Abstract:
A method and a system for processing a reflected microwave signal generated by a radar level gauge system arranged to transmit microwaves towards the material in the tank, and receive a reflection of said microwave signal as a tank signal. The tank signal is processes by a plurality of processes, each process being adapted to determine a process variable in a specific region of the tank, each specific region corresponding to a predefined propagation distance range. 
     Such multi-processing of the received tank signal has the advantage that each process can be optimized to that particular region of the tank. More specifically, a process concerned with a particular region of tank only needs to treat a portion of the tank signal.

Description:
FIELD OF THE INVENTION 
   The present invention relates to radar level gauge (RLG) systems. In such systems, microwaves are emitted into a tank and a reflected tank signal is received. Based on signal processing of this reflected signal, a process variable such as the level of a content in the tank can be determined. 
   BACKGROUND OF THE INVENTION 
   Such RLG systems typically include a signal generator, means to emit the signal into the tank, and a receiver for receiving the reflected tank signal. The received signal can for example be a time domain reflectometry (TDR) signal or a frequency domain signal, such as a frequency modulated continuous wave (FMCW) signal. The received signal typically comprises at least a surface reflection (echo) caused by an interface between different materials in the tank, typically but not necessarily a liquid surface. Normally, the received signal also includes various interfering reflections caused e.g. by the bottom and walls of the tank or the transition between the signal generator and the wave guide. 
   In order to improve the accuracy of the measurement result, the signal processing of the received signal can be adapted to compensate the received signal for such interfering reflections. However, the signal processing is typically optimized in terms of general precision in the entire tank, and is not necessarily optimal in all areas of the tank. Therefore, in addition to the RLG system, additional sensors are sometimes arranged in the tank, in order to provide information about conditions in specific regions of the tank. 
   One such region of special importance is the near zone, i.e. in a region close to the entry of microwaves into the tank. In an RLG, the measurement process can be complicated or even made impossible, when the surface reflection occurs in this near zone, which can be in the range of 0 m to 2 m depending on the type of microwave signal used. The problems are caused by an interfering reflection caused by the transition between a signal transfer medium and the emitter/receiver in the tank, in combination with the limited bandwidth limiting the resolution. RLG systems may be provided with specific signal processing to handle such problems. 
   At the same time, for security reasons it is very important to have a secure indication of if and when the surface of the contents in the tank approaches the top of the tank, i.e. some kind of overfill sensing system. Therefore, one example of an additional sensor mentioned above is an overfill sensor, arranged in the top of the tank and adapted to detect when the level exceeds a certain level. Although the normal measurement processing may be adapted to provide accurate measurements also in the top of the tank (referred to as the near zone), such a redundant sensor system is required by authorities in many countries. 
   GENERAL DISCLOSURE OF THE INVENTION 
   It is an object of the present invention to provide information about conditions in specific regions of the tank, without the need for additional sensors. 
   This and other objects are achieved by providing a radar level gauging method and system for processing a tank signal comprising consecutive tank signal portions each corresponding to a measurement cycle, wherein each portion of the received tank signal is processed in a plurality of processes, each process being adapted to determine a process variable in a specific region of the tank, each specific region corresponding to a separate predefined propagation distance range. 
   The term “measurement cycle” is here used to indicate the cyclic process of determining a measurement result. The cycles are not necessarily equal in length, and the content of the transmitted signal may also very between different cycles. In a pulsed RLG, a measurement cycle corresponds to the transmission of one pulse, while in a FMCW RLG a measurement cycle corresponds to one sweep of frequencies. As each portion of the tank signal corresponds to one measurement cycle, each of the plurality of processes will be performed on information from each measurement cycle. 
   One region may be the area closest to the signal entry into the tank (referred to as the near zone), while another region may be the area furthest away from the signal entry into the tank. In a vertically oriented tank, the regions corresponding to different propagation distance ranges will relate to vertical layers of the tank, e.g. a top region and a bottom region. 
   Such multi-processing of the received tank signal has the advantage that each process can be optimized to that particular region of the tank. More specifically, a process concerned with a particular region of tank only needs to treat a limited content of the tank signal. In the case of a time domain reflectometry signal, the process only needs to handle a limited time range, and in the case of a frequency domain tank signal, the process only needs to handle a limited frequency range. The processing is thus simplified, and such a regionally limited process can be made more robust. 
   It should be noted that the regions may overlap, so that a portion of the tank may be covered by several regions. The tank signal relating to this region will this be processed in several processes. Thus will be the case, for example, if one process handles essentially the entire tank, in order to obtain a measurement result, while other processes only handle smaller sub-regions, in order to provide more robust measurements of these specific regions. 
