Patent Description:
In filling level determining systems functioning by transmitting and receiving electromagnetic signals using a transmission line probe extending towards and into a product in a tank, pulsed electromagnetic signals are usually used. Although having the capability of providing excellent measurement accuracy, such so-called pulsed guided wave radar (GWR) systems have drawbacks.

For instance, it has proven to be relatively challenging to handle issues related to timing and temperature dependence, especially while striving for low cost and complexity.

For non-contact radar level gauge systems, pulsed signals are generally not used. Instead, various implementations of FMCW-techniques are typically used. Recently, the trend has been towards stepped measurement sweeps and higher frequencies. The reasons for increasing the frequency of the measurement signals include that the dimensions of the non-contact radar level gauge system can be reduced and that the measurement accuracy can be improved.

There have also been efforts to implement FMCW-type techniques on guided wave radar systems. For instance, <CIT> discloses an FMCW-type radar level gauge configured to transmit an electromagnetic transmit signal and receive an electromagnetic return signal reflected from the surface, the electromagnetic transmit signal having a bandwidth of at least <NUM>, a relative bandwidth (max frequency/min frequency) of less than <NUM> and an upper frequency limit less than <NUM>. The gauge according to <CIT> comprises a single conductor probe mechanically suspended in the tank and extending into the product in the tank, and a matching arrangement providing an electrically matched connection between an electrical feed-through and the single conductor probe.

Although the radar level gauge system according to <CIT> apparently has various advantageous properties, it would be desirable to provide for improved measurement performance.

<CIT> relates to an FMCW-type radar level gauge system comprising a signal propagation device; a microwave signal source; a microwave signal source controller; a mixer configured to combine a transmit signal from the microwave signal source and a reflection signal from the surface to form an intermediate frequency signal; and processing circuitry coupled to the mixer and configured to determine the filling level based on the intermediate frequency signal.

<CIT> relates to a method for lessening disturbances of a measurement signal in a radar unit for distance measurement by means of frequency-modulated radar in continuous wave operation.

In view of the above, a general object of the present invention is to provide for improved measurement performance in a radar level gauge system using a transmission line probe for guiding electromagnetic signals.

According to a first aspect of the present invention, it is therefore provided a method of determining a filling level of a product in a tank using a radar level gauge system comprising a transceiver, a transmission line probe, and processing circuitry, the method comprising the steps of: generating an electromagnetic transmit signal exhibiting a measurement sweep across a time series of piece-wise constant frequencies being within a measurement frequency range starting at a first frequency, and ending at a second frequency higher than the first frequency, a difference between frequencies in each pair of adjacent frequencies in the frequency range being equal to the first frequency; guiding the transmit signal towards and into the product in the tank; guiding an electromagnetic reflection signal, resulting from reflection of the transmit signal at impedance discontinuities encountered thereby, back towards the transceiver; mixing the reflection signal with an electromagnetic reference signal exhibiting a reference sweep across a time series of piece-wise constant reference frequencies, the reference signal being in phase with the transmit signal at a start of the measurement sweep and exhibiting a constant frequency difference in relation to the transmit signal across the measurement sweep, the mixing resulting in a mixer output indicative of a difference between the reflection signal and the reference signal; forming a measurement signal based on the mixer output; and determining the filling level based on the measurement signal.

The present invention is based on the realization that the use of a transmit signal in which the difference between adjacent frequencies is equal to the lowest frequency of the frequency range of the transmit signal allows for signal processing providing information about polarities of echo signals resulting from reflection of the transmit signal at the impedance discontinuities encountered thereby. This facilitates the identification of echo signals resulting from reflection of the transmit signal at certain impedance discontinuities, such as a reference impedance discontinuity at an interface between the transceiver and the transmission line probe and/or an impedance discontinuity at an end of the transmission line probe. This, in turn, provides for more robust and reliable filling level determination.

