US20050285777A1 - Near zone detection in radar level gauge system - Google Patents
Near zone detection in radar level gauge system Download PDFInfo
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- US20050285777A1 US20050285777A1 US10/876,240 US87624004A US2005285777A1 US 20050285777 A1 US20050285777 A1 US 20050285777A1 US 87624004 A US87624004 A US 87624004A US 2005285777 A1 US2005285777 A1 US 2005285777A1
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- 238000001514 detection method Methods 0.000 title description 7
- 230000002452 interceptive effect Effects 0.000 claims abstract description 62
- 238000000034 method Methods 0.000 claims abstract description 52
- 239000000523 sample Substances 0.000 claims abstract description 36
- 230000008569 process Effects 0.000 claims abstract description 18
- 239000000463 material Substances 0.000 claims abstract description 14
- 238000012545 processing Methods 0.000 claims abstract description 12
- 230000007704 transition Effects 0.000 claims abstract description 7
- 230000003321 amplification Effects 0.000 claims abstract description 6
- 238000003199 nucleic acid amplification method Methods 0.000 claims abstract description 6
- 238000002310 reflectometry Methods 0.000 claims abstract description 6
- 238000005070 sampling Methods 0.000 claims description 6
- 230000001419 dependent effect Effects 0.000 claims description 4
- 238000004458 analytical method Methods 0.000 description 7
- 238000005259 measurement Methods 0.000 description 6
- 238000009434 installation Methods 0.000 description 4
- 239000007788 liquid Substances 0.000 description 3
- 229920006395 saturated elastomer Polymers 0.000 description 3
- 230000009977 dual effect Effects 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
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- 230000005540 biological transmission Effects 0.000 description 1
- 230000008859 change Effects 0.000 description 1
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- 230000010355 oscillation Effects 0.000 description 1
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
- G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
- G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
- G01F23/28—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring the variations of parameters of electromagnetic or acoustic waves applied directly to the liquid or fluent solid material
- G01F23/284—Electromagnetic waves
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
- G01S13/06—Systems determining position data of a target
- G01S13/08—Systems for measuring distance only
- G01S13/10—Systems for measuring distance only using transmission of interrupted, pulse modulated waves
- G01S13/103—Systems for measuring distance only using transmission of interrupted, pulse modulated waves particularities of the measurement of the distance
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/88—Radar or analogous systems specially adapted for specific applications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
- G01S7/40—Means for monitoring or calibrating
- G01S7/4004—Means for monitoring or calibrating of parts of a radar system
Definitions
- the present invention relates to radar level gauge systems, and more specifically RLG systems of the kind where a time domain reflectometry (TDR) signal is processes to determine at least one process variable for a material in the tank.
- TDR time domain reflectometry
- Such RLG systems typically include a pulse generator, a probe extending into the tank for guiding the signal, and a detector for receiving said TDR signal.
- the TDR signal typically comprises at least a surface reflection pulse caused by an interface between different materials in the tank, typically but not necessarily a liquid surface, and an interfering pulse caused by a transition between the pulse generator and the probe.
- the measurement process can be complicated or even made impossible, when the surface reflection pulse occurs in the beginning of the probe, where the influence of the interfering pulse is significant.
- This area of the measurement range is herein referred to as the “near zone”, and can be in the range of 0 m to 0.2-0.5 m depending on the type of pulse generator used.
- the problem can be accentuated by the fact that the interfering pulse has an opposite sign compared to the surface reflection, and therefore will cause an attenuation or complete cancellation of the surface reflection pulse if they occur too close to each other.
- RLG systems may exhibit a zone immediately below the top of the tank, where measurement results can be uncertain.
- An object of the present invention is to improve RLG systems in terms of near zone detection.
- a method of the kind mentioned by way of introduction comprising obtaining a first sampled TDR signal with an amplification such that the interfering pulse is unsaturated, determining a compensation pulse signal using a previously stored pulse shape, a detected pulse amplitude and a current pulse position, subtracting said compensation pulse signal from the first sampled TDR signal, determining if a surface reflection occurs in an area where said interfering pulse will have a significant interfering effect on the surface reflection pulse, and if no surface reflection occurs in this area, updating the current pulse position.
- a compensating signal is subtracted from the TDR signal in order to improve the accuracy in the near zone.
- the compensating signal is determined using a preset pulse shape and current values for the maximum amplitude and position of the interfering pulse. When no surface reflection is determined to occur in the near zone, the position of the interfering pulse can be updated.
