level.

[0057] Figure 5 is a flow chart outlining the general steps of a method of determining a fill level of an article contained in a tank using a guided wave radar level gauge according to an embodiment of the application, and will be described with reference to the system described in Figure 1 and Figure 2 The method comprises forming (500) a transmit signal Tx signal in the form of a sequence of pulses with a pulse repetition frequency f Tx Here it is assumed that the transceiver 17 comprises all necessary circuitry for forming the Tx signal, and as mentioned above, the transceiver 17 can also be controlled by a measurement control unit (MCU) 19, i.e. by a control circuit.

[0058] The method further comprises transmitting (502) the transmit signal and receiving (504) a reflected signal resulting from reflections of the transmit signal at the surface of the article, wherein the transmit signal propagates along a probe 11 connected to the transceiver 17.

[0059] According to the described method, forming the transmit signal comprises setting the pulse repetition frequency based on the length of the probe; and the method finally determines (506) the fill level based on the received reflected signal.

[0060] When installing the radar level gauge, an operator can manually set the length of the probe in order to store the length of the probe in a storage unit of the radar level gauge accessible to the control circuit. It is also possible to remotely set the probe length using a communication interface of the radar level gauge. Furthermore, the method can further comprise automatically determining the length of the probe and setting the pulse repetition frequency based on the determined length of the probe.

[0061] Since the speed of propagation of the signal depends on the dielectric constant of the medium in which the probe is immersed, the required pulse repetition frequency depends on the dielectric properties of the material in the tank and on the filling level of the tank. Therefore, the method can comprise determining the dielectric properties of the material in the tank.

[0062] Even if the application has been described with reference to particular exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Also, it should be noted that various parts of the system and method can be omitted, interchanged or arranged in various ways without departing from the function of the application.

[0063] In addition, various modifications to the disclosed embodiments will become apparent to those skilled in the art once the application has been realized with the fore going description and accompanying drawings. In the claims, the word "comprising" does not exclude other elements or steps, and the singular "a" or "an" does not exclude a plurality. In the event of multiple dependent claims, the features of the dependent claims can be combined with the features of the independent claims other than those specifically set forth in the combination.

Claims

1. A guided wave radar level meter (1) for determining the filling level of a product contained in a storage tank, comprising: A transceiver (17) configured to provide a pulse with a controllable pulse repetition frequency f Tx a transmission signal Tx signal in the form of a pulse sequence, and receiving a reflection signal generated by reflection of the transmission signal on the surface of the object; a probe (11) connected to the transceiver and configured to propagate the Tx signal toward the surface and return the reflected signal to the transceiver, the probe having a known length; as well as A control circuit (19) is configured to determine the filling level based on the received reflection signal, wherein the control circuit is further configured to: determining the travel time of two probe tip reflections for a known length of the probe; and A pulse repetition frequency is provided to the transceiver so that the travel time of two probe tip reflections is less than the period of the Tx signal.

2. The guided wave radar level meter according to claim 1, wherein: The control circuit is configured to set the pulse repetition frequency so that a travel time of three probe tip reflections is less than a period of the Tx signal.

3. A method for determining the filling level of a product contained in a storage tank using a guided wave radar level meter (1), the method comprising: The transceiver (17) forms (500) a pulse repetition frequency f Tx a transmission signal Tx signal in the form of a pulse sequence, transmitting (502) the transmission signal, and receiving (504) a reflection signal generated by reflection of the transmission signal at the surface of the object, wherein the transmission signal propagates along a probe (11), the probe (11) is connected to the transceiver and is configured to propagate the transmission signal toward the surface and return the reflection signal to the transceiver, and the probe has a known length; determining the travel time of two probe tip reflections for a known length of the probe; and determining (506) the filling level based on the received reflected signal, The forming of the transmission signal includes providing a pulse repetition frequency so that the travel time of two reflections from the probe end is less than the period of the Tx signal.

4. The method according to claim 3, further comprising: The pulse repetition frequency is set so that the travel time of three probe tip reflections is less than the period of the Tx signal.

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