Abstract:
A guided wave radar level gauge for determining a process variable of a product in a tank, a feed through fitting, a probe extending into the tank, transceiver circuitry mounted on a circuit board, a housing having a body portion for accommodating said circuit board, and a neck portion for attachment of said housing to said feed through fitting. A rigid, essentially straight, coaxial connector is arranged in said neck portion. Said connector having a central lead portion without detachable connections, a first end of said lead portion protruding into said body portion, and a second end of said lead potion in electrical contact with said probe when said housing is attached to said feed through fitting. Said circuit board is mounted in direct contact with said first end of said lead portion, so that said connector provides electrical contact between said probe and said transceiver circuitry.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to guided wave radar level gauges, i.e. radar level gauges (RLG) where the emitted waves are guided by a structure, such as a probe extending into a tank. More specifically, the present invention relates to the connection of the guiding structure to the processing circuitry of the RLG. 
   BACKGROUND OF THE INVENTION 
   A conventional guided wave RLG is schematically illustrated in  FIG. 1 . The gauge essentially comprises two parts, a probe  100  extending into the tank, and a gauging unit  101  fixed on top of the probe. The upper end of the probe is attached in a tank connection  102 , which is mounted in the ceiling of the tank. The gauging unit has a body portion  103  for housing the processing electronic of the gauge, typically located on one or several circuit boards  104  and a neck portion  105  for attaching the gauging unit to the tank connection  102 . 
   In order to connect the processing electronics to the probe  100 , a coaxial connector  107  provided with a coaxial terminal  106  is arranged in the neck portion  105 , and adapted to be brought into electrical contact with the probe  100  when the housing is mounted. A coaxial cable  108  is then connected from the terminal  106  to a corresponding terminal on the circuit board  104 . 
   An example of such a guided wave RLG is disclosed in U.S. Pat. No. 6,778,044. 
   A disadvantage with this solution is that the coaxial cable connections are relatively expensive and bulky. 
   GENERAL DISCLOSURE OF THE INVENTION 
   It is therefore an object of the present invention to mitigate these problems, and to provide an improved connection between the probe and the processing circuitry in a guided wave radar level gauge. 
   According to one aspect of the present invention, this is accomplished by a guided wave radar level gauge comprising a feed through fitting, adapted to be securely attached in the ceiling of the tank, a probe, having a first end fixedly arranged in said fitting and a second end extending into the tank, transceiver circuitry mounted on a circuit board, said transceiver circuitry being arranged to generate an electromagnetic signal to be guided by the probe into the tank, and to receive a reflection of said signal guided back from the tank by the probe, processing circuitry connected to said transceiver circuitry and arranged to determine said process variable based on a relationship between said transmitted signal and said reflection, a housing having a body portion for accommodating the circuit board and a neck portion for attachment of said housing to said feed through fitting and a rigid, essentially straight, coaxial connector arranged in said neck portion, said connector having a central lead portion without detachable connections, a first end of said lead portion protruding into the body portion, and a second end of said lead potion in electrical contact with the probe when the housing is attached to the feed through fitting, wherein the circuit board is mounted in direct contact with the first end of the lead portion, so that the essentially straight connector provides electrical contact between the probe and the transceiver circuitry. 
   According to this solution, one single connector extends between the feed through fitting and the circuit board, thus eliminating the need for a coaxial cable and its terminals. The phrase “detachable connections” refers to connections that are designed to be detachable, such as the connection between a terminal and a coaxial cable. 
   In free propagating radar level gauges it has been know to design the tank housing in such a way as to allow for an arrangement of the circuit board in direct connection with a hollow wave guide, extending through a tank seal to an antenna in the tank. However, due to the fundamental difference between guided waves and free propagating waves, this solution has not been considered to be useful in the case of guided waves. Instead, as mentioned above, a coaxial connection has been provided by a coaxial connector and a coaxial cable. 
   The present invention lies in the design of a rigid coaxial connector that can be used to bridge the distance between the feed through fitting and the circuit board, and allow mounting of the circuit board directly onto the connector. 
   The circuit board is preferably arranged in a plane perpendicular to the axial extension of the coaxial connector. This facilitates the connection between the circuit board and the connector. 
   According to one embodiment, the circuit board is provided with a hole, adapted to receive a tip of the lead portion, enabling electrical contact between said lead portion and conducting paths on a side of the circuit board opposite said connector. By receiving the tip in the hole, a secure and reliable connection is provided. 
   According to another embodiment, a tip of the lead portion is adapted to be brought into biased contact with a conducting path on a side of the circuit board facing said connector. Preferably, the tip is yieldingly arranged. Such a connection does not require guiding the tip into a hole, and thus facilitates mounting of the circuit board. 
