Abstract:
Level gauging including an approxiamtion of a distance to the surface of a product kept in a tank. The approxiamtion is determined by relating an amplitude of a first harmonic of an IF signal with an amplitude of a second harmonic of the IF signal. 
     Basically, each harmonics represents a given distance range. By determining the received power in two or more harmonics, and correlating them to each other, the distance may be estimated. Depending on modulation and other parameters the distance dependence may be very different, and may be selected to suit the application.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a radar level gauge using electromagnetic waves to determine a distance to a surface of a product in a tank. 
       BACKGROUND OF THE INVENTION 
       [0002]    Since the radar level gauging was developed as a commercial product in the 1970&#39;s and 1980&#39;s, frequency modulated continuous wave (FMCW) has been the dominating measuring principle for high accuracy applications. An FMCW measurement comprises transmitting into the tank a signal which is swept over a frequency range in the order of a few GHz. For example, the signal can be in the range 25-27 GHz, or 9.5-11 GHz. The transmitted signal is reflected by the surface of the contents in the tank (or by any other impedance transition) and an echo signal, which has been delayed a certain time, is returned to the gauge. The echo signal is mixed with the transmitted signal to generate a mixer signal, having a frequency equal to the frequency change of the transmitted signal that has taken place during the time delay. Due to the linear sweep, this difference frequency, also referred to as an intermediate frequency (IF), is proportional to the distance to the reflecting surface. The mixer signal is often referred to as an IF signal. 
         [0003]    Although highly accurate, present FMCW systems are relatively power hungry, making them less suitable for applications where power is limited. Examples of such applications include field devices powered by a two-wire interface, such as a 4-20 mA loop, and wireless devices powered by an internal power source (e.g. a battery or a solar cell). Various methods are available to decrease power consumption. 
         [0004]    In U.S. Ser. No. 12/981,995, by the same inventor, a novel and less power hungry measuring principle was introduced, involving emitting a series of pulses having constant carrier wave frequency, each pulse being long compared to the time of transit (e.g. a pulse duration in the order of 10 us to 100 ms, compared to time of transit in the order of tenths of a μs). The method is therefore referred to as a Multiple Frequency Pulsed Wave (MFPW). 
         [0005]    The number of different carrier wave frequencies in a measurement cycle is insufficient to provide a continuous IF signal, or even an approximation of the IF frequency in the way done in so called “stepped” or “discrete” FMCW system. Instead, the small set of frequencies is chosen according to a specified frequency scheme, and a phase shift in the received pulse is determined for each frequency. 
         [0006]    The process of determining the distance to the surface involves establishing a change of phase with emitted frequency (see  FIG. 1 ). The line A represents an initial distance estimation, while line B represents an updated estimation. In theory, only two values (points x) are required to determine the rate of change (slope of line B), while in practice a larger number, e.g. a 5-20 samples, may be required. During a more complicated echo situation or during start-up a few hundred may be required. Naturally, from a power economy point of view, it is desirable to provide high accuracy and reliability with as few samples as possible. 
         [0007]    One way to increase accuracy without requiring many samples is to determine a “rough approximation” of the distance to the surface, and to use this estimate as a starting point when determining an accurate distance measure. 
       General Disclosure of the Invention 
       [0008]    Based on the above, it is an object of the present invention to provide an improved radar level gauging system of the kind with pulses of constant frequency which are long compared to the time of transit, which system is capable to provide a rough distance estimate independently of the existing measurement. 
         [0009]    According to a first aspect of the present invention, this and other objects are achieved by a method for determining a distance to a surface of a product kept in a tank, the method comprising forming a transmit signal as a pulse train of distinct carrier wave pulses having a duration greater than 1 microsecond and shorter than 100 milliseconds, each pulse being frequency modulated around a defined center frequency, transmitting the transmit signal towards the surface, receiving an electromagnetic return signal reflected at the surface, mixing the return signal with the transmit signal in a first channel (I-channel) and mixing the return signal with a 90° phase shifted transmit signal in a second channel (Q-channel), to provide two IF (intermediate frequency) signals, filtering the IF signals to provide a filtered signal corresponding to a first selected harmonic of the modulation frequency, mixing the filtered signals of each channel with the first selected harmonic of the modulation frequency, to provide two primary amplitude values (I and Q), calculating (step S 5 ) actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted, based on the primary amplitude values and determining a distance measure based on the actual phase properties, filtering the IF signals to provide a filtered signal corresponding to a second selected harmonic of the modulation frequency, mixing the filtered signals of each channel with the second selected harmonic of the modulation frequency, to provide two secondary amplitude values (I and Q), and providing an approximation of the distance based on a relationship between the primary and the secondary amplitude values. 