   Such a regionally limited process can include subtracting a compensation signal from the tank signal, which compensation signal includes background information about the region of interest, also referred to as a “signature”. Such a compensation signal can be formed using a tank signal in which the surface reflection occurs at a distance from the region of interest, and then extracting the portion of the tank signal relating to the region of interest. 
   In a pulsed, time domain, system, the relevant portion of the tank signal may be extracted by selecting a time range of the signal. 
   In a frequency domain system, such as an FMCW system, the compensation signal can be formed by low pass or band pass filtering a tank signal received when no surface reflection occurred inside the region of interest. 
   For a bottom region of the tank, a compensation signal can be formed by saving a tank signal from a measurement cycle when the surface reflection is close to the bottom. 
   In any case, an updated compensation signal CS n *(t) can be formed by combining the compensation signal CS n (t) with a previous compensation signal CS n−1 (t), according to
 
CS n *( t )= a CS n ( t )+(1 −a )CS n−1 ( t ),
 
where a is a weight factor between zero and 1.
 
   One example of a regionally limited process can be a near zone process, especially adapted for the near zone, suitable for providing an overfill detection system. This embodiment of the invention is based on the realization that while a redundant overfill sensing system is often required, the RLG system itself is in fact capable of providing such sensing, as long as the suitable signal processing is applied. By applying such signal processing in a separate measurement process, a redundant overfill detection system is provided within the RLG system, thus eliminating the need for a separate sensor. 
   The near zone process can be limited to treating the signal content originating from the near zone of the tank, and does therefore not have to handle interference from other regions of the tank. This makes the processing more robust, to the degree that it meets the requirements of an overfill sensing system. 
   According to one embodiment, a near zone process may include: for each portion of the received tank signal, subtracting a compensation signal CS n (t) based on a near zone signature of the tank from the received tank signal, and detecting a peak with an amplitude greater than a predetermined threshold in the near zone, monitoring if a peak occurs in a predetermined number of measurement cycles, and identifying a surface reflection based on said peak. 
   The compensation signal here includes information about the near zone of the tank, and is referred to as a near zone signature. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the present invention will be described in more detail with reference to the appended drawings, illustrating presently preferred embodiments. 
       FIG. 1  shows schematically a radar level gauge system according to an embodiment of the present invention. 
       FIG. 2  shows a block diagram of the signal processing according to an embodiment of the present invention. 
       FIG. 3  shows a flowchart of two of the processes in step  FIG. 2 . 
       FIG. 4  shows a flow chart of the detector step in  FIG. 3 . 
       FIG. 5  shows a state model of an alternative embodiment of the detector step. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows a schematic block diagram of a radar level gauge (RLG)  10 , in which the present invention has been implemented. The gauge  10  is arranged to perform measurements of a process variable in a tank, such as the level of an interface  2  between two (or more) materials  3 ,  4  in the tank  5 . Typically, the first material  3  is a content stored in the tank, e.g. a liquid such as gasoline, while the second material  4  is air or some other atmosphere. In that case, the RLG will enable detection of the level of the surface of the content in the tank. Note that different tank contents have different impedance, and that the electromagnetic waves will only propagate through some materials in the tank. Typically, therefore, only the level of a first liquid surface is measured, or a second liquid surface if the first liquid is sufficiently transparent. 
   The RLG  10  comprises a microwave controller  11 , a microwave emitter/receiver  12 , and a signal transfer medium  13  connecting the emitter/receiver  12  to the controller  11 . The controller  11  can comprise a transmitter  14 , a receiver  15 , a circulator  16  and any control circuitry  17  required to manage these components. Further, the controller  11  can comprise an A/D-converter  18  for digitizing a tank signal, i.e. a signal received from the tank. 
   The emitter/receiver  12  can, as shown in  FIG. 1 , include a free radiating antenna  19  in the top of the tank, or alternatively the emitter/receiver  12  can include a steel pipe acting as a wave guide, or a transmission probe (e.g. coaxial probe, single probe, or twin probe) extending into the tank. 
   The signal transfer medium  13  can be a wire or cable, but can also include more sophisticated wave guides. In case of a explosive or otherwise dangerous content in the tank  5 , the signal transfer medium  13  may include an air tight seal passing through the tank wall. It is also possible that the controller  11  is connected directly to the emitter/receiver  12  with a suitable terminal, or that the emitter/receiver  12  is arranged on the same circuit board as the controller  11 , in which case the signal transfer medium simply may be a track on the circuit board. 
   The radar level gauge  10  further includes processing circuitry  20  for communicating with the microwave controller  11  and for determining a measurement result based on a relation between transmitted and received microwaves. The controller  11  is connected to the processing circuitry  20  by a data bus  21 , and is adapted to generate a microwave signal in accordance with control data from the processing circuitry  20 . 
