Patent Publication Number: US-2022228900-A1

Title: Waveguide for a radar level gauge

Description:
FIELD OF THE INVENTION 
     The present invention relates to a waveguide for a radar level gauge system. In particular, the present invention relates to a waveguide comprising two waveguide sections. 
     BACKGROUND OF THE INVENTION 
     Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide or transmission line. 
     The transmitted electromagnetic signals are reflected at the surface of the product, 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 to the surface of the product can be determined. More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals. 
     In a radar level gauge system, many different parts must be joined together to both propagate the signal from the signal generation circuitry to the antenna and for providing a tight seal of the tank. In a high-frequency system, it is particularly important to provide a propagation path without any gaps or other interruptions in the waveguide chain. 
     PTFE (Polytetrafluoroethylene) is a commonly used material for seal and/or antenna parts, and due to the thermal expansion properties of the material it is difficult to achieve the tight tolerances of the waveguide that are often required, particularly for high frequencies, i.e. in the GHz range, where a gap-free waveguide chain becomes important to maintain signal performance. To reach the required tolerances, it may be necessary to use special assembly methods such as laser welding which complicates the assembly process. 
     Accordingly, it is desirable to find solutions where high-frequency a radar level gauge and a waveguide signal path can be achieved without resorting to complicated and specialized assembly methods while still fulfilling the required tolerance levels. 
     SUMMARY 
     In view of above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide a waveguide for a radar level gauge comprising a high-frequency waveguide addressing the drawbacks of presently known solutions. 
     According to a first aspect of the invention, there is provided a waveguide for connecting measurement circuitry to an antenna in a radar level gauge. The waveguide comprises: a first tubular waveguide section having a female connecting portion, the first tubular waveguide section having a first abutment surface on an outer surface; a second tubular waveguide section having a male connecting portion arranged within the female connection portion so that the first and second waveguide portions are movable relative each other in an axial direction and to provide a continuous tubular passage through the waveguide, the second tubular waveguide section having a second abutment surface; and a spring arranged to abut against the first abutment surface and the second abutment surface. 
     The antenna is configured to be vertically arranged in a tank or container for measuring the level of a content of the container. In particular, the antenna is typically arranged to emit a signal vertically into a tank such that the signal reaches a surface of a product in the tank and is reflected back towards the antenna where it is received such that the distance from the antenna to the surface can be determined, thereby making it possible to determine the fill level in the tank. 
     The measurement circuitry is typically arranged outside of the tank in a housing connected to a feed-trough where a vertically arranged waveguide connects the measurement circuitry to the antenna. 
     The present invention is based on the realization that issues relating to tight tolerances and the requirement of a gap-free waveguide can be overcome by the described two-part waveguide design which is spring-loaded to avoid gaps also during temperature changes when thermal expansion and contraction may occur. The described waveguide simplifies manufacturing and assembly since it consists of few parts and since machining of the two waveguide sections can be performed with high precision with existing manufacturing methods, thereby providing a cost effective and reliable solution. 
     According to one embodiment of the invention, the spring is biased between the first and second abutment surfaces when the waveguide is installed in a tank. Preferably, the spring is biased so that first and second tubular waveguide sections can move both towards and away from each other. 
     According to one embodiment of the invention, the first and second abutment surfaces are provided in the form of a first and second portion protruding from an outer surface of the first and second waveguide section, respectively. The protruding portions may for example be a respective shoulder of the waveguide section reaching around the circumference of the first and second waveguide section. 
     There is also provided a radar level gauge feed-through assembly comprising a waveguide according to any one of the described embodiments, wherein the radar level gauge feed-through further comprises: a housing; a housing connection; and a dielectric antenna body, wherein the waveguide is arranged between the housing connection and the dielectric antenna body. 
     The radar level gauge feed-through assembly may further comprise a locking ring in which the waveguide is at least partially arranged, the locking ring being configured to allow movement of the first and/or second waveguide portion only in the axial direction. Moreover, the antenna body may be a lens antenna. 
     Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing an example embodiment of the invention, wherein: 
         FIG. 1  schematically illustrates an exemplary tank arrangement comprising a guided wave radar level gauge system according to an embodiment of the invention; 
         FIG. 2  is a block diagram schematically illustrating measurement circuitry of radar level gauge system according to an embodiment of the invention; 
         FIGS. 3A-B  are schematic illustrations of a waveguide according to an embodiment of the invention in open and closed positions; and 
         FIG. 4  is schematic illustration of a radar level gauge feed-through assembly comprising a waveguide according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     In the present detailed description, various embodiments of the waveguide according to the present invention are mainly described with reference to a free-radiating radar level gauge system installed in a tank located on land. However, the described system and method is suitable for use in other areas such as in marine applications. Moreover, various embodiments of the present invention are mainly discussed with reference to a radar level gauge comprising a lens antenna even though other types of antennas are also feasible. 
       FIG. 1  schematically shows a tank level monitoring system  100  comprising an example embodiment of a radar level gauge system  102  wirelessly connected to a host system  104 . In the illustrated example, the radar level gauge system  102  is battery powered. However, the described radar level gauge system  102  may equally well be loop-powered or powered by dedicated power lines. 
     The radar level gauge system  102  comprises a measurement electronics unit  106  arranged on an outside of the tank  108 , an antenna  110  at least partly arranged on an inside the tank  108 , and a feed-through assembly  112  connecting the measurement electronics unit  106  with the antenna  110 . 
     The radar level gauge system  102  is arranged on a tank  108  containing a product  114  to be gauged. To reduce the energy consumption of the radar level gauge system  102 , at least parts of the radar level gauge system  102  may be operated intermittently and energy may be stored during inactive or idle periods to be used during active periods. 
     With reference to  FIG. 2 , the radar level gauge system  102  in  FIG. 1  comprises a measurement unit (MU)  210 , a wireless communication unit (WCU)  211  and a local energy storage unit for example in the form of a battery  212 . The wireless communication unit  211  may advantageously be compliant with WirelessHART (IEC 62591). As is schematically indicated in  FIG. 2 , the MU  210  comprises a transceiver module  213  and a measurement processor  220 . The transceiver module  213  is controllable by the measurement processor  220  for generating, transmitting and receiving electromagnetic signals having frequencies defining a frequency bandwidth, such as 75G-80 GHz. The measurement processor  220  is coupled to the transceiver  213  for determining the filling level in the tank  108  based on a relation between the transmit signal S T  and the reflection signal S R . 
     The measurement processor  220  may include a microprocessor, microcontroller, programmable digital signal processor or another programmable device. The measurement processor  220  may also, or instead, include an application specific integrated circuit, a programmable gate array or programmable array logic, a programmable logic device, or a digital signal processor. Where the measurement processor  220  includes a programmable device such as the microprocessor, microcontroller or programmable digital signal processor mentioned above, the processor may further include computer executable code that controls operation of the programmable device. 
       FIG. 3A  schematically illustrates a waveguide according to an embodiment of the invention and  FIG. 3B  is an exploded view of the waveguide  300 . The waveguide  300  is configured to connect the measurement circuitry illustrated in  FIG. 2  to an antenna. 
     The waveguide  300  comprises a first tubular waveguide section  302  having a female connecting portion  304 . The first tubular waveguide section  302  further comprises a first abutment surface  306  on an outer surface thereof. The second tubular waveguide section  308  comprises a male connecting portion  310  configured to be arranged within the female connection portion  304  so that the first and second waveguide sections  302 ,  308  are movable relative each other in an axial direction and to provide a continuous tubular passage through the waveguide  300 . The second tubular waveguide section  308  further comprises a second abutment surface  312 . 
     To enable relative axial movement in both directions of the waveguide sections  302 ,  308 , a spring  314  is arranged to abut against the first abutment surface  306  and the second abutment surface  308 . The spring  314  is thus arranged between the first and second tubular waveguide sections  302 ,  308  and biased between the first and second abutment surfaces  306 ,  312 . 
     In  FIGS. 3A-B , the first and second abutment surfaces  306 ,  312  are formed by portions of the first and second tubular waveguide sections  302 ,  308  having a larger diameter than adjacent portions which are closer to the opposing waveguide section. Accordingly, the first and second protruding portions  306 ,  312  may form a shoulder reaching around the circumference of the first and second waveguide section  302 ,  308 , respectively. However, it should be noted that the abutment surfaces  306 ,  312  could be formed in many different ways, such as in the form of a collar or in the form of a plurality of protrusions as long as it enables the compression of the spring between the two waveguide sections  302 ,  308 . Moreover, it would also be possible to mechanically attach the ends of the spring  314  to the respective waveguide sections  302 ,  308 . 