         [0010]    According to a second aspect of the present invention, this and other objects are achieved by a radar level gauging system, for determining a distance to a surface of a product kept in a tank, the system comprising a transceiver for transmitting electromagnetic transmit signals formed by a pulse train of distinct carrier wave pulses having a duration greater than 1 microsecond and shorter than 100 milliseconds, each pulse being frequency modulated around a defined center frequency, and receiving electromagnetic return signals reflected at the surface, a set of RF mixers for mixing the return signal with the transmit signal in a first channel (I-channel) and mixing the return signal with a 90° phase shifted transmit signal in a second channel (O-channel), to provide two IF (intermediate frequency) signals, a first set of filters for filtering the IF signals to provide a filtered signal corresponding to a first selected harmonic of the modulation frequency, a first set of IF mixers for mixing the filtered signals of each channel with the first selected harmonic of the modulation frequency, to provide two primary amplitude values (1 and Q), a second set of filters for filtering the IF signals to provide a filtered signal corresponding to a second selected harmonic of the modulation frequency, a second set of IF mixers for mixing the filtered signals of each channel with the second selected harmonic of the modulation frequency, to provide two secondary amplitude values (I and Q), and processing circuitry connected to receive outputs from the IF mixers and operable to calculate actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted, based on the primary amplitude values and determining a distance measure based on the actual phase properties, and providing an approximation of the distance based on a relationship between the primary and the secondary amplitude values. 
         [0011]    The level gauging thus includes an approxiamtion of a distance to the surface. The approxiamtion is determined by relating an amplitude of a first harmonic of an IF signal with an amplitude of a second harmonic of the IF signal. 
         [0012]    The frequency modulation of the transmit signal introduces a distance dependence. Two different harmonics of the modulated signal are used in two separate signal paths (sequential or parallel) to provide two different values representing different distance dependencies. By comparing these two values, e.g. dividing one with the other and forming a quotient between them, a rough approximation of the distance to the surface can be provided. 
         [0013]    Basically, each harmonics represents a given distance range. By determining the received power in two or more harmonics, and correlating them to each other, the distance may be estimated. Depending on modulation and other parameters the distance dependence may be very different, and may be selected to suit the application. 
         [0014]    The actual phase of the IF-signal of a plurality of pulses may be used in the process of providing a more exact distance measure, e.g. as disclosed in U.S. application Ser. No. 12/981,995, herewith incorporated by reference. 
         [0015]    According to one approach, the approximated distance is used as input in the accurate distance determination, e.g. to improve the statistical analysis applied to a plurality of phase properties, thereby enabling a more sensitive and reliable distance measurement. Alternatively, the rough approximation can be used as a post processing verification of the accurate measurement. If the rough approximation is significantly different than the accurate distance measure, then a new accurate distance measurement should be obtained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention will be described in more detail with reference to the appended drawings, showing currently preferred embodiments of the invention. 
           [0017]      FIG. 1  is a diagram showing phase versus transmitted frequencies. 
           [0018]      FIG. 2  is a schematic block diagram of a radar level gauge mounted on a tank. 
           [0019]      FIG. 3  is a more detailed block diagram of the transceiver in  FIG. 1 , according to a first embodiment of the present invention. 
           [0020]      FIG. 4  illustrates frequency modulation of a carrier wave. 
           [0021]      FIG. 5  is a flow chart of a method according to an embodiment of the present invention. 
           [0022]      FIG. 6  shows the first five harmonics for sinusoidal modulation. 