   Another aspect of the invention relates to a coaxial connector for use is a guided wave radar level gauge having a probe with one end attached to a feed through fitting in the ceiling of a tank, the connector comprising a central lead portion, a first end of the lead portion being adapted to be secured in said feed though fitting in electrical contact with said probe, a second end of the lead portion being adapted to be brought into direct contact with a circuit board in said radar level gauge. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention. 
       FIG. 1  shows a sectional view of a radar level gauge according to prior art. 
       FIG. 2  shows schematically a radar level gauge according to a embodiment of the present invention arranged on a tank. 
       FIG. 3  shows an exploded view of the radar level gauge in  FIG. 1 . 
       FIG. 4  shows a sectional view of an embodiment of the connector in  FIG. 3 . 
       FIG. 5  shows an exploded view of the connector in  FIG. 4 . 
       FIG. 6  is a bottom view of the circuit board in  FIG. 3 . 
       FIGS. 7   a  and  7   b  are two examples of how the connector in  FIGS. 3 and 4  is connected to a circuit board. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2  schematically shows a radar level gauge (RLG) system  1  according to an embodiment of the present invention. The RLG in  FIG. 2  is referred to as a guided wave radar (GWR) system, and is typically suitable when the transmitted signals are unmodulated DC pulses, but can also be used for transmitting high frequency (micro wave) signals. The RLG comprises a housing  2 , a tank connection  3 , and a probe  4  attached to the tank connection and extending into the tank  5 . 
   The probe can be a coaxial probe, a rigid or flexible twin probe, or a rigid or flexible single probe. A (twin or single) flexible probe is also referred to as a wire probe, while a (twin or single) rigid probe is also referred to as a rod probe. In some implementations, the probe can be replaced by a hollow wave guide, but this would require a transition between the hollow wave guide and a coaxial terminal in the tank connection. The probe may be provided with a coating, e.g. plastic, in order to protect the probe against corrosive tank content, or for hygiene reasons. 
   In particular in the case of a flexible probe, the lower end of the probe can be attached in the bottom of the tank, or attached to a weight that ensures that the end of the probe remains in the bottom of the tank. 
   The RLG  1  is arranged to determine a product level in the tank  5 , i.e. the level L of an interface  6  between two (or more) materials  7 ,  8  in the tank  5 . Typically, the first material  7  is a product stored in the tank, e.g. a liquid such as gasoline, while the second material  8  is air or some other atmosphere. In that case, the RLG will enable detection of the level of the surface  6  of the product  7  in the tank. Typically, only the level of a first liquid surface is measured, and/or a second liquid surface if the first liquid is sufficiently transparent. 
   In operation, the RLG transmits an electromagnetic signal, which is allowed to propagate along the probe  4  towards the surface  6 . The signal is reflected by the surface, and the RLG determines the level L based on a relationship between the transmitted and received signals. 
   In the case of pulsed radar gauging, the signals can be DC pulses or pulses modulated on a carrier wave of a GHz frequency (microwaves). The pulses typically have a length of about 2 ns or less, with a pulse repetition frequency in the order of MHz, at average power levels in the mW or μW area. 
   In the case of Frequency Modulated Continuous Wave, FMCW, the signal can be a continuous signal with a frequency varying over a certain range (Frequency Modulated Continuous Wave, FMCW). 
   The various components of the RLG are shown in more detail in  FIG. 3 . 
   The upper end of the probe  4  is attached to a tank seal  10  which is sealingly secured in a flange  11  adapted to be fixedly arranged to the wall of the tank, typically in the upper part of the tank. The flange  11  and tank seal  10  form the tank connection  3 . In case of stainless steel probe, the seal can be welded to the flange. For probes of other materials, the seal can be formed with a protective plate and a threaded fitting, and be secured by a nut, as indicated in  FIG. 3 . In any case, the probe is connected to the tank seal  10  so that electromagnetic signals can be transmitted through the seal  10  to and from the probe  4 . The upper side of the tank seal is provided with a coaxial connection terminal (not shown), in electrical contact with the probe  4 . An example of an electrical connection of a probe through a tank seal is disclosed in U.S. Pat. No. 6,148,681. 
   The RLG housing  2  comprises a lower part having a neck portion  16  and a body portion  12 , and a cover  13  which is mounted on top of the lower part, here by means of screws  14 . The cover  13  is preferably sealed by means of a rubber sealing  15 . The body portion  12  is designed to accommodate the at least one circuit board  19 , while the neck portion  16  is adapted for mounting the housing  2  on the tank connection  3 , here by means of a threaded sleeve  17 . 