Furthermore, the transmit signal configuration according to embodiments of the present invention may provide an unambiguous relation between the difference in phase between the reflection signal and the transmit signal, and the filling level. This provides for improved precision in the determination of the filling level, in relation to determining the filling level using frequency shift information.

According to a second aspect of the present invention, it is provided a radar level gauge system, for determining a filling level of a product in a tank, the radar level gauge system comprising: a transceiver for generating, transmitting, and receiving electromagnetic signals; a transmission line probe coupled to the transceiver and configured to guide an electromagnetic transmit signal from the transceiver towards and into the product in the tank, and guide an electromagnetic reflection signal, resulting from reflection of the transmit signal at impedance discontinuities encountered thereby, back towards the transceiver; and processing circuitry coupled to the transceiver for determining the filling level based on a timing relation between the reflection signal and the transmit signal, wherein the radar level gauge system is configured to: generate the transmit signal to include a measurement sweep across a time series of piece-wise constant frequencies being within a measurement frequency range starting at a first frequency, and ending at a second frequency higher than the first frequency, a difference between frequencies in each pair of adjacent frequencies in the frequency range being equal to the first frequency; mix the reflection signal with an electromagnetic reference signal exhibiting a reference sweep across a time series of piece-wise constant reference frequencies, the reference signal being in phase with the transmit signal at a start of the measurement sweep and exhibiting a constant frequency difference in relation to the transmit signal across the measurement sweep, the mixing resulting in a mixer output indicative of a difference between the reflection signal and the reference signal; form a measurement signal based on the mixer output; and determine the filling level based on the measurement signal.

The "transceiver" may be one functional unit capable of transmitting and receiving microwave signals, or may be a system comprising separate transmitter and receiver units. For all embodiments, it should be noted that the processing circuitry may be provided as one device or several devices working together.

In summary, the present invention thus relates to a method of determining a filling level of a product in a tank, comprising the steps of: generating an electromagnetic transmit signal exhibiting a measurement sweep across a time series of piece-wise constant frequencies being within a measurement frequency range starting at a first frequency, and ending at a second frequency higher than the first frequency, a difference between frequencies in each pair of adjacent frequencies in the frequency range being equal to the first frequency; guiding the transmit signal towards and into the product in the tank; guiding an electromagnetic reflection signal back towards the transceiver; mixing the reflection signal with an electromagnetic reference signal, resulting in a mixer output indicative of a difference between the reflection signal and the reference signal; forming a measurement signal based on the mixer output; and determining the filling level based on the measurement signal.

These 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, wherein:.

In the present detailed description, various embodiments of the radar level gauge system and method are mainly discussed with reference to a radar level gauge system comprising a transmission line probe in the form of single conductor probe, or so-called Goubau probe.

It should be noted that this by no means limits the scope of the present invention, which equally well includes radar level gauge systems and methods using other types of transmission line probes, such as a coaxial probe or a parallel wire transmission line probe.

<FIG> schematically shows a level measuring system <NUM> comprising a radar level gauge system <NUM> according to an example embodiment of the present invention, and a host system <NUM> illustrated as a control room.

The radar level gauge system <NUM> is installed to measure the filling level of a product <NUM> in a tank <NUM>. The radar level gauge system <NUM> comprises a measuring electronics unit <NUM> arranged outside the tank <NUM>, and a transmission line probe, here in the form of a single conductor probe <NUM>, extending from the measuring electronics unit <NUM> towards and into the product <NUM>. In the example embodiment in <FIG>, the single conductor probe <NUM> is a wire probe, that has a weight <NUM> attached at the end thereof to keep the wire straight and vertical.

By analyzing a timing relation between an electromagnetic transmit signal ST being guided by the transmission line probe <NUM> towards the surface <NUM> of the product <NUM>, and an electromagnetic reflection signal SR being guided back from the surface <NUM> by the transmission line probe <NUM>, the measurement electronics unit <NUM> can determine the distance between a reference position (such as a feed-through between the outside and the inside of the tank) and the surface <NUM> of the product <NUM>, whereby the filling level L can be deduced. It should be noted that, although a tank <NUM> containing a single product <NUM> is discussed herein, the distance to another material interface along the transmission line probe <NUM> (if present) may be measured in a similar manner.