           [0023]      FIG. 7  shows Fourier coefficients for even tones for triangular modulation. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]    In the present description, embodiments of the present invention are mainly described with reference to a radar level gauge system having a free propagating antenna for radiating and capturing electromagnetic signals. It should be noted that this by no means limits the scope of the invention, which is equally applicable to other signal propagating devices, including other free propagating antennas such as a rod antenna, a patch antenna, a fixed or movable parabolic antenna or a conical antenna, and wave guides, such as a still pipe, a transmission line or a probe, such as a single-line probe (including a so-called Goubau probe), a twin-line probe or a coaxial probe. 
         [0025]      FIG. 2  schematically illustrates a radar level gauge system  1  according to an embodiment of the present invention, comprising a measurement electronics unit  2 , and a signal propagating device, here a horn antenna  3 . The radar level gauge system  1  is provided on a tank  5 , which is partly filled with a product  6  to be gauged. In the case illustrated in  FIG. 1 , the product  6  is a solid, such as grain or plastic pellets, which is well-known to represent a difficult application requiring a relatively high measurement sensitivity of the radar level gauge system. By analyzing transmitted signals S T  being radiated by the antenna  3  towards the surface  7  of the product  6 , and echo signals S R  traveling back from the surface  7 , the measurement electronics unit  2  can determine the distance between a reference position and the surface  7  of the product  6 , whereby the filling level L can be deduced. It should be noted that, although a tank  5  containing a single product  6  is discussed herein, the distance to any material interface present in the tank  5  can be measured in a similar manner. 
         [0026]    As is schematically illustrated in  FIG. 1 , the electronics unit  2  comprises a transceiver  10  for transmitting and receiving electromagnetic signals, which is connected to the antenna  3  via a wave guide  9 . The unit  2  further comprises a processing circuitry  11 , which is connected to the transceiver  10  for control of the transceiver and processing of signals received by the transceiver to determine the filling level of the product  6  in the tank  5 . The processing circuitry  11  is also connected to a memory  12 , storing any software required for the operation of the gauge  1 , and also providing RAM used during operation. 
         [0027]    The processing circuitry  11  is further connectable to external communication lines  13  for analog and/or digital communication via an interface  14 . As an example, the communication between the communication interface  14  and an external control station (not shown) can be provided by a two-wire interface, which has a combined function of both transmitting the measurement result to the control station and receiving power for operation of the gauge  1 . The current in the lines is regulated in accordance with a measurement result determined by the processing circuitry  11 . 
         [0028]    Alternatively, the gauge may communicate wirelessly with the control station using e.g. a Wireless HART protocol, and use a local power supply with batteries or other means of scavenging energy for autonomous operation. 
         [0029]    Although being shown as separate blocks in  FIG. 1 , several of the transceiver  10 , the processing circuitry  11  and the interface  12  may be provided on the same circuit board. 
         [0030]      FIG. 3  schematically shows an exemplary transceiver  10  suitable for implementing an embodiment of the present invention. 
         [0031]    A signal generator  20  is arranged to generate carrier wave pulses with a well defined and accurately controlled carrier frequency. The accuracy deviation should be less than 1/1000, and preferably less than 1/10000 or better. This may be achieved with a feed back control system, and the signal generator may for example include a phase locked loop, PLL. 
         [0032]    In the illustrated example discussed herein, the pulse duration will be assumed to be about 2 ms, but significantly shorter pulses, e.g. in the order of μs are also possible. It should be noted that in a tank gauging context, pulses of a duration in the order μs or ms, as mentioned here, are much too long to enable pulsed distance measurement with time domain reflectometry, i.e. to receive a reflected pulse and determine its time of flight. The pulses according to the present invention are thus longer than pulses in conventional pulsed radar level gauges, where a typical pulse duration is in the order of ns. 
         [0033]    The average power of the pulses can be in the nW or μW area. However, the duty cycle, i.e. the relationship between the pulses and the intervals there between, should be limited to less than 50%, and is preferably significantly lower, e.g. 5% or even 1% or less. Compared to conventional FMCW, this means that the pulses may be emitted with a relatively high power, without increasing the average power of the measurement cycle. 