   Inside the housing  2  is arranged various processing circuitry, notably transceiver circuitry  18  mounted on a circuit board  19 . According this embodiment of the present invention, the circuit board is arranged essentially horizontally in the housing  2 . 
   Further, a coaxial connector  20  is fitted inside the neck portion  16 , adapted to transmit signals between the probe and the transceiver circuitry. The connector is shown in more detail in  FIGS. 4 and 5 . 
   The connector  20  includes an electrically conducting lead portion  31 , which extends along the entire length of the connector, forming a tip  32  in the upper end of the connector. A dielectric portion  33   a ,  33   b  coaxially surrounds the lead portion  31  along essentially the entire length of the connector. In order to prevent axial displacement of the lead portion  31  with respect to the dielectric portion  33   a ,  33   b , the lead portion is formed with a radially protruding girdle  38 , and the dielectric portion comprises two separate pieces  33   a  and  33   b , fitted one from each end of the lead portion  31  and formed to fixate the girder. One or several sealing members  34 , e.g. o-rings, are arranged between the two pieces  33   a ,  33   b , in order to protect the lead portion  31  from moisture and dirt. 
   In the illustrated example, the connector is further provided with a metal sleeve  35 , arranged coaxially around the lower piece  33   a  of the dielectric portion. The sleeve portion  35  has a threaded portion  36  and a nut-shaped portion  37 . The connector thus has a shape resembling a spark plug, and can be fitted in a similar way. 
   Returning to  FIG. 3 , the connector is rigidly fixed into the neck portion  16  of the housing  2 , here by means of the threaded portion  36 . The circuit board  19  is then arranged in the lower part  12  of the housing, in direct contact with the tip  32  of the connector  20 . The circuit board is preferably secured by screws in proximity to the hole  21 , in order to secure a satisfactory electrical contact. In the illustrated example, the circuit board  19  is provided with a hole  21  adapted to receive the upper tip of the connector, and enable electrical connection of the tip  32  with conducting paths on the upper side of the circuit board. The circuit board can further be provided with another hole  22 , adapted to receive a guiding pin  23  in the lower part  12  of the housing, to ensure that the circuit board is guided into place without damaging the tip  32  of the connector  20 . 
   The connection of the circuit board with the connector is facilitated if the circuit board is mounted in a plane essentially perpendicular to the axial extension of the connector. In the illustrated example, the connector is vertically arranged, while the circuit board is mounted horizontally. 
   The lower part  12  of the housing  2  can now be mounted on the tank connection  3 , so that the lower end  39  of the lead  31  is brought into electrical contact with the connection in the tank seal  10 . 
     FIG. 6  shows the under side of the circuit board  19  in  FIG. 3 . As mentioned above, the circuit board is preferably secured by screws, and for this purpose has a number of screw holes  24 . In a neighborhood  25  of these screw holes  24 , the protective lacquer layer of the circuit board has been removed, so as to expose the ground layer of the circuit board. Similarly, in a neighborhood  26  around the hole  21 , the protective lacquer layer has also been removed. This serves to ensure satisfactory electrical connection between the ground layer of the circuit board and the material in the hosing  2  to which the circuit board  19  is attached to. Preferably, these screw holes  24  are positioned symmetrically around the hole  21 , in order to avoid unbalanced surface currents. In an area  27  immediately surrounding the hole  21 , preferably within a radius essentially corresponding to the radius of the dielectric portion  33   a , the ground layer of the circuit board has been removed (etched), so as to ensure electrical isolation of the tip  32  from the ground layer. 
   The electrical connection between the circuit board and the tip  32  of the connector  20  can be accomplished in various ways. 
   In the case of connection to a conducting path on the upper side of the circuit board, the tip  32  is received by the hole  21 . As shown in  FIG. 7   a , the circuit board can be provided with a sleeve fitting  41 , adapted to snugly receive the tip  32  of the connector  20 . The sleeve fitting  41  can be similar to the inner sleeve of a conventional coaxial cable connector, and is provided in electrical contact with the conducting paths on the upper side of the circuit board (not shown). Alternatively, the protruding part of the tip  32  is simply soldered to the conducting path of the circuit board when the circuit board is in place. 
   In the case of connection to a conducting path on the side facing the connector  20 , the hole  21  may not be required. As shown in  FIG. 7   b , the tip  32  can then be yieldingly arranged in the connector, and adapted to be brought into biased contact with the conducting path of the circuit board. Alternatively, the circuit board  19  is simply secured firmly in place, pressing the conducting path against the tip  32 . 
   The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the connector  20  may be connected to the circuit board in other ways. Further, the details of the connector design can be modified depending on the application.