     As illustrated in  FIG. 3B , a first end portion  316  of the male connecting portion  310  of the second waveguide section  308 , here illustrated as an upper portion, has a smaller outer diameter compared to remaining portions of the second waveguide section  308 , and the female connecting portion  304  of the first waveguide section  302  has a portion  320  with a correspondingly smaller inner diameter configured to match said outer diameter. The female connection portion  304  and the male connecting portion  310  are thus configured be in contact to form a sliding joint between the first tubular waveguide section  302  and the second tubular waveguide section  308 . 
     To allow relative axial movement of the waveguide sections in both directions, there will be a small gap  324  in the assembled waveguide  300  as illustrated in  FIG. 3A  where the end portion  316  of the second waveguide section does not reach the bottom of the narrower portion  320  of the first waveguide section  302 . To minimize the influence on signal propagation of such a gap  324 , it is desirable to minimize the wall thickness of the end portion  316  of the second waveguide section  308 . Thereby, the end portion  316  of the male connecting element  310  preferably has a wall thickness in the range of 0.1-1 mm. It has been shown that with the aforementioned wall thickness, the gap  324  may be as large as 10 mm without noticeable reduction in the quality of the propagated signal. In an example implementation, the allowable relative movement in the axial direction is in the range of ±1 mm. However, the skilled person readily realizes that the waveguide easily can be configured to allow more movement. 
     Moreover, the illustrated pocket  322  at the top of the first waveguide section is formed as part of a reflection adaptation together with a glass feedthrough  326  for connecting the waveguide  300  to signal generation circuitry. Additionally, a second end portion of the second waveguide section  308  comprises a conical opening  318  acting as a feeder horn for providing the signal to a lens antenna. In applications where the first waveguide section is closest to the antenna, the conical opening is thus arranged in an end portion of the first waveguide section facing the antenna. 
     The inner diameter of the waveguide is preferably selected so that the main propagation mode is the TE 11  mode for a given frequency. In the TE 11  mode, the electric field is perpendicular to the direction of propagation, and for a circular waveguide, a signal in the TE 11  mode propagates with minimum degradation. For a circular waveguide, a diameter of 2.6 mm provides a single mode bandwidth (TE 11 ) for the frequency range 67.6 GHz-88.3 GHz. As discussed above and illustrated in  FIG. 3A , there is a small gap  324  which will result in a portion of the waveguide having a larger diameter than the remainder of the waveguide. In this small portion with the larger dimeter, resonances may arise for certain axial lengths of the gap and for certain frequencies, which in turn means that other modes such as the TM 01  mode may occur. However, the waveguide is preferably designed to minimize the occurrence of resonances for a given operating frequency. 
     Even though embodiments of the waveguide are described with reference to a circular waveguide, the signal propagation path of the waveguide may equally well be formed by a rectangular waveguide. Thereby, the tubular waveguide may have either a circular or a rectangular cross-section. 
     The described waveguide con be configured and adapted in many different ways to suit a given application. The first and second waveguide sections  302 ,  308  may for example be flipped so that the second waveguide section  308  is arranged above the first waveguide section  302 , i.so that the waveguide section with the male connection portion is arranged above the waveguide section with the female connection portion. The lengths and other dimensions of the different parts such as the male and female connecting portions may also be varied as long as the waveguide as a whole fulfils the required signal propagation properties. 
       FIG. 4  schematically illustrates a radar level gauge feed-through assembly  112  comprising the waveguide  300  illustrated in  FIGS. 3A-B . The feed-through assembly  112  further comprises a housing  400 , a housing connection  402 ; and a dielectric antenna body  404 , wherein the waveguide  300  is arranged between the housing connection  402  and the dielectric antenna body  404  here illustrated as a lens antenna. 
     The feed-through assembly  112  further comprises a locking ring  408  in which the waveguide  300  is at least partially arranged. The locking ring  408  is configured to hold the waveguide  300  to allow movement of the first and/or second waveguide portion  302 ,  308  only in the axial direction. 
     In  FIG. 4 , the waveguide is illustrated in connection with a free radiating antenna, but the principles of the described waveguide may equally well be implemented in a guided wave radar (GWR) system using a signal propagation device in the form of a single lead probe or the like. 
     Even though the invention has been described with reference to specific 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 parts of the system and method may be omitted, interchanged or arranged in various ways, the system and method yet being able to perform the functionality of the present invention. 
     Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.