- the compensation signal can be based on the fact that the shape of the interfering pulse is essentially constant for a given type of pulse generator.
- the amplitude can be influenced by the type of probe and the installation.
- FIG. 1 schematically shows a radar level gauge system mounted on a tank.
- FIGS. 2 a and 2 b are graphs of TDR signals obtained with a conventional RLG system.
- FIG. 3 is a flow chart of a method according to an embodiment of the invention.
- FIGS. 4 a and 4 b are graphs of TDR signals compensated according to the method in FIG. 3 .
- FIG. 5 is a graph illustrating the influence of temperature on the position of the interfering pulse.
- FIG. 6 a - b are graphs of an interfering pulse influenced by a surface reflection in the near zone.
- FIG. 7 is a flow chart of a method according to a further embodiment of the invention.
- FIG. 1 shows schematically a radar level gauge system 1 in which a method according to the invention may be advantageously used.
- the system 1 is arranged to perform measurements of a process variable such as the level of an interface 2 between two materials 3 , 4 in a tank 5 .
- the first material 3 is a liquid stored in the tank, e.g. gasoline, while the second material 4 is air.
- the system 1 comprises a probe 10 , attached to a mounting device 11 and extending into the tank 5 .
- the probe 10 can be of any known kind, such as a coaxial or dual line probe. In the presently described case, it is a dual line transmission probe.
- the mounting device 11 of the probe 10 is connected, via line 12 , for example a cable or the like, to a transceiver 13 , for generating a series of pulses to be transmitted by the probe 10 into the tank 5 , and receiving a time domain reflectometry (TDR) signal.
- line 12 for example a cable or the like
- TDR time domain reflectometry
- the transceiver 13 comprises a pulse generator 14 , preferable arranged to generate pulses with a length of about 2 ns or less, with a frequency in the order of MHz, at average power levels in the nW or ⁇ W area.
- the transceiver 13 also comprises a sample and hold circuit 15 , connected to the pulse generator 14 and to the line 12 .
- the transceiver 13 is controlled by a micro processor 17 , providing the pulse generator with a high frequency (e.g. 2 MHz) clock signal, and the sample and hold circuit 15 with an oscillation signal (e.g. 40 Hz).
- the microprocessor 17 is also, via an amplifier 18 and an A/D-converter 19 , connected to an output of the sample and hold circuit 15 , which provides the processor 17 with a digitalized, sampled TDR signal 20 .
- the signal 20 is expanded in time, allowing for use of conventional hardware for conditioning and processing.
- the processor 17 is further connected with a ROM unit 21 (e.g. an EEPROM) for storing pre-programmed parameters, and a RAM unit 22 for storing software code 23 executable by the microprocessor 17 .
- the processor can also be connected to a user interface 24 .
- FIG. 2 a - b shows two graphical examples of a TDR signal 30 a and 30 b acquired with a conventional RLG system in a tank containing oil, as well as a dashed line indicating a signal 31 for an empty tank.
- the signal comprises a first, negative pulse 32 , and a second, positive pulse 33 .
- the first pulse is caused by the transition from the cable 12 into the probe 10 in air. As the probe has higher impedance, the pulse is negative.
- the second pulse is caused by the reflection against an oil liquid. As the probe in oil has lower impedance than the probe in air, this pulse is positive.
- the first pulse can be used as a reference, as it defines the beginning of the probe. However, it also influences the appearance of the second pulse, especially in the illustrated situation, where the two pulses have opposite signs. Therefore, the first pulse is here referred to as an interfering pulse, while the second pulse is referred to as the surface reflection pulse.
- the oil surface is closer to the top of the tank, and the surface reflection pulse 33 thus appears closer to the interfering pulse 32 .
- the surface reflection pulse does not appear distinctly, but only results in a slight elevation of the positive flank of the interfering pulse 32 . This is visible in the figure when comparing the TDR signal 30 with the empty tank signal 31 . The surface reflection in this case occurs somewhere close to 475 .
- a preferred embodiment of the invention is illustrated in the flow chart in FIG. 3 .
- the process described by the flow chart can be realized by a computer program comprised in the program code 23 and running on the processor 17 .
- This control program can thus control the pulse generator 14 , the sample and hold circuit 15 , the amplifier 18 , and any other components of the system required to perform the described method.
- two sampled TDR signals are obtained for each measuring, a first sampled signal is obtained by the sample and hold circuit 15 , and amplified by amplifier 18 such that the interfering pulse. is not saturated. This is suitable for obtaining information about this pulse, enabling use of it as a reference pulse during the future processing and analysis of the TDR signal. Then, a second sampled signal is obtained and amplified using more powerful amplification. This is suitable for ensuring that the surface reflection pulse, which in some situations, depending on materials in the tank and other conditions, can be very weak, is more easily detectable.