As is schematically illustrated in <FIG>, the measurement electronics unit <NUM> comprises a transceiver <NUM>, processing circuitry <NUM>, a communication interface <NUM>, and a communication antenna <NUM> for wireless communication with the control room <NUM>. The transceiver <NUM>, the processing circuitry <NUM>, and the communication interface <NUM> are all illustrated as being enclosed in a measurement electronics unit housing <NUM>.

The transceiver <NUM> is configured to generate, transmit and receive electromagnetic signals, and is coupled to the transmission line probe <NUM> via a feed-through <NUM> through a wall of the tank <NUM>. Various suitable feed-through configurations are, per se, known in the art, and the feed-through <NUM> is schematically indicated as a simple box in <FIG>.

The processing circuitry <NUM> is coupled to the transceiver <NUM> and is configured to determine the filling level L based on a timing relation between the reflection signal SR and the transmit signal ST as will be described in greater detail further below. The communication interface <NUM> is connected to the processing circuitry <NUM> and configured to allow communication with the host system <NUM> via the communication antenna <NUM>. In the example embodiment of <FIG>, the communication between the radar level gauge system <NUM> and the host system <NUM> is indicated as being wireless communication. Alternatively, communication may, for example, take place over an analog and/or digital wire-based communication channel. For instance, the communication channel may be a two-wire <NUM>-<NUM> mA loop and the filling level may be communicated by providing a certain current corresponding to the filling level on the two-wire <NUM>-<NUM> mA loop. Digital data may also be sent across such a <NUM>-<NUM> mA loop, using the HART protocol.

Moreover, although not shown in <FIG>, the radar level gauge system <NUM> may be connectable to an external power source, or may be powered through communication lines.

<FIG> is a partial schematic block diagram of an example configuration of the radar level gauge system <NUM> in <FIG>.

The transceiver <NUM> is here shown as including a first signal generator <NUM>, a second signal generator <NUM>, a power divider <NUM>, a first mixer <NUM>, a second mixer <NUM>, and optional first <NUM> and second <NUM> analog bandpass filters. The processing circuitry <NUM> is shown as including timing circuitry <NUM>, first <NUM> and second <NUM> samplers, first <NUM> and second <NUM> optional digital bandpass filters, a measurement block <NUM>, a transformation block <NUM>, and a filling level determining block <NUM>.

As is schematically indicated in <FIG>, the timing circuitry <NUM> is coupled to the first signal generator <NUM> and to the second signal generator <NUM>. The timing circuitry <NUM> is configured to control the first signal generator <NUM> to generate a transmit signal ST, and to control the second signal generator <NUM> to generate a reference signal SREF. The first signal generator <NUM> is connected to the transmission line probe <NUM> via the power divider <NUM>, and thus provides the transmit signal ST to the transmission line probe <NUM>. The reflection signal SR guided back by the transmission line probe <NUM> is routed by the power divider <NUM> to the first mixer <NUM>, which is also connected to receive the reference signal SREF from the second signal generator <NUM>. The reference signal SREF provided by the second signal generator <NUM> and the reflection signal SR from the transmission line probe <NUM> are combined by the first mixer <NUM>, resulting in a mixer output indicative of a difference between the reflection signal SR and the reference signal SREF. In embodiments where there is a constant non-zero frequency difference between the transmit signal ST and the reference signal SREF, the portion of the mixer output that is of interest for additional processing will exhibit a frequency that is substantially equal to this non-zero frequency difference. To facilitate subsequent signal processing, the mixer output may therefore optionally be passed through a first analog bandpass filter <NUM>, as is schematically indicated by the dashed box in <FIG>.