   In use, the processing circuitry  20  controls the microwave controller  11  to generate and transmit a measurement signal to be emitted into the tank  5  by the emitter/receiver  12 . This signal can e.g. be a pulsed signal (pulsed level gauging) or a continuous signal with a frequency varying over a certain range (Frequency Modulated Continuous Wave, FMCW). The microwave emitter  12  acts as an adapter, enabling the signal generated in the controller  11  to propagate into the tank  5  as microwaves, which can be reflected by the surface of the material  3 . 
   A tank signal, i.e. the emitted signal and its echo, or a mix of emitted and reflected signals, is received by the emitter/receiver  12 , and communicated to the microwave controller  11 , where it is received by receiver  15  and A/D converted by converter  18 . The digitized signal is then provided to the processing circuitry  20  via bus  21 , and the processing circuitry  20  determines a measurement result based on a relation between the emitted and received waves. 
   According to this embodiment of the present invention, the processing circuitry is arranged to process the received tank signal in a plurality of processes, each process being adapted to determine a process variable in a specific region of the tank. This is illustrated in  FIG. 2 . It should be noted that the processes do not need to be parallel as indicated in  FIG. 2 . On the contrary, they can be performed sequentially, as long as they use the same input (tank signal portion). In  FIG. 2 , three different processes  31 ,  32  and  33  are shown, each being adapted to determine a process variable in a specific region  51 ,  52  and  53  of the tank  5  in  FIG. 1 . The results from the three separate processes are evaluated in an evaluation module  34 . 
   In the illustrated case one process  31  corresponds essentially to the conventional measurement process, and is intended to provide a measurement result, such as a tank level, that is valid in the entire tank. The process  31  thus treats the entire tank signal, and handles various types of interference that can occur in the tank. 
   The two other processes  32 ,  33  are adapted to provide measurement results such as a tank levels in a limited region, here the near zone  52  and the bottom zone  53 , respectively. As these processes are only intended to provide valid results under certain circumstances, they can be made more robust, and can replace additional sensor systems sometimes required by authorities. 
   According to a preferred embodiment, one of the processes  32  is a near zone process, intended to function as an overfill detection process. The purpose of such a process is to securely detect any surface echo in an overfill region near the top of the tank, in order to avoid an overfill situation. If a surface echo is detected in the overfill zone, the output from the overfill protection system will be received by the evaluation module  34 , and can trigger an alarm, causing a shutdown of the pumping system connected to the tank. Further, the evaluation module can be adapted to let the output from the overfill detection process  32  overrule the output from the normal measurement process  31 , as the near zone process  32  is considered to be more robust in this region of the tank. In an ideal situation, the normal process  31  will detect the same surface echo as the overfill detection process  32 , but there is a risk that the normal process has been disturbed by interferences from the tank and produces an erroneous result. 
   An overfill detection process according to an embodiment of the present invention is illustrated in more detail in  FIG. 3 , in a schematic block diagram showing examples of the processes  31  and  32 . It should be noted that this embodiment relates to a frequency modulated continuous wave (FMCW) system. However, a similar system could be implemented in a pulsed system with only minor modifications. 
   As is clear from  FIG. 3 , the process  32  is arranged to process the same input signal (tank signal) as process  31 , and also includes many of the same steps. More specifically, process  31  includes process step S 1 , for adapting the gain of the tank signal, step S 2 , for Fourier transforming the tank signal and providing a tank signal spectrum, step S 3  for locating any peak in the spectrum, step S 4  for determining distance from tank entry and amplitude, step S 5  for tracking a surface echo, and step S 6  for identifying an echo. The near zone process  32 , on the other hand, includes process step S 7 -S 11 , of which steps S 8 -S 10  essentially correspond to steps S 2 -S 4  of process  31 . 
   In step S 7 , a compensation is subtracted from the tank signal. This compensation signal includes background information from the near zone, and can be deduced from an earlier tank signal, where the surface reflection was established to be well outside the near zone. Here, as the tank signal is an FMCW signal, such a near zone signature can be generated by low pass filtering the tank signal. The low pass filtering has three purposes: first of all, it eliminates the surface reflection from the signal, secondly, it allows sampling of the compensation signal, and thirdly, it avoids high frequency content having non-stable phase. 
   In step S 8  the tank signal is Fourier transformed to create a spectrum, just as in step S 2  in process  31 , and in step S 9  a peak is located by simply finding a local maximum (a bin larger than its neighboring bins). In step S 10  the amplitude and position of this peak is determined, which is used in the following step S 11 . 
   In step S 11  it is determined if the peak represents a surface echo within the overfill region, and if so an output is generated. 