- the principle of a two-part sampling process is not novel, and the invention is by no means limited to the use of such a process.
- a first, non-saturated sampled TDR signal is obtained in step S 1 .
- This first signal does not necessarily cover the entire range of the tank, but preferably only the beginning of the probe.
- its primary purpose is to gain information about the interfering pulse.
- the amplitude of the interfering pulse A i is then determined in step S 2 , and in step S 3 compared to an amplitude value currently stored in the RAM 22 . If the detected amplitude is greater than the previously stored value, this stored value is replaced with the detected amplitude in step S 4 , before the program control moves on to step S 5 .
- step S 5 the processor 17 defines a compensation signal based on a pulse shape previously stored in the ROM 21 , and a current pulse position, P i , also stored in the RAM 22 .
- the current pulse position P i in the RAM 22 corresponds to the anticipated position of the extreme point of the interfering pulse 32 , and the compensation signal is aligned so that its extreme point coincides with the current pulse position P i .
- the compensation signal can also be rescaled in order to correspond to the amplitude of the interfering pulse, in the illustrated case using the detected amplitude value A i.
- step S 2 it can be advantageous to secure a detection of the interfering pulse amplitude before step S 5 , here in step S 2 .
- the amplitude is measured every sampling. If it could be ascertained that no surface reflection was present in the near zone during this detection, one single detection would be sufficient, as the amplitude is essentially unaffected by ambient conditions.
- the steps S 3 and S 4 will allow for a gradual increase of the stored amplitude value.
- the stored value will also increase. Basically, as soon as it has been ascertained that the amplitude of the interfering pulse has been detected without disturbance from any surface reflection, no additional detections are required. If considered advantageous, such a routine could be implemented in the program.
- the pulse shape stored in ROM 21 can be determined and stored during pre-installation calibration, or be set to a fixed curve if the expected variations are minor. It has been found that the curve shape typically only differs in terms of amplitude when comparing different types of probes, such as a twin rigid probe and a coaxial probe. Therefore, it can be sufficient to store a curve corresponding to the pulse generator to be used, or possibly several different curves in case the system can be used with different signal boards having different pulse generators. Further, installation dependent adjustments of this shape may also be stored in a user area of the ROM 21 , by providing suitable options in the user interface 24 .
- step S 6 the processor subtracts the compensation signal from the detected TDR signal, resulting in a compensated TDR signal 40 (see FIG. 4 a - b ).
- step S 7 the position of any surface reflection pulse is determined by processor 17 , using conventional techniques for TDR signal processing.
- Such techniques may include threshold analysis, derivative analysis, etc.
- step S 6 The purpose of the subtraction in step S 6 is to remove any influence from the interfering pulse from the TDR signal, in order to enable a more secure localization of the surface reflection pulse.
- the surface reflection by definition occurs during the time range after the extreme point of the interfering pulse, P i , computational power can be saved by only defining the compensation signal for this time range.
- FIG. 4 a - b show two graphical examples of compensated TDR signals 40 a and 40 b , essentially corresponding to the signals 30 a and 30 b in FIG. 2 . Note however, that the position of the surface reflections are slightly offset. From the curves it is clear that the compensation signal has been subtracted starting from the extreme point, P i .
- the surface reflection pulse 43 a is probably far enough from the interfering pulse 42 a to be detectable even without the compensation in step S 6 .
- FIG. 4 b the situation is similar to that in FIG. 2 b , where the surface reflection is less apparent and more difficult to detect due to interference from the interfering pulse.
- the subtraction of the compensation signal has removed the influence from the interfering pulse, and the surface reflection 43 b is clearly visible, and thus easy to detect correctly.
- step S 8 it is determined if any surface reflection pulse is present in the near zone.
- a quantitative definition of the near zone in terms of time distance from the extreme point P i must be defined, and stored in the RAM 22 or ROM 21 .
- This definition should be based on characteristics of the probe and tank, and preferably be such that it includes the entire area in which the influence of the interfering pulse is significant. Typically, this is the case until the interfering pulse has declined to a defined percentage of its maximum amplitude, for example about 5%, preferably only about 3%, and most preferably only about 1%. This position where the interfering pulse has declined to this predefined level thus indicates the beginning of the near zone.