         [0034]    The pulses are typically radar pulses, with a carrier wave frequency in the GHz area. The frequency range may be in the order of 10% of the operating frequency, and may be e.g. between 25 and 27 GHz, or between 9.5 and 11 GHz. The number of frequencies N may preferably be determined based on the conditions prevalent in the tank. The duration of the pulses, the duty cycle, and the frequency of the PLL are controlled by the processor  11  (see  FIG. 2 ). 
         [0035]    The PLL  20  is connected to a modulator  21 , arranged to frequency modulate the pulses from the PLL  20 . The modulation can be of various types, including periodical (such as triangular or sinusoidal) or random (noise like). With a periodic modulation, an integer number of modulation periods is preferably contained within each emitted pulse, and the modulation preferably starts at a locked position (such as start at zero for each pulse). A random modulation is preferably pseudo-random. 
         [0036]    In the illustrated case, the modulation is assumed to be periodic, with a modulating frequency f m . The frequency variation of the modulated carrier wave is shown in  FIG. 6 . The modulation frequency should be chosen with respect to the pulse duration to ensure that a complete number of modulation periods (P mod ) are included in each pulse. In the present example, where the pulse length is in the order of ms, the modulating frequency should be in the order of kHz, and may for example be 10 kHz. For a pulse duration in the range of 10 us to 100 ms, the modulating frequency may be in the order of a few kHz to one MHz. 
         [0037]    With reference to  FIG. 4 , the modulation has an amplitude selected to introduce a variation of the carrier wave frequency (modulation width) of δf, which is significantly smaller than the predetermined frequency range of the measurement. As mentioned, the predetermined range of carrier wave frequencies used in the frequency scheme may be in the order of 1 GHz, and the modulation width δf can then suitably be in the order of MHz. 
         [0038]    A circulator  22 , or a power divider such as a Wilkinson Power Divider (WPD), is provided to direct the signal from the signal modulator  21  to the antenna  3  in the tank  5  and to direct a return signal from the antenna  3  to a receiver section of the transceiver  10 . 
         [0039]    The receiver section comprises one or several RF mixers, for mixing the return signal with the transmit signal. The RF mixing may be homodyne mixing, i.e. the return signal is mixed directly with the transmit signal, but it may also be useful to apply mixing with a delayed transmit signal. 
         [0040]    In the illustrated case, the receiver section includes two channels, an in-phase channel (I) and a quadrature channel (Q), each comprising a mixer  23   a ,  23   b . The first mixer  23   a  is provided with the transmitted pulses directly from the PLL  20  (I-channel). The second mixer  23   b  is provided with the transmitted pulses from the PLL  20  via a 90 degrees phase shifter  24  (Q-channel) arranged between the modulator  21  and the mixer  23   b.    
         [0041]    The output from the mixers  23   a ,  23   b  will not be DC level signals, but instead be IF (intermediate frequency) signals including the modulation frequency f m  and harmonics thereof. The receiver further comprises two sets of band pass filters  25   a ,  25   b  and  26   a ,  26   b , respectively, each set being arranged to filter out a desired frequency content. The output from each RF mixer  23   a ,  23   b  is connected to one filter in each set, so that two different frequency contents (primary and secondary) are filtered out from each channel (I and Q). As discussed below, the desired content may be one of the first harmonics of f m . The filtered signals are amplified by amplifiers  27 . 
         [0042]    Each set of filters is then connected to a set of IF mixers  28   a ,  28   b , and  29   a ,  29   b , respectively. These mixers also receive as input the modulation frequency f m  from modulator  21 , via two multipliers  31  and  32 . The multiplier  31  is adapted to provide a multiple M of the modulation frequency f m , corresponding to the harmonic that has been filtered out by filters  25   a ,  25   b , while the multiplier  32  is adapted to provide a multiple N of the modulation frequency f m , corresponding to the harmonic that has been filtered out by filters  26   a ,  26   b . For example, if the first and second harmonics are selected, M=2 and N=3. 