   Step S 11  is preferably designed so as to avoid unnecessary alarms, as this would result in unwanted costs. In a simple case, step S 11  monitors the occurrences of peaks in the overfill region by a counter. This is illustrated in  FIG. 4 . First in step S 12  it is verified that the peak is within the overfill region. In step S 13  it is then verified that the amplitude of the peak is greater than a predefined threshold. If a valid peak is detected, a counter is increased in step S 14 , but preferably only up to a specified limit. If no valid peak is detected, the counter is decreased in step S 15 . Thus, each measurement cycle that a valid peak is detected in the overfill region, the counter is increased, and each cycle no peak is detected the counter is decreased. In step S 16  it is checked if the counter exceeds a predefined threshold, and if this is the case, the valid peak is considered as a surface echo in the overfill region, and an output is generated instep S 17 . In order to make the process more robust, an hysteresis can be introduced by providing an output until the counter falls below a second threshold, lower than the first threshold. 
   A more sophisticated process that can be implemented in step S 11  is shown in  FIG. 5  as a state model. According to this process, a pre-region is defined immediately outside the overfill zone, and in addition to the counter for counting peak detections in the overfill zone, there is a pre-counter for counting peak detections in the pre-region. The states  61 - 65  are labeled No peak, Pre-region, Enter zone, Inside zone and Leave zone. 
   The No peak state  61  is reached when both counters are equal to zero. As soon as a peak with sufficient amplitude is detected inside the pre-region the Pre-region state is reached. If, on the other hand a peak with sufficient amplitude is detected inside the overfill zone the Enter zone state is reached. 
   In the Pre-region state  62 , a process similar to the one in  FIG. 4  is run. The pre-counter is increased for each measurement cycle for which a valid peak is detected in the pre-region, and decreased for each cycle for which no peak is detected. During periods when the pre-counter exceeds a predefined threshold, step S 11  of the overfill detection will generate an output, indicating a surface echo in the pre-region. If the pre-counter reaches zero, program control returns to the No peak state  61 . If a peak instead is detected in the overfill zone, program control proceeds to the Enter zone state  63 . 
   In the Enter zone state  63 , also a process similar to  FIG. 4  is run. The counter is increased for each measurement cycle for which a valid peak is detected in the overfill zone, and decreased for each cycle for which no peak is detected. If the counter reaches zero, the program control returns to the Pre-region state  62  if the pre-counter is greater than zero, or to the No peak state  61  if the pre-counter is also zero. If the counter instead exceeds a predefined threshold, the program control proceeds to the Inside zone state  64 . This threshold can be different from the threshold in the Pre-region state  62 . 
   While in the Inside zone state  64 , step S 11  of the overfill detection process  32  will generate an output, indicating the current position of the detected peak. Program control will remain in the Inside zone state  64  as long as peaks are detected in the overfill zone, and the counter will be increased up to a predefined level, possibly equal to the threshold in the Enter zone state  63 . As soon as a measurement cycle detects no peak with a sufficient amplitude in the overfill zone, program control will proceed to the Leave zone state  65 . 
   While in the Leave zone state  65 , step S 11  of the overfill detection process  32  will generate an output, indicating the position of the last detected peak. For each cycle without peak in the overfill zone, the counter will be decreased, and when below a predefined threshold, program control will return to the Enter zone state  63 , and no output will be generated. This threshold is preferably lower than the threshold in the Enter zone state  63  thereby creating a hysteresis effect. If a new peak is detected in the overfill zone before the counter has fallen below this threshold, program control will instead return to the Inside zone state  64  and again output the current position of the peak. 
   When the near zone process  32  is implemented as an overfill detection process as described above, it may be required by regulations to ensure that the process does not fail, and various checks can be implemented for this purpose. One such check is a sweep fail check, which raises an alarm if too many measurement cycles fail, e.g. due to linearization errors or tank signal clipping. A sweep fail check can be implemented by letting a counter count each failed measurement cycle and determine a ratio between the number of failed cycles and the total number of cycles. If this ratio exceeds a given threshold, an alarm is raised. 
   Although described mainly with reference to a FMCW system, it should be realized that the present invention can be advantageously applicable to any RLG system. More specifically, the above described overfill detection process  32 , can be adapted for a pulsed, time domain, system. Such a process will not require Fourier transformation of the tank signal, and will identify peaks in the time domain instead of in the frequency domain. The near zone signature will further not be a low pass filtered tank signal, but a selected time range from the tank signal. The overall structure of the process  32  will however remain intact. 
   Further, it should be noted that the number of processes is not limited to three, as shown in  FIG. 2 . On the contrary, an implementation of the overfill detection system described only requires two processes, and it may be advantageous to implement more than three.