- the interfering pulse 32 a can be considered to be unaffected by the surface echo, and the TDR signal 30 a can in step S 9 be analyzed in order to update the current pulse position P i , stored in the RAM 22 . The details of this updating process will be described in more detail below.
- step S 10 a second, more amplified sampled TDR signal is acquired, and any desired information is extracted by processing and analyzing this signal in step S 11 . It may be advantageous to perform a compensation also of the second TDR signal, but as the surface reflection has been located outside the near zone, this may not be required. After the analysis of the TDR signal has been completed, or possibly even before this, program control returns to step S 1 to acquire a new sampled TDR signal.
- step S 8 If a surface reflection is found to be present in the near zone in step S 8 , no update of the pulse position P i will be made. However, if, as in the present example, the first sampling in step S 1 was only of a portion of the measurement range, a second sampled TDR signal, having a greater range than the first TDR signal, is acquired in step S 12 . Just like the first TDR signal, this second TDR signal is acquired with an amplification such that the interfering pulse is non-saturated. Note that, if instead a TDR signal covering the entire range was acquired already in step S 1 , no second TDR signal would be required at this stage.
- step S 13 the subtraction performed in step S 6 is again performed, in order to obtain a compensated TDR signal, covering the entire measuring range.
- This signal is processed and analyzed in step S 14 to extract any desired information from the TDR signal.
- processing steps S 11 and S 14 are essentially equivalent, the only difference being that the TDR signals to be analyzed have different amplification. Also, the TDR signal analyzed in step S 14 has been compensated in step S 13 , while the TDR signal analyzed in step S 11 may be uncompensated.
- the described method allows a better determination of a surface reflection pulse in the near zone, by subtracting a compensation signal based on information about the interfering pulse from the TDR signal.
- the position of the interfering pulse may change.
- the influence of temperature is illustrated in FIG. 5 , from which it is clear that the amplitude of the pulse is almost constant, while the position changes.
- the current position of the interfering pulse is therefore updated in S 9 each time the surface reflection is determined to be outside the near zone, i.e. when the interfering pulse is uninfluenced by any surface reflection.
- the step S 9 quite simply includes determining the extreme point of the interfering pulse, using known methods of TDR signal analysis.
- the pulse position P i is divided into a first part which is relatively unaffected by a surface reflection in the near zone, and a second part which is more affected by a surface reflection in the near zone. Consequently, the first part can be updated each time the first sampled TDR signal is acquired, while the second part is only updated if no surface reflection is present in the near zone.
- This division of the position P i can provide a more precise determination of the position of the interfering pulse, even during periods when a surface reflection is present in the near zone.
- FIG. 6 a - b show a section of a TDR signal close to the interfering pulse.
- no surface reflection is present in the near zone
- FIG. 6 b a surface reflection is in the near zone close to the top of the tank.
- the position of the extreme point 51 a , 51 b of the interfering pulse 52 a , 52 b is significantly affected by the surface reflection pulse 53 a , 53 b .
- the position P thres at which the negative slope of the interfering pulse crosses a given threshold is essentially unaffected by the surface reflection.
- This threshold can be expressed as a percentage of the maximum pulse amplitude, and in the illustrated example it is 30% of the maximum amplitude. Note that the pulse is negative, and so the amplitude is calculated from the DC level and down. The threshold is thus 30% of the amplitude below the DC level. As this position P thres is uninfluenced by ambient conditions, it can thus be used as the part of the pulse position P i not affected by the surface reflection pulse.
- this distance should only be determined when it has been determined that the surface reflection is not present in the near zone (i.e. in step S 9 in FIG. 3 ). Determining this distance is simply done by determining the actual position of the extreme point P i using conventional TDR signal analysis, and then subtraction the current value of the threshold crossing P thres .
- FIG. 7 includes the steps S 21 -S 29 , corresponding to the steps S 1 -S 9 in FIG. 3 .
- the first acquired TDR signal preferably includes the entire measurement range, as no second sampling is made. If no surface reflection is found present in the near zone in step S 28 , program control continues to step S 30 , where any additional TDR signal analysis is performed, before returning to step S 21 . If a surface reflection is found to be present in the near zone in step S 28 , the position of the extreme point, P i , is updated, before program control continues to step S 30 .
- the method has been described only with reference to probe based RLG, while it may also be used to improve the performance of free propagating RLG, i.e. systems where the pulses are transmitted into the tank without a probe.
- the interfering pulse is caused by the transition between the radar antenna and free space, and the advantages of the described method will be similar.