To provide a reference mixer output, the transmit signal ST and the reference signal SREF are provided to the second mixer <NUM>. The transmit signal ST and the reference signal SREF are combined by the second mixer <NUM>, resulting in a reference mixer output indicative of a difference between the transmit signal ST and the reference signal SREF. The main reason for doing this is that there may be drifts over time, such as due to varying temperatures, in the difference between the transmit signal ST and the reference signal SREF. In embodiments where there is a constant non-zero frequency difference between the transmit signal ST and the reference signal SREF, the portion of the reference mixer output that is of interest for additional processing will exhibit a frequency that is substantially equal to this non-zero frequency difference. To facilitate subsequent signal processing, the reference mixer output may therefore optionally be passed through a second analog bandpass filter <NUM>, as is schematically indicated by the dashed box in <FIG>.

As is schematically shown in <FIG>, the mixer output is sampled by the first sampler <NUM>, which may be controlled by the timing circuitry <NUM> to be synchronized with the operation of the first signal generator <NUM> and the second signal generator <NUM>. The sampled mixer output may optionally be passed through a first digital bandpass filter <NUM>, as is schematically indicated by the dashed box in <FIG>. In the same way, the reference mixer output is sampled by the second sampler <NUM>, and the sampled reference mixer output may optionally be passed through a second digital bandpass filter <NUM>.

The sampled values of the mixer output and the reference mixer output are provided to the measurement block <NUM>, where the amplitude and the phase of the mixer output is measured in relation to the amplitude and phase of the reference mixer output according to one of several methods per se well known to those of ordinary skill in the art. The measured values of the amplitude and phase as a function of frequency of the transmit signal ST are then further processed by the transformation block <NUM> and the level determining block <NUM> to determine the filling level L in the tank <NUM>, and provide a signal indicative thereof.

It should be noted that elements of the transceiver <NUM> may be implemented in hardware, and may form part of an integrated unit normally referred to as a microwave unit, and that at least some portions of the processing circuitry <NUM> may be embodied by software modules executed by an embedded processor. The invention is not restricted to this particular realization, and any implementation found suitable to realize the herein described functionality may be contemplated.

Exemplary operation of the radar level gauge system <NUM> described so far will be described in greater detail further below with reference to the flow-chart in <FIG> and other illustrations as indicated.

In a first step <NUM>, an electromagnetic transmit signal ST is generated. With further reference to <FIG>, the transmit signal ST exhibits a measurement sweep that is transmitted during a sweep time tsweep. As can be seen in the enlarged portion of <FIG>, the measurement sweep is across a time series of piece-wise constant frequencies fn. The frequencies in this time series of piece-wise constant frequencies fn are within a measurement frequency range starting at a first frequency f<NUM> and ending at a second frequency f<NUM>, which is higher than the first frequency f<NUM>. A difference fstep between frequencies in each pair of adjacent frequencies fn, fn+<NUM> in the frequency range is equal in magnitude to the first frequency f<NUM>. Accordingly, if the first frequency f<NUM> is, say, <NUM>, then the difference fstep is <NUM>. Referring back to <FIG>, the timing circuitry <NUM> may control the first signal generator <NUM> to generate the above-described measurement sweep.

Although a single measurement sweep is shown in <FIG>, it should be understood that the transmit signal ST may typically include a sequence of measurement sweeps. Furthermore, the frequency need not be monotonically increasing as is shown in <FIG>. In embodiments, a measurement sweep may start at the second frequency f<NUM> and end at the first frequency f<NUM>. According to other embodiments, the frequencies may be output in arbitrary order. In such embodiments, it may be desirable to sort sampled values in order of increasing frequency of the transmit signal, from the first frequency f<NUM> to the second frequency f<NUM> before certain digital processing.

In the subsequent step <NUM>, the transmit signal ST is guided by the transmission line probe <NUM> towards and into the product <NUM> in the tank <NUM>.

An electromagnetic reflection signal SR resulting from reflection of the transmit signal ST at impedance discontinuities encountered thereby is guided by the transmission line probe <NUM> back towards the transceiver <NUM>. Referring to <FIG>, the reflection signal SR is passed, via the power divider <NUM>, to the mixer <NUM>.