         [0043]    According to one embodiment, the filters  25   a ,  25   b ,  26   a ,  26   b  and multipliers  31 ,  32  are variable, to enable selection of desired frequency content. However, in some cases the filters  25   a ,  25   b ,  26   a ,  26   b  are determined by hardware, and not variable. In such a case, it is the modulator frequency f m , modulating width δf, and factor N that are varied in order to achieve the desired filtering. The practical implementation of the filters may in most cases be digital and may be combined with the mixers. 
         [0044]    The output from the mixers  25   a ,  25   b ,  26   a ,  26   b  will be DC level signals, representing primary and secondary phase values for each channel. These values are each fed to a sequence of processing elements including a low pass filter  33 , an amplifier  34 , an integrator  35  and an A/D converter  36 . 
         [0045]    The filters  33  and amplifiers  34  improve the signal-to-noise ratio of the radar level gauge system. The low pass filters  33  preferably have a bandwidth corresponding to the inverse of the pulse width. In other words, if the duration of the pulse is 2 ms, a suitable filter will be 500 Hz wide. Longer pulses can thus be used to increase the sensitivity (smaller bandwidth) at the expense of the average power. However, if a low duty cycle is desired, in order to achieve a low average power consumption, the pulses should be kept reasonable short, and the bandwidth of the low pass filters  33  must then be wider (with less sensitivity) than a conventional FMCW. The integrators  35  are adapted to integrate the signals over the length of the pulse. 
         [0046]    The order of these elements (more specifically the placement of the A/D converter), will be governed by the choice of components, i.e. if they are analogue or digital components. The digital output is supplied to the processor  11  for storage and processing which will be described below. 
         [0047]    It should also be noted that the topology of the transceiver illustrated in  FIG. 3 , with two parallel receiver channels (I and Q), each being divided into two parallel paths (N-filter and M-filter) is not to be viewed as limiting to the concept. 
         [0048]    For example, an equally useful alternative may be to provide the primary and secondary phase values in a sequential fashion, i.e. having only one filter path for each channel, and alternating the settings of this path. In  FIG. 3  that would mean that there would be only two sets of filters  25 ,  26 , amplifiers  26 ,  27 , mixers  28 ,  29 , and processing elements  33 - 36 , instead of four. 
         [0049]    Indeed, a sequential approach may be used also for the RF mixing by mixers  23   a ,  23   b , even further reducing the number of components, at the cost of increasing the number of pulses per measurement. 
         [0050]    Depending on the type of frequency modulation introduced by the modulator  21  (sinusoidal, triangular, random, etc.) the IF signals output by mixers  23   a ,  23   b  will have a distance dependence and will be spread over different harmonics. 
         [0051]    If the transmitted signal has a sinusoidal modulation with modulation width δf, the received signal after the homodyne mixing can be described by Bessel functions as harmonics of f m  as illustrated in  FIG. 6 . 
         [0052]    The argument in the Bessel functions is X=2πhδf/c, where h is the distance to the reflecting surface and c is the speed of light in vacuum. With increasing distance h (and thus increasing X) the received signal is spread over more harmonics. If, for example, the modulation width δf is 15 MHz and the distance h is 10 m, the argument X will be ˜2.0 and most received signal will occur in f m . The distance dependence of J1(X) (for X smaller than 2) has the same feature as the high-pass filtering in conventional FMCW-systems, i.e. to reduce the dynamic range ˜20 dB by having an amplitude distance dependence counteracting the 1/h-dependence. If a fixed  6   f  is used it will be limited to 8-10 MHz to measure up to 20-25 m without losing too much amplitude by spreading power to too many other harmonics. 
         [0053]    It is also possible to pick out the second tone, 2f m , which is more efficient to suppress close echoes and allows 12-15 MHz modulating width in applications to 20-25 m. The modulation width is adjustable depending on the used distance interval. Depending on the type of modulation it can either be different for different installations or be adjusted during the measurement. 
         [0054]    An alternative to sinusoidal modulation is triangular frequency modulation which has different properties, and may be more efficient than sinusoidal modulation. 