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Abstract
Description
- The present invention relates to radar level gauge systems, and more specifically RLG systems of the kind where a time domain reflectometry (TDR) signal is processes to determine at least one process variable for a material in the tank.
- Such RLG systems typically include a pulse generator, a probe extending into the tank for guiding the signal, and a detector for receiving said TDR signal. The TDR signal typically comprises at least a surface reflection pulse caused by an interface between different materials in the tank, typically but not necessarily a liquid surface, and an interfering pulse caused by a transition between the pulse generator and the probe.
- In such RLG systems, the measurement process can be complicated or even made impossible, when the surface reflection pulse occurs in the beginning of the probe, where the influence of the interfering pulse is significant. This area of the measurement range is herein referred to as the “near zone”, and can be in the range of 0 m to 0.2-0.5 m depending on the type of pulse generator used. The problem can be accentuated by the fact that the interfering pulse has an opposite sign compared to the surface reflection, and therefore will cause an attenuation or complete cancellation of the surface reflection pulse if they occur too close to each other. As a result, RLG systems may exhibit a zone immediately below the top of the tank, where measurement results can be uncertain.
- Conventional RLG systems include various processing of the TDR signal, and some of these processing methods are described in U.S. Pat. No. 5,973,637. However, none of these methods are suitable for improving near zone detection.
- An object of the present invention is to improve RLG systems in terms of near zone detection.
- This and other objects are achieved with a method of the kind mentioned by way of introduction, comprising obtaining a first sampled TDR signal with an amplification such that the interfering pulse is unsaturated, determining a compensation pulse signal using a previously stored pulse shape, a detected pulse amplitude and a current pulse position, subtracting said compensation pulse signal from the first sampled TDR signal, determining if a surface reflection occurs in an area where said interfering pulse will have a significant interfering effect on the surface reflection pulse, and if no surface reflection occurs in this area, updating the current pulse position.
- According to an embodiment of the invention, a compensating signal is subtracted from the TDR signal in order to improve the accuracy in the near zone. The compensating signal is determined using a preset pulse shape and current values for the maximum amplitude and position of the interfering pulse. When no surface reflection is determined to occur in the near zone, the position of the interfering pulse can be updated.
- The compensation signal can be based on the fact that the shape of the interfering pulse is essentially constant for a given type of pulse generator. The amplitude can be influenced by the type of probe and the installation.
- The invention will now be described in more detail, with reference to the appended drawings, in exemplifying manner illustrating a preferred embodiment of the invention.
-
FIG. 1 schematically shows a radar level gauge system mounted on a tank. -
FIGS. 2 a and 2 b are graphs of TDR signals obtained with a conventional RLG system. -
FIG. 3 is a flow chart of a method according to an embodiment of the invention. -
FIGS. 4 a and 4 b are graphs of TDR signals compensated according to the method inFIG. 3 . -
FIG. 5 is a graph illustrating the influence of temperature on the position of the interfering pulse. -
FIG. 6 a-b are graphs of an interfering pulse influenced by a surface reflection in the near zone. -
FIG. 7 is a flow chart of a method according to a further embodiment of the invention. -
FIG. 1 shows schematically a radarlevel gauge system 1 in which a method according to the invention may be advantageously used. Thesystem 1 is arranged to perform measurements of a process variable such as the level of aninterface 2 between twomaterials tank 5. Typically, thefirst material 3 is a liquid stored in the tank, e.g. gasoline, while thesecond material 4 is air. - The
system 1 comprises aprobe 10, attached to amounting device 11 and extending into thetank 5. Theprobe 10 can be of any known kind, such as a coaxial or dual line probe. In the presently described case, it is a dual line transmission probe. - The
mounting device 11 of theprobe 10 is connected, vialine 12, for example a cable or the like, to atransceiver 13, for generating a series of pulses to be transmitted by theprobe 10 into thetank 5, and receiving a time domain reflectometry (TDR) signal. - The
transceiver 13 comprises a pulse generator 14, preferable arranged to generate pulses with a length of about 2 ns or less, with a frequency in the order of MHz, at average power levels in the nW or μW area. Thetransceiver 13 also comprises a sample and hold circuit 15, connected to the pulse generator 14 and to theline 12. - The
transceiver 13 is controlled by amicro processor 17, providing the pulse generator with a high frequency (e.g. 2 MHz) clock signal, and the sample and hold circuit 15 with an oscillation signal (e.g. 40 Hz). Themicroprocessor 17 is also, via anamplifier 18 and an A/D-converter 19, connected to an output of the sample and hold circuit 15, which provides theprocessor 17 with a digitalized, sampledTDR signal 20. Thesignal 20 is expanded in time, allowing for use of conventional hardware for conditioning and processing. Theprocessor 17 is further connected with a ROM unit 21 (e.g. an EEPROM) for storing pre-programmed parameters, and aRAM unit 22 for storingsoftware code 23 executable by themicroprocessor 17. The processor can also be connected to auser interface 24. -