In the mixer <NUM>, the reflection signal SR is mixed with an electromagnetic reference signal SREF, in step <NUM>. The reference signal SREF exhibits a reference sweep across a time series of piece-wise constant reference frequencies. The reference signal SREF is in phase with the transmit signal ST at the start of the measurement sweep and exhibits a constant frequency difference in relation to the transmit signal ST across the measurement sweep. The constant frequency difference may advantageously be non-zero, but could also be zero in certain embodiments. Accordingly, the reference sweep is identical to the measurement sweep in <FIG>, with the exception of a frequency offset.

Referring back to <FIG>, the timing circuitry <NUM> may control the second signal generator <NUM> to generate the reference sweep. In embodiments with a zero frequency difference, there would be no need for the second signal generator <NUM>, but the transmit signal ST could be directly provided to the mixer <NUM> and mixed with the reflection signal SR. It may, however, be beneficial to use first <NUM> and second <NUM> signal generators and a non-zero frequency difference, since this allows for the use of relatively cheap square-wave clock generators as signal generators. Unwanted frequencies may be removed using a bandpass filter - either or both of the analog filter <NUM> and the digital filter <NUM> indicated in <FIG> - only allowing frequencies close to the non-zero frequency difference to pass. A suitable frequency difference may be in the order of kHz, such as <NUM>.

As is, per se, well-known, the output from the mixer <NUM> - the mixer output - is indicative of a difference between the signals input to the mixer <NUM>. In this case, the mixer output is thus indicative of the difference between the reflection signal SR and the reference signal SREF.

In the subsequent step <NUM>, a measurement signal Sm is formed based on the mixer output. As is schematically indicated in <FIG>, formation of the measurement signal may include sampling the mixer output, by the first sampler <NUM>, and the reference mixer output, by the second sampler <NUM>, at sampling times that are coordinated with the frequency steps of the transmit signal ST. At least one sample of each of the mixer output and the reference mixer output may be taken for each piece-wise constant frequency in the measurement sweep. To achieve this, the first sampler <NUM> and the second sampler <NUM> may, for example, be controlled by the timing circuitry <NUM> as indicated in <FIG>. The mixer output can be seen as a complex signal with a real component representing the amplitude of the mixer output and an imaginary component representing the phase of the mixer output, and the reference mixer output can be seen as a complex signal with a real component representing the amplitude of the reference mixer output and an imaginary component representing the phase of the reference mixer output. Based on the mixer output and the reference mixer output, the phase of the mixer output can be related to the phase of the transmit signal ST, so that the amplitude and the phase of the mixer output can be measured by the measurement block <NUM> in <FIG>. It should be noted that there may other ways of referencing the phase of the mixer output to the phase of the transmit signal ST. For instance, the phase of the transmit signal ST may be assumed to exhibit predefined values for the different frequency steps and may be stored in memory. The configuration in <FIG> is, however, expected to provide more accurate results when the radar level gauge system <NUM> is subjected to varying temperatures etc..

Finally, in step <NUM>, the filling level L is determined, by the level determining block <NUM>, based on the measurement signal Sm.

The step <NUM> of forming the measurement signal Sm may advantageously include forming a frequency domain measurement signal indicative of the amplitude and the phase of the mixer output as a function of the frequency of the transmit signal ST, as described above. This frequency domain measurement signal may be used directly in step <NUM> to determine the filling level L. An example of such a frequency domain measurement signal <NUM> is shown in <FIG>, where the solid line <NUM> represents samples of the above-mentioned real component of the mixer output, and the dashed line <NUM> represents samples of the above-mentioned imaginary component of the mixer output.