         [0055]    The resulting Fourier coefficients for the even harmonics (2f m , 4f m , etc) at triangular modulation are shown in  FIG. 7 . The coefficients for 1, 3 etc. are very similar to those for 2, 4 etc., but with the peaks between the peaks for even harmonics. The peaks are all close to 1 so if the closest harmonic is filtered out nearly all signal power is used. The argument X is the same as above, so a wider δf may be used in the case of triangular modulation. At 50 MHz modulation width the full axis in the diagram above (0&lt;x&lt;20) corresponds to 0-30 m distance. With 50 MHz modulation width the spectral density can be reduced 17 dB without reducing the received signal. This reduction will remain also for long pulses. The lower spectral density for the same power may be used to increase the power. 
         [0056]    By determining the amplitude of two different harmonics, in two separate processes, and comparing them with each other, a rough estimation of the distance may be provided. For example, in  FIG. 6 , it is clear that when the amplitude of the first two harmonics are the same (i.e. a quotient of 1) then X is slightly greater than 2. From a known value of X, the distance h can be calculated using the relationship above. 
         [0057]    Operation of the radar level gauge illustrated in  FIGS. 2 and 3 , and in particular the process and in particular the process to provide a distance approximation, will be further described with reference to  FIG. 5 . 
         [0058]    First, in step S 1  a pulse of a determined duration, e.g. 2 ms, is generated by the PLL  20  and transmitted as signals ST into the tank by the antenna  3 . 
         [0059]    In step S 2 , the transmitted electromagnetic signals ST are reflected at impedance transitions in the tank  5 , including the surface  7  of the product  6  contained in the tank  5 , and are returned to the transceiver  10  as echo signals SR by the antenna  3 . The return signal SR is received by the two channels (I and Q) in the receiver side of the transceiver  10 . 
         [0060]    In step S 3 , the return signal SR is mixed with the transmit signal in mixers  23   a  and/or  23   b , to form an IF signals. 
         [0061]    Then, in step S 4 -S 5 , amplitudes of two different harmonics of the IF signal are determined (in the illustrated example the processed output from mixers  28   a  and  29   a , or alternatively from mixers  28   b  and  29   b ). Finally, in step S 6  these two amplitudes are compared to each other to determine a rough distance approximation. 
         [0062]    As explained above, one useful relationship is simply the quotient between the two amplitudes. Such a quotient may indicate a specific distance, based on knowledge of the distance dependence of the two harmonics. 
         [0063]    It should further be noted that it may be advantageous to determine amplitudes for two or more different harmonics in each channel. For example, amplitudes for the first and third harmonics can be determined for one channel, and amplitudes for the second and fourth harmonics for the second channel. This may enable an even more robust approximation. 
         [0064]    The distance approximation method outlined herein may advantageously be implemented in a radar level gauge adapted to operate according to the principles disclosed in U.S. Ser. No. 12/981,995. 
         [0065]    According to that principle, the primary (or secondary) phase values discussed above are recorded in memory  13  for all frequencies in a predetermined frequency scheme. Each phase difference value (which is in the range 0-2π) is correlated to an expected phase difference value, calculated based on a previously detected distance. The difference between the expected phase and the actual, detected phase corresponds to a distance offset. In principle, one such offset, resulting from a single frequency, is sufficient to provide an updated distance detection. However, the uncertainty in such a detection will normally be too significant to provide satisfactory reliability, and some kind of statistical analysis will normally be required. 
         [0066]    A rough approximation provided by the present invention, for each transmitted pulse, can be very advantageous to make such analysis more robust and reliable. Under certain conditions, this approximation may eliminate the need for an initial distance detection as discussed in U.S. Ser. No. 12/981,995. 
         [0067]    Alternatively, or in combination, the approximation discussed herein may be used to verify a more accurate distance determination, e.g. obtained according to U.S. Ser. No. 12/981,995. 
         [0068]    The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the principles of the present invention do not require I/O processing, but are in fact applicable also to a single channel system. Further, the details of the transceiver circuitry may be modified in a number of ways, without departing from the inventive concept.