FIG. 2 a-b shows two graphical examples of aTDR signal signal 31 for an empty tank. As is clear fromFIG. 2 a, the signal comprises a first, negative pulse 32, and a second, positive pulse 33. The first pulse is caused by the transition from thecable 12 into theprobe 10 in air. As the probe has higher impedance, the pulse is negative. The second pulse is caused by the reflection against an oil liquid. As the probe in oil has lower impedance than the probe in air, this pulse is positive. During processing, the first pulse can be used as a reference, as it defines the beginning of the probe. However, it also influences the appearance of the second pulse, especially in the illustrated situation, where the two pulses have opposite signs. Therefore, the first pulse is here referred to as an interfering pulse, while the second pulse is referred to as the surface reflection pulse. - In
FIG. 2 b, the oil surface is closer to the top of the tank, and the surface reflection pulse 33 thus appears closer to the interfering pulse 32. As a consequence, the surface reflection pulse does not appear distinctly, but only results in a slight elevation of the positive flank of the interfering pulse 32. This is visible in the figure when comparing theTDR signal 30 with theempty tank signal 31. The surface reflection in this case occurs somewhere close to 475. - It is clear from
FIG. 2 a-b that conventional processing methods will be insufficient for determining the exact position of the surface reflection pulse, due to interference from the pulse caused by the transition between cable and probe. - A preferred embodiment of the invention is illustrated in the flow chart in
FIG. 3 . The process described by the flow chart can be realized by a computer program comprised in theprogram code 23 and running on theprocessor 17. This control program can thus control the pulse generator 14, the sample and hold circuit 15, theamplifier 18, and any other components of the system required to perform the described method. - In this particular embodiment, two sampled TDR signals are obtained for each measuring, a first sampled signal is obtained by the sample and hold circuit 15, and amplified by
amplifier 18 such that the interfering pulse. is not saturated. This is suitable for obtaining information about this pulse, enabling use of it as a reference pulse during the future processing and analysis of the TDR signal. Then, a second sampled signal is obtained and amplified using more powerful amplification. This is suitable for ensuring that the surface reflection pulse, which in some situations, depending on materials in the tank and other conditions, can be very weak, is more easily detectable. The principle of a two-part sampling process is not novel, and the invention is by no means limited to the use of such a process. - Returning top
FIG. 3 , a first, non-saturated sampled TDR signal is obtained in step S1. This first signal does not necessarily cover the entire range of the tank, but preferably only the beginning of the probe. As mentioned, its primary purpose is to gain information about the interfering pulse. - According to this particular embodiment, the amplitude of the interfering pulse Ai is then determined in step S2, and in step S3 compared to an amplitude value currently stored in the
RAM 22. If the detected amplitude is greater than the previously stored value, this stored value is replaced with the detected amplitude in step S4, before the program control moves on to step S5. - In step S5, the
processor 17 defines a compensation signal based on a pulse shape previously stored in theROM 21, and a current pulse position, Pi, also stored in theRAM 22. The current pulse position Pi in theRAM 22 corresponds to the anticipated position of the extreme point of the interfering pulse 32, and the compensation signal is aligned so that its extreme point coincides with the current pulse position Pi. If required, the compensation signal can also be rescaled in order to correspond to the amplitude of the interfering pulse, in the illustrated case using the detected amplitude value Ai. - As the pulse amplitude can vary significantly depending on the probe and installation, it can be advantageous to secure a detection of the interfering pulse amplitude before step S5, here in step S2. This means that the amplitude is measured every sampling. If it could be ascertained that no surface reflection was present in the near zone during this detection, one single detection would be sufficient, as the amplitude is essentially unaffected by ambient conditions. However, to handle the case when a surface reflection is present in the near zone during startup (when the first sampling is made), thus reducing the detected amplitude of the interfering pulse, the steps S3 and S4 will allow for a gradual increase of the stored amplitude value. Therefore, when the surface reflection pulse moves out of the near zone, and the detected amplitude of the interfering pulse increases, the stored value will also increase. Basically, as soon as it has been ascertained that the amplitude of the interfering pulse has been detected without disturbance from any surface reflection, no additional detections are required. If considered advantageous, such a routine could be implemented in the program.