With reference to <FIG>, the frequency domain measurement signal <NUM> may alternatively be formed by additionally adding mirrored data sets for negative frequencies. Referring to <FIG>, each mirrored data set 54a-b for a negative frequency value -fn is a complex conjugate of a data set 56a-b of the measured amplitude and phase of the mixer output for the corresponding positive frequency value +fn. When the frequency domain measurement signal <NUM> in <FIG> is transformed, by the transformation block <NUM>, to a time domain measurement signal, echo peaks with different polarities depending on the properties of the different encountered impedance discontinuities can be obtained. An example of such a time domain measurement signal <NUM> is shown in <FIG>.

Based on the time domain measurement signal <NUM>, it is straightforward to translate the time to distance D from the transceiver <NUM> (typically from some reference structure, such as the tank feed-through <NUM>) to the surface <NUM> of the product <NUM> (the "ullage"), which can easily be converted to the filling level L. In the exemplary time domain measurement signal <NUM> (converted to distance D) in <FIG>, the reflection from the tank feed-through is a negative peak <NUM>, and the reflection from the surface <NUM> is the first strong positive peak <NUM>.

The transformation to the time domain may advantageously include performing inverse digital fourier transformation on the frequency domain measurement signal <NUM>. The above-mentioned addition of the mirrored data sets is made possible by the specific property of the frequency step fstep being equal to the first frequency f<NUM> of the measurement sweep (see <FIG>). This, in turn allows for determination of the polarities of the reflection peaks, which facilitates identification of the so-called fiducial pulse (the negative peak <NUM>) and the surface reflection (which may for example be the first positive peak <NUM> that is higher than a predefined threshold). For example, inverse fast fourier transformation (IFFT) may be used.

Through a suitable choice of the first frequency f<NUM>, the phase of the measurement signal Sm will be unambiguous across the desired measurement range. The desired measurement range will depend on the application. For a longer measurement range, the first frequency may be selected to be relatively low. For instance, a first frequency f<NUM> of <NUM> will correspond to a maximum range of about <NUM>, which should be more than enough for most applications. For most tank gauging application, the first frequency f<NUM> may advantageously be in the range <NUM> to <NUM>.

Regarding the choice of value for the second frequency f<NUM>, this will typically be a trade-off between measurement time/energy consumption and accuracy. For most tank gauging application, the second frequency f<NUM> of the measurement sweep may be in the range <NUM> to <NUM>.

In certain applications, it may be desired to measure very short distances very precisely. Then the first frequency f<NUM> may be selected higher, such as <NUM>, and the second frequency f<NUM> may also be selected higher, such as <NUM>.

Claim 1:
A method of determining a filling level of a product (<NUM>) in a tank (<NUM>) using a radar level gauge system (<NUM>) comprising a transceiver (<NUM>), a transmission line probe (<NUM>), and processing circuitry (<NUM>), the method comprising the steps of:
generating (<NUM>) an electromagnetic transmit signal (ST) exhibiting a measurement sweep across a time series of piece-wise constant frequencies being within a measurement frequency range starting at a first frequency (f<NUM>), and ending at a second frequency (f<NUM>) higher than the first frequency, a difference (fstep) between frequencies in each pair of adjacent frequencies in the frequency range being equal to the first frequency (f<NUM>);
guiding (<NUM>) the transmit signal (ST) towards and into the product (<NUM>) in the tank (<NUM>);
guiding (<NUM>) an electromagnetic reflection signal (SR) resulting from reflection of the transmit signal (ST) at impedance discontinuities encountered thereby, back towards the transceiver (<NUM>);
mixing (<NUM>) the reflection signal (SR) with an electromagnetic reference signal (SREF) exhibiting a reference sweep across a time series of piece-wise constant reference frequencies, the reference signal being in phase with the transmit signal (ST) at a start of the measurement sweep and exhibiting a constant frequency difference in relation to the transmit signal (ST) across the measurement sweep, the mixing resulting in a mixer output indicative of a difference between the reflection signal (SR) and the reference signal (SREF);
forming (<NUM>) a measurement signal (Sm) based on the mixer output; and
determining (<NUM>) the filling level based on the measurement signal.