- The pulse shape stored in
ROM 21 can be determined and stored during pre-installation calibration, or be set to a fixed curve if the expected variations are minor. It has been found that the curve shape typically only differs in terms of amplitude when comparing different types of probes, such as a twin rigid probe and a coaxial probe. Therefore, it can be sufficient to store a curve corresponding to the pulse generator to be used, or possibly several different curves in case the system can be used with different signal boards having different pulse generators. Further, installation dependent adjustments of this shape may also be stored in a user area of theROM 21, by providing suitable options in theuser interface 24. - In step S6, the processor subtracts the compensation signal from the detected TDR signal, resulting in a compensated TDR signal 40 (see
FIG. 4 a-b). - In step S7 the position of any surface reflection pulse is determined by
processor 17, using conventional techniques for TDR signal processing. Such techniques may include threshold analysis, derivative analysis, etc. - The purpose of the subtraction in step S6 is to remove any influence from the interfering pulse from the TDR signal, in order to enable a more secure localization of the surface reflection pulse. As the surface reflection by definition occurs during the time range after the extreme point of the interfering pulse, Pi, computational power can be saved by only defining the compensation signal for this time range.
-
FIG. 4 a-b show two graphical examples of compensated TDR signals 40 a and 40 b, essentially corresponding to thesignals FIG. 2 . Note however, that the position of the surface reflections are slightly offset. From the curves it is clear that the compensation signal has been subtracted starting from the extreme point, Pi. - In
FIG. 4 a, thesurface reflection pulse 43 a is probably far enough from the interferingpulse 42 a to be detectable even without the compensation in step S6. - In
FIG. 4 b, the situation is similar to that inFIG. 2 b, where the surface reflection is less apparent and more difficult to detect due to interference from the interfering pulse. InFIG. 4 b, however, the subtraction of the compensation signal has removed the influence from the interfering pulse, and thesurface reflection 43 b is clearly visible, and thus easy to detect correctly. - Returning now to the flow chart in
FIG. 3 , in step S8, it is determined if any surface reflection pulse is present in the near zone. In order to make this determination, a quantitative definition of the near zone in terms of time distance from the extreme point Pi must be defined, and stored in theRAM 22 orROM 21. This definition should be based on characteristics of the probe and tank, and preferably be such that it includes the entire area in which the influence of the interfering pulse is significant. Typically, this is the case until the interfering pulse has declined to a defined percentage of its maximum amplitude, for example about 5%, preferably only about 3%, and most preferably only about 1%. This position where the interfering pulse has declined to this predefined level thus indicates the beginning of the near zone. - If no surface reflection is present in the near zone (like in
FIG. 2 a), the interferingpulse 32 a can be considered to be unaffected by the surface echo, and theTDR signal 30 a can in step S9 be analyzed in order to update the current pulse position Pi, stored in theRAM 22. The details of this updating process will be described in more detail below. - Then, in step S10, a second, more amplified sampled TDR signal is acquired, and any desired information is extracted by processing and analyzing this signal in step S11. It may be advantageous to perform a compensation also of the second TDR signal, but as the surface reflection has been located outside the near zone, this may not be required. After the analysis of the TDR signal has been completed, or possibly even before this, program control returns to step S1 to acquire a new sampled TDR signal.
- If a surface reflection is found to be present in the near zone in step S8, no update of the pulse position Pi will be made. However, if, as in the present example, the first sampling in step S1 was only of a portion of the measurement range, a second sampled TDR signal, having a greater range than the first TDR signal, is acquired in step S12. Just like the first TDR signal, this second TDR signal is acquired with an amplification such that the interfering pulse is non-saturated. Note that, if instead a TDR signal covering the entire range was acquired already in step S1, no second TDR signal would be required at this stage.
- Then, in step S13, the subtraction performed in step S6 is again performed, in order to obtain a compensated TDR signal, covering the entire measuring range. This signal is processed and analyzed in step S14 to extract any desired information from the TDR signal.
- Note that the processing steps S11 and S14 are essentially equivalent, the only difference being that the TDR signals to be analyzed have different amplification. Also, the TDR signal analyzed in step S14 has been compensated in step S13, while the TDR signal analyzed in step S11 may be uncompensated.
- As illustrated, the described method allows a better determination of a surface reflection pulse in the near zone, by subtracting a compensation signal based on information about the interfering pulse from the TDR signal. However, while the amplitude of the interfering pulse is essentially unaffected by ambient conditions, the position of the interfering pulse may change. The influence of temperature is illustrated in
FIG. 5 , from which it is clear that the amplitude of the pulse is almost constant, while the position changes. As described above, the current position of the interfering pulse is therefore updated in S9 each time the surface reflection is determined to be outside the near zone, i.e. when the interfering pulse is uninfluenced by any surface reflection. In one embodiment of this updating process, the step S9 quite simply includes determining the extreme point of the interfering pulse, using known methods of TDR signal analysis. - However, during periods when a surface reflection is present in the near zone, the extreme point of the interfering pulse is influenced by the surface reflection. Therefore, in a more elaborate embodiment, the pulse position Pi is divided into a first part which is relatively unaffected by a surface reflection in the near zone, and a second part which is more affected by a surface reflection in the near zone. Consequently, the first part can be updated each time the first sampled TDR signal is acquired, while the second part is only updated if no surface reflection is present in the near zone. This division of the position Pi can provide a more precise determination of the position of the interfering pulse, even during periods when a surface reflection is present in the near zone.
-
FIG. 6 a-b show a section of a TDR signal close to the interfering pulse. InFIG. 6 a, no surface reflection is present in the near zone, while inFIG. 6 b, a surface reflection is in the near zone close to the top of the tank. It is clear from the figures that the position of theextreme point 51 a, 51 b of the interferingpulse 52 a, 52 b is significantly affected by thesurface reflection pulse 53 a, 53 b. However, it has been established by the inventor that the position Pthres at which the negative slope of the interfering pulse crosses a given threshold is essentially unaffected by the surface reflection. This threshold can be expressed as a percentage of the maximum pulse amplitude, and in the illustrated example it is 30% of the maximum amplitude. Note that the pulse is negative, and so the amplitude is calculated from the DC level and down. The threshold is thus 30% of the amplitude below the DC level. As this position Pthres is uninfluenced by ambient conditions, it can thus be used as the part of the pulse position Pi not affected by the surface reflection pulse. - The distance from the threshold crossing Pthres to the actual extreme point of the interfering pulse will however be affected by the surface reflection pulse. Therefore, this distance should only be determined when it has been determined that the surface reflection is not present in the near zone (i.e. in step S9 in
FIG. 3 ). Determining this distance is simply done by determining the actual position of the extreme point Pi using conventional TDR signal analysis, and then subtraction the current value of the threshold crossing Pthres. - In an alternative embodiment, illustrated in the flow chart in
FIG. 7 , employs a single sample measuring process.FIG. 7 includes the steps S21-S29, corresponding to the steps S1-S9 inFIG. 3 . - In this case, the first acquired TDR signal preferably includes the entire measurement range, as no second sampling is made. If no surface reflection is found present in the near zone in step S28, program control continues to step S30, where any additional TDR signal analysis is performed, before returning to step S21. If a surface reflection is found to be present in the near zone in step S28, the position of the extreme point, Pi, is updated, before program control continues to step S30.
- It is clear to the skilled person that only the presently preferred embodiments of the invention has been described. Several variations and modifications are possible within the scope of the appended claims. For example, slight modifications of the described method may be advantageous when adopting it to a RLG system having a different electronic design. Also, the method can naturally include additional steps and routines, not described herein.
- Further, the method has been described only with reference to probe based RLG, while it may also be used to improve the performance of free propagating RLG, i.e. systems where the pulses are transmitted into the tank without a probe. In such a system, the interfering pulse is caused by the transition between the radar antenna and free space, and the advantages of the described method will be similar.
Claims (20)
Priority Applications (3)
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US10/876,240 US6972712B1 (en) | 2004-06-24 | 2004-06-24 | Near zone detection in radar level gauge system |
DE112005001439.8T DE112005001439B4 (en) | 2004-06-24 | 2005-06-23 | Improved short-zone measurement in a radar level measurement system |
PCT/SE2005/000990 WO2006001767A1 (en) | 2004-06-24 | 2005-06-23 | Improved near zone detection in radar level gauge system |
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US10/876,240 US6972712B1 (en) | 2004-06-24 | 2004-06-24 | Near zone detection in radar level gauge system |
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US20050285777A1 true US20050285777A1 (en) | 2005-12-29 |
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US10/876,240 Expired - Lifetime US6972712B1 (en) | 2004-06-24 | 2004-06-24 | Near zone detection in radar level gauge system |
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US (1) | US6972712B1 (en) |
DE (1) | DE112005001439B4 (en) |
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US6972712B1 (en) | 2005-12-06 |
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DE112005001439T5 (en) | 2007-05-16 |
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